What does the sensing bulb of a TXV/EVV do and where should it be placed?

What does the sensing bulb of a TXV/EVV do and where should it be placed?

• A. The bulb senses evaporator outlet temperature to modulate the valve; place it on the suction line near the evaporator outlet for accurate measurement.
• B. The bulb senses ambient room temperature; place near outdoor condenser.
• C. The bulb senses liquid line temperature; place at the inlet to the compressor.
• D. The bulb senses discharge line temperature; place near the compressor discharge.

Answer: (A)

The sensing bulb is the temperature sensor that controls how much refrigerant the valve lets through by tracking the temperature of the refrigerant leaving the evaporator. It should be placed on the suction line just after the evaporator outlet so it directly "sees" the evaporator conditions (gas at evaporator pressure). As the outlet temperature rises, the bulb's charge expands, raising pressure in the capillary to the valve and causing the valve to close a bit; as the outlet temperature falls, the bulb pressure drops and the valve opens more. This keeps the evaporator superheat at the desired setting. Placing the bulb elsewhere—on the ambient room side, on the liquid line, or on the discharge line—would not reflect the evaporator outlet conditions and would misdrive the metering.

The sensing bulb is the temperature sensor that controls how much refrigerant the valve lets through by tracking the temperature of the refrigerant leaving the evaporator. It should be placed on the suction line just after the evaporator outlet so it directly "sees" the evaporator conditions (gas at evaporator pressure). As the outlet temperature rises, the bulb's charge expands, raising pressure in the capillary to the valve and causing the valve to close a bit; as the outlet temperature falls, the bulb pressure drops and the valve opens more. This keeps the evaporator superheat at the desired setting. Placing the bulb elsewhere—on the ambient room side, on the liquid line, or on the discharge line—would not reflect the evaporator outlet conditions and would misdrive the metering.