A thermal expansion valve (often abbreviated as TEV, TXV, or TX valve) is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator.he condenser converts the high-pressure and-temperature gas to a high-pressure liquid by transferring heat to a lower temperature medium, usually ambient air. The high pressure liquid then enters the expansion valve where the TX valve allows a portion of the refrigerant to enter the evaporator.