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Filtration

Backflushing in comparison to siphon rotor

A view of the geometrical differences of the backflushing rotor in comparison to the siphon rotor is very easy. So the main difference is in the design of the connection between the actual filtration room and the backflushing chamber resp. siphon chamber behind the rotor bottom. While for the backflushing rotor this connection only guarantees the liquid exchange between both rotating process rooms on filter level, for the siphon rotor an immersing effect is an essential part of this design principle, because otherwise no siphon effect can be achieved.

For this reason such rotors have a siphon chamber with a significant larger diameter. The liquid exchange will be guaranteed by sloped connection holes near the rotor bottom from the real rotor area into the siphon chamber. The underlying siphon effect will be realized by the depth of immersion of the siphon peeler tube. This depth of immersion resp. lowering of the level is not necessary. So the diameter of this chamber can nearly be adapted to the diameter of the real rotor room. Process-related the edge ring diameter of the backflushing chamber is nearly on the level of the edge ring of the main rotor room. This results in interesting variation possibilities for the process. The edge ring diameter of the siphon chamber is only unessential above the filter level. This deliberately chosen design highly reduces the processing possibilities of the siphon basket

A direct comparison of the competitive horizontal peeler centrifuge concepts with siphon basket resp. backflushing rotor indicates significant distinctive features. So the siphon effect will be more than substituted by the essentially higher g-force factors of the Krettek backflushing design. Furthermore only with the backflushing rotor an effective backflushing is possible. Further this concept allows a variety of processing and constructional interesting solutions and therefore offers the user of the centrifuge a maximum of process variances / elasticity. And that „all inclusive”.

Design of backflushing chamber (left) and siphon chamber (right) at backflushing rotor and siphon basket with corresponding diving height of cake ∆h

Design of backflushing chamber (left) and siphon chamber (right) at backflushing rotor and siphon basket with corresponding diving height of cake ∆h

The Krettek horizontal peeler centrifuge with backflushing rotor has a huge number of exclusive features. Following only some features should be explained on basis of a comparison diameter of 1.250 mm. So the Krettek standard rotor presents the worldwide slimmest design and the highest g-force factor, refer to our so far possible investigations. The distinctive values are:

Z = 1500 x g as well as an excellent large filter area of A = 3, 5 m².

With a comparable diameter the siphon rotor has only a g-force factor of

Z = 1030 x g and a filter area of A = 3, 14 m².

The rather traditional siphon centrifuge design shows on the one hand a clear lower g-force factor, which cannot be compensated by the siphon effect if it is used. Also the very useful processing backflushing effect can only moderately be realized in the siphon rotor.

By the siphon rotor this chamber is design as a siphon and the diameter of edge ring of the siphon chamber is slightly less than the level of the filtration element.

The backflushing chamber of the backflushing rotor has no siphon design and the diameter of the edge ring is approx. the same as the edge ring of the rotor.

The fluid itself will generate in the backflushing rotor the necessary difference of pressure for the filtration Δp. Therefore the filtration pressure under the centrifugal force will be

formel1

That means for the Krettek horizontal peeler centrifuge ProCent type P125 B XXL with a rotor diameter of 1.250 mm and a g-force factor of 1.500 x g a pressure of approx. 23 bar will be achieved at a fluid density of 1.000 kg/m³.

Looking at the situation in the siphon rotor of a horizontal peeler centrifuge with the same rotor diameter and a g-force factor of 1.030 x g a pressure can be achieved only of

Pc = 1.000 Kg/m³. 1.030. 9, 81 m/s². 0,155 m

Pc = 15, 66. 105 N/m² = approx. 15, 5 bar

Through the siphon basket it is possible to generate an additional low pressure below the filtration element to increase the filtration performance at a constant rotor speed. Swivelling the filtrate peeler device into the siphon chamber, a level difference between the filtration element and the level of liquid in the siphon chamber will be generated. With this siphon effect it is possible to reduce the pressure until the vapour pressure of the liquid; the result is a vacuum below the filtration element. Thus the difference of the filtration pressure will be increase by the value of the low pressure of max. approx. 1 bar.

That means for example for the siphon centrifuge that with a g-force factor of 1.030 x g a fluid column of only 10 mm will be enough to create a low pressure of 1 bar.

There are two decisive restrictions of using an additional pressure in a siphon basket:

1. The limit for the additional low pressure is the vapour pressure of the fluid.

The vapour pressure depends of the boiling point of the fluid. At this point the vapour pressure of the fluid achieves the value of the ambient pressure of approx. 1 bar.

That means that only a filtration pressure of max. 16,5 bar can be achieved with the above mentioned siphon centrifuge. With our comparable Krettek horizontal peeler centrifuge a filtration pressure of approx. 23 bar will be achieved sole by a higher g-force factor .

2. The effect of the additional difference of pressure of the siphon centrifuge can only be used, as long as the area below the filter element is completely airtight to the ambient atmosphere. As soon as the first pore will be evacuated during the dewatering of the filter cake, air flows into the area behind the filter element and the siphon effect no longer exists.

 

Krettek Separation GmbH