TM 9-4120-403-14
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
the set point, it closes the liquid line solenoid valve (6) to shut off refrigerant flow to the evaporator coil (10).
As the compressor suction pressure starts to drop, the hot gas solenoid valve (9) closes and 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.
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 charging valves (13) are provided for charging, and general servicing of the high and low pressure sides
of the refrigerant system.
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.
The pressure control switch (16) overrides the fan speed control switch to place the fan in either HI or LOW
SPEED when preset pressures are reached.
1-11. 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
elements are used.
1-8
