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5.9 Gas Ballast Control
The principle of the gas ballast is described in chapter 3.4.7 Gas Ballast.
To open or close the gas ballast turn the gas ballast switch to the position open or close.
Operation with gas ballast when drawing off condensable vapours
We suggest operation with the gas ballast valve open, provided that the composition of
gas in the vacuum pump to be drawn off is not known and cannot be ruled out.
If condensable gas and vapours are to be delivered, the latter or their condensates will mix with the oil.
As a result of this, the pump performance will degrade.
5.10 Trap installation and maintenance
Where corrosive vapors or large quantities of condensable vapors are evolved from vacuum processing,
a cold trap may be used in the connecting line to the pump. It will help prevent damage to the pump
mechanism and reduce oil contamination. The cold trap, immersed in a suitable Dewier flask, is installed
so that the vapors may come in contact with the surfaces of the trap and condense. Commonly used
refrigerants are liquid nitrogen or dry ice and acetone. The refrigerant to be used depends upon the
freezing point of the contaminants. A variety of cold traps are available – see chapter 8 Accessories.
When using a cold trap the refrigerant should be maintained at a high level in the flask to keep the trap
at a uniformly low temperature. If the trap warms up, it may cause the condensate to evaporate again.
The refrigerant add tube on the liquid nitrogen trap should not be obstructed as the refrigerant boil-off
can produce dangerously high pressures. If the trap becomes saturated it should be disconnected from
the system, drained and cleaned. An increase in pressure in the vacuum system will normally indicate
that the trap has become saturated. To clean the trap, remove the trap from the system, allow the trap
to warm up and rinse off the condense with a suitable solvent in a fume hood. Thoroughly clean and
dry the trap before reinstalling in the system.
5
INSTALLATION AND OPERATION
