WD Catalog 2018

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4-2
Chapter 4 – Air Conditioner Components
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R-134a (HFC-134a) is gaining favor as the “environmentally friendly” substi-
tute for R-12 in mobile HVAC systems. Properly designed R-134a systems will
equal or surpass the performance of R-12 systems, without danger to the
earth’s ozone layer. As the production of R-12 and other CFC chemicals ceases
during the mid-1990’s, more emphasis will be placed on the development of R-
134a systems and the recovery and recycling of R-12 in existing equipment.
Refrigerants have the ability to absorb and move a lot of heat energy from
inside a truck cab to the outside. This is because the refrigerant in the system is
controlled to change state at just the right temperature and pressure. Note in
Figure 4-1 that a refrigerant changes state in the condenser and the evapora-
tor.
HOT HIGH
PRESSURE GAS LOW PRESSURE GAS
[SUCTION SIDE]
HIGH PRESSURE
LIQUID
CONDENSER
RECEIVER-DRIER
EVAPORATOR
EXPANSION
VALVE
COMPRESSOR
COLD LOW PRESSURE GAS
THERMOSTAT
CAB AIR
FLOW
OUTSIDE AIR FLOW
METERED
REFRIGERANT
Vehicle operating conditions such as engine RPM and air temperature influ-
ence actual AC system operating temperature and pressure. These factors,
along with the relative humidity, influence operator comfort (how efficiently
the system moves heat energy from the cab to outside air).
When a refrigerant changes from a high pressure liquid to a low pressure
gas in the evaporator, the refrigerant is much cooler than the air in the truck
cab. Nature's law takes effect, (remember how heat energy always moves from
a warm to a cooler environment) and the heat energy in the cab air moves into
the refrigerant in the evaporator coil. The refrigerant gets warmer and the cab
air colder. Figure 4-2 shows the refrigerant “change of state” in graphic form as
that change is related to pressure and temperature. The curved line indicates a
typical pressure/temperature range inside the evaporator and condenser.
When refrigerant enters the condenser (from the compressor) as a high
temperature high pressure gas, it is much hotter than the outside air. Again,
natures law takes effect and heat energy in the refrigerant moves into the air
as the air passes through the condenser fins.
Figure 4-1
In this drawing the AC
system is shaded to indicate
the high pressure side of the
system during AC opera-
tion. Controlled changes of
state occur inside the con-
denser and evaporator.