TM 9-4120-388-14
1-9. REFRIGERATION CYCLE - Continued.
(8) The expansion valve (8) controls the amount and pressure of Iiquid 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 ad-
justs the flow of Iiquid refrigerant to the evaporator coil (10).
(9) As the Iiquid refrigerant leaves the expansion valve (8), it passes thru a distributor (9) and enters the evapo-
rator coil (10). As the Iiquid 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 Iiquid to a gas. As the air from the conditioned spaces
comes in contact with the evaporator coil (10), the air is cooled.
(10) 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).
When the temperature control thermostat on the control panel senses that cooling conditions have reached
(1)
the set point, it closes the Iiquid line solenoid valve(6) to shut off refrigerant flow to the evaporator coil (10).
(2)
As the compressor suction pressure starts to drop, the pressure regulating valve (12) opens to allow flow
of hot gas from the compressor.
The expansion valve (quench) (11 ) senses the temperature of the gas at the suction side of the compressor.
(3)
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.
The service valves (13) are provided for charging, and general servicing of the high and low pressure sides
(4)
of the refrigerant system.
(5)
The pressure switch (low) (14), the pressure switch (high) (15), and the pressure relief valve (4) are pro-
vided to protect the unit from damage due to pressure extremes.
1-10. HEATING.
HI HEAT mode.
a.
When the mode selector is set for HI-HEAT, the six heating elements, located behind the evaporator coil, are ener-
gized. These elements are protected from overheating by a thermal cutout switch. Three of the elements are ther-
mostatically controlled by the temperature control thermostat, and the remaining three are on all of the time.
b. LOW HEAT mode.
When the mode selector switch is set for LO-HEAT, only the three thermostatically controlled heating elements
are used.