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. This extra force is essentially required to overcome the unconfined failure strength of the bulk material where the surface of 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, to encourage the mass to flow freely. 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, and problems of noise, fatigue failure of welds and possible resonance or vibration affecting other local plant, a further 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 to flake away 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 whole 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 the vibration is needed. To counter these stiffness features, the tendency is for the vibrators to be attached some distance up the hopper wall, where its influence is diminished in the crucial flow region nearest to the outlet. Instead, the vibration tends to compact material in the larger cross sectional 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’ rests upon 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 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 were developed to create a circumferential flow pattern, by replacing the normal hoop stress present in the radially converging channel of a conical hopper by a more favourable mode of tensile bulk failure that generates wall slip at significantly reduced wall inclinations. The outlet and circumferential region of the cross section that is shielded by the reed generates preferential flow, to acts as a ‘hoop stress cutter’. The residual circumference fails as a beam in bending, rather than as a thick wall cylinder subject to external pressure, generating tensile, instead of a compressive failure mode.
This weaker cross section in the cone therefore deforms more readily, to also reduce the wall contact pressures and lower the resistance to slip offered by wall friction. A larger proportion of the potential energy available for flow is therefore able to generate bulk deformation.
An alternate application of the reeds is to reduce the risk of flushing by 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 the mass and providing a preferential escape path for excess air to escape from the reducing void volume as the material settles.
For further details contact Ajax equipment Ltd. Mule Street, Bolton. BL2 2AR.
Improving Flow in Conical Hoppers
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. This extra force is essentially required to overcome the unconfined failure strength of the bulk material where the surface of 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, to encourage the mass to flow freely. 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, and problems of noise, fatigue failure of welds and possible resonance or vibration affecting other local plant, a further 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 to flake away 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 whole 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 the vibration is needed. To counter these stiffness features, the tendency is for the vibrators to be attached some distance up the hopper wall, where its influence is diminished in the crucial flow region nearest to the outlet. Instead, the vibration tends to compact material in the larger cross sectional 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’ rests upon 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 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 were developed to create a circumferential flow pattern, by replacing the normal hoop stress present in the radially converging channel of a conical hopper by a more favourable mode of tensile bulk failure that generates wall slip at significantly reduced wall inclinations. The outlet and circumferential region of the cross section that is shielded by the reed generates preferential flow, to acts as a ‘hoop stress cutter’. The residual circumference fails as a beam in bending, rather than as a thick wall cylinder subject to external pressure, generating tensile, instead of a compressive failure mode.
This weaker cross section in the cone therefore deforms more readily, to also reduce the wall contact pressures and lower the resistance to slip offered by wall friction. A larger proportion of the potential energy available for flow is therefore able to generate bulk deformation.
An alternate application of the reeds is to reduce the risk of flushing by 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 the mass and providing a preferential escape path for excess air to escape from the reducing void volume as the material settles.
For further details contact Ajax equipment Ltd. Mule Street, Bolton. BL2 2AR.
Tel. 44 (1)204 386723 or sales@ajax.co.uk. ■