When does the thermal expansion valve work most efficiently?

The thermal expansion valve (TXV) operates most efficiently when the pressure drop across the evaporator is small. This condition allows the TXV to accurately regulate the flow of refrigerant into the evaporator, matching the cooling load requirements of the system.

When the pressure drop across the evaporator is small, it indicates that the refrigerant is flowing adequately and the heat exchange process can occur more effectively. A small pressure drop ensures that there is less resistance to the flow of refrigerant, allowing the valve to maintain optimal superheat, which is essential for preventing liquid refrigerant from entering the compressor.

In contrast, if the pressure drop is substantial, it could lead to reduced refrigerant flow and potentially result in insufficient cooling or mechanical issues. The other options, such as the refrigerant being fully vaporized or the system being under high load, do not directly indicate the efficiency of the TXV operation in relation to the pressure drop across the evaporator. Efficiency is primarily linked to maintaining the correct flow and superheat conditions facilitated by a small pressure drop in this context.