A vapor-compression refrigeration system operates with a cascade arrangement. Re

Question

A vapor-compression refrigeration system operates with a cascade arrangement. Refrigerant R-22 is the working fluid in the high-temperature cycle and refrigerant R-134a is used in the low-temperature cycle. For the R-134a cycle the working fluid enters the compressor as saturated vapor at -30 F and is compressed isentropically to 50 lbf/in^2. Saturated liquid leaves the intermediate heat exchanger at 50 lbf/in^2 and enters the expansion valve. For the R-22 cycle the working fluid enters the compressor as saturated vapor at a temperature 5 F below that of the condensing temperature of the R-134a in the intermediate heat exchanger. The R-22 is compressed isentropically to 250 lbf/in^2. Saturated liquid enters the expansion valve at 250 lbf/in^2. The refrigeration capacity of the cascade system is 20 tons. Refer to the following table:

Determine the following:

I) The power input to each compressor (Btu/min)

II) the overall coefficient of performance of the cascade cycle

Explanation / Answer

I) power input to compressor 1 =77.62 Btu/min power input to compressor 2 = 81.58 Btu/min II) Co-efficient of performance =4.1

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