Re: Constant Flow Regardless Of Bin Level
I think there are far too many variables to be able to come to an agree or disagree conclusion by simply thinking about gravitational affects on flow from a container.
Anyone who has every done any shear testing of powders will know how their flow characteristics and bulk density can change at different stresses. These stresses are representative of the weight (and therefore also the height) of the powder in the container/silo/hopper. At higher stresses some powders can compact together and result in a more cohesive powder, this can then lead to bridging and funnel flow in the silo. So in turn greatly affecting the flow rate from the container.
So to conclude I would say there are far too many variables involved to be able to come to any simple conclusion is flow based independent of height in a container. I would suggest that the height is a very important influencing factor in the flow rate of the powder.
Regards
Ben ■
Untitled
Rats, that wasn't the side I was on. ■
Re: Constant Flow Regardless Of Bin Level
If dry materials would behave in a predictable and uniform manner, there would be no need for gravimetric feeders that compensate for changes in flowability and/or bulk density.
Regards, Delmar Schmidt
Melfi Technologies Houston
www.melfitechnologies.com
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Constant Flow Regardless Of Bin Level
In general, once flow has developed, the conditions of flow at the outlet of a hopper are not sensitive the head of material, any more than the pressure ultimately developes attains a limiting value independent of head of storage. Of course there is some variation when the level is or gets very low and may be an initial difference between 'first fill' discharge and that of a re-start condition.
Developed Mass Flow will generate consistent discharge conditions through an outlet, provided the head of material exceeds about three times the outlet size.
Non-Mass Flow is inherently unstable, but may be reasonably consistent with free flowing bulk materials, provided that the flow rate is not impeded by ‘dynamic structural arching’ as congested large particles jostle to approach an outlet that is relatively small.
At the other end of the particle size scale, cohesive restrictions, ‘air retarded flow’ and ‘slurping’ are causes of flow restriction or instability that are virtually independent of the head of material in the hopper, in fact 'flushing' is more probable with low heads than when there is a longer residence period of flow.
Contact lyn@ajax.co.uk for a diagram illustrating the influence on flow rate of particle size in relation to the size of the orifice. ■
Flow Rates
I think if a bin is allowed to empty without the introduction of a variable the operator is cosigning himself to factors which are assigned to the characterisitics of the materials processing through the bin.
I do not think that noninterviention by the operator, or the equipment designer is an option.
The introduction of vibration (as in the form of liquifuction on wet sand) alters the characteristic of the material and the material flows as a different consistancy than when no external force is applied.
The flow rate of a bin is obviously restricted by the discharge chute dimension. I would say for observation however that the rate of discharge of materials never reaches the optimal level of discharge due to bin design.
From what I've ovserved, bin configuration and design includes in its design ideas like angle of repose of a material, and assumes that once a material reaches this angle that external flow off the face of the material will occur.
What happens if vibrations are introduced that alter the consistancy of a product? How does this effect the frictional coefficients that, existing, restrict through frictional forces the flow of a material until a certain angle is reached.
My question goes back to: Is there a better way of designing and building transfer bins that if implemented will increase discharge rates and create increases in productivity and rates of transfer of bulk materials? ■
Re: Constant Flow Regardless Of Bin Level
I Agree with Lynn,
Granular material in a container or bin behaves similar to a block cave operation as material support is removed from the bottom.
Initially assume all material, in a container is at an equilibrium stress. No internal stress is observed which has an arch formation. As small granular material is withdrawn centrally, from an opening at the bottom, whose width is a few multiples of the grain size, a tentative arch begins to form above the opening. This tentative arch is defined by the shape of the maximum stress field trying to form a bridge or arch within the material. If small grains of material are removed from below, the critical arch shape may begin to falter as grains of material fall away, from within the arch, leading to its eventual collapse. If the grains are large enough and of interlocking angular shapes, with respect to the opening, then a stable arch will form.
This arch only needs sufficient head pressure to cause granular locking within the arch. An increase in the height does not add to the arch stress.
You can download the block cave simulation from our website to see this effect at: www.conveyor-dynamics.com,
Lawrence Nordell
Conveyor Dynamics, Inc. ■
Re: Constant Flow Regardless Of Bin Level
The less product in the silo the greater the condensation. Depending on the material this can have an efect on the rate of flow. ■
Constant Flow Regardless of Bin Level
A point of contention between collegues of mine is this. Through a fixed orifice at the bottom of say, a hopper bin, would the bin empty of its contents at a uniform rate. Say at a consistant flow of 5 ton/hr or whatever, depending on the characteristics of the medium, be it pellitized feed or grain, and the size of the restriction. The important point being does it flow at the same rate when the bin is 1 foot full as when it is 10 feet full.
Any thoughts?? ■