Page 133 - Advanced Gas Turbine Cycles

Chapter 6. ‘Wet’ gas turbine plants 105

6.6. Some detailed parametric studies of wet cycles

The general thermodynamic conclusions given above are confirmed by more
detailed parametric studies which have been made by several authors of various wet
cycles.
Macchi et al. [9] made an extensive study of water injection cycles in their two classic
papers and their results are worth a detailed study. Some of their calculations (for ISTIG,
RWI and HAT) are reproduced in Figs. 6.18-6.20, all for surface intercooling (parallel
calculations for evaporative intercooling are given in the original papers).
For the ISTIG cycle, Fig. 6.18 shows thermal efficiency plotted against specific work
for varying overall pressure ratios and two maximum temperatures of 1250 and 1500°C.
Peak efficiency is obtained at high pressure ratios (about 36 and 45, respectively), before
the specific work begins to drop sharply. Note that the pressure ratios of the LP and HP
compressors were optimised within these calculations.
Macchi et al. provided a similar comprehensive study of the more complex RWI cycles
as illustrated in Fig. 6.19, which shows similar carpet plots of thermal efficiency against
specific work for maximum temperatures of 1250 and 150O0C, for surface intercoolers.
The division of pressure ratio between LP and HP compressors is again optimised within
these calculations, leading to an LP pressure ratio less than that in the HP. For the RWI
cycle at 1250°C the optimisation appears to lead to a higher optimum overall pressure ratio
(about 20) than that obtained by Horlock [5], who assumed LP and HP pressure ratios to be
same in his study of the simplest RWI (EGT) cycle. His estimate of optimum pressure ratio

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47

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450 500 550 600 650 700 750 800 850
SPECIFIC WORK [kJlkg AIR]
Fig. 6.18. Overall efficiency and specific work of ISTIG plant (after Macchi et al. A).