An apparatus and method for compressing gas refrigerant using two
different compressors operated alternatively in a series or parallel mode
for obtaining two different compression ratios and thereby provide
efficient operation for both a relatively lower suction pressure and a
relatively higher suction pressure. This system avoids an unbalanced mass
flow rate when the compressors are operated in series by unloading a
downstream one of the compressors. When operated in the series mode,
refrigerant is bypassed back to a suction inlet of the downstream
compressor during a portion of a compression stroke of the downstream
compressor for equalizing the mass flow rate through the two compressors.