Fixed metering devices, such as pistons and capillary tubes, have a set opening and work well only within a narrow range of conditions. Adjustable devices, such as thermostatic and electronic expansion valves, open and close to hold a target superheat as load changes. The metering device is the dividing line between high side and low side, and a restriction here starves the evaporator.
It's a piston system, so charge it by superheat, not subcooling, the metering device can't adjust for you.
Also heard as
- expansion device
- throttling device
Related terms
TXV
Thermostatic expansion valve: a metering device that adjusts refrigerant flow to hold constant evaporator superheat.
Superheat
How many degrees refrigerant vapor is above its boiling point at the pressure it is at.
Starved
Describes an evaporator not getting enough refrigerant, so part of the coil runs warm and superheat is high.
Flash gas
Liquid refrigerant that turns to vapor because its pressure dropped, either at the metering device or where it shouldn't.