PURPOSE: To reduce the shaft power of a compressor by a method wherein hot- water is obtained by the high temperature gas of outlet side, cold water is produced by the hot-water through an absorption type refirgerator and the cold water is utilized as the cooling source of the gas of inlet side.

CONSTITUTION: First, second and third compressors 1, 2, 3 are provided with first, second and third coolers 11, 12, 13 at the inlet sides thereof and first, second, third heating units 21, 22, 23 at the outlet side thereof. First, second and third absorption type refrigerators 31, 32, 33 are prepared for respective stages. Hot-water HW is produced in the heater by the hot temperature gas being discharged, the cold water CW is produced in the absorption type refrigerator by the hot-water HW and the cold water CW is utilized as the cooling source in the cooler. Accordingly, the shaft power of the pump may be reduced.