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 TM 55-1905-219-14-8
4-28.
AIR CONDITIONING SYSTEM-MAINTENANCE INSTRUCTIONS (Continued).
(22)
Automatic Control Sequence.
(a) Each refrigerant circuit is fitted with a thermal expansion valve and a solenoid valve in the
liquid line. The solenoid valve is actuated by a thermostat having its thermal control bulb located in the
medium being cooled. The thermal expansion valve is actuated by its control bulb which is located at the
leaving end of the refrigerant circuit (suction line). A strainer is provided ahead of the solenoid valve to keep
the controls clean. A hand expansion bypass valve and stop valves before the strainer and at the thermal
expansion valve outlet permit servicing the controls without shutdown of the circuit. Note that each set of
cooling coils, each water chiller, or other evaporator may consist of one or more refrigerant circuits each with its
set of controls.
(b) The thermal expansion valve regulates the flow and expansion of the refrigerant in the circuit.
The valve is factory set to remain open as long as the temperature of the refrigerant gas at the point of bulb
attachment is 810 F (-13.323.3 ) or more above the refrigerant saturation temperature. This regulation
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C
supplies the circuit with a maximum amount of refrigerant while preventing liquid refrigerant from surging back
to the compressor.
(c)
When the compartment or other medium being cooled reaches the desired temperature, the
thermostat opens, the solenoid valve closes and the flow of liquid refrigerant to the thermal expansion valve
and circuit is cut off. (An open solenoid valve can be detected by observing the contacts of its thermostat,
which should be closed, or by a humming sound when the ear is pressed against the valve casing.)
(d) With the solenoid valve closed or, in the case of multiple circuits, after the last solenoid valve
closes and refrigerant flow is cut off to all circuits, the compressor continues to operate for a very short time
until the suction pressure drops to the cut-out setting of the low pressure control switch, opening the switch
contacts and stopping the compressor motor.
(e) As soon as the temperature of the compartment or other medium being cooled rises just above
the cut-in setting of the thermostat, the thermostat closes and the solenoid valve opens allowing liquid
refrigerant to flow to the thermal expansion valve. The thermal expansion valve meters the flow of refrigerant
to the circuit. The refrigerant expands (vaporizes) in the circuit and the suction pressure rises. When the
suction pressure rises above the cut-in setting of the low pressure control switch, the switch contacts close and
the compressor motor starts.
4-821
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