The invention relates to improvements in a refrigeration system having an oil cooled screw compressor from which mixed oil and compressed refrigerant issues to an oil separator via a discharge duct, and wherein a refrigerant pump draws liquid refrigerant from the high pressure receiver and delivers it to the discharge duct to cool the oil and desuperheat the refrigerant. The refrigerant pump and a hydraulic motor that drives it are in a single sealed housing. The hydraulic motor is energized with pressure oil from the oil pump whereby oil is returned to the compressor from the oil separator. Oil flow to the hydraulic motor is throttled in accordance with output from a temperature sensor at the discharge duct, to maintain a constant temperature of oil-refrigerant mixture passing to the oil separator. A standpipe arrangement prevents cavitation at the refrigerant pump.