Patent ID: 7712329

Claim:
A compressor system for circulating refrigerant in a heating or cooling system, including: a first compressor in a first shell, said first compressor having a first low side lubricant sump; a second compressor in a second shell, said second compressor having a second low side lubricant sump, said first and second compressors being connected in series, and said compressors being operable with both compressors being on or with said first compressor being off and only said second compressor being on, a low side return line for refrigerant vapor, said low side return line being connected through a lubricant trap to said first shell, and a branch conduit connected from said lubricant trap to said second shell, whereby at least a majority of lubricant entrained in system refrigerant vapor is delivered directly to said first shell when only said second compressor is on or both said first and second compressors are on; a lubricant transfer conduit connected between said first low side sump of said first compressor and said second low side sump of said second compressor; and a normally open control valve in said lubricant transfer conduit, said normally open control valve being open to permit lubricant flow in said lubricant transfer conduit from said first low side sump of said first compressor to said second low side sump of said second compressor at least when said first compressor is off and said second compressor is on, and said normally open control valve being closed to prevent flow in said lubricant transfer conduit when both said first and second compressors are on.