CHANGE OF PUMPING SPEED WITH TIME FOR CRYOSORPTION ARRAY

Abstract

Pumping speed of the cryosorption element in commercial cryopumps decreases with the amount of pumped gas, because the sticking coefficient on a cryosorbent decreases with increased coverage. The change of pumping speed is calculated for a cylindrical cryoarray using the combined view factor-Monte Carlo method-where the sticking coefficient is variable with position on the array and time. The effect of different models of sticking coefficient vs coverage was considered. The evolution of the hot pumping spots and/or the saturation front could be clearly observed. For a fixed design, the rate of decrease in pumping speed nearly conforms to the rate of change in sticking coefficient. The effect of cryoarray designs on the total capacity and regeneration period is analysed, which is quite different from the effect on the static pumping speed with clean adsorbent surface.