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Specialist article
01.01.2020  |  571x
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Overcome poor flow in hoppers with Lynflow™ ‘Tuned Reeds’.

Stimulating the flow of bulk materials in hoppers by the use of vibrators is greatly handicapped by the difficulty of applying the vibration to where it is most needed. A force is required to overcome the unconfined failure strength of the bulk material where an ‘arch’ forms over the outlet or to break down the wall of a ‘rathole’ that has developed through the central region of the stored material. Vibration applied to a confined loose solid usually results in the bulk gaining strength and then being more difficult to handle. This is particularly true if the material is not able to flow. Apart from this, problems of noise, fatigue, failure of welds and possible resonance or vibration affecting other local plant, a consequence of crudely applied vibration, is that flow is often generated at the expense of flow consistency. This is because a bulk mass in a firm condition tends to break down erratically from unstable arches or flake away in surges from the inner surface of ‘pipes’ or ‘ratholes’ that extend from the outlet opening to the top surface of the stored material.

The most common way of fitting vibrators is to attach them to the outside walls of hoppers. Their effect is to shake the contents around the fixing bracket and for some distance into the mass. Unfortunately conical hoppers are very stiff in the lower regions where the radius of curvature of the metal is smallest and the proximity of the outlet flange provides high local stiffness, but this is precisely the region where vibration is needed. To counter these stiffness features, the tendency is to attach the vibrators some distance up the hopper wall and its influence is diminished in the crucial region near the outlet so the vibration tends to compact material a region that does not initially present a difficult flow problem. Crudely applied vibration can do much more harm than good.

A Solution

The economical and efficient performance of ‘LynFlowTM Tuned Reeds’ is based on two fundamental principles; one of basic mechanics and the other of bulk technology. The first is that a cantilever mounted rod or bar can be tuned to vibrate at the resonant frequency of the applied vibration. This allows vibrations to be efficiently carried from one end of the member to the other. A vibrator mounted on a bracket external to the hopper wall thereby transfers energy to the tip of a vibrating reed projecting into a critical flow region, whilst minimising the disturbing effect elsewhere.

The second feature is that, apart from imparting vibration directly to a region where it can be most effective, rather than ‘second hand’ through the container walls, the Lynflow™ tuned reed also acts as a flow insert to alter the regime of flow near the hopper outlet. Flow inserts of this offset type develop a circumferential flow pattern, replacing the normal hoop stress present in the radially converging channel of a conical hopper by a more favourable mode of tensile failure and also stimulates wall slip at lower wall inclinations. The circumferential region of the hopper contents that is shielded by the reed acts as a ‘hoop stress cutter’. Blocking flow from this portion of the periphery leave a peripheral void that allows the residual circumference to fail as a curved beam in bending, rather than as a thick wall cylinder subject to external pressure, thereby failing in tension at a much lower stress than in the compressive mode of radial flow.
It also reduces the wall contact pressures, so lowering the resistance to slip offered by wall friction. A larger proportion of the potential energy that is available by loss of height is therefore used to generate bulk deformation.

Another application of the reeds is to reduce the risk of flushing of excessively aerated powders. Loose flow difficulties associated with fluidity are addressed to bring the powder more quickly to a stable flow state by applying oscillating forces to compact the mass whilst the underside of the inclined blade provides a preferential escape path for excess air to escape from the voids.

A word of Caution.
These are not ‘magic wands’ to cure impossible situations, but a genuine retrofit technique to improve flow and resist ‘flushing’.

For further details contact Ajax equipment Ltd. Mule Street, Bolton. BL2 2AR.
Tel. 44 (1)204 386723 or

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