Saab 900 Service Manual page 27

Heating and ventilation system, air conditioning system 854-15
:~~System
components
~cling
clutch system
This section describes the circulation ofthe refri-
gerant through the system and the function of
the various components.
Expansion valve
This valve, which meters the amount of refrige-
rant injected into the evaporator, is governed by
the pressure and temperature at the evaporator
outlet. The valve senses increases in the
temperature in the evaporator and increases the
amount of refrigerant supplied, thereby lowering
the temperature again. The refrigerant is in a
liquid state in, and immediately downstream of,
the expansion valve. However, as soon as the
pressure drops, the refrigerant starts to boil, and
this change in state removes heat from the air
passing over the fins of the evaporator. In this
way heat is removed from the ambient air. The
quantity of refrigerant entering the evaporator
must be evacuated as 100% gas or low-pressure
vapour. If too much refrigerant is metered into
the evaporator, the system will be "flooded" and
the refrigerant will not vaporize. The increase in
pressure will prevent the refrigerant vaporizing
and there will also be insufficient room for it to
expand.
If too little refrigerant is metered into the
evaporator the system will become "starved".
The unit will notthen be capable of cooling atfull
capacity; the refrigerant will have vaporized and
boiled off before reaching the evaporator. When
the correct quantity of refrigerant is metered, it
will be 100 % liquid immediately downstream of
the expansion valve and 100 % gas at the outlet.
Anti-frost thermostat
1
Expansion valve
1 From receiver shell
2 To evaporator
3 Compensating hose
4
Temperature-sensitive body
3-"-=::
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The anti-frost thermostat is a device which pre-
vents the evaporator fins from freezing and ice
from forming. .
A capillary tube is located between the fins and
senses their temperature .
The thermostat is connected in series to the
compressor's magnetic clutch. When the temp-
erature in the evaporator drops below 35°F
(1.5°C) the current to the magnetic clutch is cut
off and the compressor stops. When the
temperature in the evaporator rises to 5°C the
thermostat closes and the compressor starts.
Saab900