Thermodynamics

594 | ThermodynamicsRegenerative Rankine Cycle10–39C How do the following quantities change when thesimple ideal Rankine cycle is modified with regeneration?Assume the mass flow rate through the boiler is the same.Turbine work (a) increases, (b) decreases,output:(c) remains the sameHeat supplied: (a) increases, (b) decreases,(c) remains the sameHeat rejected: (a) increases, (b) decreases,(c) remains the sameMoisture content (a) increases, (b) decreases,at turbine exit: (c) remains the same10–40C During a regeneration process, some steam isextracted from the turbine and is used to heat the liquid waterleaving the pump. This does not seem like a smart thing to dosince the extracted steam could produce some more work inthe turbine. How do you justify this action?10–41C How do open feedwater heaters differ from closedfeedwater heaters?10–42C Consider a simple ideal Rankine cycle and an idealregenerative Rankine cycle with one open feedwater heater.The two cycles are very much alike, except the feedwater inthe regenerative cycle is heated by extracting some steam justbefore it enters the turbine. How would you compare the efficienciesof these two cycles?10–43C Devise an ideal regenerative Rankine cycle that hasthe same thermal efficiency as the Carnot cycle. Show thecycle on a T-s diagram.10–44 A steam power plant operates on an ideal regenerativeRankine cycle. Steam enters the turbine at 6 MPa and450°C and is condensed in the condenser at 20 kPa. Steam isextracted from the turbine at 0.4 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 net work output per kilogram of steamflowing through the boiler and (b) the thermal efficiency ofthe cycle. Answers: (a) 1017 kJ/kg, (b) 37.8 percent10–45 Repeat Prob. 10–44 by replacing the open feedwaterheater with a closed feedwater heater. Assume that the feedwaterleaves the heater at the condensation temperature of theextracted steam and that the extracted steam leaves the heateras a saturated liquid and is pumped to the line carrying thefeedwater.10–46 A steam power plant operates on an ideal regenerativeRankine cycle with two open feedwater heaters. Steamenters the turbine at 10 MPa and 600°C and exhausts to thecondenser at 5 kPa. Steam is extracted from the turbine at 0.6and 0.2 MPa. Water leaves both feedwater heaters as asaturated liquid. The mass flow rate of steam through theboiler is 22 kg/s. Show the cycle on a T-s diagram, and determine(a) the net power output of the power plant and (b) thethermal efficiency of the cycle. Answers: (a) 30.5 MW,(b) 47.1 percent10–47 Consider an ideal steam regenerative Rankinecycle with two feedwater heaters, one closedand one open. Steam enters the turbine at 12.5 MPa and550°C and exhausts to the condenser at 10 kPa. Steam isextracted from the turbine at 0.8 MPa for the closed feedwaterheater and at 0.3 MPa for the open one. The feedwater isheated to the condensation temperature of the extracted steamin the closed feedwater heater. The extracted steam leaves theclosed feedwater heater as a saturated liquid, which is subsequentlythrottled to the open feedwater heater. Show the cycleon a T-s diagram with respect to saturation lines, and determine(a) the mass flow rate of steam through the boiler for anet power output of 250 MW and (b) the thermal efficiencyof the cycle.Boiler5ClosedFWH64P II3798OpenFWH2y10TurbinezP I11Condenser1 – y – zFIGURE P10–4710–48 Reconsider Prob. 10–47. Using EES (or other)software, investigate the effects of turbine andpump efficiencies as they are varied from 70 percent to 100percent on the mass flow rate and thermal efficiency. Plot themass flow rate and the thermal efficiency as a function of turbineefficiency for pump efficiencies of 70, 85, and 100 percent,and discuss the results. Also plot the T-s diagram forturbine and pump efficiencies of 85 percent.10–49 A steam power plant operates on an ideal reheat–regenerative Rankine cycle and has a net power output of 80MW. Steam enters the high-pressure turbine at 10 MPa and550°C and leaves at 0.8 MPa. Some steam is extracted at thispressure to heat the feedwater in an open feedwater heater.The rest of the steam is reheated to 500°C and is expanded inthe low-pressure turbine to the condenser pressure of 10 kPa.Show the cycle on a T-s diagram with respect to saturationlines, and determine (a) the mass flow rate of steam throughthe boiler and (b) the thermal efficiency of the cycle.Answers: (a) 54.5 kg/s, (b) 44.4 percent10–50 Repeat Prob. 10–49, but replace the open feedwaterheater with a closed feedwater heater. Assume that the feed-1