Quiz 5 Solution - After compression Pr = 19.404 T = 673 °F...

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650:462 Power Plants Quiz 5 November 3, 2010 Solution You are asked to look into a gas turbine system which was put into place with a novel air preheating system. Designed by a Stevens graduate, this system has worked extremely poorly. Management wants to know whether to remove the heat exchanger. It seems to be working correctly in that the exit temperature of the flue gas drops 300 °F when the heat exchanger is operating yet the efficiency of the system seems very poor. Analyze this situation by assuming the inlet temperature is 80 °F at atmospheric pressure, the pressure ratio in the compressor is 14, the exit temperature from the combustor is 1800 °F and the isentropic efficiency of the compressor and turbine are 90%. 1. Inlet conditions: Pr = 1.386, h = 129.06 Btu/lbm
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Unformatted text preview: After compression: Pr = 19.404; T = 673 °F; h = 274 Btu/lbm Actual Enthalpy = 290 Btu/lbm Combustor exit enthalpy = 577 Btu/lbm; Pr = 287 Turbine exhaust isentropic enthalpy = (Pr = 20.5) 276, T =1140 R Turbine Actual enthalpy = 306, T = 1260 R Efficiency = (271 – 161)/(287) = 0.38 2. With increase in inlet temperature, turbine side stays the same, new compressor side: Inlet temp = 380 °F , Pr = 6.573 h = 201.56 Btu/lbm After compression h = 426 Btu/lbm Actual enthalpy = 451 Btu/lbm New efficiency = (271 – 249)/126 = .17 3. Find temperatures going through heat exchanger: Hot in = 1260 R Hot out = 960 R Cold in = 80 °F = 540 R Cold out = 380 °F = 840 R Effectiveness = (840-540)/(1260-540) = 0.417...
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