The Mechanics and Physics of Modern Grain Aeration Management

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The MECHANICS and PHYSICS of MODERN GRAIN AERATION MANAGEMENT

1355 FMFrame Page 2 Monday, August 6, 2001 1:20 PM

The MECHANICS and PHYSICS of MODERN GRAIN AERATION MANAGEMENT

Edited by

Shlomo Navarro Ronald Noyes

CRC PR E S S Boca Raton London New York Washington, D.C.


Library of Congress Cataloging-in-Publication Data The mechanics and physics of modern grain aeration management / Shlomo Navarro and Ronald Noyes, editors. p. cm. Includes bibliographical references and index. ISBN 0-8493-1355-4 (alk. paper) 1. Grain aeration—Management. I. Navarro, Shlomo. II. Noyes, Ronald T. SB190 .M42 2001 633.1′0468—dc21

2001035405

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $1.50 per page photocopied is paid directly to Copyright clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1355-4/02/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2002 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1355-4 Library of Congress Card Number 2001035405 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper


Dedication This book is dedicated to Juliet Navarro and Zona Noyes, who provided much-needed support throughout the years of development and strongly encouraged us to complete this book. Without their continued patience and understanding, this project would not have been possible.



Foreword The discontinuation of registration of many highly toxic pesticides used until recently and the ban on methyl bromide (associated with the depletion of atmospheric ozone) have generated an immediate worldwide need for alternative technologies to solve grain storage problems. This manual presents the most advanced theoretical concepts and practical solutions to grain management through the effective use of various forms of aeration as they affect grain storage science and technology. This book describes and illustrates the many variations in aeration practice that are required for effectively cooling grain, for controlling storage insects, and for improving the storability of grain commodities while minimizing residual pesticides and molds. The beginning of this collaborative writing partnership between Shlomo Navarro and Ronald Noyes began in 1991 to 1992, when Navarro submitted the original outline for the handbook to CRC Press. CRC asked Noyes to review the outline, and Noyes suggested the need for more applied chapters in the book. The authors communicated on a series of ideas related to grain storage technologies, including aeration, and on the need for this aeration handbook. An expanded outline of the handbook developed during subsequent meetings in 1993 and 1994. After the beginning of preparatory collaborations by the two authors in 1997, additional topics of major importance with other collaborating authors were identified and incorporated to make this a more complete coverage of the many aspects of forced air and gas movement that collectively fall within the scope of aeration. The expanded coverage of all aspects of aeration contained in this manual is deemed necessary to provide guidance for practicing and future grain storage engineers, grain technologists, and entomologists, their technicians, and bulk grain managers, who are destined to bear responsibility for the quality of cereals and their products throughout the world. Because the scientific community uses the metric system, metric units are employed predominantly throughout the book. However, because of its expected extensive use in the U.S., we have adopted American English as the language of the book and have periodically incorporated dual units of measure. Thus, metric units are primary; and English units are secondary in some sections (especially in the “practical” handbook core Chapters 5, 6, 7 and 8) to make working examples more user friendly to U.S. engineers, grain storage manufacturers, and commercial grain industry personnel. Dual units (metric/English) are also used where data tables were abstracted from other English unit sources such as dissertations, research reports, and technical papers. In addition to the conversion of English to metric units, another difficulty that remained was to bridge between the various units used to describe airflow, even within the metric system. As an example of dual units, we cite the conventionally accepted pressure units, inches water column (in w.c.) used by past generations of engineers, and the new generation that uses only Pascal (Pa) units. To conform to the internationally accepted SI units system, we have elected to use Pa for pressure systems. For airflow rates we used three principal units of airflow — cfm/bu, (m3/h)/tonne for practical discussions of airflow, and L/s.m3 for theoretical scientific discussion of research data and practices. Although the units cfm/bu and L/s.m3 reflect accurate definitions of airflow rates, the unit (m3/h)/tonne was included because it reflects a more meaningful unit used in daily practice in grain storage technology worldwide. This approach was adopted primarily because the grain storage manager usually knows the weight (mass) of grain in the bins but not the volume of grain. He requires additional calculations to convert weight to volume. We believe the readers will find these conversions useful in using the recommendations of the text for local use when converted to the units adopted by the reader. The editors and co-authors are convinced that this book has provided us with a unique opportunity to collectively summarize the state of the art of grain storage technology throughout the world. The book is arranged to provide the reader with an Introduction that includes a general background of grain storage technology. Chapter 1 reviews the objectives and describes what is


expected by the application of the various forms of aeration. Chapter 2 includes basic approaches adopted toward the effects of aeration on the stored grain ecosystem, then describes the physical process that occurs during heat and mass transfer in non-aerated grain bulks. Chapters 3 and 4 deal with air properties (psychrometrics) and grain bulk properties, respectively. To enhance the description and design applications of aeration systems in Chapter 5, appendices include physical design considerations that help the practicing engineer to apply his knowledge to the design and operation of a wide range of aeration systems. Chapters 6, 7, and 8 deal with experimental aeration systems, the operation of aeration systems, and specialized supplementary aeration systems. The potential importance to the grain industry of chilling grain with refrigerated air is considered separately in Chapter 9. Special attention is also given to evaluating aeration system efficiency in Chapter 10. Finally, because of the special design requirements involved, Chapter 11 is dedicated to modeling of air distribution in aeration.


Preface The preparation and release of this publication is timely because it details the most practical available technology that is designed to cope with environmental pollution resulting from historic and traditional conservation practices in grain storage facilities. Its release at the beginning of the 21st century coincides with the preferences of many consumers for grains and seeds that are free of pesticide residue. This handbook focuses on the protection of grain and other bulk products from deterioration by insects and molds through diligent sanitation and temperature management practices. It includes contributions by distinguished researchers currently active in the field of grain storage with particular expertise in aeration and cooling technologies of grain stored in bulk. Their joint experience is derived from research work on four continents and from travels and field experience throughout the world. The original impetus for the preparation of this volume was a modest, state-of-the-art publication prepared for the FAO (Food and Agriculture Organization) of the United Nations entitled Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52 (Navarro and Calderon, 1982). This FAO publication documented the current information on aeration technology available at that time. Particular emphasis was placed on the inherent advantages of using aeration in subtropical climates. The outline of the 1982 publication was prepared during an FAO mission by Shlomo Navarro to Cyprus to assist the Cyprus Grain Commission and the Cyprus Ministry of Agriculture. The objective of the mission was to disseminate the grain storage management technology to strengthen the existing infrastructure in Cyprus in the use of aeration and chilling of grain by refrigerated air. Dr. Navarro remains indebted to the late Geoff G. Corbett, senior officer, Storage of Food Crops and Inputs, FAO Agricultural Research Service, for his encouragement in the preparation of the FAO publication. Geoff Corbett will always be remembered for his contribution to disseminating advanced grain storage technologies throughout the world, particularly in developing countries.



Acknowledgments We especially thank our colleague Dr. Ezra Donahaye, Department of Stored Products, Israel Agricultural Research Organization (ARO), for his careful reading and accurate review of this publication. His excellent comments have resulted in significant improvements in the scientific accuracy and functional application of aeration throughout this book. The excellent staff of CRC Press LLC, particularly John Sulzycki, senior editor, Sara Seltzer, production manager, and Madeline Leigh, project editor, are to be commended for their diligence and painstaking attention to detail that has provided technical continuity of thought and consistency in reading style throughout the text. CRC has indeed homogenized the writings of authors from four continents to the great benefit of and better understanding by the reader. CRC has been most supportive of the authors during the duration of the writing and review phases of preparation of this progressively enlarging work in progress. We are extremely grateful to Dr. Guray Ahmet Ferizli, visiting scientist from the University of Ankara, Turkey, who contributed to the understanding of the text through his illustrative work. Dr. Ferizli prepared many figures for Chapters 1, 5, 6, 7, and 9. We also thank Dr. Sam Angel of the ARO for his invaluable help in reviewing drafts of various chapters, improving style, and retrieving literature. We are also grateful to Dr. Simcha Finkelman for helping to finalize the chapters and for annotating lists of references. Miriam Rindner, Avi Azrieli, Rafael Dias, and Dr. David HoveveySion of the Department of Stored Products, ARO, were of tremendous help during the different phases of the preparation of this publication by preparing new graphs and providing literature and data. Our special thanks particularly are extended to Dr. Svetlana Fishman, mathematician at the Department of Statistics, Israel ARO, for her patience and competence in reviewing the many mathematical equations for technical accuracy and application in various chapters. We also profoundly thank all contributing authors and reviewers of this book. Without their enthusiasm, perseverance, loyal support, and dedication to the vision of this new stand-alone worldwide handbook of aeration, this volume could not have been written.



About the Authors Dr. Shlomo Navarro, Ph.D., is a principal scientist in the Agricultural Research Organization (ARO), Volcani Center, Bet Dagan, Israel. Dr. Navarro was instrumental in organizing the Department of Stored Products and has served as head of the department. He has chaired numerous professional and scientific committees at the ARO, including the Research Projects Evaluation Committee on Food Technology and the Stored Products Advisory Board of the Ministry of Agriculture. He has held numerous positions, among them director for academic affairs and director for international cooperation at the ARO. Currently he serves as chairman of the Committee for Promoting Commercialization of R&D Applications and deputy director for international relations, ARO. He has conducted postharvest research in tropical and subtropical countries of the world. In 1972, Dr. Navarro cofounded the Permanent Working Committee for the International Conference on Controlled Atmosphere and Fumigation (CAF). He has been Secretary of CAF since its inception. He has edited two books of proceedings of the CAF Conferences and one book of the International Conference on Stored Product Protection. His Handbook of Aeration of Grain in Subtropical Climates for the FAO was the leading authority for aeration in tropical and subtropical grain stores and was the forerunner of this book. He has authored or coauthored 287 technical articles in scientific journals, conference proceedings, and books. He holds five patents on developing storage structures and technologies. Dr. Navarro is the leading authority on hermetic storage in semi-permanent plastic storage structures for use with grains, seeds, dried fruits, and other stable bulk products. Dr. Ronald Noyes, P.E., Ph.D., is professor, Stored Product Management, BioSystems and Agricultural Engineering Department, Oklahoma State University, Stillwater, OK. Since 1985 he has developed advanced grain storage automatic aeration controls and recirculation fumigation systems for grain elevators in Oklahoma and throughout the U.S. His work in adapting recirculation fumigation to steel bins and concrete silos in the U.S. has improved fumigation efficacy while minimizing required dosages. During his tenure on the faculty of Purdue University in the mid 1960s, Dr. Noyes conducted the first field research of Dryeration, a high-speed aeration process used to improve drying efficiency while maintaining higher grain quality from high-temperature grain dryers. From 1968 to 1985, Dr. Noyes was chief engineer and vice president, engineering for a U.S. grain dryer manufacturer, where he invented and patented an energy-saving process that reduced fuel consumption of continuous flow column type grain dryers by 40 to 50%. He holds six U.S. patents on grain dryers. Dr. Noyes is a member of the Permanent Working Committee of the International Conference on Controlled Atmosphere and Fumigation (CAF) and consults internationally on grain storage aeration and fumigation system design and management. Mr. David Armitage C. Biol., M.I. Biol., is a senior scientific officer and contract manager with the Ministry of Agriculture, Fisheries and Food’s Central Science Laboratory in York, where he leads a small sub-team on integrated commodity management. He has worked on aeration since 1971, initially with Mr. Norman Burrell, the pioneer of the technology in the U.K. Mr. Armitage has contributed over 60 papers on storage technology, specializing in effects of control measures on insects, mites, and fungi. He is a principal author of the U.K.’s primary advisory sources on storage, The Grain Storage Guide‚ and the integrated grain storage manager software. Currently he is on the organizing committee of the 8th International Working Conference on Stored Product Protection to be held in 2002 in York, U.K. He is also known as an aquarist, specializing in a family of air-breathing fish that he has studied in their natural habitats in Africa and Asia. Dr. Digvir S. Jayas, P. Eng., P. Ag., is associate dean for research, Faculty of Agricultural and Food Sciences, University of Manitoba, Winnipeg, Manitoba, Canada. Dr. Jayas joined the University of Manitoba in 1985 as a member in the Department of Agricultural Engineering (now Biosystems Engineering), serving from 1997 to 1999 as department head. His research objective is to reduce qualitative and quantitative losses of stored grain. As an interdisciplinary research


leader, he coordinates work of entomologists, agricultural engineers, and mathematicians into new methods of measuring, analyzing, and modeling grain properties, and heat and mass transfer in stored grain — the basis for non-chemical insect control methods. His physical process research has led to a better understanding of biotic and abiotic variable interactions. Dr. Jayas has made important progress in the use of digital image processing for grain type classification. He has authored or coauthored over 300 technical articles in scientific journals, conference proceedings, and books. He is co-editor of the book Stored-Grain Ecosystems and coauthor of Grain Drying: Theory and Practice. He has received awards from the Canadian Society of Agricultural Engineering, the American Society of Agricultural Engineers, and the Association of Professional Engineers of Manitoba. Dr. Dirk E. Maier, Ph.D., P.E., is associate professor and extension agricultural engineer in the Agricultural and Biological Engineering Department, Purdue University, West Lafayette, Indiana. In addition to maintaining an active technology transfer and continuing education program, Dr. Maier conducts research on postharvest engineering and value-added processing of agricultural crops and biological products. His research also includes ecosystem modeling, stored products protection (IPM), alternative crop storage systems, dehydration of biological products, bulk material (grain, feed) handling, facilities design and simulation, and feed manufacturing. Dr. Maier is cofounder of the Purdue Grain Quality Team and is director of Purdue University’s Postharvest Education and Research Center and Grain Quality Laboratory. He is a member of the Editorial Board of the Journal of Stored Products Research. Dr. William E. Muir, P. Eng., is a professor in the Department of Biosystems Engineering, University of Manitoba, Winnipeg, Canada. He has been teaching and conducting grain storage research at the university since 1967. Dr. Muir provided the engineering input to the grain storage ecosystem studies initiated at the Canada Department of Agriculture Research Station and at the University of Manitoba. His research led to analysis of the stored grain bulk as a man-made ecosystem impacted by several biotic variables (the living grain, several species of insects, mites, and microorganisms) and abiotic variables (time, temperature, moisture content, carbon dioxide, oxygen, aeration, etc.). He has carried out many studies on the effect of aeration on the storability of grain, on in-bin drying systems, on modeling temperature distribution in grain bulks, on the influence of ambient temperature on the successful operation of aeration systems, and on heat transfer models. His multidisciplinary approach was first presented in an international symposium in 1971 and the resulting co-edited book, Grain Storage: Part of a System. Dr. Graham Thorpe, Ph.D, D.Eng., obtained his bachelor’s degree in chemical engineering from the University of Nottingham, U.K. After working one year as a process engineer, he completed his Ph.D. at the University of Nottingham on mathematical modeling of pneumatic conveyor dryers. After obtaining his Ph.D., he held the position of research scientist at CSIRO, Division of Mechanical Engineering, Melbourne, Australia, where he designed and tested refrigerated grain storage systems with storage capacities up to 15,000 tonnes. He also designed and evaluated the performance of a fluidized bed grain dissinfestation unit that treated over 100 tonnes of grain per hour. Professor Thorpe contributed to theoretical development of stored grains engineering with Professor Stephen Whitaker at the University of California, Davis. He applied the “volume averaging” theorem to derive from first principles the rate at which moisture diffuses through bulk stored grains and the constraints that must be satisfied for grains and intergranular air in thermodynamic equilibrium. Professor Thorpe has also developed complex mathematical models of bulk stored grains, including the effects of natural convection on moisture migration. Professor Thorpe is employed by Victoria University, Melbourne, where he is developing a novel open cycle desiccant bed grain cooling system and applying the computational fluid dynamics methods to grain storage systems designs. The new cooling system is predicted to cool one tonne of grain per 0.5 kW of energy. In 1998 the University of Melbourne awarded Professor Thorpe the Doctor of Engineering in recognition of his distinguished works in postharvest technology.


Technical Reviewers of Book Chapters We are especially grateful for the excellent technical reviews of the chapters of this book by the scientists listed below. Each of the reviewers is a highly recognized professional in grain and seed drying and handling, storage research, teaching, and extension programs in their states or regions at leading universities and government research stations in the U.S. and Israel. Without their diligent and thorough reading, their in-depth insight and understanding of grain and seed storage systems, and numerous valuable suggestions for improving the clarity of the ideas and illustrations, numerous important ideas and concepts in the book would not have been as well defined or clearly understandable. We very much appreciate the great contribution in time and energy devoted to this text by Dr. Jonathan Donahaye, which required an effort far beyond his heavy research workload at the Volcani Center. A special and significant contribution was provided by the mathematical review by Dr. Svetlana Fishman, a scientist with the ARO Department of Statistics, Volcani Center. Her review of formulae and mathematical modeling equations was essential to confirming the validity and technical soundness of the text. We recognize that there are many demands on the time of these busy, well-known professionals. This section is to publicly acknowledge their sacrifices of time and willingness to assist in this important work with only the satisfaction of making a contribution to the improvement of grain storage worldwide through improved aeration systems as their primary reward. Dr. Shlomo Navarro, Editor and Author Dr. Ronald Noyes, Co-Editor and Author Mr. David Armitage, Author Dr. Digvir Jayas, Author

Dr. Dirk A. Meier, Author Dr. William. E. Muir, Author Dr. Graham Thorpe, Author

Technical Reviewers and Their Chapters General Technical Review

Individual Chapter Reviews

General Editorial Review of Chapters 3 to 9 Dr. E. Jonathan Donahaye (Emeritus) Department of Stored Products Agricultural Research Organization The Volcani Center P. O. Box 6 Bet Dagan, 50 250 Israel

Chapters 1 and 2 Dr. James L. Steele (Retired) USDA Agricultural Research Service U.S. Grain Marketing and Production Research Center 1515 College Avenue Manhattan, KS 66502 U.S.A.

Specific Technical Review Chapters 3, 4, and 11 Dr. Svetlana Fishman Department of Statistics Agricultural Research Organization The Volcani Center P. O. Box 6 Bet Dagan, 50 250 Israel

Chapter 5 Dr. Kenneth J. Hellevang, Professor Agricultural and Biosystems Engineering Department P.O. Box 5626 Fargo, ND 58105 U.S.A.


Chapter 6 Dr. Carl J. Bern, Professor Agricultural and Biological Engineering Department 217 Davidson Hall Iowa State University Ames, IA 50011 U.S.A.

Chapter 8 Dr. William F. Wilcke, Professor Biosystems and Agricultural Engineering Department 1390 Eckles Avenue University of Minnesota St. Paul, MN 55108 U.S.A.

Chapter 7 Dr. Joseph P. Harner, Professor Department of Biological and Agricultural Engineering Seaton Hall Kansas State University Manhattan, KS 66506 U.S.A.

Chapter 9 Dr. Fred Bakker-Arkema, Professor Emeritus Agricultural Engineering Department Michigan State University 211 Farrall Hall East Lansing, MI 48824 U.S.A.

Chapter 10 Dr. Christopher L. Butts USDA Agricultural Research Service National Peanut Research Laboratory 1011 Forrester Drive, S.E. Dawson, GA 31742 U.S.A.

Shlomo Navarro and Ronald Noyes, Editors


Introduction Shlomo Navarro and Ronald Noyes

1. INTRODUCTION Aeration is the most widely used and one of the safest technologies for preserving grain without the use of chemicals. However, unless the necessary knowledge is available on planning and operating grain aeration systems, this technology cannot be successfully implemented. Although aeration is widely applied, its use is often misinterpreted and the objectives of aeration are not achieved. This book deals with mechanical and physical aspects of aerating grain. But if the biological factors and the ecosystems of the grain bulk are not understood, the information on the physical design and operation of aeration presented in the book would not be complete. Therefore, this introduction presents a short overview of the grain bulk ecosystem, with particular emphasis on the biological agents — namely, insects, mites, and microflora — and how they interact with the dormant yet living grain kernels during storage.

2. HOW AERATION FITS IN THE GRAIN BULK ECOSYSTEM In the context of the grain bulk ecosystem, grain is considered as a living organism even though its biological activity is extremely low. This low level of activity is due to the prerequisite for conservation of grain in storage — that it should be stored at low moisture contents. Grain that is termed dry has moisture at a level that is safe for storage and, in consequence, it remains in a dormant condition. Although such grain may not always possess all the viable characteristics of seeds, it is still considered part of the living composition of the grain bulk ecosystem. With respect to its interaction with other biotic agents, particularly insects and microflora, it serves as a host for the development of these noxious organisms. Rodents, although part of this ecosystem, cause very little damage in modern structures with well-designed grain storage facilities. In practice, most structures exclude rodents from the ecosystem since they cannot reside within the depth of the grain bulk or survive the extreme dryness prevailing in the grain mass without a supply of water. However, for grain stored in bags, rodents do pose a serious problem, especially in inadequately constructed or poorly maintained storage facilities. Although bag storage is still the prevalent method of storing grain in developing countries, only the bulk storage of grain is considered here. In contrast to rodents, insects and mites are considered natural residents of the ecosystem because of their abilities to enter most storages easily and reproduce within the range of low humidities characteristic of the grain bulk. Most insect species can survive very low humidity conditions; and at temperatures that enable reproduction, they can become the predominant organisms of the grain bulk ecosystem. Their metabolisms are adapted to generate metabolic water, and they can live and reproduce without the assistance of an external water supply. Mites favor more humid conditions and prefer grain bulks in which the humidity of the intergranular air is close to or equivalent to the critical grain moisture content. Although similar to insects in that they can survive and reproduce freely within the grain bulk, their occurrence and economic importance is limited to countries where storage of grain is carried out at higher levels of moisture — though often below the critical moisture content. Microflora will develop only when the water activity (aw) in the grain mass is sufficiently high. An equivalent relative humidity of


70% in air is considered as a critical humidity for the development of xerophytic microorganisms in the grain. Thus, grain moisture contents that are in equilibrium with the surrounding air containing a lower relative humidity than 70% are considered safe for each grain variety. Development of microorganisms is a factor in the ecosystem that arises only when adverse storage conditions permit excessive moisture accumulation in the grain bulk or when grain is stored initially above permissible safe moisture contents required for its preservation. Consequently, the main reason for drying or reducing the moisture content of grain is to prevent the activity of the microflora. These organisms consist of fungi, yeasts, and bacteria. Fungi live and reproduce best at medium aw levels (70 to 80% relative humidity), whereas yeast and bacterial development require humidities higher than 85% in the intergranular air. Most grain insects and mites are of tropical or subtropical origin and favor the prevailing temperatures of warm climates. Therefore, a relatively rapid reduction in temperature of the immediate environment is an important intervention that tends to inhibit their biological activities. Thus, the primary objective of aeration is to alter the microclimate of the ecosystem by reducing its temperature, thereby creating micro-environmental conditions unfavorable to the development of all organisms that are noxious to grain stored in bulk. The general information that follows will help readers to understand the principles of grain storage, the background of modern storage technologies, and the different aspects of grain conservation.

3. BACKGROUND ON STORAGE OF CROPS, FOOD, AND FOODSTUFFS Growing crops and protecting them until ready for consumption have been major preoccupations of mankind since the inception of agriculture. Storage is an essential interim operation in the food pipeline that moves crops from producer to processor and processed foodstuffs from processor to consumer. It equilibrates the quantitative fluctuations or surges in supply between harvests that create the imbalance of supply and demand. As the major consumer of cereal food and pulse crops, the human population was estimated to be about 5.3 billion in 1990; and projections indicate growth to 8.1 billion in 2025. Dependence on cereals for food energy has decreased in developed countries. However, 53 developing countries still derive 40% of all food energy from cereals. The present demand has caused a serious reduction in world cereal stocks, especially in the major export countries. With the steady world population growth, global food production has scarcely kept pace with increased demand. Surpluses in industrialized countries are in striking contrast to the food shortages in many developing countries. There are still threats of famine in countries where natural disaster and internal strife combine to destroy the agricultural infrastructure. Today, hunger threatens the lives of about 800 million people in the developing world, with approximately 60% of them living in Asia. People suffer from food shortage or malnutrition — especially in the poorest countries, where agricultural production is never in surplus, where suitable grain storage facilities are inadequate or nonexistent, and in regions subject to extreme climatic fluctuations from one year to the next. Durable foodstuffs with low moisture content form the bases for most human diets precisely because these commodities can be stored for extended periods and are continuously available, provided there is no serious insect infestation or moisture damage losses. However, losses occur at every stage of food handling and storage. These losses may be quantitative, qualitative, or both. The magnitude of losses is highly variable; in severe cases they may even reach 100%. Qualitative losses are more difficult to evaluate than quantitative ones. Qualitative losses may consist of changes in physical appearance, nutritional degradation, loss of germination, insect infestation, presence of insect fragments or filth, contamination by mold, or development of mycotoxins. Some of these factors are difficult to detect visually.


In developed countries, qualitative aspects of food loss are of greater importance than the quantitative ones. In these countries cereal grains are stored in large centralized bulk storage facilities or on-farm in bulk. Under these conditions quantitative losses are generally at low levels so that further loss prevention measures are not cost effective. Losses of biological origin, such as grain or insect respiration or limited drying due to insufficient aeration of grain, are common in storage. Quantitative losses on an annual basis are usually less than 1% in developed countries. Developing countries are characterized by small-scale farming, where deficiencies in handling and storage methods and warm and humid climatic conditions often promote rapid deterioration of the stored foodstuffs. In developing countries the major portion of grain and pulses (sometimes up to 80% of the national production) is kept on the farms for home consumption. Post-harvest losses of food grain in developing countries have been conservatively estimated during the 1980s at 10 to 15% by the FAO’s Special Action Programme for the Prevention of Food Losses. For example, losses of corn due only to insects in farmers’ stores in Nigeria, Swaziland, and Kenya were in the order of 6 to 10%. In recent decades, major efforts have been devoted to improving storage conditions of cereal and pulse crops and reducing losses in tropical countries. Past attempts at introducing state-of-theart storage structures into several developing countries for this purpose have failed. Many such “white-elephants” stand empty, deteriorated and abandoned. However, storage systems that are more suitable for local climatic and farming conditions have also been widely introduced, which has enabled the successful transfer and updating of modern conservation and control technologies with consequent reduction in storage losses. Reduction of storage losses at the small-scale and subsistence farmer levels has proved to be far more difficult than in the commercial or public sectors because the available storage conservation technologies are costly and not applicable to most of the traditional storage methods unless radical changes are made. Also, it is difficult to educate and transfer new storage technology information to large numbers of farmers in remote farming districts. Therefore, new solutions must be found that are appropriate to the local conditions and acceptable to the societies into which they are to be introduced. These new storage technologies must be demonstrated to be physically and economically practical, and a means for transferring the storage methodology to users at the local level must be developed. In spite of the advances recorded in many fields of modern agriculture and particularly a changing approach to pest control, fumigation has remained a mainstay for control of stored product insects. However, it is worth noting that of the 14 fumigants listed some 20 years ago, only two remain in regular worldwide use today — namely, phosphine and methyl bromide. Methyl bromide (MB) is characterized by its lethal effect within very short exposure times, such as 4 h to 24 h. Insect resistance to this fumigant has not been recorded in the field. In contrast, phosphine is a relatively new fumigant that is extremely widespread and popular, particularly in developing countries, because of its ease of application in comparison with MB. Phosphine has the distinct disadvantage of requiring long periods of exposure, with a minimum of 5 days now recommended. A serious threat to this fumigant is the increasing number of reports of insects that have developed resistance over the last decade. MB is regarded as the main anthropogenic compound that is depleting the ozone layer. It is widely used as a fumigant in agriculture, for pest control in structures, stored commodities, and quarantine treatments. Its main uses are for soil sterilization (about 72% of total usage), disinfestation of perishables (9%), disinfestation of durables (14%), and against pests in structural fumigations (5%). Presently there is no available alternative to MB for short-exposure fumigations. Development of alternatives to MB is likely to be costly, and many developing countries will not be able to afford evaluations of these alternatives without assistance. Regulatory actions to reduce and eliminate the use of MB have been taken recently by the United Nations Environment Program (Montreal Protocol) and by the U.S. Environmental Protection Agency (EPA). In October 1998, the U.S. Congress made specific changes to the Clean Air Act to “harmonize” the U.S. phase-out of MB


with the Montreal Protocol schedule for developed countries. The EPA has taken the necessary regulatory steps to implement these changes. The new MB schedules include a 25% reduction from the 1991 baseline in 1999, a 50% reduction in 2001, a 70% reduction in 2003, and a 100% reduction in 2005. For developing countries, the agreed schedule is reduction in consumption by 20% by 2005 with total phase-out by 2015. Under present agreements, there are exemptions for all countries from controls on MB when used for quarantine, preshipment fumigations, and for some critical agricultural uses yet to be defined. Contact insecticides may provide persistent protection against reinfestation. They can be applied directly to grain, but they are not normally registered for use on processed foodstuffs. Contact insecticides include synthetic chemicals, insect growth regulators, plant extracts (botanicals), and inert dusts. One major constraint associated with their use is the presence of chemical residues in the treated commodities. Resistance also is a major problem, while the high cost of registration is a constraint to the development of new products. Among the non-chemical alternatives of physical control methods, aeration of bulk grain plays an important role. Other non-chemical alternatives include the use of modified atmospheres, heat, irradiation, and physical removal of insects. Treatment with controlled or modified atmospheres based on carbon dioxide and nitrogen offers a potential alternative to fumigation with toxic gases for insect control in all durable commodities. However, these intensive control methods are not suitable to a large percentage of existing bulk storages because of relatively high application costs and lack of sealed storages. As a general rule, except aeration and chilling by refrigerated air, cold treatments are not used for disinfestation of large masses of durables. A major problem encountered with cooling or chilling is the time needed to cool such masses. For this reason, cooling is generally used to prevent reproduction and reinvasion of pests in grain bulks by applying aeration and refrigerated aeration for cooling, rather than as a disinfestant. Heat treatment is one of the very few pest control options for grain that is capable of matching the speed of treatment afforded by MB. Fluid bed heating systems for bulk grain have been developed to a commercial prototype stage. But heat treatment is also quite expensive to apply to large bulks of relatively low value grain. The electromagnetic spectrum also offers a series of possibilities for processed foods. The two extreme ranges — longwave radio frequencies and ionizing irradiation — have detrimental effects on insects, whereas medium wavelengths, especially in the range of visible light, are used for insect monitoring purposes. Irradiation is already in use commercially for shelf-life extension of some fresh commodities and for disinfestation. The food industry is concerned about consumer acceptance of irradiated food products. The large initial capital expenditure for plant construction also poses a serious constraint. Physical removal of insects, sanitation, and improved packaging methods should all be regarded as means to assist pest control in stored commodities. Biological methods, including the use of microbiological control agents and pheromones, are at an early stage of implementation. Although pheromones are used increasingly for monitoring purposes, their widespread application as control measures is not expected in the near future.

4. THE BENEFITS OF AERATION IN PRESERVATION OF STORED GRAIN AND SEEDS The world grain industry, particularly the storage sector, is undergoing constant changes and adaptations to rapidly evolving agricultural practices as well as technological and administrative developments. Many countries have adopted deregulation processes that have significantly influenced the attitudes and decision making of grain growers regarding the subsequent handling and destinations of their newly harvested crops. In some countries, growers choose to deliver their harvests directly to the grain cooperatives or grain growers associations, centrally or regionally. In


other countries, on-farm storage and direct delivery to consumers or merchants are the preferred options. Globally, the process of grain production and storage management is under the influence of these changing realities. The endeavor to provide “food security” for all is dependent upon improved storage technologies at all levels that enable the reduction of both the quantitative and qualitative losses of grain in storage. A significant development over the past 10 years is the fast-approaching phase-out of MB. This has resulted in a significant increase in the number of publications dealing with its alternatives. In particular, the search for non-chemical methods of insect control has increased in intensity. Additionally, a public awareness has arisen with respect to pesticide residues in food and their harmful influence on the environment. Public pressure is increasing to encourage legislators to close every loophole that might enable the contamination of food with toxic materials. Consequently, future prospects for using new fumigants on stored food products remain very limited. Many research groups are now in a “rethink” mode as a direct result of pressure from national and international legislative bodies and import country grain purchase contract restrictions. These authorities are rapidly reducing the range of existing chemical options, while the development of new, friendly chemicals specifically for the stored product market has become prohibitively expensive. These constraints have led to a realization that prevention is better than cure. The emphasis is rapidly shifting to integrated pest management (IPM) or integrated commodity management (ICM), with chemical means of control as a last resort. However, in practice, chemical control still plays a dominant role, with phosphine fumigation as the mainstay of the grain storage industry — even though, as with methyl bromide, its use may also become increasingly more restricted in the future. Grain aeration technology provides many advantages and benefits when applied appropriately and when its qualifying factors are recognized in comparison to conventional chemical treatments. One of the aims of this book is to disclose all of the advantages that the aeration technology can offer. On one hand, aeration has limitations with regard to killing insects in a short time — while on the other hand, for the range of temperatures obtainable by aeration, it is possible to arrest insect development and even prevent oviposition. Widespread experience has proven that insects can develop resistance to chemicals applied at commercial levels. In addition, and contrary to the general understanding and consensus, we must question the belief that fumigation provides a complete kill. Although it is feasible to obtain complete mortality under laboratory conditions, we question the chances of undertaking a commercial fumigation that can guarantee that all life stages of all insects have been killed. Adequate sealing is essential for successful fumigation. The question then arises as to how many of the storage structures are seal-tested before phosphine is applied? The objective difficulties in achieving adequate sealing of large commercial storages for a successful fumigation must be recognized. Many bins that are sealed for fumigation purposes fail sealing pressure tests. Only a few of these failed tests in unsealed or partially sealed storages are reported, and literature on fumigation usually claims successful treatments. These partial fumigation failures undoubtedly provide the selection pressure that generates insect resistance to phosphine. The wisdom of relying upon a single chemical such as phosphine, with the hope that resistance does not develop, has been invalidated. Furthermore, improvements in aeration technologies for grain cooling provide an alternative that is becoming progressively cheaper. When properly applied, aeration cooling by itself can meet the nil tolerance for infestation in certain circumstances, and it can be selectively combined with a number of other treatments if required. In aerated storages, insects are not evenly distributed throughout the bulk; they tend to concentrate on or near the surface. Such high infestations are susceptible to other control measures. For example, it has been shown that surface infestations in bins declined to zero after an application of pirimiphos-methyl was raked into the surface. Thus, aeration plus a surface application can meet industry standards. However, it is important that the material applied to the surface has some degree


of persistence and that it is applied at an early stage of storage. A number of trials were conducted in Australia under an aeration program named Smart Aeration. In both farm bins and in commercial bins, the technique of surface application plus good aeration met commercial requirements. In comparing the benefits and advantages of aeration with alternative methods, the costs of different procedures depend on a range of factors, including inputs (wet or dry grain), facilities (sealed or unsealed storage), desired results, and market preferences. Since cooling by aeration has several advantages in addition to controlling insects that cannot be achieved using chemicals, it is clearly easier to compare fumigants with protectants than to compare chemicals with aeration. For control of insects, where a quick kill is required, chemicals and fumigants are superior to aeration. However, when a reduction in dependence on chemicals is the objective, cooling by aeration should be regarded as the primary complementary technique. In the modern technological trends of grain storage, two areas of major importance where chemicals and aeration are complementary are in the management of resistance and the implementation of IPM. Aeration is superior and without competition for the short-term storage of wet grain, for preventing moisture migration in large bulks, for suppressing insect development, and for preservation of quality in grain, in seeds, and in oilseeds. Aeration is the most widely applied and environmentally user-friendly technology in the grain industry. Its proper implementation has a significant impact on the reduction of chemical pollution and on prevention of contamination by pesticide residues of the food and feed products in daily use. The use of aeration should be maximized in the application of modern grain storage technology. This will increase our contribution toward a better and safer environment by reducing chemical residues in food and feed, and reducing the risk of development of resistance by insects. The objective of this book is to provide the relevant information to enable the reader to take full advantage of the benefits of aeration. Whoever is involved — old or new generations of grain storage managers, farmers and commercial grain facility operators, silo or warehouse engineers, grain storage systems designers, sanitation specialists, or food technologists — aeration must be the leading grain storage management tool of the future.


Contents Chapter 1 Objectives of Aeration .......................................................................................................................1 Shlomo Navarro, Ronald Noyes, David Armitage, and Dirk E. Maier Chapter 2 Stored Grain Ecosystem, and Heat and Moisture Transfer in Grain Bulks ...................................35 Shlomo Navarro, Ronald Noyes, and Digvir S. Jayas Chapter 3 Ambient Air Properties in Aeration.................................................................................................79 Graham Thorpe Chapter 4 Physical Basis of Aeration.............................................................................................................125 Graham Thorpe Chapter 5 Aeration Systems Design...............................................................................................................195 Digvir S. Jayas and William E. Muir Chapter 6 Experimental Aeration Systems.....................................................................................................251 Ronald Noyes, Shlomo Navarro, and David Armitage Chapter 7 Operating Aeration Systems ..........................................................................................................315 Ronald Noyes and Shlomo Navarro Chapter 8 Supplemental Aeration Systems ....................................................................................................413 Ronald Noyes, Shlomo Navarro, and David Armitage Chapter 9 Chilling of Grain by Refrigerated Air ...........................................................................................489 Dirk E. Maier and Shlomo Navarro Chapter 10 Evaluating Aeration System Efficiency .........................................................................................561 Shlomo Navarro and Ronald Noyes Chapter 11 Airflow Distribution in Ventilated Beds of Grain .........................................................................585 Graham Thorpe Apendix A ......................................................................................................................................625 Appendix B ....................................................................................................................................629 Index...............................................................................................................................................635


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CHAPTER

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Objectives of Aeration Shlomo Navarro, Ronald Noyes, David Armitage, and Dirk E. Maier

CONTENTS 1.1 1.2

Effects of Forced Aeration on the Preservation of Stored Grain ............................................2 Objectives of Aeration..............................................................................................................4 1.2.1 Cooling the Grain Bulk................................................................................................4 1.2.1.1 Suppression of Insect Development .............................................................4 1.2.1.2 Suppression of Mite Development ...............................................................8 1.2.1.3 Suppression of Microfloral Growth ..............................................................9 1.2.1.3.1 Wet Grain Storage and the Influence of Fungi ........................12 1.2.1.3.2 Control of Respiration and Fungi .............................................13 1.2.1.3.3 Aeration to Suppress Mold Activity .........................................14 1.2.1.3.4 Cooling Wet Grain ....................................................................15 1.2.1.4 Maintenance of Seed and Grain Quality ....................................................18 1.2.1.4.1 Maintenance of Germination ....................................................19 1.2.1.4.2 Maintenance of Grain Quality by Cooling...............................22 1.2.2 Equalization of Temperature throughout the Grain Bulk..........................................24 1.2.2.1 Prevention of Moisture Migration in the Grain Bulk ................................24 1.2.2.2 Prevention of Head-Space Water Condensation.........................................25 1.2.3 Prevention of Biological Heating...............................................................................25 1.2.3.1 Ecological Aspects of Heating of Grain.....................................................26 1.2.3.1.1 Heating in Heavily Infested Grain............................................26 1.2.3.1.2 Heating in Moist Grain .............................................................26 1.2.3.2 Means of Arresting Heating in Grain .........................................................27 1.2.4 Limited Grain Drying by High-Airflow Aeration......................................................27 1.2.5 Use of Aeration Systems in Fumigation Processes ...................................................27 1.2.5.1 Recirculation to Obtain Adequate Distribution ..........................................27 1.2.5.2 Removal of Fumigant Residues and Odors................................................28 References ........................................................................................................................................29

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THE MECHANICS AND PHYSICS OF MODERN GRAIN AERATION MANAGEMENT

1.1 EFFECTS OF FORCED AERATION ON THE PRESERVATION OF STORED GRAIN Aeration can be defined as the forced movement of ambient air of suitable quality or of suitably conditioned air through a grain bulk for improvement of grain storability (Calderon, 1972). Aeration is also called active, mechanical, low-volume, or forced ventilation, since fan power is used to deliver ambient air. Aeration should be distinguished from passive or natural ventilation due to natural or convective air currents, which take place in grain bins with open manholes or in granaries with open doors or windows. Passive aeration also takes place in corn cribs, used traditionally in tropical and subtropical climates. Wind forces ambient air to flow through corn (maize) cribs, causing slow drying of damp unshelled corn and other grains. Aeration is a widely used method for the preservation of stored grain. This technology is used to modify the grain bulk microclimate; to create unfavorable conditions for the development of harmful or damaging organisms in the grain; and to create favorable conditions for the sustained preservation of grain quality. The effects of aeration on stored grain are better demonstrated by viewing the grain bulk as an ecosystem in which grain, microflora, and insects are biotic components. Substantial storage losses are often caused by microflora due to favorable moisture conditions, and insect infestation can be destructive if preventative control measures are not taken. Monetary losses of grain in storage have been estimated to range from 1 to 50% (Sinha and Muir, 1973) and in some instances can render the grain worthless or costly for proper disposition. These losses should be considered a result of interactions among the components of the ecosystem as affected by the grain and ambient conditions. The interactions between the biotic and abiotic components of the system are in a dynamic state, with each component continuously affecting the others. The role of aeration in this ecosystem is to “condition” the stored grain to improve existing conditions in the grain bulk by moving air of suitable quality through the grain mass (Figure 1.1). Moving air of suitable quality through the system (air properties of low temperature and humidity) can create conditions that suppress the development and growth of insects and microflora and sustain quality preservation and safe storage of grain. Forced aeration is an effectively applied method in commercial-scale bulk storage of grain and takes advantage of two important physical properties of the grain bulk: 1. Porosity of the grain bulk: for most cereal grain, the intergranular void volume is 35 to 55% of the grain bulk volume. The porous nature of bulk grain permits forced air to contact almost all grain kernels. 2. Thermal insulation property of the grain bulk: due to low thermal conductivity, the grain mass is selfinsulating. This enables maintenance of a modified microclimate long after the grain bulk is aerated.

To summarize, aeration is possible because air can be forced through the grain bulk to impart desirable properties to the grain; and these properties are maintained (for prolonged storage) due to the thermal insulative nature of the bulk. Although the role of temperature has long been recognized as an important regulator of biological processes, manipulation of temperature by aeration techniques was first brought into focus in the early 1950s. Since then several authors have reported their findings on aeration carried out in temperate climates, forming the basis of present-day aeration technology (Bewer, 1957; Burges and Burrell, 1964; Holman, 1966; Hukill, 1953; Johnson, 1957; Jouin, 1963; Kreyger et al., 1960; Shedd, 1953; Shibaev and Karpov, 1969; Williamson, 1961). Understandably, grain aeration technology was developed and has been used mostly in temperate climates, primarily as a result of need and the availability of selected air of desired properties — namely, low temperature and humidity in these regions. However, from the mid 1960s, experimental


OBJECTIVES OF AERATION

Figure 1.1

3

Biotic and abiotic components of the grain bulk ecosystem and the ecosystem microclimate changes resulting from aeration.

work was also conducted in warm climates such as Australia (Griffiths, 1967; Elder, 1969), Brazil (Sartori et al., 1976), India (Bhatnagar and Bakshi, 1975), and Israel (Calderon, 1974; Navarro et al., 1969). In some of these countries, aeration technology has been put into routine practice (Elder, 1969; Navarro, 1976). The use of effective aeration can be advantageous, especially in subtropical regions that have reasonably cool winters and cool nights. Experience has shown that grain bulks cooled during winter maintained the acquired low grain temperatures for many months, continuing into the following summer (Navarro et al., 1969). The relative suitability of aeration techniques for subtropical climates and limitations for use in tropical climates will become more evident in the following chapters. However, though aeration is not widely practiced in tropical climates, two potential aspects deserve mention: use of aeration with dehumidified air, and use of refrigerated air. Trials on the use of aeration with dehumidified air have yielded promising results in warm and humid areas (Odigboh, 1976). Ambient air was forced through a sorbent bed (containing CaCl2) where the exhausting air for aeration of grain was much lower in relative humidity. This method of air dehumidification by desiccants has been developed and used for many years in seed storage practice (Justice and Bass, 1978). Refrigerated aeration involves cooling ambient air with a refrigeration unit before using it to aerate a grain bulk. Refrigerated aeration has been used for cooling dry grain in subtropical climates when ambient temperatures are too high for successful insect control by aeration with untreated air (Hunter and Taylor, 1980; Navarro et al., 1973). Refrigeration involves considerable investment; but together with the dehumidified air method, it could answer questions about the practicability of aeration for safe commercial storage in tropical climates. At present, forced aeration of grain is one of the most effective non-chemical methods in use for control of stored grain conditions, biological activity, and grain quality losses. Nevertheless, forced aeration is not the sole remedy for prevention of stored grain losses. Efforts should continue to integrate other methods with this technology, including alternative methods for control of aeration


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air qualities. However, the main contribution of aeration, in the environment-minded world of today, is the reduction in use of controversial pesticide chemicals in grain storage. Therefore, the extension and promotion of appropriate aeration technologies is recommended. The many aspects elaborated in this book should be considered during the planning stage of aeration systems to be added to existing storages or when erecting new storage installations. The aim of this book is to gather the available knowledge on grain aeration in countries with temperate and warm climates and to present a document for practical and beneficial promotion of this technology.

1.2 OBJECTIVES OF AERATION The purpose of aeration is to improve and sustain the condition of bulk grain in storage. Aeration is achieved by moving air of desired or selected properties through a grain bulk until a new microclimate is produced that will keep the stored grain from deteriorating. Although aeration is aimed at improving storage conditions, it is not generally aimed at improving the intrinsic quality attributes of the grain but rather at maintaining those quality attributes. Since aeration with air of different characteristics has different effects on the stored grain, storage conditions may be improved in various ways. The improvement depends on the properties of the air used for aeration and on the existing condition or properties of the grain. Therefore, before operating an aeration system, it is essential to understand the effect aeration will have on the grain. Without prior knowledge of the process, the benefits in improved storage conditions cannot be anticipated. The specific objectives of operating any aeration system should be clear in advance of operation. These objectives may be defined according to the effects of aeration on a grain bulk as follows: • • • • • •

1.2.1

Cooling the grain bulk Equalizing temperature throughout the grain bulk Preventing biological heating in damp grain Limiting drying Introducing and recirculating fumigant gases Removing odors and fumigant residues

Cooling the Grain Bulk

Cooling grain is the most frequently applied objective of grain aeration. If cold air is available (during fall or winter seasons, on cold nights), introducing and moving this air throughout the grain mass gradually lowers the grain temperature. Thus, a new environment is created for all biological components of the grain bulk ecosystem. The biological component responses are reviewed in the following sections. 1.2.1.1 Suppression of Insect Development Freshly harvested grain is often at a temperature favorable to the development of the common stored-product insects. These are generally of tropical or subtropical origin and require fairly high temperatures (in the range of 27 to 34°C) for development. These insects thrive at about 29 to 30°C. After several months of storage at or above 27°C, any lot of grain would probably be infected with insects if protective measures were not taken. Grain-infesting insects are sensitive to temperature. Insect development is slowed or frequently stopped below 16°C, with little survival of storedproduct insects above 42°C. In the southwestern U.S., wheat, rice, and sorghum have grain temperatures up to 40°C at harvest. During the fall harvest in the northern U.S., grain temperatures around 15 to 17°C are typical.



Table 1.1

5

Optimum Temperature for Rapid Insect Growth; the Temperature at which the Development Cycle Takes 100 Days on One of the Best Foods for Each Species; and Minimum Humidity Requirements of Some Stored-Product Insects

Cold Hardiness

Response to Humidity

Species

Minimum Optimum Relative Temperature Humidity (°C) (%)

Safe Temperature (°C) (Oviposition to the Change to Adult in a Mean of 100 Days)

Species Needing High Temperature Cold hardy

Moderately cold hardy Cold susceptible Moderately cold hardy Cold susceptible

Tolerant of low RH

Tolerant of low RH Tolerant of low RH Need moderate RH Need high RH

Khapra beetle Trogoderma granarium Rust-red grain beetle Cryptolestes ferrugineus Saw-toothed grain beetle Oryzaephilus surinamensis Confused flour beetle Tribolium confusum Rust-red flour beetle Tribolium castaneum Lesser grain borer Rhyzopertha dominica Flat grain beetle Cryptolestes pusillus

33–37

1

22

32–35

10

20

31–34

10

19

30–33

1

21

32–35

1

22

32–35

30

21

28–33

60

19

26–30

50

17

27–31

60

18

Species Thriving at Moderate Temperatures Cold hardy Moderately cold hardy

Need high RH Need high RH

Grain weevil Sitophilus granarius Rice weevil Sitophilus oryzae

From Burges, H.D. and Burrell, N.J. (1964). Cooling bulk grain in the British climate to control storage insects and improve keeping quality, J. Sci. Food Agric., 15, 32–50; and Howe, R.W. (1965). A summary of optimal and minimal conditions for population increase of some stored-product insects, J. Stored Prod. Res., 1, 177–184.

A summary of the optimum and safe temperatures for insect growth for a 100-day development cycle for several major grain pests is listed in Table 1.1. The temperatures given are transferable to microclimate and grain temperatures for stored grain. At temperatures lower than 20°C, population growth of most storage insects is significantly suppressed. This is clearly shown in Table 1.1 (Burges and Burrell, 1964). According to Table 1.1, grain and microclimate temperatures in the range of 17 to 22°C are considered “safe” for insect management, since completion of their life cycles at those temperatures takes about 3 months or more. At low temperatures, oviposition and fecundity of these insects are also much lower so that, over time, their population growth remains insignificant. Consequently, insect damage caused under these low temperature conditions is negligible. The optimum temperature and relative humidity conditions for stored-grain insects vary by species. Storage insects can develop at relative humidities below 70%, but some species reproduce successfully at relative humidity levels below 30% (Table 1.1). However, for dry grain, the equilibrium relative humidity (ERH) of the grain bulk is usually higher than 30% but does not exceed 70%. Therefore, stored-grain insects must tolerate the microclimate’s relative humidities in dry grain bulks. In most grain storages without cooling, the number of insects rapidly increases, rendering the commodity unsaleable since there is a low tolerance of live pests in grain trades of most countries. The aim of cooling is therefore twofold: to reduce the grain temperature below the development


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temperature of insects, and to cool the grain quickly enough so that an egg laid by a wandering female insect on the first day of storage will not develop to adulthood. The quoted recommendation of Burges and Burrell (1964) was that the grain should be cooled to below 17°C based on the fact that S. granarius would take more than 100 days to complete a life cycle. The researchers also recommended temperatures as low as 5°C if the stored bulk is infested. Today, quality demands and longer periods of storage dictate the need for faster cooling. The extent to which this objective can be achieved by ambient aeration only depends on the climate immediately after harvest and the airflow rate chosen. If cooling is considered as a series of discrete “fronts,” then the first target is to cool the grain to below 15°C to prevent the fastest developing insect, usually O. surinamensis, from completing a life cycle. This assumes that a life cycle would have been completed under the initial grain storage conditions, which allows only 17 days to cool the grain. The next aim is to lower the temperature to below 10°C. This should prevent breeding of the most cold hardy insect, usually, S. granarius, which takes 26 days at optimum temperature and 144 days at 15°C. Howe (1965) summarized the minimal temperatures for development of many stored-product pests. A rule of thumb for aeration is that about 1000 volumes of air are required to cool one volume of grain (Burrell and Laundon, 1967; Poichotte, 1977). This yields results similar to calculations based on the ratio of air to cooling front velocity (McLean, 1980). Based on airflow rate, the hours of aeration required to pass a cooling front through grain can be determined. A study of meteorological records reveals the average number of days after harvest before the total number of hours below a given temperature can be accumulated. This will determine whether sufficient cooling can be achieved in time to prevent insect development. Based on this principle, Armitage et al. (1991) showed that an airflow of 10 (m3/h)/tonne was the minimum rate to achieve satisfactory aeration during the warmest years in the U.K. In many climates, cool air is not available in sufficient quantity after harvest; and higher airflow rates may be required for timely aeration. For example, Harner and Hagstrum (1990) showed that airflow rates greater than 1.5 cfm/bu (90 (m3/h)/tonne) were required in Kansas in July and August to complete one cooling cycle based on limited hours of sufficiently cool air. However, this theoretical level of airflow is not considered economically feasible. Arthur et al. (1998) calculated the numbers of S. zeamais occurring in unaerated maize and maize cooled at 3 different airflow rates in 11 southern U.S. states. They recommended an airflow rate of 0.1 cfm/bu (6 (m3/h)/tonne). The researchers indicated that, while aeration reduced populations dramatically, it was not sufficient to completely prevent moderate insect population increases. Where sufficient cool air is not available immediately after harvest, fumigation may have to be applied before cooler weather arrives. Hagstrum and Flinn (1990), and Longstaff (1986) have modeled strategies based on this principle. Airflow rates of 0.1 cfm/bu are considered standard, but airflows greater than about 0.2 cfm/bu would not be considered in the realm of standard aeration — based on the original purpose of aeration of controlling moisture movement (moisture migration) in grain masses. This natural moisture movement results from convection currents caused by grain temperature differentials between cold grain along outside walls and warm grain in the center of the mass. Aeration immediately after harvest may have to be delayed for reasons other than climate. In the northern hemisphere, many malting barleys have dormancy problems and are often stored at high temperatures in order to break dormancy. This delay at high temperatures obviously makes the grain vulnerable to insect attack. Armitage and Woods (1997) and Armitage and Cook (1997) suggested dormancy should be broken below 20°C or above 40°C to discourage infestation by five species of insects, including Trogoderma granarium Everts. In colder climates, ambient aeration is used to cool grain to low temperatures — not just to prevent insect increase but also to kill the insect during prolonged storage. Fields (1992) summarized survival times of many stored-product insects at low temperatures. The lower temperature range could be divided into three bands. Between the chill coma and minimum breeding temperatures, the insects are able to sustain a low metabolic rate but are unable to repair accumulating physical



7

damage. Between the chill coma temperature and the super-cooled point, insects are unable to feed and therefore slowly starve. At and below super-cooled temperature, death occurs when the water in body fluids freezes (see Chapter 2, Table 2.2). Evans (1983) determined that the median chill coma temperature for S. granarius is between 2.7 and 5.6°C; for C. ferrugineus at 4.4 to 6.4°C; and for O. surinamensis at 5.6 to 10.0°C. Reducing grain temperatures to these values is desirable to prevent damage by the insects, which may live for extended periods at temperatures above chill coma temperatures. Smith (1970) found the supercooled point of C. ferrugineus to be –17°C without acclimation at an unspecified RH, while Robinson (1926) found that between 12 and 16% mc, the super-cooled point of S. granarius, is from –9 to –10°C. Fields (1992) gave the super-cooled point of O. surinamensis as –16°C. These temperatures are unlikely to be achieved by aeration within the mass of British grain stores or in many other locations; however, these temperatures may be achieved in climates of Scandinavia and Canada. Death by starvation at low temperatures can be retarded by increased availability of water and acclimation (Ushatiskaya, 1948, 1950; Evans, 1983). The species of insect also determines survival rates at low temperatures (David et al., 1977). Although grain beetles do not hibernate, Evans (1979) has noted how some strains lower their oxygen consumption with exposure to cold, an apparent adaptation. Thorpe and Elder (1980) incorporated the decay of chemical pesticides into a heat and mass transfer simulation model. Their objective was to determine the potential of reducing insecticide usage and delaying insect resistance by chilling bulk stored grain. They found an optimum airflow rate for chilling the grain, which slowed chemical breakdown and extended efficacy of pesticides to their maximum level. The decay rates of malathion and methacrifos were insensitive to the initial grain temperature and moisture content when the bulks were chilled quickly after the pesticide application during bin filling. As malathion was phased out in the Australian grain industry in 1977, it was replaced by other chemicals (Longstaff, 1988a). However, resistance continued to develop with time against the newer and more expensive insecticides. A major factor in reducing insect population growth rate is prolongation of the development period at lower temperatures. Longstaff (1988a) investigated the effect of temperature manipulation upon the spread of a resistant gene in an infested grain stored under Australian ambient weather conditions. Cooling of the grain had a pronounced effect upon the generation time of the insects and thus on the rate of spread of the resistant gene. He concluded that “combining grain cooling and insecticide treatments slowed the rate of development and/or spread of pesticide resistance.” In a related study, Longstaff (1988b) determined that cooling grain to 15°C was not sufficient to prevent population growth. However, aeration immediately after fumigation gave some long-term insect protection when grain was cooled quickly. The benefit of cooling depended on the type of insecticide. With pyrethroids, a beneficial effect and reduced application rate were noted. Organophosphorous insecticides, on the other hand, showed a positive temperature-toxicity relationship. Hagstrum and Flinn (1995) described the integrated pest management (IPM) approach to pest control that involves insect sampling, risk/benefit analysis, and use of multiple control tactics. IPM is a concept that is well established in crop protection and one that must be more widely understood and used by stored-grain managers. In their approach, the economic injury level (EIL) is defined as the insect density that causes reductions in market value greater than the cost of the control. A critical concept in IPM is the economic threshold (ET), an insect density at which control measures should be applied to prevent insect populations from exceeding the EIL (Hagstrum and Flinn, 1995). The ET approach to control insect populations in stored grain is illustrated in Figure 1.2. Onstad (1987) provides a detailed discussion of the economic threshold. Stored-grain IPM programs would be improved by the development of better insect sampling programs. Sampling of insect populations is critical to an IPM program, because without it the manager would not know if the population were approaching or exceeding the economic threshold. IPM programs use risk/benefit analyses to maximize profit and reduce economic losses. IPM


8

Figure 1.2


Economic threshold (ET) and the economic injury level (EIL) concept, demonstrating the population dynamics of insects over time in aerated and un-aerated stored grain. (From Hagstrum, D.W. and Flinn, P.W. [1995]. IPM in grain storage and bulk commodities, in Stored Product Management, Krischik, V., Cuperus, G., and Galliart, D., Eds., Oklahoma State University, Stillwater, OK, pp. 201–205. With permission.)

programs are based on an understanding of the ecology of insect pests and allow for a variety of control measures, such as sanitation, parasites, and aeration, to be substituted for some or all insecticide applications (Hagstrum and Flinn, 1995). 1.2.1.2 Suppression of Mite Development Mites are important pests of stored products, particularly in damper, cooler, or maritime climates. Mites can hollow out the germ of cereals or reduce oilseeds to empty shells. They can also contaminate the products with feces and impart an offensive odor. Feeding mite-infested food to animals may cause nutritional problems; handling infested grain may cause allergies in humans, and ingestion may cause clinical symptoms. In comparison with stored-grain insects, control of mites has received little attention — although mites are usually omnipresent and easier to detect than insects. Geographic variations occur, but the most common mites encountered include Acarus siro L., Lepidoglyphus destructor Schrank and Tyrophagus putrescentiae Schrank (which live off the grain and associated fungi) and predatory Cheyletus eruditus Schrank. As an example, A. siro, perhaps the most widespread species, can develop between 7 and 30°C, unless RH is below 60 to 65% (Cunnington, 1984). From this data, the crucial control parameter for these pests is not temperature, but establishing an equilibrium relative humidity (ERH) below about 65% RH (about 12.5% moisture content for wheat at 25°C), which suppresses mite development. Weekly rates of mite increases at optimum conditions of 20 to 25°C and 80 to 90% RH are about sixfold, emphasizing an ability to quickly increase and the need to surpress population dynamics. Data for L. destructor is given by Stratil et al. (1980). Although temperatures required to suppress development of mites in damp grain (14 to 16% mc wet basis) are obtainable in temperate climates, maintenance is too expensive at the bulk periphery when mean ambient temperatures are favorable for mite development (Burrell, 1974). Burrell and Havers (1976) concluded that, although cooling by aeration is unlikely to prevent moderate mite infestation, aeration may be expected to reduce the incidence of hot spots, and the heavy populations of mites associated with hot spots. The authors also recommended drying grain for prevention of mite infestations rather than cooling moist grain.



Table 1.2

9

Approximate Minimum and Optimum Temperatures at which Storage Mites Breed

Species Tyrophagus putrescentiae Glycyphagus destructor Cheyletus eruditus Carpoglyphus lactis Aleuroglyphus ovatus Rhizoglyphus echinopus Caloglyphus berlesei Acarus siro

Temperature (°C) Minimum Optimum 9–10 10–15 12 15 22 6–10 16.5 7

23–28 15–25 25–27 25–28 23–25 23–27 22–30 23–30

From Sinha, R.N. (1968). Climate and potential range of distribution of stored-product mites in Japan, J. Econ. Entomol., 61, 70–75.

Mite infestations in grain are more common in temperate climates than in subtropical climates. Aeration cooling in both of these regions should be aimed mainly at prevention of insect damage. In contrast, the predator, C. eruditus, requires a temperature minimum of about 12°C (Boczek, 1959) and rarely establishes itself in cool grain until the summer. Unfortunately mites are not very susceptible to organo-phosphate pesticides; so physical control measures — drying and cooling — must be relied upon for control. Continuous or high-temperature drying techniques are processes that are completed too quickly to permit mite development in grain. Ambient-air, slow-drying techniques may take several weeks, even with airflow rates several times those required for cooling. During slow drying, mites may develop to significant populations before reducing their development at a rate dependent on the final moisture content achieved by drying (Armitage et al., 1982). Mites favor conditions of moderately low temperatures and high relative humidities. The temperatures that prevent growth of mites vary from species to species but are generally in the range of 0 to 10°C (Smith, 1974). Most mite species found in stored grain reproduce very rapidly between 20 to 30°C (Table 1.2). However, mite survival is seriously limited at relative humidities below 60% (equivalent to about 12% moisture content for cereal grains). When moisture contents of cereal grains are higher than 14%, conditions are favorable for mite development. Therefore, for grain stored with initial moisture contents lower than 14%, mite infestation is negligible. 1.2.1.3 Suppression of Microfloral Growth The fungi that grow in the field such as Fusarium, Cladosporium, and Alternaria are replaced in store by species adapted to more xerophilic conditions, such as Penicillium and Aspergillus spp. (Christensen and Kaufmann, 1969). Some species, such as members of the Aspergillus flavus group, which produce aflatoxins, and Penicillium verrucosum, which produces ochratoxin A, create metabolites containing mycotoxins that are injurious to human, fowl, fish, and animal health. Fungi may also reduce the viability of grain as well as cause discoloration and taints. To remove moisture effectively by natural air drying, at least 10 times and preferably 20 to 30 times as much airflow should be used than airflows required for cooling grain. Unless higher than normal aeration rates (0.2 to 0.3 cfm/bu) are used, temporary storage of damp grain with normal airflow rates (0.1 cfm/bu) for cooling will lead to undesirable fungal as well as mite activity. Attention has been given to modeling strategies for cost-effective optimum drying of grain to avoid over- or under-drying (Nellist, 1988). Unfortunately, these studies neglect adequate analyses of spoilage avoidance.


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Table 1.3

Temperature and Relative Humidity Conditions of Fungi on Stored Grain

Fungus Alternaria Aspergillus candidus A. flavus A. fumigatus A. glaucus (blue eye mold) A. restrictus Cephalosporium acremonium Epicoccum Fusarium moniliforme F. graminearum (G. zeae) Mucor Nigrospora oryzae Penicillium funiculosum P. oxalicum P. brevicompactum P. cyclopium P. viridicatum

Minimum % ERH for Germinationa

% EMCb

91b 75 82 82 72 71–72 97 91 91 94 91 91 91 86 81 81 81

19 15 16–17 16–17 13.5–14.0 13.5 22 19 19 20.5 19 19 19 17 16 16 16

Growth Temp °C Min. Opt. Max. °C °C °C –3 10 6–8 12 8 — 8 –3 4 4 –3 4 8 8 –2 –2 –2

20 28 36–38 37–40 25 — 25 25 28 25 28 28 30 30 23 23 23

36–40 44 44–46 50 38 — 40 28 36 32 36 32 36 36 30 30 36

Note: = Low to moderate moisture storage fungi = High moisture storage fungi a Approximately 5% or more of the spore population can germinate at this relative humidity. b Approximate equilibrium moisture content at 25.5°C equal to minimum percent relative humidity in which fungus can germinate, but probably takes a higher moisture content for fungus to grow and compete on cereal grain (average values for wheat and corn). From Purdue University (1988). Plant Pathology Department Publications, Purdue University, Lafayette, IN; and Lacey, J., Hill, S.T., and Edwards, M.A. (1980). Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inform., 38, 19–32.

As with fast cooling models, the aim in this case is to model a drying front moving through the grain before spoilage levels (primarily fungal deterioration) become significant and detrimental. Grain should be dried fast enough so that fungi development in the slowest drying areas does not exceed acceptable levels. Although there is no definition of acceptable fungal contamination, numbers of storage fungi in grain dried with ambient air frequently exceed 100,000 colonies per gram (Armitage et al., 1982). Like mites, the equilibrium relative humidity (ERH), which constitutes the lower limit for most fungi development, is in the region of 65 to 70% (Ayerst, 1969). Low temperatures are required to prevent microfloral damage in damp grain. Table 1.3 shows that temperatures lower than 5°C (and for Penicillia molds, below 0°C) are needed for the suppression of mold development. Most fungi do not grow at relative humidities below 70%, which is equivalent to about 13% mc for cereal grains. Therefore, microfloral growth is dependent mainly upon the ambient humidity, and cooling the grain does not seem to be an efficient method for arresting development. Nevertheless, the lower the temperature, the more limited the microfloral damage. Therefore, grain with slightly high moisture content can be stored without being seriously damaged if the ambient temperature is sufficiently low. Christensen and Kaufmann (1974) reported the possibility of storing sound, in-good-condition grain of 15% moisture content for 9 to 12 months, without damage, when the grain temperature is maintained between 8 and 10°C. However, these low temperatures are difficult to attain by aeration in subtropical climates. Very often grain must be harvested under unfavorable weather conditions with a moisture content too high for safe storage. Sometimes this is a result of cold, cloudy, or rainy weather at harvest, when field crops do not receive adequate solar radiation and wind to finish field drying. Even in regions where the relative humidity is high at night, with or without the deposition of dew,



11

grain harvested in early morning may have a moisture content 3 to 5% above that harvested in midafternoon. Differences were greater than that in sorghum seeds collected from different parts of the same heads. Sorghum kernels collected at about 8 a.m. from the top of the heads of several plants had an average moisture content of 16.3%, whereas kernels from the bottom of the same heads had an average moisture content of 35.0% — a difference of almost 20% (Christensen and Kaufmann, 1969). Few people seem to be aware of this wide variation in moisture content, although at times wide-spread moistures at the beginning of harvest can have a great influence on the storability of some types of grain. Most of the maize (corn) produced and marketed in the U.S. is harvested with cylinder-concave or rotary self-propelled harvesters or combines. For best shelling results, maize is harvested at a moisture content of about 23 to 26%. Unfavorable weather at harvest time, delayed maturity, or other factors may result in maize harvested at moisture contents of 27 to 35% or more. In much of the U.S. corn belt, daytime temperatures and the resulting grain temperatures are within a range that permits rapid microbial growth, especially if the moisture content of the harvested grain is above 22%. This biological activity and favorable conditions combine to form a significant grain storage hazard. Several approaches and combinations of approaches have been developed to improve storability and maintain marketable qualities in corn and other grains harvested at susceptible temperature and moisture conditions. The principal approaches are (1) drying to a moisture content safe for storage; (2) aeration with ambient air that maintains a low, uniform temperature to prevent migration of moisture; and (3) aeration with pre-conditioned air, generally artificially cooled with refrigerantbased systems. Grain system operators must take into account the moisture content of the grain when received. In addition, they must assess grain conditions such as how long the grain can be kept before drying without losing grade or quality. Also they must determine the temperature and moisture conditions at which the grain should be conditioned to maintain quality for the required long- or short-term storage period or for immediate market. The uses of the grain, shifts in market prices and demands, plus storage and marketing costs are also important considerations. Short-term holding of grain at higher than “safe” storage moisture levels is often economically beneficial. Grain marketing systems in all countries are based on wet-basis moisture contents that are part of the grading system. For example, No. 2 commercial maize in the U.S. is based on 15.0% moisture content (wet basis). Farmers or commercial elevator operators who market No. 2 maize at moisture levels below 15.0% lose a significant amount of profit from grain moisture weight loss from overdrying. Commercial elevators often blend drier maize with wet maize to ship at the allowable 15.0% moisture, profiting from combining over- and underdried grain. Overdrying increases fuel costs; but if grain is underdried or delivered to a grain elevator without drying, drying and/or shrinkage penalties are assessed. When maize is delivered to a U.S. grain elevator at moisture levels at 15.1% or higher, grain elevator managers typically assess a drying expense discount of $0.01/bu and a moisture shrinkage factor of 0.7% weight for each 0.5% moisture above 15.0%. If maize is stored for farmers by the elevator, discounts and shrinkage start at 14.6%. Between 20.1 and 20.5% moisture, maize discounts range from about $3.98 to 4.73 per tonne ($0.10 to 0.12 per bushel), and shrinkage for moisture ranges from about 7.7 to 9.1% (Assumption Coop Grain Company, 1997). Some grain milling processes — such as wheat flour milling or wheat, oats, or maize processing for cereals — require specific moisture levels of 15 to 17% to obtain optimum milling or processing yields. The closer wheat, oats, maize, or other food grains are to the desired processing moisture level, the more efficient and profitable the milling process becomes. In summary, grain cooling by forced aeration is beneficial in preventing fungal damage through both temperature and moisture control. However, attaining effective low grain temperatures and


12


moistures by using aeration may be difficult in subtropical warm climates. In most cases, microfloral damage should be prevented by storing dry (or dried) grain rather than by aeration cooling. However, there are cases where an acceptable reduction in deterioration rate can be more easily achieved by lowering grain temperature rather than drying or artificially drying the grain. 1.2.1.3.1

Wet Grain Storage and the Influence of Fungi

Field grain carries many mold spores or fungi. Field fungi are invariably present on cereal grains at harvest either under the epidermis or on the surface of the seeds (Christensen and Kaufmann, 1974). At low temperatures, below about 5°C, the original field fungi may survive for long periods of time in both damp and dry grain. At higher temperatures, these field organisms die out; if the grain is damp, field fungi are rapidly replaced by storage molds. Mold spores can germinate and damage grain at above a minimum grain equilibrium moisture content and temperature. Several common storage fungi are listed in Table 1.3. Grain with moisture levels that result in ERH values above 80% RH may have several common field fungi, such as Aspergillus flavus and Penicillium funiculosum, which may germinate even at minimum temperatures of 6 to 8°C. The concept of “safe grain” moisture levels that sustains an ERH of 65 to 70% or less are illustrated by A. candidus, A. glaucus (blue eye mold) and A. restrictus, which have a minimum of about 72% ERH. Of these common grain fungi, several are only active at relatively high moisture levels, while others are considered “low to moderate moisture level fungi.” The low to moderate moisture storage fungi are the most dangerous to stored grain. This is because of the possibility of developing just above the critical moisture content that may be created due to uneven drying moisture, condensate moisture, or moisture migration that can result in moisture buildup in dry stored grains. These mold species that germinate in the range of 71 to 82% ERH are listed in bold type in Table 1.3. A higher grain moisture level may be required to sustain the growth of these fungi. In ventilated or refrigerated grain, odors in the air leaving the grain bulk are useful guides to its condition. A faint mustiness in the normally fresh smell of the expelled air is an indication of the beginning of fungal attack (Burrell, 1974). Such odors should be taken as a danger sign and indicate the need for remedial action. The presence of the odor in the air above a bin of unventilated grain indicates that fungi have proliferated to a considerable extent. Such activity occurs in bulk drying systems where drying is too prolonged. At a later stage of fungal growth, visible colonies of fungi may appear on isolated kernels in cold grain. Fungal growth occurs more rapidly with increasing moisture. To prevent this, knowledge of conditions that lead to fungal growth is essential. Most common storage fungi only become visible to the naked eye at counts in excess of five hundred thousand viable particles per gram of grain, but mycotoxin production may occur at levels below this. Visible growth is usually first seen on broken seeds where the endosperm or germ is exposed, but sound seed may also be attacked. Dead seeds, lacking natural defenses, are expected to succumb first to fungal attack; but seeds attacked by fungi are not necessarily dead. Tests on moldy seeds taken from damp bulks of grain demonstrate that they will often germinate. Rapeseeds, for example, may be covered with fungal colonies for weeks before they die (Burrell et al., 1980). Once fungal growth has preferentially attacked isolated individual seeds in a layer of damp grain, it then spreads to adjacent seeds until the damp layer is completely caked. During the process, the seeds are invaded by a succession of different fungi that consume dry matter, increase the moisture content, and produce carbon dioxide and heat. In caked grain, dry matter losses of 10 to 30% are common; and in extreme cases the seeds disintegrate when disturbed. Fungal growth and the production of fungal spores create a variety of problems; the product becomes musty, unpalatable, or unstable — but above all, fungi or fungal metabolites may be harmful to men and animals. For example, many fungi produce toxins that may adversely affect animals and, through animal products, gain access to the human food chain (MAFF, 1980; WHO,



Figure 1.3

13

Estimated number of weeks of freedom from visible molding for barley at a range of temperatures and moisture contents. (From Burrell, N.J. [1982]. Refrigeration, in Christensen, C.M. (Ed.), Storage of Cereal Grains and their Products, based on data by Kreyger [1972] and Burrell [1966], St. Paul, MN, pp. 407–441. With permission.)

1979). Another type of harmful effect is caused by spores and other dusts released into the air when moldy grain is disturbed by handling. Such dusts may sensitize workers and animals, causing allergies to specific fungi. Temperatures well below 0°C are needed to prevent fungal activity during sustained storage of grain above 23% moisture content. For short-term storage, particularly for moistures up to about 22%, temperatures just above freezing are adequate. The limitations of low-temperature storage are illustrated for barley in Figure 1.3; but at the same relative humidity, wheat, rye, and oats are more readily attacked by fungi than barley (Kreyger, 1972). 1.2.1.3.2

Control of Respiration and Fungi

The losses of cereal grains, from the time of maturity in the field to time of consumption, vary from 5 to 50% of the production depending on the type of cereal grain, variety, geographic region, and climate (Brooker et al., 1974). Field losses tend to increase as grain moisture decreases due to shatter losses during delayed harvest. Rain on mature grain decreases grain quality; and storms cause lodging of grain, increasing harvesting field losses. Losses and loss rates during slow drying and storing generally increase with an increase in moisture. Respiration of carbohydrates, the primary constituent of grain kernel dry matter, is a process that produces heat, water, and carbon dioxide. The combustion of a simple sugar follows the following molecular equation (Brooker et al. 1974): C6 H12O6 + 6O2 → 6CO2 + 6 H2O + 677.2 kcal mole

(1.1)

One percent dry matter loss is accompanied by the production of 14.7 grams of carbon dioxide per kg of dry matter, causing an equivalent heat production of 157.2 kJ/kg grain (Steele et al., 1969). In the U.S., much of the work on grain deterioration has focused on the dry matter loss of maize during natural and low-temperature drying operations and subsequent storage periods. In Europe, much of the work on grain deterioration has focused on the small grains such as wheat, rye, and barley. In the 1950s grain spoilage due to respiration and molding became a concern in many on-farm grain storage installations (Hall, 1980). Large-scale farm storage systems that approach the size of small country grain elevators are usually managed more like commercial elevators. These operations


14


are frequently more technically advanced than most small farm operations. Operators of large farms have more investment at risk and typically keep more up to date on grain storage technology than smaller farmers. Often, during adverse weather conditions, small farm in-bin drying systems are not capable of drying high-moisture maize rapidly enough; and serious spoilage occurs in the undried grain. Foster (1953) did early studies on the development of the design parameters for ambient- and low-temperature drying. He pointed to the deterioration of grain in the top 0.3 m (meters) of the grain pile, which dried last. He used a weighted deterioration index, which increased with time, to estimate grain spoilage. Saul and Lind (1958) and Steele (1963) developed the deterioration concept further by relating the production of CO2 to the dry matter loss of maize due to respiration and mold development. Steele and Saul (1962) proposed a relationship between the effect of temperature and moisture content. Later Steele et al. (1969) reformulated the proposed multiplicative relationship to also include mechanical damage effects on the rate of dry matter loss. The proposed relationship was given by Steele et al. (1969) as: AST = TR mT mM mD

(1.2)

The allowable storage time, AST, is in hours. The reference time, TR, is defined as 230 hours for a 0.5% dry matter loss when maize is held at a constant temperature of 15.6°C and 25% moisture content. The reference grain condition was field-shelled maize that was visually assessed to contain 30% mechanical damage based on weight at the time of assessment. The relationships of the dimensionless multipliers, mT, mM, and mD, were given in graphical form. A reference time of 58 hours was determined for a 0.1% dry matter loss, and 536 hours for a 1.0% loss. Saul (1970) reported on more studies for grain temperatures below 15.6°C. This extended the range for the temperature multiplier and lengthened the AST at grain temperatures significantly below 15.6°C. Thompson (1972) investigated the temporary storage of high-moisture shelled maize under continuous aeration and used a computer simulation to predict moisture content, temperature, and grain quality. Grain quality was calculated using the dry matter loss equation of Steele et al. (1969) and Saul (1970). Saul concluded that deterioration was minimized with higher airflow rates, later harvest dates, and lower moisture contents. Although deterioration was slowed at lower initial grain temperatures, the total deterioration was about the same over the length of storage. The quality index varied by as much as twofold depending upon the local yearly weather pattern. 1.2.1.3.3

Aeration to Suppress Mold Activity

The three possibilities for suppressing mold development with aeration are: 1. Remove the heat generated due to spontaneous heating of wet grain. In such a case, moisture and temperature of grain may remain unchanged; but further heating within the grain mass may be reduced to that in line with the present grain condition, temperature, and moisture. 2. Cool the grain mass for this purpose; high cooling rates are necessary. To hold wet grain in temporary storage for an extended period of time, the grain must be cooled quickly to a temperature that will minimize mold development. The grain temperature must be maintained uniformly at or below that level. 3. Dry the grain. Airflow rates lower than typical for drying grain are applicable and can effectively reduce grain moisture to safe storage levels.

According to Lacey et al. (1980), the number of field microorganisms found in stored grain depends largely on conditions prior to harvest but also on the conditions of storage. In wet weather, field fungi may develop abundantly and then be carried in to store.



15

If the grain is sufficiently dried and then stored well, the field microorganisms may survive for long periods. Otherwise they die out rapidly. Thus, field microorganisms found in stored grain may indicate that storage has been good although pre-harvest conditions may have been poor. In contrast, in the presence of many intermediate fungi, high water activity may indicate low temperature deterioration. If Fusarium species are present, there is a possibility of toxin production. The particular microorganisms that grow in stored grain are determined by the interaction of many factors. These include the water activity of the product, temperature of storage (including any spontaneous heating), aeration, period of storage, chemical composition of the product, presence of foreign material, insect and mite infestation, and the use of chemical preservatives. Water activity (aw) is the ratio of the vapor pressure of water in a product or solution to that of pure water at the same temperature. Water activity and ERH are numerically equivalent, but ERH is expressed as a percentage; thus, aw 0.8 = 80% ERH. At aw 1.00 free water is available in the substrate. Water activity (or ERH) is a more useful parameter than water content since it reflects the availability of water for metabolic processes. Different products with the same water activity may have very different water contents. For example, oilseeds have a higher water activity at a given water content than starchy cereal seeds (Lacey et al., 1980). Microorganisms have characteristic optimum and limiting water activities and temperatures. These are interrelated. For a given microorganism, the lowest minimum water activity for growth occurs at or slightly above the optimum temperature, while the maximum temperature for growth may be highest at rather low water activity (Ayerst, 1966). This interaction of water activity and temperature determines the ability of microorganisms to germinate as well as their rate of growth. There is a continuous spectrum of species growing in grain from –8º to 70°C and from aw 1.0 down to about 0.65, but groups with similar requirements can be recognized (Lacey et al., 1980). Water activity largely determines the amount of microbiological heating in a stored crop. The minimum for fungal growth is about 0.6 aw. At water activities slightly above 0.65, Aspergillus restrictus and species of the A. glaucus group grow slowly without increasing the grain temperature. If the water activity is somewhat higher, then growth is more rapid and the temperature soon rises, since the grain, being an insulator, cannot conduct metabolic heat away faster than it is produced. Metabolism also produces water, increasing the water activity further. Microbiological heating is thus a self-accelerating process up to a peak temperature. Nevertheless, the initial water activity largely determines the maximum temperature reached (Lacey et al., 1980), and different combinations of water activity and peak temperature produce characteristic associations of microorganisms. The conditions under which the different species predominate are not necessarily their optimum for growth in pure culture but are those at which the species compete most effectively in a mixed population (Lacey et al., 1980). Table 1.4 summarizes the limiting water activities for mold growth in relation to temperature. Since mold activity is dependent on temperature, molds appear to tolerate lower temperatures at higher water activities. Table 1.4 is based on approximate equilibrium moisture content values at 25.5°C equal to the minimum percent relative humidity in which fungus can germinate, but fungi seem to require higher moisture content to grow and compete with grain. Water activity appears to be a more accurate approach to express the humidity values in terms of water activity (decimal of relative humidity) and dependence on temperature (Lacey et al., 1980). 1.2.1.3.4

Cooling Wet Grain

Moist or wet grain is characterized as being above 70% ERH, which usually places cereal grains in a moisture range of 13 to 14% moisure content (mc) in ambient temperature ranges. Regardless of geographic region, terrain elevation, latitude, or prevailing weather patterns, designing an aeration system to keep moist grain from spontaneous biological heating due to sustained mold growth requires much higher aeration airflow rates than dry grain. This requires installing higher


16


Table 1.4

Limiting Water Activity (aw) for Mold Growtha

Temperature Tab (°C) –5 0 5 10 20 27.5 30.0 35.0 40.0 45.0

Experimental Data aw

Polynomial Values aw

0.953 0.910 0.882 0.782 0.698 0.673 0.678 0.703 0.765 0.828

0.943 0.903 0.843 0.779 0.683 0.666 0.672 0.702 0.750 0.808

a

At water activity levels below the limiting experimental values, molds do not grow, or grow very slowly. Above the limiting values, mold growth is possible. From Lacey, J., Hill S.T., and Edwards M.A. (1980). Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inform., 38, 19–32.

capacity blowers, ducts, and vents than those used for aeration of dry grain in order to provide fast cooling while gradually drying the grain to safe moisture levels. Regardless of ambient temperatures, aeration systems should be started as soon as moist grain is placed in holding tanks to keep fresh air moving through the moist grain. Aeration should be operated as the tank is being filled and run continuously until the grain is removed or the moisture reaches a safe storage level. If aeration is stopped, the interstice air RH will increase to equilibrium, which may cause germination of fungi, mold growth, and spontaneous heating. Although low humidity air will quickly become saturated, continuous airflow will minimize spontaneous heating. While aeration of dry grain typically removes from ¼ to ½ percentage point moisture for each complete cooling cycle, aeration of wet grain may remove as much as ¾ to 1 percentage point or more during the time required to complete a cooling cycle. The wet grain aeration process operates much like low-speed natural air drying. Aeration is not considered a drying process, but continuous high aeration airflow rates used for wet grain cooling slowly lower grain moisture toward safer levels while keeping fresh air moving across each kernel. Depending on mc, even in cool or cold regions, wet grain can develop spontaneous heating even at relatively low grain storage temperatures of zero to 10°C, causing mold odor, germ damage, and discoloration and resulting in a severe loss of grain quality. A review of Table 1.5 indicates that grain above 20% moisture levels is unsafe unless uniform bulk grain temperatures of at least 0°C are maintained until the grain is dried or processed. Table 1.5 lists wet holding time vs. grain temperatures during which stored maize at a range of moisture contents is known to sustain dry matter losses that will cause the grain to lose one grade level in U.S. grain markets. This chart is used by U.S. producers and elevator managers in the U.S. corn belt and other grain regions as a guide for cooling and holding high-moisture maize while it is dried to safe storage levels. For storage at high moisture, maize should be cooled to sufficiently low temperatures or should be dried to safe storage moisture levels well in advance of the time listed. Variables such as uneven cooling, high moisture zones, and condensate drainage that creates wet grain under fill spouts require that a margin of safety be applied to Table 1.5. Kuppinger et al. (1977) compared the low temperature drying of maize in Germany at field moisture contents of 35% and maize at moisture contents of 20 to 25% after pre-drying in a high-temperature dryer. Germination, dry matter loss, and spore count of microorganisms were determined to evaluate the quality of the maize. Germination was not found to be a good quality indicator because of high values even after molding was already visually detected. Dry matter loss varied from 0.6 to 3.0% for



17

Table 1.5

Approximate Allowable Storage Time (Days) for Cereal Grains

Moisture Content (& wet basis)

–1

4

14 15 16 17 18 19 20 22 24 26 28 30

* * * * * * * 190 130 90 70 60

* * * 280 200 140 90 60 40 35 30 25

Temperature (°C) 10 16 21 * * 230 130 90 70 50 30 15 12 10 5

* 240 120 75 50 35 25 15 10 8 7 5

27

200 125 70 45 30 20 14 8 6 5 4 3

140 70 40 20 15 10 7 3 2 2 2 1

* Approximate allowable storage time exceeds 300 days. Compiled from ASHRAE (1995). Based on composite of 0.5% maximum dry matter loss calculated on the basis of USDA research; Transactions of ASAE 333-337, (1972); Unheated Air Drying, Manitoba Agriculture Agdex 732-1, rev. 1986.

Table 1.6

Spore Count (Number of Spores per Gram of Dry Matter) for Bacteria, Yeast, and Molds in Harvest–Wet and Pre-Dried Maize

Microorganism Bacteria Yeast Molds

Harvest–Wet at 35% wb

Pre-dried to 19% wb

0.2106–4.3106 0.3105–1.3105 3.9103–6.5104

1.5103 2.1100 3.2102

From Kuppinger, H.V., Muller, H.M., and Muhlbauer, W. (1977). Die beluftungstrocknung vonerntefrischem und vorgetrocknetem kornermais unter thermodynamischem undmikrobiologischem aspekt, Grundl. Landtechnik Bd., 27, 119–132.

maize dried from 35% moisture, while the dry matter loss of pre-dried maize was only 0.04%. The spore count for the microorganism groups of harvest-wet and pre-dried maize is listed in Table 1.6. Grain deterioration, associated largely with fungal growth, is the principal constraint on the allowable length of time to complete drying. The maximum permissible storage time in low temperature drying systems depends on grain type and physical condition, moisture content, and temperature. Saul and Lind (1958) and later Steele et al. (1969) monitored CO2 production to indicate the amount of dry matter loss incident to microbial growth in shelled corn. From these studies, the allowable storage time was established based on a dry matter loss of less than 1% (Figure 1.4). Steele et al. (1969) calculated the permissible storage time for fieldshelled corn with 30% mechanical damage with temperature and moisture content. Storage time was determined based on 0.5% dry matter loss (Figure 1.5). Thompson (1972) incorporated these data into a deterioration index and adopted a dry matter loss of 0.5% as the major constraint in establishing minimum airflow requirements for drying grain with unheated air.


18

Figure 1.4


Permissible storage time for corn based on a dry matter loss of less than 1%. (From Foster, G.H. [1982]. Drying cereal grains, in Storage of Cereal Grains and their Products, Christensen, C.M., Ed., American Association of Cereal Chemists, St. Paul, MN, pp. 79–116. With permission.)

1.2.1.4 Maintenance of Seed and Grain Quality Historically, research has demonstrated that low kernel temperatures are desirable for better maintenance of seed and grain quality. Storage of seeds at low temperatures (cold storage) is in widespread practice today, and studies have shown that the lower the temperature (within certain limits), the longer the seeds maintain their full viability. A rule of thumb was proposed (Harrington, 1973) where the relationship between seed longevity and the seed temperature and moisture is expressed arithmetically. This rule states that a seed’s lifespan in storage is doubled for each 5°C decrease in temperature (within the range of 0 to 50°C) and for each 1% decrease in seed moisture (within the range of 5 to 14%). Hukill (1963) approached the problem of defining the relationship between viability and environmental factors in a different way. He used the data of Toole and Toole (1946; 1954) on the viability of soya beans to develop the concept of an “age index.” Roberts (1974) summarized the attempts to define quantitatively the relationship between temperature, moisture content, and period of viability. This dependence of seed life on storage temperature (and humidity) was expressed mathematically in a formula by Roberts (1974), where the seed lifespan can be predicted according to data on seed temperature and moisture. Roberts (1974) determined a series of viability constants that make possible the prediction of the time needed for viability to drop to any given level of germination and that enable the equation to be applied to many different seeds. From this work he derived many convenient nomograms. A simplified version, based on work by Linko (1960) for estimating the half viability period for wheat under a wide range of conditions, is shown in Figure 1.6. It demonstrates the extent to which low temperatures may be used to prolong storage life. Another convenient form for expressing loss of viability at higher (Kreyger, 1967) and lower (Burrell, 1966) temperatures is illustrated in Figure 1.7.



Figure 1.5

19

Permissible storage time for field-shelled corn with 30% mechanical damage with temperature and moisture content. Storage time was determined based on 0.5% dry matter loss. (From Steele, J.L., Saul, R.A., and Hukill, W.V. [1969]. Deterioration of shelled corn as measured by carbon dioxide production, Trans. ASAE, 12, 685–689. With permission.)

Germination tests as a measure of stored grain viability are important in the evaluation of grain quality in storage. Grain with reduced viability reportedly becomes vulnerable to mold attack and is more susceptible to deterioration. Consequently, seed storability (period over which seeds can be safely stored at specified references or standard conditions) becomes greatly reduced. 1.2.1.4.1

Maintenance of Germination

The terms germination and viability are often used interchangeably. The Association of Official Seed Analysts defines seed germination as “the emergence and development from the seed embryo of those essential structures which, for the kind of seed in question, are indicative of the ability to produce a normal plant under favorable conditions” (Copeland and McDonald, 1985). Viability, on the other hand, refers more directly to the ability of seeds “to survive as viable regenerative organisms” until the time and place are right for the beginning of a new generation. “Like other forms of life, seeds cannot retain their viability indefinitely and eventually deteriorate and die.” (Copeland and McDonald, 1985). Germination is important in cooling by aeration or in chilled storage — not only because the germ viability is crucial to the reproductive cycle of seed grain,


20


Figure 1.6

Estimated half-life for wheat (time taken for germination to fall to 50%) under a wide range of storage conditions. Dotted line shows half-life for wheat stored at 5°C and 20% moisture. (From Burrell, N.J. [1982]. Refrigeration, in Storage of Cereal Grains and their Products, Christensen, C.M., [Ed.], St. Paul, MN, pp. 407–441, based on Linko [1960] and Roberts [1974]. With permission.)

Figure 1.7

Estimated time for the viability of malting barley to fall by 5% at a wide range of temperatures and moisture contents. Note that at temperatures below about 10ºC, damp seed may remain dormant for many months and may also become water sensitive. (From Burrell, N.J. [1982]. Refrigeration, in Storage of Cereal Grains and their Products, Christensen, C.M. [Ed.], St. Paul, MN, pp. 407–441, based on Burrell [1966] and Kreyger [1967]. With permission.)



21

Table 1.7

Allowable Storage Time (in Days) of Seed Grain as a Function of Moisture Content (% wb) and Temperature (°C)

Moisture Content (% wb)

5°C

10°C

15°C

20°C

18% 20% 26%

80 42 15

33 20 8

25 15 6

13 9 4

From Bewer, H.E. (1957). Getreidekonservierung mit kalter Nachtluft. München Wolfrathausen, Ber. Inst. Landtechnik No. 47, Bonn.

but the germ is essential in several end uses of cereal grain, such as malting and brewing, distilling, and sprouting (Christensen, 1982). In 1927, Reimann noted that grain storage in Germany was worry-free between November and March. However, in April and May problems arose as the ambient temperatures surrounding the grain storage increased. He stated that minimal quality losses (i.e., germination) in the storage of bread grains could be achieved when moisture contents were below 14% wet basis (wb) and grain temperatures less than 10°C. No decrease in the viability or damage to milling and baking qualities were found in wheat samples stored at 5°C and moisture contents of 10 to 20% (wb) were reported by Swanson (1941). Carter and Young (1945) investigated the effect of moisture content, temperature, and storage time on the development of “sick” wheat (i.e., brown germs). The proportion of “sick” wheat increased with higher moisture content, temperature of the grain, and longer storage times. Only a small percentage of “sick” wheat developed over 32 days in wheat at 18.6% moisture stored at 5°C. The initial conditions of different batches of grain vary considerably. Shands et al. (1967) have shown that the weather before harvest can seriously affect the germination potential of cereal grains. Cool storage also may reduce the germination of grain, particularly if it has been damaged by combining during harvest (Arnold, 1963). Blum and Gilbert (1957) have suggested that inhibition of germination can result from fungal activity at high moisture content. Cooling of wet grains to subzero temperatures can be detrimental, however. For example, Agena (1961) measured the effect of refrigeration on the germination of cereals at temperatures between 6 and –24°C and at moisture contents of 20 to 26%. He found that for wheat, barley, and rye, temperatures below –6°C damaged the seed in 24 hours and that germination failure increased as the temperature was progressively reduced and as moisture contents were increased. Research has now fully established that the dormancy of some grains, particularly some varieties of barley, is maintained for long periods unless the grain is thoroughly dried and stored warm for a period of 2 to 3 weeks. Exposure of moist grain to cool storage, such as that experienced in chilling, prolongs natural dormancy. It may, however, also impose a secondary dormancy or “water sensitivity” that prevents barley from germinating after it is steeped in water during the malting process. Malting and seed barley should, therefore, be dried normally because cool, moist storage conditions adversely affect germination. Bewer (1957) stored the seeds of primary bread grains (wheat, oats, barley, and rye) at moisture contents ranging from 18 to 26% and temperatures of 5 to 20°C. The seed quality was evaluated by testing germination. He concluded that the higher the moisture content and temperature in the ranges studied, the sooner spoilage would occur. His results are summarized in Table 1.7. Papavizas and Christensen (1958) evaluated viability, brown germs, and percentage of seeds invaded by fungi. They concluded, “Evidence suggests that wheat with a moisture content up to 16% may be stored without obvious deterioration for a year at a temperature of 10°C or below.” Agena (1961) investigated the effect of storing higher moisture grain at temperatures of –24 to 6°C on the viability


22


of grain and mold development. At 0°C he recommended maximum storage times of 40 days at 26%, 60 days at 22%, and 170 days at 18% moisture content. No reduction in baking quality was observed. Christensen and Kaufmann (1969) reported that fungi-free maize stored for two years at 18% moisture and 15°C had a germination rate of 96%; but when stored at 15.6 to 15.8% moisture at 5, 10, and 15°C for two years, the germination rate was 100%. However, No. 2 maize stored at 12ºC decreased in germination from 60% at the beginning of the test to 42% after 6 months, to 36% after 12 months, and to 1% after 18 months. When maize was inoculated with a mixture of Aspergillus species and stored at 15.9% moisture content and 15°C, germination was only 48% after 2 years. The germination of 18% moisture maize inoculated with A. flavus stored at 15°C over 4.5 months was 62%. Come (1982) noted that seeds of tropical and subtropical origin retained high viability when stored in bulk silos in a dry 5°C atmosphere. Burrell (1974) noted that the conditions stated by various researchers on the safe storage with respect to seed germination varied considerably. Apparently, different batches of grain behave differently when exposed to the same set of storage conditions. He stated, “For the preservation of high germinability, drying is preferable to chilled storage” in cool and moist conditions. The estimated safe storage time to preserve the viability of barley at a 95% germination rate was determined to be a function of temperature and moisture content. Generally, the lower the moisture content and the lower the temperature, the longer the seed can be stored (Copeland and McDonald, 1985). The knowledge of the appropriate limits is the basis for the proper engineering design of a grain cooling and seed storage system. 1.2.1.4.2

Maintenance of Grain Quality by Cooling

Rates of chemical deterioration such as oxidation of fats and vitamin loss occurring in grain during storage are very slow and sometimes insignificant at low temperatures. The rate of chemical reaction taking place in stored food is halved with each decrease of 10°C in temperature. Therefore, cool storage is important for the prevention of deteriorative changes in stored grain. Converse et al. (1977) reported on changes in quality of wheat stored with and without aeration in concrete silos. Aeration of wheat significantly reduced losses in germination and development of free fatty acids. Wheat from aerated silos had a better physical appearance and a more desirable aroma than wheat from non-aerated silos. Grain in storage, even though a living component of the grain bulk ecosystem (Figure 1.1), is actually in a dormant state in which all biotic activities of the grain are imperceptibly slow. This state of inactivity should be maintained for as long a period as possible, since activation of life processes in grain leads to loss of viability, followed by deterioration in quality. Thus, low ambient temperatures, introduced into the grain bulk by aeration, are very beneficial in keeping the grain in the state of quiescence needed to maintain grain quality over long periods of storage. By the 1970s considerable practical experience with grain chilling had been obtained, and earlier recommendations were refined. A summary of current grain chilling and storage recommendations is listed in Table 1.8. The data on the relationships between moisture content, temperature, and allowable storage time of small grains are based on the research work of Bewer (1957), Agena (1961), Kosmina (1956), Scholz (1962), and Jouin (1965). The chilled storage of grains above 22% moisture was recommended in 1972, but this practice was no longer considered practical by 1989 because drying was more economical. Chilling maize proved to be more complicated than chilling small grains (Sulzer-Escher Wyss, 1980). According to research recommendations, maize above 21% moisture content should only be stored short-term under continuous chilling at 3 to 5°C without reheat. Maize at 19 to 21% moisture can be stored 3 to 6 weeks by chilling once to 8 to 10°C. By the end of the storage period, the maize needs to be dried to a safe level. Maize at 17 to 18% moisture chilled to less than 10°C



Table 1.8

MC (%) 12–15a 15–16.5a 16.5–18a 18–20a 20–22a 22–25b 25–30b >30b

23

Moisture Content Ranges (mc), Grain Temperatures (°C), and Allowable Storage Times (AST) of Grains with Different End Uses under Chilled Storage Conditions Seed Grain and Malting Barley °C AST 9–12 8–10 5–7 5 5 5 4–5 —

1.5 y< 1–1.5 y 4–6 m 2–3 m 3–4 w 1–2 w 2–3 d —

Bread Grains °C AST 10–12 9–10 8–10 8–10 6–8 5–7 4–5 —

1.5 y< 1.5 y< 5–10 m 2–7 m 4–16 w 3–8 w 5–10 d —

Feed Grains °C AST 10–14 10–12 8–10 8–10 8–10 5–8 4–5 4–5

1.5 y< 1.5 y< 6–13 m 3–9 m 5–20 w 10–25 w 14–30 d cv because, when a substance is heated at constant pressure, not only is its internal energy increased but energy also has to be supplied to work against its surroundings as it expands. The re-derivation of Equation 3.45 began with the definition of enthalpy, Equation 3.50, which can be used to express the change of enthalpy at constant pressure, namely: ∆H = ∆U + P∆V

(3.56)

∆H = mc p ∆T

(3.57)

From Equations 3.52 and 3.45:

Mixtures of air and water vapor at atmospheric temperature and pressure may be treated as an ideal gas, the enthalpy of which has the convenient property of being independent of pressure (Potter and Somerton, 1995). This becomes obvious from Equation 3.50, when Boyle’s law is expressed as PV = constant, suggesting that no matter how the pressure, P, or the volume, V, may change, the enthalpy, H, remains unchanged at a given temperature. In the case of substances such as air that do not change phase when they are heated or cooled under normal atmospheric conditions, the specific enthalpy, h, per unit mass is defined as:

(

h = h0 + cp T − T 0

)

(3.58)

in which h0 is the specific enthalpy at some reference temperature, T0. The total enthalpy, H, of a substance is calculated by multiplying its enthalpy per unit mass (i.e., its specific enthalpy, h) by its mass, m. In mathematical symbols: H = mh

(3.59)

The reference temperature is somewhat arbitrary; and in the case of water, it is set at 0.01°C at which the internal energy of liquid water at a pressure of 611.3 Pa is assigned the value of 0 J/kg (Kirillin et al., 1981). These latter conditions are known as the triple point of water at which water vapor, liquid water, and ice all coexist in equilibrium with each other. Since specific enthalpy, h, is defined as:


106


h = u + Pv

(3.60)

in which u and v are the internal energy per unit mass and volume per unit mass respectively, the specific enthalpy, h, at the reference conditions for u can be calculated as: h = 0 + 611.3 × 0.0010002 = 0.611 J kg

(3.61)

because the specific volume of liquid water at 0.01°C and 611.3Pa is 0.0010002 m3/kg. Reference states for other substances are defined under different conditions. For example, carbon dioxide is at its reference point when it has a temperature of 0°C at a pressure of 98kPa. In grain storage applications, air and water vapor behave as ideal gases so that the difference between the enthalpy, h2, at the temperature T2, and the enthalpy h1 at the temperature T1 is found from:

(

)

(

)

h2 − h1 = h 0 = c p T2 − T 0 − h 0 − c p T1 − T 0 = c p (T2 − T1 )

(3.62)

in which cp is again the specific heat at constant pressure. When calculating the enthalpy change of a single substance such as air or water, the reference enthalpies and temperatures do not have to be known. In almost all calculations involving practical processes, the reference enthalpies and temperatures cancel out. This is observed in calculating the heat load on an air conditioning system in Example 3.15 and the adiabatic saturation temperature outlined in Section 3.13. This canceling out of the reference states provides a very useful check on whether the algebra has been carried out properly. When considering a mass, m, of a substance, the change in enthalpy of the substance that does not change its phase is: H2 − H1 = mc p (T2 − T1 )

(3.63)

which is the same as the change in the sensible heat of the substance, given by Equation 3.48. When dealing with practical problems associated with the aeration of grains, it is often necessary to calculate changes of the enthalpy of a substance such as water that changes into a vapor when it is heated. The specific enthalpy hw of water vapor is defined as:

(

)

hw = hw0 + c pw T − Tw0 + hv

(3.64)

in which hv is the latent heat of vaporization of water at the temperature T, hw0 is the reference enthalpy of liquid water at temperature Tw0 , and cpw is the specific heat of liquid water at constant pressure. In this equation the enthalpy of water vapor is computed by first calculating the enthalpy of liquid water at temperature, T, and this is expressed by the terms hw0 + c pw T − T 0 . The difference between the specific enthalpy of water vapor and liquid water at the temperature, T, is the latent heat of vaporization that must be supplied to increase the internal energy, u, of the water and increase the value of Pv , which also occurs in the definition of h (Equation 3.60). To summarize, the enthalpy of water vapor is computed by first calculating the enthalpy of liquid water at a temperature T, and adding to this the increase in enthalpy required to vaporize the water also at temperature T.

(

)


AMBIENT AIR PROPERTIES IN AERATION

107

Example 3.14 Use Equations 3.49 and 3.64 to calculate the differences in specific enthalpy of water vapor when it is heated from 20 to 30°C. Take the specific heat of liquid water at constant pressure to be 4.18 kJ/kg°C. Method The specific enthalpies of water vapor at 20°C and 30°C are designated by hw1 and hw2 respectively, so: At 20°C:

(

)

(

)

hw1 = hw0 + 4.18 × 20 − Tw0 + 2501.33 − 2.363 × 20 = hw0 + 4.18Tw0 + 2537.67 and at 30°C: hw 2 = hw0 + 4.18 × 30 − Tw0 + 2501.33 − 2.363 × 30 = hw0 + 4.18Tw0 + 2555.84 The difference in specific enthalpies hw2 – hw1 is found to be 18.17 kJ/kg. Grain-storage scientists often want to calculate the changes in enthalpy of air–water vapor mixtures. For example, it may be necessary to calculate the cooling load on an air conditioning system used to cool air for aeration. At a more fundamental level, enthalpy balances are carried out in detailed analyses of heat and moisture transfer processes that occur within bulks of aerated grains. The change of enthalpy of a mixture of air and water vapor is estimated by calculating the change of specific enthalpy of air and the change of enthalpy of water vapor that is in the air and adding the two results. The procedure is illustrated by the following example. Example 3.15 A brewery operating in a tropical climate decides to cool moist barley in storage for several weeks ahead of production using chilled air. During the day the temperature of the ambient air is 31°C, and its humidity is 0.019 kg water/kg dry air. The brewer plans to cool the barley to a temperature of about 18°C. The supplier of the grain chiller advises that this can be achieved by cooling the air to 10°C, when it will have a humidity of 0.0076 kg of water per kg of dry air. After it has been chilled, this air has a relative humidity of close to 100%, which would cause molding in barley close to where the air enters the grain. To prevent the barley from molding, the chilled air has to be reheated to 20°C to reduce its relative humidity to about 50%. If the mass flow rate of the air to be cooled is 1.4 kg/s, what is the cooling capacity of the grain chiller, and how much power does the heater consume when raising the temperature of the air from 10 to 20°C? The specific heats of dry air and water at constant pressure are 1.003 and 4.18 kJ/kg respectively. Method Figure 3.15 shows the mass, enthalpy, and heat flows around the grain cooling unit. If fat represents the total mass flow rate of moist air entering the grain cooling unit — the combined flow rate of dry air and the water vapor associated with it — and fa is the mass flow rate of dry air: fat = fa (1 + win )

(3.65)


108


Figure 3.15

Mass and energy flows around the chilling and reheating sections of a refrigerated grain cooling unit.

which is rearranged to give: fa =

fat 1.4 = = 1.374 kg dry air second (1 + win ) 1.019

(3.66)

Although not needed in the forthcoming calculations, it is noted that the mass flow rate of water vapor entering the grain cooling unit is 1.4 – 1.374 = 0.026 kg/second. The flow rate, fc, of the condensate leaving the cooling unit is the difference between the rate at which water vapor flows into the system and that flowing out of the system, i.e.: fc = fa (win − wcool ) = 1.374 × (0.019 − 0.0076) = 0.01566 kg water second

(3.67)

This system does no work on the environment (except for the work associated with changes in PV) so the cooling load, Q˙ cool , is simply equal to the rate of change of enthalpy of the air and water streams entering and leaving the system (Potter and Somerton, 1995). As a result, the energy balance can be written in words as: Rate of enthalpy entering the chiller in the warm air = Rate of enthalpy leaving the chiller in the cooled air + rate of enthalpy leaving the chiller in the liquid condensate + the heat removed by the cooling coils of the chiller

(3.68)

The rate at which enthalpy (formerly known as total energy) enters the chilling unit can be expressed as the flow rate at which dry air through the unit as fa kg per second multiplied by the specific enthalpy of the air/water vapor mixture. Associated with each kilogram of dry air that enters the unit is win kilograms of water vapor. The dry air has a specific enthalpy of ha0 + ca Tin − T 0 kJ/kg, and that of water vapor is hw0 + cw Tin − T 0 + hvin . The enthalpy of the

(

)

(

)



109

(

(

)

(

)

)

mixture of dry air and water vapor is therefore ha0 + ca Tin − T 0 + win hw0 + cw Tin − T 0 + hvin kJ per kg of dry air, so the rate at which enthalpy enters the chiller is fa times this quantity. The rate at which enthalpy leaves the chiller is calculated in a similar way. The enthalpy of the liquid condensate is hw0 + cw Tc − T 0 , the enthalpy of liquid water at a temperature of Tc. It has already been established that its mass flow rate, fc, is fa (win − wcool ) . The temperature, Tc, of the condensate is assumed as that of the air leaving the cooling section of the chiller, in this case 10°C. Equation 3.68 can therefore be expressed algebraically as:

(

{

)

(

)

(

(

)

(

)

fa ha0 + ca Tin − T 0 + win hw0 + cw Tin − T 0 + hvin

{

(

(

)} )

= fa ha0 + ca Tcool − T 0 + wcool hw0 + cw Tcool − T 0 + hvcool

{

(

+ fa (win − wcool ) hw0 + cw Tc − T 0

)}

(3.69)

)} + Q˙

cool

This can be rearranged so that the required cooling load, Q˙ cool , of the chiller can be calculated explicitly, thus:

{

Q˙ cool = fa ca (Tin − Tcool ) + cw win Tin + win hvin − cw wcool Tcool − wcool hvcool − cw (win − wcool )Tcool

}

(3.70)

The values of the standard enthalpies and temperatures do not occur in the final equation. If they did, they would indicate that an error had been made in the algebra. The cooling load, Q˙ cool , is 68.75 kW. From a practical point of view, it should be recognized that the electrical power consumption of air conditioners is usually only about one third of their cooling duty. In this case the power consumption of the compressor would be about 23 kW. In this grain chilling unit, the air leaving the evaporator section is saturated with water vapor; so it must be heated by 10°C to reduce its relative humidity to minimize mold development on grain at the air inlet duct. This is a sensible heating process because the humidity of the air remains constant at 0.0076 kg/kg while it is being heated; in mathematical symbols, wout = wcool. The enthalpy balance can again be stated in words as: Rate at which enthalpy enters the heater in the cool air stream + rate at which enthalpy leaves the heater in the air stream = rate of heat entering the heater through the heating element

(3.71)

Mathematically, these enthalpy and heat values are:

{

(

(

)

)} + Q˙ (h + c (T − T ) + h )} (

)

fa ha0 + ca Tcool − T 0 + wcool hw0 + cw Tcool − T 0 + hvcool

{

(

)

= fa ha0 + ca Tout − T 0 + wout

0 w

heat

(3.72)

0

w

out

vout

This equation is easily rearranged to enable the load on the heating element to be calculated, i.e.:

{

}

Q˙ heat = fa ca (Tout − Tcool ) + cw (wout Tout − wcool Tcool ) + (wout hvout − wcool hvcool )

(3.73)

Inserting the appropriate values into this equation, and again noting that wout = wcool, results in:


110


Q˙ heat = 1.374 × {1.003 × (20 − 10) + 4.18 × 0.0076 × (20 − 10) + 0.0076 × 2.363 × ( −20 + 10)} ≈ 13.92 kW

(3.74)

To reduce the relative humidity of the chilled air to a sufficiently low value to prevent it from causing grains to become moldy requires approximately 14 kW more power to heat the air. A further 3 kW of power would be consumed by the fans used to cool the condenser, so the total power consumption of this grain chiller would be about 40 kW. 3.12.1

How to Quickly Calculate Enthalpy Changes

Post-harvest technologists and air conditioning engineers often need quick rules of thumb to perform calculations when away from their offices without access to data on properties of materials, or when they need to perform approximate calculations quickly. Quick estimates provide a good idea of the magnitudes of quantities such as mass flow rate, heating loads, and humidity changes. A quick, approximate method of calculating the change in enthalpy of moist air is presented below. When air is heated at constant moisture content, the change in its specific enthalpy is almost equal to the number of degrees Celsius that the air is heated. Hence, if air is heated from 9 to 31°C and its humidity stays constant, its enthalpy increases by (31 – 9) = 22 kJ/kg of dry air. This is purely sensible heat because no change in moisture content has occurred. When air is humidified at constant temperature, its enthalpy increases by about 2.555 kJ/kg of dry air for every 1 g/kg increase in moisture content. (Conversely, if the moisture content increases by 0.3914 g/kg, the associated increase in enthalpy of dry air is 1 kJ/kg.) For example, if air is humidified from a moisture content of 10 g/kg to 21 g/kg at constant temperature, its enthalpy increases from 2.555 × (21 – 10) = 28.1 kJ/kg. This is primarily the result of a change in latent heat. When the air changes in both temperature and moisture content, the total enthalpy change is the sum of the sensible and latent heat changes. For example, if air with a temperature of 19°C and a moisture content of 10 g/kg is heated and humidified to 31°C and 19 g/kg, the change in its enthalpy is calculated as: total change in enthalpy = sensible change + latent change = (31 – 19) + 2.555 × (19 – 10) = 12 + 22.995 ≈ 35 kJ/kg. A more accurate value calculated from thermodynamic tables is 35.27 kJ/kg, which indicates that the error of the approximate calculations is about 0.75%. 3.12.2

Enthalpy Measurement Using the Psychrometric Chart

The modification of environments in stored grains usually results in a change in the enthalpy of the intergranular air. The psychrometric chart provides the stored-grain technologist and design engineer with a convenient tool for quickly calculating the differences in enthalpy of air at different conditions. Figure 3.16 shows how enthalpy is represented on the psychrometric chart. The enthalpy scale is usually drawn along a diagonal. The full psychrometric chart has enthalpy values at 1 kJ/kg intervals as shown in Figure A.1 in Appendix A. Example 3.16 Using Figure 3.16, find the difference in the enthalpies of air for the conditions 40°C dry-bulb temperature and a humidity of 11.6 g water/kg dry air, and air with a dry-bulb temperature of 15°C and a humidity of 5.9 g water/kg of dry air.



Figure 3.16

111

Lines of constant enthalpy represented on a psychrometric chart.

Method Using the method described above, the enthalpy of air that has a dry-bulb temperature of 40°C and a humidity of 11.6 g water/kg of dry air, point A, is 70 kJ/kg. Air with a dry-bulb temperature of 15°C and a humidity of 5.9 g/hg, point B, has an enthalpy of 30 kJ/kg. The difference in enthalpy of air at these two conditions is (70 – 30) = 40 kJ/kg.

3.13 THE ADIABATIC SATURATION TEMPERATURE The air used to aerate a bulk of stored grain usually increases or decreases in humidity as it passes through the grain bulk. In this section a simpler system will be considered that consists of air flowing through an apparatus containing water. This simple system, shown schematically in Figure 3.17, is considered sufficiently large for the air to enter the system, to reach equilibrium at the same temperature as that of the water, and to become saturated with water vapor. The temperature of the air leaving the system (and that of the water with which it is in equilibrium) is known as the adiabatic saturation temperature, which is numerically very close to the wet-bulb temperature of the air. This closeness is a peculiar feature of the air/water system. The wet-bulb and adiabatic saturation temperatures are not the same for other systems, such as the ethanol/air system for example. An understanding of this system is a useful precursor to understanding the air/water/grain system because the analysis of both systems requires a sound understanding of enthalpy balances. It can be seen from Figure 3.17 that air with a humidity ratio, w, flows into the system at an airflow rate, fa, whereupon it comes to temperature and moisture equilibrium with water at the adiabatic saturation temperature, T*. The air leaves the system with a temperature, T*, and it is saturated with a humidity ratio of w*. Because the air gains moisture as it passes through the system, a mass flow rate, fw, of water with a temperature of T* is added to the system as shown in the figure. No heat crosses the walls of the system, hence its appellation adiabatic; and no work is


112


Figure 3.17

A schematic diagram of the equipment used to produce air at its adiabatic saturation temperature.

done by the system. As a result, the first law of thermodynamics can be applied to this system in the very simple form of an enthalpy balance, i.e., the rate at which enthalpy enters the system equals the rate at which enthalpy leaves the system (Potter and Somerton, 1995). In addition to satisfying the enthalpy balance, mass balances on the dry air and the water must also be met. 3.13.1

Mass Balances

The mass balance on dry air flowing into and out of the system is quite straightforward; it is fa in both cases. The mass balance on the water is calculated as follows: The rate of water in the air entering the system in the air + the rate of make-up water = the rate of water in the air leaving the system

which is expressed algebraically as: fa w + fw = fa w *

(3.75)

which with slight rearrangement becomes: fw = fa (w * − w)

(3.76)

This equation signifies that the rate of make-up water that flows into the system to maintain a constant quantity of water in the system is exactly equal to the amount of water that evaporates into the air stream. 3.13.2

Enthalpy Balance

The rate at which enthalpy (energy) enters the system in the air stream + the rate at which enthalpy enters the system in the make-up water = the rate at which enthalpy of the moist air exits the system.

Mathematically, this becomes:

{

(

)

(

(

)

fa ha0 + ca T − T 0 + w hw0 + cw T − T 0 + hv

{

(

= fa h + ca T * −T 0 a

0

) + w * (h

0 w

(

)} + f {h

+ cw T * −T

0 w

w

0

) + h )} * v

(

+ cw T * −T 0

)}

(3.77)



113

in which w* and h*v are the saturation humidity and latent heat of vaporization water at the saturation temperature, T*. Substituting Equation 3.76 into Equation 3.77 results in:

{

(

)

(

(

)

(

)

)

(

(

fa ha0 + ca T − T 0 + w hw0 + cw T − T 0 + hv + (w * − w) hw0 cw T * −T 0

{

(

(

)

= fa ha0 + ca T * −T 0 + w * hw0 + cw T * −T 0 + hv*

))}

)}

(3.78)

Inserting the values of 1.003 and 4.18 kJ/kgK for ca and cw , respectively, and making use of Equation 3.49 for the values of the latent heat of vaporization of water results in: 1.003 (T − T *) + w (2501.33 + 1.814T − 4.18T *) − w * (2501.33 − 2.363T *) = 0

(3.79)

The humidity, w, and temperature, T, of the air entering the system are given, and the task is to find the adiabatic saturation temperature, T*. From Equation 3.9 finding w*, thus: w* = 0.622

ps patm − ps

(3.80)

where ps is the saturation vapor pressure of water at T*. Both ps and h*v are both functions only of the adiabatic saturation temperature, T*; hence, when Equation 3.80 is inserted into Equation 3.79, the only unknown in the resulting expression is the adiabatic saturation temperature, T*. Just as in the calculation of wet-bulb temperature, Newton’s method (Stein, 1987) can be used to solve Equation 3.79, which is expressed in the form: f (T *) = 1.003 (T − T *) + w (2501.33 + 1.814T − 4.177T *) − w * (2501.33 − 2.363T *) (3.81)

( )

The task is to find the value of T*, which makes f T ∗ = 0 so that Equation 3.79 is satisfied. Following the same procedure as that described in Section 3.8, the solution algorithm is expressed as: T * p+1 = T * p −

( ) ( )

f T *p

df T * p

(3.82)

dT * p in which:

df (T *) dw * = −1.003 − 4.177 w + 2.363 w * − (2501.33 − 2.363T *) dTw dTw

(3.83)

dw * dw * dps = dT * dps dT *

(3.84)

Now:

From Equation 3.9:


114


dw * 0.622 patm = 2 dps ( patm − ps )

(3.85)

 6800  dps ps − 5 =  dT * (T * + 273)  (T * + 273) 

(3.86)

and, as is Equation 3.22:

The implementation of this algorithm is illustrated in the annotated program presented in Appendix III of this chapter. The above program calculates the adiabatic saturation temperature of air with a dry-bulb temperature of 17°C and a relative humidity of 50% to be 11.3°C, which is the same as the wetbulb temperature calculated in Section 3.8.

3.14 SPECIFIC VOLUME The specific volume, v, of an air–water vapor mixture is the volume (cubic meters) of the mixture that contains one kilogram of dry air. It is the reciprocal of the density (i.e., one divided by density). Hence, if air has a specific volume of 0.8 m3/kg of dry air, its density is 1/0.8 = 1.25 kg of dry air/m3. Note that the density is not the same as the density based on one kilogram of moist air per cubic meter. The reason for using definitions of specific volume based on the mass of dry air is because it makes calculations easier when quantities such as enthalpy and humidity are also related to the mass of dry air. For example, the rate of enthalpy change of a volume flow rate, V˙ , of moist air (which is readily measured) can be determined by calculating the mass flow rate, fa, of dry air. Thus: fa =

V˙ m 3 of moist air per second 3 v m of moist air per kg of dry air

(3.87)

V˙ kg dry air per second v

(3.88)

that is: fa =

The rate, Q˙ , at which heat must be added to or removed from the air stream when its specific enthalpy (kJ/kg dry air) changes by ∆h is therefore given by: V˙ Q˙ = ∆h kJ s (≡ kW ) v

(3.89)

It can be seen that by expressing quantities on the basis of the mass of dry air, the calculations become quite cogent and coherent. Air and water vapor close to atmospheric conditions behave like ideal gases. This means Dalton’s law of additive pressures can be applied, which states: The pressure of a gas mixture is equal to the sum of the pressures that each gas would exert if it existed alone at the same temperature and volume of the mixture.



115

Applying this law to a mixture of air and water vapor that occupies a volume V at a temperature T, the pressure, pa, of the air can be calculated from the ideal gas law, i.e.: pa =

ma RaT V

(3.90)

in which ma kg is the mass of the air and Ra kJ/(kg·K) is the specific gas constant of air. In the same volume and at the same temperature, the vapor pressure of water vapor, pw , in the air is calculated using the ideal gas law as if the air molecules were not present. This is written as: pw =

mw Rw T V

(3.91)

If one kilogram of dry air is considered so that ma = 1, the mass of water vapor in the air is w kg, since the definition of humidity, w, is the mass of water per kilogram of dry air. In this case, atmospheric pressure, patm, is the sum of the pressures of air and water vapor. Thus: patm = pa + pw

(3.92)

By using Equations 3.90 and 3.91 and noting that it is the specific volume, v, being calculated, this can be written as: patm =

1 ⋅ RaT w ⋅ Rw T + v v

(3.93)

T {R + wRw } v a

(3.94)

or: patm =

Hence, the volume of moist air that contains 1 kg of dry air — the specific volume — is calculated using the equation: v=

T {R + wRw } patm a

(3.95)

The values of Ra and Rw are 0.2870 kJ/(kg·K) and 0.4615 kJ/(kg·K), respectively. Example 3.17 Calculate the specific volume of air at atmospheric pressure, 101.325 kPa, at a temperature of 36°C when its humidity is 0.0174 kg/kg. Method Inserting these values into Equation 3.95 results in: v=

(36 + 273.15) 0.287 + 0.0174 × 0.4615 ( ) 101.325

i.e., the specific volume, v, of the moist air is 0.900 m3/kg of dry air.


116


Figure 3.18

A simplified psychrometric chart showing lines of constant specific volume.

3.14.1 Specific Volume and the Psychrometric Chart Lines of constant specific volume on a psychrometric chart are illustrated in Figure 3.18. Air with a dry-bulb temperature of 11°C and a humidity ratio of 4 g/kg of dry air, point A, has a specific volume of 0.81 m3/kg of dry air. If the air is heated to 32°C and its moisture content is maintained at 4 g/kg of dry air, point B, the specific volume increases by thermal expansion to 0.87 m3/kg. Now, if the air temperature is constant at 32°C and the moisture content is increased to 26.0 g/kg, point C, the specific volume increases to 0.90 m3/kg. This increase in specific volume results from the extra water vapor in the moist air.

3.15 DESIGN CALCULATIONS USING THE PSYCHROMETRIC CHART The psychrometric chart enables grain-storage technologists to carry out a range of estimates and calculations very quickly. When users become thoroughly familiar with its use, its graphical nature provides a variety of insights into how stored grains are likely to be affected by aerating with air under a range of conditions. Examples of calculations that can be carried out using the psychrometric chart are presented in the following sections. 3.15.1

Heating and Cooling

Before stored grains are cooled by aeration, it is sometimes beneficial to dry them with warm air. Note, however, that the airflow rate needed to dry grains using natural air is at least an order of magnitude greater than that required to cool grains. A task that often confronts grain-storage technologists involves calculating the capacity of the heater required to warm ambient air before it enters the silo. The psychrometric chart can be used for this calculation.



117

Example 3.18 A farmer wishes to dry 55 tonnes of grain stored in a silo. The farmer has been advised that the grain will reach its target moisture content in about one month if it is ventilated continuously with 0.9 m3 per second of air that has been heated by 5°C, when its average temperature will be 25°C and its relative humidity will be 50%. Use the psychrometric chart to calculate the capacity of the heater the farmer needs to specify. Method The average temperature of air entering the heater is (25 – 5) = 20°C. When the air is heated its moisture content stays the same; hence, the humidity of the air entering the heater is the same as that leaving it at 25°C and 50% relative humidity. From the psychrometric chart, observe that this humidity is 10 g/kg (≡ 0.01 kg of water/kg of dry air). The specific volume of the air entering the heater is seen from the psychrometric chart to be about 0.843 m3 of moist air/kg of dry air; hence, its density is 1/0.843 = 1.167 kg of dry air/m3 of moist air. Given that the farmer proposes to dry the grain using 0.9 m3 of ambient (moist) air per second, the mass flow rate of dry air required is 1.186 × 0.9 = 1.067 kg/s. The enthalpy of air with a humidity of 10 g/kg at temperatures of 20°C and 25°C is found from the psychrometric chart to be 45.5 and 50.5 kJ/kg of dry air, respectively. In this case the capacity of the heater required to heat the air by 5°C is the mass flow rate multiplied by the change in enthalpy; i.e., heating capacity = 1.067 × (50.5 – 45.5) = 5.33 kJ/s ≡ 5.33 kW. 3.15.2

Cooling and Dehumidification

The storability of food grains is closely related to their temperature and moisture content. Aeration is used to modify these two variables.To be effective, it is sometimes necessary to modify the conditions of the air used to aerate grain. Air conditioning (chilling) is used to change both the temperature and humidity of the air. The psychrometric chart is very useful in providing both quantitative and qualitative insights into these processes. Example 3.19 A seed merchant located in the humid tropics wants to store large quantities of grains that are to be used for seed. He proposes to aerate the seeds with air that has a temperature of 15°C and a relative humidity of 50%. To do this he proposes to purchase a grain chilling system that cools 0.6 m3/s of ambient air, with an average temperature of 30°C and a relative humidity of 70%, to 5°C and a relative humidity of 100%. This cool air is then heated to 15°C to prevent seeds in the vicinity of the aeration duct from becoming moldy. Calculate the cooling capacity of the air conditioner required to cool the air to 5°C, and the capacity of the heater required to reheat the air to 15°C. Method The required cooling and heating capacities are calculated from the formula: Cooling capacity = mass flow rate of air × change in enthalpy of the air

From the psychrometric chart, the enthalpy of air at 30°C and 70% is 78.5 kJ/kg; and the enthalpy of air at 5°C with a relative humidity of 100% is 18.5 kJ/kg. The specific volume, v, of air at the entrance to the cooler is 0.884 m3/kg dry air; hence, its density ρ ( = 1 v) is 1.131 kg of dry air/m3. The mass flow rate of the air is therefore 0.6 m3/s × 1.131 = 0.6786 kg dry air/s.


118


Now: The cooling load = mass flow rate × specific enthalpy change = 0.6786 × (78.5 − 18.5) = 40.72 kJ s = 40.72 kW. In this calculation the rate at which the enthalpy of the condensed water leaves the system is ignored. This enthalpy is negligibly small. The relative humidity of air leaving the cooling unit is 100%, which would cause the grain in the vicinity of the aeration duct to become moldy. Its relative humidity can be reduced by heating it by 10 to 15°C, whence its relative humidity is reduced to 50%. The specific enthalpy of this heated air is 28.5 kJ/kg; hence, the heating load = mass flow rate × specific enthalpy change = 0.6786 × (28.5 – 18.5) = 6.79 kJ/s = 6.79 kW. This heat could be provided by the condenser of the grain chiller.

3.16 MEASURING THE PROPERTIES OF MOIST AIR To ensure that aeration systems are working properly, grain-storage technologists often have to measure the temperature and moisture content of the intergranular air within the stored grains and the conditions of the air used during the aeration process. 3.16.1

The Measurement of Temperature

Temperature is more reliably measured than humidity, and thermocouples made from two different metals are often convenient to use in grain storage applications because: 1. They are sufficiently strong to be forced into bulks of grains, although care must be taken not to form kinks in them which adversely affect their accuracy. 2. Thermocouples are relatively inexpensive, and they are disposable should they be destroyed during grain handling operations of grain stores in which they happen to be installed. 3. Thermocouples can be quite accurate (±0.5°C) when they are calibrated. 4. A wide variety of instruments are capable of measuring the voltage outputs of thermocouples, converting them to temperatures and recording the results. Care should be taken to ensure that the conversion process does not lead to unacceptably high errors (greater than 1°C).

Thermocouples operate as a result of the Seebeck effect, which results in a voltage forming in a circuit made up of two dissimilar metals, such as copper and constantan, as shown in Figure 3.19. One junction is maintained at a reference temperature, and the other junction (used to measure temperature) is known as the test junction. If the circuit is broken as shown in the figure, the voltage may be measured using a potentiometer or voltmeter; and the temperature of the test junction can be calculated from this reading. In modern equipment, the cold junction is usually replaced by a block of metal that is such a good conductor of heat that both terminals of the thermocouple may be assumed to be at the same temperature. The block of metal is not held at a reference temperature, but its temperature is measured; and this is used in calculating the temperature of the test junction. Thermocouples that consist of one wire fabricated from copper and the other from the alloy constantan are commonly used in grain storage applications, and it is known as a type T thermocouple. In one experiment, over 10km of type T thermocouple wire was installed during the instrumentation of a grain storage shed (Thorpe et al., 1980).



Figure 3.19

3.16.2

119

A schematic diagram of a thermocouple formed from copper and constantan wires. One junction is held at a known temperature, and the other junction is used to sense the temperature that is to be measured.

The Measurement of Humidity

The measurement of humidity within bulks of grain and in the air used to aerate them is often more problematic than the measurement of temperature, particularly if the humidity sensors have to be left unattended because they become damaged by dust or go out of calibration. Four methods often used by grain-storage technologists to measure grain interstitial air humidity are: • • • •

Battery-powered hygrometer or psychrometer (wet- and dry-bulb thermometers) Sensors that rely on the electrical conductivity of hygroscopic salts Dew-point temperature meters Capacitance measurements of samples of stored grains

Wet- and dry-bulb temperatures are generally used to ascertain the humidity of ambient air and the humidity of air flowing in ducts. Instruments are now readily available that measure humidity by measuring the change in electrical resistance of a hygroscopic salt. Such hygrometers have an accuracy of about 1% RH. They can be provided with saturated salts that, when they are in equilibrium with air, result in relative humidities known to be accurate to better than 0.5%. The instruments have built-in calibration programs that correct for room temperatures in a typical range of 15 to 30°C. A feature of these hygrometers is that they are also programmed to display the psychrometric properties discussed above — i.e., the wet- and dry-bulb temperature, the dew point temperature, relative humidity, the humidity ratio, specific enthalpy, and the vapor pressure of water vapor. McBea Services (1998) has developed a hygrometer especially for measuring the conditions within stored grains. This commercially available device is based on a hygroscopic salt hygrometer described above, and it displays a range of psychrometric properties. It consists of a nylon tube that is tipped by a perforated steel probe, enabling it to be inserted into bulks of grain. Air from deep within the grain is aspirated over the sensor of the hygrometer, and its conditions are displayed digitally. They can be transmitted via a communications port to a computer or data logger. Dew point meters are among the most accurate instruments for measuring the moisture content of air, because temperature and the amount of light reflected from a mirror can both be measured accurately. They can be used in the field, but the air for which humidity is to be measured needs to be forced through the meter using a small fan. Experience has shown that the mirrors in such devices tend to lose their reflectivity, and to be effective they need to be cleaned periodically. Notwithstanding these drawbacks, dew point meters are recommended when accurate measurements are to be made.


120


REFERENCES Ahmad, M. and Thorpe, G.R. (1997). The VUT solar desiccant system for cooling grains, Proceedings of the Australian and New Zealand Solar Energy Society Annual Conference, Canberra, December 1997. Çengel, Y.A. and Boles, M.A. (1998). Thermodynamics — An Engineering Approach, 3rd ed., WCB/McGrawHill, Boston. Hunter, A.J. (1987). An isostere equation for some common seeds, J. Agric. Eng. Res., 37, 93–107. Kirillin, V.A., Sychev, V.V., and Sheindlin, A.E. (1981). Engineering Thermodynamics, Translated by Semyonov, S., Mir Publishers, Moscow. McBea Services (1998). Wet-Bulb Temperature Instrument for Grain Quality Assessment, Horsham, Victoria, Australia. Potter, M.C. and Somerton, C.W. (1995). Thermodynamics for Engineers, Schaum’s Interactive Outline Series, McGraw Hill, New York. Stein, S.K. (1987). Calculus and Analytic Geometry, 4th ed., McGraw Hill, New York. Thorpe, G.R., La Fontaine, R.F., and Elder, W.B. (1980). The acquisition and analysis of data from a remotely sited 15,000 tonne capacity refrigerated grain store, Agricultural Engineering Conference, September/October, The Institution of Engineers, Geelong, Australia.



121

APPENDIX I A listing of a program to compute the wet-bulb temperature of air given its dry-bulb temperature and relative humidity. REM To calculate the wet-bulb REM temperature of air of a given dry-bulb REM temperature, T deg C, and relative humidity, rh%. REM INPUT " Dry-bulb temperature of air:"; T INPUT " Relative humidity of air, %:"; rh REM REM The first task is to calculate the humidity, w, REM of the air at temperature T and known rh. REM REM patm = 101325 REM REM Calculate the saturated vapor pressure, psa, of air REM at temperature T by means of equation 3.1 psa = 6E+25 / (T + 273.15) ^ 5 * EXP(-6800 / (T + 273.15)) REM REM Calculate the humidity, w, of the ambient air REM using equations 3.11 and 3.12. pw = .01 * rh * psa w = .622 * pw / (patm - pw) PRINT "humidity of ambient air:"; w a = .0004042 b = 5.816E-07 REM Make an initial guess of tw - let it be T,say. tw = T FOR I = 1 TO 20 REM From equation 3.1 ps = 6E+25 / (tw + 273.15) ^ 5 * EXP(-6800 / (tw + 273.15)) REM Equation 3.20 wsat = .622 * ps / (patm - ps) REM REM Equation 3.33 dwdps = .622 * patm / (patm - ps) ^ 2 REM REM Equation 3.34 dpsdt = ps / (tw + 273.15) * (6800 / (tw + 273.15) - 5) dwdt = dwdps * dpsdt REM REM Equations 3.26 and 3.27 ftw = w - wsat - (a + b * wsat) * (tw - T) dftw = -dwdt - a + b * T* dwdt - b * (tw * dwdt + wsat) REM REM Update wet-bulb temperature using equation 3.30 twnew = tw - ftw / dftw REM REM Check if the result is sufficiently accurate IF ABS(twnew - tw) < .000001 THEN GOTO 10 tw = twnew NEXT I 10 tw = twnew PRINT "Wet-bulb temperature:"; tw END


122


APPENDIX II An annotated listing of the Newton-Raphson search for the dew point temperature. CLS INPUT w tdp = 17 patm = 101325 FOR kk = 1 TO 10 REM Equation 3.1 ps = 6E+25 / (tdp + 273.15) ^ 5 * EXP(-6800 / (tdp + 273.15)) REM Equations 3.42 and 3.43 dfps = .622 * patm / (patm - ps) ^ 2 dpsdt = ps / (tdp + 273.15) * (6800 / (tdp + 273.15) - 5) REM Equations 3.39 and 3.41 ft = .622 * ps / (patm - ps) - w dft = dfps * dpsdt REM Calculation of the updated dew point temperature using equation 3.40 tdpnew = tdp - ft / dft REM Has the desired accuracy been achieved? IF ABS(tdpnew - tdp) < .000001 GOTO 1 tdp = tdpnew NEXT kk 1 tdp = tdpnew PRINT " Dew point temperature is:"; tdp; " degrees C"



123

APPENDIX III A program listing to calculate the adiabatic saturation temperature of air given its dry-bulb temperature and relative humidity. REM To calculate the adiabatic saturation REM temperature, TS, of air of a given dry-bulb REM temperature, T deg C, and relative humidity, rh%. REM INPUT " Dry-bulb temperature of air:"; T INPUT " Relative humidity of air, %:"; rh REM REM The first task is to calculate the humidity, w, REM of the air at temperature T and known rh. REM REM Calculate the saturated vapor pressure of air REM at temperature T REM REM Equation 3.1 ps = 6E+25 / (T + 273.15) ^ 5 * EXP(-6800 / (T + 273.15)) REM Calculate the vapor pressure, p, of the air p = rh * ps / 100 REM Find the humidity ratio of the air at temperature T using equation 3.8. patm = 101325 w = .622 * p / (patm - p) REM REM Now calculate the adiabatic saturation temperature, TW, REM REM Make an initial guess of TS (Choose the dry-bulb temperature) TS = T REM Implement Newton's method to calculate TS FOR I = 1 TO 20 ps = 6E+25 / (TS + 273.15) ^ 5 * EXP(-6800 / (TS + 273.15)) wstar = .622 * ps / (patm - ps) REM Equation 3.81 ftw = 1.003 * (T - TS) + w * (2501.33 + 1.814 * T - 4.177 * TS) ftw = ftw - wstar * (2501.33 - 2.363 * TS) REM Equations 3.83 to 3.86. dwdps = .622 * patm / (patm - ps) ^ 2 dpsdt = ps / (TS + 373.15) * (6800 / (TS + 273.15) - 5) dwdt = dwdps * dpsdt dftw = -1.003 - 4.177 * w + 2.363 * wstar - dwdt * (2501.33 - 2.363 * TS) REM Equation 3.82 TSNEW = TS - ftw / dftw REM Check that successive estimates of TS do not vary by more than 0.0001 degrees C IF ABS(TSNEW - TS) < .0001 THEN GOTO 10 TS = TSNEW NEXT I 10 TS = TSNEW PRINT "Adiabatic saturation temperature:"; TS END



CHAPTER

4

Physical Basis of Aeration Graham Thorpe

CONTENTS 4.1 4.2

Introduction...........................................................................................................................126 The Hygroscopic Nature of Food Grains ............................................................................127 4.2.1 The Interactive Effect of the Hygroscopic Nature of Food Grains and Atmospheric Humidity on Aeration Systems ...................................................129 4.2.2 Expressions for Grain Moisture Content Used in Commerce and Science............130 4.3 Sorption Isotherms................................................................................................................131 4.3.1 Determining the Differential Heat of Sorption using a Graphical Method ............134 4.3.2 Sorption Isotherms for Oilseeds...............................................................................139 4.3.2.1 An Algebraic Method of Calculating the Differential Heat of Sorption.................................................................................................141 4.4 The Integral Heat of Wetting of Grains...............................................................................144 4.5 The Specific Enthalpy of Moist Grains ...............................................................................145 4.6 Calculating the Conditions in Aerated Grain Stores ...........................................................148 4.6.1 Fronts and Zones ......................................................................................................148 4.6.2 Speeds of Fronts .......................................................................................................152 4.7 Calculating the Mass of Grains in a Grain Store ................................................................155 4.8 The Thermal Conductivity of Stored Grains .......................................................................157 4.8.1 The Specific Heat of Moist Grains ..........................................................................159 4.9 A Numerical Analysis of Aerated Grain Beds ....................................................................161 4.9.1 Mass Balances ..........................................................................................................161 4.9.1.1 Dry Air ......................................................................................................161 4.9.1.2 Moisture ....................................................................................................163 4.9.2 Energy Balance.........................................................................................................163 4.9.2.1 The Energy and Moisture Balances Expressed in Terms of the Temperature and Moisture Content of Stored Grains ..............................165 4.9.2.2 Analytical Methods of Calculating ∂hv ∂T and ∂HW ∂T ....................168 4.9.2.3 A Brief Excursion into Numerical Methods ............................................169 4.9.3 Numerical Solution of the Equations that Govern Heat and Moisture Transfer in Aerated Grain Bulks ..............................................................................171 4.10 The Effects of Respiration on Heat and Mass Transfer in Aerated Beds of Grains ..........174 4.10.1 Mass Balances ..........................................................................................................175 0-8493-1355-4/02/$0.00+$1.50 © 2002 by CRC Press LLC

125


126


4.10.2 4.10.3 4.10.4 4.10.5 4.10.6

Moisture Balance......................................................................................................175 Carbon Dioxide ........................................................................................................175 Oxygen......................................................................................................................176 Nitrogen and Other Non-Reacting Gases ................................................................176 Mass Balance on the Solid Substrate.......................................................................176 4.10.6.1 Thermal Energy Continuity ......................................................................176 4.10.7 A Slumping Bed of Grain ........................................................................................180 4.10.8 The Heat of Respiration ...........................................................................................182 4.11 Conclusions...........................................................................................................................183 References ......................................................................................................................................183 Appendix I A BASIC Computer Program used to Calculate the Ratio, hs hv , of the Heat of Sorption to the Heat of Vaporization of Free Water................................186 Appendix II An Implementation of the Analytical and Numerical Schemes to Evaluate ∂HW ∂T .................................................................................................187 Appendix III A Listing of the Program used to Calculate Heat and Moisture Transfer in an Aerated Bed of Grains ......................................................................................188 Appendix IV A Listing of a Numerical Procedure to Solve the Coupled Heat and Mass Transfer Equations that Govern Heat and Mass Transfer in Beds of Ventilated Grains ...............................................................................................191

4.1 INTRODUCTION The physical properties of the grain bulk are interrelated and are affected by other physical and biological variables. Knowledge of the physical properties of a bulk of grain provides the rational basis for the beneficial use of aeration. These physical grain properties are reviewed in this chapter. Grain-storage technologists help to maintain the quality of stored food grains and to reduce the likelihood of damage by insect pests, mites, and molds. To do this successfully, they must be able to manipulate the stored grains’ ecosystem so that the physical and chemical conditions within the grain kernels and in the spaces between the grains (the interstitial or intergranular spaces) are: • Hostile to harmful biological agents • Conducive to maintaining grain quality

The conditions in the intergranular spaces relate to humidity and temperature of the air, to the presence of poisonous gases such as fumigants, and to the presence of contact insecticides. Conditions within the intergranular spaces impact strongly on the conditions of the grain kernels. The temperature and humidity of the intergranular air are the key variables that are manipulated in the aeration process. The combination of these two variables has a profound effect on the rates at which biologic agents develop. It was pointed out in Chapter 3 that the humidity and dry-bulb temperature correspond to a wet-bulb temperature of air. Desmarchelier (1988) has developed a very useful and simple correlation between the wet-bulb temperature and the rates at which insect populations grow. Wilson and Desmarchelier (1994) show graphically how the propensity for molds to develop in stored grains is also a function of grain temperature and humidity. It has been pointed out that other determinants of the stored grain ecosystem, such as the presence of toxic gases (phosphine or carbon dioxide) also have a definite influence on the stored grain ecosystem. The physical processes involved in distributing fumigants around a bulk of stored grains are closely related to those that govern the cooling processes in an aerated grain store. Although the movement of fumigants is not specifically considered, an understanding of the mathematical descriptions of aeration leads naturally to an understanding of the movement of fumigants in stored grains.


PHYSICAL BASIS OF AERATION

127

The aim of this chapter is to show how to calculate the physical parameters operating in an aerated grain store. When bulks of grains are aerated, the grains can adsorb (take up) or desorb (give up) moisture. As a consequence of the hygroscopic nature of grain: • The temperature and moisture content of the intergranular air have a profound effect on the grain temperature and moisture content of grains stored in an aerated store. • The temperature and moisture content of the air used for aeration also have a strong effect on conditions within an aerated grain store. It is by no means impossible for aerated grains to increase to a temperature that exceeds the temperature of the air used for aeration.

A simple graphical method of calculating the conditions inside an aerated grain store will be presented, and it will be shown how to calculate the rate at which stored grain cools. A more rigorous mathematical analysis is presented, and it is shown how this can also be programmed.

4.2 THE HYGROSCOPIC NATURE OF FOOD GRAINS One of the most important properties of food grains is that they take up moisture from the atmosphere in which they are placed. Hence, grains that are harvested in a climate that has a high relative humidity are likely to have higher moisture content than those grains that are harvested in a climate with a low relative humidity. More importantly, when the grains harvested in a climate with a high humidity are stored, the intergranular air also has a high relative humidity. If this relative humidity exceeds 70%, molds are likely to develop when the grains are stored; and the grains may increase in temperature as a result of the molds respiring. This could lead to complete grain spoilage or to mycotoxins forming that are poisonous to humans and animals. High moisture contents can also lead to populations of mites developing, and insect pests usually develop more rapidly in moist grain than in arid grains. A high relative humidity of the intergranular air, and the resulting high grain moisture content, is a variable that should be controlled by stored-grain technologists. Grain contains dry matter and a quantity of water. Water in grain is found to be held in three main forms: absorbed water, adsorbed water, and chemically bound water (Hunt and Pixton, 1974). Absorbed water is held loosely within the tissues of the grain by capillary forces. The forces of capillary attraction, which depend on the capillary dimensions, lower the vapor pressure of the held water to below that of free water. The absorbed water is associated with water-soluble constituents such as sugars, mineral salts, organic acids, and some vitamins that the grain contains. These constituents form a concentrated solution, depending on the amount of moisture present, and determine the level of the vapor pressure. Adsorbed water consists of the gas molecules of water held to the surface of porous particulate materials by electrostatic forces. The vapor pressure of such adsorbed water is much lower than that of free water since it is bound to grain constituents by powerful forces. Chemically bound water, or water of composition, is combined in chemical union with grain constituents during developmental growth and maturation of the seeds. The proteins present in grain may suffer irreversible changes in their properties when this water is removed. If grain is kept in a sealed vessel, the water present in the grain exerts a vapor pressure less than the saturation value of free water. The more tightly the sorbed water is bound in the grain, the less it affects the relative humidity of the intergranular air space. There is an exchange of water with the surrounding atmosphere to maintain a balance between the moisture in the grain and that in the atmosphere. When the water vapor pressure of the grain is equal to that in the intergranular air spaces, the moisture content of the grain is described as its equilibrium moisture content. Each grain has a characteristic equilibrium curve, which is obtained by plotting a graph of moisture contents against different relative humidities of the air. The equilibrium values vary depending on the species and varieties of grain, temperature, and the previous moisture level of the grain.


128

Figure 4.1


Lines of constant grain moisture content superimposed on a simplified psychrometric chart. Lines of constant relative humidity are also shown.

Adsorption describes the process of condensation of a vapor onto a solid surface at pressures less than saturation (Hunter, 1991). Food grains and seeds adsorb moisture from the atmosphere at all relative humidities exceeding zero, and as a result they are described as hygroscopic. Grain kernels or seeds are thus quite different from glass beads or steel ball bearings, which remain completely dry even in atmospheres with relative humidities up to 100%. However, if ball bearings are colder than the dew point temperature of the air in which they are in contact, then water condenses on the surfaces of the ball bearings. This continues until the ball bearings warm up to the dew point temperature. Water also condenses on the surface of grains or seeds that are colder than the dew point temperature of the air surrounding them, but the results can be disastrous; the grains germinate or go moldy. The hygroscopic nature of food grains is illustrated by considering what happens when a kernel of 12% moisture content wheat (wet basis) is placed in 80% relative humidity air. Over an extended period, the moisture content of the wheat kernel increases to and then remains at about 16%. At this level of moisture content, the grain kernel and the surrounding air are in equilibrium. Each type of grain — wheat, barley, canola — reaches a fixed moisture content that depends on the temperature and relative humidity of the surrounding air. This moisture content is referred to as the equilibrium moisture content (EMC). Many authors refer to the equilibrium relative humidity (ERH), which is the relative humidity that air reaches when it is in thermodynamic equilibrium with grain at a given moisture content and temperature. Figure 4.1 shows lines of constant equilibrium moisture content of wheat drawn on a psychrometric chart. These EMC curves are described in detail by mathematical equations in Section 4.3. Experiments have clearly shown that food grains are hygroscopic. Attempts have been made to explain why this should be so. One answer lies in the fact that food grains are not solid bodies like ball bearings. The molecular structure of grain kernels is characterized by countless numbers of small pores. Even the pericarp or outer layer of the kernel varies by grain type and variety as to its moisture permeability. As a result, moisture vapor is able to diffuse by varying rates from the outer surface of a kernel through the pores and to penetrate the entire kernel. The diameters of grain kernel pores are very small — typically 0.01 mm. Their large number means that in total they have a large surface area. Water molecules are attracted to the surfaces of



129

the pores, where they condense. As the water vapor pressure increases from zero, water molecules form a monomolecular layer on the grain kernel substrate. Further increases in the vapor pressure of moisture result in more water molecules condensing to form multimolecular layers. Eventually the surface moisture content is sufficiently high for liquid water to fill successive layers of pores. Thus, moisture gradually penetrates to the center of grain kernels and equalizes out. Readers who want a deeper understanding of sorption processes in capillary porous media such as food grains may wish to refer to the work of Luikov (1980) that presents a comprehensive view of transport phenomena in capillary porous media. Hunter’s (1991) work presents useful insights into the mechanisms of sorption in step-by-step detail. Food grains and seeds adsorb moisture from the atmosphere at all relative humidities exceeding zero. It must be emphasized that, while a single wheat kernel may absorb moisture from a high humidity atmosphere, it may take many months for a bulk of grains to adsorb the same level of moisture from the atmosphere. Such long periods of time were measured by Pixton and Griffiths (1971), and their work has been interpreted in a detailed scientific framework by Thorpe (1981, 1982) and Thorpe et al. (1991a, 1991b). 4.2.1

The Interactive Effect of the Hygroscopic Nature of Food Grains and Atmospheric Humidity on Aeration Systems

The hygroscopic nature of food grains has a profound effect on long-term storage and the performance of grain aeration and drying systems. Some of these effects are: 1. Most of an aerated grain bulk does not usually attain the temperature of the air used for aeration. This is because the grain EMC differs than the air ERH used to aerate the bulk. Grain moisture content near the air inlet may become drier or more moist, depending on the initial grain moisture content and the ERH of the inlet air. A practical example illustrates this point. Consider a bulk of wheat that has an initial moisture content and temperature of 10% (wet basis) and 30°C, respectively, and which is aerated with air with an average temperature of 10°C and a relative humidity of 80%. In a typical aeration system, if aeration time to cool grain is 200 hours, most of the wheat would have cooled to only about 17°C during this time. Further operation of the aeration fan has little effect on the temperature of most of the grain. The grain with a temperature of 17°C would have dried to about 9.5% (wet basis). A small fraction of the grain — that located near where the air enters the grain bulk — would have cooled to 10°C and increased in moisture content to about 18%. In other words, the grain near the air inlet duct would be at the equilibrium moisture content (EMC) associated with the air. Most of the grain would not cool to 10ºC because moisture adsorbed by the grain near the air inlet releases heat of sorption. This heat is the latent heat of the water vapor that condenses onto the grains. This heat prevents the grain downstream of the air inlet cooling to 10ºC. The reason why the average moisture content of the grain falls only by about 0.5% is that the absolute humidity of the integranular air in the warm grain is initially 10 g/kg, whereas the absolute humidity of the aeration air entering the grain is only 6 g/kg. Hence, as the air is forced through the grain, more moisture is blown out in the air leaving the grain bulk than entering the grain bulk. Since mass is neither created nor destroyed, this difference can only arise by the grain bulk losing moisture — i.e., becoming drier. The grains near the air inlet may become moist after extended aeration because the air used for aeration has a higher ERH than the grain. For this reason, once the grain bulk has cooled, aeration should be stopped to prevent increased moisture content of the grain around the aeration duct. Open-cycle desiccant grain cooling systems, such as that developed by Thorpe and Ahmad (1998) and Thorpe (1998), are effective at both drying and cooling stored grains. 2. The rate at which a bulk of aerated grain cools is strongly affected by the hygroscopic nature of food grains. In the case discussed above, all of the grain bulk eventually cools to 10ºC; but because the food grains are hygroscopic, it takes 100 times longer to cool to this temperature than if the food grains


130


were not hygroscopic. When the grains have cooled to 10ºC, the moisture content of the entire bulk will be about 18%. 3. The hygroscopic nature of food grains determines the degree to which they can be dried by air of a given humidity and temperature. If wheat is aerated continuously with air with a temperature of 30ºC and a relative humidity of 40%, it will eventually dry to a moisture content of about 10% (wet basis). This occurs when all of the grain is in equilibrium with the air entering the grain store. However, it would take many months for all of the grain bulk to dry to this low moisture content if normal aeration airflow rates were to be used. Grain-storage technologists should bear this in mind when requested to design an aeration system that is to act also as a grain dryer. 4. The hygroscopic nature of grains has a determining effect on the stored grain ecosystem. The relative humidity and temperature of the intergranular air profoundly influence biological and chemical phenomena that occur in stored grains. The relative intergranular air relative humidity depends on the grain moisture content. Fungi do not become active until the relative humidity of the intergranular air exceeds about 70%, depending on temperature (Wilson and Desmarchelier, 1994), and populations of mites become viable only when the intergranular relative humidity exceeds about 70% (Christensen and Kaufmann, 1969). This again underscores the desirability of storing dry grains. Many insect pests that infest stored grains cease breeding when the wet-bulb temperature of the intergranular air falls below about 12ºC (Desmarchelier, 1988). Manipulating the wet-bulb temperature of the intergranular air in a mass of stored grains through aeration provides a simple and direct method of temperature control.

4.2.2

Expressions for Grain Moisture Content Used in Commerce and Science

Grain moisture contents are expressed in several ways. In the world of commerce and grain trading, moisture contents are expressed in terms of the percentage wet basis, Mw , defined by: Mw = 100

Wwater Wdry matter + Wwater

(4.1)

in which Wwater is the weight of water that is associated with a weight, Wdry matter, of dry matter of the grains. In other words, the moisture content expressed on a wet basis is the weight of water divided by the total weight of wet grains. The moisture content of grains is sometimes expressed on a fractional wet basis, Ww , which is simply: Ww =

Wwater Wdry matter + Wwater

(4.2)

The moisture content of grains expressed on a percentage dry basis, M, is calculated from the ratio of the weight of water to the weight of dry matter and it is defined as: M = 100

Wwater Wdry water

(4.3)

This can be expressed as a fractional dry-basis water content, W, as follows: W=

Wwater Wdry matter

(4.4)



131

This definition is preferred when performing theoretical calculations because the mass of dry matter in a given region of a grain bulk is often considered to be constant. As a result, changes in the amount of water in that region of grain are linearly related to changes to the moisture content (dry basis) of the grain. The relationships between fractional dry-basis and wet-basis moisture contents are readily found by dividing the numerator and denominator of Equation 4.2 by Wdry matter, and using the definition of the fractional dry-basis moisture content given by Equation 4.4, i.e.: Wwater Wdry matter W Ww = = Wdry matter Wwater 1+ W + Wdry matter Wdry matter

(4.5)

Note that a grain moisture content expressed on a wet basis has a lower numerical value than when the same moisture content is expressed on a dry basis. For example, if the grain moisture content, W, on a dry basis is 0.2 (20%), its moisture content, Ww , on a wet basis is found from equation: Ww =

0.2 W = = 0.1667 1 + W 1 + 0.2

(4.6)

i.e., the moisture content expressed on a wet basis is 0.1667. It is a simple matter to rearrange Equation 4.5 so that the fractional dry-matter moisture content can be expressed in terms of the fractional wet-basis moisture content, thus: W=

Ww 1 − Ww

(4.7)

When dealing with moisture contents expressed in percentage terms, the relationships between the wet- and dry-basis forms are: 100 M 100 + M

(4.8)

100 Mw 100 − Mw

(4.9)

Mw = and M=

4.3 SORPTION ISOTHERMS As the relative humidity of air surrounding a grain kernel increases from zero, the water molecules first form a monomolecular layer on the grain substrate. Then a multimolecular layer may form; and as the relative humidity increases further, free water condenses in the micropores of the kernel. The surfaces of the free water are not flat or planar but have curved menisci with small radii in the small pores. More energy is required to evaporate a given quantity of water from a grain kernel than it takes to evaporate free water with a flat surface. The reasons for this can be gleaned from a study of surface chemistry, such as that presented by Moore (1963).


132


Water molecules in the body of the water are attracted in all directions to other water molecules. However, net forces acting on molecules close to the surface of the water attract them into the body of the water. Hence, work must have been done on these near-surface molecules in moving them to the surface — their energy is greater than those molecules deeper in the water. The area-to-volume ratio is smallest in a spherical drop of water. When water droplets form in air, they tend to become spherical to minimize their surface energy, because work must be done when the surface area of a body of water is increased. Solid surfaces also have energy associated with them, which explains why work must be done in comminution (grinding) processes that result in the increased surface areas of solids. The surface energy of water is higher at concave surfaces because surface tension energy is inversely proportional to the radius of the water surface in contact with its vapor (Luikov, 1980). The vapor pressure of water vapor is also lower when the radii of the menisci are smaller. The total energy required to remove a unit mass of water from grain kernels, the differential heat of sorption, hs, is conveniently partitioned into two components — namely, the latent heat of vaporization of free water, hv , and the differential heat of wetting, hw . The differential heat of wetting, hw , is the decrease in energy of absorbed moisture expressed as J/kg of moisture that is adsorbed at constant grain moisture content. Therefore, hw can also be defined as the energy reduction that would occur if one kilogram of water could be added uniformly to such a large mass of grains that its moisture content would change by a negligible amount: hs = hv − hw

(4.10)

In Chapter 3 it was noted that, when water evaporates into a stream of dry air, the temperature of the water decreases to the wet-bulb temperature. The decrease in temperature results from the fact that the water molecules with the highest velocities near the surface tend to escape into the air stream, and slower moving molecules are left behind. If it is desired to maintain the temperature of the water constant, more heat would have to be added to the liquid water. This is the latent heat of vaporization of water. A similar situation exists when drying grains — heat must be added to the grains to transform the water in the grains into water vapor. However, in this case, the water molecules are adsorbed onto the surfaces of the pores within the grain kernels; and as a result, their kinetic, rotational, and vibrational energies are all lower than those that occur in free water. The energy of the molecules of water vapor that has been evaporated from the grain kernels is exactly the same as that vapor that would have arisen from the evaporation of free water. As a result, more energy must be supplied to vaporize a given mass of water adsorbed to grains than to free water. Consider a large mass of grains with a moisture content W and one kilogram of saturated water vapor. Both the water vapor and the grains are at the same temperature, T. First, heat is removed from the system so that the water vapor condenses at the temperature T; and since the mass of water is one kilogram, the amount of heat removed is the latent heat of vaporization of water, hv kJ kg. Figure 4.2 shows this to be negative, which is in keeping with the fact that heat that leaves a system is considered to be negative in an energy balance. Observe what happens when the liquid water is added uniformly to the grains. When the water vapor has been condensed, there remains one kilogram of liquid water and a mass of grains so large that its moisture content is barely changed when the one kilogram of moisture is added to it. For example, if the initial moisture content of the grain were 0.1 kg of water/kg of dry grain, and the mass of dry matter in the grains is 10 tonnes, then the addition of the 1 kg of water increases the grain moisture content to 0.1001 kg of water/kg of dry grains. The point is this: when water is added to the grains, their moisture content remains virtually unchanged. As noted when the liquid water is adsorbed by the grains, the water molecules become bound to the surfaces within the pores of the grain kernel, and they lose energy. The grains and water increase in temperature, and the



Figure 4.2

133

Energy exchanges associated with the condensation of water vapor to liquid and the wetting of grains by water.

original temperature, T, of the system is restored by removing an amount of heat, hw, from the grain/water system. This quantity of heat is called the differential heat of wetting of the grain at the temperature T and moisture content W. Because heat is lost by the system, it is conventional for hw to be automatically assigned a negative value; hence it is not necessary to write – hw when heat is rejected from a system. This is different from the latent heat of vaporization hv , which is thought of as a positive quantity when it enters a system. Hence, when latent heat of vaporization is rejected, it must be written as –hv. The change in enthalpy, ∆h, of the grain/water system in the process of going from 1 kg of water vapor and a large mass of grains to the situation in which the moisture has been adsorbed by the grains is –hv + hw . At 25°C the latent heat of vaporization of


134


Figure 4.3

The variation of the differential heat of sorption as a function of the moisture content of paddy.

water is 2442.3 kJ kg: The total heat transferred to the surroundings during the condensation and wetting processes is the total heat of wetting; and when the moisture content is 0.1, the heat liberated is 3116.4 kJ kg of water that is adsorbed. This simple calculation illustrates that heat must be removed during both stages of the process; and the fact that hw is deemed to be negative results in its having to be added to the negative value of the latent heat of vaporization. When the above adsorption process is reversed, as occurs in the drying of grains, heat must be added to the moist grains to liberate the water as vapor. The energy is used to “shake” the water molecules free from the solid surfaces of the grain kernels so that they can notionally become liquid water, and then further energy is required to vaporize the water. The amount of energy required to remove 1 kg of water from a very large mass of grains at effectively constant moisture content and temperature is called the differential heat of sorption, hs (Equation 4.10). Since the differential heat of wetting, hw , is a negative quantity, the differential heat of sorption exceeds the latent heat of vaporization of free water. The variation of the differential heat of sorption as a function of moisture content for paddy is shown in Figure 4.3. The quantity of energy that must be added to moist grain to liberate one kilogram of moisture vapor is hw . As the moisture content increases, hw decreases, which is consistent with the previous discussion on the surface energy of moist grain. The differential heat of wetting, hw , has a high absolute value when the grain moisture content is low, and it is reduced as larger pores fill with moisture. 4.3.1

Determining the Differential Heat of Sorption Using a Graphical Method

The differential heat of sorption can be found by comparing the vapor pressures of water in equilibrium with moist grains and free water. This indicates that the differential heat of sorption is uniquely related to the sorption isotherm. Othmer (1940) has presented a simple method of calculating the differential heat of wetting based on the Clapeyron equation, written as: hl dp = dT (V − v) T

(4.11)

where p is the equilibrium vapor pressure of water above either liquid water or a grain kernel at temperature T, hl is the latent heat of vaporization or the heat of sorption, V is the volume per unit mass of water vapor, and v is the volume of a unit mass of water. Under typical atmospheric



135

conditions, the ideal gas laws can be used to express the volume V of water vapor in terms of pressure p and T, thus: V=

RT p

(4.12)

The volume, V, of a unit mass of water vapor is about 1700 times greater than the corresponding volume, v, of condensed water vapor at the normal boiling point. Hence, v can be ignored without significantly affecting the accuracy of Equation 4.11. Ignoring v and substituting Equation 4.11 into Equation 4.12, the equation then takes the form of the Clausius-Clapeyron equation (Moore, 1963) which is written as: hp dp = l dT RT 2

(4.13)

Equation 4.13 can be rearranged with the result: dT 1 dp = hl p RT 2

(4.14)

Equation 4.14 applies to any substance to which the underlying assumptions apply; and if it is applied to liquid water, the latent heat of vaporization is hv . Hence, Equation 4.14 becomes: 1 dps dT = hv ps RT 2

(4.15)

in which ps is the saturation vapor pressure of free water, which has been reported by Hunter (1987) to be the following function of temperature: ps =

 −6800  6.0 × 10 25 exp   5 Tabs  Tabs 

(4.16)

If grain kernels with a fixed moisture content are held at the same temperature, T, as free water, the differential heat of sorption is hs, the corresponding vapor pressure of water is p′, and Equation 4.15 can be expressed as: 1 dp′ dT = hs p′ RT 2

(4.17)

As the temperatures of the grain kernels and free water are the same, Equations 4.15 and 4.17 can be equated, thus: 1 dp′ 1 dps = hs p′ hv ps

(4.18)

dp′ hs dps = p′ hv ps

(4.19)

or by rearranging:


136

Figure 4.4


A plot of ln p′ against ln ps of samples of wheat at three different moisture contents. Data taken from Tables 4.5 and 4.6: a) 10% mc, b) 12% mc, and c) 16% mc.

which may be integrated to yield: ln p′ =

hs ln ps + const hv

(4.20)

Since the ratio hs hv is independent of grain moisture content, W, and ps and p′ (for a given grain moisture content) are functions only of temperature. Hence, plotting ln p′ against ln ps, a straight line of slope hs hv is obtained. This slope will be different for each grain moisture content, W. It is pointed out above that the hygroscopic nature of food grains profoundly interacts with the air humidity to affect the performance of grain aeration and drying systems. If such systems are designed and operated accurately, equations are needed that relate the moisture content of grains and the conditions of the intergranular air. When moist air and grains are in equilibrium at atmospheric pressure, the grain moisture content is dependent on only two variables that describe the thermodynamic state. In practice, stored-grain technologists usually express grain moisture content as a function of the temperature, ºC, and relative humidity of the atmosphere surrounding a grain kernel. These equations are usually called sorption isotherms (although they are occasionally called isostere equations), and a typical set of isotherms is shown in Figure 4.1. The figure shows lines of constant grain moisture content plotted as a function of dry-bulb temperature, absolute humidity and relative humidity. Most of the equations are semi-empirical, and Sun and Woods (1993) have reviewed many of them. Although the equations have the same form, the constants in the various equations have to be modified to account for whether moisture is desorbed (removed from the grains) or adsorbed (added to the grains). Two isotherm equations approved by the ASAE are the Chung-Pfost (1976) and the modified Henderson (Henderson, 1952, Thompson, 1972) equations. The Chung-Pfost equation takes the form: A r = exp  − exp ( − BW )  T +C 

(4.21)



137

Table 4.1

Constants in the Chung-Pfost Isotherm Equation for a Number of Grain Types

Grain Type Barley Beans (edible) Corn (Yellow Dent) Peanut (kernel) Peanut (pod) Rice (rough) Sorghum Soybean Wheat (Durum) Wheat (hard) Wheat (soft)

A

B

C

761.74 671.78 312.31 254.98 521.99 594.65 1099.68 138.45 921.69 529.45 725.89

19.889 14.964 16.958 29.243 17.903 21.733 19.644 14.967 18.077 17.609 23.607

91.323 120.098 30.205 33.892 12.354 35.703 102.849 24.576 112.350 50.998 35.662

From Pfost, H.B., Rengifo, G.E., and Sauer, D.B. (1976). High temperature, high humidity grain storage, Paper No. 76-3520, ASAE, St. Joseph, MI.

in which r is the fractional relative humidity of air in equilibrium with the grains, and W is the moisture content of the grains, fractional dry basis. A, B, and C are constants that are specific to the grain under consideration. The moisture content, W, is expressed in terms of the relative humidity and temperature, thus: W=−

1  (T + C )  ln  − ln (r )  B  A

(4.22)

The values of the constants in Equation 4.22 are given in Table 4.1. The modified Henderson equation is:

(

r = 1 − exp − K (T + C)(100W )

N

)

(4.23)

in which C, K, and N are grain-specific constants. The grain moisture content, W, is expressed in terms of the fractional relative humidity r; and the temperature, T, is given by: 1

 ln (1 − r )  N W = 0.01    − K (T + C ) 

(4.24)

The constants used in the modified Henderson equation are given in Table 4.2. Example 4.1 Using the Chung-Pfost and modified Henderson equations, calculate the moisture content of Yellow Dent corn in equilibrium with air that has a dry-bulb temperature of 25°C and an absolute humidity of 14 g/kg. The atmospheric pressure is 101,325 Pa. Express the resulting grain moisture contents in terms of percentage wet basis. Method The Chung-Pfost and modified Henderson Equations 4.21 and 4.23 are expressed in terms of temperature and relative humidity, r; but in the example the humidity is expressed in terms of the


138


Table 4.2

Constants in the Modified Henderson Isotherm Equation for a Number of Grain Types

Grain Type

K × 105 *

N

C

Barley Beans (edible) Corn (Yellow Dent) Peanut (kernel) Peanut (pod) Rice (rough) Sorghum Soybean Wheat (Durum) Wheat (hard) Wheat (soft)

2.2919 3.5953 8.6541 65.0413 6.6587 1.9187 0.8532 50.3633 2.5738 2.3008 1.2300

2.0123 1.7639 1.8634 1.4984 2.5362 2.4451 2.4757 1.3628 2.2110 2.2857 2.5558

195.267 193.091 49.810 50.560 23.318 51.161 113.725 43.016 70.318 55.815 64.346

* The value of K in the case of barley is 0.000022919. From Pfost, H.B., Rengifo, G.E., and Sauer, D.B. (1976). High temperature, high humidity grain storage, Paper No. 76-3520, ASAE, St. Joseph, MI.

absolute humidity, w, or humidity ratio discussed in Section 4.3. To calculate the relative humidity, the vapor pressure, pw , of the water vapor in the intergranular air is determined by rearranging Equation 3.16, namely: w = 0.622

pw patm − pw

i.e.: pw =

wpatm 0.622 + w

In this case: pw =

0.014 × 101325 = 2244.5 Pa 0.622 + 0.01

The saturation vapor pressure of water vapor at 25ºC is determined from Equation 3.1, i.e.: ps =

6 × 10 25 6800  6 × 10 25 6800    = = 3168.75 Pa 5 exp − 5 exp −    T + 273.15 25 + 273.15  (T + 273.15) (25 + 273.15)

The relative humidity is defined by Equation 3.11, i.e.: r=

pw ps

which in this case is: r=

2244.5 = 0.708 3168.75

Using these data in the Chung-Pfost Equation 4.22 as it applies to Yellow Dent corn results in:



139

W=− −

1 25 + 30.205 ln − ln (0.708) 16.958  312.31  1 ln [( −0.17676) ( −0.3453)] 16.958

= 0.1649 This moisture content is expressed on a fractional dry basis. To convert it to percentage wet basis, Mw , Equation 4.8 is used thus: Mw = 100

W 0.1649 = 100 = 14.15% 1+ W 1 + 0.1649

The fractional dry basis of Yellow Dent corn is obtained from the modified Henderson Equation 4.24 as follows: 1

1

  1.8634  ln(1 − r )  N ln 0.292 W = 0.01    = 0.01  −5  − K (T + C )   −8.6541 × 10 (25 + 49.81)  1.231  = 0.01   0.006474 

0.53665

= 0.1671

i.e., the modified Henderson equation predicts the moisture content (fractional dry basis) of Yellow Dent corn in equilibrium with air with a humidity of 10 g/kg and at a temperature of 25°C is 0.1671, or 14.3% on a wet basis. 4.3.2

Sorption Isotherms for Oilseeds

The amount of oil in oilseeds such as canola and sunflower can constitute over half the weight of the seeds. The oils adsorb a negligible amount of water, and this has a profound effect on the sorption isotherm. For example, if an oilseed consists of 50% by weight of oil, and 10% of its weight is moisture, the dry matter of the seed constitutes 40% of the seed mass. The grain moisture content is therefore 10/40 = 25% (dry basis) of the dry matter. This is reflected by the relative humidity of the integranular air sufficiently high for the grains to become moldy, and possibly to ignite spontaneously. In this case, the relative humidity of air in equlibrium with the oilseeds with a moisture content of 10% would be more likely to mirror the relative humidity of air in equililibrium with cereal grains that have a moisture content of about 25%. This illustrates why moisture contents required for the safe storage of oilseeds are lower than those of grains. Pollio et al. (1987) expressed the relationship between equilibrium relative humidity of soybeans as a function of the moisture content based on the oil-free dry matter and the temperature of the seeds. This approach has been used by Steele (1990) to evaluate the isotherms of canola and sunflower seeds. He defined an oil-free dry matter moisture content, Wofdm, as: Wofdm =

100 M (100 − OC)

(4.25)

in which OC is the percentage of oil in dry matter of the seeds and M is the moisture content of the seeds expressed as a percentage dry basis. Steele’s experiments were carried out with oilseeds


140


Table 4.3

Values of A and B in Equation 4.26 for Canola as Functions of Temperature

Temperature range °C T ≤ 15 15 < T ≤ 25 25 < T ≤ 35 35 < T ≤ 45 T > 45

Table 4.4

A

B

40.5 40.5 – 0.93 (T – 15) 31.2 – 0.70 (T – 25) 24.2 + 0.04 (T – 35) 24.6

1.66 1.66 – 0.007 (T – 15) 1.59 – 0.008 (T – 25) 1.51 + 0.002 (T – 35) 1.53

Values of A and B in Equation 4.26 for Sunflower Seeds as Functions of Temperature

Temperature range °C T ≤ 15 15 < T ≤ 25 25 < T ≤35 35 < T ≤45 T > 45

A

B

107.3 107.3 – 3.09 (T – 15) 76.4 – 2.66 (T – 25) 49.8 + 1.27 (T – 35) 62.5

2.04 2.04 – 0.01 (T – 15) 1.94 – 0.014 (T – 25) 1.80 + 0.009 (T – 35) 1.89

that had oil contents ranging from 37 to 50%, relative humidities in the range of 10 to 95% and at temperatures of 15°C, 25°C, 35°C, and 45°C. Steele (1990) found that an isotherm that is mathematically similar to the Henderson (1952) equation (which has two empirically determined constants) accurately fitted his data at a given temperature. Here a modified form of the isotherm is proposed that interpolates the isotherms between the temperatures measured by Steele. No data are available for temperatures less than 15ºC and above 45ºC; hence, the isotherms that prevail at 15ºC and 45ºC, respectively, are taken at temperatures lower or greater than these temperatures respectively. The equations should be treated with caution outside the range tested by experiment. The proposed isotherm equation is:

(

−B r = exp − AWofdm

)

(4.26)

Expressions for A and B as functions of temperature for canola and sunflower seeds are listed in Tables 4.3 and 4.4 respectively.

Example 4.2 The oil content of a batch of canola seeds is 44%. Calculate the equilibrium relative humidity of the canola if its moisture content is 7% (wet basis) and its temperature is 27°C. Method The moisture content, M, of the canola on a dry basis is found from Equation 4.9, i.e.: M=

100 Mw 100 × 7 = = 7.53%dry basis 100 − Mw 93



Table 4.5

141

Temperature and Relative Humidity Data for Wheat with Moisture Contents (Wet Basis) of 10%, 12%, and 16%

Moisture Content 10% (Wet Basis) Temperature Relative °C Humidity 15 25 35

Moisture Content 12% (Wet Basis) Temperature Relative °C Humidity

0.29 0.34 0.40

15 25 35

0.53 0.57 0.60

Moisture Content 16% (Wet Basis) Temperature Relative °C Humidity 15 25 35

0.80 0.81 0.82

The moisture content, Wofdm, on a percentage oil-free dry basis is therefore given by Equation 4.25, i.e.: Wofdm =

100 M 100 × 7.53 = = 13.44% (100 − OC) 100 − 44

Equation 4.26 therefore leads to the determination of the equilibrium relative humidity thus:

(

)

(

− 1.59 − 0.008 ( 27− 25 ) ) −B r = exp − AWofdm = exp −(31.2 − 0.7 (27 − 25))13.44 (

)

= 0.6072 The equilibrium relative humidity of air in equilibrium with canola with oil content of 44% and moisture content of 7% wet basis at 27ºC is 0.6072. Example 4.3 The following sorption data were obtained for wheat. Estimate the values of the differential heats of sorption of water on the wheat at each of the conditions given in Table 4.5. Method The saturation vapor pressures of water at 15°C, 25ºC, and 35ºC can be calculated from Equation 4.16, and from Equation 3.11 the equilibrium vapor pressures, p′, of water vapor surrounding the grain kernels, can be found, thus: p′ = rps

(4.27)

The values of ln p′ and ln ps are given in Table 4.6, and they are given for the three grain moisture contents. It can be observed from Table 4.7 that the values of hs hv increase with decreasing grain moisture content, and the results indicate that hs hv = 1.27, 1.1, and 1.01 when the grain moisture contents are 10%, 12%, and 16% wet basis, respectively. Equation 3.49 can be used to calculate the latent heats of vaporization hv of water at the three temperatures, and this allows the differential heats of sorption to be calculated under the specified conditions. 4.3.2.1 An Algebraic Method of Calculating the Differential Heat of Sorption The relationships established above enable one to obtain mathematical expressions for the ratio hs hv from the sorption isotherm. Equation 3.11 defines the relative humidity of air, i.e.:


142


Table 4.6

Values of ln p′ and ln ps for Various Moisture Contents and Temperatures

Temperature °C

ln ps

15 25 35

7.44 8.06 8.64

Table 4.7

Grain Moisture Content, % Wet Basis 10 12 16 ln p′ ln p′ ln p′ 6.20 6.98 7.72

6.81 7.50 8.13

7.23 7.85 8.44

Values of hs / hv Calculated from an Othmer Plot

Temperature °C

hv , J/kg

15 25 35

2467 2443 2419

Grain Moisture Content, % Wet Basis 10 12 16 hs / hv = 1.10 hs / hv = 1.01 hs / hv = 1.27 hs, J/kg hs, J/kg hs, J/kg 3133 3103 3072

2714 2687 2661

2492 2467 2443

From Othmer, D.F. (1940). Correlating vapor pressure and latent heat data — a new plot, Ind. Eng. Chem., 32, 841–856.

p = rps

(4.28)

may be differentiated using the product rule to give the result: dp = rdps + ps dr

(4.29)

When this equation is substituted into Equation 4.19: rdps + ps dr hs dps = p hv ps

(4.30)

and making use of Equation 4.28, Equation 4.30 becomes: rdps + ps dr hs dps = rps hv ps

(4.31)

which can be rearranged into the form: dr r

h  dp =  s − 1 s h  v  ps W

(4.32) W

This equation has been written in a form that emphasizes that the changes in r and ps are restricted to take place along lines of constant grain moisture content. This condition is imposed



143

because the algebra is based on Equation 4.18, which also requires that the grain moisture content is constant. The saturation vapor pressure, ps is a function of only temperature; hence: ∆ps ≈

dps ∆T dT

(4.33)

Likewise, when a point moves along a line of constant grain moisture content drawn on a psychrometric chart, it can be seen that any change, ∆r, in the relative humidity is also a function only of temperature; hence: ∆r ≈

∂r ∆T ∂T W

(4.34)

Using these ideas in Equation 4.32: hs p dT ∂r −1 = s hv r dps ∂T

(4.35) W

This equation enables one to calculate hs hv , provided dps dT and ∂r ∂T W can be evaluated. The Equation 4.16 for the saturated vapor pressure, ps, can be differentiated to yield:   dps ps 6800 = − 5  dT (T + 273.15)  (T + 273.15) 

(4.36)

The derivative of r with respect to T, ∂r ∂T ∂ M depends on the sorption isotherm equation used. In the case of the modified Chung-Pfost isotherm, Equation 4.21: ∂r ∂T

= M

Ar

( T + C )2

exp ( − BM )

(4.37)

Example 4.4 Write a BASIC program using the modified Chung-Pfost equation to calculate hs hv for Durum wheat at temperatures of 10°C, 20°C, and 30°C, and moisture contents of 9%, 12%, and 15% (wet basis). Comment on the results. Method A list of the program used to calculate hs hv for Durum wheat is given in Appendix I. Output It can be seen from Table 4.8 that the output hs hv increases with decreasing grain moisture content, as would be expected. It can also be noted that hs hv is calculated to be dependent on temperature, but the dependence is small. This dependency may be real. Alternatively, it may have arisen as a result of experimental errors in measuring the sorption isotherm; or it may have resulted from the form of the sorption isotherm equation not exactly fitting the data.


144


Table 4.8

Output of the Program used to Calculate hs / hv

Temperature ºC

MC % w.b.

hs/hv

10 20 30 10 20 30

10 10 10 13 13 13

1.122975 1.113704 1.106104 1.061514 1.056876 1.053075

Reproduced with permission of CSIRO Entomology, Technical Report No. 76, 1988.

Hunter (1987) developed a sorption isotherm equation that guarantees that hs hv is independent of temperature, and it appears to be accurate. The equation enables the relative humidity of air in equilibrium with grains of a given moisture content and temperature to be calculated easily. However, it is not possible to revert the equation so that grain moisture content can be expressed explicitly as a function of relative humidity and temperature. Hunter’s equation can be reverted using numerical methods (Thorpe, 1994).

4.4 THE INTEGRAL HEAT OF WETTING OF GRAINS When grains are very dry, their differential heats of wetting have comparatively large (negative) values; and as they become more moist, the magnitude of the heat of wetting diminishes. At very high moisture contents, the differential heat of wetting of grains becomes zero because any water that condenses on the grains is not influenced by the solid surfaces; and it behaves exactly the same as free water. A procedure is required to determine the total amount of heat of wetting liberated when grain is wetted from a moisture content of zero to a final moisture content. This can be viewed as summing the product of the differential heats of wetting and small increases in moisture content over the range of moisture contents 0 to W. A natural way of doing this is to define the integral heat of wetting, HW , as: HW =

∫

W

0

hw dW

(4.38)

in which we recall that W is the moisture content of grain expressed on a fractional dry basis. If the integral heat of wetting is to be calculated accurately, reliable expressions for the differential heat of wetting hw in the moisture content range for 0% to the moisture content of interest must be available. This requires that sorption data be obtained over this range. Absolute values of the integral heats of wetting are often not needed. Only differences in integral heats of wetting are needed as in the equilibrium theory of heat and mass transfer in grains presented by Sutherland et al. (1971). Thorpe et al. (1990) calculated the integral heats of wetting from Hunter’s (1987) isotherm for nine grains, but the resulting expressions are mathematically complicated. The modified Chung-Pfost isotherm is far more tractable, and differences in the integral heats of wetting of grains are readily determined. (If the Chung-Pfost equation is accurate down to zero moisture contents, the absolute value of the integral heat of wetting can be calculated.) Making use of Equations 4.21 and 4.35: hs − hv dT ps Ar = exp ( − BM ) 2 hv dps (T + C) r and, making use of Equation 4.21, 4.10, and 4.38:

(4.39)



145

HW = − hv

Aps dT 2 dps (T + C)

∫

W

0

exp ( −100 BW ) dW

(4.40)

The pre-multiplier, 100, of B was introduced because the limits of integration are carried out between limits of fractional dry basis moisture contents. The integrated form of Equation 4.40 is: HW = hv

ps A dT 2 {exp ( −100 BW ) − exp ( 0 )} 100 B dps (T + C)

(4.41)

Example 4.5 Calculate the integral heat of wetting of 14% (wet basis) moisture content wheat at 25°C. Method The first task is to convert the moisture content of the grains on a fractional dry basis, noting that Ww = 14 / 100 = 0.14 , which is then used in Equation 4.7, i.e.: W=

0.14 = 0.1628 kg water kg dry grain 1 − 0.14

The saturation pressure at 25°C is calculated from Equation 4.16 as follows: ps =

 6.0 × 10 25 −6800  5 exp   = 3168.75 Pa (25 + 273.15)  (25 + 273.15) 

and dps dT is given by Equation 4.36:   dps ps 6800 = − 5 = 189.26 Pa °C dT (25 + 273.15)  (25 + 273.15)  The latent heat of vaporization, hv , of free water is found from Equation 2.25, thus: hv = 2501.33 − 2.363 × 25 = 2442.26 kJ kg The integral heat of wetting, HW , from Equation 4.41, can now be calculated, thus: HW = 2442.26

529.45 1 3168.75 2 {exp ( −100 × 0.1628) − exp ( 0 )} 100 × 18.077 189.26 (25 + 112.35)

= −104.7 kJ kg of water adsorbed by the grains

4.5 THE SPECIFIC ENTHALPY OF MOIST GRAINS If water were not adsorbed by grains, the specific enthalpy of moist grains could be calculated simply by adding the enthalpies of one kilogram of dry grains and the W kg of liquid water associated with them. However, water is adsorbed, and as a consequence its energy is lower than that of free


146


water. The amount that the energy is lower than that of free water is the integral heat of wetting, HW , which is always assigned a negative value as discussed above. The specific enthalpy, H kJ kg dry grains, of moist grain is therefore given by:

(

)

(

(

))

H = hσ0 + cσ T − T 0 + W h10 + c1 T − T 0 + HW

(4.42)

Again, as mentioned above, care must be taken when using Equation 4.42 because Hw can be obtained only by measuring sorption data to moisture contents below those that are usually measured. Given its importance in calculating the rates at which bulks of grain dry and cool, it is suggested that such sorption data be obtained. Example 4.6 Calculate the heat required to dry 1 kg of moist wheat that has an initial moisture content of 20% (wet basis) and a final moisture content of 10% (wet basis) at a constant temperature of 25°C. Assume that, after the water has been removed from the grains, it is in liquid form. How much heat would be required if the water were to be in the form of a vapor? Take the specific heat of liquid water at constant pressure as 4.18 kJ kg K. Method The heat, Q, required to liberate the moisture is the difference in enthalpies of the moist grain and free water at the end and start of the heating process. At the start of the process, all of the water is bound to the grains; and at the end of the process, the grains have been dried somewhat and the water that has been removed from them is free water. To accomplish this drying, the heat of wetting must be added to “shake” the molecules free from the surfaces of the pores of the grain kernels which provides them with the energy they have in liquid water. The enthalpy Hi of the moist grains at the start of the process is given by: Hi = Wdry matter H1 in which H1 is the specific enthalpy, kJ kg dry matter, of the moist grain at the beginning of the heating process; and Wdry matter is the mass of the dry grains. At the end of the process the total enthalpy, Hf , is the sum of the enthalpies of the grains and the free water, i.e.: H f = Wdry matter H2 + mh1 in which H2 is the specific enthalpy of the moist grains, m is the mass of water liberated, and h1 is its specific enthalpy. The amount of heat, Q, that must be supplied to the grains to provide the energy to release the water is given by: Q = Hf − Hi One task is to calculate the mass of dry matter, Wdry matter , in the grains. It is known that there is 1 kg of moist grain, hence: Wtotal = Wdry matter + Wwater = 1.0 kg



147

Initially, the moisture content Ww of the grains is 0.2 on a fractional wet basis, which can be expressed algebraically as: Ww =

W Wdry matter + W

or: W=

Ww 0.2 = = 0.25 (1 − Ww ) 0.8

hence, it is found that the mass of dry matter in the 1 kg of moist grains is 0.8 kg. The final moisture content of the grain, 10% wet basis, is equivalent to a fractional dry-basis moisture content of 0.1111. It is now possible to calculate the mass, m, of the water removed from the grains by means of the expression: m = Wdry matter (W1 − W2 ) in which W1 and W2 are the moisture contents of the grain at the start and end of the drying process; i.e., they have values of 0.25 and 0.1111 respectively. The mass of water removed is readily calculated to be 0.1111 kg. The specific enthalpies defined above are calculated as follows:

(

)

(

)

(

)

(

))

(

H1 = hσ0 + cσ 25 − T 0 + 0.25 hw0 + cw 25 − T 0 + HW 1

(

))

(

H2 = hσ0 + cσ 25 − T 0 + 0.1111 hw0 + cw 25 − T 0 + HW 2 h1 = h10 + c1 25 − T 0

When these expressions for the enthalpies are used in the equation for the heat, Q, supplied to the grains, there results:

(

)

( (

( (

)) ))

 h 0 + c 25 − T 0 + 0.1111 h 0 + c 25 − T 0 + H  σ σ 1 1 W2 Q = Wdry matter  0   − h − c 25 − T 0 − 0.25 h 0 + c 25 − T 0 − H  1 1 W1   σ σ

(

(

(

+ 0.1389 h10 + c1 25 − T 0

)

))

= 0.8 ( HW 2 − HW 1 ) As mentioned in Chapter 3, the fact that the reference enthalpies and temperatures cancel out gives us confidence that the algebra was performed correctly. The difference in the two integral heats of wetting can be found by integrating Equation 4.41 between the limits W1 and W2, thus: HW 2 − HW 2 = hv

{

}

ps A dT 2 exp ( −100 BW2 ) − exp ( −100 BW1 ) 100 B dps (T + C)

From Equations 3.25, 4.16, and 4.36, hv = 2442.26 kJ kg, ps = 3168.75 Pa, and dps dT = 189.26 Pa °C which, when substituted into the expression for Q, results in:


148


0.8 ( HW 2 − HW 1 ) = 0.8 × 13.62 = 10.9 kJ kg dry grains In other words 10.9 kJ of energy would have to be supplied to liberate 0.1111 kg of water from moist grains containing initially 0.8 kg of dry matter and 0.20 kg water. If only one quarter of this amount of moisture were released from grains initially with the same moisture content, it would require only 0.84 kJ of energy because, at high moisture contents, the water molecules are very loosely bound to the grains. As a consequence this water has an enthalpy that is very similar to that of free water. When the moisture is liberated as vapor from the grains, its enthalpy (or energy content) is higher than that of liquid water; and this is accounted for by using the expression for the enthalpy of water vapor, Equation 3.64. In this case it is found that:

(

(

(

)

))

(

(

(

)

H f = 0.8 hσ0 + cσ 25 − T 0 + 0.1111 hw0 + cw 25 − T 0 + HW 2 + 0.1389 h10 + c1 25 − T 0 + hv

(

)

(

(

))

))

Hi = hσ0 + cσ 25 − T 0 + 0.25 h10 + c1 25 − T 0 + HW 1 Hence, 282.2 kJ of energy is required to liberate 0.1111 kg of water from grain with an initial moisture content of 20% (wet basis). Far more energy is required to vaporize the water than to remove it simply as liquid water. It can also be noted that the latent heat of vaporization of free water at 25°C is calculated to be 2442.26 kJ kg. The average heat of sorption of water to dry grains from 20 to 10% moisture content (wet basis) is 282.2/0.1111, i.e., 2540 kJ kg of moisture evaporated from the grains.

4.6 CALCULATING THE CONDITIONS IN AERATED GRAIN STORES We have already noted that the hygroscopic nature of grains has a profound effect on the temperature and relative humidity of aerated grains, and that these conditions have a decisive impact on the storability of grains. We have also noted that the wet-bulb temperature of the air used to aerate the grain and the initial grain moisture content are the two most important variables that affect the conditions that prevail in an aerated grain store. Due to the importance of this process, it is also discussed in Section 6.1.2.1, to give experimental data on grain temperature profiles, and in Section 7.1.6 to select ambient air for aeration. In this section we shall quantify the effects of these two important variables. In the next section we shall discuss how long it takes for a bulk of aerated grains to cool. 4.6.1

Fronts and Zones

When air is forced through a bulk of grains, three zones — each with different temperatures and moisture contents — are formed. The zones are designated A, B, and C. Each zone is separated by a front or change zone as shown in Figure 4.5. These fronts or zones move through the bed of grains in exactly the same direction of the airflow. The most rapidly moving part of a zone (which may also be called a wave) is called the leading edge, and the slowest moving part is the trailing edge. Further presentations of grain temperature profiles are shown in Sections 6.1.2.1 and 7.1.6. The grain in zone A is temperature and moisture equilibrium with the air used to aerate the grain, and the grain moisture content can be found from an appropriate isotherm equation or a psychrometric chart. The aeration process has not affected the grain in zone C as it has not been reached



Figure 4.5

149

Three zones in a bed of aerated grain; zone A is in equilibrium with the air used to aerate the grain, and the grain in zone C has not been affected by aeration and is at its initial conditions. The grain in zone B is at the dwell state, the state that ultimately prevails throughout most of a bed of aerated grains. Zones A and B are separated by a moisture front, and zones B and C are separated by a temperature front.

by a change front; hence, these grains are at their initial temperature and moisture content. Temperature fronts travel relatively rapidly — about 10–3 times the air velocity through the grain — compared with moisture fronts, which have velocities some 100 times slower. It is clear that after a grain bulk has been aerated for some time, the temperature front has moved completely through the grain bed; and most of the grain is in zone B and a small amount of the bed is in zone A. Zone C has been completely expelled from the grain bulk. The aeration system should be designed so that the intergranular wet-bulb temperature and grain moisture content in this zone are sufficiently low to ensure safe long-term storage conditions. How do we calculate the grain conditions in zone B? Detailed mathematical analyses of heat and mass transfer processes in bulk stored grains have led to the formulation of the following design steps: 1. Assume that the moisture content in zone B is the same as the initial moisture content of the grain that was loaded into the grain store — the same as that in zone C. 2. Estimate the temperature in zone B by finding the point of intersection of the inlet air wet-bulb temperature line and the line corresponding to the initial grain moisture content on a psychrometric chart, as shown in Figure 4.6. The dry-bulb temperature at the point of intersection is a first approximation to the grain temperature in zone B. 3. Estimate the difference in moisture contents between zones B and C by using the rule for cereal grains that states that the grain moisture decreases by 1% (wet basis) for every 28°C of cooling from zone C calculated from step 2. Since the main aim of aeration is to cool grains from their initial temperature, it follows that the grain moisture content in zone B is generally a little lower than the initial moisture content. Should the grain temperature in zone B be greater than that in zone C, the difference in grain moisture content must be added to that of zone C.


150


Figure 4.6

A graphical method of estimating the conditions of grain at the dwell state. Each figure highlights the effect of initial grain state and conditions of the air used to aerate the grains.

4. Refine the temperature estimate of zone B by calculating the intersection of the line of constant wet-bulb temperature of the inlet air with the grain moisture content line corresponding to the moisture content estimated in step 3, as shown in Figure 4.6.

The effects of aerating grain on the formation of fronts and zones is explained in Section 7.1.6 also. The examples given here contain important information on how to calculate the changes in different zones in the grain bulk. Example 4.7 A bulk of wheat initially with a temperature of 35°C and a moisture content of 12% (wet basis) is aerated with ambient air that has a dry-bulb temperature of 11°C and a relative humidity of 90%. What are the grain temperature and moisture contents in zones A, B and C, provided the temperature wave has not travelled sufficiently far to exit the grain bulk? Method The temperature and moisture content in zone C are those of the grain as it was loaded into the silo, i.e. 35°C and 12% (wet basis) respectively. The temperature of the grain in zone A is that of the inlet air, (11°C); and since it is in equilibrium with air with a relative humidity of 90%, it can be seen from Figure 4.6 that its moisture content is about 18%. The wet-bulb temperature of the air used to aerate the grain is about 10°C. This is determined by observing that a line of constant enthalpy (which corresponds closely to a line of constant wetbulb temperature) passes through the point on the psychrometric chart defined by a dry-bulb temperature of 11°C and a relative humidity of 90%. This line intersects the saturation line at 10°C. This 10°C wet-bulb line intersects the 12% grain moisture content at a dry-bulb temperature of about 14.5°C. This is the first estimate of the dry-bulb temperature in zone B.



151

A more refined estimate of the temperature and moisture content in zone B is obtained by applying the rule that, in the case of cereal grains, the grain moisture content in zone B reduces by 1% for every 28°C of cooling. In this case the grain has cooled from 35°C to 14.5°C; hence, the grain moisture content in zone B is (35-14.5)/28 = 0.73% lower than in zone C. That is, the moisture content at the dwell state is 12-0.73 = 11.27% (wet basis). The dry-bulb temperature at which the 10°C wet-bulb line intersects a line of constant grain moisture of 11.27%. Since this line is not shown on the psychrometric chart, it is necessary to interpolate between the 11% and 12% lines, and find that the dry-bulb temperature in zone C is about 15.5°C. It has been found that grain in zone C has a moisture content of about 11.3% (wet basis) and a temperature of 15.5°C as indicated by point B in Figure 4.6. 4.6.1.1 The Effects of Initial Grain Moisture Content on the Conditions within a Bulk of Grain It is noted above that the moisture content of the grains as they are loaded into a silo and the wet-bulb temperature of the air used to aerate the grains have the most significant effect on the conditions within most of a bulk of aerated grains. It will be seen just how important these two variables are by carrying out a few examples. Example 4.8 Grain Moisture Content — 9% Wet Basis We repeat the above worked example, but consider a bulk of grain that has an initial moisture content of 9% as opposed to 12% as considered above. All other conditions remain the same. Method Zone A — As above; i.e., the grain has a dry-bulb temperature of 11°C and a moisture content of about 18% wet basis. Zone C — The grain temperature is 35°C and its moisture content is 9% wet basis, as specified. Zone B — The intersection of the 10°C wet-bulb temperature line with the 9% moisture content line corresponds to a dry-bulb temperature of 18.5°C. Hence the first estimate of the grain conditions in zone B are defined by a grain moisture content of 9% and a temperature of 18.5°C. Refined values are 8.4% and a temperature of 19.5°C. This is point B1 in Figure 4.6.

This example shows the significant effect that the initial moisture content of the grain has on the dwell temperature. When grain with a temperature of 35°C is aerated with air with a wet-bulb temperature of 10°C and its initial moisture content is 12%, the dwell temperature is 15.5°C. When the initial moisture content of the grain is only 9%, its dwell temperature would be about 19.5°C. Even though the dwell temperature in this second example is much higher than that of the first example, the aridity of the grain results in its long-term storability is likely to be similar in both cases. It should be noted that the initial intergranular wet-bulb temperatures in the 12% and 9% moisture content bulks at 35°C are about 28°C and 22°C respectively. Because aeration reduces the wet-bulb temperature in each case, it is likely that aeration will be beneficial to good grain storage. 4.6.1.2 The Effects of the Wet-Bulb Temperature of the Air Used for Aeration on the Conditions within a Bulk of Grain The wet-bulb temperature of the air has a very significant effect on the conditions within aerated grain stores — the lower the wet-bulb temperature, the lower the grain dwell temperature.


152


Example 4.9 Repeat the worked example 4.7, but consider that the aeration air has the same dry-bulb temperature, namely 11°C, while the relative humidity is 20%. These conditions correspond to a wet-bulb temperature of about 3°C. The initial grain temperature and moisture content are 35°C and 12% respectively. Method Zone A — The grain has a dry-bulb temperature of 11°C and a moisture content of about 8% wet basis. Zone C — The grain temperature is 35°C and its moisture content is 12% wet basis, as specified. Zone B — The intersection of the 3°C wet-bulb temperature line with the 12% moisture content line corresponds to a dry-bulb temperature of 7°C. Hence the first estimate of the grain conditions in zone B are defined by a grain moisture content of 12% and a temperature of 7°C. More refined estimates of the moisture content and grain temperature at the dwell state are 11.0% and 7.5°C respectively. This is point B2 in Figure 4.6.

This example highlights the significant effect that the wet-bulb temperature of the inlet air has on the dwell temperature. Reducing the relative humidity from 90% to 20% results in a dwell temperature reduction from 15.5°C to 7.5°C. In the latter case the dry-bulb temperature of the stored grain is actually lower than that of the air used for aeration, and this phenomenon is exploited in open cycle desiccant bed cooling systems (Thorpe and Ahmad, 1998).

4.6.2

Speeds of Fronts

We now have a simple method of calculating the moisture contents and temperatures in bulks of aerated grains. The next important step is to calculate how long it takes to cool a bulk of aerated grains. It is observed that when a bulk of grain is aerated with high relative humidity air, that results in zone A having a higher moisture content than that in zone B. Since the temperature of the grain in zone B is lower than that in zone C, both the moisture and temperature waves widen as they pass through the grains. This case, which is the one most often encountered in practice, is depicted diagrammatically in Figure 4.7. If the grain is aerated with air that dries the grain in zone A, the trailing edge of the wetting zone or wave is calculated to travel faster than the leading edge. This is physically impossible; and if there were no resistance to moisture transferring from the grain kernels to the grain, a shock wave would develop as shown diagrammatically in Figure 4.8. The ratio of the speeds of the temperature and moisture waves to the speed of the air flowing through the grain can be calculated from the work of Sutherland et al. (1971). Figure 4.9 shows the ratio of speeds on the temperature wave. It can be seen that the ratio is sensitive to the temperature of the wave and less sensitive to the grain moisture content. Figure 4.10 shows the ratio of the speeds of points on the moisture wave to the speed of air flowing through the grains. These ratios are about two orders of magnitude smaller than those of the temperature wave. Somewhat more recently, Hunter (1987) has presented analytical expressions for the speed of temperature and moisture fronts in aerated grain bulks. Example 4.10 Mary-Lou is a farmer who wishes to aerate a circular silo that has a diameter of 4 m and that is 5 m high and full of wheat. The silo is fitted with a perforated floor, and it is aerated uniformly with an airflow rate of 1 liter per second per tonne. The initial temperature of the grain is 35ºC, and its moisture content is 12% wet basis, and the bulk density of the grain is 800kg/m3. If the



153

Figure 4.7 An illustration of the spread of temperature waves through beds of dry grain that are aerated with air that has a high relative humidity. Moisture fronts spread in an analogous manner.

Figure 4.8

In the idealized case of thermodynamic equilibrium between the air and the grains, moisture fronts are sharp when wet grains are aerated with low relative humidity air. Temperature fronts exhibit similar behavior.

aeration air has a relative humidity of 90% and a dry-bulb temperature of 11ºC, advise the farmer on the following points: 1. Project the total volume flow rate of air and its velocity through the grains. 2. Advise the time it will take for the farmer to first begin to detect that the aeration system is doing its job if she measures the temperature of the grain that is near the upper surface of the bulk of grain. 3. Calculate the time it will take for all of the farmer’s grain to have been cooled by aeration, as indicated by the grain near the upper surface having cooled to the dwell temperature. 4. Estimate the distance the leading edge of the wetting wave has penetrated the bed of grain just as the trailing edge of the temperature wave is leaving the bed, as calculated in 3. above.


154


Figure 4.9

The ratio of the speed of a temperature wave to the speed of air through a bulk of grain.

Figure 4.10

The ratio of the speed of a moisture wave to the speed of air through a bulk of grain.

Method 1. Before we can find the volume flow rate of the aeration air, we must find the volume of the grain, thus:

Volume = area of the base of the silo × the height of the silo i.e.:

Volume = π d 2 4 × height = ( π × 4 × 4 4) × 5 = 62.8 m3



155

Now the mass of the grains = density × volume = 800 × 62.8 = 50 240 kg ≡ 50.24 tonnes. Since the volume flow rate of air has been specified as 1 liter/second/tonne of grain cooled, the total volume flow rate of air = 50.24 liters/second ≡ 0.05024 m3/second. The velocity is given by the formula:

Velocity = volume flow rate area over which the air flows i.e.:

Velocity = 0.05024 ( π × 4 × 4 4) = 0.004 m s 2. In principle, the farmer would begin to detect that aeration is doing its job by measuring the temperature of the grain near the upper surface of the bulk of grain. This fall in temperature occurs as the leading edge of the cooling wave first reaches the upper surface. From Figure 4.9 we observe that the ratio of the velocity of a point on a temperature wave traveling through grain at 35°C with a moisture content of 12% wet basis to the velocity of aeration air is 0.0045. The velocity of the leading edge of the temperature wave is therefore 0.004 × 0.0045 m/s; and because the grain bulk is 5 m high, the time it takes to reach the upper surface of the grain is 5/(0.004 × 0.0045) ≈ 277,777 seconds or 277,777/3600 ≈ 77.16 hours. 3. All of the grains have been cooled to the dwell temperature or less when the trailing edge of the temperature wave leaves the upper surface of the grain. We know from the example above that the dwell temperature and moisture contents in the case considered are 16°C and 11.3% respectively. From Figure 4.9 the ratio of the temperature front speed to the air speed is about 0.002; hence, the time taken for the trailing edge to leave the top of the grain bulk is 5/(0.004 × 0.002) = 625,000 seconds, or 173 hours. 4. We now wish to calculate how far the leading edge of the wetting wave travels into the grain bulk in 625,000 s. From the example above we know that the grain moisture content is 11.3% and its temperature is 16°C. Hence, from Figure 4.10 we can see that with these conditions a moisture front would travel at about 90 × 10–6 times that of the air through the grains. Since the velocity of the air is 0.004 m/s and the time under consideration is 625,000 seconds, the distance traveled by the leading edge of the moisture front is:

0.004 × 90 × 10 −6 × 625, 000 = 0.225 m. These calculations indicate that, after the aeration fan has been operating for 173 hours, most of the grain is at the dwell temperature of 16°C, and that the leading edge of the wetting wave has penetrated a relatively short distance, 0.225 m, into the bed of grain.

4.7 CALCULATING THE MASS OF GRAINS IN A GRAIN STORE The airflow rate in a grain aeration system is often specified on the basis of a certain volume flow rate per tonne of grain stored. An airflow rate of 1 liter of air per second per tonne of grain results in grain cooled to the dwell state after about 100 hours of fan operation. Given a silo, the volume of which is specified, the mass of the stored grain needs to be calculated. This can be determined if the bulk density, ρb, and the angle, α, of repose of the grains are known. This latter variable determines the volume of the peak of grain. Both the bulk density and the angle of repose depend upon the type of grain stored and its moisture content, and they vary with the time of storage. As time progresses the grain bulk settles, its bulk density increases somewhat, and its angle repose may reduce. Some typical values of these parameters are given in Table 4.9. The values given are a fairly crude average of those reported by a number of workers and where an appropriate range of values is given.


156


Table 4.9

Bulk Density and Angle of Repose for Various Types of Commodities

Grain Type Barley Corn (shelled) Flaxseed Grain sorghum Oats Peas Rice Rye Soybeans Wheat

Figure 4.11

Bulk Density, kg /m3 Mean Value Range 640 746 704 710 480 800 576 691 737 762

614–691 704–768 691–717 640–742 410–563 — — 666–717 736–738 736–832

Angle of Repose, ° Mean Value Range 24.4 23.4 21.4 26.5 26.5 25.1 31.5 21.5 26.0 24.6

16–28 16–27.5 14–25 20–33 18–35 — 20–36 17–26 16–30 16–27

The angle of repose of grains is the angle between the upper surface of a peak of grain and the horizontal.

Example 4.11 A farmer has a silo that has a diameter of 4 m, and it has a sloping conical floor that is at an angle of 35° to the horizontal as shown in Figure 4.11. The height of the cylindrical section is 4 m. If the grain is filled through a hatch placed in the center of the silo roof to within 30 cm of the top of the cylindrical section, calculate the mass of the grain the silo holds if it is filled with (1) rice and (2) peas. Method The first task is to calculate the volume of the grain, which is conveniently considered in three sections, namely:



157

• The volume of grain in the conical base • The volume of grain in the cylindrical section • The volume of grain in the peak

If the diameter of the silo is designated as d and the angle of the conical base to the horizontal is γ, the height of the conical base is (d 2) tan γ . The volume of the grain is the conical base = 1 3 area of the base × height of the cone = 1 3 π d 2 4 × d 2 tan γ = 1 3 π 4 2 4 × 4 2 tan 35 = 5.866 m3. The cylindrical section of the silo is filled to within 30 cm of where it joins the silo roof. The volume of the grain in this section is therefore given by: area of the base of the cylinder × the height of grain in the cylindrical section =

πd 2 πd 2 × height of grain = × ( 4 − 0.3) = 46.495 m 3 4 4

The volume of grain in the upper peak of the grain depends on the angle of repose, α; and it is found in a way that is analogous to that used to find the volume of the grain in the conical base. The volume of the peak =

1 area of the base × height of the peak 3

In the case of rice, the angle of repose, α, is 31.5°; so the volume of grain in the peak of the grain bulk is 1 3 π 4 2 4 × 4 2 tan 31.5 = 5.133 m3. The total volume of the grain is therefore 5.866 + 46.5 + 5.133 = 57.5 m3. The mass in the silo if found from bulk density, ρb, and its volume = ρb × volume = 576 × 57.5 = 33120 kg ≡ 33.12 tonnes. Peas have a higher bulk density than rice, but their angle of repose is a little less. The volume of the peak is therefore 1 3 π 4 2 4 × 4 2 tan 25.1 = 3.924 m 3 . Hence, the total volume of the peas is 5.866 + 46.495 + 3.924 = 56.3 m, slightly less than the volume of the rice because the angle of repose of the peas in not as high as the angle of repose of rice. The bulk density of peas is 800 kg/m3; hence, the mass of peas in the silo is 800 × 56.3 = 45040 kg ≈ 45 tonnes. It is interesting to note that the same silo is a 33-tonne silo when it contains rice, but a 45-tonne silo when it contains peas.

4.8 THE THERMAL CONDUCTIVITY OF STORED GRAINS Aeration enables a bulk of grain to be cooled after typically 100 to 200 hours operation of the aeration fan. Once the grain has been cooled, it may begin to increase in temperature as a result of heat conducted through the surfaces of the grain store. The effects of heating are normally most pronounced at the periphery of the store. The behavior of insects in this warm region might dominate the performance of aeration as an insect control measure. It is therefore important to have some understanding of the factors that affect how quickly the grain heats up in the store periphery. The conduction of heat into and within a bulk of stored grains is of crucial importance when the performance of aerated grain stores must be predicted. This is particularly apparent at the edges of a grain store fabricated from sheet metal, where the grains respond rapidly to the outside conditions (Thorpe, 1997). The rate, q, at which heat flows in the x direction through grain is proportional to the area, A, normal to the direction in which the heat flows and the temperature gradient, dT dx in the x direction. q∝A

dT dx

(4.43)


158

Figure 4.12


Calculation of the rate at which heat flows through a slab of grains.

When the area is one square meter and the temperature gradient is 1 K/m (≡ 1°C/m), the rate of heat flowing, Watts (≡ Joules/second), is numerically the same as the thermal conductivity, k, which depends on the material. Some materials such as metals have very high thermal conductivities. Mild steel, which is often used in the construction of grain silos, has a thermal conductivity of about 40 W/mK. Concrete, also used to construct grain stores, has a thermal conductivity of about 0.85 W/mK. Bulk grain has a thermal conductivity of about 0.15 W/mK, and that of air is 0.023 W/mK. The equation that governs the rate of heat flowing is expressed mathematically as: q = − kA

dT dx

(4.44)

and the meanings of the symbols have been discussed above. This equation is sometimes known as Fourier’s law of heat conduction, and its derivation is described in detail by Serway (1992). The application of Equation 4.44 is well illustrated by means of an example. Example 4.12 One side of a 20 mm thick region of grains has a temperature of 40°C, while the other side has a temperature of 10ºC as shown in Figure 4.12. Assuming that heat conduction is the only mechanism of heat transfer in the grains, and the temperature gradient across the grains is constant, calculate the rate of flow of heat per square meter normal to the temperature gradient. Take the thermal conductivity of the grains to be 0.15 W/mK. Method The rate of heating is strongly influenced by the thermal conductivity of the grains, their density, and specific heat. The thermal conductivity of grains also influences heat transfer by natural convection. Natural convection occurs because, when the air between the grains is heated, it decreases in density and begins to flow upward through the grain. Air is drawn from cooler regions of the grain store, and it is replaced by warmer air so that the entire bulk of grain eventually heats up. The mechanisms of heat and moisture transfer by natural convection have been described in



159

detail by Thorpe (1996) and Singh and Thorpe (1993a, 1993b). A working computer program written in FORTRAN, which calculates the rate of natural convection in bulks of stored grains that have simple geometries, has been described by Thorpe (1996) and is available from the author. The fact that the temperature gradient is constant indicates that the grain has had sufficient time for the heat transfer to become steady. To simplify, assume that moisture transfer and natural convection have negligible effects on the rate of heat transfer, and that the rate of heat transfer is uniform across the entire face of the grain normal to the direction in which the heat flows. The temperature gradient, dT dx , is calculated as follows: dT Tx +∆x − Tx 10 − 40 = = = −1500° C m ∆x 0.02 dx The gradient is negative because, as the distance in the direction of x increases, the temperature decreases; so to plot temperature vs. distance in the direction of heat flow, the slope of the line, or the gradient, would be negative. To find the rate of heat flow per square meter, assume the area, A = 1. Now Equation 4.44 can be used to calculate the rate at which heat flows through the grains, q = − kA

dT = −0.15 × 1 × ( −1500) = 225 Watts dx

It is always difficult to obtain precise values of the thermal conductivities of bulk grains from the literature for two reasons. First, grains are biological materials that vary from batch to batch. Second, there appear to be systematic errors in the ways that thermal conductivity has been measured. Gray (1998) has carried out a comprehensive critical review of data on the thermal conductivity of grains. He observed that measurements have standard errors of about 14%. There are grounds to suppose that these errors arise from how the samples are prepared for measurement in terms of their packing, for example, which is likely to have a significant effect on the thermal conductivity. This can be appreciated from the theoretical research carried out by Nozad and Whitaker (1985a, 1985b) that clearly shows that, as the void fraction (fraction of air spaces) increases, the thermal conductivity decreases in materials such as bulk grains that have voids filled with air. Gray (1998) also alludes to the fact that the alignment of the grains is also likely to affect the thermal conductivity of grains; i.e., the thermal conductivity in one direction is likely to be different from that in a direction normal to the first. This is known as anisotropy. Gray (1998) cites research (Chang 1986, for example) that indicates that thermal conductivity is largely independent of the variety of grains. Kustermann (1981) found that seasonal variations also have little effect on the thermal conductivity of bulk grains. Many authors have found that the thermal conductivity of grains is affected by temperature and grain moisture content, and the results summarized by Gray are presented in Table 4.10. 4.8.1

The Specific Heat of Moist Grains

When stored-grain technologists wish to calculate the amount of heat required to raise the temperature of one kilogram of moist grains, they need to be able to calculate the specific enthalpy of the grains. Chapter 3 illustrates that performing calculations on a dry basis ultimately makes life simpler. This is why enthalpy changes that occur in moist grains are calculated on the basis of one kilogram of dry grain. Before enthalpy changes can be determined, the specific heat, cg , of the dry matter of stored grains must be known. The specific heat of water, cw , is a well-established quantity, and methods of calculating the integral heat of wetting have been discussed. To calculate the rate at which the specific enthalpy of moist grains change with temperature, ∂H ∂T , simply differentiate Equation 4.42 with respect to temperature, thus:

Barley, Harrington Canola, Torch

Corn, Stauffer

Gram

Lentils, Laird Oats Pea, Trapper Rice, Calrose

Sorghum Soybean Pigeon pea Wheat, hard red

Chang (1986)

Dutta et al. (1988)

Alagusundaram et al. (1991) Kustermann et al. (1981) Alagusundaram et al. (1991) Putranon et al. (1980)

Sarma and Thompson (1973) Chuma et al. (1981) Shepherd and Bhardwaj (1986) Chandra and Muir (1971)

Grain, Variety

24–49 30 27 1–20

(–28)–29 Not measured (–28)–29 20–35

10–39

1–22.5 10.2–49.4 7–21 4.4–22.5

10.20–21.2 Not measured 10.20–21.3 12.1–18.8

10.30–21.4

12.00–18.2

9.70–22.8 0.75–19.6

Moisture Content % Wet Basis

0.966 0.810 0.997 0.800

0.907 0.980 0.918 0.930

0.998

0.870

0.660

0.770

0.690

0.936 0.920

721–752 633.2–792 782 Not measured

790 Not measured 795 Not measured

Not measured

731–826

673 660–740

Bulk Density Correlation kg/m3 Coefficient R2

k = 0.173 + 7.51 × 10–4T + 1.51 × 10–3Mw When T = 19.4°C k = 0.0994 + 1.41 × 10–3Mw When T = 1.7°C k = 0.0991 + 8.86 × 10–4Mw When Mw = 12.0% k = 0.07257 + 1.209 × 10–4 ρ When Mw = 15.6% k = 0.09480 + 1.026 × 10–4 ρ When Mw = 18.2% k = 0.09020 + 1.165 × 10–4 ρ k = –0.507 + 0.00255(273+T ) – 2.13 × 10–6 (273+T )2 + 0.00424 M – 6.56 × 10–6 M 2 + 6.48 × 10–6 M (273 + T ) k = 0.193 + 1.0 × 10–3 T + 1.52 × 10–3 Mw k = 0.104 + 1.82 × 10–4 T + 2.21 × 10–3 Mw k = 0.168 + 8.4 × 10–4 T + 3.05 × 10–3 Mw When T = 25°C k = 0.0913 + 2.42 × 10–3 Mw When Mw = 12.1% k = –0.1423 + 8.88 × 10–4 (273 + T ) When Mw = 18.8% k = –0.1786 + 10.6 × 10–4 (273 + T ) k = 0.0976 + 1.48 × 10–3 Mw k = 0.139 + 1.23 × 10–3 Mw k = 0.0991 + 3.1 × 10–3 M When T = 20°C k = 0.140 + 1.41 × 10–3 Mw When T = 5°C k = 0.144 + 9.54 × 10–4 Mw When T = 1°C k = 0.136 + 1.36 × 10–3 Mw

Thermal Conductivity of Bulk Grains, W/mK

160

22

(–28)–29 1.7–19.4

Temperature °C

The Thermal Conductivity of Bulk Grains as Collated by Gray (1998)

Alagusundaram et al. (1991) Moysey et al. (1977)

Author

Table 4.10





161

∂H ∂H = cg + cwW + W J kg of dry grain ∂T ∂T

(4.45)

One of the difficulties of using Equation 4.45 is that ∂HW ∂T may be hard to determine with the required degree of accuracy because it relies on sorption data that are obtained down to low levels of moisture content. To avoid this problem, many authors have expressed the variation of the specific heat, cm, of moist grains with moisture content as follows: cm = cg0 + c1 M

(4.46)

where cg0 is the specific heat of dry grains, c1 is an empirical coefficient, and M is the moisture content of grain, expressed typically on a percent dry basis. The literature is somewhat ambiguous on whether the specific heat relates to one kilogram of dry grains or wet grains. The variation of ∂H ∂T with moisture content is subsumed in the coefficient c1. The values of cw and ∂HW ∂T in Equation 4.45 are 4186 J/kg and, from Hunter’s isotherm equation, on the order 100 J/kg, respectively. Now ∂ HW ∂T is also a function of grain moisture content; and if the functional relationship is linearized, the following approximation for the specific heat of grains is obtained: cm = cg0 + c2 M

(4.47)

in which c1 is a constant that accounts for the variation of ∂HW ∂T with M, the grain moisture content.

4.9 A NUMERICAL ANALYSIS OF AERATED GRAIN BEDS The graphical analysis of the behavior of aerated beds of grain is very useful because it highlights the physical processes that occur. It also enables reasonable estimates to be made of the grain temperatures and moisture contents. However, the temperature and humidity of the aeration air usually vary continuously throughout the aeration process; and design methods are needed that can deal with this variability. A convenient way of achieving this is to develop a simple computer program that simulates heat and moisture transfer in bulks of aerated grains. A step-by-step derivation of the heat and mass balance equations will be presented to show how they can be simply encoded into a computer program. Many of the ideas discussed in connection with the air/water system and enthalpy in Chapter 3, and on the properties of grains discussed in this chapter, will be used. The analysis will be restricted to one-dimensional systems, in which it is assumed that airflows with a uniform velocity through a bed of grains and heat also flows only in the direction of the airflow. This differs from real systems that are three dimensional; and the velocity of the aeration air varies with position, as indicated by the work of Thorpe (1997), for example. The reason for confining the analysis to one-dimensional flows is that it illustrates most of the principles of the solution procedure, and it ensures that the algebra is kept as simple and uncluttered as possible. 4.9.1

Mass Balances

4.9.1.1 Dry Air Consider an element of the bed of grain, length ∆x, and with an area of 1m2 normal to the flow as shown in Figure 4.13. The law of conservation of mass states that matter can be neither created


162


Figure 4.13

An element of a bed of grain on which mass balances are carried out.

nor destroyed. As it relates to the flow into and out of the element, the law can be expressed in words, thus: Rate of mass flowing into element

=

Rate of mass flowing out of element

+

Rate of mass accumulation

(4.48)

The rate at which dry air enters the element is ρaua, and after a length ∆x the density of the air changes incrementally to ρa + ∂ρa ∂x ∆x and the velocity changes to ua + ∂ua ∂x ∆x . If its density changes with time, air may also accumulate in the element, which has a total volume of 1.∆x m3. If the volume fraction of air within the element is ε, which typically has a value of about 0.4, the volume of air in the element is ε·1·∆ x m3. The remainder of the volume, (1 − ε ) 1 ⋅ ∆ x m3, is occupied by the grain kernels. If the rate of change of the density of dry air in the element is ∂ρa ∂t , the mass balance Equation 4.48 can be expressed in mathematical symbols as: ∂u ∂ρ ∂ρ    uaρa =  ua + a ∆x  ρa + a ∆x + ε a ∆x    ∂x ∂x ∂t Rate of air flowing into element

=

Rate of air flowing out of element

+

(4.49)

Rate of accumulation of air

Multiplying out the brackets results in: uaρa = uaρa + ua

∂ρa ∂u ∂u ∂ρa ∂ρ ∆x + ρa a ∆x + a ∆x∆x + ε a ∆x ∂x ∂x ∂x ∂x ∂t

(4.50)

Collecting terms and dividing by ∆x, and letting ∆x → 0 results in: ε

∂ρa ∂ρ ∂u + ua a + ρa a = 0 ∂t ∂x ∂x

(4.51)



163

∂ ∂

∂ ∂

∂ ∂ ∂

Figure 4.14

∂

An enthalpy balance on an element of a bulk of grains.

This is the mass conservation equation for a fluid, in this case dry air flowing in one dimension through a ventilated bulk of grain. 4.9.1.2 Moisture When the density of dry matter in the grain kernels is ρσ, the mass of dry matter in the element is ρσ (1 − ε ) ⋅ 1 ⋅ ∆ x ; and if the moisture content of the dry matter is W, then the mass of water in the grains in the element is ρσ (1 − ε ) W∆ x . If the porosity, ε, and the density of the grain kernels are both constant, then the rate at which moisture accumulates in the element is ρσ (1 − ε ) ∂W ∂t . The mass balance on the moisture flowing into and out of the element is carried out in exactly the same way as the balance on the dry air with the result: ε

∂ρ1 ∂ρ ∂u ∂W + u1 1 + ρ1 1 + ρσ (1 − ε ) =0 ∂t ∂x ∂x ∂t

(4.52)

where ρ1 is the density of moisture vapor in the interstitial air. The last term in Equation 4.52 accounts for the accumulation of moisture in the grain kernels. 4.9.2

Energy Balance

The law of the conservation of energy states that energy can be neither created nor destroyed. When an energy balance is carried out on the element of length ∆x, this implies that the energy entering the element must equal that leaving the element plus any energy that might accumulate within the element. When the first law of thermodynamics is applied to a flow process, such as aerating stored grains, changes in potential energy and kinetic energy of the air can often be ignored; and since no net work is done by the system, the energy balance reduces to an enthalpy balance. Further details on how to deal with energy balances can be obtained from standard engineering thermodynamics texts, such as that by Çengel and Boles (1998). However, while analyzing the performance of grain aeration systems, only enthalpy balances will be considered. An important point to remember about enthalpy is that it is closely related to the energy contained in a substance as a result of the motion of its constituent molecules. The enthalpy balance over a small element of length ∆x, illustrated in Figure 4.14, may be expressed in words as:


164


Rate of enthalpy flowing into element

=

Rate of enthalpy flowing out of element

+

Rate of accumulation of enthalpy

(4.53)

This statement can be refined by writing it as: Rate of enthalpy  Rate of enthalpy flowing   Rate of enthalpy flowing     into element in dry air   out of element in dry air   accumulating in dry air        + + +             Rate of enthalpy flowing Rate of enthalpy flowing Rate of enthalpy          into element in out of element in accumulating in     = +  (4.54) moisture vapor moisture vapor moisture vapor             + + +       Rate of enthalpy  Rate of energy flowing   Rate of energy flowing          into element by out of element by accumulating in       grains  thermal conduction   thermal conduction    This thermal energy equation can be expressed in mathematical symbols as follows: ∂ρ u h   ∂ρ h  ρaua ha   ρaua ha + a a a ∆x   ε a a ∆x  x ∂ ∂t  +      + +      ρ u h   ρ u h + ∂ρ1u1h1 ∆x   ∂ρ1h1 + ε ∆x  1 1  1 a =  ∂x ∂t     −   − +  ∂T    ∂T ∂ ∂T    ∂H    keff ∂x  keff  +    ∆x  (1 − ε )ρσ ∆x   ∂x ∂x  ∂x     ∂t 

(4.55)

The above energy balance includes thermal energy conducted into the element along the temperature gradient ∂T ∂x. The thermal conductivity is designated keff , and its numerical values have been discussed in Section 4.9. A more fundamental approach to the question of thermal conductivity in porous media such as food grains has been presented by Nozad et al. (1985a, 1985b). The rules for differentiating a product can be used to obtain expressions such as: ρaua ha +

∂ (ρaua ha ) ∂x

= ρaua ha + ha

∂ (ρaua ) ∂x

+ ρaua

∂ (ha ) ∂x

(4.56)

When such expansions are used in constructing an enthalpy balance around an element, the following equation is obtained: ρσ (1 − ε ) = keff

∂ (ρaua ) ∂ (haρa ) ∂ (ρ1h1 ) ∂ (ρ1u1 ) ∂h ∂h ∂H +ε + ha + ρaua a + h1 + ρ1u1 1 +ε ∂t ∂t ∂x ∂x ∂x ∂t ∂x

∂2T ∂x 2

The mass balance on dry air, Equation 4.51, can be expressed as:

(4.57)



165

ε

∂ρa ∂ (ρaua ) =0 + ∂x ∂t

(4.58)

which, when multiplied by the enthalpy ha of air, becomes: εha

∂ (ρaua ) ∂ρa =0 + ha ∂x ∂t

(4.59)

and the moisture balance, Equation 4.52, yields: h1ε

∂ (ρ1u1 ) ∂ρ1 ∂W + h1 = −(1 − ε ) ρσ h1 ∂t ∂x ∂t

(4.60)

Using these relationships in the enthalpy balance, Equation 4.57: ρσ (1 − ε )

∂h ∂h ∂h ∂h ∂H ∂W ∂2T + ερa a + ρaua a + ερ1 1 + ερ1u1 1 − (1 − ε ) ρσ h1 = keff 2 ∂t ∂t ∂t ∂x ∂x ∂t ∂x

(4.61)

4.9.2.1 The Energy and Moisture Balances Expressed in Terms of the Temperature and Moisture Content of Stored Grains Our goal is to derive equations that will enable us to calculate how the temperature, T, and moisture content, W, of the grains vary with time along the bed of grain. Before we can use Equation 4.61 to obtain the temperature, we must express the enthalpies of the air and the grains in terms of T and W. Given these two values, we can use a sorption isotherm such as Equation 4.23 to calculate the corresponding humidity, w, of the air in the voids between the grains. The enthalpy of moist grain is a function of temperature, T, and moisture content, W, i.e.: H = H (T , W )

(4.62)

By the chain rule of differentiation (Stein, 1987 and Fraleigh, 1990), the equation can therefore be written: ∂H ∂H ∂W ∂H ∂T = + ∂t ∂W ∂t ∂T ∂t

(4.63)

∂H ∂H = h10 + c1 T − T 0 + W ∂W ∂W

(4.64)

Differentiating Equation 4.42:

(

)

and fundamental properties of integrals (Stein, 1987 and Fraleigh, 1990) applied to Equation 4.38 enable the differential of HW to be expressed with respect to W as the differential heat of wetting, thus: ∂HW = hw ∂W

(4.65)


166


which, when inserted into Equation 4.64, results in: ∂H = h10 + c1 T − T 0 + hw ∂W

(

)

(4.66)

From Equation 4.42 by differentiation with respect to temperature, T: ∂H ∂H = cσ + c1W + W ∂T ∂T

(4.67)

Using Equations 4.66 and 4.67 in Equation 4.63 results in: ∂H  ∂T ∂H ∂W  = h10 + c1 T − T 0 + hw + c + c W + W  ∂t ∂t  σ 1 ∂T  ∂t

{

(

}

)

(4.68)

If the assumption is made that the specific heats of air and water vapor are constant, expressions of the form of Equation 3.58 can be differentiated with the results: ∂ha ∂T = ca ∂t ∂t

(4.69)

∂ha ∂T = ca ∂x ∂x

(4.70)

and:

The latent heat of vaporization of water is a function only of temperature, which enables the chain rule of differentiation to be applied to Equation 3.64 with the results: ∂h1 ∂T ∂hv ∂T = c1 + ∂t ∂t ∂T ∂t

(4.71)

∂h1 ∂T ∂hv ∂T = c1 + ∂x ∂x ∂T ∂x

(4.72)

and:

The enthalpy balance, Equation 4.61, may thus be written as:

{

(

)

ρσ (1 − ε ) h10 + c1 T − T 0 + hw

} ∂∂Wt + ρ (1 − ε)c σ

σ

+ c1W +

∂HW  ∂T  ∂T  ∂t

∂h  ∂T  ∂h  ∂T  + ε ρaca + ρ1c1 + ρ1 v  + ρ u c + ρ u c + ρ u v  ∂T  ∂t  a a a 1 1 1 1 1 ∂t  ∂x 

{

(

)

− ρσ (1 − ε ) h10 + c1 T − T 0 + hv

}

∂W ∂2T = keff 2 ∂t ∂x

(4.73)



167

This equation appears to be much more useful to stored-grain technologists because it is possible to recognize familiar variables such as grain temperature, T, and grain moisture content, W. These are also associated with the time, t, that the aeration system has been operating and the distance, x, from the air inlet. Equation 4.73 contains two velocities, that of water vapor, u1, and the velocity, ua , of the dry air that contains the moisture vapor. Intuitively one might think that these two velocities are the same — after all, the dry air and vapor are well mixed. For all practical purposes this intuition would be correct — but not quite, because it is always possible for moisture vapor to flow down concentration gradients and therefore travel faster or slower than the dry air in which it is being transported. This phenomenon is treated quite fully by Bird et al. (1960b). However, the two velocities can be treated as being the same as that of the dry air, i.e., u1 = ua. The humidity, w, of the air in the intergranular pores may be approximated by: w=

ρ1 ρa

(4.74)

Also noting that the mass flow rate, fa, of dry air through the one-dimensional bulk of grain with a cross-sectional area of one square meter is given by the following expression: fa = ρaua

(4.75)

Collecting terms in the thermal energy balance Equation 4.73, and making use of Equations 4.59, 4.60, and 4.75 leads to:  ∂H  ∂T ∂h   ∂T   ρσ (1 − ε )cσ + c1W + W  + ερa ca + w  c1 + ρ1 v    ∂T  ∂t ∂T   ∂t    ∂h   ∂T ∂W ∂2T  + fa ca + w  c1 + v   − ρσ (1 − ε )hs = keff 2  ∂t ∂T   ∂x ∂x 

(4.76)

At last an equation is expressed in terms of the grain temperature, T, the grain moisture content, W, and the humidity, w, of the intergranular air. The moisture balance, Equation 4.52, can also be expressed in terms of fa , by making use of Equation 4.74 with the result: εw

∂ρa ∂ρ ∂u ∂W ∂w ∂w + ερa =0 + ρaua + ua w a + wρa a + (1 − ε ) ρσ ∂t ∂t ∂t ∂x ∂x ∂x

(4.77)

and, by making use of Equation 4.51: ερa

∂W ∂w ∂w =0 + ρaua + (1 − ε ) ρσ ∂t ∂t ∂x

(4.78)

Again noting that fa = ρaua, the moisture balance 4.78 reduces to:

(1 − ε) ρa

∂W ∂w + fa =0 ∂t ∂x

(4.79)


168


One of the assumptions made in deriving Equation 4.79 is that the moisture vapor and the air both have the same velocities, and this is quite reasonable. However, Thorpe (1995) has shown in a rigorous fashion that any additional transport of moisture through the bed of grains that might arise from the diffusion of moisture vapor can be accounted for simply by adding an extra term, the mass balance equation, thus:

(1 − ε) ρσ

∂W ∂w ∂2 w + fa = ρa Deff 2 ∂t ∂x ∂x

(4.80)

in which Deff is the effective diffusion coefficient through beds of grain (Thorpe, 1981 and Thorpe et al., 1991a, 1991b). The diffusion term is negligible in the direction of flow, even when fa is as small as 0.0001 kg/sm2; but it may become important in beds of grain in which there is no airflow. 4.9.2.2 Analytical Methods of Calculating hv T and HW T Equations 4.75 and 4.79 are beginning to look more useful because they contain variables that affect the rate at which molds grow, the rate at which insect populations increase, the rate at which seeds lose their viability, and so on. However, Equation 4.75 still contains some terms such as ∂hv ∂T and ∂Hw ∂T that must be calculated. The first expression is easy to evaluate since from Equation 3.49: hv = 2501.33 − 2.363T

(4.81)

from which it follows immediately that: ∂hv = −2.363 ∂T

(4.82)

The other term, ∂HW ∂T , is a little more complicated. Perhaps the most transparent way of evaluating the differential is from Equation 4.41, namely: ps A dT 2 {exp ( −100 BW ) − exp ( 0 )} 100 B dps (T + C)

(4.83)

HW = f1 (T ) f2 (T ) f3 (T ) f4 (T ) (exp ( −100 BW ) − 1) A 100 B

(4.84)

HW = hv in the following form:

in which: f1 (T ) = hv f2 (T ) = 1 (T + C)

2

f3 (T ) = (T + 273.15)



169

f4 (T ) = 1 (6800 (T + 273.15) − 5) By the product rule of differentiation (Stein, 1987 and Fraleigh, 1990): ∂HW = (a1 + a2 + a3 + a 4) (exp ( −100 BW ) − 1) A 100 B ∂T

(4.85)

in which: a1 =

∂f1 (T ) f (T ) f3 (T ) f4 (T ) ∂T 2

a2 = f1 (T )

∂f2 (T ) f (T ) f4 (T ) ∂T 3

a3 = f1 (T ) f2 (T )

∂f3 (T ) f (T ) ∂T 4

a 4 = f1 (T ) f2 (T ) f3 (T )

∂f4 (T ) ∂T

From the expression for the latent heat of vaporization as a function of temperature, Equation 3.49: ∂f1 (T ) = −2.363 ∂T ∂f2 (T ) 3 = − 2 (T + C ) ∂T ∂f3 (T ) =1 ∂T ∂f4 (T ) 6800 = 2 2 ∂T (T + 273.15) (6800 (T + 273.15) − 5) 4.9.2.3 A Brief Excursion into Numerical Methods Since ∂HW ∂T can be directly expressed in terms of grain temperature, T, and moisture content, W, it is considered as a closed form expression. The combined moisture and energy balances, Equations 4.80 and 4.76 respectively, do not have such solutions; so equations for the grain temperature and moisture content cannot be derived as a function of time and distance along the bed of grain from the air inlet. Instead, a branch of applied mathematics called numerical analysis must be used with computers to do most of the computational work.


170


Figure 4.15

The variation of the integral heat of wetting, HW , of grain with temperature, T.

To help get a feel for numerical analysis, numerical methods will be used to calculate ∂HW ∂T . Consider the plot of HW against temperature shown in Figure 4.15, when the grain moisture content is 0.14 fractional dry basis. Suppose we wish to calculate ∂HW ∂T at some temperature T, which by definition is equivalent to saying we wish to calculate the gradient to the curve at when the temperature is T. Geometrically it is apparent that the tangent is closely parallel to a line that is drawn from a point T – ∆T 2 to T + ∆T 2 on the curve that represents the dependence of HW on T. Now: ∂HW ∆HW = as ∆T → 0 ∂T ∆T

(4.86)

For a given change in ∆T, the corresponding change in ∆HW is given by: ∆HW = HW

T + 0.5 ∆T

− HW

T − 0.5 ∆T

− HW

T − 0.5 ∆T

hence: ∆HW HW = ∆T

T + 0.5 ∆T

∆T

(4.87)

It follows that, as ∆T approaches 0, we have: ∂HW HW = ∂T

T + 0.5 ∆T

− HW

∆T

T − 0.5 ∆T

(4.88)

Example 4.13 Write a BASIC program to calculate ∂HW ∂T using a numerical method and verify the result analytically. The numerical solution is obtained from Equation 4.88. The program shown in Appendix II calculates the integral heat of wetting of wheat with a temperature of 25°C and a moisture content of 0.14 (fractional dry basis) to be –101.7 kJ kg dry grain. The analytical value of ∂HW ∂T



Figure 4.16

171

The discretization of a bed of grains.

calculated from Equations 4.83 and 4.85 is 0.80132 kJ °C . For the numerical solution, set ∆T = 0.1°C, which results in a calculated value of ∂HW ∂T of 0.80132 kJ °C . Both values are in agreement. The fact that two quite different methods have been used to give the same results is a good, but not infallible, check of the correct solution. However, the results should still be treated with skepticism because they were obtained by carrying a large number of differentiations of functions that are merely empirical fits to data. Such fits can be quite accurate, but large errors can occur in the differentials of experimentally determined functions. 4.9.3

Numerical Solution of the Equations that Govern Heat and Moisture Transfer in Aerated Grain Bulks

The partial differential equations that govern heat and mass transfer in bulk stored grains, Equations 4.75 and 4.79, are coupled — that is, the solution of one affects the solution of the other. Furthermore, the source term ρσ (1 − ε ) hs ∂W ∂t is non-linear; and it is impossible to obtain closed form solutions so that the grain temperature and moisture content can be expressed directly as a function of distance along the grain bed and time from the start of aeration. The mathematical difficulties are made more severe because of the fact that the temperature and the humidity of the aeration air vary arbitrarily with time. The key to obtaining the temperature- and grain moisturetime responses of a bulk of aerated grain is to use numerical analysis to solve the governing partial differential equations. Kreyszig (1998) is a widely used engineering mathematics text that provides a useful introduction to the simple numerical methods used to solve Equations 4.76 and 4.80. The bed of grains is divided into N – 1 sections in the direction of the airflow as shown in Figure 4.16. The height of the bed is L, so that each section has a length, ∆x, given by the equation: ∆x =

L N −1

(4.89)

Points, called nodes, are separated by the distance ∆x, so there is a total of N nodes. At the first node, the temperature and humidity of the air used for aeration are known because they correspond to those of the air entering the grain. Assuming mass and thermal equilibrium between the air and the grains, Equation 4.22 enables one to calculate the grain moisture content at the first node. At the start of the aeration, the distributions of grain moisture contents and temperatures within the bed of grain (they need not be uniform throughout the grain bed) are known. Therefore, temperatures and moisture contents can be assigned to nodes 2,3,4,....,N – 1,N. The next step is to calculate how the grain temperatures and grain moisture contents at each node change with time. To do this, make simple approximations to the first and second derivatives that appear in Equations 4.76 and 4.80. For example: ∂wi wi − wi −1 ≈ ∂x ∆x

(4.90)

in which wi and wi–1 are the humidities of the intergranular air at nodes i and i – 1, respectively. This particular form of differencing, known as upwind differencing (Patankar, 1980), can be used


172


because it makes the solution of the equations quite stable. The partial differential fw fx has been approximated by making use of the values of w at the point of interest and at a point upwind from this point — that is, in the direction against which the air flows. In the numerical approximation of ∂HW ∂T , values of HW at T – ∆T 2 and T + ∆T 2 were used to calculate the derivative. Because points were chosen that were equally spaced behind and ahead of the point T, the principle used to calculate the partial differential was called a central differencing scheme. It is more accurate than the upwind scheme, but central difference can cause solutions to become unstable if it is used in terms that involve velocities. However, one can safely use a central difference scheme to approximate the second order derivative that governs the diffusion of moisture through the bed of grains. In this case, the definition of a second derivative as the rate of change of a rate of change can be used.

∂2 wi = ∂x 2

∂wi +1 2 ∂x

−

∂wi −1 2

∆x

∂x

as ∆x → 0

(4.91)

where ∂wi −1 2 ∂x and ∂wi +1 2 ∂x are the average gradients of humidity between the nodes i – 1 and i and i and i + 1, respectively. If the following approximations are made: ∂wi −1 2 ∂x

≈

wi − wi −1 ∆x

(4.92)

≈

wi +1 − wi ∆x

(4.93)

and: ∂wi +1 2 ∂x

which, when substituted into Equation 4.91, yields the result: ∂2 wi wi −1 − 2 wi + wi +1 = ∂t 2 ( ∆x )2

(4.94)

By analogy, the following approximations can be made for the derivatives of temperature with respect to x: ∂Ti Ti − Ti −1 ≈ (4.95) ∂x ∆x and: ∂2T1 Ti −1 − 2Ti + Ti +1 = ∂x 2 ( ∆x )2

(4.96)

The grain moisture content and temperature vary continuously from point to point within the aerated grain bed, and this fact is captured by Equations 4.76 and 4.80; but in the above treatment, these variables can only be calculated at the nodes. In other words, the conditions in the bed of aerated grains can be calculated only at discrete points; and the problem is said to have been discretized. As ∆x gets smaller and smaller, the numerical solution generally becomes a better approximation of the continuous Equations 4.76 and 4.80. The grain moisture content and temperature also vary continuously with time, but in this approach they can be evaluated only at certain discrete time intervals. If the grain moisture contents



173

and temperatures are calculated after time intervals of ∆t, the values are updated at the times ∆t. 2∆t, 3∆t,…, (p–1)∆t, p∆t, (p+1)∆t, … after the aeration system has been turned on. Here p represents the pth update. This idea of calculating the variables only at certain times is used to discretise the time derivatives in Equations 4.76 and 4.80, thus: ∂Wi Wi p+1 − Wi p = ∂t ∆t

(4.97)

in which Wi p+1 is the grain moisture content at the ith node after the (p+1)st time step. The initial moisture contents are designated Wi 0 . The differential of temperature with respect to time is treated in exactly the same way, thus: ∂Ti Ti p+1 − Ti p = ∂t ∆t

(4.98)

Equations 4.75 and 4.79 are solved by inserting into them the discretized forms given by Equations 4.90, 4.94, 4.95, and 4.96. The moisture conservation Equation 4.79 then takes the form: ρa Deff fa Wi p+1 − Wi p  wi − wi −1  + (w − 2wi + wi+1 )  = ∆t ρσ (1 − ε )  ∆x  (1 − ε ) ρσ ∆x 2 i −1

(4.99)

The simplest method of solving Equation 4.99 is to update the grain moisture contents explicitly, thus: Wi p+1 = Wi p −

ρa Deff fa ∆t wip − wip−1 + w p − 2 wip + wip+1 ρσ ∆x (1 − ε ) ρσ ∆x 2 (1 − ε ) i −1

(

)

{

}

(4.100)

At the start of the solution procedure, the values of wi0 and Wi 0 (p = 0) are known, hence Equation 4.100 can easily be used to calculate Wi1 after the time ∆t. The method is called explicit because the only unknown, Wi p+1 , occurs on the left-hand side of the equation; and all of the variables on the right-hand side of the equation are known. The unknown is given explicitly. The grain temperatures at the interior nodes may be similarly calculated after the next time step only at the same time that the grain moisture content is updated, i.e.:  ∂h   ∂W ∆t   Ti p+1 = Ti p + ρc ρσ (1 − ε )hs − fa  ca + w c1 + v    ∂ ∂T   t p  

(4.101)

 T p − Ti −p1   Ti −1 − 2Ti + Ti +1   * i  + keff    ∆x 2  ∆x  where: ρc p = ρ (1 − ε )  c + c W + ∂HW  + ε  ρ c + ρ c + ρ ∂hv  σ  a a 1 1 1 ∂T   σ 1 ∂T  The solution method has been programmed in BASIC, and a listing is given in Appendix III. The principal difference between the computer program and the analysis is that the program does not contain an expression for ∂HW ∂T , first because its value cannot be calculated accurately. Second, it has been claimed by Close and Banks (1972) that its value is negligible.


174


Appendix III shows the implementation of a program to solve the heat and moisture balance Equations 4.100 and 4.101.

4.10 THE EFFECTS OF RESPIRATION ON HEAT AND MASS TRANSFER IN AERATED BEDS OF GRAINS When the relative humidity of intergranular air exceeds 70%, fungal activity is likely to increase. The storage fungi consume the grain kernels; and as they do so, their respiration leads to the generation of heat, moisture, and carbon dioxide. The resulting higher grain temperatures and moisture contents are likely to promote increased mold activity, hence the process is auto-catalyzing. Molds not only increase the dry matter loss and destroy the useful properties of the grains, but they can also produce mycotoxins that are poisonous in very low concentrations — typically parts per billion — to humans and domestic animals. It is therefore good agricultural practice to ensure that grains are stored under conditions such that their intergranular moisture contents are initially less than 65%. This criterion alone will not prevent mold activity, because temperature gradients in the grain bulk can cause natural convection currents and molecular diffusion to transport moisture from warm regions to cooler regions of a grain bulk. Detailed mathematical descriptions of these phenomena have been given by Thorpe (1996) and Singh and Thorpe (1993a, 1993b). Moisture migration has the effect of locally raising the grain moisture content, which may result in mold activity. Like the rate of insect population growth, loss of seed viability, discoloration, and other indicators of storage quality, the rate of mold activity appears to be a function of the enthalpy of the intergranular air. The lower the enthalpy, the lower the deleterious activity. Ventilation of stored grain is therefore a useful management tool to reduce or prevent mold activity by reducing the temperature, which is beneficial. Lower temperatures also reduce the propensity for moisture migration to occur because the intergranular vapor pressure of water is reduced. Ventilation may also be used to remove the heat of respiration, and this may be exploited to enable damp grains to be stored temporarily between harvest and drying. It is a surprising fact that the rate of respiration can be exploited to provide energy for drying grains; although in practice this should be treated with great caution, as storing damp grains is generally poor agricultural practice. The analysis of heat and moisture transfer in respiring bulks of grain differs from that presented above for nonrespiring grains, in which the grain substrate disappears as a result of the oxidation reaction. As the grain kernels oxidize and the dry matter is consumed, water is formed as a result of respiration and water that is bound to the grain substrate is also released. Furthermore, when the grain substrate is consumed, the amount of energy released is lower than that expected if the grains were perfectly dry. This is because the bound water reduces the surface energy of the grain substrate; hence, when it is consumed by fungi, the amount of energy liberated is reduced. The analysis presented in this chapter will highlight these issues. Although the actual physical and chemical processes that occur in respiring bulks of grains are very complicated, simplifying assumptions can be made. At this stage the assumptions are no more than hypotheses, but they provide an excellent starting point for further research and analysis. For example, as grain kernels are consumed, an assumption can be made that the bed bridges and does not collapse; but the void fraction of the air increases. Extreme cases of bulks of paddy that bridge are observed when grain stores containing paddy are being unloaded. It is not uncommon for the angle of repose of the grain to be 90°, possibly as a result of a high coefficient of friction between the grain kernels and extraneous matter in the grain bulk. The other extreme that might be considered is that the grain slumps as it rots away, a phenomenon occasionally observed close to grain stores when small piles of grain have been exposed to the weather. These are real situations, and both of them may occur simultaneously. The respiration process in stored grains is represented by the oxidation of hexose, i.e.:



175

C6 H12O6 = 6CO2 + 6 H2O, ∆H = q0

(4.102)

where q0 is the heat of reaction of one kilogram of hexose at 25°C. This specification of temperature implies that the moisture is formed in its condensed state — liquid water. If the heat of reaction had been written such that the water formed as a vapor, it would assume a lower value because it would not have included the heat of condensation. Analysis This work first addresses the problem of the bed of grain that bridges as the substrate of the grain kernels is consumed. 4.10.1

Mass Balances

As this problem solution deals with a chemical reaction, namely the oxidation of cellulosic material, all of the chemical species involved must be accounted for. In the intergranular spaces these are four — namely, moisture vapor, carbon dioxide, oxygen, and non-reacting elements such as nitrogen, the inert gases, and so on. These four components are designated by the subscripts 1, 2, 3, and 4, respectively. It is also recognized that the grain substrate also reacts, and any water associated with the substrate as it disappears must be accounted for. The appropriate mass continuity equations may be written as:

( )

(

)

∂ ∂ ε σρσ W ) + ε ρ + ∇ ⋅ ε γ ρ1v1 = S1 ( ∂t ∂t γ 1 4.10.2

(4.103)

Moisture Balance

Compared with the analysis of heat and moisture transfer in a uniformly aerated (one-dimensional) bulk of non-respiring grains, notice three features of the equation. First, the void fraction, εγ , of the grain bulk is not assumed to be constant. For this reason it has been included in the quantities to be differentiated. When εγ is constant, it can be moved outside of the brackets. Second, three-dimensional grain bulks are involved. For this reason vector notation is used; that is little more than a shorthand way of writing what would otherwise take up a lot of space. It is easy to see what this means when written as:

(

∂ ε ρu ∂ ε ρv ∂ ε ρw ) ( ∂x ) + ( ∂y ) + ( ∂z ) γ 1 1

∇ ⋅ ε γ ρ1v1 =

γ 1 1

γ 1 1

(4.104)

in which u1, v1, and w1 are respectively the components of the velocity of water vapor in the x, y, and z directions. Third, moisture liberated by the respiring grains is represented by S1, which has the units of kg/m3/s. 4.10.3

Carbon Dioxide

The equation that governs the transport and liberation of carbon dioxide is:

( ) + ∇ ⋅ (ε ρ v ) = S

∂ ε γ ρ2 ∂t

γ 2 2

2

(4.105)


176


in which S2 is the rate at which carbon dioxide is liberated. 4.10.4

Oxygen

( ) + ∇ ⋅ (ε ρ v ) = S

∂ ε γ ρ3

γ 3 3

∂t

(4.106)

3

Here S3 is the rate of liberation of oxygen; but since oxygen is consumed, S3 is a negative quantity. 4.10.5

Nitrogen and Other Non-Reacting Gases

Nitrogen and other inert gases do not participate in the respiration of the molds, so:

( ) + ∇ ⋅ (ε ρ v ) = 0 = S

∂ ε γ ρ4

γ

∂t

4.10.6

4 4

(4.107)

4

Mass Balance on the Solid Substrate

The solid substrate of the grain kernels is deemed to be consumed at a rate of S5 kg/m3/s:

(

∂ ε γ ρσ ∂t

)=S

(4.108)

5

In the above equations, the various densities, ρ1, ρ2, ρ3, and ρ4, refer to the masses of the corresponding chemical species per unit mass of air flowing through the intergranular pores of the grain bulk. The density ρσ refers to the density of the dry matter in the grain kernels. 4.10.6.1 Thermal Energy Continuity The enthalpy balance in three-dimensional systems is derived in the same way as in the case of one-dimensional sysems described in detail above. However, instead of carrying the enthalpy over a small element, the balance is carried out over a small cube, as described by Bird et al. (1960a). The enthalpy balance is expressed as: ∂ (ε ρ H ) + ∂t σ σ

4

∑  ∂∂t (ε ρ h ) + ∇ ⋅ (ε ρ v h ) = k γ i i

σ i i i

eff

∇2T

(4.109)

i =1

Again, the product rule of differentiation may be used to expand Equation 4.109 as follows:

H

∂(ε σρσ ) ∂t

+ ε σρσ

4

+

∑ ε ρ

γ i

i =1

∂H + ∂t

4

∑ i =1

( )

 ∂ερ  γ i + hi ∇ ⋅ ε γ ρi vi hi t ∂ 

∂hi  + ε γ ρi vi ⋅ ∇hi  = keff ∇2T ∂t 

(

)

   

(4.110)



177

The heat of oxidation, Qr , of the substrate is defined as the difference between the sum of the enthalpies of products of reaction and the reactants. If the molecular weight of the grain substrate is taken to be 180, and the molecular weight of carbon dioxide, for example, is 44, then for every kilogram of substrate that is oxidized heat is liberated together with:  6 × 44  = 1.47 kg of CO 2  180 

(4.111)

 6 × 18  = 0.6 kg of water  180 

(4.112)

 6 × 32  = 1.07 kg of oxygen is consumed  180 

(4.113)

The heat of oxidation, q0, of one kilogram of cellulosic material is thus: q0 = 1.47 × h2 + 0.6 (h1 − hv ) − 1.07 h3 − h4

(4.114)

In Equation 4.114, the heat of vaporization, hv , is subtracted because the enthalpy, h1, is of water vapor, whereas q0 is defined on the basis of liquid water formed. The rate of liberation of heat per unit volume, Qr , is therefore given by: 4

−Qr =

∑h S − h S i i

(4.115)

v 1

i =1

If each of the mass balance Equations 4.103 and 4.105 to 4.108 are multiplied by the enthalpy of the species to which the equations relate, Equation 4.115 can be written as: 4

Qr = −

∑ i =1

( )

 ∂ ε ρ  γ i + ∇ ⋅ ε γ ρi vi hi    ∂t

+ hv S1 − h1

(

 ∂ ε ρ )  − ( ∂t ) (h σ σ

 

0 σ

(

+ cw T − T 0

)

γ

+ HW

)

(4.116)

∂ (ε ρ W ) ∂t σ σ

As in the case of non-respiring grains: ∂H ∂H ∂W ∂H ∂T = + ∂t ∂W ∂t ∂T ∂t

(4.117)

∂H  ∂T ∂H ∂W  = cw (T − T0 ) + HW + c + Wcw + W  ∂t ∂t  σ ∂T  ∂t

(4.118)

or:

{

}

which enables us to write the overall enthalpy balance as:


178


{W (h

0 w

(

+ cw T − T 0

∂ε ρ ))} ( ∂t ) + ε ρ {h σ σ

+ cw (T − T0 ) + hw

0 w

σ σ

} ∂∂Wt

∂(ε σρσ ) ∂H  ∂T  + ε σρσ cσ + Wcw + W  + hv S1 − hw0 + cw T − T 0 + hv W ∂t ∂ ∂ T t  

{

{

(

(

}

)

(4.119)

4

)

− ε σρσ hw0 + cw T − T 0 + hv

} ∂∂Wt + ∑ ε ρ ∂∂ht + ε ρ v ⋅ ∇h  i

γ i

γ i i

i

i =1

= keff ∇2T + Qr This equation can be simplified to: ∂ε ∂HW  ∂T ∂W  − hs ε σρσ − ρσ σ cσ + Wcw +  ∂ T t ∂ t ∂ ∂t   4

+

∑ i =1

∫

W

0

hs dW

∂h   2  ε γ ρi i + ε γ ρi vi ⋅ ∇hi  = keff ∇ T + Qr − hv S1   ∂t

(4.120)

In arriving at the above equation, the identity for the differential heat of sorption used was: hs = hv − hw

(4.121)

and the identity used was: HW − hvW =

∫

W

0

hw dW − hv

∫

W

dW = −

0

∫

W

0

hs dW

(4.122)

Again, there is confidence about accuracy because all of the quantities defined at standard conditions have canceled out. Now: ∂hi ∂T = ci ∂t ∂t

i = 2, 3, 4

(4.123)

and: dh1 ∂T ∂hv ∂T = cw + ⋅ dt ∂t ∂T ∂t

(4.124)

Similarly:

( )

ε γ ρi vi ⋅ ∇hi = ε gρi v c p ⋅ ∇T i

i = 2, 3, 4

(4.125)

and:

( )

ε γ ρ1v1 ⋅ ∇h1 = ε γ ρ1v1 c p ⋅ ∇T + ε γ ρ1v1 1

∂hv ⋅ ∇T ∂T

(4.126)



179

Bird et al. (1960b) point out that the mass average velocity, vγ , is defined as follows: 4

ργ v γ =

∑ρ v

(4.127)

i γ

i =1

and ργvγ represents the rate at which mass passes through a unit area of the intergranular pores. Since the composition of the intergranular fluid is dominated by air: ρ γ v γ ≅ ρa v a

(4.128)

v1 = v a + u1

(4.129)

Now:

where u1 is the diffusion velocity of water vapor through the intergranular spaces, which is several orders of magnitude less than va. The enthalpy balance may then be written as: ∂ε ∂HW  ∂T ∂W  − ρσ σ − hs ε σρσ cσ + cwW +  ∂T  ∂t ∂t ∂t 

∫

W

0

hs dW

∂h   ε γ ρa v a ⋅ ∇T + ε γ ρa v a w  cw + v  ⋅ ∇T = Keff ∇2T + Qr − hv S1  ∂T 

(4.130)

At this point an effective thermal dispersivity, Keff , has been introduced in place of the effective thermal conductivity keff . This serves to emphasize that thermal energy is also transported by hydrodynamic dispersion, which arises from the random flow paths of the air between the grain kernels that augment the apparent thermal conductivity of the grain bed. Whitaker (1991) has used this device, and specifically Thorpe and Whitaker (1992a, 1992b) have used the thermal dispersivity in studies of heat and mass transfer in ventilated bulks of grain. The one-dimensional form of this equation becomes: ∂ε ∂HW  ∂T ∂W  − hs ε σρσ − ρσ σ cσ + cwW +  ∂ T ∂ t ∂ t ∂t   faca

∫

W

0

hs dW

∂h  ∂T ∂T ∂2T  + fa w  cw + v  = Keff 2 + Qr − hv S1  ∂x ∂T  ∂x ∂x

By stoichiometry: S4 = −1.66 S1

(4.131)

so that the moisture balance equation becomes: ε σρσ

∂W + ε γ ρ AvA ⋅ ∇w = S1 + 1.66 S1W = S1 (1 + 1.66 W ) ∂t

The one-dimensional form is:

(4.132)


180


ε σρσ 4.10.7

∂W ∂w + fa = S1 (1 + 1.66 W ) ∂t ∂x

A Slumping Bed of Grain

Analysis of the slumping bed of grains is similar to that of the bridging bed, but the grains fall under the effects of gravity as the grain substrate disappears as a result of it being consumed by fungi. As the bed of grain slumps (or collapses) the grain kernels have a velocity, vσ, that depends on their position in the grain bed. One might expect those grains very near to the floor of a grain bulk to have a smaller velocity than those at the top of the bulk because they are located on top of grains that are disappearing. The mass balance equation on the grain kernels is therefore expressed as: ∂(ε σρσ ) ∂t

+ ∇ ⋅ (ε σρσ v σ ) = S4

(4.133)

The moisture in a bulk of slumping grains is transported not only by water vapor in the intergranular air but also by the bulk movement of the grain kernels which contain about ten thousand times more moisture on a weight-per-volume basis than does dry air. The overall moisture conservation equation is therefore expressed as: ∂ (ε ρ W ) + ρεσvσ ⋅ ∇W + ρεσW ∇ ⋅ vσ = S1 ∂t σ σ

(4.134)

while the mass conservation equations on the carbon dioxide and oxygen remain the same as for the bridging case. The one-dimensional case is governed by the somewhat simplified equation: ∂ (ε ρ W ) + fσ ∂∂Wx + W ∂∂fxσ + fa ∂∂wx = S1 ∂t σ σ Thermal Energy Conservation The thermal energy conservation equation for the slumping bed of grains is written as: ∂ (ε ρ H ) + ∇ ⋅ (εσρσvσ H ) + ∂t σ σ

4

∑  ∂∂t (ε ρ h ) + ∇ ⋅ (ε ρ v h ) = k γ i i

σ i i i

eff

∇2T

i =1

into which may be inserted the definitions of the enthalpies hi and H to obtain:

(

(

)

ε σρσ hw0 + cw T − T 0 + hw

) ∂∂Wt

∂H  ∂T  ε σρσ  cσ + Wcw + W   ∂T  ∂t

(

(

)

{

(

+ ε σρσ hσ0 + cσ T − T 0 + HW + W hW0 + cw T − T 0

)})∇ ⋅ v

σ

(4.135)



{

(

181

}

)

ε σρσ v σ ⋅ hw0 + cw T − T 0 + hw ⋅ ∇W ∂H   + ε σρσ v σ cσ + Wcw + W  ⋅ ∇T ∂T   4

+

∑ i =1

( )

 ∂ερ  γ i + hi ∇ ⋅ ε γ ρi vi hi t ∂ 

(



4

) + ∑ ε ρ ∂∂ht + ε ρ v ⋅ ∇h  = K 

γ i

i

γ i i

i

eff

∇2T

(4.136)

i =1

The definition of the heat of reaction, Equation 4.116, may be used in Equation 4.136 to obtain the thermal energy conservation equation for a slumping bed of grains, namely: ∂H  ∂T ∂W  ε σρσ cσ + Wcw + W  − ε σρσ hs ∂T  ∂t ∂t  − ε σρσ ∇ ⋅ v σ

∫

W

0

hs dW − ε σρσ hs v σ ⋅ ∇W

∂H   + ε σρσ cσ + WCw + W v σ ⋅ ∇T ∂T   ∂h   + ε γ ρa v a ⋅ ∇T + ε γ ρa v a w ⋅  cw + v  ⋅ ∇T  ∂T  = Keff ∇2T + Qr − hv S1

(4.137)

in which the assumption is made that the velocities of all the chemcial species are the same as that of the dry air, namely va. The hydrodynamic dispersion of thermal energy is subsumed in the effective thermal dispersion coefficient, Keff . Expressing the above equation in one-dimensional form results in: ∂H  ∂T ∂W  ε σρσ cσ + Wcw + W  − ε σρσ hs ∂T  ∂t ∂t  − ε σρσ ∇ ⋅ v σ

∫

W

0

hs dW − fs hs

∂W ∂x

∂H  ∂T  + fσ cσ + Wcw + W  ∂T  ∂x  + faca

∂h  ∂T ∂T  + fa w ⋅  cw + v   ∂x ∂T  ∂x

= Keff

∂2T + Qr − hv S1 ∂x 2


182


4.10.8

The Heat of Respiration

A method of calculating the heat of respiration of the fungi is needed. These appear to be obtainable only by fitting mathematical relationships to experimental data. Such models have been presented by Thompson (1972) and, more recently, by Lacey et al. (1994). As noted above, respiration may be regarded as the complete combustion of carbohydrates to form carbon dioxide and water, along with the liberation of heat. The oxidation of 1 kg of the grain substrate liberates 15,778 kJ of heat and forms 1.47 kg of carbon dioxide and 0.6 kg of water. Several workers (Thompson, 1972; Seib et al., 1980; and Lacey et al., 1994) have quantified the factors that affect the rate of respiration. In this chapter use is made of Thompson’s (1972) work on the respiration of maize. He determined that the dry matter loss is time dependent, and after a time t seconds the fractional loss, dm, of dry matter is given by:

( (

) )

dm = 8.83 × 10 −4 exp 1.667 × 10 −6 × t − 1 + 2.833 × 10 −9 t

(4.138)

Equation 4.138 applies to shelled maize at 15.5°C and 25% moisture content (wet basis) with 30% damage. Because the temperature and moisture content of the grains vary with location in the grain store, it is necessary to map the value of the real time, t, into some physiological time, tp. Real-time and physiological time are related by: tp =

t MM MT

(4.139)

in which MM and MT modify the elapsed time (or life) of the grains depending on their moisture content and temperature. Converting Thompson’s (1972) expression to SI Units results in the following: 1. Temperature Modifier When T ≤ 15.5°C or M ≤ 19%:

MT = 32.2 exp ( −0.1044T − 1.856)

(4.140)

When T > 15.5°C and 19 < M < 28%:

MT = 32.2 exp ( −0.1044T − 1.856) + {( M − 19) 100} exp (0.0183T − 0.2847)

(4.141)

When T > 15.5°C and M > 28%:

MT = 32.2 exp ( −0.1044T − 1.856) + 0.09 exp (0.0183T − 0.2847)

(4.142)

2. Moisture Modifier The moisture modifier, MM, is given by the expression:

( (

)

)

1.53 MM = 0.103 exp 455 MDB − 0.00845 MDB + 1.558

(4.143)

in which MDB is the grain moisture content, % dry basis. Equations 4.138 to 4.143 are used to calculate the dry matter loss, dm, after the physiological time, tp, by means of the expression:



183

{ (

)}

dm = 8.83 × 10 −4 exp 1.667 × 10 −6 t p − 1 + 2.833 × 10 −9 t p

(4.144)

from which it is readily determined that the rate of dry matter loss is given by:

(

{ (

) }

ddm = 14.72 × 10 −10 exp 1.667 × 10 −6 t p − 1 + 2.833 × 10 −9 dt

) (M M ) M

T

(4.145)

These empirical equations have been included in the computer program listed in Appendix IV to this chapter. 4.11 CONCLUSIONS Equations that govern heat and mass transfer in bulks of ventilated grains have been derived. Mass and energy source terms associated with the moisture bound to the grains substrate appears in the equations. Two extreme cases are analyzed — namely, bulks of grain that bridge and bulks that slump as the substrate is consumed.

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Thorpe, G.R. (1995). More complete mathematical descriptions of heat and moisture transfer in ventilated bulks of respiring grains, J. Soc. Eng. Agric., 24(2), 10–15. Thorpe, G.R. (1994). Heat and moisture transfer in ventilated bulks of respiring grains — a theoretical analysis and technological application, Report, Department of Civil and Building Engineering, Victoria University of Technology, Melbourne, Australia. Thorpe, G.R. (1981). Moisture diffusion through bulk grain, J. Stored Prod. Res., 17(1), 39–42. Thorpe, G.R. (1982). Moisture diffusion through bulk grain subjected to a temperature gradient, J. Stored Prod. Res., 18(1), 9–12. Thorpe, G.R. and Ahmad, M. (1998). The performance of a solar desiccant cooling system for stored grains, in Stored Grain in Australia, Proceedings of the Australian Post-harvest Technical Conference. Banks, H.J., Wright, E.J., and Damcevski, K.A., Eds., CSIRO Entomology, Canberra, 26th–29th May, 1998, 209–216. Thorpe, G.R. (1981). The modeling and potential applications of a simple solar regenerated grain cooling device, Postharvest Biol. Technol., 13, 151–168. Thorpe, G.R., Stokes, A.N., and Wilson, S.G. (1990). The integral heats of wetting of food grains, J. Agric. Eng. Res., 46, 71–76. Thorpe, G.R., Ochoa, J.A., and Whitaker, S. (1991a). The diffusion of moisture in food grains. I. The development of a mass transfer equation, J. Stored Prod. Res., 27, 1–9. Thorpe, G.R., Ochoa, J.A., and Whitaker, S. (1991b). The diffusion of moisture in food grains. II. Estimation of the effective thermal diffusivity, J. Stored Prod. Res., 27, 11–30. Thorpe, G.R. and Whitaker (1992a). Local mass and thermal equilibria in ventilated grain bulks. Part II. The development of heat and mass conservation equation, J. Stored Prod. Res., 28(1), 15–27. Thorpe, G.R. and Whitaker, S. (1992b). Local mass and thermal equilibria in ventilated grain bulks. Part I. The development of constraints, J. Stored Prod. Res., 28(1), 29–54. Whitaker, S. (1991). Some improved estimates for the principle of local thermal equilibrium, Industrial Eng. Chem. Res., 30, 983–997. Wilson, S.G. and Desmarchelier, J.M. (1994). Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30, 45–60.


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APPENDIX I A BASIC Computer Program used to Calculate the Ratio, hs/hv , of the Heat of Sorption to the Latent Heat of Vaporization of Free Water REM To calculate differential heat of sorption algebraically REM by making use of modified Chung-Pfost equation. REM Constants apply to Durum wheat. A = 921.65 B = .1808 C = 112.35 CLS PRINT TAB(20); "------------------------------------------" PRINT TAB(20); " Temp.(C) M. c.(%wb) hsbyhv " PRINT TAB(20); "------------------------------------------" FOR w = 9 TO 15 STEP 3 FOR t = 10 TO 30 STEP 10 REM Convert moisture content to % dry basis m = 100 * w / (100 - w) REM Saturation vapour pressure of water REM Equation 3.24 ps = 6 * 10 ^ 25 / (273.15 + t) ^ 5 * EXP(-6800 / (t + 273.15)) REM Derivative of saturation vapour pressure with respect to temperature REM Equation 3.37 dpsdt = ps / (t + 273.15) * (-5 + 6800 / (t + 273.15)) REM Relative humidity REM Equation 3.10 r = EXP(-A / (t + C) * EXP(-B * m)) REM Derivative of relative humidity with respect to temperature REM Equation 3.38 drdt = A * r / (t + C) ^ 2 * EXP(-B * m) REM The required result REM Equation 3.36 hsbyhv = 1 + (ps / r) * drdt / dpsdt PRINT TAB(26); t; TAB(41); w; TAB(50); hsbyhv NEXT t NEXT w END



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APPENDIX II An Implementation of the Analytical and Numerical Schemes to Evaluate HW T REM To calculate integral heat of sorption algebraically REM by making use of modified Chung-Pfost equation. REM Constants apply to Durum wheat. a = 921.65 b = .1808 C = 112.35 CLS w1 = .14 t = 25 t0 = t hv = 2501.33 - 2.363 * t ps = 6 * 10 ^ 25 / (273.15 + t) ^ 5 * EXP(-6800 / (t + 273.15)) dpsdt = ps / (t + 273.15) * (-5 + 6800 / (t + 273.15)) intheat1 = a * ps * hv / (100 * b * dpsdt * (t + C) ^ 2) * (EXP(-100 * b * w1) - 1) PRINT "Temperature:"; t PRINT "Moisture content (dry-basis):"; w1 PRINT "Integral heat of wetting:"; intheat1 REM t = t0 chi = a / (100 * b) * (EXP(-100 * b * w1) - 1) f1 = hv f2 = 1 / (t + C) ^ 2 f3 = t + 273.15 f4 = 1 / (6800 / (t + 273.15) - 5) df1 = -2.363 df2 = -2 / (t + C) ^ 3 df3 = 1 df4 = 6800 / (t + 273.15) ^ 2 / (6800 / (t + 273.15) - 5) ^ 2 a1 = df1 * f2 * f3 * f4 a2 = f1 * df2 * f3 * f4 a3 = f1 * f2 * df3 * f4 a4 = f1 * f2 * f3 * df4 sum = a1 + a2 + a3 + a4 dhwdt = sum * chi PRINT "Analytical value of dhwdt:"; dhwdt REM REM A numerical evaluation of DHWDT deltat = .1 REM Calculate integral heat of wetting when the temperature is REM t0 - 0.5*deltat t = t0 - .5 * deltat hv = 2501.33 - 2.363 * t ps = 6 * 10 ^ 25 / (273.15 + t) ^ 5 * EXP(-6800 / (t + 273.15)) dpsdt = ps / (t + 273.15) * (-5 + 6800 / (t + 273.15)) intheat1 = a * ps * hv / (100 * b * dpsdt * (t + C) ^ 2) * (EXP(-100 * b * w1) - 1) REM Calculate integral heat of wetting when the temperature is REM t0 + 0.5*deltat t = t0 + .5 * deltat hv = 2501.33 - 2.363 * t ps = 6 * 10 ^ 25 / (273.15 + t) ^ 5 * EXP(-6800 / (t + 273.15)) dpsdt = ps / (t + 273.15) * (-5 + 6800 / (t + 273.15)) intheat2 = a * ps * hv / (100 * b * dpsdt * (t + C) ^ 2) * (EXP(-100 * b * w1) - 1) REM Carry out the numerical differentiation PRINT "Numerical value of dhwdt:"; (intheat2 - intheat1) / deltat END


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APPENDIX III A Listing of the Program used to Calculate Heat and Moisture Transfer in an Aerated Bed of Grains 10 REM ********************************************* 20 REM 30 REM TO SIMULATE THE PERFORMANCE OF A BED 40 REM OF AERATED GRAINS 50 REM 60 REM ********************************************* 100 DIM T(100), W(100), WNEW(100), TNEW(100), H(100), TEQ(100) 110 REM DT = TIME STEP 120 REM W = LENGTH OF GRAIN BED, M. 130 REM NX = NUMBER OF NODES 140 REM DX = STEP LENGTH 150 REM EPS = VOID FRACTION OF THE BED 160 REM RHOS = DENSITY OF GRAIN KERNELS 170 REM RHOA = DENSITY OF AIR 180 REM CG = SPECIFIC HEAT OF DRY GRAIN, J/KG/K 190 REM CW = SPECIFIC HEAT OF LIQUID WATER, J/KG/K 200 REM CA = SPECIFIC HEAT OF AIR, J/KG/K 210 REM DHVDT = DIFFERENTIAL OF LATENT HEAT W.R. TEMPERATURE 220 REM T(1) = INLET AIR TEMPERATURE 230 REM H(1) = HUMIDITY OF INLET AIR 240 REM VEL = FACE VELOCITY OF AIR THROUGH THE GRAIN, M/S. 250 REM HV = LATENT HEAT OF VAPORIZATION OF WATER, J/KG 260 REM N = RATIO OF MOLECULAR WEIGHTS OF WATER VAPOUR AND AIR. 270 REM PATM = ATMOSPHERIC PRESSURE, PA. 280 REM T(I) = TEMPERATURE OF GRAINS 290 REM H(I) = HUMIDITY OF INTERSTITIAL AIR 300 REM KEFF = EFFECTIVE THERMAL CONDUCTIVITY OF GRAIN BED. 310 REM DEFF = EFFECTIVE DIFFUSIVITY OF MOISTURE VAPOUR THROUGH GRAIN. 320 REM FA = MASS FLOW RATE PER UNIT AREA OF BED, KG/S/M^2 330 CLS 340 REM *** 360 REM SET COEFFICIENTS IN THE MODIFIED CHUNG-PFOST EQUATION 370 ACP = 921.65 380 BCP = 18.08 390 CCP = 112.35 470 REM *** 480 TIME = 0 490 DT = 3600 495 REM LENGTH OF BED IS W, AND NUMBER OF NODES IS NX 500 W = 5 510 NX = 21 520 NXM1 = NX - 1 530 DX = W / (NXM1) 540 EPS = .4 550 RHOS = 1300 560 RHOB = (1 - EPS) * RHOS 570 RHOA = 1.2 580 CG = 1300 590 CW = 4180 600 CA = 1000 610 KEFF = .15 620 DEFF = .000005 630 DHVDT = -2377



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APPENDIX III (continued) A Listing of the Program used to Calculate Heat and Moisture Transfer in an Aerated Bed of Grains 640 INPUT "HUMIDITY OF AIR USED FOR AERATION:"; HINA 650 INPUT "TEMPERATURE OF AIR USED FOR AERATION (DEGREES C) "; TINA 652 H(1) = HINA 653 T(1) = TINA 660 GOSUB 1530 665 REM AN OPERATIONAL HINT: 666 REM IF YOU WISH TO INCREASE THE FLOW RATE BY 10, SAY, TO 667 REM SIMULATE A DRYING OPERATION REDUCE THE TIME STEP, DT, BY 10. 670 FA = .01 680 HV = 2502390! 690 N = .622 700 PATM = 101325! 710 PRINT 720 PRINT 721 INPUT "INITIAL GRAIN TEMPERATURE (DEGREES C) :", TIN 722 INPUT "INITIAL GRAIN MOISTURE CONTENT (% WET BASIS) :"; WIN 730 REM INITIALISE MOISTURE CONTENT AND TEMPERATURE 740 FOR I = 2 TO NX 750 T(I) = TIN 760 W(I) = WIN / (100 - WIN) 770 NEXT I 780 FOR KKK = 1 TO 25 790 FOR KK = 1 TO 50 800 REM 810 REM UPDATE TIME 820 REM 830 TIME = TIME + DT 840 FOR I = 2 TO NXM1 850 GOSUB 1360 860 REM SOLVE MOISTURE CONSERVATION EQUATION ALONG THE BED 900 DHDX = (H(I) - H(I - 1)) / DX 910 DWDT = -FA / RHOB * DHDX + RHOA * DEFF / (RHOB * DX ^ 2) * (H(I - 1) - 2 * H(I) + H(I + 1)) + DDMDT * .6 915 REM CALCULATE THE MOISTURE CONTENT AT END OF TIME STEP 920 WNEW(I) = W(I) + DWDT * DT 940 REM 950 REM SOLVE THERMAL ENERGY EQUATION 960 REM 970 GOSUB 1380 1030 DENOM = (CG + CW * W(I)) * RHOB + EPS * RHOA * (CA + H(I) * (CW + DHVDT)) 1050 A = RHOB * hs * DWDT 1040 DTDX = (T(I) - T(I - 1)) / DX 1100 B = -FA * (CA + H(I) * (CW + DHVDT)) * DTDX 1110 C = KEFF * (T(I - 1) - 2 * T(I) + T(I + 1)) / DX ^ 2 1120 REM CALCULATE THE TEMPERATURE AT END OF THE TIME STEP 1140 TNEW(I) = T(I) + DT * (A + B + C) / DENOM 1160 REM PRINT" HS";HS;" HVAP:";HVAP;" HW:";HW;" RH:";RH;" H(I):";H(I) 1170 REM CALCULATE BOUNDARY CONDITIONS AT EXIT OF THE BED 1180 TNEW(NX) = TNEW(NX - 1) 1190 H(NX) = H(NX - 1) 1200 WNEW(NX) = WNEW(NX - 1) 1210 NEXT I 1220 FOR I = 2 TO NX


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APPENDIX III (continued) A Listing of the Program used to Calculate Heat and Moisture Transfer in an Aerated Bed of Grains 1230 T(I) = TNEW(I) 1240 W(I) = WNEW(I) 1250 NEXT I 1270 NEXT KK 1280 PRINT "TIME:"; TIME / 3600; " HOURS" 1290 FOR I = 1 TO NX 1300 M = W(I) / (1 + W(I)) 1310 PRINT I; T(I); W(I); H(I); M 1320 NEXT I 1340 NEXT KKK 1350 END 1355 REM*************************************************** 1360 REM TO CALCULATE SATURATION VAPOUR PRESSURE OF WATER 1370 PS = (6E+25 / ((T(I) + 273) ^ 5)) * EXP(-6800 / (T(I) + 273)) 1380 REM CALCULATE RELATIVE HUMIDITY OF INTERSTITIAL AIR 1390 DPSDT = PS / (T(I) + 273.15) * (-5 + 6800 / (T(I) + 273.15)) 1400 RH = EXP(-ACP / (T(I) + CCP) * EXP(-BCP * W(I))) 1410 DRDT = ACP * RH / (T(I) + CCP) ^ 2 * EXP(-BCP * W(I)) 1420 hsbyhv = 1 + (PS / RH) * DRDT / DPSDT 1460 HVAP = HV + DHVDT * T(I) 1470 hs = HV * hsbyhv 1490 P = PS * RH 1500 REM CALCULATE INTERSTITIAL HUMIDITY OF AIR. 1510 H(I) = (P * N) / (PATM - P) 1520 RETURN 1532 REM 1535 REM************************************ 1530 REM TO CALCULATE GRAIN MOISTURE CONTENT *1540 REM AT THE INLET OF THE BED. 1550 PATM = 101325! 1560 N = .622 1570 P = PATM * H(1) / (N + H(1)) 1580 PS = (6E+25) / ((T(1) + 273) ^ 5) * EXP(-6800 / (T(1) + 273)) 1590 RH = P / PS 1600 W(1) = -1 / BCP * LOG(LOG(RH ^ (-(T(1) + CCP) / ACP))) 1870 RETURN



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APPENDIX IV A Listing of a Numerical Procedure to Solve the Coupled Heat and Mass Transfer Equations that Govern Heat and Mass Transfer in Beds of Ventilated Grains 10 REM ********************************************* 20 REM 30 REM TO SIMULATE THE PERFORMANCE OF A BED 40 REM OF AERATED RESPIRING GRAINS 50 REM 60 REM ********************************************* 100 DIM T(100), W(100), WNEW(100), TNEW(100), H(100), TEQ(100) 110 REM DT = TIME STEP 120 REM W = LENGTH OF GRAIN BED, M. 130 REM NX = NUMBER OF NODES 140 REM DX = STEP LENGTH 150 REM EPS = VOID FRACTION OF THE BED 160 REM RHOS = DENSITY OF GRAIN KERNELS 170 REM RHOA = DENSITY OF AIR 180 REM CG = SPECIFIC HEAT OF DRY GRAIN, J/KG/K 190 REM CW = SPECIFIC HEAT OF LIQUID WATER, J/KG/K 200 REM CA = SPECIFIC HEAT OF AIR, J/KG/K 210 REM DHVDT = DIFFERENTIAL OF LATENT HEAT W.R. TEMPERATURE 220 REM T(1) = INLET AIR TEMPERATURE 230 REM H(1) = HUMIDITY OF INLET AIR 240 REM VEL = FACE VELOCITY OF AIR THROUGH THE GRAIN, M/S. 250 REM HV = LATENT HEAT OF VAPORIZATION OF WATER, J/KG 260 REM N = RATIO OF MOLECULAR WEIGHTS OF WATER VAPOUR AND AIR. 270 REM PATM = ATMOSPHERIC PRESSURE, PA. 280 REM T(I) = TEMPERATURE OF GRAINS 290 REM H(I) = HUMIDITY OF INTERSTITIAL AIR 300 REM KEFF = EFFECTIVE THERMAL CONDUCTIVITY OF GRAIN BED. 310 REM DEFF = EFFECTIVE DIFFUSIVITY OF MOISTURE VAPOUR THROUGH GRAIN. 320 REM FA = MASS FLOW RATE PER UNIT AREA OF BED, KG/S/M^2 330 CLS 340 REM *** 360 REM SET COEFFICIENTS IN THE MODIFIED CHUNG-PFOST EQUATION 370 ACP = 921.65 380 BCP = 18.08 390 CCP = 112.35 470 REM *** 480 TIME = 0 490 DT = 3600 495 REM LENGTH OF BED IS W, AND NUMBER OF NODES IS NX 500 W = 5 510 NX = 11 520 NXM1 = NX - 1 530 DX = W / (NXM1) 540 EPS = .4 550 RHOS = 1300 560 RHOB = (1 - EPS) * RHOS 570 RHOA = 1.2 580 CG = 1300 590 CW = 4180 600 CA = 1000 610 KEFF = .15 620 DEFF = .000005 625 REM LATENT HEAT OF VAPORIZATION OF WATER, HVAP = HV + DHVDT*T


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APPENDIX IV (continued) A Listing of a Numerical Procedure to Solve the Coupled Heat and Mass Transfer Equations that Govern Heat and Mass Transfer in Beds of Ventilated Grains 630 DHVDT = -2377 640 INPUT "HUMIDITY OF AIR USED FOR AERATION:"; HINA 650 INPUT "TEMPERATURE OF AIR USED FOR AERATION (DEGREES C) "; TINA 652 H(1) = HINA 653 T(1) = TINA 660 GOSUB 1530 665 REM AN OPERATIONAL HINT: 666 REM IF YOU WISH TO INCREASE THE FLOW RATE BY 10, SAY, TO 667 REM SIMULATE A DRYING OPERATION REDUCE THE TIME STEP, DT, BY 10. 670 FA = .01 680 HV = 2502390! 690 N = .622 700 PATM = 101325! 710 PRINT 720 PRINT 721 INPUT "INITIAL GRAIN TEMPERATURE (DEGREES C) :", TIN 722 INPUT "INITIAL GRAIN MOISTURE CONTENT (% WET BASIS) :"; WIN 730 REM INITIALISE MOISTURE CONTENT AND TEMPERATURE 740 FOR I = 2 TO NX 750 T(I) = TIN 760 W(I) = WIN / (100 - WIN) 770 NEXT I 780 FOR KKK = 1 TO 25 790 FOR KK = 1 TO 50 800 REM 810 REM UPDATE TIME 820 REM 830 TIME = TIME + DT 840 FOR I = 2 TO NXM1 850 GOSUB 1360 855 GOSUB 1880 860 REM SOLVE MOISTURE CONSERVATION EQUATION ALONG THE BED 900 DHDX = (H(I) - H(I - 1)) / DX 910 DWDT = -FA / RHOB * DHDX + RHOA * DEFF / (RHOB * DX ^ 2) * (H(I - 1) - 2 * H(I) + H(I + 1)) + DDMDT * .6 * (1 + 1.66 * W(I)) 915 REM THIS IS EQUATION 4.132 WITH A TERM THAT ACCOUNTS FOR THE 916 REMDIFFUSION OF MOISTURE. 920 WNEW(I) = W(I) + DWDT * DT 940 REM 950 REM SOLVE THERMAL ENERGY EQUATION 960 REM 970 GOSUB 1380 1030 DENOM = (CG + CW * W(I)) * RHOB + EPS * RHOA * (CA + H(I) * (CW + DHVDT)) 1050 A = RHOB * hs * DWDT 1040 DTDX = (T(I) - T(I - 1)) / DX 1100 B = -FA * (CA + H(I) * (CW + DHVDT)) * DTDX 1110 C = KEFF * (T(I - 1) - 2 * T(I) + T(I + 1)) / DX ^ 2 1120 D = RHOB * DDMDT * (1.5778E+07 - .6 * HVAP) 1130 REM EQUATION 4.131 1140 TNEW(I) = T(I) + DT * (A + B + C + D) / DENOM 1160 REM PRINT" HS";HS;" HVAP:";HVAP;" HW:";HW;" RH:";RH;" H(I):";H(I) 1170 REM CALCULATE BOUNDARY CONDITIONS AT EXIT OF THE BED 1180 TNEW(NX) = TNEW(NX - 1)



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APPENDIX IV (continued) A Listing of a Numerical Procedure to Solve the Coupled Heat and Mass Transfer Equations that Govern Heat and Mass Transfer in Beds of Ventilated Grains 1190 H(NX) = H(NX - 1) 1200 WNEW(NX) = WNEW(NX - 1) 1210 NEXT I 1220 FOR I = 2 TO NX 1230 T(I) = TNEW(I) 1240 W(I) = WNEW(I) 1250 NEXT I 1270 NEXT KK 1280 PRINT "TIME:"; TIME / 3600; " HOURS" 1290 FOR I = 1 TO NX 1300 M = W(I) / (1 + W(I)) 1310 PRINT I; T(I); W(I); H(I); M 1320 NEXT I 1340 NEXT KKK 1350 END 1355 REM*************************************************** 1360 REM TO CALCULATE SATURATION VAPOUR PRESSURE OF WATER 1370 PS = (6E+25 / ((T(I) + 273) ^ 5)) * EXP(-6800 / (T(I) + 273)) 1380 REM CALCULATE RELATIVE HUMIDITY OF INTERSTITIAL AIR 1390 DPSDT = PS / (T(I) + 273.15) * (-5 + 6800 / (T(I) + 273.15)) 1400 RH = EXP(-ACP / (T(I) + CCP) * EXP(-BCP * W(I))) 1410 DRDT = ACP * RH / (T(I) + CCP) ^ 2 * EXP(-BCP * W(I)) 1420 hsbyhv = 1 + (PS / RH) * DRDT / DPSDT 1460 HVAP = HV + DHVDT * T(I) 1465 REM CALCULATE DIFFERENTIAL HEAT OF SORPTION 1470 hs = HV * hsbyhv 1490 P = PS * RH 1500 REM CALCULATE INTERSTITIAL HUMIDITY OF AIR. 1510 H(I) = (P * N) / (PATM - P) 1511 MCDCP = W(I) / (1 - W(I)) 1513 HW = ACP * PS / BCP / DPSDT / (T(I) + CCP) ^ 2 * (EXP(-BCP * MCDCP) - 1) 1520 RETURN 1532 REM 1535 REM************************************ 1530 REM TO CALCULATE GRAIN MOISTURE CONTENT 1540 REM AT THE INLET OF THE BED. 1550 PATM = 101325! 1560 N = .622 1570 P = PATM * H(1) / (N + H(1)) 1580 PS = (6E+25) / ((T(1) + 273) ^ 5) * EXP(-6800 / (T(1) + 273)) 1590 RH = P / PS 1600 W(1) = -1 / BCP * LOG(LOG(RH ^ (-(T(1) + CCP) / ACP))) 1870 RETURN 1875 REM CALCULATE RATE AT WHICH DRY MATTER, DDMDT, IS LOST. 1876 REM THIS USES EQUATIONS 3.138 TO 3.145 1880 MCWB = W(I) * 100 / (1 + W(I)) 1890 MCDB = W(I) * 100! 1900 AR = 32.3 * EXP(-.1044 * T(I) - 1.856) 1910 IF (T(I) 19) AND (MCWB 15) AND (MCWB > 28) THEN TMOD = AR + 9.000001E-02 * EXP(.0183 * T(I) - .2847) 1940 WMOD = 1!


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APPENDIX IV (continued) A Listing of a Numerical Procedure to Solve the Coupled Heat and Mass Transfer Equations that Govern Heat and Mass Transfer in Beds of Ventilated Grains 1950 IF (MCWB > 13) AND (MCWB 0.715 For sudden expansion in duct diameter, the friction loss is calculated as:

∆P = ρ

v12 2

 D12  1 − 2   D2 

2

(5.12)

where: subscript 1 refers to locations upstream, and subscript 2 refers to locations downstream of the contraction or the expansion.

5.4 AIR DISTRIBUTION THROUGH GRAIN BULKS 5.4.1

Pressure Patterns

The performance efficiency of an aeration system depends primarily on the uniformity of the airflow distribution in different regions of the grain bed. If the direction of airflow is assumed to

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be perpendicular to the lines of constant static pressure, then the pressure pattern due to airflow in the grain bed must be known. In other words, knowledge of pressure patterns in grain beds due to airflow is essential for the efficient design of forced ventilation systems. 5.4.2

Experimental Studies

The best way to determine the pressure pattern is to conduct experiments under field situations to measure static pressures at various locations in a large grain bed. Field testing, however, is expensive. Several experimental studies have been conducted to determine the airflow distribution in large grain bulks. Collins (1953), Hukill (1954), and Hukill and Shedd (1955) studied the air distribution and drying patterns of oats in a Quonset 16 grain dryer (Great Lakes Steel Corp., U.S.), a conicalshaped, galvanized steel structure with a semicircular on-floor duct running along the length of the structure. Brooker (1958) and Williamson (1965) studied the airflow distribution through wheat in rectangular storage containers when the air was introduced through different on-floor duct systems (semicircular, rectangular, and inverted v shapes). Boyce and Davies (1965) studied the effect of air escape areas on the air distribution through 1.22 m deep bulk barley. Later, Barrowman and Boyce (1966) extended the work of Boyce and Davies (1965) to evaluate the effect of duct spacing, air escape areas, and bed depths of barley on the air distribution. Alagusundaram et al. (1994) determined the pressure patterns in flat-bottom bins equipped with three different partially perforated floors (straight, square, and cross) and filled with wheat, barley, or canola. 5.4.3

Mathematical Predictions

5.4.3.1 Need for Mathematical Models The resistance of a product to airflow is affected by several factors such as the moisture content of the product, the shape and size of grain kernels, the configuration of voids, the amount, size, and distribution of foreign material, the method of filling the bin, grain depth, compaction from overburden (depth and vibration), and the direction of airflow. To consider the effect of all these factors while conducting experiments is practically impossible. Furthermore, the experimental data are empirical and do not apply to conditions other than those under which the experiments are conducted. On the other hand, mathematical models can predict pressure patterns in a grain bed by considering all the important parameters. Once the model is validated against measured data, simulations can be run for various floor configurations and airflow rates to determine a system capable of providing the most uniform airflow pattern for cooling a particular type of grain efficiently and evenly in a specific shape and size of storage structure. Mathematical models of non-linear airflow through grain have been developed and solved using analytical or numerical methods. The solution to models, using analytical methods, gives exact results once the problem has been formulated in terms of partial differential equations. Spencer (1969) predicted pressures in stored grain by solving a linear Laplace equation using complex analytic functions. However, under most circumstances the analytical methods of solving partial differential equations become difficult due to the complexity of the problem. Numerical methods are useful in obtaining an approximate solution when an analytical solution cannot be obtained. 5.4.3.2 Finite Difference Models The finite difference method (e.g., Incropera and DeWitt, 1990; Smith, 1985) has been widely used to predict pressure patterns in grain beds (Brooker, 1961, 1969; Jindal and Thompson; 1972). In this method, the approximate values of the derivatives of pressure at a point are obtained by Taylor series expansion around the point in terms of the values of pressures at the surrounding


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mesh points. Brooker (1961) developed a finite difference solution for the partial differential equation that describes the pressure field when air velocity is assumed to be proportional to the pressure gradient raised to an exponent. Predicted and measured pressures were in good agreement in a rectangular bin containing wheat. Brooker (1969) extended his work to numerically predict the pressure patterns in a rectangular bin containing maize. Using maize, the predicted pressures did not compare well with measured values. Jindal and Thompson (1972) followed Brooker’s approach to predict two-dimensional pressure patterns and airflow paths in a three-dimensional triangular cross-section-shaped pile of sorghum. Pierce and Thompson (1975) modified the model of Jindal and Thompson (1972) to predict pressure patterns for conical shaped piles of grain. Lai (1980) calculated three-dimensional pressure patterns and velocity distributions in a circular grain bin using a method of lines, which is similar to the finite difference method. 5.4.3.3 Finite Element Models A more powerful numerical method, the finite element method, is advantageous over the finite difference method when the problem is non-linear (Segerlind, 1976; 1984). Marchant (1976), Segerlind (1982), Miketinac et al. (1986), and Miketinac and Sokhansanj (1985) solved the mathematical model for two-dimensional non-linear airflow using the finite element method. Khompis et al. (1984) predicted pressure patterns and velocity distributions in circular grain bins using the finite element method. Smith (1982) applied the two-dimensional model of Marchant (1976) to determine the pressure patterns and velocity distribution for a three-dimensional rectangular grain bulk. Most of the previous studies were done with clean grains. Haque et al. (1981) considered the effect of distribution of fines in the bin in their prediction model. Jayas et al. (1990) developed an axisymmetric model using the finite element method to predict pressure patterns in bins of canola. They considered the effect of foreign material distribution, moisture content, direction of airflow, and filling method on the predicted pressure patterns. Alagusundaram et al. (1989) solved the governing differential equation of flow of air for predicting three-dimensional pressure patterns in a grain bed using the finite element method.

5.5 DESIGN OF AERATION SYSTEMS 5.5.1

Design Criteria

One of the main critera for aeration system design is to select the proper fan for a given bin, set of bins, silos, or other storage units. The size (capacity) of an aeration fan is selected to move a temperature front through the bulk within the design time. The lowest powered fan that can accomplish this objective is usually selected because it normally requires the lowest capital and electrical energy costs. A larger capacity fan may be necessary if the number of available hours of cool ambient air is a limiting factor or if several bins are to be aerated with one fan. 5.5.2

Fan Characteristics

Fan performance is measured by several variables: the rate of airflow supplied against a specified airflow resistance, rate of decrease in airflow as airflow resistance increases, maximum static pressure developed by the fan, energy efficiency of the fan, energy efficiency of the fan motor, temperature rise of the air as it passes through the fan, noise level from fan and motor, and projected working life of the fan and motor. The performance of fans is graphically represented by plotting airflow rate on the ordinate and static pressure on the abscissa. Fan performance curves for various fans sold for commercial elevator


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Figure 5.14 Measured performances of three 2.2 kW, 450 mm diameter axial flow fans superimposed with the calculated system curve (dashed line) for ventilating 105 m3 of wheat, 3 m deep in a bin with 35 m2 floor area. (From Metzger, J.F., P.D. Terry, and W.E. Muir. [1981]. Performance of several axialflow fans for grain bin ventilation, Can. Agric. Eng., 23, 11–16.)

or farm storage use can be obtained from the fan suppliers or can be measured in the laboratory using standard procedures (e.g., Metzger et al., 1981). Some curves supplied by manufacturers tend to indicate higher airflow rates than independently measured fan performance curves at a given static pressure (Metzger et al., 1981). A few fan manufacturers use certified laboratories for fan testing (Sukup, 1994). Therefore, if possible, measured fan performance curves should be used for designing grain ventilation systems. The performance of similarly sized fans from different manufacturers can vary widely (Figure 5.14). For example, against a resistance of 600 Pa, fan A provides a measured flow rate of 800 L/s; fan B provides 1300 L/s; and fan C provides 2600 L/s, which at this airflow resistance is more than three times higher than the airflow rate of fan A (2600/800 = 3.25). Total resistance to airflow increases as the rate of airflow through grain increases. The graph of this relationship between airflow rate and airflow resistance for a specific bin is called the system curve. The system curve for the ventilation of 105 m3 of wheat at a depth of 3 m in a bin having a floor area of 35 m2 is superimposed onto the performance curves for the three 2.2 kW fans (Figure 5.14). The intersection of the system curve and the fan performance curve indicates how the fan will perform on the specified bin. Fan A provides an airflow of 1300 L/s, fan B provides 1400, and fan C provides 1800 L/s. Thus, on this particular bin, fan C supplies about 40% more air than fan A (1800/1300 = 1.38). For a specific bin of grain, the differences among the three fans are not as great as indicated by comparing airflows at the same airflow resistance due to static pressure increasing with increased airflow. The differences between these three fans, however, are large enough to result in significantly different aeration rates for the bin of wheat. Measured fan performances of different types of fans from one manufacturer are given in Figure 5.15 (Metzger et al., 1981). The performance of axial fans depends on their design and



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Figure 5.15 Comparison of the measured performances of different types of fans from one manufacturer. (From Metzger, J.F., P.D. Terry, and W.E. Muir. [1981]. Performance of several axial-flow fans for grain bin ventilation, Can. Agric. Eng., 23, 11–16.)

construction. Tube-axial fans may be the best selection for high airflows operating against relatively low airflow resistance or static pressure (Figure 5.15). They usually have the lowest cost but also tend to have the lowest efficiency and produce the loudest noise. Axial fans are easily connected to circular ducts. Vane-axial fans have vanes attached to the inside of the fan tube on the downstream side of the fan to straighten the airflow as it leaves the fan blades. The vanes reduce the turbulence in the air and convert velocity energy into static pressure. Because of this conversion, vane-axial fans maintain higher airflows at higher static pressures than tube-axial fans of the same size and power (Figure 5.15). Centrifugal fans usually maintain moderate to high airflows against high static pressures (Figure 5.15). Compared with axial fans, centrifugal fans typically deliver lower airflow at low static pressures. However, performance of axial fans drops off rapidly at high static pressures. The airflow delivery for centrifugal fans may be 50 to 75% as much at medium to high static pressures as at low pressures (Figure 5.15). If a fan has to handle many different grain types or different depths of grain in a bin or silo, then a centrifugal fan is often the best selection. 5.5.2.1 Fan Sound Levels Centrifugal fans tend to be quieter and more efficient than axial fans, but they are usually more expensive at the same performance level. Sound levels produced by fans are a function of tip speed or the peripheral velocity of the wheel. While slow-speed (1100 to 1800 rpm) direct-drive fans are relatively quiet, high-speed (3000 to 3600 rpm) fans are very loud, especially axial blowers with diameters of 600 to 750 mm. Anyone working near such fans for an extended period should wear ear protectors. Larger centrifugal fans use belt drives that reduce the wheel blade speed. They are, therefore, not as loud as axial blowers even at much higher power inputs. Around a grain elevator with many tall curved structures, sound waves can bounce off structural sidewalls and may be redirected or focused toward sensitive areas, such as a residential housing area. Fan noise control in aeration systems is discussed in Section 7.4.4. 5.5.2.2 Fan Efficiency Because the efficiencies of centrifugal fans are usually higher and their direct drive motors are usually not mounted in the air stream, the temperature rise of the air passing through the fan is


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Figure 5.16 Total energy efficiencies of the three 2.2 kW fans of Figure 5.14. (From Metzger, J.F., P.D. Terry, and W.E. Muir. [1981]. Performance of several axial-flow fans for grain bin ventilation, Can. Agric. Eng., 23, 11–16.)

less than for axial fans at the same static pressure level. But for centrifugal fans operated at high static pressures, the heat of compression is higher than for axial fans operated at low static pressures, even with the higher fan efficiencies of centrifugal fans. Total efficiency of the fan and motor combination is the amount of power in the air leaving the fan divided by the amount of electrical power entering the electric motor connected to the fan, multiplied by 100%. Efficiencies increase from 0% at zero airflow to a peak of about 70% at maximum airflow and then decrease at higher static pressures with reduced airflow rates. Metzger et al. (1981) measured the total efficiencies of 11 small fans. The peak efficiencies for each of the 11 fans varied from 50% for a 1.1 kW fan to 25% for a 0.25 kW fan. Peak efficiencies of the three fans in Figure 5.14 were 33% for fan B, 42% for fan A, and 45% for fan C (Figure 5.16). When operating at the conditions of the bin of wheat of Figure 5.14, fans A and B would have efficiencies of 28%; and fan C would have an efficiency of 40%. 5.5.2.3 Fan Power Requirement and Temperature Rise Peak power demands measured by Metzger et al. (1981) were greater than the rated power. The average ratio of maximum operating power to rated power ranged from 2.0 for 300 mm diameter fans to 1.2 for 600 mm diameter fans. The design and selection of the electric components supplying power to such fans must allow for the higher power. For axial fans the electrical power that is not used to move air (electric resistance heat in the motor’s wiring, friction in motor and fan bearings, and air turbulence) is given off as heat that warms the air as it passes by the direct drive motor mounted in the air stream and through the fan. The amount of heat given off by the fan and motor increases as efficiency decreases. The temperature rise, however, also depends on the airflow rate, decreasing as airflow rate increases. Temperature rises for vane-axial fans can vary from about 0.5°C to as high as 5°C. For centrifugal fans, the temperature rise will be slightly lower at the same static pressure. At high static pressures, the temperature rise can be up to10°C. When high-pressure fans exceed their high-pressure operating limits and stall, with very little air movement, temperatures as high as 18°C have been observed. For some axial fans that have the motor mounted outside the air stream, the temperature rise will be less because the motor heat is lost to the surroundings. In-line centrifugal fans have their motors installed in the fan housing; consequently, temperature rises include motor heat like axial fans.



5.5.3

221

Design of Aeration Ducts

5.5.3.1 Air Velocity in Supply and Aeration Ducts The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) recommends that air velocities in ducts should be equal to or less than 7.5 m/s (7500 (L/s)/m2) to minimize friction losses and noise (Agric. Can., 1990; Holman, 1960). Agric. Can. (1990) recommends that air velocities in ducts for grain ventilation should follow this recommendation. Other factors such as storage type may also affect the design of grain aeration ducts. If the ducts are short, friction losses may be small or negligible compared with the pressure losses in the grain. 5.5.3.2 Design of Aeration Duct Systems The equation for airflow along the duct has to be solved together with the equation for flow through the grain. To solve the equations they must be simplified, and therefore the results are only approximately correct. Probably the most accurate method for aeration duct design is the finite element method that was used by Marchant and Nellist (1977) and Burrell et al. (1982). Simulated results are sensitive to the coefficient for pressure regain in the duct as well as the grain resistance (Burrell et al., 1982). When air is sucked down through shallow grain bulks into an on-floor duct and the air velocities at the fan end are high, the static pressure in the duct close to the fan can be up to 20 times that at the distant end (Burrell et al., 1982). Consequently, large differences in airflow through the grain along the duct can occur. Airflow distribution through the grain bulk is much better if the air is blown into the duct, although a regain of static pressure occurs at the distant end of the duct of up to 4 times that at the fan end (Burrell et al., 1982; Fick et al., 1990). Distribution is further improved by reducing the diameter of the duct in stages along its length (similar to reducing cross-section areas of air-conditioning ducts when air passes into lateral ducts, and air volume is thus reduced in the main duct); by increasing the grain depth; by decreasing the fan end velocity; by increasing the friction loss along the duct; and by inserting constrictions in the duct (Burrell et al., 1982). To deliver the same mass of air per unit of time, the required fan pressure is much larger if the air is sucked through rather than blown into the ducts. Many of the recommendations on the design and operation of ducts for grain aeration systems are empirical rules for duct spacing and air velocities in the ducts (Holman, 1960; Burrell, 1974; Foster and Tuite, 1982; and Hellevang, 1997). There are several ways to lay out an aeration duct system. Often more than one duct is required. The aim is to keep the air paths through the grain as nearly equal in length as possible. If a path is significantly shorter than the others, an excessive amount of the air will flow in that direction. The longest path should be less than 1.5 times the length of the shortest path (Holman, 1960), though larger variations in path lengths may be used with satisfactory results in small stores of dry grain (Burrell, 1974). Multiple ducts should not be farther apart than the depth of the grain bed, and the distance from the wall to the nearest duct should not exceed half of the grain depth. Resistance to airflow in grain is less in the horizontal direction than in the vertical direction. The air that flows horizontally out of the ducts provides a more uniform airflow distribution than could be expected if air resistance were the same in both directions. 5.5.4

Design of Long-Perforated Ducts and Manifolds

Long-perforated ducts and branching manifolds can be designed using the static pressure regain method. If a constant depth of grain covers a long-perforated duct, then to obtain the same airflow


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through all the grain, the static pressure along the ducts should be kept constant. If the duct diameter is constant, as air flows out of the duct into the grain, the velocity of the air in the duct decreases. As air velocity (or velocity pressure) decreases, the velocity energy is partially converted to static pressure. Therefore, to maintain the same static pressure throughout the duct, the duct diameter must be reduced to increase the velocity. If there were no friction losses in the pipe and in the conversion from velocity energy to static pressure energy, then the duct diameter would be designed to maintain a constant air velocity. In aeration systems, however, energy is lost in the conversion from velocity to pressure energy and there are friction losses along the duct. The system is therefore designed so that the pressure regain is equal to the friction losses along the duct. Although a gradual reduction in duct cross-sectional area should provide the most uniform airflow, step decreases are usually more practical. The static pressure regain can be calculated by:

(

∆P = ( R 2) v12 − v22

)

(5.13)

where: ∆P = regain in static pressure, Pa R = regain coefficient v1 = initial velocity, m/s v2 = reduced velocity, m/s The coefficient R can range between 0.9 for smooth ducts to 0.5 or less in rough, poorly constructed systems. In aeration systems, however, approximately 75% of the velocity energy is converted to static pressure energy, and an R of 0.75 may be used (Brooker et al., 1974). 5.5.5

Computerized Aeration System Models

Computerized models can be classified into two groups: 1. The heat and mass transfer models that simulate the aeration process for linear or nonlinear airflow and can be used for system evaluation and optimization 2. The models that can be used in designing physical systems — for example, selecting a fan for a bin filled with a certain grain

Many different models of the first type have been reported in the literature, and their usefulness has been demonstrated (e.g., Sharp, 1982; Morey et al., 1979; and Metzger and Muir, 1983). Such models have been used to assess the uniformity of cooling and to compare different control strategies. The models of the second type are scarce in the literature. This is probably because the fan characteristics data are localized in nature and can change frequently as new fans become available on the market.

5.6 PLANNING AN AERATION SYSTEM Quantities of different types of grains to be stored, geographic location and associated weather conditions, segregation of grain types and grades, drying method (if used), power supply limitations, physical layout of facility, interface of grain storage with other business options, initial costs of the bins or silos, grain handling system requirements, and speed of grain movement are the major factors to be considered when selecting bins or silos. Because sanitation is such an important component of integrated pest management, bins or silos should be designed for ease of internal and external cleaning.



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Large bins that require vertical structural wall stiffeners should have the stiffeners mounted on the exterior of the bin wall, where they will collect less debris and will be accessible for cleaning. Internal stiffeners are very difficult to keep clean and may rust out prematurely because of contact with moist grain. The gaps between the corrugated wall metal and vertical interior stiffeners trap grain, dust, and trash; and they harbor insects. Rusty stiffeners can create both a serious sanitation problem and a major structural defect. Aeration systems should be installed in most if not all bins or silos. To allow for as much flexibility as economically possible, future uses of bins, such as for Dryeration and near-ambient drying, should always be considered during the planning stage of grain storage facilities.

5.7 PRACTICAL AERATION SYSTEM DESIGNS AND PROCEDURES In this section, procedures for designing aeration systems are presented. In the preparation of this section, the handbook prepared by Hellevang et al. (1997) for Midwest Plan Service was consulted, and selected materials were integrated with the information developed in the previous sections of this chapter and with materials selected from other publications (e.g., Navarro and Calderon, 1982). The selected airflow rate and direction of airflow plays an important role in the design of an aeration system (Chapters 6 and 7). The selected airflow rates given in this section are examples of typical designs. In the preparation of this section, medium (250 to 1000 tonnes) to large (3000 to 8000 tonnes and higher capacity) aerated bins were considered. Because rapid temperature changes occur in small bins, less emphasis was placed on their design. The background information in this chapter should be sufficient to design aeration systems for all sizes and types of bulks (e.g., flat storages, steel bins, and concrete silos). 5.7.1

Aeration System Design Recommendations

5.7.1.1 Airflow Rates In temperate climates, airflow rates of 3 to 6 (m3/h)/tonne (0.05 to 0.10 cfm/bu) are typically used; and for regions with limited cooling time, airflow rates of 12 to 15 (m3/h)/tonne (0.20 to 0.25 cfm/bu) are typically used. For upright storages, airflow rates of 3 to 6 (m3/h)/tonne (0.05 to 0.10 cfm/bu) are typically used; and for horizontal storages, airflow rates of 6 to 12 (m3/h)/tonne (0.10 to 0.20 cfm/bu) are typically used. Higher airflow rates than these will cool grain faster but often are not economical. Because airflow and power requirements for grain depths exceeding 30 m (100 ft) become excessive, reduced airflow rates of 2 to 3 (m3/h)/tonne (0.03 to 0.05 cfm/bu) should be considered. Doubling the airflow rate triples the required static pressure, while fan power is increased by over four times. For wheat and other small grains, airflow rates of 3 to 6 (m3/h)/tonne (0.05 to 0.10 cfm/bu) are used; and for corn and soybeans, airflow rates of 6 to 12 (m3/h)/tonne (0.10 to 0.20 cfm/bu) are common for medium (15 to 30 m) grain depths. 5.7.1.2 Air Velocities in Ducts To minimize friction loss in a duct, a compromise between the duct diameter and air velocity has to be made. In an aeration duct, maximum velocity should be at or below 600 m/min (2000 fpm). For transition and supply ducts up to 6 m (20 ft) long, velocity could be 750 m/min (2500 fpm). Transition duct taper should be 20° or less. Right-angle elbows should be assembled from two pieces of 45° elbows with a short, straight section between them (Figure 5.6) instead of one 90°


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elbow. Their radius of curvature should be at least 1.5 times and preferably 2.0 times the duct diameter. 5.7.1.3 Air Distribution Systems The ratio of the length of the longest airflow path to the length of the shortest airflow path should be 1.5:1. Positive pressure systems have a more uniform airflow distribution and are preferred over negative pressure systems in horizontal storages. The exit velocity from the perforations should not exceed 9 m/min (30 fpm). 5.7.1.4 Intakes and Exhaust In general, roof vents should be equally spaced around the circumference of the roof at about ⅓ to ½ the distance up the slope from the lower edge. One or more vents should be located near the peak to minimize moist air condensation in downspouts used for filling the storage. The vent cross-section area should be sized preferably for an air velocity of 300 m/min (1000 ft/min) with a maximum velocity of 450 m/min (1500 ft/min). The pressure difference between the head-space of a storage bin or silo and outside should not exceed 30 Pa (⅛ of an inch water column) during either pressure or suction aeration. Higher pressure differences may cause structural damage and are indicative of an inadequate exhaust area. Louvered exhaust fans (mounted in the end wall near the roof peak) — capable of moving six to eight head-space (with the structure filled) air changes per hour — should be used in horizontal storages. Screened inlet louvers should be mounted at the opposite end of the flat storage to provide adequate inlet air entry. Design velocity for the louvered area should be the same as for roof vents (i.e., 300 to 450 m/min). End wall exhaust fans help minimize buildup of humid air above the grain mass by providing movement of ambient air through the head-space at selected times. This humid air may cause roof condensation and dripping of moisture from roof trusses onto the grain surface. The exhaust fans should generally operate during the night, when roof temperatures drop below the head-space air dew point. A 24-hour timer can be set to operate exhaust fans from late evening until sunrise. Nighttime operation will help keep surface grain relatively cool by removing the daily buildup of head-space heat as well as humid air from under the roof. Although end wall fans supplement roof vents, roof vents should be sized to vent the aeration airflow. Example If a 30 m (100 ft) by 100 m (330 ft) flat storage had a head-space air volume of 6000 m3 (211,888 ft3), one or more end wall exhaust fans that can deliver 36,000 to 48,000 m3/h, or 600 to 800 m3/min (21,000 to 28,000 ft3/min) should be used. An inlet louver area of 2.0 to 3.0 m2 (22 to 32 ft2) should be used. 5.7.2

Aeration System Design Procedures and Examples

The following procedure is recommended for aeration system design. 5.7.2.1 Calculate the Amount of Grain to be Aerated Vertical (upright) storage is defined as any structure where grain depth is greater than the structure diameter or width; otherwise, the structure is known as horizontal or flat storage. In calculating the amount of grain, do not ignore sloped floors, grain surfaces, and walls.



225

In most parts of the world, the grain mass is often a more readily available unit of measure than the volume of the bulk because when filling a storage, the mass of each load is recorded. The capacity of the storage in these regions is referred to on a mass basis rather than on a volume basis. At full capacity, the same storage can hold different weights of different commodities because their bulk densities differ. This can create confusion. In the U.S. storage system, bushel is used to define the amount of the commodity in storage. Although the bushel is a volume unit, it is calculated by measuring mass and dividing it by a standard bulk density — which differs for each commodity. In this system, it is difficult to estimate the actual grain mass in storage because the depth of grain, drop height, and length of the storage time affect bulk density. The U.S. grain elevators use a compaction factor based on stored grain depth to account for consolidation in storages. The storage volume of a cylindrical bin is calculated by using the following equation: 2

D Volume ft 3 or m3 = π ×   × h  2

(

)

(5.14)

where D = diameter, and h = height. In the U.S. storage system, 1 ft 3 = 0.80 bu For rectangular stores (warehouses) in U.S. storage systems, the total storage volume (TSV) is calculated as:

( )

Total Storage Volume (TSV) ft 3 = ( ft length × ft width × grain depth in ft )

(5.15)

To obtain capacity in bu: Capacity in bu = TSV × (0.8 bu/ft3) 5.7.2.2 Select the Airflow Rate and Determine the Air Volume Airflow rates are discussed in detail in Chapters 7 and 8; but as a general rule, apply information given above in Section 5.7.1, Aeration System Design Recommendations. 5.7.2.3 Estimate Static Pressure Requirements To select the proper aeration fan for the system to be operated at a certain airflow rate [(m3/h)/tonne], knowledge of static pressure requirements is essential (refer to Section 5.3). Figures 5.17 to 5.21 provide static pressure (Pa) and power requirements (kW/100 tonne) vs. depth (m) for wheat, maize (shelled corn), sorghum, soybeans, and cottonseed (delinted), respectively. The airflow rates given in Figures 5.17 to 5.21 are based on the bulk densities mentioned in these figures. The same data, static pressure (inches of water), and power requirements (hp/1000 bu) vs. depth (ft) are given in Figures 5.22 to 5.26 for the U.S. system. For wheat, maize, sorghum, and soybeans, relationships are based on data from Shedd (1953) for loose grain modified for packed fill. Basic values for loose grain were increased as follows: wheat 30%, maize 34%, and sorghum and soybeans 41%. For delinted cottonseed, relationships are from Smith (1975) based on field observations and laboratory studies.


226


Figure 5.17 Static pressure developed at different airflow rates and fan power requirements for aerating wheat (bulk density 0.830 tonnes/m3, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

To compensate for pressure losses inside the aeration duct, static pressure values were further increased based on recommended maximum velocities of 600 m/min (2000 ft/min) for vertical storages, and on lower velocities as shown in Table 5.4 (Table 5.5 in the U.S. system) for horizontal storages. Basic pressures were increased also for supply ducts for a maximum velocity of 750 m/min (2500 ft/min) having lengths of 3 m (10 ft). For longer supply ducts with elbows and increased velocities, Figure 5.27 and Table 5.6 (Table 5.7 in the U.S. system) will serve as guides for calculating additional pressure. Calculations based on the method proposed by Henderson (1958) show that curves given in Figures 5.17 to 5.26 are applicable for vertical storages when the maximum velocity in the grain near the duct surface is 9 m/min (30 ft/min). For horizontal storages where the longest air path is close to l.5 times the shortest path, the maximum velocity in the grain near the duct surface should be 6 m/min (20 ft/min). Figures 5.17 to 5.26 provide only estimates of static pressure and fan power for selected depths. For many storage conditions — such as grain and seeds stored over long periods, stored near railroads where vibrations may cause consolidation, for grain with high levels of fine material, or excessive lint in cottonseed — static pressure and power requirements would exceed the values



227

Figure 5.18 Static pressure developed at different airflow rates and fan power requirements for aerating maize (shelled corn) (bulk density 0.764 tonnes/m3, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

given in Figures 5.17 to 5.26. In these cases, additional resistance factors need to be estimated and applied (refer to Section 5.3). The static pressure values given in Figures 5.17 to 5.26, calculated based on the above assumptions, compared favorably with observations under field conditions. These graphs simplify computations and minimize the possibility of calculation errors. For design purposes, any inaccuracies caused by the above assumptions are negligible. Static pressures for other seeds or grains that are similar in size, shape, density, and surface texture to one of these five seeds can also be approximated from respective charts (Figures 5.17 to 5.26). Pea beans are very close to soybeans. Barley, oats, popcorn, and rough rice static pressures are close to each other and fall approximately halfway between wheat and corn. Where these assumptions are not applicable, more detailed calculations can be made using Section 5.3. Example Estimate the static pressure that will develop in an aeration system: 1000 tonnes of wheat, grain depth of 10 m, airflow rate of 6 (m3/h)/tonne, supply duct of 300 mm diameter, 20 m long with one 90° (3-piece) elbow (with a centerline radius of 1.5 times the duct diameter) (Figure 5.6).


228


Figure 5.19 Static pressure developed at different airflow rates and fan power requirements for aerating grain sorghum (bulk density 0.800 tonnes/m3, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

The estimated static pressure (Pa) to move the specified volume of air will be the sum of: Basic aeration system (Figure 5.17) Supply duct (Figure 5.27) Elbow (Table 5.6) Total Pressure

814 392 118 1324

5.7.2.4 Estimate Fan Power Requirements Once the static pressure and the air volume required to aerate the grain bulk are determined, the fan power (kW or hp) required can be estimated using the methods described in this section. Since the minimum theoretical power (W) required to move any amount of air (m3/s) against any resistance, (Pa) can be determined as:

(

)

Air power (W ) = Air volume m 3 s × pressure ( Pa )

(5.16)



229

Figure 5.20 Static pressure developed at different airflow rates and fan power requirements for aerating soybeans (bulk density 0.800 tonnes/m3, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

Using Equation 5.16 and the definition of fan efficiency (Section 5.5.2.2), actual fan power is: Fan power (W ) =

(

)

Air volume m 3 s × pressure ( Pa ) Fan static efficiency

(5.17)

where Fan static efficiency is given in decimal. Basic aeration system static pressure and fan power requirements can be determined from graphs given in Figures 5.17 to 5.26. A static efficiency of 50% was assumed in the calculation of fan power. However, these values should be adjusted when the operating static efficiencies of fans are significantly different than 50%. In such cases, the fan powers given in Figures 5.17 to 5.26 should be adjusted according to Equation 5.18 where static efficiency is given in decimal: Adjusted kW 100 tonnes =

kW 100 tonnes ( Figures 5.17 to 5.21) × 0.50 (Static efficiency) Static efficiency of selected fan

(5.18)


230


Figure 5.21 Static pressure developed at different airflow rates and fan power requirements for aerating cottonseed (delinted) (bulk density 0.471 tonnes/m3, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

For example, if the aeration system requires 0.28 kW per 100 tonnes of wheat (Figure 5.17) and the fan has a static efficiency of 60%, the adjusted power requirement of the selected fan is computed as: Adjusted kW 100 tonnes =

0.28 × 0.50 = 0.23 kW 100 tonnes 0.60

For the given example of aerating 1000 tonnes of wheat, the estimated fan power is 2.3 kW. Where long supply ducts with high air velocities result in pressure losses greater than the allowances incorporated in Figures 5.17 to 5.26, the additional pressure loss should be added to the values read from Figures 5.17 to 5.26. Example Using the example given in the previous Section 5.3, for the basic aeration system at a static pressure of 814 Pa at 6000 m3/h ≈ 1.66 m3/s and for a fan having a static efficiency of 60%, fan power is:



231

Figure 5.22 Static pressure developed at different airflow rates and fan power requirements for aerating wheat (bulk density 60 lb/bu, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

Fan power (W ) =

(

)

1.66 m 3 s × 814 ( Pa ) 0.60

= 2, 252 W ≅ 2.3 kW

When the static pressure is increased due to friction loss in the supply duct and the elbow as in the previous example to 1324 Pa, the fan power is: Fan power (W ) =

(

)

1.66 m 3 s × 1, 324 ( Pa ) 0.60

= 3, 663 W ≅ 3.7 kW

5.7.2.5 Choose the Aeration Fan The aeration fan is an important component of the system. Details related to fan characteristics were given in Section 5.5.2. If static pressure and airflow rate are known, an appropriate fan can


232


Figure 5.23 Static pressure developed at different airflow rates and fan power requirements for aerating maize (shelled corn) (bulk density 56 lb/bu, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

be selected using fan performance data for available fans. Typical fan performance data are given in Tables 5.8 and 5.9, which can be used for selecting the size of fan. 5.7.2.6 Choose the Aeration System Type Basic considerations in choosing the air distribution system are: 1. Should permit periodic cleaning of the enclosed duct or underfloor space to remove foreign material, grain, and insect or rodent debris 2. Should have sufficiently large duct area to properly carry the design airflow without exceeding the maximum design air velocities 3. Should be located in areas that create the least obstacle for machinery movement in and around the grain storage facility

The reader should check Section 5.1 for more details on designing storage facilities and aeration systems.



233

Figure 5.24 Static pressure developed at different airflow rates and fan power requirements for aerating grain sorghum (bulk density 56 lb/bu, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

5.7.2.7 Decide How Many Ducts are Required and Where to Locate Them Duct location and spacing is an important step in designing an aeration system. The layout of the duct system should facilitate the ease of operation, especially in flat storages where the floor is used as a working area for grain transfer equipment during unloading grain from the storage. Above-floor ducts may require dismantling when unloading equipment is operated. Determine the air distribution duct system that meets the required 1.5:1 airflow path ratio. The distance from the wall to the beginning of the perforated section of the ducts is determined by centering the perforated portion of the duct in the bin. The number of duct locations needed and the spaces between ducts are calculated using the following equations: Number of ducts =

Building width or bin diameter ( m) Grain depth ( m)

(5.19)


234


Figure 5.25 Static pressure developed at different airflow rates and fan power requirements for aerating soybeans (bulk density 60 lb/bu, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

Duct spacing ( m) = Grain depth ( m) =

Building width or bin diameter ( m) Number of ducts

Distance from sidewall to duct wall ( m) =

Duct spacing ( m) 2

(5.20)

(5.21)

Example Design the duct spacing layout pattern for a 30 m wide by 70 m long flat storage with a sidewall grain depth of 8 m; the surface of the grain bulk is leveled; the estimated size of half-round ducts is width of 1.0 m and height of 0.5 m. From Equation 5.19: Number of ducts = 30/8 = 3.75 ≈ 4 ducts



235

Figure 5.26 Static pressure developed at different airflow rates and fan power requirements for aerating cottonseed (delinted) (bulk density 32/lb/bu, a fan static efficiency of 50% was assumed in the calculation of fan power). (Adapted from Navarro, S. and Calderon, M. [1982]. Aeration of Grain in Subtropical Climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

Table 5.4 Recommended Maximum Allowable Air Velocities (m/min) within Aeration Ducts of Up to 15 m in Length to be Used in Suction Systems Airflow Rate [(m3/h)/tonne] Grain depth (m)

4 6 8 10 15

Corn and Soybeans 3 6 12 15 230 260 270 290 360

230 270 310 350 460

270 320 380 450 600

280 340 420 500 600

Wheat and Sorghum 3 6 12 15 350 400 450 500 600

390 460 540 600 600

440 560 600 600 600

470 600 600 600 600

From Equation 5.20: Duct spacing = 30/4 = 7.5 m From Equation 5.21: Distance from sidewall to duct centerline = 7.5/2 = 3.75 m


236


100

50

D ct

30

Du

00 30 00 28 0 0 26 0 0 24 0 0 22 00 20

i am

00

17

m)

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m r, (

15

ete

00 15 0 0 14 0 0 13 0 0 12 0 0 11 00 10 0 90

20

0

80

5

0

70

4

0

60

8

.4

7 6

.3

5

.2

22

14

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12

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.5

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20 18

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0

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1.0

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0 50 0 45

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4

AIRFLOW (m3 / sec)

3

0

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3 0

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2

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0

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1.5

90 80

.05

70

.04

60

.03

50

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.03 .04 .05

.07

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.15 .2 .25 .3 .4 .5 .6 .7 .8.9 1

1.5

2 2.5 3

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5 6 7 8 910

FRICTION LOSS mm W.C. PER m DUCT LENGTH Figure 5.27 Friction loss due to airflow in round ducts (based on standard air of 1.2 kg/m3 density flowing through round galvanized metal ducts having approximately one joint each 80 cm duct length). (Adapted from Navarro, S. and Calderon, M. [1982] Aeration of Grain in Subtropical Climates, FAO, Agricultural Services Bulletin No. 52. With permission.)

Lengthwise ducts are used, with the perforated metal starting about 4 m (13.1 ft) from the end wall and the centerline of the first duct placed 3.75 m (12.3 ft) from the building sidewall. Then ducts 2, 3, and 4 are spaced 7.5 m (24.6 ft) apart, from centerline across the building width, leaving 3.75 m from the centerline of duct 4 to the far sidewall. Check the air path length ratio for 1.5:1: The longest path from duct 1, 3.75 m –0.5 m (half duct width) + 8 m height = 11.25 m. Shortest distance to the grain surface is 8 m – 0.5 m = 7.5 m. Air path length ratio 11.25/7.5 = 1.5 meets the design guideline.



237

Table 5.5 Recommended Maximum Allowable Air Velocities (ft/min) within Aeration Ducts of Up to 50 ft in Length to be Used in Suction Systems Airflow rate (ft3/min/bu) Grain depth (ft)

0.05

13 20 26 33 49

Corn and Soybean 0.09 0.19 0.23

755 853 886 951 1181

755 886 1017 1148 1509

886 1050 1247 1476 1969

919 1115 1378 1640 1969

Wheat and Sorghum 0.05 0.09 0.19 0.23 1148 1312 1476 1640 1969

1280 1509 1772 1969 1969

1444 1837 1969 1969 1969

1542 1969 1969 1969 1969

Table 5.6 Pressure Loss (Pa) in Accordance with Air Velocity (m/min) and Duct Diameter (mm) in 3-Piece Round Cross-Section Elbows (90°) Where Centerline Radius Is Equal to 1.5 Times the Duct Diameter Airflow (m3/h)

200 m/min Pa

1000 1500 2000 3000 4000 5000 6000 8000 10000 15000 20000 25000 30000

531 796 1061 1592 2122 2653

20 39 59 147 255 392

Duct Diameter (mm) 300 400 m/min Pa m/min Pa

472 707 943 1179 1415 1886 2358

10 29 49 78 118 196 314

398 531 663 796 1061 1326 1989 2653

10 20 29 39 59 98 226 392

500 m/min Pa

424 509 679 849 1273 1698 2122 2546

10 20 29 39 88 167 255 363

Compiled from data from ASHRAE, 1997. Fundamentals Handbook, ASHRAE Inc., New York. Table 5.7 Pressure Loss (Inches of Water) in Accordance with Air Velocity (ft/min) and Duct Diameter (inch) in 3-Piece Round Cross-Section Elbows (90°) Where Centerline Radius Is Equal to 1.5 Times the Duct Diameter Airflow (ft3/min) 589 883 1177 1766 2354 2943 3531 4709 5886 8829 11772 14715 17657

8 ft/min

in w.c.

1742 2612 3481 5223 6962 8704

0.079 0.157 0.236 0.591 1.024 1.575

Duct Diameter (inch) 12 16 ft/min in w.c. ft/min in w.c.

1549 2320 3094 3868 4642 6188 7736

0.039 0.118 0.197 0.315 0.472 0.787 1.260

1306 1742 2175 2612 3481 4350 6526 8704

0.039 0.079 0.118 0.157 0.236 0.394 0.906 1.575

20 ft/min

in w.c.

1391 1670 2228 2785 4177 5571 6962 8353

0.039 0.079 0.118 0.157 0.354 0.669 1.024 1.457

Compiled from data from ASHRAE, 1997. Fundamentals Handbook, ASHRAE Inc., New York.


238


Table 5.8 Example of Fan Performance Data Power (hp)

Diameter Speed (in) (RPM)

1

Air Flow Rates (ft3/min) at Indicated Static Pressure Static Pressure (Inches of Water Column) 2 3 4 5 6 7 8 9

10

Axial 0.50 0.75 1 1.5 1.5 1.5 1.5 3 5 7.5 10

12 12 14 12 14 16 18 18 24 24 26

3,450 3,450 3,450 3,450 3,450 3,450 3,450 3,500 3,500 3,500 3,500

1,500 630 1,700 750 2,880 1,050 2,200 950 2,800 1,300 3,500 2,400 1,300 4,350 3,000 1,400 5,700 4,600 2,650 10,500 9,000 7,000 12,500 11,100 9,450 15,500 14,000 12,250

1,400 4,600 2,900 6,550 3,900 9,500 5,800 3,400

Low-Speed Centrifugal 3 5 7.5 10

— — — —

1,750 1,750 1,750 1,750

4,580 4,320 3,820 3,350 7,800 7,000 6,250 5,550 10,500 9,750 8,950 8,000 13,300 12,400 11,500 10,500

2,550 4,600 3,300 7,400 6,100 9,550 8,500 7,300

High-Speed Centrifugal 3 5 7.5 10

— — — —

3,500 3,500 3,500 3,500

3 5 7.5 10

18 24 28 28

3,500 3,500 3,500 3,500

— — — —

2,950 4,350 5,700 6,800

— — — —

2,550 3,850 5,100 6,300

— — — —

2,120 3,200 4,500 5,750

— — — —

1,650 2,200 3,800 5,100

— — — —

1,000 1,800 2,900 4,450

In-line Centrifugal 3,800 5,500 6,200 7,700

3,600 5,000 6,000 7,300

3,400 4,400 5,700 6,800

3,000 4,100 5,500 6,500

2,500 3,900 5,200 6,300

1,900 3,600 2,800 1,800 4,800 4,500 4,000 3,500 3,000 6,000 5,400 5,100 4,800 4,400

Note: Data presented are a composite from several manufacturers. Consult a comparable table or performance curve for the actual fans compared. From Hellevang et al. (1997). Dry Grain Aeration Systems Design Handbook, 1st ed., Midwest Plan Service, Ames, IA.

5.7.2.8 Determine Aeration Duct Length Duct length is based on the longer of the two criteria: (1) getting air to the ends of the bulk, and (2) providing enough perforated surface area for reasonable exit velocity. For bulks exceeding 30 m (100 ft) length, place a fan on both ends of the duct, split the ducts with a gap between ducts at the center and fans on both ends, or use a manifold at the midpoint of the building. For dual fan systems, such as the first and second options of separating the building length into halves with fans on each end, size each fan and duct for half of the total required airflow. Before calculating the perforated duct length (PDL), it is first necessary to calculate the distance from end wall to the perforated duct (E): E ( ft ) = (0.7) (Distance from sidewall to duct wall, ft )

(5.22)



239

Table 5.9 Example Fan Performance Data in Metric Units Power (kW)

Diameter (cm)

Speed (RPM)

0.25

Air Flow Rates (m3/h) at Indicated Static Pressure Static Pressure (kPa) 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25

2.50

Vane Axial 0.37 0.56 0.75 1.12 1.12 1.12 1.12 2.24 3.73 5.59 7.46

30.5 30.5 35.6 30.5 35.6 40.6 45.7 45.7 61 61 66

3,450 3,450 3,450 3,450 3,450 3,450 3,450 3,500 3,500 3,500 3,500

2549 2888 4893 3738 4757 5947 7391 9684 17840 21238 26335

1070 1274 1784 1614 2209 4078 5097 7815 15291 18859 23786

2209 2379 4502 11893 16056 20813

2379 7815 11128 16141

4927 6626 9854

5777

Low-Speed Centrifugal 2.24 3.73 5.59 7.46

— — — —

1,750 1,750 1,750 1,750

7781 13252 17840 22597

7340 11893 16565 21068

6490 10619 15206 19539

5692 9429 13592 17840

4332 7815 12573 16225

5607 10364 14442

12403

— — — —

3602 5437 7646 9769

— — — —

4248 6626 8835 10704

3228 6116 8155 10194

4757 7646 9175

High-Speed Centrifugal 2.24 3.73 5.59 7.46

— — — —

3,500 3,500 3,500 3,500

— — — —

5012 7391 9684 11553

2.24 3.73 5.59 7.46

45.7 61 71 71

3,500 3,500 3,500 3,500

6456 9345 10534 13082

6116 8495 10194 12403

— — — —

4332 6541 8665 10704

2803 3738 6456 8665

— — — —

1699 3058 4927 7561

In-line Centrifugal 5777 7476 9684 11553

5097 6966 9345 11044

3058 6796 5947 5097 8665 8155 7476

Note: English to metric conversions at 1.699 m3/h = 1.0 ft3/min. Data presented are a composite from several manufacturers. Consult a comparable table or performance curve for the actual fans compared. From Hellevang et al. (1997). Dry Grain Aeration Systems Design Handbook, 1st ed., Midwest Plan Service, Ames, IA.

Maximum distance between the wall and end of the perforated duct, E, is 0.7 times the distance between the outside duct and the sidewall (one half grain depth). The factor 0.7 is used for making distances equal for an angle of repose of 25°, thus helping to preserve the airflow path ratio design. Burrell (1974) and Foster and Tuite (1982, 1992) used a factor of 1.0. An analysis of grain volumes served by the end of the duct indicates that a factor of 0.5 may be appropriate. The factor 0.7 is used by Hellevang et al. (1997) in their design examples because this factor was judged most appropriate. Calculate the perforated duct length (PDL) using the following equation: PDL = L – 2 ( E )

(5.23)


240


where: PDL = Perforated duct length, ft or m L = Building length, ft or m E = Distance from end wall to perforated duct, ft or m 5.7.2.9 Calculate Duct Cross-Section Area The minimum cross-section area (CSA) (m2) can be calculated by dividing airflow rate (Q) (m3/min) by air velocity (V) (m/min) in the duct:

( )

CSA m 2 =

(

Q m3 min

)

V ( m min)

(5.24)

Ducts should be large enough to carry the airflow without exceeding the maximum design air velocity. High air velocities increase the fan power required. Size the CSA of aeration ducts in bins for a maximum air velocity (V) (m/min) of 600 m/min (2000 fpm). Use reduced air velocity in ducts placed in flat storage to improve the airflow uniformity. For further details, use Tables 5.4 and 5.5. Example Design the duct spacing layout pattern for a 30 m wide by 70 m long flat storage with a sidewall grain depth of 8 m. This flat storage building contains 30 m × 70 m × 8 m = 16,800 m3 of grain sorghum. Thus, the storage will hold 16,800 m3 × 0.75 tonne/m3 = 12,600 tonnes of sorghum. Because of the relatively shallow, uneven grain depth and the fact that the site is near Brownsville, TX, 26° north latitude, 98° west longitude, in a coastal subtropic climate, a suction system design airflow of 9 (m3/h)/tonne (0.15 cfm/bu) is selected. The total airflow needed is 12,600 tonnes × 9 (m3/h)/tonne = 113,400 m3/h or 1890 m3/min (67,700 ft3/min). Because of the building length and width, the ducts will be run in separate manifolds approximately half the length of the building with blowers on each end. So the airflow for half of the structure is 1890/2 = 945 m3/min, and each of the four ducts needs to handle 945/4 = 236 m3/min. The CSA = Q/V = 236/600 = 0.393 m2. Since a half-round duct is used, the diameter is that of a circular duct with an area of 0.393 × 2 = 0.786 m2. The area of a circle, Ac = 3.1416 D2/4 = 0.785 D2, then D2 = Ac/0.785, and D = (Ac/0.785)0.5. If Ac = 0.786, then D = (0.786/0.785)0.5 = (1.00)0.5 = 1.00 m. So the half-round duct base width needed is 1.0 m; the duct height is 0.5 m. 5.7.2.10 Calculate Duct Surface Area The maximum allowable velocity through the perforated surface should be 9 m/min (30 fpm). Aeration ducts should have at least 10% of the surface area open (perforated). Perforations should be as large as practical without allowing whole seeds or grain kernels to pass through. Perforations of 2.0 to 2.4 mm (0.078 to 0.094 inch) diameter are an industry standard for wheat, maize, sorghum, soybeans, and other cereal grains. For small seeds like flaxseed, canola, etc., perforations need to be smaller; or a wire or cloth screen should be used over a duct with standard perforations. For negative pressure systems, it is recommended to reduce this velocity to 6 m/min (20 fpm). Duct surface area (SA) (m2) can be calculated using the following equation:

( )

SA m 2 =

(

Q m3 min

)

V ( m min)

(5.25)



241

The surface area for a duct is the surface area per linear meter (linear foot) times the duct length. About 80% of the total surface area of round ducts is considered usable when the duct sits on the floor. Example For horizontal storages with shallow depth, long ducts, and suction aeration systems, the recommended surface exit velocity is 6 m/min. The example given in the previous section with 30 m wide by 70 m long flat storage is used. The distance from the perforated duct to the end wall is 3.75 – 0.5 = 3.25 m (11 ft). So the total perforated duct length is 70/2 – (3.25 × 2) = 35 – 6.5 = 28.5 m (93.5 ft). From Equation 5.25:

(

SA = 236 m 3 min

) (6 m min) = 39.3 m

2

The surface area for the half-round duct is Ad = (3.14 D/2) × L = (3.14 × 1.0 m/2) × 28.5 m = 44.7 m2. Since this is slightly larger than the recommended 39.3 m2, it meets the guidelines. For economic considerations the duct diameter may be slightly reduced. Assuming uniform air discharge along the length of the duct, the average duct exit velocity is (236 m3/min)/44.7 m2 = 5.3 m/min. This is lower than the suggested 6 m/min for suction systems, which is acceptable. Where perforated round ducts laid on the floor are used, it is recommended to reduce the duct surface area to 80% to compensate for restriction caused by the floor. Therefore, the round duct surface area (RDSA) is calculated as:

(

)

RDSA ft 2 ft = (0.80 × ( D × π) × ft ) ft

(5.26)

where D is the diameter in m or ft, and maximum air velocity through the perforated duct surface is 9 m/min (30 ft/min). Then, based on the airflow per duct (APD), the minimum perforated duct length (PDLmin) can be determined: PDLmin =

APD ( RDSA)(30)

(5.27)

where: PDLmin = Minimum perforated duct length, m or ft RDSA = Surface area, m2/m (ft2/ft) length APD = Airflow per duct, m3/min (ft3/min) 5.7.2.11 Determine Dimensions of the Supply Duct Maximum air velocity in short supply ducts (5 to 7 m) should be maintained at 750 m/min (2500 fpm). For long ducts, manifolds, and elbows, consult Figure 5.27 or Table 5.7. Example In the Brownsville, TX, flat storage warehouse, the length of the transition duct from the wall to the perforated duct is 3.25 m inside and 1.5 m outside, including a wall thickness of 0.4 m (the


242


total length is 5.15 m). Each duct will handle 236 m3/min, so the supply duct area, As = (236 m3/min)/(750 m/min) = 0.315 m2, or a supply duct of 0.63 m. 5.7.2.12 Size the Roof Vent Area The velocity through the roof vents should not exceed 450 m/min (1500 ft/min), although 300 m/min (1000 ft/min) is preferred. The static pressure between the head-space of the storage bin and the ambient should not exceed 30 Pa (one eighth inch water column), when the fan is operated. The total cross-sectional area for the roof vents in the horizontal warehouse discussed above is (1890 m3/min)/(300 m/min) = 6.3 m2. 5.7.3

Examples of Planning Aeration Systems

The examples given in this section are based on design procedures presented in Section 5.7.2 in a step-by-step process as described by Hellevang et al. (1997). To avoid confusion in using both metric and U.S. units, the first two examples are in U.S. units and the third example is in metric units. 5.7.3.1 Example 1: Aeration System for a Cylindrical Bin in U.S. Units Information on the system: • • • •

Storage diameter (D): 30 ft Level Grain depth (h): 25 ft Grain type: wheat Design airflow rate: 0.20 cfm/bu

5.7.3.1.1

Step 1: Calculating the Storage Volume

The storage volume is calculated by using Equation 5.14: 2

2

D 30 Volume ft 3 = π ×   × h = 3.1416 ×   × 25 = 17, 672 ft 3  2  2

( )

17, 672 ft 3 × 0.80 bu ft 3 = 14,138 bu Calculate the total airflow required: Total airflow = 14,138 bu × 0.2 cfm bu = 2,828 cfm The estimated static pressure is determined using Figure 5.22: for grain depth = 25 ft, static pressure = 3.5 inches of water 5.7.3.1.2

Step 2: Choosing a Fan

Choose a fan using fan manufacturers’ data such as in Table 5.8. From Table 5.8, for static pressure = 3.5 in w.c., the 18-inch diameter, 3 hp axial fan, that delivers 2650 cfm at a static pressure of 3 inches, does not deliver the capacity required at the design static pressure.



243

The 18-inch diameter, 3 hp in-line centrifugal fan, rated at 3000 cfm at 4 inch static pressure, exceeds the design airflow by 6%, at a higher static pressure. This choice meets the aeration design performance requirements. Design engineers should source vane-axial and centrifugal fans from more than one supplier, unless one supplier covers the range of aeration airflow needs satisfactorily. 5.7.3.1.3

Step 3: Choosing the System Type

A perforated in-floor duct system will be designed. See Section 5.5.3.2 for design details of alternative types of aeration floor ducts. 5.7.3.1.4

Step 4: Determining Duct Locations

Using the design criterion of 1.5:1 airflow path length ratio, the maximum allowable distance between the wall and duct perforations is one half the grain depth, 12.5 ft. The minimum distance between the perforated duct and the grain surface is the grain depth, 25 ft. The distance from the wall to the beginning of the perforated duct is determined by centering the perforated duct(s) in the bin (see Step 6). 5.7.3.1.5

Step 5: Calculating the Perforated Area

Determine the required perforated area using Equation 5.25:

( )

SA ft 2 =

(

Q ft 3 min

) = 2, 828

V ( ft min)

ft 3 min = 94.3 ft 2 30 ft min

The layout of the duct system can now be determined based on the required perforated area of 94.3 ft2. A single duct will not meet the required 12.5 ft maximum distance between the wall and the duct perforations because the bin radius is 15 ft. A parallel duct system with a manifold or Y-shaped duct system could provide the required perforated area, meet the spacing criteria, and use only a single fan. A Y-shaped duct system will be used in this example, Figure 5.7. 5.7.3.1.6

Step 6: Sizing the Ducts

Determine the perforated duct length and width to provide 94.3 ft2. Use a duct width of 2.5 ft, a common width for perforated duct covers. The required length is calculated as 94.3 ft2/2.5 ft = 37.7 ft. For the Y-shaped duct system, each leg would be half of the total length, 37.7 ft/2 = 18.8 ft. The duct length is rounded to a whole number, 19 ft. The required distance from the wall to the start of the perforated duct, Step 4, is determined by centering the perforated ducts in the bin, using caution not to violate the 1.5:1 air path criteria in Step 4. (All points on the bin floor within 12.5ft of the perforated duct.) Determine the minimum duct cross-sectional area. Each leg will carry 1414 ft3/min, half of the total required airflow. Using the maximum allowable duct air velocity of 2000 ft/min; A = (1414 ft3/min)/(2000 ft/min) = 0.71ft2. Using a duct width of 2.5 ft, the minimum duct depth = 0.71 ft2/2.5 ft = 0.28 ft ≈ 0.3 ft ≈ 4 inches. The dimensions of the two perforated ducts are 19 ft × 2.5 ft × 4 inches. Determine the non-perforated supply duct cross-sectional area (CSA) to limit the maximum allowable non-perforated supply duct air velocity to 2500 ft/min using Equation 5.24:


244


( )

CSA ft 2 =

(

Q ft 3 min

) = 2, 828 ft

V ( ft min)

3

min = 1.1 ft 2 2, 500 ft min

The width is chosen to match the perforated duct width of 2.5 ft, and the minimum depth is 1.1 ft2/2.5 ft = 0.44 ft ≈ 6 inches. The dimensions of the non-perforated supply duct are 2.5 ft × 6 inches. To avoid errors during construction, the perforated and non-perforated supply ducts could have the same depth. The non-perforated supply duct depth is greater and should be used. This reduces the velocity in the perforated duct by increasing the cross-sectional area. 5.7.3.1.7

Step 7: Sizing the Roof Vent Area

The minimum roof vent area = (2828 ft3/min)/(1000 ft/min) = 2.8 ft2. 5.7.3.2 Example 2: Level-filled flat storage in U.S. units Information on the system: • • • •

Grain depth (h): 11 ft Storage dimensions: 60 × 80 ft Grain type: Shelled Corn Design airflow rate: 0.10 cfm/bu

Because the building is 80 ft long, it is more economical to use lengthwise ducts. This is temporary storage, so above-floor round ducts will be used. 5.7.3.2.1

Step 1: Calculating Duct Spacing

Using Equation 5.19, the number of ducts = building width/grain depth = 60 ft/11 ft = 5.5 ducts (normally round the answer up to the next integer); so use 6 ducts. Using Equation 5.20, duct spacing is 60 ft/6 = 10 ft. Using Equation 5.21, the distance from side wall to duct is 10 ft/2 = 5 ft. Use Equation 5.22 to calculate the distance from end wall to the perforated duct (E): The distance from end wall to the perforated duct = 0.7(5) = 3.5 ft. Calculate the perforated duct length (PDL) using Equation 5.23: PDL = L − 2 ( E ) i.e.: PDL = 80 ft − (2(3.5 ft )) = 73 ft 5.7.3.2.2

Step 2: Calculating the Storage Volume

Estimate the amount of grain in the storage, and the amount of grain served by each duct.

(

Total Storage Volume (TSV) = (length × width × grain depth) × 0.8 bu ft 3 = (80 ft × 60 ft × 11 ft ) × 0.80 = 42,240 bu Volume of grain aerated by each duct is 42,240 ÷ 6 = 7040 bu.

)



5.7.3.2.3

245

Step 3: Calculating Airflow

Airflow per duct (APD) = airflow rate (AR) × amount of grain aerated per duct (GAD) = 0.10 cfm/bu × 7040 bu/duct = 704 cfm/duct. Using depth of grain and design airflow, estimate the static pressure from Figure 5.23 as 0.5 inches of water. 5.7.3.2.4

Step 4: Choosing a Fan

Choose a fan based on fan performance data such as in Table 5.8. An axial fan, 12 inch diameter, 0.5 hp, will provide 1500 cfm at a static pressure of 1 inch of water. Each fan of this size can be connected to two ducts through a manifold. 5.7.3.2.5

Step 5: Choosing Ducts

Select the duct sizes from the information presented in Sections 5.1 and 5.7. To size the round ducts, use the flow rate provided by the fan, i.e., 1500/2 = 750 (ft3/min)/duct. The perforated duct area (DA) = airflow per duct (APD) duct velocity (DV)

(

= 750 ft 3 min

) (2000 ft min) = 0.38 ft

2

Which gives a duct diameter of 8.3 inches for a round duct. Because an 8.3-inch round duct is not commonly available, use a 10-inch diameter duct. Supply duct area = (1500 ft3/min)/(2500 ft/min) = 0.6 ft2, use a 12-inch round duct (0.79 ft2). 5.7.3.2.6

Step 6: Determining perforated duct length

Determine the minimum perforated duct length using the criterion for maximum velocity for the perforated section of the ducts. It is recommended to reduce the duct surface area to 80% to compensate for restriction caused by the floor when using round ducts. Therefore, using Equation 5.26, the duct surface area per ft length (RDSA) is calculated as:

(

)

RDSA ft 2 ft = (0.80 × ( D × π) × 1.0 ft ) ft = (0.80 × (1.0 ft × π) × 1.0 ft ft ) = 2.51 ft 2 ft

(5.28)

where D is the diameter in ft and the maximum air velocity through the perforated duct surface is 30 ft/min. Calculate the minimum Perforated Duct Length using: PDLmin =

APD 750 ft 3 min = = 9.95 ft ( RDSA)(30) 2.51 ft 2 ft × 30 ft min

(

)

(5.29)

where: PDLmin = Minimum perforated duct length, ft RDSA = Surface area, ft2/ft length APD = Airflow per duct, ft3/min The minimum perforated duct length from Equation 5.27 is 10 ft. Note that the length for perforated section calculated in Step 1, 73 ft, exceeds the design criterion.


246


Figure 5.28 Simple geometry of the values on duct distances and peak height of the bulk, less half duct diameter.

5.7.3.2.7

Step 7: Calculating Total Airflow

Determine the total required airflow (RQ) as airflow per duct (APD) times number of ducts (ND); i.e., RQ = 750 ft3/duct × 6 ducts = 4500 ft3/min. 5.7.3.2.8

Step 8: Calculating Roof Vent Area

The minimum roof vent area needed for the grain storage building = (4500 ft3/min)/(1000 ft/min) = 4.5 ft2. 5.7.3.3 Example 3: Peak-Filled Flat Storage in Metric Units Information on the system: • • • •

Storage dimensions: 30 × 70 m Sidewall grain depth: 8 m Grain type: Sorghum Design airflow rate: 6 (m3/h)/tonne

Design the duct layout using half-round ducts having a diameter of 1 m — a base width of 1.0 m and a peak height of 0.5 m. From Equation 5.19: number of ducts = 30/8 = 3.75 = 4 ducts. From Equation 5.20: duct spacing = 30/4 = 7.5 m. From Equation 5.21: distance from sidewall to the duct centerline = 7.5/2 = 3.75 m. Lengthwise ducts will be used with the perforated metal starting at 4.0 m from the end wall (half of the grain depth), and the centerline of the first duct placed 3.75 m from one building sidewall. Then ducts 2, 3, and 4 are spaced 7.5 m apart across the building width, leaving 3.75 m from the centerline of duct 4 to the far sidewall. Check that the air path length ratio does not exceed 1.5:1. From duct 1, 3.75 m –0.5 m (half duct width) + 8 m = 11.25 m. Grain sorghum, with a surface slope of 23°, is stored in the flat storage. The shortest distance to the sloped grain surface is a line perpendicular to the grain surface through the center of duct 1 or duct 4 unless this line crosses the sidewall. In this case, the shortest distance is the line between the center of the duct and the corner where grain meets the sidewall. This distance is calculated based on the simple geometry (Figure 5.28) of the values on duct distances and peak height of the bulk, less half duct diameter.



247

This configuration would result in a distance of 8.83 – 0.5 = 8.33 m. Thus, 11.25/8.33 = 1.35:1, less than 1.5:1, which meets the design guideline. The grain peak height at the center of the building is tan 23 × 15 m (half width) + sidewall depth = 0.424 (15) + 8 = 6.37 + 8 = 14.37 m. From duct 2, the longest path is 3.75 – 0.5 = 3.25 m to the center of the building, then 14.37 m to the peak, or 17.62 m. The shortest distance from duct 2 to the grain surface is a line perpendicular to the grain surface through the center of duct 2. This distance is calculated based on the simple geometry (Figure 5.28) of the values on duct distances and peak height of the bulk, less half duct diameter. This configuration would result in a distance of 11.76 – 0.5 = 11.26 m. Thus, 17.62/11.26 = 1.56:1, which slightly exceeds the 1.5:1 guideline; but since this is only an engineering guideline, the design is acceptable. Another consideration is that the static pressures on each duct vary based on how the air supply is designed. If one fan supplies ducts 1 and 2, the airflow through duct 1 is greater than through duct 2 by the ratio of the static pressure in each duct, which is based on the relative grain depth for each duct. The shortest distance to the surface of duct 2 is 11.26 m, compared to the perpendicular distance from the grain surface to duct 1 of 8.33 m. This ratio is 11.26/8.33 = 1.352, so duct 1 has about 35% less static pressure than duct 2. Thus, one fan will deliver about 1.35/(1.0 + 1.35) = 1.35/2.35 = 0.574 or 57.4% of its airflow to duct 1, and 1.0/2.35 = 42.6% of its air to duct 2. If each duct has a separate but equal capacity blower, fan 1 will deliver more airflow than fan 2 by about the same ratio.

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Metzger, J.F., P.D. Terry, and W.E. Muir. (1981). Performance of several axial-flow fans for grain bin ventilation, Can. Agric. Eng., 23, 11–16. Miketinac, M.J. and S. Sokhansanj. (1985). Ventilation-pressure distribution in grain bins — Brooker’s model, Int. J. for Num. Methods in Eng., 21, 1067–1075. Miketinac, M.J., S. Sokhansanj, and D.S. Jayas. (1986). Graphical analysis of airflow distribution in grain bins using finite element method, Can. Agric. Eng., 28, 23–30. Moody, L.F. (1944). Friction factors for pipe flow, Transactions of the ASME, 66, 671–684. Morey, R.V., H.A. Cloud, R.J. Gustafson, and D.W. Peterson. (1979). Evaluation of the feasibility of solar energy grain drying, Transactions of the ASAE, 22, 409–417. Navarro, S. and Calderon, M. (1982). Aeration of Grain in Subtropical Climates. FAO Agricultural Services Bulletin No. 52, Rome. Pabis, S., D.S. Jayas, and S. Cenkowski. (1998). Grain Drying: Theory and Practice, John Wiley & Sons, New York. Pierce, R.O. and T.L. Thompson. (1975). Airflow patterns in conical-shaped piles of grain, Transactions of the ASAE, 18, 946–934, 938. Segerlind, L.J. (1976). Applied Finite Element Analysis, John Wiley & Sons, New York. Segerlind, L.J. (1982). Solving the nonlinear airflow equation, Paper No. 82–3017, ASAE, St. Joseph, MI. Segerlind, L.J. (1984). Applied Finite Element Analysis, 2nd ed., John Wiley & Sons, New York. Segerlind. L.J. (1983). Presenting velocity-pressure gradient data for use in mathematical models, Transactions of the ASAE, 26, 1245–1248. Sharp, J.R. (1982). A review of low temperature drying simulation, J. Agric. Eng. Res., 27, 169–190. Shedd, C.K. (1953). Resistance of grains and seeds to airflow, Agric. Eng., 34, 616–619. Singh, R.P. and D.R. Heldman. (1984). Introduction to Food Engineering, Academic Press, New York. Smith, E.A. (1982). Three-dimensional analysis of air velocity and pressure in beds of grain and hay, J. Agric. Eng. Res., 27, 101–117. Smith, G.D. (1985). Numerical Solution of Partial Differential Equations: Finite Difference Method, 3rd ed., Oxford University Press, Oxford, UK. Smith, L.L. (1975). Aeration of cottonseed in storage, Agric. Res. Serv. Marketing Res. Rep. No. 1020, U.S. Department of Agriculture, Washington, D.C. Spencer, H.B. (1969). Pressure drop in on-floor-duct drying systems, J. Agric. Eng. Res., 14, 165–172. Sukup. (1994). Sukup Centrifugal Fans-Heaters, Drying or Aeration, 21101-94G, Sukup Manufacturing Company, P.O. Box 677, Sheffield, IA 50475. Williamson, W.F. (1965). Pressure losses and drying rates in grain ventilated with various on-floor duct systems, J. Agric. Eng. Res., 10, 271–276.



CHAPTER

6

Experimental Aeration Systems Ronald Noyes, Shlomo Navarro, and David Armitage

CONTENTS 6.1

Review of Experimental Laboratory and Pilot-Plant Studies .............................................252 6.1.1 Ideal Aeration Model — Constant Heat Transfer, No Loss of Mass......................253 6.1.2 Aeration Models with Moisture Transfer ................................................................254 6.1.2.1 Grain Temperature Profiles during Aeration ............................................255 6.1.2.2 Results of a Large-Scale Laboratory Aeration System Research Study......256 6.1.2.3 Airflow Rate as a Significant Variable in Cooling Grain.........................259 6.1.2.4 Temperature Gradients and Fluctuations ..................................................262 6.1.2.5 Final Grain Temperature Profiles after Aeration ......................................264 6.1.2.6 Time Required for Cooling Dry Grain.....................................................267 6.1.2.7 Changes in Grain Moisture Content during Cooling of Grain ................271 6.1.2.8 Movement of Drying Fronts during Cooling of Grain ............................272 6.1.2.9 Grain Mass Temperature Distribution ......................................................273 6.1.2.10 Deep-Bed Aeration Control Strategies .....................................................276 6.2 Review of Field Trials With Aeration Systems ...................................................................277 6.2.1 Physical Effects of Aeration.....................................................................................277 6.2.1.1 Temperature and Moisture Fronts during Aeration..................................278 6.2.1.2 Changes in Moisture Content of Grain during Aeration..........................279 6.2.1.3 Temperature Distribution during Cooling at Different Airflow Rates .....280 6.2.1.4 Grain Temperatures during Extended Aerated Storage............................280 6.2.1.5 Grain Temperatures Obtained with Ambient Cooling..............................285 6.2.1.6 Prevention of Moisture Migration ............................................................288 6.2.1.7 Aeration Time Required............................................................................290 6.2.1.8 Energy Consumption.................................................................................292 6.2.2 Biological Effects of Aeration..................................................................................295 6.2.2.1 Effects on Suppressing Insect Development ............................................296 6.2.2.2 Effects on Heavily Infested Grain Bulks..................................................299 6.2.2.3 Insect Movement Affected by Cooling Fronts .........................................301 6.2.2.4 Effects on Mite Populations......................................................................303 6.2.2.5 Controlling Microfloral Growth................................................................306 References ......................................................................................................................................308

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252


6.1 REVIEW OF EXPERIMENTAL LABORATORY AND PILOT-PLANT STUDIES In the previous chapters, the physical basis and theory of aeration, and the description of the aeration system components were discussed. This chapter reviews experimental and laboratory pilot-plant studies relating to aeration. The description of the physical processes and the empirical data are needed to support and validate the theoretical basis of aeration. Mechanical aeration of grain is described in Chapters 1 and 5. This process using fans or blowers is a relatively young technology for grain storage, the benefits of which have not yet been fully investigated. The practice of aerating stored grain in the U.S. was first developed during the early 1950s for large, flat storage warehouses built to hold the U.S. grain reserve following World War II. Aeration of grain stores in other parts of the world was also being developed during that time. Robinson et al. (1951) explained that aeration was used to cool the grain mass and keep temperatures constant in order to prevent moisture migration. The experience with aeration in maintaining grain in condition in U.S. flat storages was so favorable that aeration was soon adapted to silos and large bolted steel grain tanks at grain elevators, followed by small grain bins on farms. Due to the influence that cooling has on limiting insect and microflora development, cooling of grain has become the primary objective of aeration. However, this objective can be achieved only under favorable climatic conditions. These exist in cold and temperate regions of the globe and in subtropical regions with a cold season. This technology is not feasible during hot summer months in temperate and subtropical zones or in the warm tropical belt. Aeration to control insect populations in grain stores was investigated by Burgess and Burrell (1964) in Britain; Sutherland (1968) in Australia; Smith (1974) and Arthur (1994) in the U.S.; Lasseran and Fleurat-Lessard (1990) in France; and Navarro et al. (1969) in Israel. In the U.S., over 20 million tonnes of hard red winter wheat are harvested and stored each year during hot summer months in the temperate Southern Great Plains region of the U.S. (Nebraska, Kansas, Oklahoma, Texas, New Mexico, and Colorado). Wheat in this region typically enters storage at 35 to 40°C. In this hot, high-risk storage region, where insects pose the main storage problem, storage systems are primarily regulated by temperature. Although residual pesticides and fumigation were the primary pest management control methods for decades, aeration has become a major pest management tool since the late 1980s due to increasing public resistance to pesticide residues. In view of the public concern over the use of pesticides in the post-harvest sector, the grain industry has generally accepted the comprehensive Integrated Pest Management (IPM) philosophy, which includes aeration (Cuperus et al., 1993). IPM can be defined as a systems approach to commodity protection based on the desire to reduce, as much as possible, the use of chemical pesticides that endanger the environment as well as consumers and pesticide applicators. Cuperus et al. (1993) indicated that temperature was the most critical variable in stored-product pest population dynamics; and according to their field observations, storage insect populations could be successfully suppressed by aeration. This is also self-evident in the findings of many earlier studies mentioned elsewhere in this chapter on the relationship between insect development and temperature. In most regions of the U.S. and in other major grain production and storage regions, particularly in Europe and Australia, aeration is now a major pest management tool. Aeration has become increasingly important during the past decade as consumer awareness of pesticide residues in stored food and feed grains increases. As export and processor demands in many countries grow for identity preserved (IP) marketing of food grain, more emphasis is being placed on aeration. In some climatic regions grain is harvested above its permissible moisture content for storage — that is, the moisture content above which microflora can develop. High-capacity aeration (involving the use of high airflow rates) has been proposed as a solution for preventing mold formation under such conditions to hold the moist grain until it can be dried to safe storage moisture levels. Experimental data on preservation of moist grain has been reported by Burrell and Laundon (1967), Jouin (1961), and Thompson (1972).


EXPERIMENTAL AERATION SYSTEMS

253

In the U.S. corn belt — from Ohio to Nebraska and Minnesota to New York — maize is the major crop. From 300 to 400 million tonnes of high-moisture maize are harvested annually in the U.S. from midsummer through late fall. Before storage, grain in that region needs to be dried to prevent mold damage (Henneberry, 1998). Although aeration is used to hold wet maize and keep it from molding until it can be dried, insects are still a major storage problem in the northern U.S. Therefore, aeration can effectively be used to control insects in the cool season in these regions. A related technology based on high airflow rate aeration was developed to finish drying slightly moist hot grain transferred directly from grain dryers without cooling in the dryer. This process was termed Dryeration (Foster, 1973). Preservation of slightly moist grain by aeration is possible when the ambient temperatures enable removal of metabolic heat that develops during storage. Such aeration installations have been proposed for use in warm climates as well. However, experimental work to describe the benefits of aeration under these climatic conditions still needs to be documented. The use of aeration to prevent moisture condensation by equalizing grain bulk temperatures as a secondary objective to that of cooling has been partially documented by Muir (1973) in Canada, Converse et al. (1969), and Yaciuk et al. (1975) in temperate climates. Limited data have been gathered for aerated grain bulks stored under subtropical conditions (Navarro and Calderon, 1982; White, 1988). In contrast, experimental data on the beneficial use of aeration to prevent moisture condensation in bulk stored grain in tropical climates are still lacking, in spite of the fact that aeration installations have been proposed for use in such climates. Aeration systems that have been installed in the tropics appear to remain largely unused. Additional benefits of aeration, such as removal of storage odors and fumigants, have been sporadically mentioned in the literature (Burrell, 1974; Holman, 1960; Navarro and Calderon, 1982), but documented data is not available. Some documentation of the use of low volume airhandling systems for use in recirculation of fumigants is provided by Noyes, et al. (1992, 1996, and 1998). In the following sections, data from pilot experimental installations and commercial-scale field trials and experiments are reviewed. 6.1.1

Ideal Aeration Model — Constant Heat Transfer, No Loss of Mass

When considering the cooling rate of a grain mass, there are two general approaches to the use of models. In the first, it is assumed that no mass (moisture) transfer takes place between the grain and the cooling medium. In the second, latent moisture transfer is taken into account. The following sections describe these two approaches. In an ideal aeration model with constant heat transfer and no moisture transfer, the cooling zone is of negligible thickness; and the maximum temperature changes in the cooling air occur throughout the cooling period (Foster, 1967). The air temperature rise is equal to the temperature drop in the grain. When the heat-balance equation is written, air and grain temperature terms are equal and drop out. The unit amount of air that is required to cool a unit amount of grain is simply a ratio of specific heat of grain to specific heat of air. In an early important paper on heat transfer in a fixed bed, Schumann (1929) analytically solved the cooling rate of a bed of broken solids through which air passed at a constant rate. He developed a two-equation heat transfer model that predicted the temperature history of any point in the bed. His model assumed constant thermal properties, non-compressible fluids, and solid particles small enough to ensure no temperature gradient within the particles at any time. Furnas (1930) extended Schumann’s (1929) model to larger airflow rates and bed-depths and applied the solutions to beds of iron balls. Bakker-Arkema and Bickert (1966), working with deep-bed cooling of beets, developed a model based on Schumann’s (1929) analysis to predict temperatures in a deep bed as a function of time and position. The model assumed that no mass transfer took place between the beets and the cooling


254


source, there was no temperature gradient in individual beets, and interstice air velocity was constant. When they compared theoretical and experimental cooling rates, real cooling rates were considerably higher than predicted rates. They stated that moisture loss and evaporative cooling of the beets resulted in faster cooling than the theoretical model projected. The Schumann (1929) deep-bed analysis is acceptable for cooling systems where thermal conductivity of bed particles is large and particle size is small. These conditions do not apply to most biological particles (Bakker-Arkema et al., 1974). 6.1.2

Aeration Models with Moisture Transfer

Aeration models that include latent heat transfer were discussed by several researchers (Boyce, 1966; Burrell and Laundon, 1967; Foster, 1967; Moysey, 1969; and Person et al., 1966). Foster explained that during the cooling of grain, air temperature must increase while air absolute humidity (mass of water per mass of air) and relative humidity also increase. Cooling the grain supplies the heat for evaporating grain moisture. Moisture released during cooling reduces the amount of sensible heat exchange required, lowering cooling air volume. Early drying models included moisture changes (Barre et al., 1971; Baughman et al., 1971; Henderson and Henderson, 1968; and Bloome and Shove, 1971, 1972). Several researchers derived grain cooling models from drying models which included grain moisture change (Boyce, 1966; Sutherland et al., 1971; Ingram, 1979; and Bakker-Arkema et al., 1967). Boyce (1966) designed a thin-layer drying model for grain and used the same equations for grain cooling. His model, incorporating semi-equilibrium deep-bed drying, used the summation of heat and mass transfer rates of several thin layers. In these models, grain and air in each layer are assumed to reach temperature equilibrium in the simulation time interval, while moisture transfer is predicted by a thin layer diffusion equation. Sutherland et al. (1971), using an equilibrium analysis, assumed that heat and mass transfer coefficients between air and grain are infinite, there is no diffusion of heat or mass in the airflow direction, grain interstice voids are straight parallel channels, and air moves uniformly through a cylindrical bed of grain. Their model predicted leading and trailing edges of cooling or heating fronts and grain wetting or drying. They claimed that their computer model could predict times and front shapes for equilibrium fairly well; but by including the effects of finite transfer and diffusion coefficients, it should be possible to improve their computer program performance. Ingram (1979) used the method of characteristics to solve heat and mass transfer equations for cooling and drying. He found that his results compared closely with those of finite difference solutions. Best results were obtained using low airflow rates in the range of 0.061 and 0.122 kgm–2s–1 (2.9 to 5.7 (m3/h)/tonne). Results were less satisfactory at higher airflow rates. Bakker-Arkema et al. (1967) conducted numerous studies on the cooling of a wet bed of cherry pits using a three-equation analysis, with equations for product temperature, air temperature, and specific humidity of the air. Using this model, the effect of several parameters (airflow, convective mass and heat transfer coefficients, inlet air conditions, porosity, specific heats, and heat of vaporization) on cooling rates were studied. Non-equilibrium simulation models describe both heat and mass transfer through rate equations, with no simplifying equilibrium assumptions. A widely used model is the Michigan State University grain drying simulator (MSU model), by Bakker-Arkema et al. (1974) and Brooker et al. (1974). In this model, one equation was used to determine grain temperature and moisture content, and a second equation predicted air temperature and absolute humidity. Heat transfer was solved using heat transfer coefficients. Mass transfer was predicted using either a mass transfer coefficient with an empirical thin layer drying rate equation or a theoretical diffusion rate equation. Morey et al. (1978) stated that the high-temperature MSU grain drying simulator model is not suitable for low-temperature, low-airflow simulation because cooling simulation requires the solution of partial differential equations involving short simulation time intervals. A validation test by Keener



255

et al. (1978) compared performance of the MSU model with a new moisture transfer equation and a new partial differential equation. All three models predicted moisture content to within about 1.0 percentage point wb over a 100-hour drying test using airflow rates of 10 to 15 m3/min.tonne. Schultz (1984) conducted a comparison of simulation techniques for wheat aeration. He examined the factors of solution method, hysteresis, and simulation time interval, developing simulation models for each combination of factors. Model predictions were compared to wheat aeration field data to determine grain moisture content and temperature prediction accuracy. His equilibrium simulation was patterned closely after methods presented by Thompson (1972) and semi-equilibrium simulation was patterned after Thompson et al. (1968). 6.1.2.1 Grain Temperature Profiles during Aeration Due to the importance of temperature profiles in aeration, the subject is also discussed in Section 4.6, to calculate the speed of fronts moving through grain bulks, and in Section 7.1.6, to select ambient air for aeration. When cool air is drawn or pushed through a bin of warm grain, all the grain does not cool uniformly. A cooling front moves through the grain in the direction of airflow. The cooling front is the leading edge of a cooling zone, the region in the grain mass where grain temperatures are changing. Leading and trailing edges define top and bottom boundaries of a cooling zone when the direction of airflow is upward (Figure 6.1). These edges are reversed for suction or down-flow cooling as opposed to pressure or up-flow cooling, as shown. Ahead of the cooling front or the leading edge of a cooling zone, grain temperatures have not changed. Behind the trailing edge, grain temperatures are almost the same as the cooling air temperature. Figure 6.1b provides a schematic representation of a cooling front, illustrating the leading and trailing edge of a cooling zone in a dimensionless bulk of grain. In this example an airflow rate of 5.36 Ls·m3 (25.6 (m3/h)/tonne) was illustrated. The leading edge (9 hours) shows almost a proportional drop when it reaches the top of the grain mass (X/L = 1.0 depth). The temperature of the leading edge, shown with a heavy curved black line as it approaches the top, is within 1 to 2°C (1.8 to 3.6°F) of the maximum grain temperature, which indicates that the leading edge has reached the grain surface. After additional aeration of 29 hours (total of 38 hours), the trailing edge reaches the top as shown with a horizontal dashed line coinciding with 19°C at all depths (X/L = 0 to 1.0). This results in the bin profile shown in Figure 6.1b. Heat of compression of air (which heats pressure cooling air 1 to 3°C or more) and evaporative cooling were ignored in this presentation. The times for the leading and the trailing edges to reach the top, Figure 6.1, were taken from Epperly’s (1989) work for an airflow rate of 5.36 Ls·m3 (25.6 (m3/h)/tonne). Air relative humidity was considered constant and in equilibrium with grain moisture content. In practice, factors like fluctuation in ambient air temperature may lead to variations in achievable grain temperatures that are different than the theoretical values shown below. Simplified temperature profiles, which are a sequence of curved lines of the same shape that advance in time, were not shown in Figure 6.1. Results of experimental work carried out at the same flow rates (5.36 Ls·m3 or 25.6 (m3/h)/tonne) are shown in Figure 6.2 (Epperly, 1989). It appears from the data in this illustration that surface grain temperatures dropped between 9 hours for the leading edge and 38 hours for the trailing edge of the cooling zone from the beginning of the tests for this airflow rate. With different aeration airflow rates, the elapsed time required for the leading and trailing edges of the cooling zone to exit the grain mass differ. These “edges” were well defined over a distance of 25 to 50 cm, depending on cooling air velocity (Epperly, 1989). As airflow rates increase, the time needed for the trailing edge to reach the surface of the grain bulk is shorter. To determine the leading and trailing edges, Epperly (1989) plotted the temperature profile vs. time in a column of wheat for an aeration rate of 5.36 Ls·m3 (25.6 (m3/h)/tonne). Results of this experimental temperature profile are shown in Figure 6.3 for comparison with Figure 6.2 at the same airflow rate.


256


(

°

)

°

° °

AT

° Figure 6.1

Schematic presentation of the leading edge and the trailing edge in a column of grain, and graph showing cross-section temperatures in relation to times to reach leading and trailing edges in upward (pressure) airflow. (Based on data from Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University.)

6.1.2.2 Results of a Large-Scale Laboratory Aeration System Research Study To evaluate the characteristics of aeration cooling zones under varied airflow rates, Epperly (1989) conducted a large-scale laboratory study of the effect of cooling and heating grain using six aeration airflow rates ranging from 0.04 to 0.64 (m3/min)/m3 (0.05 to 0.8 cfm/bu) in a corrugated steel bin cooling hard red winter wheat.



257

Figure 6.2

Temperature profile vs. location in grain mass at different instances of time for a column of wheat at an aeration rate of 5.36 Ls·m3 (25.6 (m3/h)/tonne) used for determining times for the leading and trailing edges to exit the grain mass. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

Figure 6.3

Temperature profile vs. time in a column of wheat for an aeration rate of 5.36 Ls·m 3 (25.6 (m3/h)/tonne) used for determining times for the leading and trailing edges to exit the grain mass. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)


258


Objectives of this study were to: 1. Determine the rate at which the leading and trailing edges of a cooling front propagate through a grain mass in relation to different airflow rates. 2. Develop a model to predict the temperature profile in the cooling zone of aerated wheat.

Six airflow rates were replicated three times by aerating the three vertical tubular columns within the main bin simultaneously at aeration rates (Qa) of 0.04, 0.08, 0.16, 0.32, 0.48, and 0.64 (m3/min)/m3 (0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 cfm/bu). The grain mass was initially warmed to 35 to 36°C (95 to 96.8°F), with air within 5 to 10 percentage points of the wheat equilibrium relative humidity (ERH). Conditioned inlet air dry-bulb temperatures (Tdbc) for the cooling tests typically ranged from about 17 to 22°C (62.6 to 71.6°F) for the six test airflows. Conditioned inlet wet-bulb temperatures (Twbc) ranged from about 12 to 15°C (21.6 to 27°F) for the six tests. However, the temperatures varied only about 1 to 2°C (1.8 to 3.6°F) during a test, with a lower Tdbc accompanied by a lower Twbc value. After each cooling test, the wheat was rewarmed with conditioned inlet air dry-bulb temperatures that typically ranged from about 34 to 38°C (93.2 to 100.4°F) for the six test airflows. Conditioned inlet wet-bulb temperatures for warming ranged from about 28 to 32°C (82.4 to 89.6°F) for the six tests. The temperature varied only about 1 to 2°C (1.8 to 3.6°F) during any test, with a lower Tdbc accompanied by a lower Twbc value. Initially, the hard red winter wheat used in the study had about 12.5% moisture content (wb). Although the moisture removed and absorbed during sequential cooling and rewarming tests varied between tests, a 95% confidence interval for the average change in moisture content while cooling was 0.78 ± 0.37 percentage points. For warming, the moisture change average was 0.79 ± 0.54 percentage points. Thus, the tests showed that almost all the moisture lost during aeration cooling tests was regained during warming tests (Epperly, 1989). The times for the cooling zone leading, ΘL, and trailing, ΘT, edges to exit the grain mass were determined from temperature vs. time data shown in Table 6.1. The leading edge was defined as the time at which the top thermocouple readings first started to drop. The trailing edge was difficult to determine because of the exponential behavior of the temperature vs. time curve at the last stages of aeration. Therefore, the trailing edge time was defined as when: T − To ≥ 0.95 ∆T

(6.1)

where: T = trailing edge grain temperature,°C To = initial grain temperature,°C ∆T = differential between initial and final grain mass average temperatures,°C The times for leading and trailing edges of the cooling fronts to move out of the grain mass for each aeration rate and test column are given in Table 6.1. The average amount of time for fall cooling of grain through one complete cycle, which is the same as ΘT, is shown in Table 6.1. Where ∆mc is the average moisture loss, ∆T is the temperature drop during aeration, and ΘL is the time for the leading edge of the cooling zone for aeration rates from 0.04 to 0.64 (m3/min)/m3 or 3.2 to 51.2 (m3/h)/tonne. The grain mass temperature drop, ∆T, was determined by taking the difference between the mean of all column temperatures before aeration began and at the time the trailing edge exited the grain mass. The moisture content change, ∆mc, was determined by averaging moisture contents of all column sample points before and after aerating and then taking the difference of the two averages. The rewarming moisture content gain was determined in the same manner. The temperature drop (∆T) and moisture losses (∆mc) are given in Table 6.1.



Table 6.1

Approximate Cooling Times for Different Aeration Rates

(m3/min)/m3 0.64 0.48 0.32 0.16 0.08 0.04

259

Qa Airflow Ratea (m3/h)/tonne Ls·m3 51.2 38.4 25.6 12.8 6.4 3.2

10.72 8.04 5.36 2.68 1.34 0.67

cfm/bu

T Trailing Edge Time (h)

L Leading Edge Time (h)

mc (% w.b.)

T drop (°C)

0.8 0.6 0.4 0.2 0.1 0.05

20.7 32.5 38.0 90.5 146.7 273.0

4.0 5.2 9.0 31.5 47.0 106.2

0.66 0.89 0.86 0.76 0.66 0.87

15.1 18.2 16.4 18.5 19.0 16.9

a

airflow rate was calculated using a mean bulk density value of 0.754 kg/m3. From Epperly, D.R. (1989). Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.

6.1.2.3 Airflow Rate as a Significant Variable in Cooling Grain The only variable that had any statistically significant effect on the trailing edge time, ΘT, and the leading edge time, ΘL, was the airflow rate. Airflow rate data were plotted (Figures 6.2 and 6.3) and fitted to a regression equation for aeration rate only. The best fit was in log-log format. The equations for ΘL and ΘT are: Θ L = 72.2 ⋅ Qa−1.212

(6.2)

ΘT = 195.8 ⋅ Qa−0.914

(6.3)

where: ΘL = time for leading edge of cooling front to move out of grain mass, h (Figure 6.4) ΘT = time for trailing edge of cooling front to move out of grain mass, h (Figure 6.5) Qa = aeration rate, Ls·m3 Mathematical models were then developed for predicting the time for the leading and trailing edges of the cooling front to reach a specified location (X/L) in the grain mass. The equations that predict the time for the leading, ΘLL, or trailing, ΘLT, edge of the temperature front to reach a given location at a certain aeration rate are: Θ LL = 72.12 ⋅ Qa−1.205 ⋅

X 1.258 L

Θ LT = 204.34 ⋅ Qa−0.9769 ⋅

X 0.6493 L

(6.4)

(6.5)

where: ΘLL = time for leading edge of cooling front to reach a given location at a certain aeration rate, h ΘLT = time for trailing edge of cooling front to reach a given location at a certain aeration rate, h Qa = aeration rate, Ls·m3 Results of other investigators that observed ΘT cooling times during aeration are listed in Table 6.2 along with Epperly’s calculated ΘT values using Equation 6.3. Calculated results are


260


Figure 6.4

Plot of time (h) to reach leading edge (ΘL) at various airflow rates (Ls·m3) Qa with the best fit equation for the data given in Equation 6.2. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

Figure 6.5

Plot of time (h) to reach the trailing edge ΘT at various airflow rates (Ls·m3) Qa with the best fit equation for the data given in Equation 6.3. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)



Table 6.2

261

Comparison of Cooling Time with Other Investigators’ Results

Investigators Sanderson et al. (1988a)

Burrell and Laundon (1967) McCune et al. (1963) Sorenson et al. (1967) a

Qa (m3/min/m3)

Observed T (h)

Calculated T (h)

1.38 0.73 0.41 0.20 0.05 0.28 0.10 0.09

8–10 15–17 30–35 40–60 260 60a 125 140

9.5 17.6 31.0 61.8 240.0 45.2 128.0 137.3

Extrapolated data.

within 8% of observed values, with the exception of Burrell and Laundon’s (1967) data, which were extrapolated. Sutherland et al. (1971) developed a computer model using equilibrium theory and simulated front interactions when cooling with high-humidity air. They state that a temperature front travels typically at about ¹⁄₄₀₀ of the air unit face velocity or average airflow transit velocity through the grain mass. Using an experimental grain bin, Miller (1965) used an air velocity of 1.2 Ls·m3 at different grain depths to determine the velocities of leading and trailing edges of a cooling zone in aerated sorghum. He approximated the times required for the fronts to pass through the grain mass to be (with airflow rates, Qa, in cfm/bu): Θ L = 3.9 Qa−0.94

(6.6)

ΘT = 29 Qa−0.65

(6.7)

where: ΘL = time for leading edge of cooling front, hours ΘT = time for trailing edge of cooling front, hours Qa = aeration flow rates, cfm/bu Example For 0.1 cfm/bu (4.8 m3/m3.h), calculate ΘL and ΘT using Equations 6.6 and 6.7; ΘL = 3.9 (0.1) –0.94 = 3.9/(0.1)0.94 = 3.9/0.115 = 33.9 hours; or 34 hours ΘT = 29 (0.1)–0.65 = 29/(0.1)0.65 = 29/0.224 = 129.5 hours; or 130 hours To convert the above equations from cfm/bu to m3/m3.h, Equations 6.6 and 6.7 can be rewritten as: Θ L = 3.9 (0.0207.Qa ) ΘT = 29 (0.0207.Qa ) where: ΘL = time for leading edge of cooling front, hours ΘT = time for trailing edge of cooling front, hours Qa = aeration flow rates, m3/m3.h

−0.94

−0.65

(6.8) (6.9)


262

Figure 6.6


Temperature profile vs. location in grain mass at different instances of time for a column of wheat and an aeration rate of 10.72 Ls·m3 (51.2 (m3/h)/tonne) used for determining times for the leading and trailing edges to exit the grain mass. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

Example For 4.8 m3/m3.h (0.1 cfm/bu), calculate ΘL and ΘT using Equations 6.8 and 6.9; ΘL = 3.9 (0.0207 × 4.8)–0.94 = 3.9/(0.0994)0.94 = 3.9/0.1142 = 34.2 hours ΘT = 29 (0.0207 × 4.8)–0.65 = 29/(0.0994)0.65 = 29/0.223 = 130 hours 6.1.2.4 Temperature Gradients and Fluctuations From the start of aeration until the leading edge exits the grain mass, the temperature curves as a function of depth tend to be primarily concave downward as shown in Figures 6.6 through 6.9. When the leading edge exits the grain mass, the temperature gradient is nearly a diagonal line between the initial grain temperature to the lowest grain temperature from the bottom to the top of the grain mass. From the time the leading edge exits until the trailing edge exits, the temperature gradients are curved slightly upward with progressively decreasing slopes as time increases, to the point where the temperature profile is essentially horizontal when the trailing edge exits. An initial decrease of 2 to 4°C (3.6 to 7.2°F) and then an increase in grain temperatures was observed near the bottom of the grain mass at the end of each test (Figures 6.6 through 6.9). This temperature fluctuation occurred in a layer in the range of 13 to 36 cm (5 to 14 in) from the bottom of the grain mass, depending on the test, and then moved higher in the grain mass with time based on the loss of evaporative cooling. This temperature change phenomenon near the bottom of grain bins or silos during pressure aeration has been observed by other researchers (Sanderson, et al., 1988a; and Burges and Burrell, 1964). It is attributed to evaporative cooling occurring from the moisture front movement. There was also a slight temperature hump near the bottom of the grain mass in some aeration tests before aeration was started, as shown in Figures 6.6 and 6.7. The temperature rise was generally 2 to 3°C in magnitude and occurred about 13 to 55 cm (5 to 22 in) from the bottom of the grain



263

Figure 6.7


Figure 6.8



264

Figure 6.9



mass. From observing the order in which the tests were performed, the first two tests (Figures 6.6.and 6.7) had a slight temperature rise, while no temperature rise was observed the later the test was performed (Figure 6.8 and 6.9). The temperature rise could be attributed to the moisture content change, or heat of adsorption, which was more pronounced in the lower section of the grain mass. The final time temperature profiles were relatively flat for the higher airflow rates (Figures 6.6 and 6.7). But there appeared to be an upward temperature gradient for the final time reading in the upper part of the bin, which became much more pronounced as the airflow rate dropped (Figures 6.8 and 6.9). The aeration should have been extended longer for tests in Figures 6.8 and 6.9. 6.1.2.5 Final Grain Temperature Profiles after Aeration Aeration systems in temperate climates are generally designed for typical commercial aeration rates of from 0.02 to 0.04 (m3/min)/m3 for vertical bins, compared to 0.04 to 0.08 (m3/min)/m3 (0.05 to 0.1 cfm/bu) for horizontal bins. An aeration cycle may take from about 150 to 275 hours to completely push or pull a cooling zone through a grain bin, according to Table 6.1. Using high airflow rates of 0.32 (m3/min)/m3 (0.4 cfm/bu) can reduce cooling time to about 38 to 40 hours (doubling the aeration rate cuts cooling time in half). With lower aeration rates, final temperature profiles are less uniform than with higher aeration rates. Table 6.1 lists a time of 146.7 hours for a cooling zone to exit the grain mass with 0.08 (m3/min)/m3 (0.1 cfm/bu) aeration rates, compared to 38 hours for 0.32 (m3/min)/m3 (0.4 cfm/bu) and 273 hours for 0.04 (m3/min)/m3 (0.05 cfm/bu). Table 6.1 shows approximate cooling times for cool fronts to move through a clean, level grain mass and total cooling time for grain at aeration airflow rates of 0.04 to 0.8 (m3/min)/m3 (0.05 to 1 cfm/bu). Grain with trash, dockage, fines, foreign material, and peaked surfaces may require substantially longer fan operation times. Even with level, clean grain, aeration times increase with



Figure 6.10

265

Final grain temperatures compared to the entering air conditions for an aeration rate of 10.72 Ls·m3 (51.2 (m3/h)/tonne) used for determining times for the leading and trailing edges to exit the grain. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

consolidation or packing due to vibration and shrinkage (making the grain mass denser and decreasing the interkernel air space). Shedd (ASAE, 1993) suggests an increase of 50% in resistance to airflow based on grain consolidation or packing. Grain in storage facilities (concrete silos, steel bins, or flat storages) near railroad tracks with frequent train traffic is reported to pack more, due to earth vibrations, than grain stored several hundred feet from an active railroad (Epperly and Noyes, 1988). Final grain temperatures in each test were compared to the aeration inlet air dry-bulb (Tdbc) and wet-bulb (Twbc) temperatures. Tdbe is the aeration exit air dry-bulb, and Twbe is the wet-bulb temperature. Four representative examples are shown in Figures 6.10 through 6.13 for four airflow rates in research conducted by Epperly (1989). Final grain temperatures, Tfinal, closely followed inlet aeration air dry-bulb temperatures. For higher airflow rates, the final grain temperature was slightly less than the inlet air dry-bulb temperature, but final grain temperature was greater than inlet air dry-bulb temperatures for the lower airflow rates. In Figures 6.10 through 6.13 below, Tdbc is the aeration inlet air dry-bulb, Twbc is the wet-bulb, Tdbe is the aeration exit air dry-bulb, and Twbe is the wet-bulb temperature. X/L is the proportion of distance from air inlet with 0 at the base and 1 at the grain surface. Table 6.3 shows the difference between the final grain temperatures and inlet air dry-bulb temperatures at three different locations in the grain mass for each airflow rate. As airflow rate decreased, the temperature difference and slope of the final temperature gradient increased. The temperature difference between the final grain and inlet air dry-bulb temperatures varied from a few degrees less than the inlet air dry-bulb temperature for the highest airflow rate to about 10°C greater than the inlet air dry-bulb temperature for the lowest airflow rate. There was a nearly flat gradient when aerating at 0.64.(m3/min)/m3 and a gradient of about 6°C at an airflow rate of 0.04.(m3/min)/m3 (Epperly, 1989).


266


Figure 6.11

Final grain temperatures compared to the entering air conditions for an aeration rate of 8.04 Ls·m 3 (38.4 (m3/h)/tonne) used for determining times for the leading and trailing edges to exit the grain mass. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

Figure 6.12




Figure 6.13

267


6.1.2.6 Time Required for Cooling Dry Grain Time to cool the grain is the time needed to aerate the grain mass to reduce the temperature of the grain to the cooling capacity of the available ambient air. To determine the time required for cooling grain, studies were carried out on temperature fronts of aerated bulks experimentally (Ingram, 1976, 1979; Sorenson et al., 1967) and theoretically (Sutherland et al., 1971; 1983; Bowden et al., 1983). Most of these findings were based on computer simulations with limited data from field experiments. Epperly (1989) and Sanderson et al. (1988a) conducted experimental pilot plant studies to determine the physical changes in wheat during aeration. The primary limitations in these experimental facilities are their bin sizes. In Epperly’s (1989) work, the influence of external ambient temperatures was isolated by using insulation of a larger grain mass aerated at the same rate as the test columns and the temperature fronts in the entire grain mass to reduce the effect of radial temperature gradients. Sanderson et al. (1988a) showed that grain loaded in a small diameter (0.61 and 1.22 m) bin follows the ambient temperature more closely than grain in a larger bin that has a smaller surface area to volume ratio. Also, the particular weather conditions prevailing in the geographical location of Winnipeg, Manitoba (Latitude 49°54'N) affected grain temperatures at the center of the bins by radial heat transfer. Two approaches to determine the times for cooling grain are discussed in this section. The approach to develop a mathematical model to predict the temperature profile in the cooling zone by Epperly (1989) was reviewed previously in Sections 6.1.2.2 to 6.1.2.5. In this section we compare the approach developed by Sanderson et al. (1988a) with results presented by Epperly (1989). Sanderson et al. (1988a) defined cooling times as the duration of forced aeration required to lower the temperature in the top layer of the bins they used (3.33 m above the bin floor), from its initial temperature to where it approaches a value dictated by incoming air conditions during


268


Table 6.3

Final Grain Temperatures (Tfinal) in the Grain Mass as Related to Inlet Air Dry-Bulb Temperature (Tdbc) at Three Different Bin Locations, under the Influence of Various Airflow Rates (see also Figures 6.10 through 6.13)

Aeration Rate (m3/min)/m3

Bin Location

0.64

bottom middle top bottom middle top bottom middle top bottom middle top bottom middle top bottom middle top

0.48

0.32

0.16

0.08

0.04

Tfinal–Tdbc (°C) –1 –2 –2 –1.5 –1 3 –1 0 4 2 5 6 3 6 9 4 7 10

From Epperly, D.R. (1989). Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.

aeration. The final temperatures were higher or lower than the incoming ambient air due to temperature changes through the fan and ducting, evaporative cooling, and weather changes. Reported cooling times were approximations; it was difficult to obtain an exact cooling time due to fluctuating ambient air conditions and the exponential nature of the cooling curve. Due to wide variations in weather from one year to the next, an accurate comparison between cooling times from year to year could not be made. Also, comparing bins aerated at different airflow rates but with the same aeration start date was questionable because of diurnal fluctuations in ambient air temperatures. It was observed that doubling the aeration rate resulted in less than twice the speed of the initial cooling zone at airflow rates greater than 3 Ls·m3 (13.6 (m3/h)/tonne). Cooling times at an airflow rate of 0.85 Ls·m3 (3.8 (m3/h)/tonne) could not be compared with cooling times at the higher airflow rates because of the different weather during the increased cooling times (Sanderson et al., 1988a). A simple method for calculating the approximate time to cool a grain bulk by aeration was presented by Navarro and Calderon (1982): F = ( M × DT × c) (Q × Sw × CF × DH ) where: F = cooling time hours (h) M = mass of grain to be cooled (kg) ∆T = difference in initial and final grain temperatures (°C) c = specific heat of grain (kcal/kg/°C) Q = air volume flowing through the grain (m3/h)

(6.10)



Table 6.4

Airflow Rate Ls·m3 23.2 12.2 12.2 6.9 3.4 0.85 0.85

269

Measured and Calculated Cooling Times for Grain Bulks Subjected to Various Airflow Rates in Experimental Bins in Winnipeg, Manitoba, during 1983 and 1984

Airflow Rate (m3/h)/tonne 104.9 55.1 55.1 31.2 15.4 3.8 3.8

Grain Temperature (°C) Initial Final 33 33 33 30 30 33 30

20 21 21 17 17 15 19

Cooling Times (h)

Measured 8–10 15–17 15 30–35 40–60 260 150–170

Calculated using CF 0.5a 6.9 12.4 12.4 25 46 210 150

Calculated using CF 0.4a

Calculated using Epperly’s Equationb

8.6 15 15 31 57 267 187

11 20 20 34 64 227 227

a

Equation 6.10 (Sanderson et al., 1988a; Navarro and Calderon, 1982). Equation 6.3 (Epperly, 1989). Adapted from Sanderson D.B., Muir, W.E., and Sinha, R.N. (1988a). Intergranular air temperatures of ventilated bulks of wheat, J. Agric. Eng. Res., 40, 33–43. b

Table 6.5

Measured and Calculated Cooling Times for Grain Bulks Subjected to Selected Airflow Rates in Experimental Bins by Epperly (1989) Compared to Calculated Cooling Times Using Equation 6.10 with Correlation Factor of 0.4

L/s/m3 (m3/h)/tonnea 10.72 8.04 5.36 2.68 1.34 0.67 a b c

51.2 38.4 25.6 12.8 6.4 3.2

Average Initial Grain Temperature (°C)

Average Final Grain Temperature (°C)

35.0 38.0 35.5 37.0 35.3 35.5

20.0 19.0 19.5 17.2 17.5 18.4

Calculated Using Equation 6.3 Measured by (Epperly, 1989) Epperly (1989)b 22.4 29.1 42.2 79.5 149.8 282.3

Calculated using Equation 6.10 with CF 0.4c

20.7 32.5 38.0 90.5 146.7 273.0

20.9 30.7 38.1 86.0 146.1 273.1

Airflow rate was calculated using a mean bulk density value of 0.754 kg/m3. See Table 6.1. Equation 6.10 (Adapted from Sanderson et al., 1988a; Navarro and Calderon, 1982).

Sw = average specific weight of aeration air (kg/m3) ∆H = maximum enthalpy difference of air entering and leaving the grain bulk (kcal/kg) CF = correction factor for enthalpy term (dimensionless) The correction factor (CF) in Equation 6.10 was used as a multiplier to obtain an average enthalpy difference between air entering and leaving the grain mass. This CF is required because the leading edge of a cooling front travels faster than the trailing edge, which results in a gradual decrease in the enthalpy of the exhaust air. An aeration temperature front (heating or cooling) is defined as that zone within the grain mass where the temperature changes from an initial value to a new value, caused by aeration. A CF value of 0.5 was suggested by Navarro and Calderon (1982) based on assumptions proposed by Sorenson et al. (1967). Measured aeration air conditions were used with two CF values of 0.5 and 0.4 (Equation 6.10) to predict cooling times for the various storage conditions (Table 6.4). Cooling grain data supplied by Epperly (1989) was analyzed using Equation 6.10 (Sanderson et al., 1988a; Navarro and Calderon, 1982) for a correction factor of 0.4 as suggested by Sanderson et al. (1988a) (Table 6.5). It should be noted that in Epperly’s (1989) work, there was always a


270

Table 6.6


Measured Average Initial and Final Grain Temperatures Compared with Final Lowest Temperatures to Develop Calculated ∆T Proportion Coefficients for Use in Determining Trailing Edge Times Using Equation 6.1

L/s/m3

(m3/h)/tonnea

Average Initial Grain Temperature (°C)

10.72 8.04 5.36 2.68 1.34 0.67

51.2 38.4 25.6 12.8 6.4 3.2

35.0 38.0 35.5 37.0 35.3 35.5

Average Final Grain Temperature (°C) 20.0 19.0 19.5 17.2 17.5 18.4

Final Lowest Grain Temperature (°C) 19.5 18.5 18.0 15.0 14.5 15.5 Average =

Calculated Proportion using Equation 6.1 0.968 0.974 0.914 0.900 0.856 0.855 0.911

a Airflow rate was calculated using a mean bulk density value of 0.754 kg/m 3. Based on Epperly, D.R. (1989). Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University (Figures 6.10 through 6.13).

difference between air inlet dry-bulb temperature and grain temperatures (Table 6.3). This difference became larger as the airflow rate was reduced and the distance from the air inlet increased. The calculations using Equation 6.10 were based on average initial and final grain temperatures, and for the air cooling capacity, on the enthalpy difference of air entering and leaving the grain bulk given by Epperly (1989). Parts of the data used in these calculations are shown in Figures 6.10 through 6.13. Table 6.5 shows that the calculated cooling times using Equation 6.10 gave very close cooling time values to those measured by Epperly (1989) (Table 6.1). But when Equation 6.3 is used, the calculated cooling times of Epperly (1989) depart markedly from his measured values. The differences in the cooling times calculated using Equation 6.10 and measured values by Epperly (1989) are consistent and gave a correlation of r2 = 0.9999, statistically highly significant to justify the use of the methods considering the range of their accuracy (Table 6.5). Epperly (1989) equations give a mathematical approximation of the cooling times for wheat, but they lack accuracy when cooling times need to be calculated for different grain and ambient wet-bulb temperature conditions. Although the Sanderson et al. (1988a) approach can predict cooling times over a wide range of grain and ambient conditions, the calculations must be based on the averages of the varying ambient conditions. From this comparative analysis it appears that the mathematical models proposed by Epperly (1989) and by Sanderson et al. (1988a) can reasonably be used to estimate cooling times within the limitations described above. The model analyzed by Sanderson et al. yield more aeration hours than Epperly’s model, when calculations are based on the average grain temperatures closer to the average air inlet temperatures (Table 6.5). But for calculations of the time for the trailing edge reaching the top of the bulk (in pressure systems — leaving a gradient temperature between the different layers), Epperly’s equation results in a reduction of calculated cooling hours (compared to Sanderson et al. data). This is because not all of the available enthalpy differential is used during the cooling process. To overcome the exponential behavior of temperature vs. time curve, Epperly (1989) proposed the use of Equation 6.1 to define the cooling zone trailing edge temperature as 0.95 value of the difference between the initial and final average grain temperatures (∆T). Since Equation 6.10 is dependent on enthalpy differences, it is of interest to analyze the correction factor obtained using Epperly (1989) data. This is important in order to establish a base line that will permit a reasonable determination of the ∆T proportion under controlled experimental aeration. The data in Table 6.6 give a high calculated proportion at high airflow rates and progressively lower values as the airflow rates decrease. If an average is considered, then the values of 0.911 obtained as experimental results are lower than the set value of 0.95. This average proportion coefficient is useful in estimating the suggested cooling times in the operation of aeration systems discussed in Chapter 7.



Figure 6.14

271

Schematic psychrometric chart showing air conditions during aeration for cooling grain with an equilibrium relative humidity higher than air entering the grain bulk (point 1, air entering the grain bulk; point 2, air leaving the drying front within the grain bulk; and point 3, air leaving the grain bulk).

6.1.2.7 Changes in Grain Moisture Content during Cooling of Grain In the work reported by Epperly (1989), the relative humidity of the three lowest aeration rates (0.67, 1.34, and 2.68 Ls·m3) ranged from 67 to 72%; and the relative humidity of the three higher aeration rates (5.36, 8.04, and 10.72 Ls·m3) ranged from 50 to 54%. Using these relative humidities, the 95% confidence interval for the change in moisture content while cooling with aeration was 0.78 ± 0.37% (wb) percentage point. When comparing this to the moisture gain from rewarming, which had a 95% confidence interval of 0.79 ± 0.54% (wb) percentage point, nearly all the moisture lost while cooling was regained by rewarming. Therefore, Epperly concluded that relative humidity differences in these tests do not affect the change in moisture content during aeration. Foster (1967), Sorenson et al. (1967), McCune et al. (1963), and Sanderson et al. (1988a, 1988b), also experienced moisture losses and gains consistent with the ranges reported by Epperly (1989). For all cases where air RH is different than the interstitial grain bulk equilibrium RH, heat transfer is accompanied by moisture transfer. Although ambient air entry into the bulk is accompanied by cooling and wetting or heating and drying processes, in practice these effects have rather limited but significant effects on grain moisture content. In the model described by Sutherland et al. (1971), the cooling zones (fronts) were found to widen when grain was cooled with high-humidity air, compared to cooling with air in equilibrium with the grain humidity. In a study on the effects of aeration rates, initial moisture contents of grain, varying weather conditions, and fan control strategies on movement and shapes of moisture fronts within wheat bulks, Sanderson et al. (1988b) used eight 0.61 m bins and one 1.22 m diameter bin, all 3.66 m in height. For the aerated conditions studied, a mean drop in moisture content of 0.9 percentage points (based on 26 observations with s.d. = 0.44) occurred throughout the bins as the initial cooling front passed through the grain bulks. The potential for this moisture loss in the initial cooling front is demonstrated in Figure 6.14. Ambient air entering the aerated grain bulk at point 1 in Figure 6.14 removes moisture; and, thus, evaporative cooling occurs that removes energy from the grain. After the initial cooling zone (front) has traversed the grain bulk (trailing edge exits the grain bulk), air exits the grain bulk at temperature


272


T2 (point 2 in Figure 6.14). By continuing to aerate, the only drying that will occur is in the drying zone, shown as the difference in absolute humidities of the ventilation air between points 1 and 2 in Figure 6.14 (W2–W1). The moisture loss during cooling occurs throughout the grain bulk because it results from energy in the grain rather than energy in the air (as used in high-temperature, high-airflow grain drying). As cooling air absorbs energy from grain, its relative humidity decreases; and grain tends to equilibrate its moisture content with the air moisture content, which is at a lower level than the grain relative humidity. Moisture removal during aeration cooling is similar to (but not as dramatic as) that used in Dryeration, where in-bin tempering allows rapid cooling of heated grain to remove 1 to 3 percentage points throughout the grain mass. In Dryeration, high enthalpy from hot grain allows high airflow to exhaust at or near saturation at the high grain temperatures. Thus, evaporative cooling is optimized in the Dryeration process. Starting from the conditions in zone 1 (where the grain has reached equilibrium with the entry air) and zone 3 (where grain temperature and moisture have not changed), continued integration produces two characteristic lines, the intersection of which gives the grain condition in zone 2 as shown in Figure 6.14. The temperature front line lies practically along a constant grain moisture content line, whereas the moisture front line nearly follows a constant air enthalpy line (parallel to the wet-bulb temperature). The spread of the cooling and the drying front until the trailing edge exits the grain bulk is schematically illustrated in Figure 6.14. The drying effect illustrated in Figure 6.14 is restricted to the air entry area of the bulk at the initial cooling of grain. Further aeration has an additional drying effect on the grain bulk as demonstrated by Sanderson et al. (1988b). This aspect will be discussed separately. 6.1.2.8 Movement of Drying Fronts during Cooling of Grain Sanderson et al. (1988b) observed that moisture profiles within continuously aerated experimental bins showed the development and movement of drying fronts. The drying zone was defined as the depth of grain where the moisture content changed from the initial value to one in equilibrium with the ventilation air. The leading edge of the drying front moved faster than the trailing edge. This effect was also shown in drying tests by Bowden et al. (1983), by Anderson and Kline (1986), and theoretically by Rouvet et al. (1979). But these experimental and test results disagreed with simulations presented by Ingram (1979), who stated that, once established, the depth of a drying zone remained constant. The airflow rates applied by Sanderson et al. (1988b) varied from 3.0 to 23.2 Ls·m3 (14.3 to 111 (m3/h)/tonne), and the aeration time varied from 1272 to 96 hours. The upper end range of these tests used higher than normal airflow rates (approximately double), which are somewhat beyond the normally acceptable ranges used for aerating grain storages. However, under certain conditions where grain depths are relatively shallow, such as in flat storages or shallow steel bins, the highest rate may be economically practical. Similarly, the extended aeration hours generated by the lower than normal airflow rates are beyond the practical allowable time limits for grain aeration. Therefore, this study should be analyzed as generally designed to study the drying effect on grain rather than aeration for cooling only. Results indicated no appreciable differences in drying front depths for airflow rates of 3 to 23 Ls·m3 (average air velocities of 7.5 to 60 mm/s). Drying front depths varied from 0.7 to 4 m depending on initial moisture contents. Higher moisture contents generally resulted in deeper drying fronts, but the gradients of the fronts were similar to those experienced at other moisture contents. Barre et. al. (1971); Ingram (1976, 1979); and Sutherland et al. (1971) suggested that the depth of a drying front is determined mainly by the airflow rate. They stated that the effect of doubling the airflow rate almost doubles the drying front depth.



Figure 6.15

273

Curves taken from Schumann (1929) showing graphical solution to Equation 6.11.

The following observations were made by Sanderson et al. (1988b), comparing speeds of drying fronts as affected by airflow rates and control strategies: 1. The speed of drying fronts was not linearly proportional to airflow rate. Doubling the airflow resulted in up to a 2.5 times increase in the drying rate over the same drying period. 2. Continuous aeration with high-humidity air forced the drying front through the grain bed. The drying period was extended when the humidistat control was used, compared with drying time using continuous ventilation at an identical airflow. The speed of the drying fronts for bins with and without the humidistat control were equal. The use of humidistat control caused increased over-drying throughout the grain mass.

6.1.2.9 Grain Mass Temperature Distribution Work done by Schumann (1929) and Furnas (1930) gave theoretical solutions to the temperature history of a cold bed of broken solids heated by a hot fluid. Their solutions involved differential equations with modified Bessel functions and were presented in graphical form. Bakker-Arkema and Bickert (1966) developed a model for cooling sugar beets using this model. They developed Equation 6.11 for use in their cooling model: Φ( y, z ) = e − y

∫ e [2 yz ] dz z

o

−z

o

(6.11)

where Φ is the temperature variable related to the grain, and z is related to the position in the grain mass, X/L. Although their results were not very accurate for beet cooling, adaptations of their equations to grain cooling in this study yielded favorable results. The graphical solution for Equation 6.11 is given in Figure 6.15. The temperature distributions found in Epperly’s (1989) study (Figures 6.2 and 6.6 through 6.9) are in the same form as that in Figures 6.15 through 6.18. A temperature prediction model was then developed using Equation 6.11 and Figure 6.15. The variable y is related to the time in the aeration period, Θ. To determine the relationship between y and Θ, graphs of the temperature distributions were superimposed on Figure 6.15. The values of y were determined by an analogy between the experimental data shown in Figures 6.2 and 6.6 through 6.9 with Equation 6.11. A least squares regression analysis was used


274


Figure 6.16

Measured and calculated temperature gradients for an aeration rate of 10.72 Ls·m3. (From Epperly, D.R. [1989]. Effects of Airflow Rates on Cooling Time and Temperature Distribution in Aerated Wheat, unpublished doctoral dissertation, Oklahoma State University. With permission.)

Figure 6.17


to determine the functional relationship between y and Θ. The conversion from time in aeration, Θ, to y had to be separated into two equations related to whether the time was after or before the leading edge exited the grain mass. The aeration rate, Qa, was a factor during the time after the leading edge of the cooling front exited the grain mass. The regression equations were:



Figure 6.18

275


y = 5 ⋅ Θc − 1.1 for Θ < Θ L

(6.12)

y = (7.04 + 1.09.Qa ) ⋅ Θc + 4.32 for Θ < Θ L

(6.13)

and

where the time correction, Θc, was: Θc =

Θ ΘL

for Θ < Θ L

(6.14)

and Θc =

Θ − ΘL ΘT − Θ L

for Θ > Θ L

(6.15)

The variable z was related to the position in the grain mass, X/L; z is equal to X/L times a factor of 10. The variable Φ is related to grain temperature where: Φ=

T − To ∆T

(6.16)

The model using these equations was applied to the temperature distributions measured in the tests. Figures 6.16 through 6.18 compare the measured and predicted temperature gradients for the aeration rates used in this study. The model fits the actual data relatively well. Most of the temperature predictions are within 10% of measured grain temperatures.


276


6.1.2.10 Deep-Bed Aeration Control Strategies A simulation model for grain cooling is presented and verified against experimental data by Sun and Woods (1997). The model is applied to grain cooling during a typical season at a location in the southeast U.K. Fan control strategies are investigated, and the use of differential control based on the grain-to-air temperature difference is found to be effective. In the simulation, barley placed in storage at 16% wb moisture content (0.19 d.b.) and 30°C (86°F) in late July was cooled to 15°C (59°F) in one month using a 2°C (3.6°F) differential, whereas for malting barley it was necessary to use a 6°C (10.8°F) differential to reduce rewetting. Rewetting of the bed at the air entry region was significant in all situations simulated; for malting barley, the heat of absorption significantly affected cooling. The simulations for wheat and barley were very similar. The results indicate that cooling, even during this warm period, can achieve temperatures that greatly decelerate insect development. The simulation model was used to investigate a number of areas: 1. Control aeration by temperature measurements alone, as temperature is a very reliable and economic measurement. 2. The most difficult situation in which to cool stored grain is with an early harvest at a warm location. It is particularly important to investigate how long it will take to reach about 15°C (59°F) and to prevent insect development under these conditions. 3. Due to the high humidities associated with cool conditions in the U.K., and the impracticality of controlling aeration by relative humidity (RH) or by temperature, cooling grain inevitably involves the risk of some rewetting. Malting barley at 12% wb is particularly susceptible (Burges and Burrell, 1964). This absorption process needs to be investigated, as the wet region generated is vulnerable to mites, fungi, and loss of germinative capacity. 4. Associated with rewetting is the release of the heat of absorption. From data on the heat of absorption (Gallaher, 1951) and specific heat (Disney, 1954), the calculated adiabatic temperature rise for a one percentage point moisture addition is around 15°C (27°F). Clearly, the transfer of moisture can have a significant effect on cooling.

The mathematical model used in this study consists of a set of four partial differential equations in four independent variables (Sharp, 1982; Sun and Woods, 1997): air humidity H(t,x), air temperature T(t,x), grain temperature G (t,x), and grain moisture content M(t,x). Sun and Woods (1997) concluded that: 1. A low-temperature deep-bed model based on recently obtained thin-layer data has been developed and verified against experimental data for low-temperature drying. The model should be compared with cooling data as they become available. 2. The simulation was used to investigate the cooling of winter barley in the southeast U.K. The results showed that barley stored at 16% wb and 30°C (86°F) was cooled to 15°C (59°F) within one month and lost around one percentage point moisture content using the differential temperature control method with dT = 2°C (3.6°F). Malting barley stored at 12% wb and 25°C (77°F) was cooled to 17.6°C (63.7°F) within one month using the differential temperature control method with ∆T = 6°C (10.8°F) to reduce rewetting and the associated heat release. 3. The cooling rate was not sensitive to the position of the grain temperature sensor used to measure the differential relative to ambient temperature. 4. The fan running times were greatly reduced as the temperature differential was increased, without a loss in cooling performance. 5. The simulation for wheat was very similar to that for barley. 6. The results demonstrate that, even under difficult climatic conditions due to harvest date and location, grain can be cooled quickly enough to decelerate insect development until ambient temperatures fall, enabling more rapid cooling. 7. The simulation of long-term cooling over a six-month winter period predicts a gradual moisture content rise of the order of one percentage point. 8. Rewetting in the entry region was significant for all simulations.



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9. Recirculation of air can disperse moisture from the rewetted region, but it is a slow process. 10. The simulation model should be applied to a wider range of control strategies and climatic conditions.

6.2 REVIEW OF FIELD TRIALS WITH AERATION SYSTEMS Aeration system field trial research data has great use for direct application to solving engineering problems or for use in the future design of new aeration systems. Field data are also of vital use to engineering researchers involved in grain system modeling, such as the OPI systems (Crompton, 1998) and other computer-based aeration control and monitoring systems discussed in Chapter 7. Aeration system models are excellent tools to aid in facility design and to use in predicting possible alternative systems based on analysis of many variables that are difficult to assess. However, there is no replacement for live field research data. For models to be a useful engineering or entomological design tool, they must be validated with field tests that result in actual system performance data. Models must be correlated with real-world experience from grain storage facilities, either at government institute bulk storage facilities or at university grain storage research sites. 6.2.1

Physical Effects of Aeration

Aeration, the forced movement of ambient air through a grain bulk or other granular commodity with interstice air spaces, is conducted to preserve or maintain the physical qualities or to improve the physical condition of the product. The normal objective of aeration is to initially cool the bulk to a desired temperature level to remove harvest heat for purposes of preservation from insects or molds. Aeration is used to recool previously cooled grain to maintain temperature uniformity throughout the bulk. Aeration helps to avoid or minimize concentrated moisture accumulation through condensation on cool grain from convection air movement caused by uneven grain mass temperature distributions. In general, the physical effects of aeration are known to be beneficial as an IPM tool to control or minimize pest damage. The use of aeration will receive more attention as a principal grain management tool in the 21st century as consumers, governments, processors, and importers of grain reject the use of residual chemicals on food grains. To that point, use of aeration can help maintain the purity of grain as a healthy food or feed product. However, on a minute scale, the specific effect of the heat and mass transfer processes of aeration on the biochemical structural properties of individual kernels of grain (or on specific varieties of grain) has not been reported. Keeping wheat, maize, rice, barley, or oat kernels from exceeding specific hot or cold temperature limits, from changing temperature too rapidly and exceeding molecular limitations, or from exchanging grain moisture too rapidly may be significant in maintaining the milling or processing properties to reach maximum use of the product. Research indicates that rice and maize fracture more severely when moisture is exchanged too rapidly, not only during drying but also during rapid cooling when kernels are hot. In both cases, tempering warm kernels followed by slower cooling at specifically designed aeration airflow rates is known to dramatically improve market and processing quality. Humidity control is very important for sunflower cooling and in popcorn storage for processing. Proper aeration and storage helps minimize rancidity in oilseeds, such as peanuts (ground nuts), safflower, canola, soybeans, cottonseeds, and sunflowers by maintaining cool bulk temperatures. Chilled aeration is becoming a more important quality control tool for high-value grain commodities such as edible beans and food grains. Chilling has economic advantages over drying of some food grains. It also may relieve or eliminate molecular and biochemical stresses on the


278


product, especially if chilled storage is maintained until time for processing. In some cases, milling yields may be improved or processing parameters enhanced. Inexpensive methods of storage, including economical aeration systems, are needed in many developing and industrialized countries to minimize storage and handling losses from pests and fungi. To meet the demands of an expanding world population, continued research is needed to study the physical effects of aeration and storage conditions on grain and seed products used for human food and fiber consumption. The following sections discuss field and laboratory aeration research that has been conducted to provide some of the answers needed by grain storage engineers, scientists, technologists, and grain managers throughout the world. The research results and data on a variety of aeration-related problems are discussed by leading grain storage researchers from field sites in several countries. The following discussions provide insight to field research of aeration problems from tropical, subtropical, and temperate climatic regions. 6.2.1.1 Temperature and Moisture Fronts during Aeration Temperature fronts tend to be deep and indistinct. The depth of cooling zones depends primarily on airflow rates and temperature differentials (Muir et al., 1987). Sanderson et al. (1988a) found that cooling zone depths were usually greater than 1.75 m (5.75 ft) and often spanned the entire bed depth 3.5 m (11.5 ft). They found no noticeable differences in temperature front depths for different aeration rates in their 3-year study. Sutherland et al. (1983) and Ingram (1979) found that, when nominal grain mass air velocity is doubled, the temperature zone depth almost doubled. Burrell and Laundon (1967) observed a cooling zone of a depth of 4.3 m (14.1 ft) using an air velocity of 3 m/min in 16.8% moisture wheat, but only 3.05 m (10 ft) in higher moisture wheat. Several researchers studied grain aeration temperature fronts experimentally (Sorenson et al., 1967; McCune et al., 1963; Miller, 1965; Sanderson et al., 1988a, 1988b) and theoretically (Hunter, 1988; Sutherland et al., 1971, 1983). Most of these findings are based on computer simulations with limited field test data. Sutherland et al. (1983) found that grain moisture has little influence on the speed of a cooling front, but grain temperature plays a primary role in the speed of a cooling front. They found that the leading edge of each cooling front moves at a uniform speed regardless of grain conditions on each side of the front. McCune et al. (1963) aerated 16.2% moisture content sorghum at an aeration rate of 1.6 Ls·m3 (0.12 cfm/bu) with air at about 100% RH. They required 125 hours to lower grain temperature 25°C (77°F) (2°C [3.6°F] above cooling air temperature) and 168 hours (⅓ more hours) to cool the grain mass to the inlet air temperature, 23°C (73.4°F). Hunter (1988) used a relationship between relative humidity and saturation pressure to estimate the dwell state (the grain condition after the temperature front passage and ahead of the moisture front) from the initial seed state and the inlet state. He developed equations for approximating the front speeds and logarithmic slopes for temperature and moisture, obtaining similar results to Sutherland et al. (1983). Sanderson et al. (1988a) conducted pressure up-flow cooling field studies in 1983 and 1984 with wheat at initial moisture contents from 15 to 25% (wet basis) and aeration rates from 0.85 to 23.2 Ls·m3 (0.06 to 1.73 cfm/bu). Cooling times were defined as the time required to cool the top grain layer to a stable temperature dictated by air passing through adjacent cooled grain layers. They approximated cooling completion times due to variable ambient air conditions and the exponential nature of the cooling curve. They modified Navarro and Calderon’s (1982) method to approximate cooling time as shown in Equation 6.10. By changing the correlation factor from 0.5 to 0.4, they improved the accuracy of their observed data. However, Epperly (1989) considered the accuracy of this method was limited by the unpredictability of ambient air conditions; so calculated cooling times can vary considerably.



279

Several factors are thought to change the time required to cool grain and to determine the final temperature of the grain mass. Although aeration rate and grain moisture content are the two primary factors affecting cooling time, grain type and bin fill conditions are also very important modifying factors. While initial moisture content affects cooling time, initial and final grain temperatures do not seem to affect cooling time (Moysey, 1969; Burrell and Laundon, 1967; and Navarro et al., 1973). When grain at initial temperatures of 30°C (86°F) and 15°C (59°F) was cooled simultaneously, the time required to cool all grain to near aeration air temperature was approximately the same (Holman, 1960). Boyce (1966), Person et al. (1966), Sorenson et al. (1967), and Sutherland et al. (1971) found that cooled grain did not reach cooling air dry-bulb temperature; but high-moisture grain almost reaches the wet-bulb temperature of the entering air. Person et al. (1966) added that, within certain limits of dry-bulb air temperature and grain moisture, grain temperature can be controlled entirely by the specific humidity of the conditioned air entering the grain mass. Temperature drop of the grain mass is not affected by cooling time. Pabis and Henderson (1962) discovered that within a short time from cooling air entry, the temperature of a grain kernel approached the air-stream temperature. Burrell and Laundon (1967) found that heat exchange between individual kernels of grain and the air passing through it at 2.7 to 3.0 m/min (8.8 to 9.8 ft/min) is almost instantaneous. 6.2.1.2 Changes in Moisture Content of Grain during Aeration Investigators report an inevitable initial drop in moisture content of the grain during cooling regardless of the entering air relative humidity (Sorenson et al., 1967; McCune et al., 1963; Foster, 1967; and Sanderson et al., 1988a, 1988b). The amount of moisture reduction usually ranges between 0.2 and 1.0 percentage point in moisture content, depending on grain and aeration air properties. Burrell and Laundon (1967) reported an average fall of about 0.5 percentage point in the moisture content of grain per 24 hours of refrigeration aeration chilling. They found that when evaporative moisture losses increase, total cooling time is reduced. They stated that a moisture reduction of 0.5 percentage point will produce one third of the total heat loss. Kline and Converse (1961) found it took 160 hours to cool wheat to 8.3°C (47°F), with a moisture content reduction of 0.3 percentage point in summer, compared to 310 hours with a 0.1 percentage point moisture loss in winter, at the same relative airflow rate of approximately 3 (m3/h)/tonne or 2.4 m3/m3.h (0.05 cfm/bu). Metzger and Muir (1983) found airflow rates of 0.5 to 3.0 Ls·m3 (0.037 to 0.22 cfm/bu) resulted in moisture content reductions of 0.5 to 0.7 percentage points. Moysey (1969) calculated that for most cases, the cooling process will produce a grain moisture content reduction of 0.25 to 0.75 percentage point. Armitage (1980), Armitage and Stables (1984), and Armitage and Llewellin (1987) conducted various tests in the British climate and reported a 0.2 to 0.5 percentage point drop in mc during cooling. Sanderson et al. (1988b) observed a mean drop in moisture content of 0.9 percentage point as the cooling front passed through a grain mass. Figure 6.14 demonstrates the potential for moisture loss in the cooling front. Aeration air entering at condition 1 removes energy and moisture from the grain as it travels through the grain mass, and the leading edge of the cooling front exits in equilibrium with conditions of the initial grain mass (point 3 in Figure 6.14). Sorenson et al. (1967) and Sanderson et al. (1988b) stated that the variables affecting the magnitude of the drop in moisture content during cooling include initial grain temperature and moisture content as well as wet-bulb temperature of the aeration air. Foster (1967) stated that the moisture change during aeration is proportional to the temperature change. He further stated that the heat used to evaporate moisture accounts for about half of the cooling and the time required for cooling the grain is reduced proportionately.


280


Moysey (1969) and Sanderson et al. (1988a) determined that another factor affecting total cooling time is the loss of efficiency during the final stages of cooling. Burrell and Laundon (1967) found that, as the cooling zone moves out of the grain, the rate of heat removal and moisture loss drops; so cooling time increases as an exponential curve. Foster (1967) cooled and then warmed small bins of 11 to 13.5% initial moisture wheat. In three tests, wheat moisture content was reduced 0.44 to 0.52 percentage point when cooled from 27 to 10°C (80.6 to 50°F). When warmed from 10 to 27°C (50 to 80.6°F) by aeration, its moisture content was increased 0.50 to 0.67 percentage point. Moisture changes were calculated from absolute humidity differential of entering and exhaust air. Epperly (1989) conducted a large-scale laboratory study of the effect of cooling and warming hard red winter wheat using six aeration airflow rates ranging from 0.04 to 0.64 (m3/min)/m3 (.05 to 0.8 cfm/bu) in a 1.8 m (6 ft) diameter by 3.0 m (9.8 ft) high corrugated steel bin. He observed an average moisture loss during aeration cooling ranging from 0.66 to 0.89 percentage point, with an overall average of 0.78 percentage point (Table 6.1). During aeration warming, he obtained an average moisture gain of 0.79 percentage point, about the same as for cooling moisture losses. 6.2.1.3 Temperature Distribution during Cooling at Different Airflow Rates Studies at Oklahoma State University (Cuperus et al., 1986) and in Australia (Evans, 1983) indicate that effective use of aeration systems to rapidly lower grain mass temperature during fall months not only produces an environment that discourages infestations but can be fatal to most insects. They believe that aeration becomes effective in reducing existing stored grain insect populations when the temperature drop in the grain mass exceeds a threshold level in either amount or rate of temperature drop. Based on 30-year averages, cooling hour accumulations below 12.8, 15.6, and 18.3°C from September to November were estimated for the state of Georgia in the U.S. The temperature accumulations indicate that aeration could be used to cool corn stored in Georgia. Airflow rates of 0.234 (m3/min)/m3 may be necessary to shorten the time required to complete an aeration cycle to take advantage of short-duration cold fronts (Arthur and Johnson, 1995). Temperatures recommended for fall cooling of wheat stored in the midwestern U.S. are below 12.8 and 15.6°C (55 and 60°F). Recorded data from weather stations in 11 southern states were used to determine the optimum initial activation temperatures for immediate cooling of corn that is stored after harvest. A bin-cooling model was integrated with a model for maize weevil development to predict weevil population growth in unaerated corn and corn aerated at 12.3, 15.6, and 18.3°C with airflow rates of 0.078, 0.156 and 0.234 (0.1, 0.2, and 0.3 cfm/bu). Aeration at 15.6 or 18.3°C, depending on the geographic zone at an airflow rate of 0.078 (m3/min)/m3 resulted in the lowest number of maize weevils. In all zones, aeration dramatically reduced the predicted number of maize weevils compared to population levels in unaerated corn (Arthur et al., 1998). Cold weather frontal systems reaching the U.S. southern plains in October and November typically produce temperatures below 10°C (50°F) during the nighttime. These cold weather systems usually last two to four days, with 40 to 80 hours of available aeration time per frontal system. Typical commercial aeration at 0.08 (m3/min)/m3 (0.1 cfm/bu) requires 125 to 150 hours or more to cool the grain. Epperly et al. (1989) felt that, with the use of aeration rates of 0.15 to 0.20 (m3/min)/m3 (0.18 to 0.25 cfm/bu) or higher, grain could be cooled during one 2- or 3-day span of cold weather. They felt that cooling times using these higher airflow rates could be more accurately predicted. 6.2.1.4 Grain Temperatures during Extended Aerated Storage A 30-month wheat storage field study was conducted from wheat harvest during the summer of 1987 to the winter of 1989 by OSU researchers on a farm in central Oklahoma (Epperly et al.,



Figure 6.19a

281

North–south bin cross-section — six temperature profiles from an Oklahoma steel farm storage bin holding 53.5 tonnes of wheat including two aeration periods from September through mid December, 1987. Note that the grain surface was leveled to obtain homogenous temperature distribution.

1989). A 5.5 m (18 ft) diameter round corrugated steel grain bin, equipped with a pressure aeration system, was filled with 71.3 m3 (2023 bu) of winter wheat to a depth of about 3.0 m (9.8 ft). The aeration system airflow rate was approximately 7 to 8 (m3/h)/tonne (0.12 to 0.13 cfm/bu). A set of 29 thermocouple cables was installed in a symmetrical pattern in the grain mass to monitor grain temperatures. Grain temperatures were monitored weekly throughout the 30-month storage period. Twelve graphic plots in Figures 6.19a and 6.19b show samples of grain temperature data of a north–south cross-section of the grain mass during the first and second years of the research from September, 1987, through the spring of 1988. During September, 1987, most of the grain temperatures ranged from 32 to 43°C (90 to 110°F) with some cooler temperatures near the bin walls — especially around the north wall, which received no direct sunlight. The automatic aeration system was started September 24, 1987. By October 1, grain temperatures ranged from 16°C (60°F) near the floor to 21°C (70°F) near the surface. On October 22, when the first stage of aeration was complete, grain temperatures were mostly in the 10 to 16°C (50 to 60°F) range, with 10°C (50°F) grain temperatures near the floor and around the north wall.


282


Figure 6.19b North–south bin cross-section — six temperature profiles from an Oklahoma steel farm storage bin holding 53.5 tonnes of wheat including two aeration periods from December, 1987, through late October, 1988. Note that the grain surface was leveled to obtain homogenous temperature distribution.

Figure 6.19a illustrates six graphic views of the north–south temperature profile of the grain mass at 6 times from early September through early December, 1987. This was before, during, and after the initial aeration and about the midpoint of the second aeration cycle. The scale along the bottom of each figure is the distance from the north wall; so the left side of each figure is the north wall. In November, 1987, most of the grain stayed in the 10 to 16°C (50 to 60°F) range. The bin was aerated a second time starting in early December; and grain temperatures cooled to 4 to 10°C (40 to 50°F) by December 17 (Figure 6.19b, first graphic cross-section profile), when the cooling cycle was completed and with cooler temperatures along the floor and walls. During the winter, the grain mass gradually warmed along the north–south cross-section of the bin and was between 4 and 10°C (40 and 50°F) on March 11, 1988. The grain warmed along the south sidewall, top surface, and through the perforated floor. By May 5, most of the grain had warmed to the 10 to 21°C (50 to 70°F) range, with warmer grain temperatures around the edges of the bin. The grain continued to warm throughout the summer (Figure 6.19b, July 1, 1988); and by October 13, the center of the mass was primarily about 38 to 40°C (100 to 105°F). Across the bin profile, temperatures generally ranged from 21 to 27°C to above 41°C (70 to 80°F to above



283

100°F), with a grain layer down the north wall of 16 to 21°C (60 to 70 F). Aeration was started October 14; and by October 20, 1988, the lower and outer portions of the grain mass had cooled to 15 to 25°C (60 to 75°F). Rewarming during the spring and summer, 1988, was accelerated due to the shallow depth and full perforated drying floor area (compared to much larger storage tanks with narrow aeration ducts). Convection air movement in this 3 m (9.8 ft) deep, 5.5 m (18 ft) diameter bin pulled warm air through the wheat, accelerating rewarming (Epperly and Noyes, 1988). The average grain temperature during the study at major compass points and the center of the mass is shown in Figure 6.20a. This provides a graphic look at temperature change and correlates with grain moisture samples shown in Figure 6.20b. Figure 6.20b shows the average moisture profile of the bin at various depths during this storage period. The wheat was harvested in the 12.0 to 12.8% range; but by November, 1988, after the third aeration cycle, the grain moisture had dropped to 9.2 to 10.4%. As the grain warmed during the winter of 1988 to 1989, the grain absorbed moisture from convection air movement; and by March, 1989, the moisture ranged from about 10.3 to 10.8% mc. The moisture drop during the aeration cycle in October, 1988, was the most dramatic. This was probably due to excessive aeration fan operation until early November, 1988. Although this was a small grain mass compared to most commercial storage facilities, it illustrates bin wall boundary effects due to ambient air temperatures and solar heat gain through southeastern to southwestern exposure wall sections. A high percentage of wheat in larger bins remain cool longer in the spring and summer because of the thermal insulation properties of the grain mass. Sealing of fan and auger openings in storages can help to minimize cold air drainage and convection currents warming the grain. Concrete silo walls also act as temperature buffer zones, or insulators, that help keep the outer layers of the grain bulk cooler in warm weather. Wind blowing across roof vents and eave openings create negative pressures and induce convection air movement through base openings, warming cool grain in the center of the mass. Therefore, sealing blowers, augers, and other base openings to minimize gravity convection air movement up through the structure, especially in bins with full drying floors, are especially important when trying to maintain cool grain temperatures. With a large base opening, dense cool air will drain from the bin and pull in warm light air through roof eaves and vents. Typical grain temperatures obtainable from large-scale commercial grain storages were reported under the subtropical climate of Israel (Ben-Ami and Dayagi, 1967). Temperatures obtained in two sizes of aerated bins of 1180 tonnes and of 2350 tonnes of wheat stored in metal bins of 10 m high were recorded. The longest storage period recorded for the 2350 tonnes of locally grown wheat was 22.5 months (Ben-Ami and Dayagi, 1967). The initial grain temperatures of 32°C and 10 to 13% moisture content were recorded immediately after harvest in May. Aeration was carried out at an average airflow rate of 8.5 (m3/h)/tonne. Aeration of 113 hours during summer months and early autumn until the end of September reduced the grain temperature to 28°C. The most effective aeration started in October; and using an average of 105 h/month of fan operation, the average wheat temperature was reduced from 28 to 15°C at the end of December. An additional 50 hours of fan operation was necessary to maintain the same temperature of 15°C until the end of April. During the summer months there was a progressive increase in temperature up to 20°C in August and 24°C in October. Aeration of 145 hours of fan operation was necessary to reduce the temperature from 24 to 12°C in January. The same wheat was stored for an additional two months until the end of March (total of 22.5 months) with excellent germination capacity and without insect infestation. Similar results were obtained for the bin containing 1180 tonnes of wheat stored for 18 months (Ben-Ami and Dayagi, 1975). Imported hard red winter wheat of 12% moisture content was received


284

THE MECHANICS AND PHYSICS OF MODERN GRAIN AERATION MANAGEMENT 120 110

Temperature (°F)

100

North

90

South 80

East

70

West Center

60 50 40 30

1-Mar-1989

1-Dec-1988

1-Sep-1988

1-Jun-1988

1-Mar-1988

1-Dec-1987

1-Jun-1987

1-Sep-1987

20

Date Figure 6.20a

Grain temperature profile for small Oklahoma farm bin showing average grain temperature change from the averaged data, about 0.5 m from walls on major compass points and the bin center, during storage from September 1, 1987, through March 1, 1989.

Moisture content (%)

13

12

11 North South East West Center

10

9

1-Mar-1989

1-Dec-1988

1-Sep-1988

1-Jun-1988

1-Mar-1988

1-Dec-1987

1-Sep-1987

1-Jun-1987

8

Date Figure 6.20b Grain moisture profile for small Oklahoma farm bin showing average grain moisture change, about 0.5 m from walls on major compass points and the bin center, during storage from September 1, 1987, through March 1, 1989.

at 30°C in September. Aeration of 117 hours reduced the wheat temperature to an average of 14°C in January. A progressive temperature increase up to 24°C was noted from April to September. A second cycle of aeration for 130 hours cooled the grain during the second winter of storage to 11°C at the end of January. This wheat was stored without the need to use chemical treatments for an additional two months until it was unloaded.



Figure 6.21

285

Effect of aeration on temperature drop of 4500 tonnes of wheat in a horizontal storage (8 m high) in relation to hours of aeration and ambient conditions. (From Navarro, S. and Bulbul, 0. [1970]. Observations on forced aeration of stored wheat, Israel Minist. Agric. Dep. Plant Prot. Prog. Rep. 1969/70, Stored Prod. Res. Lab., 85–92 [Hebrew, with English summary].)

6.2.1.5 Grain Temperatures Obtained with Ambient Cooling In various tests in the U.K. (Armitage, 1980; Armitage and Stables, 1984; Armitage and Llewellin, 1987; Armitage et al., 1991; and Armitage et al., 1994), it was possible to reduce temperatures to below 5°C and hold them there for 6 months using between 350 and over 1000 hours of aeration, depending on the fan control used. Figure 6.21 shows the reduction in grain temperature that was obtained in a grain bulk compared to the distribution of aeration hours during the cooling season and the seasonal availability of temperatures in different ranges (Navarro and Bulbul, 1970). It is clear that aeration (blowing air) during summer had a slight cooling effect on the bottom layers of the wheat bulk. However, the high temperatures in the center were forced upward to the top layers. Aeration at an airflow rate of 2.5 (m3/h)/tonne (0.04 cfm/bu) starting in mid November gradually reduced wheat temperatures to a range of l2 to l5°C (53.6 to 59°F) throughout the bulk during a period of several weeks, while operating the fan 742 hours. From an analysis of the ambient temperature data, most of these hours were carried out at temperatures in the range 5 to 15°C (41 to 59°F). Comparing these data with theoretical values,


286

Figure 6.22


Effect of aeration on the cooling of 1142 tonnes of wheat. (From Navarro et al. [1969]. Observations on prolonged grain storage with forced aeration in Israel, J. Stored Prod. Res., 5, 73–81.)

whereas the required theoretical aeration time was 427 hours, the observed fan operation time was extended to nearly 1.7 times the expected time. This extension of hours of operation can be partially explained by the fact that the thermostat that controlled the aeration system was not efficiently managed. The thermostat was set on 20°C (68°F) from 14 November until 19 December; and then it was adjusted to 15°C (59°F), requiring additional hours of fan operation. In another trial (Figure 6.22), 1142 tonnes of wheat were stored in a metal bin equipped with an aeration system in which the fan was operated by suction air at a flow rate of 7.6 (m3/h)/tonne (Navarro et al., 1969). During this trial, the fan was operated manually at night. Aeration for 708 hours from September to February reduced grain temperature from a range of 27 to 32°C (80.6 to 89.6°F) to 10 to 14°C (50 to 57.2°F). Wheat could not be aerated during the summer months because of high ambient temperatures. This resulted in a grain temperature increase to an average 19°C (66.2°F) during the summer. However, 532 hours of aeration during the second winter reduced grain temperature to 11 to 15°C (51.8 to 59°F). In this trial, extended aeration hours were also required to obtain the desired temperatures because an automatic aeration controller was not used. Trials conducted in Australia indicate that efficient aeration can be achieved using the timeproportioning controller developed by CSIRO (Elder, 1972). The time-proportioning controller functions in a manner very similar to the time switch. It operates the fan for a preset number of hours per day or per week, but fan operation occurs during the periods of lowest ambient temperatures sensed by a thermostat. The set point of the thermostat is varied by means of a clock-type electric motor. The set point rises at a rate of 1.7ºC per week when the fan is not in operation, and falls at a rate of 2.8ºC per week when the fan is in operation. In a trial to test the efficiency of the CSIRO controller compared with a thermostat, the number of aeration hours was reduced by 38% (Navarro et al., 1978) (Figure 6.23). To reduce the initial average grain temperature (960 tonnes of wheat) from 28 to 19°C (82.4 to 66.2°F), the fans were operated for 145 hours using the CSIRO



Figure 6.23

287

Average grain temperature, temperature setting of the thermostat, setting of the CSIRO timeproportioning aeration controller and cumulative hours of aeration for two similar bins containing 960 and 980 tonnes of wheat. (From Navarro et al. [1978]. Comparison of two techniques for controlling grain aeration systems, Israel Agric. Res. Org. Prog. Rep. 1977/78, Stored Prod. Div. Special Pub. No. 117, 81–91 [Hebrew, with English summary].)

time-proportioning controller. To obtain a similar reduction in temperature in another bin (980 tonnes of wheat), the fans were operated 236 hours using a thermostat. This upflow aeration trial, with an average airflow rate of 6.4 (m3/h)/tonne (0.1 cfm/bu), shows the significant reduction in aeration time that can be achieved by using adequate aeration control equipment. Other experience gained in Australia on grain aeration has shown similar levels of temperature reduction (Sutherland, 1968). In a bulk of 2500 tonnes of wheat, the average grain temperature was reduced from 32 to 9°C (89.6 to 48.2°F) in about 1100 hours of aeration. The fans were controlled by wet-bulb temperature control with a relative humidity limit set to 75%. In another trial (Sutherland,


288


1968), the relative humidity limit was not employed; and more effective setting limits resulted in a reduction in fan operation hours needed. For trials conducted under similar conditions during the 1964 and 1965 seasons, initial high summer harvest temperatures recorded in February were reduced by the end of May to below 10°C (50°F) by operating the fans for 800 and 700 hours, respectively. In these trials the average airflow rate was 6.2 (m3/h)/tonne (0.1 cfm/bu) (Sutherland, 1968). The results of these Australian trials indicate that aeration is effective in cooling grain in subtropical climates that have a cool season. Cooled grain temperatures depend on available ambient temperatures. However, the number of hours per day having ambient temperatures suitable for aeration plays an important role in the aeration schedule. As a general rule, average grain temperatures should be around 10°C (18°F) above the minimum ambient air temperature. In practice the average ambient wet-bulb temperature should be used as a measure for predicting the cooling capacity of the air. 6.2.1.6 Prevention of Moisture Migration A primary objective of aeration is to prevent moisture migration. Grain temperatures of unaerated bulks gradually cool around the outside of the mass, creating a temperature differential from the outside to the center when stored for long periods of time. Therefore, aeration to equalize grain temperature with that of the ambient is essential to prevent the formation of temperature gradients in the grain bulk. Aeration, starting during late summer and continuing into the fall, reduces grain temperatures from 35 to 40°C to 20 to 25°C (95 to 104°F to 68 to 77°F) or below. In steel bins or silos, under-roof condensation may occur during the process of moisture migration. Moisture that collects under the roof often drips on the grain, wetting the grain surface and extending the process of mold and crusting of grain on the surface. To minimize or prevent roof condensation, additional ambient air must be circulated periodically or continually through the head-space. This may be done by the use of additional roof vents or by the installation of exhaust fans on the roof. Although 25°C (77°F) is considered favorable for insect reproduction, uniform grain temperatures of 20°C (68°F) or below will inhibit insect reproduction and minimize temperature gradients that begin to develop as ambient temperatures drop in autumn. Aeration of grain by stages, as described in the previous section, should be performed during autumn, and then in winter, to reduce the risk of moisture migration. However, using multiple aerations for progressive or stepped temperature reduction can greatly increase the aeration hours and the amount of grain moisture removed, compared to cooling directly to the desired temperature in minimum cooling time. The formation of temperature gradients and consequently moisture migration is a well-known phenomenon in unaerated grain bulks — especially in corrugated steel bins, where the high thermal conductivity of metals causes rapid heat transfer from grain and moisture migration problems — compared to thicker and less conductive walls of concrete silos. To predict the extent of the damage, ambient minimum daily temperatures should be examined carefully. Then, using a psychrometric chart (Figure A.1, Appendix A) and the equilibrium moisture content of the grain (Figure A.4 and Table 5.1), the conditions under which moisture condensation is expected can be estimated. For example, for wheat grain stored at 25°C and 11% moisture content, the intergranular equilibrium relative humidity is approximately 55% (Figure A.4 and Table 5.1). When average ambient temperatures drop below 24°C, the saturation temperature is reached (Figure A.1). Cooling of metal roofs below 24°C results in moisture condensation on the under-roof surface, since the head-space of a bin may hold approximately the same moisture as the intergranular air space of the grain bulk — especially with inadequate roof venting or in a climate with little wind. With recommended levels of roof ventilation (Chapter 8, Section 8.8), designing for aeration exhaust velocities of 300 to 400 m/min in temperate regions, head-space relative humidities will be more like ambient air than interstitial grain humidities. This is especially true in regions with



Figure 6.24

289

Temperature records (same times for both bins) of inner side of metal roof (A), surface grain at 20 cm (B), and 100 cm deep (C) in identical bins, each containing 2800 tonnes of wheat, one nonaerated and one aerated. (From Carmi, Y. and Navarro, S. [1967]. Observations on water condensation in grain silos caused by temperature differences between the grain and the external temperature, Israel Minist. Agric. Dep. Plant Prot. Prog. Rep. 1966/67; Stored Prod. Res. Lab. Rep. No. 6 [Hebrew, with English summary].)

strong prevailing winds because of the ambient air movement through roof-eave junctions and vents, which cause periodic air changes. As a result of the low ambient temperature, the most rapidly cooled sections of the bulk are those at the periphery, such as the roof and the walls of the bin. Therefore, the first place to expect moisture condensation is the roof because of the tendency of warm, humid, less dense air to move vertically through the grain mass into the bin head-space by convection. Later, as the grain surface cools by a further drop in ambient temperature, moisture condensation will take place on the grain surface also. This is a typical phenomenon for unaerated bins holding warm grain during cool seasons. In Figure 6.24, records of two identical bins (one non-aerated, the other aerated) demonstrate the temperature differences between the wheat and the metal roof of the bin affected by ambient conditions in December (winter). In the non-aerated bin, the wheat moisture content at the surface layer rose from 11.7 to 15.9% in approximately one month of storage, and water condensation was apparent on the underside of the metal roof. In the aerated bin (with upward aeration), wheat moisture content at the surface layer rose from 11.3 to 12.5% (Carmi and Navarro, 1967). Although no condensation on the bin roof was apparent in the aerated bin, the increase (from 11.3 to 12.5%) in moisture content of wheat could be attributed to the delayed aeration performed during December. In this case ambient air blown through the deep warm layers of the bulk reached the naturally cooled surface layer. As a result, a slight moisture deposit was observed on the surface layer. A well-established method to prevent moisture migration is to equalize grain temperature periodically as the ambient temperature drops. In subtropical climates where condensation is especially acute in metal silos, aerating once in autumn and continuing aeration intermittently during winter is a recommended procedure. The temperature gradients obtained before aeration, and the progressive equalization of temperatures throughout 1100 tonnes wheat under the subtropical climate of Israel, are shown in Figure 6.25 (Calderon, 1972). Although this procedure calls for more aeration hours than is necessary for initial cooling of the grain, it solves the problem of moisture migration. In general, the savings through reduced grain spoilage offset the additional aeration costs. To maintain grain condition during a seasonal cooling change in temperature, aeration should be used to prevent moisture migration. For the previously mentioned example of grain at 35°C and


290

Figure 6.25


Temperature distribution measured through the cross-section of 1100-tonnes wheat bulk, showing the even cooling pattern produced by downward aeration from a single longitudinal duct. (From Calderon, M. [1972]. Aeration of grain — benefits and limitations, EPPO Bull., 6, 83–94.)

11% moisture content, aeration should be started as soon as ambient temperatures begin to fall (generally within 4 to 8 weeks of harvest) to minimize condensation problems. The process of aeration itself, which may take 6 to 8 weeks to complete 150 to 200 hours of cooling at 6 (m3/h)/tonne, keeps convection air currents from becoming established. Thus, moisture migration is prevented during the aeration period. Some grain managers and scientists believe that moisture migration can occur when air is warm and grain is cold. The classic and most documented process of moisture migration is when outside temperatures are low and the grain mass is warm. The process of “reverse moisture migration” is not common and has not been well documented. Although convection currents can occur with cold air settling to the bottom of a bin, for condensation to occur at the bottom of the grain mass, the floor and grain temperatures adjacent to the floor should be much colder than the grain bulk. However, since the floor and the grain at the bottom of the storage are likely to be at about the same temperature, reverse moisture migration is not possible. Regarding the penetration of warm air from the top of storage structures with a metal roof, moisture deposition at the top of the grain mass is not a common process. In this case cold intergranular air tends to settle, and warmer air entering from the surroundings of the storage structure tends to rise. These contradicting currents are apparently not sufficient to create moisture migration to the top layers of the grain mass. The only instance where moisture may be added to the grain mass is when aeration with warm air is applied through a pressure system into cold grain. Even if this is done accidentally, the effect of aerating cold grain in warm weather would only result in a restricted moistening of the grain around the aeration duct. If the system is operated by suction, this limited moistening effect may occur at the top layer. 6.2.1.7 Aeration Time Required Under constant uniform conditions, 600 to 800 volumes of air are required to cool one volume of grain (Jouin, 1965; McCune et al., 1963; Poichotte, 1977). However, this generality does not take into consideration non-uniform air distribution within the bulk — the state of different biological factors affecting grain temperature and fluctuations in the cooling capacity of the ambient air. Analysis of results obtained by Epperly (1989) reveal that for a wide range of airflow rates tested (0.04 to 0.64 (m3/min)/m3 (0.05 to 0.8 cfm/bu)), the total volume of air required was between 660 and 864 m3 air to cool one m3 of grain. The lower airflows required smaller total volumes of air to cool the grain mass. The emperical results obtained by Epperly (1988) are 8 to 10% higher than those quoted earlier for other researchers.



Figure 6.26

291

Calculated and experimental total aeration hours to cool wheat grain given by different authors. The Armitage et al. (1991) curve was calculated by using 1000 volumes to cool one volume of grain; the Epperly (1989) curve was plotted using Equation 6.3; the Navarro and Calderon (1982) curve was plotted using Equation 6.10 (with a correction factor of 0.4 and ∆T multiplier of 0.911 (Table 6.6)) to cool wheat temperature from 36 to 19°C at 68% relative humidity; the Miller (1965) curve was plotted using Equation 6.7; and the Sanderson et al. (1988a) curve was plotted using experimental results and extrapolated for airflow rates lower than 3.8 (m3/h)/tonne. (From Armitage et al. [1991]. The cost and effectiveness of aeration in the British climate, Proc. 5th Int. Working Conf. on Stored Prod. Prot., Bordeaux, September 1990 [F. Fleurat-Lessard and P. Ducom, Eds.] III, 1925–1933; Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agric. Serv. Bull. No. 52, Rome; Miller, J.A. [1965]. Effects of aeration time for various airflow rates on properties of effluent air from grain aerated in storage, unpublished M.S. thesis, Texas A & M University, College Station, TX; Sanderson et al. [1988a]. Intergranular air temperatures of ventilated bulks of wheat, J. Agric. Eng. Res., 40, 33–43.)

Burrell and Laundon (1967) concluded the minimum airflow required to cool a volume of grain was 1000 volumes of air per one volume of grain. Based on this air-to-grain volume ratio, Armitage et al. (1991) calculated the number of aeration hours for five commonly employed airflow rates. These cooling times were plotted on a graph (Figure 6.26) to compare with the calculated and experimental cooling times given by other workers (Epperly, 1989; Navarro and Calderon, 1982; Miller, 1965; and Sanderson et al., 1988a). Although these researchers used different approaches that resulted in different aeration times to cool grain, the nature of the curves shown in Figure 6.26 remains exponential. However, the presentation in Figure 6.26 shows the difficulty of selecting cooling times using a single equation. From Figure 6.26 it appears that two extreme ranges were obtained using data of Armitage et al. (1991) for the upper extreme levels and that of Sanderson et al. (1988a) and Miller (1965) for the lower extreme levels. Note that results given by Epperly (1989) and Sanderson et al. (1988a) describe pilot experiments, whereas data of Armitage et al. (1991) and Navarro and Calderon (1982) reflect field experiments resulting in extended aeration hours. These data will be compared and discussed in Chapter 7.


292


Suitable daily aeration hours for cooling in subtropical climates are usually rather limited. However, as mentioned in previous sections, aeration is performed not only to cool grain but also to equalize grain temperature progressively with decreases in ambient air temperature. All these factors increase the theoretical number of aeration hours needed to cool the grain. In practice, many more aeration hours may be needed than those given in theoretical calculations to obtain desired grain temperatures. Also, the aeration regime and the monitoring methods chosen by the storage operator affect total aeration time. The time required to cool the grain will vary with the airflow rate, cleanliness and compaction of the grain, depth, duct and vent designs, the cooling capacity of the ambient air, and the number of available aeration hours at the selected air temperatures. Grain can be cooled from a very warm condition to a very cold condition during one aeration cycle if suitably cold ambient conditions exist for long enough time periods. From trials described in the previous section, if airflow rates are low, it sometimes takes several months before the grain is cooled to the desired temperatures. Grain stored immediately after harvest is hot from summer sun and weather. In subtropical climates, low ambient aeration temperatures are not available immediately after harvest; high grain temperatures must be reduced by stages. The practical ranges that can be adopted are reduction by about 5°C stages to about 20°C, then to 15°C, then possibly to 10°C. From typical ambient temperature ranges given in Figure 6.21, an average of 13 hours per day were available below 20°C (68°F) in autumn (October, northern hemisphere); about 9 hours per day below 15°C (59°F) for November; and about 10 hours per day below 10°C (50°F) during December/January in low-altitude subtropical regions. Therefore, assuming that the pre-set temperatures could be obtained after 250 hours of aeration at an airflow rate of 3 (m3/h)/tonne (0.05 cfm/bu), the expected temperature drop could be obtained after 20 to 25 days aeration during the period October to December/January. In practice, from one year to the next, cooling may require even more time because the desired cooling air temperatures may not be available due to seasonal weather variations. Although the theoretical amount of aeration hours needed to cool the grain are much less, actual fan operation hours could be in the range of 500 to 600 hours over a period of 2 to 3 months at an average airflow rate of 6 (m3/h)/tonne (0.1 cfm/bu)(Sutherland, 1968; Navarro et al., 1969). Noyes et al. (1992) reported that extensive hours of aeration at 0.1 cfm/bu (6 (m3/h)/tonne) were used because of aeration mismanagement by the elevator grain managers. During a field study, one operator recorded about 1400 hours of aeration time including summer, fall, and winter aeration in 1988 to 1989 on a 250,000 bu. (7000 tonne) wheat bin. The next year, after he cleaned his aeration controller and avoided summer aeration, aeration totaled about 400 hours during two cycles in early fall and midwinter. 6.2.1.8 Energy Consumption If fan energy consumption and aeration hours are known, the energy required for cooling grain can be estimated. In the present section, the energy consumed will be expressed in kW.h required to cool each tonne of grain. Fan energy consumption may be estimated from Figures 5.17 through 5.26 in Chapter 5. Armitage et al. (1991) used a similar approach and estimated static pressure requirements using the relations developed by Matthies and Petersen (1974). By using Equation 5.17 for a fan efficiency ratio of 66%, Armitage et al. (1991) calculated the power requirement for 3 m and 10 m deep bulks. They also calculated the hours of aeration required to pass three cooling fronts, achieving first 15, then 10, and finally 5°C through grain. This was compared to the time required to achieve these temperatures in six 20-tonne farm-scale bins and also to the hours of aeration and fan power used in 17 commercial (ca. 1000-tonne) stores. The calculations were based on meteorological data from two widely separated locations in the U.K. and included 20-year averages and extreme years. Hours spent below 15, 10, and 5°C in the



293

immediately post-harvest months were acquired; and thus it was possible to estimate how many days it would take until a cooling front passed through the grain. The conservative estimates showed that, based on 1000 volumes of air required to cool one volume of grain, 83 to 417 hours were required for each cooling front for airflows between 17 and 3.4 (m3/h)/tonne (10-2 cfm/tonne). However, aerating the 20-tonne bins with a 0.2 kW fan at 9.9 to 13.3 (m3/h)/tonne required maximum running hours of only 60, 140, and 190 hours using a differential thermostat set at 4°C or 90, 420, and 730 hours using a 2°C differential. The commercial stores used 120 to 463 hours aeration using fan powers between 1.1. and 3.8 W/tonne. Multiplying the estimated or actual W/tonne by the hours of aeration and assuming the fans to be only 66% efficient yielded energies of 0.10 to 0.22 kW.h/tonne for the tested airflow ranges and a grain depth of 3 m, typical of farm storage. For 10 m deep beds, it was calculated as 0.24 to 1.05 kW.h/tonne, more typical of commercial storage. In the farm test, cumulative kW.h/tonne of 0.09, 0.21, and 0.28 were required to achieve the three cooling fronts of 15, 10, and 5°C using a 4°C differential; the corresponding values using a 2°C differential were 0.13, 0.63, and 1.10 kW.h/tonne. The commercial stores used 0.19 to 0.57 kW.h/tonne. Armitage et al. (1991) included biological factors in their calculations to predict the effects of cooling on insects. They assumed that initially, the warmest grain would be 30 to 35°C, optimal for the three commonest British grain beetles: Sitophilus granarius, Oryzaephilus surinamensis, and Cryptolestes ferrugineus. While the first cooling front progressed, the insects would reproduce rapidly at an optimum rate; so calculations were based on the maximum infestation in the slowest cooling areas. By knowing the days available for rapid breeding at optimum temperatures until the cooling front reached these regions, it was possible to calculate if the insects can complete their development and how many progeny would be produced. The predicted results of the calculated cooling times for the 15°C front were that no development of S. granarius could occur at airflow rate of 6.8 (m3/h)/tonne. However, calculated predictions suggested that O. surinamensis and C. ferrugineus might be able to develop at 6.8 (m3/h)/tonne, when aeration was started in July or August. However, in the farm test, using six 20-tonne bins aerated at 10 (m3/h)/tonne when the three insects were present at a combined infestation of 2.5/kg, and with initial temperatures close to 30°C in September, no live insects were detected in traps after February; and none were found in samples taken during outloading of the bins. The energy per tonne required to pass each cooling cycle through the grain was estimated by multiplying aeration hours required for each of the five airflow rates, for two grain depths (3 m for farms and 10 m for commercial storage depths), and by W/tonne (Table 6.7). Costs per tonne could then be calculated using the local power cost per kW.h. In trials carried out in Victoria, Australia (Sutherland, 1968), the measured energy consumption for cooling 2500 tonnes of wheat bulks over a 3-year period varied from 2.6 to 3.8 kW.h/tonne. In another trial carried out in Israel, the effective cooling of 1142 tonnes of wheat from 27 to 32°C to 10 to 14°C was obtained at an energy input of 2.8 kW.h/tonne (Navarro et al., 1969). In the same trial the energy requirement to reduce the bulk temperature from 19°C to 11–14°C was only 2.0 kW.h/tonne. From Figures 5.17 to 5.26 (Chapter 5) and Figure 6.26, the theoretical energy requirements can be estimated for cooling wheat grain to 15°C in vertical or horizontal storage. In these calculations the line recommended by Armitage et al. (1991) was used, based on 1000 volumes of air to cool one volume of grain. This line, compared to other authors, results in longer fan operation hours to cool grain. At 6 (m3/h)/tonne, this results in 235 hours of aeration. Using the higher value, the power consumption estimates would be based on maximum values, which provide conservative aeration time values for peaked grain. Calculated values for cooling wheat stored in 10 m high bins and 25 m high bins at an airflow rate of 6 (m3/h)/tonne are 0.68 kW.h/tonne and 3.38 kW.h/tonne, respectively. However, in practice, based on 10 years of trials carried out on different structures and with different operation procedures, the average values for cooling commodities such as wheat, soybeans, and maize,


294

Table 6.7


Calculated Hours of Aeration and Fan Power Required, Dates to Achieve 15 and 10°C, and Number of Insects Developing from a Single Ovipositing Female for a Range of Airflows. Data Assumes Aeration Starts in July on the Warmest Year from 20-Year Averages in the U.K. 3.4

cfm/tonne h Aeration W/tonne 3m 10 m Date 15°C Achieved Date 10°C Achieved O. surinamensis S. granarius

2 417 0.24 0.57 18 August 20 November 1319 46

6.8 4 208 0.59 2 24 July 11 October 49 12

Airflow — (m3/h)/tonne 10.2 13.6 6 139 1.07 4.73 16 July 29 September

8 104 1.68 8.19 12 July 21 September

17 10 83 2.68 12.74 09 July 21 September

3

From Armitage et al. (1991). The cost and effectiveness of aeration in the British climate, Proc. 5th Int. Working Conf. on Stored Prod. Prot., Bordeaux, September 1990 (F. Fleurat-Lessard and P. Ducom, Eds.) III, 1925–1933.

are estimated — for horizontal bins, 1.8 kW.h/tonne; and for vertical bins, 3.2 kW.h/tonne (Navarro and Calderon, 1982). These values are higher than those given in Table 6.7 because the fan operation times for cooling grain were based on values obtained in a subtropical climate, probably with low fan efficiencies and using differential thermostats that increased operation efficiency compared to the fan operation times given by Armitage et al. (1991) in Figure 6.26. However, the energy requirement for horizontal storages with 10 m depth estimated by Navarro and Calderon (1982) (1.8 kW.h/tonne) is close to the value estimated by Armitage et al. (1991) (2 kW.h/tonne) for storages with 10 m depth. To compare the costs of aeration with those of commonly used fumigants for bulk treatment, current prices based on dosage and energy requirement (variable operating costs) are given in Table 6.8. In this table the capital investment in equipment and cost of labor involved in the application of each treatment (for fumigation or aeration) was not considered. Since local practices and prices vary from one country to another, values given in Table 6.8 should be used as general guidelines. However, this comparison demonstrates that aeration can be competitive with fumigation when the objective is preservation of grain from insect damage. If the objective is complete mortality of insects, fumigants are still better than aeration. In many situations the cooling effect of aeration suppresses their development effectively to control insects, but a complete kill is not always achieved. An added bonus of aeration is the prevention of moisture migration, an objective that cannot be achieved using fumigation. Experimental data on fan operation hours to equalize grain temperatures is lacking. Table 6.6 shows calculated values of ∆T proportion coefficients based on the measured average initial and final grain temperatures obtained from experimental data (Epperly, 1989). As the ∆T coefficient approaches unity, the final grain temperatures also approach uniformity. It would appear that, even under controlled conditions, the aeration hours needed to achieve uniform temperatures are slightly longer than those recommended by Epperly (1989) using Equation 6.3, as indicated by the slope of the bottom lines in Figures 6.8 and 6.9. Figure 6.25 shows that, in a field trial, about 28% more aeration time was needed for equalizing the temperatures after half of the bulk was cooled to below 15°C and the lower half to below 20°C. Therefore, to compensate for time needed to equalize grain, temperature values given in Table 6.8 were already increased by 30%. From field trials shown in Figures 6.19, 6.20, 6.21, and 6.22, it is evident that early aeration to remove high temperatures from immediately harvested grain may increase the fan operation hours. It is stressed, however, that these energy requirements can be reduced by employing efficiently designed aeration systems and by effective selection of desirable ambient air using suitable control-monitoring equipment.



Table 6.8

295

A Cost Comparison between Aeration and Fumigation to Control Insects in Stored Grain; Values Based on Energy Requirements for Aeration and Fumigant Required for Adequate Control of Insects; Capital Investment of Equipment and Cost of Work Not Included Funigation Methyl Bromide Phosphine

Dosage or Energy Requirement

62 g/tonne

3 g/tonne

Cost U.S.$/Tonne

0.27–0.41b

0.15–0.18

Aeration Horizontal Storagea Vertical Storagea kW.h/tonne 0.83–1.56 0.07–0.012c

kW.h/tonne 2.22–4.39 0.18–0.35c

a

Power requirement in horizontal storage was calculated for a depth of 10 m, and for vertical storages for a depth in of 25 m of wheat. Airflow rates for subtropical climate (Table 7.10) were increased by 30% to compensate for equalizing temperatures. b Cost of fumigant only (methyl bromide or phosphine) in the U.S. market, excluding preparations needed for fumigation like silo sealing and operator costs. c Cost of power only based on an average of U.S.$ 0.08/kW.h. Adapted from Navarro, S. and Calderon, M. (1982). Aeration of grain in subtropical climates, FAO Agric. Serv. Bull. No. 52, Rome.

6.2.2

Biological Effects of Aeration

Cooling grain by ambient or refrigerated aeration is gradually replacing residual insecticides in grain as the universally preferred strategy for grain storage. This is partly because the costs of ambient aeration are cheaper than using insecticides (costs of refrigeration are comparable), because aeration is regarded as environmentally preferable, and because it is a more versatile tool than chemical application. Aeration can even out temperature gradients and prevent condensation of water on the surface of the grain, which can eliminate sprouting and fungal growth. In properly designed systems, aeration can cool the grain fast enough to prevent insect development. Where insects are already present, grain can be often cooled to a temperature that may cause them to dehydrate and die within a normal storage season. Cooling can also prolong germination, preserve baking, and other biochemical qualities of grain for specific markets. Many fungi and mites can still grow at temperatures close to freezing; but with sufficient moisture, cooling can delay their development and provide benefits compared with uncooled storage. The relative humidity in equilibrium with a given grain moisture content varies with temperature. As the temperature falls, the relative humidity in equilibrium with a given moisture content of grain also drops. For instance, at 25°C, Maris Huntsman English wheat desorbing moisture is in equilibrium with 70% RH at 15.6% mc (ISO method); but for 15.6% mc wheat at 5°C, the equilibrium RH is between 60 and 65%. Maintaining cool grain temperatures has serious implications for the control of mites and molds, which are largely dependent on adequate moisture for their growth. Also, this helps to explain why some northern hemisphere grain, stored at temperatures and moistures that are marginally safe at moderate temperatures, often spoils rapidly when it heats up on arrival at tropical ports. The suitability of regional climates for ambient air cooling of grain dictates the potential use of aeration technology for grain protection. In tropical climates, ambient aeration may only be useful for evening out temperature gradients and preventing moisture translocation and associated hot spots if temperatures below 20°C cannot be achieved after harvest. Alternatively, integrated pest management strategies — such as using intermittent fumigations until the arrival of cooler weather — may have to be employed. If grain can be quickly cooled below 20°C, the growth of most pests may be avoided. However, grain weevils can still be a threat down to 10°C. In temperate northern climates, grain may be cooled to temperatures near and below freezing; so cooling can help prevent mite increase and kill residual insect populations during the course of a normal storage season.


296


6.2.2.1 Effects on Suppressing Insect Development Burges and Burrell (1964) concluded that ambient cooling grain was successful in reducing grain temperatures to 17 to 22°C in the British climate and in preventing insects from invading and developing in the grain storage. Burrell (1967) reported on the effect of chilling on infested grain bulks. The development of an infestation was prevented in 23.2% moisture barley after chilling it from 17 to 18°C to 3 to 4°C. Although the high moisture content caused molding at the top of the pile, about 97% of the insects disappeared from the grain. Some insects survived for 6 months in the cold grain. It was suggested that the stimulation for the insects to leave the grain was not only due to the low temperatures but also due to the air movement. From trials with cold-air aeration, Burrell (1967) concluded that cooling was sufficient to inactivate and possibly kill insects when 140 tonnes of barley were cooled from 35°C to below 15°C. He warned, however, that cold grain was more difficult to fumigate since methyl bromide vaporized less readily. Also, insects showed more resistance to fumigants at lower temperatures. He recommended to fumigate first and then cool the grain. Insect activity in Minnesota was reported as severe despite cool temperatures during much of the year (Subramanyam and Harein, 1989). A survey of maize facilities in 1977 showed that 90% were infested with several adult insect species. In 1983, 43% of wheat samples, 83% of maize samples, and 60% of oat samples collected from farm storages were infested with adult insects. Samples taken from barley storages in 1985 and 1986 showed on average 85 to 100% infestation. The barley temperatures in July and August ranged from 15.6 to 35°C, and moisture contents from 11 to 20%. During the months of June, July, and August, temperatures of non-aerated grain stored under Oklahoma conditions reached 35 to 38°C (Cuperus et al., 1986). During September, October, and November, grain temperatures ranged from 24 to 30°C. Those were optimal levels for stored grain insect activity and allowed populations to reach high levels. Although aeration with ambient air was shown feasible and effective as a grain management tool, the grain temperatures were not reduced to less than 15°C before late October to early November. Cuperus et al. (1986) noted high mortality of insects when the grain temperatures dropped below 15°C for extended periods of time. Epperly et al. (1987) concluded that reducing the temperatures of grain below 10 to 13°C during the early fall in Oklahoma grain storages produced an unfavorable environment for stored-grain insects. By maintaining low grain temperatures through the spring and summer, stored grain insect activity was significantly reduced. Chemical insect control was not needed in the bins with low grain temperatures. Metal silos with perforated floors were filled with 35 tonnes of barley at 28.1°C and 13.3% moisture content in France (Lasseran and Fleurat-Lessard, 1990). In five aeration cycles with cold night air, the temperature of the bulk was reduced to –1.2°C between August and the following January. Due to metabolic heating, the grain temperature rose to 13.8°C by the following June. At the beginning of the tests, six boxes with adult insects, larvae, and eggs were placed into the grain. At the end of the storage period, more insects were found in the core of the bulk than near the wall and toward the top of the pile rather than the bottom. They observed that only 1% of the insects survived the cold treatment, and they claimed cold treatment was superior to chemical treatment. It was concluded that cold-air aeration was effective in disinfecting grain and that temperatures below 5°C over a period of at least 2.5 months can kill insects. Customers have grown increasingly reluctant to accept commodities that contain chemical residues (Longstaff, 1988). However, not only customers should be concerned. There is increasing pressure on the grain industry throughout the world to reduce its dependence on chemicals in the preservation of bulk commodities. “Persons engaged in the bulk handling and storage of agricultural foodstuffs (e.g., grains, peanuts) for human and animal consumption may incur occupational exposure associated with cancer risk…” noted Alavanja et al. (1990). The primary methods of pest



297

control are the application of insecticides to the grain as a bin is filled and by fumigating the sealed storage structure. Several investigations have shown a beneficial effect by combining the application of chemical insecticides with lower grain temperatures, which can be achieved with grain chillers independent of ambient conditions. In discussing the use of aeration to suppress insects, two important aspects of insect population dynamics are the temperatures that influence insect metabolic activity and the temperatures that affect rate of insect population growth. Suppression of insect activity will be discussed using these two parameters. After Sutherland (1968) cooled 2700 tonnes of heavily infested wheat to 8.3°C (47°F), less than one live insect per 45 kg of grain was found during sieving. His research showed that with good aeration management, aeration could reduce insect activity, moisture migration, and mold formation, eliminating the need for insecticides. Aeration cooling to manage stored grain insect populations was investigated in Australia (Sutherland et al., 1971; Ghaly, 1984), in England (Burges and Burrell, 1964; Burrell, 1967; Armitage and Stables, 1984), in Israel (Navarro et al., 1969, 1973; Donahaye et al., 1974), and in the U.S. (Johnson, 1957; Cuperus et al., 1986). Burges and Burrell (1964) found that cooling 6500 tonnes of malting barley to 20°C (68°F) greatly reduced insect risk, and 17°C is safe if cumulative daily insect heat production is negligible. Navarro et al. (1969) cooled 1142 tonnes of wheat from 26.8 to 32.2°C (80 to 90°F) to 10.2 to 13.8°C (50 to 57°F). They reported that most grain-infesting insect species were dead after 22 months of storage. Armitage and Stables (1984) quickly cooled two 30-tonne bins of infested wheat to below 5°C (20°F) and compared results with two non-aerated bins. The number of live insects were much lower in the aerated than the non-aerated bins, showing the value of cooling grain quickly. In a 3-year study on aerated and non-aerated farm grain bins, Cuperus et al. (1986) found that insect numbers in aerated bins were significantly lower than in non-aerated bins. Insect mortality occurred when grain temperatures dropped below 15°C for extended times. They found that in grain cooled to –7°C (20°F) in February, the mean grain temperature was 2°C (36°F) in April, 13°C (55°F) in July, and 18°C (64°F) in October without additional aeration. Grain samples in April contained no live insects, and aeration provided continued residual pest control into the warm weather season. Aeration cooling of grain as an insect control measure was investigated by several researchers. Cuperus et al. (1986, 1990, 1993) investigated the use of aeration for the prevention of insect population growth in stored grain in temperate climates. In their trial studies the objectives were to cool grain rapidly (within 4 to 6 weeks) to 2 to 5°C (36 to 41°F) and to maintain such a temperature long enough to control development of the rice weevil (Sitophilus oryzae), lesser grain borer (Rhyzopertha dominica), rusty grain beetle (Cryptolestes ferrugineus), and red flour beetle (Tribolium castaneum) populations. Another technique in the utilization of aeration cooling is to lower the temperature rapidly enough to prevent insect acclimatization, thus slowing insect population growth. Desmarchelier et al. (1979) examined the influence of chilling fumigated grain in Australia. Populations of insects that were fumigated but not cooled, recovered to detection levels after just 10 days of grain storage, while those subjected to cooling after fumigation did not become detectable for 3 weeks. Fields (1990) demonstrated that if cooling is carried out rapidly, during the fall when insects are not previously exposed to cool temperatures, all adult rusty grain beetles (Cryptolestes ferrugineus) die. But if cooling occurs during midwinter, there is a 60% survival rate. Thus, the rapid cooling of warm grain to prevent insect acclimatization to cold appears to be an effective pest management tool. Although freezing of grain by aeration is not a recommended practice, DeJean (1992) reported the killing of insects in grain by aerating with air at –18°C (–0.4°F) in northern Minnesota.


298

Table 6.9


Grain Condition and Insect Infestation after 22 Months of Storage, in a Bulk of Wheat Cooled by Aeration with Ambient Air

Sampling Area Surface layer 0.2 to 5.0 m depth 5.0 to 9.5 m depth Aeration duct

Mean Number of Free Insects per kg Grain and Percent of Survival (Shown in Parentheses) Wheat Moisture Content (%) T.g.a R.d. S.o. T.sp. O.sp. C.c. C.sp Ac. 12.3 12.0 10.5 12.4

3(0) 3(5) 15(3) 25(0)

1(0) 14(0) 2(0) 7(0) 2(0) 0(0) 0(0) 0(0)

3(0) 1(0) 0(0) 2(0)

2(0) 0(0) 0(0) 1(0)

14(0) 0(0) 0(0) 1(0)

1(0) 0(0) 0(0) 0(0)

1(100) 0(0) 0(0) 50(100)

InsectDamaged Grain (%) 16.3 0.4 0.5 2.8

a

T.g. = Trogoderma granarium; R.d. = Rhyzopertha dominica; S.o. = Sitophilus oryzae; T.sp. = Tribolium sp.; O.sp. = Oryzaephilus sp.; C.c. = Cadra cautella; C.sp. = Cryptolestes sp.; and Ac. = Acaridae. From Navarro et al. (1969). Observations on prolonged grain storage with forced aeration in Israel, J. Stored Prod. Res., 5, 73–81.

Hagstrum and Flinn (1990) simulated the effects of grain moisture and temperature on insect populations in grain aerated over selected target dates. These projections closely fit the results of experiments in wheat storages of the U.S. southern high plains by Cuperus et al. (1990). In tests carried out by Arthur (1994) with 2.5-tonnes capacity bins in Savannah, Georgia, an aeration rate of 0.08 cfm/bu (4.8 (m3/h)/tonne) was used to cool corn to 11.5 to 15.5ºC compared with natural cooling to 15.3 to 18.7ºC in unaerated bins. The average numbers of trapped maize weevils, red flour beetles, Indian meal moths, and almond moths were significantly greater in unaerated bins than in aerated bins. Heat liberated by the metabolism of insects developing in grain stored in subtropical climates contributes to the already high temperatures of harvested grain taken into storage (Sinha, 1974). High post-harvest temperatures of the grain enable initial insect infestations to rapidly build up populations that necessitate prompt control measures. Temperatures for wheat immediately after harvest are usually in the range of 30 to 35°C (86 to 95°F), which is optimal for development of many of the storage insect species. Furthermore, the grain moisture content does not present a limitation to the development of most of these insects. Therefore, aeration is advocated as soon as night temperatures begin to decrease and effective aeration hours become available for cooling the grain bulks, since cooler grain is much safer than warm grain from insect damage (Table 1.1). In the following section, a review of field trials is given on the use of aeration to limit insect development with special reference to subtropical climates. Several authors have given 17°C (63°F) as the lower threshold limit at which the development rate of most injurious stored-product insects is significantly inhibited (Burges and Burrell, 1964; Sinha and Utida, 1967). In grain bulks with initially low or negligible insect infestations, reduction of grain temperature to below 17°C (63°F) results in creating unfavorable conditions for insect development; and the buildup of insect populations can be prevented. Once the grain temperature is lowered, the majority of the bulk is effectively protected for long periods without the need for chemical treatments. A trial conducted with 1142 tonnes (252,000 bu) of wheat in flat storage structures exemplifies this situation (Navarro et al., 1969). Samples taken at the start of the trial showed that a light insect infestation was present. However, wheat temperatures that were reduced progressively from 27 to 32°C (81 to 90°F) to 10 to 14°C (50 to 57°F) using aeration during the first winter (Figure. 6.22) suppressed insect development. During the second winter of storage, aeration cooled the grain bulk to an average of 11 to 14°C (52 to 57°F). Data on the insects found at the end of the trial, after 22 months of storage, showed that most of the insects were dead (Table 6.9). Williams (1973) reported on aeration of farm silos in Victoria, Australia, containing 44 tonnes (1670 bu) of wheat. After 18 months of storage, wheat unloaded from an aerated silo was in good condition. Williams concluded that insect development could be controlled in that region of Australia using aeration. The above two trials serve to strengthen the deduction that, by changing the



Figure 6.27

299

Progressive increase in insect populations (dead and alive) in initially infested non-aerated stored wheat from time of harvest. (From Navarro, S. and Donahaye, E. [1975]. Conservation of wheat grain in butyl rubber/EPDM containers during three storage seasons, Israel Agric. Res. Org. Prog. Rep. 1974/75, Stored Prod. Div., 69–89 [Hebrew, with English summary]. With permission.)

microclimate of the grain bulk by uniform cooling, aeration is effective in arresting the development of stored-grain insects common to subtropical climates. The introduction of aeration systems in the late 1960s into all grain storage structures in Israel resulted in a significant reduction in the need to fumigate wheat to control storage insects. From a survey carried out on the national grain reserve, the percentage of wheat requiring fumigation was reduced progressively from 25% during 1964/1965 (March to April) to 3% during 1970/1971 (Calderon, 1972; Navarro, 1976). 6.2.2.2 Effects on Heavily Infested Grain Bulks The time period from when grain is loaded into bins until it has been cooled below 17°C (63°F) is often long enough to allow considerable development of insect populations. The intensity of infestation depends mainly on the level of sanitary measures taken in the grain storage environment to clean up residual infestations, the initial infestation in the harvested grain, and its temperature and moisture content. Under most conditions it is difficult to maintain a storage site completely free from infestation. Therefore, it is possible and even probable that insect populations start to develop as soon as grain is stored after harvest. Figure 6.27 illustrates the number of insects found in storage trials of unaerated wheat (Navarro and Donahaye, 1975). Under such conditions, the number of live insects per kg of grain rapidly increases to a level that necessitates prompt treatment. Even though aeration lowers the temperatures of grain bulks containing a heavy initial infestation, if the overall temperature drop is insufficient, a subsequent temperature rise may occur due to insect metabolism. The heating effect in insufficiently cooled wheat due to insect activity is clearly illustrated in Figure 6.28 (Gonen and Kashanchi, 1977). Although the grain temperature of 21°C (69°F) — required to delay development of the lesser grain borer (Rhyzopertha dominica), the dominant insect species encountered in this trial — was achieved. The insects were neither immobilized nor killed. As shown in Figure 6.28, there was an apparent upward insect migration toward the temperature and grain sampling locations in the upper part of the grain mass at depths of from 2 to 7 m (6.5 to 23 ft) in the bulk. Although the bulk was cooled to 17 to 20°C (63 to


300

Figure 6.28


Changes in grain temperature in a bulk of wheat affected by naturally developing hot spots due to insect activity (Rhyzopertha dominica, dominant species). (From Gonen, M., and Kashanchi, Y. [1977]. Changes in temperature, composition and dispersion of an insect population in a naturally occurring hot spot deep in a wheat bulk, Israel Agric. Res. Org. Prog. Rep. 1976/77, Stored Prod. Div. Publication No. 105, 87–93 [Hebrew, with English summary].)

68°F), accumulation of metabolic heat and the consequent increase in grain temperature was not prevented. Therefore, if a heavy insect infestation is present in the grain bulk to be cooled, the temperature should be lowered to below 17°C (63°F) (Burges and Burrell, 1964). Temperatures as low as 5°C (41°F) are effective in causing mortality of certain insect species after relatively short exposure periods. However, since these temperatures are seldom available in subtropical climates, the objective of grain cooling by aeration should be to prevent insect infestation development in the grain bulk rather than to cause mortality of an existing population (Burrell, 1967). When Burrell (1967) cooled an infested bin in December (and maximum temperatures fell) from a maximum of 35 to 16°C, O. surinamensis numbers dropped from about 290 live to 180/kg for an efficacy of about 41%. Armitage and Burrell (1978) cooled two infested 40-tonne bins. In one bin, temperatures fell from a maximum 22°C to below 10°C while insect numbers per kg fell from a mean of 165 to 73. In the second bin, the temperature fell from a maximum of 30°C to below 15°C, while O.surinamensis fell from about 305 to 145/kg. The possible fate of the missing insects is discussed below under insect movement. It is not sufficient to cool infested grain only once because the remaining insects are likely to cause reheating. Burrell (1967) mentioned that O.surinamensis infested grain cooled from 37 to 15°C containing 125 to 450 insects per kg heated surrounding grain by 3°C in February when aeration was discontinued. In December, grain in the same bin location warmed from 24 to 37°C, when aeration was discontinued. An area of one bin containing 675 S.granarius per kg heated from 11 to 13°C in January to 18 to 21°C in less than a month. Clearly, further measures need to be taken after cooling heavily infested grain. Armitage and Llewellin (1987) showed that O.surinamensis and S. granarius could both be killed at the center of bins cooled to 5°C but that they were likely to survive at the grain surface, where grain temperatures may fluctuate (Figure 6.29).



Figure 6.29

301

Survival of S. granarius (top) and O. surinamensis (below) in a 20-tonne bin aerated at an airflow rate of 10 (m3/h)/tonne with manual cooling (1) and a second bin cooled with a differential thermostat (2). (Control held at insects’ sub-breeding temperature). Note: Moisture content = ca 15%; Temperatures: 1 = mainly above 6°C, 2 = mainly below 4°C. Arrows show the second and fourth sampling dates (From Armitage, D.M. and Llewellin, B.E. [1987]. The survival of Oryzaephilus surinamensis [L.] [Coleoptera: Silvanidae] and Sitophilus granarius [L.] [Coleoptera: Curculinidae] in aerated bins of wheat during British winters, Bull. Entomol. Res., 77, 457–466.)

From observations carried out in aerated bins, Navarro et al. (1980b) concluded that when grain temperatures were reduced to 18°C (64°F) before infestation became apparent, insect penetration and development in the bulk was negligible. However, for grain stored in a heavily infested storage installation, insect penetration into the freshly loaded grain bulk at a suitable temperature for insect development was significant. This infestation necessitated fumigation of the grain to control the insects before the aeration system was operated. Therefore, to prevent insect development in the grain, aeration should be preceded and followed by a rigorous sanitation program around the storage structures involving thorough cleaning and the application of surface sprays with contact insecticides. 6.2.2.3 Insect Movement Affected by Cooling Fronts When insects are present when grain is cooled, some insect species are likely to move in response to changes in temperature and/or humidity caused by the aeration. Armitage and Stables (1984) and Armitage et al. (1983) suggested that O. surinamensis moved from cooled grain, but


302

Figure 6.30


Comparison of S.granarius caught in pitfall traps (n = 16) in surface treated (lower line) and untreated (upper line) aerated bins of wheat (n = 3). (From Armitage et al. [1994]. Integrated pest management in stored grain: combining surface insecticide treatments with aeration, J. Stored Prod. Res., 30, 303–319.)

S. granarius was unlikely to do so. Armitage et al. (1994) showed that O. surinamensis migrated to the surface during cooling and that S. granarius were likely to survive and breed at the surface. Thus, a generation of insects might emerge during the coldest part of the succeeding year. Elder and Ghally (1983) considered the surface of aerated bins to be the most likely place for insects to breed in Australia because it was the warmest. Mathlein (1961) recorded large numbers of O. surinamensis at the surface of an aerated bin in March at 5 to 10°C. Therefore, the surface of aerated bins are likely to require surface pesticide treatment (Armitage et al., 1994; Nickson et al., 1994) (Figures 6.30 and 6.31). Although insects are often blown out of heavily infested bins by aeration fans (Armitage and Burrell, 1978), the velocities required to lift them from the grain can only occur very close to the ducts (Armitage, 1981) which might occur if they are fleeing from a cooling front. Thus, this situation is most likely to occur in suction cooled bins. A frequently encountered situation in aerating heavily infested bulks in upward airflow systems is the dispersal of storage insects with the air leaving the exhaust ports or vents. On two occasions the spread of infestation was investigated by Navarro (personal information, 1996). On these two occasions, aerated grain silos equipped with pressure systems assisted in the dispersal of storage insects. It appeared that primarily adult insects tended to migrate to the hot, humid top section of the grain bulk. Once they reached the exhaust port located on the roof, they were lifted by the exhaust air and spread randomly by wind. Sticky traps were installed at various distances from the silo to identify the distances insects were carried by wind. The occurrence of O. surinamensis adult population, which does not fly, together with other flying storage insects (like T. castaneum and R. dominica), supported the view that insect movement from the infested bin is by wind. Insect populations encountered at a distance of 200 to 300 m from the aerated silo bins (containing cereal grain) were mainly O. surinamensis, T. castaneum, and R. dominica. Adults of these three insect species were blown out of the silo and were responsible for infesting an adjacent food processing plant. Although adults of O. surinamensis do not fly, after being blown from the infested silos and landing at the proximity of the food processing plants, they apparently crawled



Figure 6.31

303

Comparison of O.surinamensis numbers caught in pitfall traps (n = 16) in surface treated (lower line) and untreated (upper line) aerated bins of wheat (n = 3). (From Armitage et al. [1994]. Integrated pest management in stored grain: combining surface insecticide treatments with aeration, J. Stored Prod. Res., 30, 303–319.)

into the packaged processed cereals. Although adult R. dominica can fly, their numbers were not greater than O. surinamensis and T. castaneum. In one of these cases, the food processing plant was moved to a location safe from infestation. Research observations have been recorded on the composition and direction of dispersion of insect populations in wheat bulks under the influence of aeration (Navarro et al., 1980b). A preference for “comfortable” temperatures for insect development was pronounced, with insects accumulating in regions of optimum temperatures available. Although the insects in these trials were not marked to identify the movement of specific individuals, their presence in different locations of the bulk was determined by regular sampling at fixed locations. In bins aerated by blowing air upward, the insect population tended to accumulate in the upper layers of the bulk; whereas in bins aerated by a downward flow, insects tended to migrate to the lower layers of the bulk. However, this movement was found to be peculiar to fast-moving insects such as the red flour beetle (Tribolium castaneum) and saw-toothed grain beetle (Oryzaephilus surinamensis). In contrast, large numbers of the lesser grain borer (Rhyzopertha dominica), a slow-moving insect, congregated in a localized region causing the temperature to increase from 23 to 28°C (73 to 82°F) in 4 weeks. The characteristic dispersion of insects found in an initially infested bin was different from that of insects that penetrated grain bulks from the surroundings. Figure 6.32 illustrates the dispersion of red flour beetle adults (T. castaneum) throughout a wheat bulk, while lesser grain borer (R. dominica) adults found at a certain level tended to accumulate there, generating a hot spot. In this trial, chilled aeration was used to reduce the wheat temperature during summer. Although the temperature drop prevented further insect development, insects remained alive. In similar trials where an initial infestation was not significant, aeration alone was sufficient to prevent further insect development (Navarro et al., 1980b). 6.2.2.4 Effects on Mite Populations In the 1960s it was thought that the storage life of damp barley at 16 to 22% mc could be extended by refrigeration, but Burrell and Laundon (1967) found 7000 mites per kg after only


304

Figure 6.32


Changes in size of live Tribolium castaneum and Rhyzopertha dominica populations found in wheat samples taken from different depths of a grain bulk (883 tonnes) aerated with refrigerated air during summer in relation to average grain temperature recorded. Size of shaded areas in the circles indicates the number of insects found per kg. Arrows indicate grain sampling date. (From Navarro, et al. [1980b]. Dispersion of insect populations in stored grain bulks, Israel Agric. Res. Org. Prog. Rep. 1979/80. Stored Prod. Div. Special Publication No. 181, 127–157. Bet Dagan [Hebrew, with English summary].)

2 months in barley at 19 to 20% mc held at 11°C. In a field sampling survey, Burrell and Havers (1970) found mites levels up to 27,000/kg in 10 farms with aerated grain at up to 20.7% mc. Eight out of 19 bins they observed had mean mite infestations in excess of 1000/kg. These data indicate the folly of trying to store damp grain, even using aeration. Burrell and Havers (1976) found in 2 consecutive years that numbers of all mites were lower in an aerated bin holding grain at 16% mc, than in an unaerated bin, with one exception. This was with a bin infested by Lepidoglyphus, which was more numerous in the aerated bin during the first year, possibly because of a 70-fold greater presence in the unaerated bins of the predatory mite, Cheyletus. It can be concluded that Cheyletus in unaerated bins reduced the total mite population,



Figure 6.33

305

Effect of cooling on populations of three mite genera in 10-tonne bins of rapeseed aerated at an airflow rate of 10 (m3/h)/tonne. Mean minimum cooled (aerated) rapeseed winter temperature was 2°C and mean minimum uncooled (unaerated) winter temperature was 7°C. (From Armitage, D.M. [1980]. The effect of aeration on the development of mite populations in rapeseed, J. Stored Prod. Res., 16, 93–102.)

but aerated bins had insufficient Cheyletus population to provide predatory control. Therefore, Lepidoglyphus numbers remained relatively high. Hurlock et al. (1980) used airflow rates 3 to 6 times those normally recommended for aeration and increased the mc of grain in aerated bins by about 1%. Despite this, aeration was responsible for lower mite populations during the winter; but when summer came, their numbers rose to equal or even higher than those under aerated conditions. Armitage (1980) achieved reductions in mite populations during aeration of rapeseed. At 8% mc (63 to 70% RH), Lepidoglyphus mites did not exceed 1/kg in aerated bins until the temperature rose above 5°C and were usually lower by a factor of 10 compared to unaerated bins. At 9% mc (over 70% RH), Acarus mites did not exceed 1000/kg in two aerated bins until March, when unaerated numbers were about 5000 to 35,000/kg (Figure 6.33). In grain with moisture content in equilibrium above 70% RH, there is a difference between the distribution of mites in aerated and unaerated bins (Armitage, 1984). In damp grain, conditions are suitable for mite development throughout the bulk, providing the grain is warm enough. However, when the grain is aerated to 0 to 5°C — which is normal in the northern regions of the northern hemisphere — they will only find conditions suitable at the surface, where ambient temperature fluctuates. This surface distribution of mites may give the impression that they have been driven to the surface; but in reality, it is likely that they are just breeding there more rapidly than elsewhere. Even the relatively small mites are not blown out of the bin by the air that flows at low velocities through the grain bulk (Armitage, 1981). In bins of dry grain, there is no difference between aerated and unaerated bulks because the mites can only exist at the surface, when grain at the surface of the bulk accumulates moisture in the winter. The mite population rises with the increase in mc and falls as the surface dries out in the summer. This post-winter drop in numbers may be misinterpreted as an effect of aeration, but in reality it is due to a fall in equilibrium relative humidity. Mite population declines are sometimes


306


Comparison of Acarus/Kg. (n=5) at the surface of surface treated (t) and untreated (u) aerated bins of wheat (n=3) 10000

1000

Acarus/Kg.

100

10

1 t 0.1

0.01

u 3

7

11

15

19

23

27

31

55

60

65

69

73

77

81

weeks Figure 6.34

Comparison of numbers of Acarus/Kg. (n = 5) at the surface of surface treated (upper line) and untreated (lower line) aerated bins of wheat (n = 3). (From Armitage, D.M., Cogan, P.M., and Wilkin, D.R. [1994]. Integrated pest management in stored grain: combining surface insecticide treatments with aeration, J. Stored Prod. Res., 30, 303–319.)

assisted by predators. Armitage et al. (1994) showed that a surface treatment of suitable organophosphate pesticides was necessary to prevent mites occurring at the surface of aerated bins, which otherwise could exceed 1000 mites/kg (Figures 6.34 and 6.35). 6.2.2.5 Controlling Microfloral Growth When Hurlock et al. (1980) inadvertently increased the moisture content of their aerated grain by 0.5 to 1.0%, to about 15%, compared with unaerated wheat, they found that storage fungi, particularly members of the Aspergillus restrictus group and Wallemia sebi, increased. It is difficult to distinguish between the interactions of mites and fungi because some fungi are toxic to mites (Solomon et al., 1964), while many fungi are less numerous in the presence of mites (Armitage and George, 1986). Cooling can delay the succession from field to storage fungi as shown in airtight bins holding barley at 20% mc by Burrell et al. (1978). In practice, the increase in fungal numbers during ambient air drying can be determined by comparing the numbers near the duct, which dry soonest, with grain near the surface, which does not dry until the drying front breaks through the surface (Armitage et al., 1982; Armitage, 1986). The occurrence of ochratoxins in northern Europe (MAFF, 1994) may well be an indication that there are shortcomings in the ambient air drying strategies currently employed, because the lower mc for the fungus responsible for producing the mycotoxin, P.verrucosum, is about 17% mc — well above recommended storage levels. Based on the spore counts for bacteria, yeasts, and molds, ambient air drying of 35% moisture content maize was not recommended. It was concluded that based on the microbiological developments in maize, two-stage drying was preferred.



Figure 6.35

307

Comparison of numbers of Glycyphagus/Kg. (n = 5) in surface treated (upper line) and untreated (lower line) aerated bins of wheat (n = 3). (From Armitage, D.M., Cogan, P.M., and Wilkin, D.R. [1994]. Integrated pest management in stored grain: combining surface insecticide treatments with aeration, J. Stored Prod. Res., 30, 303–319.)

The high temperatures in the continuous-flow dryer reduced the initial spore count significantly. On the other hand, high-temperature drying reduces storability due to damage to kernels of corn. Mühlbauer et al. (1981) investigated the microbiological activity during the ambient drying of wheat under German conditions. Microorganisms, dry matter loss due to respiration, germination, and baking quality were used as quality parameters. It was concluded that wheat up to a moisture content of 22% could be dried without lowering the quality significantly. Germination of the wheat was 85%, baking quality was acceptable, and dry matter loss was 0.1% at the end of drying. However, spores of field fungi remained viable until the completion of drying, while storage fungi increased with drying time. The onsets of grain spoilage in the upper layers of the pile during ambient aeration and lowtemperature drying were quantified by Eimer and Morcos (1985). Drying and rewetting of wheat at 30°C aerated at 0.4 m/s, as well as the development of surface molds, were determined to be a function of time, moisture content, and relative humidity. Most of the molds developed at relative humidities above 85% — the levels generally found in the upper layers of grain aerated with ambient air, or chilled air, and during low-temperature drying. Eimer and Morcos (1985) noted the trade-off between mold development and dry matter loss. Molding occurred earlier in non-aerated grain, while dry matter losses were higher in aerated grain. Germination decreased and spore count increased as the relative humidity of the interstitial air increased. Friday et al. (1989) investigated low-temperature drying of different maize hybrids. They verified that so-called mold-resistant hybrids indeed were less susceptible to molding despite a higher moisture content, greater damage index, and greater visual damage. Friday et al. (1989) related the mold resistance of different hybrids to the CO2 production of dry matter loss and proposed to add a hybrid multiplier to the dry matter loss equation of Steele et al. (1969) (Equation 1.2). (Table 1.5 in Chapter 1 and Table 7.9 in Chapter 7). Coenen (1987) investigated deterioration during the temporary cold storage of wheat. He defined allowable storage as the time until a maximum spore count of 10,000 per gram of dry matter is reached. With increasing time and moisture content, the spore count increases. Spoilage-free storage time was found to be a function of low moisture content for wheat temperatures of 5 to 15°C. Kernel discoloration, odor development, and germination losses were lower in cold, aerated storages than in warm, non-aerated storages. Coenen (1987) determined the specific airflow rates necessary to keep spore counts below 10,000 per gram of dry matter as a function of moisture content in aerated low-temperature storages. Mold spores develop into fungi, which cause grain spoilage. In addition to the contamination due to the production of mycotoxins, the major losses due to fungi in stored grain are (1) dry matter


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loss due to the conversion of starch and sugar to carbon dioxide, water, and heat; (2) oxidation of fat resulting in the production of fatty acids; (3) decrease in germination; and (4) spoilage of grain kernels due to excessive amounts of microorganisms (Kuppinger et al., 1977). “Unless the temperature or moisture content, or both, of the stored grain is lowered enough to stop the growth of storage fungi, they will continue to grow…” noted Bailey (1994). Contamination of grain due to fungi-produced toxins, such as Aflatoxin, has recently become a concern to government officials, grain merchants, grain producers, and consumers. Food containing more than 20 parts per billion (PPB) of Aflatoxin is unsafe for human consumption under FDA rules (Meronuck, 1995). However, there is much misunderstanding in the grain industry about the nature of fungi. In the following paragraphs, some of the basic microbiological aspects of fungi are summarized. Studies in Britain (Burrell, 1974) showed that visible microfloral growth rarely occurs during a storage period of up to 8 months in bulks of cooled grain held at 5 to 10°C (41 to 50°F) if the moisture content is below about l8%. However, these temperatures can hardly be attained in subtropical climates. An alternative way of preventing microfloral damage to moist grain in warm climates could be by aerating when the ambient relative humidity is low and hence the temperature is high. Although this procedure has been proposed by Esmay et al. (1979) for preventing damage to moist stored paddy in the tropics, it requires verification by field investigations before it can be generally recommended. Even with favorable research support data, skilled grain management would be required to safely store moist grain in warm climates. The effect of aeration on quantitative changes in the microfloral population of a soybean bulk was investigated by Paster (1974). Aeration carried out in two stages reduced the temperature of soybeans from 38 to 40°C (100 to 104°F) to 20 to 22°C (68 to 72°F). The soybean moisture content was in the range of 11 to 13%, and the initial microfloral population was 25 to 325 CFUs (colonyforming units) per g of soybeans. The population after the aeration period was 25 to 250 CFUs per g of soybeans. Additionally, aeration was shown to suppress the self-heating process of the soybeans by preventing a temperature rise. This trial, and others carried out in subtropical climates (Benvenisti et al., 1971), indicate that aeration can be effectively used to reduce the risks of selfheating of stored soybeans, especially during winter. Heated-air aeration to prevent microfloral activity was investigated by Navarro et al. (1980a). The objective was to evaluate the possibility of supplying very low supplemental heat to the aeration system to reduce the air humidity and hence to obtain limited drying of peanuts. The aeration system was equipped with electrical resistance heating elements controlled by a differential thermostat. Under the climatic conditions of the trial, use of this system led to a 3.3°C increase in the air temperature. Heatedair aeration for 1088 hours during 68 days of storage reduced the average moisture content of the unshelled peanuts from 8.9 to 8.0%. There were significantly fewer microfloral colonies in peanuts stored in the bin aerated with heated air than in the bins aerated with ambient air. Under certain conditions, if the aeration system is suitably manipulated (to supply low-humidity air), conditions that depress microfloral activity may be achieved. The examples given of selfheating of soybeans, and the prevention of microfloral activity in peanuts with supplemental heated aeration systems, suggest that these possibilities should be further investigated. However, in subtropical climates, it is necessary to dry the commodities to below their critical moisture content (13% for wheat, 8% for unshelled peanuts, and 11% for soybeans) soon after harvest in order to prevent microfloral growth. REFERENCES Alavanja, M.C.R., Blair, A., and Masters, M.N. (1990). Cancer mortality in the U.S. flour industry, J. Natl. Cancer Inst., 82, 848–849. Anderson M.E. and Kline, G.L. (1986). Field comparison of faldry — a low-temperature, corn drying model, Paper No. 86–6505, American Society of Agricultural Engineers. St. Joseph, MI.



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Noyes, R.T., Kenkel, P., Criswell, J., and Cuperus, G. (1996). Installation and sealing of phosphine recirculation systems in silos at U.S. grain elevators, in Proceedings, 6th International Conference on Controlled Atmosphere and Fumigation, Nicosia, Cyprus. Noyes, R.T., Phillips, T., Cuperus, G., and Bonjour, E. (1998). Advances in recirculation fumigation technology in the U.S., in Proceedings, 7th International Conference on Stored Product Protection (Donahaye, E., Navarro, S., and Varnava, A., Eds.), Beijing, PRC, October 21–26,1996. Pabis, S. and Henderson, S.M. (1962). Grain drying theory. III. The air/grain temperature relationship, J. Agric. Eng. Res., 7(1), 21. Paster, N. (1974). Cooling of soybean bulks by aeration: A. Effect of aeration on quantitative changes in the microfloral population of the bulk, Israel Agric. Res. Org. Prog. Rep. 1973/4. Stored Prod. Div. 101–112, Bet Dagan (Hebrew, with English summary). Person, N.K., Jr., Sorenson, J.W., Jr., and McCune, W.E. (1966). Thermodynamic considerations in designing controlled storage environments for bulk grain, Trans. of the ASAE, 9(4), 520–523. Poichotte, J.L. (1977). La conservation des grains par la ventilation, Fermes Mod., 51, 43–46. Robinson, R.N., Hukill, W.V., and Foster, G.H. (1951). Mechanical ventilation of stored grain, Agric. Eng., 32, 606–608. Rouvet, J.C., De Backer, L.W., and Persons, E. (1979). An analytical-numerical method for simulating water transfer in a ventilated deep bed of cereals, Trans. of the ASAE, 22(6), 1444–1450. Sanderson, D.B., Muir, W.E., and Sinha, R.N. (1988a). Intergranular air temperatures of ventilated bulks of wheat, J. Agric. Eng. Res., 40, 33–43. Sanderson, D.B., Muir, W.E., and Sinha, R.N. (1988b). Moisture contents within bulks of wheat ventilated with near-ambient air: experimental results, J. Agric. Eng. Res., 40, 45–55. Schultz, L.J. (1984). A comparison of simulation techniques for wheat aeration, unpublished M.S. thesis, Oklahoma State University, Stillwater, OK. Schumann, T.E.W. (1929). Heat transfer: a liquid flowing through a porous prism, J. Franklin Inst., 208(5), 405. Sharp, J.R. (1982). A review of low temperature drying simulation models, J. Agric. Eng. Res., 27, 169–190. Sinha, R.N. (1974). Climate and the infestation of stored cereals by insects, pp. 117–141, in Proc. First Int. Working Conf. Stored Prod. Entomol., Savannah, GA. Sinha, R.N. and Utida, S. (1967). Climatic areas vulnerable to stored product insects in Japan, Appl. Entomol. Zool., 2, 124–132. Smith, L.B. (1974). The role of low temperature to control stored food pests, pp. 418–430, in Proc. First Int. Working Conf. Stored Prod. Entomol., Savannah, GA. Solomon, M.E., Hill, S.T., Cunnington, A.M., and Ayerst, G. (1964). Storage fungi antagonistic to the flour mite (Acarus siro L.), J. Appl. Ecol., 1, 119–125. Sorenson, J.W., Jr., Person, N.K., Jr., McCune, W.E., and Hobgood, P. (1967). Design method for controlledenvironment storage of grain, Trans. of the ASAE, 10(3), 366–369. Steele, J.L., Saul, R.A., and Hukill, W.V. (1969). Deterioration of shelled corn as measured by carbon dioxide production, Trans. of the ASAE, 12, 685–689. Subramanyam, Bh. and Harein, P.K. (1989). Insects infesting barley stored on farms in Minnesota, J. Econ. Entomol., 82, 1817–1824. Sun D.W. and Woods, J.L. (1997). Deep-bed simulation of the cooling of stored grain with ambient air: a test bed for ventilation control strategies, J. Stored Prod. Res., 33(4), 299–312. Sutherland, J.W. (1968). Control of insects in a wheat store with an experimental aeration system, J. Agric. Eng. Res., 13(3), 210–219. Sutherland, J.W., Banks, P.J., and Griffiths, H.J. (1971). Equilibrium heat and moisture transfer in airflow through grain, J. Agric. Eng. Res., 16, 368–386. Sutherland, J.W., Banks, P.J., and Elder, W.B. (1983). Interaction between successive temperature or moisture fronts during aeration of deep grain beds, J. Agric. Eng. Res., 28, 1–19. Thompson, T.L. (1972). Temporary storage of high moisture corn using continuous aeration, Trans. ASAE, 15, 333–337. Thompson, T.L., Peart, R.M., and Foster, G.H. (1968). Mathematical simulation of corn drying — a new model, Trans. ASAE, 11, 582–586. White, G.G. (1988). Temperature changes in bulk stored wheat in subtropical Australia, J. Stored Prod. Res., 24, 5–11.


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Williams, P. (1973). Grain insect control by aeration of farm silos in Australia, Ann. Technol, Agric., 22(3), 557–561. Yaciuk, G., Muir, W.E., and Sinha, R.N. (1975). Selection of locations for long term storage of wheat, Paper 75-4057, Am. Soc. Agric. Eng., St. Joseph, MI.


CHAPTER

7

Operating Aeration Systems Ronald Noyes and Shlomo Navarro

CONTENTS 7.1

7.2

Selecting Ambient Air for Aeration ....................................................................................317 7.1.1 Effects of Ambient Air on Living Organisms of the Storage Ecosystem ..............317 7.1.2 Aeration by Regions ................................................................................................317 7.1.3 Aeration in the Tropics ...........................................................................................318 7.1.4 Selection of Aerating Air in High-Altitude Climates .............................................319 7.1.5 Surveying Meteorological Data ..............................................................................320 7.1.6 Heat and Moisture Transfer in Airflow Through Grain .........................................325 7.1.7 Effect of Cooling at High Humidities ....................................................................327 7.1.8 Effect of Cooling at Low Humidities .....................................................................329 Aeration Methods ................................................................................................................331 7.2.1 Direction of Airflow ................................................................................................331 7.2.1.1 Up-Flow (Pressure) vs. Down-Flow (Suction) Aeration Airflow ...........331 7.2.1.2 Advantages of Pressure Airflow ..............................................................332 7.2.1.3 Disadvantages of Pressure Airflow ..........................................................334 7.2.1.4 Advantages of Suction Airflow ...............................................................335 7.2.1.5 Disadvantages of Suction Airflow ...........................................................335 7.2.1.6 Conclusions About Airflow Direction .....................................................336 7.2.2 Safe Storage of Dry Grain by Aeration ..................................................................337 7.2.2.1 Cooling Dry Grain ...................................................................................337 7.2.2.2 Calculation of CWBT ..............................................................................340 7.2.2.3 The Mold Envelope .................................................................................348 7.2.2.4 Effect of Wetter Regions of the Grain Bulk ...........................................348 7.2.2.5 Using CWBT to Estimate Insect Population Growth Rate ....................349 7.2.2.5.1 Threshold Commodity Wet-Bulb Temperature to Prevent Insect Population Growth ......................................................349 7.2.2.5.2 Rates of Insect Population Growth ........................................354 7.2.2.6 Managing Aeration Systems by Using CWBT to Control Insect Populations ....................................................................................355 7.2.2.6.1 Moisture Content, CWBT, and the Corresponding Dry-Bulb Temperature ...........................................................355 7.2.2.6.2 Target CWBT for Controlling Insect Populations ................356

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316

7.3

7.4

7.5


7.2.2.6.3 Operating the CWBT Aeration Controller ............................357 7.2.2.6.4 Requirements for CWBT Control of Aeration Systems .......357 7.2.2.6.5 Advantages of CWBT Control for Warm Temperate Climates ................................................................357 7.2.2.7 Conclusions in Using the CWBT Method ..............................................358 7.2.3 Selecting Cooling Rates ..........................................................................................358 7.2.3.1 Time Required for Cooling .....................................................................358 7.2.3.2 Selecting the Airflow Rate for Dry Grain ...............................................364 7.2.3.3 Equalizing Grain Temperatures to Prevent Condensation ......................365 7.2.4 Safe Storage of Wet Grain by Aeration ..................................................................368 7.2.4.1 Cooling to Suppress Microfloral Activity ...............................................370 7.2.4.2 Cooling to Maintain Quality of Moist Grain ..........................................371 7.2.4.3 Selecting Cooling Rates for Wet Grain ...................................................372 7.2.4.4 Airflow Rates for Maintaining Grain Condition .....................................374 Aeration System Operating Strategies ................................................................................376 7.3.1 Operational Settings for Aeration Control Systems ...............................................376 7.3.1.1 Multiple Staged Set-Point Strategies .......................................................376 7.3.1.2 Wet-Bulb Temperature Control Strategy .................................................377 7.3.1.3 Aeration to Prevent Moistening or Drying of Grain ..............................377 7.3.2 Alternative Aeration Methods .................................................................................378 7.3.2.1 Early Fall Aeration to Prevent Insect Infestation ....................................378 7.3.2.2 Winter Aeration to Prevent Insect Infestation .........................................379 7.3.2.3 Minimizing Damage to Infested or Damp Grain ....................................379 7.3.2.4 Eliminating Spontaneous Heating of Grain ............................................380 7.3.3 Long-Term Storage of Grain ..................................................................................381 7.3.4 Aeration by Types of Storage .................................................................................381 7.3.4.1 Aeration of Upright and Vertical Structures ...........................................382 7.3.4.2 Aeration of Horizontal Structures ...........................................................382 Selection of Aeration Fans ..................................................................................................384 7.4.1 Axial-Flow Fans ......................................................................................................384 7.4.2 Centrifugal Fans ......................................................................................................385 7.4.2.1 Forward-Curved Fans ..............................................................................385 7.4.2.2 Radial-Bladed Fans ..................................................................................386 7.4.2.3 Backward-Curved Fan .............................................................................386 7.4.3 Operational Characteristics of Fans ........................................................................387 7.4.3.1 Estimating Static Pressure Requirements ................................................388 7.4.3.2 Estimating Fan Power Requirements ......................................................388 7.4.3.3 Fan Efficiency and Slippage ....................................................................389 7.4.3.4 Selecting Fans Based on Storage Conditions ..........................................389 7.4.4 Fan Noise .................................................................................................................390 7.4.4.1 Controlling Fan Noise by Fan Selection .................................................391 7.4.4.2 Controlling Fan Noise by Fan Position and Sound Diversion ...............391 7.4.4.3 Controlling Fan Noise with Acoustical Silencers ...................................392 7.4.5 Heat of Compression ...............................................................................................392 Aeration Control Equipment ...............................................................................................393 7.5.1 Simple Mechanical Controller without Relative Humidity Control ......................394 7.5.1.1 Single Thermostat Controllers .................................................................394 7.5.1.2 Upper and Lower Limit Temperature Span Control ...............................394 7.5.1.3 Humidity Control Settings .......................................................................395 7.5.1.4 Automatic Timers for Aeration Control ..................................................395


OPERATING AERATION SYSTEMS

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7.5.2

Complex Electromechanical Controller with Humidity Control ...........................396 7.5.2.1 Temperature Difference Controller ..........................................................397 7.5.2.2 Wet-Bulb Controller .................................................................................398 7.5.2.3 Proportional Time Controller ...................................................................399 7.5.3 Microprocessor and Computer-Based Control and Monitoring Systems ..............400 7.5.3.1 Microprocessor-Based Aeration Controllers ...........................................400 7.5.3.2 Computer-Based Monitoring and Control Systems ................................402 7.5.4 Selecting Aeration Controllers ................................................................................406 7.5.5 Computer Aid to Predict Aeration System Performance .......................................407 7.6 Economic Impacts from Aeration .......................................................................................408 References ......................................................................................................................................409

7.1 SELECTING AMBIENT AIR FOR AERATION 7.1.1

Effects of Ambient Air on Living Organisms of the Storage Ecosystem

Air temperature is a regulating factor in the survival, development, and reproduction of all living organisms of the storage ecosystem, including microflora, insects, and mites. This effect of air temperature is further influenced by the level of water activity (aw) or relative humidity (RH) of the atmospheric air. In considering air characteristics suitable for aeration, those properties should be selected that will prevent or suppress the development of damaging organisms. Generally storedproduct insects develop progressively fastest from 20 to 35°C (68 to 95°F) and are suppressed or arrested in their development within 5 to 7°C (9 to 13°F) below or above this range. The optimum temperatures for development of common storage fungi are within a wider range of 20 to 50°C (68 to 122°F). 7.1.2

Aeration by Regions

To determine the most appropriate ambient air properties that can be obtained under varying climatic conditions, the general ambient temperature limits in different regions of the world should be understood. A classification system with five global climate regions was discussed by Gunn (1968). These world climatic regions are: 1. 2. 3. 4. 5.

Tropical belts — 12 months above 20°C (68°F) Subtropical belts — 4 to 11 months above 20°C, 1 to 8 months from 10 to 20°C (50 to 68°F) Temperate belts — 4 to 12 months from 10 to 20°C (50 to 68°F) Cold belts — 1 to 4 months from 10 to 20°C (50 to 68°F); 8 to 11 months below 10°C (50°F) Polar belts — 12 months below 5°C (41°F)

Because most major stored-product insect species do not multiply rapidly enough to be considered as pests below 20°C (68°F), they generally do not infest stored products in cold regions and are not a problem in polar belts. They thrive and multiply best in the tropical belt and to a lesser extent in subtropical and temperate regions. Humid middle-latitude prairie grasslands (in subtropical or temperate regions) — which comprise 7% of the land area of the earth and in which about 40% of the world’s population lives — produce most cereal crops of the world such as wheat, barley, oats, maize (corn), millet, sorghum, rice, and rye. In general, locally grown stored cereals in these latitudes are not often subjected to year-round heavy insect infestations on a scale similar to that of the humid tropical belt. However, the fact that grain in subtropical and temperate climates is harvested in summer (often at temperatures above ambient in the shade) followed by bulk storage means that insects can develop into heavy


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infestations year-round. The climates of the areas of the world in which stored products are infested by cosmopolitan insect species are the same as those of the farmlands of the world. Climatic descriptions and examples of insect species characteristic of the major farming regions of the world are given below, according to Sinha (1974): Crop producing and storing areas classified by climate: 1. Semiarid lands — summers hot; winters in low latitudes hot year-round, in middle latitudes cool to cold. Dry seasons with less than 38 cm (15 inches) rainfall. Typical countries: Egypt, Sudan, Niger, Somali Republic, Pakistan. Major cereal crops: cereals, especially millet and sorghum; often with irrigation. Typical major storage insects: Lasioderma serricorme (F.), Tribolium castaneum (Herbst), Sitotroga cerealella (Oliv.). 2. Low-latitude wetlands and drylands — all seasons warm or hot; wet season preceded by dry seasons; slightly cooler in the low-sun period of the year. Typical countries: India, Thailand, Brazil, Ghana, Sri Lanka, some regions of Mayanmar and Indonesia. Major cereal crops: rice, maize, and other cereals. Typical major storage insects: Sitophilus oryzae (L.), Rhyzopertha dominica (F)., and Corcyra cephalonica (Stainton). 3. Mediterranean lands — warm to hot year-round; summers dry; winters cool to mild, humid. Typical countries: Italy, Greece, Turkey, Portugal. Major cereal crops: winter crops, maize, wheat, barley, and oats. Typical major storage insects: Ephestia cautella (Walker), S. oryzae (L.), S. granarius (L.), R. dominica, T. castaneum. 4. Humid tropical lands — hot and moist year-round. Typical countries: Congo, Columbia, Venezuela, some regions of Burma-Mayanmar and Indonesia. Typical cereal crops: Rice and maize. Typical major storage insects: S. zeamais Motschulsky, S. oryzae, S. cerealella. 5. Middle-latitude semihumid lands — summers warm to hot; winters cool to cold; four distinct seasons; frost-free period more than 90 days, varying with latitude; humid much of the year. Typical countries: U.S. (portions), USSR, China, Australia, West Europe, Canada, Japan. Major cereal crops: Wheat, barley, maize, and all other cereals except tropical crops. Typical major storage insects: S. granarius, Oryzaephilus surinamensis (F.), R. dominica, Tribolium spp. Group 5 has four subdivisions: a. Humid continental with short summers — major cereal crops: spring wheat, barley, and oats; major storage insects: Cryptolestes ferrugineus (Steph.), O. surinamensis. b. Humid continental with long summers — major cereal crops: corn and winter wheat; major storage insects: S. oryzae, Tenebroides mauritanicus (L.), T. castaneum., R. dominica. c. Humid subtropical — major cereal crops: wheat and rice; typical major storage insects: O. surinamensis, S. granarius., R. dominica, S. oryzae. d. West coast marine — major cereal crops: wheat and rice; typical major storage insects: O. surinamensis, S. oryzae.

7.1.3

Aeration in the Tropics

Some grain storage structures in the tropics are equipped with modern aeration systems. However, the objectives of aeration, the expected benefits to be derived, and actual research data on aeration of tropical grain storages are not well documented in the literature. Condensation problems have been reported for metal structures in the tropics in which the daily rise in temperature of the head-space is accompanied by a limited cooling at night (Hall, 1970). It seems probable that by using adequate aeration, the heat accumulated in the head-space can be exhausted by upward aeration during nighttime and reduce the intensity of condensation. Since one of the principal objectives in the application of aeration is to cool the grain, this seems achievable only where temperatures drop below 18 to 20°C (64 to 68°F). According to Threwartha (1943), these conditions can be encountered in low-latitude deserts and steppes. Temperatures during the two distinct seasons that are characteristic of the deserts average around 35°C (95°F) in summer and about 12°C (54°F) in winter. However, the day-to-night temperature differential and the duration of suitable cooling temperatures must be known. If at least 5 to 8 hours of



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cooling air temperatures below 20°C (68°F) are consistently available at night, then aeration in dry, hot climates is feasible. 7.1.4

Selection of Aerating Air in High-Altitude Climates

In tropical climates, the feasibility of using ambient aeration to cool grain is very limited. However, altitude can attenuate climatic conditions (though this is not included in the above climatic classification). Consequently, lower temperatures prevail in the higher elevations of the tropics, in the highland (upland) climates. For example, in typical upland Savannah climate (near Sao Paulo, Brazil) at about 800 m (2625 ft) altitude, temperatures of 10 to 12°C (50 to 54°F) prevail during several months of the year. Uplands exist in different tropical regions. Lower temperatures prevail with increasing height above sea level because of the less favorable radiation balance in the free air and because rising air (whether lifted by convection currents above a relatively warm surface or forced up over mountains) undergoes a reduction of temperature and air pressure. In the lower atmosphere the average rate of temperature decrease with altitude is 0.6 to 0.7°C (1.0 to 1.2°F) per 100 m (330 ft). Thus, in the upland areas of Kenya at between 1300 and 1800 m (4265 and 5900 ft) altitude, minimum temperatures of 5 to 11°C (41 to 52°F) are available during the storage season. Consequently, favorable conditions exist for aeration cooling of grain. In general, when sea level temperatures are 30 to 35°C (86 to 95°F), it is possible to obtain aeration cooling temperatures of 15 to 20°C (59 to 68°F) in tropical and subtropical regions of Central and South America, Africa, and Asia to cool grain in storage located at site elevations of about 2500 m (8200 ft) and higher. Although favorable conditions for the operation of aeration systems exist in tropical and subtropical belts, its use in these regions has not been well documented. At high altitudes, air has a lower density due to lower atmospheric pressure. Consequently, the vapor pressure of the air is much lower. Due to lower air density at high altitudes, air volumes and time required to aerate grain is greater than with normal atmospheric pressures at or near sea level. Thus, when using two identical aeration systems, a unit operating at 2000 m (6500 ft) elevation has a much lower capacity than one operating at sea level. This is because aeration fans are volumetric airmoving devices. Both fans move the same volume of air; but due to the lower dry-air density, the highaltitude fan moves fewer kg of dry air per minute. But grain at both locations can be cooled at the same rate if both fans can deliver the same number of kg of the same dry-air quality per unit of time. Therefore, when designing ambient air aeration systems at various altitudes, extended aeration times or increased airflow rates must be considered. When the number of hours per day suitable for operating the system is limited, the airflow rate must be increased to compensate for the lower air density and vapor pressure. Belt-drive aeration fans should be used in storage systems at high elevations so that increased fan motor loading, fan efficiency, and cooling capacity can be obtained. If a storage site is located at an elevation where the air density is 30% lower than at sea level, the fan speed and air velocity must be increased by 30% to achieve similar performance to sea level. Because of lower air density, the increase in air velocity through air ducts and the grain mass should provide about the same frictional resistance as the lower air velocity at sea level. The time required to reduce the temperature at different altitudes and the relationships among altitude, atmospheric pressure, air density, and aeration time required are shown in Table 7.1. Theoretical values in this table were calculated based on the principle of the cooling energy of dry air required to cool grain. From Table 7.1, about 1.3 times the cooling time (and cooling air volume) is required to cool grain from 27 to 15°C (80 to 59°F) at an altitude of 2000 meters (6560 ft), compared to the same amount of cooling at sea level (Calderon, 1974). In large areas of the tropics, favorable conditions for the operation of aeration systems exist. However, a thorough meteorological data analysis survey at every planned grain storage site is essential before deciding on the installation of grain aeration systems. Based on technical knowledge


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Table 7.1

Estimated Time Required to Reduce 12% Moisture Content (Wet Basis) Wheat from 27 to 15°C at Different Altitudes at an Airflow Rate of 6.0 (m3/h)/tonne. Theoretical Aeration Times Based on 63% Average Ambient Air Relative Humidity and 15°C

Atmospheric Pressure (mmHg) 760 720 600 510 420

Altitude (m)

Air Density (kg/m3)

Aeration Time Required (h)

0 1000 2000 3000 4000

1.22 1.16 0.95 0.82 0.67

100 105 127 149 182

From Calderon, M. (1974). The possible role of aeration in the control of stored product insects in warm climates, pp. 77–84. Proc. First Int. Working Conf. Stored Prod. Entomol., Savannah, GA.

gained from studying aeration field installation operations and aeration field research in subtropical climates, field experiments should be promoted in tropical climates for the optimization of the use of aeration for preventing storage losses. 7.1.5

Surveying Meteorological Data

Most countries have a meteorological service from which climatic data can be obtained. Such data may then be transformed for use in climatographs that indicate the potential of the climate for aeration. The mean weekly or monthly temperature and humidity data throughout the year can be plotted on a graph of temperature against relative humidity, as points representing each month of the year. The points are then connected and a climatograph developed. Examination of temperature variation on a general geographical basis indicates that the 30° latitude line (both north and south of the equator) form the rough boundaries for aeration using ambient air. To the north and south of these boundaries, favorable ambient conditions increase progressively until the outer boundaries of temperate climates are reached (Lamb, 1972). Figure 7.1 illustrates examples of these boundaries, regions of warm climates on the north and south belts of the tropical regions, where aeration is possible only at high altitudes or using chilled air. Below and above those boundaries are the regions with moderate or temperate climates that are most suitable for aeration. These areas are typically characterized as grain-growing regions of the world. Beyond these regions are the polar belts, where insect infestation does not pose a problem. To assess the practicality of cooling dry grain by aeration, more detailed climatological analyses have been made by several authors. Burges and Burrell (1964) constructed maps to show the hours available at 75% relative humidity and below, in relation to geographical position in Britain. Bailey (1968) noted the air temperatures in the Australian wheat belt and their relationship to the aeration of stored grain. To do this, the number of hours per month during which air temperatures were below 26.7°C, 21.1°C, and 15.6°C (80°F, 70°F, and 60°F) were recorded using instruments especially constructed for collecting the data. Noyes et al. (1991) recorded the number of hours of available cooling time above 13°C (55°F) during October and November for Oklahoma, U.S., based on 30 years of U.S. Weather Service data. Arthur and Flinn (2000) conducted simulation studies to determine temperature accumulations below defined thresholds and to show the impact of controlled aeration on populations of the rusty grain beetle. Recorded data from weather stations in Texas, Oklahoma, Kansas, eastern New Mexico, and eastern Colorado were used to determine hours of temperature accumulation below 23.9°C in June and July, 15.6°C in September and October, and 7.2°C in December. An airflow rate of 0.1 cfm/bu (6.4 (m3/h)/tonne) was required to complete summer cooling in most of the hard red winter wheat zone except southern Texas. Simulation results of insect control showed summer



Figure 7.1

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World map showing the zone that is generally unsuitable for ambient aeration of grains in the central latitudes, and geographic regions of north and south latitudes where ambient aeration is likely to be effective.

aeration was also effective in suppressing rusty grain beetle throughout the red winter wheat region except for southern Texas. Table 7.2 shows monthly means of daily minimum and maximum dry-bulb temperatures and relative humidities for three typical locations in the U.S. San Antonio, TX, located at a latitude of 29° 32'N, is characterized by elevated temperatures and a long warm season. It is located near the boundary latitude where the use of aeration is marginal, because it has a short winter period with minimum temperatures above freezing that are favorable for aeration. The central U.S. location for this aeration temperature comparison is Topeka, KS, at a latitude of 39° 04'N. Normal minimum winter temperatures are below freezing and reach a mean of –8.4°C in January. Low temperatures suitable for aeration can be obtained even in May. Summer in Topeka, compared to San Antonio, is relatively short. The northern grain belt location selected for this comparison in Table 2 is Minneapolis, Minnesota, where below-freezing winter temperatures extend for over 5 months. In this location, aeration is possible most of the year. Data shown in Table 7.2 require the comparison of temperature and humidity to determine if ambient air is suitable for aeration. For this comparison, climatographs show the combination of temperatures and humidity values for two different climatic locations (San Antonio and Topeka), Figures 7.2 and 7.3. These figures combine the values for monthly mean daily maximum temperatures with minimum relative humidities and mean daily minimum temperatures with maximum relative humidities on graphs. Comparison of the climatographs in Figures 7.2 and 7.3 reveals a typical feature of ambient conditions. When suitable ambient aeration temperatures prevail, high relative humidities may cause concern to the aeration system operator, related to the possibility of wetting the grain. The reverse condition occurs when air temperatures become much warmer and relative humidities drop, creating ambient conditions that may be unsuitable for aerating grain. The operation of ambient aeration systems will be compared with aeration systems based on wet-bulb temperature. Figures 7.2 and 7.3 compare climatographs from two different geographic


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Table 7.2 Monthly Mean Daily Minimum and Maximum Dry-Bulb Temperatures (°F and °C) and Relative Humidities (%) for Three Typical Locations in the U.S. Months

Max Temp (°F)

Min Temp (°F)

Max Temp (°C)

Min Temp (°C)

Max RH (%)

Min RH (%)

San Antonio, TX, Latitude 29° 32'N: 52-Year Temp. Average; 51-Year RH Average; Elev. 788 ft (240 m) January February March April May June July August September October November December

60.8 65.7 73.5 80.3 85.3 91.8 95.0 95.3 89.3 81.7 71.9 63.5

37.9 41.3 49.7 58.4 65.7 72.6 75.0 74.5 69.2 58.8 48.8 40.8

16.0 18.7 23.1 26.8 29.6 33.2 35.0 35.2 31.8 27.6 22.2 17.5

3.3 5.2 9.8 14.7 18.7 22.6 23.9 23.6 20.7 14.9 9.3 4.9

80 80 79 83 88 88 87 86 86 84 81 80

57 52 47 51 55 52 45 45 51 52 56 57

Topeka, KS, Latitude 39° 04'N: 47-Year Temp. Average; 30-Year RH Average; Elev. 877 ft (267 m) January February March April May June July August September October November December

37.4 42.1 53.8 66.5 75.9 84.6 89.3 88.2 82.3 71.3 54.8 42.7

16.9 21.5 31.2 42.7 53.3 63.1 67.4 65.3 57.2 45.2 32.4 22.3

3.0 5.6 12.1 19.2 24.4 29.2 31.8 31.2 27.9 21.8 12.7 5.9

–8.4 –5.8 –0.4 5.9 11.8 17.3 19.7 18.5 14.0 7.3 0.2 –5.4

77 78 79 80 84 87 86 87 88 83 81 80

64 60 53 51 55 58 56 57 60 57 64 68

Minneapolis, MN, Latitude 44° 53'N: 55-Year Temp. Average; 34-Year RH Average; Elev. 834 ft (254 m) January February March April May June July August September October November December

20.7 26.6 39.2 56.5 69.4 78.8 84.0 80.7 70.7 58.8 41.0 25.5

2.8 9.2 22.7 36.2 47.6 57.6 63.1 60.3 50.3 38.8 25.2 10.2

–6.3 –3.0 4.0 13.6 20.8 26.0 28.9 27.1 21.5 14.9 5.0 –3.6

–16.2 –12.7 –5.2 2.3 8.7 14.2 17.3 15.7 10.2 3.8 –3.8 –12.1

74 76 76 75 76 79 81 84 85 81 80 78

68 66 61 51 50 52 53 56 61 60 69 73

Compiled from Wood R.A. (Ed.) (1996) Weather of U.S. Cities, 5th ed., Gale Research (Int. Thomson Publ. Co.). With permission.

locations to show the extreme differences in mean daily temperature and humidity conditions. Daily temperature and humidity variations can be further understood by analyzing the thermohygrograph shown in Figure 7.4. From Figure 7.4 it is clear that when the temperature falls to below 20°C, the relative humidity rises above 80%. When this situation occurs, inexperienced grain storage operators often find it difficult to decide which ambient temperatures and relative humidity conditions are suitable for aerating grain. To compare these variations in temperature and humidity, data for San Antonio was plotted on a psychrometric chart to demonstrate the cooling capacity of ambient air (Figure 7.5).



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Figure 7.2

Climatograph showing monthly mean daily maximum dry-bulb temperature and minimum relative humidity, and minimum dry-bulb temperature and maximum relative humidity for San Antonio, TX. (Based on data from Wood, R.A. [Ed.] [1996]. Weather of U.S. Cities, 5th ed., Gale Research; Int. Thomson Publishing Co.)

Figure 7.3

Climatograph showing monthly mean daily maximum dry-bulb temperature and minimum relative humidity, and minimum dry-bulb temperature and maximum relative humidity for Topeka, KS. (Based on data from Wood, R.A. [Ed.] [1996]. Weather of U.S. Cities, 5th ed., Gale Research; Int. Thomson Publishing Co.)

Figure 7.5 shows extreme monthly mean daily minimum temperature and maximum humidity and maximum temperature and minimum humidity for San Antonio. Data in this figure is based on averages of 52 years. The diagonal line at 15°C wet-bulb temperature line was selected to coincide with 64% air relative humidity and 19°C equivalent dry-bulb temperature. This diagonal line was arbitrarily chosen as an example to indicate the potential cooling effect that the ambient air may have. These diagonal lines are parallel to the enthalpy values of air on the psychrometric chart shown in Figure 7.5 (for more details on the cooling capacity of air, refer to Chapter 3). The climatograph plot of climatic conditions in San Antonio shows that for December to February,


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Figure 7.4

Daily changes in temperature and relative humidity of the ambient, characteristic of a subtropical climate during October, northern hemisphere. (Data from Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome. With permission.)

Figure 7.5

Monthly mean daily minimum temperature (°C) and maximum relative humidity (%), and daily maximum temperature (°C) and minimum relative humidity (%) values plot on a psychrometric chart. For San Antonio, TX, latitude 29° 32'N. The diagonal 15°C wet-bulb temperature coincides with 12% grain moisture content in equilibrium with 64% air relative humidity. (Based on data from Wood, R.A. [Ed.] [1996]. Weather of U.S. Cities, 5th ed., Gale Research; Int. Thomson Publishing Co.)

aeration would be beneficial for most times during the day. However, aeration time could be extended for limited times of the day during November and March through April. To determine whether the available ambient air is suitable for aerating grain, it is necessary to locate the state point condition of the air on the psychrometric chart, either by measuring the wetbulb and dry-bulb temperatures, or by measuring the RH and dry-bulb temperature (Figure 3.8 in



325

Chapter 3). Wet-bulb temperature is the best means of assessing the ability for air to cool the grain; so wet-bulb temperature is a good means for controlling aeration systems as illustrated in the following example. Example 7.1 The cooling ability of air with a wet-bulb temperature of 15°C (59°F) is equivalent to the following sets of dry-bulb temperature and humidity: 16°C (60.8°F) and 90% RH; 30°C (86°F) and 17% RH; or 36°C (96.8°F) and 6% RH. If a grain bulk contains 30°C (86°F) wheat at 12% moisture content, the intergranular air equilibrium RH (ERH) will be approximately 64%. (Refer to Appendix A, Figure A1 (Psychrometric Chart) and A.4 (ERH graph)). A wet-bulb temperature of 15°C (59°F) will satisfactorily cool 30°C (86°F), 12% wheat to 19°C (66.2°F), no matter what the equivalent air temperature and RH may be. Further cooling or heating will depend on the combination of temperature and RH of the ambient air (see discussion below and in Chapters 3 and 6). (Note: Example 7.1 is discussed throughout Sections 7.16–7.18.) The beneficial effects of aeration in subtropical climates are known. However, the shortage of sufficient cool air soon after harvest may seriously limit the usefulness of aeration in certain regions. A study of detailed weather data should be made as a precautionary measure before new storage sites are selected. This site selection analysis should also take into consideration regional climatic fluctuations that may have a bearing on available aeration hours during the aeration season. The choice of new storage sites is usually decided based on economic, political, logistic, or demographic data; rarely are climatic factors taken into consideration. Seldom is the storage expert consulted at the planning stage; but when his opinion is taken into consideration, the selection of storage sites and their orientation on the basis of local climatic factors and variations within a region can provide significant financial benefits. 7.1.6

Heat and Moisture Transfer in Airflow Through Grain

Because of the importance of the subject, heat and moisture transfer is also discussed in Section 4.6, to calculate the speed of fronts moving through grain bulks; in Section 6.1.2.1, to give experimental data on grain temperature profiles; and in this section, to select ambient air for aeration. Sutherland et al. (1971) proposed a procedure to describe the equilibrium heat and moisture transfer in airflow through grain. They described the formation of three zones (A, B, and C) separated by two temperature and moisture fronts that move through the grain bed in the direction of airflow. When cool air is forced through a warm grain mass, the entire grain mass does not cool at once. A rapid step-by-step change in the state of air that takes place as it flows through grain results in the creation of the zones, A, B, and C. See also descriptions and figures in Sections 4.6.1 and 6.1.2.1. A temperature front (heating or cooling) is defined as the leading edge of the zone within a grain mass where kernel temperatures begin to change from the energy exchange of the aeration mass airflow, from an initial value to a new value. Leading and trailing edges are the top and bottom portions of this zone. These edges can be thought of as a thin layer of grain along a plane through the bulk, beyond which grain temperatures are unchanged because the air flowing past that point has stabilized with the grain temperatures ahead of and at that point. The leading edge of a cooling zone represents the line of air mass in equilibrium with the grain mass. Zone A (Figure 7.6) is described as the zone where the grain reaches equilibrium with the entry air, and no further change takes place. Relative humidities of air entering the interstitial air space of the grain mass equalize, and grain moisture slowly reaches equilibrium with the air humidity. In Zone A, the temperatures of the grain, the entering air, and the intergranular air are all equal. In this model the relative humidities of the intergranular air and the entering air are also equal, and the grain moisture content has assumed the value which is in equilibrium with this humidity.


326

Figure 7.6


Schematic presentation of the cooling front with constant grain moisture content (wheat 12% in equilibrium with 64% RH air). The grain temperature profile shows zones A, B, and C describing cooling without moisture transfer.

The cooled grain behind the trailing edge has stabilized with the incoming air wet-bulb temperature, so there is no further energy exchange at this point. With lower airflow rates, the cooling zone between the leading and trailing boundary planes or edges becomes thinner. When airflow rates are higher, the cooling zone becomes wider or thicker because of the reduced contact time of each particle of air with an individual grain kernel. In Zone C, grain temperature and moisture content do not change from the start of aeration. Interstice and exhaust air temperatures are equalized with Zone C grain temperatures. Air relative humidity is in equilibrium with the Zone C grain moisture content. The temperatures of the intergranular air and the air leaving the grain are both equal to the grain temperature. The relative humidity in this zone has the value that is in equilibrium with the grain moisture content in Zone C. Figure 7.6 illustrates the leading edge of the cooling process where the grain moisture content (mc) is 12%, in equilibrium with 64% air relative humidity. Air entering the grain mass with a wetbulb temperature of 15°C (59°F) and air RH of 64% has a dry-bulb temperature of 19°C (66.2°F). By definition of a leading edge, at this stage of the aeration process, the grain temperature at the bottom has equalized with that of the air; but the top of the grain mass remains unchanged. Therefore, a gradient of temperatures between 19°C at the bottom and 30°C (86°F) at the top of the grain mass is obtained. The leading edge is where 30°C grain starts to cool. Zone B is described as bounded by the temperature and moisture fronts of Zones A and C. Sutherland et al. (1971) presented a theory that enables one to predict the conditions in Zone B. Also, based on the two programs they developed, the front velocities and profiles — covering all possible cases of cooling and heating (in the temperature front) and wetting and drying (in the moisture front) — could be predicted. The way in which the fronts spread out in passing through a grain bed produces the leading and trailing edges. In their model, for the temperature front, the leading edge traveled 1.96 times the speed of the trailing edge; and at the time when the leading edge of the temperature front first emerges from the bed, the width of the front is 49% that of the bed.



Figure 7.7

327

Schematic presentation of the cooling and wetting front in a grain bulk, at initial temperature of 30°C and 12% moisture content wet basis (in equilibrium with 64% relative humidity). Ambient air dry-bulb temperature is 16°C, and relative humidity is 90%. Grain temperature and moisture profile shows cooling with moisture increase near the aeration duct.

According to Sutherland et al. (1971), Zone B, bounded by trailing and leading temperature and moisture fronts, is the area where temperature, humidity, and grain moisture changes gradually occur. Zone B conditions are not easily derived. In the faster moving temperature front, the major effect is a significant change in grain temperature, with a minor change in grain moisture. The slower moving moisture front causes a secondary grain temperature change. It is important to note that a thickening cooling front was observed, which they attributed to the effects of finite heat and mass transfer coefficients and other dispersive mechanisms. Further evidence of the passage of a thickening cooling front has been obtained in a field trial of refrigerated aeration of wheat in northern Australia (Sutherland et al., 1970). 7.1.7

Effect of Cooling at High Humidities

If cooling air has a wet-bulb temperature of 15°C (59°F) when the ambient relative humidity is 90%, the dry-bulb temperature is approximately 16°C (60.8°F) (Figure 7.7). As this air is blown into a bulk of 30°C (86°F) grain, the air will be heated from 15 to 19°C. When air is heated, the absolute humidity remains unchanged (horizontal lines in Figure A.1), but the heat content, relative humidity, and temperature will change until equilibrium is reached. As long as the grain is higher than 19°C (66.2°F) dry-bulb temperature at 64% RH (Figure 7.7 and Figure A.1), no moisture change will be detected in the grain (Sutherland et al., 1971). At 19°C (66.2°F) dry-bulb and 64% RH, both grain wet-bulb temperature and air wet-bulb temperature reach equilibrium at 15°C (59°F). After equilibrium is reached at the air entry area and as aeration continues, the grain surface moisture content will start to increase, gradually rising from 12 to 18% moisture content. The result is the formation of a relatively thin layer of moist grain around the aeration duct (Sutherland et al., 1983). Quantitative data to demonstrate the intensity of this wetting front is very scarce. The final stages of the grain bulk temperature and moisture content in a wetting front is shown schematically in Figure 7.8. At the end of the cooling stage, grain moisture content increased to 18% at the air inlet area to equilibrate with 90% ambient air relative humidity. In the rest of the


328

Figure 7.8


Schematic presentation of the grain temperature and moisture content variation through the grain bed for the cooling and wetting front shown in Figure 7.7.

bulk, the initial wheat moisture content remained almost constant at 12% wet basis (in equilibrium with 64% RH). The trailing edge temperature as grain cooled near the top of the grain bulk was 19°C — compared to the grain temperature at the air inlet to the bulk, which was the ambient drybulb air temperature used for aeration (16°C). Figure 7.8 illustrates the limited moistening that occurs during trailing edge cooling. In the example shown in Figure 7.7, initial grain moisture content is equivalent to 12%, which is in equilibrium with 64% air RH. Air entering the grain mass has a wet-bulb temperature of 15°C (59°F), but exhaust air RH is 90% at 16°C (61°F). The grain moisture content at the top of the grain mass remains unchanged because the exhaust air is in equilibrium with the ERH of the grain mass; but at the bottom, a restricted region of grain gains moisture to 18% moisture content in equilibrium with 90% RH. In this region grain dry-bulb temperatures tend to equalize to 16°C (61°F) of the air. If aeration continues using constant conditions, it may be possible to achieve a leading edge grain mc between 18% at the bottom and 12% at the top. But in practice, constant ambient air quality is seldom available. A slight moisture increase of 1 to 2% is observed in suction systems because of the high RH conditions that prevail, usually at night. It is common to observe moistening effects that usually develop in regions where aeration air meets the grain mass over a depth of 50 cm to 100 cm. In practice, the daily changes in relative humidity and temperature (Figure 7.4) compensate or offset, to a large extent, the moisture increase that may occur when aeration is used at high ambient air humidities. Continuous supply of high-humidity air at low temperatures is uncommon when only ambient air is used. However, when using refrigerated air for chilling grain, it is possible to obtain a continuous supply of high-RH air. Data on the limited advance of a moisture front using refrigerated air is given in Chapter 9 on chilling grain. In the experimental data supplied in Chapter 9, the advance of the moisture front remained restricted to the area around the aeration duct. Since the grain temperature beyond the moisture front was cooled to a temperature slightly higher than the air entry into the bin, this in turn caused air humidity to equalize with that of the intergranular air humidity. In Figure 7.7 the grain bulk on the left of the figure shows temperature and moisture content conditions after the trailing edge reached the top of the grain bulk. The grain moisture content at



Figure 7.9

329

Schematic presentation of the cooling and drying front in a grain bulk, at initial temperature of 30°C and 12% moisture content wet basis (in equilibrium with 64% relative humidity). Grain temperature and moisture profile shows cooling with moisture decrease near the aeration duct.

the air inlet area increased to 18% in equilibrium with 90% ambient air relative humidity. The wheat initial moisture content remained constant above the air inlet section at 12% wet basis (in equilibrium with 64% RH). The trailing edge reached the top of the grain bulk with a temperature of 19°C. Assuming no fan heat of compression, the grain temperature at the air inlet to the bulk would remain at or near the ambient air temperature used for aeration, 16°C. As shown in Figure 7.8, the grain moisture content increased at the air inlet area to a state of equilibrium with 90% ambient air relative humidity. Like the previous example from Figure 7.7, this example also assumes a constant initial wheat moisture content of 12% wet basis, in equilibrium with 64% RH, above the air inlet section. The trailing edge temperature was 19°C as it reached the top of the grain bulk. Because the assumption of no heat of compression, the grain bulk temperature is shown as being equalized with the ambient temperature of 16°C. 7.1.8

Effect of Cooling at Low Humidities

The amount of moisture removed in the leading edge of the cooling front is the difference in air moisture ratio of the aeration air between points B and C, Figure 7.9, and is limited by the thermal energy available in the grain at the start of aeration. After the trailing edge of the cooling front has passed through the grain mass, the air exits the grain at temperature B (Figure 7.9). By continuing to aerate, the only drying that will occur is in the drying front (between points A and B) and is the difference in absolute air moisture ratio of the aeration air. The final moisture content of grain will be in equilibrium with the equivalent air relative humidity of point A. See also Sections 4.6 and 6.1.2.1. The moisture carried out per unit mass of aeration air can be higher during the cooling process (process points C to B) than for the adiabatic drying in the drying front (process points B to A)


330

Figure 7.10


Schematic presentation of the grain temperature and moisture content variation through the grain bed for the cooling and drying front shown in Figure 7.9.

but is limited by the thermal energy available in the grain initially. Cooling moisture removal occurs throughout the grain mass because it is dependent on energy in the grain rather than energy in the air, as is the case with drying in the drying front. These effects are illustrated in Figures 7.9 and 7.10. Figure 7.9 shows the limited drying that occurred during cooling. In this example, initial grain mc is equivalent to 12%, in equilibrium with 64% air RH. Air entering the grain mass has a wet-bulb temperature of 15°C (59°F), but ambient air RH is 40% with a dry-bulb temperature of 23.5°C (74.3°F). Therefore, the evaporative cooling that takes place results in drying a restricted region in the lower grain mass to 9.5% mc, which is in equilibrium with 40% RH. But while some drying occurs in the lower grain mass, the grain mc at the top of the grain mass remains unchanged. In this lower grain region, grain dry-bulb temperature tends to equalize to 23.5°C (74.3°F) with the incoming air. If aeration is continued using the same constant conditions, it is possible for the leading edge grain mc to reach about 9.5% in a thin layer at the bottom while remaining at about 12% throughout the grain mass up to the top. This occurs from excessive aeration under conditions where a slight drying of 1 to 2% of moisture occurs using pressure systems. Because of the slight heating, typically 3 to 5°C (5.4 to 9°F), that occurs using centrifugal fans, it is common to observe this drying effect, especially if aeration continues after the cooling cycle is complete. The evaporative cooling effect that can occur by using the wet-bulb temperature is shown in Figure 7.9. Although the dry-bulb temperature lies at 23.5°C (74.3°F), the wet-bulb temperature at 15°C (59°F) is sufficiently low to reduce the grain temperature from 30°C (86°F) to 19°C (66.2°F), with grain mc at 12%. Since the moisture front closely follows a constant air enthalpy (parallel to the wet-bulb temperature) line, wet-bulb temperature is considered a more satisfactory criterion than dry-bulb for the control of grain aeration systems (Griffiths, 1967). The value of using the wet-bulb temperature in controlling aeration systems will be discussed later in this chapter. The variation of the grain bulk temperature and moisture content in a drying front is shown schematically in Figure 7.10. At the end of the evaporative cooling stage, grain moisture content decreased at the air inlet area to 9.5% moisture content, as it reached equilibrium with 40% ambient air relative humidity. In the rest of the bulk and in the upper layers, the initial wheat moisture content remained almost constant above the air inlet section at 12% wet basis (in equilibrium with



331

64% RH). The trailing edge temperature as it reached the top of the grain bulk was 19°C. Note the grain temperature at the air inlet to the bulk is the ambient dry-bulb air temperature used for aeration (23.5°C). If aeration under these conditions continues to a state of equilibrium, the grain will gradually stabilize at 23.5°C and 9.5% mc as illustrated in the dried layer of grain at the bottom of the grain bulk. This aspect of drying effect is discussed in Chapter 6, Figure 6.14, and illustrated in Figure 7.9. This example illustrates that significant grain moisture (market weight loss) can be removed during long periods of aeration when air conditions are below the equilibrium RH conditions of the grain mass. The conditions are relevant where humidity and temperature are constant in the ambient air — a state that normally does not occur for more than a few hours at a time in nature, but can be developed in controlled laboratory tests or with commercial chilled aeration refrigeration units for indefinite periods. However, the above examples emphasize the disadvantages of aerating grain at extreme temperature and humidity conditions. In practice, as shown in Figure 7.4, both relative humidity and temperature levels in the ambient air generally fluctuate continually, masking these effects of moistening or drying the grain when the wet-bulb temperature of ambient air is well below that of the grain. However, even with daily cyclic temperature and humidity conditions, as long as the air moisture condition remains stable, the wet-bulb temperature will remain the same. Therefore, although the wet-bulb temperature can be used to define the cooling capacity of the ambient air, the effects of extreme temperature and humidity conditions over extended periods of aeration should also be considered.

7.2 AERATION METHODS 7.2.1

Direction of Airflow

7.2.1.1 Up-Flow (Pressure) vs. Down-Flow (Suction) Aeration Airflow The question of whether air should be blown or sucked through the grain is a subject of controversy that has caused more heated discussions than any other aspect of aeration. Arguments for and against the two procedures were reported by Burrell (1974), Holman (1960), Lasseran (1981), and Hellevang et al. (1997). There are well-established practices based on know-how in different countries, where some are in favor of blowing and others in favor of suction (Shove, 1968; Elder, 1969; Noyes, et al., 1991). Several points should be clarified before the advantages and disadvantages are considered for each aeration procedure. In adequately planned aeration systems, especially in vertical storages, almost the same airflow rate can be obtained by blowing or suction. With three-phase axial fans, the direction of rotation of impellers can be changed by reversing any two phases, though only about 70 to 80% of their “forward” volume can be obtained. The fan blades do not operate as efficient air foils when rotating with the thin, trailing edge of the blade cutting the air and the thick edge trailing. Thus, the procedure of reversing fan blade rotation is not recommended for permanent changes in direction of airflow. Changing the direction of centrifugal fan wheel rotation must not be done to reverse the direction of airflow under any condition. Reversing the direction of centrifugal fan wheels results in a complete change of the fan characteristics, resulting in very little air movement. It is necessary to turn the fan housing so the aeration duct is attached to the opposite fan opening to reverse the airflow direction. For pressure fan systems, the fan outlet is connected to the supply duct of the aeration system. When installing suction fans, the fan inlet is connected to the supply duct. Vane-axial fans usually have identical flanges on each end, so they can be turned around and rebolted to the supply duct. But centrifugal fans require major revisions of the supply duct connection to reverse airflow direction.


332


Figure 7.11

Section of psychrometric chart schematically showing the effect of heat of compression on reducing air relative humidity.

The arguments for and against blowing or suction are discussed in the following section. Both procedures should be considered if maximum advantage is to be taken of ambient conditions to improve grain storability. 7.2.1.2 Advantages of Pressure Airflow 1. All fans operating on pressure airflow experience an air temperature rise from heat generated by the work of compressing the ambient air. Since weather conditions for cooling grain are available in temperate climates, and most pressure fans heat ambient air by an estimated 2 to 5°C (4 to 9°F), pressure airflow aeration systems are generally suitable for these regions.

Example 7.2 Centrifugal fans on a corrugated steel tank with 18 m (59 ft) of sorghum may develop 2.5 to 3.0 kPa (10 to 12 in w.c.) of static pressure and add 3 to 5°C (4 to 9°F) temperature rise to outside air. A vane-axial or tube-axial direct drive propeller fan on a 9 m (30 ft) tank depth may develop 0.75 to 1.0 kPa (3 to 4 in w.c.) static pressure. “Air-over” motor heat on direct drive vane-axial fans will add 0.5 to 0.75°C (0.9 to 1.4°F) and an additional 0.5 to 1.7°C (0.9 to 3.1°F) from “heat of compression” for a 1.0 to 2.7°C (1.8 to 4.9°F) total air temperature rise. Pressure systems operating with a temperature rise of 3 to 5°C (4 to 9°F) can be used to cool grain, but the added heat reduces ambient air relative humidity. For example, with a 4°C (7.2°F) temperature rise, air at 15°C (59°F) and 90% RH will increase its temperature to 19°C (66.2°F); and the air RH will drop to about 70%. This air condition has the capacity of reducing the air relative humidity as shown in Figures 7.11 and 7.12. In this example ambient air conditions were 15°C dry-bulb temperature and 90% relative humidity (Figure 7.11). The estimated heat of compression was 4°C as a result of static pressure of 2.8 kPa by aerating at an airflow rate of 6 (m3/h)/tonne in a steel tank 18 m high containing sorghum. Note that as a result of 4°C of heat of compression, air moisture content



Figure 7.12

333

Schematic presentation of temperature profile of a bin containing sorghum showing the effect of heat of compression on reducing air relative humidity from 90 to 70%. Note that moisture content of sorghum near the aeration duct is not affected (same ambient air conditions as in Figure 7.11).

remained constant at 9.5 g/kg, while the air relative humidity was reduced from 90 to 70% and the wet-bulb temperature increased from 14.1 to 15.6°C. Figure 7.12 is a schematic presentation of the same ambient air conditions given in Figure 7.11 that shows the progressive change in temperature profile of a bin containing sorghum. Due to the heat of compression of the fan, ambient air relative humidity is reduced from 90 to 70% in the aeration duct. As a result of the lowered relative humidity, the sorghum moisture content near the aeration duct is now in equilibrium with the entering air at 70% RH; thus, the grain is not affected.

2. 3.

4.

5.

6.

As shown in Example 7.2, heat of compression can provide enough sensible heat to lower air relative humidities to a point of equilibrium with grain at safe storage levels, so moisture is not added. Also, compression heat may reduce air RH below the grain ERH so that grain with marginal moisture levels for safe storage can be dried slowly to a safer level. If warm grain is loaded on top of cool grain, rewarming of the lowest section is prevented by blowing air upward. Moisture increase of the cool layer of grain is also prevented. To decide when to stop aerating, it is necessary to know when the cooling front has passed through the grain. This is done by temperature measurement at the last (top) layer of grain to be cooled. Since the last cooled layer in upward aeration is located near the surface of the bulk, temperature measurement can be easily done using simple temperature probes. This aspect is very important in large bulks not equipped with remote temperature-control systems. Uniform airflow is desirable as it is directly related to uniform cooling of the grain. In large, flat storages with long aeration ducts, it was shown by Burrell (1970) that more uniform airflow along the ducts and distribution through the grain is obtained by upward (pressure) than by downward (suction) airflow. A fan operated on pressure is slightly more efficient than when the same fan is operated on suction. When the volume is moved by pressure, the compressed air is denser than air under suction, so there are more kg/min moved under pressure than under suction by about 2 to 4%. Since warm air is less dense than cold air, upward airflow is in the direction of natural convection currents that needs less energy for operation of the system (Foster and Tuite, 1992). Convection currents deriving from temperature gradients are indirectly detectable by the moisture translocation


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in the grain of mass. Cold air in the outer grain layers move vertically downward near the walls of bins and then rise upward in the center, causing moisture translocation. However, this movement is so slow that it is usually not directly detectable using a conventional thermoanemometer. Although the advantage of convection currents in upward aeration is noted (Driscoll and Srednicki, 1998), from a practical standpoint of system design they have no noticeable effect on direction of airflow during aeration. Experimental data to assess the benefits of this aspect of aeration is lacking in literature.

7.2.1.3 Disadvantages of Pressure Airflow 1. For grain bulks stored at high temperature that are to be cooled during cold weather — if aeration is delayed to allow natural cooling of the grain surface — at the start of operating the aeration system, warm air will be blown through the cool layer of surface grain. As a result, moisture will condense on the cold grain and a layer of damp, moldy grain may form. 2. A primary problem encountered in pressure systems is condensation dripping from the underside of the roof onto the top grain surfaces and top sidewall grain. Roof condensation (not cool surface grain) is the primary grain surface moisture source in pressure-aerated bins. 3. An equally serious problem can be moist air that is pushed up downspouts and into elevator distributors or turn-heads, where condensation drains down the spout (and down other tank or silospouts and the leg casing) and causes deep pockets of wet moldy grain under fillspouts. 4. A significant problem of pressure aeration is “heat of compression” and motor heat (of “air over” motors in propeller or vane-axial fans) added to the cooling air in the fan. Heating air due to heat of compression in pressure systems may exceed desirable grain temperature limits, especially in tall silos. This is a serious concern in subtropical and tropical regions, where ambient cooling temperature conditions are marginal. 5. Roof exhaust fans are often not adequately designed to control under-roof condensation. More roof vent area should therefore be recommended for pressure aeration systems in subtropical regions than in temperate regions to prevent roof condensation during pressure aeration. 6. In some aeration systems the fan heat is negligible. Shallow depths such as in flat storages and low-height farm bins; low resistance to airflow of large kernels or seeds, like maize and soybeans; and low airflow rate are situations characterized for preventing development of significant fan heat in pressure systems. In these cases, if prolonged aeration is undertaken regardless of air humidity, it is possible for grain to absorb moisture around the duct. Thus, the average relative humidity of the ambient air dictates the equivalent grain moisture increase. 7. If fan inlets are located where grain is being handled, and prolonged aeration is performed near a dusty area, duct perforations could become blocked by grain dust. It may become necessary to temporarily stop aeration during grain handling to prevent fine dust and trash particles from accumulating on the inside surface of the duct. 8. During filling and unloading operations in pressure aeration systems, fines and f.m. (foreign material) sift down through the duct perforations. Then high-velocity air pushes the light fine material, grain dust, and dockage to the end of the ducts, where it gradually fills up the duct volume from the end of the ducts toward the middle of the bins. This happens due to inadequate cleanout and maintenance of the ducts because most duct covers are not designed by bin manufacturers for easy removal for housekeeping and sanitation. As ducts fill up, those sections of the bin near the plugged ducts do not receive adequate airflow and therefore may not cool satisfactorily. 9. In ducts, airflow speeds are sometimes high. Under excessive air velocities, the grain dust and f.m. in the duct are fluidized, causing a stack-up at the end sections that blocks the duct. This blockage constricts the duct and results in a gradual rise in static pressure. Over time, as the ducts fill up and static pressure increases, total airflow becomes noticeably reduced and aeration time and expense is increased, and grain heating and spoilage may occur. 10. Weather shielding of fan inlets cannot be overemphasized. For fans located in the open, a rain shield should be installed above the fan inlet to prevent sucking rainwater into the fan. Rainwater sucked into the aeration fan system can result in standing water in the ducts, rusting and corrosion of duct materials, mold development in broken grain fines which attracts insects, and increased moisture levels of the bottom grain. During long periods of rainy weather, all aeration should be stopped until the rain stops.



335

7.2.1.4 Advantages of Suction Airflow 1. Suction aeration is frequently used, particularly in cool weather regions, to prevent possible condensation of moisture in the exhaust air on the undersurface of the bin roof (Holman, 1960; Kline and Converse, 1961). Downward aeration should also be used in subtropical climates when grain is warm at the beginning of the cool season. 2. Suction airflow provides rapid “early cooling” of the top 1 to 3 m (3 to 10 ft) of grain where insect populations are heaviest. Rapid cooling of the surface and upper grain mass may provide immediate control of insects, minimizing damage and reducing or eliminating the need for fumigation and the use of residual pesticides. Early cooling should be a high priority process in grain management. 3. Suction systems used in tropical and subtropical regions, where the cooling capacity of ambient air is usually marginal, provide full ambient air cooling potential. Full advantage can be taken of the cooling capacity of the air because fan heat is not added to the air. In tall silos, suction air is preferred in cases where the unavoidable high grain resistance to airflow results in excessive fan heat added to the ambient air.

Example 7.3 When 12% moisture wheat at 30°C (86°F) is suction cooled with ambient air at 19°C (66°F) and 64% RH, the wheat should cool close to the cooling temperature of the ambient air. For 19°C (66°F) dry-bulb and 64% RH, the wet-bulb temperature of the ambient air is about 15°C — very close to the grain bulk interstitial air wet-bulb temperature reading of about 14.7°C. Fan compression heat in such conditions would reduce the cooling capacity of air 2 to 5°C or more. For example, with pressure systems, additional fan heat of 5°C will increase the ambient air wet-bulb temperature to 17°C, thus reducing the cooling capacity of air. 4. If high humidity ambient air is used, the consequent moistening effect takes place over the whole upper grain surface area; therefore, the moisture increment per unit of grain will be less significant. (This is in contrast to aeration by pressure airflow, where all moisture transfer is concentrated on those grains in close proximity to the aeration duct.) 5. Air can be directed to desired regions of the bulk by covering the low-resistance areas with plastic sheets. This procedure is applicable before the ducts are completely covered by grain in horizontal storages. Uncovered ducts can be temporarily blocked by covering them with plastic sheets. Fan suction holds the plastic against the duct, sealing it. Also, to increase airflow rates in slow-cooling areas of the grain bulk, cover the grain surface with plastic sheets in areas with high-cooling airflow, increasing airflow through grain surfaces that are left exposed.

7.2.1.5 Disadvantages of Suction Airflow 1. The beneficial effect of fan compression heat in reducing air relative humidity is lost. It is difficult to control the relative humidity of air that enters the grain bulk, especially in subtropical climates, as air relative humidity tends to be high when the lowest air temperatures are available. Attempts to aerate at lower relative humidities usually result in decreasing the daily hours of aeration. Aeration performed by suction without a humidistat control has shown that a slight increase in moisture could be detected to a depth of 120 cm (4 ft) below the grain surface (Table 7.3). Grain stored for long periods at these moisture contents may deteriorate if the grain is stored at marginally safe moisture levels, i.e., at 68 to 72% ERH. However, if the stored grain moisture level is 1 to 2% lower than the upper moisture content limit for safe storage, these slight increases in moisture content are not considered significant; and aeration without humidistat control is usually an acceptable practice. 2. The slowest area to cool is located near the bottom of the grain bulk, where temperature monitoring is usually difficult and often impossible using portable temperature probes. Yet without this knowledge, a decision on whether to discontinue aeration cannot be made safely. With temperature thermocouples installed in the storage, the lowest level thermocouple readings can provide information showing that bottom grain is cooled and the aeration fans can be stopped.


336


Table 7.3

Final Wheat Moisture Contents (% mc) of the Upper Layers of a Suction-Cooled Bulk with Initial mc of 11.5%

Depths from surface (cm) 0 to 5 20 40 60 80 100 120 140

Point A

Point B

Point C

11.8 13.1 13.7 13.5 13.4 12.9 12.9 12.8

11.6 13.3 13.4 13.1 13.2 12.8 12.2 12.3

11.8 12.6 13.7 13.8 13.5 13.0 12.9 12.6

Notes: 1. Wheat aerated for 1794 hours during 22 months storage. 2. Sample Points A, B, and C were 3.8 m (12.5 ft) apart. From Navarro, S. and Calderon, M. (1982). Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome.

3. For warm grain loaded on top of cool grain, downward airflow should not be applied. This runs the risk of rewarming the cool layer and causing moisture condensation if the mass of warm grain is large compared to the cool grain and the temperature differential between cool and warm grain is large — 10 to 15°C (18 to 27°F) or more. If there is a question of moisture condensation problems, aeration fans should be operated an additional period of time to gradually move the adsorbed surface moisture. 4. A fan operated on suction is slightly less efficient than the same fan operated on pressure, due to its reduced ability to move the equivalent mass of air per minute. In pressure systems, the compressed air is denser than air under suction where the air is expanded and the air density is lower, so there are more kg/min moved under pressure than under suction by about 2 to 4%. 5. Airflow paths for pressure vs. suction aeration are initially similar, but a major disadvantage of suction systems is that the suction airflow may channel fines, foreign material, and trash onto perforated duct or false floor surfaces. In some cases, the duct surface may gradually seal over, greatly restricting airflow. Indication of duct surface sealing problems would be a gradual rise in static pressure over time and noticeably reduced airflow. 6. Suction can cause roof collapse of bins if roof vents freeze over during winter storms in cold climates.

7.2.1.6 Conclusions About Airflow Direction As with most processes, there are significant advantages and disadvantages in selecting a specific aeration method. The designs of aeration systems involve many variables; it is important to recognize when the advantages of upflow vs. downflow, or pressure vs. suction, outweigh the disadvantages. The decision to use pressure or suction aeration should be made after considering the factors listed above, which are summarized graphically in Figure 7.13. The decision should also be based on the geographic region, prevailing weather conditions, and the product stored. Either pressure or suction airflow could be used in most grain storage structures, and most aeration systems can be adapted for pressure or suction airflow depending on the specific situation. WARNING: There is one condition where pressure airflow should be used that must not be ignored — aeration in regions where aeration roof vents can become iced over due to freezing rain or heavy snow. For example, suction systems are not used for storage bins in the central and northern U.S. corn belt because of the many roof collapses that occurred from 1950 to 1970 before the grain industry recognized that suction airflow was not satisfactory.



Figure 7.13

337

Different effects of ambient temperature and relative humidity on the grain bulk as a result of operating the aeration system in pressure or suction airflow. (Data from Navarro, S. [1976]. Aeration of bulk-stored grain in commercial facilities, Israel Agric. Res. Org. Special Publication No. 56 [Hebrew, with English summary]. With permission.)

Because only limited cooling weather conditions prevail in the tropical and subtropical regions of the world, the situations that are frequently encountered conform to the guidelines listed in Table 7.4 and Figure 7.13. Suction Airflow • Suction airflow provides very quick early cooling of the top of grain where insect populations are heaviest. • Suction airflow should be used to aerate warm grain when aeration is started during cool weather, especially for grain stored in metal bins. • Suction airflow should be used in tropical or subtropical humid climates when cool weather conditions are marginal.

Pressure Airflow • Pressure airflow should be preferred in large flat storages for uniform airflow. • Pressure airflow can usually be performed regardless of the air humidity. • Pressure airflow minimizes or eliminates the risk of roof collapse from icing of aeration vents.

7.2.2

Safe Storage of Dry Grain by Aeration

7.2.2.1 Cooling Dry Grain A primary objective of aeration is to maintain the moisture content of dry grain at a safe level. Based on an understanding of the air humidity and grain moisture interrelations discussed in


338

Table 7.4


Summary of Advantages and Disadvantages of Upward (Pressure System) and Downward (Suction System) Airflow

Advantages of Upward (Pressure) Airflow

Advantages of Downward (Suction) Airflow

1. Air temperature rise from heat generated by the fan warms (but dries) the cool air 2. Enables adding warm grain to the top of the grain mass 3. Enables easier temperature monitoring of the last (top) layer of grain to be cooled

1. Condensation on the under-surface of the bin roof is minimized 2. Rapid early cooling of the top of grain, where insect populations are heaviest, is achievable 3. Provides full ambient air cooling potential where cooling capacity of ambient air is usually marginal in tropical and subtropical regions 4. High air humidity is distributed to the whole upper grain surface, keeping moisture increment per unit of grain at acceptable levels 5. Air can be directed to desired regions of the bulk by covering the low-resistance areas with plastic sheets

4. More uniform airflow distribution is obtained in large flat storages with long ducts 5. A fan operated on pressure is slightly more efficient than when the same fan is operated on suction Disadvantages of Upward (Pressure System) Airflow

Disadvantages of Downward (Suction System) Airflow

1. Condensation dripping from the roof undersurface frequently increases grain surface moisture 2. In delayed aeration, moisture condenses on the cold grain causing mold damage and crusting

1. Difficult to avoid grain moisture increase at surface of the bulk, when aeration is in humid climates

3. Moist air is pushed up down-spouts and into elevator distributors or turn-heads, condenses, and can drain into several bins, causing deep pockets of wet moldy grain under fill-spouts 4. Heat of compression in fans may increase cooling air temperature above desirable limits, especially in tall silos and when cooling is the only objective 5. Larger roof vent area is recommended in tropical and subtropical regions than in temperate climates 6. Where fan compression heat is negligible, special precaution is needed to avoid moisture increase around the ducts

2. The slowest area to cool is located near the bottom of the grain bulk where temperature monitoring is usually difficult without temperature cable sensors that reach the bottom of the bin 3. Warm grain cannot be loaded on top of cool grain, because of the risk of rewarming the cool layer and causing moisture condensation 4. Fan operated on suction is slightly less efficient (estimated 2 to 4% reduction) than when the same fan is operated on pressure 5. Suction airflow may channel fines or foreign material and trash onto perforated duct or false floor surfaces; duct surface may gradually seal over, restricting airflow 6. Warning: In cold latitudes where snow accumulates on bin roofs and freeze vents, suction airflow should not be used as negative pressure can cause roof collapse, even with small fans

What Special Precautions? 1. When aeration fans are operated in dusty, trashy areas, duct and perforations could become blocked 2. In narrow and open aeration ducts, high-velocity air may push light material, trash, and dockage to the end of the duct, blocking airflow in that part of the duct 3. Rain shields should be installed above fan inlets to prevent rainwater from being sucked into fans

Chapter 4, an aeration strategy can be adopted that will prevent the grain moisture from increasing to unsafe levels. While developing an aeration system to control insect populations, Wilson and Desmarchelier (1994) developed the critical control term seed wet-bulb temperature (SWBT). Although Wilson and Desmarchelier (1994) developed the SWBT term, the authors of this chapter feel that this term implies that this technique is unique for seed aeration and its use is only for the preservation and cooling of seeds. In practice, the same control method can also be used for the aeration of grains other than seeds and for commodities that have not been classified as seeds. When aeration is considered for oilseeds, beans, and nuts — such as sunflower seeds, rapeseeds, soybeans, and peanuts — the term grain also is misleading. However, since aeration is largely used for grain such as wheat, barley, sorghum, and maize, it appears that the use of the term commodity instead of seed



339

would be better for use with aeration. Therefore, to give the SWBT term a wider meaning for the control of insects in all types of grain including seeds, hereafter in this text, the term CWBT will be used to describe commodity wet-bulb temperature. In spite of making this change in terminology from SWBT to CWBT for this text, we give full credit for the development of the SWBT insect control term to Wilson and Desmarchelier (1994). The CWBT is the wet-bulb temperature value of air at equilibrium within the interstitial air spaces of a seed or grain bed. The CWBT value is obtained when seed or grain mass interstitial air is blown at 300 m/min or 5 m/sec across a wet-bulb thermometer. Air state conditions that are in equilibrium with the moisture content of the grain is believed to be maintained for air flowing between the seeds or the grain kernels at the low flow rates used for aeration. But CWBT cannot be maintained at the high airflow rates typically used for seed and grain drying. After aeration is terminated in dry grain, if grain interstitial air is blown over a wet bulb thermometer, the thermometer gives a value for the commodity wet-bulb temperature (CWBT) of the interstitial air. Sutherland et al. (1971) and Navarro and Calderon (1982) described the equilibrium heat and moisture transfer in airflow through grain. These air conditions are best shown when the characteristic temperature and moisture fronts are plotted on a psychrometric chart (Figures 7.6, 7.7, and 7.9). The temperature of the leading edge of the aeration cooling front lies along the grain ERH line, whereas the grain moisture content line closely follows a wet-bulb temperature line. Using grain ERH values, the temperature and moisture content changes that occur during aeration of grain can be closely predicted. The majority of the grain bulk is not cooled to the inlet air dry-bulb temperature, but it reaches a temperature that can be plotted on the air and grain moisture content lines of the psychrometric chart (Figures 7.6, 7.7, and 7.9). Therefore, since the moisture front closely follows a constant wetbulb temperature (and air enthalpy) line, wet-bulb temperature is more satisfactory than dry-bulb temperature for controlling grain aeration systems (Griffiths, 1967). Ambient air is subject to marked fluctuations in temperature and humidity between day and night. During the day, higher temperatures with lower humidities prevail. To determine whether the ambient air is suitable for aerating the grain, it is necessary to locate the air state point on the psychrometric chart. This can be done either by measuring any two state points (i.e., the wet bulb and dry-bulb temperatures) or the relative humidity and dry-bulb temperature. Since wet-bulb temperature is the best means of assessing cooling air quality, this value should be used for controlling aeration systems designed to cool grain. However, the use of wet-bulb temperature for aeration control is not widely understood by commercial grain industries throughout the world. CWBT aeration controllers are therefore used very little by world grain industries. The only CWBT aeration controller known to be offered commercially is by Winks (1998a, 1998b). That unit is discussed later in this chapter. To understand the principals of CWBT aeration control, a sound understanding of the interactive relationships between wet-bulb, dry-bulb, and the equilibrium relative humidity conditions of the grain mass must be developed (see Chapter 3). If the selected air has a wet-bulb temperature several degrees below the grain bulk wet-bulb temperature and a moderate to high RH, warm grain will be cooled as the cool air passes through warm grain during the aeration. Since cool air blown into a warm grain bulk will be heated, its immediate or instantaneous absolute humidity remains unchanged; but the RH and temperature will change until equilibrium is reached. As long as grain temperature is higher than the dry-bulb air temperature and the RH intersection on the psychrometric chart (Figure 7.5), no moisture increase will be detected in the grain. At point A on the chart (Figure 7.6), both wet-bulb temperatures of air and grain reach equilibrium with the dry-bulb temperature. This is the theoretical point where cooling of grain should be discontinued to prevent the grain moisture content from increasing. If aeration is continued, the grain surface that comes into contact with the air will start to change in moisture content toward equilibrium with the air RH.


340

Figure 7.14


Section of psychrometric chart to show the relationship between the moisture content (wet basis) of British Wheat shown in Figure 7.15 (Wilson and Desmarchelier, 1994) in comparison to the critical air relative humidity for mold growth (Lacey et al., 1980) and wet-bulb temperature lines. (Data from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32.)

It is very difficult to calculate exactly when grain temperature has reached equilibrium since a number of variables are involved in this process. These variables include air velocity, air distribution in the aeration duct, changing air temperature and humidity, and nature of the grain. Therefore, by operating aeration systems when the wet-bulb temperature is the lowest (usually at night when the dry-bulb temperature is lowest and air RH is the highest), grain moisture content will increase slightly due to absorption of released moisture from the air as it enters the grain mass. Thus, in suction systems, the moisture content of surface grain will increase slightly; and in pressure systems, the moisture content of grain around the aeration duct will increase slightly — but the amount of grain affected is negligible. In practice, when sophisticated aeration control means are not available, a practical rule of thumb is that as long as the air dry-bulb temperature is at least 6°C (10.8°F) below the grain temperature, the majority of the grain bulk can be aerated at all RHs. However, this operating method lacks the advantage of precision operation of the system using a wet-bulb aeration controller. 7.2.2.2 Calculation of CWBT To utilize CWBT to manage an aeration system and control insect populations, aeration practitioners need to be able to determine a CWBT value for the stored grain. Because it is usually inconvenient and difficult to measure CWBT by actually extracting an air sample from within the grain bulk, the CWBT is calculated from known moisture equilibrium properties for the type of grain and from the grain’s dry-bulb temperature and moisture content (Figure 7.14). However, this calculation is not simple. The inability to readily determine CWBT has been a barrier to its adoption as an index for controlling insect population growth. Figures 7.15 through 7.23 remedy this problem by allowing



Figure 7.15

341

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for British wheat. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

CWBT to be readily determined for nine types of stored products. To determine CWBT from these figures, the user need only know the grain type, its dry-bulb temperature, and its moisture content. The method used to calculate the CWBT data for Figures 7.15 through 7.23 is summarized as follows: The grain condition is specified from its type (e.g., wheat), its dry-bulb temperature, Tdb,°C, and its moisture content, W’, (fractional, wet-basis). From these data Hunter’s isostere equation (Hunter, 1987) is used to determine the equilibrium water activity, aw , the decimal basis relative humidity of the interstitial air in moisture equilibrium with the commodity. From aw , the absolute humidity, w, kg/kg (dry-basis) of the interstitial air is calculated. Then the Tdb and w values are used in an iterative procedure to find the CWBT value. The method for calculating CWBT begins by converting the wet-basis grain moisture content, W’, kg/kg, to dry-basis, W, kg/kg, using:

W=

W′ 1− W′

(7.1)

There are many different grain equilibrium isotherm relationships available for calculating water activity. The isostere equation is used because it applies to nine different types of grain. The isostere equation is: a ln bW − (W w0 ) c ln dW hs −1 = n hv 1 − (W w0 ) n

(7.2)


342

Figure 7.16


Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for barley. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

where: a, b, c, d, n, and w0 are constants for each grain type hs is the latent heat of water bound in the grain kJ/kg–1, and hv is the latent heat of vaporization of free water, kJ/kg–l water A value for (hs hv ) − 1 is obtained from Equation 7.2 and then used to calculate the equilibrium water activity, aw , from: p  aw =  s   p0 

hs ,hv −1

(7.3)

where p is a constant for each grain type, and pw , Pa, the saturation vapor pressure of water, is calculated from the absolute (dry-bulb) grain temperature, TK, Kelvin (where TK = Tdb + 273.15) using (Hunter, 1987): ps =

 −6800  6.0 × 10 25 exp   5 TK  TK 

(7.4)

Next, aw and ps are used to calculate the partial pressure, p, Pa, of the moisture in the interstitial air, from: p = aw ps

(7.5)



343

Figure 7.17

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for maize. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

Figure 7.18

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for peanuts. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)


344


Figure 7.19

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for rapeseed. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

Figure 7.20

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for paddy. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)



345

Figure 7.21

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for sorghum. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

Figure 7.22

Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for soybeans. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)


346

Figure 7.23


Insect development zones for the corresponding wet- and dry-bulb temperatures and mold envelope (defining microflora development zone) for sunflower seed. (Redrawn from Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

Then, the absolute humidity, w, kg/kg (dry-basis) is calculated using: w = 0.622

p PA − p

(7.6)

where PA, Pa, is the atmospheric pressure, and PA ≡ 101325 Pa. At this stage of the calculation procedure, the equilibrium absolute humidity, w, of the interstitial air at the grain dry-bulb temperature, Tdb, can be determined. The desired CWBT value is the wetbulb temperature of this air. This value cannot be calculated directly, so an iterative method is used. The method adopted here, as diagrammed in Figure 7.14, assumes that a line of constant air enthalpy through the point (Tdb, w) on a psychrometric chart is parallel to a line of constant wet-bulb temperature through the same point. Inspection of a psychrometric chart will show that this is a reasonable approximation. To facilitate this inspection, based on values given in Figure 7.9 for British wheat by Wilson and Desmarchelier (1994), Figure 7.14 was prepared that constitutes a section of a psychrometric chart. The selected values for moisture contents of 13%, 14%, and 15% can be compared with the critical air relative humidity for mold activity (this is also discussed as the mold envelope in the next section), and the 70% relative humidity values. The 70% RH values are shown here because this value is considered as the critical RH limit for grain conservation. Figure 7.14 clearly compares the three different criteria used for estimating the storability of grain — namely, the critical equilibrium RH for grain storage (70%), the critical air RH for mold activity, and the grain moisture content. It is important to note the proximity of the lines for the 70% RH values and for the 14% mc values that are the acceptable critical limits for the storability of grain.



347

The iterative procedure calculates the dry-bulb temperature at which the constant enthalpy line intersects the air-water saturation curve. As the enthalpy and wet-bulb lines are assumed to be parallel, this temperature is also the dry-bulb temperature at which a constant wet-bulb line intersects the air-water saturation curve. Because the wet-bulb and dry-bulb temperatures are the same at the saturation curve, this temperature is also the required grain wet-bulb temperature, CWBT (Wilson and Desmarchelier, 1994). To begin the iterative procedure, the enthalpy of the interstitial air at the point (Tdb, w), is calculated using (Sutherland el al., 1971; Sutherland, 1983): h = hconstant = CaTdb + w (Cw Tdb + hv )

(7.7)

where h is the air enthalpy, kJ kg–l dry air, Ca is the specific heat of dry air, kJ kg–1 (°C)–l, Cw is the specific heat of liquid water, kJ kg–1 (°C)–l, and hv is calculated from:  dh  hv = 2502.39 +  v  Tdb  dTdb 

(7.8)

where (Wilson, 1987): Ca = 1.0048 kJ kg −1 (°C)

−1

Cw = 4.1868 kJ kg −1 (°C)

−1

and: dhv dTdb = 2.3768 kJ kg −1 (°C)

−1

the datum for all enthalpies is 0°C, and all specific heats are at constant pressure. Equation 7.7 is rearranged to obtain an expression for the constant enthalpy line: wh =

hconstant − 1.0048Tdb 2502.39 + 1.81Tdb

(7.9)

where the subscript h of wh indicates that this value has been obtained from the enthalpy function, Equation 7.7. To determine the CWBT, the temperature at which Equation 7.9 intersects the airwater saturation curve, defined by Equation 7.4, needs to be found. Equation 7.4 enables calculation of ps as a function of the dry-bulb temperature, and from Equation 7.6, the saturation absolute humidity, ws, kg/kg, dry-basis, corresponding to ps, is: ws = 0.622

p PA − ps

(7.10)

The intersection of Equation 7.9 with the saturation curve occurs when wh = ws. To find the temperature at which this occurs, a difference function: ∆ w = wh − ws

(7.11)


348


is formed; and Brent’s method (Brent, 1973) is used to find the temperature that makes this function zero. This temperature is the grain wet-bulb temperature, CWBT. This method has been used to calculate the CWBT data for nine grain types as presented in Figures 7.15 through 7.23. 7.2.2.3 The Mold Envelope When using CWBT to manage an aeration system, it is important to avoid conditions that promote mold growth. Figures 7.15 through 7.23 also indicate limiting grain conditions under which molds can grow — these limiting conditions are referred to as the mold envelope. Experimental data for limiting water activities that allow growth of molds have been taken from Table 1.4 (Chapter 1) of Lacey et al. (1980). At water activities below the limiting experimental values, molds do not grow or grow very slowly. Above the limiting values, progressively more rapid mold growth is possible with increased water activity levels. For the limiting water activity data to be useful, equilibrium grain conditions corresponding to these water activities are needed. To enable these equilibrium conditions to be determined, the following polynomial has been fitted through the experimental data of Lacey et al.: aw* = A + BTdb + CTdb2 + DTdb3 + RTdb4

− 5 < Tdb < 45

(7.12)

where aw* is a limiting water activity calculated from the polynomial, Tdb is the dry-bulb temperature,°C, and A, B, C, D, and E are polynomial coefficients having the following values: A B C D E

= = = = =

+0.902763 –0.105872E – 01 –0.385927E – 03 +0.227389E – 04 –0.221648E – 06

Table 1.4 compares the experimental data against values from the polynomial showing that the polynomial is generally accurate to within about 1.5% of the experimental data. An exception occurs at temperatures around 5°C, where the polynomial gives values that are about 4.5% lower than that for the experimental data. More importantly, the polynomial is always conservative; that is, it gives values of water activity that are lower than those for the experimental data. Equations 7.2 through 7.4 have been used to find the dry-bulb temperature and equilibrium grain moisture content (Tdb , W′ ), that correspond to the limiting water activity values from the polynomial. These calculations have been done over the range 5 < Tdb , < 35 for nine types of commodities, and a curve has been plotted for the resulting (Tdb, W′ ) values. The mold envelope is indicated as a dotted line in Figures 7.15 through 7.23. On the left-hand side of the mold envelope, molds do not grow, or grow very slowly. The right-hand side of the envelope shows conditions where more rapid mold growth can progressively occur, so conditions on the right-hand side of the mold envelope should be avoided where possible. Even on the right-hand side of the envelope there is a delay before grain damage due to molding becomes visible, and the rate of mold growth may be quite slow for conditions near to the envelope. It is this delay that allows moist grain to be dried before mold damage becomes visible. 7.2.2.4 Effect of Wetter Regions of the Grain Bulk On the basis of the mold envelope data, the grain in a storage may appear to be safe from molds if the average grain condition is on the left-hand side of the mold envelope. However, this could



Table 7.5

349

Maximum Commodity Moisture Content that is Likely to Avoid Mold Activity at any Temperature (Based on the Mold Envelope)

Seed type

Maximum Commodity Moisture Content (% Wet Basis)

British wheat Barley Maize Peanuts Rapeseed Paddy rice Sorghum Soybeans Sunflower seed

14.0 14.4 12.9 8.5 7.8 14.2 13.3 10.6 8.2

From Wilson, S.G. and Desmarchelier, J.M. [1994]. Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30[1], 45–60. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32.

be misleading if there are localized regions of wetter grain that lie on the right-hand side of the mold envelope. If molding begins in the wetter regions, it can spread to other parts of the grain bulk. Consequently, the occurrence of molding is determined by the condition of the wettest grain, not by the average moisture content. Wet grain should be avoided, particularly if the average grain conditions are near the mold envelope. Maximum Grain Moisture Content that Avoids Molding From Figures 7.15 through 7.23 it can be seen that based on the mold envelope, there is a maximum grain moisture content that is likely to avoid mold activity at all temperatures — it is the lowest moisture content in the mold envelope. Table 7.5 lists these maximum commodity moisture content values for the nine commodity types. To avoid molding regardless of temperature, commodity moisture content should be less than the tabulated maximum values. 7.2.2.5 Using CWBT to Estimate Insect Population Growth Rate 7.2.2.5.1

Threshold Commodity Wet-Bulb Temperature to Prevent Insect Population Growth

Desmarchelier (1988) has shown that there is a threshold CWBT, T0, that prevents insect population growth. Values of T0 for eight grain-infesting insect species are given in Table 7.6. At, or below T0, insect population growth rates are zero; above T0 insect populations increase exponentially. The thresholds given in Table 7.6 deserve discussion from two aspects: (1) the capability of the combinations of temperature and grain moisture content of the CWBT thresholds to provide adequate insect control; and (2) the use of the CWBT as a tool for management of insect control. The following discussion attempts to clarify these aspects of CWBT. Since one of the objectives in aeration is to create temperature and humidity conditions that will prevent a specific insect population growth, advance knowledge of the composition of the infesting insect population is essential. It is common for a grain bulk to be infested by a specific dominant insect population, but it is also common that a grain mass is infested by a mixture of insect species. Each case needs to be analyzed separately to determine the specific threshold for the target wet-bulb temperature.


350


Table 7.6

Desmarchelier’s (1988) Data (Revised for R. dominica) for Estimating the Rate of Growth of Insect Populations

Species

Commodity

T0 °Cwb (note a)

Rhyzopertha dominica Sitophilus oryzae Sitophilus granarius Sitophilus zeamais Trlbolium castaneum Oryzaephilus mercator Oryzaephilus surinamensis Lasioderma serricorne

Wheat Wheat Wheat Wheat Sorghum and wholemeal flour Wholemeal flour Wholemeal flour Wholemeal flour

12.0 9.0e 8.5 14.0 16.3e 12.1 16.4 13.9

a

b c d e

T m. °Cwb (note b) >31 >23 >23.5 >21 >30 >28.4 >28.4 >25.5

k (°Cwb-week)–1 (note c) 0.0257d 0.052 — 0.018 0.123 0.048 0.073 0.122

Threshold commodity wet-bulb temperature (CWBT), T0,°Cwb, the highest commodity wet-bulb temperature that prevents growth of insect populations. Tm,°Cwb, the CWBT at which the insect population growth rate is a maximum. Rate coefficient, k, (°Cwb-week)–1. This value revised by Desmarchelier in 1990. Where Desmarchelier (1988) gives more than one T0 value, the one with the lower T0 value has been used.

For bulks infested with a dominant insect species, it is possible to determine the target CWBT and set the combination of temperature and RH that will give the equivalent wet-bulb temperature. However, when a mixture of insects infests the bulk, the relative abundance of the dominant species in the population may also be considered in setting the CWBT. In such case, it would be reasonable to set the minimum CWBT to the lowest value that has the capacity to prevent insect growth. Before continuing to select the preferred combination of temperature and humidity that provide insect control, the relationship between threshold CWBT and the temperature humidity combinations should be further clarified. To illustrate the relationship between the threshold CWBT that controls four primary storedgrain insect species lesser grain borer (Rhyzopertha dominica); rice weevil (Sitophilus oryzae); saw-toothed grain beetle (Oryzaephilus surinamensis); and red flour beetle (Tribolium castaneum), T0 values given in Table 7.6 were plotted on a psychrometric chart as shown in Figure 7.24. The wet-bulb temperature lines shown in Figure 7.24 represent any combination of temperature and RH the CWBT may have in Figures 7.15 through 7.23. To those familiar with the psychrometric charts, this type of presentation illustrates the value of the CWBT shown in those Figures. Although the wet-bulb temperature lines shown in Figure 7.24 are the critical CWBT that prevents growth of insect populations, there are combinations of temperature and air RH that do not have equivalent effects on the rate of population growth. These particular cases are for the same three insect species illustrated in Figures 7.25 through 7.27. For the development limits of Rhyzopertha dominica and Sitophilus oryzae, the finite rate of increase data supplied by Birch (1953) were plotted on a linear scale that gives the combination of RH and dry-bulb temperature values (Figures 7.25 and 7.26). For comparison with the highest CWBT that prevents growth of R. dominica and S. oryzae, as given in Table 7.6, the lines of T0 equivalent to 12.0°C and 9.0°C were drawn as a combination of RH and dry-bulb temperatures. The critical limits for mold development (the mold envelope described above in Figures 7.15 through 7.23) in terms of equilibrium relative humidity values (ERH) were also included in both Figures 7.25 and 7.26. This comparison reveals that for RH values below 30% in air or at equivalent conditions, development would not be feasible according to Birch (1953). But based on the CWBT limits recommended by Wilson and Desmarchelier (1994), development of R. dominica and S. oryzae is theoretically possible. Considering grain temperature conditions in the range of 25 to 35°C immediately after harvest as a possibility, it would appear that the boundaries described using the CWBT method for R. dominica and S. oryzae are more restricting than the data provided by Birch (1953).



Figure 7.24

351

Psychrometric chart to show lines for saturation temperature, 70% RH, critical air RH for mold activity, and threshold wet-bulb temperature lines for insect activity. The mold envelope was based on data from Lacey et al. (1980), and the threshold wet-bulb temperature lines for the three insect species were based on data given by Desmarchelier (1988). (Data from Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. Desmarchelier, J. M [1988]. The relationship between wet-bulb temperature and intrinsic rate of increase of eight species of stored-product Coleoptera, J. Stored Prod. Res., 24, 107–113.)

However, RH values below 30% in equilibrium with grain moisture content are extremely rare conditions. This ERH value would be equivalent to wheat below 8% mc (wet basis) (see Chapters 3 and 5). Thus, it appears that the CWBT limits given for R. dominica and S. oryzae are a safe recommendation as indicated in the shaded areas of Figures 7.25 and 7.26. Similar plots of data for Oryzaephilus surinamensis and Tribolium castaneum reveal a different situation than R. dominica and S. oryzae. Accordingly, data provided by Howe (1956a, 1956b) was used in the presentation of limits of finite rate of development for preparing Figures 7.27 and 7.28 for O. surinamensis and T. castaneum. For these two insect species, the CWBT values at the lower humidities were shown in Figure 7.24 to limit development, whereas data provided by Howe (1956a, 1956b) (Figures 7.27 and 7.28) indicate survival below the limits indicated by the CWBT method. These discrepancies may result in the improper use of aeration systems using the CWBT method when O. surinamensis and T. castaneum are present. Therefore, although the CWBT method is a practical approach for the operation of aeration systems in the suppression of these insect species, it is important to know the low humidity limitations which O. surinamensis and T. castaneum can tolerate. The limiting CWBT values given in Table 7.6 are lower than the combinations of temperatures and RH shown in Figures 7.25 and 7.26. It would appear that insects on flour or whole meal flour could reproduce at lower temperatures than they could on whole grain. Although data given in Table 7.6 for O. surinamensis and T. castaneum are relevant for modeling studies under laboratory conditions, its justification for the operation of the aeration remain questionable. Therefore, a closer look at the insect survival data obtained on whole grain kernels at various grain moistures and


352


100 Mold development line

90

Air relative humidity (% RH)

80 70 60

Safe storage conditions

50 40 30

Threshold CWBT to prevent development of R. dominica

20

Limits of finite rate of increase (λ=0) for R. dominica population

10 0 0

10

5

15

20

25

30

35

40

45

50

Dry-bulb temperature (°C) Figure 7.25

Critical thresholds for the combined effect of relative humidity and temperature for development of Rhyzopertha dominica, of mold development and shaded area to show safe storage conditions. (Data from Birch, L.C. [1953]. Experimental background to the study of the distribution and abundance of insects, Ecology, 34, 678. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32.)


90


80 70 60 50


40 30

Limits of finite rate of increase (λ=0) foS. oryzae population

20 Threshold CWBT to prevent development of S. oryzae

10 0 0

5

10

15

20

25

30

35

40

45

50

Dry-bulb temperature (°C)

Figure 7.26

Critical thresholds for the combined effect of relative humidity and temperature for development of Sitophilus oryzae, of mold development and shaded area to show safe storage conditions. (Data from Birch, L.C. [1953]. Experimental background to the study of the distribution and abundance of insects, Ecology, 34, 678. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32.)



353



90 80 70


60 50 40

Limits of finite rate of increase (λ=0) for O. surinamensis population

30 Threshold CWBT to prevent development of O. surinamensis

20 10 0 0

5

10

15

20

25

30

35

40

50

45


Critical thresholds for the combined effect of relative humidity and temperature for development of Oryzaephilus surinamensis, of mold development and shaded area to show safe storage conditions. (Data from Howe, R.W. [1956a]. The biology of two common storage species of Oryzaephilus [Coleoptera, Cucujidae], Ann. Appl. Biol., 44, 341–355. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)

100


90

Mold development line

80 70


60 50 40 30

Threshold CWBT to prevent development of T. castaneum

20

Limits of finite rate of increase (λ=0) for T. castaneum population

10 0 0

5

10

15

20

25

30

35

40

45

50


Critical thresholds for the combined effect of relative humidity and temperature for development of Tribolium castaneum, of mold development and shaded area to show safe storage conditions. (Data from Howe, R.W. [1956b]. The effect of temperature and humidity on the rate of development and mortality of Tribolium castaneum [Herbst] [Coleoptera, Tenebrionidae], Ann. Appl. Biol., 44, 356–368. Lacey J., Hill, S.T., and Edwards, M.A. [1980]. Microorganisms in stored grains: their enumeration and significance, Trop. Stored Prod. Inf., 38, 19–32. With permission.)


354


temperatures would lead to different values than those given in Table 7.6. These limitations of the data provided in Table 7.6 are further discussed in Section 7.2.2.5.2 below. In principle, the recommended boundaries for safe storage of grain in terms of ERH and temperature values were those based on the limits of finite rate of mold growth developed by Howe (1956a, 1956b), not the CWBT method. This information partially clarifies the limitations to the use of the CWBT method. Similar careful comparative examinations are recommended in the presence of other insect species infesting stored grain. In many parts of the world these four insect species, together with S. granarius and S. zeamais, are the most common, damaging species. Lasioderma serricorne and O. mercator are typically less known to infest large grain bulks, though their presence is reported occasionally (Navarro et al., 1991). 7.2.2.5.2

Rates of Insect Population Growth

Desmarchelier (1988) has also correlated the intrinsic rate of growth, r, (week)–1, of an insect population by a rate equation: r = k (T − T0 ), T0 ≤ T ≤ Tm

(7.13)

where: k, (°Cwb-week)–1, is a rate coefficient T,°Cwb, is the CWBT; T0,°Cwb, is the threshold CWBT for preventing population growth Tm,°Cwb, is the CWBT at which the insect population growth rate is a maximum The population multiple, Nt N0 , where Nt is the population after a time t, weeks, and N0 is the population at t = 0, is calculated from: Nt = exp (rt ) N0

(7.14)

Equation 7.13 only applies where population growth rate is positive and increasing with CWBT (that is, for CWBTs below Tm). Further, it does not apply below the threshold CWBT, T0, but below T0, it can reasonably be expected that the population growth rate is negative (Evans, 1987). At T0, the intrinsic rate of increase is zero. Consequently, population increase can be prevented by temperatures at or below T0. Between T0 and Tm, the population increases exponentially with the temperature difference (T – T0). Near T0, when this difference is small, the rate of population increase is close to zero. This means that the rate of population increase can be controlled at a low value by temperatures just above T0. Above Tm, the population growth rate falls with increasing CWBT. Worst-case values for k and T0 are given in Table 7.6. Rather than a specific Tm value, Table 7.6 indicates values above which Tm occurs. Values in Table 7.6 should be treated with caution as there are anomalies or limitations: 1. Data for some species are very restricted, and the data in Table 7.6 are limited as to the type of commodity used to rear the insects. Only wheat and whole meal flour, and in one case sorghum, were used. Further, use of Equations 7.13 and 7.14 for different commodities assumes that those commodities are universally infestible by insects. This may not always be the case; for example, rapeseed is unsuitable for most insects and those insects that do infest rapeseed reproduce more slowly. Equations 7.13 and 7.14 therefore indicate an upper limit for the rate of population increase where the commodity is not a limitation.



355

2. Some of the underlying data suggests a slight dependence of T0 on moisture content, showing T0 decreasing with increasing grain aridity. That is, lower T0 values are needed to prevent insect population growth in drier grain. 3. The relationship between CWBT and intrinsic rate of increase is generally correct, but there are some situations where it is inconsistent. For example, a CWBT of 14°C for wheat is equivalent to 24°C (dry-bulb) and 10% moisture content (wet basis), and to 17°C and 14% moisture content. However, these two grain conditions do not have equivalent effects on the rate of population growth of Sitophilus oryzae and Rhyzopertha dominica. Sitophilus oryzae will reproduce successfully at 17°C and 14% moisture content but not at 24°C and 10%. The reverse applies for Rhyzopertha dominica (B. C. Longstaff, unpublished).

In summary, Equations 7.13 and 7.14 need to be verified for a greater variety of grains. Also, there are temperature and moisture content effects upon the intrinsic rate of increase that are not accounted for. Equations 7.13 and 7.14 should therefore be regarded as useful empirical tools for managing aeration systems in temperate climates but not relied on for absolute accuracy. However, at present, these equations are the only simple means of estimating both temperature and moisture content effects on the rate of insect population increase in stored grains. Population Growth and CWBT Values in Figures 7.15 through 7.23 As discussed above, Equations 7.13 and 7.14 indicate that population increase is controlled at a low value by CWBT values near T, and the population increases exponentially with the temperature difference above T0. As the CWBT values for T0 in Table 7.6 range from 8.5 to 16.4°Cwb, this means that insect populations are controlled by the conditions represented in the bottom left-hand corner of Figures 7.15 through 7.23. Moving away from that corner, CWBT values progressively increase and insect populations become correspondingly uncontrolled. 7.2.2.6 Managing Aeration Systems by Using CWBT to Control Insect Populations In the following discussion the term control insect population means achieving a very low (or near zero) rate of population increase, which ensures that increasing insect population will not become a problem during the intended commodity storage time. The term CWBT control means the use of CWBT to manage aeration systems and control insect populations. 7.2.2.6.1

Moisture Content, CWBT, and the Corresponding Dry-Bulb Temperature

Inspection of the data in Figures 7.15 to 7.23 shows that for a particular value of CWBT the corresponding dry-bulb temperature is higher for dry grain than it is for wet grain. This means that dry grain does not need to be cooled to as low a dry-bulb temperature as wet grain to achieve the same control of insect populations. Example 7.4 At a CWBT of 14°C, the corresponding dry-bulb temperature for British wheat (Figure 7.15) is about 23°C at a moisture content of 10%, and about 18°C at a moisture content of 14%. Consequently, the use of a 14°C CWBT to control insects in 10% moisture content wheat requires a drybulb temperature of only 23°C; but for 14% moisture content wheat, the dry-bulb temperature must be reduced to 18°C. We have known since biblical times that insect populations grow more slowly in dry grain than in wet grain. However, Equations 7.13 and 7.14 and the data in Figures 7.15 through 7.23 now


356


Table 7.7

Rate of Insect Population Growth: Weekly Multiples at Commodity Wet-Bulb Temperatures between 20 and 9°C

Insect Species

Commodity Wet-Bulb Temperatures °C (Wet Basis) 20 17 15 13 11 9

R. dominica S. oryzae S. zeamais T. castaneum O. mercator O. surinamensis L. serricorne

1.228 1.772 1.114 1.576 1.461 1.301 2.100

1.137 1.516 1.055 1.090 1.265 1.045 1.460

1.080 1.366 1.018 — 1.149 — 1.144

1.026 1.231 — — 1.044 — —

— 1. 110 — — — — —

— 0.000 — — — — —

From Wilson, S.G. and Desmarchelier, J.M. (1994). Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30(1), 45–60.

Table 7.8

Rate of Insect Population Growth: 3 Monthly Multiples at Commodity Wet-Bulb Temperatures between 20 and 9°C

Insect species R. dominica S. oryzae S. zeamais T. castaneum 0. mercator 0. surinamensis L. serricorne

Commodity Wet-Bulb Temperatures°C (Wet Basis) 20 17 15 13 11 9 14.430 1696.000 4.071 371.000 138.300 30.460 15908.000

5.303 223.200 2.018 3.063 21.28 1.767 136.500

2.721 57.740 1.264 — 6.108 — 5.724

1.396 14.940 — — 1.753 — —

— 3.865 — —

— 0.000 — —

— —

— —

From Wilson, S.G. and Desmarchelier, J.M. (1994). Aeration according to seed wet-bulb temperature, J. Stored Prod. Res., 30(1), 45–60.

allow determination of the CWBT value (and corresponding grain dry-bulb temperature and grain moisture content) needed to control insect populations. 7.2.2.6.2

Target CWBT for Controlling Insect Populations

In Australia, Desmarchelier’s empirical relationship (Equations 7.13 and 7.14) is increasingly used to set conditions to control insect populations in aerated wheat stores. In the warm conditions that prevail at wheat harvest in Australia, it is impractical to use aeration to kill insects. For grain intended for domestic consumption, where some small live insect populations can be tolerated, aeration alone can be used to manage insect populations. Australia requires that no live insects are present in export wheat. This requires a different insect-management strategy. Treatments such as fumigation are used to kill insects, followed by aeration cooling to control population growth of any residual insects and to prevent reinfestation after treatment. Cooling followed by fumigation is now possible for dry wheat. Before use of CWBT control, it was believed that a commodity dry-bulb temperature of about 15°C was needed to control insect populations. At these temperatures fumigation only works slowly. However, CWBT control for dry wheat requires a commodity dry-bulb temperature substantially above 15°C, making fumigation after aeration a practicality. To control the rate of insect population growth in stored commodities, a target CWBT is selected based on local knowledge of problem storage-insect species. Data on insect population growth rate vs. CWBT from Tables 7.7 and 7.8 are also evaluated. These data were calculated from Equations 7.13 and 7.14. As these equations do not apply below the threshold CWBT, T0, Tables 7.7. and 7.8 do not indicate values of population growth rate value for CWBTs below T0.



357

A strategy used successfully in Australia is to select a target CWBT about 2°C above the lowest threshold temperature, T0, for the insect species present in the seed bulk. For example, in grain stores in Australia, the common insect species are Rhyzopertha dominica, Tribolium castaneum, and Oryzaephilus surinamensis. Of these, from Table 7.6, Rhyzopertha dominica requires the lowest threshold temperature, T0 = 12.0, to prevent insect population growth. The target CWBT is therefore selected as 2°C above this value, 14°C. Interpolating from the values in Table 7.8, this target CWBT gives a 3-month multiple of about two. Wheat aeration trials with a target CWBT of 14°C have resulted in undetectable insect populations at outloading after a 6-month storage period. 7.2.2.6.3

Operating the CWBT Aeration Controller

To operate an aeration system, users need to select an aeration controller setting that will achieve the target CWBT. In Australia this controller setting is now selected using a scientifically developed PC computer program. For specified initial commodity conditions, geographic location, time of year, and aeration flow rate, this program utilizes Australian meteorological data (Australian Government Publishing Service, 1988) to calculate the required aeration cooling time and the CWBT after aeration for nine types of commodities and for five types of aeration controllers. This allows the interactions among the aeration system, commodity type, controller type and setting, and local weather conditions to be used in selecting the target CWBT. It also allows the program user to discover whether there is a controller setting that will achieve the target CWBT for the weather conditions likely to occur at an aeration site. A more detailed explanation of how to select the target CWBT and how CWBT is used to control aeration systems in Australia can be found in Wilson (1990). 7.2.2.6.4

Requirements for CWBT Control of Aeration Systems

Although a CWBT controller has particular advantages for dry grain, in its broadest concept, CWBT control can be applied: 1. Wherever the mean ambient air wet-bulb temperature falls below the target CWBT needed to control insect populations 2. For a sufficient period of time to allow all of the grain bulk to be cooled down to the target CWBT temperature 3. Before growth of insect populations becomes a problem (for example, in Australia, very low insect numbers are found in wheat if the grain bulk is cooled to the target CWBT within 5 weeks of receipt)

Without performing detailed calculations, it is usually quite difficult to judge whether the above criteria can all be met. This is why the computer program was developed. It is interesting to note that the existing use of dry-bulb temperature to control insect populations in cool climates where the grain is harvested wet — for example, in Europe — is effectively the same as using CWBT control. This is because the CWBT and the grain dry-bulb temperature are very close for wet grain. Use of CWBT control has a particular advantage in warm temperate climates since it is appropriate to both wet and dry grain. 7.2.2.6.5

Advantages of CWBT Control for Warm Temperate Climates

From experience in Australia, the use of CWBT control appears to be most effective in climates with large daily temperature variations. Low temperatures at night can be used to cool the grain down to the target CWBT. Warm conditions during the day allow grain to be harvested at a low moisture content. Low moisture content ensures that when the grain is cooled down to the target


358


CWBT, its dry-bulb temperature is close to the mean ambient dry-bulb temperature. Example 7.5 illustrates this concept. Aerating with a CWBT that controls insect populations at a dry-bulb temperature close to the mean ambient temperature may seem difficult to achieve; but as illustrated by the examples below, in temperate Australia this occurs at normal wheat reception moisture contents of about 10% wet basis. Example 7.5 By cooling the grain to the target CWBT so that the grain dry-bulb temperature is near the mean ambient dry-bulb temperature, there are several benefits: 1. The aeration system only needs to cool the grain from its warm harvest temperature (typically 25 to 35°C in Australia) down to a dry-bulb temperature that is near the ambient dry-bulb temperature — not to a temperature that is significantly below ambient. 2. Problems of the grain bulk rewarming are minimized when the aeration system is switched off, because the dry-bulb temperature of the grain bulk is already near the mean ambient temperature. This makes it possible to cool the grain bulk by aeration and then leave the aeration system switched off. 3. Moisture migration is minimized because there is only a small temperature difference between the interior of the grain bulk and the mean ambient temperature.

Warm temperate regions where CWBT control offers these advantages lie between latitudes 25° and 40°, especially latitudes 30° to 35°, and many elevated or highland regions closer to the equator. In addition to parts of Australia, this includes parts of the U.S., southern Italy, southern Spain, China, Northern India, the Middle East, North and South Africa, and South America (Figure 7.1). 7.2.2.7 Conclusions in Using the CWBT Method There are particular strategies and benefits arising from using commodity wet-bulb temperature (CWBT) to control insect populations in an aerated grain store. To use CWBT aeration, insect population growth rate data are used to select a target CWBT for controlling insect populations. A computer program may be used to select an aeration controller setting that will achieve the target CWBT for the climatic conditions of the aeration site. Essential CWBT data (and limiting conditions for mold growth) for nine types of commodities are provided in Figures 7.15 through 7.23. CWBT control of aeration systems has particular benefit in warm temperate climates that provide: (1) low temperatures at night that can be used to cool the grain down to the target CWBT; and (2) warm conditions during the day that enable the grain to be harvested with a low moisture content. When low moisture content grain is cooled down to the target CWBT, its dry-bulb temperature is near to the mean ambient dry-bulb temperature. Consequently, the commodity will not tend to rewarm when the aeration system is turned off. 7.2.3

Selecting Cooling Rates

7.2.3.1 Time Required for Cooling The time required to cool grain to the approximate temperature of the ambient air depends greatly on the airflow rate. The relationship of cooling time with heat and mass transfer supplied by the airflow rate and the specific heat of the grain to be cooled was discussed in Chapter 6. Depending on the method used, different cooling times can be calculated for the same airflow rate (Figure 6.26). This process is a function of the temperature and moisture content of the air, and thus the amount of evaporative cooling that takes place. Increasing airflow rate proportionally



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decreases cooling time. Therefore, in choosing the airflow rate, basic considerations are the time required to reduce grain temperature and the acceptable fan power levels and costs. As shown in Figure 7.6, as air at 19°C is forced through the warm 30°C grain, the air begins to cool successive thin layers of grain in the cooling zone over the temperature range from 19 to 30°C. Because of the difference in mass/unit volume in grain bulks, cooling air traveling at a relative mean air velocity of about 3 to 5 m/min will equalize with the grain temperature within a few seconds after reaching grain that is warmer than the air. Because of the large difference in mass/unit volume (mass density) of air compared to grain, research shows that 700 to 800 unit volumes of air are required to cool one unit volume of clean grain to near the dry-bulb ambient air temperature. Since wheat typically has an interstitial air space of about 40%, the specific air velocity between kernels is on average at least 2.5 times the mean air velocity of a given storage configuration. At a normal aeration airflow rate of 6 (m3/h)/tonne, the dwell time (entrance to exit time) of air passing through any storage is about 5 minutes. But the actual mean air velocity may vary from about 1.4 to 2.6 m/min for flat storages with grain depths of 7 to 13 m, about 3 to 4 m/min for steel bins with 15 to 20 m of grain, and 6 to 9 m/min for silos that range from 30 to 45 m deep. At 6 (m3/h)/tonne (0.1 cfm/bu), 700 to 800 volumes of air would be delivered in 116 to 132 hours. If the airflow delivery rate is doubled to 12 (m3/h)/tonne (0.2 cfm/bu), the same mass flow is delivered in half the time; and grain is cooled in 58 to 66 hours. When aeration airflow is very slow, the vertically advancing cooling zone in the grain mass is very broad or thick, with a leading edge and a trailing edge, as described in Chapter 6. As airflow rates increase and cooling is faster, the cooling zone or cooling bandwidth becomes narrower by the approximate ratio of the changes in airflow rates. Epperly (1989) found that at 6 (m3/h)/tonne (0.1 cfm/bu), the leading edge of the cooling zone required about 40 hours to move through the grain mass from the entrance to the exit grain surface. The trailing edge (grain almost at average air temperature) began to advance through the grain after about 70 hours; and cooling was completed in about 110 hours. From the discussion in this chapter and experimental data given in Chapter 6, an attempt was made to establish cooling times that represent a reasonable range of aeration conditions both for ambient air and grain. For this purpose a comparison of cooling times was made using Equations 6.3 and 6.10 (Chapter 6) to demonstrate the similarities in the empirical (Epperly, 1989) and theoretically calculated (Navarro and Calderon, 1982) values (Figure 7.29). The calculated values in Figure 7.29 are based on a simple heat-balance equation with the assumption that air enters and leaves the grain bulk at constant relative humidity in equilibrium with the grain moisture content during the entire cooling period (Navarro and Calderon, 1982). This assumption does not reflect the true dynamics of heat and mass (moisture) balance that occur during most normal aeration processes, as it is known that aeration reduces grain moisture by a small but significant percentage (from 0.2 to 0.5% typically) during most aeration cycles in grain at safe storage moistures (Epperly, 1989; Holman, et al., 1960). The amount of moisture removed is a function of grain moisture and temperature and ambient cooling air conditions. Thus, more moisture is removed when aerating warm, moist grain in late summer than cooler, drier grain during the fall or winter. The calculated total aeration hours were obtained using the equation given by Navarro and Calderon (1982) to cool wheat of dry-bulb temperatures from 36 to 19°C at 68% relative humidity (13% wet basis moisture content). Note that a correction factor of 0.5 (Table 6.5) and ∆T multiplier of 0.911 (Table 6.6) was used in the model. Experimental data from Epperly (1989) is based on a pilot scale research study to reduce wheat temperatures from 36 to 19°C (at about 13% wet basis moisture content). The level of moisture removal is directly related to the cooling time differences between summer, fall, and winter aeration. But since this constant moisture model is instructive and does not cause


360

Figure 7.29


Calculated and experimental aeration hours required to cool grain. Lines were obtained using equations given by Navarro and Calderon (1982) and Epperly (1989) to cool wheat temperature from 36 to 19°C. (Data from Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52. Rome. Epperly, D.R., Clary, B.L., Noyes, R.T., and Cuperus, G.W. [1989]. Predicting temperature gradients during stored grain aeration, International Summer Meeting of ASAE/CSAE, Quebec, PQ, Canada, 34 p. June 25–28. With permission.)

significant errors in cooling time or final grain temperatures, initial discussions will be based on constant moisture cooling. The heat exchange equation listed as Equation 6.10 in Chapter 6 is repeated here for the convenience of the reader: Φ = ( M × ∆T × c) (Q × Sw × CF × ∆H )

(6.10)

Since the enthalpy, ∆H, decreases as the cooling front passes through grain, Navarro and Calderon (1982) recommended a correction factor of 0.5, while Sanderson et al. (1988b) recommended 0.4. The correction factor 0.5 used by Navarro and Calderon (1982) was based on an average value obtained from field tests. The variations in field tests derive from the lack of uniformity in the distribution of temperature and the daily variations of the ambient temperature. Data gathered by Epperly et al. (1989) was based on pilot scale temperature changes of a grain bulk under controlled conditions. For airflow rates below the range of 15 (m3/h)/tonne, analysis of data indicates that the average correction factor of 0.5 in Equation 6.10 would result in closer results to the empirical curve developed by Epperly (1989). Since widely used airflow rates range below 15 (m3/h)/tonne, for comparison purposes Equation 6.10 was plotted on a graph together with data reported by Epperly et al. (1989) for total aeration hours required to cool a bulk of wheat from 36 to 19°C (Figure 7.29). To compute the curve in Figure 7.29, Equation 6.10 was used with a correction factor of 0.5 and a ∆T multiplier of 0.911, which closely profiles the empirical curve developed by Epperly (1989). To help the reader become familiar with the computing procedure needed in using Equation 6.10, a section of the psychrometric chart was prepared in Figure 7.30. This figure shows the grain dry-bulb



Figure 7.30

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Section of psychrometric chart to demonstrate the dry-bulb temperature, relative humidity, wetbulb temperature lines, air density, and enthalpy differences of the interstitial grain air space and that of ambient air.

temperature and the ambient dry-bulb temperature for the interstitial air relative humidity of 68% in equilibrium with 13% moisture content. In Figure 7.30 the enthalpy values and air density are shown to facilitate the calculation of the cooling time. For computing cooling times that are in ranges other than the values given in Figure 7.30, the reader is referred to the use of the psychrometric charts in Chapter 3. This information was used in Example 7.6 for calculating the cooling time to reduce grain temperature from 39 to 19°C based on Equation 6.10. Example 7.6 Estimate the number of hours of aeration necessary to cool 1000 kg (1 tonne) of wheat at a moisture content of 13% from 36 to 19°C (97 to 66°F), at an airflow of 6 (m3/h)/tonne, using the following data: • Specific heat of wheat at 13% moisture content is 0.40 kcal/kg/°C (Figure 9.5) • Relative humidity in equilibrium with 13% moisture content is 68% (Figure A.4) • Enthalpy at 68% relative humidity for a temperature change in the air from 36 to 19°C (97 to 66°F) is ∆H = 102 – 41 = 61 kJ/kg (Figures 7.30 and A.1); 0.50 ∆H = 0.50 × 61.0 = 30.5 kJ/kg × 0.239 kJ/kcal = 7.29 kcal/kg • Average specific weight of air for the given conditions is (1.194 + 1.095)/2 = 1.14 kg/m3 (reciprocal of specific volume from Figure A.1)

Depending on the expected temperature reduction, a ∆T multiplier was selected to adjust for the differences between the ambient temperature and the target final grain temperature. In the present example a ∆T multiplier of 0.911 is used (Table 6.6). Therefore, the adjusted ∆T value will be:

(36° − 19°C) × 0.911 = 15.5°C Using Equation 6.10, calculate the required aeration time as follows:


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Figure 7.31

Calculated family of curves showing the aeration time needed for reducing wheat (at 12% moisture content wet basis) temperatures from 25, 30, 35, and 40°C to ambient temperatures of 10, 15, and 20°C at 64% relative humidity. Calculations assume no moisture mass transfer; total aeration hours are from Equation 6.10 with 0.5 correction factor and 0.911 ∆T multiplier (Table 6.6.). (Data from Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome.)

F = Aeration hours = (1000 kg × 15.5°C × 0.40 kcal kg °C)

(6 m

3

)

hr × 1.15 kg m 3 × 7.29 kcal kg = 123.2 ≈ 123 hr

Using Equation 6.10 with the correction factor of 0.50, Figure 7.31 presents a family of curves to describe several variations of temperature changes from 25°, 30°, 35°, and 40°C to ambient temperatures of 10°, 15°, and 20°C at 64% relative humidity. This family of curves clearly indicates that by reducing or increasing the airflow rate beyond certain limits, the aeration time needed to cool grain may exceed practical limits. At a low range of airflow rate, below 1 (m3/h)/tonne, the aeration time exceeds 500 hours, which makes grain cooling very difficult — especially in geographical regions with marginal ambient temperature conditions. Figure 7.31 also shows that if airflow rates are increased above 10 (m3/h)/tonne, the cooling capacity becomes progressively less effective. At the high airflow rates, for each increment in airflow rate the cooling time becomes less pronounced (the lines are asymptotic). Analysis of different situations that enables calculation of expected total fan operation hours has a practical monetary value. Although useful, data in Figure 7.31 should be used with caution



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since it is based on the assumption that no moisture mass transfer occurs in the grain bulk — i.e., these data describe sensible cooling where the ambient air relative humidity is considered in equilibrium with the grain moisture content. Although this situation cannot be achieved in practice because of the daily variation in relative humidity and temperature of ambient air, Figure 7.31 has a practical application as it enables estimation of the time needed to cool grain under several scenarios of grain temperature — from 25 to 40°C to ambient temperatures of 15 to 20°C. The inverse proportional asymptotic curves shown in Figure 7.31 describe a specific condition for cooling wheat at 12% moisture content. The cooling time developed from Equation 6.10 will serve as a guide. Although it represents the airflow rate and time relationship to cool the grain, other significant factors such as heat loss by conduction, latent heat exchange, non-uniform airflow effect due to peaked grain, and the core of foreign material below fillspouts have been neglected. Due to these factors, results of Equation 6.10 or Figure 7.31 to estimate cooling time may vary from the actual time required for cooling. To estimate the cooling time for other situations, it is also necessary to describe the grain conditions and other storage parameters such as grain moisture content, specific heat, ambient conditions, and the airflow rate. By assuming that the other grain parameters are constant, the initial grain temperature and ambient air conditions are the primary factors that influence the curves shown in Figure 7.31. For the same ambient temperature and humidity conditions, as the grain temperature increases, Figure 7.31 shows that less cooling hours are needed to reach equilibrium with ambient air because the enthalpy difference decreases between grain and ambient temperature. It is expected that the correction factor values (CF of 0.50 for wheat in the above example) will vary for different types of grain and possibly by grain bulk configuration (vertical vs. horizontal structures). Therefore, to describe such conditions, additional pilot plant experimental work similar to the study of Epperly (1989) would be necessary to accurately determine the possibly significant differences in different grain types and storage condition scenarios. Except for chilled aeration, where uniform air temperature and humidity conditions are maintained, cooling front movement through grain bulks using ambient aeration is intermittent when automatic aeration controllers with thermostat limits are used. The daily progress of cooling fronts may vary widely as aeration fans cycle on and off and as ambient temperatures vary below and above the controller thermostat set-point due to cyclic local weather conditions. Based on values in Figure 7.31, approximately 525 to 800 m3 of air per m3 (volumes of air per volume of grain) are calculated for cooling times for wheat in the range of 12 to 13% moisture content. An alternative method for analyzing cooling time is to analyze the amount of cooling air to grain on a mass ratio basis. This can be done by using the density of air for a given volume of air found by researchers to complete a cooling cycle. Based on the 800 volumes of air per volume of grain, if cooling air was 55°F at 65% RH with an air density of 13.1 lb/ft3 (0.0763 lb/ft3), for 1.0 ft3 of wheat at 48 lb/ft3, the mass flow of air would be 800 ft3 × 0.076 lb/ft3 = 61.07 lb of air. The cooling mass ratio would be 61.07/48 = 1.27 lb air/lb wheat, or 1.27 kg air/kg wheat. With a large grain mass and variations in ambient temperature that are difficult to control, the value of about 800 m3 of air per m3 of grain may be considered for practical applications. Then the time required to deliver 800 m3 of air per m3 (volumes of air per volume) of grain will be: t = (800 × V ) Q where: t = time, hours V = volume of grain, m3 Q = volume of air, m3/h

(7.15)


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Example 7.7 Using Equation 7.15, calculate the time to cool 1000 tonnes of wheat aerated at an airflow rate of 6 (m3/h)/tonne. The volume of the wheat bulk is: 1000/0.78 = 1282 m3, where 0.78 tonne/m3 is the bulk density of wheat at 780 kg/m3. The volume of air per hour required for 1000 tonnes of wheat will be: 1000 tonnes × 6 (m3/h)/tonne = 6000 m3/h. The estimated aeration hours needed to cool the wheat will be: t = (800 × 1282) 6000 = 170.9 ≈ 171 h This simplified method of calculating the cooling times based on a mass ratio basis or using a volume-per-volume ratio can provide quick estimates, but the accuracy is limited. This equation does not accurately estimate seasonal cooling time requirements. USDA research data indicates that, for cooling clean level grain at ¹⁄₂₀ cfm/bu (3 (m3/h)/tonne), it takes about 170 hours to complete a summer cooling cycle, 240 hours during the fall, and 340 hours in the winter (Holman, 1960). The primary differences in cooling times between seasons are based on the higher enthalpy of cooling air and higher rates of evaporative cooling in warmer months compared to cooler months. If the same grain is cooled once during consecutive cooling seasons, the grain will be drier at the start of each successive season. Thus, evaporative cooling potential is lower in the fall than the summer, and lower in the winter than the fall. 7.2.3.2 Selecting the Airflow Rate for Dry Grain In selecting the airflow rate, a basic consideration is the time required to reduce grain temperature. In Section 7.2.3.1 and Chapter 6 (Equation 6.10), a simple heat-balance equation was given to estimate the time in hours of aeration. As a general guideline in helping to choose the desired airflow rate, it is clear that at an airflow rate of 3 (m3/h)/tonne (0.05 cfm/bu), the time required for cooling the wheat, for example, may be extended to 275 hours or more (Figure 7.31). The available cooling periods (below 20°C or 68°F) during early autumn (from September through November for the northern hemisphere) range from 5 to 10 hours per day (Figure 7.5). From this it is clear that aeration would be intermittent and would be expected to last 28 to 55 days or longer, based on the frequency and length of intermittent cooling weather fronts in each geographic region. If the airflow rate is reduced to 2 (m3/h)/tonne (0.033 cfm/bu), the cooling time will be extended to 400 hours, or to about 40 to 80 days without considering that further reduction in ambient temperature will take place. During this prolonged aeration period, although the temperature of most of the bulk remains high, the surface temperatures drop; and warm air blown from the inner grain bulk may result in moisture condensation on cool grain near the periphery. Field trials in subtropical climates have revealed the inefficiency of airflow rates lower than 3 (m3/h)/tonne. It is recommended that for subtropical climates, aeration airflow should be maintained above this rate — for example, within the range of 5 to 8 (m3/h)/tonne (0.083 to 0.13 cfm/bu), which is normally economically feasible. Increased airflow results in a proportional reduction in cooling time. But although airflow rates of 6 (m3/h)/tonne are effective in achieving a 50% reduction in aeration hours compared to an airflow rate of 3 (m3/h)/tonne, higher airflow rates are accompanied with a high static pressure and power requirement increase due to the increased resistance to airflow. For example, doubling the airflow rate will result in a reduction in cooling time by about half; but the static pressure will increase by a factor of about 3 times and power consumption will be approximately 4 times the pressure and power required at the lower airflow rate.



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An important objective of aeration is to cool grain to desirable temperatures before economically damaging insect infestation starts to develop. This implies that stored grain should be cooled as soon as possible. From field observations, Navarro et al. (1980b) indicated the desirability of cooling the grain within the first month of storage. To do this, the required airflow rate must be estimated on the basis of the limited number of suitable aeration hours available at the season. For this purpose either Figure 7.31 or the 800 volumes of air per volume of grain rule of thumb can be used. Example 7.8 If the average number of suitable cooling hours at a given season is 10 hours per day and the duration of the planned cooling period is about 30 days, the required airflow rate can be estimated using the 800 m3/m3 rule of thumb method as:

) (

(

(

)) available cooling hours wheat )) (10 × 30) =

Required airflow rate, m 3 h tonne = 800 × specific volume of grain m 3 tonne

(

(

= 800 × 1.28 m 3 tonne for 790 kg m 3

(

Required airflow rate = 1024 300 = 3.4 m 3 h tonne

) (0.057 cfm bu)

If it is desirable to complete the cooling in two weeks, the airflow rate would need to be:

(

) (

(

Required airflow rate, m 3 h tonne = 800 × 1.28 m 3 tonne for 790 kg m 3 wheat

(

= 1024 140 = 7.3 m 3 h tonne

)) (10 × 14)

) (0.122 cfm bu)

This airflow rate is commonly used and economically practical in the grain industry. However, if only 5 hours per day were suitable for aeration and the elevator or mill operator intended to cool the grain in two weeks, the required airflow rate would need to be two times the volume listed above, or 14.6 (m3/h)/tonne (0.24 cfm/bu), which may not be practical. Calculations may show that airflow rates may be too high (as in Example 7.8) to match available aeration hours in some regions, or during seasons when aeration hours are limited. In such cases, alternative insect control methods should be considered. Insect population monitoring as well as other integrated pest management (IPM) methods may be employed until suitable aeration conditions become available. For high-value crops, grain chilling may be a desirable alternative so that cooling can be conducted on a 24-hour-per-day basis immediately after the crop is stored, without having to wait for suitable intermittent weather required for ambient aeration. 7.2.3.3 Equalizing Grain Temperatures to Prevent Condensation The most important site of moisture accumulation in the grain bulk is the exposed top surface. The mechanism of moisture transfer by air convection currents and by diffusion is called moisture migration (Figure 7.32). Moisture condensation in bulks is a common and significant problem in grain storage systems. In order to eliminate moisture migration, it is essential to equalize temperatures within the grain bulk. Aeration is the important tool needed to minimize or eliminate moisture migration. When cool evening conditions occur outside a warm grain bulk, usually there is a large temperature difference between the outside and the center of the bulk. This temperature difference drives the two processes that cause moisture migration: natural convection air currents and moisture diffusion. Both of these processes tend to move moisture from warm regions to cool regions located in the upper part of the store, creating wet spots where mold can occur (Nguyen, 1987).


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Figure 7.32

Moisture migration to the cold surface of a warm bulk of grain.

Without aeration, the amount of moisture migration that occurs: (1) increases with time; (2) increases with storage structure height — the speed of the natural convection currents that transport the moisture, and hence the amount of moisture transported, increases with the height of the store; and (3) increases markedly with combined high grain temperatures and high grain moisture concentrations (Nguyen, 1987). This is because the amount of moisture in the natural convection air currents, and hence the amount of moisture transported, is much greater when both grain temperature and moisture concentration are high. Griffiths (1964) describes the effect of moisture migration on an unprotected, uncooled grain bulk. He found that caking normally occurs in nonaerated grain stores but not in aerated stores. Aeration works in several ways to reduce molding and moisture migration: 1. Aeration prevents significant moisture migration by lowering the temperature difference between the grain bulk and outside ambient temperatures. Aeration removes the temperature driving force that causes natural convection currents and moisture diffusion and, hence, moisture migration. 2. Aeration controls insect populations, preventing them from generating heat and moisture that contribute to moisture migration. 3. Aeration lowers grain temperatures, thereby moving the grain bulk away from the mold envelope region indicated in Figures 7.15 through 7.23. Unfortunately, if the grain has a high moisture content, lower temperatures than for dry grain are required to avoid the mold region.

A second critical source of concentrated moisture in grain is moisture liberated by insects and molds, which is then absorbed by the grain. This is an important aspect of the grain bulk ecosystem (Pixton and Griffiths, 1971). A further major and problematic moisture source in grain bulks is from condensation that occurs in fillspouts or conveyors that drains into the core of fines and trash in the discharge line under the bin or silo fill-point. This condensation is caused when warm aeration air flows up through the head-space into cold downspouts or into fill conveyors, where the air cools and water condenses on the metal surfaces. The condensed water drains by gravity into the aerated storage and other storage units. This moisture source often runs vertically deep into the grain mass, providing an environment where mold and spontaneous heating attracts insects to points much deeper in the grain mass than grain surface moisture from roof condensation.



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Air convection currents are the most important cause of moisture migration. The density of air increases with decreasing air temperature; therefore, cold, dense air tends to sink and displace warm, less dense air (Figure 7.32). Wheat stored after harvest at 30°C (86°F) and at an equilibrium RH of 60% is surrounded by air with a specific volume of 0.88l m3/kg. When the outer surfaces are cooled by heat loss to the atmosphere during the cool season, the air surrounding the grain at these surfaces may cool to 15°C (59°F), which at the same relative humidity has a specific volume of about 0.825 m3/kg (Figure A.1). The velocity of the convection air currents produced by this differential intergranular air temperature are approximately proportional to the temperature difference. However, these currents move so slowly that they are undetectable. As air cools, its moisture-carrying capacity rapidly decreases. Moisture condensation occurs in unaerated bins, where warm air comes into contact either with cool grain layers internally in the grain mass or with grain layers cooled by surface exposure to low ambient winter temperatures. This is characteristic of unaerated bins, where convection moisture transfer occurs. Equalizing the temperature of all levels of the grain bulk prevents this in-bin moisture movement phenomenon. In temperate and subtropical climates, late or inadequate aeration of warm grain during late winter may result in accelerating the moisture migration process in the naturally cooled grain layers. In this case, at the start of aeration, a considerable increase of moisture content is detectable in the grain. For pressure-aerated silos with airflow rates insufficient to give a complete cooling pass through the grain, or when hot grain is stored for prolonged periods of time and then aerated late in the cool season, dampening may be encountered in the upper surface layers of the grain. Providing the airflow rate is within the recommended range, this problem can be overcome by continuing aeration until the trailing edge of the cooling zone reaches the upper layer and dries the condensed moisture. Although the purpose of aeration is not grain drying, for this particular case the drying of a moist layer by aeration can and must be done. Therefore, monitoring of grain surface conditions during both pressure and suction aeration is a high priority in aeration system management. When aeration is performed by sucking air downward, warm air comes into contact with a relatively cool layer of grain at the bottom of the bulk. If the silo floor is in contact with earth, the grain temperature is affected by earth temperature. Observations have shown that normally the earth temperature below a silo is relatively constant throughout the year; although it is lower than the high initial grain temperature, it is much higher than the minimum ambient temperature obtained during winter. Therefore, suction (downward) aeration in silos aerated in late winter is likely to result in a slight moisture accumulation in the grain compared with that obtained in pressure (upward) aeration. One problem encountered in suction aeration is the difficulty of early detection of moist grain damage at the bottom of the bulk. Moist grain is located along the side wall and concrete floor junction of steel bins, where ambient temperatures are much colder than bin floor temperatures. If the aeration fan openings are not sealed before aeration, winds may blow cold air into the ducts that can cool grain adjacent to (and to a distance of one or two meters from) the ducts. When suction cooling is started, moisture condenses from the warm air flowing through the cooler grain adjacent the cold floor ducts, creating a moist grain zone. If aeration is continuous, the grain and ducts gradually warm to the temperature of the exhaust air, and the condensation condition is eliminated. If suction aeration systems exhibit this problem annually in the same bins, aeration should start as early as suitable ambient temperatures are available. In order to remove previously condensed moisture, additional aeration may be required beyond the time when the trailing edge of the cooling zone passes through the bottom grain. However, this procedure is only suitable for small, localized moisture increases at the bottom of the bin and is not intended for drying grain. It should be emphasized that the best method to overcome moisture migration is to equalize grain temperature periodically as the ambient temperature drops. Starting aeration in autumn and continuing it for one to two days at 4- to 6-week intervals during winter is recommended. Although


368


this procedure calls for more aeration operation hours than is necessary for initial cooling of the grain, it solves the problem of moisture migration. Additional aeration is needed in climatic regions with large seasonal temperature differentials, such as high elevations in subtropical regions and in temperate zones that are farther from the equator. Since the temperature differential between the grain mass and ambient air in these regions is larger than in warmer regions, it provides the major driving force for moisture migration (the greater the temperature differential within the grain mass, the greater the moisture migration). In cold climates where ambient temperatures in the winter are at or lower than 0°C, the top surface and sidewall grain in the bin tend to reach subfreezing ambient temperatures (–10°C and lower). These cold surfaces present significant temperature gradients, with the cool grain (10 to 15°C) in the center of the bin. This type of moisture migration is observed in cold climates of the U.S., Canada, and similar cold regions of the world. Subfreezing surface and sidewall grain moisture content may increase. Grain deterioration occurs only when grain temperature warms sufficiently for microflora development in the warm season. Although this phenomenon is not well documented, it is well known in the cold climates of the U.S. and Europe. The solution to cold weather moisture migration, like moisture migration in temperate climates, is aeration to equalize the grain temperature as the weather begins to moderate to stabilize the grain mass temperatures uniformly at about 10 to 15°C. Use of temperature readout cables to determine when the surface grain temperature has been equalized is a major benefit in predicting and controlling moisture migration. A second management option to reduce the intensity of moisture migration is to level the grain surface as the bin is filled initially. Level grain is easier to equalize than peaked grain when re-aerating during winter. The procedure of coring the bin to lower the bin peak by ⅓ to ½ of the peak height partially levels the surface, and it reduces steel bin volumes by about 2 to 3%. Lowering the peak is advantageous because it improves the ability to aerate the bin by a substantial amount due to the reduced grain depth and the removal of part of the core of fines. Coring loosens the center grain mass and improves the ability to control moisture migration by allowing faster aeration. There is no need to try to physically level the grain; the peak reduction process can be done by simply operating the unload conveyor to remove the center of the peak. Coring is discussed in detail in Section 8.3.1, Chapter 8. A further consideration that grain managers should be aware of is that whenever aeration systems are operated for cooling, grain moisture — and thus market weight — is removed. In most countries including the U.S., grain is marketed on a moisture basis. Thus, the additional moisture removed during excessive aeration results in a substantial grain market weight loss that can be very costly. Under moisture basis market conditions, grain managers should evaluate moisture migration spoilage losses vs. market weight losses from excessive aeration. If spoilage is minimal or not a factor, one aeration cycle in the early fall may be adequate if grain is held for short-term storage (3 to 6 months). If grain is stored longer than 6 months, additional aeration may be desirable. For grain bulks cooled to temperatures in the range of 10 to 15°C (50 to 59°F), the moisture migration phenomenon under subtropical summer conditions has not been observed. Once grain is cooled, generally it can be kept safely during the warm summer — though this does not imply any relaxation from the requirement that cool grain should be carefully monitored during warm summer conditions. 7.2.4

Safe Storage of Wet Grain by Aeration

Often grain must be harvested under unfavorable weather conditions at moisture contents too high for safe storage. Sometimes this is a result of cold, cloudy, or rainy weather at harvest when field crops do not receive adequate solar radiation to field dry them to normal harvest moistures or to safe storage levels. In regions where the relative humidity is high at night, with or without



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the deposition of dew, grain harvested in early morning (even in relatively dry weather) may have a moisture content 5 to 10% above that harvested in midafternoon. Researchers found great differences in moisture content of sorghum kernels collected in the field from different parts of the same heads at the same time. The grain collected about 8:00 A.M. from the top of the heads of several plants had an average moisture content of 16.3%, whereas kernels from the bottom of the same heads had an average moisture content of 35.0% — a difference of almost 20% (Christensen and Kaufmann, 1969). Few people seem to be aware of this source of variation in moisture content, although at times it can have a great influence on the storability of a given lot of grain. Most of the maize marketed in the U.S. is harvested with combines. For best results, combine harvesters require a maize kernel moisture content of about 23 to 26%. Unfavorable weather at harvest time, delayed maturity, or other factors may result in maize harvested at moisture contents of 30 to 35% or more. In much of the U.S. corn belt, the daytime temperatures during harvest are high enough to promote rapid growth of fungi. If the moisture content is above 22%, yeast and bacteria may also develop rapidly. Clearly, the harvesting of moist grain poses a serious hazard during storage. Several approaches have been developed to improve storability and maintain marketable quality in maize and other grains harvested at high moisture content. The principal approaches are: 1. Drying — to a moisture content safe for storage (ERH = 70% or lower) 2. Aeration — to maintain a low and uniform temperature to reduce mold activity and to prevent later migration of moisture 3. Chilled aeration — aeration with artificially cooled air

The operator receiving harvested grain must take into account its moisture content at reception, its condition, and how long it can be kept before drying without losing grade and quality. He should also consider the temperature and moisture content it should be dried to for a given length of storage or for immediate marketing during the harvest season. The uses for the grain, shifts in market prices and demands, and above all, costs must also be analyzed. Short-term holding of grain at higher than safe moisture levels (but below the critical moisture content) is often economically beneficial. Grain trading in the international market is based on wet basis moisture contents that are part of the grading system. For example, No. 2 commercial maize (corn) in the U.S. is based on 15.5% moisture content, which is not safe for storage. Farmers or commercial elevator operators who market No. 2 maize at moisture levels below 15.5% lose a significant amount of profit from grain moisture weight loss as a result of overdrying. Commercial elevators blend drier grain with wet grain to ship at the allowable 15.5% moisture, thus making more profit from the overdried grain. Overdrying increases fuel costs; but if grain is underdried or delivered to a grain elevator without drying, drying and/or shrinkage penalties are assessed. When maize is sold to a typical U.S. grain elevator at moisture levels of 15.1% or higher, grain elevator managers assess a monetary drying expense discount of $0.01/bu and a moisture shrinkage factor of 0.7% weight for each 0.5% moisture above 15.0% moisture content (Assumption Coop Grain Company, 1997). If maize is stored for the farmer in open-elevator storage, discounts and shrinkage starts at 14.6% moisture content. Elevators charge the moisture discounts to pay for drying the grain to storage level. In addition, physical shrinkage of the load of maize, based on the weight of water that will be removed during drying, is also applied. The percentage weight removed by drying is calculated by the equation: Percent Weight Removed in Drying = Shrinkage Shrinkage % = 100 ×

[(IM-FM) (100-FM)]

(7.16)


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where: FM = final moisture content (wet basis) IM = initial grain moisture content (wet basis) For maize that is delivered to the elevator at between 20.1% and 20.5% moisture content, moisture discounts range from about $3.98 to $4.73 per tonne ($0.10 to $0.12 per bushel). The company also applies shrinkage changes ranging from 7.7 to 9.1% for moisture content ranges that include the grain moisture from 20.1% and 20.5 to 14.6%, for 5.5 to 5.9% actual moisture (Assumption Coop Grain Company, 1997). Example 7.9 To shrink the weight of the wet maize at an Illinois Elevator to a dry maize, if the delivered maize tested 20.5%, the percent of moisture content reduction would be 20.5 – 14.6 = 5.9% moisture. The percentage of water removed during drying, or physical shrinkage, is calculated by Equation 7.16 as follows:

[

]

Shrinkage = 100 (20.5 – 14.6 100 – 14.6) = 100 (5.9 85.4) = 6.91% Shrinkage of the grain mass, or the percentage weight of water removed when drying wheat that weighs 790 kg/m3 from 20.5 to 14.6%, a removal of 5.9 percentage points of moisture, is 6.91% weight loss. If the final moisture content (FM) is constant, the percent weight removed per percent moisture content removed also remains constant; thus, for the FM = 14.6%, the shrinkage for each percent mc is 6.91/5.9 = 1.171% wt/1.0% mc. For comparison and confirmation, for an initial moisture of 28.2% dried to 14.6%, a reduction of 13.6 percentage points of moisture, the weight loss is 100[(28.2 – 14.6)/100 – 14.6)] = 15.925. The shrink ratio is 15.925/13.6 = 1.17096, or 1.171% wt/1.0% mc. Some elevators increase their shrinkage charge percentages above the true shrinkage values calculated by Equation 7.16. This practice seems to be typical among grain elevators that dry maize in U.S. corn belt states, where they use more than the actual moisture shrinkage, then charge drying costs to farmers who market maize above 14.5 to 15.0% mc. Some grain milling processes — such as wheat flour milling or wheat, oats, barley, or maize processing for breakfast cereals — require specific moisture levels of 15 to 17% to obtain optimum milling or processing yields. The closer wheat, oats, maize, or other food grains are to the desired processing moisture level, the more efficient and profitable the milling process becomes. Because wheat, oats, and barley are normally stored and marketed at safe moisture levels of 12 to 13% or less, the moisture of these grains and maize must be “tempered” by increasing the moisture level before processing. 7.2.4.1 Cooling to Suppress Microfloral Activity Grain storage microflora pose serious problems for the storage of wet grain. Three possibilities for suppressing microfloral development are: 1. To remove the heat generated by the spontaneous heating of wet grain using continuous aeration — in such cases the moisture and temperature of the grain may remain unchanged; but further heating is prevented using aeration continuously until grain is processed, dried, or cooled. 2. To cool the grain mass in order to hold wet grain — holding moist grain presents a much more serious aeration condition than aerating dry grain. Thus, high cooling rates of at least 5 to 10 times normal aeration, or 30 to 60 (m3/h)/tonne, are necessary, depending on climatic conditions (see Tables 7.10 and 7.11 below). To hold wet grain in temporary storage for an extended period of



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time, grain must be quickly cooled to a temperature that minimizes mold development. The cooling air wet-bulb temperature is a very important factor when cooling wet grain for mold control. Adiabatic cooling may lower grain temperatures below the ambient air temperatures, but the grain must be cooled to temperatures that suppress or arrest mold growth (Figures 7.15 through 7.23). The grain temperature must be maintained uniformly at or below that level until the grain is dried, processed, transferred, or the moisture content is lowered by high-capacity aeration to a level safe for storage. Moist grain should be monitored during holding aeration to check for spontaneous heating that aeration is unable to control. 3. To dry the grain to safe storage levels — much higher airflow rates than standard aeration (typically about 6 (m3/h)/tonne) are required. Although drying systems use similar components to those of aeration systems, standard aeration airflow rates are usually much lower than rates used in drying grain; thus, the aeration duct cross-section and surface areas are too small for use with high-drying airflow rates. Natural air drying requires airflow rates that start at the upper end of the airflow rates recommended for holding wet grain. Drying airflow rates should range from 60 to 300 (m3/h)/tonne. Full false drying floors used in bin-drying systems are required to minimize high duct resistances and grain entry losses for drying.

Table 1.4 summarizes the limiting water activity levels necessary for mold growth in relation to temperature. For mold development, water activity (decimal value of relative humidity) and its relationship to temperature is a more common expression of the humidity values. Storage molds are tolerant to lower water activity levels than other microflora such as yeasts and bacteria (Christensen and Kaufman, 1974). Since mold activity is dependent on temperature, research has shown that molds that develop at lower temperatures tolerate higher water activities. Table 1.3 is based on approximate equilibrium moisture content values at 25.5°C (78°F) that are equal to the minimum percent relative humidity in which fungus can germinate. Although these relative humidities are sufficient for germination, molds require a higher moisture content to grow and develop on grain. Regardless of the ambient temperatures, the aeration system should be started as soon as moist grain is placed in wet holding bins in order to keep low-humidity air moving through the moist grain. The aeration should operate as the bin is filled and run continuously until the grain is removed or the moisture reaches a safe storage level. If aeration is stopped, the interstitial air relative humidity will increase to reach equilibrium with the grain moisture content. Several hours without aeration in high-moisture grain may cause spore germination of molds, mycelial growth, and spontaneous heating. While aeration of dry grain typically removes from ¼ to ½ percentage point moisture for each complete cooling cycle, aeration of wet grain may remove as much as ¾ to 1 percentage point or more per cooling cycle since the wet grain aeration process operates much like low-speed natural air drying. Although aeration is not considered to be a drying process, the continuous high-aeration airflow rates used for wet-grain cooling serve to slowly lower grain moisture toward safer levels while keeping fresh air moving across each kernel. 7.2.4.2 Cooling to Maintain Quality of Moist Grain Moist grain is generally above 70% ERH, equivalent to 13 to 14% mc for most cereal grains (Figure 5.1 and Table A.4). Regardless of geographic region, terrain elevation, latitude, or prevailing weather patterns, designing an aeration system to keep wet grain from self-heating due to sustained mold growth requires much higher aeration airflow rates than for dry grain. Wet grain holding requires high-capacity fans, ducts, and vents to provide rigorous cooling volumes. Depending on its mc, wet grain can develop spontaneous heating — even in cool or cold regions, and even at relatively low grain storage temperatures of 0 to 10°C — causing germ damage, mold odor, and discoloration, resulting in a severe loss of grain quality. A review of Table 7.9 indicates that maize above 20% moisture content is unsafe for storage unless uniform bulk grain temperatures of at least 0°C are maintained until the grain is dried or processed.


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Table 7.9 Grain Temp. °C 0 4 10 16 21 27b 32 38

Estimated Allowable Storage Timesa (Days) for Moist Shelled Maize

15

Moisture Content (% wet basis) 16 17b 19b 21 23

c

c

c

c

c

448 155 85 49 28 15 9

491 275 154 86 48 27

265 148 83 47 26 15

377 197 69 39 22 12 7 4

206 108 39 22 12 7 4 3

131 68 26 16 8 4 2 1

25 92 48 21 10 5 3 2 1

a

Estimated storage time for shelled maize held at constant temperature and moisture during which 0.5% dry matter loss is expected to occur. b Continuous aeration is required at recommended airflow rates of 30 to 60 (m3/h)/tonne (0.5 to 1.0 cfm/bu) during the wet holding of shelled maize at or above 18% moisture content and with grain and/or air temperatures above 27°C (81°F). c More than 2 years. From ASAE Standard S-535 and is calculated from equations presented in reference sources, Steele et al. (1969), Thompson (1972), and Friday et al. (1989).

Holding wet grain temperatures at or just below the freezing level provides safe conditions against most grain molds. Table 7.9 lists holding time vs. grain temperatures during which stored maize at a range of moisture contents is known to sustain 0.5% dry matter losses that cause it to drop one grade level in U.S. grain markets. These data are used by many grain producers and elevator managers in the U.S. corn belt and other grain regions as a guide for cooling and holding high-moisture maize until it can be dried to safe storage levels. For storage at high moisture, maize should be cooled to sufficiently low temperatures or should be dried to safe storage moisture contents well in advance of the time listed. Variables such as uneven cooling, high moisture zones, or condensed moisture drainage that creates wet grain under fillspouts require that a margin of safety be applied to values in Table 7.9. 7.2.4.3 Selecting Cooling Rates for Wet Grain Wet grain that is to be held without drying for several days or weeks must be cooled rapidly to prevent self-heating and damage to the grain mass. Uniform air movement through the wet grain mass is vitally important. If overall airflow is adequate, but air movement in some portions of the storage unit is low, mold development and heating can occur in that region due to the inability of air to remove the heat generated by the grain. Foster and Tuite (1982) recommended aeration airflow rates for cooling 13 to 15% moisture content grain in horizontal and vertical storage in temperate and subtropical climates (see Table 7.10). These airflow rates of 1.6 to 3.3 (m3/h)/tonne (about 0.05 to 0.1 cfm/bu) for steel tanks are minimally adequate for dry-grain aeration, for which they are recommended; but they are much too low for removing the heat generated by wet grain. Table 7.11 lists airflows in the order of 30 to 60 (m3/h)/tonne (0.5 to 1.0 cfm/bu) that are recommended for holding and cooling moist grain in tropical and subtropical climates. These are minimum airflow rates recommended for in-bin natural air drying of grain with moisture levels of 16 to 20% in temperate climates. Holding wet grain above 24% in subtropical regions or above 22% in tropical regions is not recommended.



Table 7.10

Storage Horizontala Verticalb

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Recommended Airflow Rates for Cooling 12 to 15% Moisture Content Grain by Aeration Temperate (m3/min)/m3 (m3/h)/tonne 3–6 1.5–3

Climate Subtropical (m3/h)/tonne (m3/min)/m3

0.04–0.08 0.02–0.04

6–12 3–6

Tropic (m3/h)/tonne (m3/min)/m3

0.08–0.16 0.04–0.08

12–24 6–12

0.16–0.32 0.08–0.16

a

Flat bunkers and corrugated steel bins. Concrete silos. From Foster, G.H. and Tuite, J. (1982). Aeration and stored grain management, in Storage of Cereal Grains and their Products, 3th ed., (Christensen, C.M., Ed.), Am. Assoc. Cereal Chem., St. Paul, MN, pp. 117–143. b

Table 7.11

Recommended Aeration Airflow Rates for Cooling and Holding Grain Above Safe Storage Levels and Wet Grain in Subtropical and Tropical Climates

Moisture Content (% wb) Subtropic Tropic 13 14 15 16 17a 18a 20a 22a 24a

— 13 14 15 16a 17a 18a 20a 22a

Estimated Times (m3/h)/tonne 6 12 18 24 30 36 45 60 60

Airflow Rates (m3/min)/m3 0.08 0.16 0.24 0.32 0.40 0.48 0.60 0.80 0.80

ft3/min/bu

Travel Timea min.

0.10 0.20 0.30 0.40 0.50 0.60 0.75 1.0 1.0

5.0 2.5 1.7 1.3 1.0 0.8 0.7 0.5 0.5

Cooling Timea h (minimum) 150 75 50 40 30 25 20 14b 12b

a

Travel time is the approximate residence time that a unit of air is in the grain mass from entrance to exit; cooling time is the total projected time required to pass a cooling cycle through the grain mass. b Evaporative cooling from free surface moisture. From McKenzie, B.A. and Foster, G.H. (1966). Holding grain at various moistures with aeration. From lecture at Purdue University.

Once wet grain (above 16 to 18% mc) is placed in an aeration holding bin, aeration fans should be operated continuously until the grain has cooled below 15°C (59°F), has dried below 14% uniform moisture content, or has been transferred for processing or drying. At grain moisture contents above about 20%, free surface moisture begins to influence cooling requirements. Although the cooling rate increases due to adiabatic (evaporative) cooling, airflow rates should be used that ensure that the air exhausts from the bulk at below the ERH of the wet grain. By continuous high airflow movement, the kernel surfaces are less likely to support mold spore germination since this is mainly a surface phenomenon. In some subtropical regions and in most tropical regions, holding high-moisture grain with high-airflow aeration may still be difficult to manage, especially in regions with sustained high ambient air humidity. From Table 7.11 it is apparent that cooling times are very short with the recommended airflow rates necessary for holding higher moisture wet grain. Although initial cooling is the first objective, holding the grain at that moisture and gradually reducing the moisture level is the primary objective. If the grain to be stored is a high-value commodity in subtropical or tropical climates, a grainchilling unit matched to a relatively fast aeration fan system may be a better alternative than highspeed aeration. Even when chilling is used, if moisture levels are 18 to 20% or higher, a cooling front should be moved through the entire grain mass within one to two days to keep mold spores from germinating before the grain is chilled. Chilling speed is more critical if grain temperatures are above 20°C (68°F) when loading into storage.


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Table 7.12


Recommended Airflow Rates for Natural Air Drying and Cooling/Holding Wet Grain in Temperate Climates

Grain Type Wheat

Shelled maize and grain sorghum Oats/barley

a

b

Grain Natural Air Dryinga Cooling/Holdingb Moisture 3 3 3 3 3 (%) (m /h)/tonne (m /min)/m ft /min/bu (m /h)/tonne (m3/min)/m3 ft3/min/bu 20 18 16 25 20 18 16 25 20 18 16

180 120 60 180 120 90 60 180 120 90 60

2.4 1.6 0.8 2.4 1.6 1.2 0.8 2.4 1.6 1.2 0.8

3.0 2.0 1.0 4.0 3.0 2.0 1.0 4.0 3.0 2.0 1.0

36 24 12 48 36 24 12 48 36 24 12

0.48 0.32 0.16 0.64 0.48 0.32 0.16 0.64 0.48 0.32 0.16

0.6 0.4 0.2 0.8 0.6 0.4 0.2 0.8 0.6 0.4 0.2

Recommendations for drying with either unheated or heated air. (Compiled from Farmers Bulletin [1965]. Drying shelled corn and small grains, Farmers Bulletin 2114, U.S. Department of Agriculture, Washington, D.C.) McKenzie, et al. 1966; R. Noyes field experience, 1966–68. (Compiled from McKenzie, B.A. and Foster, G.H. [1966]. Holding grain at various moistures with aeration. From lecture at Purdue University.)

7.2.4.4 Airflow Rates for Maintaining Grain Condition When the purpose of aeration is other than cooling the grain bulk, alternative airflow rates may be required. The use of high-aeration airflow rates for maintenance of condition has been suggested by Jouin (1963) to extend the storage period of recently harvested damp grain. For this purpose, airflow rates in the range of 15 to 30 (m3/h)/tonne (0.25 to 0.5 ft3/min/bu) were recommended. These airflow rates coincide with the rates listed in Table 7.10, Table 7.11, and Table 7.12 for tropical, subtropical, and temperate climate aeration of moist grain. Grain deterioration has been correlated with the respirative (metabolic) evolution of carbon dioxide by the grain (Hall, 1970; Steele and Saul, 1969). Carbon dioxide relates directly to dry matter loss, combined with water and heat generation. Teter (1979) proposed a method for determining the airflow rate requirement to remove heat generated by moist grain, based on respiration rates given by Hall (1970). This method assumes an arbitrary 30°C (86°F) temperature of the aeration air. Knowing the grain moisture content (.m = decimal, wet basis) and using figures obtained for carbon dioxide output (mg C02/day/100 g dry matter) by grain at different moisture contents (Hall, 1970), the heat generated by a tonne of grain can be calculated using Equation 7.17 as follows: heat = kcal tonne day = 25.51 (mg CO2 day 100 g dry matter ) (1 – .m )

(7.17)

Then, based on the assumption that moist grain heats from respiration at a rate of 3°C (5.4°F)/day and causes the aeration air temperature to rise at the same rate, the volume of air needed per tonne of grain can be calculated. Since 0.24 kcal heats 1 kg of air 1°C (1.8°F), for each degree rise, the volume of air needed per tonne can be calculated by dividing each kcal/tonne/day by 0.24 or multiplying by 4.19. The permissible air relative humidity for removing heat of respiration can be determined knowing the moisture content and equilibrium relative humidity of the grain (Figure A.4). The daily number of available hours of ambient air with a relative humidity equal to or lower than the equilibrium relative humidity of the grain should be determined. For this purpose the daily temperature and relative humidity records need to be analyzed. The grain interstitial air relative humidity level to intersect the design daily relative humidity curve should be compared to determine the



375

number of hours needed to remove heat of respiration (Figure 7.4). Then, using Equation 7.18, the airflow rate ((m3/h)/tonne) can be calculated:

(m h) tonne = [(kcal tonne day) × Sv × 4.19] (∆T 3

res

× h day)

(7.18)

where: h/day = allowable hours of operation of aeration system Sv = specific volume of air (m3/kg) ∆Tres = grain temperature increase due to respiration of moist grain (°C) Example 7.10 To calculate the airflow rate required to maintain the condition of 15% moisture content in maize (corn), first estimate the CO2 production by the maize. From Hall’s (1970) data, 16% maize will produce 11.8 mg CO2 /day/100 g dry matter. This respiration data can be used to calculate the heat generated per tonne per day. Alternatively, the comparison of grain interstitial air and ambient air wet-bulb temperatures may simplify the analysis for determining the suitable number of hours for aeration to remove heat of respiration. Heat generated = 25.51 × 11.8 × (1 − 0.16) = 253 kcal tonne day. Next, estimate the specific volume of air using Figure A.1 — for example, 0.84 m3/kg. Then determine the daily number of available hours suitable for aeration. From Figure A.4, it is clear that for maize at 16% moisture content, any air at less than 81% relative humidity could be used. Using a thermohygrograph chart as shown in Figure 7.4, and assuming grain temperature is equal to or greater than ambient temperature, the climatic conditions in this case would allow aeration for 15 to 20 h/day. Use the previous assumption that moist grain respiration heats the grain at a ∆Tres =3°C/day. For a 20 h/day aeration, calculate the respiration heat airflow rate as follows:

(m h) tonne = (253 × 0.84 × 4.19) (3 × 20) = 16.8 (m h) tonne 3

3

For a 12 h/day the airflow rate to remove respiration heat would be:

(m h) tonne = (253 × 0.84 × 4.19) (3 × 12) = 24.7 (m h) tonne 3

3

Since the appropriate airflow rate to be used depends on the number of aeration hours available per day, Figure 7.33 was prepared for a 20 h/day using the above calculations. From Figure 7.33 it is clear that when the moisture content is no higher than 16% and the daily available hours of aeration are less than 20, airflow rates required to prevent heating of sorghum, maize, wheat, and soybeans are higher. However, it should be emphasized that these airflow rates are necessary until the temperature of the commodity is reduced to a level where respiration processes of the grain are significantly reduced. USDA research (Farmers Bulletin, 1965) was conducted to develop guidelines for drying wet grain with natural air in the U.S. while minimizing required fan power. These airflow rates were designed to provide minimum airflow that would dry the grain under normal ambient RH conditions while keeping the wet grain from going out of condition during the early stages of drying. These airflow rates, listed for wet-grain holding in all climates, are listed in Table 7.11 and Table 7.12. The application of these airflow rates, under suitable ambient conditions, provides the capacity of partially drying the grain, while grain cooling is also achievable. However, the application of


376


Figure 7.33

Theoretical airflow rates (m3/h)/tonne) for removing heat of respiration of different commodities in relation to their moisture content. Curves were calculated on the basis of 20 available aeration hours per day. (Data from Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome.)

such high airflow rates is more feasible in very small temporary storage bins equipped with adequate air distribution systems. The method is not intended for use in large silos or warehouses with aeration systems designed for cooling dry grain. In Example 7.10, aerating 16% mc maize to control respiration heating fits well within the recommended airflow rates in Table 7.12 for 16% mc maize based on the assumption that respiration heat should be removed using almost continuous aeration. The 14.8 (m3/h)/tonne calculated for 20 h/day in the previous example for 16% maize falls between the airflow rates of 12 (m3/h)/tonne (at 24 h/day) listed in Table 7.12 for 16% maize and 24 (m3/h)/tonne for 18% maize, but provides a conservative comparison to the research data in Table 7.12.

7.3 AERATION SYSTEM OPERATING STRATEGIES 7.3.1

Operational Settings for Aeration Control Systems

Operating aeration control systems requires a thorough knowledge of the control device, storage structure, and aeration system characteristics. Essentially, aeration controllers are electrical system control devices designed to provide automatic starting and stopping of aeration fans based on selected temperature and humidity levels deemed suitable for the aeration program. 7.3.1.1 Multiple Staged Set-Point Strategies Grain managers can use an adjustable or multiple aeration controller set-point strategy that involves starting aeration at a higher set-point than the final desired cool grain temperatures. When cooling weather begins to develop, aeration should begin cooling the grain as soon as an adequate amount of ambient air temperatures are 8 to 10°C lower than grain temperatures. After 2 to 4 weeks of intermittent cooling, when weather conditions are cooler, the thermostat can be set to 15°C (59°F) or lower final temperature setting for the remainder of the cooling cycle. As aeration progresses, and by using the second-staged, lower set-point to further limit the overall temperature of the grain mass, the upper-level grain layers that cooled to 20 to 22°C (68 to 71°F) or lower initially will then cool down to the target temperature when cooler weather



377

conditions are available later in the fall. This staged set-point concept may result in a total aeration period with 15 to 25% more fan operating hours, compared to aeration using only one thermostat setting throughout the cooling; but it will result in much faster suppression of insect populations, drastically reducing risk of moisture migration. 7.3.1.2 Wet-Bulb Temperature Control Strategy In some regions, 3 to 6 weeks or longer following harvest may be required to obtain the low temperatures suitable to accumulate 150 to 200 hours aeration. If outside air temperatures drop to only 3 to 5°C (5.4 to 9°F) below the grain mass temperature, it may be advisable to compare the ambient air wet-bulb temperature with that of the interstitial grain bulk. Very often the dry-bulb temperature does not indicate the cooling capacity of the ambient air. Where sufficiently low ambient (dry-bulb) temperatures occur, as in Canada and Europe, the thermostat set-point can be initially set to the desired low setting, such as 7 to 10°C (13 to 18°F) below the grain mass temperature. In the warm Australian climate, low grain dry-bulb temperatures were difficult to achieve until winter and often required that the aeration system run for long periods. Consequently, in Australia, aeration was either burdened with such high running costs that other methods of insect population control were regarded as more cost effective, or was given up as impractical (Wilson and Desmarchelier, 1994). Desmarchelier (1988) has shown that existing data on rates of insect population growth can be empirically correlated against only one parameter, the commodity wet-bulb temperature (CWBT). This means that the influence of both dry-bulb temperature and grain moisture content in the control of insect populations can be accounted for using the CWBT. If interstitial air in moisture equilibrium with the grain is extracted from the grain bulk and blown over a wet-bulb thermometer, the thermometer gives a value for the wet-bulb temperature of the interstitial air. It is this value which is referred to as the commodity wet-bulb temperature (CWBT) and detailed in Section 7.2.2.1. 7.3.1.3 Aeration to Prevent Moistening or Drying of Grain In theory, if air — with wet-bulb temperature (w.b.t.) and RH characteristics that are identical to the characteristics of the interstitial air of the grain bulk — is passed through the grain bulk, the moisture content of the grain will not change. However, during normal weather conditions in most regions of the world, the ambient air is subject to substantial day and night variations in temperature, humidity, and enthalpy. Therefore, in practice, without humidity-controlled chilled aeration, it is virtually impossible to operate aeration systems without moisture transfer with a constant w.b.t. and RH setting. No matter what aeration system is used, whether manual or automatic, air blown into the aeration system will tend to either moisten or remove moisture from the grain. The reasoning behind the selection of practical air properties in aerating grain was given in Sections 7.2.2.1 and 7.3.1.7. If a system is to be operated based on w.b.t., then a method must be selected for determining the w.b.t. of the grain bulk. The values can be determined by making calculations based on grain temperature and moisture or by using automatic sensors. The w.b.t. of the ambient air should be lower than the interstitial air of the grain bulk to accomplish the target w.b.t. of the grain bulk. In the w.b.t. system, dry-bulb temperature is neglected; but a comparison of the dry-bulb temperature with that of the ambient reveals that if the ambient temperature is lower than the grain temperature, the end result of aeration is a slight moistening of the grain. If the dry-bulb temperature is higher than the grain temperature, the end result is a slight drying of the grain. From practical observations, moistening or drying of grain can occur at the ambient air entry interface into the mass of grain in the bin. In temperate climates this cyclic effect can be caused by operating the aeration system continuously under fluctuating daily humidity conditions. Because


378


this is a cyclic effect between wetting and drying, as the ambient d.b.t. and RH fluctuate throughout the day and night, this cyclic effect has not provided grounds for much concern in temperate climates. In contrast, aeration in subtropical climates is carried out mainly at night during suitable periods of the year. Therefore, grain at the air entry port is mostly affected by low w.b.t. and high RH ambient air, resulting in a limited additive moistening in this region of the grain bulk. This limited moistening effect around the aeration duct was identified by Navarro et al. (1969) and further verified by Navarro and Calderon (1982) as providing a location for development of insect and mite species favoring high humidities and warm climates. The operation of the system at a w.b.t. below that of the grain bulk achieves the desired cooling effect, but lengthy aeration periods and/or high inflow rates can have a minor drying effect on the grain. Such drying may have a limited but marked effect, depending on the parameters employed. Under normal operating conditions, drying of one percentage point or more may occur during one cooling season with one to three aeration cycles. In the U.S. and other countries, a 1 to 2 moisture percentage point loss represents a significant (1.2 to 2.3% in weight) marketable grain weight reduction and loss in profit margin. For grain stored at the upper permissible levels of moisture content for storage (up to 15% for British Wheat), the drying effect can be more pronounced. For grain stored in particularly dry conditions, such as semi-arid climates of the Mediterranean region and Western Australia, the grain may have been harvested at as low as 9% moisture content. Therefore, a further reduction in grain moisture by aeration is unlikely or less pronounced than for grain stored at a relatively high moisture content and subjected to aeration. 7.3.2

Alternative Aeration Methods

The decision to use a specific method of aeration depends on the availability of suitable climatic conditions to fulfill the planned aeration objectives. These objectives were detailed in Chapter 1. The use of ambient conditions characterized by dry summers and cool winters with intermittent rain will be discussed as examples in the following paragraphs. As previously described in Section 7.1, when relating to grain storage technology, a classification that refers to the variations in the temperature and relative humidity of the ambient is the most satisfactory. For warm, seasonal climates, also described as Mediterranean climate (Mackay and Jamieson, 1970), the diurnal temperature variation is 5 to 10°C (9 to 18°F) throughout the year. Maximum day temperatures in summer are around 30°C (86°F), while at night they drop to about 22°C (72°F). In winter the maximum day temperatures reach 15°C (59°F), while at night they fall to 5°C (41°F). The annual rainfall in winter is 500 to 1000 mm. The air relative humidity changes very little with the season and has a mean of about 60 to 65% during the day, rising to 75 to 80% at night. However, in areas bordering warm, dry regions, the annual rainfall may be less. RH during the hot summer days usually ranges from around 20% to above 80% at night. 7.3.2.1 Early Fall Aeration to Prevent Insect Infestation With the climatic conditions described above, the wheat harvest takes place in early summer, during May and June in the northern hemisphere. Initial post-harvest ambient temperatures are in the range of 30 to 40°C (86 to 104°F) in the shade throughout the summer months, and grain temperatures in storage may reach these temperature levels. Under these conditions there is a real risk that damaging insect populations will develop within a few months. Therefore, aeration of grain is encouraged as soon as possible after harvest, particularly if aeration of the freshly harvested stored grain can reduce grain temperatures to a moderate level of 20 to 22°C (68 to 72°F).



379

For example, if nighttime air temperatures drop to 18 to 22°C (64 to 72°F) when daytime air temperatures are 35°C (95°F) or below, several hours per night of beneficial aeration can be done. During very hot weather, however, usually from mid-June through mid-August, the available cooling time is quite limited. In many countries, harvested grain entering storage is already infested — either as a result of field infestations of the ripe grain by stored-product insects or infestations initiated by residual insect populations in the grain harvesting, transport, and cleaning machinery before it is taken into storage. There is not normally a serious problem of insect populations in grain coming into storage at harvest time in the U.S. But an incident that occurred at a southern U.S. country elevator immediately after harvest during the mid-1990s illustrates a problem that may occur if aeration is started too soon. At this elevator, aeration fans were turned on a few days after harvest in late June. The smell of freshly harvested wheat from the exhaust air of the aeration fan moved downwind as a large odor plume within 24 hours after aeration was started. A mass influx of flying stored-grain insects from the adjacent fields and creeks followed the fresh wheat smell upwind. They covered the sides and roofs of the four 5000-tonne storage bins, and entered roof vents and eave gap openings. The 30,000 tonnes of grain had to be fumigated immediately. This may suggest the importance of sanitation measures taken around the storage facilities throughout the year, but particularly when fresh, uninfested grain is to be aerated immediately after harvest. The sooner cooling weather arrives after the harvest is completed, the more control can be provided by aeration over insect populations that are starting to develop. Aeration should not be operated during the hottest part of the summer but should be started as soon as nighttime temperatures begin to fall in late August and early September. As hot summer weather moderates, insect activity enters its optimal range. In the northern hemisphere, populations begin to accelerate in late August and early September. Therefore, aeration should be started as soon as suitable cooling weather is available in late summer and early fall. 7.3.2.2 Winter Aeration to Prevent Insect Infestation After early fall cooling is completed, the grain and storage facilities should be monitored periodically (at 2- to 4-week intervals) for insect, moisture, and mold problems. If grain is to be held for longer than 6 to 9 months after harvest, further cooling can be undertaken during the winter to prevent or control such problems should they arise. During fall aeration, using an efficient aeration system, grain temperatures can be reduced to 14 to 16°C (57 to 61°F) or lower in temperate climates and 18 to 22°C (64 to 71°F) in subtropical regions. Winter aeration can further reduce grain temperatures to 5 to 9°C (40 to 48°F) in temperate regions, and 12 to 18°C (54 to 64°F) in subtropical regions with minor loss of grain weight from aeration moisture removal. 7.3.2.3 Minimizing Damage to Infested or Damp Grain Although the immediate risk of insect damage can be delayed by cooling under subtropical conditions, a subsequent temperature rise may occur due to accumulation of metabolic heat liberated by insects (Burrell et al., 1967; Navarro, 1974). With heavy infestations (30 to 40 insects per kg of grain), cooling is not likely to destroy all insects unless the grain can be kept below 10°C (50°F) for many months (Burrell et al., 1967). These cool grain temperatures will suppress further insect population development. If the initial insect population is relatively low (less than 5 insects per kg of grain), cooling by aeration can be effective in preserving grain from insect damage. Grain at moisture contents in the range of 13.5 to 15% can be damaged by fungal activity if a prolonged time elapses prior to cooling (Christensen, 1955). However, in bulks of grain cooled to


380


at 5 to 10°C (41 to 50°F) (Burrell, 1974), if the moisture content is below 18%, visible mold rarely occurs during a storage period of up to 8 months. For subtropical and tropical climatic conditions, long-term storage of grain above its critical safe storage ERH of 65 to 70% (13.0–l3.5% moisture content, for wheat) is risky. Although the locally harvested grain may reach storage centers in dry conditions, grain imported into certain warm countries may have relatively high moisture contents. Moisture levels of 14% for wheat, 15% for corn, or 14% for soybeans are not unusual for export consignments delivered from temperate countries. Under these circumstances, great care should be taken to ensure that such moist grains are only imported during winter to enable rapid cooling using aeration; otherwise, alternative precautions should be taken. Even when moist grains are cooled during winter, the planned storage period should be limited to the cool months because of the possibility of reheating due to fungal activity. Otherwise, moist grain should be dried and cooled or transferred for food processing or feed production before the relatively long hot season when aeration with ambient air is ineffective. Ferreira et al. (1979) used a computer simulation model to study the feasibility of storing maize using aeration at several locations in Brazil. The feasibility study indicated that maize at 16% moisture content could not be safely stored for more than 155 days using blowing (pressure) aeration at any of the studied locations in Brazil. Table 7.9 lists the approximate number of available days for safe storage of maize at several moisture and temperature levels before a quality reduction to the next lower grade (in the U.S. grading system caused by the loss of ½% dry matter) can be expected. Table 7.12 lists recommended airflow rates for holding and cooling several types of moist grain. Table 7.11 lists aeration rates for holding moist and wet grain above safe storage moisture levels in subtropical and tropical climates. 7.3.2.4 Eliminating Spontaneous Heating of Grain Some commodities are more sensitive than others to development of hot spots in the bulk. Maize and soybeans are particularly susceptible to spontaneous development of heating foci, especially when they are stored at moisture contents close to or above critical ERH levels. The way these heating foci develop has not been thoroughly investigated. However, high dockage concentrations contribute markedly to this phenomenon (Benvenisti et al., 1971; Navarro et al., 1979). A possible explanation is that dockage provides a substrate readily invaded by molds and other microorganisms compared with the relative resistance to mold development of sound grain kernels (Paster, 1972). These heating foci can be encountered in different depths of the grain; but they are more active in layers of up to a few meters from the surface, possibly due to higher levels of moisture accumulations in this region of the bulk. If the heating occurs during winter, aeration at an adequate airflow rate is satisfactory. However, under summer conditions, alternative methods should be considered — for example, aeration with refrigerated air or removal of peak grain. In crops with relatively high oil contents such as maize, soybeans, and cottonseed, the occurrence of heating foci is much more common than in wheat and barley. In subtropical climates, especially where large diurnal temperature fluctuations prevail, there is apparently a twice-daily quantitative air interchange between the external atmosphere and the bulk. This occurs due to the pumping effect caused by pressure release as head-space and intergranular air heats and expands during the day. Air is sucked into the silo by contraction as head-space and intergranular air temperatures drop at night, causing a drop in pressure. A tendency for moisture increase in dockage accumulation locations has been observed due to the capacity of the dockage to absorb moisture faster than the grain. This moisture increase phenomenon, facilitated by daily diurnal air interchange, is apparently due to the much smaller particle size and porous molecular structure of the dockage. An additional safeguard against heating foci is to spread or distribute dockage evenly during loading or to clean the grain to remove dockage prior to loading.



7.3.3

381

Long-Term Storage of Grain

The procedure for storing wheat and soybeans for several years in the subtropical climate of Israel was reported by Ben-Ami and Dayagi (1967). This procedure has been successfully applied in metal horizontal bins for storage capacities of 1000 to 2400 tonnes of grain or soybeans. The general storage method for wheat in these climatic regions is to apply aeration immediately after harvest (May and June) before a residual insect population starts to develop from the storage structure, the handling facilities, or from adjacent bins. A second aeration is applied in early autumn (late September) until grain temperature is lowered to 12 to 14°C (54 to 57°F). During the following summer, grain temperatures rise to 18 to 20°C, which is still below the threshold for insect development. Using this grain temperature management procedure that relies on aeration and sanitation, locally grown wheat was stored safely without using residual chemical grain protectants for 4 years. During this period rigorous sanitation procedures were applied. Chemical treatments using residual contact insecticides were restricted to surface application of external bin walls as a protective barrier to minimize infiltration of insects into the bins. A similar procedure might be applied for the safe storage of imported wheat, barley, and corn. However, the success of this long-term storage procedure has been restricted to fresh wheat placed in storage immediately after harvest. If these grains were received and were infested during the warm summer months, they could not have been successfully maintained without a fumigation treatment. Those central storages that have adopted this aeration and sanitation system very seldom needed fumigation treatments. Holding soybeans in storage for extended periods of time (2 to 3 years) has been a practice in Israel. Although insect infestation is not as limiting for long-term soybean storage, soybean heating in large bulks has been a restricting factor. Excessive accumulation of dockage and moisture migration causes heating of soybeans in storage. If soybeans did not heat during the warm summer months after harvest, aeration for the long-term preservation of cooled soybeans became an accepted practice in Israel. The aeration regime to cool soybeans is similar to that applied for cereal grain with the exception that the aeration system may need to be operated to overcome occasional slight reheating of soybeans. Grain destined for long-term storage (more than one year) should be as clean as economically practical and stored initially at moistures with ERH levels at or slightly below those normally used for safe short-term storage (Table 7.5). Clean grain is stored better than grain with significant levels of dockage and other foreign material (f.m.). Absence of dockage and f.m. minimizes insect food supplies for secondary storage pests or those species that are unable to penetrate sound grain. By recooling the bulk late in the winter or early spring while the weather is still cool enough, a cool-grain temperature uniformity is established through the bulk that helps maintain the grain in a cool condition well into the warm summer months. Epperly et al. (1989) found that, for level grain in large steel bins that were cooled to –7°C (20°F) in February, the mean grain temperature was 2°C (36°F) in April, 13°C (55°F) in July, and 18°C (64°F) in October without additional aeration. Grain samples in April contained no live insects, and the cool grain provided continued residual pest control well into the warm weather. 7.3.4

Aeration by Types of Storage

There are basic differences in the methods of aeration between short upright structures (bins), tall vertical structures (silos), and horizontal structures (flat storages). Upright, vertical, and horizontal structures also differ in their airflow rate requirements. These differences are discussed in the following sections. Steel grain bins in the U.S. are typically bolted, corrugated steel galvanized bins that normally have sidewall heights that are less than their diameters. However, bolted steel and welded steel grain bins may range from a diameter-to-sidewall height ratio of 1:0.3 to 1:1.5. Silos in the U.S.


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Table 7.13

Variable Airflow Rates in 12 m (40 ft) Deep Bins Based on Grain Depths for 0.08 m3/min/m3 (0.10 cfm/bu) Aeration Systems

Fan Type Centrifugal fan

Vane-axial fan

Relative Grain Depth in Bin Full ¾ Full ½ Full ¼ Full Full ¾ Full ½ Full ¼ Full

m3/min/m3

cfm/bu

0.08 0.13 0.20 0.40 0.08 0.16 0.26 0.60

0.10 0.16 0.25 0.50 0.10 0.20 0.33 0.75

% Airflow Increase 0 62.5 250 500 0 200 325 750

Times Increase 0 0.6 2.5 5.0 0 2.0 3.3 7.5

From Noyes, R.T., (1997). Critical aeration management factors in Texas coastal grain storage. Presented at the South Texas Country Elevator Conference, Sheraton Beach Hotel, South Padre Island, TX, May 15–16.

are typically concrete or bolted steel (smooth sidewall) storage units that are much taller than their diameters. The diameter-to-sidewall height ratios for silos normally range from 1:2 to 1:7. However, what is called a steel bin in the U.S. may also be called a steel silo or silo bin in many other countries, while the U.S. silo (concrete) is also called a silo in other countries. 7.3.4.1 Aeration of Upright and Vertical Structures Upright structures are those with overall heights less than their diameters. These are bolted steel bins or welded steel tanks. Vertical structures — those with an overall height greater than their diameter — are concrete or steel silos. In upright and vertical structures, as soon as the aeration ducts are covered with grain to a height that permits adequate air distribution, aeration can be performed. In some cases where maximum airflow rate is needed, the bulk depth can be kept lower than the full bin, thus reducing resistance to airflow and increasing the air volume. The higher airflow obtained at shallower grain depths is especially important if grain moisture is above safe storage moisture levels. When aeration is used to hold wet grain in storage structures equipped with systems designed to give normally recommended dry grain aeration airflow rates, the grain depths should be kept shallow (¼ to ½ of bin depth) to keep airflow at higher than normal rates. For example, with an aeration system that delivers the design airflow when full, both the relative airflow rate and actual airflow volume increase as grain depth decreases, as shown in Table 7.13. It may appear from Table 7.13 that vane-axial fans are better than centrifugal fans. Vane-axial fans provide more airflow at shallow grain depths but are limited in static pressure, based on grain depth for different grain types. A vane-axial fan suitable for aerating maize at 12 m (40 ft) depths would be inadequate for grain sorghum at 12 m (40 ft). Table 7.13 provides a comparison of relative airflow and percent increase of total airflow vs. grain depth for a system designed to deliver 0.08 m3/min/m3 (0.10 cfm/bu) (Noyes, 1997). 7.3.4.2 Aeration of Horizontal Structures In horizontal structures the loading schedule in relation to duct arrangement is an important design aspect. In large bins or warehouses, the loading process may take several weeks. Where longitudinal ducting exists, the grain may be loaded in shallow layers along the length of the building to cover all of the duct surface area. With this method of loading, aeration can begin immediately after covering the aeration duct with 1.5 to 2 m of grain (Figure 7.34). Some long warehouses with longitudinal ducts use two separate ducts for each half of the warehouse, with



383

Figure 7.34

Longitudinal tubular perforated ducting is shown along the centerline of a flat storage warehouse.

Figure 7.35

A duct pattern using shorter ducts is shown where the pattern is laid across the width of a flat storage warehouse.

fans manifolded on each end. Thus, after half the warehouse is filled, the aeration system can be started on the half that is full. A duct pattern using shorter ducts laid across the width of the warehouse (Figure 7.35) allows filling to the full depth starting at one end. Then loading can continue progressively to full depth for the entire length of the structure. This is a less labor-intensive method than the full-length duct system described above in Figure 7.34. If each duct has its own fan, any part of the warehouse where the ducts are already well covered can be aerated as desired. Aeration can begin when each duct in Figure 7.35 is covered with grain and is ready for the aeration system to be operated. However, the system works better with at least 1.5 to 2 m of grain coverage over each duct, so the airflow distribution is uniform along the length of each duct. This duct pattern is specified for relatively wide warehouses where unloading from one end may progress for several months. Thus, ducts that are still well covered can be aerated while other ducts have been uncovered and removed. A hybrid option of the lengthwise or crosswise duct systems (Figure 7.36) involves the use of cross-ducts that extend only about 75% of the width of the floor from one side of the building. On the other side of the building, several lengthwise ducts that are T connected to fans along that side provide the aeration for the other 25% of the building. A distance of 3.5 to 5 m (12 to 16 ft) between the ends of cross-ducts and the sides of the longitudinal ducts provides a suitable vehicle alley along the full length of the building. This alley can be used for access by bucket unloaders, tractor-mounted augers, portable pneumatic conveyors,


384


Figure 7.36

Shows a hybrid option of the lengthwise or crosswise duct systems.

and other vehicles. In this system, vehicles can maneuver between the short ducts to push grain to floor conveyor-receiving hoppers or to unload the structure without removing any cross-ducts. This concept saves much labor and damage to above-floor ducts. The relatively short sections of longitudinal ducts along the side of the driveway can be removed for easy cleaning of that side of the storage. An alternative method to enable aeration before the storage structure is full is to provide the aeration duct with sliding ports inside the ducts that can be used to seal off the uncovered portion and permit aeration of the grain-covered sections of the ducts. Other methods of efficient use of aeration ducts can be considered, provided that even air distribution is maintained along the graincovered ducts. For suction systems, plastic sheeting can seal exposed ducts for fan operation.

7.4 SELECTION OF AERATION FANS An overview on fan types and their characteristics is given in Section 5.14 of Chapter 5. The present section addresses the selection of fan types appropriate to the purpose of aeration. Once airflow rates have been decided, the next step is to select the correct fan type or design. Fan selection should be made based on a review of the following important grain storage system factors: type of storage, grain type, grain depth and condition, grain moisture, climatic conditions, and direction of airflow. These factors are generally listed in the order of their significance to fan performance. Two basic types of fans that are commonly used for aeration are axial flow fans and centrifugal fans. 7.4.1

Axial-Flow Fans

Airflow through axial-flow fans is generally parallel with the tubular fan housing centerline. The primary types of axial fans used for grain aeration are vane-axial fans and tube-axial fans, which have a tubular casing enclosing the fan motor and fan impeller wheel. Axial fans are considered low-pressure fans that are suitable for horizontal storage and grain bins with relatively shallow depths (Figure 7.37). They are characterized by intense noise caused by higher fan bladetip speeds when delivering the same volume delivered by centrifugal fans. Vane-axial fans are more efficient and more expensive than tube-axial fans because their design includes straightening vanes that are bolted or welded to the tubular housing beside the motor (on direct-drive fans) which straighten the spiraling airflow pattern from the fan. Straightening vanes recover energy from turbulence, which allows the fan to deliver more airflow for the same power compared to tube-axial fans. Most of the axial aeration fans are of the vane-axial design.



385

Figure 7.37

Typical performance curves for tube-axial or vane-axial fans. Redrawn from ASHRAE (2000). Fans, Chapter 18, Systems and Equipment Handbook, Am. Soc. Heat. Refrig. Air Cond. Eng. Inc., New York.

Figure 7.38

Centrifugal fan and blade forms.

7.4.2

Centrifugal Fans

A centrifugal fan consists of a bladed wheel that rotates in a spiral housing. Air flows axially into the wheel and is then moved from the center to the periphery of the fan by the centrifugal force of the blades on the rotating wheel. Centrifugal fans are commonly used for relatively high pressures needed in upright storages. There are three basic types of centrifugal fan wheel-blade weldments — forward-curved, straight-bladed, and backward-curved (Figure 7.38). As shown in Figures 7.37 and 7.39, fan power for centrifugal backward-inclined and axial-flow fans is related to either English units of Brake Horsepower (BHP), as shown on the power curve, or to kilowatts (kW) as the unit of power in metric systems (1.0 kW = 0.746 BHP). 7.4.2.1 Forward-Curved Fans Forward-curved centrifugal fans develop a higher air volume than a radial- (straight) bladed or backward-curved fan of the same diameter and power (Figure 7.38a). But each fan wheel is primarily designed for use on a specific low-pressure air handler application such as air conditioning systems, furnaces, and other specialized applications. Because of its low static pressure range capability and overloading characteristics, it is not suitable for use in aeration systems.


386

Figure 7.39


Typical performance curves for a backward-curved centrifugal fan. Redrawn from ASHRAE (2000). Fans, Chapter 18, Systems and Equipment Handbook, Am. Soc. Heat. Refrig. Air Cond. Eng. Inc., New York.

7.4.2.2 Radial-Bladed Fans The radial-blade centrifugal fan (straight-bladed or paddlewheel fan) is suitable for high upright or vertical low-capacity structures where high pressures are needed to produce relatively small airflows. In the straight-bladed radial fan, the blades lie along the radius of the wheel (Figure 7.38b). Radial-blade fans can generate a lot more pressure than the forward-curved fan, but is not very efficient and is generally not suitable for aeration applications. This fan is best suited for material handling or “dirty” air conditions because of its self-cleaning blade design. A characteristic feature of the straight-bladed fan is that the power requirement is greatest when the fan is delivering the maximum air volume. Therefore, to prevent overloading, a restriction on the discharge or intake side of the fan should be fitted (in the form of a shutter). This reduces the airflow rate when the fan is operated in an aeration system at a static pressure lower than that for which it was designed. 7.4.2.3 Backward-Curved Fan The backward-curved (inclined) centrifugal fan is usually best suited for large, relatively deep aeration systems where a large variation of airflow may occur. The blades of the impeller are curved plates, airfoils, or flat plates that are inclined backward (Figure 7.38c). The point of maximum power absorbed by this type of fan is usually within its normal working range, so it is generally non-overloading. In backward-inclined centrifugal fans, the airfoil-blade fan is generally the most efficient design but is more expensive. The thin steel curved-blade centrifugal fan is not as efficient or as expensive as the airfoil but is more efficient and more expensive than the flat-blade. The backward-inclined flat-blade centrifugal fan is the least efficient but most economical. The variations in efficiency between these three types of backward-inclined centrifugal fans are relatively minor. All three designs will perform well in aeration systems if properly specified for the pressure and air volumes required. If the air volume increases due to operation against lower static pressure, then the power absorbed decreases (Figure 7.39). This would be like operating the fan in a system with a partially filled bin or aerating grain with a lower resistance to airflow than that for which the system was originally planned.



387

In addition to the security provided by the non-overloading characteristics of this fan, the motor rating need not be excessively high when high air volumes are delivered against low static pressures. 7.4.3

Operational Characteristics of Fans

A fan is characterized by its ability to move air against a certain range of pressures while revolving at a nominal speed. To cause air to flow through an aeration system and maintain a certain velocity, static pressure must be built up by the fan to overcome the frictional resistance of the system to airflow. Additional work in the form of pressure generated by the fan is required to develop the needed velocity of the air. This extra pressure is called the dynamic pressure. It is measured by taking the difference between the system total pressure and the static pressure. Fan efficiency is a measure of the power output of a fan in relation to power input. The fan efficiency and power input vary with the operating load. The peak efficiency commonly occurs when the fan is operating near the maximum pressure and generally ranges from 40 to 80%. Fan efficiency can be expressed either as total efficiency or static efficiency:

Fan total efficiency =

{Air volume (m s) × Fan total pressure Pa}

(7.19)

Fan static efficiency =

{Air volume (m s) × Fan static pressure Pa}

(7.20)

3

Power input ( kW ) 3

Power input ( kW )

By plotting airflow as a function of static pressure, total pressure, power, and total (mechanical) and static efficiency, the main characteristic curves of a typical fan can be described. Typical fan performance curves for a backward-curved centrifugal fan are shown in Figure 7.39. The operating point of the fan is based on establishment of equilibrium between the fan output and the resistance of the aeration system to airflow. It is where the fan stabilizes its static pressure in correlation with the airflow rate for a given duct system, grain type (and condition), grain bulk depth, and aeration system. Fans are rated by the manufacturer according to the ability to deliver air against a range of static pressures. If the aeration system characteristic curve (composed of the resistance to flow of the aeration installation and that of the grain bulk) has been accurately determined, the point of intersection of the aeration system curve and the fan performance curve (fan static pressure) determines the actual airflow. The airflow rate through the aeration system in a given installation varies when different types of grain and grain depths are aerated. In some concrete elevator silo annexes with a group of silos, one fan may be mounted on an aeration duct manifold to aerate several silo bins with different grain depths. Valves on the manifold can be operated to control the volume of air to each bin. The aeration system resistance for different situations involving different combinations of bins should be within the recommended performance range of the fan. Aeration system resistance curves can be determined knowing the dimensions of the storage bin, the commodity to be aerated, the aeration duct or manifold configuration, and the desired airflow rates (see Section 7.4.3.1, Estimating Static Pressure Requirements). Figure 7.40 shows the resistance of three types of aeration system curves (A, B, and C) plotted with a fan performance curve. These curves were normalized, and the 100% design volume of the aeration system curve (A) was arbitrarily selected to intersect at 60% of the maximum volume of the fan.


388

Figure 7.40


Characteristic fan static pressure curve and the aeration system curves fitting the recommended selection range. System A intersects with fan curve at the system design point.

The system design point should lie on the intersection of curve A and the fan static pressure curve within the recommended fan selection range. The resistance for aeration systems represented by curves B and C (Figure 7.40) provides the boundaries of the recommended selection range on fan curve (between points 2 and 3). Fans should be selected on the basis of performance ratings and the recommended fan selection range supplied by the manufacturer. Fans manufactured in the U.S. that are Air Movement and Control Association (AMCA) certified indicate that they have been tested, meet all AMCA rating standards, and deliver the airflow for all conditions listed in the fan manufacturers’ literature. This is a performance guarantee. Other countries or groups of countries (such as the EU) may have similar fan rating associations or agencies with performance standards. Fans supplied by manufacturers that do not have a rating certification by an industry-recognized independent fan performance testing laboratory or rating agency may or may not perform as rated in their manufacturers’ performance curves or rating charts. 7.4.3.1 Estimating Static Pressure Requirements To select the proper aeration fan for the system to be operated at a designated airflow rate, knowledge of static pressure requirements is essential. Figures 5.17 to 5.26, which give the fan requirements for wheat, shelled corn, soybeans, sorghum, and cotton, have been prepared using the metric system. In practice, the quantity of grain stored is a more readily available figure than the volume of the bulk, which necessitates further measurements after loading. Therefore, the airflow rates given in Figures 5.17 to 5.26 are based on bulk density and are given in cfm/bu and in (m3/h)/tonne. The static pressure requirement can be obtained from the information supplied in Section 5.7. 7.4.3.2 Estimating Fan Power Requirements Once the static pressure and the air volume required to aerate the grain bulk is determined, the required fan power can be calculated based on fan characteristics. Fan power requirements can be calculated in either kilowatts (kW) or horsepower units (0.746 HP). Although many manufacturers supplying fans still use horsepower (HP), in the present publication kW is preferred as the standardized unit in the International System of Units.



389

Chapter 5 provides details related to fan performance and estimating fan power requirements with appropriate equations to calculate adjusted fan power. Grain mass characteristics were discussed in Chapter 4. In general, small grains such as wheat, oats, barley, rice, millet, and sorghum, with their relatively small kernels, pack tightly and provide much smaller interstitial air passages than coarse grains and seeds such as maize, soybeans, peanuts (ground nuts), and field peas. Even though some small grains have larger porosity values (total air [volume] void or interstice space between kernels in the grain or seed bulk) — and some coarse grains have smaller porosity values — small, tightly packed kernels generally create more total system resistance to airflow because of the large exposed surface area in these small interstitial air passages. 7.4.3.3 Fan Efficiency and Slippage Centrifugal fans are usually much more efficient across the entire system curve or static pressure operating range than most vane-axial fans. Vane-axial fans operate best as high-volume, low-pressure air movers for uses such as bin roof exhaust fans or farm bins with depths of below 6 to 9 m (20 to 30 ft) for small grains or 12.5 to 15 m (40 to 50 ft) for larger kernel grains. Tube-axial fans (Figure 5.2, Chapter 5) with no airflow straightening vanes may be suitable for farm storage where grain depths are relatively shallow, 3 to 5 m (10 to 16.4 ft), but they are not suitable for commercial storage with greater grain depths. Vane-axial fans are more efficient because of the airflow-straightening vanes that recover some of the fan energy (Figure 7.37). As static pressure or grain resistance to airflow increases for vane-axial fans, air slips back between or past the blades and the housing, causing slippage. With centrifugal fans, the centrifugal or throwing action of the fan on the air — and the close tolerance on the clearance between the tapered inlet orifice and wheel inlet opening — results in less slippage. As total airflow increases and grain depth decreases, both the relative airflow rate and actual airflow values increase (Table 7.13). Both tube-axial and vane-axial fans should be checked for close blade-tip to fan housing clearance gap by rotating the fan wheel by hand and gauging or measuring the blade-tip clearance all the way around. The blade-tip air gap allows some air to flow back to the outside. This lost airflow is called slippage. The smaller or tighter the tip clearance and the less slippage that is allowed, the more efficient a vane-axial fan will be and the higher the static pressure it will deliver. Typical bladetip clearances for 30 cm (12 inch) to 70 cm (27 inch) diameter fans are 0.3 cm (0.12 inch) to 0.5 cm (0.2 inch). Fan housings are difficult to roll and keep concentric or perfectly round, so fan blades are machined to smaller diameters to make up for slightly eccentric fan housings. Fan blades are mounted on drive motor shafts. Then the entire motor/fan blade is shifted to center the fan blade so that the gap is equally spaced around the inlet orifice or tubular fan housing. Fans with larger than 0.5 cm (0.2 inch) fan blade-tip clearance lose excessive amounts of air by back-flow around the fan blade tips. For example, in an elevator near Corpus Christi, Texas, a hopper tank aeration fan was observed with a 1.5 cm to 1.8 cm (0.6 to 0.7 inch) tip clearance gap. Checks of several other tanks with the same fans revealed that the aeration fan on the first tank had the wrong fan blade. The other fan tip clearances were about 0.3 cm (0.12 inch) (Noyes, 1997). 7.4.3.4 Selecting Fans Based on Storage Conditions A centrifugal fan design should be selected if grain depth exceeds 6 to 9 m (20 to 30 ft) and small grains are stored. If coarse grains are stored, a vane-axial aeration fan (Figure 5.2) is generally satisfactory for stored product depths up to 12.5 to 15 m (40 to 50 ft). Centrifugal fans are required for very tall steel storage tanks with 15 to 25 m (50 to 80 ft) sidewalls and steel or concrete silos from 20 to 40 m (65 to 130 ft) in height. Medium-pressure, high-volume centrifugal fans (with backward-inclined flat blades) (Figure 7.38c) are generally suitable for steel storage tanks with


390


Table 7.14

Typical Motor Speed (RPM), Power (HP/kW), Airflow Rate (m3/h), and Noise (dBA) of Centrifugal Fans

Speed (RPM)

HP

kW

m3/h

dBA

2780 2800 2820 2820 2870

1.5 2.0 3.0 4.0 5.5

1.1 1.5 2.2 3.0 4.1

2080 2170 3040 3210 3685

81 86 91 93 96

From S&P industries.

coarse grains. But high-pressure, medium-volume centrifugal blowers (with backward-inclined airfoil blades) (Figure 7.38c) should be used to handle high static pressures in silos and small grains stored in tall steel tanks. For marginal situations, where the storage unit involves depths and bulk grain products which create conditions that exceed a vane-axial fan system performance curve by only 20 to 30%, bolting two vane-axial fans in series may provide economically viable alternatives to high-volume, mediumpressure centrifugal fans. Two vane-axial fans in series will deliver approximately the same airflow at twice the grain depth and twice the static pressure as one vane-axial fan at half the depth. Two vane-axial fans will usually be much less expensive than a centrifugal fan that delivers the same airflow and static pressure. Keep in mind that high-altitude storage with low air density will require a fan to operate at higher speeds to provide the same weight of dry air compared to sea level. Fans for high-altitude use and for countries with 50-cycle (hertz) power should be belt-driven, unless they are direct-drive fans that are designed for use in those countries and conditions. 7.4.4

Fan Noise

Aeration fan noise is generated primarily as a function of fan wheel diameter, number of fan blades, RPM, and blade-tip speed. Therefore, for fans with the same wheel diameter, slower speed fans are inherently quieter than higher speed fans. With fans operating at the same RPM, larger diameter fans are louder than smaller diameter fans. Table 7.14 illustrates higher fan noise caused by increasing fan speeds. If an axial fan has the same fan blade diameter as a centrifugal fan wheel, the centrifugal fan is much quieter because the fan housing mass and inlet orifice ring that surrounds the fan inlet is smaller than the fan wheel diameter, so sound from the centrifugal fan wheel is partially contained within the fan scroll housing. Vane-axial fan blade tip speed fan noise is emitted directly outward from the fan inlet or outlet orifice. Fan blade-tip speed is the primary source of fan noise (Table 7.14). Fan blades create noise due to turbulence from wind shear as the high blade speed passes through slow-moving air. Fan tip speed and blade passage frequency (BPF) creates the total sound power level from the fan. The BPF, as a function of fan sound level, is based on the number of fan blades and the rotational speed (RPM) of the fan wheel. The BPF is the primary frequency of the fan. Example 7.11 Compute the BPF for a vane-axial fan wheel with 6 blades operating at 1750 RPM. BPF =

1750 RPM × 6 blades 1750 = = 175 blade cycles sec = 175 Hz. 60 sec min 10



391

If the fan wheel is 0.75 m (30 in) in diameter, the tip velocity is: 1750 RPM × 0.75 × 3.1416 = 4123 m min or 68.7 m sec (229 ft sec) Fan sound level or noise is measured electronically by acoustic sound power meters that register fan sound power levels in decibels (dB) through three sound filter scales — dBA, dBB, and dBC. The dBA scale is acoustically very similar to the sound frequency profile received by the human ear. Sound levels are measured at the center of eight frequency or octave bands, with each successive band double the previous band. Bands 1 through 8 are centered at 63, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz (cycles/second). A physical law of acoustics is that, if one sound source produces a given sound level when a second, identical sound source emits the same dB level, the combined sound increases by 3 dB. Thus, doubling the sound level results in a 3 dB increase in sound power level. Adding more identical sound sources does not increase the sound level beyond the 3 dB increase. That means that doubling the sound power level results in a 3 dBA sound level increase to the human ear. Example 7.12 If a 5 kW vane-axial aeration fan generates 100 dBA at 5 meters, when a second identical aeration fan (positioned adjacent and parallel to the first fan) is operated at the same time as the first fan, the sound level recorded at 5 meters should be 100 + 3 = 103 dBA. If a third 5 kW fan is operated beside these two fans, the sound level recorded 5 meters in front of all three blowers is still 103 dBA. Another physical law of acoustics is that if a true point source of sound emits a power level that is recorded at a specific distance, doubling the distance cuts the sound level in half, or reduces the sound power level by 3 dBA. Thus, if a fan emits a sound level of 100 dBA at 5 meters, doubling the distance to 10 meters for the meter reading should result in reducing the sound power level in half — or 100 – 3 = 97 dBA. At 20 meters, a reading from the aeration fan operating as a true sound point source with no obstructions (in an open field) or near reflective surfaces such as grain bins should be 97 – 3 = 94 dBA. At 40 meters a reading of 91 dBA would be expected. 7.4.4.1 Controlling Fan Noise by Fan Selection Aeration fan noise can be controlled by several physical methods. From the previous examples, distance is a very important deterrent of sound from aeration fans. Doubling the distance reduces the effective noise level by half — 3 dBA less. Reducing fan speed (by using a low-speed fan to provide the same airflow rate instead of a high-speed fan) results in substantial reductions in sound level. Using several smaller fans instead of one larger fan may reduce the sound as two or more fans will only double the existing sound level of one fan. Selecting centrifugal fans of the same airflow rate as vane-axial fans results in quieter operation as centrifugal fans are much quieter — typically ⅛ to ¼ as loud as vane-axial fans. A centrifugal fan that delivers the same airflow as a vane-axial blower may operate at 88 to 91 dBA at 5 meters, compared to the vane-axial blower at 100 dBA at the same distance. 7.4.4.2 Controlling Fan Noise by Fan Position and Sound Diversion Aeration fans should be positioned so that their inlets or outlets are directed away from or in the opposite direction from residential, business, or commercial areas where people live or work. For existing storage units with noisy aeration fans that are a sound nuisance, several options can be considered. One option is to add a short tube section of the same diameter as the fan or larger, install a 90° elbow, and add a vertical air supply or exhaust tube to point the sound vertically


392


upward into the sky. If rain protection is needed for the vertical sound tube, an additional 90° elbow pointed away from inhabited areas can be added. If use of a vertical tube is not desirable, the air-supply tube can be turned 90° or more at ground level to point the sound away from the front of the blower inlet (the sides of centrifugal blowers). The air-supply tube can also be routed around the bin or silo so the outlet is pointed away from neighbors. Warning: If ground-level air-supply tubes are used for sound diversion, they should be securely sealed with a grate to keep children, animals, or pests from entering the tube. 7.4.4.3 Controlling Fan Noise with Acoustical Silencers If the use of air-supply tubes is not feasible, cylindrical, square, or rectangular acoustical silencers, made with perforated walls (like grain dryer grain columns) packed with insulation (that look and operate like the inside of phone booth sound boxes) can be attached to the fan inlet to block, absorb, and reduce sound emissions. The most effective inlet silencers are solid wall tubes with perforated insulated tube liners that are slightly oversized on the inside tube diameter. They have a smaller perforated, insulated acoustical torpedo-shaped pipe or tube suspended in the center of the main silencer housing. This insulated cylindrical sound diverter tube breaks up and traps the sound waves before they can reach the inlet or outlet of the silencer. When there are other large structures nearby, sound control becomes more difficult. If several grain bins are in a cluster, fan sound can be reflected or bounced from these structures. Where sound is difficult to control, the best option is often to use sound absorbers. Steel plating is an excellent material for the outside of a sound box. The inside must contain perforated material with rock wool or fiberglass packing material. The denser the packing material is compressed between the perforated inner liner and the steel outer shell, the better it absorbs the sound. A “torpedo” center absorber or divider panel down the middle of a square or rectangular sound absorber helps to trap long wavelength sounds. 7.4.5

Heat of Compression

Centrifugal fans and vane-axial fans operating on pressure or up-flow aeration move air through the grain mass by compressing air in the transition and duct system to overcome the resistance to airflow by the grain, broken kernels, foreign material (f.m.), dockage, plus duct and roof vent system restrictions. This is similar in concept to water pumps that develop pressure to overcome piping system component resistance. Some of the mechanical energy used in forcing the air through the grain and ducting systems translates into sensible mechanical heat energy in the airstream that warms the ambient air. Small air passages between grains such as wheat and sorghum are easily blocked by grain fines and broken kernels. These small grains have high static resistance to airflow compared to coarse grains like soybeans and maize. As shown on Shedd’s airflow resistance charts (1993), pushing air at 0.08 m3/m3·min through wheat or sorghum takes about 2 to 2.5 times the pressure than through maize or soybeans (Figures 5.17 to 5.26). In pressure systems, some of the fan mechanical energy used to compress the air as it is forced through the grain is converted into heat. As fan static pressure increases, more heat is generated. Knowing the fan heat derived from friction loss and the energy loss in the fan, the increase in air temperature can be estimated (Osborne, 1977) by the following equation: Temperature rise °C = ∆T =

kPa ρ×c×ε

(7.21)



where: ∆T = kPa = ρ = c = ε =

393

temperature rise °C static pressure (kPa) air density (kg/m3) specific heat of air (kJ/kg/°C) total fan efficiency (decimal)

Example 7.13 Calculate the temperature rise for a fan delivering air at a static pressure of 2.5 kPa and with a total efficiency of 0.60, based on Equation 7.19, for air density of 1.2 kg/m3 and specific heat of 1 kJ/kg/°C. ∆T = 2.5 1.2 × 1.0 × 0.6 = 2.5 0.72 = 3.47 = 3.5 °C As a result of the pressure loss by friction and fan heat added to air, the temperature increase in the aeration system may be as high as 9°C (16°F) (Navarro, et al., 1978b). Although this arrangement results in waste of fan power in producing heat rather than in moving air, it has a beneficial aspect. Where the fan compression temperature increase is within the range of 3 to 4°C (5 to 7°F), the decreased RH reduces the danger of increasing grain moisture regardless of the ambient air humidity. Use of slightly heated air is particularly suited to aeration in subtropical climates, where the lowest temperatures are recorded at night when air humidity is close to saturation. Although heat of compression is related to static pressure, fan efficiency also has a direct effect. Not all fan systems develop the same heat of compression when delivering the same static pressure. Inefficient fans or blowers generate more heat at the same static pressure compared to efficient units. But a specific fan should develop the same heat of compression each time it reaches a specific static pressure, regardless of grain type or weather conditions. Direct-drive vane-axial fans also develop heat of compression; but due to their lower maximum static pressure rating, this is usually limited to 0.8 to 1.0 kPa (3 to 4 in w.c.) SP, which is considerably lower than centrifugal fans that develop 2.5 to 4.0 kPa SP. Heat from direct-drive fan motors usually adds 0.25 to 0.75°C (0.4 to 1.4°F) to the airstream, depending on motor load and the waste heat lost to the airstream. A vane-axial fan and a centrifugal fan operating at 1 kPa SP will develop about the same heat of compression. A field study by Noyes (1988) was made to document heat of compression data for four 6.4 m (21 ft) diameter × 41 m (135 ft) concrete silos where three of the silos contained stored wheat. Grain depths of two silos were 40 m (132 ft), while the third silo wheat depth was 24 m (78 ft) and the fourth silo was empty. Average static pressures of the four silos, respectively, were 4000, 4250, 3750, and 600 Pa (16", 17", 15", and 2.4" water column). Temperature rises of 6.7 to 7.2°C, 5.3°C, and 2.8°C (12 to 13°F, 9.6°F, and 5.0°F) respectively were recorded.

7.5 AERATION CONTROL EQUIPMENT Automatic aeration system controllers are one of the most cost-effective grain storage management tools available to grain managers. An automatic controller allows grain managers to optimize the use of suitable cooling weather to cool stored grain because the controller starts and stops aeration fans exactly when preset conditions occur. The principal use of aeration controllers is to sense and operate aeration fans during appropriate conditions for efficient cooling of stored grain, usually following harvest or delivery from farm to elevators. Furthermore, some complex electronic aeration controllers are designed to monitor grain conditions such as grain mass, exhaust, and ambient air conditions to minimize moisture gain or loss.


394

7.5.1


Simple Mechanical Controller without Relative Humidity Control

7.5.1.1 Single Thermostat Controllers Some simple aeration controllers operate with one temperature set-point activated by an upperlimit dry-bulb thermometer that switches the fan system on when ambient air conditions cool to the thermostat setting. When the primary aeration objective is efficient grain cooling in temperate climates, simple low-cost aeration controllers operated by one thermostat with an hour meter can effectively handle aeration control requirements at many elevator and farm grain storage facilities. A high-limit thermostatic control operates aeration fans when ambient temperatures reach the selected grain cooling set-point. The time (hour) meter records cumulative fan operation time so managers can record progressive fan operating time accumulation and monitor cooling progress during the aeration cycle. Some operators using a single thermostat may set the thermostat to provide the highest desired cooled grain temperature. For example, a wheat farmer in the south-central U.S. (southern Kansas or Oklahoma) may want to cool his wheat as early in the fall as possible using only one cooling cycle to minimize his grain moisture loss. From weather data, he knows that cool weather fronts of 3 to 5 days’ duration begin to occur about mid-September and continue with a few days’ separation through October and November. Another option is to set the aeration controller thermostat only one time during the season to provide the desired final grain temperature, say at 15°C (59°F). If he is using a suction airflow system, that setting would be made directly on the thermostat dial. If a pressure aeration system is used, he needs to know the fan compression heat temperature rise developed by the grain and ducting system resistance to the airflow. If the compression heat rise is 2°C (3.6°F), the thermostat set-point should be 13°C (55.4°F) to provide 15°C (59°F) or cooler grain. Ambient temperatures and relative humidities are usually highly variable with daily fluctuations as well as seasonal changes. Although aeration systems can be operated manually, this requires constant daily attention and monitoring of ambient conditions. To select only the favorable ambient conditions to cool grain, thermostats and humidistats can be used. In subtropical climates, the days of autumn tend to be warm with low RH, while nights are usually cooler and more humid. It is possible, therefore, to aerate at night and utilize the lowest temperatures available — but this runs the risk of dampening the grain. To reduce this risk, as a rule of thumb, aeration systems should be operated only when ambient air temperature is at least 6 to 8°C (11 to 14°F) lower than the average grain temperature. A high-limit thermostat can be used to prevent fan operation when ambient temperatures are above the thermostat setting (Figure 7.41). As the ambient temperature decreases progressively during the season, the thermostat setting should be lowered. However, difficulties may be experienced since the grain bulk temperature is not homogenous and there is no compensation for the grain moisture content. Although thermostats perform better than repeat-cycle timers, they also operate the fan regardless of air relative humidity. 7.5.1.2 Upper and Lower Limit Temperature Span Control Simple mechanical controllers used in colder parts of temperature regions may use two thermostats to bracket upper and lower temperatures to cool the grain within a narrow temperature range (such as 5 to 15°C) to avoid extreme grain storage temperature variations. A farmer or grain elevator manager in a northern geographical latitude such as the northern U.S. or Canada might want to limit the grain temperatures to within a narrow limit of 10°C (18°F) to minimize temperature gradients. Suction airflow is seldom used in colder climates due to the risk of roof collapse if roof vents become blocked by blowing snow and ice. Assuming the same



Figure 7.41

395

Automatic aeration controller wiring diagram showing high-limit thermostat. (Data from Noyes et al. [1992]. Elevator Manager’s Summary Report, Oklahoma Wheat Elevator Electrical Energy Reduction Demonstration Project, OSU Grant Contract No. 3881 OIL/SECP 88. With permission.)

heat of compression temperature rise of 2°C (3.6°F), he would set the upper limit thermostat to 13°C (55°F) and the lower limit thermostat to 3°C (37°F) to achieve his objective of cooling his wheat to 5 to 15°C (41 to 59°F). 7.5.1.3 Humidity Control Settings The typical daily cycle of relative humidity moves through rather wide extremes. The purpose of humidistats is to prevent high-humidity air above a preselected value from entering the grain bulk. Where high humidity is a problem in tropical or subtropical climates, an aeration controller circuit may include an upper-limit thermostat switch connected in series with a humidistat switch. This provides a situation where both the air temperature and the humidity must be below the desired set-points before the aeration fans are started. Some aeration controllers include upper and lower limit thermostats, and upper and lower limit humidistats. The concept is to provide precise control of grain moisture during aeration. However, caution should be used when trying to bracket the aeration cooling system using four set-points, such as upper and lower humidistat settings of 70% and 50%, and upper and lower thermostat settings of 20°C and 10°C (68°F and 50°F). The use of tight controller set-points can greatly limit the amount of cooling time compared to the total amount of suitable aeration weather that could be used with less restrictive settings. Example 7.14 An up-flow system that was operated in a 24 m (78 ft) diameter corrugated steel tank in the south central U.S., with four set-points during the fall of 1988, developed a massive layer of moldy, crusted grain that gradually sealed off all airflow. At the time of investigation, the moldy layer was about 0.3 to 0.5 m (1.0 to 1.5 ft) thick and covered most of the bin cross-section area near the grain surface, so air passage of both centrifugal fans was almost completely blocked by the thick layer of molded grain. Fan heat of compression temperatures of 10.6°C and 17.2°C (19°F and 31°F) were recorded.


396


This system had high and low thermostat settings of about 13°C and 7.5°C (55°F and 45°F) plus high and low humidistat settings of about 80% and 60%. The grain mass received an estimated 2 to 3 hours per day of pressure aeration. These short bursts of warm, moist air that cooled in the upper grain with long stagnant conditions steadily raised the moisture of the grain near the grain surface. The moldy crust gradually spread across the peaked surface. Fan static pressure and fan heat of compression escalated exponentially until the crusted area covered most of the 24 m (78 ft) bin diameter. This is a classic example of the forced translocation or migration of grain moisture due to poor aeration fan system management. With only an upper temperature control, this problem would not have developed because the aeration system would have operated for much longer periods of time with much shorter periods between when the fans were not operating. 7.5.1.4 Automatic Timers for Aeration Control Aeration systems can be controlled using 24-hour repeat-cycle timers to operate aeration fans during a set period of the day or night, with the nightly hours of fan operation, such as 6:00 pm to 9:00 am daily, selected on the basis of current weather conditions. Management of aeration using repeat-cycle timers may require periodic resetting. To develop a database for improved timer control of aeration fans to use for reevaluation of the expected desirable cooling hours, recent temperature and humidity fluctuations can be recorded on a thermohygrograph in the vicinity of the aeration fans. Automatic timers are cheap, simple, and reliable; but since they operate regardless of whether prevailing ambient conditions are favorable or unfavorable, their cumulative cooling performance is generally less efficient than the simple thermostatic controller with a recording hour meter. However, a well-managed cycle timer can be better than manual aeration fan operation, especially during weekends when employees are not working. 7.5.2

Complex Electromechanical Controller with Humidity Control

Complex electromechanical controllers that operate aeration fans on the basis of upper and lower dry-bulb temperatures and upper and lower relative humidities can provide precise control of grain temperatures as well as grain moisture conditions. However, inadequate aeration fan time may result if this type of controller or an electronic version with similar limit-setting capability is set too close between both upper and lower temperature and upper and lower humidity limit set-points. As illustrated earlier at an elevator in Oklahoma, overcontrol of both temperature and humidity can severely limit fan operation to the point that moisture buildup in the grain mass can occur even though desirable ambient air conditions are selected. An advantage of electromechanical aeration controllers is that they can be designed, assembled, and maintained by a competent local electrician or elevator maintenance technician who understands electrical control systems and aeration system operation. This enables him to control the aeration fans at a complete grain storage facility using one master thermostat/humidity control system. Large grain storage facilities may involve as many as 30 to 40 fans with total power ranging from 150 to 500 kw. To minimize inrush or starting current and related voltage drop, it is relatively simple to develop branch fan control circuits with time-delay relays that start several small fans in one group in sequence with several large fans individually or two at a time. If a facility uses all suction or all pressure aeration, then one master aeration control box can operate the entire facility, with slave time-delay control boxes interconnected for temperature and humidity control of all aeration systems at the same set of operating temperature and humidity conditions. But if different grains require different cooling conditions, or if part of a system has pressure aeration and another part uses suction aeration, use of two separate controllers may be desirable for precise control of both systems. This permits selection of target temperature settings independently to offset the temperature increase from heat of compression of the cooling air in the pressure fans.



Figure 7.42

397

A complex electromechanical aeration controller circuit design that incorporates high/low thermostat and high/low humidity control functions as well as time-delay relays. The “ladder” schematic circuit design uses only one power-consuming device per horizontal line or “ladder rung” on the diagram. (Data from Noyes et al. [1992]. Elevator Manager’s Summary Report, Oklahoma Wheat Elevator Electrical Energy Reduction Demonstration Project, OSU Grant Contract No. 3881 OIL/SECP 88. With permission.)

Figure 7.42 illustrates a complex electromechanical aeration controller schematic circuit design that incorporates high/low thermostat and high/low humidity control functions as well as timedelay relays. The “ladder” schematic circuit design uses only one power-consuming device per horizontal line or “ladder rung” on the diagram. Complex controllers with less input weather control can be developed simply by deleting the low-limit humidistat, the low-limit thermostat, or both humidistats when humidity control is not desired. More time-delay relays (TDRs) can be added to control any number of fans by adding more horizontal lines to the schematic control circuit. In this situation, the TDR coil, instantaneous, and time-delay switch symbols are added with appropriate fan motor starter or motor coils identified. If fan motors are small, the TDR switch contacts are often rated to handle the start and run amperage through the timer electrical switch contacts, thereby minimizing the cost for additional motor starters or power contactors. 7.5.2.1 Temperature Difference Controller Temperature difference controllers consist of a temperature sensor that monitors ambient air integrated with a second sensor that monitors grain bulk temperatures. This controller operates the fans when a temperature differential greater than a predetermined setting is recorded. When a cooling front is passed completely through the grain bulk, the ambient air temperature setting can be lowered if additional cooling is needed. The main difficulty experienced with temperature difference controllers is in placing the temperature sensor in the grain bulk at a location that represents the average temperature of the grain bulk. A logical design is to place the remote grain sensor in the section of the grain bulk that will cool last. If multiple storage units are controlled by one controller, then one remote grain temperature sensor has to represent all grain being cooled (unless the controller design will accept additional remote grain sensors). Also, the remote sensor connection must be of adequate length to reach the


398

Figure 7.43


Schematic of combinations for recommended, aeration possible, and aeration of no value conditions in using differential thermostats. The differential thermostat is set for the difference in temperature between grain and ambient air at different ambient air relative humidities. (Data from Lasseran, J.C. [1981]. Special ventilation des grains, Perspectives Agricoles, Numero hors serie, Institut Technique des Cereales et des Fourrages, Paris.)

central grain mass in very large structures, which could be 40 to 50 m (130 to 165 ft) from the aeration control panel. Figure 7.43 provides a schematic of combinations for recommended, aeration possible, and aeration of no value conditions in using differential thermostats. This approach of controlling grain temperature is beneficial at temperate climates where the aeration system can be operated at significant temperature differentials (Lasseran, 1981). It simplifies the controller setting and operation procedures where more sophisticated control methods are not feasible. 7.5.2.2 Wet-Bulb Controller The best means of assessing the capacity of air to cool the grain is by using the wet-bulb temperature (Griffiths, 1967; Elder, 1971). This method takes into account the effect of air relative humidity on the grain temperatures reached. In principle, any dry-bulb temperature controller can be converted to wet-bulb simply by covering the temperature sensing element with a wet wick and providing it with adequate ventilation. However, in practice, problems of maintenance and reliability pose a limitation on the use of such controllers. Although these controllers have been reported as being developed for Grain Elevators Board storages in Victoria, Australia (Griffiths, 1967), there are no other reports on their use.



Figure 7.44

399

CSIRO time-proportional aeration controller (right) connected to fan operation hours counter (middle) and a thermostat (left) for comparing aeration operation efficiency mounted on a wall.

Several manufacturers recently have introduced electronic data loggers with the capability of measuring temperature and humidity of the ambient air. Some of these are equipped with the necessary software to convert the two thermodynamic properties of air into wet-bulb temperature readings. They also have the optional controller setting capacity. The only commercially available electronic wet-bulb temperature (w.b.t.) controller is an Australian product introduced commercially in 1998 that has similar features described above and marketed under the trademark of mDhT Aeration Controller™ (WRC Technology, Nambour, Queensland). It is a patent-pending controller capable of independently controlling the operation of fans in up to four silos or four zones of a shed. One sensor monitors ambient conditions. Limit values may be set in each mode — in drying mode a minimum RH may be set, while in cooling mode a minimum temperature may be set (Winks, 1997, 1998a, 1998b). In essence the controller examines the difference between the w.b.t. measured by a sensor suspended in the air outside the silo and compares that measured by another sensor inserted into the grain bulk. If the w.b.t. of the ambient air is less than that of the intergranular air, the electronic circuit activates the fan through a relay. The developers of this controller have experimented drying canola and Macadamia nuts stored in bulk with successful results. In differential dry-bulb mode, an mDhT Aeration Controller achieved a reduction in temperature in 1000 tonnes of canola from 22.3°C to about 16°C from February to early June 1998, under the hot climate of Australia. In this process the moisture content of the aerated canola was lowered from 8 to 6.3% (Winks, 1998a, 1998b). Field research tests with bulk grain products were being planned by the developer. 7.5.2.3 Proportional Time Controller The proportional time controller, also named CSIRO time-proportioning controller (Figure 7.44), functions both as time switch and thermostat (Elder, 1972). It operates the fan for a preset number of hours per day or per week whenever the ambient temperature is below the setpoint of a thermostat connected to the time switch. The thermostat set-point is varied by means of a clock-type electric motor — it raises the temperature (3°C or 5.4°F per week) when the ambient temperature is higher than the thermostat set-point, and it reduces the set-point temperature (at a rate of 3°C or 5.4°F per week) when the thermostat set-point is higher than the ambient. There is a cumulative change of up to 3°C per week depending upon the length of time the ambient temperature is above or below the set-point setting.


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The time-proportioning controller has been applied successfully in a variety of practical situations in several subtropical climate conditions. Results indicate two advantages in using this controller over manually adjusted thermostats: (1) there is a reduction in the number of hours of fan operation; and (2) manual temperature setting is not required. Since this is a self-adjusting controller, the fan operates during the periods of lowest atmospheric temperatures. The CSIRO controller is designed to aerate grain stored in dry, temperate, or semi-arid climates to select the optimum cooling weather in a specified time period and to operate the fan system to accumulate a predetermined amount of time during each period. If not enough suitable weather is available, a deficit time log allows the system to operate the prescribed fan time plus the backlog time to catch up during the next time block. During each time block (e.g., four weeks), the fan run-time can be spaced out so that not all the time is accumulated during the first week, even though an adequate number of hours of suitable weather may be available during that time. Figure 6.23 shows results obtained with two identical bins — one aerated using a thermostat with manual adjustment, and the other using a time-proportioning controller (Navarro et al., 1978b). A reduction of 38% in fan operation hours was obtained with the time-proportioning controller compared to the thermostat for a similar decrease in grain temperature. These controllers are available commercially in Australia as a solid-state electronic circuitry unit with digital readouts. They are used to operate aeration fans to maintain uniformity of grain temperatures throughout the bulk during very long periods of time (months to years). Their use in Australia and other semiarid regions is appropriate, where grain is usually marketed at a very low moisture content (9 to 11% for wheat). The drying effect of aeration is less pronounced than in regions where grain is marketed at relatively higher moisture contents. In a grain market system such as that in the U.S., where commodities like wheat are marketed at moisture contents of 12 to 13%, continuous long periods of fan operation would reduce the grain moisture and, thus, the marketable weight of the grain. 7.5.3

Microprocessor and Computer-Based Control and Monitoring Systems

During the past two decades, microprocessors and computers have revolutionized monitoring, data collection, and system control processes from industry to producer levels. Electronic equipment components have been revolutionized by the development of the microprocessor or computer chip, which can store complex preset control and processing algorithms. Advanced programmable chip and integrated circuit design allows for the storage, replacement, and manipulation of data so that prepackaged systems can be adjusted through input data devices to modify and manage processes to fit a variety of operational needs. Remote access and highspeed delivery systems allow operation and access of processes for management of systems at multiple remote sites by a central manager. These advancements in technology are discussed in depth in the following sections. 7.5.3.1 Microprocessor-Based Aeration Controllers Microprocessor-based aeration controllers have evolved since the early to mid 1980s in several countries. The primary function of these programmable microprocessor-based controllers is to control the fan state of an aeration system based on the ambient (air temperature and relative humidity) and grain (grain temperature and moisture content) conditions. A primary difference between microprocessor-based controllers and electromechanical units is the use of microchips that can be preprogrammed with the equilibrium relationships between the air and grain conditions for numerous stored products. The utilization of the equilibrium moisture content (EMC) or equilibrium relative humidity (ERH) relationship allows programming of the aeration, drying, and conditioning of grains based on temperature (both air and grain), air relative humidity, and grain moisture content or ERH.



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Electromechanical systems are limited to control based on operator decisions of manual settings for the temperature and relative humidity of the air only. Originally, microprocessor-based controllers were developed as stand-alone units that could be programmed via a user interface that was part of the controller box. The storage of data related to controller performance was limited to a few key parameters such as total fan run time; minimum, average, and maximum temperature; relative humidity; and EMC values. Most microprocessorbased aeration controllers today can be interfaced with computers for the purpose of programming control parameters and for the delayed or real-time collection of key performance data. In some cases the user interface is limited to a readout device, while settings can only be changed via the computer interface. In a few systems the microprocessor resides on an electronic board that is installed in an open expansion slot of a dedicated computer that must remain connected to the control system at all times. Microprocessor-based aeration control systems have been programmed with a large diversity of fan control strategies to achieve a number of often diverging objective criteria such as minimum power consumption, minimum moisture loss, or optimum safe storage temperature for a variety of grains and oilseeds. Fan control based on the commodity wet-bulb temperature (CWBT) has been incorporated into the microprocessor-based mDhT Aeration Controller (see Section 7.5.2.2 above). This strategy has proven successful for warm and dry climates as a means of cooling low moisture content grain to control insect populations (Wilson and Desmarchelier, 1994). It is calculated based on the equilibrium relative humidity relationship of the stored grain with the help of integrated temperature and relative humidity sensors that are connected to the controller and are strategically placed in the grain mass. The CWBT is discussed in detail in Section 7.2.2. The controller incorporates the CWBT concept into three of its four cooling modes: • Continuously variable differential wet-bulb temperature control • Wet-bulb temperature set-point control with automatic switching to differential wet-bulb temperature control after a preset period • Selected fixed differential between the ambient wet-bulb temperature and the grain wet-bulb temperature

One widely adopted stand-alone microprocessor-based aeration controller in the U.S. is the Sentry Programmed Aeration Controller (SentryPAC by Sentry Technologies, Chico, California, U.S.). It has seven built-in strategies for the purpose of storage, drying, and rewetting of stored grains and oilseeds. The most unique aspects of this patented controller are (1) its adaptive temperature and equilibrium moisture content bands that are self-adjusting based on the given weather conditions; and (2) the operator-selected minimum daily fan run time. In the storage mode the controller attempts to maintain the grain temperature based on the 21-day average ambient temperature while maintaining a desired target moisture content in the grain mass. When the minimum daily fan run time is limited due to unfavorable weather conditions, the hours during which the fan is unable to operate are added to the so-called backlog. The adaptive EMC band is widened by 0.15% around the target moisture content for every 4 hours; and the temperature band is widened by 0.5° around the 21-day average temperature for every 8 hours of accumulated backlog. Ambient conditions are monitored every 15 minutes to determine whether to turn the aeration fan on or off, while the adaptive bandwidths are adjusted every 24 hours. Some units also incorporate strategies to control a fan and burner unit for the purpose of inbin grain drying. The basic control algorithm involves two RH set-points. The upper RH set-point switches the fan and burner off when the RH of the air exceeds this limit to prevent excessively humid air from rewetting the grain. The lower RH set-point switches the fan and burner off when the RH of the air is below this limit to prevent excessively dry air from overdrying the grain. When the air RH is between the upper and lower RH set-points, the fans and burners are switched on. A


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more advanced controller will also incorporate a burner RH set-point. When the air exceeds this limit but is less than the upper RH set-point, the fan and burner are switched on. If the burner can be modulated, the microprocessor calculates how much heat energy to add in order to achieve a desired inlet RH. When the air is between the lower RH and the burner RH set-points, only the fan is operated. One commercial microprocessor-based controller available in the U.S. utilizes the advanced fan and burner control strategy with EMC-based set-points and modulating burner heat (Bullseye by Choretime Brock, Milford, Indiana, U.S.). 7.5.3.2 Computer-Based Monitoring and Control Systems In the past, microprocessor-based grain temperature monitoring systems had stand-alone readout boxes that were not integrated with aeration control. With the advent of personal computers, computer-based temperature monitoring systems became commercially available in the early 1990s. Currently, only one commercial system integrates grain temperature monitoring with aeration control. It is the OPI 2000 Monitoring Alarm and Control system (OPI 2000 by OPIsystems, Calgary, Alberta, Canada), which is now used at major grain terminals and storage facilities in several countries. The OPI 2000 system is an extremely versatile hardware system that can access thousands of digital or analog sensing devices that coordinate diverse operations to manage the handling, drying, storage, and processing of grain at farm and commercial facilities. The primary sensing devices are digital-based temperature cables (thermistor and thermocouple can also be used) and level sensors. Equipment such as grain dryers, bearing temperature monitoring, relays, diodes, and sampler feedback devices can also be incorporated into the system. For control applications, the OPI 2000 can monitor other operations including ambient temperature, relative humidity, fan operation, and plenum static pressure. Field devices called remote terminal units (RTUs) collect information from the sensing network. The RTU network is connected with a common communication cable to a central controller such as a master control unit (MCU) for monitoring alarm-only applications. The RTU is also connected with a programmable logic controller (PLC) when interfacing with third-party platforms or with the OPI Graphical Interface Monitoring Alarm and Control (OPIGIMAC) system, which is the PC-based software interface between the OPI 2000 hardware sensing network and the storage facility manager. The OPIGIMAC software is built with the Labview graphical language program, which is supported by National Instruments (Austin, Texas, U.S.). This makes it relatively fast and easy for OPI technicians and other scientists to develop custom programs for many specific applications. The OPIGIMAC software is based on the universal point-and-click format and includes extensive on-line help support. The program has three main screen areas. The display area contains a graphic of the facility layout. Each system can handle up to ten remote sites with up to 50 structures per site and 32 cables per storage structure. The tool bar area provides access to the system’s monitoring, alarm, and control functions. The main working area displays information relevant to the function selected. In the configuration screen, hardware components are described, RTUs and data logging are activated, and diagnostics are performed. The software provides several monitoring options. The view by cable shows the temperature profile for a cable compared to the minimum, average, and maximum temperature for all cables of a selected storage structure. The view by layer compares all sensors of a selected layer compared to the minimum, average, and maximum temperature for all layers of a selected storage structure. The view by table option allows data to be displayed in a tabular format over a user-selected time period. A two-dimensional graph displays trends over time and transforms thousands of data points into meaningful management information. The effect of user-selected control strategies on storage quality can be assessed based on grain temperature profiles, ambient temperature, relative humidity, calculated equilibrium moisture content for the stored grain, and fan run times.



Figure 7.45

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OPIGIMAC user interface showing cross-section of the site and a cross-section view of one bin with four temperature cables.

In the alarm screen, high limits and rate of rise limits can be set for each sensor or sensor group to alert operators when thresholds have been reached. When an alarm is triggered, the user is notified about the location of the problem sensor, and possible solutions are recommended. These same alarm threshold functions can also be set to operate safety equipment if desired. The control screen allows for selection of customized aeration, reconditioning, natural air drying, and low temperature heater control strategies specific to each storage structure, grain type, and target objectives. Other process control systems can be easily incorporated into the OPIGIMAC user interface. The OPIGIMAC system computer can be accessed via modem from anywhere in the world. Operations managers in large grain storage complexes that may involve a central logistic hub facility with several satellite facilities — such as an inland terminal and country elevators — can provide a higher order of integrated grain management control. The central OPIGIMAC computer can communicate with remote microprocessor data storage units at each satellite site, which may consist of large steel bins, concrete silo annexes, large flat storage buildings, grain dryers, or other mill process-control systems. Consequently, most temperature-based functions at a grain terminal, processing plant, or a group of facilities in a geographic area can be operated by one or two managers using a central computer that communicates via modem with the remote satellite systems. Additionally, the modem allows OPI systems to diagnose, service, and update the OPIGIMAC system remotely. An OPIGIMAC system was recently installed to control the aeration fans and to monitor local weather conditions and grain temperatures in 16 pilot bins, each with a capacity of 11.7 tonnes, at the Purdue University Post-Harvest Education and Research Center (PHERC — West Lafayette, Indiana, U.S.). Figure 7.45 illustrates the user interface of the site layout of the facility and a crosssection view of one bin showing four temperature cables. Figure 7.46 shows the air temperature and relative humidity as recorded by the OPI ambient sensor over a period of four weeks. The graph also shows the equilibrium moisture content for maize adjusted for the heat gain from the fans as calculated by the system software. Grain temperatures recorded at three depths on the center cable in a bin filled with about 9.8 tonnes of food maize that was not aerated remained between 22.1 and 27.2°C (Figure 7.47).


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Figure 7.46

Air temperature and relative humidity as recorded by the OPI ambient sensor over a period of four weeks.

Figure 7.47

Grain temperatures recorded by the OPI system at three depths on the center cable in a bin that was not aerated.

Figure 7.48 shows grain temperatures at the same locations as well as the fan status in a pilot bin aerated intermittently with about 120 (m3/h)/tonne and a modified adaptive EMC-based aeration control strategy. Before July 24, grain temperatures ranged from 18.4 to 25.7°C. The adaptive bands self-adjusted based on weather conditions and minimum daily fan run times. Once the adaptive EMC and temperature bands had widened sufficiently, the controller was able to take advantage of suitable weather conditions to initiate several nighttime cooling cycles from late July to early August. Grain temperatures were lowered to 13.7 to 15.1°C; the backlog was used up, and the



Figure 7.48

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Grain temperatures recorded by the OPI system as well as the fan status in a pilot bin aerated with a modified adaptive EMC-based aeration control strategy.

bands were narrowed. The backlog accumulated the hours the fans were off, which widened the bands proportionally.

Figure 7.49

Grain temperatures recorded by the OPI system for a pilot bin aerated with chilled air. The computer-controlled chiller operated when weather conditions were warm, maintaining grain temperatures between 10° and 15°C.

Grain temperatures increased during the next two weeks to 16.7 to 22.2°C while the backlog reaccumulated. Several aeration cycles followed, resulting in rewarming the grain to 22.7 to 25.2°C. Rewarming occurred because the maximum allowable operating temperature was set too high. In contrast, Figure 7.49 shows the same data for a maize-filled pilot bin aerated intermittently with about 60 (m3/h)/tonne of chilled air during the same time period. The computer-controlled chiller operated almost every night when weather conditions were warm, maintaining grain temperatures between 10 to 15°C.


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The flexibility and versatility of the OPIGIMAC system based on the above application is illustrated by the following facts: 1. The system is powerful enough to meet the needs of intensive research involving high-frequency data collection and extensive data storage. 2. In the chilled bin, one grain temperature sensor can be programmed as the set-point above which the chiller is automatically turned on by the system. 3. The system can be programmed to operate the chiller only during a user-specified time period (i.e., between 8 o’clock in the evening and 8 o’clock in the morning unless the grain temperature drops below the set-point earlier) in order to minimize energy use during power company-specified peak demand times. 4. A complex aeration strategy can be programmed outside of the OPIGIMAC system using commercially available software (Labview by National Instruments) and subsequently linked into the system as a new user-defined fan control strategy.

7.5.4

Selecting Aeration Controllers

Use of automatic aeration controllers that minimize excessive aeration results in savings by reduction in grain market weight loss, grain damage due to spoilage (self-heating) and insect infestation, end-use quality loss, and aeration fan electrical operating costs. As long as grain temperature control is the primary objective, one simple low-cost electromechanical aeration controller may suffice to control all the fans at one installation (assuming all fans are either suction or pressure). The payback on such a low-cost aeration controller ($500 to $1500 U.S.) is usually less than one year. For systems where grain has to be dried in storage (in-bin drying) conditioned to a specific end use (popcorn to optimize popping volume) or market moisture content (soybeans harvested too dry), or where weather conditions are highly variable, a microprocessor-based aeration controller is preferred. The payback on such an aeration controller ($1500 to $3000 U.S.) is usually less than one year when critical end-use quality factors are considered. In temperate regions where the relative humidity remains closely above and below the grain ERH each day, humidity sensors may not be needed. In addition to adding cost to aeration controllers, humidity sensors add operating and service complexity and can cause unnecessary restrictions of aeration cooling during suitable cooling weather. However, in regions where humidity tends to cycle more significantly above or below the grain ERH for lengthy periods of time, humidity sensors may provide a valuable service. When a humidity sensor is properly used and maintained in conjunction with a microprocessor or computer-based aeration control system, it allows the aeration controller to calculate the grain EMC and grain wetbulb temperature. Thus, fans can be operated based on ambient wet-bulb temperature or based on the combination of dry-bulb temperature, RH, and corresponding ERH. This allows a closer control of grain moisture and avoids excessive weight loss by drying. Grain storage facilities are dusty environments for sensitive instruments, and maintenance is difficult because accumulation of dust causes the response accuracy and repeatability of humidity sensors to gradually drift out of calibration. If RH sensors drift, accuracy change could seriously affect end-use quality of stored products. If sensor accuracy drifts ± 3 to 5%, grain equilibrium moisture content could vary by equivalent percentages. If conditions persist for long periods, serious overdrying due to moisture shrink or spoilage problems due to rewetting could result. Thus, microprocessor-based controllers require that operators implement a preventive maintenance program to assure accurate sensor readings. In colder climates, use of both high- and low-level thermostat control may be desirable. Dual thermostats are used to control cooling temperatures within limits to prevent overcooling the grain. However, operating aeration based on humidity controls may not be encouraged in subtropical regions since the aeration fan operating time can be severely restricted.



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One primary reason why microprocessor-based aeration control systems have not been more widely adopted is the fact that stored grain managers are overwhelmed with the apparent complexity of controllers that are intended to help them manage these biological processes more effectively. Automatic aeration controllers have often been abandoned by stored grain managers soon after installation due to the apparent inadequacy of the fan control strategy given local weather conditions. Before implementing any automatic control strategy, the local historic weather records should be evaluated to determine whether a planned strategy guarantees sufficient fan run time to achieve the desired control objective. Ten years of historic weather records are a minimum for evaluation, although 20 to 30 years is recommended. 7.5.5

Computer Aid to Predict Aeration System Performance

Computers are an ideal platform on which to model grain storage management systems and strategies. Computer models can be utilized to study the physical and biological parameters involved in grain storage and to establish realistic operating parameters to implement the best stored grain quality management practices. Numerous computer programs have been developed throughout the world for this purpose. Most of these programs have their roots in the in-bin drying and aeration model developed by Thompson (1972) and the in-bin grain storage model developed by Muir et al. (1980). A comprehensive stored-grain ecosystem model should contain the following sub-models: 1. Location-specific historic weather conditions (dry-bulb temperature, relative humidity, wind speed, snow cover, and solar radiation) 2. A grain aeration sub-model to predict the grain temperatures and moisture contents in a bin due to forced convection 3. A three-dimensional grain storage sub-model to predict the grain temperatures and moisture contents in a bin due to conduction, free convection, and diffusion 4. A grain deterioration sub-model 5. An insect development sub-model 6. Relevant stored-product quality sub-models — for example, residual pesticide breakdown, fumigation effect, mycotoxin development, and end-use quality change

One computer aeration model that incorporates most of these sub-models is the Purdue University Post-Harvest Aeration & Storage Simulation Tool (PHAS2T). It simulates two-dimensional heat conduction, insect development, and dry matter loss during non-aerated storage and allows for the study of a range of different in-bin drying, conditioning, and aeration storage strategies (Zink, 1998; Adams, 1994; Maier, 1992). PHAS2T utilizes a 30-year weather database for over 200 U.S. stations. It has been experimentally verified for the drying, conditioning, and storage of a number of crops and control strategies. The PHAS2T code is currently being expanded by incorporating more sophisticated finite element analysis of the boundary conditions for larger scale storage structures. To maximize its usefulness, PHAS2T will be incorporated into a computer-based commercial grain management system in order to interlink empirical and practical data for future systems’ operation and control management. Maier et al. (1996) used PHAS2T to quantify the effect of eight common maize management practices on dry matter loss and maize weevil development (Sitophilus zeamis) using ambient weather conditions in three key regions of the U.S. Aeration was found to be desirable for control in all situations. Chilling maize with refrigerated air immediately after the fall harvest proved to be the most effective aeration strategy. A combination of controlled ambient aeration in the fall and chilled aeration during summer storage had significant potential as a non-chemical preservative pest management technique for all locations and years. Zink et al. (1997, 1998) used PHAS2T to evaluate the optimum combination of drying/conditioning and aerated storage management strategies to maximize moisture uniformity


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and popping volume in popcorn while minimizing energy consumption, insect development, and dry matter loss. For the two midwestern U.S. locations investigated, chilled aeration with air cooled to 13°C and 67% RH yielded the best overall results in the extreme western corn belt. A microprocessor-based aeration controller with a modified adaptive equilibrium moisture content band was best for the eastern corn belt. Another computer model that aids in predicting the performance of an aeration system is the Stored Grain Advisor (SGA), a decision-support system for stored-grain management. It is based on the model developed by Muir et al. (1980) and can be used to make decisions about managing insect pests in stored wheat. SGA does this by predicting the likelihood of insect infestation and by recommending appropriate preventative and remedial action. Sub-models for insect population growth allow SGA to predict future insect population growth as well as the degradation of insecticides, effects of fumigation, and cooling of wheat with aeration.

7.6 ECONOMIC IMPACTS FROM AERATION The most significant economic impact of aeration is in preventing moisture migration in grain bulks. There is no alternative solution to this phenomenon. Therefore, an aeration system is an integral part of modern storages that creates a unique solution with a highly remarkable economic impact. Additional benefits that derive from aeration, such as grain quality preservation at low temperatures, are difficult to compare with alternative solutions like the use of modified atmospheres. The only treatment that may have a comparable parameter is the energy cost to prevent insect damage. If fan energy consumption and aeration hours are known, the energy required for cooling grain can be estimated in the number of kW hours required to cool each tonne of grain. From Figure 7.31 and Table 7.10, the theoretical aeration hours and energy requirements can be estimated for cooling wheat to 15°C in vertical or horizontal storages. Experimental energy consumption values on aeration were given in Section 6.2.1.8. Table 6.8 also summarizes cost comparisons between aeration and fumigation to control insects. Accordingly, power requirements for aerating vertical storages is higher than for aerating horizontal storages. For example, energy values for cooling wheat stored in 10 m (33 ft) high bins and 25 m (82 ft) high bins at an airflow rate of 6 (m3/h)/tonne were calculated as 0.33 kW·h/tonne and 2.22 kW·h/tonne, respectively. Since practical field research experience has shown that to equalize grain temperatures, 30 h more energy is required, the values given in Table 6.8 include this additional operation time. Reasons for the difference in power and energy required among upright, vertical, and horizontal storages are related to the higher fan power requirement due to greater grain resistance as grain depths increase. However, specific energy increase may also relate to grain cleanliness, whether aeration ducts are properly designed and located, and if grain is peaked in bins. The duration of grain in storage also has a significant effect on static pressure because grain tends to compact from a loose initial fill to a more dense mass as vibrations of the structure and pressures from the grain mass above cause grain kernels to shift, settle, and compact tightly. These energy requirements can be reduced by employing efficiently designed aeration systems and by effective selection of desirable ambient air using suitable controls and monitoring equipment. The unique features of aeration makes it an indispensable technology in modern storages. Its strong economic impacts are principally in preventing moisture migration and in quality preservation at low temperatures. Additional features of aeration such as in situ drying have alternative solutions. When aeration is compared with conventional fumigation, the adverse impact of chemicals on the operator, consumer, and the environment should be considered. Therefore, an extremely important economic impact of aeration is its environmentally user-friendly feature. Currently only modified atmosphere storage and hermetic storage can provide comparable solutions.



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Navarro, S. (1976). Aeration of bulk-stored grain in commercial facilities, Israel Agric. Res. Org. Special Publication No. 56 (Hebrew, with English summary). Navarro, S., Kashanchi, Y., and Pisarev, V. (1980b). Dispersion of insect populations in stored grain bulks, Israel Agric. Res. Org. Prog. Rep. 1979/80, Stored Prod. Div. Special Publication No. 181, 127–157. Bet-Dagan (Hebrew, with English summary). Navarro, S. and Calderon, M. (1982). Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome. Navarro, S. (1974). Aeration of grain as a non-chemical method for the control of insects in the grain bulk, pp. 341–353. Proc. First Int. Working Conf. Stored Prod. Entomol., Savannah, GA. Navarro, S., Donahaye, E., and Dias, R. (1991). Insect Pests of Storage, Joint publication of the Ministry of Agriculture, ARS, and the Ministry of Health, Centre for Public Health (in Hebrew). Navarro, S., Donahaye, E., and Calderon, M. (1969). Observations on prolonged grain storage with forced aeration in Israel, J. Stored Prod. Res., 5, 73–81. Navarro, S., Kashanchi, Y., and Frandji, H. (1978a). Drying of wheat seeds with refrigerated air, Israel Agric. Res. Org. Prog. Rep. 1977/78. Stored Prod. Div., Special Publication Bet-Dagan (Hebrew, with English summary). Navarro, S., Kashanchi, Y., and Gonen, M. (1978b). Comparison of two techniques for controlling grain aeration systems, Israel Agric. Res. Org. Prog. Rep. 1977/78, Stored Prod. Div., Special Publication No. 117, 81–91, Bet-Dagan (Hebrew, with English summary). Navarro, S., Kashanchi, Y., and Frandji, H. (1979). Physical and biological causes of loss in stored grain in Israel, Israel Agric. Res. Org. Prog. Rep. 1978/9, Stored Prod. Div., Special Publication No. 140, 45–49 (Hebrew, with English summary). Nguyen, T.V. (1987). Natural convection effects in stored grains — a simulation study, Drying Technol., 5, 541–560. Noyes, R.T., (1997). Critical aeration management factors in Texas coastal grain storage. Presented at the South Texas Country Elevator Conference, Sheraton Beach Hotel, South Padre Island, TX, May 15–16. Noyes, R.T., Clary, B.L., and Cupperus, G.W. (1991). Maintaining quality of stored grain by aeration, OSU Extension Facts No. 1100. Noyes field experience, 1966–1968. Noyes, R.T. (1991). Aeration of Texas coastal region grain storage: critical management decisions, p. 104, in Proceedings, Texas Grain Improvement Conference, May 22–23, Texas Agricultural Extension Service, Corpus Christi, TX. Noyes, R.T. (1988). Unpublished office file records from Farmers Coop Elevator, Mooreland, OK. Noyes, R.T., Epperly, D.R., Clary, B.L., and Cuperus, G.W. (1992). Elevator Manager’s Summary Report, Oklahoma Wheat Elevator Electrical Energy Reduction Demonstration Project, OSU Grant Contract No. 3881 OIL/SECP 88. Osborne, W.C. (1977). Fans, International Series in Heating, Ventilation and Refrigeration, Pergamon Press, Oxford. Paster, N. (1972). Observations on the constituents of bulk stored soybeans, Israel Agric. Res. Org. Prog. Rep. 1971/2, Stored Prod. Res. Lab., pp. 74–81 (Hebrew, with English summary). Pixton, S.W. and Griffiths, H.J. (1971). Diffusion of moisture through grain, J. Stored Prod. Res., 7, 133–152. Poichotte, J.L. (1977). La conservation des grains par la ventilation, Fermes Mod., 51, 43–46. Sanderson, D.B., Muir, W.E., and Sinha, R.N. (1988a). Moisture contents within bulks of wheat ventilated with near-ambient air: experimental results, J. Agric. Eng. Res., 40, 45–55. Sanderson, D.B., Muir, W.E., and Sinha, R.N. (1988b). Intergranular air temperatures of ventilated bulks of wheat, J. Agric. Eng. Res., 40, 33–43. Shedd, C.K. (1953). Resistance of grain and seeds to airflow, Agric. Eng., 34(9), 616–619. Shove, G.C. (1968). Aerating stored dry grain, University of Illinois, Cell. Agric. Coop. Ext. Serv. Circular 984. Sinha, R.N. (1974). Climate and the infestation of stored cereals by insects, pp. 117-141, in Proc. First Int. Working Conf. Stored Prod. Entomol., Savannah GA. Steele, J.L. and Saul, R.A. (1969). Deterioration of shelled corn as measured by carbon dioxide production, Trans. Am. Soc. Agric. Eng., 5, 685–689. Sutherland, J.W., Pescod, D., and Griffiths, H.J. (1970). Refrigeration of bulk stored wheat, Aust. Refrig. Air Cond. and Heat, 24(8), 30–34; 43–45.


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CHAPTER

8

Supplemental Aeration Systems Ronald Noyes, Shlomo Navarro, and David Armitage

CONTENTS 8.1

8.2

Special Aeration Uses and Processes that Enhance Aeration .............................................414 8.1.1 Push-Pull Aeration....................................................................................................414 8.1.2 Cross-Flow Aeration Systems ..................................................................................417 8.1.2.1 Advantages and Disadvantages of Cross-Flow Aeration .........................424 8.1.2.2 Cross-Flow Aeration Design.....................................................................425 8.1.2.3 Future Research Needs for Cross-Flow Aeration.....................................427 8.1.3 Chilled Aeration........................................................................................................428 8.1.4 Dryeration.................................................................................................................428 8.1.4.1 Increased Drying Capacity with Dryeration ............................................429 8.1.4.2 Tempering During Adiabatic Cooling Moisture Removal .......................430 8.1.4.3 Dryeration Airflow Design .......................................................................431 8.1.4.4 Dryeration Cooling Bin Design ...............................................................434 8.1.4.5 The Ideal Dryeration Bin .........................................................................437 8.1.4.6 Higher Grain Quality from Dryeration ....................................................438 8.1.4.7 Dryeration Facility Operation Management ............................................438 8.1.4.8 Dryeration Research Data Results ...........................................................439 Recirculation Fumigation Systems ......................................................................................439 8.2.1 Methyl Bromide Recirculation.................................................................................440 8.2.2 Carbon Dioxide Recirculation..................................................................................440 8.2.3 Phosphine Recirculation...........................................................................................441 8.2.3.1 Benefits of Recirculation Fumigation of Phosphine ................................442 8.2.3.2 Recirculation Fumigation Procedures.......................................................443 8.2.3.3 Recirculation Fumigation vs. Aeration Airflow Rates .............................444 8.2.3.4 Gas Manifold Piping Design Alternatives................................................444 8.2.3.5 Recirculation Blowers ...............................................................................445 8.2.3.6 Sealing Leaks and Structures....................................................................449 8.2.3.7 Phosphine Recirculation in Large Steel Bins...........................................449 8.2.3.8 Piping Designs for Multiple CLF Steel Bin System Models ..................451 8.2.3.9 Design of Phosphine Recirculation in Silos.............................................453 8.2.3.10 CLF Combined with Low-Airflow Suction Aeration for Silos................457 8.2.3.11 Flat Storage CLF Systems ........................................................................458 8.2.3.12 CLF Installation and Operating Economics .............................................458

0-8493-1355-4/02/$0.00+$1.50 © 2002 by CRC Press LLC

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414


8.3

Bulk Aeration Systems .........................................................................................................460 8.3.1 Bunker Aeration........................................................................................................460 8.3.2 Vertical Aerators for Grain Bulk Peak and Ridge Aeration....................................462 8.3.2.1 Portable Spear Aerators ............................................................................462 8.3.2.2 Pedestal Aerators.......................................................................................462 8.4 Supplemental Processes and Systems that Improve Aeration .............................................466 8.4.1 Coring Bins and Silos ..............................................................................................467 8.4.1.1 Coring Improves Aeration in Steel Bins and Silos ..................................470 8.4.1.2 Bin and Silo Fill and Discharge Offset Problems....................................472 8.5 Cleaning Grain to Improve Aeration ...................................................................................472 8.6 Grain Temperature Monitoring ............................................................................................473 8.6.1 Monitoring Grain Temperatures for Molds and Insects ..........................................473 8.6.2 Temperature Monitoring in Wet Grain Holding and Dry Grain Cooling Bins...........473 8.6.3 Grain Temperature Data Analysis ............................................................................474 8.6.4 Understanding and Interpreting Temperature Profile ..............................................475 8.6.4.1 Temperature Profiles during Aeration.......................................................475 8.6.4.2 Interpreting Grain Temperature Data........................................................476 8.6.4.3 Problems Encountered with Thermocouple Readings .............................478 8.7 Aeration to Remove Odors and Fumigant Gases and Maintain Pesticide Life in Grain Bulks ......................................................................................................................478 8.7.1 Musty Odor Removal ...............................................................................................478 8.7.2 Exhausting Fumigant Gases .....................................................................................479 8.7.3 Maintaining Pesticide Strength ................................................................................479 8.8 Roof Ventilation Systems .....................................................................................................481 8.8.1 Steel Bin Roof Venting.............................................................................................481 8.8.2 Roof Exhaust Fans to Minimize Condensation .......................................................482 8.9 Multistage Aeration Systems................................................................................................484 8.10 Manifold Operated Large Blowers.......................................................................................484 References ......................................................................................................................................485

8.1 SPECIAL AERATION USES AND PROCESSES THAT ENHANCE AERATION Special forms of aeration: in addition to normal pressure or suction aeration, several aeration methods are in use to provide specific benefits. These specialized forms of aeration are push-pull aeration, cross-flow aeration, chilled aeration, Dryeration, and recirculation fumigation or closedloop fumigation. Chilled aeration is discussed in Chapter 9. Special uses of aeration: aeration to remove mustiness and odors, to clear or ventilate fumigant gases from grain at the end of a fumigation, to cool grain to minimize residual pesticide volatility, and to control pesticide decay are special uses of aeration that enhance grain storage management. These special applications of aeration will be discussed in detail in sections of this chapter. Processes that improve aeration: three supplemental processes that enhance grain storage aeration systems include: (1) cleaning grain to remove trash, foreign material (f.m.), and dockage; (2) using grain spreaders or levelers in bins to level the grain surface and distribute f.m. and dockage; and (3) coring peaked grain in bins to remove part or all of grain peaks and the center core concentrations of f.m. and dockage under the fill spout line. 8.1.1

Push-Pull Aeration

Push-pull aeration can only be used in sealed concrete or steel silos with base- and roof-mounted fans operating in series. This specialized aeration process can be considered for grain bed depths


SUPPLEMENTAL AERATION SYSTEMS

Figure 8.1

415

Schematic presentation of push-pull aeration in a concrete silo with identical centrifugal fans used for pressure and suction.

of more than 20 m and should be designed to develop lower static pressures than either pressure or suction fans alone can deliver at the same airflow rate. Push-pull aeration should not be used on bolted or welded steel grain bins with sealed roofs, since most steel bin roof structures are not designed to withstand the suction pressures. Roof collapses are likely to occur if push-pull systems are used on steel bins. This process allows the use of two fans of smaller power and/or lower operating pressure range (Figure 8.1). It can produce the airflow of one larger pressure or suction fan while operating at lower static pressures and higher individual fan efficiency. In push-pull systems each fan only has to move the air through approximately half of the grain depth. Identical pressure and suction fans should be used to assure that the unit airflow displacement is well matched. At about mid-depth of the grain mass, the static pressure changes from positive to negative pressure and the suction fan on the roof produces the remaining flow. The pressure fan produces slightly more static pressure at the common airflow because it moves denser compressed air compared to the suction fan, which generates a negative static pressure when handling the warm moist air in the upper part of the silo. Thus, the neutral static pressure point will likely be about 53 to 55% of the distance from the base of a silo. The primary benefit of push-pull aeration is that compression heat is lower due to the reduced effective grain depth for each fan and the lower static pressure required of the pressure fan. This factor is significant in subtropical and temperate regions where cooling capacity of ambient air is marginal. Another benefit is that push-pull systems may use two vane-axial fans with reduced power per fan to match the capacity and pressure range of one centrifugal fan. The two vane-axial fans usually


416


are less expensive, even with two small motor starters and electrical wiring, compared with one centrifugal fan with heavier electrical service and larger motor and protective equipment. Example 8.1 Condition: A grain elevator in Corpus Christi, Texas (MSL elevation) has 7.6 m Internal Diameter (ID) by 30 m high silos that hold about 1350 m3 or 1000 tonne/silo. The elevator stores wheat from May to December and maize from August to April. The manager cools the wheat quickly after harvest to minimize germ damage from harvest heat, then aerates in September or early October to suppress insect populations. Rapid aeration of maize in September is needed to cool and hold it at ideal moisture contents of 14.5 to 15.0% moisture content (wet basis) for processing and to minimize market weight loss and additional drying costs. Requirement: Design a push-pull aeration system that will deliver 6.0 (m3/h)/tonne through wheat at 30 m grain depth. Using the same silo and aeration system components, estimate the airflow delivered when maize at 30 m depth is aerated. Assume that suction and pressure airflow at 15 m (half depth) is equal. Method: Use identical roof and base fans for push-pull aeration. Check Figure 5.17 (wheat) for static pressure and power vs. depth values. Solution: From Figure 5.17 for wheat, the static pressure required for 6.0 (m3/h)/tonne at 15 m, half the silo depth, is about 1.55 kPa pressure. The wheat aeration design requires 6.0 (m3/h)/tonne × 1000 tonne/silo = 6000 m3/h from the silo base and roof aeration fans. From Figure 5.17, estimated fan power required is 0.5 kW/100 tonne × 10 = 5.0 kW/fan at a static pressure of 1.55 kPa/fan. From Table 5.9, a 5.6 kW high-speed centrifugal fan delivers about 6466 m3/h at 2.0 kPa. A second fan option is the 5.6 kW in-line centrifugal fan, which delivers about 6796 m3/h at 2.0 kPa. The high-speed centrifugal is selected because it provides the desired airflow and is usually less expensive than an in-line centrifugal fan of the same power. Because maize is to be held at a moisture level of 14.5 to 15.0%, the preferred airflow rate for maize is at least 50% higher airflow rate than that for wheat, or 9.0 (m3/h)/tonne. Thus, the desired airflow rate is at least 9000 m3/h. Checking Figure 5.18 for maize for 9.0 (m3/h)/tonne, at 15 m depth, the static pressure is 0.6 kPa. The 5.6 kW high-speed centrifugal fan is rated at 9685 m3/h at 0.5 kPa and 8665 m3/h at 1.0 kPa. Interpolating for 0.6 kPa provides a rated capacity of: 9685 9685 9685 9685

– – – –

{(9685–8665) × [(0.6–0.5)/(1.0–0.5)]} = [1020 × (0.1/0.5)] = (1020 × 0.2) = 204 = 9481 m3/h.

This airflow of 9.48 (m3/h)/tonne, slightly higher than the design airflow rate of 9.0 (m3/h)/tonne, is acceptable for cooling the maize. The use of Figure 5.17 and similar figures for other types of grain enables a person to estimate fan power requirements based on assumptions explained in Chapter 5. The estimated fan power in Figure 5.17 is based on 50% static efficiency. The fan power requirement needs to be adjusted after additional pressure losses in ducts and the correct fan efficiency factor is known. Table 5.9 is based on fans commercially available, principally in the U.S. market. In Table 5.9, fan power is listed according to ratings specified by the manufacturer. Fan efficiency varies from one type of fan to another as well as among manufacturers. Another restriction of Table 5.9 is that



417

airflow rates are not specified at all consecutive static pressures. Therefore, interpolating or extrapolating is necessary to select the correct fan. The fans listed in Table 5.9 should be viewed as examples of what may be available in the market in selecting a specific fan when the aeration system design requires fans with power from 0.38 kW to 7.5 kW. In many cases, fans listed in Table 5.9 do not meet the aeration system design requirements as fans in the table are too small. For larger fans, consult catalogs of commercial fan manufacturers. The fan power given in Figure 5.17, Figure 5.9, and other grain pressure vs. airflow and power charts may approximate the fan power in Table 5.9, but they are not expected to accurately match specific fan sizes in the table. Often the required airflow may fall between fan selections in the table. In Example 8.1 above, two conventional 5.6 kW base-mounted high-speed centrifugal pressure or suction fans could not have been used in parallel (side by side) because the static pressure would have been 6.0 kPa, which exceeds fan operating pressure limitations. Had these fans been capable of delivering this pressure, compression heat would have been about three times as high. Also, the total delivered airflow would have been less than the airflow produced by the two high-speed centrifugal fans operating as push-pull fans in series close to their optimum performance near the middle of their operating pressure range. The key to effective operation of the push-pull or pressure-suction series fan aeration is that the storage unit must be well sealed. When a push-pull system is used, the storage unit must be able to sustain the high negative suction pressure generated by the roof-mounted suction fan. Push-pull systems are normally used only on tall concrete silos where the concrete roof decks are normally well reinforced to withstand the high negative pressures generated by the suction fan of the aeration system. Warning: High negative pressures from push-pull systems may cause steel grain bin roofs to collapse. Suction systems are only suitable for concrete or steel silos where roof structures are sufficiently strong. Steel bins or steel silos with diameters equal to or greater than one third to half their sidewall height are usually not suitable for push-pull systems with sealed roofs. Their roof panels are generally too thin and weak to provide the needed resistance to collapse from suction pressure. Sloped-roof steel silos may be able to withstand moderate suction pressures, but the silo manufacturer should be consulted about roof strength before installing a push-pull system. 8.1.2

Cross-Flow Aeration Systems

Cross-flow aeration — moving air horizontally by pressure, suction, or pressure and suction through a grain bed — has great potential for substantially reducing fan power while increasing the airflow rate. This method of aeration has not been researched or used much since early investigations were made in the 1950s and 1960s. Holman (1960) pointed out that the advantage of cross-flow aeration for concrete silos is rapid cooling at low operating costs. The disadvantages are complexity of installation and operation and initial cost. Figure 8.2 illustrates a cross-flow aeration design for silos. Nelson et al. (1966) states that “Cross-flow aeration systems seem potentially suitable for drying or aerating grain stored in tall, upright, cylindrical bins. The shorter air circulation path reduces air resistance compared to vertical circulation from bottom to top. As a result, blowers that operate against lower pressures can be used in cross-flow systems.” Hunter (1985) developed a system to estimate airflow patterns from a variety of aeration duct patterns in storage silos using conformal mapping techniques. He developed flow field pressure patterns for a two-duct cross-flow aeration system in silos with the airflow field divided into 10 equal sections. He also developed equations for estimating the pressure difference across an aerated seed bulk by adapting the Ergun (Hunter, 1985) equation:


418

Figure 8.2


Cross-flow aeration system in upright storage showing solid and perforated duct designs for both entrance and exhaust ducts. (From Holman, L.E. (1960). Aeration of Grain in Commercial Storages, Marketing Research Report No. 178, USDA, ARS, Washington, D.C., 46p.) With permission.

dp dx = Rv + Sv 2

(8.1)

where: p = static pressure of air, Pa x = distance coordinate, m v = face velocity of the air, m/s R = first Ergun coefficient, Pa s/m, which depends on seed type S = second Ergun coefficient, Pa s2/m, which depends on seed type For cross-flow aeration systems in a cylindrical storage of diameter, D, using semicircular ducts of radius, r, Hunter (1985) developed the equation: ∆p = ∆p1 + ∆p2 + ∆p3

(8.2)

2QR  D  ln −1 r  πl

(8.3)

∆p1 =



419

∆p2 =

2 SQ2 πl 2 Pp

(8.4)

∆p3 =

SQ2 l2 D

(8.5)

and:

where: Q = airflow in m3/s l = length of duct, m D = inside diameter of silo, m r = radius of half round duct, m Hunter (1985) gives values for R and S for a variety of seeds. He suggests that allowance for packing of loose seeds should be accounted for by increasing the values of R and S by 20%. Loo (1985) reported research using a two-duct cross-flow aeration system for rice cooling on 25.9 m high concrete silos which held 750 tonne of 14% paddy. The high flow rate (HFR) crossflow aeration system designed by Loo involved two push-pull vane-axial blowers connected to semicircular ducts attached to the inside walls (Figure 8.3). Loo (1985) compared the HFR cross-flow system to a medium airflow rate (MFR) vertical suction system using two ducts on the 45° hopper slope, and a low airflow rate (LFR) vertical suction system with one duct on the 45° hopper slope. These tests compared flow rates of 0.3, 0.1, and 0.03 (m3/min)/tonne for the HFR, MFR, and LFR systems, respectively. The HFR push-pull system used two 10 HP vane-axial blowers; the MFR used one 10 HP centrifugal suction blower; and the LFR system used a 0.5 HP centrifugal blower. Loo (1985) reported that the MFR aeration system was preferred over the HFR and LFR aeration systems because of lower capital costs, lower operating costs, more uniform temperature distribution throughout the mass, and better quality and milling yields of the long-grain paddy. He indicated that the high velocity of air across the center of the mass with much lower flow near the silo walls, which left the paddy inadequately cooled, was a problem that needed to be resolved before crossflow aeration would be suitable. Maintenance of the vertical ducts was also a serious concern. During a three-year span from 1963 to 1965, Converse (1967) conducted field research on wheat and sorghum using a two-duct cross-flow aeration system installed in a 5.2 m (17 ft) diameter by 42.7 m (140 ft) tall concrete silo in central Kansas. He installed two vane-axial fans, one on each duct. In part of the tests, he used both fans as a push-pull cross-flow system. In the other tests he used only one of the fans per test. Parts of the tests were conducted using the pressure fan and part with the suction fan. Converse (1967) documented pressure and suction levels at 6.1 m intervals along each of the two ducts during each test. He monitored grain temperatures in the mass, duct inlet and exhaust temperatures, average RH, initial and final moisture contents, and moisture removed during the tests. Power use and costs were documented for all tests. The system was developed to cool stored grain rapidly and to accomplish the small amount of drying needed to condition newly harvested grain having a moisture content 1 to 2% too high for safe storage. In 3 years of tests with grain sorghum and wheat, the grain was cooled to near ambient air temperatures in 24 to 48 hours, and grain moistures were reduced 1% in about 100 hours of aeration. According to Converse (1967), promising features of this system are satisfactory installation and maintenance of perforated and solid vertical duct sections on the walls of an upright concrete bin; satisfactory airflow distribution through grain 33.5 to 42.7 m (110 to 140 feet) deep; and an


420

Figure 8.3


Plan and elevation of tower silo experimental aeration system comprising push and suck cross-flow aeration system at Ulu Tiram Buruk, Malaysia. (From Loo, K.F. [1985]. Silo storage in Malaysia, in Preserving grain quality by aeration and in-store drying, Proceedings of an international seminar held at Kuala Lumpur, Malaysia, 9–11 October 1985. Editors B.R. Champ and E. Highley. pp 165–172. With permission.)

airflow rate as much as 10 times the normal rate that is practical for bins of similar size having floor-duct aeration systems. Two 7.5 HP vane-axial fans were used, one on the roof of the test bin to push air through the grain and one on the opposite side of the bin near the bottom to pull air through the grain and to exhaust it to the outside. The system provided airflow rates of about 30 (m3/h)/tonne (½ c.f.m. per bushel) with wheat or grain sorghum in the test bin. Horizontal airflow was reasonably uniform throughout the entire grain depth in the various tests (Converse, 1967). Day and Nelson (1964) studied cross-flow natural air drying effects on wheat in model bins using dimensionless analysis. They studied three vertical duct pattern options — two-duct (bin type I), four-duct (bin type II), and six-duct (bin type III) bins, using three sizes of model bins. Prediction equations were developed for a 14.9 cm (5.875 in) diameter bin. Validation studies were conducted on two bins, a 0.61 m (24 in) diameter bin and a 1.42 m (56 in) diameter bin. Tests of the three cross-flow duct patterns were conducted on the 14.9 cm (5.875 in) diameter and a 0.61 m (24 in) diameter bin. Only the two-duct system was tested on the 1.42 m (56 in) diameter bin. Bin type I had one inlet duct with one outlet duct directly opposite at 180° of circumference (Figure 8.4). Bin type II, with four ducts, had two inlet ducts directly opposite and two exhaust ducts at 90° circumference positions from inlet ducts. Bin type III had three inlet ducts spaced 120° apart around the circumference and three alternating exhaust ducts spaced 120° apart, offset from the inlet ducts by 60° around the circumference. A pressure fan system with a manifold connected to supply air to all inlets was used for each of the tests. Airflow patterns were developed on the 0.61 m (24 in) diameter bin for all three-duct patterns. Static pressure distribution patterns on cross-section views of the bin duct layout pattern in Figure 8.4 indicate the relative airflow at locations in the bin for all three sets of ducts. Lines of



Figure 8.4

421

Static pressures (inches water column) along lines of equal air pressure throughout a typical crosssection of the bins for type I bins with two ducts, type II bins with four ducts, and type III bins with six symmetrically spaced ducts. (From Day, D.L. and G.L. Nelson. [1964]. Drying effects of crossflow air circulation on wheat stored in deep cylindrical bins, Technical Bulletin No. T-106, Oklahoma Agricultural Experiment Station, Oklahoma State University, January. With permission.)

equal pressure in each of the three silo cross-section views, bin types I, II, and III, and the direction of the inlet and outlet airflow arrows indicate the direction (but not the relative velocity) of the air. As the length of each line varies, the relative air velocities in that section of the bin vary with the distance the air at each point along the line has traveled, with airflow lines perpendicular to pressure lines. Since lines of equal velocity were perpendicular to lines of equal pressure, the highest velocity airflow in type I would be a straight line (shortest path) between the inlet and outlet. The slowest airflow line would be the longest curved air path against each wall. In type II, the fastest air velocity would follow a straight line between each inlet and each outlet, very close to the wall on each side of the inlet ducts. Slowest moving air would be along a line directly between the two inlets, which split and turned toward the two outlets near the center of the bin where the two 0.16 inch w.c. lines intersect. As this airflow path was about twice the length of the shortest path between inlet and outlet, the slowest air velocity would be about half that of the shortest air path. It is likely that there was a zone around the center of type II — where the two straight pressure lines intersect and where little or no airflow occurs, especially if the center of the bin contained a higher percentage of grain fines, broken kernels, and dockage. Bin type III further illustrates the phenomenon of a dead air zone in the center of the bin. With three inlets and outlets — the short circuit of air directly from inlet to outlet — the cooling pattern in this bin was very poor, especially with a dense core of fines in the center. In cross-flow aeration systems, less air pressure is required on type II than type I, and even less pressure on type III. This cross-section view of duct patterns of the three bin types illustrates the magnitude of static pressures for each duct system based on a Reynold’s number term of 80 (Day and Nelson, 1964). The pressure drop across bin type I was 0.5 inches w.c; for bin type II, the pressure drop was approximately 0.15 inches w.c.; and on type III ducts, the differential pressure was only about 0.10 inches w.c. In terms of airflow patterns for a common airflow volume, these differential pressures can be compared by shortest and longest air paths. The shortest air path changes from 1.0 diam for type I to 0.7 diam for a type II duct pattern, and to 0.50 diam for type III ducts. The longest air path, which followed the inner bin wall from the center of the inlet duct to the center of the outlet duct for the type I duct pattern, was 1.57 diam. The longest paths for type II and type III approach the center of the bin. With type II ducts, the longest air path is essentially the same as for type I except that the path makes a 90° turn at the center to reach the exit duct. Type III ducts seem even less likely to be successful because the longest air path would need to move to the center, then make an acute 60° turn to reach the exit, again with the same theoretical path length as duct types I and II. In Figure 8.5, Day and Nelson (1962) show three bin cross-sections that represent the zones that dry slowest. The same is expected to occur in an aeration system with these duct patterns. However,


422

Figure 8.5


Three cross-flow aeration duct patterns for cylindrical bins or silos showing the zones (shaded area) of the bin with minimal cooling and drying using full-time aeration with symmetrical inlet and exhaust duct patterns. (From Day, D.L. and G.L. Nelson. [1964]. Drying effects of cross-flow air circulation on wheat stored in deep cylindrical bins, Technical Bulletin No. T-106, Oklahoma Agricultural Experiment Station, Oklahoma State University, January. With permission.)

there is reason to question the slow drying pattern of bin type I. Logic does not suggest that much cooling or drying would occur near the center of a bin or silo with duct patterns type II or III. In an aeration system, both type II and III bins are expected to have a diamond-shaped central dead zone or core (Figure 8.5) that does not cool sufficiently, or cools much more slowly. The center core of grain may cool slowly by conduction due to the differential temperature between the center of grain mass and the cooled grain 1 to 2 m away. However, it may not cool to a level to prevent deterioration. Because of the central grain mass that may not cool adequately, type I cross-flow aeration systems need to be carefully designed. If the bin diameter is too large for satisfactory aeration, a type II silo should be considered. Thus, for cross-flow aeration to work with any degree of success, these stagnant air zone problems must be overcome. One way to overcome these inherent cross-flow cooling (or drying) deficiencies is to use alternating airflows. For example, in a Type II, four-duct system, if one of the two inlet ducts is used as an exhaust duct (one inlet and three exhaust ducts) part of the time, then the center dead zone can be aerated with direct airflow across the silo. For example, using the four-duct system, Figure 8.6, a separate fan is installed on each of the two opposite ducts. All four ducts are used all the time, with two opposite ducts used alternately as the inlet duct. When the system is designed for pressure aeration, one duct is used as the inlet duct with the opposite duct used as the center exhaust duct. For example, during half of the aeration time, duct A would serve as the inlet duct with fan A pushing air into the grain mass with fan C turned off. When duct A is pressurized, ducts B, C, and D are exhaust ducts. When duct C is the inlet duct and fan C is turned on, the inlet airflow is reversed; and ducts A, B, and D are the exhaust ducts. Reversing airflow eliminates the dead zones in the cooling pattern, as air will flow through all four areas between the inlet duct and the adjacent exhaust ducts. Since the direct distance between the inlet and adjacent exhaust ducts is a ratio of 0.7 D to 1.0 D, exhaust duct C will only receive about 70% as much exhaust air per duct as each of the ducts B and D, based on their airflow path lengths. The direct distance from the center of duct A to the center of duct C is 1.0 diam while the direct distance from duct A to ducts B and D is only 0.7 diam. Therefore, the grain aerated by ducts B and D receive more airflow initially than the flow through the center of the grain mass to duct C. However, if the supply airflow is reversed from duct A to duct C, more air flows along path C–D and C–B than the previous situation in which airflow was from duct A. The longer airflow distances along the paths A–C and C–A are compensated by having continuous alternating airflow throughout the aeration process.



Figure 8.6

423

A four-duct cross-flow aeration strategy to provide complete cooling zones by alternating the inlet between opposite ducts A and C with three exhaust ducts.

Periodically the inlet duct should be switched from duct A to duct C to allow cooling of the grain along the walls in the region between ducts C and B and ducts C and D. Thus, the four-duct cross-flow aeration system can be physically designed with all four ducts the same size, so that the only mechanical adjustment is reversing inlet or supply air ducts A and C across the silo. These ducts should be valved such that when the fan is switched, the former inlet duct becomes vented to atmosphere as the opposite exhaust duct when the second inlet duct is pressurized. An advantage of reversing inlet airflows with one inlet and three exhaust ducts is that air moves along all sides as well as across the center of the mass. The key element is that air must be forced along all sides of the silo, as demonstrated in the four-duct example above. This four-duct aeration process using one entrance and three exhaust ducts with alternate entrance ducts has not been reported in the research literature. However, the work reported by Day and Nelson (1964) and others on a four-duct model using two opposite entrance and two opposite exhaust ducts clearly shows that grain in the dead zones between the center and side exhaust ducts in Figure 8.6 will be satisfactorily cooled using the alternating flow inlet ducts with three exhaust ducts. Also, airflow directly across the center of the silo to the opposite exhaust duct will solve the problem of the center dead zone. However, this new approach will require research trials to demonstrate its effectiveness.


424


It should be noted that the aeration system can be designed as a suction airflow system with negative air pressure on exhaust ducts A or C, so the other three ducts act as inlet supply ducts. Using cross-flow suction aeration eliminates compression heat for maximum cooling in tropic and subtropic regions. 8.1.2.1 Advantages and Disadvantages of Cross-Flow Aeration Kumar and Muir (1986) tested airflow resistance in cleaned and uncleaned wheat and barley in a test bin with pressure sampling points that were 1.53 m apart in vertical and horizontal directions using an airflow rate of 0.077 m3/s. They observed that the filling method comparing layered (spreader) vs. end (spout filling) changed the bulk density and calculated airflow resistance in each direction. Horizontal airflow resistances were only 61% and 64% as high for vertical uncleaned and cleaned wheat, respectively, for end filling at an airflow rate of 0.077 m3/s. At the same airflow rate for barley, the horizontal airflow resistances were for vertical uncleaned and cleaned, 47% and 62%, respectively, for end filling vs. 75% and 83% for layer filling, respectively. Two of their conclusions were that “Airflow resistance of a bed of the test grain was considerably less for air flowing horizontally than for air flowing vertically for the same filling method,” and “Layer filling in comparison to end filling caused an increase in bulk density and airflow resistance.” They partially attributed this difference in resistance to the bulk densities being 3 to 6% higher for layer than for end filling (Kumar and Muir, 1986). Thus, one major advantage of horizontal airflow in a mass of wheat is that it only develops about 60% (61% for uncleaned and 64% for cleaned wheat) of the resistance compared to vertical airflow, either up-flow or down-flow. These tests were carried out for wheat transferred to bins from grain downspouts or conveyor discharges without mechanical spreaders. Airflow resistance values for horizontal compared to vertical flow varies considerably between grain types. For example, for barley, Kumar and Muir (1986) found that horizontal resistance was only 47% compared to vertical airflow for end filling and 62% for layer filling. Thus, airflow resistance appears to be significantly less for horizontal flow compared to vertical flow for each tested grain (Kumar and Muir, 1986). It is expected that this relationship will be true for most long and narrow shaped cereal grains. This is a highly significant development for aeration. This means that through the same grain distance, horizontal airflow results in only about 50 to 60% of the static pressure at the same vertical airflow (Kumar and Muir, 1986; Jayas and Muir, 1991; Jayas and Mann, 1994). But far more important than the pressure and power differential between vertical and horizontal airflow is the fact that horizontal distances across silo grain masses (in tall storage units) are generally much shorter than the vertical distance (6 to 9 m diameter vs. 30 to 40 m tall). Therefore, power required for aerating grain horizontally may only be about 10 to 15% of the power to aerate vertically at the same airflow rate because of the much shorter airflow path and the lower resistance to airflow through grain horizontally compared to vertically. Also, pressure cross-flow cooling creates very little heat of compression in the 6 to 9 m diameters of most silos. Advantages of cross-flow aeration compared with conventional vertical aeration include: 1. 2. 3. 4. 5. 6.

Horizontal airflow has about 50 to 60% of the resistance of vertical airflow in cereal grains. Horizontal airflow paths are generally about 20 to 30% as long as vertical air paths. Smaller aeration fans with lower capital costs and operating costs can be used. Much lower power is required with lower pressure heat rise. Total airflow rates can be much higher with shorter cooling times and less power. Horizontal airflow can provide higher airflow through the center core grain of the silo or bin (depending on ducting pattern) for fast cooling required for grain with high dockage or f.m. content. 7. Square or rectangular silos can be designed with false plenum (opposite) walls as part of the initial design.



425

Disadvantages of cross-flow aeration compared with conventional vertical aeration include: 1. 2. 3. 4. 5.

Multiple inlet/outlet ducts and manifold design are more complex. Securing ducts to silo walls due to grain movement and pressures is difficult. The complex full height inside multiple inlet/outlet ducts leads to higher installation cost. Storage must be full of grain to aerate (under current designs). Aeration with only partially filled bins requires control of duct openings, which makes the design more complicated.

8.1.2.2 Cross-Flow Aeration Design The physical laws of air movement, or fan laws, state that: (1) airflow is proportional to fan speed; (2) static pressure is proportional to fan speed squared; (3) fan power is proportional to fan speed cubed; and (4) static pressure and fan power are proportional to air density (Brooker et al., 1992). Analysis of these laws in tall silo bins reveals the significant advantage of cross-flow aeration. The advantage of cross-flow aeration is demonstrated by proper application of the fan laws comparing vertical and horizontal aeration (assuming horizontal airflow in a tall silo bin), which results in the same airflow at much lower static pressure of vertical airflow. The following example illustrates this comparison. Example 8.2 Use the cylindrical 7.6 m diameter by 30 m high concrete silos in Example 8.1 to develop static pressure and fan power requirements for cross-flow aeration fan systems for 1000 tonne capacity silos. Use four equally spaced vertical ducts with cross-flow aeration and alternate the supply duct. 1. Develop fan designs for 6 and 12 (m3/h)/tonne for wheat. 2. For these two cross-flow airflow rates, compare cross-flow static pressure and power with values for push-pull and full-depth aeration for wheat.

Method Use the four-duct cross-flow aeration system, where one duct serves as the supply inlet duct and the other three ducts are exhaust outlet ducts. Airflow distances vary from about 0.75 D to 0.9 D for airflow from the two adjacent ducts, compared with 1.0 D to 1.3 D for the longest path to the opposite duct (Figure 8.6). For approximate calculations, assume that aeration ducts are flush with the bin walls and assume an average airflow distance of 1.0 D, or 7.6 m for all three exhaust ducts. From Figure 5.17, check for 7.6 m distances for wheat depth and multiply by a static pressure or airflow resistance coefficient of 0.6 based on research by Jayas and Mann (1994) for lower grain resistance to horizontal component of airflow. From Table 5.9, select fan type and size for 6 and 12 (m3/h)/tonne for wheat. Solution 1. Cross-flow aeration: At 6 (m3/h)/tonne For 6 (m3/h)/tonne at 7.6 m × 0.6 (horizontal flow coefficient) = 4.6 m is the effective depth. From Figure 5.17, static pressure is 0.33 kPa; the estimated power is about 0.13 kW/100 tonnes, or 0.13 × 10 (100 tonnes) = 1.3 kW. Total airflow for 1000 tonnes at 6 (m3/h)/tonne is 6000 m3/h. Fan selection should be made by checking the airflow rate and computed static pressure from the fan manufacturer’s catalog or literature ratings. Fan power should be close to 1.3 kW for 6000 m3/h at 0.33 kPa if the fan is about 50% efficient.


426


Table 8.1

Comparison of Calculated Fan Power (kW) and Static Pressure (kPa) Requirement for Three Aeration Methods in 30 m Tall Silo Containing Wheat (Described in Example 8.2) Airflow Rates

Aeration Method Cross-flow Push-pull Full depth

Effective Depth (m) for Calculating Fan Power 4.6 15 30

6 (m3/h)/tonne kW kPa

12 (m3/h)/tonne kW kPa

1.3 5.0 20

3.0 20 —

0.33 1.5 5.86

0.45 3.13 —

At 12 (m3/h)/tonne For 12 (m3/h)/tonne, at 7.6 m × 0.6 (horizontal flow coefficient) = 4.6 m is effective depth. From Figure 5.17, static pressure would be about 0.45 kPa, and the estimated power would be about 0.3 kW/100 tonnes. Power required is 0.3 × 10 (100 tonnes) = 3.0 kW. Total airflow required is 12 × 1000 tonne = 12,000 m3/h at 0.45 kPa. 2. Vertical push-pull aeration fan comparison At 6 (m3/h)/tonne For the push-pull aeration system design, the half grain depth (30/2 = 15 m) and the grain mass of 1000 tonnes will be used. From Figure 5.17 (wheat) at 6 (m3/h)/tonne (6000 m3/h) airflow at 15 m grain depth requires 1.55 kPa. The estimated power is about 0.50 kW/100 tonnes, or 0.50 × 10 (100 tonnes) = 5 kW, or two 2.5 kW fans. At 12 (m3/h)/tonne From Figure 5.17 (wheat) at 12 (m3/h)/tonne (12,000 m3/h) airflow at 15 m grain depth requires 3.13 kPa. The estimated power is about 2.0 kW/100 tonnes, or 2.0 × 10 (100 tonnes) = 20 kW or two 10 kW fans. 3. Vertical aeration full-depth fan comparison At 6 (m3/h)/tonne From Figure 5.17 (wheat) at 6 (m3/h)/tonne (6000 m3/h) airflow at 30 m grain depth requires 5.86 kPa. The estimated power is about 2.0 kW/100 tonnes, or 2.0 × 10 (100 tonnes) = 20 kW. This exceeds the pressure ratings of the fans in Table 5.9. But higher pressure fans than those mentioned in Table 5.9 are available in the market. At 12 (m3/h)/tonne To aerate 30 m depth wheat bulk at 12 (m3/h)/tonne is not practical. Fan and static pressure requirements exceed practical limits.

These scenarios demonstrate that in one 7.6 m diameter by 30 m high silo, cross-flow aeration has great potential for increasing the rate of cooling of tall grain masses in silos or tall steel bins while keeping the operating cost low. Research has demonstrated that the static pressure in horizontal airflow aeration is effectively reduced by about 40% compared to vertical aeration of the same grain type and depth — for grains that have elongated kernels such as wheat, maize, paddy, barley, and oats. Elongated kernels tend to orient themselves with the long axis of the kernel laying generally horizontal, which may reduce airflow resistance horizontally. But grains with kernels that are generally spherical in shape, such as soybeans, sorghum, and millet, do not exhibit much difference in static pressure with horizontal vs. vertical airflow. Packing factor is a primary difference between horizontal and vertical airflow for round kernel grains. Table 8.1 was developed by summarizing the results from Example 8.2. As shown in Table 8.1, cross-flow aeration of wheat at 6 (m3/h)/tonne requires about only ¹⁄₅ of the fan power and static pressure as the same airflow rate used in vertical push-pull aeration. Wheat aerated at 12 (m3/h)/tonne with in-line or push-pull aeration required about seven times as much fan power and static pressure as the same wheat using cross-flow aeration.



427

When full-depth push or pull aeration at 6 (m3/h)/tonne was used, the fan power was almost identical to the power required at 12 (m3/h)/tonne using push-pull aeration; but the pressure was 87% higher. From a practical standpoint, power and pressure requirements using 12 (m3/h)/tonne airflow rates were too high to push air through 30 m of wheat; however, it is physically possible. Thus, full-depth vertical aeration using fans at either the bottom or top only at rates significantly higher than 6 (m3/h)/tonne does not appear feasible. Example 8.3 For a 10 m diameter silo with 40 m depth of wheat, what is the static pressure and fan power required to aerate at 15 (m3/h)/tonne? Compare cross-flow aeration with full-depth aeration airflow rates using the same fan power. For simplicity, assume that the average cross-flow air path is the effective inside diameter of the silo. Solution For horizontal airflow in a cross-flow aeration system, the effective distance across the silo is 10 m × 0.6 = 6 m. Using Figure 5.17, for an airflow rate of 15 (m3/h)/tonne, the static pressure is 0.75 kPa and the power is about 0.65 kW/100 tonnes. The grain volume in this silo is 3141 m3. At 0.769 tonnes/m3 × 3141 m3, the mass is 2415 tonnes. The estimated total power for cross-flow aeration is 0.65 kW/100 tonnes × 24.15 = 15.7 kW. At 15 (m3/h)/tonne, the airflow rate is 36,225 m3/h. If that same fan power were used for vertical aeration of the same silo, on Figure 5.17, following the 0.65 kW/100 tonne line diagonally upward to 40 m grain depth, the airflow rate would be about 2.7 (m3/h)/tonne at a pressure of about 4.5 kPa. The total airflow delivered would be 6520 m3/h — 18% of the airflow with the same fan power used in cross-flow aeration. Thus, cooling is about five times faster with cross-flow aeration using the same power. If the cooling cycle required 300 hours at 2.7 (m3/h)/tonne, it would require only about 55 to 60 hours with cross-flow aeration. Using cross-flow aeration, cooling might be completed during the first early season cold weather front. Early control of insect populations by rapid cooling of the mass using cross-flow aeration might minimize or eliminate the need for other grain management practices, such as turning grain or fumigation. 8.1.2.3 Future Research Needs for Cross-Flow Aeration As indicated in Example 8.1 above, it is estimated that cross-flow aeration can deliver about twice the airflow at less than half the power of vertical push-pull aeration. It should be seriously considered for aeration in subtropical regions, where rapid cooling is needed to maximize use of limited favorable cooling weather. Suction airflow to eliminate compression heat further enhances cross-flow aeration. Additional research should be devoted to optimizing four-duct cross-flow aeration systems for silos to minimize installation and maintenance costs, while improving the operational characteristics to overcome the disadvantages pointed out earlier. Two-duct, vertical wall plenums on opposite walls of square, rectangular, six-sided, or octagonal storage silos could simplify the design of new storage silos by providing relatively uniform airflow horizontally across stored grain bulks. Plenum outlets could be controlled to provide optimum airflow for various grain depths by sectioning plenums into separate chambers based on the grain level in the silo. One common inlet or outlet plenum could service two silos that share a common wall. Because of the potential for increased airflow with less power compared to full-depth aeration, cross-flow aeration warrants additional research. Field trials are needed to evaluate improvement in uniformity of cooling that can be achieved by cross-flow air movement in round silos with four-duct


428


systems. Field trials on efficiency of cross-flow aeration that alternate between opposite inlet ducts with three ducts used for exhaust at all times in achieving this objective are lacking. In these tests, when not used as an inlet duct, the duct opposite the inlet duct exhausts air along with the two dedicated exhaust ducts positioned adjacent the two alternating inlet ducts. The efficiency of this method in eliminating the dead zone that occurs (Figure 8.6 a, b) needs to be demonstrated in the laboratory and in commercial-scale trials. The advantages mentioned in the previous sections, together with the reasons this method have not been widely applied, need to be analyzed and evaluated. 8.1.3

Chilled Aeration

Chilled aeration, which uses refrigerated air with humidity control, provides predictable, precise cooling under weather conditions that are unsuitable for ambient aeration. This special type of aeration provides significant benefit in tropical and subtropical regions. Chilling can be used effectively in temperate regions where sensitive and/or high-value products such as seeds, soybeans, maize, popcorn, milled rice, peanuts, and macadamia nuts require cooling for several weeks or months in storage. Chapter 9 provides a complete discussion of the research background, design, and application of chilled aeration. 8.1.4

Dryeration

Dryeration, a specialized version of aeration, is a drying and cooling process that derives its name from the words drying and aeration. It uses high aeration flow rates to enhance hightemperature drying capacity and efficiency. The basic process of Dryeration is that hot grain is transferred from the dryer into two or more separate holding bins (Figure 8.7), (or into one large continuous flow holding bin) where it tempers or sweats for several hours before cooling. Depending on the type of grain and the normal drying temperature, hot grain is removed from the dryer 1 to 3% moisture higher (Table 8.2) than in normal drying. The remaining moisture is removed during tempering and slow cooling processes in Dryeration bins. Dryeration can increase the capacity of continuous flow or batch dryers by 75 to 100%. Grain quality is maintained at high levels and drying efficiency is improved (saving fuel) by stopping the drying process 1 to 3% above the target dry grain moisture. This is achieved by moving the hot grain (55 to 65°C; 130 to 150°F for maize) into cooling bins, preferably hopper-bottom bins for efficient gravity transfer, to temper for 4 to 12 hours (Figure 8.8). After tempering, high-speed ambient aeration adiabatically cools the hot grain close to ambient air temperatures in 6 to 12 hours. Cool dry grain is transferred directly to storage bins at a moisture usually 2 percentage points lower than when it left the dryer, depending on the temperature difference between the hot grain and cooling air. Advantages of Dryeration are: • • • •

Grain quality maintained at high level Drying capacity increased 50 to 100% with minimal capital investment Drying fuel efficiency improved by 25 to 40% Cooling bins provide added grain handling and storage volume after drying

Disadvantages of Dryeration are: • • • •

Continuous dryers require a burner in the cooling section for maximum drying Additional handling equipment is required One extra handling operation to transfer hot grain, then cooled grain Increased management required due to the extra grain transfer operation



Table 8.2

429

Moisture Reduction from Maize during Cooling at 30 (m3/h)/tonne (0.5 cfm/bu)

Hot Maize Temperature °C °F 54 61 67

(128) (142) (152)

Moisture Reduction (6 Tests) Mean %

Range %

1.7 2.1 2.5

1.5–1.9 1.7–2.3 2.0–3.1

From McKenzie, B.A., G.H. Foster, R.T. Noyes, and R.A. Thompson. (1967). Dryeration — Better Corn Quality With High Speed Drying. AE-72; Agricultural Engineering Department, Purdue University, Lafayette, IN.

Figure 8.7

Schematic flow diagram of the Dryeration process.

Figure 8.8

Corrugated steel bin with hopper-mounted aeration fan and duct typical of Dryeration tempering/cooling bin system design.

8.1.4.1 Increased Drying Capacity with Dryeration Conventional dryer capacity is increased by: • • • •

Eliminating the cooling zone from continuous dryers, using the entire grain column for drying Eliminating the cooling cycle from batch dryers Stopping high-temperature drying at higher moisture levels Increasing normal drying temperatures by 10 to 15°C (18 to 27°F)


430


Table 8.3

Dryeration — Showing Increased Dryer Capacity

Drying Air Temperature °C °F 88 116 144

190 240 290

Drying Capacity — 25 to 14% Moisture Maize (Wet Basis) Conventional Dryeration Dryeration Drying/Cooling Temper/Cool Increase tonne/h bu/h tonne/h bu/h % 2.0 2.6 3.5

77 102 136

3.8 4.9 6.4

150 191 250

94.8 87.2 83.8

From Thompson, R.A. and Foster, G.H. (1963). Stress Cracks and Breakage in Artificially Dried Corn, Marketing Research Report No. 631, USDA Agricultural Marketing Service, Transportation and Facilities Research Division, Washington, D.C., 0ctober 1963. Foster, G.H. (1966). Personal conversations on Dryeration

and 35 mm Dryeration slides. With permission.

In continuous dryers, an additional burner is added to heat the cooling zone, increasing the overall drying zone by 25 to 35%. To return to conventional cooling, the cooling zone burner is shut off. In batch dryers, eliminating the cooling cycle decreases the overall conventional batch drying/cooling time by 20 to 40%. Since the final bound moisture dried from grain at the end of the drying cycle is the most difficult to remove in any grain product, removing the final 2 to 3% of moisture in conventional dryers often requires as much time as the initial 5 to 10%. Removing this final moisture outside the dryer reduces the conventional drying effort by 25 to 40%, depending on the grain type and total moisture to be removed. Because high-temperature drying stops at higher moisture levels, dryer plenum temperatures can be increased by 10 to 15°C (18 to 27°F) without significantly decreasing the product quality. The increase in plenum heat level can further increase the drying rate by 10 to 15% compared with conventional drying. When these three Dryeration functions — external cooling and added drying column, removing the hardest drying, and increased drying temperature — are incorporated into the drying process, the drying capacity can be increased by 84 to 95% (Table 8.3). At the same time, drying efficiency (cost per kg or bu of grain dried) is reduced by 25 to 40% because the hardest moisture removal (the final 1.5 to 2.5%) takes place outside the dryer. This is achieved by efficient use of the residual kernel heat through tempering and evaporative cooling in the Dryeration tempering/cooling bins (Table 8.3). 8.1.4.2 Tempering during Adiabatic Cooling Moisture Removal The main principle of Dryeration is that the cooling air exhausts saturated at very high temperatures. During normal aeration, ambient air warms and picks up a relatively small amount of moisture (grain typically loses 0.1 to 0.5% moisture content) as the air temperature drops along the sloped wet-bulb depression (adiabatic cooling) line. The cooling air exhausts at or near warm grain temperatures, 30 to 35°C (85 to 95°F), partially saturated, before the leading edge of the cooling zone reaches the grain surface. The high moisture removal that occurs during Dryeration can best be visualized and understood by analyzing Dryeration exhaust air conditions on a high-temperature psychrometric chart, Figures A.2 and A.3, Appendix A. These two charts cover the range of temperatures involved in cooling 50 to 65°C (120 to 150°F) grain with specifically designed aeration airflow rates. In Dryeration, cooling air exhausts at saturation at hot grain temperatures, typically about 55 to 65°C (130°–150°F) for maize. The high temperature psychrometric chart in Figures A.2 and A.3, Appendix A, are used to illustrate the normal aeration process compared to Dryeration cooling. Foster and Thompson (1966) found that the theoretical limit of Dryeration moisture removal was about 3.5%. But due to convection and conduction heat losses during transfer in noninsulated



431

conveyors and tempering in noninsulated bins, 2 to 2.5 percentage points of moisture removal was typical during maize drying in commercial elevator or farm drying systems. Example 8.4 Determine the ratio of moisture in saturated exhaust air from Dryeration at 60°C (140°F) with ambient air at 15°C (60°F), 80% RH compared to conventional aeration exhaust at 30°C (86°F), 70% RH from 32.5°C (90°F) 15% mc maize. Then determine the moisture removal ratio of hot maize at 17% mc from Dryeration compared to ambient aeration of dry maize at 15% mc. Method Check the high-temperature psychrometric chart, Figure A.2, Appendix A, for the saturated exhaust air moisture level from Dryeration bins at 60°C (140°F), the moisture level of aeration exhaust air from 15% maize stored at 32.5°C (90°F) with exhaust air at 30°C (86°F), 70% RH. Check water content of the ambient air conditions, 15°C (60°F), 80% RH (Figure A.1). Solution From Figure A.2, Appendix A, at 53°C (140°F) saturated exhaust air contains about 0.10 kg moisture/kg dry air. Ambient air at 15°C (60°F), 80% RH has a water content of 0.0085 kg/kg (Figure A.1). Exhaust air from cooling dry 15% mc maize, which exhausts at 30°C (86°F), 70% RH, has a water content of 0.0188 kg/kg. Exhaust moisture ratio =

0.10 kg kg = 5.3 0.0188 kg kg

Dryeration exhaust air carried 5.3 times as much moisture per kg of dry air as conventional aeration exhaust. But consider the actual ratio of moisture removal by each process, for ambient air at 15°C (60°F), 80% RH: Total moisture removal ratio =

0.10 − 0.0085 0.0915 = = 8.9 0.0188 − 0.0085 0.0103

Maize temperatures from high-temperature driers often reach 60°C and higher. Under the common condition of saturation at 60°C, the airborne moisture of 0.15 kg/kg (not shown on the chart in Figure A.2) is 50% more than at 53°C. As illustrated in Example 8.4, during maize drying with separate cooling by Dryeration, hot saturated exhaust air typically carries much more moisture than exhaust air from dry grain cooling. The difference in moisture removal capacity of the cooling air between normal dry grain cooling and Dryeration cooling is derived from the high temperature differential between the hot grain and ambient air that occurs in the Dryeration process. This high moisture removal from adiabatic cooling reduces cooling times by half to two thirds of conventional cooling at the same airflow rates. 8.1.4.3 Dryeration Airflow Design Standard aeration airflow of 6 (m3/h)/tonne (0.1 cfm/bu) will normally cool dry grain in about 125 to 150 hours during the fall season in temperate climate zones such as the U.S. During Dryeration, cooling time is greatly reduced because part of the cooling is by evaporation (adiabatic


432


cooling) as well as by convection. Dryeration cooling is completed in 8 to 12 hours, or 10 to 15 times faster than conventional aeration. However, Dryeration airflow is only about 1 to 2% of the airflow used in cooling sections of high-capacity grain dryers. Foster and Thompson (1966) developed the Dryeration cooling rates through laboratory benchscale and full-scale drying research. They found that the ideal aeration airflow rate of 30 to 45 (m3/h)/tonne (0.5 to 0.75 cfm/bu) cooled the grain to ambient temperatures in 8 to 12 hours while removing 1.75 to 2.5 percentage points of final grain moisture. The Dryeration cooling rate heat balance equation is listed by Brooker et al. (1992) as:

[

(

)

)]

(

(

t 60 Qa paCa Ta − Tg + 60 Qa pa h fg Wg − Wa = GpgCg Tg − Ta

)

(8.6)

where: t = hours to cool grain to Ta G = volume of grain cooled (m3) Ta = inlet air temperature (°C) Tg = hot grain temperature (°C) Wa = humidity ratio of inlet air (kg/kg) Wg = saturation humidity ratio at hot grain temperature (kg/kg) Qa = airflow rate (m3/min) pa = air density (kg/m3) pg = bulk grain density (kg/m3) Ca = specific heat of air (kJ/kg°C) Cg = specific heat of grain (kJ/kg°C) hfg = heat of vaporization (kJ/kg) Rearranging Equation 8.6, a Dryeration cooling time equation is developed:

t=

[

(

(

GpgCg Tg − Ta

)

)

(

60 Qa paCa Ta − Tg + 60 Qa pa h fg Wg − Wa

)]

(8.7)

Further simplifying Equation 8.7 yields the following Dryeration cooling time equation:

t=

(

) 60 Q p [C (T − T ) + h (W − W )] GpgCg Tg − Ta

a a

a

a

g

g

fg

(8.8)

a

Converting the equation to reflect total airflow in m3/h yields Equation 8.9 as follows:

t=

(

) (W − W )]

GpgCg Tg − Ta

[ (

)

Qa′ pa Ca Ta − Tg + h fg

where: Qa′ = m 3 h

g

a

(8.9)



433

Example 8.5 Determine the time required to cool 500 tonnes of 60°C, 16.7% mc (wet basis) maize using 15°C, 60% RH ambient air at an airflow rate of 30 (m3/h)/tonne by solving Equation 8.9 as follows: G Ta Tg Wa Wg Q′a pa pg Ca Cg hfg

= = = = = = = = = = =

500 tonne/0.7207 tonne/m3 = 693.8 m3, or 694 m3 inlet air temperature = 15°C hot maize (exhaust air) temperature = 60°C humidity ratio of inlet air at 15°C, 60% RH = 0.0065 kg/kg saturation humidity ratio at 60°C maize temperature = 0.153 kg/kg airflow rate = 15,000 m3/h (airflow rate/tonne = 30 (m3/h)/tonne) inlet air density = 1.20 kg/m3 bulk density of maize = 721 kg/m3 specific heat of air = 1.01 kJ/kg°C specific heat of maize = 2.01 kJ/kg°C heat of vaporization for maize = 2640 kJ/kg, for 17% maize at 60°C

Using Equation 8.9 in solving for cooling time t: t=

694 × 721 × 2.01(60 − 15) 15, 000 × 1.2 [1.01(15 − 60) + 2640 (0.153 − 0.0065)]

t=

694 × 721 × 2.01( 45) 18, 000 [1.01( −45) + 2640 (0.1465)]

t=

45, 258, 828 45, 258, 828 = = 7.37 hours = 7 hours 22 minutes 18, 000 [−45.45 + 386.76] 6,143, 580

Equation 8.9 can be further simplified for most grains that are dried as the factor pa is relatively stable and can be factored in as a constant. Ca = 1.01 is essentially unity across the normal range of ambient cooling conditions and can be dropped out. Thus, Equation 8.9 can be written for all grains discharged hot from dryers as:

t=

( ) 1.2 Q ′ [(T − T ) + h (W − W )] GpgCg Tg − Ta

a

a

g

fg

g

(8.10)

a

For hot maize, in addition to pa being factored in, Cg, pg, hfg also have relatively stable numerical values and can be factored in as constants without introducing a significant error. Modifying Equation 8.10 using these factors provides the simpler Equation 8.11 for cooling times for hot maize discharged from dryers as follows:

t=

[(

(

1207.67 G Tg − Ta

)

(

)

Qa′ Ta − Tg + 2640 Wg − Wa

)]

(8.11)

When the desired cooling time is known, Equation 8.10 can be rewritten to determine the design airflow rate, Q′a, for any grain as Equation 8.12:


434


Qa′ =

[(

(

GpgCg Tg − Ta

)

(

)

1.2 t Ta − Tg + h fg Wg − Wa

)]

(8.12)

Modifying Equation 8.11 for hot maize gives Equation 8.13:

Qa′ =

[(

(

1207.67 G Tg − Ta

)

(

)

t Ta − Tg + h fg Wg − Wa

)]

(8.13)

Example 8.6 A grain manager plans to dry 500 tonne (694 m3) of maize per day. Determine the fan capacity needed for cooling 60°C maize at 17% moisture in 10 hours using 15°C cooling air. Select the appropriate fan type and size, assuming that the depth of maize requires the fan to operate at 1.2 kPa. Method The design engineer can determine the fan capacity needed by solving Equation 8.13, using the data listed above: Solution Qa′ =

1207.67 × 694 (60 − 15) 37, 715, 534 = = 10, 937 m 3 h 10 [(15 − 60) + 2652 (0.1534 − 0.0064)] 3448.44

From Table 5.9, a 7.46 kW, 71 cm diameter vane-axial fan operating at 1.2 kPa will provide 10,772 m3/h (98.5% of design airflow rate). 8.1.4.4 Dryeration Cooling Bin Design In Dryeration, hot grain is normally transferred into one of two Dryeration bins. Each bin is designed to hold all the hot grain from the dryer during a one-day (24-hour cycle) drying period. Dryeration cooling bin capacity should be designed to hold at least two times the conventional drying system capacity designed for drying moisture removal range from 25 to 15% mc (wet basis). For example, if a dryer normally dries 200 tonnes (7867 bu) of maize per 24 hours from 25 to 15%, each Dryeration cooling bin should be designed to hold 400 tonnes (15,733 bu) when drying from 21 to 22% to 17 to 18% with the final cooled grain moisture level at 15.0%. Equipment specifications for designing Dryeration bins and the associated aeration fan and duct equipment are listed in Tables 8.4 and 8.5. Once the static pressure is determined, the type and size of fan should be selected from Table 5.9 based on the airflow required. Sizes and volumes for the tempering/cooling bins are nominal dimensions. Two-hopper bottom steel grain bins that hold 2 to 2.5 times the conventional mid-season drying capacity of the dryer are recommended. But for very large grain dryers or multiple dryers at one elevator site, larger steel flat-bottom grain bins with full drying floors and sweep unloaders may be a more economical choice. Use of hopper-bottom bins with gravity unloading decreases mechanical equipment maintenance and the potential for mechanical failure. For daily unloading and cleanup, hopper bins (Figures 8.6



Table 8.4

Bin Grain Dia Depth ft. ft. 12 10 15 20 30 40 50 14 10 15 20 30 40 50 18 10 15 20 30 40 50 21 10 15 20 30 40 50 24 10 15 20 30 40 50

435

Dryeration Specifications for Maize in Hopper-Bottom Bins (British Units)

Grain Volume bu 900 1,300 1,800 2,700 3,600 4,500 1,200 1,800 2,400 3,600 4,800 6,000 2,000 3,000 4,000 6,000 8,000 10,000 2,800 4,200 5,600 8,400 11,200 14,000 3,600 5,400 7,200 10,800 14,400 18,000

Design Requirements for Airflow Air Volume Static Pressurea 0.50 0.75 1.0 0.50 0.75 1.0 cfm inches w.c. 450 675 900 0.8 1.0 1.3 675 1,012 1,350 1.3 1.9 2.5 900 1,350 1,800 2.0 3.2 4.7 1,350 2,025 2,700 4.6 7.7 11.5 d 1,800 2,700 3,600 8.9 15.2 d d 2,250 3,375 4,500 14.0 600 900 1,200 0.8 1.0 1.3 900 1,350 1,800 1.3 1.9 2.5 1,200 1,800 2,400 2.0 3.2 4.7 1,800 2,700 3,600 4.6 7.7 11.5 d 2,400 3,600 4,800 8.9 15.2 d d 3,000 4,500 6,000 14.0 1,000 1,500 2,000 0.8 1.0 1.3 1,500 2,250 3,000 1.3 1.9 2.5 2,000 3,000 4,000 2.0 3.2 4.7 3,000 4,500 6,000 4.6 7.7 11.5 d 4,000 6,000 8,000 8.9 15.2 d d 5,000 7,500 10,000 14.0 1,400 2,100 2,800 0.8 1.0 1.3 2,100 3,150 4,200 1.3 1.9 2.5 2,800 4,200 5,600 2.0 3.2 4.7 4,200 6,300 8,400 4.6 7.7 11.5 d 5,600 8,400 11,200 8.9 15.2 d d 7,000 10,500 14,000 14.0 1,800 2,700 3,600 0.8 1.0 1.3 2,700 4,050 5,400 1.3 1.9 2.5 3,600 5,400 7,200 2.0 3.2 4.7 5,400 8,100 10,800 4.6 7.7 11.5 d 7,200 10,800 14,400 8.9 15.2 d d 9,000 13,500 18,000 14.0

Rates of 0.5, 0.75, and Fan Powerb 0.50 0.75 1.0 HP 0.12 0.21 0.36 0.28 0.60 1.1 0.57 1.4 2.7 1.9 4.9 9.8 d 5.0 12.9 d d 9.9 0.15 0.28 0.49 0.36 0.80 1.4 0.76 1.8 3.6 2.6 6.5 13.0 d 6.7 17.2 d d 13.2 0.25 0.47 0.80 0.60 1.3 2.4 1.3 3.0 5.9 4.3 10.9 21.7 d 11.2 28.7 d d 22.0 0.36 0.66 1.1 0.85 1.9 2.5 1.8 4.2 8.3 6.0 15.3 30.4 d 15.7 40.2 d d 30.8 0.45 0.85 1.5 1.1 2.4 4.2 2.3 5.4 10.6 7.8 19.6 39.1 d 20.2 51.7 d d 39.6

1.0 cfm/bu Duct Areac 0.50 0.75 1.0 ft2 15 22 30 22 34 45 30 45 60 45 68 90 d 60 90 d d 75 20 30 40 30 45 60 40 60 80 60 90 120 d 80 120 d d 100 33 50 67 50 75 100 67 100 133 100 150 18.59 d 133 200 d d 167 47 70 93 70 105 140 93 140 187 140 210 280 d 187 280 d d 233 60 90 120 90 135 180 120 180 240 180 270 360 d 240 360 d d 300

a

ASAE standard data for clean dry maize increased 50% for fine material and compaction plus 0.5 in w.c. pressure loss for duct restrictions. b Calculated on basis of 50% static fan efficiency. c Minimum perforated area for duct surface based on air velocity of 30 ft/min entering maize. d Requirements are above practical limits. From McKenzie, B.A., G.H. Foster, R.T. Noyes, and R.A. Thompson. (1967). Dryeration — Better Corn Quality With High Speed Drying, AE-72; Agricultural Engineering Department, Purdue University, Lafayette, IN.

and 8.8) perform best as Dryeration cooling bins. A perforated aeration duct is normally placed down the hopper slope starting near the top of the hopper with the open lower end stopping about 0.5 to 1.0 m (20 to 40 inches) from the discharge opening. On larger hopper bins, large fans should be mounted on the concrete base with a duct connected to the aeration duct inside the hopper. Depending on bin size, one or two aeration fans are used. The type of fan depends on the depth of the bin, type of grain being dried, and the static resistance to airflow. For maize, soybean, or other coarse grain cooling, vane-axial blowers usually develop adequate static pressure. Because of higher resistance to airflow, centrifugal blowers are usually required for small grains. Large hopper bins may use one large fan manifold connected to two to four ducts inside the hopper. Some Dryeration bin systems may work better with two or more independent fans and aeration ducts per bin. A decision on the number of fans and ducts needed may be based on calculating the fan design for cooling a half-full bin. This allows cooling the hot grain when the bin is partially


436

Table 8.5


DRYERATION Specifications for Maize in Hopper-Bottom Bins (Metric)

Design Requirements for Airflow Air Volume Static Pressurea Bin Grain Grain 30 45 60 0.50 0.75 1.0 Dia Depth Volume m m tonnes m3/h (×100) kPa 3.65 3.1 22.8 7.6 11.5 15.3 0.20 0.25 0.32 4.6 34.3 11.5 17.2 22.9 0.32 0.47 0.62 6.1 45.7 15.3 22.9 30.6 0.50 0.80 1.17 9.1 68.6 22.9 34.4 45.9 1.14 1.92 2.86 d 12.2 91.4 30.6 45.9 61.2 2.22 3.79 d d 15.2 114.3 38.2 57.4 76.4 3.49 4.25 3.1 30.5 10.2 15.3 20.4 0.20 0.25 0.32 4.6 45.7 15.3 22.9 30.6 0.32 0.47 0.62 6.1 61.0 20.4 30.6 40.8 0.50 0.80 1.17 9.1 91.4 30.6 45.9 61.2 1.14 1.92 2.86 d 12.2 121.9 40.8 61.2 81.5 2.22 3.79 d d 15.2 152.4 51.0 76.4 102.0 3.48 5.5 3.1 50.8 17.0 25.5 34.0 0.20 0.25 0.32 4.6 76.2 25.5 38.2 51.0 0.32 0.47 0.62 6.1 101.6 34.0 51.0 68.0 0.50 0.80 1.17 9.1 152.4 51.0 76.4 102.0 1.14 1.92 2.86 d 12.2 203.2 68.0 102.0 136.0 2.22 3.79 d d 15.2 254.0 85.0 127.4 170.0 3.49 6.4 3.1 71.1 23.8 35.7 47.6 0.20 0.25 0.32 4.6 106.7 35.7 53.5 71.3 0.32 0.47 0.62 6.1 142.2 47.6 71.3 95.2 0.50 0.80 1.17 9.1 213.3 71.3 107.0 142.7 1.14 1.92 2.86 d 12.2 284.4 95.2 142.7 190.3 2.22 3.79 d d 15.2 355.6 118.9 178.4 237.9 3.49 7.3 3.1 91.4 30.6 45.9 61.2 0.20 0.25 0.32 4.6 137.1 45.9 68.8 91.7 0.32 0.47 0.62 6.1 182.8 61.2 91.7 122.3 0.50 0.80 1.17 9.1 274.3 91.7 137.6 183.5 1.14 1.92 2.86 d 12.2 365.7 122.3 183.5 244.7 2.22 3.79 d d 15.2 457.1 152.9 229.4 305.9 3.49

Rates of 30, 45, and Fan Powerb 0.50 0.75 1.0 kW 0.09 0.16 0.27 0.21 0.45 0.82 0.42 1.04 2.01 1.42 3.65 7.31 d 3.73 9.62 d d 7.38 0.11 0.21 0.36 0.27 0.60 1.04 0.57 1.34 2.68 1.94 4.85 9.69 d 5.00 12.82 d d 9.84 0.19 0.35 0.60 0.45 0.97 1.79 0.97 2.24 4.40 3.21 8.13 16.18 d 8.35 21.4 d d 16.40 0.27 0.49 0.82 0.63 1.42 2.46 1.34 3.13 6.19 4.47 11.41 22.67 d 11.71 29.98 d d 22.97 0.34 0.63 1.12 0.82 1.79 3.13 1.72 4.03 7.90 5.82 14.62 29.16 d 15.06 38.55 d d 29.53

60 (m3/h)/tonne Duct Areac 0.50 0.75 1.0 m2 1.39 2.04 2.79 2.04 3.16 4.18 2.79 4.18 5.58 4.18 6.32 8.36 d 5.58 8.36 d d 6.97 1.85 2.79 3.72 2.79 4.18 5.58 3.72 5.58 7.43 5.58 8.36 11.15 d 7.43 11.15 d d 9.29 3.07 4.65 6.23 4.65 6.97 9.29 6.23 9.29 12.36 9.29 13.94 18.59 d 12.36 18.59 d d 15.52 4.37 6.51 8.64 6.51 9.76 13.01 8.64 13.01 17.38 13.01 19.52 26.02 d 17.38 26.02 d d 21.65 5.58 8.36 11.15 8.36 12.55 16.73 11.15 16.73 22.30 16.73 25.09 33.46 d 22.30 33.46 d d 27.88

a

ASAE standard data for clean dry maize increased 50% for fine material and compaction plus 0.124 kPa pressure loss for duct restrictions. b Calculated on basis of 50% static fan efficiency. c Minimum perforated area for duct surface based on air velocity of 9.1 m/min entering maize. d Requirements are above practical limits. From McKenzie, B.A., G.H. Foster, R.T. Noyes, and R.A. Thompson. (1967). Dryeration — Better Corn Quality With High Speed Drying, AE-72; Agricultural Engineering Department, Purdue University, Lafayette, IN.

loaded and avoids the use of manifolds for simplicity and reduced cost. Example 8.7 illustrates a realistic Dryeration bin and fan sizing problem. Example 8.7 A country elevator operator decides to upgrade the capacity of his continuous flow grain dryer from 15 tonne/h (550 bu/h) at 25 to 15% mc to 30 tonne/h by installing a burner in the cooling section and adding a Dryeration cooling bin and conveyor system, similar to the system shown in Figure 8.7. Dryeration cooling bins should hold 200% of the conventional dryer capacity at 10 points of moisture removal. Thus, cooling bin capacity = 15 tonne/h × 24 h × 2.0 = 720 tonne (26,460 bu). The elevator manager plans to cool the grain in 8 hours, from about 55°C (135°F) to near ambient temperature in the cooling bins. The recommended airflow rates are 30 (m3/h)/tonne (0.5 cfm/bu) to cool hot



Table 8.6

437

Influence of Cooling at Various Airflow Rates in Conventional Drying and Cooling Time and in Dryeration on Stress Crack Formation during Dryeration Cooling of Maize

Dryeration Cooling Airflow Rate (m3/h)/tonne cfm/bu 12 21 30 60

Conventional Aeration Cooling Time hours

Dryeration Cooling Time hours

Checked Kernels %

75 55 30 15

30 18 12 6

9.4 13.4 12.8 16.2

0.2 0.35 0.5 1.0

Compiled from Thompson, R.A. and Foster, G.H. (1963). Stress Cracks and Breakage in Artificially Dried Corn, Marketing Research Report No. 631, USDA Agricultural Marketing Service, Transportation and Facilities Research Division, Washington, D.C., 0ctober 1963, Foster, G.H. (1966). Personal conversations on Dryeration and 35 mm Dryeration slides.

Table 8.7

Effect of Dryeration on Percent Checked Kernels and Breakagea Compared to Conventional Drying

Drying Method Conventional drying and cooling Dryeration

Percent Checked Kernels

Percent Breakage

43.6 7.6

11.3 6.7

a

Based on three tests by each method of drying maize from 25 to 14% moisture. Breakage percentages determined on small samples in a laboratory breakage tester. Compiled from Thompson, R.A. and Foster, G.H. (1963). Stress Cracks and Breakage in Artificially Dried Corn, Marketing Research Report No. 631, USDA Agricultural Marketing Service, Transportation and Facilities Research Division, Washington, D.C., 0ctober 1963, Foster, G.H. (1966). Personal conversations on Dryeration and 35 mm Dryeration slides.

maize in 12 hours and 60 (m3/h)/tonne (1.0 cfm/bu) to cool in 6 hours (Table 8.6). The manager selects an airflow rate at 80% of the 6-hour airflow, or 48 (m3/h)/tonne. So, his design airflow is 720 tonne × 48 (m3/h)/tonne = 34,560 m3/h. It is important to remember that the Dryeration process is very flexible. Although tempering maize and other grains for 8 to 12 hours is ideal, tempering for only 3 to 4 hours still provides a major part of the tempering process that allows significant internal heat and moisture transfer and reduces kernel stress crack formation. Extended tempering times for 18 to 24 hours — compared to the standard recommendation of 8 to 12 hours of tempering — is also logistically an acceptable practice. 8.1.4.5 The Ideal Dryeration Bin: The ideal Dryeration bin is a live-bottom bin that provides continuous uniform (plug) flow of grain through the bin, using a metering floor in the bin or some alternative method that provides plug flow of grain from the bin. This specialized bin design is feasible from an economic standpoint since one bin sized to handle the total daily volume of grain from the dryer can be controlled to discharge grain at the same rate as the increased dryer output. Thus, the tempering and cooling zones remain stationary and the grain moves through these zones with tempering of grain taking place in the upper half of the bin and cooling in the lower half of the bin. The cost savings by eliminating the second bin (in a two-bin system) and the additional conveyors pays for the live bottom. The overall system is much simpler as an in-line continuous drying, tempering, and cooling process.


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8.1.4.6 Higher Grain Quality from Dryeration Grain kernel stress cracks normally occur when hot grain kernels are suddenly exposed to high flow rate of cool air (in conventional dryers) as shown in Tables 8.6 and 8.7. During Dryeration, cooling air exhausts at high temperatures near saturation during most of the cooling period, so moisture removal efficiency is very high. Dryeration airflow has the potential to absorb and remove up to 3.5% of the grain moisture, depending on the hot grain temperature. Tables 8.6 and 8.7 list research results from Dryeration field tests. A primary quality deterioration aspect of maize drying, stress crack formation, is illustrated in Table 8.6, which compares cooling time and percent of checked kernels at four airflow rates. Table 8.7 shows the advantage in Dryeration cooling of hot maize vs. aeration of warm maize under normal storage conditions, with 43.6% checked kernels and 11.3% breakage under conventional drying/cooling compared to 7.6% and 6.7% with Dryeration. Checked kernels are those kernels that contain multiple stress cracks that result in severe breakage or shattering during handling impacts. Checked kernels indicate seriously stress-damaged kernels. Single stress cracks or minor fissures, which often occur in the field during natural drying, create minimal handling damage. Table 8.6 illustrates that 12 hours are required to cool the grain at a nominal design airflow rate of 30 (m3/h)/tonne (0.5 cfm/bu), with an acceptable level of 12.8% checked kernels. At the lower airflow rates of 12 (m3/h)/tonne (0.2 cfm/bu) and 21 (m3/h)/tonne (0.35 cfm/bu), the percentage of checked kernels, 9.4 and 13.4% is not much different. By eliminating the sudden thermal shock from high-speed cooling in external grain dryers, which causes kernel surfaces to contract and shrink, stress cracking and subsequent breakage during later handling are greatly reduced as shown in Tables 8.6 and 8.7. 8.1.4.7 Dryeration Facility Operation Management After high-temperature drying, hot maize is typically tempered for 8 to 12 hours in the cooling bin. Then the 6- to 12-hour cooling process is started, leaving an hour or two for unloading. The entire process is designed to work well in a 24-hour cycle. If cooling requires 12 hours and it takes three hours to unload, cooling should start after 9 hours of loading and tempering. If drying other grains besides maize, when drying air temperatures are lower than 60°C and grain temperatures after drying are lower, less moisture is removed due to reduced enthalpy (low drying energy differential); so cooling time with the same airflow rate is longer. To operate within a 24-hour cycle, when grain temperatures are lower than 60°C, the cooling cycle should be started after 4 to 6 hours of tempering. Cool air quickly heats to grain temperatures and reaches saturation in the lower part of the grain mass. Thus, even though cooling has started, additional hot grain transferred into the bin later in the day tempers adequately before the cooling front reaches the middle to upper levels of the cooling bin. The Dryeration process typically operates as follows: Bin 1 is filled on Day 1, and on Day 2 while Bin 1 tempers, cools, and is unloaded, Bin 2 is filled. Then by Day 3, Bin 1 has been emptied and is ready to be filled again, and the process continues on a daily 24-hour cycle. Thus, each bin fits a 24-hour cycle, with 4 to 12 hours of tempering, 6 to 12 hours of cooling, and the balance for unloading and preparing for the next refill. Once hot grain has tempered in the bin for 4 to 6 hours or more, cooling can begin while bin filling continues if the cooling rate is marginal — less than 30 (m3/h)/tonne (½ cfm/bu). Compared to normal drying, Dryeration requires one additional handling step (an extra elevator or transfer conveyor is required) and two Dryeration bins. The two bins provide additional storage and working capacity for other storage or handling uses after the drying season. Loewer et al. (1994) showed that if the hot maize temperature is 60°C (140°F) after leaving the dryer at 17% moisture content when dried with the dryer plenum temperature at 105°C (220°F),



Table 8.8

Dryeration Time Required to Cool to Two Temperatures with Two Airflow Rates

Cooling Airflow Rate cfm/bu (m3/min)/tonne 30 48

439

0.5 0.8

Time Required to Cool 60°C (140°F) Maize to: 24°C (75°F) Hours

15°C (60°F) Hours

7.6 6.0

12.6 11.5

Compiled from Thompson, R.A. and Foster, G.H. (1963). Stress Cracks and Breakage in Artificially Dried Corn, Marketing Research Report No. 631, USDA Agricultural Marketing Service, Transportation and Facilities Research Division, Washington, D.C., 0ctober 1963, Foster, G.H. (1966). Personal conversations on Dryeration and 35 mm Dryeration slides.

it does not lose much heat by conduction. If it tempers and then cools at the prescribed rate of ½ cfm per bushel to 21°C (70°F), 3.0 points of moisture can be removed. If the maize is cooled to 10°C (50°F) instead of 21°C (70°F), about 3.75 points could be removed. However, convective heat losses occur and grain cools in bins without insulated sidewalls, so a general rule of thumb is that about 2.0 points of moisture removal is typical for drying outside the dryer. Crops dried at lower drying temperatures with lower temperatures going into the tempering bin have reduced Dryeration capacity and moisture removal compared to maize. Cooling airlfow may need to be increased with lower hot grain temperatures and less evaporative cooling with grains dried at lower temperatures than maize. 8.1.4.8 Dryeration Research Data Results When the pericarp or outer skin of maize kernels cools rapidly, it shrinks faster than the mass of the kernel, causing high tension in the pericarp. Multiple stress cracks or fissures develop when the pericarp cannot withstand the shrinkage forces. Table 8.7 illustrates the reduction in stress cracks and kernel breakage using high-temperature drying plus Dryeration vs. conventional drying and cooling. Dryeration, with its much slower cooling design, solves the problem of “cooling shock” as grain is allowed to temper and equalize internal stresses before cooling. Foster (1966) studied the effect of cooling to a lower ambient temperature on Dryeration cooling systems. Table 8.8 shows that grain cooled to 24°C (75°F) cooled 65.7% faster than grain cooled to 15°C (60°F) when cooling at the lower design level of 0.5 (m3/min)/tonne (0.5 cfm/bu). When using 48 (m3/min)/tonne (0.8 cfm/bu), the cooling ratio to 24°C was 91% faster than cooling to 15°C. In summary, Dryeration is a powerful grain management tool. Drying capacity can be increased by 50 to 75% or more without investing in a new grain dryer. The disadvantage is that one additional handling circuit into and out of the Dryeration cooling bins is required. However, the additional hopper-bottom bins are useful at other times during the year to handle a variety of other granular products. Few commercial or industrial processes provide as much efficiency with so many advantages and so few disadvantages as Dryeration.

8.2 RECIRCULATION FUMIGATION SYSTEMS Mechanical recirculation of fumigant gases improves the speed with which fumigant gases can be released into a sealed storage and enables the rapid development of a uniform gas distribution throughout a bulk of granular materials such as grain or of bagged products in the store. Recirculation is also used to fumigate spaces or confined space such as a tarped pallet, but the discussion in this text will be confined to fumigation of products such as bulk grain in sealed storages.


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8.2.1


Methyl Bromide Recirculation

A recirculation fumigation process for methyl bromide (MB) fumigation was researched and developed in the U.S. in the early 1920s. Prior art for fumigation was to spray hydrocyanic acid directly into the grain through a pipe with orifices that was inserted into the grain. This method caused grain close to the chemical discharge manifold to absorb a high concentration of the fumigant, which left this grain with an objectionable level of the absorbed liquid chemical. Other researchers introduced the fumigant directly to the grain as a full-strength vapor. But the grain close to the gas inlet point also absorbed a high concentration of the slow-moving gas, where the gas movement was caused by the gas release over a limited period of time and by random convection air currents in the structure. Moffet (1927) developed a process where the desired grain fumigant (gaseous hydrocyanic acid) was released into an airstream so that it was diluted and forced through the grain mass in a sealed grain storage structure. The air/gas mixture was recovered in the head-space and returned to the base of the structure through a suction pipe to be forced by the recirculation fan through the grain on a continuous recirculation basis until a substantially uniform desired level of the fumigant gas was achieved. Then recirculation was discontinued through the remaining fumigation period. When the fumigation time was completed, the recirculation pipe was disconnected and the blower was then used to ventilate the structure, clearing the grain of the fumigant gas. Moffet (1927) did not indicate or claim a specific recirculation flow rate, which is typical of a process or method patent where broad general claims give stronger patent protection. Dawson (1963) described a portable, plastic recirculation device fumigation system designed to release methyl bromide into the head-space air of a sealed grain storage structure where it vaporized prior to reaching the grain surface. The gas recirculation apparatus of this system was designed for down-flow through the bulk, with a pressure return pipe to the top of the bin or silo head-space. The system did not disclose or claim a specific range of air or gas flow rates in which the recirculation process was claimed to operate at an optimum condition. But the total required fumigant was applied from the outside of the grain mass through tubing and circulated through the grain several times until there was even distribution of fumigant gas throughout the grain mass. 8.2.2

Carbon Dioxide Recirculation

Bond (1984) recommends that the air and fumigant mixture be recirculated at least two and preferably four or five times to obtain thorough blending of air-fumigant mixture in 15 to 20 min. A flow rate of 1 L (liter) of air per min for each 50 L of grain (approximately 3 m3/h/m3 of void space for porosity of 40%, or 1.54 (m3/h)/tonne of grain for bulk density of 0.78 tonne/m3) is considered adequate in the design of fumigant recirculation systems. The airflow rate is the basic factor needed for the design of such a system. Using the existing knowledge for the design of aeration systems, it is possible to calculate the resistance to flow through the commodity, the friction loss in ducts, elbows, and air distribution systems, and to subsequently determine the fan capacity required in a given grain bin (Shedd, 1953; Navarro and Calderon, 1982). Cook (1980) indicated that low airflow recirculation can be used effectively with less sorptive fumigants to obtain even penetration and distribution in grain bulks. With this method, one air change in 8 to 12 hours or approximately 0.10 m3/h/m3 of void space or 0.05 (m3/h)/tonne was found to be satisfactory in recirculating phosphine gas to attain and maintain a uniform gas concentration. Carbon dioxide can be used to modify atmospheres in grain storage facilities to control stored product insects (Jay, 1980). Since carbon dioxide is 50% heavier than air, after a certain time diffusion of carbon dioxide results in a higher concentration in the bottom layers of the bin. Therefore, after the structure is purged with carbon dioxide, it is necessary to recirculate the mixture in order to obtain an even carbon dioxide concentration for effective insect control (Wilson et al.,



441

1980). Banks and Annis (1980) recommend recirculating the storage atmosphere from the base of the bin into the head-space via external pipework with a small sealed blower to maintain an adequate carbon dioxide–air mixture. They concluded from unpublished data that a recirculation rate of about 0.1 volumes of the bin volume per day (approximately 0.01 m3/h/m3 void space) is adequate for this purpose. Fumigation has been in use many years and will continue as one of the most potent and reliable insect control methods. Most design requirements for CO2 recirculation systems are viewed as a major barrier to investment in the fumigation technology. In addition, CO2 recirculation systems have been avoided simply because of complications in construction and restrictions in space for the large piping and fan equipment. Future developments of the recirculation process may bring about a reappraisal of the system requirements. Modified atmospheres, especially the application of carbon dioxide, is a technology that is becoming established in commercial storages throughout the world. The time is ripe for completion of silo system design requirements that include recirculation when silos are converted to the modified atmosphere technology. Navarro et al. (1986) reported on recirculation rate requirements needed when carbon dioxide is used to attain a uniform gas mixture for the control of stored product pests. They used a gastight experimental bin of 666 L capacity containing wheat and equipped with a recirculation system. A cyclic pressure increase and decrease was caused by the adsorption and desorption processes taking place during gas recirculation in the bin. An index based on the ratio of lowest (Clow) to highest (Chigh) concentration of carbon dioxide at a given time was used to determine the distribution of carbon dioxide. An equation based on this index was developed as a means of predicting the time needed to attain uniform distribution based upon the recirculation rates used. Cpt = 1 − (1 − Cpo ) e

[

]

− 0.9692 Q0.454 t

(8.14)

where: Q = the volumetric flow rate in (m3/h)/m3 Cpt = time to achieve a concentration ratio of the lowest concentration (Clow) to the highest concentration (Chigh) at time (t) then:

Cp = Clow/Chigh At t = 0, Cp = Cpo . When uniform distribution is achieved, Clow = Chigh, and Cp = 1. Temperature gradients, diffusion rate, and rates of adsorption and desorption during the recirculation period were found to influence the recirculation rate. For rates varying from 0.1 m3/h/m3 to 3.7 m3/h/m3, Equation 8.14 was found to accurately predict the time to reach uniform distribution of carbon dioxide. Uniformity in carbon dioxide concentration was reached in 7 h at a recirculation rate of 0.1 m3/h/m3. This means that relatively low fan power requirements are needed to recirculate carbon dioxide in treated bins. For large bins where duct size in recirculation systems is a critical economical aspect, the possibility of using small ducts proportional to the chosen recirculation rate may reduce the cost of using a carbon dioxide modified atmosphere. 8.2.3

Phosphine Recirculation

Aluminum or magnesium phosphide or phosphine (PH3) has been used as a fumigant in pellet or tablet form since the early 1950s in the U.S. In steel storages (bins and flat buildings), phosphine pellets or tablets have been traditionally probed 1 or 2 meters deep in the grain. During conventional


442


fumigation, grain surfaces are either plastic tarped, or roof openings are sealed. In steel bins, about 75% of the dosage is surface probed and 25% is placed in base aeration ducts. In silos, automatic dispensers drop a calculated number of pellets or tablets per 35 m3 (1000 bu) onto grain conveyed by belt conveyors or elevator legs during grain transfer to storage units. Because phosphine gas is explosive in sealed compartments at concentrations above 17,900 ppm, researchers have been reluctant to test phosphine in recirculation systems like those used for methyl bromide. In the mid-1970s, James Cook, a Houston, Texas commercial applicator, conducted field tests on a new phosphine gas recirculation fumigation system later termed closed loop fumigation (CLF) (Noyes et al., 1989). Cook’s systems were designed to use low airflow rate centrifugal fans connected to closed piping or duct systems to recirculate phosphine gas released from pellets or tablets in the top of storage units (Cook, 1980). Cook received a U.S. patent, “low airflow fumigation method” (called the J-System) in April, 1980, on his phosphine recirculation system. Cook established upper and lower limits of air–gas flow rates for phosphine recirculation fumigation. The J-System recirculated phosphine to achieve uniform distribution throughout the grain bulk much faster and more predictably than was possible with conventional probe fumigation relying on diffusion and convection air currents. In his tests, Cook used a 0.075 kW (0.10 HP) recirculation blower that delivered 353 m3/h (210 cfm) through grain in an 8100 tonne (300,000 bu) welded steel storage bin. He concluded that this flow rate, 0.043 (m3/h)/tonne (0.0007 cfm/bu), about one gas exchange per 12 hours, provided satisfactory mixing and relatively uniform gas distribution with 4 to 6 air exchanges in 2 to 3 days (Cook, 1980). In his U.S. letters patent, Cook (1980), referring to the time required to achieve homogenous phosphine concentration, stated, “The rate at which the air is circulated is less than approximately 0.006 cfm/bu and is preferably maintained approximately between 0.0015 cfm/bu and 0.0008 cfm/bu.” The rate of 0.36 (m3/h)/tonne (0.006 cfm/bu) is about 1.4 hours per air exchange or 17 air changes per day. Cook (1980) preferred to use between 0.090 and 0.048 (m3/h)/tonne (0.0015 to 0.0008 cfm/bu), which are equivalent to 5.6 to 10.4 hours per air exchange, or 2.3 to 4.3 air changes per day. These airflow rates were considered as the upper and lower limits of gas flow to provide economical but effective uniformity of gas distribution in sealed storage units. By specifying that gas flow was less than 0.36 (m3/h)/tonne, he seemed to indicate that a substantially higher gas flow rate was not necessary — that flow rates between 0.090 and 0.048 (m3/h)/tonne were satisfactory. Through his recirculation fumigation work, Cook established a safer, faster, more effective, and more economical method of phosphine fumigation that required less fumigant compared to conventional fumigation methods when storages were adequately sealed. Degesch America purchased the J-System patent rights from Cook in the early 1980s and worked with Cook and commercial fumigators to install recirculation fumigation systems at grain elevators along the southern Texas coastal area from Houston to Corpus Christi. They tested phosphine recirculation systems in storage holds of grain tankers for “in-transit ship fumigation” from the U.S. to Russia and Japan. Inexpensive 10-cm (4-in) ID flexible plastic perforated drainage hoses were installed along floors, wall junctions, and across the center of ship holds. Hoses were connected to recirculation fans on the grain surface, then hold covers were sealed. J-System recirculation performed satisfactorily during long in-transit fumigations. 8.2.3.1 Benefits of Recirculation Fumigation of Phosphine Compared to conventional probe/tarp fumigation methods, or automatic pellet dispensing into concrete silos to fumigate while turning grain, closed-loop fumigation (CLF) is performed without removing the grain from the silo (Noyes et al., 1995). Aeration fans, vents, conveyors, and downspouts are primary openings to be sealed prior to use of CLF. In dosing for CLF, fumigant can be



443

spread on grain surfaces or added to a small percentage of grain conveyed from one bin into another bin, with dosage added to conveyors during transfer of a portion of the grain (2 to 5% of bin capacity) into the treated bin. Recirculation fumigation benefits and advantages are: 1. Reduced grain handling damage and shrinkage by eliminating the process of moving or “turning” grain between silos while dosing 2. Better gas uniformity and efficacy, thus reducing failures and the need for repeating the fumigation 3. Faster fumigant distribution and uniformity 4. Reduced labor compared with conventional probe or pellet dispensing 5. Worker safety improved as CLF minimizes confined space entry 6. Reduced potential of insect resistance to phosphine by providing a complete and uniform kill 7. Reduced fumigation cost by commercial fumigators and more efficient and safer work; commercial fumigators can service more customers with fewer workers

8.2.3.2 Recirculation Fumigation Procedures In conventional probe fumigation, phosphine pellets are preferred over tablets because of quicker gas release. For CLF, pellets are recommended where grain is relatively cool and dry to maximize release rate. When grain is warm and moist, tablets are recommended for slower, more uniform gas release rates. Tablets require fewer containers for faster, more efficient dosage application (1 tablet = 5 pellets = 1 gram of phosphine). Pellets or tablets can be spread on the grain surface and “walked” into the grain 0.3 to 0.6 m (1 to 2 ft). If it is necessary or desirable to recover the spent ash after the fumigation, packets, blankets, or sachets can be placed on the surface or buried in shallow trenches with pull cords laid through the roof entry hatch for easy recovery. The dosage can be placed in a shallow layer on a canvas tarp, or on a plastic or metal sheet that is placed on the grain surface. After placing the dosage and sealing the structure, the blower should be started within 1 to 2 hours and run for 1 to 2 days until uniform gas levels are measured. Direction of gas flow Phosphine gas could be recirculated in either direction. But the typical method is for the blower to pull the gas/air mixture from the storage head-space through an external metal pipe, PVC pipe, or plastic tubing connected to a centrifugal blower inlet. Typical external pipe sizes are 10-, 12.4-, or 15-cm (4-, 5-, or 6-inch) ID, depending on the bin size, the selected airflow rate, and the design pressure losses due to pipe friction. The blower pushes the gas through a piping duct or manifold into the aeration duct located at the base of the storage, forcing it up through the grain back to the storage head-space. The blower is initially operated 1 to 2 days to thoroughly mix the gas, then is shut off for a day or two. It is then operated only 3 to 6 hours (about one gas exchange) every day or two to maintain sufficient head-space gas levels until the fumigation is complete. With CLF piping in place on bins or silos sealed except for fans, vents, and conveyors, the response time to begin fumigation of a large number of silos or storage bins can be reduced to less than one day, using two or three people. This is compared to several days preparation time by several workers for each conventional fumigation. The dosage for all storage units can be placed in the grain the same day that the decision is made to fumigate, the storage units can be sealed, and all storage units can be fumigated simultaneously. By fumigating all storage units at the same time, reinfestation of fumigated storages by insects migrating from adjacent non-fumigated bins or silos is eliminated. In well-sealed silos or grain bins, CLF fumigant dosages are typically reduced by 35 to 70% of amounts used in conventional fumigations with less stringent sealing requirements.


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8.2.3.3 Recirculation Fumigation vs. Aeration Airflow Rates Aeration airflow is an order of 20 to 100 times that of recirculation fumigation or CLF. Therefore, grain depth, interstitial surface area, and number of passages is much more critical in aeration compared to fumigation. The amount of interstitial air volume in the two 5000-tonne bins provides a typical grain storage example. When a steel bin is filled with grain to the top of the sidewall, grain fills about 95% of the bin volume. The head-space accounts for only 5% of the empty bin volume. Using 5% headspace volume and combining head-space volume with the interstitial air in the grain mass of the two 5000-tonne bins, the total air volume is 2 × 5000 × 1.28 m3/tonne × (0.40 + 0.05) = 12,800 × 0.45 = 5760 m3 of air volume. The CLF fan delivers 1070 m3/h, the gas recirculation rate is 1070 m3/h/5760 m3 = 0.186 m3/h/m3 of void space. Compared to aeration, based on the grain volume, the airflow rate is 1070/10,000 = 0.107 or about 0.11 (m3/h)/tonne. This is only 1.8% (¹⁄₅₅th) of the normal aeration airflow rate of 6 (m3/h)/tonne (0.1 cfm/bu) for commercial steel bins. Aeration airflow rates in standard steel bins are typically 50 to 100 times as high as gas flow rates for recirculation fumigation. At 6 (m3/h)/tonne (0.1 cfm/bu), one complete air exchange requires about 5 minutes in a full grain bin. At a CLF gas flow rate of 1070 m3/h, to circulate the 5760 m3 of air volume requires 5760/1070 = 5.38 or 5.4 hours per cycle. The objective of CLF is uniform gas distribution; so with gas distribution through aeration ducts or perforated drying floors, only 4 to 8 gas exchanges are usually required to obtain a uniform gas mix through the grain mass. At 0.11 (m3/h)/tonne, 5.4 hours per air exchange, or 4.4 air exchanges per day, gas distribution should be uniform in 1 to 1.5 days. Once phosphine gas levels are uniform, recirculation blowers should be shut off to prevent pumping the fumigant out at leak points in the system, especially if the grain storage is not sufficiently gas-tight. Research Products Corporation (RPC), Salina, Kansas, promoted the use of a CLF blower timer control panel that could be preset to operate the CLF blower at selected on/off intervals. RPC researchers reported that recirculation blower cycle timing produced better results than with continuous or manual CLF blower operation. Bigler and Bigler (1998) stated that periodic CLF blower operation allowed better gas diffusion (during fan off times), which improved gas uniformity and stability and allowed the use of lower dosages with higher levels of efficacy. After an initial period of several hours of continuous operation, RPC recommended a ratio of 1 hour on and 2 hours off; so the CLF blower operated only about ⅓ of the time during the balance of the fumigation (Bigler and Bigler, 1998). 8.2.3.4 Gas Manifold Piping Design Alternatives Typical CLF pipe sizes are 10 cm (4 inches) ID connected to the inlet and outlet of 0.19- to 0.56-kW (0.25- to 0.75-HP) centrifugal blowers, and 10- to 15-cm (4- to 6-inch) suction piping for 0.75- to 1.12-kW (1.0- to 1.5-HP) blowers. On pressure piping manifolds of 0.75- and 1.12-kW blowers, 12.5- and 15-cm (5- and 6-inch) ID pipes are connected from the blower outlet to the first tee, then 10-cm (4-inch) ID pipes are used for lateral conduits carrying reduced gas flows. Smaller CLF blowers use 10-cm ID pipes for the main pipe manifold with 5- and 7.5-cm (2- and 3-inch) ID lateral piping. Recirculation gas flow from CLF, typically 0.06 to 0.3 (m3/h)/tonne (0.001 to 0.005 cfm/bu), are so low in grain that there is little flow resistance through the grain. Tube, hose, or pipe sidewall flow resistance produces most of the blower pressure loss. Permanent pipe and fittings are recommended for the hard piping, except when connecting suction and pressure pipe manifolds to the centrifugal blower. Construction costs vary widely based on the difficulty of installing piping through roofs, manhole covers, exterior sidewall vents, securing vertical suction pipes to the sidewalls, etc. External pipe mounting on steel bins is relatively simple. Brackets can be spaced 3- to 6-m (10- to 20-ft)



445

apart. Self-tapping screws or self-threading bolts can be used to fasten pipe-mounting brackets on corrugated steel bins with thinner steel walls. Galvanized angles, channels with standard U-bolts or formed all-thread U-bolts make an economical anchoring scheme. Piping half-bands welded to angles for bolting to bins or silos are another CLF pipe-mounting alternative. Used 10-, 12.5-, or 15-cm (4-, 5-, or 6-in) diameter pipe may be available at salvage prices in some areas for CLF systems use. Non-perforated 10- and 15-cm (4- and 6-in) ID black plastic flexible drainage hose is a very inexpensive material that works well for low-volume gas handling and can eliminate several elbows. This black flexible plastic hose appears to hold up well over several years of permanent outdoor exposure to sunlight. Hose pipe adapters make good physical connections between hose and pipe of the same nominal ID. Duct taping these joints makes a sufficiently strong, tight connection. For small diameter laterals from the main line to connect with individual concrete silos, 5- or 7.5-cm (2- or 3-in) ID “schedule 40” PVC and soft, flat nylon-covered water hose (that can be clamped for shut-off) make an inexpensive pressure system. These systems can handle misalignment of hard piping and save labor. Using inexpensive ball valves instead of soft water hose and stainless steel hose clamps provides a permanent installation with easy shut-off of gas flow through empty silos. Use of rubber connectors between pipe fittings that are permanently installed is not recommended. Most rubber adapters or fittings deteriorate rapidly when exposed to sunlight, resulting in leaks. However, they work well when used for short-term connections of the CLF blower to the permanent CLF suction and pressure piping manifolds for 7 to 10 days use for a fumigation. Then the blower and adapters should be disconnected and stored inside. Suction and pressure pipe manifold connectors should be sealed to prevent insect, bird, or rodent entry into the storage structure.

8.2.3.5 Recirculation Blowers Blowers used for phosphine gas handling should be made from chemically resistant materials. Aluminum or plastic wheels and housings are preferred because they are spark resistant. Stainless steel centrifugal blowers are excellent choices where economically competitive. Carbon alloy steel blower wheels and housings should be coated with epoxy or other tough spark- and chemicalresistant materials. Centrifugal blowers have assembly joints that should be leak-tested during operation to make sure the seams are completely sealed. Blower motor shaft seals are a major source of leaks. C-face mounted blower motors may leak through the windings and housing. A leaky CLF blower running continuously can pump significant amounts of fumigant gas out of the storage structure during fumigation. Gas flow rates of 0.12 to 0.6 (m3/h)/tonne (0.002 to 0.010 cfm/bu) are recommended to provide a total air change every 50 to 250 minutes or about 6 to 24 changes per day. Lower gas flow rates can still be effective — they merely require more fan operating time to develop uniform gas levels. According to Cook (1980), 0.042 (m3/h)/tonne (0.0007 cfm/bu) from a 0.0746-kW (¹⁄₁₀-HP) blower delivering 5.9 m3/min (210 cfm) in an 8000-tonne (300,000-bu) bin, or about one air change every 12 hours was satisfactory. Normal aeration airflow at 6 (m3/h)/tonne (0.1 cfm/bu) displaces one air change in a full bin of grain with 40% air void space in about 5 minutes — 20 times faster than a CLF blower delivering 0.3 (m3/h)/tonne (0.005 cfm/bu), which requires about 100 minutes per air exchange. Tables 8.11 and 8.12 list a range of blower sizes, power requirements, and air flows for a series of blowers suitable for use in gas recirculation systems. For example, a 0.25-kW blower (model PB-9) will deliver 269 m3/h at a static pressure of 1.0 kPa (Table 8.11). The same model PB-9 with a different wheel design and a ⅓-HP motor will deliver 160 cfm at a static pressure of 4 inches water column (Table 8.12).


446

Table 8.9


Straight, Smooth, Round Pipe Diameter, Area, Airflow, and Friction Lossa for Pipe Velocities — Metric Units

Diameter (cm) Area (m2) Velocity (m/min)

10 0.0079 FLa m3/h

Pipe Diameter (cm) and Cross-Sectional Area (m2) 12.7 15.2 17.8 20.3 0.0126 0.0182 0.0248 0.0325 m3/h FLa m3/h FLa m3/h FLa m3/h FLa

25.4 0.0507 m3/h FLa

792 853 914 975 1036 1067 1097 1128 1158 1219 1280 1341 1372 1463 1524 1585 1676 1707 1768 1828 2134

381 412 440 469 499 511 528 541 558 588 616 645 658 702 731 763 805 823 848 880 1026

596 642 687 734 780 800 826 845 870 917 963 1008 1028 1099 1142 1193 1257 1284 1326 1374 1604

2386 2570 2752 2937 3116 3205 3301 3388 3478 3669 3847 4032 4121 4402 4578 4771 5035 5141 5310 5504 6418

0.82 0.94 1.08 1.23 1.39 1.47 1.56 1.65 1.74 1.92 2.12 2.31 2.42 2.76 3.00 3.25 3.67 3.81 4.07 4.32 5.90

0.65 0.75 0.86 0.98 1.11 1.18 1.25 1.31 1.39 1.54 1.70 1.85 1.94 2.21 2.42 2.62 2.91 3.01 3.24 3.46 4.72

858 924 988 1055 1122 1152 1188 1218 1253 1337 1386 1452 1482 1586 1646 1717 1811 1848 1910 1976 2310

0.54 0.63 0.72 0.82 0.93 0.84 1.04 1.10 1.16 1.28 1.41 1.54 1.62 1.84 2.00 2.16 2.42 2.51 2.70 2.88 3.91

1168 1257 1347 1436 1529 1571 1616 1660 1710 1798 1890 1976 2019 2157 2243 2335 2468 2513 2602 2695 3147

0.46 0.54 0.62 0.70 0.80 0.84 0.89 0.94 0.99 1.10 1.21 1.32 1.39 1.58 1.72 1.86 2.08 2.15 2.31 2.47 3.38

1527 1641 1761 1878 1996 2053 2113 2169 2229 2345 2464 2580 2639 2816 2932 3054 3226 3283 3400 3521 4108

0.41 0.47 0.52 0.62 0.69 0.74 0.78 0.82 0.87 0.96 1.06 1.16 1.21 1.39 1.50 1.62 1.82 1.89 2.02 2.16 2.95

0.32 0.40 0.42 0.49 0.56 0.59 0.62 0.66 0.70 0.77 0.87 0.93 0.97 1.11 1.20 1.30 1.45 1.51 1.62 1.73 2.35

a

FL = Friction Loss in kPa per 30.5 m of pipe length From Cincinnati Fan and Blower Company, Cincinnati, OH.

If bins are well sealed with good aeration ducts, CLF systems with 0.37 to 0.56 kW (0.5 — 0.75 HP) blowers work satisfactorily on 4000 to 8000 tonne (150,000 to 300,000 bu) bins. A 0.75-kW (1.0-HP) blower provides adequate gas flow for two 5000-tonne (180,000-bu) bins (Noyes, 1993) or 0.75 kW for 10,000-tonne (360,000-bu) bins. For standard bolted bins without intensive sealing, recirculation airflow rates should be in a relatively high range of 0.42 to 0.6 (m3/h)/tonne (0.007 to 0.010 cfm/bu). Example 8.9 Select a blower for CLF in an existing 1500-tonne silo containing wheat. Silo height is 30 m and diameter is 9 m. Use 10-cm ID pipe. The desired recirculation rate is one air change every 2 hours. Head-space volume is 5% of the grain mass volume. Assume that the grain storage facility is at sea level. Method Using Table 8.9 (Table 8.10 provides British Units), find the estimated static pressure, then use the airflow for that static pressure to select the appropriate blower model from Table 8.11 for metric units (Table 8.12 provides British Units). Solution Since the mass of grain is 1500 tonnes, the estimated grain bulk volume is 1500 tonnes ÷ 0.78 tonne/m3 = 1923 m3. The interstitial air volume is 1923 m3 × 0.40 = 769 m3. Adding the 5%



Table 8.10

447

Straight, Smooth Round Pipe Diameter, Area, Airflow and Friction Lossa for Pipe Velocities — British Units

Diameter (cm) Area (sq ft) Velocity (fpm

4 0.087 cfm FL

Pipe Diameter (in.) and Cross-Sectional Area (sq ft) 5 6 7 8 0.136 0.196 0.267 0.349 cfm FL cfm FL cfm FL cfm FL

10 0.545 cfm FL

2600 2800 3000 3200 3400 3500 3600 3700 3800 4000 4200 4400 4500 4800 5000 5200 5500 5600 5800 6000 7000

227 245 262 279 297 304 314 322 332 350 367 384 392 418 435 454 479 490 505 524 611

355 382 409 437 464 476 492 503 518 546 573 600 612 654 680 710 748 764 789 818 955

1.63 1.89 2.08 2.47 2.78 2.95 3.12 3.30 3.48 3.85 4.25 4.63 4.86 5.55 6.02 6.50 7.28 7.55 8.10 8.66 11.80

1420 1530 1638 1748 1855 1908 1965 2017 2070 2184 2290 2400 2453 2620 2725 2840 2997 3060 3161 3276 3820

Airflow m3/ha Static Pressure (kPa) 0.75 1.0 1.25 1.5

1.75

2.0

— — 381 482 638 677 857 1070 1265 1315 1830 2196 2905 3923 4827

— — — — — — 482 507 — 855 1147 1640 2370 3419 4274

— — — — — — — — — 687 924 1495 2344 3284 4136

3.26 3.76 4.33 4.93 5.56 5.89 6.23 6.59 6.95 7.69 8.48 9.26 9.70 11.05 12.02 13.00 14.68 15.25 16.27 17.30 23.60

2.60 3.01 3.46 3.94 4.45 4.71 4.98 5.26 5.55 6.15 6.78 7.41 7.77 8.85 9.67 10.50 11.64 12.05 12.95 13.85 18.90

511 550 588 628 668 686 707 725 746 796 825 864 882 944 980 1022 1078 1100 1137 1176 1375

2.17 2.52 2.88 3.28 3.71 3.93 4.15 4.38 4.62 5.13 5.65 6.18 6.48 7.38 8.02 8.66 9.68 10.05 10.78 11.52 15.65

695 748 802 855 910 935 962 988 1018 1070 1125 1176 1202 1284 1335 1390 1469 1496 1549 1604 1873

1.86 2.15 2.47 2.82 3.18 3.37 3.56 3.76 3.97 4.40 4.85 5.30 5.55 6.32 6.88 7.44 8.31 8.61 9.25 9.89 13.50

909 977 1048 1118 1188 1222 1258 1291 1327 1396 1467 1536 1571 1676 1745 1818 1920 1954 2024 2096 2445

a

FL = Friction Loss in inches w.c. per 100 ft. of pipe length. From Cincinnati Fan and Blower Company, Cincinnati, OH.

Table 8.11

Recirculation Blower Specifications — Metric Units

Model

kW

Inlet/Outlet ID/OD (cm)

0.25

0.5

A-3b A-4Bc B-8c PB-9b B-9b PB-9b PB-9b PB-10Ab A6Cc PB-10Ab PB-12Ab PB-12Ab PB-14Ab PB-15b PB-15b

0.06 0.25 0.25 0.25 0.37 0.37 0.56 0.75 1.12 1.12 1.5 2.24 3.7 5.6 7.5

10/10 10/10 10/10 10/10 12.5/10 12.5/10 12.5/10 15.2/12.7 15.2/12.7 15.2/12.7 17.8/15.2 17.8/15.2 20/15.2 20/20 20/20

334 571 576 652 823 865 991 1230 1673 1488 2115 2429 3157 4234 5102

235 420 494 573 756 773 932 1142 1487 1403 1986 2313 3036 4074 4964

a

b c

— — 210 269 521 583 773 979 949 1203 1670 2053 2775 3788 4689

— — — — 336 477 692 850 — 1087 1510 1929 2641 3662 4549

— — — — — 353 613 712 — 979 1339 1781 2506 3545 4412

60 hertz, 3450 RPM; for 50 hertz, blower speed will be 2875 RPM. For blower capacity using 50 hertz power, multiply m3/h by 0.83. Example: PB-9, 0.56 kW @ 0.75 kPa static pressure = 857 m3/h × 0.83 = 711 m3/h. Compiled from Cincinnati Fan and Ventilator Co, Inc., Cincinnati, OH. Compiled from Degesch America, Inc., Weyers Cave, VA.

1.30 1.61 1.73 1.97 2.22 2.35 2.49 2.63 2.78 3.08 3.49 3.71 3.89 4.43 4.82 5.21 5.81 6.03 6.47 6.92 9.41


448


Table 8.12

Model c

A-3 A-4Bc B-8c PB-9b B-9c PB-9b PB-9b PB-10Ab A6Cc PB-10Ab PB-12Ab PB-12Ab PB-14Ab PB-15b PB-15b a

b c

Recirculation Blower Specifications — British Units

HP

Inlet/Outlet ID/OD (in)

1"

2"

¹⁄₁₂ ⅓ ⅓ ⅓ ½ ½ ¾ 1 1.5 1.5 2 3 5 7.5 10

4/4 4/4 4/4 4/4 4/4 5/4 5/4 6/5 6/5 6/5 7/6 7/6 8/6 8/8 8/8

199 340 343 388 490 515 590 732 996 886 1259 1446 1879 2520 3037

140 250 294 341 450 460 555 680 885 835 1182 1377 1807 2425 2955

Airflow CFMa Static Pressure (in w.c.) 3" 4" 5" 6" — — 227 287 380 403 510 637 753 783 1089 1307 1729 2335 2873

— — 125 160 310 347 460 583 565 716 994 1222 1652 2255 2791

— — — — 200 284 412 506 — 647 899 1148 1572 2180 2708

— — — — — 210 365 424 — 583 797 1060 1492 2110 2626

7"

8"

— — — — — — 287 302 — 509 683 976 1411 2035 2544

— — — — — — — — — 409 550 890 1330 1955 2462

60 hertz, 3450 RPM; for 50 hertz, blower speed will be 2875 RPM. For blower capacity using 50 hertz power, multiply CFM by 0.83. Example: PB-9, 3/4 HP @ 3 in. static pressure = 510 CFM × 0.83 = 425 CFM. Cincinnati Fan and Ventilator Co, Inc., Cincinnati, OH. Degesch America, Inc., Weyers Cave, VA.

head-space volume (1923 m3 × 0.05 = 96 m3), the free air volume is (769 m3 + 96 m3 = 865 m3). Since one air change every 2 hours was recommended, the recirculation airflow rate is 865 m3 ÷ 2 h = 432 m3/h. The velocity in the existing 10-cm pipe is 432 m3/h ÷ 60 = 7.20 m3/min; 7.20 m3/min ÷ 0.007854 m2 = 917 m/min. From Table 8.9, the friction loss for a gas flow of 432 m3/h for 30 m = 1.04 kPa. Assuming the pipe length is about 30 m, the estimated total FL is 1.04 kPa. From Table 8.11, model B-9 delivers 521 m3/h at a static pressure of 1.0 kPa. The 0.37-kW model B-9 output of 521 m3/h (Table 8.11) exceeds the 432 m3h requirement by 18.5%. Example 8.10 If the blower in Example 8.9 was operated at a grain elevator located at 1500 m (4920 ft) above mean sea level (MSL), calculate the airflow delivery of the 0.37-kW constant speed B-9 blower at 1.04 kPa pressure. Method Check Table 7.1 for altitude air density adjustments to the rated airflow. Solution From Table 7.1, for 1500 m (4920 ft) MSL elevation, use straight-line interpolation between 1000 m with a standard air density of 1.16 kg/m3 and 2000 m at 0.95 kg/m3. The density for 1500-m elevation is (1.16 + 0.95)/2 = 2.11/2 = 1.06 kg/m3. Because the fan has a uniform volumetric displacement, its performance is reduced directly proportional to the difference in air density between sea level and 1500 m elevation, or 1.06/1.22 = 0.869, or 87%. At 1500-m altitude the airflow rate of air equivalent to sea level (at normal temperature and pressure conditions) will be 521 m3/h × 0.87 = 453 m3/h.



449

8.2.3.6 Sealing Leaks and Structures Sealing bin or silo openings is key to successful CLF system operations. Welded steel and concrete bins are normally sealed better during construction than bolted steel bins unless the bolted bin wall sheet and roof panel joints were well caulked during construction. Roof/sidewall air gaps and the space between roof panel ridges and fill rings are critical sealing areas in corrugated steel bins. The open roof panel ends under the fill ring flashing, collects grain dust, and makes a natural breeding place for insects. These openings should be treated with a high-density foam sealer with at least 50% expansion. They should be coated with a PVC or urethane paint for good sealing. Roof/sidewall air gaps are serious problems on concrete silos because many roof decks were cast separately and then set on top of the concrete silo walls. Often, these roof/sidewall junctions are not caulked or sealed. Sealing silo roof/wall cracks and external under-roof wall vent openings is easiest to do when the silo is filled to within 1 to 2 m (3 to 6 feet) of the roof so the grain surface can be used as a work platform. Eaves and fill ring/roof panel gaps should be foam sealed. Base/sidewall junctions should be sealed with silicone caulk, a flexible elastomer paint, or other flexible urethane-type polymer seals that retain their plasticity over long periods of time and withstand ultraviolet (UV) radiation from sunlight. If the roof deck/wall gap is very close, 0.15 to 0.5 cm (¹⁄₁₆ to ³⁄₁₆ in), a food-grade silicone caulking may be the most suitable sealant. If the gap is larger, an expandable foam sealant coated after curing with urethane or PVC paint is preferable to minimize cost. Use expandable foam to fill the exterior vent openings by building the foam up by layers from the bottom of the vent opening. An alternative is to cut plywood or steel plates with “J” bolts that can hook around the vertical bars, be pulled up tight, then sealed with silicone around the edges. Or cut a close or pressfitting wood plate that can be hammered into place and use a silicone bead around the edges to hold it in place and seal air gaps. It is important that CLF be used in well-sealed structures that can maintain 150 to 200 ppm of phosphine gas for at least 96 hours (preferably 120 hours or more) so the gas has time to create a lethal exposure time for insect control. Low (10 to 15°C) grain/air temperatures cause slow gas release, which can extend a CLF operation from 5 to 8 days or longer, which makes sealing more critical. To prevent gas loss, sidewall seams, eave gaps, roof vents, aeration blowers, unload conveyors, sidewall entry doors, and bolted joints of corrugated steel bins should be well sealed. Leaks cause fumigant loss, especially in high winds, increasing inflow of air and outflow leakage of air and gas mixture, further diluting and reducing gas levels. New bolted steel bins to be used with CLF should be carefully sealed for gas leaks at all wall and roof panel joints with a closed-cell, adhesive-backed foam strip, silicone, or other suitable joint sealing materials during construction. 8.2.3.7 Phosphine Recirculation in Large Steel Bins Following James Cook’s successful field test on PH3 gas recirculation in the early 1980s and Degesch America’s installation of the “J-System” at elevators from Houston to Corpus Christi, TX, field testing of recirculation fumigation was conducted in 1986 to 1988 by the Union Equity Cooperative Grain Exchange, Enid, OK. They installed a “J-System” on an 8150-tonne (300,000-bu) welded steel storage bin and one 815-tonne (30,000-bu) concrete silo at their Fairfax Elevator, Kansas City. Two 0.063-kW (¹⁄₁₂-HP) blowers that developed about 294 m3/h (175 cfm) each were used to recirculate gas on the 8150-tonne bin. About 0.072 (m3/h)/tonne (0.0012 cfm/bu) or one grain mass and head-space air exchange in 8.4 hours, or about 2.8 air changes per day, was applied. The airflow of 0.072 (m3/h)/tonne (0.0012 cfm/bu) was within Cook’s test guidelines for the 8150tonne steel bin (Noyes et al., 1989). A second 8150-tonne welded steel bin at Fairfax Elevator, Kansas City, was fumigated by conventional probe method. The dosage was 30 flasks of phosphine (49,800 pellets) applied to each


450


Table 8.13

Comparison of Gas Levels in Two 8150-Tonne Welded Steel Tanks using Alternative Fumigation Methods

Dates (1987) Cum. Hours Conditions m ft Head-space 3.0 10 6.1 20 9.1 30 12.2 40 13.5 Fan Average

Dates (1987) Cum. Hours Conditions m ft Head-space 3.0 10 6.1 20 9.1 30 12.2 40 13.5 Fan Average

No Recirculation (300,000 bu 30 Flasks — 24 Probed in, 6 in Aeration Ducts) 10/27 10/27 10/28 10/28 10/29 10/29 10/30 11/2 19 25 43 49 67 73 93 162 — Windy — — — — — — Sample Location (Depth or Distance from Surface) Concentration, PPM 300 325 175 400 500 475

525

325

20 0 125 275 400 187

75 20 250 575 1000 408

400 375 325 300 1000 454

10/27 19 —

0 0 100 400 175 167

15 0 200 425 625 240

0 0 175 400 550 254

10 0 200 450 2100 543

20 0 200 450 1125 378

Recirculation (300,000 bu 30 Flasks — Broadcast on Grain Surface) 10/27 10/28 10/28 10/29 10/29 10/30 25 43 49 67 73 93 Windy — — — Fans Off —

11/2 162 —

Sample Location (Depth or Distance from Surface) Concentration, PPM 200 425 675 800 1000 950

1050

350

125 190 210 300 300 221

825 875 925 1100 900 946

650 625 525 310 150 435

125 275 275 425 400 321

650 575 600 450 700 608

500 600 650 750 700 667

850 850 900 950 950 917

775 825 875 950 925 883

From Noyes, R.T., M.E. Stringer, and B.L. Clary. (1989). Closed loop fumigation, Table 2, Comparison of Recirculation Vs. Non Recirculation on 300,000 Bu. Welded Steel Tank, Section V. 1989 Oklahoma Grain Elevator Workshop Manual.

bin. In the conventional fumigation, 24 flasks were probed into the grain surface throughout the head-space area and six flasks were placed in the six aeration ducts. In the recirculation bin, the 30 flasks were spread in a thin layer on the grain surface. Label rate dosages for steel bins is typically 150 to 300 pellets per 35 m3 (1000 bu). The dosage used in this field test in welded steel bins was 49,800/300 = 166 pellets per 35 m3 (1000 bu), or near the lower recommended dosage level (Noyes et al., 1989). Table 8.13 lists the test results of these two identical side-by-side welded steel bins containing the same volume and type of wheat fumigated simultaneously (Noyes et al., 1989). Gas sample tubing was installed to pull samples from the head-space, then at 3-m (10-ft) intervals from the center of the mass to a depth of 12.2 m (40 ft) and from the fans/aeration ducts, about 13.7 m (45 ft) below the grain surface. Results of the 19-hour readings show the value of gas recirculation from the relative uniformity of gas levels throughout the mass. This is expressed in gas readings ranging from 125 ppm at 3 m (10 ft) to 400 to 425 ppm at the fan and head-space when two air exchanges were completed after 25 hours. After 2 days and 4 gas exchanges, it is evident that gas release continued as levels ranged from 500 ppm at 3 m (10 ft) to 800 ppm at the surface. In the recirculation bin, the two blowers were operated continuously for the first 73 hours and then shut off for the remainder of the fumigation. After 93 hours and 11.4 gas exchanges, gas release appeared to be leveling off, with a difference of 225 ppm between the highest reading, 1100



451

ppm (at 12.2-m or 40-ft) depth and 825 ppm at 3-m (10-ft) below the surface. Based on CLF (phosphine recirculation) management guidelines (Noyes et al., 1998), the blowers were shut off at about the right time, just before gas readings peaked. If the blowers had not been shut off, it is estimated that the 7-day gas readings in the grain mass may have dropped substantially lower. In the non-recirculated bin, lethal gas levels did not develop at the 3- and 6-m (10- and 20-ft) grain depths until the fourth day. If the fumigation had ended after 7 days, insects in those sections of storage would have been inadequately exposed. Gas release in the aeration duct continued to build, peaking at 2100 ppm after 67 hours, then leveling off at 1125 to 1000 ppm for the balance of the test. From a review of the head-space and fan readings, it appears that the non-recirculation bin had roof leaks due to the relatively low head-space readings but high fan readings. In contrast, the recirculation bin fan readings dropped off sharply between the 93- and 162-hour readings. An average of the 7-day readings showed a mean value of 454 ppm in the non-recirculated bin compared to 435 ppm in the recirculated storage. However, the loss of gas in the recirculation bin may have been substantially reduced by shutting off the blowers after 2 to 3 days and four to six gas exchanges. Even though the gas levels increased after 2 days in this example, with higher gas exchanges recommended in CLF designs, recirculation should be discontinued after 24 to 36 hours. The non-recirculation bin losses may have been less because of the slow release of the gas with pellets probed into cool grain. The fact that the gas was not recirculated means that it was not forced to flow past leak points in the head-space such as leaking roof joints, vents, and downspouts, and past recirculation fan and aeration fan leaks. Results of comparative tests demonstrated that relatively uniform gas readings were reached in all parts of the bins using recirculation (Table 8.13). Gas quickly reached lethal levels throughout the recirculation bin at least three days earlier than in the probed, non-recirculated bin. From a concentration and time (C × t) standpoint, in recirculation systems that are properly sealed, a better distribution of C × t is achieved at all depths of the bulk compared with conventional probe fumigation. When using recirculation, gas levels should be monitored periodically at key levels (at least mid-point) in the grain mass as well as the aeration ducts and bin head-space. If the head-space gas level drops below 100–200 ppm, a short period of 1 to 2 hours (¼ to ⅓ of one air exchange) of recirculation blower operation to redistribute gas and increase concentrations in top grain levels, head-space, and aeration ducts might be desirable to maintain adequate gas levels in all parts of the structure. This may require the CLF fan to be operated once per day, or it may be 2 or 3 days between recirculation fan operations. 8.2.3.8 Piping Designs for Multiple CLF Steel Bin System Models In 1988 Oklahoma State University (OSU) researchers inspected the steel bin phosphine recirculation system at the Fairfax Elevator, Kansas City, Kansas, and began designing CLF installations for use on several steel storage bins in Oklahoma in 1988 and 1989 (Noyes et al., 1989). CLF systems were installed at several Oklahoma elevators from 1989 through 1993 as demonstration projects funded by the Oklahoma Department of Agriculture (ODA) and Region VI, U.S. Environmental Protection Agency (USEPA). Kenkel and Noyes (1993) and Kenkel et al. (1994) developed design and economic data on CLF system installations and operations during the OSU project. New methods of installing CLF piping and blowers were designed into these commercial grain elevators. CLF demonstrations increased operating flexibility and reduced installation and operating costs. These new CLF system designs used one blower to recirculate phosphine gas simultaneously through two or more large steel bins. Portable CLF blower mounting bases allowed the blower to be moved from one elevator facility to another by grain companies or by commercial fumigators. OSU scientists recommended that to obtain effective insect control at a grain storage site, all grain storage volumes should be fumigated simultaneously. In a CLF system, airflow rates can vary from 0.048 to 0.36 (m3/h)/tonne (0.0008 to 0.006 cfm/bu), a 7.5:1 ratio of airflow between maximum


452

Figure 8.9


Two 5000-tonne steel bins operate on one 0.75-kW blower.

and minimum airflow rates (Cook, 1980). This flexibility in gas flow rate allows several bins to be manifolded to operate as a single unit with one CLF blower. On 20- to 30-m (70- to 105-ft) diameter steel bins with multiple aeration fan and transition positions, a CLF piping system design using one blower connected to each aeration fan duct on multiple bins is the preferred piping method. Where two, three, or four steel bins are close together, one blower can be manifolded with suction and pressure piping to circulate gas to all bins simultaneously or to any combination of bins. Empty bins can be disconnected from the system. Flexible tubing can be used to connect bin manifolds where bins are 5 to 10 m (16 to 33 ft) apart. These tubes can then be removed between fumigations for service vehicle traffic. Getting a uniform gas distribution is more difficult in large-diameter bins than in tall silos. Although lower gas recirculation rates may be successful, to obtain uniform concentrations within 24 to 36 hours, recirculation rates for well-sealed silos and bins should be in a recommended range of 0.18 to 0.48 (m3/h)/tonne (0.003 to 0.008 cfm/bu). If bin sizes in a manifolded group vary significantly, it may be desirable to use metering orifices on lateral piping to each bin to improve gas distribution. The piping design used for distribution of phosphine by Winks and Russel (1996) included metering orifices in each lateral pipe. OSU researchers designed multiple bin installations to use one blower at facilities where storage bins are close together to minimize CLF installation costs. Gas recirculation in two 5000-tonne (185,000-bu) bins in central Oklahoma (Figure 8.9) was provided by one 0.75-kW (1-HP) blower (Figure 8.10) delivering 1176 m3/h (700 cfm) or 0.12 (m3/h)/tonne (0.0019 cfm/bu) — a suitable recirculation rate. Figure 8.9 shows that the suction pipe from the roof head-space is connected to the inlet of the centrally located blower. Pressure pipes line up with two aeration fan ducts for minimum piping. If one bin is empty, that bin is disconnected from the blower and gas is circulated only to the filled bin. These two bins were connected with 15-cm (6-in) PVC suction from the bin head-space to the blower inlet, and 10-cm (4-in) PVC pipe distributed the gas back to two aeration fan ducts on each bin (Noyes et al., 1998). Multi-bin models now use 10-cm suction and pressure pipe for improved installation economy, as 10-cm piping is adequate for the gas flow from individual bins. The airflow vs. pressure rating of the 0.75-kW (1.0-HP), model PB-10A blower is listed in Tables 8.11 and 8.19. At 3 inches static pressure, this blower delivers 1070 m3/h (637 cfm). When the blower is operated on both bins, a 10-cm (4-in) diameter suction pipe would be sufficient. Each suction pipe would handle about 534 m3/h (318 cfm) with a pipe velocity of about 1113 m/min (3650 feet per minute) or 18.5 m/sec (61 feet per second). Use of higher velocity and reduced gas flow is considered negligible when compared with the higher investment cost for larger diameter pipe. When the system is operated with only one bin, higher gas velocities and additional friction from elbows and tees increase the static pressure from



Figure 8.10

453

A 0.75-kW centrifugal CLF blower connected to 15-cm suction pipes from the roof head-space and 10-cm pressure pipes connected to two aeration fan ducts of two 5000-tonne bolted steel bins.

0.75 kPa to 1.25 to 1.5 kPa (3 inches w.c. to 5 to 6 inches w.c.). This pressure increase will result in reducing the blower capacity from 1070 m3/h (637 cfm) to 715 to 840 m3/h (425 to 500 cfm). Using 10-cm PVC pipe, at 755 m3/h, the pipe velocity will be about 1575 m/min (5170 fpm) or 26.2 m/sec (86 fps), acceptable for both suction and pressure piping. With both bins in use, the airflow rate for 10,000 tonne (370,000 bu) at 1070 m3/h (637 cfm) is 0.11 (m3/h)/tonne (0.0017 cfm/bu), one air exchange in 4.9 hours or about 5 air changes per day. With only one bin in use and blower delivery reduced to 755 m3/h, the airflow rate is 0.14 (m3/h)/tonne, one air exchange in 3.5 hours, or 6.8 changes per day. For either one or two bins, with fast gas release at warm ambient and grain conditions, a fan operation of 1.0 to 1.5 days should be sufficient to develop initial uniform gas levels. Head-space and fan duct gas levels should be monitored so that CLF blowers can be operated again when gas levels in either area drop to 100 to 150 ppm. A second management strategy is to operate the CLF blower long enough for about one air exchange (in this case, 3.5 to 5.0 h) once per day or about 15 to 20% of the time starting the third or fourth day of the fumigation. A third approach is the use of a cycle timer that turns the fan on for 1 hour and off for the next 2 to 4 hours. This helps keep gas levels more uniform while minimizing leaks. Tables 8.9 and 8.10 list pipe inside diameter (ID), cross-section area (A), and acceptable duct velocities for manifold piping normally used in CLF systems. The blowers listed in Tables 8.11 and 8.12 have 20-cm down to 10-cm (4-in) inlets and outlets. After selecting the recirculation rate, friction loss for an existing pipe system can be determined from Table 8.9 or 8.10. If a new installation is designed, pipe diameter may be kept at a standard size such as 10-cm or 15-cm diameter pipes. Since larger pipes are more expensive, the reduction of friction loss by using larger pipes should be weighed against using a blower that will have sufficient static pressure to recirculate the gas at the desired rate. For most applications, even with silos of about 15,000 m3, recirculation pipes of 10- or 15-cm diameter have been found to perform satisfactorily. The blower output can be determined, and a blower model can be selected from Table 8.11 or 8.12. To check a blower operating in a CLF system, the blower operating point can be obtained by combining inlet and outlet pressures to obtain total static pressure for use in the tables. 8.2.3.9 Design of Phosphine Recirculation in Silos The first concrete silo fumigation recirculation system observed in the U.S. was an 815-tonne (30,000-bu) concrete silo at Fairfield Elevator, Kansas City, KS (Noyes et al., 1989). This demonstration unit used one 0.063-kW (¹⁄₁₂-HP) blower that delivered about 254 m3/h (150 cfm), or


454


0.31 (m3/h)/tonne (0.005 cfm/bu) through 30 m (100 ft) of wheat. This airflow resulted in one air exchange in 1.67 hours or 14.4 air changes per day. In the summer of 1988, a CLF system was designed and installed in four 9.1-m (30-ft) diameter × 40-m (130-ft) high, 2000-tonne (75,000-bu) concrete silos at an elevator in western Oklahoma. A 10-cm (4-in) diameter PVC pipe was installed inside each silo from the silo head-space to the silo hopper. The outlet of a 0.063-kW (¹⁄₁₂-HP) CLF blower with explosion-proof motor and wiring rated at 280-m3/h (165 cfm) at 1 kPa was connected to the vertical pipe inside the head-space. Placing the blower in the head-space eliminated the need to penetrate roof or sidewalls with suction and pressure piping to reduce installation labor costs. The PVC circulation pipe was clamped against the inside silo ladder side rail at 1- to 2-m intervals to withstand grain flow pressures during filling and unloading. This CLF system delivered 0.14 (m3/h)/tonne (0.0023 cfm/bu), 3.6 hours/air change, 6.7 air changes per day. Many concrete silos at U.S. elevators that were built in the 1950s and 1960s were typically 5to 6-m ID by 30- to 35-m height. A continuous concrete pouring process was used, usually with two parallel rows of full-sized silos to build groups of silos called annexes. Smaller diamond or star-shaped interstice bins were formed between each group of four full-sized cylindrical silos. To provide head-space ventilation to all the silos, including star bins, rectangular 10-cm × 20-cm (4-in × 8-in) under-roof vent openings were formed as notches at the top of all walls, directly below the roof slab. Between most roof deck slabs and the top surface of the silo walls there is an irregular gap that is usually unsealed. Both the roof deck/side wall gap and the exterior under-roof vents must be sealed to secure the silo annex head-space cavity. To reduce cost of installation in concrete silos, Noyes et al. (1998) designed CLF systems that manifold-connected two to twenty or more silos as one storage unit for fumigation. In a CLF silo system design, fifteen 1000-tonne (36,700-bu) silos manifold-connected to one CLF blower are equivalent to one 15,000-tonne (550,000-bu) steel bin or three 5000-tonne (185,000-bu) steel bins. The suction manifold design for concrete silos was relatively simple. It consisted of sealing all exterior under-roof vents with expandable foam insulation while leaving all interior under-roof vents open as gas accumulation passages. Then the suction pipe from the head-space of any convenient silo in the manifolded group was connected to the CLF blower inlet. Three head-space suction pipe entry options were developed: 1. Manhole cover — cut a hole through a steel or cast-iron manhole cover and silicone seal a piece of PVC pipe through the holes or weld a 10- or 15-cm steel pipe nipple to the lid that can be connected by flex hose to the vertical blower suction pipe. 2. Roof deck — cut a 10- or 15-cm hole through the roof deck of the selected silo and seal a piece of PVC pipe through the opening. Connect to vertical pipe with a flexible hose. 3. Exterior under-roof wall vent — firmly connect a 10-cm suction pipe (PVC or steel) with a 90° elbow against the vertical reinforcing bars in the exterior vent of a silo (preferably one with a vertical outside ladder from the silo roof deck to the ground, so the vertical pipe can be clamped to the ladder side rail) and foam seal the vent around the PVC pipe. A steel plate (that is slightly smaller than the rectangular vent) with a pipe nipple or coupling welded to the plate can be bolted to the rebars, then sealed with silicone around the edges.

The bottom of the suction pipe is connected to the blower inlet at ground level. Alternatively, the blower can be mounted on the silo roof deck, and the pressure pipe can be run to the pressure distribution pipe manifold at ground level. Where ladders are available, suction pipes are attached to the side rails of exterior ladders down the outside of a silo to bucket elevator legs or to interior ladders beside manlifts. If these options are not available, the pipe is installed down the side of a silo mounted with clamp brackets bolted into the concrete at 3- to 4-m (10- to 14-ft) intervals. Some elevator managers prefer a rigid, permanent pipe connection for the suction and pressure pipe manifolds. Others like the idea of storing flexible hose manifolds, which can be quickly



455

connected to permanent pipe adapters to the silo head-space and to a shorter manifold pipe system on the pressure side. Except for mounting the vertical suction pipe to the concrete silos when no external ladders are available, the pressure pipe manifold is the most difficult part of designing and building CLF systems in concrete silos. Unless the silos have an aeration manifold system around the outside of the silos, the pressure pipe must be connected individually to the base of each active silo in the group. If some star or interstice bins are not used, they do not need to be connected to the manifold because the phosphine gas passing across the top of the empty interstice silos fumigates them. Four types of CLF manifold base connections are used that depend on the design of the silo annex: 1. External silo aeration pipe manifolds connected to exterior and some interior silos to which the CLF pressure pipe can be connected. 2. Discharge spouts from the bottom of each silo that discharge to a central belt conveyor or drag conveyor to which small lateral pipes can be connected. 3. Exterior CLF pipe manifold around the outside of the annex and connecting a 5-cm (2-in) hose or pipe lateral to a hole cut in the base entry manhole cover on the outside of each silo base. 4. CLF manifold connected to the inlets or transitions of individual silo aeration fans.

At some installations, a combination of these four connection methods may be needed, and possibly connections to the sides of downspouts of over-driveway bins. Sealing the slide gates and discharge spout outlets in tunnels of concrete elevators is a major factor. Silicone caulking is a primary resource for sealing rack-and-pinion slide gates. Some outlet spouts in tunnels and driveways may have to be bagged and taped below the rack-and-pinion gates using large 6- to 8-mil thick gas-impervious plastic bags. Tunnel ventilation fans should be run continuously during fumigation as a precaution to make sure that phosphine leakage into the tunnel does not build up to a dangerous concentration level. Although some facilities were built using 15-cm (6-in) PVC pipe for the suction piping, 10-cm (4-in) pipe is adequate for most installations that use 0.75-kW (1.0-HP) or smaller CLF blowers. A 10-cm ID pipe has a cross-sectional area of 78.5 cm2 (12.6 sq in or 0.088 sq ft). The 15-cm (6-in) ID PVC pipe, with a cross-section area of 176.72 cm2 (28.27 sq in or 0.196 sq ft), reduces gas velocities to about 20 m/s (66 ft/s) for a 0.75-kW blower capacity of 1344 m3/h (800 cfm). Pneumatic conveying design duct velocities of 18 to 24 m/s (60 to 80 ft/s), typical for carrying bulk products, are acceptable for gas recirculation designs. CLF manifold systems that connect from 3 to 17 concrete silos that operate as one storage volume using one CLF recirculation blower have been installed in Oklahoma and Texas grain elevators. These concrete facilities with combined grain volumes of 5000 to 10,000 tonnes (185,000 to 370,000 bu) are fumigated simultaneously with one blower. This is equivalent to the volume of one or two 5000-tonne (185,000-bu) steel bins. Blowers are moved from site to site to minimize capital investment. Results have been documented on operating procedures and costs. A CLF system has been designed for a U.S. elevator to fumigate four rows of hexagonal-shaped silos containing 54 bins totaling 1.25 million bushels of storage with one 2.25-kW (3-HP) blower. This 54-bin quadrant design constitutes ⅓ of one section, or ¹⁄₁₂ of a 400,000-tonne (15-million bushel) concrete elevator annex. Silos built as an annex with parallel rows of silos are more economical to manifold with several silos as a unit than to install CLF systems on each individual silo. Figure 8.11 and Figure 8.12 illustrate two CLF systems in double-row concrete silo annexes. Figure 8.11 shows a top view of parallel rows of concrete silos with gas flow patterns through interior under-roof vents with the suction piping connected to only one silo through a roof inspection manhole cover. The side view shows the CLF system suction pipe mounted beside an exterior ladder and the pressure pipe connected to the silo manhole access opening plates near the base. Figure 8.12 shows a CLF system


456

Figure 8.11


Top view of a CLF system in a double-row concrete silo annex connected to a roof inspection manhole cover. Side view shows the pressure pipe connected to the silo manhole access opening plates near the base.

installed through an exterior under-roof vent that has been sealed. The side view shows the CLF system suction pipe mounted beside an exterior ladder and the pressure pipe routed into the unload conveyor tunnel, where a piping manifold connects to each silo discharge spout above the rackand-pinion slide gate. In the top view of both figures, the interior under-roof connecting vents are used as ducts for gas flow between silos. Also, the suction pipe is connected to one silo through a roof manhole cover (Figure 8.11), or through an exterior under-roof vent (Figure 8.12). In both designs, all other exterior under-roof vents are sealed while all interior vents are left unsealed. The suction piping is positioned so that it can be attached to the side rail of an exterior ladder or other vertical means of support to minimize construction costs such as crane or bucket truck rental. In the second concrete CLF model, the suction pipe is connected to the head-space of the silo through the external under-roof vent; and then the space between the suction pipe and the vent are foam-sealed (Figure 8.12). The pipe must be securely braced so no movement is allowed to dislodge the foam seal of the vent. In this model, the blower outlet pressure piping is routed into the unload conveyor tunnel. A 5-cm diameter flexible pressure hose connects laterally to a thin, perforated sheet sandwiched under the manifold mounted on the side of each silo discharge spout above the rack-and-pinion slide gates.



Figure 8.12

457

Top view of a CLF system in a double-row concrete silo annex connected to an exterior underroof vent that has been sealed. Side view shows the pressure pipe manifold connected to silo discharge spouts in the unload conveyor tunnel.

The flexible hose connection between the pipe and manifold allows for misalignment between the main manifold pipe and the silo spout manifold. Each silo outlet can be shut off when empty to keep gas from short-circuiting by clamping plates across the soft hose. Another shut off design for each silo is to hard-plumb each l5-cm lateral pipe with an inexpensive ball valve. Care should be taken to cap open ends of the pressure and suction pipes when the blower is stored. However, provide a 1.0- to 1.5-mm opening at the bottom edge of the pipe cap or cover to allow condensate to drain from the pipes. Warning: PVC pipe should not be used inside silos due to static electricity that may be generated by direct contact with sliding grain, unless each pipe is grounded by a knowledgeable electrician. Continuous grounding must be installed in PVC plastic pipe to continually discharge static voltage to a positive ground such as metal water piping or steel reinforcing bars. PVC pipe can be used inside concrete elevator basements where grain does not contact the pipe and no static electricity is generated. PVC pipe is a popular duct material for use on the outside of storages due to light weight, chemical resistance, low cost, and ease of fabrication and assembly. 8.2.3.10 CLF Combined with Low Airflow Suction Aeration for Silos The basis for combining a high gas flow rate CLF fan with a relatively low-airflow rate suction aeration system has merit for several reasons. Down-flow CLF will work as well in silos as up-flow


458


if the structure is sealed satisfactorily. Early U.S. concrete silo aeration systems were operated as suction systems at an airflow rate of ¹⁄₅₀ cfm/bu (1.2 (m3/h)/tonne). A cooling rate of 1.2 (m3/h)/tonne requires about 500 to 600 hours of aeration during fall cooling. The suggested rate for suction CLF as an aeration system is 0.60 to 0.48 (m3/h)/tonne (¹⁄₁₀₀–¹⁄₁₂₅ cfm/bu). This airflow rate provides fall cooling in about 1000 to 1500 hours, or about 40 to 60 days of continuous fan operation. A major constraint for adding aeration to silos is installing the required amount of aeration duct surface area for proper air velocity as the air leaves the grain and enters the aeration duct. The recommended air velocity entering or leaving the air duct is about 9 m/min (30 fpm), which provides negligible pressure loss. By using a higher entry velocity, such as 20 to 25 m/min (66 to 82 fpm), with a small increase in static resistance to the fan and using much lower airflow rates, relatively small aeration ducts can be installed in the bottom of concrete silos. This practice should only be used in climates where suitable quantities of cool temperatures are available, not for subtropical or tropical climates except at high elevations. Example 8.11 At 0.6 (m3/h)/tonne (¹⁄₁₀₀ cfm/bu), the airflow for a 540-tonne (20,000-bu) silo would be 5.67 m3/min (200 cfm). The required amount of duct surface area would be 5.67/9 = 0.63 m2 (6.67 sq ft), which would be difficult to add to existing silos. By using an airflow entry velocity of 20 m/min, the required surface area would be 5.67/20 = 0.28 m2 (3.0 sq ft). Assuming that a small, round perforated duct has a 75% effective surface area when laying against a floor or wall, the perforated duct area could be supplied by a 15-cm (6-in) diameter perforated tube approximately 0.77 m (2.5 ft) in length. This size perforated duct can easily be secured on the hopper-sloped bottom of concrete or steel silos. Manifolds connecting the main suction air duct to the individual aeration duct would be similar to those CLF systems operating at typical CLF gas flow rates of 0.2 to 0.1 (m3/h)/tonne (¹⁄₃₀₀ to ¹⁄₆₀₀ cfm/bu). This perforated duct should have a tapered (pointed) top end for good cleanout. Even though the total grain mass would require 1000 hours or more of cooling, the grain where the major part of the insect population resides could be cooled during the first week or two of lowairflow suction aeration with the CLF/aeration system. 8.2.3.11 Flat Storage CLF Systems CLF was installed in two flat storages in Oklahoma in 1998. These two buildings required about 80 man-hours each of labor for sealing. These flat storages ranged from 20,000 to 25,000 tonnes. One unit was very tightly sealed — gas levels by day 2 were 1900 ppm and 7 days later were 850 ppm. The manager reduced the dosage by 50% in 1999. An 80,000-tonne (3 billion bu) flat storage at the Tulsa Port of Catoosa received CLF in 2000. Six 1.5 kW blowers (3 per side) were installed with 6 roof suction pipes. The dosage was applied in 3 hours by dumping pellets in the fill conveyor outside the storage in about 20,000 bu of new grain. The fill conveyor dumped onto the reversible distributor conveyor along the top of the building, which spread the grain and pellets along the 150-meter length of the building. The 20,000and 25,000-tonne storages each used two 1 kW CLF fans, which were used for both flat storages. All three storages had good efficacy with CLF. 8.2.3.12 CLF Installation and Operating Economics For steel bins, construction and installation cost estimates were about $2500 U.S. to $3000 U.S. for a 5000- to 8000-tonne (185,000- to 300,000-bu) steel bin, or about $0.30 U.S. to $0.37 U.S./tonne ($0.008 U.S. to $.01 U.S./bu). Because concrete silo volumes can be combined



Table 8.14

459

Installation Costs (in $U.S.) of a Recirculation Fumigation System at an Oklahoma Country Elevator with Four 5000-Tonne (185,000-bu) Steel Bins in 1992

Two — 0.75-kW (1-HP) TEFC centrifugal blowers @ $420/each 15-cm (6-in) suction and 10-cm (4-in) pressure Schedule 40 PVC piping Roof outlet flashing PVC pipe brackets Miscellaneous hardware (rubber pipe transition boots, bolts, screws, etc.) Aerial bucket truck rental for installation of roof and sidewall piping Millwright labor (construction foreman and electrician)a Total installation costs Cost per 5000-tonne (185,000-bu) bin Cost per bushel

$840 $1,080 $101 $598 $428 $1,411 $1,015 $4,633 $1,158 $0.00626

a

Note: Does not include minor amounts of elevator personnel labor. From Kenkel, P. and R.T. Noyes. (1993). Costs and benefits of installing closed loop fumigation systems in commercial elevators, OSU fact sheet No. 219, Cooperative Extension Service, Oklahoma State University, Stillwater, July, 1993.

but piping is more complex, installation cost per unit volume may be somewhat higher for concrete than for steel. But when one or more grain turning operations are eliminated in concrete facilities, and grain damage and grain dust losses are reduced significantly, the net result for concrete silos can be a payback as fast or faster than for steel, where grain damage by additional handling to fumigate is not a factor (Kenkel and Noyes, 1993; Kenkel et al., 1994). Table 8.14 lists the actual installation costs for a CLF system installed on four 5000-tonne (185,000-bu) corrugated steel grain bins at a central Oklahoma grain elevator in 1992. This installation used 15-cm (6-in) Schedule 40 PVC pipe for the suction pipe connected from the bin roof head-space to the blower inlet, and 10-cm (4-) ID Schedule 40 PVC pipe for the pressure connections from the blower to two aeration transitions per bin. The total installation cost was $4633 for 20,000 tonnes (740,000 bu) of storage. The net cost was $0.23 U.S./tonne ($0.0063 U.S./bu). The elevator manager calculated he paid for the system costs in less than two years from reduced fumigant cost, timeliness of fumigations, and improved efficacy (Kenkel et al., 1994). Using one blower manifold connected to several bins reduced the overall cost of CLF systems. Inexpensive flexible black plastic drainage hose, which costs 25 to 30% as much as Schedule 40 PVC plastic pipe, was used as an economical means of connecting the CLF blower simultaneously to two or more tanks. These design innovations reduced the material costs of installation, making CLF affordable for most elevators. Table 8.14 provides actual installation costs of the CLF system installed on four 5000-tonne (185,000-bu) bolted steel bins, with one 1-HP (0.75-kW) blower for two bins. The CLF installation material list outlined in Table 8.14 is for the four 5000-tonne bin elevator facility shown in Figure 8.9. This figure illustrates the CLF system on two of the four 5000-tonne bins, which demonstrates the use of one CLF blower for two bins. Figure 8.10 shows a close view of the 1.0-HP (0.75-kW) centrifugal blower installed on the motor mount with the CLF blower connected to six-inch suction pipes and four-inch pressure pipes to two 5000-tonne (185,000-bu) bolted steel bins. This size blower can provide adequate recirculation for all four of the 5000-tonne tanks, or 20,000 tonnes. CLF systems on concrete silo facilities with three to 17 silos are installed in Oklahoma and Texas. Based on field experiences to date, capital investment to install CLF systems in concrete silos were calculated (in $U.S.) to range from $0.025/tonne to $0.92/tonne ($0.0007 to $0.025/bu) at mid-1990 prices. Costs to install CLF in the 20,000- to 80,000-tonne flat storages in 1998 and 2000 ranged (in $U.S.) from $0.27/tonne to $0.34/tonne ($0.0075/bu to 0.0095/bu) using in-house elevator labor. CLF operating costs are very low. Costs to run 0.38- to 1.2-kW (0.5- to 1.5-HP) blowers range from about 10- to 30-kW·h per day, or $1.20 to $3.00/day per blower at $0.10/kW·h for electricity.


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A 5000-tonne (185,000-bu) bin with 0.38-kW (0.5-HP) blower costs $3.50 to $10.00 to operate for 3 to 5 days. In comparing fumigation in silos with automatic dispensers to operation of concrete silo CLF systems, shrink losses (from grain dust and moisture losses), conveyor operating costs, and additional labor costs associated with grain turning should be considered as additional costs. Preliminary estimates indicate that CLF systems installed in steel bins will pay back in 2 to 4 years. By eliminating grain turning in concrete silo CLF system operations, payback is usually 2 to 4 years. Private application using CLF that also reduces the cost of commercial application provides additional savings to the elevator. Added benefits of timeliness, labor saving, easier management, and safety are factors that are difficult to assess monetarily, but they provide significant capital advantages.

8.3 BULK AERATION SYSTEMS Cooling of grain bulks that are relatively shallow presents aeration design problems that must be carefully studied. The primary challenge is how to provide adequate airflow distribution throughout a long, wide, shallow grain mass with the unique condition that the grain mass is contained or sealed on all surfaces. The following section provides guidance for aeration system designs for grain bunkers. 8.3.1

Bunker Aeration

Bunkers for grain storage are used routinely in Australia, where grain production is characterized by large seasonal variations in crop size. In Australia, bunkers were originally developed by the Bulk Handling Authorities to address large grain crop variations by providing a low capital cost semi- permanent grain storage option (Connell et al., 1992; Morris, 1984; Yates and Sticka, 1984). Bunkers have also been adopted in countries like Cyprus (Varnava et al., 1995), Turkey (Navarro, 2000), Israel (Navarro et al., 1994), and the U.S. (Siebenmorgen et al.,1989), where the permanent storage capacity of many regions is easily exceeded in favorable years. Grain storage in bunkers is principally based on sealed storage that is suitable for using phosphine fumigation (Banks and Sticka, 1981) or to obtain a self-generated modified atmosphere (Navarro et al., 1984). However, some of these types of storages have been regarded as inferior compared to permanent stores as a result of handling losses, admixing of non-grain matter, water leaks, and storage losses deriving from moisture migration. Diurnal temperature fluctuations, accentuated by solar radiation, and followed by rapid cooling at night cause successive moistening and heating cycles on the upper grain surface. This may result in gradual moisture accumulation and mold development, particularly during the transient seasons between summer and winter when temperature fluctuations are greatest. Thus, during storage dry grain may gradually rise to above critical levels, enabling limited microfloral spoilage to occur. This becomes particularly accentuated along the peaks of bunkers where warm air rising on convection currents tends to concentrate moisture condensation in confined areas of the surface sealed by the tarpaulin (Navarro et al., 1984). Bunker design improvements and better bulk grain storage management have reduced operational and insect damage losses, but little progress was reported about reducing peak grain moisture migration related losses for many years. However, in an attempt to reduce moisture condensation on paddy in the Phillipines and wheat in Israel and Cyprus, Navarro and Donahaye found that rounding the ridge peak was beneficial. Lowering the ridge peak elevation by even 1 to 2 meters before tarping bunkers of various sizes produced significant condensate moisture reduction as indicated by period moisture samples from bunker ridge areas of sharply peaked vs. rounded ridge



Figure 8.13

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Aeration layout for the 1996/1997 chickpea trial bunker at Murtoa, Victoria. (From Darby J.A. [1999]. Bunker aeration-maintenance trial, 1996/97. Technical Rep.No. 84, Nov. 1999, CSIRO Entomology, Canberra, Australia. With permission.)

bunkers during the storage period (Navarro et al., 1994). Their recommendation is that, by rounding bunker surfaces to lower ridge heights by at least 2 meters, moisture condensation is spread over a much wider area, reducing the relative rate of moisture accumulation and thus mold development. Darby (1999; 1998) reported a bunker aeration system and described an initial trial investigating the performance of an aeration-maintenance system installed on a bunker of chickpeas at Murtoa, Victoria. The aeration system demonstrated that a suction-based aeration system could effectively aerate grain stored in a conventional bulk handling bunker (Figure 8.13). A leakage rate approaching 40% of the total aeration capacity was encountered, but the apparent even distribution of leaks across the bunker envelope meant that their influence was not considered a significant factor. The Murtoa trial demonstrated that aeration-maintenance systems can prevent crusting with a moderate aeration airflow rate of 0.15 L/sec/tonne (0.54 (m3/h)/tonne). The moisture migration and condensation processes at Murtoa were eliminated once the grain was cooled by a cooling front. Darby (1999) did not develop the complete design of an appropriate aeration layout in the trial. Positioning of ducts across the equipment entrance area of the bunker proved to be particularly obstructive to front-end loaders. Positioning ducts closer to the bunker walls at the unopened end of the bunker was less obstructive but was still not considered very satisfactory. This preliminary field trial indicated that bunker aeration layouts could include inlet air ductwork positioned adjacent to the bunker perimeter to avoid interference with the central operating thoroughfare of the bunker and not incur leakage problems (Darby, 1999). Aeration is not commonly used in bunker storages across Australia today. According to Darby (1999), it appears that an efficient technical design for bunker dedicated aeration systems has not yet been well developed. A bunker of paddy rice, equipped with aeration in the U.S. in Arkansas, was described by Siebenmorgen et al. (1989). The primary design objective of the aeration system was to “aerate the grain mass during the cooler months to alleviate the problems associated with moisture migration.” A relatively high aeration airflow rate of 1.77 L/sec/tonne (6.37 (m3/h)/tonne) was used to cool 12 to 13% mc paddy. Grain temperatures were reduced to about 10°C. The duct layout consisted of three supply ducts on the base with a return duct running along the ridge. Inducing the air under suction through the grain in the bunker is the most appropriate method of applying aeration to a store with a flexible surface membrane. The suction system holds the bunker cover onto the grain surface to seal the grain pile and so force the air through the grain bulk. Suction is also important to anchor the tarp securely against the grain surface during high winds.


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The aeration layout used in the trials at Murtoa is illustrated in Figure 8.13. An important factor in the bunker aeration system design layout was to determine an appropriate balance between aeration air leakage against a user-friendly operational arrangement. Positioning ductwork with high negative pressures close to bunker walls was unacceptable due to leakage risk, whereas positioning ductwork in the central regions of the bunker obstructs loading operations. The Ysuction duct arrangement was considered a reasonable trade-off for the initial trial (Darby, 1999). 8.3.2

Vertical Aerators for Grain Bulk Peak and Ridge Aeration

Aeration of relatively shallow and circular grain bulk peaks or long ridges in flat grain storage structures requires specialized aeration. As peaks and ridges form under fill conveyors, they tend to build up a high proportion of grain fines, dockage, and foreign material. These small particles fill the interkernel air voids, forming a core of fines that restricts airflow. With this core of fines, combined with the natural tendency for air to take the shortest airflow path directly to the surface, peaks and ridges are very difficult to cool. The following sections describe equipment that can be used to dramatically improve peak and ridge aeration. A relatively new development in aeration design over the last 25 years has been the vertical aeration system, sometimes described as Pedestals. Typically these comprise a 1-m diameter vertical perforated section of duct connected by extendable necks to a centrifugal fan which usually exhausts the air; so the system is often based on suction, although conversion to blowing is easily arranged. These vertical spot aeration systems are basically permanent installations of aeration spears or hot spot coolers (Shove, 1968; Mathlein, 1961; Armitage and Burrell, 1978), but they are able to ventilate larger volumes and greater depths of the stored crop. 8.3.2.1 Portable Spear Aerators Armitage and Burrell (1978) described a small portable spear aerator used for cooling grain ridges and peaks that consisted of a tube of 0.1 m diameter (Figure 8.14). The open end of the perforated tube was connected to the suction side of a small fan. The other end of the tube, which had a spiral ridge (like auger flighting), was formed into a pointed tip. This enabled the tube to be screwed into the grain bulk by rotating the blower end using two handlebar extensions. The aeration spear was screwed into the grain perpendicular to the surface of the bulk through the center of 2-m-square polyethylene sheet. The plastic sheet (Figure 8.15) keeps air from short-circuiting along the tube. The presence of the plastic sheet forces suction air to flow down through the grain 4 to 5 m below the grain surface through grain not covered by the sheet — at least 0.65 m in all directions beyond the sheet. This greatly enlarged aeration pattern spreads the cooling effect much wider and deeper, distributing air more effectively through the grain mass than the spear without the sheet (Figure 8.15). One manufacturer’s recommendations suggest pedestals of 10 to 20 cm (4 to 8 in) in diameter cooling 400 to 500 tonnes of grain each using fans of 0.3 to 2.2 kW. These aerator spears deliver 575 to 2870 m3/h at maximum grain depths of 2 to 10 m. The suggested distance between pedestals varies with moisture contents from below 15% to above 20% and ranges from 3 to 4.5 m for 10–cm (4-in) ducts and 575 m3/h to 7.5-10 m for 20-cm (8-in) ducts at 2870 m3/h airflow rates (Figure 8.16). The capital costs of spot aerator installations are around £2/tonne ($2.8 U.S./tonne) per unit. These low costs and the reduced chance of damaging vertical above-floor ducts compared with horizontal on-floor ducts have contributed to their increased use, particularly but not exclusively in the U.K. 8.3.2.2 Pedestal Aerators Large welded steel or concrete grain tanks with relatively low sidewall heights of 10 to 20 m compared to diameters of 40 to 50 m have a serious problem in providing adequate airflow to the



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Figure 8.14

Screw-in aeration spear for vertical aeration. (From Armitage, D.M. and Burrell, N.J. [1978]. The use of aeration spears for cooling infested grain, J. Stored Prod. Res., 14, 223–226. With permission.)

Figure 8.15

Distribution from aeration of a 40-tonne bin of barley; isobars in cm water gauge (WG). Left view, plastic sheet used; right view, no sheet. • = Sampling positions (same for both views). (From Armitage, D.M. and Burrell, N.J. [1978]. The use of aeration spears for cooling infested grain, J. Stored Prod. Res., 14, 223–226. With permission.)

peak and central parts of the grain mass. When large tanks or bins have a diameter-to-sidewall height ratio of 2.5:1 to 3:1, standard floor-level aeration systems cannot provide adequate airflow to the vertical center or peak region of the mass. Airflow from floor ducts take a more direct path perpendicular to the grain surface, rather than flowing vertically upward through the grain peak. Thus, peak grain does not cool satisfactorily. Vertical airflow to the peak is further restricted due to the high percentage of fines and dockage that accumulates in the grain spout line directly below the bin fill-point when grain spreaders or distributors are not used. This dense core of fines, which may occupy a full-depth cylindrical section of the bin of 5- to 10-m diameter in large bins, partially restricts vertical air movement. Removal of part of the peak by coring the bin — operating the unload conveyor to withdraw grain from the center of the bin — loosens the mass and removes part of the high concentration of fines directly over the unload conveyor slide gate. Although this will improve aeration in the center, coring does not resolve the problem of inadequate air distribution from floor ducts.


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Figure 8.16


Uninstalled portable spot aerator (left), and spot aerator installed on a bulk of a flat storage (right). (From Martin Lishman, U.K. With permission.)

One solution is to provide a vertical pedestal aerator at the center of the bin that provides a vertical aeration duct of sufficient cross-section and height to release the necessary volume of air right at the center of the mass to cool the peak grain. This tall pedestal aerator physically shortens the distance from the duct to the peak. The release of a high volume of air from the vertical duct causes a cylinder of air to move vertically in the center of the mass directly up through the peak grain. The pedestal aerator performs satisfactorily with either suction or pressure aeration systems. A common size of grain storage tank design used by large grain corporations in many countries is a welded steel tank with a 42- to 52-m (138- to 170-ft) diameter, 15- to 20-m (50- to 65-ft) sidewall height, and a 30° roof slope. Example 8.12 An elevator surperintendent must cool grain in a welded steel tank with a 42-m diameter, 15-m sidewall height (2.8:1 ratio), and 27-m bin peak height. The grain peak for the 25° grain surface slope is about 10 m above the sidewall grain intercept. If grain is 14 m deep at the sidewall, the peak height is about 24 m from the floor. To improve aeration from previous years, a 2-m diameter × 7.5-m perforated cylinder pedestal aerator is installed next to the center unload conveyor slide gate (Figures 8.17 and 8.18). Note that the 45° steel cone top of the pedestal aerator is not perforated. As shown in Figure 8.17, the perpendicular distance from the center of the bin at the floor to the 25° grain surface slope is 21 m. The perpendicular distance to the grain surface from the top of the 7.5-m vertical perforated duct is 14 m. The superintendent knows that a significant improvement in aeration airflow through the vertical cylindrical grain mass in the center of this bin can be made by coring the bin, removing about two 1000-bu semi truckloads (62 m3) of grain. This volume of cored grain forms an inverted cone of 8-m diameter and lowers the peak about 4–5 m (to bottom of the inverted cone). This creates a slightly shorter air path to the inverted cone surface than to



Figure 8.17

465

Cross-section of large grain tank with pedestal aerator added to floor ducts.

the main surface slope. Thus, with the peak lowered and fines removed from the core, an adequate volume of air can flow vertically throughout the central part of the grain mass. In the example above, the competing airflow pressure from the two parallel floor ducts passing on both sides of the pedestal aerator causes the airflow from the vertical aerator to assume a slightly elliptical shape. However, the overall result is an excellent airflow rate and pattern in the central peak section of the grain mass. Although grain depths vary along the 7.5-m sidewall height of the pedestal aerator, the depth variation causes only minor changes in static pressure and the airflow along the vertical height of the aerator sidewall. Based on USDA recommendations of 9 m/min entrance or exit velocity from perforated aeration ducts, with 47.1 sq meters of surface area × 9 m/min velocity, the design airflow of the pedestal aerator would be 424 m3/min or 25,440 m3/h. At a design aeration flow rate of 6.25 (m3/h)/tonne of grain, the pedestal aerator would service 4070 tonnes or 5240 m3 of grain. If the average depth of grain serviced by the aerator were 21 m, the vertical cylinder would aerate a cylindrical column of grain with an area of about 250 m2 and a diameter of about 18 m (60 ft). The total loose grain volume (not considering packing or consolidation) of the grain mass in the 42-m diameter tank above is 24,014 m3 (18,742 tonnes). If the pedestal aerator services 5240 m3, the floor duct system would have to aerate the balance, 24,014 – 5240 = 18,774 m3 of grain. At 6.25 (m3/h)/tonne or 4.8 m3/h/m3, the duct airflow system would require an airflow rate of 90,115 m3/h. If the pedestal aerator and floor ducts were supplied by a common air source, the combined airflow to be supplied would be 90,115 + 25,440 = 115,555 m3/h. For new tanks, an under-floor supply duct with openings into the cross ducts at spaced distances across the centerline of the tank should be installed. The grain pressures against the sides and top of pedestal aerators are very high. The internal structure of this vertical cylindrical duct must be ruggedly designed to withstand the high consolidation pressures during storage and high grain forces during loading and unloading. Pedestal aerator structures should be built by experienced grain aeration duct suppliers. The ideal position of the pedestal aerator is at the center of the tank. However, most grain tanks have a center unload gate. To avoid conflict with the unloading system, the pedestal aerator should


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Figure 8.18


Plan view of large grain tank with pedestal aerator and parallel floor ducts supplied by two centrifugal fans. An optional centrifugal fan (as shown) can be added to supply the pedestal aerator if the pedestal aeration system is added to the center of a tank with existing parallel floor ducts.

be offset by 1 to 2 m along the bin centerline beside the center unload slide gate so unloading is not restricted (Figure 8.18). Because of the need for a high airflow rate near the center of the tank, the aerator can be supplied by a horizontal duct from the side of the tank that is perforated. This perforated duct can also continue from the opposite side of the pedestal aerator to aerate along the centerline of the tank to the far wall of the tank. If a perforated supply duct is used, it must be sized with sufficient cross-section to provide the required pedestal aerator airflow as well as the design airflow for horizontal distribution. If an aeration duct continues from the opposite side of the pedestal aerator to the far wall, that extension duct can be much smaller than the aeration duct supplying the pedestal aerator. It should be designed to provide the desired cross-section area and surface area for the airflow to be distributed only for that duct extension. For the additional centerline aeration, the duct must carry 25,440 m3/h for the pedestal aerator, plus the additional airflow for the full-length aeration duct across the tank centerline. The vertical aerator supply fan must also be oversized for the center line aeration duct.

8.4 SUPPLEMENTAL PROCESSES AND SYSTEMS THAT IMPROVE AERATION Some grain managers use grain spreaders to distribute fines and trash and to level the grain to enhance aeration. As an alternative to using spreaders, coring bins removes part of the fines, dockage, and foreign material that accumulates in the spout line in line with grain flow from the fillspout or conveyor discharge. It also lowers peak height and reduces the grain depth that air must penetrate. Removing part of the core of fines loosens the grain mass, reduces restrictions to airflow, and eliminates part of the ideal insect habitat. Besides making grain less attractive to insects, grain cleaning enhances aeration by removing broken pieces of grain, dust, trash, and foreign material (f.m.), which plug the interstice air passages and restrict airflow. Blending grain can enhance aeration by lowering the average grain moisture to a drier, safer level, thereby reducing the need for trying to use aeration to remove grain moisture.



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Roof vents, roof exhaust fans, temperature monitoring systems, and static pressure readout units are also important supplemental grain management tools that support and enhance aeration. 8.4.1

Coring Bins and Silos

Because of their small particle size and low mass, broken grain kernels and particles of grain, or fines, and some trash and small foreign material particles (f.m.) have shorter trajectory and higher surface friction than whole kernels when transferred into bins and silos. Whole kernels with more mass have a higher energy and trajectory, which allows them to bounce on impact with the grain surface and slide down the sloped surface of the grain peak. By contrast, the much smaller fines drop near the center of the grain flow impact area and settle between whole kernels, forming a vertical cylindrical zone of high concentration under the point of impact with the grain surface. This cylinder of dense grain and fines extends from the bottom of the grain mass to the center of the peak at the surface. Without using a grain spreader or distributor to spread grain fines and level grain surfaces, a high percentage of all broken kernels and fines in a grain mass tends to settle at or near the center of the storage in round steel bins and tall concrete or steel silos. The concentration density of this cylinder or core of fines is highest at its center and gradually reduces in density with increased distance from the center of the core. This core of fines varies in diameter with different grains, storage structures, and handling systems; but the diameter of the core that is difficult to aerate in most structures is estimated to be between 1.5 m and 3 m in diameter. Leveling large steel bins by hand is impractical, but coring, operating the unload conveyor which draws down this center core of small grain, fines, and dockage to lower the peak height, provides much of the benefit of leveling grain in steel bins. However, coring to partially level grain surfaces in tall, slender silos with height-to-diameter ratios of 5:1 to 7:1 (6- to 8-m diameter vs. 30- to 40-m height) or in low walled flat storage structures is not practical. During normal unloading or coring, if the fill-point is directly over the outlet of the unload conveyor, the grain drawn from the bin by the unload conveyor causes vertical grain flow-down through the center core of the bin. This creates an imaginary cylindrical column of grain that flows downward, removing part of the fines core as grain flows into the unload conveyor. This flow creates an inverted cone or funnel in the grain surface that gradually increases in diameter. As unloading continues, the grain on the inverted cone slopes gradually sloughs off and slides into the bottom of the cone, where it funnels down the center flow path to the conveyor or gravity spout inlet (Figure 8.19). Bins can be cored to check bin or silo grain moisture profiles by running the unload conveyor just long enough to pull a full core sample from top to bottom. This requires about 45 to 90 seconds, depending on bin depth and conveyor capacity. Use confetti as a marker material to show when surface grain is discharged. However, there is a major difference in withdrawing a small sample of grain by removing a small vertical cylinder in the center of the core of fines, compared to coring the bin to remove major portions of the total core of fines. When coring for the purpose of cleaning the grain, a significant part of the grain peak must be removed, either periodically, using multiple cores (one to two times daily) during filling (Figure 8.19), or after the bin is completely filled using a single peak draw-down process (Figure 8.20). Coring to clean the grain may require 30 to 60 minutes of conveyor operation instead of 1 to 2 minutes to sample the grain profile from the center of the core of fines. During coring, a grab sample large enough for a moisture meter test should be taken at about 5-second intervals. Seal each sample in a sealable plastic bag to test for moisture and develop the moisture profile at all levels in bins or silos. The preferred method of coring the bin is done while the bin is being filled (Figure 8.19). The unload conveyor should be operated periodically, such as once or twice per day, after every 6 to


468


Figure 8.19

Coring a grain bin to remove the center core of f.m. and dockage by operating the unload conveyor after each 1 to 2 meters of fill. (Redrawn and adapted from Noyes, R.T. and P. Kenkel. [1999] Storing moist wheat at commercial elevators in Oklahoma, OSU Current Report, CR-1741, Cooperative Extension Service, Oklahoma State University.)

Figure 8.20

Coring after the bin is filled reduces peak height by ⅓ to ½, improves aeration through the center of the mass, and reduces cooling times. This is beneficial but still less effective than coring during filling. (Redrawn and adapted from Noyes, R.T. and P. Kenkel. [1999] Storing moist wheat at commercial elevators in Oklahoma, OSU Current Report, CR-1741, Cooperative Extension Service, Oklahoma State University.)



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8 hours of fill or after every meter or two meters of grain depth is added (Figure 8.19). Coring during initial filling removes a major part of the fines and f.m. to improve aeration management because a new funnel and draw-down cone is formed each time with a diameter of 2 to 3 meters across the top for each additional 1- to 2-meter fill layer. Removing this inverted cone periodically removes a large portion of the fines in a bin. This grain should be cleaned, then transferred back into the storage unit at the end of the filling operation as the final fill. Using this process, the final peak can be clean grain with very few fines. Coring after the bin is filled (Figure 8.20) removes some f.m. and grain fines from the spout line, but much less than the amount of fines removed by coring while filling as shown in Figure 8.19. However, lowering the peak and reducing the dense pack of fines loosens the center of the grain mass and improves aeration airflow. When coring a steel bin after filling is complete, remove about one third to one half the bin diameter or peak height as shown in Figure 8.20. An estimate of coring time can be calculated by using the core diameter as the average of the length and width dimension of the unload conveyor hopper in steel bin floors or gravity spout openings in silos. If the bin or silo has a 30-cm by 45-cm (12-in by 18-in) unload hopper, the average dimension is 37.5 cm (15 in); so a 37.5-cm (15-in) core diameter could be assumed. For a 30-cm (12-in) square unload conveyor hopper or spout, the round grain core can be estimated at 30-cm (12-in) diameter. The equation for the volume of a cylinder can be used to calculate approximate core volumes. Vc = 0.785 × ( Dcr ) × h 2

where: Vc = 0.785 = Dcr = h =

(8.15)

volume of cylinder the coefficient for area of a circle = π/4 diameter of the core height of core sample

Example 8.13: Estimating Core Volumes Assume a 24-m (78-ft) diameter steel bin has 15-m (50-ft) sidewalls, a 20-m (65-ft) grain peak height, and a 37.5-cm (15-in) square unload auger hopper. Assume a 37.5-cm (15-in) diameter grain core, and determine the volume per m (and per ft) of core. Using Equation 8.15 for metrics, Vc = 0.785 (0.375 × 0.375) × l = 0.11 m3/m. For English, Vc = 0.785 (1.25 × 1.25) × 1 = 1.23 ft3/ft or about 1.0 bu/ft. Another important consideration in coring is what volume of grain should be removed to improve aeration. A recommended grain removal is to form an inverted cone with a base diameter (top of the inverted cone) about half the bin diameter. This places the bottom of the cone level with the surface grain slope at the sidewall. The formula for a cone is: Vcn = where: Vcn = cone volume in m3 (or ft3) hcn = height of cone in m (or ft) Dcn = diameter of cone in m (or ft)

0.785 h × Dcn2 = 0.2618 h × Dcn2 3

(8.16)


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Example 8.14: Calculating the Volume of Grain Removed while Coring Peaked Bins A 20-m (65.6-ft) diameter bin is filled with wheat that has a peak height, h = 4 m (13.1 ft) and a 22° grain slope. How much wheat must be transferred from the bin by operating the unload conveyors to form a cone of half the peak diameter? Using Equation 8.16, the bin peak volume = Vcn = Vp = 0.2618 × 4 (20)2

(

Vp = 0.2618 × 4 × 400 = 0.2618 × 1600 = 418.9 m 3 14, 793 ft 3 or 11,887 bu

)

The diameter (Dcn) of the inverted cone with half the peak height is half the bin diameter, or 10 m. To determine the volume of wheat to be transferred to reduce the peak height by half, use Equation 8.16 with 0.5 D = Dcn. As the peak is drawn down, an inverted cone forms. When the peak height is reduced in half, the bottom of the inverted cone should be even with the grain surface at the sidewall. The total grain volume removed is double the amount calculated for a simple cone using Equation 8.16, since the peak and inverted cone volumes are essentially equal. Grain volume to lower peak by 50% = 2V0.5p = 2 × 0.2618 × 2(10)

(

2V0.5 p = 2 × 0.2618 × 200 = 104.7 m 3 3700 ft 3 or 2974 bu

2

)

Reducing the peak height by half removes only 104.7/418.9 = 0.25 = 25% of the peak volume, and only reduces the tall steel bin volume by 2 to 3%. If it were desired to reduce the peak by 33% (or some other percent), the same procedure is used; but the inverted cone base diameter would be 33% instead of 50% of the bin diameter. 8.4.1.1 Coring Improves Aeration in Steel Bins and Silos Most elevator managers and producers fill steel bins to the peak. Peaked grain is hard to manage. It is hazardous when grain is stored above safe moisture levels (12.5 to 13.0% moisture content for most grains). It is especially important to core and level bins filled with moist grain where aeration is more critical, since few large steel storage bins have powered grain spreaders to level the surface and spread the fines and trash. Removing the dense core of fines, broken kernels, dust, and trash that impedes airflow loosens the center, making vertical air movement easier. About ¼ to ½ of the peak height in all bins should be removed by coring the bin when grain spreaders or distributors are not used. Removing part of the dense core and reducing the peak height by ¼ to ½ reduces aeration cooling times by 20 to 30% compared to the hours of cooling required to cool the grain with the complete peak. WARNING: Moist grain respires at a higher rate than dry grain. During respiration, oxygen (O2) is converted to carbon dioxide (CO2), which can create a safety problem for workers entering storage. Extreme caution should be used when entering bins containing moist grain. Bins with moist grain should be checked for low oxygen or high carbon dioxide levels. Operate the aeration fan system for an hour before entry to exchange the air in the grain and head-space with fresh air. If bin entry is made to add confetti markers or probing grain for moisture samples, use the buddy system with the inside man wearing a safety harness and rope in case of a cave-in or bridging.



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Example 8.15: Coring Steel Bins Assume the 24-m (78-ft) diameter and 20-m (65.6-ft) peak height steel bin in Example 8.13 has a 37.5-cm (15-in) square unload auger hopper with a 210 m3/h or 3.5 m3/min (6000 bph or 100 bu/min) unloading rate. Assume a 37.5-cm (15-in) diameter grain core, with a core volume of 0.11 m3/m (1.0 bu/ft). Calculate the velocity of grain flowing down the core to the conveyor. At 20 m (65.6 ft) peak height, handling 3.5 m3/min (99.3 bu/min), core grain velocity = 3.5 m3/min/0.11 m3/m = 31.8 m/min (99.3 bu/min/0.952 bu/ft = 104.3 ft/min). The core flow time, metric units: 20 m = 0.63 min; 0.63 min × 60 sec min = 37.8 or 38 seconds 31.8 m min The core flow time, English units: 65.6 ft = 0.63 min; 0.63 min × 60 sec min = 38 seconds 104.3 ft min During coring, when the full 20-m (65.6-ft) depth of core is removed, at a core volume of 0.11 m3/m (1 bu/ft), about 2.2 to 2.3 m3 (65 to 70 bushels) will be removed. This grain can be cleaned and recycled back into the same bin. To determine precisely the core volume for a given bin, a marker material such as a heavy confetti (so it won’t float on the vortex funnel surface) should be mixed in the surface grain on the peak so that it feeds immediately into the inverted cone vortex. Then the total volume of grain can be transferred into a truck or wagon and weighed. The important factor is knowing the time when the surface grain has reached the bottom of the bin or silo to establish that the core has been uniformly sampled. In Example 8.15, the grain coring/sampling period starts as soon as grain starts to discharge (it takes 5 to 7 seconds for the core grain to reach the conveyor discharge) and continues for about 40 seconds. If samples are pulled at 5-second intervals, 9 samples from top to bottom will be taken at 2.2-m (8-ft) intervals. Example 8.16: Coring Concrete Silos Base grain flow rates on tunnel belt and leg capacities: assume a 6-m (20-ft) diameter, 40-m (130-ft) grain depth, unloading at 168 m3/h or 2.8 m3/min (4800 bu/h or 80 bu/min) through a 35.6-cm (14-in) gravity spout. Determine the vertical grain velocity and the core volume during coring. Metric Solution Assume a 35.6-cm diameter grain core, volume/m = 0.785 × 0.356 × 0.356 × 1 m = 0.0995 m3/m, or about 0.10 m3/m. Vertical grain velocity is 2.8 m3/min/0.1 m3/m = 28 m/min. For a 40-m grain depth, the core flow time is 40 m/28 m/min = 1.42 min; 1.42 min × 60 sec/min = 85.2 or 85 seconds. The core volume = 0.1 m3/m × 40 m = 4.0 m3 can be cleaned and recycled into the same silo.


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English Solution Assume a 14-in or 1.17-ft diameter grain core, volume/foot = 0.785 × 1.17 × 1.17 × 1 = 1.08 cu ft/ft; 1.08 cu ft × 0.8 bu/cu ft = 0.86 bu/ft. Vertical grain velocity is 80 bu/min/0.86 bu/ft = 93 ft/min. For 130-ft grain depth, the core flow time is 130 ft/93 ft/min = 1.4 min, or 60 sec/min × 1.4 min = 84 seconds. The core volume = 0.86 bu/ft × 130 ft = 111.8 or 112 bu can be cleaned and recycled into the same silo. 8.4.1.2 Bin and Silo Fill and Discharge Offset Problems The grain flow filling a steel bin or silo quite often enters the storage structure at an angle due to downspout discharge slopes or from the horizontal belt conveyor trajectory. When grain flow is not vertical and centered in the structure, the core of fines often develops in a skewed or slanted position. Because of the entering grain trajectory, the cylindrical column of fines and trash tilts toward the fill-point; so near the grain peak of full bins, the core of fines is near or at the center of the structure. But the bottom of the skewed cylinder of fines is not centered over the center unload point in the bin or silo. In such cases, the small diameter cylinder of discharge grain pulled into the unload conveyor may not pull fines from the column of fines until near the grain surface. A solution to this problem that enhances coring results may be to install a vertical section of discharge spouting. This spouting will cause grain to change direction and direct grain flow vertically downward in the center of the structure, directly over the unload conveyor hopper. Concrete silos built in parallel rows are usually unloaded onto a conveyor belt or drag conveyor in a tunnel with the conveyor centered between the centerlines of the two rows of adjacent silos. The silos usually have one-way sloped hopper bottoms with square spouts that discharge directly over the centered tunnel conveyor. Because the spout openings are at the lower sloped edge of the silo hopper bottom, the grain flow vortex is not centered in the middle of the silo. Although it is obvious that grain flow from silos with offset discharge spouts does not flow as a symmetrical concentric vortex down the center of the silo, little research or field experience is available on how the grain actually flows. Some practitioners postulate that these silos unload as plug flow — grain flowing from the bottom hopper with no vortex or funnel flow as occurs in concentric center unload flat-bottom structures. Thus, coring may not be as effective in concrete silos that are not filled or unloaded from the center as in storage units that are center filled and unloaded. But when concentric coring is possible, it can enhance aeration and storage quality of the grain.

8.5 CLEANING GRAIN TO IMPROVE AERATION Some aeration research engineers argue that cleaning grain decreases the aeration airflow rate compared to grain with high levels of dockage and f.m. Their logic is that cleaning makes the grain mass more dense, therefore the “porosity” of the grain must be lower. Cleaning lightweight trash, dockage, and f.m. from grain increases the test weight of grain. But higher test weight and denser grain does not equate to lower porosity or less interstitial air space. On the contrary, lightweight trash and dockage that partially fills kernel interstitial air space can impede aeration airflow, compared to clean, higher test weight grain. Light trash usually slides down the grain slope and accumulates along the walls of steel bins and flat storages. Small broken grain particles and weed seeds tend to fill up the void space between kernels toward the center of the grain mass below the spout line, fill line, or central core of the grain mass. These small particles block air movement through the core, resulting in slow cooling or no cooling. Storages with trashy grain may have less back pressure and higher fan output than the



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same bins where the grain has been cleaned; but the bins with trash may suffer from short-circuiting of air around the outside of the grain mass and lower airflow through the center of the mass due to the core of grain fines, dockage, and f.m., resulting in reduced cooling in the center of the bulk. Grain with a high percentage of fines develops steeper peak grain slopes compared to clean grain, which normally has a lower coefficient of friction than trashy or dirty grain. Clean peaked grain should have relatively low resistance to airflow through the center of the mass even with a peak, compared to dirty or trashy grain with a core of fines under the fill-point. Also, because of lower frictional resistance, clean grain peak heights in grains with relatively smooth outer coatings, such as wheat, maize, oats, popcorn, sorghum, and barley, tend to be lower in clean grain than dirty grain. Thus, clean peaked grain can be aerated with minimum additional cooling time required compared to uncleaned peaked grain. Grain cleaning improves storability by reducing the fines and f.m., which is an attractant and food supply for some storage insects and an excellent substrate for mold development. Thus, cleaning grain further enhances aeration as a non-chemical method of grain protection. When combined with aeration, grain cleaning may be considered as a synergistic integrated pest management (IPM) treatment.

8.6 GRAIN TEMPERATURE MONITORING Millions of tonnes of grain go out of condition annually in many countries because grain condition and temperatures are not monitored. Temperature control is an important phase of grain inspection. Grain temperatures provide a direct indication of the health of a grain mass. Monitoring grain temperatures is by far the most efficient way to manage grain cooling effectively. 8.6.1

Monitoring Grain Temperatures for Molds and Insects

Moisture migration, as described in several sections in this book, occurs when grain temperatures vary widely in bins. It usually begins when warm grain stored in the summer is not cooled in the fall. Then grain against outside walls and along the top surfaces cools rapidly during cold fall and winter temperatures, compared to the center of the bulk. Although aeration and grain turning can be used to prevent or break up moisture migration, temperature monitoring can help confirm conditions when aeration or turning is necessary. Grain stored early during harvest often has higher moisture levels than are safe for long-term storage. Grain moistures throughout a bin may vary widely. Loads of warm wet (above 13.5%) grain transferred into storage may develop storage molds that show up on temperature cable readouts as hot spots in the grain mass. Also, spontaneous heating of grain caused by insect populations in infested grain can be detected by temperature monitoring. 8.6.2

Temperature Monitoring in Wet Grain Holding and Dry Grain Cooling Bins

Use of temperature monitor cable systems is vitally important in wet grain cooling or slowspeed natural air drying bins. By using a well-designed thermocouple or thermistor temperature cable system, grain managers can detect hot spots that are usually a good indication of low airflow zones, insect infestations, or wet grain heating. Using this warning, the manager can transfer the hot grain to another holding bin, breaking up developing hot spots. During transfers, limited cooling of the hot grain occurs by conduction and convection. Analysis of grain temperature profiles helps managers to decide when to operate aeration systems manually or to monitor performance of automatic aeration controllers. Monitoring leading and trailing edges of cooling fronts provides a higher level of precision to aeration system time management that allows managers to minimize grain moisture loss from overcooling during aeration.


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Figure 8.21

Typical movement of a hot spot recorded in thermocouple number 8 (at a depth of 46 feet from surface) during aeration of a 105-foot deep bin.

Automatic controllers use selected cooling air temperatures on an ongoing basis but do not usually inform the manager when aeration is complete. Managers must use temperature readings to determine when cooling is complete to minimize overcooling expense, prevent stopping aeration before the completion of cooling, and monitor hot spots as they are detected in a storage after aeration. 8.6.3

Grain Temperature Data Analysis

It is advisable to monitor grain temperatures often when sensor-equipped temperature cables exist. Grain temperature readouts are of little value unless they are analyzed and used to make correct grain management decisions. The grain manager must also be able to determine if the temperature value is a false reading. Temperatures should be closely monitored from the time grain is stored until it is transferred or shipped to detect hot spots developed by molds or insects. When grain managers are working with moist grain, crusting grain in an advanced stage of moisture migration, or storage with a hot spot, daily temperature monitoring is necessary. For newly stored dry grain or grain with non-uniform temperature, monitoring weekly or at two-week intervals is advisable. With dry grain that has uniform temperatures, recording temperatures once in 2 to 4 weeks is recommended. If a grain temperature reading is about 5°C higher than the average grain mass readings, or has increased by 2 to 5°C from previous readings at the same sensor location, the grain requires immediate attention. If the insect population is high and ambient conditions are not suitable for grain cooling, fumigation may be advisable. If heating occurred due to localized excessive grain moisture or moisture migration, aerating grain may be possible until temperatures are stabilized and no further heating occurs (Figure 8.21). However, if heating is in a localized accumulation of f.m., coring before aeration is advised. If aeration does not cool the hot spot, the grain must be transferred. Figure 8.21 shows biological or spontaneous heating with temperatures of 137 to 122°F on thermocouples (T/Cs) 8, 7, 6, 5, and 9 on November 3, 1989. By November 16, most of the heat



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had been moved to the bottom (T/Cs 1–3). Within 6 days (11/19/89) the warmest reading was T/C 1 at 86°F. During grain aeration, grain temperatures should be monitored and logged as a permanent record to determine the positions of the leading and trailing edges of the cooling zone, and when the cooling front has passed completely through the grain mass. Maintaining a logbook or a computerized spread sheet to record grain temperatures is an excellent stored grain management tool. These logbook entries should be kept at regular intervals such as twice per month during the storage season. During aeration, readings should be taken once or twice per week until the aeration period is nearing completion. When aeration is 85 to 90% complete, temperature readings should be logged daily to stop aeration when the trailing edge of the cooling zone exits the grain mass and the grain is uniformly cool. After fall cooling and throughout the winter period, grain temperatures should be checked at bi-weekly or monthly intervals. With suction-type aeration systems, it is also useful to determine when cooling appears to be nearing completion by monitoring the temperature of the aeration fan exhaust air. Using manual temperature probes to check the completion of the cooling cycle with pressure systems is more difficult because it requires checking the temperature from the top of the grain bulk. With either type of aeration system, the best way to determine when to stop aerating is by analyzing the temperature readings during aeration. Some people use the excuse for not using their temperature cable system that some of the temperature sensors are malfunctioning. Occasionally a sensor may fail, but the remaining data are still useful. Malfunctions are usually termination problems at junctions such as terminal strips or deterioration of the analog selector switch contacts. Replacement of older readout units with a new unit and/or replacement of terminal strips with new terminals often eliminates corrosion buildup that causes millivolt signal loss. One company can service another company’s thermocouple equipment. Small service companies are available that can service thermocouple systems manufactured by former manufacturers. 8.6.4

Understanding and Interpreting Temperature Profile

Grain temperature monitoring should be a key part of the Integrated Pest Management (IPM) program. Managers should take full advantage of their temperature readout systems to read and interpret the data. Temperature readings can be taken while aeration fans are running. However, grain temperatures may change 1 or 2° before the fans are turned off and for a few minutes after. Although temperature sensors and the temperature readout device may have a combined error of 0.5 to 1.5°C, this is within the acceptable limits to obtain a consistent idea of the temperature profile. 8.6.4.1 Temperature Profiles during Aeration When cool air is drawn or pushed through a bin of warm grain, the entire grain mass does not cool uniformly. A cooling front, the leading edge of the cooling zone, slowly moves through the grain mass. Grain temperatures ahead of the front are stable, but temperatures behind the cooling front gradually decrease almost to the temperature of the cooling air. The trailing edge of the cooling zone defines the boundary where grain has cooled to a level near the ambient temperature. Thus, the leading and trailing edges define the top and bottom boundaries of the cooling zone. Ahead of the cooling front or leading edge, grain temperatures have not changed. Behind the trailing edge, the grain is almost the same temperature as the cooling air. Graphical representations of grain cooling temperature profiles are shown in Figure 8.22 (Epperly and Noyes, 1991). As shown, for aeration airflow rates of 3, 6, and 24 (m3/h)/tonne (¹⁄₂₀, ¹⁄₁₀, and ⁴⁄₁₀ cfm/bu) in a 3-m depth research


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Figure 8.22


Grain temperature profiles in a 3-m deep research bin cooled at three airflow rates (¹⁄₂₀, ¹⁄₁₀, and ⁴⁄₁₀ cfm/bu equivalent to about 3, 6, and 24 (m3/h)/tonne).

bin, the difference in elapsed time required for the leading and trailing edges of the cooling zone to exit is inversely proportional to the airflow rates. At 3 (m3/h)/tonne (¹⁄₂₀ cfm/bu), the leading edge of the cooling zone exited the grain mass in about 75 hours, while the trailing edge required another 200 hours. Leading and trailing edge times at 6 (m3/h)/tonne (¹⁄₁₀ cfm/bu) were about 50 and 150 hours. For 24 (m3/h)/tonne (⁴⁄₁₀ cfm/bu), these time values were shortened to about 10 and 38 hours. Grain temperature profiles show that cooling times responded proportionately to airflow rates. Leading edges required about 25% of aeration time in clean level wheat, compared to trailing edges at all three airflow rates. Table 6.1 shows approximate times for cooling fronts to move through any clean grain and total cooling time for grain at aeration airflow rates of 3.1 to 49.8 (m3/h)/tonne (¹⁄₂₀ to 0.8 cfm/bu). Grain with trash, fines, foreign material (f.m.), dockage, and peaked surfaces require substantially longer cooling times. Even with clean level grain, aeration times increase with consolidation or packing due to vibration and shrinkage (making the grain mass denser and decreasing inter-kernel void space). Although unpublished data and the experience of the authors indicates that unclean grain and compaction cause significant reductions to airflow rate (and increases in cooling times), experimental data are lacking in the literature. Research to provide experimental data on these topics is needed. 8.6.4.2 Interpreting Grain Temperature Data Grain temperature data from a U.S. grain elevator is analyzed to show how to interpret the data. The temperature data can be used to improve management of stored grain. The discussion below shows warning signals to look for to detect hot spots and insect infestations. Temperature data from a 36.4-m (120-ft) tall concrete silo in Oklahoma filled with freshly harvested wheat are shown in Figure 8.23. The aeration system was manually controlled, and the data in Figure 8.23 reflect the grain temperature profiles typically found at periodic intervals in commercial silos under ambient conditions of Oklahoma between July, 1988, and March, 1989. The numbers on the right- and left-hand columns indicate the thermocouple (T/C) number, with #1 located 0.3 to 0.6 m (1 to 2 ft) from the floor. Thermocouples were spaced 1.8 m (6 ft) apart. Reviewing each temperature data column gives a vertical temperature profile through the grain mass at that thermocouple cable location (Epperly and Noyes, 1991).



Figure 8.23

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Typical grain temperature data recorded in a 36.4-m (120-ft) deep silo at different times of pressure (upflow) aeration in October and November, 1988, and in January, 1989.

When analyzing grain temperature data, use temperature averages at selected time intervals to identify data trends. However, the entire temperature map should be scanned to avoid overlooking unusual details or activities. Plotting temperatures in graphic form as shown in Figure 8.23 illustrates variations at specific times, such as during the early part of the storage, compared to the aeration period during the fall and the winter or during the natural warm-up period from warm ambient temperatures in the spring. Initially, except for T/C#1 and T/C#10, the freshly harvested wheat temperatures all ranged between 38.3ºC (101ºF) and 48.3ºC (119ºF). Manually operated pressure aeration was started in early October. By 10/7, T/C#1 through T/C#5 showed a strong temperature gradient from 19.4ºC (67ºF) up to 39.4ºC (103ºF), with grain temperature in the upper 70% of the grain above 37.8ºC (100ºF). Significant cooling occurred by 10/16 as shown by the marked shift of the steep temperature gradient from T/C#3 at 16.1ºC (61ºF) to T/C#11 at 36.1ºC (97ºF). During that 9-day period, T/C#5 dropped 13.8ºC from 39.4ºC (103ºF) to 25.6ºC (78ºF). On 11/7, all T/Cs through T/C#12 were reading below 32.2ºC (90ºF). The operation of the aeration system continued to January 16, 1989, when the highest recorded grain temperatures were around 4.4ºC (40ºF) and two T/Cs registered –6.7ºC (20ºF). The final temperature readings on 3/11/89 showed a gradual warming of about 1.7 to 2.8ºC (3 to 5ºF) throughout most of the grain, but the general temperature profile remained about the same. This small temperature increase illustrates the excellent insulation properties of stored grain and lack of convection air currents or drainage of cold air from the silo. Notice that gradual cooling occurs throughout the grain mass behind the leading edge of the cooling front. Most T/C readings at each level drop from one reading date to the next, with the differential temperature change increasing toward the leading edge (lower part of the bin or silo on pressure cooling systems) of the cooling zone. Another aspect of analyzing grain bin or silo temperature data is through grain temperatures in the bottom of the bin or silo. T/C#1 tended to be relatively cooler than most of the grain mass in the summer and warmer than the grain in the bottom third of the silo within 1 to 2 meters above


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the floor (and higher in the winter). This can be seen by comparing T/C#1 with T/C#2 through 6 for all dates listed in Figure 8.23. This is especially true in cone-bottom bins or silos where the hopper is below the ground surface because the air or ground temperatures around the cone area affect the grain temperature in the cone. Outside air may also leak into the bin through the unsealed fan and aeration ducts, changing the lower grain temperatures. That is one reason why sealing fan and auger openings are important. For manual control, fan operation decisions could be based on actual air temperatures or weather predictions, or the fans may have been turned on late in the day and turned off the following morning. A 7-day, 24-hour repeat cycle timer can be used if fans are started and stopped at a specific time each day. If a cold weather front kept ambient temperatures low for several days at a time, fans may have run continuously day and night until weather warmed during the day. These are typical methods of manual control of aeration systems. 8.6.4.3 Problems Encountered with Thermocouple Readings Thermocouples consist of two wires of different types of metal, such as copper-constantan or iron-constantan. When the two metals are connected, a very small (milli-amp) current is continuously generated. The voltage drop across the connection between the two metals changes with the temperature at the connection. Temperature readout devices convert these small linear voltage measurements into meaningful temperatures when one end of the connection serves as reference (see also Section 3.1.6.1 on temperature measurement). The two wires making the thermocouple may be welded or even twisted tightly together to make the connection. The connection is then usually coated with a protective material to prevent corrosion. If the circuit is broken or loose anywhere from the readout device to the thermocouple, incorrect readings or no temperature reading will result. Inaccuracies in readout devices and in thermocouples can result in differences in readings — even when there is no temperature change. This is due to the accuracy of the readout device ±0.56°C (1°F). Thermocouples have a typical error range of 1.1°C (2°F) to 1.7°C (3°F). Even with only a limited number of valid temperature readings on a given thermocouple cable, the general condition of the grain can still be learned in relation to the cooling effect of aeration on the grain mass.

8.7 AERATION TO REMOVE ODORS AND FUMIGANT GASES AND MAINTAIN PESTICIDE LIFE IN GRAIN BULKS 8.7.1

Musty Odor Removal

Grain stored at moistures that are above safe storage levels develops a musty odor from mold development. Roof and downspout condensation also increases grain moisture that results in moldy grain. Smut from field infestations can also cause grain to smell musty. Aeration is often required to remove musty grain odors and improve market quality of the grain. To check if mustiness has been removed, grain samples should be smelled with the sample held up against the nose, permitting inhaled air to pass through the grain. The amount of aeration fan time required to remove odors varies with the amount of grain that is affected and the cause of the odor. Some odors are persistent and may require 50 to 100 air changes, while others may be cleared with 5 to 10 air changes. Approximately 5 minutes is required per air change at 6 (m3/h)/tonne, so one hour of fan operation would provide about 12 air changes at that rate. Other airflow rates provide proportional air change times, such as 10 minutes per air change at 3 (m3/h)/tonne.



8.7.2

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Exhausting Fumigant Gases

The removal of fumigant gases at the end of fumigations is a very important use of aeration systems. Grain fumigants are often exhausted using manually controlled aeration to rapidly ventilate the grain mass. Pressure aeration systems are preferred for ventilating grain so exhaust gases exit the bin through roof vents, high in the air where the gas rapidly dilutes in the wind, well away from workers. Aluminum phosphide pellets or tablets that produce phosphine gas is by far the most common stored grain fumigant of choice, worldwide. Suction is not the recommended method for gas removal after fumigation. This is because of the potential hazard to worker safety by exhausting the fumigant into the silo workspace. If a suction fan system is used, the roof vents should be opened and the fan covers removed to enable gas emission to the atmosphere by wind movement across bin or silo roofs. Upward convection currents are induced which pull the gas out the roof vents. Natural ventilation should be used for 1 to 2 days before suction fans are turned on for forced removal of remaining gases. It is highly recommended that gas concentrations are monitored in the silo workspace until a safe atmosphere is detected. To determine if the storage is adequately ventilated, a 3–5 to mm ID plastic tube should be connected to the fan transition to monitor the gas concentration levels during gravity or natural ventilation. Aeration fans should be shut off for an hour or two before taking gas samples from the transition duct and other locations in the storage unit. When a suitable reading is obtained (below 0.3 ppm of phosphine for worker exposure limits in the U.S.), additional readings should be taken 6 to 8 hours after the fans have been turned off to see if more gas has desorbed from the grain. In addition to sampling from the fan transition, bin or silo head-space readings should also be taken. WARNING: Once base fans are unsealed, keep all personnel and animals away from that area until venting of gas is complete. Mounting reports indicate that phosphine bonds to grain molecules, maintaining low levels of phosphine for several weeks under certain conditions. Monitor for phosphine gas periodically for several days before shipping grain from a fumigated storage.

8.7.3

Maintaining Pesticide Strength

The useful life of grain pesticides depends on grain temperature and grain air space humidity — useful life decreases as temperature increases above normal ambient levels of 25 to 30°C (Snelson, 1987; Wohlgemuth et al., 1987). A literature survey conducted by Snelson (1987) produced very little useful information on the reaction of insects to insecticides. Most of the available data refer to studies involving the degradation of insecticide under varying temperature and moisture regimes. A major factor obscuring the correct chemical interpretation of insecticide breakdown on grain is the widespread use of moisture content as a measure of the water present in grain. Moisture content is arbitrarily defined in terms of weight loss under the influence of heat and is not linearly related to water activity. Water activity is derived from the laws of chemical thermodynamics, which describes the activity of a chemical in equilibrium in two phases — e.g., water in grain and interstitial relative humidity. If water activity is used as a measure of water present, the breakdown rate of insecticides is found to be independent of grain type (Snelson, 1987). When application of stored grain insecticides was common, a grain management strategy advocated in Australia was the integration of aeration with reduced application rates of pesticide (Longstaff, 1981). This strategy was based on the assumption that pesticides remain active for increased periods at lowered grain temperatures. In a paper that analyzed different grain management strategies, Hunter (1981) compared the economics of different strategies for grain protection. These strategies included conventional aer-


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ation, high airflow rate aeration, and refrigerated aeration compared with no aeration and with use of the stored grain insecticide methacrifos. Cooling of grain using conventional aeration was shown to be economically attractive. In 1977, when dichlorvos and malathion were still widely used as grain protectants, they were applied directly to grain in a spray or dust formulation. Desmarchelier et al. (1977) showed that there was a marked increase in efficiency of dichlorvos under forced airflow conditions. In one of their experiments, a grain bulk of about 3000 tonnes was surface-sprayed with dichlorvos. Thirty minutes after application, the grain was aerated for 46 hours using downward aeration. Natural infestation of Ephestia cautella and Rhyzopertha dominica was controlled, but Cryptolestes spp. survived. Takimoto et al. (1978) studied the degradation of fenitrothion applied to rice. They showed that decomposition proceeded more rapidly at 30°C than at 15°C, with the respective half-lives of about 4 months and more than 12 months independent of the applied concentrations. Desmarchelier (1979) developed a mathematical model for the loss of carbaryl on grains in storage based on relative humidity and temperatures and demonstrated that this model held true under widely varying commercial conditions in various parts of Australia. Desmarchelier (1980) explained how the loss of bioresmethrin, carbaryl, and d-phenothrin on wheat during storage was related quantitatively to wheat temperature and equilibrium relative humidity. Desmarchelier et al. (1979) pointed out that combining cooling and/or drying of grain with appropriate use of insecticides provided flexible procedures of pest control that are generally superior to and cheaper than the control given by cooling alone, or drying alone, or insecticides alone. They discussed the effect of drying and cooling on both pest biology and the chemistry of protectants. Desmarchelier et al. (1980) quoted the half-life of fenitrothion at 30°C and 50% relative humidity as 16.2 weeks. Desmarchelier (1985) reviewed the work carried out on the high insecticidal potency of the vapors of slightly volatile insecticides and developed the concept of using aeration to distribute dichlorvos vapors throughout the grain mass. He also pointed to the practical importance of regulating the storage temperature in preserving the effectiveness of the insecticide deposit while keeping the final residue level under control. Temperature-labile insecticides can be most useful if the storage temperature can be lowered to extend their persistence at an effective level throughout the storage period. By the end of the storage period, the residue level is comparatively low and, because of its labile nature, is degradable during processing and cooking. This concept has been illustrated using experimental data for methacrifos and fenitrothion. Thorpe and Elder (1980) discussed the use of mechanical refrigeration to improve the storage of pesticide-treated grain. They cited models to show that the loss of methacrifos on treated wheat initially at 30°C and 11% moisture content could be reduced during 6 months storage from 90% to about 30% by prompt cooling to 20°C. The cost of such cooling was more than offset by the savings in insecticide. Thorpe and Elder (1982) showed how aeration is able to reduce the rate of degradation of insecticides applied to stored grain and to render the rate of decay relatively insensitive to initial grain conditions. In the temperate and subtropical wheat-growing regions of Australia, aeration can reduce usage of methacrifos by factors of 7 and 4, respectively. Desmarchelier (1983) discussed several ways of maximizing the benefit obtained from cooling grain soon after it is put in storage. Such cooling not only reduces the degradation of the insecticide but also reduces the reproductive rate of insect pests. This enables the amount of insecticide to be reduced considerably. Rapid cooling of insecticide-treated grain has the additional advantage that the grain is available for immediate shipment, if that is required. On the other hand, should the grain have to be held over, the residual insecticide deposit remains effective during the extended storage, thus obviating the need for retreatment or the risk of infestation. Arthur and Throne (1994) studied the effect of aeration on corn treated with 8 ppm pirimiphosmethyl. The corn was infested artificially with adults of the red flour beetle, the maize weevil, and



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eggs of the Indian meal moth to determine the effects of aeration on insect control in southeastern Georgia. During the storage period of October through August, pirimiphos-methyl degraded more quickly in unaerated than in aerated bins. No live insects were recovered from either unaerated or aerated treated bins. The percentage of insect-damaged kernels and number of beetles was greater in unaerated untreated than in aerated untreated bins. It was concluded that aeration potentially can reduce insect pest population levels and subsequent damage in corn stored in southeastern Georgia. During the last decade many countries have reduced pesticide usage, particularly usage of insecticidal protectants (Matteson, 1995). Australia, for example, has enforced residue-free standards on its grain for many years. The U.S. Environmental Protection Agency has announced a partnership among the EPA, the USDA, the U.S. Food and Drug Administration, state grain associations, and utility companies to promote environmental stewardship in pesticide use. Several state agencies in the U.S. no longer recommend insecticidal protection on grain; instead, they are relying on other, non-chemical methods of maintaining commodity quality. Some large food processing companies in the U.S. have committed themselves to moving toward a less chemical-intensive pest-management program. This is due to litigation about illegal residues as well as to public opinion (Hegele, 1994). All of this has contributed to more frequent insect infestations in grain stores, and this trend is likely to contribute to implementation of suitable IPM strategies (Zettler, 1997).

8.8 ROOF VENTILATION SYSTEMS 8.8.1

Steel Bin Roof Venting

Venting of steel bin head-space is a very important aspect of aeration system design for good management. An inadequate vent cross-section area, for either pressure or suction aeration, results in reduction of airflow and aeration efficiency. An often overlooked aeration system feature is to design the size and location of roof vents to minimize under-roof condensation dripping on the surface grain. A very important aspect of roof venting is keeping moist air from flowing up gravity fillspouts or downspouts. Moisture condenses inside cold spouts and runs back onto the surface grain. Installing one or two vents close to the center fill-point helps to minimize condensation around the bin fill-pipe discharge opening. This is especially critical in cool or cold regions. Downspout condensation can be greatly reduced or eliminated by installing spring-loaded or gravity-actuated (counter-balanced) flaps that seal downspouts and are then opened by slight pressure when grain flows down the spout. On bins that use pressure aeration and have no roof exhaust fans, vents near the fill-point help to minimize condensation in downspouts or conveyor outlets by creating a low-pressure zone and path of least resistance, so moist exhaust air can flow out through the vent. On bins with roof exhaust fans, the peak vent will provide an inflow of fresh air near the downspout, blocking moist air entry into the downspout. Also, ambient air will be drawn down the spout into the bin and out through the roof exhaust fans. In pressure aeration, the roof vent system must be designed with sufficient cross-sectional area to allow adequate exhaust or inlet air volume to maintain vent throughput velocities of 300 to 400 m/min (1000 to 1500 fpm). A practical but less accurate roof vent area sizing guideline that provides approximate results is to design 0.1 m2 of duct cross-section area per kW (or 1 ft2/HP) of aeration fan power. This guideline may significantly differ from the recommended vent throughput velocities, since the fan power required for aeration is strongly dependent on the grain type. This approach is somewhat more conservative than the air velocity method. This design approach provides air velocities of about 250 m/min and results in a larger exhaust area than in the design based on air velocity, which might be helpful to minimize roof condensation when roof exhaust fans are not used.


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The vent area should be divided into several equally spaced vent units based on the customary design practices in the area. For safety, when vents are to be sealed for fumigation, walkways with safety rails should be installed. One or two vents should be installed near the bin roof peak or fillpoint to minimize exhaust air entering the downspout or fill conveyor outlet, which can result in condensation and dripping onto the grain. Although bolted steel bins normally have an air gap of 1.2 to 1.8 cm (½ to ¾ in) between the roof and sidewall that serves as a ventilation air inlet or outlet, this air gap should not be included in the ventilation system open area design. If a gas recirculation system is installed in the future, this sidewall-to-roof gap will need to be sealed. Example 8.17 A 22-m (72-ft) diameter, 5000-tonne (180,000-bu) bolted, corrugated steel bin has an aeration airflow rate of 6 (m3/h)/tonne (0.1 cfm/bu). Design the vent system. Solution: Total airflow at 6 (m3/h)/tonne × 5000 tonne = 30,000 m3/h. Total vent area = 30,000 m3/h ÷ 60 min/h = 500 m3/min ÷ 300 m/min = 1.67 m2 vent area. Using 0.5-m square roof vents with a cross-sectional area of 0.25 m2/vent, 1.67 m2 ÷ 0.25 m2/vent = 6.7 or 7 vents. For seven vents, place one vent near the peak and space six vents equally (about 60° apart) around the roof about ⅔ of the roof slope down from the peak. Example 8.18 For the same size 5000 tonne bin in Example 8.17, suppose that the selected design point of the aeration fan is 22.5 kW (29.6 HP). This fan is suitable for aerating wheat at about 5 (m3/h)/tonne and corn at about 9 (m3/h)/tonne. Using the rule-of-thumb design approach of 0.1 m2/kW of fan power, the roof vent area required would be 22.5 kW × 0.1 = 2.25 m2 of vent area. If 0.25 m2 vents are used, 2.25 ÷ 0.25 = nine vents recommended. Place one vent near the peak, then space eight vents symmetrically (45° apart, radially) around the roof about ⅓ of the roof slope down from the peak. 8.8.2

Roof Exhaust Fans to Minimize Condensation

To minimize humid exhaust air roof condensation during upflow or pressure aeration, highcapacity propeller type roof fans can be installed. Roof exhausters should be sized to provide a total air volume of at least two to three times the aeration fan system airflow in order to draw in excess ambient air to dilute moist exhaust air, lowering the dew point of the total air mass exhausting through the roof fans. When roof fans are used, fresh air is pulled in through the roof vents, mixed with the cooling air moving upward through the surface grain, and exhausted through the roof fans. Thus, the drier, diluted air mass that contacts the underside of bin roof sheets is less likely to experience condensation. One or more roof exhaust fans may be used depending on bin size, aeration fan air volume, and roof exhauster capacities. If roof vents are mounted about ⅔ of the roof slope distance from the peak, roof exhaust fans should be spaced about ⅓–¼ of the roof slope distance from the peak and mounted symmetrically around the roof. If two fans are used, they should be placed opposite each other on the roof. Three fans should be spaced 120° apart; four fans, 90° apart; 6 fans at 60° intervals; and 8 fans at 45° angular spacing. To obtain good air mixing, the use of half as many fans as roof vents allows each exhaust fan to be centered between two vents. This arrangement of offsetting the fans from the roof vents provides good mixing of cool, dry ambient air from the vents with warm, moist air rising from the grain surface.



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Exhaust fan air volume should be at least 200% of the aeration fan volume. For roof fans with 200% of the aeration fan capacity, roof vents should be designed to match the airflow capacity of the aeration fans. If roof fans are sized for more than 200% of aeration fan airflow, roof vent area should be increased to match the excess airflow. Roof fans are high-volume, low-pressure fans. If the roof vent area is too small, restricting fresh air inlet, roof exhaust fan capacity may be greatly reduced; and the roof exhaust fans may fail to provide adequate ambient airflow to keep moist air from condensing on the roof (Noyes, 1991). Hutchison and Holman (1968) conducted studies to determine if fan ventilation provides better control of air conditions in the overspaces of flat grain storages than natural ventilation. The study was carried out to find an alternative for frequent grain turning (moving grain from one bin to another). In the southern regions of the U.S., aerated grain remaining in storage during the summer often has temperatures of 10 to 15.5°C (50 to 60°F). Air temperatures in the head-space under galvanized steel roofs may build up to 60ºC (140°F) or more when roof exhaust or aeration fans are not in use. During cool weather, air in these flat storages moves upward through warm grain; and moisture may condense on the grain surface and on the underside of the roof. This problem can become serious during nights and cool days. Conventional roof ventilators leave pockets with little or no ventilation in the overspace of these flat storages. If the grain is to be fumigated, these roof ventilators need to be tightly sealed. Because of their relatively large numbers, sealing these ventilators requires extra time and materials. The most common method of sealing ventilators is to seal them with a gas-tight plastic sheet that is usually fastened with masking tape. Hutchison and Holman (1968) found that installing fans with air inlet in one end and air outlet in the other end of flat storages provides better control of air conditions in the overspaces above grain than natural ventilation through ventilators. Flat storages with this type of fan arrangement have only a few openings, which can be easily sealed if a fumigation is required. Tests were carried out with fans mounted at one end of the overspace, and results in reducing the overspace air temperature were compared with ridge-installed ventilators (Hutchison and Holman, 1968). Airflow rates, providing an air change every 3 minutes in the overspace, kept the space 16.7ºC (30°F) lower than natural ventilation during April and May. Overspace ventilation using roof or gable ventilation fans improved performance of pressure system aeration. Overspace fans should be operated continuously during aeration to remove warm, moist air to minimize condensation on the underside of the roof. Overspace fans can be operated during rainy periods if rain is not pulled into the storage through louvers, cracks, or other openings. The prevailing wind direction during operation of fans mounted at one end of the overspace has a significant effect on the airflow rate. If fans discharge into the wind, the amount of warm air removed from the overspace can be reduced by 50% or more. If the fans discharge downwind, the wind aids the fans and much more air is moved through the overspace. If louvers are used, they can be closed by wind blowing against them, and the amount of warm air discharged from the storage is reduced. The direction the flat storage is erected on the site with respect to wind direction can have an effect on air movement through the overspace. Example 8.18 Using the 5000-tonne bolted steel bin in Example 8.17, with seven vents at 0.25 m2 vents, design a roof exhaust fan system that handles 200% of aeration system airflow. This bin has upflow aeration of 6 (m3/h)/tonne, for a total airflow of 30,000 m3/h. For this 22-m (72-ft) diameter bin with six vents mounted symmetrically around the roof and one vent at the peak, three exhaust fans are recommended. The roof exhaust fans must each handle (2 × 30,000) ÷ 3 = 60,000 ÷ 3 = 20,000 m3/h (11,900 cfm). The exhaust fans need to be spaced about ⅓ of the roof slope distance down from the peak,


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with one fan spaced between each pair of roof vents so ambient air does not short-circuit directly from vent to fan without mixing with the exhaust air from the grain. An important practice used by some U.S. grain elevators that use roof exhaust fans on steel bins with pressure aeration systems operated by an aeration controller is to continue the operation of the roof fans for a short time (30 minutes to an hour) after the base fans shut off. This allows the roof exhaust fans to remove the moist exhaust air that has exited the grain surface and is in the bin head-space and to minimize condensation. A timer that controls the roof exhausters can be started by the normally closed (NC) contact on the thermostat. When the thermostat switch opens to stop aeration, the roof fan timer activated by the NC contact will restart the roof fans and operate them for a preset time, then shut them off until the next aeration fan cycle.

8.9 MULTISTAGE AERATION SYSTEMS Multistaging or series mounting of aeration fans is a useful and economical method of developing a desired aeration airflow rate. For example, if the aeration design objective for a deep grain storage unit, either a bolted steel bin (at 20 to 25 m) or silos at 30 to 40 m depths, the obvious fan selection is centrifugal fans. However, depending on the product to be stored, two vane-axial fans mounted in series or in line may produce the desired airflow at a much lower cost. Two properly sized vane-axial fans may be bolted together in series to deliver the same airflow at the desired static pressure as one centrifugal fan. The initial cost of two vane-axial fans mounted in line may be only 40 to 50% of the cost of a centrifugal blower that provides equivalent airflow at the desired pressure. The two vane-axial fans may cost more to operate than the single centrifugal fan, but the combined capital and operating costs may favor the vane-axial fans for 12 to 15 years of operation. Keep in mind that heat of compression may be a significant problem on two-staged fans. Another method of multistaging fans, either vane-axial, standard centrifugal or in-line centrifugal, that reduce heat of compression, is the push-pull method. Push-pull, as implied, involves using identical fans with one fan at the storage structure base and one fan on top of the silo (see also Section 8.1.1).

8.10 MANIFOLD-OPERATED LARGE BLOWERS Some grain storage facilities are well suited to aeration where one blower provides the airflow for several storage units. An example is concrete silos grouped in one concrete elevator annex (a group of silos built as a unit in one, two, or more rows), or flat storage warehouse units with multiple parallel floor ducts uniformly spaced across the width of the warehouse. One large blower has several benefits compared to multiple individual fans with one fan per duct (Figures 8.24 and 8.25). For example, a flat storage unit with one fan per duct could easily be modified using one large fan to replace the individual fans on each aeration duct. Thus, one fan would supply all aeration ducts from one continuous supply manifold. The design of one large blower connected to a manifold, which has valves at each lateral duct, is advantageous. If there is a problem of overheating grain in one section of the storage structure, the airflow through one or more ducts near the hot grain can be dramatically increased by closing off several of the other ducts to increase the flow through the selected duct or ducts. A similar situation might occur in a group of concrete silos, where one silo may have a serious heating problem. With a large blower manifolded to deliver 4 (m3/h)/tonne to 8 to 12 silos (Figures 8.24 and 8.25), if one silo needs rapid cooling, airflow to several silos can be shut off to increase the airflow to 8 to 10 (m3/h)/tonne to one silo. This may provide 2 to 2.5 times as much air through



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Figure 8.24

A manifolded suction aeration system on a concrete silo annex.

Figure 8.25

A manifolded pressure suction aeration system on a concrete silo annex.

the bin that needs rapid cooling as it would normally receive. Instead of needing 200 to 220 hours to cool, with the increased airflow, a cooling front can be moved through the silo with the hot grain mass in 70 to 100 hours, or only 3 to 4 days of continuous operation. This quick cooling response could not be delivered if individual fans rated at 4 (m3/h)/tonne had been installed on each silo. In summary, important design aspects of one large blower on a supply manifold are: • Replaces several individual blowers on a large bin, a flat storage, or on several concrete silos • Eliminates moving individual blowers from one bin to another • Conveys outside air to inside silos by running lateral ducts through restricted areas such as unload conveyor tunnels. • Increases airflow to selected lateral aeration ducts by closing valves to adjacent ducts, dramatically increasing the cooling rate near hot spots

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Hunter, A.J. (1985). Design of air distribution systems and fan selection for grain aeration, in Preserving grain quality by aeration and in-store drying, Proceedings of an international seminar held at Kuala Lumpur, Malaysia, 9–11 October 1985. Champ, B.R. and Highley, E., Eds. 108–113. Hunter, J. (1981). Economics of combined cooling and pesticide treatment, in First Australian Stored Grain Pest Control Conference (Williams, P. and Amos, T.G., Eds.), May 1981, Melbourne, Victoria. 4, 20–26. Hutchison, R.S. and Holman, L.E. (1968). Fan ventilation of air space above grain in flat storages, USDA, ARS 52-27. Jay, E.G. (1980). Methods of applying carbon dioxide for insect control in stored-grain insects, U.S. Dept. Agric., Sci. and Ed. Adm., AAT-S-13. Jayas, D.S. and Mann, D. (1994). Presentation of airflow resistance data of seed bulks, Appl. Eng. Agric., 10(1), 79–83. Jayas, D.S. and Muir, W.E. (1991). Airflow-pressure drop data for modelling fluid flow in anisotropic bulks, Trans. ASAE, Am. Soc. Agric. Eng., 34(1), 251–254. Kenkel, P. and Noyes, R.T. (1993). Costs and benefits of installing closed loop fumigation systems in commercial elevators, OSU fact sheet No. 219, Cooperative Extension Service, Oklahoma State University, Stillwater, July, 1993. Kenkel, P., Noyes, R.T., Cuperus, G.W., and Criswell, J.T. (1994). Updated Estimates of the Costs and Benefits of Closed Loop Fumigation Systems: Field Results from an Oklahoma Country Elevator, in Proceedings, 1994 Texas High Plains Grain Elevator Workshop, Texas A&M University Extension Center, Amarillo, TX, January 27. Kumar, A. and Muir, W.E. (1986). Airflow resistance of wheat and barley affected by airflow direction, filling method and dockage, Trans. ASAE, American Society of Agricultural Engineers, Vol. 29, SeptemberOctober, 1423–1426. Longstaff, B.C. (1981). An analytical investigation of the combined use of natural aeration and pesticides, in First Australian Stored Grain Pest Control Conference, (Williams, P. and Amos, T.G., Eds.) May 1981, Melbourne, Victoria. 4, 16–20. Loo, K.F. (1985). Silo storage in Malaysia, in Preserving grain quality by aeration and in-store drying, Proceedings of an international seminar held at Kuala Lumpur, Malaysia, 9–11 October 1985, B.R. Champ and Highley, E., Eds., 165–172. Lower, O.J., Bridges, T.C. and Bucklin, R.A. (1994). On-Farm Drying and Storage Systems, Pam DeVoreHansen, Acquisitions Books & Journals. American Society of Agricultural Engineers, St. Joseph, MI. Mathlein, R. (1961). Experiments with fresh-air treatment for the control of grain storage pests, Statens Vaxyskyddanstalt Meddelaonden 12, 87 125. Matteson, P.C. (1995). The 50% pesticide cuts in Europe: a glimpse of our future? Am. Entomol., 41, 210–220. McKenzie, B.A., Foster, G.H., Noyes, R.T., and Thompson, R.A. (1967). Dryeration — Better Corn Quality With High Speed Drying, AE-72; Agricultural Engineering Department, Purdue University, Lafayette, IN. Moffett, E.C. (1927). Fumigating Process, U.S. Patent No. 1,613,186. U.S. Patent Office, Washington, D.C., January 4, 1927. Morris, W.J.T. (1984). Bunker storage in Victoria, in Workshop on grain handling: Challenges, shortcomings, improvements, solutions, Melbourne, 6–7 August 1984, Barton, ACT, Institution of Engineers Australia. Section 1.3. Navarro, S. (2000) — Personal observation from a trip to Central Anatolia, Turkey. Navarro, S. and Calderon, M. (1982). Aeration of grain in subtropical climates. FAO Agricultural Services Bulletin, Rome, No. 52. Navarro, S., Donahaye, E., and Fishman, S. (1994). The future of hermetic storage of dry grains in tropical and subtropical climates, Proc. 6th Int. Working Conf. Stored-Product Protection, (Highley, E., Wright, E.J., Banks, H.J., and Champ, B.R., Eds.), Canberra, Australia, 17–23 April 1994, CAB International, Wallingford, Oxon, U.K., 130–138. Navarro, S., Donahaye, E., Kashanchi, Y., Pisarev, V., and Bulbul, O. (1984). Airtight storage of wheat in a PVC covered bunker, in Practical aspects of controlled atmosphere and fumigation in grain storages: Proceedings of an international symposium, l 1–22 April,1983, Perth, Western Australia, Amsterdam, Elsevier, 601–614. Navarro, S., Jay, E.G., and Leesch, J.G. (1986). Recirculation rate requirements for adequate distribution of carbon dioxide in grain silos, Tran. ASAE 29(5), 1348–1354.


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Nelson, G.L., Day, D.L., Welch, G.B., and Hamilton, H.E. (1966). Analysis of static pressures for cross-flow air circulation in cylindrical grain bins. Bulletin No. B-645, Oklahoma Agricultural Experiment Station, Oklahoma State University, March. Noyes, R.T. (1991). Aeration of Texas coastal region grain storage: Critical management decisions, in Proceedings, Texas Grain Improvement Conference, May 22–23, Texas Agricultural Extension Service, Corpus Christi, TX, 104. Noyes, R.T. (1993). Closed Loop Fumigation System: Design and Management, presented at Indiana Stored Grain Pest Management Workshop, Purdue University, Lafayette, Indiana, September 9. Noyes, R.T. and Kenkel, P. (1999). Storing moist wheat at commercial elevators in Oklahoma, OSU Current Report, CR-1741, Cooperative Extension Service, Oklahoma State University. Noyes, R.T., Stringer, M.E., and Clary, B.L. (1989). Closed loop fumigation, Table 2, Comparison of Recirculation Vs. Non Recirculation on 300,000 Bu. Welded Steel Tank, Section V. 1989 Oklahoma Grain Elevator Workshop Manual. Noyes, R.T., Kenkel, P., and Tate, G. (1995). Closed loop fumigation systems. Chapter 20 in Stored Grain Management, OSU Circular E-912, First Revision. Oklahoma State University, Stillwater, OK, 153–161. Noyes, R.T., Phillips, T.W., Cuperus, G.W., and Bonjour, E.L. (1998). Advances in recirculation fumigation technology in the U.S.A., in Proceedings, 7th International Working Conference on Stored Product Protection, Beijing, PR China, 14–19 October, 1998. (I) 454–462. Shedd, C.K. (1953). Resistance of grain and seeds to airflow, Agric. Eng., 34(9), 616–619. Shove, G.C. (1968). Aerating stored dry grain. University of Illinois, College of Agriculture, Cooperative Extension Service. Siebenmorgen, T.J., Freer, M.W., Benz, R.C., and Loewer, O.J. (1989). Temperature and relative humidity data in bunker stored rice. Applied Eng. Agric. 5, 259–264. Snelson, J.T. (1987). Grain protectants, ACIAR Monograph No. 3. Takimoto, Y., Ohshima, M., and Miyamoto, J. (1978). Degradation and fate of fenitrothion applied to harvested rice grains. J. Pestic. Sci., Japan, 3, 277–290. Thompson, R.A. and Foster, G.H. (1963). Stress Cracks and Breakage in Artificially Dried Corn, Marketing Research Report No. 631, USDA Agricultural Marketing Service, Transportation and Facilities Research Division, Washington, D.C., 0ctober 1963. Thorpe, G.R. and Elder, W.B. (1980). The use of mechanical refrigeration to improve the storage of pesticide treated grain, Int. J. Refrig., 3(2), 99–106. Thorpe, G.R. and Elder, W.B. (1982). Modelling the effects of aeration on the persistence of chemical pesticides applied to stored bulk grain, J. Stored Prod. Res., 18, 103–114. Varnava, A., Navarro, S., and Donahaye, E. (1995). Long-term hermetic storage of barley in PVC covered concrete platforms under Mediterranean conditions, Postharvest Biol. & Technol. 6, 177–186. Wilson, A.D, Banks, H.J., Annis, P.C., and Guiffre, V. (1980). Pilot commercial treatment of bulk wheat with CO2 for insect control: the need for gas recirculation, Aust. J. Exp. Agric. Anim. Husb., 20, 618–624. Winks, R.G. and Russel, G.F. (1996). Active fumigation systems: Better ways to fumigate grain, in Proc. Int. Conf. Controlled Atmosphere and Fumigation in Stored Products, (Donahaye, E.J., Navarro, S., and Varnava, A., Eds.) (1997). 21–26 April 1996, Printco Ltd., Nicosia, Cyprus, 293–303. Wohlgemuth, R., Harnisch, R., Thiel, R., Buchholz, H., and Laborius, A. (1987). Comparing tests on the control and long-term action of insecticides against stored product pests under tropical climate conditions. Post-Harvest Project, Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH, Hamburg, Germany. Yates, C.J. and Sticka, R. (1984). Development and future trends in bunker storage, in Practical aspects of controlled atmosphere and fumigation in grain storages: Proceedings of an international symposium, 11–22 April, 1983, Perth, Western Australia. Amsterdam, Elsevier, 589–600. Zettler, J.L. (1997). Influence of resistance on future fumigation technology, in Proc. Int. Conf. Controlled Atmosphere and Fumigation in Stored Products, (Donahaye, E.J., Navarro, S., and Varnava, A., Eds.) 21–26 April 1996, Printco Ltd., Nicosia, Cyprus, 445–454.


CHAPTER

9

Chilling of Grain by Refrigerated Air Dirk E. Maier and Shlomo Navarro

CONTENTS 9.1 Introduction...........................................................................................................................491 9.2. Chilling Unit and its Components .......................................................................................493 9.2.1 Refrigeration Unit.....................................................................................................493 9.2.1.1 Mechanics of Refrigeration ......................................................................493 9.2.1.2 Removal of Condensed Water ..................................................................495 9.2.1.3 Adjusting the Relative Humidity of Refrigerated Air..............................495 9.2.1.4 Ice Formation in the Evaporator...............................................................495 9.2.2 The Refrigeration Process ........................................................................................496 9.2.2.1 Description of the Refrigeration Process..................................................496 9.2.3 Selecting the Refrigeration Unit Capacity...............................................................496 9.2.3.1 Grain Cooling and Heat Removal Loads .................................................496 9.2.3.2 Selecting the Cooling Rate .......................................................................498 9.2.4 Airflow Control ........................................................................................................500 9.2.5 Modeling the Refrigeration System of a Grain Chiller...........................................500 9.2.5.1 Compressor................................................................................................501 9.2.5.2 Condenser..................................................................................................501 9.2.5.3 Reheater.....................................................................................................502 9.2.5.4 Evaporator .................................................................................................502 9.2.5.5 Chiller Performance Data .........................................................................503 9.2.5.6 The Chiller Refrigeration Cycle Model....................................................504 9.2.5.7 Evaporator Heat Transfer Coefficient .......................................................504 9.2.5.8 Reheater Heat Transfer Coefficient ..........................................................506 9.2.5.9 Condenser Heat Transfer Coefficient........................................................506 9.2.6 Alternative Cold-Air Systems ..................................................................................507 9.2.6.1 Dessicant Cooling System ........................................................................507 9.2.6.2 Heat-Pump Cooling Systems ....................................................................507 9.2.6.2.1 Dehydrofrigidation ..................................................................507 9.2.6.2.2 Air-to-Air Heat Pumps............................................................509 9.2.6.2.3 Water-to-Air Heat Pumps........................................................509 9.2.6.3 Evaporative Cooling Systems ...................................................................510 9.2.6.4 Chilled Water Systems ..............................................................................510 0-8493-1355-4/02/$0.00+$1.50 © 2002 by CRC Press LLC

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490


9.3

Operation of a Grain Chiller ................................................................................................510 9.3.1 Grain Chiller Controls..............................................................................................512 9.3.1.1 Control of Evaporator Temperature..........................................................513 9.3.1.2 Airflow Controller .....................................................................................513 9.3.1.3 Control of Reheat Temperature ................................................................514 9.3.1.4 Control of Relative Humidity ...................................................................515 9.3.2 Set-Point Selection and Control...............................................................................518 9.3.2.1 Set-Point for Airflow Control in a Grain Chiller of Known Capacity ...................................................................................518 9.3.2.2 Set-Point for Airflow Control in a Grain Chiller of Various Capacities ................................................................................519 9.3.2.3 Use of Microprocessors for Automatic Set-Point Selection and Control................................................................................................521 9.3.3 Practical Aspects of Operating Chilling Units.........................................................521 9.3.3.1 Removal of Condensed Water ..................................................................521 9.3.3.2 Dust Filter .................................................................................................522 9.3.4 Cooling-Front Movement through Grain .................................................................522 9.3.5 Chilling of Hot Grain after Transfer from a Dryer .................................................523 Biological Considerations ....................................................................................................524 9.4.1 Control of Insects and Other Pests ..........................................................................525 9.4.2 Control of Respiration and Fungi ............................................................................527 9.4.3 Control of Other Quality Parameters .......................................................................527 Costs and Benefits of Grain Chilling...................................................................................529 9.5.1 Storage Structure Type and Size ..............................................................................529 9.5.2 Frequency of Rechilling and Insulation of Storage.................................................530 9.5.3 Recirculation of Chilling Air ...................................................................................532 9.5.4 Energy Requirements and Operating Costs .............................................................532 9.5.6 Net Present Cost Analysis of Grain Chillers ...........................................................534 9.5.6.1 Case Study of Popcorn Chilling ...............................................................535 9.5.6.2 Case Study of Wheat Chilling ..................................................................535 Overview of Applications of Grain Chilling .......................................................................535 9.6.1 Historical Development of Grain Chilling...............................................................535 9.6.2 Grain Chilling in Europe..........................................................................................538 9.6.2.1 Germany ....................................................................................................538 9.6.2.2 Great Britain..............................................................................................538 9.6.2.3 France ........................................................................................................538 9.6.2.4 Italy............................................................................................................539 9.6.2.5 Spain..........................................................................................................540 9.6.2.6 Yugoslavia .................................................................................................541 9.6.2.7 Hungary .....................................................................................................541 9.6.3 Grain Chilling in the U.S. ........................................................................................541 9.6.3.1 Chilling Low-Moisture Maize and Wheat in Michigan, 1988–1991.......542 9.6.3.2 Chilled vs. Ambient vs. No Aeration .......................................................542 9.6.3.3 Chilling Low-Moisture Rice in Louisiana and Michigan, 1991..............544 9.6.3.4 Chilling Low-Moisture Wheat in Kansas, 1991.......................................544 9.6.3.5 Chilling Low-Moisture Wheat in Carrolton, Texas, 1992 .......................544 9.6.3.6 Chilling of Low-Moisture Seeds in Texas since 1997 .............................544 9.6.3.7 Chilling Low-Moisture Maize in Indiana, 1992 ......................................544 9.6.3.8 Chilling Low-Moisture Popcorn in Indiana, 1994–1995 .........................546 9.6.3.9 Simulated Chilling of Low-Moisture Popcorn in Indiana and Kansas .......547 9.6.3.10 Simulated Chilling of Low-Moisture Maize in Indiana, South Carolina, and Texas ........................................................................547

9.4

9.5

9.6


CHILLING OF GRAIN BY REFRIGERATED AIR

491

9.6.4 9.6.5

Grain Chilling in Australia.......................................................................................548 Grain Chilling in the Middle East and Africa .........................................................549 9.6.5.1 Israel ..........................................................................................................549 9.6.5.2 Cyprus .......................................................................................................550 9.6.6 Grain Chilling in Asia ..............................................................................................550 9.6.6.1 Malaysia ....................................................................................................550 9.6.6.2 Thailand.....................................................................................................550 9.6.6.3 China .........................................................................................................552 9.6.7 Grain Chilling in Latin America ..............................................................................553 9.6.7.2 Argentina ...................................................................................................553 9.6.7.3 Brazil .........................................................................................................553 9.6.7.4 Columbia ...................................................................................................554 9.7 Conclusions...........................................................................................................................554 References ......................................................................................................................................555

9.1 INTRODUCTION Aeration to cool and improve storage conditions of grain with selected ambient air has been employed successfully under conditions where ambient air is effective during the cold hours of the day or night and in cold weather seasons. However, there are many storage situations where ambient air conditions are not sufficient to cool grain. To control fungi on moist grain, protect grain against mites and insects, control self-heating, preserve the germination capacity and quality of stored grain in warm climates, or when warm grain is stored immediately after harvest, aeration using ambient air may not be sufficient. In answer to these situations, refrigerated air units for chilling grain have been developed for commodities that can justify the added expense of refrigerated aeration cooling. In this type of aeration process, ambient air is conditioned by passing it through the evaporator coil and a secondary reheat coil of the refrigeration unit and then blowing the chilled air into the grain bulk via the existing aeration system (Figure 9.1). Passage through the secondary reheating coil is designed to adjust the air relative humidity to 60 to 75% to match the target moisture content of the dry grain (Figure 9.2). The amount of reheating and the final air temperature are adjustable by the operator to achieve the desired aeration conditions. In this chapter the term refrigeration is used to describe air passing through an evaporator coil of a refrigeration unit and thereby reaching a cold air temperature that may be near or below the freezing point of water. The term chilling is used for the cooling of grain with refrigerated air. The term grain chilling (or chilled aeration) should not be confused with ambient aeration, which uses selected ambient air temperatures or selected temperatures and relative humidities to cool grain. In both processes, chilled or ambient aeration, the system uses the same air distribution ducts inside storage structures. Slight adaptation may be necessary to connect the air supply duct of the refrigeration unit to the fan inlet. If the chilling unit fan is capable of developing the static pressure to push the design airflow through the grain mass, it may be best to remove the original bin aeration fan from the aeration duct. Often, storage tanks have separate aeration ducts with multiple fans. This requires that the chilled air supply duct be manifolded to multiple inlets of the storage structure. If the chiller is connected to only one inlet, the other inlets need to be sealed off to keep cold air from escaping from the storage structure. Where bulk grain is stored in bins, tanks, silos, and flat storages that are equipped with conventional aeration systems, cooling is possible only to a few degrees above the minimum ambient temperature. In contrast, chilling enables grain to be cooled below the minimum ambient temperature. In large grain bulks, once the grain has been initially cooled, grain temperatures are maintained for long time periods due to the insulating properties of the grain, and rechilling for short time periods may be required only occasionally.


492


Figure 9.1

Schematic of the grain chilling process.

Figure 9.2

Safe storage conditions of grains under various time–temperature–moisture combinations. (From Burrell, N.J. [1982]. Refrigeration, 407–441, in Storage of Cereal Grains and their Products, [Christensen, C.M., Ed.], St. Paul, MN. With permission.)

In many grain growing areas of the world, the moisture content of grain at harvest is too high for safe storage; damp grain is readily attacked by molds unless it is protected in some way. The most widely used method of preventing mold growth is to dry the grain to a safe level. Burrell (1982) argued that there is a balance between safe moisture content and safe temperature; the lower the temperature of bulk grain, the damper it can be safely stored (Figure 9.2). This leads to the conclusion that drying need not be so stringently applied, since the power required to evaporate moisture from a bulk of grain is far greater than that required to cool the same bulk. For example, the energy required to evaporate 6 percentage points of moisture from a grain mass is at least six times greater than that required to cool the same bulk from 25 to 5°C temperature by using refrigerated air. Therefore, chilled storage may have an economic advantage over drying. The advantage is easily lost through grain spoilage caused by storing excessively damp grain, which requires frequent rechilling, or by excessively high capital equipment costs.



493

Drying alone is not successful against all organisms, as dry grain may still be attacked and damaged by insects, especially if the grain temperature is above 15°C. Protecting grain with pesticides has been a widely accepted practice recommended for stored grains in many countries. As grain is marketed and moves through various facilities, the identity of a specific lot of grain is usually lost and additional pesticide treatments may occur. As the number of pesticide applications increases, potentially toxic residues may accumulate in the final food and feed products. Reports from the midwestern U.S. indicate that the lesser grain borer, Rhyzopertha dominica (F.), was found to have become resistant to chlorpyrifos-methyl (Reldan) (Beeman and Wright, 1990; Zettler and Cuperus, 1990). As a result, R. dominica was removed from the label. Also, the label for pirimiphos-methyl (Actellic) now specifies that it will suppress but not control R. dominica, even if it is applied at the maximum recommended rate. As additional insects develop resistance to currently available pesticides, greater emphasis must be placed on alternative control methods for protecting the food supply. Aeration for cooling grain is a technology dependent on the availability of suitable ambient air conditions. The advantage of aeration may be maximized with additional integrated pest management (IPM) strategies (Hagstrum and Flinn, 1992). Although the quality of harvested grain can never be improved with storage time, quality can be maintained or the rate of deterioration can be slowed with an integrated post-harvest management system such as S.L.A.M. (Maier et al., 1994). This approach combines Sanitation of harvesting, handling and processing equipment, and storage structures, with proper Loading of storage structures, including pre-cleaning of grain, spreading, coring, and leveling, which are the initial bases for excellent grain storage management. Controlled ambient or chilled Aeration, combined with effective Monitoring (including inspection, insect sampling with pheromone and pitfall traps, risk-benefit analyses of control strategies, insect resistance, and biological methods) are the additional IPM management tools and strategies needed to maintain grain quality at highest levels with minimum use of pesticides. In a major study on Enhancing the Quality of U.S. Grain for International Trade (U.S. Congress, 1989a), maintaining low temperature and moisture levels in bulk-stored grain was identified as the primary way to preserve grain quality and prevent damage from molds and insects. Grain chilling is a technology that can be successfully applied under many climatic conditions to preserve grain quality during storage when commodity value and profit margins are sufficient. It permits shortto long-term storage of grain independent of the ambient conditions. Grain chilling has been applied commercially in over 50 countries during the past 40 years but has only recently gained recognition in the U.S. (Maier, 1994). It is estimated that over 80 million tonnes of grain are cooled annually worldwide with grain chilling systems. Grain chilling is accepted as a grain conditioning technology in much of Western Europe; currently most new units appear to be marketed in Southeast Asia. In the 1960s grain chillers were primarily used as a means of preserving high-moisture (moist, damp) grain. Later, grain chilling was applied to improving storability of sensitive commodities subject to development of heat foci (hot spots), i.e., for soybeans and maize, and preserving the quality of high-value dry grain, seeds, and edible beans, primarily against mites and insects.

9.2 CHILLING UNIT AND ITS COMPONENTS 9.2.1

Refrigeration Unit

9.2.1.1 Mechanics of Refrigeration The primary components of a typical refrigeration unit are illustrated in Figures 9.3 and 9.4. The main purpose of the refrigeration unit is to remove both heat and moisture from the air before it is blown into the grain bulk. In the refrigeration cycle, a refrigerant evaporates from the liquid


494


Figure 9.3

Principle of the refrigeration system.

Figure 9.4

Components of a typical refrigeration unit. (Data from Navarro, S. and Calderon, M. [1982]. Aeration of grain in subtropical climates, FAO Agricultural Services Bulletin No. 52, Rome.)



495

state and condenses from the gaseous state at designed levels of temperature and pressure (Meacock, 1979). A liquid refrigerant placed in a vessel absorbs heat from its surroundings as it evaporates. A refrigeration unit contains a compressor, typically composed of a piston inside a cylinder (Figure 9.3), which compresses and heats the refrigerant gas. The hot compressed refrigerant gas is forced under pressure into a cooling coil called the condenser. The condenser coil cools the compressed hot refrigerant gas by using a fan to draw atmospheric air over the coil to remove heat from the system. In a warm climate where there is no danger of the water freezing, the condenser may be water cooled. Metal fins on the tubes of the condenser permit rapid heat exchange between the hot gas inside the coil and the cooling medium flowing over it. As the cooling medium removes latent heat of condensation from the compressed refrigerant gas, the gas condenses back to a liquid collected in a reservoir (Figure 9.3). The liquid refrigerant is forced under pressure through a small diameter tubing and an expansion valve, and then is sprayed into a larger diameter evaporator coil. The evaporator coil, operated at a lower pressure, is connected to the suction side of the compressor. At lower pressure, the temperature of the liquid is above its boiling point. The boiling liquid refrigerant converts into a gas, absorbing heat from the coil. The latent and sensible heats required for boiling the refrigerant into a gas are absorbed from ambient air passing over the fins of the evaporating coil, cooling the air. Depending on the chiller operating set-point, the ambient air usually reaches its dew point as it flows across the evaporator coil of a grain chiller (Figure 9.4). After reheating the air slightly in a secondary condenser coil to achieve the desired chilled air relative humidity, the chilled air is blown into the grain bulk through the aeration system manifolds and ducts. Three aspects of refrigeration that take place in the evaporator are discussed in the following sections. 9.2.1.2 Removal of Condensed Water Cooling in the evaporator may increase the air relative humidity to saturation, resulting in water condensation on the coil. The condensed water draining from the cold evaporator coil must be discharged from the refrigeration unit. This precaution is sometimes overlooked, causing condensed water to accumulate in the grain chilling unit at the start of operating it at a new site. Thus, the condensate must be drained. 9.2.1.3 Adjusting the Relative Humidity of Refrigerated Air To adjust the relative humidity of the air to a desired level (typically 65 to 70%), chilled air should be slightly reheated before it enters the grain bulk. Because of its importance in the chilled aeration process, this subject will be discussed in more detail later in this chapter. 9.2.1.4 Ice Formation in the Evaporator As air passes over the cold evaporator coil surfaces, condensate droplets often change to frost or ice accumulation on the evaporator coil fins. As ice forms on the evaporator coil, the refrigeration capacity is reduced because the ice restricts heat flow between the air and evaporator. At an advanced stage, the accumulated ice may also restrict or block the path of the refrigerated air. If near water-freezing temperatures (below 5°C) are reached at the evaporator, an automatic defrost system may be necessary to melt the ice. Defrost systems increase the capital and operating costs of refrigeration units. To avoid defrosting costs, grain chilling units can be controlled by an alternative system which combines a throttle valve and a thermostat to cut out the compressor and interrupt refrigeration whenever the air temperature falls below 5°C. The throttle valve controls the amount of refrigerant


496


directed to the evaporator. Thus, it functions as a differential pressure valve that regulates the amount of chilling that occurs in the evaporator. 9.2.2

The Refrigeration Process

9.2.2.1 Description of the Refrigeration Process The terms adiabatic and isothermal are frequently used in thermodynamics. In an adiabatic process, no heat is added to or removed from a substance while its psychrometric state point changes during a thermal process. Cooling of grain due to moisture release by evaporation of water, without changing the heat content of grain, is an example of an adiabatic process. An isothermal process indicates a change taking place at constant temperature. In this process, the dry-bulb temperature of grain remains constant, but its heat content changes due to moisture increase or decrease. In gases, a characteristic change of temperature is accompanied by a change in volume. A system composed of a vapor and a liquid of the same chemical composition and maintained at constant pressure remains in phase equilibrium only at one temperature. When a liquid and its vapor are in phase equilibrium, they are said to be at the boiling point. The boiling point varies with pressure, so a condition was established to ensure uniformity, known as Standard Atmospheric Pressure (S.A.P.). S.A.P. is defined as a state where atmospheric conditions will support a column of mercury with a mass of 13.60 grams per cubic centimeter 760 mm high when subject to a gravitational acceleration of 980.7 cm per second squared. This is equivalent to 1013 millibars, 14.696 lbs per square inch, or 29.921 inches of mercury. The temperature at which evaporation and condensation of a refrigerant takes place varies with pressure. An increase in temperature corresponds to an increase in pressure, and vice versa. As the gas pressure rises, so does the gas temperature until a state of equilibrium is reached and no further cooling takes place. If the rate of evaporation of the gas is controlled, the temperature and the concurrent rate of cooling of the surroundings can be controlled. If the gas is compressed, its temperature increases to a level higher than the surrounding atmosphere. The gas is then allowed to cool and liquefy, providing a source of liquid refrigerant at a higher pressure than when it evaporates in the evaporator. The addition of a throttling device to reduce the pressure of the hot liquid to that of the evaporating vessel provides the basis of a closed-cycle continuous refrigeration process. The withdrawal and compression of the gas from the evaporating vessel can be accomplished by several means. For large loads, particularly in the air conditioning field, the most common method is the use of a piston compressor. Centrifugal, axial-flow, and screw-type compressors are also used based on designer and manufacturer preferences. The most important factor in any refrigeration cycle is the pressure–temperature relationship of the refrigerant. 9.2.3

Selecting the Refrigeration Unit Capacity

9.2.3.1 Grain Cooling and Heat Removal Loads If chilled air from a refrigeration unit is blown into a warm, moist grain mass, moisture is absorbed from the grain by the air due to its lower water vapor pressure compared to that of the grain. Evaporation results in grain being cooled below the dry-bulb temperature of the air and close to its wet-bulb temperature. In grain chilling, a considerable quantity of energy and moisture is transferred from the grain to the air. In cooling moist grain with chilled air, moist exhaust air leaving the grain bulk has a high latent heat load. In cooling dry grain with chilled air, the exhaust air has a low humidity and thus a low latent heat content. When cooling dry grain, it is usually more economical to recirculate



Figure 9.5

497

Specific heat of various commodities at different moisture contents (1 kcal/kg/°C = 4.1868 kJ/kg/°C) (Disney, 1954 [wheat]; Rao and Pfost, 1980 [shelled peanuts, soybeans, and cottonseed]). (Data from Disney, R.W. [1954]. The specific heat of some cereal grains, Cereal Chem., 31, 229–239. Rao, V.G. and Pfost, H.B. [1980]. Physical properties related to drying 20 food grains, Am. Soc. Agric. Eng. Pap. No. 80-3539 presented at the 1980 winter meeting.)

part of the dry exhaust air through the refrigeration unit rather than to use all ambient air because the recirculated air usually has a lower enthalpy than the ambient air. The quantity of energy to be removed from a grain bulk depends on the temperature drop required and the specific heat of the grain, which varies according to its moisture content (Figure 9.5). Disney (1954) showed that the specific heat of wheat at 23% moisture content was about 16% higher than at 15% moisture content. In addition to a larger heat load for moist grain, the production of metabolic heat is very pronounced at 23% moisture content but is negligible at 15% moisture content. These two factors result in an increase of the heat load while cooling moist grain. This is partially offset during chilling by the evaporation of moisture from the grain through evaporative cooling. The total energy (Ht) removed from a given mass of grain during chilling is calculated as: Ht = Hm + Hr + Hl

(9.1)

where: Ht = total heat to be removed in the cooling process Hm = heat to be removed from a given mass of grain Hr = metabolic heat generated by respiration Hl = ambient and solar heat gain within the storage structure The H values have units of kcal or kJ. Hm = m (to − t1 ) c p

(9.2)


498

Figure 9.6


Mean daily loss of dry matter for cereals. B = barley; W = wheat; V = variable temperature in cooled bins (7.5ºC mean, range 0 to 17ºC). (From Burrell, N.J. [1982]. Refrigeration, 407–441, in Storage of Cereal Grains and their Products, [Christensen, C.M., Ed.], St. Paul, MN, and based on data from Scholtz, 1962; Mattei, 1968; Kreyger, 1967; Steele et al., 1969; Hyde and Burrell, 1973. With permission.)

where: to = the initial temperature (in °C) t1 = the temperature after cooling (in °C) m = the grain bulk mass (in kg) cp = specific heat (in cal • g–1 • °C–1) Example 9.1 If 1 tonne (1000 kg) of wheat at 23% moisture content (cp = 0.5 cal • g–1 • °C–1) is to be cooled from 30 to 5°C in one day, then Hm = 1000 (30 − 5) 0.5 = 12, 500 kcal The heat to be removed from the grain (Hm) must be added to the mean heat production due to respiration (Hr) of a tonne of grain at 23% moisture content at a temperature of 30°C reduced to 5°C. The dry matter loss of a tonne of wheat at 23% moisture content is about 0.09% per day at 30°C (Figure 9.6) and 0.01% per day at 5°C. Assuming the bulk to cool evenly from 30 to 5°C, a mean dry matter loss of 0.05% per day, which is equivalent to a daily heat production of 1880 kcal is assumed. This increases the heat load by about 15% to a total of 12,500 + 1880 = 14,380 kcal. Additional heat gain (Hl) through the walls, floor, roof, and ducting must be estimated from thermal conductivity tables for the materials used. Models that estimate heat gain into the storage structure were developed and described in Chapter 2. Although it is important to be able to estimate heat gain through the structure, the insulation properties of the grain bulk considerably reduce this effect, particularly in large bulks. 9.2.3.2 Selecting the Cooling Rate Ambient conditions, the cooling rate, the specific airflow rate, and grain conditions need to be considered in selecting the refrigeration capacity of the grain chilling unit. To select the refrigeration unit capacity, the first step is to plot the average operational ambient temperature and relative



499

humidity conditions on a psychrometric chart (Appendix A, Figure A.1 Psychrometric chart). On the chart, the acceptable range of refrigerated air conditions can be described. The enthalpy difference between the refrigerated air conditions and the ambient air is the heat removal requirement of the refrigeration unit. Operation of a refrigeration unit attached to an existing aeration system depends on the maximum airflow rate of the aeration system, and the refrigeration unit airflow should be adapted to this rate. If the chilled airflow rate is higher than the aeration system design flow rate, a pressure loss results that decreases the grain chilling efficiency. Under extreme conditions where the aeration system cannot efficiently absorb the full airflow of the refrigeration unit, capacity is wasted as cooled air is heated due to the frictional pressure losses in the aeration system. Once the ambient conditions and the airflow rate are known, the size of the refrigeration unit required to deliver a predetermined air temperature is calculated as: Refrigeration unit capacity = ∆h ⋅ Q ⋅ Sw

(9.3)

where: ∆h = the enthalpy difference — i.e., the difference in heat content of the ambient air and the air leaving the refrigeration unit, in kcal/kg or kJ/kg (Figure A.1) Q = the air volume flowing through the unit in m3/h Sw = the specific weight of ambient air in kg/m3 (reciprocal of specific volume, from Figure A.1). Example 9.2 For average summer conditions of 30°C and 50% RH, the heat content of air is 15.41 kcal/kg (64.5 kJ/kg). If the evaporator is to be designed to cool the air to 5°C, the heat content of the air will be 4.42 kcal/kg (18.5 kJ/kg). Thus, the refrigeration unit should be sized to remove ∆h = 15.41 – 4.42 = 10.99 kcal/kg (46 kJ/kg) from the air. To chill 1000 tonnes of wheat with a design airflow rate of 3.0 (m3/h)/tonne, the total volume flowing through the refrigeration unit will be 3000 m3/h. The specific weight of ambient air at 30°C and 50% RH is 1.14 kg/m3. For the given conditions: Refrigeration unit capacity = 10.99 × 3000 × 1.14 = 37, 586 kcal h

(or 157, 364 kJ

h = 43.71 kW )

The above calculation refers to the total heat output for the refrigeration cycle. This value is different from the heat equivalent of work. To calculate the heat equivalent of work, the coefficient of performance (COP) must be considered. The COP, which expresses the effectiveness of a refrigeration system, is the ratio of useful refrigeration output to the net energy supplied from external sources, such as electrical power (Threlkeld, 1970). The COP depends on the refrigerant, head pressure, suction pressure, superheated conditions, and environmental conditions (Stoecker, 1958). For a typical refrigeration unit, the coefficient of performance ranges between 2.5 and 3.5 in subtropical climates, depending on the ambient conditions and the refrigerated air volume involved. Elder et al. (1975), using a standard commercial chiller of 70 kW nominal capacity to cool wheat to 15°C, recorded an average COP of 3.0 with the unit working at 80% of capacity. Using the above example, with a coefficient of performance of 3.0, the work supplied to the compressor will be: 43.71 kW ÷ 3.0 = 14.6 kW


500


To this value the equivalent power requirement for the aeration fan and condenser fan must be added to determine the total connected load requirement. In a trial using a refrigeration unit equipped with a 14.9 kW compressor, a 3.7 kW radial fan was used to cool the condenser, and a 7.5 kW centrifugal fan was used for blowing the air (Navarro et al., 1973a). The cooling capacity of the compressor at 0°C evaporation temperature and 30°C refrigerant condensation temperature was rated at 42,500 kcal/h (177,939 kJ/h or 49.4 kW). Under the operational conditions of the test, the total average consumption recorded was 17.7 kW. This illustrates the difference between the net energy supplied by the whole unit (17.7 kW) and the total refrigeration capacity (49.4 kW) at a coefficient of performance of 2.8. 9.2.4

Airflow Control

A major problem encountered in the operation of refrigeration units is the control of airflow and the resulting effect on air temperature leaving the refrigeration unit. The chilled air temperatures the refrigeration unit can deliver under prevailing ambient air conditions can be calculated. When the ambient conditions are constant, an increase in airflow of relatively warm air results in an increase in the chilled air temperature. A decrease in such airflow causes a decrease in the achievable chilled air temperature. For a given refrigeration unit, under constant airflow, the expected chilled air temperatures are also influenced by variations in daily and seasonal ambient conditions. A critical step in selecting a suitable refrigeration unit is to ensure that the fan characteristics of the unit are properly matched with the aeration system of the storage structure. Generally, the fan characteristics of a refrigeration unit are predetermined and rated for operation within a wide range of applications (i.e., for cooling large grains such as maize and soybeans or small grains such as sorghum and wheat in silos, bins, tanks, or flat storages). Thus, the blower capacity of a specific chiller model may not effectively fit the static pressure requirements of the existing aeration system. An option in new installations is to design the aeration system according to the fan characteristics of the selected refrigeration unit, assuming the chilling unit airflow cools the grain in an acceptable length of time. For practical purposes, grain chilling units are rated according to the mass of grain they are capable of cooling within 24 hours at standard ambient conditions. Although this is of value in guiding the user, a unit may not achieve its rated capacity if the flow of chilled air cannot be obtained due to excessive static pressure and/or warm ambient conditions. Older refrigeration units were supplied with manual airflow control or with an automatic damper that restricts airflow according to the chilled air temperature. These flow controls are needed to reduce the airflow rate when ambient air is warm and the refrigeration unit is not capable of reducing the chilled air temperature to the set-point. Recent developments enable the use of accurate variable speed drives (VSDs) that electronically control the fan motor revolutions (Maier et al., 1996a). 9.2.5

Modeling the Refrigeration System of a Grain Chiller

For economic reasons, grain chillers are not built with unlimited refrigeration and fan capacities. Grain chiller manufacturers have built several models with a range of airflow and refrigeration capacities to handle different ambient and grain cooling loads. In order to achieve the desired grain chilling effect, the refrigeration capacity of a system needs to be properly chosen as indicated in the previous Section 9.2.3. In current commercial grain chillers, the operator needs to select the grain type, the desired chilled grain temperature, and moisture content. The built-in microprocessor-based controller adjusts the cold-air temperature across the evaporator, the necessary reheat temperature to match the equilibrium moisture content of the stored grain, and the airflow rate through the chiller to maximize the refrigeration load.



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For certain ambient conditions (cold spells in the early fall and spring), the set-point temperature at the chiller outlet is maintained by built-in electrical heaters. Thus, grain chillers can operate in three distinct modes: (1) maximum refrigeration (during high ambient heat load the airflow is throttled); (2) maximum airflow (during low ambient heat load the refrigeration capacity is throttled); and (3) assisted electrical heating (when the reheating system is not sufficient during lowtemperature ambient conditions). The single-stage refrigeration cycle of a grain chiller can be modeled by steady-state internal energy balances across the compressor, condenser, and evaporator. This requires knowledge of the thermodynamic properties of the refrigerant, i.e., the entropy, enthalpy, and specific volume as functions of temperature and pressure at each state point in the operating cycle. Also, the refrigerant mass flow rate, which is a function of the design of the compressor, must be known. However, the internal energy and mass balances of the refrigeration cycle are not of direct relevance to the modeling of the chilled grain aeration system. Thus, they are not considered here. In contrast, the external energy balances across the condenser, reheater, and evaporator coils are essential for grain chilling system modeling. They can be established in terms of the heat transfer between the refrigerant and the air flowing across the heat exchangers. Since the airflow and the air temperature and relative humidity into the bin are significant in the simulation of a chilled aeration system, these factors should be considered in the modeling of a grain chiller. 9.2.5.1 Compressor Compressors are the major energy-consuming units in the chilling system. Their design affects the characteristic performance from which energy consumption under different operating conditions can be estimated. Threlkeld (1970) described theoretical compressor equations that are widely used in the design of refrigeration cycles (Cleland, 1991; Rumsey, 1989; Smith et al., 1987; Cleland, 1983). However, a characteristic performance curve for a specific commercial compressor can only be obtained from the manufacturer. Maier (1992) modeled a grain chiller that represents an existing design (Granifrigor KK140, Sulzer-Escher Wyss, Germany) and used its characteristic compressor curves to calculate and design the compressor, evaporating, and condensing capacities of the refrigeration cycle. The compressor of the grain chiller uses refrigerant R-22 and operates with 4°C subcooling and 10°C superheating. The equations describing the compressor curves of the KK140 grain chiller were presented by Maier and Bakker-Arkema (1992b). 9.2.5.2 Condenser The condenser of the chilling unit can be analyzed externally as a dry, finned-tube, cross-flow heat exchanger. The heat transfer, qc(W), from the chiller condenser to the externally flowing ambient air is (Holman, 1986): qC = UC AC ∆TC

(9.4)

where: UC (W/m2/°C) = the overall condenser heat transfer coefficient AC (m2) = the total surface area for heat exchange between the condenser coil and the air The log-mean temperature difference (LMTD), ∆TC (°C), can be used for cross-flow heat exchanger calculations when one of the fluid temperatures remains constant. In the vapor compression cycle of the grain chiller, the refrigerant condenses at constant temperature. The LMTD for the condenser is (Holman, 1986):


502


∆TC =

To − Ta T −T  ln  o c   Ta − Tc 

(9.5)

where: To (°C) = the condenser outlet air temperature Ta (°C) = the ambient air temperature Tc (°C) = the condensing refrigerant temperature 9.2.5.3 Reheater The theoretical analysis of the reheater is similar to that of the condenser. The condensing capacity of the reheater in terms of the external heat exchange is qH(W): qH = UH AH ∆TH

(9.6)

where UH(W/m2/°C) is the overall reheater heat transfer coefficient, and AH(m2) is the total heat exchange surface area of the reheater coil. The log-mean temperature difference of the reheater, ∆TH(°C), is: ∆TH =

T2 − T3  T3 − Tc  ln    T2 − Tc 

(9.7)

where: T2 (°C) = the chilled air temperature before the reheater T3 (°C) = the chilled air temperature after the reheater and before the connecting duct Tc (°C) = the condensing refrigerant temperature 9.2.5.4 Evaporator The external evaporator analysis of the grain chiller must account for dehumidification of the air flowing across the heat exchanger. During dehumidification, the air-side surface is wetted with water (or frost). The heat transfer occurs due to sensible cooling and condensation (latent cooling). The heat transfer from the wet chiller evaporator to the external airflow can be calculated as a function of the enthalpy difference qE(W) (Threlkeld, 1970): qE = UEw AE ∆hE

(9.8)

where: UEw (W/m2/(kJ/kg)) = the overall wet evaporator heat transfer coefficient AE (m2) = the total heat exchange surface area of the evaporator coil If the refrigerant temperature remains essentially constant during evaporation (as in the vapor compression cycle of a grain chiller) and if the energy lost by the air due to condensation is assumed negligible, the following approximate log-mean enthalpy difference (LMHD), ∆hE (kJ/kg), applies (Threlkeld, 1970):



∆hE =

503

h1 − h2  h −h  ln  1 s, R   h2 − hs, R 

(9.9)

where: h1 (kJ/kg) = the enthalpy of the air before the evaporator h2 (kJ/kg)

= the enthalpy of the air after the evaporator

hs,R (kJ/kg) = a fictitious enthalpy of saturated air at the evaporating temperature For certain combinations of cold-air set-points and ambient cooling loads, no condensation occurs in the evaporator. Then, the evaporator heat transfer analysis is similar to the condenser and reheater dry-fin analysis. The heat transfer from the dry chiller evaporator, qE (W), to the external airflow is: qE = UE AE ∆TE

(9.10)

where: UE (W/m2/°C) = the overall dry-evaporator heat transfer coefficient AE (m2) = the total heat exchange surface area of the evaporator coil The log-mean temperature difference of the dry evaporator, ∆TE (°C), is: ∆TE =

T1 − T2 T −T  ln  2 e   T1 − Te 

(9.11)

where: T1 (°C) = the temperature of the air before the evaporator T2 (°C) = the temperature of the air after the evaporator Te (°C) = the evaporating refrigerant temperature To solve the steady-state external energy balances across the condenser, reheater, and evaporator, as shown in Equations 9.4, 9.6, 9.8, and 9.10, the overall heat transfer coefficients must be known. Expressions for UC, UH, UE, and UEw for a commercial grain chiller were determined empirically from experimental performance data supplied by the manufacturer (Maier, 1992). The formulation of the expressions requires the analysis of the experimental data in terms of the compressor capacities and the psychrometric state points of the air before and after the condenser, reheater, and evaporator, respectively. 9.2.5.5 Chiller Performance Data The experimental data collected during performance tests of a grain chiller (Maier, 1992) included the ambient air temperatures and relative humidity; the air temperature after the centrifugal fan (and before the evaporator); after the evaporator, and after the reheater; the airflow into the centrifugal fan at a given static counterpressure; the temperatures of the evaporating and condensing refrigerant; and the total electrical energy consumption of the compressor, centrifugal fan, condenser fan, and the electrical controls of the chiller. Based on the experimental data, the psychrometric state points for the air passing through the chilling unit were established over a range of ambient cooling loads. At every state point at least


504


two psychrometric air properties were known, allowing the remaining properties to be calculated. From the experimental psychrometric state points and the compressor curves, expressions for the overall heat transfer coefficients of the evaporator, condenser, and reheater of the chilling unit were determined. 9.2.5.6 The Chiller Refrigeration Cycle Model After developing external energy balances for the components of the refrigeration cycle of the KK140 grain chiller, Maier (1992) developed a numerical procedure to simulate the steady-state performance under any ambient and grain cooling load. The output from the grain chiller model (i.e., the temperature, relative humidity, and flow rate of the air) were used as the inputs to a grain aeration and storage model. Additionally, the total power consumption and the operating hours of the chiller are of interest in the analysis of the chilling and storage of cereal grains. The algorithm of the grain chiller simulation model differentiates between the three distinct operating modes of the chiller — maximum refrigeration, maximum airflow, and electrical heating. Details of each mode were described by Maier (1992). The grain chiller operates within a specific airflow range. The minimum airflow occurs with the fan throttle closed. The closed position is not at zero airflow but rather when the airflap allows only about 25% of the maximum airflow to pass. This is a safety precaution to prevent the fan capacity from being restricted excessively, thereby causing ice formation on the evaporator and possibly total blockage of air. If the minimum airflow is too low for the evaporative cooling capacity, the chiller controller shuts off the system automatically within a few minutes. The maximum airflow occurs at fully open fan throttle (or, in the case of a VSD, at maximum motor rpm) and is determined by the binchiller operating point as described below. As the chiller fan forces air into the storage structure and through the grain, resistance to the airflow (i.e., pressure drop) develops as a result of air friction and turbulence. The pressure drop through grain depends on the rate of airflow, surface and shape of the grain, porosity of the grain bulk, depth of the pile, and settling of the grain due to structural vibration (Brooker et al., 1992). In addition to the pressure drop through the grain, pressure drop occurs in the aeration duct or the perforated floor and in the expansion of the air from the duct into the bin. Maier et al. (1993c) assumed these additional losses to add about 20% to the total pressure drop. The pressure drop in the chiller includes the resistance of the duct connected to the bin, the reheater, the evaporator, the throttle controlling the airflow across the evaporator, and the filter screen before the centrifugal fan. The commercial grain chiller used in their study added 1250 Pa of pressure drop at a fully open throttle, and 200 Pa at a closed throttle, to the total pressure drop of the system. From the pressure drop through the grain and the additional pressure losses, a system curve of the static pressure as a function of the airflow can be established. When plotted on the same diagram, the intersection of the system curve and the fan curve yields the operating point for the combined bin-chiller system at full throttle. An algorithm similar to the one suggested by Brooker et al. (1992) was implemented to calculate the operating point in the grain chiller simulation model. 9.2.5.7 Evaporator Heat Transfer Coefficient To calculate values of the overall wet evaporator heat transfer coefficient, Equation 9.8 is solved for UEw. The evaporator capacities were calculated from the compressor curve as a function of the experimental evaporating and condensing refrigerant temperatures. The evaporator heat transfer surface area (AE) was known. The LMHD values are calculated from Equation 9.9. The enthalpies before (h1) and after (h2) the evaporator are calculated from the psychrometric state points. The fictitious enthalpy hs,R is calculated for saturated air at the evaporating refrigerant temperature.



Figure 9.7

505

Experimental overall heat transfer coefficients and corresponding regression line for the wet evaporator as a function of the face velocity. (From Maier, D.E. [1992]. The chilled aeration and storage of cereal grains, Ph.D. dissertation, Michigan State University, East Lansing. With permission.)

The overall heat transfer coefficient for the wet evaporator of the grain chiller can be expressed as a function of the airflow through the evaporator coil. The face velocity, VfE (m/s), at the inlet to the evaporator is: VfE =

m˙ a v1 A fE

(9.12)

where: · (kg/s) is the dry air mass flowrate through the chiller m a v1 is the specific volume (m3/kg) of the air before the evaporator Af E (m2) is the face area of the evaporator heat transfer coil Figure 9.7 shows the values of the overall wet evaporator heat transfer coefficient, calculated from Equation 9.8, vs. the face velocity, calculated from Equation 9.12, for a range of experimental cooling loads. From a linear regression analysis, the following equation can be determined from the data. UEw = 13.0 + 1.41VfE

(9.13)

Figure 9.8 shows the values of the overall dry-evaporator heat transfer coefficient, calculated from Equation 9.10 after solving for UE, vs. the face velocity, calculated from Equation 9.12, for a range of experimental cooling loads. From the experimental data, the following equation was derived for the dry evaporator heat transfer coefficient: UE = 14.1 + 2.78 VfE

(9.14)


506

Figure 9.8


Experimental overall heat transfer coefficients and corresponding regression line for the dry evaporator as a function of the face velocity. (From Maier, D.E. [1992]. The chilled aeration and storage of cereal grains, Ph.D. dissertation, Michigan State University, East Lansing. With permission.)

9.2.5.8 Reheater Heat Transfer Coefficient The calculation of the reheater capacity in Equation 9.6 requires knowledge of the overall reheater heat transfer coefficient. The analysis of the experimental data (see Figure 9.9) yielded the following equation: UH = 15.4 + 2.73 VfH

(9.15)

The face velocity into the reheater, VfH (m/s), was calculated as: VfH =

m˙ a v2 A fH

(9.16)

· (kg/s) is the dry air mass flow rate through the chiller, v is the specific volume (m2/kg) where m a 2 of the air before the reheater, and Af H (m2) is the face area of the reheater heat transfer coil. 9.2.5.9 Condenser Heat Transfer Coefficient The calculation of the condenser capacity in Equation 9.4 requires knowledge of the overall condenser heat transfer coefficient. Since the condenser fan speed is constant, the air velocity through the condenser is finite. Thus, the overall condenser heat transfer coefficient is constant. The analysis of the performance data yielded the following value: UC = 27.8 W m 2 °C

(9.17)

The face velocity, Vfc (m/s), into the condenser was: Vfc = 2.25 m s

(9.18)



Figure 9.9

9.2.6

507

Experimental overall heat transfer coefficient and corresponding regression line for the reheater as a function of the face velocity. (Data from Maier, D.E. [1992]. The chilled aeration and storage of cereal grains, Ph.D. dissertation, Michigan State University, East Lansing. With permission.)

Alternative Cold Air Systems

9.2.6.1 Dessicant Cooling System Thorpe (1998) patented a grain cooling system using a desiccant, which cools atmospheric air by drying it at night (Figure 9.10). The dry air is blown through the stored grains, cooling them to their wet-bulb temperature. During the day, the desiccant is dried using air that is heated by a gas burner or by a solar collector. Warm humid air leaving the desiccant is vented to the atmosphere. In extensive field trials, the prototype chiller reduced the wet-bulb temperature of ambient air by more than 6°C, which resulted in effective cooling of the stored grains. In terms of energy consumption, one tonne of grain can be cooled using about 20 mega joules (MJ) of heating energy. He estimated that if domestic gas was used, the cost of gas would be less than 20 Australian cents ($0.12) per tonne of grain cooled. Solar energy can be used in conjunction with the gas burner or as a sole source, depending on the climatic conditions. Beery (1998) has evaluated desiccant wheel dehumidification systems using enhanced starchbased (ESA) desiccant adsorbents. Moist air passes through half of a rotating wheel packed with desiccant. The air emerges drier and can be cooled more efficiently than humid air. On the other half of the wheel, warm air passes through the desiccant and regenerates it. Maize grits are an ESA that have been used in a Pressure Swing Adsorption System (PSA) to produce dry (low dew point) air. When operated with 1.0 mm maize grits, the PSA system dried moist air to a dew point of –69°C; modified 1.0 mm maize grits dried moist air to a –80°C dew point. The maize grits were stable over at least 250,000 cycles of operation in the PSA system. The unique aspect of utilizing a desiccant wheel dehumidification system is the potential for chilling grain with a grain-based desiccant. 9.2.6.2 Heat-Pump Cooling Systems 9.2.6.2.1

Dehydrofrigidation

Shove (1966) developed a refrigeration process called dehydrofrigidation to dry and chill grain at low temperatures in an insulated dome structure using a refrigeration system. The first phase


508

Figure 9.10


Solar-assisted desiccant grain cooling system developed in Australia. (Thorpe, G.R. (1998). Personal communication, Victoria University of Technology, Melbourne.)

consisted of cooling the shelled maize from its harvest temperature to –1 to 10°C within 24 hours. Assuming a moisture content above 22%, about 30 (m3/h)/tonne were needed to cool the grain. No recirculation was proposed during the cooling phase since the enthalpy of the exhaust air was generally higher than the inlet air to the refrigerator. A unit of 35 kW refrigeration capacity was considered sufficient to cool a daily load of 102 to 122 tonnes in the corn belt of the U.S. Once the grain was chilled, the drying phase began. Shove (1966) designed a closed system to dry maize to 15.5% moisture content by dehumidifying the air in the evaporator and reheating it slightly in the condenser to control the relative humidity. The design goal of the tested system was in the range of 40 to 120 kW·h/tonne per 10 points of moisture removal, including cooling. The system had only a small daily drying rate due to the short allowable storage time of wet maize. Due to several experimental test problems encountered, including molding of the grain in the top layers and inadequate controls to properly condition the air, dehydrofrigidation was never commercialized. In subtropical climates a different situation is encountered than in the U.S. corn belt. The grain moisture content after harvest is usually relatively low but occasionally may remain several points above the safe storage moisture content (whenever the interstitial relative humidity remains above 65 to 70%). The ambient temperatures after harvest usually remain high (25 to 30°C). Drying at low



509

temperatures using refrigeration units may appear inefficient when compared with high-temperature drying, yet the overall rate of power consumption for chilling is significantly lower; and machinery and handling costs are less. For seeds requiring special care to maintain varietal purity and prevent occasional mixture with other varieties, the in-situ aspect of dehydrofrigidation may present an advantage. Tests conducted in drying wheat seeds with refrigerated air indicated the feasibility of this approach (Navarro et al., 1978). A refrigeration unit of 42,500 kcal/h was used to deliver air at a temperature of 15°C and relative humidity of 50% under summer conditions. The initial seed temperature of approximately 30°C was reduced to 14°C after 162 hours of dehydrofrigidation. Seed moisture content of 14.2% was reduced to 12.3% at a power consumption of 12 kW·h/tonne of seeds. Although this method has been adopted by a seed company in Israel, it requires further research. 9.2.6.2.2

Air-to-Air Heat Pumps

Air-to-air heat pumps are widely used in domestic and commercial space heating and cooling applications. They are also used for water heating and for specialty uses such as curing lumber and drying grain. The attractiveness of heat pumps lies in their high coefficient of performance (COP) and low energy demand. Foster (1984) reported using a heat pump for low-temperature maize drying as early as 1975. The heat pump achieved a system COP of 2.36, and electrical demand was 52% less than for a comparable electrical-resistance system. The heat pump was permanently installed on a 100-tonne grain bin and could have been utilized for grain chilling during the summer by reversing the airflow through the unit. Cunney et al. (1983) used a fuel-engine-driven heat pump to chill undried barley intermittently in order to extend its storage life. The barley was simultaneously dried in batches using heat recovered from the engine, ambient air, and dryer exhaust. The 20.5% moisture content barley had to be rechilled every 2 to 3 weeks as its temperature began to rise as soon as chilling was finished. The system had an energy absorption rate of 10 to 12 kW; and, at typical winter temperatures in Ireland, attained a COP of 5.0 to 5.5. Drying air temperatures up to 60ºC were used. Steele (1998) proposed and validated a new concept in using a heat pump grain drying and cooling system. The theoretical performance potential of the new concept was determined to be less than 698 kJ/kg of water removed when artificially drying shelled maize. The experimental closed-loop heat pump system was connected to a stationary grain drying bin (6.4 m diameter). The heat pump system components included a trim condenser, a counter-flow air-to-air heat exchanger, a fan, and a data acquisition and control unit to monitor grain and system operating parameters. The final concept calls for a combination of continuous counterflow drying through one part of the bin followed by chilling of the grain through the other half of the bin. The application of air-to-air heat pumps may have a potential use in drying systems where the grain is slightly moist (up to 15% mc for cereals) and for relatively small bulks. The main reason for these limitations is the capacity of the chilling unit that is a restricting factor for which a large capital investment is necessary. This technology may be economically justified when operated as described by Steele (1998) in grain drying and cooling systems. Because of the limited sizes of the chilling units and the corresponding airflow delivery, their drying rate may remain limited. Therefore, they may fit for drying and cooling of small bulks in the range of about 100 to 200 tonnes. 9.2.6.2.3

Water-to-Air Heat Pumps

Water-to-air heat pumps are also used to supply the heating and cooling needs of domestic and commercial spaces. Both open- and closed-loop systems are available. Open-loop systems require large quantities of water from a well or a stream and are not considered environmentally friendly because the heat exchange raises the temperature of the water returned to the well or stream. Closed-loop


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(geothermal) systems recirculate a fixed amount of water through a long loop buried several meters below the ground surface. A primary advantage of water-to-air heat pumps is that the temperature of the ground, through which the water is recirculated, generally stays relatively constant — which makes the heat exchange more efficient compared to the seasonal and daily fluctuations of ambient air used with air-to-air heat pumps. No reports have been found in the literature on the use of waterto-air heat pumps for the cooling or chilling of grain. 9.2.6.3 Evaporative Cooling Systems Evaporative cooling systems are used extensively in the greenhouse industry. They function based on the psychrometric principle that the dry-bulb temperature of relatively unsaturated air is reduced along its wet-bulb temperature line when humidity is added to the air. Evaporative cooling systems move large volumes of dry air across humidification pads in order to saturate the air and reduce its dry-bulb temperature. No application of evaporative cooling systems to the cooling or chilling of grains has been found in the literature. However, the principle of cooling grain to its wet-bulb temperature has been exploited in the desiccant cooling system described by Thorpe (see Section 9.2.6.1) and in the seed wet-bulb temperature (SWBT) aeration control scheme described by Desmarchelier (1988) (see Section 7.2.2). 9.2.6.4 Chilled Water Systems In the north of Buenos Aires province (Argentina), a seed conditioning plant utilizes a 28 m double-belt drying (3 zones) and chilling (1 zone) unit to ensure that seed is dry and cool after a water-based seed treatment to avoid loss of germination and spoilage in bags. Drying takes place with liquid petroleum (LP) gas burners, which heats ambient air from the drying fans to a maximum temperature of 40°C. The refrigerated cooling unit at the last section of the belt dryer reduces seed temperatures from 28 to 3°C with 4.2 m3/h of airflow. After transferring the seed into the bags, seed temperatures are reportedly still around 10°C. The waste air is recirculated through a watercooled finned-heat exchanger in order to maximize cooling efficiency. Due to the confidential nature of the application, no additional system performance details are available (Caramangiu, 1999).

9.3 OPERATION OF A GRAIN CHILLER Two basic objectives are considered for using refrigeration systems: 1. Cooling of moist grain without changing the moisture content: Cooling of moist grain must be considered separately from dry-grain cooling. For moist cereals the main objective is to cool the grain and store it at a higher moisture content. In the past this was an accepted practice for the temperate European and British climates, where the objective was to cool cereals of up to 17% moisture content to a temperature of 12°C and below (Burrell, 1974). McCune et al. (1963) used conditioned air to maintain the moisture content of grain at any desired level. He suggested that overdrying, occurring in bulks of grain ventilated with fresh air, could be avoided by refrigerating with air conditioned to the equilibrium relative humidity of the grain. Attempting to store grain by refrigeration for more than a few weeks at moisture contents above 22% for cereals does not seem feasible because of the need for frequent refrigeration, heavy thermal insulation, and the high capital and operating costs incurred (Burrell, 1982). There are practical reasons for storing damp grain for various periods in addition to the desire to reduce fuel costs during drying and eliminate expensive drying machinery that may stand idle



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for a large proportion of the year. For example, damp grain is preferable to dry grain for livestock feed because it is more easily rolled and crushed and more efficiently digested. Dust is less of a problem when damp grain is handled, thereby reducing the chances of dust inhalation, dust explosion, and fire. For milling, wheat is usually required to be processed at 15 to 17% moisture content. At this moisture content, damp grain can heat unless periodic cooling is applied. Therefore, one of the purposes of grain chilling is to reduce power consumption by storing grain at higher moisture content than would otherwise be safe. According to Burrell (1992), the main aim of grain chilling in Europe has been to cool the grain and to store it at a higher moisture content than was previously considered safe. Observations in France and Belgium indicate that grain arriving at up to 17% moisture is often chilled and undried, and that damper grain is usually dried to below this level before chilling. Typical grain temperatures achieved by chilling after harvest in the warmer areas of France indicate that temperatures of 12 to 16°C are considered acceptable for storing grain at up to 16 to 17% moisture content. During chilling, cold air from the refrigeration unit is heated slightly, sometimes by a secondary condenser or an extra heat source, or merely by fan heat, to reduce the relative humidity of the air. The air humidity is usually lowered to a level of 70 to 80% RH that is in equilibrium with the moist grain. This air is then injected into the base of the grain bin floor through perforated ducts. The grain at the base is cooled first. Above this is a zone of cooling grain, while higher in the bin, the grain remains warm. If the grain is freshly harvested and damp, the exhaust air from the top of the bin normally has a higher enthalpy than that of the outside air, particularly at night. Calculations based on the temperature and humidity of the exhaust air showed that its enthalpy, when chilling is started, is usually above that of the ambient air. On British farms, it is often the practice to start chilling at a high rate of airflow, and at a temperature only a few degrees below ambient, in order to pass a cold front completely through each bin on the day following harvest. At a later date a longer chilling period is completed at a lower rate of airflow and, consequently, with a larger temperature drop (Burrell, 1992). These practices are based on the use of limited heating of refrigerated air, without special precautions to condition the air. This assumes that the grain at 16 to 17% moisture content has an equilibrium relative humidity in the range of 80 to 85%. At the airflow rates applied for chilling, changes in moisture content of grain inside the bulk, due to the reduced relative humidity of air that encounters grain at higher temperature, is not significant. Conditioned air to maintain the relative humidity and temperature at any required level can be obtained by using a variable-speed fan for refrigerated air supply and a sufficiently effective reheating element. However, for practical reasons this accuracy may not be required for chilling moist grain, since the most significant changes in moisture content are expected to take place on the layers around the aeration duct. Experimental field work on the use of this method of chilling moist grain without changing its moisture content is scarce. 2. Cooling of dry grain by evaporative cooling with moisture loss: For dry grain, the efficient control of the relative humidity of the chilled air becomes critically important. The objective of cooling dry grain using refrigerated air is applicable to subtropical climates (Navarro et al., 1973a; Sutherland et al., 1970). In this case the use of refrigerated air to cool dry grain is aimed at insect rather than microflora control. Refrigeration of dry grain can be applied directly by connecting the refrigeration unit to an existing aeration system of the bin. Several modifications were made to improve chilling such as thermal insulation of the whole bin structure (Elder et al., 1975), recirculation of the exhaust air from the top of the bin (Thorpe, 1976) or both. These aspects will be discussed in the following sections. The simulated cool-down of a 579-tonne bin of dry wheat (14% moisture content) is compared in Figure 9.11 for central Michigan for the 1988 through 1991 seasons (Maier, 1992). Under the same grain conditions, bin dimensions, and chiller settings, the cooling times varied by as much as 3 days due to variations in weather conditions during the four years. The fastest and slowest cool-downs were predicted in back-to-back years. In July of 1988 it required 207 hours to chill the wheat from 30°C to below 15°C compared with 270 hours in 1989, 225 hours in 1990, and 258 hours in 1991. The predicted power consumption was 4.55 kW·h/tonne in 1988,


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Figure 9.11

Simulated chilled aeration of a 579-tonne bin of wheat in early July for the 1988, 1989, 1990, and 1991 seasons in central Michigan. Initial wheat temperature was 30ºC and moisture content 14%. (Data from Maier, D.E. [1992]. The chilled aeration and storage of cereal grains, Ph.D. dissertation, Michigan State University, East Lansing. With permission.)

6.07 kW·h/tonne in 1989, 4.93 kW·h/tonne in 1990, and 5.76 kW·h/tonne in 1991 for the 15°C grain temperature reduction during the four summer seasons. The simulated performance of the grain chiller is shown in Figure 9.12 for the 1989 cool-down (Maier and Bakker-Arkema, 1992a). Given the cyclical behavior of the ambient temperature and relative humidity, the chiller opens and closes the air throttle to maintain the bin inlet air conditions at a constant 14°C (13°C at the chiller plus 1°C heat gain in the duct) and 67% RH. The variable airflow is a unique characteristic of a chilled aeration system. In comparison, ambient aeration maintains a constant airflow but has variable air inlet conditions.

9.3.1

Grain Chiller Controls

A chilling unit needs to be equipped with several air temperature and airflow controllers to optimize its performance. These controllers should be strategically located in a visible area mounted on the chilling unit.



Figure 9.12

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Simulated performance of a German-built grain chiller when cooling 579 tonnes of wheat in July of 1989 in central Michigan. Bin inlet air temperature 14ºC and 67% RH. (Data from Maier, D.E. [1992]. The chilled aeration and storage of cereal grains, Ph.D. dissertation, Michigan State University, East Lansing. With permission.)

9.3.1.1 Control of Evaporator Temperature A thermostat to control the compressor temperature is essential to avoid ice formation on the evaporator coil. Figure 9.13 shows the evaporator unit where the thermostat sensor is located to control the compressor temperature. The minimum temperature setting should be 4°C to limit ice formation. A common problem encountered in the evaporator is the partial blockage of the evaporator coil fins from dust deposits on the coils due to lack of maintenance or a missing air filter. The partial blockage reduces the airflow through the evaporator and the heat transfer efficiency of the coil fins and reduces the air temperature in the evaporator. 9.3.1.2 Airflow Controller A thermostatic control is needed to reduce the airflow rate when ambient air is warm and the refrigeration unit is not capable of reducing the chilled air temperature to the set-point. A motorized


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Figure 9.13


Location of thermostat on the evaporator to control the compressor temperature of the grain chiller refrigeration system.

damper or variable-speed drive that electronically controls the motor speed is typically used in commercial grain chillers. The thermostat should be located immediately after the evaporator and before the reheating system (Figure 9.4). The selection of the set-point of this thermostat is discussed in Section 9.3.2. 9.3.1.3 Control of Reheat Temperature A differential thermostat is needed to control the operation of the reheater. The set-point of this thermostat should be adjusted to obtain a relative humidity in equilibrium with the moisture content of the grain to be chilled. The differential set-point for chilling dry grain requires a higher set-point than for wet grain. Assuming that for most operations air reaches saturation at the evaporator discharge, a safe grain equilibrium relative humidity of 65 to 70% RH should be used for chilling dry grain. Analysis of a psychrometric chart reveals that for an evaporator temperature of 4°C, an increase by 5°C results in a reduction in air humidity from almost saturation to 70% RH. Before setting the differential thermostat, it is important to know the temperature increase that may occur in the supply duct and the aeration duct of the storage structure to be chilled. In a trial carried out by Navarro et al. (1971), examination of data from a chilling unit of 42,500 kcal/h showed that for a 4-m long and 40-cm diameter flexible non-insulated supply duct, an increase in temperature of 2.5°C was obtained during the winter and 4.2°C during the summer. Figure 9.14 shows the temperature increase of chilled air during flow through a duct connected to a 1227-tonne wheat bin (Maier and Berruto, 1996). Each bin had two duct openings for aeration fans. The chiller was connected to both with a metal T-junction and a total of 15 m of black ducting material. Up to a 20°C ambient temperature, an increase of only about 1°C or less of the chilled air was observed in the non-insulated duct. Above 20°C ambient temperature, heating of the air increased exponentially up to 11°C at 32.5°C ambient temperature. Most of this increase was attributed to solar radiation, which reached its maximum at solar noon. During a second trial an inexpensive white foam-like material was wrapped around the connecting ducts and junction box to limit the temperature increase. A significant improvement was observed, which ranged from an increase of as little as 1°C at 25°C ambient temperature to



Figure 9.14

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Air temperature increase of the chilled air due to duct heating. (Data from Maier, D.E. and Berruto, R. [1996]. Grain chilling demonstration project, summer 1995 wheat chilling trials, Progress Report, Electric Power Research Institute, Agricultural Technology Alliance [EPRI-ATA], Palo Alto. With permission.)

as high as 4°C at 32.5°C. The average temperature increase was 2.7°C at 26°C ambient temperature compared to 3.7°C at 25°C ambient temperature when the duct was not insulated. For chilling dry grain, the expected increase in temperature due to heat gain in the duct should be deducted from the differential set-point. For the above example, if the desired temperature increase after the evaporator is 5°C, then the differential thermostat should be set to control at an average of (5 – 2.5) = 2.5°C in the winter and (5 – 4.2) = 0.8°C in the summer. Although it is assumed that the air humidity reaches saturation in the evaporator, chilling units may be operated when ambient air has a low water content. Under low-humidity cooling conditions, the increase in temperature in the reheater is a waste of energy. A recent U.S. design incorporates a temperature and relative humidity sensor that can be installed at the duct inlet of the storage structure. The sensor adjusts the temperature increase across the reheater as a function of the desired chilled air inlet temperature and relative humidity and the variable heat load on the connecting ducts. Figure 9.15 illustrates the concept during a 2-day operating period of a grain chiller connected to a 1227-tonne wheat bin (Maier and Berruto, 1996). The bin inlet temperature of the chilled air was maintained at an average temperature of 15.3°C, with a range from 12.6 to 18.5°C. During periods of high ambient temperatures, the exhaust air (or reheat) temperature was throttled at the outlet of the chiller. Unfortunately, the limit of the reheater was set at 9°C and could not be overridden by the controller. Thus, the bin inlet air temperature rose about 1.5°C on August 11 and about 3.5°C on August 12 during the high ambient temperature periods. These temperature rises above the desired bin inlet temperature of 15°C were also reflected in a reduction of the bin inlet RH. If the reheater limit had been understepped, the RH could have been maintained at the desired 65% limit. The data proved the feasibility and importance of optimizing the bin inlet air conditions by placing the controller sensor near the air inlet to the storage structure instead of at the outlet of the chiller. 9.3.1.4 Control of Relative Humidity For air blown into the grain bulk at any relative humidity higher than the equilibrium moisture content of the grain, moisture is deposited on the grain surrounding the duct. A practical way to eliminate this problem is to reheat the air leaving the evaporator to a temperature that reduces its relative humidity to a level lower than the equilibrium moisture content of the grain. Figure 9.16 illustrates the progressive


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Figure 9.15

Air temperatures and relative humidities through a grain chiller with the control sensor placed at the inlet to the wheat storage bin (Ambient = ambient air temperature; Evap LATB = evaporator air temperature; Final LAT = air temperature after reheater; Bin Inlet = bin inlet air temperature; RH% = bin inlet air relative humidity). (Data from Maier, D.E. and Berruto, R. [1996]. Grain chilling demonstration project, summer 1995 wheat chilling trials, Progress Report, Electric Power Research Institute, Agricultural Technology Alliance [EPRI-ATA], Palo Alto. With permission.)

Figure 9.16

Wheat moisture content around aeration duct before and after 67 hours of cooling (refrigeration unit operated without reheater). (Data from Navarro, S., Donahaye, E., and Calderon, M. [1971]. Aeration of grain with chilled air, Israel Min. Agric. Dep. Plant Prot. Prog. Rep. 1970/71, Stored Prod. Res. Lab. 77–108, Jaffa, Tel-Aviv [Hebrew, with English summary]. With permission.)

increase in moisture content of wheat around the duct of a bulk cooled with refrigerated air at 86% relative humidity that was not reheated sufficiently (Navarro et al., 1971). This resulted in an increase in the wheat moisture content up to 16.5% after 67 hours of cooling. For a reheating system that resulted in a temperature increase from 4.5 to 13°C (8.5°C increase in temperature), the average relative humidity was 56% (Donahaye et al., 1974). In this system, heating was caused by the fan only (which was located between the evaporator and the supply duct) without using the reheater.



Figure 9.17

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Design of a chilling unit for cooling dry grain. Note the fan located between the evaporator and the bin. Chilling unit uses a combination of a secondary condenser (reheater) and fan heat to reduce the relative humidity of the cooled air. (Data from Navarro, S. [1971]. Suggested alterations to the chilling unit to adapt it to local conditions, Rep. Stored Prod. Res. Lab. Dep. Plant Prot. Israel Min. Agric. Prog. Rep. 1970/71, pp. 22–25, Jaffa, Tel-Aviv [in Hebrew with English Summary]. With permission.)

The heat used to reheat the refrigerated and humid air may be either from a secondary condenser receiving part of the hot refrigerant gases or from the aeration fan using compression heat (as in the above example). Or, an extra heat source such as electrical resistance elements or a separate heat exchange loop may be used. Refrigeration units designed for cooling damp grain may be equipped with a secondary condenser that reheats the cooled air. However, this type of reheating system, when used alone, may not be capable of reducing air humidities to levels required for cooling dry grain. An alternative method is to place the fan after the evaporator, thereby the chilled air is reheated by the fan (Navarro, 1971; Donahaye et al., 1974; Sutherland et al., 1970). Figure 9.17 shows the design of such a setup, in which the centrifugal fan is placed after the evaporator and air drawn through it is forced through the supply duct and the bin. The advantages of this design are: (1) the cooling capacity is increased since additional heat derived from the fan (friction and compression heat) is avoided and only ambient air is refrigerated; (2) fan heat is used in the reheating process to reduce air relative humidity; (3) effective control of the reheater temperature using hot refrigerant gas is achieved. Under certain operational conditions (especially in cold weather), the heat of the refrigerant alone may not be sufficient for reheating the air. In this case full advantage is taken of the aeration fan heat as a supplement to the hot refrigerant. Although this method is efficient in adding heat to cooled air, several problems may be encountered. It has not always been possible to control the level of temperature increase; and sometimes, when operating the fan at low static pressures, the aeration fan heat alone may not be sufficient. The combination of a secondary condenser and fan heat seems a feasible solution.Where the secondary condenser alone is sufficient to provide the necessary reheating effect, this method seems to be more suitable as it can be controlled by a differential thermostat.


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An electrical heating element controlled by a differential thermostat can provide efficient humidity control. The main disadvantage of this method is that it increases the energy consumption required to operate the controlled humidity refrigeration system. Example 9.3 A grain chiller operating at 3000 m3/h is designed to reheat the saturated refrigerated air from 5 to 10.5°C in order to obtain a relative humidity reduction from 95 to 65% (Figure A.1). Calculate the energy required for an electrical heating element to increase air temperature from 5 to 10.5°C. Based on the psychrometric state points, the enthalpy at 5°C and saturation level is 18.5 kJ/kg, and at 10.5°C and 65%, relative humidity is 24 kJ/kg. This would result in an enthalpy difference of 5.5 kJ/kg (24 – 18.5 = 5.5 kJ/kg). Average air density at this temperature range is 1.247 kg/m3. Since the chiller is designed to supply 3000 m3/h, the mass of air to be moved is 3000 m3/h × 1.247 kg/m3 = 3741 kg/h. Therefore, the total energy required to increase the air temperature from 5 to 10.5°C is 3741 kg/h × 5.5 kJ/kg = 20,575.5 kJ/h or 4914 kcal/h or 5.7 kW. 9.3.2

Set-Point Selection and Control

Operating a grain chilling system in conjunction with different storage structure sizes or grain types results in differences of resistance to airflow, thereby influencing the achievable low temperature. Figure 9.18 shows an example for estimating the expected cooling temperature achievable with a 42,500 kcal/h refrigeration unit (Navarro and Donahaye, 1971). If airflow or the equivalent static pressure is known for a given wet-bulb temperature, the expected air temperature leaving the refrigeration unit can be determined. In Figure 9.18 the static pressure equivalent values for airflow are given based on the fan characteristics of the refrigeration unit. It is easier to measure static pressure than airflow by connecting a manometer to the air duct or plenum. 9.3.2.1 Set-Point for Airflow Control in a Grain Chiller of Known Capacity It is important to determine the set-point of the thermostat after the evaporator coil to regulate the airflow when using a damper or a variable-speed drive. Figure 9.18 illustrates this approach using the following steps: Step 1. Determine the ambient conditions based on daily average maximum and minimum wet-bulb temperatures. For example, when the average ambient maximum dry-bulb temperature is 28°C and RH is 62%, and the average minimum temperature is 22°C and RH is 83%, the wet-bulb temperature ranges between a minimum of 20°C and a maximum of 22.5°C. The minimum daily wet-bulb temperature should be used as the set-point of the thermostat to control the chilled air temperature. Selection of the lowest average daily temperature is necessary to permit the damper or the variable-speed drive (VSD) to reduce the airflow and allow the ambient air to cool to the set-point. It is not advisable to set the airflow for the daily average or daily highest temperatures because, when such set-points are selected, the chilled air temperatures may not remain constant. At high ambient temperatures the VSD operates to reduce the airflow rate, thus cooling ambient air to the lowest daily temperature setting. Step 2. Determine the airflow rate either by measuring it directly or by measuring the static pressure and referencing the fan manufacturer’s airflow charts. For example, at a static pressure of 3800 Pa, the airflow rate is 4250 m3/h based on the fan curve of one commercial grain chiller. Step 3. By plotting the coordinates of airflow rate (4250 m3/h) and minimum ambient wet-bulb air temperature (20°C) in Figure 9.18, the thermostat should be set to 7°C. The refrigeration unit should adjust the airflow to obtain a constant wet-bulb temperature even during the warmer hours of the day. From Figure 9.18 it is possible to estimate that when the ambient wet-bulb temperature rises to 22.5°C, the fan airflow rate should adjust to 3400 m3/h.



Figure 9.18

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Chart for determining wet-bulb air temperatures obtained in the refrigeration process with a unit of 42,500 kcal/h. (Data from Navarro, S. and Donahaye, E. [1971]. A method for setting controls on the cooling unit for the chilling of stored grain, Rep. Stored Prod. Res. Lab. Dep. Plant Prot. Israel Min. Agric. Prog. Rep. 1970/71, pp. 19–22, Jaffa, Tel-Aviv [in Hebrew with English summary]. With permission.)

Using similar charts for different refrigeration capacities, it is possible to determine the operation of a chiller at the upper and lower ambient heat load limits. For example, in extreme warm weather, if the coolest hours of the day are at a wet-bulb temperature of 27°C, using Figure 9.18, set the evaporator thermostat at 12°C as explained in Step 1 above. Although air at this point usually reaches saturation, air moisture may not condense on the evaporator coil during extreme dry weather and at high airflow rate. Alternatively, using Figure 9.18, a decision can be made to stop operating the chiller in cold weather. For example, if the ambient wet-bulb temperature is below 10°C, there is little benefit in operating the unit in cold weather because the unit will frequently reach the freezing temperature in the evaporator. 9.3.2.2 Set-Point for Airflow Control in Grain Chillers of Various Capacities In the previous section an example was presented for the set-point airflow control using a known refrigeration capacity. In this section the set-point will be determined when the system has more than one refrigeration unit rated at different capacities.


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Figure 9.19


Chart for determining wet-bulb air temperatures which can be obtained from refrigeration units of varying capacities. Data from Navarro, S. and Donahaye, E. (1971). A method for setting controls on the cooling unit for the chilling of stored grain. Rep. Stored Prod. Res. Lab. Dep. Plant Prob. Israel Minist. Agric. Prog. Rep. 1970/71, pp. 19–22, Jaffa, Tel-Aviv (in Hebrew with English summary.)

Figure 9.19 lists a range of refrigeration capacities suitable for chilling grain. The lines in the figure refer to refrigeration unit capacities from 2 to 20 kcal/kg of air. It is important to know the airflow rate of the refrigeration unit. The following section summarizes the steps for determining the thermostat set-point for the evaporator coil. Step 1. Determine the mass of air that flows through the refrigeration unit. For example, if the minimum ambient air temperature is 24.5°C and 80% RH, the air density is 1.163 kg/m3. For an airflow rate of 3010 m3/h, the mass of air is 3500 kg/h. Step 2. Determine the refrigeration unit capacity in kcal/kg. For example, a unit with a capacity of 42,000 kcal/h has a unit capacity of (42,000 ÷ 3500 =) 12 kcal/kg for the mass of air of 3500 kg/h. Step 3. Determine the maximum and minimum wet-bulb temperatures (w.b.t.). For example, following the 12 kcal/kg refrigeration capacity line to the minimum ambient w.b.t. of 22°C yields an evaporator air w.b.t. set-point of 3°C. The minimum daily w.b.t. should be used to determine the set-point in order to enable the thermostat to control the air throttle or VSD so that, during the warm hours of the day, the evaporator w.b.t. can be maintained at the same temperature as during the coolest part of the day.

The difference in using Figure 9.19 instead of Figure 9.18 is that Figure 9.19 can be used for a wide range of refrigeration unit capacities. However, a chart like Figure 9.18 minimizes calculations when a specific refrigeration unit is utilized for multiple applications. Grain chillers should be operated using the correct evaporator thermostat set-point to avoid temperature fluctuations at the transition or supply duct, thereby preventing an increase of moisture in the first layers of grain.



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9.3.2.3 Use of Microprocessors for Automatic Set-Point Selection and Control Maier (1992) evaluated several critical chilling parameters which showed the importance of adjusting the cold-air and reheater set-points properly to match the grain conditions desired in the bin. Proper adjustment of the reheating of the air is critical when crops with different moisture contents are chilled and rechilled. If this is not done properly, the cooling times can be extended significantly without reaching the desired grain temperatures. Thus, a microprocessor-based controller for a grain chiller should incorporate (1) the automatic adjustment of the chiller temperature set-points as a function of the desired grain and air conditions, (2) a sensor to indicate when rechilling of the bin is to occur, and (3) automatic shut-off of the chiller when the desired temperature is reached. The automatic adjustment of the cold-air and reheater set-points has been incorporated into the microprocessor-based controllers of several commercial grain chillers. The automatic adjustment is generally achieved by requiring the operator to specify (1) the desired grain temperature, (2) the grain storage moisture content, and (3) the grain type. A control scheme like the following is generally implemented. The reheater temperature is set equal to the desired grain temperature (i.e., the air temperature at the bin inlet). The cold-air temperature setting at the evaporator is determined by subtracting the amount of reheating needed from the reheater set-point. The amount of reheating depends on the temperature increase to lower the cold-air temperature after the evaporator to the desired equilibrium relative humidity (ERH) of the air at the bin inlet. The ERH can be determined from a preprogrammed look-up table as a function of the selected grain temperature, moisture content, and the grain type. Many grain chilling applications require summer operation of the chiller. Thus, it may be assumed that the cold air after the evaporator is saturated. Under those conditions only a temperature sensor at the bin inlet is needed to make the psychrometric calculations to determine the needed cold air temperature set-point. If the chiller is operated during low ambient temperature periods, or in very arid regions, the cold air may not be saturated and an additional relative humidity sensor may be needed after the evaporator. Today’s technology has made electronic controllers readily available for applications in agriculture. It appears more intuitive from an operator’s point of view to specify the bin inlet conditions in terms of the grain temperature, moisture content, and grain type than in terms of the cold-air and reheater set-points. Adoption of the approach of using of microprocessors for automatic setpoint selection and control by grain silo and storage designers should result in improved and effective operation of the chilling system. 9.3.3

Practical Aspects of Operating Chilling Units

9.3.3.1 Removal of Condensed Water Chilling units have a collection tray under the evaporator coils to drain the water condensed from the refrigerated air. Before operation of the chiller, drainage piping should be installed to dispose of the condensed water. During trials carried out by Navarro et al. (1971) a 42,500 kcal/h chiller operated on two different bins delivered an average rate of condensed water (L/h) of 4.9 L/h during winter and 5.9 L/h during early spring. Analysis of data revealed that the ambient air water content for winter conditions averaged 7.4 g/kg and 8.4 g/kg during early summer conditions. The chilling unit was operated at an evaporator set-point of 5°C for an air water holding capacity of about 5.5 g/kg. Assuming that excess of water is condensed, the water condensation for each kg of air was 7.4 – 5.5 = 1.9 g/kg in winter and 8.4 – 5.5 = 2.9 g/kg in early summer. The measured air mass flow was 4430 kg/h in winter and 3180 kg/h in early summer. However, the chilling unit was


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Figure 9.20

Condensed water (L/h) (continuous line) collected from a chilling unit of 42,500 kcal/h in relation to ambient air water content (g/m3) (dotted line) and damper position (100% is full open). (Data from Navarro, S., Donahaye, E., and Calderon, M. [1971]. Aeration of grain with chilled air, Israel Min. Agric. Dep. Plant Prot. Prog. Rep. 1970/71, Stored Prod. Res. Lab. 77–108, Jaffa, Tel-Aviv [Hebrew, with English summary]. With permission.)

operated with a damper that controlled the air mass flowing through the unit. Therefore, a water balance calculation could not be made (Figure 9.20). The amount of condensed water collected from refrigerated air reaches impressive volumes, and the drainage of this water is a prerequisite in planning the operation of chilling units. 9.3.3.2 Dust Filter Chilling units are usually operated in a dusty environment. Therefore, they should be equipped with efficient air filters to prevent dust deposits on the evaporator coils. The filters should be selected to cause minimum airflow pressure drop through the unit. Dust deposits on wet refrigeration coils can cause a sticky layer that may block air passage through the evaporator. Periodic cleaning of dust filters is recommended as part of routine maintenance of the chilling unit. The filters should have the following features: large surface area to prevent the need for frequent cleaning, easy removal for cleaning, and durability for easy cleaning with a water hose. Most commercial grain chillers have a U-tube manometer installed at the air inlet to the fan to monitor the pressure drop across the filter. Under normal operating conditions, the pressure drop should not exceed 0.5 cm water column. 9.3.4

Cooling-Front Movement through Grain

One of the primary advantages of grain chilling is the predictability of moving a cooling front through a grain mass. As discussed in Chapter 6, the time to move a cooling front through grain correlates strongly with the airflow rate. For example, at the typical aeration airflow rate of 6.0 (m3/h)/tonne, it takes approximately 160 hours to move a front through a grain mass compared to 19 hours at 60 (m3/h)/tonne (Epperly, 1989).



Figure 9.21

523

Simulated movement of a cooling front through a grain mass aerated with a grain chiller. (From Maier, D.E., Bakker-Arkema, F.W., and Ilangantileke, S.G. [1993c]. Ambient and chilled paddy aeration under Thai conditions, Agric. Eng. J., 2[1,2], 15–33. With permission.)

With ambient air, the inlet conditions into a grain mass vary as a function of the weather conditions. An automatic controller has to accumulate 160 hours at an airflow rate of 6.0 (m3/h)/tonne (by turning the fan on and off over several days) to achieve the desired final grain temperature in the grain mass. With manual control, fans are often left running when ambient temperatures rise so that multiple cooling fronts have to move through the grain before the desired final grain temperature is reached. With chilled air, the airflow rate varies in order for the grain chiller to maintain near-constant inlet conditions. Figure 9.21 illustrates the simulated cooling front movement through a 1000-tonne bin of paddy within 144 hours when aerated with a grain chiller operating with an average airflow rate of 6.0 (m3/h)/tonne (Maier et al., 1993c). 9.3.5

Chilling of Hot Grain after Transfer from a Dryer

Maize chilling following high-temperature drying provides major economic and grain quality benefits. Less stress cracking of kernels is observed when maize is first dried to 19 to 20% and then chilled in-bin while the maize is still hot (Dryeration). The evaporative cooling effect removes an additional 2.5 to 3.5% moisture content and reduces the grain temperature to a safe storage level (i.e., less than 10°C) according to Sulzer-Escher Wyss (1980). This two-stage drying process is a modified version of Dryeration, which is widely implemented in the midwestern U.S. maize belt utilizing cool ambient air during the fall harvest. The benefit of chilled aeration is the possibility of extending the Dryeration process into warmer climates, where the evaporative cooling effect of warmer air is smaller. For example, typical average weather conditions in the northern U.S. maize belt during the fall harvest may be 10°C and 65% relative humidity. The water-holding capacity of this air increases during Dryeration from 4.93 g of water per kg of dry air to about 16.73 g/kg, assuming the air comes into contact with 50°C maize, cools it at the wet-bulb temperature, and is exhausted in the saturated state. This is a 239% increase in water-carrying capacity. In comparison, in the southern maize belt, typical weather conditions during harvest may be 25°C and 55% relative humidity. In this case, the water-carrying capacity of the air during Dryeration increases from 10.86 g/kg to 21.19 g/kg — only a 95%


524


Table 9.1 Moisture Content Ranges (mc), Grain Temperatures (T), and Allowable Storage Times (AST) of Grains with Different End Uses under Chilled Storage Conditions mc (%) 12–15a 15–16.5a 16.5–18a 18–20a 20–22a 22–25b 25–30b >30b

Seed Grain and Malting Barley T(°C) AST 9–12 8–10 5–7 5 5 5 4–5 —

>1.5 y 1–1.5 y 4–6 m 2–3 m 3–4 w 1–2 w 2–3 d —

Bread Grains T(°C) AST

Feed Grains T(°C) AST

10–12 9–10 8–10 8–10 6–8 5–7 4–5 —

10–14 10–12 8–10 8–10 8–10 5–8 4–5 4–5

>1y >1y 5–10 m 2–7 m 4–16 w 3–8 w 5–10 d —

>1y >1y 6–13 m 3–9 m 5–20 w 10–25 w 14–30 d 6Pp and H > 0.3W, and within 1% if W > 6Pp and H > 0.6W when compared with results given by Equation 11.74 for the half-round duct.


608


Example 11.2 Wheat is stored in a shed that has a width of 10 m and a length of 10 meters. The wheat is aerated with an airflow rate of 0.5 m3/s using a half-round duct with a radius of 0.5 m (a diameter of 1 m), which is located 2 m from the center of the grain store. If the upper surface of the grain bulk is 5 m above the duct, calculate the pressure drop across the grain bulk. Sketch the upper surface of the grain bulk. From Table 11.1 the linear resistance term for wheat is observed to be 3131 Pa s m–2. Using Equation 11.76, the equation for the pressure drop between the top of the half-round aeration duct, where y = r, the radius of the duct, and y = H, the height of the grain above the duct, is given by: ∆p1 = p (r ) − p ( H ) Making use of Equation 11.70 and substituting in the appropriate values of the variables results in: p(0.5) = −

3131 × 0.5  0.5π 2π  0.5π   ln sinh 2 − 2 sin 2 cosh −1    2 π × 10 10 10 10 

Note that: sinh

0.5π exp (0.5π) − exp ( −0.5π) = 10 2

or: exp (0.157) − exp ( −0.157) 1.170 − 0.8547 = 2 2 i.e.: sinh 0.157 = 0.1577 Also note that: cosh

0.5π exp (0.157) + exp ( −0.157) = 10 2

or: exp (0.157) + exp ( −0.157) 1.170 + 0.8547 = 2 2 i.e.: cosh 0.157 = 1.01236 Substituting sin 2 π 10 = 0.5878 and 3131 × 0.5 (2 π × 10) = 24.9157 into Equation 11.70 results in:


AIRFLOW DISTRIBUTION IN VENTILATED BEDS OF GRAIN

609

(

)

p(0.5) = −24.9157 ln 0.15772 − 2 × 0.58782 (1.01236 − 1) i.e.: p(0.5) = −24.9157 ln (0.016328) = 102.5 Pa The next task is to evaluate: p(5) = −

3131 × 0.5  5π 2π  5π   ln sinh 2 − 2 sin 2 cosh −1  2 π × 10 10 10 10   

and sinh 1.5708 = 2.3013 and cosh 1.5708 = 2.5092, which when substituted into Equation 11.70 results in p(5) = –36.1. The pressure drop resulting from the linear or Darcian term is therefore given by: ∆p1 = p (0.5) − p (5) = 102.5 + 36.1 = 138.6 Pa The constriction pressure drop resulting from the non-linear term is given by: ∆p2 =

10756 × 0.5 × 0.5 SQ2 = 5.4 Pa = 2 πl Pp π × 10 2 × ( π × 0.5)

and the pressure drop resulting from the non-linear term when the flow is parallel is given by Equation 11.69, i.e.: ∆p3 =

10756 × 0.52 × 5 = 1.3 Pa 10 2 × 10 2

The total pressure drop across the grain bulk is 138.6 + 5.4 + 1.3 = 145.3 Pa. The shape of the isobar at a height of 5 m above the duct when the duct is placed 2 m from the center of the store is determined by solving Equation 11.73. When x = 2, y = 5, in this particular case b = 2. Inserting these values into Equation 11.73 yields the value of C, thus: 2

5π 2π 2π 2π 2π sin  − cos2 cos2 C = cosh − sin 10 10 10 10 10  

(

)

C = 2.50918 − 0.587782 − 0.80912 4 = 4.68157 − 0.42839 = 4.253 Having calculated C, Equation 11.74 can be solved for y for values of x = –5, –4, –3,…, 3, 4, 5. This can be accomplished explicitly by making use of the identity:

(

cosh −1 ψ = ln ψ + ψ 2 − 1

)

(11.77)


610


Figure 11.10

A sketch of the surface of the grain bulk considered in the example.

In this case: 1

πx πb  πx πb  2 cos2 ψ = sin sin + C + cos2  W W  W W

(11.78)

hence: y=

W ψ π

(11.79)

Substituting the appropriate values into Equations 11.73, 11.78, and 11.79 results in the following values of y: x: y:

–5.00 2.99

–4.00 3.12

–3.00 3.43

–2.00 3.81

–1.00 4.23

0.00 4.56

1.00 4.82

2.00 5.00

3.00 5.11

4.00 5.17

5.00 5.19

These data allow the isobar that passes through the point (2, 5) to be sketched. Note that x is measured from the centerline of the grain store. Figure 11.10 shows a sketch of the surface of the grains. 11.6.2

Round Duct

The linear pressure drop, ∆p1, that arises from the linear resistance term when a bulk of grain is aerated with a circular aeration duct with a diameter, d, is found by means of the equation: p( y) = −

2 2 RQ  2 πb  πy πd  πy πd   2 πb  − cosh − sinh ln sin  sinh    cosh  + cos 4 πl  2W  2 W   W W W W

2 2 RQ  2 πb  πy πd  πy πd   2 πb  + sinh − − cosh ln sin  cosh  + cos  sinh   4 πl  W W 2W  W W 2 W  

(11.80)



611

In the case of a round duct the perforated perimeter, Pp , is πd. Example 11.3 Repeat the above example, but in this case a circular aeration duct with a diameter of 1 m is used in place of a half-round duct. To calculate the contribution of the linear term to the pressure drop, simply insert the appropriate values of the variables into Equation 11.80; thus at the top of the duct, which is 1 m above the ground: p(1) = −

2 2 π π  π π   3131 × 0.5  2 2 π  2 2π  ln sin  cosh − cosh  + cos  sinh − sinh   4 π × 10 10  10 10 2  10  10 10 2   

−

2 2 3131 × 0.5  2 2 π  π π  π π   2 2π  ln sin  + cos    cosh − cosh  sinh + sinh 4 π × 10 10  10 10 2  10  10 10 2   

The terms in the above equation can be evaluated, which for completeness are given as: sin

2π = 0.5878 10

sinh

π = 0.3194 10

cos

2π = 0.8090 10

cosh

cosh

π = 1.4098 10

cosh

π = 1.0248 10 2

π = 0.2240 10 2

When these values are substituted into Equation 11.80: p (1) = −12.458 (ln [0.006169] − ln [0.19343]) = 83.9 Pa is obtained. When y = 5 cosh

5π = 2.5092 10

sinh

5π = 2.3013 10

which results in: p (5) = −12.458 (ln [3.5856] − ln [4.9351]) = −35.8 Pa The total pressure drop due to the linear or Darcy term is therefore 83.9 – (–35.8) = 119.7 Pa. The pressure loss due to the square law term being important in the vicinity of the duct is calculated from Equation 11.66, and in this case Pp , is π × 1: ∆p2 =

10756 × 0.5 × 0.5 = 2.7 Pa π × 10 2 × ( π × 1)


612


and the square law term parallel flow resistance term is the same as in the half-round duct case, namely 1.3 Pa. The total pressure drop is therefore 119.7 + 2.7 + 1.3 = 123.7 Pa. Note that the overall pressure drop using circular ducts is 17.5% lower than when half-round ducts are used. The principal reason is that round ducts offer a larger perforated area per unit length, and this reduces the velocity of the air as it leaves the duct surface. The air is nonetheless constricted, and air leaving the lower half of the duct must turn through an angle exceeding 90°. 11.6.3

Perforated Floor Ducts

Perforated floor ducts enable grain stored to be aerated, while avoiding above-floor ducts that cause obstructions during out-loading operations. In such an instance, Hunter (1983) gives the pressure, p(y), at a height y above the floor as the following equation:  2 2 4 2  φ + φ + 4 sin ( πd W ) cos ( πb W ) sinh ( πy W ) RQ −1  p( y) = sinh  πl 2 cos ( πb W ) sin ( πd W )  

[



]  1 2

 

(11.81)

in which:

[

]

φ = cos2 ( πb W ) sinh 2 ( πy W ) − sin 2 ( πd W )

[

]

+ sin 2 ( πb W ) cosh ( πy W ) − cos ( πd W )

2

In this case, d is the half-width of the floor duct, hence it follows that the perforated area, Pp , of the duct per unit length is 2d. Because the duct is flush with the floor, its height is zero by definition, hence p(0) = 0. Hence, the pressure drop, ∆ p1 across the grain bulk of height H above the center of the duct, and resulting from the linear resistance term, is simply p(H). Example 11.4 Repeat the example that involves the half-round duct, but in this case consider the grain to be aerated using a perforated floor duct that has a width of 2 m. The half-width of the duct is 1 m. Many of the quantities in Equations 11.81 have already been calculated in the above problems, but for completeness the relevant values will be repeated. Inserting the appropriate values into Equation 11.81 leads to:

{

}

φ = cos2 (2 π 10) sinh 2 (10 π 10) − sin 2 (1.0 π 10)

+ sin 2 (2 π 10){cosh (10 π 10) − cos (1.0 π 10)}

2

= 4.242 The pressure, p(5), at a height of 5 m above the aeration duct is therefore calculated from:  4.242 + 4.242 2 + 4 sin 2 (2 d 10) cos 4 (2 π 10) sinh 2 (5π 10)  3131 × 0.5 sinh −1   10 π 2 cos (2 π 10) sin (2 π 10)   = 140.1 Pa

p (5) =



Figure 11.11

613

Flow field in grain stores aerated using semi-circular, round, and in-floor ducts.

i.e., the pressure, ∆p1, that arises from the linear term that involves the constant R is 140.1 Pa. The pressure drop, ∆p2, that results from the quadratic term, Sv2, which is important near the aeration duct, is calculated as follows: ∆p2 =

10756 × 0.5 × 0.5 SQ 2 = = 4.28 Pa πl 2 Pp 100 π × 2

and, as in the previous examples, the quadratic component of the pressure drop, ∆p3, occurring as a result of the grain flowing uniformly through the grain, is 1.34 Pa. The total pressure drop across the grain bulk is therefore 145.72 Pa. The airflow fields can also be calculated using Hunter’s (1983) analysis, as can be seen in Figure 11.11. It can be seen from the figure that the analysis results in an approximation to a circular duct, but Hunter shows that the resulting error in the linear pressure term is only a few percent.

11.7 CONCLUDING REMARKS A knowledge of the flow field in a bed of grain and the pressure drop is essential if grain storage systems are to be designed with confidence. It has been shown that mathematical results on the flow of air through bulks of grain stem from work that extends beyond the traditional domain of stored-grain technologists. Indeed, some of the early results obtained by agricultural engineers working in this area were quite cumbersome and not physically based. More general, and useful, results have been obtained by chemical engineers and those working in the area of flow-through porous media. It has been shown how to solve the equations that govern pressure distributions in stored grains that have arbitrary shapes. Elegant analytical expressions have been given that enable pressure drops across grain bulks fitted with a variety of aeration ducts.


614


REFERENCES Antohe, B.V. and Lage J.L., “A general two-equation macroscopic turbulence model for incompressible flow in porous media,” International Journal of Heat and Mass Transfer, 1997, 40, 3013–3024. Brooker, D.B. (1969). Computing air pressure and velocity distribution when air flows through a porous medium and nonlinear velocity-pressure relationships exist, Trans. ASAE, 12(1), 118–120. Carnahan, B., Luther, H.A., and Wilkes, J.O. (1969). Applied numerical methods, Wiley, New York. Churchill, R.V. and Brown, J.W. (1995). Complex variables and applications. 6th ed., McGraw-Hill. Darcy, H.P.G. (1856). Les fontaines publique de la ville de Dijon. Exposition et application à suivre et des formules a employer dans les questions de distribution d’eau, Victor Delamont, Paris. Ergun, S. (1952). Fluid flow through packed columns, Chem. Eng. Progr., 48, 89–94. Forchheimer, P.H. (1901). Wassebeuregung durch boden, Zeit. Ver. Deutsch. Ing, 45, 1782–1788. Hood, T.J.A and Thorpe, G.R. (1992). The effects of the anisotropic resistance to airflow on the design of aeration systems for bulk stored grains, Agricultural Engineering Australia, 21(1 and 2), 18–23. Hukill, W.V. and Ives, N.C. (1955). Radial airflow resistance of grain, Agric. Eng., 36(5), 332–335. Hunter, A.J. (1983). Pressure difference across an aerated seed bulk for some common duct and store crosssections, J. Agric. Eng. Res., 28, 537–450. Kozeny, J. (1927). Über kapillare Leitung des Wassers im Boden, Ber. Wien Akad., 136A, 271–277. Kreyszig, E. (1998). Advanced Engineering Mathematics, John Wiley & Sons. Peaceman, D.W. and Rachford, H.H. (1955). A numerical solution of parabolic and elliptic differential equations, J. Soc. Ind. Appl. Math., 3, 28–41. Samarskii, A.A. and Andreyev, V.B. (1963). On a high accuracy difference scheme for elliptic equations with several space variables, USSR Comput. Math. and Math. Phys. 3, 1373–1382. Shedd, C.K. (1953). Resistance of grains and seeds to airflow, Agric. Eng., 34(9), 616–619. Singh A.K., Leonardi, E., and Thorpe, G.R. (1993b). Three-dimensional natural convection in a confined fluid overlying a porous layer, J. Heat Transfer, 115, 631–638. Singh, A.K. and Thorpe, G.R. (1993a). The application of a grid generation technique to the numerical modelling of heat and moisture movement in peaked bulks of grain, J. Food Proc. Eng., 16, 127–145. Singh, A.K. and Thorpe, G.R. (1993b). A solution procedure for three-dimensional free convective flow in peaked bulks of grain, J. Stored Prod. Res., 28, 221–235. Singh, A.K., Leonardi, E., and Thorpe, G.R. (1993a). A solution procedure for the equations governing threedimensional free convection in bulk stored grains, Trans ASAE, 36(4), 1159–1173. Smith, E.A. (1982). 3-Dimensional analysis of air velocity and pressure in beds of grain and hay, J. Agric. Eng. Res., 27, 101–117. Spencer, H.B. (1969). Pressure drop in on-floor duct drying systems. J. Agric Eng. Res., 14(2), 165–172. Thomas, L.H. (1949). Elliptic problems in linear differential equations over a network, Watson Scientific Computing Laboratory, Columbia University, New York. Thorpe, G.R. (1997). Modelling ecosystems in ventilated conical bottomed farm grain silos, Ecological Modelling, 94, 255–286. Wilson, S. G, Thorpe, G.R., and Nguyen, T.V. (1988). Finite difference simulation of thermal and moisture boundary layers occurring during cereal grain drying with condensation, Proc. Int. Symp. on Comp. Fluid Dynamics, August 24–27, 1987, 761–772, Elsevier Science Publishers BV (North Holland).



615

APPENDIX 11.A.1 Solution of a Set of Linear Equations that has a Tridiagonal Coefficient Matrix Equation 11.48 is solved for the intermediate values of p* implicitly in the x-direction. It was shown that when this is done, a system of linear equations with a triangular coefficient matrix results, and for values of j = 2,3,4,…, n–2,n-1.  b1  a2            

c1 b2 a3

c2 b3

c3 ai

bi

ci

aNX −3

bNX −3 aNX −2

  p2∗   d1    p∗   d2    3∗      p4   d3            pi∗+1  =  di               ∗    cNX −3   pNX −2   dMX −3  bNX −2   p∗NX −1   dNX −2 

(11.A.1.1)

in which: ai = −

∆t 1 ; 2 ∆x 2

i = 1, NX − 2

∆t ; ∆x 2

i = 1, NX − 2

bi = 1 +

ci = −

∆t 1 ; 2 ∆x 2

di = p + +

i = 2, NX − 2

∆t  Rx pi +1, j −1 − 2 pi +1, j + pi +1, j +1    2  Ry ∆y 2  ∆t  ∂v ∂u Rx Sy ∂ v Rx Sy ∂v  + Sx v + + v  v + Sx u 2  Ry Ry ∂x ∂x ∂y ∂y 

d1 = d1 − a1 p1 dNX −2 = dNX −2 − cNX −2 pNX The equations are solved by manipulating the above matrix so that it has only two bands of coefficients, thus:


616


β1 c1  β2         

c2 βi

ci β NX −3

  p2   δ1   p   δ   3   2            pi +1  =  δ i          cNX −3   pNX −2  δ NX −3  β NX −2   pNX −1  δ NX −2 

(11.A.1.2)

in which the asterisk denoting the value at the half-time step is omitted. After the equations have been transformed into the above form, it is a simple matter to determine all of the updated pressures by back substitution, i.e.: pNX −1 = δ NX −2 β NX −2

(11.A.1.3)

Using this value of pNX–1 in the penultimate equation enables one to find pNX–2 thus: pNX −2 = (δ NX −3 − cNX −3 pNX −1 ) β NX −3

(11.A.1.4)

In general the p i of the ith node is given by: pi = (δ i −1 − ci −1 pi +1 ) βi −1

(11.A.1.5)

Examination of the original set of equations indicates that the first equation is already in the desired form with: β1 = b1 and δ1 = d1

(11.A.1.6)

The first two equations may then be written as: β1 p2

+ c1 p3

= δ1

(11.A.1.7)

a2 p2

+ b2 p3 + c2 p4

= d2

(11.A.1.8)

The second equation may be manipulated into the desired form by eliminating the term involving a2, which is accomplished by multiplying the first equation by a2 β1 and subtracting the result from the second equation, i.e.:  c1a2  δ1a2  b2 − β  p2 + c2 p3 = d2 − β  1 1 

(11.A.1.9)



617

Hence, defining: β2 = b2 −

c1a2 β1

(11.A.1.10)

δ 2 = d2 −

δ1a2 β1

(11.A.1.11)

and:

the second equation becomes: β2 x2 + c2 x3 = δ 2

(11.A.1.12)

which is of the desired form. The remaining leading coefficients can be eliminated in an exactly analogous manner. For example: β3 = b3 −

c2 a3 β2

(11.A.1.13)

δ 3 = d3 −

δ 2 a3 β2

(11.A.1.14)

ci −1ai βi −1

(11.A.1.15)

and:

For the general ith node these equations are: βi = bi −

Solve for the NN unknowns x1, x2 ,......., x NN −1, x NN . . In this case the system of equations to be solved is: β1 c1  β2         

c2 βi

ci β NN −1

  x1   δ1   x   δ   2   2            xi  =  δ i          cNN −1   x NN −1  δ NX −3  β NN   x NN   δ NN 

The Thomas algorithm may be formulated in the form shown in Figure 11.A.1.1.

(11.A.1.17)


618


ALGORITHM THOMAS This algorithm represents a scheme to solve a set of linear equations, the coefficients of which form a tridiagonal matrix. INPUT:

Number of equations, NN. Coefficients ai , bi and ci Values on right-hand side of equation, di For i = 2,3,........,NN, do: ε = ai βi −1 βi = bi − εci −1 δ i = di − εδ i −1 End x NN = δ NN β NN For i = 2,3,…,NN, do: j = NN+1–i

(

)

x j = δ j − c j x j +1 β j End OUTPUT: xj . Stop END THOMAS Figure 11.A.1.1

The Thomas algorithm (1949).



APPENDIX 11.A.2 A Computer Listing of the Algorithm Used to Calculate the Pressure Field in a Bulk of Grains 10 REM PROGRAM IMPLIC2DP 20 REM ********************************************************* 30 REM 40 REM FLOW IN A TWO-DIMENSIONAL BED OF GRAINS USING AN 50 REM ALTERNATING DIRECTION IMPLICIT METHOD. 60 REM 70 REM THE PROGRAM CAN ACCOUNT FOR NON-LINEAR AND ANISOTROPIC 80 REM RESISTANCE TO FLOW 90 REM 100 REM***************************************************************** 110 DIM P(21,21),PH(21,21),PN(21),A(21),B(21),C(21),D(21),BETA(21),DELTA(21) 120 DIM U(21,21),V(21,21),VABS(21,21) 130 OPEN “OUTPUT.DAT” FOR OUTPUT AS #1 140 NX = 11 150 REM NX = NUMBER OF NODES IN THE X DIRECTION 160 REM NY = NUMBER OF NODES IN THE Y DIRECTION 170 REM NDL = NODE ON THE LEFT HAND SIDE OF THE DUCT 180 REM WX = WIDTH OF THE REGION, M 190 REM WY = HEIGHT OF THE REGION, M 200 RX = 2000 210 SX = 10000 220 RY = 2000 230 SY = 10000 240 RATIO = RX/RY 250 NX = 11 260 NY = 11 270 NDL = 1 280 NDU = 11 290 NXM1 = NX-1 300 NYM1 = NY-1 310 NXM2 = NX-2 320 NYM2 = NY-2 330 REM DIMENSIONS OF BULK 340 WX = 3! 350 WY = 3! 360 REM PDUCT = AIR PRESSURE AT THE DUCT 370 PDUCT = 300! 380 REM SPACING OF NODES IN THE X AND Y DIRECTIONS. 390 HX = WX/(NXM1) 400 HY = WY/(NYM1) 410 REM MAKE USE OF A FALSE TRANSIENT FACTOR, ALPHA 420 ALPHA = 1! 430 REM NOTIONAL TIME STEP 440 DT = .2*25/(NXM1*NYM1) 450 REM SET INITIAL PRESSURE THROUGHOUT THE BULK 460 FOR I = 1 TO NX 470 FOR J = 1 TO NY 480 P(I,J) = 0 490 PH(I,J) = 0 500 NEXT J 510 NEXT I 520 REM BOUNDARY CONDITIONS AT THE AERATION DUCT 530 FOR I = NDL TO NDU

619


620


APPENDIX 11.A.2 (continued) A Computer Listing of the Algorithm Used to Calculate the Pressure Field in a Bulk of Grains 540 P(I,1) = PDUCT 550 PH(I,1) = P(I,1) 560 NEXT I 570 KU = 50 580 KL11 = 6 590 PRINT” ----WORKING----” 600 FOR KL1 = 1 TO KL11 610 REM START OF ITERATIONS 620 REM IMPLICIT SOLUTION IN X DIRECTION 630 REM THE LOOP FIRST SETS J = 2 AND WE THEN SOLVE FOR THE PRESSURES 640 REM PH(2,2),PH(3,2),PH(4,2),…PH(NX-2,2), PH(NX-1,2) AFTER THE HALF 650 REM STEP, DT/2. IT IS VERY SIMILAR TO SOLVING ONE-DIMENSIONAL 660 REM HEAT FLOW IN A BAR USING THE IMPLICIT METHOD. NEXT THE VALUE 670 REM OF J IS INCREASED, AND THIS TIME WE SOLVE FOR PH(2,3),PH(3,3) 680 REM PH(4,3),…,PH(NX-1,3). EACH TIME WE SOLVE THE EQUATIONS 690 REM IMPLICITLY IN X, AND EXPLICITLY IN Y. THAT IS, THE PRESSURES 700 REM ON THE RIGHT HAND SIDE OF THE EQUATION RETAIN THEIR VALUES 710 REM THROUGHOUT THE TIME HALF TIME STEP. 720 FOR KH = 1 TO KU 730 FOR J = 2 TO NYM1 740 FOR I = 2 TO NXM1 750 REM WE ARE GOING TO SOLVE NX-2 EQUATIONS FOR EVERY VALUE OF J 760 REM THE SUBROUTINE THAT SOLVES THE NX-2 EQUATIONS HAS TO 770 REM BE SUPPLIED WITH COEFFICIENTS THAT RANGE FROM 1 TO NX-2. 780 REM FOR A GIVEN VALUE OF J, THE INTERIOR PRESSURES TO 790 REM BE DETERMINED RANGE FROM PH(2,J) TO PH(NX-1,J). WE 800 REM THEREFORE INTRODUCE THE NEW VARIABLE (AN INDEX OF THE 810 REM COEFFICIENTS A,B,C AND D) THAT RANGES FROM 1 TO NX-2. 820 II = I-1 830 DUDX = (U(I+1,J)-U(I-1,J))/2/HX 840 DUDY = (U(I,J+1)-U(I,J-1))/2/HY 850 DVDY = (V(I,J+1)-V(I,J-1))/2/HY 860 DVDX = (V(I+1,J)-V(I-1,J))/2/HX 870 DVABSDY = .5*VABS(I,J)^.5*(2*U(I,J)*DUDY+2*V(I,J)*DVDY) 880 DVABSDX = .5*VABS(I,J)^.5*(2*U(I,J)*DUDX+2*V(I,J)*DVDX) 890 SOURCE = +RX*(VABS(I,J)*DUDX+U(I,J)*DVABSDX)+SY*RATIO*(VABS(I,J)*DVDY+V(I,J)* DVABSDY) 900 A(II) = –ALPHA*DT/2/HX^2 910 B(II) = 1+ALPHA*DT/HX^2 920 C(II) = –ALPHA*DT/2/HX^2 930 D(II) = P(I,J)+(ALPHA*DT/2)/HY^2*(P(I,J-1)-2*P(I,J)+P(I,J+1))*RATIO 940 D(II) = D(II)+SOURCE*ALPHA*DT/2 950 REM WE ARE GIVEN AS A BOUNDARY CONDITION, OR WE 960 REM CALCULATE THE VALUE OF P(1,J). IT IS THEREFORE 970 REM KNOWN AND IT THEREFORE APPEARS ON THE RIGHT 980 REM HAND SIDE OF THE EQUATIONS FOR PH(I,J). 990 IF II = 1 THEN D(II) = D(II)-A(1)*P(I-1,J) 1000 IF II = NXM2 THEN D(II) = D(II)-C(II)*P(I+1,J) 1010 REM WE ALSO KNOW, OR CAN CALCULATE, THE VALUES 1020 REM OF P(NX,J), BECAUSE THEY ARE THE PRESSURES 1030 REM OF THE RIGHT HAND WALL OF THE ENCLOSURE. 1040 REM THIS ACCOUNTS FOR THE FORM OF THE FOLLOWING LINE. 1050 REM PRINT II,A(II),B(II),C(II),D(II)



621

APPENDIX 11.A.2 (continued) A Computer Listing of the Algorithm Used to Calculate the Pressure Field in a Bulk of Grains 1060 NEXT I 1070 REM THE SUBROUTINE THAT SOLVES THE EQUATIONS 1080 REM HAS TO BE GIVEN THE NUMBER, NN, OF EQUATIONS 1090 REM TO BE SOLVED. THIS IS NXM2 1100 NN = NXM2 1110 REM TAKE THE VALUES OF A(I), B(I), C(I), D(I) 1120 REM D(I) AND NN TO THE SUBROUTINE TO SOLVE THE 1130 REM EQUATIONS AND RETURN WITH THE VALUES 1140 REM PN(1), PN(2), PN(3),…, PN(NXM2). 1150 REM THESE SOLUTIONS CORRESPOND TO PH(2,J), 1160 REM PH(3,J),…,PH(NX-2,J), PH(NX-1,J). 1170 GOSUB 2760 1180 FOR I = 1 TO NXM2 1190 II = I+1 1200 PH(II,J) = PN(I) 1210 REM PRINT P(II,J) 1220 NEXT I 1230 NEXT J 1240 REM IMPLICIT SOLUTION IN Y DIRECTION 1250 FOR I = 2 TO NXM1 1260 REM FOR THE SECOND HALF TIME STEP WE SET I = 2, 1270 REM 3,4,…,NX-1 AND FOR EACH VALUE OF I WE 1280 REM SOLVE FOR P(I,2),P(I,3),P(I,4),…,P(I,NY-1). 1290 REM IN THIS CASE THE DERIVATIVES WITH RESPECT 1300 REM TO Y ARE TREATED IMPLICITLY, AND THOSE 1310 REM WITH RESPECT TO X (WHICH ARE EXPRESSED IN TERMS 1320 REM OF PH(I,J)) ARE TREATED EXPLICITLY. 1330 REM THIS TIME WE TREAT THE REGION AS A SERIES OF 1340 REM ONE-DIMENSIONAL RODS, BUT THIS TIME IN THE VERTICAL DIRECTION. 1350 FOR J = 2 TO NYM1 1360 REM IN THIS CASE WE HAVE TO SOLVE NY-2 EQUATIONS 1370 REM FOR EACH VALUE OF I. THERE ARE THEREFORE NY-2 1380 REM COEFFICIENTS A(JJ), B(JJ), C(JJ) AND VALUES OF 1390 REM RIGHT HAND SIDE, D(JJ) AS WE SOLVE FOR P(I,2) 1400 REM P(I,3), P(I,4),…,P(I,NY-1). 1410 DUDX = (U(I+1,J)-U(I-1,J))/2/HX 1420 DUDY = (U(I,J+1)-U(I,J-1))/2/HY 1430 DVDY = (V(I,J+1)-V(I,J-1))/2/HY 1440 DVDX = (V(I+1,J)-V(I-1,J))/2/HX 1450 DVABSDY = .5*VABS(I,J)^.5*(2*U(I,J)*DUDY+2*V(I,J)*DVDY) 1460 DVABSDX = .5*VABS(I,J)^.5*(2*U(I,J)*DUDX+2*V(I,J)*DVDX) 1470 SOURCE = +SX*(VABS(I,J)*DUDX+U(I,J)*DVABSDX)+SY*RATIO*(VABS(I,J)*DVDY+V(I,J)* DVABSDY) 1480 JJ = J-1 1490 A(JJ) = –ALPHA*DT/2/HY^2*RATIO 1500 B(JJ) = 1+ALPHA*DT/HY^2*RATIO 1510 C(JJ) = –ALPHA*DT/2/HY^2*RATIO 1520 D(JJ) = PH(I,J)+(ALPHA*DT/2)/HX^2*(PH(I-1,J)-2*PH(I,J)+PH(I+1,J)) 1530 D(JJ) = D(JJ)+SOURCE*ALPHA*DT/2 1540 REM WE KNOW THE VALUE OF PH(I,1) BECAUSE 1550 REM IT IS ON THE BOUNDARY (THE FLOOR) OF THE ENCLOSURE. 1560 REM IT IS THEREFORE INCLUDED IN THE RIGHT HAND 1570 REM TERMS OF THE EQUATIONS TO BE SOLVED.


622


APPENDIX 11.A.2 (continued) A Computer Listing of the Algorithm Used to Calculate the Pressure Field in a Bulk of Grains 1580 IF JJ = 1 THEN D(JJ) = D(JJ)-A(1)*PH(I,J-1) 1590 REM WE ALSO KNOW, OR CAN CALCULATE THE VALUES 1600 REM OF PH(I,NY), THE VALUES ON THE UPPER SURFACE 1610 REM OF THE ENCLOSURE. THEY ALSO APPEAR ON THE 1620 REM RIGHT HAND SIDE OF THE EQUATIONS. 1630 IF JJ = NYM2 THEN D(JJ) = D(JJ)-C(JJ)*PH(I,J+1) 1640 NEXT J 1650 REM WE HAVE TO SOLVE NY-2 EQUATIONS BECAUSE 1660 REM IN THE Y-DIRECTION THIS IS THE NUMBER OF 1670 REM INTERIOR NODES. THE EQUATION SOLVING SUBROUTINE 1680 REM REQUIRES THE NUMBER OF EQUATIONS TO BE SOLVED. 1690 REM IT IS NN( = NY-2). WE ALSO FEED INTO THE EQUATION 1700 REM SOLVING ROUTINE VALUES OF A(JJ), B(JJ), C(JJ) 1710 REM AND D(JJ), JJ = 1 TO NY-2. THE SOLUTIONS TO THESE 1720 REM EQUATIONS ARE CONTAINED IN PN(1),PN(2),…,PN(NY-2). 1730 NN = NYM2 1740 GOSUB 2760 1750 FOR J = 1 TO NYM2 1760 JJ = J+1 1770 REM HERE WE SET THE PRESSURES, P(I,J), AT THE 1780 REM END OF THE TIME STEP TO TN(I,J). THESE PRESSURES 1790 REM NOW BECOME THE STARTING ONES FOR THE NEXT ITERATION. 1800 REM (OR ADVANCEMENT IN TIME). 1810 P(I,JJ) = PN(J) 1820 PH(I,J) = P(I,J) 1830 NEXT J 1840 NEXT I 1850 TIME = TIME+DT 1860 REM WE SHOULD NOTE THAT WE SOLVE ONLY FOR THE PRESSURES 1870 REM OF THE INTERIOR NODES. THE PRESSURES OF THE BOUNDARY 1880 REM ARE GIVEN AS BOUNDARY CONDITIONS. FOR EXAMPLE, 1890 REM AT THE TOP OF THE ENCLOSURE THE PRESSURESS ARE SET. 1900 REM IN THIS EXAMPLE THEY ARE SET AT 20. EXCEPT AT THE DUCT 1910 REM THE BOUNDARIES ARE IMPERMEABLE, AND WE MUST THEREFORE 1920 REM FIND THE BOUNDARY PRESSURES BY EXTRAPOLATING 1930 REM FROM THE INTERIOR PRESSURE FIELD. THE CORNER NODE 1940 REM PRESSURES ARE TAKEN AS THE ARITHMETIC MEANS 1950 REM OF THEIR NEIGHBORING NODES. 1960 REM CALCULATE THE PRESSURES IN THE LOWER CORNERS 1970 P(1,1) = .5*(P(1,2)+P(2,1)) 1980 P(NX,1) = .5*(P(NX,2)+P(NX-1,1)) 1990 REM THE PRESSURES AT THE UPPER SURFACE 2000 REM ARE CONSTANT (0 IN THIS EXAMPLE) 2010 REM IMPERMEABLE WALLS 2020 FOR J = 2 TO NYM1 2030 P(1,J) = (4!*P(2,J)-P(3,J))/3 2040 PH(1,J) = P(1,J) 2050 P(NX,J) = (4!*P(NX-1,J)-P(NX-2,J))/3 2060 PH(NX,J) = P(NX,J) 2070 NEXT J 2080 REM IMPERMEABLE FLOOR 2090 FOR I = 2 TO NXM1 2100 P(I,1) = (4*P(I,2)-P(I,3))/3



APPENDIX 11.A.2 (continued) A Computer Listing of the Algorithm Used to Calculate the Pressure Field in a Bulk of Grains 2110 PH(I,1) = P(I,1) 2120 NEXT I 2130 FOR I = NDL TO NDU 2140 P(I,1) = PDUCT 2150 PH(I,1) = P(I,1) 2160 NEXT I 2170 REM CALCULATE VOLUME FLOW RATE OF AIR THROUGH THE SYSTEM 2180 REM PERMEABILITY IS KP, IE V = –KP*DPDY 2190 REM WHERE DPDY IS THE PRESSURE GRADIENT IN THE Y DIRECTION. 2200 Q = 0 2210 J = NYM1 2220 FOR I = 1 TO NX 2230 DPDY = (P(I,J+1)-P(I,J-1))/(2*HY) 2240 V(NX,J) = –DPDY/(RY+SY*VABS(I,J)) 2250 IF I = 1 THEN Q = Q+.5*HX*V(I,J) 2260 IF I = NX THEN Q = Q+.5*HX*V(I,J) 2270 IF I>1 AND I

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