Page 121 - Advanced Gas Turbine Cycles

Chapter 6. ‘Wet’ gas turbine plants 93
0.8

0.7

0.6
b
0
z
g 0.5
0
LL
LL
w 0.4
2
0.3
W
I
I-
0.2

0.1

0
1 1.5 2 2.5 3 3.5
ISENTROPIC TEMPERATURE RATIO
Fig. 6.7. Air standard thermal efficiencies of various dry plants with reversible recuperators.

(a = 4), but with rip = 0.9 for the van Liere cycle. The thermal efficiency 7 of each cycle
is highest for x- 1.0, for which it is equal to [l - (l/a)]. For the [CBT],XR cycle 7
decreases rapidly with increasing pressure ratio; the efficiency of the other cycles drops
less rapidly. Reheating and intercooling raises the efficiency but the thermal efficiency of
the van Liere cycle is highest. It drops slowly with x, but its efficiency is almost matched
by the cycle with both reheating and intercooling.
In practice, however, the heat exchanger effectiveness will not be unity for these dry
cycles, but the above analysis does suggest that for practical plants:
(i) the optimum pressure ratio for a [CBTXII plant will be low (as was illustrated in
Fig. 3.15, for a realistic heat exchanger effectiveness of 0.75);
(ii) the introduction of intercooling and reheating will increase the efficiency in the
recuperative cycles and also raise the optimum pressure ratio.

6.3.2. The simple EGT plant with water injection

The discussion of the last section is then useful in considering the evaporative cycles.
We shall see that the effect of water injection downstream of the compressor (and possibly
in the cold side of the heat exchanger) may lead towards the [CBT]1XR type of plant, with
increased cold side effective specific heat and hence increased heat exchanger
effectiveness. Water injection in the compressor may lead to a plant with isothermal
compression.