Batch discharge centrifuges

In general, soHds-retaining batch and batch automatic machines are limited to low feed concentrations to minimize the time required to unload the soHds. Continuous disk centrifuges can have higher feed concentration. The limit is the underflow concentration. Conveyor discharge centrifuges can handle high feed concentration and are limited only by the volume of soHds displacement, or torque capacity. 

The first three types, including the manual-discharge disk, are batch-feed centrifuges, whereas the latter three, including the intermittent and nozzle-discharge disks, are continuous centrifuges. 

Although the purification techniques used with the semicontinuous system are similar to that for the batch system, much less wastewater is produced because much less acid is retained on the nitrocellulose that is discharged from the centrifuge (135—137). 

Centrifugal Sedimentation Equipment. Commercial sedimentation centrifuges are characterized priacipaHy by how soHds are discharged, and the general dryness of these soHds. There are batch and automatic batch soHd bowl machines which coUect the soHds at the bowl wall. SoHds are removed very dry. Almost any soHd is coUectable, even those that are very soft and compressible. 

ScroU centrifuges discharge soHds continuously and usuaUy drier than disk and imperforate batch types. GeneraUy disk centrifuges have the highest values of Z or for a given size and therefore the best abUity to coUect fine particles at a high rate. 

Table 18-15 summarizes the several types of commercial centrifuges, their manner of liquid and solids discharge, their unloading speed, and their relative volumetric capacity. When either the liquid or the solids discharge is not continuous, the operation is said to be cycHc. Cyclic or batch centrifuges are often used in continuous processes by providing appropriate upstream and downstream surge capacity. 

Batch centrifuges with imperforate bowls are used either for producing an accelerated separation of solid particles in a liquid, or for separating mixtures of two liquids. In the former case, the solids are deposited on the wall of the bowl and the liquid is removed through an overflow or skimming tube. The suspension is continuously fed in until a suitable depth of solids has been built up on the wall this deposit is then removed either by hand or by a mechanical scraper. With the bowl mounted about a horizontal axis, solids are more readily discharged because they can be allowed to fall directly into a chute. 

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