Oil Change

Question: My question is in regard to changing oil on two Copeland semi-hermetic compressors. I take care of a walk-in freezer that holds a box temp of -20°F. I've noticed that the oil sightglass is turning black and it is hard to check the level of the oil. The compressor is about 10 years old.

Do you recommend changing the oil?

-Patrick Kessler, Chico, CA

Answer: Under normal conditions, the oil in a compressor should never need to be changed. You state that the oil is turning black and is becoming hard to se in the compressor's oil sightglass. In your case I do recommend that the oil be changed. But before you change the oil, the cause of the oil discoloration should be found. Oil can only turn “black” if the system is dirty as a result of a previous burnout not properly cleaned or because of an insufficient vacuum pulled on the system.

When a sufficient vacuum – 250 microns or lower – has been pulled on the system, one can be assured that the moisture and air has been removed from the system. If the moisture and air is still in the system and the compressors discharge temperature reaches 200F or higher, the oil will turn dark.

I am assuming that with a system operating at -20F, refrigerant R-503 is being used. Although R-502 is a good and proper refrigerant for the application, if the return gas temperature is allowed to get too high, the compressors discharge temperature will rise very quickly above 200F.

After the cause of the oil discoloration has been found and corrected, change the oil using a good quality 150-viscosity oil such as 3GS.

-George Story PE, CMS, Copeland