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. Thermal expansion valves are often referred to generically as "metering devices".
Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb filled with a similar gas as in the system that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring causing the valve to close. An air conditioning system with a TX valve is often more efficient than other designs that do not use one.
A thermal expansion valve is a key element to a refrigeration cycle; the cycle that makes air conditioning, or air cooling, possible. A basic refrigeration cycle consists of four major elements, a compressor, a condenser, a metering device and an evaporator. As a refrigerant passes through a circuit containing these four elements, air conditioning occurs.

The cycle starts when refrigerant enters the compressor in a low pressure gas form.
The refrigerant is compressed by the compressor to a high pressure gas state.
The high pressure gas then enters the condenser. The condenser precipitates the high pressure 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 valve allows a portion of the refrigerant to enter the evaporator. In order for the high pressure liquid to cool, the flow must be limited into the evaporator to keep the pressure low and allow expansion back into the gas phase.
An evaporator is used in