TM 9-4120-395-14&P
Section III. PRINCIPLES OF OPERATION
Para.
Cooling Cycle ............................................................................................................................................................. 1-13
Fan Cycle ................................................................................................................................................................... 1-15
Heating Cycle.............................................................................................................................................................. 1-14
a.
When the thermostat lever Is set to about 10 degrees Fahrenheit (5.6 degrees Centigrade) below room
temperature and system switch is set to COOL, the compressor starts up. If the fan switch is set to AUTO,
fans start up too. Fans run continuously when fan switch is set to ON.
b.
To prevent compressor overload and damage during startup, a check valve (opens prior to start of cooling
cycle to equalize pressure on both sides of the compressor.
c.
The compressor takes low pressure refrigerant gas and compresses it to a high temperature, high pressure
gas. This gas flows through the metal tubing to the outdoor coil.
d.
The fan draws outside ambient air over and through the coil. The high temperature, high pressure gas from
the compressor is cooled by the flow of air and is condensed into a high pressure liquid.
e.
The liquid refrigerant passes through a capillary tube, which reduces pressure. The low pressure liquid then
enters the indoor coil. 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 blowers circulate the warm air from the
conditioned space over and through the coil. Refrigerant absorbs heat when it changes from a liquid to a
gas. As the air from the conditioned space comes in contact with the indoor coil, the air is cooled.
f.
The refrigerant gas is then drawn back to the compressor through the accumulator and the cycle is
repeated.
a.
The heat cycle operates the same as the cooling cycle except the refrigerant flow and heat transfer is
reversed. When the normal switch is set to NORM; the system switch on the thermostat is set for HEAT,
and the thermostat lever is set at about 1 0 degrees Fahrenheit (5.6 degrees Centigrade) above room
temperature, the compressor and heater element start up. If the fan switch is set to AUTO, fans start up,
too; fans run continuously when fan switch is set to ON. Air is heated as it passes through the indoor coil
and further heated as it passes through the heater element.
NOTE
Only one stage of the heater element will operate if the temperature is within 1 to
21/2 degrees Fahrenheit (0.55 to 1.39 degrees Centigrade) of the thermostat setting.
b.
When the normal switch is set to EM ERG. HEAT (Emergency Heat), the compressor is bypassed and only
the heater element provides heat
1-15. FAN CYCLE. When the fan switch is turned to ON, the fan should run continuously. When the fan switch is in the
AUTO position, fan operation is controlled by the heating or cooling system.
1-6