Si39-303 Installation
General Information 31
3.2.6 Vacuum Drying
What is vacuum
drying?
Vacuum drying is:
“The use of a vacuum pump to vaporize (gasify) the moisture (liquid) inside the pipes and expel it leaving
the pipes completely dry inside.”
At 1 atm (760 mmHg) the boiling point (evaporating temperature) of water is 100°C but if a vacuum is
created inside the pipes using a vacuum pump then the boiling point is rapidly reduced as the degree of the
vacuum is increased. If the boiling point is reduced to a level below that of the ambient temperature then
the moisture in the pipes will evaporate.
The evacuation of air conditioner piping provides the following effects.
1. Vacuum drying
2. Removes air and nitrogen (used in air-tightness test) from the inside of pipes.
Therefore, it is necessary to ensure that the both purposes have been achieved in the vacuum drying
operation.
<Example>
When outside temperature is 72°C
As shown in the table on the right, the degree
of vacuum must be lowered below –75.2mmHg.
Boiling point
of water (°C)
Pressure
∗
mmHg Pa Torr
40 –705 7333 55
30 –724 4800 36
26.7 –735 3333 25
24.4 –738 3066 22
22.2 –740 2666 20
20.6 –742 2400 18
17.8 –745 2000 15
15.0 –747 1733 13
11.7 –750 1333 10
→
7.2 –752 1066 8
0 –755 667 5
Above figures (mmHg) are gauge pressure readings.
Key points
Lower the degree of vacuum to below –755mmHg
(V1216)
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