mech3020-s11-hw08

# mech3020-s11-hw08 - to the compressor necessary to provide...

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MECH 3020 Thermodynamics II: Homework set #8, Due 2/25 1. A simple vapor refrigeration cycle uses R-134a. The evaporator is maintained at a temperature of -10 C, and the minimum temperature in the condenser is 2 C above the ambient temperature. Assuming the compressor is isentropic, calculate the compressor work per unit mass, and the cycle coeﬃcient of performance, for ambient temperatures of 10, 20, and 30 C. 2. A heat pump using R-134 operates between the pressures of 200 kPa and 1.2 MPa. The compressor has an isentropic eﬃciency of 0.8. Calculate the required mass ﬂow of refrigerant, and the power input
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Unformatted text preview: to the compressor, necessary to provide a heating rate of 30 kW. 3. A cascade vapor refrigeration system is designed to remove ˙ Q L = 20 kW from a cooled space. The temperature of the evaporator in the bottom stage is-20 ◦ C, and the condenser in the top stage is at 1 MPa. Assuming that both compressors are isentropic, and that the bottom stage condenser pressure is equal to the upper stage evaporator pressure, determine the optimum bottom stage condenser pressure which minimizes the power input to the cycle. 1...
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