Cold and dry machine is based on the principle of freezing dehumidification using closed compression refrigeration system for cooling the compressed gas is cooled by the compressor discharge, so a lot of saturated water vapor contained therein, mist condensation droplets, after separation by soda discharged from the auto drain, compressed gas entering saturation higher temperature refrigeration dryer pre-cooler, in pre-cooler with cold, dry gas from the evaporator heat exchange, lowering the temperature of the evaporator into the cooling system, and refrigerant vapor for the second heat exchange itself temperature dropped close to the evaporation temperature of refrigerant in the cooling process twice, the compressed gas is water vapor to condense into liquid water droplets into the air and gas-water separator, down liquid water through automatic drainage from the plane, lower temperature drying compressed gas into the pre-cooler with water saturated gas just entering the heat exchange, so that the temperature itself is improved, resulting in cold and dry vent mixture containing lower water (i.e., lower dew point), the relative humidity is low drying compressed gas. Since the freeze dryer is reliable and easy to manage, low operating costs and become the industry Air Station house purification equipment preferred model.