Almost all refrigeration compressors contain oil, which is necessary to lubricate the compressor's internal bearing surfaces. As part of the normal compression cycle, some oil will be pumped out of the compressor with the refrigerant. Any oil that leaves the compressor must return to it; otherwise enough oil can be pumped out of the compressor to cause internal damage.

As part of any maintenance or service inspection, the oil level of the compressor should be checked. Unfortunately, this is not always possible since not all compressors have an oil level sight glass. An oil level that covers one-fourth to one-half of the sight glass is normally acceptable, but always follow a compressor manufacturer's recommendations for the proper oil level on a particular compressor.

If a technician does discover a low oil level, what should be done? Should he immediately add oil to the compressor? The correct answer is no. He should first ask the question, "Where did the oil go?"

Did it leak out of the system or is it trapped out in the system? More than likely the oil is trapped out in the system and simply adding oil to the compressor will likely do more harm than good. The trapped oil may return to the compressor, and with the additional oil added the compressor may become overfilled and damaged.

It is possible for the oil loss to be the result of a major system leak. However, caution should be taken since normally not much oil would leak out with refrigerant. The system leak should be repaired and the system re-charged with refrigerant. Then a decision as to whether to add oil should be made.

If there was no refrigerant leak then the oil must still be within the system. It may not be in the compressor, but it is somewhere in the system. (A common location is either in the suction line or in the evaporator.) A technician must find out where the oil is and why it became trapped.

If oil has become trapped in the evaporator, the warming of the evaporator may help return the oil to the compressor. This can easily be done with systems such as those in low temperature applications that have a defrost circuit. A technician can initiate a defrost cycle to warm the evaporator and aid in the return of the oil. Sometimes more than one defrost will need to be initiated.

Piping Practices

Poor piping practices can also lead to poor oil return. Suction lines should be sloped toward the compressor, normally using a pitch of 1/2 inch per 10 feet of run. Systems designed with the compressor located above the evaporator may require a trap or traps to be installed in the suction line to facilitate oil return. The normal recommendation is to install a trap at the base of a suction riser - if the riser is more than three to four feet. Additional traps may be required for every 20 feet of additional vertical lift.

When oil is added to a system, the oil level of the compressor should be monitored to verify that it is correct. This may require re-turning to the jobsite the next day. A technician can then determine if the oil level is adequate.

Joe Marchese is owner of Coldtronics, Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 07/04/2005