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I am trying to find the COP of a VCR system. I can reach the COP with using the EES(Engineering Equation Solver), without dealing with the UA values, i.e. just with the energy balances for the cycle. How the COP can differ, if I incorporate the UA values of the Heat Exchangers into the model? Is there any need for this?

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My opinion is, no, there's not any need to incorporate UA into your calculations. Think about it this way: The cooling provided by the evaporator really only comes from boiling refrigerant. The heat absorbed by vapor is almost negligible. To absorb more heat, you need to boil more refrigerant rather than getting refrigerant to boil sooner. To boil more refrigerant, you need to run your compressor at a higher speed, which means adding more work. Since: $$COP = \frac{Q_{evaporator}}{W_{compressor}}$$, you won't gain much clarity adding UA to the model.
Caveat: if refrigerant is not exiting the condenser as pure liquid, efficiency improvements can be made by increasing UA of the condenser.

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  • $\begingroup$ a great answer. thank you very much! A second question; what would be the difference between steady(with energy balances only, which I did already) and the quasi-steady (dynamic) model? And do you know how should I dynamically model a chiller system? A third; as you know there are unique COP's for the different loads, how can I obtain the unique COP's for the part loads? And according to your answer, the COP won't change in the part-loads since as Q is reduced the W also will be reduced, doesn't this then make the part-load analysis redundant? $\endgroup$
    – user5026
    Apr 13, 2017 at 19:59
  • $\begingroup$ A couple of thoughts on "pseudo-dynamic" models: $ COP_{carnot} = \frac{T_{H}}{T_{H}-T_{C}} $. Also, EES will give you a P-h diagram with isotherms for many refrigerants. You want to boil your refrigerant at a pressure where the saturation dome is widest. In other words, so that you receive maximum enthalpy of vaporization. If you have a compressor map, you'll have a better idea of what sort of pressure ratio you can maintain, and what it'll cost you in work. I don't think you should do a dynamic model, these systems are very nonlinear and difficult to control. $\endgroup$
    – EMiller
    Apr 14, 2017 at 14:28

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