TM 9-4120-403-14
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 (1) starts.
To prevent compressor overload and damage during startup, equalizer solenoid valve (9) is open at start
of cooling cycle to equalize pressure on both sides of the compressor.
The compressor (1) takes cold, low pressure refrigerant gas and compresses it to a high temperature,
high pressure gas. This gas flows through the metal tubing to the condenser coil (2) and receiver (3).
The condenser fan draws outside ambient air over and through the condenser coil (2). The high
temperature, high pressure gas from the compressor (1) is cooled by the flow of air and is changed into a high
pressure liquid.
The dehydrator (filter drier) (5) removes any moisture (water vapor) or dirt that may be carried by the
liquid refrigerant.
The sight glass (liquid indicator) (7) indicates the presence of moisture and quantity of refrigerant in the
system.
The liquid line solenoid valve (6) 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 (8) controls the amount of pressure of liquid refrigerant to the distributor 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).
As the liquid refrigerant leaves the expansion valve (8) it passes thru a distributor 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.
The refrigerant gas is then drawn back to the compressor (1) and the cycle is repeated.
1-6