Conducting The Dehydration Process; Figure 66 - Saturation Curve - York YVAA Installation Operation & Maintenance

Form 201.47-ICOM1
Issue date: 17/11/2022
the temperature of the vessels, the capacity and effi-
ciency of the vacuum pump, the room temperature, and
the quantity of water present in the system. You can
use an external heat source to shorten the dehydration
time, as discussed in the previous paragraph. If you use
a vacuum gauge as suggested, the corresponding satu-
ration temperature can be used as a reference. If the
system has been pressure tested and found to be tight
before evacuation, then the saturation temperature re-
cordings should follow a curve similar to the typical
saturation curve in Figure 30.
The temperature of any trapped water in the chiller
drops as the pressure decreases, until the boiling point
is reached. At this point, the temperature levels off and
remains at this level until all of the water in the shell is
vaporized. When this final vaporization has occurred,
the pressure and temperature continue to drop until
eventually a temperature of 35°F (1.6°C) or a pressure
of 5,000 μm is reached. Because vacuum pumps have
the capacity to overcome the boiling rate of the trapped
moisture, do not go below this pressure at this point.

Figure 66 - Saturation curve

When this point is reached, practically all of the air has
been evacuated from the system, but a small amount of
moisture still remains. In order to provide a medium for
carrying this residual moisture to the vacuum pump,
nitrogen must be introduced into the system to bring
it to atmospheric pressure and the indicator tempera-
ture will return to approximately ambient temperature.
Close off the system again, and start the second evacu-
ation. The relatively small amount of moisture left is
carried out through the vacuum pump and the tempera-
ture or pressure shown by the indicator drops uniform-
ly until it reaches a temperature of 35°F (1.6°C) or a
pressure of 5,000 μm.
JOHNSON CONTROLS
When the vacuum indicator registers this temperature
or pressure, it is a positive sign that the system is de-
hydrated to the recommended limit. If this level cannot
be reached, it indicates that there is a leak somewhere
in the system. Any leaks must be corrected before you
can evacuate the system to 35°F (1.6°C) or 5,000 μm.
During the primary or dehydration evacuation, close-
ly monitor the vacuum level. Do not let it fall below
5,000 μm or the equivalent 35°F (1.6°C).
If the pressure temperature relationship falls to 32°F
(0°C), the water in the system freezes, and the result is
a faulty pressure reading. Use the following procedure
as a guide to the steps for dehydration:

Conducting the dehydration process

1. Connect a high-capacity vacuum pump with indi-
cator, to the charging valves as shown in Figure
27 and start the pump.
2. Ensure all the system valves including manual
valves and EEVs are 100% open. Make sure that
all valves to the atmosphere are closed with flare
caps in place, if applicable.
3. Operate the vacuum pump until a pressure of
5,000 μm is reached. To avoid freezing any
trapped moisture in the system, see Table 28 on
page 166 for corresponding pressure and tem-
perature values.
4. To start the 8 hour vacuum hold test, close the
system charging valve connection to the vacuum
pump. Note the time and pressure.
5. Hold the vacuum obtained in Step 4 in the system
for 8 hours. Even a slight rise in pressure can in-
dicate a leak, the presence of moisture, or both. It
is important to check for pressure change with the
chiller at the same temperature. Pressure changes
proportional to temperature and can affect results.
6. To determine if it is moisture or a leak, conduct
a pressure rise test. Evacuate the system to 5,000
μm again and perform another hold test.
• If the pressure rise goes to 0 psig, it indicates that
a leak is present.
• During the hold period, if moisture is present,
the pressure stabilizes at some level below atmo-
sphere and should correspond to the room ambi-
ent temperature or the heat applied to the vessel.
See see Table 28 on page 166 for values.
7. An acceptable vacuum pressure rise is 150 μm for
the first 60 minutes.
SECTION 9 - MAINTENANCE
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