Dry vacuum pumps eliminate oil contamination and the need for fluids, traps and filters. They have had the disadvantages of low compression ratios, limited pump life, very high initial cost, and noise but the gap between dry and oil sealed pumps keeps getting smaller.
The Fuji Technology Inc. Nonlubricated Mechanical rough Vacuum Pump is a piston type pump with a compression ratio close to 50000:1.
"The single three stage piston pump can reach 2 X 10e-02 torr. When two such pumping units are connected in series,pressures of 3X10e-04 torr can be achieved without oil or other sealing liquids."
-Single stage screw-rotor vacuum pump -Reduced maintenance compared to hook & claw pumps. -Vacuum to 4 millitorr -Kashiyama claim about 30% of the Japanese dry pump market.
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