14 and evaporator 16 evaporator 16 is positioned within an enclosure 18 for conditioning a space 20 and is provided with a thermal expansion valve 22 the components are connected in a closed loop refrigeration circuit comprising a main or first conduit means or line 24 with the screw compressor 10 positioned intermediate of the evaporator 16 and the condensor 14 and with the oil separator 12 connected to the discharge side 26 of the screw compressor opposite the suction or inlet side 28 of that compressor the helical screw rotary compressor may comprise a hermetic compressor unit or the like including a casing 30 which casing carries a oil injection port 32 for the injection of oil into the screw compressor helical screw rotors not shown for cooling lubricating and sealing purposes the main or first line 24 includes a first check valve 34 downstream of the discharge side 26 of the compressor and upstream of the oil separator 12 line 24 further comprises a second check valve 36 between the oil separator and the condensor 14 each of these valves permitting refrigerant flow respectively from the screw compressor to the oil separator and from the oil separator to the condensor but not refrigerant flow in the opposite direction positioned within the first or main refrigerant line 24 upstream of the screw.