Refrigeration Compressor Rated Return Gas Temperature

[mdahlhausen]

From @lymereJ:
I noticed that the prototype models using “Low Temp” compressors end up in the IDF file being setup with a rated return gas temperature of 18.3 deg. C and a rated subcooling delta C of 0. According to AHRI Standard 540, the latter is expected but, if I am not mistaken, the former should be 4.44 deg. C (40 deg. F) as the compressors are setup as subcritical. I looked to see if we were using the setRatedReturnGasTemperature() method in the code base but it looks like we are not, so I’m guessing that it’s the default value assumed by OpenStudio. Unless the curves were indeed derived at a rated return gas temperature of 18.3 deg. C or if I am not missing something else, we should probably modify the code to set that value based on the actual rated conditions. This does impact simulation results a bit. The DOE Commercial Prototype Models follow a slightly different approach to model refrigeration systems so I’m not sure how these curves were developed.

[mdahlhausen]

Looks like we don't set the return gas temperature in the compressor constructor:
https://github.com/NREL/openstudio-standards/blob/master/lib/openstudio-standards/prototypes/common/objects/Prototype.refrigeration.rb#L352-L379

Two options are:

1. In the constructor method, set a default return gas temperature, which is possibly determined from other properties of the compressor.

2. Add a column to the compressor data that indicates the return gas temperature.

I'm fine with either. How common is it that this will be customized? If not often, I lean towards a default temperature. However it is nice to document assumptions like these in the data, rather than having them be set in the code.