How Does a Decanter Centrifuge Work?
A decanter centrifuge works by exerting a high centrifugal force on the slurry of solids and liquids. A rotating conveyor pushes out the heavier solids that settle on the decanter wall. The lighter liquid exits the decanter through the liquid outlet port on the other end of the bowl.
Continuous process decanter centrifuges can process a large volume of fluids with sludge. Centrifugal force (upwards of 3,000 g) causes graded settling and separates the fluid from small sediment (50µ).
The rotating bowl has a concentric scroll that rotates at a differential speed to that of the decanter bowl. This speed differential allows the scroll to transfer the separated sludge.
The separation of fluid from the sludge occurs inside the rotating decanter bowl. One can explain the internal working as follows.
Feed Introduction: The slurry is fed into the machine through an axial feed tube, injecting it into the conveyor. The tube conveys the slurry to the center of the conveyor feed chamber.
Acceleration / Dispersal: The injected slurry strikes against an accelerator in the conveyor feed zone. This action accelerates the flow outward through the feed ports into the bowl, forming a 'pond.'
Separation: The solids of higher density displace the liquid and settle to the bowl's wall. The conveyor continuously scrolls the solids up the angled conical part (beach).
Auger Rotation: The bowl-mounted gearbox causes a differential rotation between the conveyor and the bowl.
Sludge Discharge: The flights push the separated sludge up the bowl's conical part (beach). The solids exit the bowl through 360° solids discharge ports located at the rear end.
Fluid Discharge: The clarified liquid rises to the pond's surface and flows over the dams located in the front bowl hub. At this point, the liquid discharges from the bowl.
Decanter Bowl Speed: The bowl speed or RPM signifies the effective centrifugal force generated by the decanter. Depending on the fluid and type/density of solids, the centrifugal force is the key factor affecting performance.
Auger or Conveyor Speed: The relative rotation of the auger versus the bowl relates to the speed at which the separated sludge is conveyed out of the bowl. High sludge content applications require a fast conveyor speed to maximize efficiency.
Pond Depth: The thickness of the fluid layer formed around the bowl's inner wall is the pond depth. Deeper ponds lead to clearer centrate and wetter solids. Shallower ponds lead to dryer solids and murkier centrate.
Weir plate adjustments allow pond depth changes. Example weir plates are shown below.
Beach Angle: The angle of the conical part of the bowl is known as the beach angle. Varying the beach angle has a direct impact on the separated sludge dryness.
Conveyor Pitch: The distance between the conveyor flights is the conveyor pitch. A shorter pitch enhances the decanter's solids' separation capacity and increases the torque on the gearbox.
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