8
2.30 Traps
2.31 The need for a Trap
The pumps will handle humid air. All wetted aluminum parts are treated for corrosion protection
from moisture. All other wetted parts are stainless steel. If there is a chance liquid may be
drawn from the process under evacuation, Welch recommends a liquid trap be placed between
the process and the pump.
A simple liquid trap is a fi ltering fl ask.
When a heavy load of water vapor is evolved from the vacuum process, a cold trap is
recommended to help prevent damage to the pump mechanism. The cold trap, immersed in a
suitable Dewar fl ask, is installed so that the water vapor may come in contact with the surfaces
of the trap and condense. Commonly used refrigerants are liquid nitrogen or dry ice and
acetone or alcohol. Dry ice provides suffi
cient cooling to freeze out most heavy water vapor
loads. A variety of cold traps are available from Welch.
The symptom of a high water vapor load is if you have droplets coming out of the exhaust
port. If you see droplets of water, Welch recommends you use a cold trap to capture the water
before entering the pump or dilute the water vapor stream by adding dry nitrogen to the gas
fl ow. Please call our customer service department for additional information at (847) 676-8800.
2.32 The care of a 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 re-warmed 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.
Summary of Contents for WELCH 2567B-01
Page 12: ...12 6 11 Pumping Speed Curves...
