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Figure 5: Sample Preparation of Powder in Shear Cell
This particular event identifies the failure strength of the powder, for example, the friction level at which the powder particles are finally able to slide against each other. This same test methodology is repeated at increasingly higher compression loads applied by the vane lid. The resultant graph from the complete test (see Figure 7) shows how the failure strength of the powder changes with increasing consolidation stress.
Pressure buildup in powder stored in a storage vessel is the phemomenon that shear cells are able to simulate. The amount of pressure correlates directly with the height of the powder. The self-weight of the powder bearing down on itself is what causes consolidation in the vessel. The vane lid presses down on the powder sample, simulating the self-weight condition that causes the settling action. This is the key difference in test approach between the shear cell and simpler test devices, which only measure flow behaviour for loosely consolidated powders.
Figure 6: Vane Lid Used to Compress and Shear Powder Sample
One of the food processors in the development project organised by The Wolfson Centre actively tested a variety of flavourings with the new generation shear cell instruments. The results turned out to be especially promising for a number of reasons. Small batches of powder for new formulations (different tastes, slight changes in ingredients) could now be tested for flowability before delivery to the customer for evaluation. Scale up to production quantities could be investigated and qualified using a lab instrument to verify whether modifications would impact processibility. The obvious gain from these developments is the reduce cycle time for launching new products.
Figure 7: Flow Function Graph for Multiple Powder Samples
One important piece of data that is generated by the shear cell along with the Flow Function is the density graph for the powder. As the consolidation pressure in the sample increases, the density of the powder goes up as well. Figure 8 shows a typical density curve for a powder sample. The increase in density is most noticeable at the beginning of the consolidation process. Compressibility of the powder is a characteristic that indicates potential for flow behaviour problems. Typically, powders that are compressible to 50% of their original loose-fill volume will experience flow behaviour issues. The data points on the density graph at the highest consolidation stress values may be correlatde with the data from another test discussed earlier, the Tap Density Test, as can be seen in Figure 8. The advantage of the shear cell test is that repeatability is more predictable.
Figure 8: Density graph for Powder Sample
Challenges facing the bulk solids industry today still include the unforeseen jams and stoppages that can bring operations to a halt. Key to a major step forward is the real need for education in order to deliver companies from the simpler, but traditional, practices of flow cups, angle of repose, and tap density.
The new era of powder processing demands a test method that truly models the flow conditions for powder stored in any type of container, whether a large silo, a small feed device, or stacked sacks of final product in shipment.
Shear cell technology is the new solution for today’s processors because the test conditions correctly simulate what happens to powders.
Interested in publishing a Technical Article?
Contact Gwyneth Astles on +44 (0)1727 855574 or email:
gwyneth@intlabmate.com
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