The characteristics of the fine fractions of a bulk solid usually determine the property values required for hopper design. Whilst these may comprise a relatively small proportion of the whole in a feed stream the effects of segregation or local preponderance of fines would cause the predicted flow circumstances to arise and potential risks would be incurred if the hopper were not constructed to accommodate all circumstances of operation. The shear measurements secured are used to calculate the critical orifice size for flow, rather than the slope of the hopper walls, which depends mainly on the wall friction value. Wall friction results for the fine fractions and the coarse should not differ if they are composed of the same material, so over-steep walls are more likely to be a consequence of a conservative design interpretation than the screening out of coarse fractions in the sample tested.

Addressing the basic question of how coarser particles will influence flowability is more involved. A crucial question is whether there are sufficient fines in the bulk to fill the voids. Crudely, it may be said that coarse lumps tend to increase the bulk density if the voids are full of fines, thereby increasing the flow pressure and the consolidating pressures as if the whole material were fines of the higher density value. If the voids are not full of fines there are spaces of weakness within the mass that reduces the ability of the mass to sustain an arch, but the prospect of structural arching should be considered if the lump sizes are large in relation to the orifice size.

Reverting to the original query, the position of a tester is initially placed in an awkward position if asked to design a hopper for a material that has only a small proportion of fines. His riskless option is to test under the worst conditions, but point out that these may not occur and that a different decision based on an experienced judgement may be made on a holistic assessment. An invitation for others to carry the responsibility will usually focus minds sharply. The obligation is to set out the risk of ignoring the possibility of fine accumulation. ■

Dear Subramanyam:

A small experience in flow properties meassurement in sugarcane bgasse (this is a set of individual particles, each one thin and long). This bulk solid have coarse particles related to a mesh number 12 (in sugar cane argot). The general review says that relation between coarse/fine particles have influece on mass flow; if the coarse material is high, the mas flow is no posible and the hopper mut be more vertical and including a big discharge orifice. The shear stresses, that promote continuos strain, start in the fine material non in coarse material. On the worts case I obtained mass flow in that case when the relation was 50% of coarse-fine (r=1); better results were obtained in fine and dry material. I dident found a rule related to the proportion or the sample size. The true is that coarse material is in detriment of the mass flow (agricutural fibrous materials), and is worst when the bulk solid is cohesive and have high moisture content ■