Thermodynamics

322 | ThermodynamicsIf the air surrounding the refrigerator is at 25°C, determinethe minimum power input required for this refrigerator.Answer: 0.623 kW6–88 An air-conditioning system operating on the reversedCarnot cycle is required to transfer heat from a house at a rateof 750 kJ/min to maintain its temperature at 24°C. If the outdoorair temperature is 35°C, determine the power required tooperate this air-conditioning system. Answer: 0.46 kW6–89E An air-conditioning system is used to maintain ahouse at 72°F when the temperature outside is 90°F. If thisair-conditioning system draws 5 hp of power when operating,determine the maximum rate of heat removal from the housethat it can accomplish.6–90 A Carnot refrigerator operates in a room in which thetemperature is 25°C. The refrigerator consumes 500 W ofpower when operating and has a COP of 4.5. Determine (a) therate of heat removal from the refrigerated space and (b) thetemperature of the refrigerated space. Answers: (a) 135 kJ/min,(b) 29.2°C6–91 An inventor claims to have developed a refrigerationsystem that removes heat from the closed region at 12°Cand transfers it to the surrounding air at 25°C while maintaininga COP of 6.5. Is this claim reasonable? Why?6–92 During an experiment conducted in a room at 25°C, alaboratory assistant measures that a refrigerator that draws2 kW of power has removed 30,000 kJ of heat from therefrigerated space, which is maintained at 30°C. Therunning time of the refrigerator during the experiment was20 min. Determine if these measurements are reasonable.25°CRefrig.–30°C30,000 kJFIGURE P6–922 kW6–93E An air-conditioning system is used to maintain ahouse at 75°F when the temperature outside is 95°F. Thehouse is gaining heat through the walls and the windows at arate of 800 Btu/min, and the heat generation rate within thehouse from people, lights, and appliances amounts to 100Btu/min. Determine the minimum power input required forthis air-conditioning system. Answer: 0.79 hp6–94 A heat pump is used to heat a house and maintain it at24°C. On a winter day when the outdoor air temperature is5°C, the house is estimated to lose heat at a rate of 80,000kJ/h. Determine the minimum power required to operate thisheat pump.6–95 A heat pump is used to maintain a house at 22°C byextracting heat from the outside air on a day when the outsideair temperature is 2°C. The house is estimated to lose heat ata rate of 110,000 kJ/h, and the heat pump consumes 5 kW ofelectric power when running. Is this heat pump powerfulenough to do the job?22°CHPOutdoors2°C110,000 kJ/hFIGURE P6–955 kW6–96 The structure of a house is such that it loses heat at arate of 5400 kJ/h per °C difference between the indoors andoutdoors. A heat pump that requires a power input of 6 kW isused to maintain this house at 21°C. Determine the lowestoutdoor temperature for which the heat pump can meet theheating requirements of this house. Answer: 13.3°C6–97 The performance of a heat pump degrades (i.e., itsCOP decreases) as the temperature of the heat sourcedecreases. This makes using heat pumps at locations withsevere weather conditions unattractive. Consider a house thatis heated and maintained at 20°C by a heat pump during thewinter. What is the maximum COP for this heat pump if heatis extracted from the outdoor air at (a) 10°C, (b) 5°C, and(c) 30°C?6–98E A heat pump is to be used for heating a house inwinter. The house is to be maintained at 78°F at all times.When the temperature outdoors drops to 25°F, the heat lossesfrom the house are estimated to be 55,000 Btu/h. Determinethe minimum power required to run this heat pump if heat isextracted from (a) the outdoor air at 25°F and (b) the wellwater at 50°F.