Thermodynamics HW Solutions 69

Thermodynamics HW Solutions 69 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-131 A refrigerator consumes 600 W of power when operating, and its motor remains on for 5 min and then off for 15 min periodically. The average thermal conductivity of the refrigerator walls and the annual cost of operating this refrigerator are to be determined. Assumptions 1 Quasi-steady operating conditions exist. 2 The inner and outer surface temperatures of the refrigerator remain constant. Analysis The total surface area of the refrigerator where heat transfer takes place is A total 2 m + × + × = 21 8 1 2 1 8 08 1 2 08 91 2 (. .) (. .) . Since the refrigerator has a COP of 2.5, the rate of heat removal from the refrigerated space, which is equal to the rate of heat gain in steady operation, is && QW e = ×= COP W) W 600 2 5 1500 But the refrigerator operates a quarter of the time (5 min on, 15 min off). Therefore, the average rate of
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This note was uploaded on 01/14/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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