A TXV has three operating pressures...

A TXV has three operating pressures...

• A. Evaporator pressure
• B. Condenser pressure
• C. Suction line temperature
• D. Ambient pressure

Answer: (A)

The key idea is that a thermostatic expansion valve meters refrigerant based on the pressure signal it senses from the evaporator. The sensing bulb, located at the evaporator outlet, communicates evaporator pressure to the valve diaphragm. As evaporator pressure rises, this sensed pressure pushes the diaphragm to reduce the valve opening, cutting refrigerant flow; when evaporator pressure falls, the valve opens more to increase flow. This pressure signal from the evaporator is what the TXV uses to maintain the desired superheat and stable operation. Condenser pressure acts on the high side of the system but does not directly drive the valve opening. Suction line temperature is a temperature, not a pressure signal, and ambient pressure is atmospheric pressure that doesn’t control the valve. So the evaporator pressure is the relevant operating pressure for the TXV.

The key idea is that a thermostatic expansion valve meters refrigerant based on the pressure signal it senses from the evaporator. The sensing bulb, located at the evaporator outlet, communicates evaporator pressure to the valve diaphragm. As evaporator pressure rises, this sensed pressure pushes the diaphragm to reduce the valve opening, cutting refrigerant flow; when evaporator pressure falls, the valve opens more to increase flow. This pressure signal from the evaporator is what the TXV uses to maintain the desired superheat and stable operation.

Condenser pressure acts on the high side of the system but does not directly drive the valve opening. Suction line temperature is a temperature, not a pressure signal, and ambient pressure is atmospheric pressure that doesn’t control the valve. So the evaporator pressure is the relevant operating pressure for the TXV.