3.17 Traps
3.18 When to Use a Liquid Trap
When pumping gases or low vapor loads, a trap is not necessary unless you want to capture
vapors evolved from the process. When a heavy load of water or organic vapor is evolved from
the vacuum process, condensation of vapor in the pump mechanism may occur. The reason
is the pump is compressing the vapor as it passes through the pump. If the vapor is dense
enough, the compression will cause condensation of the vapor in the pump mechanism. The
valve system in the pump mechanism is designed to pass the small quantity of liquefi ed vapor
formed when pumping heavy vapor loads. The liquid formed will be ejected from the exhaust
port. The ejected liquid from the exhaust port can either be collected in a liquid trap attached
to the exhaust port or the vapor trapped in a cold trap placed in-line between the pump and
the vacuum chamber. A simple, inexpensive liquid trap can be made from a fi ltering fl ask. The
fi ltering fl ask is connected by 3/8”ID hose to the exhaust port by using the loose hose barb
provided.
3.19 When to Use a Cold Trap
The use of a cold trap when pumping heavy vapor loads will eliminate the need for a liquid
trap attached to the exhaust port. The cold trap is installed between the pump and the vacuum
chamber. The water or organic vapors evolved from the vacuum process will upon entering
the cold trap will come in contact with the surfaces of the trap and condense. Commonly used
refrigerants are liquid nitrogen or dry ice with alcohol slurry. Dry ice provides suffi cient cooling
to freeze out most heavy water vapor loads. A variety of cold traps are available from Welch.
Please call our customer service department. For additional information at (847) 676-8800.
3.20 The Care of a Liquid Trap
A liquid trap needs no refrigerant. The key maintenance issue when pumping high vapor loads
is to regularly drain the trap of liquid ejected from the dry vacuum pump.
3.21 The Care of a Cold Trap
When using a cold trap the refrigerant should be maintained at a high level in the fl ask to keep
the trap at a uniformly low temperature. If the trap is rewarmed it may allow re-evaporation of
the condensate. 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 and allow the trap to warm up and rinse
off the condensate with a suitable solvent in a fume hood. Thoroughly clean and dry the trap
before reinstalling into the system. If a liquid nitrogen trap is used, the refrigerant add tube
on the liquid nitrogen trap should not be obstructed as the refrigerant boil-off can produce
dangerously high pressures.
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