I am just wondering: does the TEV superheat setting change with evaporation temperature? It seems logical to me if it does, presuming that the same refrigerant is used in the thermostatic element.

Say, the TEV manufacturer adjust the TEV for 4K/7F superheat at -20C / -4F evaporation on R404a. At that moment, the additional closing force from the spring equals 0.48 bar / ~7psi vapour pressure.

Now, when evaporating at -50C, R404a, same TEV, same spring setting, that same spring induced closing force would cause the superheat setting to rise to 9.5K / 17F.

The resemblance of these calculated numbers and actual numbers of TD between SST and secondary liquid temperature, measured in my R507 liquid chiller, is scary. I always assumed that the increasing TD came from less efficient heat transfer at low temperatures, but this might be another cause.

Can somebody enlight me? Or am I just kicking in open doors, and didn't I do my homework correctly?