Centrifuges axial-flow conveyor

Practical operation of a sludge separator, of course, requires that the solids be heavier than the liquid and not be resuspended by the action of the conveyor. A modification known as an axial-flow conveyor centrifuge (described later) is more effective in separating fine light solids than the full-scroll centrifuge shown in Fig. 30.44. Even so, the liquid effluent from these machines is usually not completely free from solids and may require subsequent clarification. Within these restrictions sludge separators solve a wide variety of problems. They separate fine particles from liquids, dewater and wash free-draining crystals, and are often used as classifiers. 

AXIAL-FLOW CONVEYOR CENTRIFUGES. In 1970 Schnittger showed that the actual residence time in a conveyor centrifuge is much less than that given by Eq. 30.81). He proposed a model in which liquid flows in a thin boundary layer over an essentially stagnant liquid pond. The critical separation takes place in this layer, for once a particle moves from the boundary layer into the pond, it is effectively removed unless later resuspended mechanically. A thin boundary layer gives better separation than a thick one. The total pond volume is not important the pond needs to be no deeper than necessary to ensure that the compacted solids are well below the surface. 

The Imperforate Bowl Centrifuge. In an imperforate bowl centrifuge the flow of the continuous liquid phase is nominally axial, except for areas immediately adjacent to the feed inlet and effluent oudet. Tubular solid-bowl basket and imperforate bowl conveyor-discharge centrifuges satisfy this definition. 

In a full-scroll centrifuge (Fig. 30.44) the liquid flows in a spiral path against the motion of the conveyor. Under these conditions the boundary layer is as much as 10 times as thick as it is when the layer flows only in an axial direction. Furthermore, spiral flow induces eddies and turbulence in the boundary layer, impeding sedimentation with axial flow the boundary layer is in laminar flow, and sedimentation is unimpeded. Willis and Shapiro give E values for axial-flow centrifuges. The addition of vanes can increase E by 3 to 4 times. 

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