WD Catalog 2018

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Chapter 1 – Air Conditioning / Heating Function
1-2 G - -
The ideal cab environment has a modest humidity level. The temperature
should reach and then remain in the ideal range, 70 to 80 degrees. The cab
should reach this temperature range after a few minutes of system operation
with the windows closed. Air within the cab should be exchanged every few
minutes to remove smoke, products of respiration and other odors.
Most air conditioner systems cycle on and off by the action of thermostatic
and/or pressure sensitive devices. It is this on-off, open- closed action that
maintains a comfortable temperature range for the driver and any passengers.
Heat Sources
Heat is a form of energy. The control of heat energy is what air conditioning and
heating is all about. In summer a vehicle cab absorbs heat from various sources
such as the sun, the road surface, engine, transmission, hot outside air, and
even the people in the cab. In winter the cab looses heat to the cold outside air.
Figure 1-2 illustrates a truck cab in two situations—operating in summer
and winter.
H
H
H
H HH
H
H
H
H
H
H
H
WINTER
20°
SUMMER
100°
32°
70° 80°
120°
0°
CAB
COMFORT
RANGE
Heat Movement
Heat always moves from a warm area to a cooler one until both areas are at the
same temperature. You know from experience that on a cold day, when you
drive somewhere with the heater on, your vehicle is comfortable. But if you
park it for awhile with the engine off, the cab and the engine will both
eventually reach the same temperature as the outside air. Figure 1-3 illus-
trates this in a parked vehicle.
Figure 1-2
This drawing shows how
heat moves; one of nature’s
laws. Heat always moves
from a warm to a cool
area—heat flows into the
cab in hot weather and
flows out in cold weather.