The thermal expansion valve is a variable orifice valve located on the inlet pipe of the air conditioning (A/C) evaporator. The thermal expansion valve controls the rate at which refrigerant flows into the A/C evaporator, the speed of evaporation, and the A/C evaporator temperature.

A sealed metal bulb and tube are connected to one side of a diaphragm that moves the control valve. This bulb is held against the A/C evaporator outlet pipe and senses the outlet temperature. As the outlet temperature rises, the gas expands in the bulb and the tube. The expanding gas then causes the A/C expansion valve to open, allowing more refrigerant to enter the A/C evaporator. The increased flow of refrigerant in the evaporator lowers the evaporator temperature. When the evaporator temperature is just above freezing, the thermal expansion valve partially closes, and the evaporator temperature rises again, due to decreased refrigerant flow and evaporation. A balance tube that is connected to the A/C evaporator outlet pipe senses outlet pressure on the opposite side of the diaphragm from the metal bulb and tube.