Vibrating Sieves for powder and bulk solids
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|Round vibrating sifter||The most common type of vibrating sieve. The sieving deck has a circular shape and equipped with a single screen. They are designed for removal of foreign bodies.|
|Rectangular vibrating sifter||Rectangular sieve present the advantage to fullfill 2 functions : sieving, but also transporting laterally the powder. They can be an economical solution to avoid a setup (screw conveyor + round vibrating sieve). Their operation need however to be well mastered. They are designed for removal of foreign bodies.|
|Classifier||Classifiers are generally round sieve, with several sieving decks of different mesh opening. They are designed to select particles of a specific size.|
Round vibrating sieve are very widespred in process industries. They provide a compact solution for checking all types of bulk solids, from raw materials to finished product, regarding the presence of Foreign Bodies (contaminants).
Round sieves are generally made of the following parts :
- Cover, including inlet, inspection ports and rejection outlet
- Sieve deck, the part actually performing the sieving
- Cone / underpan, located right below the sieve and collecting the powder sieved to direct it to the next process step
- Frame and clamping system, allowing to keep firmly together the different components despite the vibration
- Unbalanced Motor (1 or 2), the driving force for the vibration
- Supporting and springs (sometimes rubber dampers) allowing to stabilize the sifter during the vibration
The sieve is made in 2 parts for round vibrating sifters : a supporting ring and the actual sieve, which can take the form of a wire mesh or a perforated plate, welded or bonded on top of it.
Chosing the detail design of the sieve will depend on the
- For non hygienic applications (chemical...) : welding the wire mesh or perforated plate to the supporting ring is sufficient
- For hygienic applications (food, chemicals...) : a plastic bonding (epoxy most of the case) is generally more hygienic has it will avoid that material is trapped in between the sieve and the ring. However, the epoxy must be accepted by the loca regulation (FDA approved...), should be durable and not be brittle.
Defining if a perforated plate or a wire mesh should be used will mainly be dependent on the throughput expected from the sieve. Indeed, perforated plate are much more durable and safer (less risk or breakage leading to a pollution of the product) but the reduced passing area vs a wire mesh means that at constant throughput, the sieve has generally to be larger (costs, difficulties of implantation...). If wire mesh needs to be chosen, some mitigations to the risks of breakage can be done by chosing a durable material, magnetizable (Stainless Steel 430 or 318), so that it can be retriebed by a magnet positionned after the sieve in case of incident.
The sieve side is generally covered by a gasket which allowed to avoid that any foreign body bypass the sieve.
The cover of vibrating sifters play a bigger role than usually perceived. It ensures the dust tightness, but also helps to distribute the powder properly to the sieve decks. Inlet of product should not be close to the rejection, otherwise it will lead to the rejection of some acceptable material due to the fact that the actual sieving area will be reduced. Inlet should be positionned at the opposite of the rejection, or at the center of the sieve. If still not sufficient, it is possible to include a deflector welded to the cover that will direct the flux of material away from the rejection. The deflector can also have an effect to protect the sieve from the impact of the product and thus increase its life span.
Inspection ports should also be positionned on the cover. To have a good vision of the sieve from the inspection port, a minimum of 3 ports are necessary. They are closed by rubber pads when not in use.
The last fitting important on the cover is the rejection, which can be more or less obstructed by a wear plate. The wear plate leaves a little space only for foreign bodies to exit to the rejection. Wear plate are used mainly in case the sieve is not properly sized and product accumulates on the sieve desk. The wear plate then avoids to have too much rejection. But it can only be viewed as a mitigation measure and design should done considering enough sieving area to let all the product go through without flooding. The rejection should lead the overs to a tight container, a BFM connection is good tool for this purpose.
The cover must be clamped strongly to the sifter in order to avoid any friction that could lead to heating, damage, or leakage of product. Different designs exist, depending on the application of the sifter : quick clamp can be used when cleaning requirement are high, otherwise, a simple ring clamp will be efficient.
The vibration is performed by an unbalanced motor (sometimes 2). Changing the unbalance will have an effect on the amplitude and direction (when 2 motors) of the vibration. Unbalance should only be changed under the direction of the manufacturer to ensure that the sieving is optimized.
Vibrating sieves' vibrations is strong, and amplitude is particularly important when the sifter stops (although some system now exist to reduce such vibrations). The support must be designed to handle such vibration and avoid to transmit it to the rest of the installation.
Due to vibrations, static electricity can build up. As a consequence, the sieve must be well grounded (manufacturer provide grounding cables). Usually metallic clips are added on 3-4 points of the gasket wrapped around the sieve deck in order to ensure this electrical continuity. Having those clips in place is very important and operators must have a checklist to make sure they have been mounted.
Vibrating sieves are a key components in a powder process to ensure reliability, safety of the installation and safety of the product.
Please follow the link to get access to vibrating sieves design details : All you need to know on industrial vibrating sieves for powder checking
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