Thermodynamics

and the condenser pressure is 2 psia. Steam leaves theprocess heater as a saturated liquid. It is then mixed with thefeedwater at the same pressure and this mixture is pumped tothe boiler pressure. Assuming both the pumps and the turbinehave isentropic efficiencies of 86 percent, determine (a) therate of heat transfer to the boiler and (b) the power output ofthe cogeneration plant. Answers: (a) 6667 Btu/s, (b) 2026 kW10–69 Steam is generated in the boiler of a cogenerationplant at 10 MPa and 450°C at a steady rate of 5 kg/s. Innormal operation, steam expands in a turbine to a pressure of0.5 MPa and is then routed to the process heater, where itsupplies the process heat. Steam leaves the process heater asa saturated liquid and is pumped to the boiler pressure. In thismode, no steam passes through the condenser, which operatesat 20 kPa.(a) Determine the power produced and the rate at whichprocess heat is supplied in this mode.(b) Determine the power produced and the rate of processheat supplied if only 60 percent of the steam is routed to theprocess heater and the remainder is expanded to the condenserpressure.10–70 Consider a cogeneration power plant modified withregeneration. Steam enters the turbine at 6 MPa and 450°Cand expands to a pressure of 0.4 MPa. At this pressure, 60percent of the steam is extracted from the turbine, and theremainder expands to 10 kPa. Part of the extracted steam isused to heat the feedwater in an open feedwater heater. Therest of the extracted steam is used for process heating andleaves the process heater as a saturated liquid at 0.4 MPa. Itis subsequently mixed with the feedwater leaving the feedwaterheater, and the mixture is pumped to the boiler pressure.6Chapter 10 | 597Assuming the turbines and the pumps to be isentropic, showthe cycle on a T-s diagram with respect to saturation lines,and determine the mass flow rate of steam through the boilerfor a net power output of 15 MW. Answer: 17.7 kg/s10–71 Reconsider Prob. 10–70. Using EES (or other)software, investigate the effect of the extractionpressure for removing steam from the turbine to be used forthe process heater and open feedwater heater on the requiredmass flow rate. Plot the mass flow rate through the boiler as afunction of the extraction pressure, and discuss the results.10–72E Steam is generated in the boiler of a cogenerationplant at 600 psia and 800°F at a rate of 18 lbm/s. The plant isto produce power while meeting the process steam requirementsfor a certain industrial application. One-third of thesteam leaving the boiler is throttled to a pressure of 120 psiaand is routed to the process heater. The rest of the steam isexpanded in an isentropic turbine to a pressure of 120 psiaand is also routed to the process heater. Steam leaves theprocess heater at 240°F. Neglecting the pump work, determine(a) the net power produced, (b) the rate of process heatsupply, and (c) the utilization factor of this plant.10–73 A cogeneration plant is to generate power and 8600kJ/s of process heat. Consider an ideal cogeneration steamplant. Steam enters the turbine from the boiler at 7 MPa and500°C. One-fourth of the steam is extracted from the turbineat 600-kPa pressure for process heating. The remainder of thesteam continues to expand and exhausts to the condenser at10 kPa. The steam extracted for the process heater is condensedin the heater and mixed with the feedwater at 600kPa. The mixture is pumped to the boiler pressure of 7 MPa.Show the cycle on a T-s diagram with respect to saturationlines, and determine (a) the mass flow rate of steam that mustbe supplied by the boiler, (b) the net power produced by theplant, and (c) the utilization factor.BoilerTurbine6759Processheater85BoilerProcessheater7Turbine8P I43FWHCondenser3Condenser2P I14 2P II 1P IFIGURE P10–70FIGURE P10–73