Question

7-7 Consider the same compressor as problems 7-5 and 7-6. Calculate the rate of entropy generation (in kW/K) of the real compressor.

Solutions:

Problem 7-1 & 7-2

7-1 Your company wants to buy a compressor that will operate adiabatically to compress 2 kg/s of saturated R-134a vapor at -15°C to a pressure of 1000 kPa. What is the minimum work (in kW) required? Use the right sign convention for work. Hint: The minimum work is that of the reversible process. Start by simplifying the entropy balance!

Ans: W = -75.92

7-2 Consider the same situation as problem 7-1. Your company wants to buy a compressor that will operate adiabatically to compress 2 kg/s of saturated R-134a vapor at -15°C to a pressure of 1000 kPa. What is the minimum outlet temperature (in°C) required? Hint: just like before, the minimum outlet temperature occurs when the process is reversible.

Ans: T2 = 47

7-5 You build the compressor from Participation Problems 7-1 and 7-2 and find that the exit temperature is actually 60°C. If the real compressor is still adiabatic, what is the real work (in kW) ? Use the right sign convention for work.

Ans: -105.38

7-6 Consider the same compressor from Participation Problems 7-5 with a real exit temperature of 60°C. What is the efficiency (expressed as a %) of the compressor?

Ans: 71.4

Only need 7-7

Answer 1