Thermodynamics

Chapter 8 | 479Water15°C4.6 kg/sSat.vapor0.23 kg/sReview Problems8–95 Refrigerant-134a is expanded adiabatically in anexpansion valve from 1.2 MPa and 40°C to 180 kPa. Forenvironment conditions of 100 kPa and 20°C, determine(a) the work potential of R-134a at the inlet, (b) the exergydestruction during the process, and (c) the second-law efficiency.8–96 Steam enters an adiabatic nozzle at 3.5 MPa and300°C with a low velocity and leaves at 1.6 MPa and 250°Cat a rate of 0.4 kg/s. If the ambient state is 100 kPa and 18°C,determine (a) the exit velocity, (b) the rate of exergy destruction,and (c) the second-law efficiency.8–97 A 30-L electrical radiator containing heating oil isplaced in a well-sealed 50-m 3 room. Both the air in the roomand the oil in the radiator are initially at the environmenttemperature of 10°C. Electricity with a rating of 1.8 kW isnow turned on. Heat is also lost from the room at an averagerate of 0.35 kW. The heater is turned off after some timewhen the temperatures of the room air and oil are measuredto be 20°C and 50°C, respectively. Taking the density and thespecific heat of oil to be 950 kg/m 3 and 2.2 kJ/kg °C, determine(a) how long the heater is kept on, (b) the exergydestruction, and (c) the second-law efficiency for thisprocess. Answers: (a) 2038 s, (b) 3500 kJ, (c) 0.04610°CMixingchamberFIGURE P8–94RadiatorRoomFIGURE P8–97Mixture45°C8–98 Hot exhaust gases leaving an internal combustionengine at 400°C and 150 kPa at a rate of 0.8 kg/s is to beused to produce saturated steam at 200°C in an insulated heatexchanger. Water enters the heat exchanger at the ambienttemperature of 20°C, and the exhaust gases leave the heatQExh. gas400°C150 kPaSat. vap.200°Cexchanger at 350°C. Determine (a) the rate of steam production,(b) the rate of exergy destruction in the heat exchanger,and (c) the second-law efficiency of the heat exchanger.8–99 The inner and outer surfaces of a 5-m 6-m brickwall of thickness 30 cm are maintained at temperatures of20°C and 5°C, respectively, and the rate of heat transferthrough the wall is 900 W. Determine the rate of exergydestruction associated with this process. Take T 0 0°C.20°CHEATEXCHANGERFIGURE P8–98BRICKWALLQ30 cmFIGURE P8–995°C350°CWater20°C8–100 A 1000-W iron is left on the ironing board with itsbase exposed to the air at 20°C. If the temperature of the baseof the iron is 150°C, determine the rate of exergy destructionfor this process due to heat transfer, in steady operation.8–101 One method of passive solar heating is to stack gallonsof liquid water inside the buildings and expose them tothe sun. The solar energy stored in the water during the day isreleased at night to the room air, providing some heating. Considera house that is maintained at 22°C and whose heating isassisted by a 350-L water storage system. If the water is heatedto 45°C during the day, determine the amount of heating thiswater will provide to the house at night. Assuming an outsidetemperature of 5°C, determine the exergy destruction associatedwith this process. Answers: 33,548 kJ, 1172 kJ8–102 The inner and outer surfaces of a 0.5-cm-thick, 2-m 2-m window glass in winter are 10°C and 3°C, respectively.If the rate of heat loss through the window is 3.2 kJ/s,determine the amount of heat loss, in kJ, through the glassover a period of 5 h. Also, determine the exergy destructionassociated with this process. Take T 0 5°C.8–103 An aluminum pan has a flat bottom whose diameteris 20 cm. Heat is transferred steadily to boiling water in thepan through its bottom at a rate of 800 W. If the temperatures