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Filtration

Filtration Equipment Selection Criteria

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This technique also has another major advantage. The residual heel remains as a layer of sediment in the basket after the product has been scraped out, because the blade must not come into contact with the filter medium. Over time, very fine particles accumulate in the heel. On the siphon version, it is possible to rinse out the heel from below. In contrast to the perforated basket version where a significant amount of effort must be invested periodically to remove the heel, the heel can remain in the basket for a much longer period.

The pusher centrifuge

Fig. 2 shows the continuous process inside a pusher centrifuge. The product is introduced through the feed pipe concentrically to the basket’s rotational axis and it is distributed on the floor of the basket. It forms a cake ring on the basket perimeter, and the oscillating motion of the pusher bottom forces the product towards the open end of the basket. As more product flows in, a new ring forms which pushes the previous ring further along. The individual rings move along the basket shell until they exit from the basket and centrifugal field.

If you look closely at the two techniques, you notice differences not only between the way the machines work, but also in their suitability for different types of product. Because cake formation must take place within a fraction of a second to ensure that the cake is pushed along, filtration speed in the cake and the filter medium must be high. Speed tends to increase if the solid particles and filter media are coarser. If the amount of fluid at the input is low, lower filtration speeds can be tolerated.

Fig. 6 shows the required relationship between solid content and particle size as well as the operating range of both types of centrifuges.

Besides differences in product requirements, there are also differences in the performance data for the two machine types. Horizontal centrifuges are more tolerant to variations at the input. If for example the mode of operation of the entire system or even only the upstream equipment goes through certain cycles, the mode of operation of the centrifuge can be modified accordingly. Because the cake remains statically resident in the basket, there is a wider choice of filter media, especially fine media. This minimizes solids loss which enhances the productivity of the centrifuge and prevents solids from entering the filtrate which could cause problems downstream.

Due to the very small particle size as well as the high filtration resistance, long residence times do not cause problems in peeler centrifuges, and cycle times can be varied as needed.

By direct comparison, requirements relating to product characteristics and constant feed conditions on peeler centrifuges are more demanding, but solids throughput rates are high on these machines, and they support continuous operation. The price-performance ratio from the solids throughput perspective is hard to beat in the filtration centrifuge product space.

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