Process Equipment Update 2 1
Figs. 1 and 2. The Promoflow unit consists of an expanded metal screen which is mounted on the inside of the hopper wall.
Promoting flow in storage bins
Gerry Wetton outlines the problems and solutions to managing flow in storage bins.
T
he 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
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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,
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