Subject : Adiabatic and Isentropic processes, AKA Heat of Compression - How to deal with it.


, Peter Kesselman writes:
>What exactly is heat of compression or adiabatic (sp?) heat.

Pete, your spelling is good. Ideal compression is actually an 'isentropic' process (a process at constant entropy), which happens to be a type of adiabatic process. An adiabatic process is basically a concept for people who like studying thermodynamic processes. It's a process in which heat is not gained or lost across the system being defined. 'Real' thermodynamic processes can only approach 'adiabatic' processes. In the case of compressors, actual discharge temperatures will generally exceed the temperatures predicted by the isentropic process.

Another adiabatic process is the throttling process, which occurs at constant entropy. The TEV throttling process is a good example of this.

>I understand that R23 has a higher adiabatic heat than R13 or R503. We are currently doing a retrofit on a cascade system with dual 2 HP hermetics (with POE), R23 in low side, HP80 in high side. The system works great, with plenty of capacity, except the R23 discharge line is running up to 300 deg C. I think this has something to do with the heat of compression. We are going to install a CPR valve to try to limit the load. Any suggestions?

The high discharge temperatures of R-23 (and R-22 and R-717--ammonia) is the nature of these refrigerants. The use of a desuperheating valve (and an appropriately sized condenser coil) will go a long way to resolving high discharge temperatures.

Andy Schoen

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