Oil Compatibility

QUESTION:

From Charles Nunes Punta Gorda, FL My question deals with R-408A as a replacement for R-502, especially in terms of oil compatibility.

I have used 408A in an ice machine and a freezer system. I have no problem with the ice machine. But I’m exper-iencing some difficulties with the freezer system. The oil seems to be freezing at the service valve on the suc-tion line at the compressor. This valve also accumulates ice — sometimes 111/22 in. thick to the compressor shell.

I used a triple evacuation method. But I did not remove the existing oil. Should I have done so? Why does one system (the ice machine) operate so well and the other (the freezer) have problems?

ANSWER:

By Matt Ricketts Atofina Philadelphia, PA

R-408A is capable of being used with mineral oil, alkylbenzene oil, or polyolester lubricants. Systems that operate with R-502 and mineral oil, and show adequate lubricant return to the compressor, may continue to use mineral oil with 408A. The majority of our 408A customers who retrofit 502 equipment leave the existing mineral oil in the system and do not experience any oil-related system problems. As always, Atofina recommends following the equipment manufacturer guidelines.

The miscibility (how well refrigerant and oil mix) of 408A with mineral oil should be sufficient to allow the return of the lubricant back to the compressor, given the typical flow in standard compact systems and systems designed with oil separators or oil traps.

However, for those systems where oil return has been a problem, miscibility can by enhanced with the addition of alkylbenzene lubricant. We have found that replacing 25% to 50% of the existing mineral oil with alkylbenzene (usually changing out the oil in the compressor crankcase) will solve most refrigerant-related oil return problems.

You stated in your letter that the oil in the system seems to be freezing at the service valve on the suction line to the compressor. From this description, it seems that oil return may not be the problem. These “freeze-ups” lead me to wonder if you may have water or another contaminant in the system that could freeze. Mineral, alkylbenzene, and polyolester oils continually circulate throughout most refrigeration systems and are designed to flow (not freeze) at extremely low temperatures. The correct viscosity (ability of oil to flow) is very important, and I would recommend making sure that the oil being used is correct for your application. R-408A will have the same effect on oil viscosity as would R-502, so no changes would be observed.

The ice buildup you are talking about on the suction line is very common for low-temperature R-502 systems. If the ice buildup pattern is different after a retrofit, I would suggest examining the superheat. An improperly adjusted expansion valve may cause ice to build up and also could cause refrigerant floodback to the compressor and severe damage.

Publication date: 03/05/2001