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4.2.16 Evacuation and dehydrating
After repairing the refrigerant system, vacuum-
dehydrate the system before charging the
refrigerant.
Vacuum-dehydrating is the process to make the
circuit dry by purging the moisture (liquid) in the
circuit to outside in state of vapor (gas) using the
vacuum pump.
As the pressure lowers below normal atmosphere
(760mmHg), the boiling point of water rapidly
drops. If the boiling point drops beyond the
atmospheric temperature, water will be vaporized.
Example: If the atmospheric temperature is 7.2 ˚C
(45 ˚F), vacuum-dehydrating will be
impossible unless the vacuum degree is
lower than –752mmHg. For vacuum-
dehydrating, it is important to select and
maintain the vacuum pump.
0
60
160
260
360
460
560
660
670
680
690
700
710
720
730
740
Freezing
755
point
760
–20 –10 0
10
20
Temperaturte (°C)
(1) Vacuum pump selection
Select a vacuum pump considering the
following two points.
qSelect a vacuum pump whose vacuum
achievability is excellent.
(A vacuum degree of –755mmHg or lower
can be achieved.)
wThe displacement must be relatively large
(approx.
40r/min. or more).
Before vacuum-dehydrating work, be sure
to confirm that the pump achieves the
vacuum degree of –755mmHg or lower by
using the vacuum gauge.
760mmHg
700
600
500
400
300
200
100
90
70
60
50
Required
40
vaccum
30
degree
20
10
Ambient
temperature
range
5
0mmHg
30
40
50
60
70 80
Boiling point of water (˚C) Atmospheric pressure(mmHg) Vacuum degree(mmHg)
40
30
26.7
24.4
22.2
20.6
17.8
15.0
11.7
7.2
0
(Reference) Kinds of vacuum pumps and achievable vacuum degree
Achievable vacuum degree
Type
Displacement
Oil rotary type
–759.98mmHg
(oil-necessary type)
–750mmHg
Oilless rotary type
(oil-unnecessary type) –759.98mmHg
Take care that this type is often used as the
most convenient type.
With the pump of an oil rotary type, it is
important to replace the oil and check the
achievability every 1 to 2 months.
(2) Vacuum-dehydrating method
There are two method of vacuum-
dehydrating of normal vacuum-dehydrating
and special vacuum-dehydrating. In general,
the normal vacuum-dehydrating is applied. If
any moisture is enters the circuit, apply the
special vacuum-dehydrating method.
[normal vacuum-dehydrating]
qVacuum-dehydrating(first time)
Connect the gauge manifold to the service
ports of the liquid line and the outlet of
discharge pressure regulator. Run the
vacuum pump for 2 hours or longer. (The
achievable vacuum degree must be –755
mmHg or lower)
If a pressure of –755mmHg or lower can not be
achieved even after pump operation of 2 hours,
moisture or leakage may exist in the system. In
this case, run the pump another hour or more.
If a pressure of –755mmHg or lower can not be
achieved even after operation of 3 hours or
more, check for leakage.
Note: Evacuate the system from the
service ports r of both liquid and
outlet of the check valve t,
because the system is blocked on
the way since the liquid solenoid
valve is provided on the way of the
system.
4-22
55
–705
36
–724
25
–735
23
–737
20
–740
18
–742
15
–745
13
–747
10
–750
8
–752
5
–755
Application
For vacuum-dehydrating For air exhausting
Applicable Applicable
100r/min.
Inapplicable Inapplicable
50r/min.
Applicable Applicable
40r/min.
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