Thermodynamics

518 | Thermodynamics10Regenerator51CombustionchamberReheater4678 9CompressorICompressorIITurbine ITurbine IIw net23IntercoolerFIGURE 9–43A gas-turbine engine with two-stage compression with intercooling, two-stage expansion withreheating, and regeneration.T43q regen6 8q in597210q regen = q saved1 q outplace between states 5 and 6. The gas enters the first stage of the turbine atstate 6 and expands isentropically to state 7, where it enters the reheater. It isreheated at constant pressure to state 8 (T 8 T 6 ), where it enters the secondstage of the turbine. The gas exits the turbine at state 9 and enters the regenerator,where it is cooled to state 10 at constant pressure. The cycle is completedby cooling the gas to the initial state (or purging the exhaust gases).It was shown in Chap. 7 that the work input to a two-stage compressor isminimized when equal pressure ratios are maintained across each stage. Itcan be shown that this procedure also maximizes the turbine work output.Thus, for best performance we havesP 2P 1 P 4P 3and P 6P 7 P 8P 9(9–26)FIGURE 9–44T-s diagram of an ideal gas-turbinecycle with intercooling, reheating, andregeneration.In the analysis of the actual gas-turbine cycles, the irreversibilities that arepresent within the compressor, the turbine, and the regenerator as well as thepressure drops in the heat exchangers should be taken into consideration.The back work ratio of a gas-turbine cycle improves as a result of intercoolingand reheating. However, this does not mean that the thermal efficiencyalso improves. The fact is, intercooling and reheating alwaysdecreases the thermal efficiency unless they are accompanied by regeneration.This is because intercooling decreases the average temperature atwhich heat is added, and reheating increases the average temperature at whichheat is rejected. This is also apparent from Fig. 9–44. Therefore, in gasturbinepower plants, intercooling and reheating are always used in conjunctionwith regeneration.