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Abstract
In this article, I investigate the consequence of using different methods to evaluate the performance of refrigerants. For demonstration purposes, I have compared the refrigerants R22 and R404a.
Refrigerant Properties
You can’t just compare the refrigerants based on properties, since this would give you the idea that R404a will give less duty.
The reason for this is that the refrigerating effect is lower.
For example:
Refrigerant = R22
Isentropic Efficiency = 0.72
Refrigeration Effect = 159.3 kJ/kg
Coefficient of Performance = 3.93
Refrigerant = R404a
Isentropic Efficiency = 0.72
Refrigeration Effect = 109.3 kJ/kg <-- R404a is lower by 31 percent
Coefficient of Performance = 3.46
The problem with looking at refrigerant properties is that you are fixing the suction and discharge pressure.
Coil Performance
The next approach is to look at the coil performance, and the TechniSolve DXC program provides an ideal tool to do this. Choosing an arbitrary coil at typical air-conditioning conditions gives the following duty:
R22: Total Duty 32.7 kW
R404a: Total Duty 32.2 kW <-- R404a is lower by 1.5 percent
This is also unrealistic, since we also fix the suction temperature and the effect of the compressor performance is not included.
Refrigeration Cycle Simulation
Using the TechniSolve RefSim program, it is possible to simulate the performance at any condition, with actual compressor performance.
R22 Air-Cooled Air-conditioned
Evaporating 5.2 °C
Condensing 46.4 °C
Cooling Duty 19.7 kW
Heat Rejection 25.7 kW
R404a Air-Cooled Air-conditioned
Evaporating 4.7 °C
Condensing 48.0 °C
Cooling Duty 20.1 kW <-- R404a is higher by 2 percent
Heat Rejection 27.2 kW
Note that the balance of the compressor and the coil performance forces a different suction and discharge.
Conclusion
Changing from R22 to R404a results in an overall performance increase of 2 percent. You will agree that this is not too significant but it is interesting to see that the trend is different to a simple refrigerant property study.
References
1. SAIRAC 2004 Technical Data Manual, Wernick B.J. Ed.
2. ASHRAE 2001 Fundamentals Handbook
3. Stoecker, W.F. and Jones, J.W. “Refrigeration & Air Conditioning”, 2nd Ed. McGraw Hill 1982.
4. Reay, D.A and Macmichael, D.B.A “Heat Pumps: Design and Applications”, Pergamon Press, First Ed. 1979.
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