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Dry Vacuum Pumps
The use of dry pumps is growing, replacing workhorse steam jets and liquid-ring pumps. Here is a comprehensive selection guide.
The term "dry vacuum pump" is used to describe a positive-displacement vacuum pump that discharges continuously to atmospheric pressure and in which the swept volume is free of lubricants or sealing liquids. Dry vacuum pumps were originally introduced in the Japanese semiconductor industry in the mid1980s to address reliability problems associated with oil-sealed pumps and contamination caused by back-migration of vacuum pump oil. The success of these pumps revolutionized semiconductor processing. Dry vacuum pumps were introduced into the U.S. chemical process industries (CPI) in the late 1980s. In the next ten years, it is anticipated that they will completely displace oil-sealed pumps, and will make significant inroads into traditional markets for steam jets and liquid-ring vacuum pumps - workhorses for the high-throughput mainstream processing operations in the CPI.
Dry pumps are compact and energy efficient, and do not contribute to air pollution, a problem with oil-sealed pumps, or water pollution, a problem with steam jets and water-sealed liquid ring pumps. Dry pumps are unique among CPI vacuum pumps, because they do not require a working fluid to produce vacuum, so nothing contacts the load being pumped. Solvents or products aspirated from the process can be discharged to an aftercondenser. Contamination is not a concern, and the condensate can be recycled directly to the process.
Rough vacuum
Subatmospheric pressures can be divided into four regions:
Rough vacuum is the region of greatest interest to the CPI, because it is where polymer reactors, vacuum distillation columns and vacuum dryers normally operate. Medium vacuum is used in molten metals degassing, molecular distillation and freeze drying. High and ultrahigh vacuum are used in the production of thin films, mass, spectrometry, low-temperature research, surface-physics research, nuclear research and space simulation.
Semiconductor applications span rough to ultrahigh vacuum, but the capital-intensive, precision-technology operations that so characterize the industry are high and ultrahigh vacuum operations. Semiconductor processing is characterized by corrosive gases (e.g., HCl), and the condensation/precipitation of hard solids (for example, AlCl^sub 3^ and Si02) from the process gas stream in the pump (1). These challenges guided the early development of dry vacuum pumps. The dry...