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11.1.4 The suction level
The compressor suction level establishes the fluid
phase change conditions inside the evaporator; this
is a crucial parameter for heat exchange and cooling
performance.
The suction pressure (low pressure of the A/C loop) is
initiated by the compressor to ensure the appropriate
evaporation process inside the evaporator (not less
than 3 bars for R134a refrigerant).
Control of the suction level:
Less compressor suction implies:
g
Higher evaporator pressure, thus higher refrigerant
boiling temperature
g
Higher evaporator temperature
g
Reduced thermal exchanges with the ambient air
More compressor suction implies:
g
Lower evaporator pressure, thus lower refrigerant
boiling temperature
g
Lower evaporator temperature
g
Increased thermal exchanges with the ambient air
AC High pressure (HP)
A/C Low pressure (LP)
Condenser
Expansion
valve
Evaporator
Receiver drier
Compressor
Suction port
11.1.5 Suction pressure limits
Increasing the fluid suction reduces the pressure
inside the evaporator.
g
Too low a pressure would generate too low a
temperature at the evaporator surface and a risk
of freezing.
g
Freezing at evaporator surface would block the air
flow, jeopardising heat exchanges and thus the
fluid evaporation.
g
This would lastly put the compressor in danger, as
it could digest liquid refrigerant via the suction port.
Absolute
pressure (bar)
Boiling Temp.
R134a
(°C)
Working zone
Suction pressure
forbidden zone
1,57
-16
1,71
-14
1,85
-12
2,01
-10
2,17
-8
2,34
-6
2,53
-4
2,72
-2
2,93
0
3,15
2
3,38
4
3,62
6
3,88
8
4,43
12
5,04
16
5,72
20
6,46
24
Evaporator freezing
Cooling capacity control
3 bars
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