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 Ext. 1.
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
ondensate. 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 2580B-01
Page 12: ...12 6 11 Pumping Speed Curves...
