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Dehydration

To make sure that there is confidence in the vacuum decay related to moisture boiling off,

only perform the dehydration process after the system has been thoroughly leak checked. The

dehydration process is only needed if the following occurs:

1.  The nitrogen holding charge on shipments other than Form 1 has been lost.
2.  The system has been open to the atmosphere for any length of time.
3.  Tube leaks have introduced moisture to the refrigerant circuit.
4.  Indications of moisture contamination have appeared in any of the sight glasses

Dehydration of a refrigerant system can be obtained by the evacuation method because the water

present in the system reacts much as a refrigerant would. However, the vacuum pressure in the

system cannot always be pulled down to a point where its saturation temperature is considerably

below that of the equipment room temperature due to low ambient room conditions and other

factors. As a result, you might need to use an external heat source or flow warm water through

at least one vessel to raise the vessel internal temperature. This ensures that heat flows into the

system and helps to vaporize the water, so that a large percentage of it can be removed by the

vacuum pump.
The length of time necessary for the dehydration of a system depends on the size or volume of the

system, the temperature of the vessels, the capacity and efficiency 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 saturation 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 recordings should follow a curve similar to the typical saturation curve in Figure 53.
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 53: 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 temperature will return to approximately ambient temperature. Close

off the system again, and start the second evacuation. The relatively small amount of moisture left

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