Vapour pumping characteristics of gas ballast pumps

Abstract

the mechanism of contamination when vapours are pumped by oil-sealed rotary pumps is described and a method is discussed for calculating the degree of protection afforded by gas ballast in terms of the maximum mass flow rate at which oil-insoluble vapours can be pumped without significant condensation of vapour in the pump. It is shown experimentally that, though this calculated maximum flow rate does correspond approximately to the practical limit for no condensation, it can often be greatly exceeded without cumulative contamination or serious impairment of pump performance. The somewhat different characteristics when pumping oil-soluble vapours are discussed and illustrated by experimental results, and it is concluded that, although gas ballast can generally provide considerable protection in such cases, a high proportion of the volumetric and mass pumping rates theoretically possible on the basis of pump displacement is more likely to be achieved in larger and hotter-running than in smaller and cooler-running pumps.