To achieve a seal between the rotors, oil is injected into the compressor; to prevent this being carried into the system the oil separator is larger and more complex than the normal delivery oil separator associated with a reciprocating compressor.
Also, because some of the heat of compression is transferred to the oil, a larger oil cooler has to fitted , winch may be ither water or refrigerant cooled.
Since a.c. motor driven compressors are usually single speed, some form cylinder unloading gear is necessary to reduce the compressor capacity.
This unloading gear usually comprises a means of holding the suction valves open