The problem of promoting flow from storage bins should not be underestimated. While the use of various flow aid devices is well established in many industries, their effectiveness in many applications has been unsatisfactory. Consequently, it is not surprising for process operators to encounter problems, such as 'bridging' and 'rat-holing'. And, therefore, insufficient flow rates.
The basic problem
We know that material in silos and storage bins develop strength as pressure is imposed upon it. The greatest pressure occurs on the walls of the hopper near its attachment to the bin. 'Bridging' for example usually occurs in the lower region of the hopper, because the area for most hopper sections decreases faster than the diminution of material strength as it moves towards the outlet.
Most flow aid devices are vibratory in nature. This random motion to the particles makes it difficult for bulk materials to develop any serious strength and as the particles are in a state of mobility, they have little difficulty of travelling to the hopper outlet.
A more serious problem occurs within the hopper when the bulk material is unable to discharge, while being vibrated or if the duration and/or magnitude of vibration are too great. This can result in material strength being increased to abnormally high values in the lower regions of the hopper and lead to compacting. Getting flow to start again may require considerable manual effort.[Page Break]
Popular flow aids
The most popular flow aid is the Bin Vibrator; either Rotary Pneumatic or Electric Out of Balance motors, due to its low cost and simplicity of installation.
Unfortunately, the bin vibrator is very inefficient owing to the way it is installed. This is because the Bin Vibrator is mounted directly to the wall and consequently most of the energy is absorbed by the walls and not by the material in the hopper. What vibration intensity does reach the material is often insufficient to be of benefit.
As vibration from a Bin Vibrator cannot be focused within a particular area of the hopper, vibration can be generated in the upper section of the hopper, causing additional compaction of material in the lower region of the hopper. These units are also rather noisy, as they cause structural vibration of the hopper/storage bin.
Aeration is another solution used to overcome material flow problems. This is where air is introduced into dry normally free flowing products. These systems can be in the form of: Aeration Pads; Magic Mushrooms and Air Canons.
The disadvantages of aeration products are that the control of product discharge can be difficult. Aerated materials can act more like liquids than solids and can easily flood process systems such as screw or belt conveyors. When feeding bagging systems with aerated products a greater volume of material/air is introduced into the filling bags, rather than the required product weight.
The Live Bottom Bin Activator is widely used as it imparts vibration up through the material mass and allows a controlled discharge which is determined by the size of its internal dome/cone width.
The disadvantages are: the units are ideally suited as a 'built in' unit on new silos; in the case of square or rectangular silos, the Bin Activator has to be oversized to take into account the dimensions over the diagonals of the silo; the Bin Activator does not lend itself to being installed retrospectively, as it would involve major and costly structural modification to the silo.[Page Break]
The ideal discharge flow aid device should have:
- A controlled amount of material agitation in the lower regions of the hopper, but not provide agitation in the upper regions.
- Maximum efficiency is achieved by transferring energy into the material, not into the hopper walls.
- It would provide the correct amount of vibration so as to provide sufficient particle mobility to assure flow, but not cause compaction. It would be quiet in operation.
- The unit would be serviceable externally to the hopper.
- The unit could be retrospectively fitted to any size, shape or configuration of hopper within a few hours.
Expanded metal screen
The Promoflow unit consists of an expanded metal screen which is mounted on the inside of the hopper wall. The shaker support frame is rigidly mounted to the metal screen using interconnecting bolts and spacer assemblies. The assemblies pass though the hopper, allowing the shaker to operate external to the hopper. Options exist for water cut stainless steel 'Rib Cage' screens for use with larger particle materials.
The screen movement is typically 3mm to 4mm which is sufficient to generate a shearing action. This undercuts the base of any 'bridging' formation within the hopper.
Because the screens are open to the passage of material, the hopper walls and not the screen take the majority of material load and allow efficient utilisation of energy.
The internally mounted screen would extend from the outlet up to approximately one third of the length of the sloping wall of the hopper. Various screen shapes and sizes are available to satisfy the requirements of most hoppers.
The Promoflow unit is an ideal retro-fit solution where product discharge is problematic. Its versatility and application flexibility allows it to be fitted to any size, shape or configuration of hopper within a few hours; without any major structural modification and without loss of hopper/silo volume.
When used in conjunction with a control timer unit, product discharge can be adjusted to suit the requirements of the downstream process. This also reduces the air usage and hence the running cost of the unit.
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Gerry Wetton is with Process Components Ltd, Macclesfield, Cheshire, UK. www.pro-components.com