Vacuum Dehydration; Operation; Figure 13 - Saturation Curve - York YR TB TB T0 Installation Operation & Maintenance

FORM 160.81-NOM1
ISSUE DATE: 10/3/2020

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 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 (vacu-
um), (see Pressure/Temperature Conversion Tables on
page 73) and a vacuum pump capable of pumping a
suitable vacuum on the system.

OPERATION

Dehydration of a refrigeration 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
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 tem-
perature recordings should follow a curve similar to
the typical saturation curve shown as Figure 13 on
page 29.
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 (2°C) or
a pressure of 5mm Hg. is reached.
JOHNSON CONTROLS
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-
cator 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 temper-
ature or pressure shown by the indicator should drop
uniformly until it reaches a temperature of 35°F (2°C)
or a pressure of 5mm Hg.
When the vacuum indicator registers this temperature
or pressure it is a positive sign that the system is evacu-
ated and dehydrated to the recommended limit. If this
level can not 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
(2°C) or 5mm Hg. in the primary evacuation. During
the primary pull down keep a careful watch on the wet
bulb indicator temperature, and do not let it fall below
35°F (2°C). If the temperature is allowed to fall to 32°F
(0°C) the water in the test tube will freeze, and the re-
sult will be a faulty temperature reading.

FIGURE 13 - SATURATION CURVE

SECTION 1 – INSTALLATION
LD00474
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