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Section 9 - Maintenance
Table 31 - System pressures
Gauge
Inches of mercury
psia
(in.Hg) below one
standard atmosphere
0 in.
14.6960
10.240 in.
9.6290
22.050 in.
3.8650
25.980 in.
1.9350
27.950 in.
0.9680
28.940 in.
0.4810
29.530 in.
0.1920
29.670 in.
0.1220
29.720 in.
0.0990
29.842 in.
0.0390
29.882 in.
0.0190
29.901 in.
0.0100
29.917 in.
0.0020
29.919 in.
0.0010
29.9206 in.
0.0002
29.921 in.
0
Note:
• One standard atmosphere = 14.696 psia = 760 mm Hg absolute pressure at 32°F = 29.921 in. Hg absolute at 32°F
• psig = pound per square inch gauge pressure = pressure above atmosphere
• psia = pound per square inch absolute pressure = sum of gauge plus atmospheric pressure
• Shell volume = L*π * r2= (Length in feet)*( 3.1416)*(radius squared) = cubic feet. To keep the units consistent, round the
length and radius to the nearest tenth of a foot.
• μm = Micron
Dehydration
To make sure that there is confidence in the vacuum
decay related to moisture boiling off, only perform the
dehydration process after the system has been thor-
oughly leak checked. The dehydration process is only
needed if the following occurs:
1. The nitrogen holding charge on shipments has
been lost.
2. The system has been open to the atmosphere for
any length of time.
3. Tube leaks have introduced moisture to the refrig-
erant circuit.
4. Indications of moisture contamination have ap-
peared in any of the sight glasses Dehydration of
a refrigerant system can be obtained by the evacu-
ation method because the water present in the
system reacts much as a refrigerant would. How-
ever, the vacuum pressure in the system cannot
164
Absolute
Millimeters of
Microns
mercury (mmHg)
760.00
760,000
500.00
500,000
200.00
200,000
100.00
100,000
50.00
50,000
25.00
25,000
10.00
10,000
6.30
6,300
5.00
5,000
2.00
2,000
1.00
1,000
0.50
500
0.10
100
0.05
50
0.01
10
0
0
always be pulled down to a point where its satura-
tion temperature is considerably below that of the
equipment room temperature due to low ambient
room conditions and other factors. As a result, you
might need to use an external heat source or flow
warm water through at least one vessel to raise
the vessel internal temperature. This ensures that
heat flows into the system and helps to vaporize
the water, so that a large percentage of it can be
removed by the vacuum pump.
The length of time necessary for the dehydration of a
system depends on the size or volume of the system,
the temperature of the vessels, the capacity and effi-
ciency of the vacuum pump, the room temperature, and
the quantity of water present in the system. You can
use an external heat source to shorten the dehydration
time, as discussed in the previous paragraph. If you use
a vacuum gauge as suggested, the corresponding satu-
ration temperature can be used as a reference. If the
system has been pressure tested and found to be tight
Form QTC4-NM2
Issue date: 05/12/2023
Boiling temperatures
of water, ⁰F (⁰C)
212 (100)
192 (88.9)
151 (66.1)
124 (51.1)
101 (38.3)
78 (25.6)
52 (11.1)
40 (4.4)
35 (1.7)
15 (-9.4)
1 (-17.2)
-11 (-23.9)
-38 (-38.9)
-50 (45.6)
-70 (-56.7)
–
Quantech
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