TXV sensing bulb location

Question: My question involves the best location for a capillary bulb on a TXV. What size line would be best for the 10 o'clock and 2'o'clock positions and why? What size line would be best for the 4 o'clock and 8 o'clock positions and why? Alos, is it preffered to be clamped on a vertical or horizontal line and why?

-John West Jr., Virginia Chapter, Richmond Va.

Answer: The location of the bulb is extremely important to proper system performance. Always follow the equipment manufacturer's recommendations regarding the sensing bulb location on their specific application. If information regarding its location is not available, the following guidelines may be used.

On horizontal suction lines 5/8-inch O.D. and smaller, the bulb may be located in any position around the circumference of the tube except the 6 o'clock position. On horizontal suction lines 7/8-inch O.D. and larger, the recommended location is 4 or 8 o'clock.

The recommended bulb location takes into consideration the differing rate of conductivity between the small and larger diameter copper tubing should there be unwanted liquid refrigerant moving through the bottom of the horizontal line. On a small suction line, the temperature difference from the 4 o'clock position to the 12 o'clock will not be significantly different. On larger diameter suction lines, should the unwanted liquid refrigerant be in the bottom of the tube, a temperature difference would be realized around the O.D. of the tube, with 12 o'clock being the warmest.

Any bulb location above 4 or 8 o'clock may decrease the thermostatic expansion valve reaction time. Locating the bulb lower on the tubing is not recommended because of oil that may be returning to the compressor. (Keep in mind that oil picks up and loses heat at a slower rate than the refrigerant and probably will not reflect the temperature of the superheated refrigerant.)

The reason the bulb location is recommended on a horizontal over a vertical section of suction line is that on a horizontal suction line, gravity separates the vapor from the liquid (refrigerant or oil) allowing for consistency of bulb placement and valve operation. When clamping a sensing bulb to a vertical suction line, it is generally recommended that it be a suction drop, not a suction riser.

The reason for this is many risers incorporate traps to facilitate oil return. In some applications the evaporator by design may be a trap. A sensing bulb placed downstream of a trap on a suction riser will permit the evaporator to fill with a liquid to the same height on the riser as the bulb before liquid is sensed.

Sensing bulbs placed on vertical suction drops do not have the advantage of gravity to separate liquids. Therefore, the sensitivity of the bulb may be degraded and overall valve operation adversely affected by various factors which may include the oil charge of the system, poor refrigerant velocities, the turbulence of the suction line, refrigerant distribution, ect.

Good thermal contact of the sensing bulb is essential for proper valve operation in any thermostatic expansion valve application. Additional information regarding the installation and adjustment may be found in the Sporlan Valve Bulletin 10-11 or SD-14 instructions shipped with ach TEV.

-Steve Esslinger, application engineer/refrigeration, Sporlan Valve Co.