The expansion valve plays the role of throttling and depressurization. When the high-pressure refrigerant liquid condensed by the condenser passes through the throttling valve, the pressure drops due to obstruction, which causes part of the refrigerant liquid to vaporize, and at the same time absorbs the latent heat of vaporization, and its own temperature It also decreases accordingly, becoming wet steam with low temperature and low pressure, and then enters the evaporator.