HVACR Metering Device Basics
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Metering devices act as pressure droppers; they drop the pressure of liquid refrigerant before it enters the evaporator coil. It’s one component that separates the high side of the system from the low side. (On the other side of the system, the compressor marks the end of the low side and the beginning of the high side.) The pressure drop may also cause some of the liquid to “flash off” and become a vapor.
HVACR metering devices come in a variety of types, including capillary tubes, pistons, TXVs, and EEVs. Each type of metering device allows us to control the refrigerant flow and drops the pressure by forcing the refrigerant through a small opening; pressure is built up on the inlet side (typically between 80 and 100 PSI), and there’s a pressure drop on the outlet side. The outlet side leads to an area with a larger volume, and increasing the volume decreases the pressure.
There are two common metering devices that are valves: the thermostatic expansion valve (TXV) and the electronic expansion valve (EEV). Both of these valves measure superheat at the evaporator outlet and modulate the size of their orifice to maintain constant superheat. (TXVs use a sensing bulb that applies pressure, and EEVs may use electronic temperature sensors or transducers.) The TXV is a common mechanical metering device that’s often wrongfully blamed for low suction pressure.
Other metering devices are called fixed orifices, which don’t modulate at all. These are capillary tubes (thin tubes) and pistons (brass that can seat and unseat itself depending on the operating mode).