By T.C. Mann
My understanding is that crankcase heaters in compressors are used to prevent the refrigerant from migrating to the oil due to low temperatures. This would cause increased foaming and loss of oil viscosity on startup.
It has also been stated that mineral oil will separate out and will pool up in the evaporator core if used with R-134a. This is due to a decrease in miscibility caused by decreased temperatures in the evaporator core. Just as in a cold compressor crankcase, it would seem that refrigerant-to-oil miscibility would increase in the evaporator core due to the cooler temps, just like in a cold compressor crankcase.
If this holds true, more — not less — refrigerant will be dissolved in the oil, and this should allow the oil to be safely returned to the compressor.
Are these assumptions correct or am I missing something?
From Dan Kramer, P.E.
Specialist Grade Member of RSES
Your letter addresses two different problems. The first is directed to refrigerant migrating to and condensing in the crankcase when the compressor is in a colder environment than the evaporator. That’s what the crankcase heater is intended to prevent. However, crankcase heaters are rarely used on residential systems because the compressors are rarely called on to run when the weather is cold. By the time the weather is warm enough for the resident to turn on the air conditioner, the compressor will have warmed up and the refrigerant that had been condensed in the crankcase will have evaporated and condensed in the cooler indoor evaporator.
However, in commercial air conditioning systems that may be called on to run anytime, either a crankcase heater or, preferably, an auto-recycling pumpdown cycle should be provided.
The pumpdown should have a liquid solenoid actuated by the indoor thermostat and a low-pressure switch set to cut in at a pressure corresponding to a temperature lower than the minimum expected outdoor temperature. Naturally, the liquid solenoid and compressor discharge valves must seat tightly or the compressor will short cycle. If the compressor does short cycle, the source of leakage into the low side should be found and fixed.
Your second question is about mineral oil “pooling” in a cold evaporator when the refrigerant is R-134a. Actually, I have been an advocate for the continued use of R-134a with either mineral oil or with alkylbenzene.
While there may be a small amount of oil hang-up in close-coupled air conditioning or heat pump systems using R-134a and mineral oil, most service people (and some major companies) who have tried it, have informally reported that the mineral oil returns normally.
With respect to pooling of oil in the evaporator, most air conditioning evaporators are high-velocity types where pooling is not likely to occur. In shell-and-tube-type or flooded evaporators, the situation is different; the vapor velocity is likely to be low and the reduced solubility of the oil in the 134a could cause it to collect on top of the liquid refrigerant surface (if the compressor pumps oil at a high rate and there is no properly operating oil separator).
I would avoid using R-134a with mineral oil in those situations without careful testing.
Publication date: 10/06/2003