MODERN POWER STATION PRACTICE: V. 3

2

MODERN

POWER

STATION PRACTICE

'1

tSA'

w II:: ~ ~ W IL.

~ ...

t2

ENTROPY

.

a. b - CONVERSla-I OF HEAT ENERGY TO KINETIC ENERGY b.c - 'REABSORPTla-I OF KINETIC ENERGY TO HEAT

ENERGY

WATER

:-

i W

,\

Pl fI I' I

SUPERHEATED>

STEAM

VAPOUR

P,I-

9

SPEOFIC VOLUME V

e SUPERHEATED ST'EAM

x

)0' IL. oJ area (see Chapter 4). e

g

e g SUPERHEATED StEAM w cr :J I« cr w

Do ~ W I-

ENTROPY

~

ENTROPY

AVERAGE TEMPERATURE

~ OF

b.c.d.e. AND Lg. HAS INCREASED, DUE TO REHEATING

b.e.d.e. e.f. f.g. g.h.i. i.a. a.b.

- HEATING AT CONSTANT PRESSURE -IDEAL EXPANSION AT CONSTANT ENTROPY BEFORE REHEATING - REHEATING AT CONSTANT PRESSURE -IDEAL EXPANSION AT CONSTANT ENTROPY AfTER REHEATING - EXTRACTION OF LATENT HEAT IN CONDENSER -IDEAL PRESSURE INCREASE AT CONSTANT ENTROPY IN FEED PUMP

FIG. 1.1.1B. Effect of reheating

On large turbines (i.e. 100 MW and over) it becomes economic to increase the cycle efficiency by using reheat, which is a way of partially overcoming temperature limitations. By returning partially expanded steam to a reheater, the average temperature at which heat is added is increased and, by expanding this reheated steam through the remaining stages of the turbine, the exhaust wetness is considerably less than it would otherwise be (Fig. 1.1.IB). Conversely, if the maximum tolerable wetness is allowed, the initial pressure of the steam can be appreciably increased. Regenerative heating of the boiler feed-water is widely used in modern power plant. the effect being to increase the average temperature at which heat is added to the cycle, thus improving the cycle efficiency (see Chapter 3).

4

MODERN

POWER

STATION

PRACTICE

1.1.2. The Nozzle When steam is-allowed to expand through a narrow orifice, it assumes kinetic energy at the expense of its enthalpy. When this kinetic energy is extracted by turbine blades, the result is an isentropic expansion, modified by the effect of frictional reheating (Fig. 1.1.2A(a». If, however, the steam expands into a chamber, the whole of the generated kinetic energy will be reabsorbed as frictional reheat and the final enthalpy wHl be the same as the original (Fig. 1.1.2A(b». This process is known as throttling and is inherently wasteful Po

Po

o

CL

>CL ..J ~

..J ~

DISSIPATED

~ i5

F z w

BY INTERNAL

REHEAT

LOSSOF LOSSOF

AVAILABILITY

AVAILABILITY

ENTROPY

~

ENTROPY

~

(b)

(0)

COMPLETE DISSIPATION OF KINETIC ENERGY (1:HROTTLING)

USEFUL EXTRACTION OF KINETIC ENERGY (TURBINE BLADING)

a-b - CONVERSION OF HEAT ENERGY TO KINETIC ENERGY b-c - REABSORPTIONOF KINETIC ENERGY TO t fA ENERGY FIG. 1.1.2A. Extraction

and dissipation

of kinetic

energy

since the kinetic energy is irretrievably thrown away; this is reflected by the large rise in e..ntropy.(Rise in entropy may be regarded as loss of availability of the energy.) Throttling is used where it is necessary to dispose of energy in the form of enthalpy~ e.g. in governing valves at partial loads, labyrinth glands and blade tip seals. Figure 1.1.2B(a) illustrates the expansion process. Two chambers are connected by a small orifice or nozzle of cross-sectional area a ft2; the left-hand chamber A is supplied with steam at pressure Pa and temperature fa; the right chamber B is fitted with an exhaust pipe and valve, to enable its pressure Pb to be varied. When the valve is closed Pb

and the flow

= Pa

G=O

As the valve is opened, Pb will fall and the pressure difference (Pa-Pb) will cause a flow through the nozzle, the steam assuming kinetic energy at the expense of its enthalpy.

5

TURBINES

.,I )

1#

~