What does the pressure in the control tubing of a TXV depend upon?

The pressure in the control tubing of a thermostatic expansion valve (TXV) is directly related to the temperature of the refrigerant in the tail coil of the evaporator. This temperature reflects the conditions of the refrigerant exiting the evaporator, which is crucial for the TXV's operation.

As the refrigerant absorbs heat within the evaporator, it evaporates and changes state from liquid to gas. The temperature measured at the tail coil, which is usually the point where the refrigerant is primarily in its gaseous state before entering the compressor, gives an indication of the heat load and how much refrigerant needs to flow into the evaporator to maintain proper cooling efficiency.

When the temperature at the tail coil becomes higher, it indicates an increase in the pressure within the control tubing. This feedback mechanism allows the TXV to adjust its valve opening, controlling the flow of refrigerant into the evaporator. If more cooling is needed, the TXV opens more to allow additional refrigerant in, thus enhancing the system's heat absorption capabilities.

In contrast, the other options relate to different components of the refrigeration cycle but do not directly influence the pressure in the control tubing of a TXV like the tail coil temperature does.