Design and performance of oil-free pumps

The development of high vacuum pumps and systems during the last 35 yr can be characterized by a trend of gradual elimination of liquids exposed to vacuum environment. Only 30 yr ago almost all vacuum systems contained oil-sealed mechanical pumps and vapor-jet pumps (diffusion pumps) with oil as the working fluid. The need for high purity vacuum process equipment precipitated the development of new pumping techniques: ion-gettering pumps, turbo-molecular pumps and cryopumps. During the last 5 yr, a number of dry coarse-vacuum pumps have been developed. Some are entirely oil-free, and some still use lubricants at the seals separating the driving mechanism from the vacuum side. This review contains general design and performance characteristics of four classes of equipment: hermetically sealed pumps using bellows to isolate the driving mechanism, a variety of multistaged positive displacement blowers, pumping trains based on turbo-drag pumps, and multistaged reciprocal piston pumps. It is entirely practical to achieve pressure ratios of 105-106 while discharging to atmosphere. Despite a complete absence of conventional lubrication in some of these pumps, periods exceeding 10,000 h without maintenance are obtained, providing a viable technology for clean vacuum systems.