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| Fluid machinery and parts |
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Special spiral rotated mechanism which cause the flow of the gas.
Weakness:Not chemical resistOnly for clean and dry gasesNot good gas tightExpensive
Benefits:High capacity and low ultimate vac ... 
The centrifugal forces press the sealing edges of the vanes onto the surface of the housing.The volume between the vanes increases and this creating vacuum
Weakness:Not chemical resist ... 