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 TM 55-1905-219-14-8
4-28. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(d) The remote thermal bulb assumes the temperature of the suction gas at the point of bulb
installation. Any change in suction gas superheat tends to move the valve in a compensating direction to
restore the superheat to a predetermined value (valve setting). For example, if too much liquid refrigerant is
fed into the evaporator, the suction gas superheat decreases and the remote bulb is lowered in temperature.
The resultant pressure decrease in the remote bulb and power assembly allows the valve pin to be moved in
the closing direction by the combined evaporator and valve spring pressures. On the other hand, if not enough
liquid refrigerant is fed into the evaporator, the suction gas superheat increases and the remote bulb
temperature rises. The resultant pressure increase in the remote bulb and power assembly overcomes the
combined evaporator and valve spring pressures and moves the valve pin in the opening direction.
(2)
Adjustment of Thermal Expansion Valves.
(a) The thermal expansion valves are factory set to maintain the suction gas leaving the
evaporator at 8to -10F (13.3to -23.3 ) superheat.
C
(b) To adjust superheat setting remove seal cap on side of valve and turn adjusting stem. Turning
stem to right decreases refrigerant flow and raises superheat. Turning stem to left increases refrigerant flow
and lowers superheat. Two turns of stem will change superheat about 1 F (17.2 ). Adjust two turns at a
C
time.
(c) Adjust each expansion valve separately and wait between adjustments to observe results.
Always tighten any loose connections and replace seal cap after adjustments.
(d) To adjust a pilot operated thermal expansion valve, adjust the pilot valve in the same manner as
above.
j. Solenoid Valves.
(1)
Solenoid valves are piston type valves operated by the magnetic action of an electric coil on a
moveable steel core or plunger. An evaporator solenoid valve is electrically actuated by a thermostat
responsive to temperature changes in the medium being cooled. A king solenoid valve, is acutated from the
compressor motor controller. Normal valve operation is automatic, but a manual opening stem is provided in
the bottom of valve.
(2)
Direct Acting Solenoid Valves.
When the electrical circuit is completed by the thermostat (or motor controller), the solenoid valve
coil is energized, the magnetic field produced lifts the steel plunger, and the valve snaps open. When the
thermostat or motor controller breaks the circuit, the solenoid coil is de-energized and the weight of the plunger
and stem closes the valve.
4-855
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