Page 401 - Advanced thermodynamics for engineers

17.2 SIMPLE GAS TURBINE CYCLE ANALYSIS 391

Maximum 4
Temperature
Temperature, T _ B

T

3
5

Heat
exchange
2 6
_
T
A

1

Entropy, s
FIGURE 17.10
Mean temperatures of energy addition and rejection for gas turbine with heat exchanger.

The effect of the heat exchanger is obviously to reduce the amount of energy required from the fuel
for the same maximum temperature, while the actual work output of the gas turbine (1-2-4-5-1)
remains constant. However, another way to consider this is to look at the mean temperatures of energy
addition and rejection (see Fig. 17.10). The values depicted by T B and T A for the heat exchange cycle
can be compared with those for the non-heat exchange one: these are shown by dotted lines. Hence,
heat exchange raises the mean temperature of energy addition and lowers the mean temperature of
energy rejection.
Figure 17.9 also shows how heat exchange is more effective with low-pressure ratio gas turbines
than high ones. As the pressure ratio increases for the same maximum temperature the difference
between T 5 and T 2 diminishes until, at some limiting pressure ratio they become equal. Obviously it is
not possible to use a heat exchanger for this cycle. A convenient way of analysing gas turbines is to plot
parameters against the isentropic temperature ratio defined as

s r ¼ r ðk 1Þ=k (17.22)
p
where r p is the compressor pressure ratio. This relationship is shown in Fig. 17.11.