Answer: A vapor-compression refrigeration system operates

Chapter 10, Problem 28P

(choose chapter or problem)

A vapor-compression refrigeration system operates with the cascade arrangement of Fig. 10.9. Refrigerant 22 is the working fluid in the high-temperature cycle and Refrigerant 134a is used in the low-temperature cycle. For the Refrigerant 134a cycle, the working fluid enters the compressor as saturated vapor at \(-30^{\circ} \mathrm{F}\) and is compressed isentropically to \(50 \text { lbf/in. }{ }^{2}\) Saturated liquid leaves the intermediate heat exchanger at \(50 \text { lbf/in. }{ }^{2}\) and enters the expansion valve. For the Refrigerant 22 cycle, the working fluid enters the compressor as saturated vapor at a temperature \(5^{\circ} \mathrm{F}\) below that of the condensing temperature of the Refrigerant 134a in the intermediate heat exchanger. The Refrigerant 22 is compressed isentropically to \(250 \text { lbf/in. }{ }^{2}\) Saturated liquid then enters the expansion valve at \(250 \text { lbf/in. }{ }^{2}\) The refrigerating capacity of the cascade system is 20 tons. Determine

(a) the power input to each compressor, in Btu/min.

(b) the overall coefficient of performance of the cascade cycle.

(c) the rate of exergy destruction in the intermediate heat exchanger, in Btu/min. Let \(T_{0}=80^{\circ} \mathrm{F}, \ p_{0}=14.7 \mathrm{\ lbf} / \mathrm{in} .^{2}\)

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