I am reading different stories about the system performance gain when using suction gas <-> liquid line heat exchangers. Most people agree that with HC refrigerants (R600a is widely used here in Europe) there is a gain in both performance and COP when using suction gas <-> liquid line heat exchange. But for HFC refrigerants like R134a the opinions differ. Some people say that there is an increase in both performance and COP, some say that COP increases but performance does not, some say that both performance and COP decreases.

I understand that SG <-> LL heat exchange increases liquid subcool and thus decreases/eliminates flashing in the expansion device. This fact alone might increase performance.

I also understand that SG <-> LL heat exchange increases suction gas superheat, and therefore discharge line temperatures. This might be a problem when operating at low evaporating temperatures/pressures.

But say that we have an R134a system, and enough subcool on the liquid to prevent flashing at the expansion device. Does adding a SG <-> LL heat exchanger increase performance and/or COP?

My theory is that it does at least increase performance. My theory is als follows: increased subcool on the liquid increases the amount of energy taken from the evaporator per kg. of refrigerant (less energy needed to cool the boiling liquid refrigerant down to evaporating temperatures). The extra subcool appears as extra suction gas superheat at the compressor (due to the heat exchange). This does not decrease performance since with most (at least simple piston type) compressors the volume displaced per second is the same due to the fixed motor speed. The extra superheat appears as an increase in discharge heat, which is removed by the condenser. However, amp draw of the compressor probably increases due to the higher load on the compressor. Therefore, I would say that the the COP stays approximately the same.

Can anyone point me to the flaws in this theory and elaborate on the subject? I would like to understand why things happen instead of just accepting them.