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Rotary / Centrifugal Sifters

High capacity rotary sifters for bulk solids materials (powder, pellets, granules, pills...)

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Section summary
1. Introduction
2. Position in the process
3. Important design considerations
4. Product impact
5. Applications of rotary sifters
6. Common problems with rotary sifters
7. Rotary sifters manufacturers


Rotary sifters are used in bulk solids industries to sieve powders at high capacity, allowing to let through the sieve good product and reject foreign bodies larger than the sieve size. Although very efficient, those equipment must be operated with specific safety precautions. This page is focusing on the design and operation of centrifugal sifters.

1. Introduction

What is a rotary sifters for bulk solids ?

Rotary sieves are composed of a feeding section, most often a short screw conveyor, a sieving area, and 2 material discharges, one for the material on specification and one for the rejected material.

The shaft of the metering screw is prolonged through the sieving section and fitted with long paddles. Those paddles, close to the sieve, are moving the material along the sieve and force the product through the sieve. At the undriven side of the sieve, no more material should be present, only eventual foreign bodies which are then pushed to a rejection chute. Good product that goes through the sieve is falling to a conical discharge, to the next unit operation.

Rotary sieves are quite popular for sieving powders at high capacity in a relatively small footprint compared to vibrating sieves, they also allow to, through the high velocity of its rotary paddles, to break small clumps of product.

The working principle of a rotary sifter is summarized on the figure below :

Rotary sifter for bulk powders

Figure 1 : typical design of a centrifugal sifter

2. Position in the process

For what purpose are used rotary sifters ?

Rotary sifters are most often placed at the beginning of material handling processes in order to reject foreign bodies that may be present in the powder. They thus allow to protect the downstream process and ensure the quality of the final product.

Some factories place them on the contrary at the end of the process, to check the final product. It is however only possible if the final product characteristics are not sensitive to the mechanical stress that rotary sifter apply to the powder (it can lead to particle breakage).

Rotary sifters can deagglomerate materials, if the agglomerate is not too hard, those machines are thus sometimes designed and used for this purpose. It should however be noted that the lumps should not be too hard nor present in too high proportion in the feed. If it is the case, dedicated lump breakers can be more adapted.


3. Important design considerations

3.1 Sieve / Grid

Rotary sifters can be equipped with wire mesh sieves of different sizes (smaller sizes are typically in the range 1-4 mm, with some applications requiring < 1 mm) and can be made of different materials like plastics (nylon) or stainless steel.

It is also possible to have a perforated grid instead of a wire mesh, which allows to have more mechanical resistance and thus reduces the risks of sieve breakage, but which also reduces the passing area and thus the throughput of the sieve.

The selection of the sieve type and material must be discussed with the supplier and done in accordance with the application (product, capacity, size required).

3.2 Easy access

In order to inspect and clean the rotary sifters, accesses are foreseen. Side hatches are used for quick inspection, while front access, on the undriven side, allow to extract the sieve and the shaft (for some models) which makes easy a complete inspection and cleaning of the sifter's rotating elements, sieves and body. For this purpose, rotary sieves allowing an easy extraction have often a cantilevered shaft.

3.3 Safety

Occupational Safety

Rotating sieves must never be accessed in operation, the high speed rotation of the shaft and paddles present a hazard. All access must therefore be guarded by electromechanical locks that are cutting the power of the drive if opened. Guarding must also be in place at the inlet of the sifter, and at the outlet (good product and rejection) to avoid that an operator manages to introduce its hand in the machine running, for example in the event of a material blockage.

Dust explosion prevention

As they rotate at high speed, centrifugal sifters are subjected to risks of powder explosion. Those risks can be of different nature :

  • Mechanical spark due to the presence of foreign body in the feed : as the paddles are most of the time rotating with a tip speed >1 m/s, a metal foreign body that would be trapped in between the rotor and the sieve could generate sparks. It is quite difficult to prevent as the sieve's purpose is basically to remove foreign bodies. One way can be to have a strong magnet at the inlet of the sieve to block metal to enter the sifter, and to ensure a gap sufficiently big in between the rotor and the sieve. Such measure efficiency must of course be confirmed through a proper dust explosion risk assessment.
  • Autoheating : In case of blockage in the sieve (at the product outlet or at the rejection outlet), the rotor can heat up the material trapped in the sifter and create a fire. This should be prevented by a proper instrumentation allowing to detect a blockage : level sensor in both outlets, amp meter on the drive to detect that product resist too much the paddles, sign of a blockage

Regular good practices (air purged bearing seals, grounding, use of electrical equipment certified for the zoning determined...) must of course be applied. All measures must be documented in the risk assessment of the installation.

3.4 In line sifting

Some manufacturers have designed special version of their sifters to be implemented in pneumatic conveying system, pressure or vacuum, dilute phase. It allows to gain space as the sieve can be placed along the conveying line and can be used for several products feeding the conveying line. Precautions related to dust explosion explained above are all the more important when the sieve is in line (risk assessment mandatory)

3.5 Sifter capacity

The capacity of rotary sifters is highly dependent on the material and mesh opening as well as the size of the sifter (screen area). There is therefore a wide range of capacities reported by suppliers, from few hundred kg an hour, to 100 t/h.

4. Product impact

Rotary sifters are using paddles rotating at high speed to make the material go through the sieve. As a consequence the stress applied may lead to deagglomeration of the particles in the feed, which may be desired since quite often rotary sifters are used to deagglomerate bulk solids, or undesirable if it breaks down too much fragile materials to the point of making it lose some specific properties.

5. Applications of rotary sifters

Rotary sifters can be applied to any powdered material that must be checked for the removal of potential foreign bodies :

  • Food processing : Flour sifting, milk sifting, starch sifting, spices, bakery mix delumping, dried fruits delumping, sugar sifting
  • Chemicals : cosmetics sifting, nylon deagglomeration, detergent sifting, phosphates sifting,

6. Common problems with rotary sifters


The following problems may be experienced by centrifugal sifters :

Table 2 : common problems with rotary sifters

Issue Root cause and action
Too much rejection Sieve opening size too small
Sieve is clogged
Feeding throughput is too high
The material has changed
Sieve breakage Sieve used for a too long time
Sieve not inspected
The sieve is getting clogged which is increasing the pressure on the sieve

7. Rotary sifters manufacturers

In order to get quotes, prices of centrifugal sifters, the reader can refer to the following companies :

https://www.palamaticprocess.com/bulk-handling-equipment/centrifugal-sieve

http://www.stolz.fr/en

(Note that PowderProcess.net has no link with those companies)