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SECTION 7 - MAINTENANCE
EVACUATION AND DEHYDRATION OF UNIT
Make sure the power is removed from the
input side of the VSD at all times when
the chiller is under vacuum (less than at-
mospheric pressure). The VSD maintains
voltage to ground on the motor when the
chiller is off while voltage is available to
the VSD. Insulating properties in the mo-
tor are reduced in vacuum and may not
insulate this voltage sufficiently.
Vacuum Dehydration
If the chiller is opened to the atmosphere for lengthy
repair or service, follow these guidelines to ensure that
all air, moisture and non-condensable gases are re-
moved prior to placing the chiller into service.
The equipment required to complete this method of de-
hydration consists of:
• a wet bulb indicator or vacuum gauge
• and a vacuum pump capable of pumping a suit-
able vacuum on the system.
TABLE 45 - SYSTEM PRESSURES
*GAUGE
In OF
MERCURY (HG)
BELOW ONE
STANDARD
ATMOSPHERE
0
10.24"
22.05"
25.98"
27.95"
28.94"
29.53"
29.67"
29.72"
29.842"
29.882"
29.901"
29.917"
29.919"
29.9206"
29.921"
*One standard atmosphere
= 14.696 psia
= 760 mm Hg. absolute pressure at 32°F
= 29.921 in Hg. absolute at 32°F
140
ABSOLUTE
MILLIMETERS OF
PSIA
MERCURY (HG)
14.696
760.
9.629
500.
3.865
200.
1.935
100.
0.968
50.
0.481
25.0
0.192
10.0
0.122
6.3
0.099
5.0
0.039
2.0
0.019
1.0
0.010
0.5
0.002
0.1
0.001
0.05
0.0002
0.01
0
0
Refer to Table 45 on page 140 to see the relation be-
tween dew point temperature and system pressure in
inches of mercury (vacuum).
Dehydration of a refrigerant system can be completed
by this method because the water present in the system
reacts like refrigerant. Pulling down the pressure in the
system where its saturation temperature is below room
temperature, heat will flow from the room through the
walls of the system and vaporize the water, allowing a
percentage of it to be removed by the vacuum pump.
The length of time necessary to dehydrate a system de-
pends on the following:
• Size or volume of the system
• Capacity and efficiency of the vacuum pump
• Room temperature
• And quantity of water present in the system.
DEW POINT
TEMPERATURES
MICRONS
WATER
760,000
212
500,000
192
200,000
151
100,000
124
50,000
101
25,000
10,000
6,300
5,000
2,000
1,000
500
–11
100
–38
50
–50
10
–70
0
NOTE:
PSIG = Lb per sq. in. gauge pressure
= Pressure above atmosphere
PSIA = Lb per sq. in. absolute pressure
= Sum of gauge plus atmospheric pressure
FORM 201.30-ICOM1 (519)
ISSUE DATE: 05/22/2019
OF
°F
78
52
40
35
15
+1
0
JOHNSON CONTROLS
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