Vacuum Dehydration; Operation - York R-134a Operation And Maintenance Manual

FORM 160.76-O1
ISSUE DATE: 05/22/2019
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 obtained in Step 3 in the system
for 8 hours; the slightest rise in pressure indicates a
leak or the presence of moisture, or both. If, after 24
hours the wet bulb temperature in the vacuum indi-
cator has not risen above 40°F (4.4°C) or a pressure
of 6.3 mm Hg, the system may be considered tight.
7. If the vacuum does not hold for 8 hours within the
limits specified in Step 6 above, the leak must be
found and repaired.
Be sure that 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.

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 dehydrating
a system, we are recommending the following, as it pro-
duces one of the best results, and affords a means of ob-
taining accurate readings as to the extent of dehydration.
The equipment required to follow this method of de-
hydration 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;
see Table 2 on page 24), and 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 system
reacts much as a refrigerant would. By pulling down
the pressure in the system to a point where its satu-
ration temperature is considerably below that of room
temperature, 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
JOHNSON CONTROLS
the system, the capacity and efficiency of the vacuum
pump, the room temperature and the quantity of water
present in the system. By the use of the vacuum indi-
cator as suggested, 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, and 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 temperature
recordings should follow a curve similar to the typical
saturation curve as shown as Figure 12 on page 26.
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.
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 indicator temperature
will return to approximately ambient temperature. Close
off the system again, and start the second evacuation.
The relatively small amount of moisture left will be
carried out through the vacuum pump and the tem-
perature or pressure shown by the indicator should
drop uniformly until it reaches a temperature of 35°F
(1.6°C) or a pressure of 5 mm Hg.
When the vacuum indicator registers this tempera-
ture or pressure, it is a positive sign that the system is
evacuated and dehydrated to the recommended limit. If
this level cannot be reached, it is evident that there is a
leak somewhere in the system. Any leaks must be cor-
rected before the indicator can be pulled down to 35°F
(1.6°C) or 5 mm Hg in the primary evacuation.
During the primary pulldown, keep a careful watch on
the wet bulb indicator temperature, and do not let it fall
below 35°F (1.6°C). If the temperature is allowed to
fall to 32°F (0°C), the water in the test tube will freeze,
and the result will be a faulty temperature reading.
SECTION 6 - MAINTENANCE
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