As refrigerant is discharged from a compressor, so is a small amount of refrigerant oil. If the refrigerant piping is properly sized and configured, this oil will eventually return to the compressor. However, there will always be some oil in circulation with the refrigerant. On a new system start-up, it may be necessary to add refrigerant oil to a compressor to compensate for the amount of oil in circulation.

The required oil charge for a system is basically a factor of the amount of refrigerant in the system and the compressor’s oil pumping rate. The most common method of determining if oil needs to be added to a compressor is to observe the compressor’s oil sight glass. During a new start-up, a technician can observe the oil level within the sight glass and determine if oil needs to be added to the compressor.

Generally, an oil level that covers about one-half of the sight glass is deemed acceptable. If the oil level falls below the sight glass and stays below, oil should be added to the compressor. However, as with any component in our industry, always follow the manufacturer’s recommendations as they may differ from generally accepted values.

But what if the compressor does not have an oil sight glass? Not every compressor does. For example, many hermetic compressors do not have one. So how do you know if oil needs to be added to these compressors?

A technician can approximate if oil needs to be added to these compressors by multiplying the amount of refrigerant in the system by 2.2 percent, and then multiplying this number by 16 fluid ounces per pound. Then subtract from this value 10 percent of the oil charge stated on the compressor nameplate.

For example, suppose a system holds 60 pounds of refrigerant and the compressor’s nameplate shows an oil charge of 118 fluid ounces. The formula is:

60 pounds x 0.022 x 16 fluid ounces/pound = 21.12 fluid ounces

Then multiply the oil charge of the compressor by 0.10. In this example, it is:

118 fluid ounces x 0.10 = 11.8 fluid ounces

Then subtract the answer in the second formula from the first:

21.12 fluid ounces – 11.8 fluid ounces = 9.32 fluid ounces

The 9.32 fluid ounces is the amount that should be added to the compressor to compensate for the amount of oil in circulation.

This is not normally an issue for refrigeration technicians working on systems using hermetic compressors.

For the most part hermetic compressors without an oil sight glass are used on smaller refrigeration systems whose refrigerant charge does not exceed the amount that would require adding oil to the compressor. However this may not always be the case, and technicians must be able to determine when oil needs to be added to these compressors.

When adding oil to a compressor, always follow the compressor manufacturer’s guidelines for the type and grade of oil to use. There are several different types and grades of oil used on different types of refrigeration systems and their compressors. Adding the wrong oil to a compressor can be detrimental to the operation of the compressor and cause it to fail prematurely.

Publication date:04/05/2010