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

The primary function of a thermostatic expansion valve (TXV) in a refrigeration system is to regulate the flow of liquid refrigerant into the evaporator to maintain a constant level of superheat at the evaporator outlet. Superheat is the temperature of the refrigerant vapor above its saturation temperature at a given pressure. Maintaining proper superheat ensures that only vapor refrigerant enters the compressor, preventing liquid refrigerant from entering and potentially damaging the compressor. The TXV achieves this by sensing the superheat at the evaporator outlet using a sensing bulb. This bulb is filled with a refrigerant and connected to the valve's diaphragm. As the superheat increases, the pressure in the bulb increases, causing the valve to open and allow more refrigerant to flow into the evaporator. Conversely, if the superheat decreases, the pressure in the bulb decreases, causing the valve to close and restrict refrigerant flow. This modulation of refrigerant flow allows the TXV to match the refrigerant flow rate with the cooling load requirements, maximizing evaporator efficiency and protecting the compressor. For example, if the desired superheat is 10°F, the TXV adjusts the refrigerant flow to ensure that the refrigerant vapor leaving the evaporator is 10°F above its saturation temperature at that pressure.