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Unformatted text preview: Assignment 6
2nd Law, Heat Engine, Refrigerator D36 A steam power plant with a power output of 150 MW consumes coal at a rate of 60 tons/h.
If the heating value of the coal is 30,000 kJ/kg, determine the thermal efﬁciency of this plant. Answer: 30.0 percent , D37An automobile engine consumes fuel at a rate of 28 L/h and delivers 60 kW of power to the
Wheels. If the fuel has a heating value of 44,000 kJ/kg and a density of 0.8 g/cm3, determine efﬁciency of this engine.
Answer: 21.9 percent D38A household refrigerator with a COP of 1.5 removes heat ﬁ'om the refrigerated space at a
rate of 60 kJ/min. Determine (a) the electric power consumed by the refrigerator and (b) the rate of heat transfer to the kitchen air.
Answer: (a) 0.67 kW, (b) 100 kJ/min D39An air conditioner removes heat steadily from a house at a rate of 750 kJ/min while drawing
electric power at a rate of 6 kW. Determine (a) the COP of this air conditioner and (b) the rate of heat transfer to the outside air.
Answer: (a) 2.08, (b) 1110 kJ/min D40A Carnot heat engine receives 650 M of heat from a source of unknown temperature and
rejects 200 kJ of it to a sink at 17°C. Determine (a) the temperature of the source and
(b) the thermal efﬁciency of the heat engine.
Answer: (a) 942.5 K‘ (b) 0.69 D41An inventor claims to have developed a heat engine that receives 750 kJ of heat from a
source at 400 K and produces 250 kJ of net work while rejecting the waste heat to a sink at 300 K. Is this a reasonable claim? Why?
Answer: No. Actual efﬁciency is higher than Carnot efﬁciency. D42An airconditioning system is used to maintain a house at 75°F when the temperature outside
is 95°F. The house is gaining heat through the walls and the windows at a rate of 750 Btu/min,
and the heat generation rate within the house from people, lights, and appliances amounts to
150 Btu/min. Determine the minimum power input required for this air—conditioning system. Answer: 0.79 hp D43A heat pump is used to heat a house and maintain it at 24°C. On a winter day when the outdoor
air temperature is 5°C, the house is estimated to lose heat at a rate of 80,000 kJ/h. Determine the minimum power required to operate this heat pump.
Answer: 2.17 kW E . .
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 Spring '09
 Davis

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