Final answer:
The superheat on a TXV decreases when the evaporator load decreases, reducing the flow of refrigerant and allowing it more time to absorb heat from the evaporator.
Step-by-step explanation:
The superheat decrease on a thermostatic expansion valve (TXV) is influenced by several factors. The superheat is the temperature difference between the refrigerant vapor and its saturation point at a given pressure. When the TXV senses a drop in the evaporator load, it reduces the flow of refrigerant into the evaporator, leading to a decrease in superheat. This is because the reduced flow allows more time for the refrigerant to absorb heat from the evaporator, causing a smaller temperature difference.
For example, if the evaporator load decreases due to a lesser cooling demand, the TXV will respond by closing partially or fully, reducing the flow of refrigerant. As a result, the refrigerant spends more time absorbing heat from the evaporator, which decreases the superheat.
It's important to note that the superheat decrease on a TXV can also be affected by other factors such as changes in ambient conditions or changes in the system's refrigerant charge.