Air Conditioner Basics Part III - Metering Devices

The metering device is the component in an air conditioning system that drastically reduces the temperature and pressure of refrigerant coming from the condenser before that refrigerant enters the evaporator.

A metering device can be as simple as the capillary tube, which is a two-foot long spiral of copper tubing that is narrower than the liquid line feeding into it. The capillary tube is commonly used as a system's metering device because of its simplicity and low cost.

A more sophisticated type of metering device is the thermal expansion valve (TXV or TEV). The top of the TXV has a sensing bulb that is attached to the suction line (the line connecting the evaporator with the compressor). The sensing bulb contains a substance (usually the same as the system's refrigerant). Temperature and pressure drop in the sensing bulb is translated to pressure drop in the TXV, which releases tension against an internal spring that is designed to restrict refrigerant passing through the TXV. This behavior optimizes the flow of refrigerant into the evaporator, which is necessary to maintain proper heat transfer within the evaporator, and also to prevent liquid refrigerant from entering the compressor.

References

Hand, Martin. “AC Evaporator, Condenser & Related Components”. MDH Motors.“Thermal Expansion Valve”. Wikipedia.