The Design of Rotary Pumps and Systems to Provide Clean Vacua

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Abstract
The oil sealed rotary pump is the most common component used in vacuum systems but such pumps when charged with mineral oil can be a major source of organic backstreaming. Work in our laboratory has been concerned with studying (i) the mechanisms by which volatile organic material is formed within and released from working pumps and (ii) backstreaming under conditions where gaseous diffusion, gas flow and pipeline conductance can determine its flow rate into the vessel. Arising from these studies has been the possibility of showing that backstreaming can be reduced to negligible levels by the use of adsorption traps to eliminate oil molecules from the vacuum atmosphere. Unfortunately, such traps may impair the ultimate vacuum achieved by also trapping water vapor. This can be overcome by using a trapped oil sealed pump to back a dry vacuum stage. The dry stage can be of the blower type (out-of-contact). Higher compression ratios are possible with in-contact vane pumps which depend on use of materials with a low coefficient of friction and a pump is herewith described. In the conclusion, some of the characteristics of an alternative lubricating fluid (fluorocarbon) are described.

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