Appendices
Polycold Cryochiller
Appendix E: Water Vapor Cryotrapping
Installation and Operation Manual
Brooks Automation
12-8
214072 Revision B
Appendix E: Water Vapor Cryotrapping
and
show the relationship between cryosurface temperature and water vapor
partial pressure for different cryotrapping efficiencies.
Cryosurface Pumping Efficiency
For satisfactory pumping results, cryosurfaces should be operated at temperatures cold enough to pro-
vide 90% or greater trapping efficiency.
0% efficiency occurs when the cryosurface is in equilibrium with water vapor. At this efficiency, satura-
tion is reached and as many water vapor molecules escape from the cryosurface as stick to it. The
right-hand column of
gives the temperatures at which zero pumping occurs at the stated
pressure.
To find the best cryosurface temperature for a vacuum application, find the pressure at which the sys-
tem operates in the left column of
and then follow across to the column showing 90% effi-
ciency. This is the warmest temperature that the cryosurface must be for efficient water vapor pumping.
In general, the deeper the vacuum level, the colder the cryosurface must be in order to trap water vapor
at optimum efficiency. For example, for an application requiring a vacuum pressure range of 1 x 10
-5
torr, the cryosurface needs to reach a temperature of -112°C or colder to achieve a water vapor trapping
efficiency of 90% or better. For an application requiring 5 x 10
-7
torr, the cryosurface must be cooled to
-124°C to obtain the same efficiency.
Figure 12-5: Water Vapor Cryotrapping
