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
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
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.
The Sensing Bulb
May 7, 2007