WD Catalog 2018

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2-7 - -
Air Conditioner – System Operation
Figure 2-8
The evaporator coil as
shown is of fin and tube
construction. The thermo-
stat probe is positioned in
between the evaporator fins
and senses the temperature.
THERMOSTAT
Note: Moisture in the air (humidity) condenses on the fins
of the evaporator as water droplets which drain out of
the evaporator through a drain hose. This action de-
humidifies the air in the cab as part of system opera-
tion, and contributes to operator comfort.
Cab air forced across the evaporator coil gives up heat energy to the cold
refrigerant inside the coil. The cooled air circulates in the cab for occupant
comfort. Refrigerant continues to expand and absorb heat energy in the
evaporator coil. Refrigerant changes from liquid to gas before it leaves the
evaporator on the way back to the compressor. The refrigerant gas moves
to the compressor through a low pressure (suction) hose. When the com-
pressor is operating, it sucks the refrigerant gas back inside, compressing
and raising its temperature and pressure.
Some of AC system operation is controlled by the operator, and some is auto-
matic. The operator can turn the system on and off, regulate the air velocity
with the blower control, and in some designs adjust the thermostat control. The
system and component operating range settings automatically cycle the clutch
on and off. The operation of the expansion valve or other refrigerant metering
device at the inlet to the evaporator is automatic.
Individual system features may differ, but the basic system function re-
mains the same. Variations in components and controls are described in Chap-
ters 4 and 5. The engine provides the power for both air conditioner and heater
operation. It drives the AC compressor and the cooling system water pump.
Engine RPM affects the efficiency of both the heater and air conditioner. The
slower the engine RPM, the less capacity a heater or AC system will have.
WARNING When an AC system is operating, the high pressure side
components, fittings and high pressure lines or hoses can
be hot enough to burn your skin if you touch them. This
includes the compressor, clutch, hoses, condenser, re-
ceiver-drier, and any control devices or metal tubing. The
low pressure side will be cool to the touch. In operation
the AC system is under load and high side pressures
normally range between 150 and 250 pounds per square
inch for R-12 and higher for some other refrigerants.