Editor's Note: CR4 thanks Dick Cawley of GEA Consulting for contributing this blog entry.

Copeland Bulletin AE4-1331 R3 covers the need to ensure that adequate lubricant always remains in the compressor sump. For instance, they recommend the addition of 1 oz oil for every five (5) lb of refrigerant over 20 lb of refrigerant charge.

A scroll compressor is equipped with about 65 oz lubricant charge. One can quickly calculate the lubricant residing in a system having, say, and 1% lubricant in refrigerant. If the system has 20 lb of refrigerant, the oil in continuous circulation (away from the compressor) would be 20*.01 = 0.2 lb, or 3.2 oz. A system charged with, say, 50 lb refrigerant would have 8 oz lubricant in circulation. This is a substantial portion of the oil charge (12.3%) that cannot be in the sump. Six (6) oz. additional oil, as recommended by the compressor manufacturer, is needed in the system. Note, too, that these calculations are for steady state operation. Many times during transient operation, the circulation rate can be higher, which emphasizes the need for sufficient run time after the compressor is started.

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- Dick Cawley