Thermodynamics

586 | ThermodynamicsSteam cycle:(a) The ratio of mass flow rates is determined from an energy balance on theheat exchanger:Thus,That is, 1 kg of exhaust gases can heat only 0.131 kg of steam from 33 to500°C as they are cooled from 853 to 450 K. Then the total net work outputper kilogram of combustion gases becomesw net w net,gas yw net,steam 1210.41 kJ>kg gas2 10.131 kg steam>kg gas2 11331.4 kJ>kg steam2 384.8 kJ>kg gash 2 144.78 kJ>kg1T 2 33°C2h 3 3411.4 kJ>kg1T 3 500°C2w net 1331.4 kJ>kgh th 40.8%m # s 13411.4 144.782 m # g 1880.36 451.802Therefore, for each kg of combustion gases produced, the combined plantwill deliver 384.8 kJ of work. The net power output of the plant is determinedby multiplying this value by the mass flow rate of the working fluid inthe gas-turbine cycle.(b) The thermal efficiency of the combined cycle is determined fromh th w netq inE # in E # outm # g h 5 ¿ m # s h 3 m # g h 4¿ m # s h 2m # s 1h 3 h 2 2 m # g 1h¿ 4 h¿ 5 2m # sm # y 0.131g384.8 kJ>kg gas 0.487 or 48.7%790.6 kJ>kg gasDiscussion Note that this combined cycle converts to useful work 48.7 percentof the energy supplied to the gas in the combustion chamber. This valueis considerably higher than the thermal efficiency of the gas-turbine cycle(26.6 percent) or the steam-turbine cycle (40.8 percent) operating alone.TOPIC OF SPECIAL INTEREST*Binary Vapor CyclesWith the exception of a few specialized applications, the working fluid predominantlyused in vapor power cycles is water. Water is the best workingfluid presently available, but it is far from being the ideal one. The binarycycle is an attempt to overcome some of the shortcomings of water and toapproach the ideal working fluid by using two fluids. Before we discuss thebinary cycle, let us list the characteristics of a working fluid most suitablefor vapor power cycles:*This section can be skipped without a loss in continuity.