An increase in superheat on the bulb of a TXV causes what to happen?

When there is an increase in superheat at the bulb of a Thermal Expansion Valve (TXV), it leads to the valve remaining open or opening further. This occurs because the TXV is designed to regulate the flow of refrigerant based on superheat levels. Superheat is defined as the temperature of the refrigerant vapor above its saturation temperature at a given pressure.

In practical terms, an increase in superheat indicates that the refrigerant has gained more heat and is moving further away from the saturation point. To ensure that the evaporator coil is fully utilized and that no liquid refrigerant enters the compressor, the TXV responds by allowing more refrigerant to flow into the evaporator. This action helps maintain the desired evaporator cooling and ensures efficient operation of the refrigeration system.

The other options do not correctly describe the behavior of a TXV in response to increased superheat since the valve's main function is to adapt to the vapor conditions at the bulb to ensure proper refrigerant flow. Thus, when superheat increases, the valve's position adjusts to facilitate the required amount of refrigerant into the evaporator.