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 TM9-4120-423-14&P
7
The expansion valve (6) controls the amount and pressure of liquid refrigerant to the evaporator
coil (8). The expansion valve (6) 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 (8).
8
As the liquid refrigerant leaves the expansion valve (6) it passes thru the distributor (7) and
enters the evaporator coil (8). 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 blower 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 evaporator coil (8),
the air is cooled.
9
To prevent compressor overload and damage during startup, control, head pressure (16) is open
at start of cooling cycle to equalize pressure on both sides of the compressor.
BYPASS CYCLE
1
This unit has a bypass cycle, which allows cooling operation at low cooling loads without
cycling the compressor 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 (8).
2
When the temperature selector on the control panel senses that cooling conditions have reached
the set point, it closes the solenoid valve (9) to shut off refrigerant flow to the evaporator coil (8).
3
As the compressor suction pressure starts to drop, the discharge bypass valve (14) opens to allow
flow of hot gas from the compressor.
4
The liquid quench expansion valve (15) senses the temperature of the gas at the suction side of
the compressor. To prevent excessively hot gas from reaching the compressor, the liquid quench
expansion valve (15) opens to allow liquid refrigerant to mix with the hot gas.
5
The service valves (10) are provided for charging, and general servicing of the high and low-
pressure sides of the refrigerant system.
6
The low-pressure switch (11), the high pressure switch (12) and the pressure relief valve (13) are
provided to protect the unit from damage due to pressure extremes.
When the MODE SELECTOR SWITCH is set for HIGH HEAT, heating elements, located behind the
evaporator coil, are energized. These elements are protected from overheating by a thermal cutout
switch. The TEMPERATURE CONTROL thermostatically controls nine of the elements, and
remaining six are on all of the time. When set for LOW HEAT, only the thermostatically controlled
elements are energized.
0003 00-2
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