FORM 160.77-O1
ISSUE DATE: 10/22/2020
JOHNSON CONTROLS
24
SECTION 3 - MAINTENANCE
and out the condenser vent. To avoid the possi-
bility of causing leaks, the temperature should be
brought up slowly so that the tubes and shell are
heated evenly.
5. Close the system charging valve and the stop
valve between the vacuum indicator and the vac-
uum pump. Then disconnect the vacuum pump
leaving the vacuum indicator in place.
6. Hold the vacuum pressure obtained in
Step 3
in
the system for 8 hours; the slightest rise in pres-
sure indicates a leak or the presence of moisture,
or both. If after 24 hours the wet bulb temperature
in the vacuum indicator has not risen above 40°F
(4.4°C) or a pressure of 6.3 mm Hg, the system
may be considered tight.
Be sure the vacuum indicator is valved off
while holding the system vacuum and be
sure to open the valve between the vacuum
indicator and the system when checking
the vacuum after the 8 hour period.
7. If the vacuum does not hold for 8 hours within the
limits specified in
Step 6
, the leak must be found
and repaired.
VACUUM DEHYDRATION
To obtain a sufficiently dry system, the following in-
structions have been assembled to provide an effective
method for evacuating and dehydrating a system in the
field. Although there are several methods of dehydrat-
ing a system, we are recommending the following, as
it produces one of the best results, and affords a means
of obtaining accurate readings as to the extent of de-
hydration.
The equipment required to follow this method of dehy-
dration consists of
• a wet bulb indicator or vacuum gauge.
• a chart showing the relation between dew point
temperature and pressure in inches of mercury
(vacuum), (refer to
).
• a vacuum pump capable of pumping a suitable
vacuum on the system.
OPERATION
Dehydration of a refrigerant system can be obtained
by this method because the water present in the sys-
tem reacts similar to refrigerant. By pulling down the
pressure in the system to a point where its saturation
temperature is considerably below that of room tem-
perature, heat will flow from the room through the
walls of the system and vaporize the water, allowing
a large percentage of it to be removed by the vacuum
pump. The length of time necessary for the dehydra-
tion of a system is dependent on the size or volume of
the system, the capacity and efficiency of the vacuum
pump, the room temperature and the quantity of water
present in the system. By using a vacuum indicator,
the test tube will be evacuated to the same pressure as
the system, and the distilled water will be maintained
at the same saturation temperature as any free water in
the system. This temperature can be observed on the
thermometer.
If the system has been pressure tested and found to be
tight prior to evacuation, then the saturation tempera-
ture recordings should follow a curve similar to the
typical saturation curve shown in
The temperature of the water in the test tube will drop
as the pressure decreases, until the boiling point is
reached, at which point the temperature will level off
and remain at this level until all of the water in the
shell is vaporized. When this final vaporization has
taken place, the pressure and temperature will continue
to drop until eventually a temperature of 35°F (1.6°C)
or a pressure of 5 mm Hg. is reached.
LD00474
FIGURE 11 -
SATURATION CURVE
When this point is reached, practically all of the air
has been evacuated from the system, but there is still
a small amount of moisture left. In order to provide
a medium for carrying this residual moisture to the
vacuum pump, nitrogen should be introduced into the
system to bring it to atmospheric pressure and the indi-
