WD Catalog 2018

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1-7 G - -
In an air conditioner, the refrigerant is trapped inside a closed system and
circulates under pressure. When we put any substance under pressure, the
pressure changes the way it acts. We can control the amount of pressure at
different locations within an air conditioner in order to change the temperature
level wherever a “change of state” occurs. Thus we use “change of state”, one of
nature’s laws, to add or subtract heat in large quantities from a substance—
fast! Figure 1-10 shows the points where “changes of state” occur in the truck
AC system. Refrigerant, in this case R-12, evaporates inside the evaporator coil
and condenses inside the condenser.
Under normal operating conditions there is no change of state within the
heater system.
COMPRESSOR
RAISES PRESSURE
FROM 20-40 PSI
TO 150-180 PSI LIQUID TO GAS
CHANGE OF STATE
IN EVAPORATOR
H
H
H
H
H
EXPANSION VALVE:
PRESSURE DROPS HERE
FROM 150-180 PSI
TO 20-40 PSI
GAS TO LIQUID
CHANGE OF STATE
IN CONDENSER
TYPICAL HIGH SIDE
PRESSURE IS
150-180 PSI
TYPICAL LOW SIDE
PRESURE IS 20-40 PSI
Air Conditioner/Heater Functions — What They Do
These systems are designed to provide comfort to the cab occupants as quickly
as possible. The systems change the cab temperature and maintain it. They use
refrigerant and engine coolant (anti-freeze) to move heat energy. The refriger-
ant in air conditioning systems removes heat from the cab. The engine coolant
takes heat from the engine and transfers some of it to the cab through the
heater core.
HVAC systems also control humidity or moisture as the air in the cab is
circulated. The system components may act together or independently, switch-
ing on or off to meet present control requirements. The driver or passengers can
change system control settings to meet their own personal comfort needs. The
controls can affect temperature and humidity levels, air direction and speed
(CFM). System controls are described in Chapters 3 (heater) and 4 (air condi-
tioner) of this manual, and explained in more detail in Chapter 5.
It is important to remember that all systems are designed to maintain a
temperature range. This range takes care of variations in outside temperature
which cause heat gain when it is hot, or heat loss when it is cold.
Air Conditioner/Heater Functions
Figure 1-10
This illustration highlights
the evaporator and con-
denser in a cutaway view of
the typical HVAC system.
The R-12 changes from
liquid to vapor or gas at the
evaporator, and back to a
liquid again at the con-
denser.