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Inverting filter – the wrong concept ?

In the pharmaceutical industry is a trend to use for dewatering preferable the sieve basket. The basket walll is perforated in clear defined distances, so that short flow paths of the separated filtrate through the basket wall can be guaranteed and so almost no solids can be deposited at the basket wall during spinning off the filtrate. Furthermore there is the possibility of an almost heel cake-free cleaning of filter cloth. To reach this effect nitrogen with a pressure of 6 – 7 bars will be blowed into the bore holes in the basket wall via nozzles located at the backside of the basket wall, in which the heel cake breaks and come off from the cloth. The removed residual heel cake will be conveyed into the product outlet and will be discharged. Favourably Doing this the sieve basket favourably is rotating only moderate.

rotor design pharma

Increased cake level for the purpose of identically throughput at inverting filter centrifuges with the effect of strongly increased cake resistance and hereby extended dehumidification time – shown on the example of Krettek horizontal peeler centrifuge PharmaCent PhS and inverting filter centrifuge at comparable diameter The more efficient Krettek backflushing design has a fully formed rotor wall and the filtrate, separated by the filter medium, flows over the unperforated rotor wall through bore holes, which are connecting both process rooms, into the backflushing chamber.

The different design of sieve basket and backfushing rotor is not only process-determind but also take influence on the g-force factor, reached during the centrifugation. The wall of the sieve basket is weakened by the perforation. So sieve baskets with same or similar dimensions cannot be operated with the same high speeds as the unperforated backflushing rotors. Nevertheless it succeeded to increase sustainable the g-force factor of the Krettek sieve basket by an optimized basket design,too.

In this context a residue-free cleaning specified by the CIP-guidelines also requires the absolute cleaning of the backside of the basket resp. rotor. This provides, that the rear basket bottom has to be freely visible. With identical design of the hinged housing the length of the basket resp. rotor cannot be realized in the same length. So the consciously extreme slim design chosen for lower cleaning requirements cannot be realized for pharma centrifuges.

Furthermore both designs are also different in placing the cloth. While in the sieve basket the cloth is integrated in a drainage element, in the backflushing rotor it is supporting on a slotted bridge. This provides a ring-shaped gap between filter cloth and rotor wall with its slotted bridges, via the filtrate can flow into the backflushing chamber.

Existing sources for germ formation and contamination at conventional fixings will be eliminated because there are no well-known clamping rings resp. dovetail grooves for fixing the filter elements.


Sectional drawing of the inverting filter centrifuge in discharge position

Sectional drawing of the inverting filter centrifuge in discharge position

This is completely different for the inverting filter centrifuge design. Due to the system, the filtration basket has a multi-piece design. For emptying the basket the filter cloth has to make a turn-up process under considerable fulling work. So a great number of seals will be stressed not only rotatory but also translational which implicate causally in an increased abrasion and product contamination. Due to its design the inverted filter baskets have to be provided with thread. This automatically results in a great number of gaps, undercuts and dead spots. Also a visual inspection in the area of basket bottom as well as translational influenced hollow shaft cannot be made without dismanteling of the inverting filter centrifuge.

Krettek Separation GmbH