LynFlowTM Tuned Reeds Help Discharge from Storage Bins
by Lyn Bates, AJAX Equipment Ltd., U.K.
Many bulk materials offer flow problems such that efficient production is impeded by flow stoppages, erratic discharge, ‘flushing’ of fine materials or variable density condition on discharge. ‘Hammer rash’ is still common in many solids handling installation, where deformed hopper makes discharge even more difficult. A common method of stimulating flow in hoppers is by the use of vibrators, but their effectiveness is greatly handicapped by the difficulties of applying the vibration to where it is most needed. Usually this is wanted where the surface of an ‘arch’ forms over the outlet, or to break down a ‘pipe’ through the central region of the stored material so that the mass to deform freely. Vibration applied to materials that are confined often results in the bulk gaining strength and then being even more difficult to discharge. This is particularly true if the material is not in a condition of movement as vibration then tends to consolidate the bulk and makes it more difficult to flow. Apart from structural damage that can follow crudely applied vibration, a further consequence 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 causes material to flake away from the inner surface of ‘ratholes’ that extend from the hopper outlet up 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. To counter these 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. Excessive energy is often applied to counter this effect. Instead, the vibration can compacts material in the larger cross sectional region that does not initially present a difficult flow problem. Crudely applied vibration usually does more harm than good. Brute force is not the answer.
The operation of ‘LynFlowTM Tuned Reeds’ rests upon two fundamental principles, one of basic mechanics and the other of powder 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 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 effect elsewhere. The second feature is that the reed acts as a flow insert to alter the regime of flow near the hopper outlet. Flow inserts of this offset type were developed by Ajax to create a circumferential flow pattern in conical hoppers by replacing the normal hoop stress present in a radially converging channel by a more favourable mode of bulk failure that generates wall slip at significantly reduced wall inclination. The difference is akin to the compressive failure of a thick wall cylinder of bulk material that is subjected to the external pressure of passive stress acting from the hopper wall compared to the bending of a deep curved beam subjected to the same forces as a uniform distributed load. Consequence are that flow takes place at lower stresses and mass flow is secured with less steep walls, so enabling the flow benefits of this mode of discharge whilst gaining extra storage capacity.
There are also other advantages. One is that passive wall contact pressures are reduced, and hence the resistance to slip offered by wall friction and causing less wear. A further benefit offered is that the reed accelerates de-aeration in powders in a fluidised condition. Flow difficulties that are associated with fluidity are countered by bringing the powder more quickly to a stable flow state, as the reed not only stimulates the expression of excess air, but also provides a preferential path for air to escape from the changing void volume as the material settles. It should be emphasised that these devices are not ‘magic wands’ that can cure the most intransigent flow problem but, by applying vibration directly to the region of flow resistance for effective arch breaking and also modifying the flow pattern to a more effective form, ‘LynFlowTM Tuned Reeds’ offer an efficient and economical way of improving the reliability and consistency of flow behaviour from bulk storage bins.
For further details contact sales@ajax.co.uk. Note that Ajax supply an advisory service on flow and powder behavioural problems and offer powder testing facilities to provide appropriate design data.
LynFlowTM Tuned Reeds Help Discharge from Storage Bins
LynFlowTM Tuned Reeds Help Discharge from Storage Bins
by Lyn Bates, AJAX Equipment Ltd., U.K.
Many bulk materials offer flow problems such that efficient production is impeded by flow stoppages, erratic discharge, ‘flushing’ of fine materials or variable density condition on discharge. ‘Hammer rash’ is still common in many solids handling installation, where deformed hopper makes discharge even more difficult. A common method of stimulating flow in hoppers is by the use of vibrators, but their effectiveness is greatly handicapped by the difficulties of applying the vibration to where it is most needed. Usually this is wanted where the surface of an ‘arch’ forms over the outlet, or to break down a ‘pipe’ through the central region of the stored material so that the mass to deform freely. Vibration applied to materials that are confined often results in the bulk gaining strength and then being even more difficult to discharge. This is particularly true if the material is not in a condition of movement as vibration then tends to consolidate the bulk and makes it more difficult to flow. Apart from structural damage that can follow crudely applied vibration, a further consequence 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 causes material to flake away from the inner surface of ‘ratholes’ that extend from the hopper outlet up 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. To counter these 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. Excessive energy is often applied to counter this effect. Instead, the vibration can compacts material in the larger cross sectional region that does not initially present a difficult flow problem. Crudely applied vibration usually does more harm than good. Brute force is not the answer.
The operation of ‘LynFlowTM Tuned Reeds’ rests upon two fundamental principles, one of basic mechanics and the other of powder 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 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 effect elsewhere. The second feature is that the reed acts as a flow insert to alter the regime of flow near the hopper outlet. Flow inserts of this offset type were developed by Ajax to create a circumferential flow pattern in conical hoppers by replacing the normal hoop stress present in a radially converging channel by a more favourable mode of bulk failure that generates wall slip at significantly reduced wall inclination. The difference is akin to the compressive failure of a thick wall cylinder of bulk material that is subjected to the external pressure of passive stress acting from the hopper wall compared to the bending of a deep curved beam subjected to the same forces as a uniform distributed load. Consequence are that flow takes place at lower stresses and mass flow is secured with less steep walls, so enabling the flow benefits of this mode of discharge whilst gaining extra storage capacity.
There are also other advantages. One is that passive wall contact pressures are reduced, and hence the resistance to slip offered by wall friction and causing less wear. A further benefit offered is that the reed accelerates de-aeration in powders in a fluidised condition. Flow difficulties that are associated with fluidity are countered by bringing the powder more quickly to a stable flow state, as the reed not only stimulates the expression of excess air, but also provides a preferential path for air to escape from the changing void volume as the material settles. It should be emphasised that these devices are not ‘magic wands’ that can cure the most intransigent flow problem but, by applying vibration directly to the region of flow resistance for effective arch breaking and also modifying the flow pattern to a more effective form, ‘LynFlowTM Tuned Reeds’ offer an efficient and economical way of improving the reliability and consistency of flow behaviour from bulk storage bins.
For further details contact sales@ajax.co.uk. Note that Ajax supply an advisory service on flow and powder behavioural problems and offer powder testing facilities to provide appropriate design data.
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