The amount of water vapor in the compressed air is determined by the temperature of the compressed air: in keeping the compressed air pressure is almost the same, the lower the temperature of the compressed air compressed air can be reduced by the water vapor content, and the excess of water vapor condenses into liquid. This principle is used to dry compressed air by refrigeration technology. Therefore, there is a cooling system for the cold dryer. The refrigeration system of refrigerating dryer belongs to compressive refrigeration, which consists of four basic components, including refrigeration compressor, condenser, evaporator and expansion valve. Between them in turn connect pipe to form a closed system, the refrigerant in the system constantly circulating, change state and heat exchange with compressed air and cooling medium. Compressed-air dryer and adsorption dryer and solvent dryer.