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QUESTION 3 a) The food compartment of a refrigerator is maintained at -5 °C by removing heat from it at a rate of 400 kJ/min. The air surrounding the refrigerator is at 22 °C. If the required power input to the refrigerator is 3 kW: i) Determine the coefficient of performance (COP) of the refrigerator and compare with the COP of reversible refrigeration cycle operating between reservoirs at the same temperatures. (4 Marks) ii) Calculate the rate of heat rejection to the room that houses the refrigerator. (2 Marks) iii) Determine how many kilowatts of power this refrigerator will require to operate if the COP is reduced to 1.35. (2 Marks) iv) In a refrigerator, heat is transferred from a lower temperature medium to a higher temperature one. Is this a violation of the second law of thermodynamics? Explain. (2 Marks) b) A heat engine receives a heat at a rate of 1 MW at a high temperature of 550 °C and rejects energy to the ambient surrounding at temperature of 300 K. Work is produced at a rate of 450 kW. Determine the amount of energy discarded to the ambient surroundings and the engine efficiency. Compare both of these to a Carnot heat engine operating between the same two reservoirs. Justify your answer. (6 Marks) c) Carnot heat engine receives heat from a reservoir at 900 °C at a rate of 48000 kJ/min and reject the waste heat to the ambient air at 27 °C. The entire work output of the heat engine is used to drive a refrigerator that remove heat from the refrigerated space at -5 °C and transfer it to the same ambient air at 27 °C. Determine the maximum rate of heat removal from the refrigerated space and the total rate of heat rejection to the ambient air. (9 Marks)

QUESTION 3

a) The food compartment of a refrigerator is maintained at -5 °C by removing heat from it at a rate of 400 kJ/min. The air surrounding the refrigerator is at 22 °C. If the required power input to the refrigerator is 3 kW:

i) Determine the coefficient of performance (COP) of the refrigerator and compare with the COP of reversible refrigeration cycle operating between reservoirs at the same temperatures.
(4 Marks)

ii) Calculate the rate of heat rejection to the room that houses the refrigerator.
(2 Marks)

iii) Determine how many kilowatts of power this refrigerator will require to operate if the COP is reduced to 1.35.
(2 Marks)

iv) In a refrigerator, heat is transferred from a lower temperature medium to a higher temperature one. Is this a violation of the second law of thermodynamics? Explain.
(2 Marks)

b) A heat engine receives a heat at a rate of 1 MW at a high temperature of 550 °C and rejects energy to the ambient surrounding at temperature of 300 K. Work is produced at a rate of 450 kW. Determine the amount of energy discarded to the ambient surroundings and the engine efficiency. Compare both of these to a Carnot heat engine operating between the same two reservoirs. Justify your answer.
(6 Marks)

c) Carnot heat engine receives heat from a reservoir at 900 °C at a rate of 48000 kJ/min and reject the waste heat to the ambient air at 27 °C. The entire work output of the heat engine is used to drive a refrigerator that remove heat from the refrigerated space at -5 °C and transfer it to the same ambient air at 27 °C. Determine the maximum rate of heat removal from the refrigerated space and the total rate of heat rejection to the ambient air.
(9 Marks)

QUESTION 3

a) The food compartment of a refrigerator is maintained at -5 °C by removing heat from it at a rate of 400 kJ/min. The air surrounding the refrigerator is at 22 °C. If the required power input to the refrigerator is 3 kW:

i) Determine the coefficient of performance (COP) of the refrigerator and compare with the COP of reversible refrigeration cycle operating between reservoirs at the same temperatures.
(4 Marks)

ii) Calculate the rate of heat rejection to the room that houses the refrigerator.
(2 Marks)

iii) Determine how many kilowatts of power this refrigerator will require to operate if the COP is reduced to 1.35.
(2 Marks)

iv) In a refrigerator, heat is transferred from a lower temperature medium to a higher temperature one. Is this a violation of the second law of thermodynamics? Explain.
(2 Marks)

b) A heat engine receives a heat at a rate of 1 MW at a high temperature of 550 °C and rejects energy to the ambient surrounding at temperature of 300 K. Work is produced at a rate of 450 kW. Determine the amount of energy discarded to the ambient surroundings and the engine efficiency. Compare both of these to a Carnot heat engine operating between the same two reservoirs. Justify your answer.
(6 Marks)

c) Carnot heat engine receives heat from a reservoir at 900 °C at a rate of 48000 kJ/min and reject the waste heat to the ambient air at 27 °C. The entire work output of the heat engine is used to drive a refrigerator that remove heat from the refrigerated space at -5 °C and transfer it to the same ambient air at 27 °C. Determine the maximum rate of heat removal from the refrigerated space and the total rate of heat rejection to the ambient air.
(9 Marks)

Added by Gooi E.

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