Weighing and dosing products in industry is a separate, quite extensive department of engineering knowledge. The issues of feeding loose materials, weighing in non-constant mode or building a precise weighing device can pose many problems for those who have to implement such projects. Using our experience, we meet the expectations of customers and provide ready-made, modern solutions that will work in any type of industry.
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Bulk material - definition and division due to loseness
Bulk material is a granular or brittle product in a state that enables to transport it mechanically. The most important properties of bulk material are bulk density (expressed e.g. in [kg/m3]), tipping angle (angle between forming and basis of cone created during free infilling of bulk material), granulation fraction, shape of granules, cohesion, and adhesion. In addition, bulk material can behave like a fluid, i.e. it can flow if the activation energy is exceeded. However, product segments do not change their shape during transport and in fact are preserved. All properties contribute to one, exceptionally important parameter – material looseness.
Bulk material can be classified as:
- well-flowing product (e.g. salt, corn, sugar)
- medium-flowing product (e.g. flour, soda)
- products that easily aerates with tendency for uncontrollable flow (e.g. carbonates, fly ash, gypsum)
- fluidizable product (e.g. cocoa)
- cohesive (compact) product (e.g. titanium white, stearates)
- adhesive (viscous) product (e.g. soot, pigments)
- abrasive product (e.g. quartz sand, silicon carbide)
- compressible product (e.g. chalk)
- brittle product (e.g. flakes, soluble coffee)
Comparison of dosing systems
|Type of feeder|
|Belt weighfeeder||Screw feeder||Vibrating feeder|
|Feed rate||20 – 40 000 [l/h]||1 – 1000 [t/h]||0,5 – 30 000 [l/h]||0,1 – 1000 [l/h]|
|Feeding accuracy||±0,5% of set value||±0,5% of set value||±2..3% of set value (volumetric feeder)
±0,5% of set value (gravimetric feeder)
|±0,5% of set value|
|Cleaning & hygiene||+++||+||+++||+++|
Volumetric or gravimetric screw feeder?
Volumetric feeders control flow by measuring a constant volumetric amount in time using the change of feeding tool velocity. In case of screw feeders, it is the rotational velocity of screw with center rod or spiral.
Gravimetric feeders are devices that can measure a weighed amount of material in time. Thanks to a feedback in the form of a scale, the controller receives a precise information on the fed amount of material. Gravimetric feeder is able to appropriately control the material feed regardless of change of bulk density or other parameters of a product.
Thanks to the application of modern control systems, the feeder operating in a batching mode will not only divide feeding into two stages: coarse feeding (e.g. 90% of a batch, material feed with constant velocity) and precise feeding (e.g. last 10% of a batch, material feed with velocity equal to 30% of full velocity), but also will take the results of previous weighing results in next batches and will disable feeding appropriately earlier. The use of constant adjustment algorithms is related to application of PID regulators and real-time control of material feed that aim to achieve appropriate flow.
Spiral or full screw (with center rod)? What kind of feeding tool choose in a screw feeder?
Spiral screw – the most often used solution preferred for hard-flowing, cohesive, and adhesive material. It is characterised by small surface of contact with product, which prevents depositing of the material on feedeng tool.
- hard-flowing material (e.g. flour, soda, granulates)
- small and viscous material and material that can collect on leading pin (e.g. cocoa, titanium white)
- material consisting of big segments that may collect and cause a great friction between leading pin and surface of through pipe
- material that requires much better hygienic conditions (e.g. food material)
Full screw (with center rod) – solution preferred for well-flowing material with high bulk density (above 1,200 [kg/m3]) and highly abrasive material. Center rod reduces the risk of uncontrollable flow occurrence. Additionally, it is possible to apply bearings on both ends of feeding tool.
- well-flowing and very loose material (e.g. salt, sugar)
- material that easily aerates with tendency for uncontrollable flow (e.g. carbonates, gypsum)
- highly abrasive material (e.g. quartz sand)
- brittle material (e.g. flakes, soluble coffee)