a refrigeration system for use in an automotive air conditioner comprises a compressor for compressing vapor - phase refrigerant , a condenser connected to the outlet of the compressor , an expansion valve connected to the outlet of the valve , and a vapor pressure - adjusting valve mounted in the passage extending from the evaporator to the compressor . when the vapor pressure inside this passage is less than a certain value , the adjusting valve reduces the flow of refrigerant from the evaporator to the compressor to maintain the vapor pressure constant . a part of the liquid - phase refrigerant on the downstream side of the condenser is added to the refrigerant evaporated by the evaporator , by means of the adjusting valve .