The low temperature and low pressure gaseous refrigerant (the refrigerant pressure is about 0.3MPa, the temperature is about 0 ℃) passing through the evaporator changes into the high pressure and high temperature refrigerant gas after passing through the compressor (the refrigerant pressure is about 1.7MPa, the temperature is about 70 ℃ when the outside temperature is 30 ℃). The compressed refrigerant enters the condenser through the exhaust pipe, and the high pressure and high temperature refrigerant gas enters the condenser. Under the forced cooling of the engine cooling fan, The refrigerant dissipates a large amount of heat into the air through the condenser to condense the gaseous refrigerant to form a refrigerant liquid with high pressure and medium temperature (at this time, the temperature of the liquid refrigerant is about 60 ℃, and the pressure is about 1.6MPa), and then enters the drying reservoir. The refrigerant after moisture absorption and filtering in the drying reservoir then arrives at the evaporator through the expansion valve. Due to the small hole effect of the expansion valve (only about 1mm in diameter), The pressure and temperature of the liquid refrigerant decrease sharply (the refrigerant pressure is about 0.3MPa, and the temperature is about 0 ℃). At the same time, a small part of the liquid refrigerant (about 15-20%) flashes into a gas state, and then enters the evaporator for heat exchange with the air in the cab (about 0.1MPa, 30 ℃) to absorb the heat in the cab (transmitted through air). After absorbing the heat, the refrigerant becomes a gas state (the refrigerant gas pressure is 0.2MPa, and the temperature is about 5 ℃), Then the low temperature and low pressure gaseous refrigerant returns to the compressor. This is done circularly in the sealing system, constantly absorbing the heat in the car to achieve refrigeration until the required set temperature in the car is reached.