The Sensing Bulb

May 7, 2007

The correct placement and installation of a thermostatic expansion valve’s (TXV’s) sensing bulb is important to its overall performance and reliability. An incorrectly installed or placed sensing bulb can lead to two major system problems: Refrigerant flooding back to the compressor, or the evaporator being starved for refrigerant and not operating at its designed capacity.

In general, the sensing bulb should be placed on a horizontal section of the suction line exiting the evaporator. If this is not possible, then it may be installed on a descending vertical section of suction line. Avoid locating the sensing bulb on a vertical riser. If a vertical suction riser is needed, then a trap should be installed at the base of the riser and the sensing bulb installed on a horizontal section of pipe before the trap. Do not install the sensing bulb on the trap or after the trap. Liquid refrigerant or a mixture of liquid refrigerant and oil boiling out of the trap will falsely influence the temperature sensed by the bulb.

In general, the sensing bulb should be attached at the 12 o’clock position on suction lines less than 7/8 inch, at the 10 or 2 o’clock position for 7/8 inch to 1 5/8 inches suction lines, and at the 4 or 8 o’clock position for suction lines greater than 2 inches.  Never place the sensing bulb at the 6 o’clock position. The returning refrigerant oil tends to travel along the bottom of the suction line and could cause the sensing bulb to read an inaccurate refrigerant temperature.

The sensing bulb should also be located at least 18 inches from the point where the suction line exits the cabinet. If needed, the suction line should be extended within the cabinet to ensure the proper location of the sensing bulb.

When installing a new TXV, care should be taken not to overheat the sensing bulb. This means attaching the sensing bulb after any brazing or soldering of the copper lines close to the sensing bulb’s location. Allow the lines to cool and then attach the sensing bulb. Also, make sure the sensing bulb is securely fastened to the suction line. Avoid locating the sensing bulb on any fittings, as this may not allow the bulb to sense an even pipe temperature. Use metal clamps to secure the bulb to the suction line. Do not use any type of tape or wire ties as this will lead to a loose connection over time and not allow the sensing bulb to read an accurate refrigerant temperature.

On multiple evaporator installations, the piping should be configured so that the refrigerant flow from any one valve does not influence the sensing bulb of another valve.  Always follow the established piping procedure for the installation of multiple evaporators on a single system.

As with many other refrigeration components used in our industry, if a manufacturer’s recommendation differs from the suggestions offered above, always follow the manufacturer’s installation guidelines first, as they take precedence over any of these general recommendations.

Publication date: 05/07/2007