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 TM 55-1905-219-14-8
4-28.
AIR CONDITIONING SYSTEM-MAINTENANCE INSTRUCTIONS (Continued).
Oil Pressure Differential Switch Data
Manufacturer
Penn Controls
Carrier Part No.
HKO6UL012 (5F20-212)
Open Setting (differential)
16 to 19 psi
(110.3 to 131.0 kPa)
Close Setting (differential)
11 to 15 psi
(75.8 to 103.4 kPa)
Type Reset
Manual pushbutton
(15) Thermal Expansion Valve.
The thermal expansion valve is an automatic device which controls the flow of liquid refrigerant to the
evaporator according to changes in superheat of the refrigerant gas leaving the evaporator. The thermal
expansion valve maintains a relatively constant degree of superheat in the refrigerant gas leaving the
evaporator regardless of suction pressure. Thus, the valve has a dual function: (a) automatic expansion
control, and (b) prevention of liquid refrigerant return to compressor.
(16) Solenoid Valve.
The solenoid valve located in the liquid line at the evaporator is actuated by a temperature control
switch or thermostat. This valve closes and cuts off the supply of liquid refrigerant to the thermal expansion
valve when the space or other medium being cooled reaches the desired temperature (thermostat cut-out
setting). When the thermostat calls for cooling (cut-in setting) the solenoid valve opens and refrigerant flow is
restored.
(17) Hand Expansion Valve.
The hand expansion valve is located in the bypass for the liquid strainer, solenoid valve and thermal
expansion valve of each refrigerant circuit. Thus, the hand expansion valve permits plant operation to be
continued manually when the automatic controls or the strainer are being serviced. Hand expansion valves
must be cracked open with care to avoid the return of liquid refrigerant to the compressor, but enough to supply
sufficient refrigerant to the circuit.
4-818
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