Thermodynamics

600 | Thermodynamics120 MW. Steam enters all three stages of the turbine at500°C. The maximum pressure in the cycle is 15 MPa, andthe minimum pressure is 5 kPa. Steam is reheated at 5 MPathe first time and at 1 MPa the second time. Show thecycle on a T-s diagram with respect to saturation lines, anddetermine (a) the thermal efficiency of the cycle and (b) themass flow rate of the steam. Answers: (a) 45.5 percent,(b) 64.4 kg/s10–93 Consider a steam power plant that operates on aregenerative Rankine cycle and has a net power output of 150MW. Steam enters the turbine at 10 MPa and 500°C and thecondenser at 10 kPa. The isentropic efficiency of the turbineis 80 percent, and that of the pumps is 95 percent. Steam isextracted from the turbine at 0.5 MPa to heat the feedwater inan open feedwater heater. Water leaves the feedwater heateras a saturated liquid. Show the cycle on a T-s diagram, anddetermine (a) the mass flow rate of steam through the boilerand (b) the thermal efficiency of the cycle. Also, determinethe exergy destruction associated with the regenerationprocess. Assume a source temperature of 1300 K and a sinktemperature of 303 K.Boiler4P II5OpenFWH3FIGURE P10–9326yP ITurbine7Condenser11 – y10–94 Repeat Prob. 10–93 assuming both the pump and theturbine are isentropic.10–95 Consider an ideal reheat–regenerative Rankine cyclewith one open feedwater heater. The boiler pressure is 10MPa, the condenser pressure is 15 kPa, the reheater pressureis 1 MPa, and the feedwater pressure is 0.6 MPa. Steamenters both the high- and low-pressure turbines at 500°C.Show the cycle on a T-s diagram with respect to saturationlines, and determine (a) the fraction of steam extracted forregeneration and (b) the thermal efficiency of the cycle.Answers: (a) 0.144, (b) 42.1 percent10–96 Repeat Prob. 10–95 assuming an isentropic efficiencyof 84 percent for the turbines and 100 percent for thepumps.10–97 A steam power plant operates on an ideal reheat–regenerative Rankine cycle with one reheater and two feedwaterheaters, one open and one closed. Steam enters thehigh-pressure turbine at 15 MPa and 600°C and the lowpressureturbine at 1 MPa and 500°C. The condenser pressureis 5 kPa. Steam is extracted from the turbine at 0.6 MPa forthe closed feedwater heater and at 0.2 MPa for the open feedwaterheater. In the closed feedwater heater, the feedwater isheated to the condensation temperature of the extractedsteam. The extracted steam leaves the closed feedwater heateras a saturated liquid, which is subsequently throttled to theopen feedwater heater. Show the cycle on a T-s diagram withrespect to saturation lines. Determine (a) the fraction ofsteam extracted from the turbine for the open feedwaterheater, (b) the thermal efficiency of the cycle, and (c) the netpower output for a mass flow rate of 42 kg/s through theboiler.5BoilerReheaterClosedFWH6984Pump II37High-PTurbine11OpenFWHFIGURE P10–9710y12Low-PTurbine2zPump ICondenser1 – y – z10–98 Consider a cogeneration power plant that is modifiedwith reheat and that produces 3 MW of power and supplies 7MW of process heat. Steam enters the high-pressure turbineat 8 MPa and 500°C and expands to a pressure of 1 MPa. Atthis pressure, part of the steam is extracted from the turbineand routed to the process heater, while the remainder isreheated to 500°C and expanded in the low-pressure turbineto the condenser pressure of 15 kPa. The condensate from thecondenser is pumped to 1 MPa and is mixed with theextracted steam, which leaves the process heater as a com-131