15.06.2010 | Author / Editor: E. McGee / Marcel Dröttboom
Sometimes, process changes require switching from gravity discharge hoppers to bin discharge or metered feeding. A thoughtful approach to screw discharging and feeding presents significant opportunities for engineers when faced with such changes.
Flexible plant operation is important in enabling companies to ‘flex’ their production to process new materials. In these cases a hopper previously used for gravity discharging a material may be required to store a new material and discharge it in a controlled fashion with either bin discharge or metered feeding. This article reviews the materials handling challenges.
Hopper discharging is where material is simply emptied into another vessel under gravity. Mass flow discharge is preferred where the flow pattern is such that there is a total slip of the contents on all wall contact surfaces during flow; and flow takes place over the entire cross-section of the hopper outlet. In practice flow is either mass flow or it is not. A key feature of mass flow is that no regions of storage remain static during a cycle of discharge. In the vast majority of cases, the hopper will need to have been designed to achieve mass flow for the particular material stored in the hopper.
However, discharge can be a prime source of segregation. Whatever the homogeneity of the bulk material filling the hopper, the flow pattern has a major effect on the product emerging from the outlet. Apart from relative motion between different zones within the hopper or indeed the major differences when a narrow central flow channel develops whilst the peripheral regions remain static until a near empty condition prevails, the flow conditions provide dilation of the bulk and opportunities for segregation to occur.
Screw feeders used to discharge from bulk storage should normally be designed as an integral feature of the hopper, in this case either a bin discharge screw or metering screw feeder. Their essential function is to provide a reliable discharge of the stored contents at the rate required. The favourable flow form offered by wedge-shaped hoppers, facilitated by a slot outlet, is often a main reason for using a screw feeder.
A common objective when fitting bin discharge screws is the enhancement of storage capacity by means of extending the outlet of a storage hopper to a long slot. An important feature of the design of a bin discharge screw, is the pattern of flow that it generates through the interface to the storage container. For a bin of mass flow design, it is essential that the total area of the outlet is active when the screw is extracting material, otherwise there may be a dead area that prevents a zone within the hopper from moving, whatever the hopper form.
To provide continuous extraction over the full axial length of the feeder, the screw geometry must be such as to...
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