Saab 900 Service Manual page 22

854-10 Heating and ventilation system, air conditioning system
Saab 900
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VIR unit
1 From evaporator
2 Use 8 mm 'socket
3 Sealing ring
4 Suction throttling valve and expansion
mounting bolts
5
a
ring
6 Expansion valve. Blow clean with R 12.
7 a-rings
8 Suction throttling valve
9 To compressor
10 From evaporator, compensating hose
11 Valve, compensating pressure
12 From condenser
13
a
ring
14
a
ring
15 Sight glass
16 Sealing ring
17 Desiccant bag
18 Filter
19 Receiver shell, blow clean with R 12
The valve is actuated by the outlet pressure from
the evaporator (the low- pressure side). When
the pressure has dropped to 29 Ib/in 2 (2,1 kg!
cm 2
),
the valve restricts or throttles the flow of
vapour to the compressor, thereby maintaining a
constant pressure in the evaporator.
The valve thus prevents the pressure in the
evaporator from dropping too low and, con-
sequently, the evaporator becoming too cold,
allOWing frost to form on the evaporator fins.
Expansion valve
This valve, which meters the flow of refrigerant
into the evaporator, is actuated by the outlet
pressure and temperature from the evaporator. If
the temperature in the evaporator rises, the
valve senses this and increases the amount of
refrigerant entering the evaporator, thereby low-
ering the temperature.
The refrigerant is in a liquid state in, and im-
mediately downstream of, the expansion valve.
However, as soon as the pressure drops, the re-
frigerant starts to boil, taking heat from the air
passing over the fins of the evaporator and
thereby removing heat from the ambient air. The
quantity of refrigerant entering the evaporator
must leave as 100 per cent gas or low-pressure
vapour.
This will cause the compressor to become
"flooded", with a resulting risk of damage. If too
little refrigerant is metered into the evaporator,
the system will become "starved". The unit will
not then be capable of cooling at full capacity.
The refrigerant will have vaporized or boiled off
before reaching the evaporator outlet. When the
exactly correct quantity of refrigerant is metered,
the refrigerant will be in a liquid state im-
mediately downstream of the expansion valve
and will change to saturated vapour having a
temperature difference of between 5 and 10
D
C
(overheating) at the outlet.
Suction throttling valve
VIR-system
Valves in Receiver (VIR) assembly
The VIR assembly is the system control unit and
is fitted direct to the evaporator. The assembly
houses the following components.
System components
This section describes the circulation ofthe refri-
gerant through the system and the function of
the'various components.
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