Tuesday, May 22, 2007

HW #9, P5 - Helium Gas Refrigeration Cycle - 6 pts

A gas refrigeration cycle with a pressure ratio of 3 uses helium as the working fluid. The temperature of the helium is -10oC at the compressor inlet and 50oC at the turbine inlet. Assuming adiabatic efficiencies of 82% for both the turbine and the compressor, determine...

a.) The minimum temperature in the cycle.
b.) The coefficient of performance.
c.) The mass flow rate of the helium in kg/s for a refrigeration load of 12 kW.

8 comments:

Anonymous said...

The tables in the back of the book don't go any lower than 298 K for helium, but the problem has temperatures lower than 263 K. Is there a way we can specify IG assumption in NIST?

Dr. B said...

Giro:
Please use constant Cp and Cv for this problem. CP = 5.1926 kJ/kg-K and CV = 3.1156 kJ/kg-K. Do not use the tables in the back of the book. There is no table for Helium anyway.

Anonymous said...

Do we need a value for gamma?

Anonymous said...

I tried solving for T1 using 0=CpLn(T1/T4)-(R/MW)Ln(P1/P4) with kevlin but am getting T1 = -65 C.

Anonymous said...

anonymous:
I think that's the T1s but not T1.
I thought i have the efficiency of turbine and H4 and H1s....since T1s is out of range in Thermophysical Properties of Fluid Systems , I cannot find H1s...any hint?Dr. B

Dr. B said...

Anon:
CP = 5.1926 kJ/kg-K and CV = 3.1156 kJ/kg-K and gamma = Cp/Cv.

Dr. B said...

Anon & Thermo:
T1S = -65 degC
Your method is correct. Thermo correctly points out that what you found was T1S (the temperature of the stream that would come out of a hypothetical isentropic turbine. Use T1S to determine T1 using the isentropic efficiency.

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