BigHvac.com - How to Service Thermal Expansion Valves

How to Service Thermal Expansion Valves

Proper Sizing, Installation, and Adjustment
Help Keep Down Callbacks

 




Metering the refrigerant flow to the evaporator
is the sole function of a thermal expansion valve (TEV). What’s
critical is that it must meter that flow at the same rate
it is being vaporized by the heat load.





To do this, it keeps the coil supplied with
the proper amount of refrigerant to maintain the right superheat
of the suction gas leaving the evaporator.




TEV operation

• At point A, hot, high-pressure liquid refrigerant enters the TEV.


• At B, cold, low-pressure liquid plus flash gas enter the
  evaporator.



• At C, the entire liquid refrigerant has been boiled off
  or vaporized by the heat load (latent heat).


• Between C and D, the vapor temperature increases dramatically
  as further heat load is applied (sensible heat). At this
  point, the gas is superheated above its saturation temperature.
  


• At D, the suction line temperature of the superheated
  gas is monitored by the sensing bulb, which signals the
  TEV to open or close accordingly.





Because good superheat control is the criterion of TEV performance,
accurate measurement is vital.




How to measure superheat accurately


Measuring superheat involves four steps.





Step A: Determine the suction pressure at the evaporator
outlet with an accurate gauge. If there is no gauge connection,
a tee installed in the valve’s external equalizer line can
be used.




Step B: Refer to a temperature-pressure chart for
the refrigerant used in the system, and determine the saturation
temperature at the observed suction pressure.




Step C: Measure the temperature of the suction line
at the remote sensing bulb location. This can be accomplished
by a strap-on thermometer or an electric device similar to
an “Annie” or “Simpson” meter.





Be certain the spot chosen for measurement is clean to help
ensure accurate readings.




Step D: Subtract the saturation temperature determined
in Step B from the suction gas temperature measured in Step
C. The difference is the operating superheat.




Which valve to use

Some service technicians seem rather uncertain as to when
to use an internally equalized thermostatic expansion valve
and when to use an externally equalized one.





Our experience has shown that an externally equalized valve
should be used whenever pressure drop through the evaporator
reaches:

• 3 psi in a 3-ton (or 3-hp) air conditioning system (evaporator
  temperature range of 30° to 50°F);


• 2 psi in a 2-ton commercial refrigeration system (evaporator
  temperature of 10° to 30°); or



• 1 psi in a 1-ton low-temperature system (evaporator temperature
  of 0° or below).





On this basis, an externally equalized valve would automatically
be the selection for any system in excess of 3 tons, regardless
of the application.




Where to place the bulb

Good temperature feedback to the TEV is vital for control.
Place the bulb where it will provide the best possible feedback.






Never put the bulb at 6
o’clock because it may sense the temperature of the oil flowing
through the pipe, rather than the temperature of the refrigerant.





Finally, be sure the bulb location is on a free-draining
suction line.




Choosing the proper TEV for the specific system remains the
first step in good service — and no callbacks.