The thermostatic expansion valve (TXV) is a refrigerant metering device that constantly modulates and meters just enough liquid refrigerant from the liquid line into the evaporator to satisfy all heat load conditions on the evaporator. The TXV also simultaneously controls a set amount of evaporator superheat while, under these same conditions, assuming the range and capacity of the valve is not exceeded.
By controlling the evaporator superheat, TXVs keep the entire evaporator just full enough of refrigerant to enable higher net refrigeration effects. The TXV also prevents liquid floodback of refrigerant to the compressor by controlling evaporator superheat, which, in turn, ensures the presence of compressor superheat. TXVs do not control box temperature or compressor running time, nor do they cycle the compressor or control evaporator pressure.