Industrial Vacuum Specifications

Positive Displacement Type Vacuum Producers:
Operating Principles of a Rotary Lobe Exhauster

Principles of Operation

A rotary lobe exhauster is a positive displacement type.  Its performance is characterized by relatively constant air flow with a variable vacuum, where the horsepower requirements increase as vacuum increases.  These machines may function either as vacuum producers or as compressors, depending on which end is connected to process. 

In Figure 1, two "Figure 8" lobe impellers mounted on parallel shafts are synchronized to rotate in opposite directions.  As each impeller rotates past the inlet, a finite volume of air is trapped and carried around the casing toward the outlet .

Given a a constant speed of rotation, the displaced volume of air remains constant regardless of pressure, temperature, or barometric pressure.  Vacuum is created literally by "scooping out" air from the system.  

A minimal clearance is maintained between the lobes, so internal lubrication is not required.  Note that Tri-lobe shaped impellers are now popular because the air pocket becomes smaller, so noise is significantly reduced.

Within a given unit frame size, the volume of air is dictated by the speed of rotation; a higher rpm equals higher flow.  The maximum vacuum is usually 15 to 16" Hg.  This limit is imposed by heat.  Many lobe type exhausters are limited to about 300F to 320F maximum discharge temperature. (The heat of compression gets serious at 16" Hg!).  Because the lobes expand with heat, over temperatures will cause the units to seize.  

Cooled systems allow operation down to approximately 27" Hg.  Cooling can be by water (for instance the GARDNER DENVER Series 6000's), or more recently by "air injection" (See for example the SIAV Series by HIBON ).

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title: Rotary Lobe Type Vacuum Producer - Principles of Operation