R134a TXV bellow gas with R12 system charge big deal?
SCT: 125 - 180F
TXV SH delta 5-8F
The compressor is a variable capacity type with constant target suction pressure.
I'd like to get some input on drop-in installation of R134a bellow TXV into a R12 system.
The vapor pressure difference between HFC134a and CFC12 in the evaporator temperature range given above will cause TXV to respond as if sensing bulb is 2F cooler than it really is compared to a matching bellow charge & system charge pair.
This MVAC TXV has an adjustment screw at the liquid line entry but to tweak it, the system has to be opened. Since getting it adjusted right on first try isn't guaranteed, so this would be just as, if not more time consuming than replacing the bellow charge with R12.
Anyways, uncompensated, my speculation is that R134a TXV would cause the SH to increase from 5-8 to 7-10(as measured at evaporator outlet, not compressor).
If my speculation is right how much relative capacity loss can it be expected?
Two things to consider when applying TEVs:
1 - Capacity, which is based on the TEV port size & stroke, the thermodynamic properties of the refrigerant, and the conditions of the application. A TEV that is selected for R-12 will have approximately 20% more capacity when using R-134A...this due to the thermodynamic properties of R-134A.
2 - What thermostatic charge will provide the proper bulb pressure (applied to the top of the TEV diaphragm) to counteract the evaporator + spring pressure (applied to the bottom of the TEV diaphragm) such that the valve can maintain an operational superheat.
The characteristics of the R-12 thermostatic charge are close enough to what is required for R-134A to operate properly. In fact, a valve manufactured for R-134A is simply an R-12 valve, with the capacity rating adjusted for the new refrigerant. The factory R-12 thermostatic charge is identical to the factory R-134A charge.