What is the primary function of a thermal expansion valve (TXV) in a refrigeration system?

The primary function of a thermal expansion valve (TXV) in a refrigeration system is to regulate the flow of liquid refrigerant into the evaporator. The evaporator is the component where the refrigerant absorbs heat and changes from a liquid to a gas. The TXV maintains a constant superheat at the evaporator outlet. Superheat refers to the temperature of the refrigerant vapor above its saturation temperature, which is the temperature at which it begins to boil at a given pressure. Maintaining proper superheat ensures that all the liquid refrigerant has evaporated before entering the compressor. This prevents liquid refrigerant from entering the compressor, which could cause damage due to liquid compression, a phenomenon where the compressor attempts to compress a non-compressible liquid, leading to potential mechanical failure. The TXV achieves this by modulating its opening based on the evaporator pressure and temperature. If the superheat is too low, the TXV restricts refrigerant flow, allowing more of the liquid to evaporate in the evaporator before reaching the outlet. If the superheat is too high, indicating that not enough refrigerant is entering the evaporator, the TXV increases refrigerant flow to ensure the evaporator is adequately supplied and prevents excessive superheating. In essence, the TXV acts as an automatic metering device, optimizing the evaporator’s efficiency and protecting the compressor.