Page 132 - Entrophy Analysis in Thermal Engineering Systems

Irreversible engines—Open cycles 125

Fig. 8.12 Comparison of the efficiency and SEG of the combined cycles of Figs. 8.7
and 8.11 at TIT¼1273K.

efficiency takes place at an optimum pressure ratio of 5.7 in the former and
15.9 in the latter. So, the combined regenerative gas turbine and steam cycle
is expected to be 3.7 percentage points more efficient with smaller turbo-
machinery in the topping cycle compared to the cycle of Fig. 8.7.
The performances of the two designs of the combined cycle at maximum
efficiency are compared in Table 8.5. The results suggest that recovering the
thermal energy of the exhaust gases is more efficient in the modified cycle
(Fig. 8.11) than the original design (Fig. 8.7). The modified cycle is expected

Table 8.5 Performance comparison of the combined cycles of Figs. 8.7 and 8.11.
Parameter Combined cycle Modified cycle
Optimum pressure ratio 15.9 5.7
HRSG inlet temperature (K) 709.6 587.3
Steam temperature (K) 694.6 572.3
Minimum SEG (J/K mole fuel) 1346 1253
Efficiency of combined cycle 0.527 0.564
Efficiency of topping cycle 0.360 0.445
Efficiency of bottoming cycle 0.343 0.328
a
Power output (kW) 422.8 452.4
Power output of topping cycle (kW) a 289.1 356.9
a
Power output of bottoming cycle (kW) 133.7 95.5
Steam flowrate (mol/s) a 8.20 6.88
a
Per unit molar flowrate of the fuel.