PURPOSE: To suppress fluctuation in the degree of dryness of refrigerant and get a superior controlling characteristic of a discharged gas temperature by a method wherein an intermediate expansion tank is connected between a condenser and an evaporator, an intermediate suction pipe above a standard liquid surface in the tank is connected to a compressor and a liquid pipe is connected below the standard liquid surface in the tank.

CONSTITUTION: An intermediate expansion tank 4 is connected to an outlet port of a condenser 2 through a liquid pipe 5 having a high temperature expansion valve 6. This tank 4 is connected to an evaporator 3 through a low temperature expansion valve 8, and an intermediate suction pipe 9 is connected to an intermediate suction port of the compressor 1 above the standard liquid surface of the tank 4. In addition, the liquid pipe 5 is connected below the standard liquid surface of the tank 4, so that liquid refrigerant flowing from the liquid pipe 5 into the tank 4 flows while flashing. Accordingly, even if the standard liquid surface in the tank 4 is varied, variation on the liquid rate of refrigerant flowing out of the intermediate suction pipe is reduced and the variation of the degree of dryness of the refrigerant is reduced.