TM 9-4120-360-14
e. Cut insulation away from sensing bulb and band clamp. Remove clamping screw from band clamp. and
remove sensing bulb.
5.78. Testing
Both expansion valves are tested in the same manner. Proceed as follows:
NOTE
Because the condenser fan impeller and the top panel were removed for access, the condenser coil will be
without airflow. Provide temporary airflow for the following test, by placing a high-velocity fan or centrifu-
gal blower directly in front of the condenser coil, and as close to it as possible.
a. With the air conditioner stopped, let the suction line warmup to ambient temperature.
b. Remove the sensing bulb from its location against the suction line, and place it in a container of ice water or
crushed ice (32F or 0C.
Do not let liquid refrigerant flood back into the compressor any longer than 1-2 seconds. The expan-
sion valve will be wide open during the following procedure. Excessive flood-back of liquid refrige-
rant will damage the compressor.
c. Start the air conditionerby setting the selector switch at COOL, and the temperature control thermostat
maximum DECREASE. Remove the sensing bulb from the ice water, and hold it in one hand to warm it while feeling the
suction line. If the suction line temperate drops, the valve is operating properly. Stop the air conditioner at once, and
re-install the sensing bulb. If the temperature of the suction line does not drop, stop the air conditioner and replace the
expansion valve.
5.79. Adjusting Superheat
A refrigerant gas is said to be superheated when its temperature is higher than the evaporating temperature corresponding
to its pressure at saturation. When a thermal expansion valve is set for optimum superheat (in this case 6F or 3.3C above
the evaporating temperature of the refrigerant at a given pressure) the evaporator coil operates at maximum efficiency.
That is, the refrigerant gas does not become warm before reaching the end of the coil, which would reduce the coil's
cooling capacity, and the refrigerant does not remain in the liquid state after passing completely through the coil, which
could result in severe damage to the compressor. When the expansion valve is properly adjusted, the temperature
difference (TD) across the evaporator should be 18-22 degrees F while the air conditioner is operating normally.
Compare the dry bulb temperatures of air entering and leaving the evaporator to find the TD. The temperature at the
evaporator discharge grill will depend upon the return air temperature. The superheat setting of a thermal expansion
valve can be adjusted by varying the setting of a compression spring (7, figure 5-9) in the power assembly of the valve.
This spring tends to hold the valve closed against the pressure in the sensing bulb and capillary tube; therefore, the greater
the spring pressure, the higher the superheat. Check superheat, and adjust if necessary, in accordance with the following
procedure:
Adjusting the expansion valve to increase refrigerant flow lowers evaporator discharge temperature;
however, this could cause a floodback and damage the compressor. If refrigerant is fed too quickly,
the evaporator will flood, causing "sweatback" or "frostback" down the suction line.
a. Remove insulation from a spot on the suction line near the sensing bulb of the thermal expansion valve to be
adjusted.
b. Install an accurate thermometer or the probe of a thermocouple on the bare spot, using a small gob of thermal
mastic, if available to improve conductivity. Tape the thermometer bulb or thermocouple junction in position, and cover
with insulating material.
c. Connect a suitable pressure gauge to the suction service valve, and open the valve.
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