FORM 160.73-O2 (605)
27
YORK INTERNATIONAL
VACUUM DEHYDRATION
To obtain a suffi ciently dry system, the following in struc -
tions have been assembled to provide an ef fec tive meth od
for evacuating and dehydrating a system in the fi eld. Al-
though there are several methods of de hy drat ing a sys tem,
we are recommending the fol low ing, as it produces one of
the best results, and af fords a means of obtaining accurate
readings as to the ex tent of de hy dra tion.
The equipment required to follow this method of de-
hy dra tion consists of a wet bulb indicator or vacuum
gauge, a chart showing the relation between dew point
tem per a ture and pressure in inches of mercury (vac u um),
(See Table 2) and a vacuum pump capable of pumping
a suit able 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 sat u -
ra tion temperature is considerably below that of room
tem per a ture, heat will fl ow 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 de hy dra tion
of a system is dependent on the size or volume of the
sys tem, the capacity and effi ciency of the vacuum pump,
the room temperature and the quantity of water present
in the system. By the use of the vacuum in di ca tor 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 tem per a ture as any
free water in the system, and this tem per a ture 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 per a ture
recordings should follow a curve similar to the typ i cal
saturation curve shown as Fig. 12.
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 fi nal 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 vac u um pump,
nitrogen should be introduced into the system to bring it
to atmospheric pressure and the in di ca tor tem per a ture will
return to approximately am bi ent temperature. Close off
the system again, and start the second evac u a tion.
The relatively small amount of moisture left will be car-
ried out through the vacuum pump and the tem per a ture
or pressure shown by the indicator should drop uni-
form ly until it reaches a temperature of 35°F (1.6°C)
or a pres sure of 5 mm Hg.
When the vacuum indicator registers this temperature
or pressure, it is a positive sign that the system is evac-
u at ed 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 rect ed
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 re sult will be a faulty temperature reading.
FIG. 12 –
SATURATION CURVE
LD00474
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