Thermodynamics

entire process, including the conversion of electric energy toheat energy. Answers: (a) 1.507 kg, (b) 689 kJ8–128 What would your answer to Prob. 8–127 be if heatwere supplied to the pressure cooker from a heat source at180°C instead of the electrical heating unit?8–129 A constant-volume tank contains 20 kg of nitrogenat 1000 K, and a constant-pressure device contains 10 kg ofargon at 300 K. A heat engine placed between the tank anddevice extracts heat from the high-temperature tank, produceswork, and rejects heat to the low-temperature device. Determinethe maximum work that can be produced by the heatN 220 kg1000 K1.2 in.Oven, 1300°FBrassplate, 75°FFIGURE P8–133EChapter 8 | 483HEAr10 kg300 KQ LQ HFIGURE P8–129engine and the final temperatures of the nitrogen and argon.Assume constant specific heats at room temperature.8–130 A constant-volume tank has a temperature of 800 Kand a constant-pressure device has a temperature of 290 K.Both the tank and device are filled with 20 kg of air. A heatengine placed between the tank and device receives heat fromthe high-temperature tank, produces work, and rejects heat tothe low-temperature device. Determine the maximum workthat can be produced by the heat engine and the final temperaturesof the tank and device. Assume constant specific heatsat room temperature.8–131 Can closed-system exergy be negative? How aboutflow exergy? Explain using an incompressible substance asan example.8–132 Obtain a relation for the second-law efficiency of aheat engine that receives heat Q H from a source at temperatureT H and rejects heat Q L to a sink at T L , which is higherthan T 0 (the temperature of the surroundings), while producingwork in the amount of W.8–133E In a production facility, 1.2-in-thick, 2-ft 2-ftsquare brass plates (r 532.5 lbm/ft 3 and c p 0.091 Btu/lbm· °F) that are initially at a uniform temperature of 75°F areWheated by passing them through an oven at 1300°F at a rate of300 per minute. If the plates remain in the oven until theiraverage temperature rises to 1000°F, determine the rate of heattransfer to the plates in the furnace and the rate of exergydestruction associated with this heat transfer process.8–134 Long cylindrical steel rods (r 7833 kg/m 3 andc p 0.465 kJ/kg · °C) of 10-cm diameter are heat-treated bydrawing them at a velocity of 3 m/min through a 6-m-longoven maintained at 900°C. If the rods enter the oven at 30°Cand leave at 700°C, determine (a) the rate of heat transfer tothe rods in the oven and (b) the rate of exergy destructionassociated with this heat transfer process. Take T 0 25°C.8–135 Steam is to be condensed in the condenser of a steampower plant at a temperature of 60°C with cooling water froma nearby lake that enters the tubes of the condenser at 15°C ata rate of 140 kg/s and leaves at 25°C. Assuming the condenserto be perfectly insulated, determine (a) the rate of condensationof the steam and (b) the rate of exergy destruction in thecondenser. Answers: (a) 2.48 kg, (b) 694 kW8–136 A well-insulated heat exchanger is to heat water(c p 4.18 kJ/kg · °C) from 25°C to 60°C at a rate of 0.4 kg/s.The heating is to be accomplished by geothermal water (c p 4.31 kJ/kg · °C) available at 140°C at a mass flow rate of0.3 kg/s. The inner tube is thin-walled and has a diameter of0.6 cm. Determine (a) the rate of heat transfer and (b) therate of exergy destruction in the heat exchanger.Brine140°C60°CFIGURE P8–136Water25°C