FTFS Chap06 P040 - Chapter 6 The Second Law of...

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Chapter 6 The Second Law of Thermodynamics Refrigerators and Heat Pumps 6-40C The difference between the two devices is one of purpose. The purpose of a refrigerator is to remove heat from a cold medium whereas the purpose of a heat pump is to supply heat to a warm medium. 6-41C The difference between the two devices is one of purpose. The purpose of a refrigerator is to remove heat from a refrigerated space whereas the purpose of an air-conditioner is remove heat from a living space. 6-42C No. Because the refrigerator consumes work to accomplish this task. 6-43C No. Because the heat pump consumes work to accomplish this task. 6-44C The coefficient of performance of a refrigerator represents the amount of heat removed from the refrigerated space for each unit of work supplied. It can be greater than unity. 6-45C The coefficient of performance of a heat pump represents the amount of heat supplied to the heated space for each unit of work supplied. It can be greater than unity. 6-46C No. The heat pump captures energy from a cold medium and carries it to a warm medium. It does not create it. 6-47C No. The refrigerator captures energy from a cold medium and carries it to a warm medium. It does not create it. 6-48C No device can transfer heat from a cold medium to a warm medium without requiring a heat or work input from the surroundings. 6-49C The violation of one statement leads to the violation of the other one, as shown in Sec. 6-4, and thus we conclude that the two statements are equivalent. 6-50 The COP and the refrigeration rate of a refrigerator are given. The power consumption and the rate of heat rejection are to be determined. Assumptions The refrigerator operates steadily. Analysis ( a ) Using the definition of the coefficient of performance, the power input to the refrigerator is determined to be Kitchen air 6-21 kW 0.67 = = = = kJ/min 40 1.5 kJ/min 60 , R L in net COP Q W & & ( b ) The heat transfer rate to the kitchen air is determined from the energy balance, kJ/min 100 = + = + = 40 60 , in net L H W Q Q & & & cool space R COP L Q &
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Chapter 6 The Second Law of Thermodynamics 6-51 The power consumption and the cooling rate of an air conditioner are given. The COP and the rate of heat rejection are to be determined. Assumptions The air conditioner operates steadily. Analysis ( a ) The coefficient of performance of the air-conditioner (or refrigerator) is determined from its definition, Outdoors 6-22 2.08 = = = kJ/min 60 kW 1 kW 6 kJ/min 750 , in net L R W Q COP & & ( b ) The rate of heat discharge to the outside air is determined from the energy balance, () ( )( ) kJ/min 1110 = + = + = kJ/min 60 6 kJ/min 750 , in net L H W Q Q & & & 6-52 The COP and the refrigeration rate of a refrigerator are given. The power consumption of the refrigerator is to be determined. Assumptions The refrigerator operates steadily.
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FTFS Chap06 P040 - Chapter 6 The Second Law of...

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