Expansion valve (TXV)
Also known as: thermostatic expansion valve, TEV, TXV
Short definitionThe valve that throttles liquid refrigerant at the evaporator inlet. It drops the pressure and feeds the evaporator as much refrigerant as the load requires.
This valve separates the high-pressure side of the cycle from the low-pressure side. As the liquid passes through the narrow opening, its pressure and temperature fall, part of it evaporates, and a cold mixture of liquid and vapour enters the evaporator. The thermostatic type (TXV) tracks superheat through a sensing bulb fixed to the evaporator outlet pipe and opens just enough to hold it steady, usually between 5 and 8 K. It needs no electricity.
The valve is selected for the refrigerant and the evaporator capacity; each refrigerant has its own orifice. An oversized valve hunts, meaning it opens and closes without rest. An undersized valve starves the evaporator and room capacity drops. If it overfeeds, liquid reaches the compressor. A sensing bulb fixed in the wrong place also gives a false superheat reading. At commissioning, superheat is measured and the valve is adjusted to suit.