The expansion valve is a device that allows the liquid refrigerant to pass through the receiver drier, causing the liquid to expand quickly and turn into low-temperature, low-pressure refrigerant in a mist form. In this way, it acts as a check and balance system between the heat load and cooling efficiency of the evaporator. The expansion valve is usually located near the evaporator.

Common types include the Block Valve, Elbow Valve and Flow Control Valve (Orifice Tube).