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TM 5-4120-377-14
l The expansion valve (8) controls the amount of pressure of liquid refrigerant to the distributor (9) and
evaporator coil (10). The expansion valve (8) senses the temperature and pressure of the refrigerant as it
leaves the evaporator coil. By use of a sensing bulb and “external equalizer line” the valve constantly adjusts
the flow of liquid refrigerant to the evaporator coil (10).
l As the Iiquid refrigerant leaves the expansion valve (8) it passes thru a distributor (9) and enters the
evaporator coil (10). As the liquid enters the coil at a reduced pressure, the reduction in pressure and the
warmer air being forced across the tubes of the coil cause the refrigerant to boil and change to a gas (vapor).
The evaporator fan circulates the warm air from the conditioned space over and through the evaporator coil.
Refrigerant absorbs heat when it changes from a liquid to a gas. As the air from the conditioned spaces comes
in contact with the evaporator coil (10), the air is cooled.
l The refrigerant gas is then drawn back to the compressor (1) and the cycle is repeated.
b. Bypass cycle. This unit has a bypass cycle which allows cooling operation at low cooling loads without
cycling the compressor (1) on and off. In bypass, the refrigerant is piped from the discharge (high side) to the
suction (low side) of the compressor, bypassing the evaporator coil (10).
l When the temperature control thermostat on the control panel senses that cooling conditions have
reached the set point, it closes the liquid line solenoid valve (6) to shut off refrigerant flow to the evaporator coil
(10).
l As the compressor suction pressure starts to drop, the pressure regulating valve (12) opens to allow flow
of hot gas from the compressor.
l The expansion valve (quench) (11) senses the temperature of the gas at the suction side of the
compressor. To prevent excessively hot gas from reaching the compressor, the expansion valve (quench) (11)
opens to allow liquid refrigerant to mix with the hot gas.
l The service valves (13) are provided for charging, and general servicing of the high and low pressure
sides of the refrigerant system.
l The pressure switch (low) (14), the pressure switch (high) (15), and the pressure relief valve (4) are
provided to protect the unit from damage due to pressure extremes.
1-12. HEATING
a. HI HEAT mode. When the mode selector switch is set for HI HEAT the six heating elements, located
behind the evaporator coil, are energized. These elements are protected from overheating-by a thermal cutout
switch. Three of the elements are thermostatically controlled by the temperature control thermostat, and
remaining three are on all of the time.
b. LO HEAT mode. When the mode selector switch is set for LO HEAT, only the three thermostatically
controlled heating elements are used.
1-7/(1-8 blank)
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