System charging via superheat/subcooling

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I am on the mechanical engineering field but no refrigeration engineer. I am trying to put some missing pieces together in this puzzle in my head to get a better understanding.

I will use a for instance on my own system I have worked on. I charged the system to about 15* super heat on the suction. The data sheets for the system do not list super heat, just pressures at temperature and compressor amperage. Everything is spot on and running perfect.

HOWEVER, just as a case of education for me, I need to better understand why charging with these methods is used? It was my humble understanding that super heat is a target value to achieve proper cooling but maintain a certain number of degrees above saturation to ensure vapor is returning to the compressor, not liquid? In which case, it would seem 15* would be a lot?

And on the subcooling side of this, I am simply guessing that we need to carefully look at the liquid pressure and the line temp to ensure that there is enough refrigerant to maintain fluid state all the way to the evap coil?

In both of these, it would seem that measurements taken right at the evap would be more ideal than at the compressor?

IN any case, please let me know if I have this straight or way off cue. The way I charge is by including as much data as possible such as weighing in the charge, pressure chart from OEM, and SH/SC calculations.


I feel like I must be missing something because if we were only concerned with the refer temp at pressure, there would be no need for ambient dry bulb and evap wet bulb data. I still need to firm that up in my head.

I realize many people just use the provided values or equation and move on with life but I am sure any engineers here can understand that it "needs to make perfect sense in my head first"