The Refrigeration system use the external energy for transfering substance heat of lower temperature to another substance with higher temperature. Its working principle is the refrigerator which in the evaporator absorb the heat from lower temperature substance and vaporize into steam, then the compressor will draw out and compress the steam that produced from evaporator. The steam with higher temperature and higher pressure will be sent to the condenser after it was compressed. After it's cooled by the cooling medium, this high pressure liquid will be depressurization in the throttle mechanism and into the evaporator to absorb the heat, so a cooling cycle is formed. During heating, the refrigerant will changes the direction of the flow by four-way valve, which is opposite to the refrigeration, the refrigerant will go through the evaporator first and then back to the condenser and finally back to the compressor.

According to the type of cooling medium of the condenser, the refrigerating units can be divided into air-cooled, water-cooled and evaporative cooling (comprehensive coolant).