52
YORK INTERNATIONAL
VACUUM DEHYDRATION
To obtain a sufficiently dry system, the following instruc-
tions 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), (see Table 3) 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 sys-
tem reacts much as a refrigerant would. By pulling down
the pressure in the system to a point where its satura-
tion 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 Fig. 22.
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 pres-
sure 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 in-
dicator temperature will return to approximately ambi-
ent temperature. Close off the system again, and start
the second evacuation.
The relatively small amount of moisture left will be car-
ried out through the vacuum pump and the tempera-
ture 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 temperature
or pressure, it is a positive sign that the system is evacu-
ated 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
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.
FIG. 22
SATURATION CURVE
LD00474
