An automatic expansion valve maintains a constant what in the evaporator?

• A. Evaporator superheat
• B. Condenser pressure
• C. Suction pressure
• D. Refrigerant charge

Answer: (A)

An automatic expansion valve works to keep the evaporator operating at a fixed evaporator superheat by modulating how much refrigerant can enter the evaporator. It uses a sensing element, usually a bulb at the evaporator outlet, to measure the temperature of the exiting vapor. That temperature is compared to the evaporating pressure’s saturation temperature to determine the superheat. If the superheat is too low (too much liquid may be entering the evaporator), the valve reduces flow; if it’s too high (not enough refrigerant vaporizing), the valve opens more to allow more refrigerant in. Keeping the superheat at the setpoint prevents liquid refrigerant from reaching the compressor, protects the compressor, and ensures efficient heat absorption in the evaporator.

Condenser pressure, suction pressure, and refrigerant charge are controlled or affected differently in the system and are not the function of the automatic expansion valve.