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TM 9-4120-389-14
SECTION III TECHNICAL PRINCIPLES OF OPERATION
1-10. REFRIGERATION CYCLE.
a. Cooling cycle. Unit operation with mode selector switch set on COOL and the
temperature control thermostat set to DECREASE.
Compressor (3) Figure 1-3 starts.
The compressor (3) takes cold, low pressure refrigerant gas and compresses it to a high temperature,
high pressure gas. This gas flows to the condensor. coil (5) and receiver (8).
The condenser fan draws outside ambient air over and through the condenser coil (5). The high
temperature, high pressure gas from the compressor (3) 's cooled by the flow of air and is changed into a high
pressure liquid.
The cooler, high pressure liquid refrigerant flows through the dehydrator (filter drier) (9) and removes
any moisture (water vapor) or dirt that may be carried by the liquid refrigerant and to the sight glass (11), which
indicates the presence of moisture and quantity of refrigerant in the system.
The liquid line solenoid valve (10) is controlled by the temperature control thermostat on the control
panel. This valve will shut off the flow of refrigerant to the evaporator section when the temperature in the
conditioned area reaches the set point.
The expansion valve (12) controls the amount of pressure of liquid refrigerant to the restrictor (13)
and evaporator coil (14). The expansion valve (12) senses the temperature and pressure of the frigerant 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 (14).
As the liquid refrigerant leaves the expansion valve (12) it passes thru a restrictor (13) and enters the
evaporator coil (14). 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 as (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 (14), the air is cooled.
The low pressure refrigerant gas is then drawn back through the accumulate (1) to the compressor (3)
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 (3) 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 (14).
1-8
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