Saab 900 Service Manual page 19

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Heating and ventilation system, air conditioning system 854-7
Air conditioning system (AC)
The function ofthe air conditioning system in the
car is to reduce the temperature inside the carto
a comfortable level when the outside tempera-
ture is high.
In wet weather, the AC system can also keep the
windscreen and side windows demisted, with the
fan running at low speed, even under the most
difficult conditions.
The AC system does not produce cold air but ex-
tracts heat from the air inside the car.
Since heat always flows from a hotter body to a
cooler one, a medium whose temperature is
lower than that of the air inside the car is used to
carry the heat away.
The medium used is a liquid (Freon or refrigerant
R12) which boils and vaporizes at a low tempera-
ture (-30°C at atmospheric pressure).
At a given volume of refrigerant, there is a con-
stant relationship between pressure and
temperature, which means, for instance, that if
there is a change in pressure there will be a cor-
responding change in temperature. It is this
property that is utiIized by the AC system.
The refrigerant is circulated round a closed sys-
tem and a reduction in the pressure in the system
raises the temperature of the refrigerant, caus-
ing itto boil (vaporize). Atthe pressure prevailing
in this system, the refrigerant vaporizes at an ap-
proximate temperature of 0-4°C (32-39°F).
However, to change its state from a liquid to a
gas, the refrigerant must be supplied with heat.
This heat is taken from the air surrounding the
evaporator in which the stated change takes
place. Because this heat is being absorbed by
the refrigerant, the surrounding air becomes col-
der. This same chilled air is then blown into the
car by the ventilation fan. The heat absorbed by
the refrigerant inside the evaporator is carried to
the engine compartment where it is dissipated in
the air by a condenser, which is cooled by the
ram air and/or the cooling fan.
The AC system is of the compressor type, which
means that a compressor forces the refrigerant
to circulate through the various system compo-
nents.
Saab900