TM 9-4120-388-14
5-23. EXPANSION VALVE - Continued.
The following test must be conducted with the power on. Exercise extreme caution.
Use care to not damage or kink the capillary.
(4)
Place the sensing bulb in a container of ice water or crushed ice so that it is reduced to a temperature near
32F (0C).
Set the TEMPerature control thermostat knob to fully DECREASE (counterclockwise), place one hand on
(5)
the exposed suction return line, and start the air conditioner in COOL mode. If a drop in temperature is felt
on the suction return line, the expansion valve is not closing fully and should be replaced. If the return line
temperature remains constant, check the pressure gate; it should indicate approximately 58 2 psi (40
1.4 kg/mm2). If the pressure is not within the above limits, test the operation of the quench valve (para 5-20)
and the pressure regulator valve (para 5-25) before attempting adjustment of the evaporator expansion
valve.
When performing the next test, turn the air conditioner to OFF as soon as a definite
drop in temperature is felt on the suction return line. If the test conditions are con-
tinued more than a few seconds the expansion valve will fully open and an exces-
sive flood-back of liquid refrigerant may damage or destroy the compressor.
With one hand still on the suction return line, remove the sensing bulb from the container and warm it in
(6)
the other palm. If a temperature drop is not felt in the suction return line by the time the sensing bulb no
longer feels cold to the hand, the expansion valve is not opening and should be replaced. As soon as a
temperature drop is felt, turn the air conditioner to OFF.
NOTE
The optimum superheat setting for the evaporator expansion valve is 10F (5.55C)
above the saturation temperature of the refrigerant at operating suction line pressure.
This setting will provide maximum efficiency of the evaporator coil.
(7)
Slip the sensing bulb into its mounting loop clamps. Be sure the bulb is installed in its original position and
tighten the nuts and screws. Be sure the sensing bulb is making metal-to-metal contact with the suction line.
(8)
Attach an accurate thermometer to an exposed surface of the suction return line adjacent to the sensing
bulb; use a small gob of thermal mastic to improve conductivity.
Rewrap insulation on the suction return line, being sure to cover the sensing bulb.
(9)
(10) Start the air conditioner in the COOL mode with the thermostat set at fully DECREASE (counterclockwise)
and allow it to run about 30 minutes, then be sure the temperature in the suction line has stabilized and
the thermometer reading remains unchanged for at least 2 minutes.
(11 ) Note the pressure on the gage connected to the low pressure (suction) service valve. Find the saturation
temperature for the pressure gage reading (see Table 5-1) and compare with the thermometer reading.
The thermometer temperature should be 10 1F (5.55 0.55C) higher than the saturation temperature
found on the chart.
(12) If the superheat setting is not within the limits shown above, adjust the expansion valve as follows:
(a) Remove the hexagonal seal cap from the side of the power assembly.
(b) Turn the adjusting stem two complete turns to change the super heat setting by 1F (0.55C). Turn
the stem clockwise to increase superheat span, counterclockwise to decrease it. Do not change more
than two full turns at one time, then wait at least 2 minutes for temperature to stabilize and recheck
pressure and temperature before further adjustment.
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