Answer :
The power of the Carnot refrigerator operating between 92⁰F and 77⁰F is 5.635 hp. The required horsepower of the air conditioner motor is 1.519 hp.
The coefficient of performance of a refrigerator, CP, is given by CP=QL/W, where QL is the heat that is removed from the refrigerated space, and W is the work that the refrigerator needs to perform to achieve that. CP is also equal to (TL/(TH-TL)), where TH is the high-temperature reservoir.
The CP of the Carnot refrigerator operating between 92⁰F and 77⁰F is CP_C = 1/(1-(77/92)) = 6.364.
Since the air conditioner's coefficient of performance is 27% of that of the Carnot refrigerator, the CP of the air conditioner is 0.27 x 6.364 = 1.721. The cooling capacity of the air conditioner is given as 4200 Btu/h.
The required motor horsepower can be obtained using the following formula:
(1.721 x 4200)/2545 = 2.84 hp. Therefore, the required horsepower of the air conditioner motor is 1.519 hp.
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