Thermodynamics

Chapter 10 | 583E # in E # outorm # 4h 4 m # 5h 5 Q # p,out m # 7h 7whereThus 26.2 MWQ # p,out m # 4h 4 m # 5h 5 m # 7h 7m # 4 10.12115 kg>s2 1.5 kg>sm # 5 10.72115 kg>s2 10.5 kg>sm # 7 m # 4 m # 5 1.5 10.5 12 kg>sQ # p,out 11.5 kg>s213411.4 kJ>kg2 110.5 kg>s212739.3 kJ>kg2 112 kg>s2 1640.09 kJ>kg2Discussion Note that 26.2 MW of the heat transferred will be utilized in theprocess heater. We could also show that 11.0 MW of power is produced inthis case, and the rate of heat input in the boiler is 43.0 MW. Thus the utilizationfactor is 86.5 percent.10–9 ■ COMBINED GAS–VAPOR POWER CYCLESThe continued quest for higher thermal efficiencies has resulted in ratherinnovative modifications to conventional power plants. The binary vaporcycle discussed later is one such modification. A more popular modificationinvolves a gas power cycle topping a vapor power cycle, which is called thecombined gas–vapor cycle, or just the combined cycle. The combinedcycle of greatest interest is the gas-turbine (Brayton) cycle topping a steamturbine(Rankine) cycle, which has a higher thermal efficiency than either ofthe cycles executed individually.Gas-turbine cycles typically operate at considerably higher temperaturesthan steam cycles. The maximum fluid temperature at the turbine inlet isabout 620°C (1150°F) for modern steam power plants, but over 1425°C(2600°F) for gas-turbine power plants. It is over 1500°C at the burner exitof turbojet engines. The use of higher temperatures in gas turbines is madepossible by recent developments in cooling the turbine blades and coatingthe blades with high-temperature-resistant materials such as ceramics. Becauseof the higher average temperature at which heat is supplied, gas-turbinecycles have a greater potential for higher thermal efficiencies. However, thegas-turbine cycles have one inherent disadvantage: The gas leaves the gasturbine at very high temperatures (usually above 500°C), which erases anypotential gains in the thermal efficiency. The situation can be improvedsomewhat by using regeneration, but the improvement is limited.It makes engineering sense to take advantage of the very desirable characteristicsof the gas-turbine cycle at high temperatures and to use the hightemperatureexhaust gases as the energy source for the bottoming cycle suchas a steam power cycle. The result is a combined gas–steam cycle, as shown