be followed to support reliable (gravity) flow. Whatever the feeding device, the flow channel it generates must also suit this critical dimension to support mass flow discharge from the vessel (i.e. when a screw, the width of the screw should be slightly wider than the minimum outlet dimension used on the vessel). In the case of a metering screw feeder this could entail the use of a larger than standard diameter unit – the drawback of which being that the delivery of material from the screw would exhibit a greater instantaneous fluctuation. A reduction in the magnitude of pulsation is possible
through double flighting at the delivery end of the screw. For many processes, this may be acceptable but, for higher accuracy applications, there may be a need for a multiple screw set, which is arranged 180° out of phase for a dual arrangement, and which would deliver smaller fluctuations in dosed weight. For many systems, economics will be driving the decision- making so the most common types of screw feeder tend to feature large cross-sectional area inlets in which flow is maintained through agitation of the bed. However, although the volume of the chamber above the screw is swept to prevent consolidation of material, the flow channel may still be dictated by the available transport capacity of the screw feeder.
A COMPLETE DRAW-DOWN Since virtually all modern industrial screw feeders feature a constant pitch spacing, this dictates that irrespective of how large the outlet of the specific vessel is, the proportions of the flow channel will be dictated by the available capacity of the feeder involved. The establishment of a preferential flow channel effectively ensure that the vessel will operate in core flow, even if designed for mass flow so this will encourage a draw from the upper region of the vessel. Thus, not only does material that has had a minimal residence time get drawn into the screw, this material will be in a highly dilated state. These two effects result in poor particle packing in the screw and thus, a low (and variable) bulk density fill of the pitch – giving rise to dose inconsistency. As inventory levels reduce in the feed vessel it is not unusual to find that the bulk density can drop significantly. In order to obtain an activation of the cross-sectional area of a vessel the feeder should be able to offer an increase in transport capacity along the length of the outlet from which it is drawing material. In the case of a screw feeder, this is achieved through the construction of design that combines and increase in pitch spacing in conjunction with a reducing shaft diameter. The simple design feature of increasing the transport capacity enables a complete draw-down to develop –
a pre-requisite for supporting mass flow. This is a mass flow vessel interface: the flow pattern will allow first in – first out stock rotation and by virtue of the flow channel now being the full area of the outlet, bulk density variation is minimal and segregation effects are significantly reduced.
An important and significant benefit of mass flow discharge on to a screw feeder is that the particle packing will potentially be more consistent than it would for material fed through a system that relies on agitators rotating along the axis of the screwfeeder. An example of the potential for inconsistent particle packing would exist where a variable speed screw is used in conjunction with a fixed-speed agitator. In such a situation the proximity of the agitator blade to the material already being transported within the screw can transmit additional stress into the powder. At high feed rates the frequency of compaction effects will be relatively low, but at trickle feed conditions the compaction frequency will be higher – giving rise to weight fluctuations. This cyclic additional packing of material into the feed
screw is likely to contribute to a build up of material on the screw which leads to the need to run the screw faster to obtain a comparable transfer rate over time, which decreases the consistency of particle packing if the draw is preferential – giving rise to greater inaccuracy. It is hoped that this very brief look at some of the
problems associated with accuracy in feeder operations may provide an insight into some of the more common issues in processes. Some of the principles discussed are transferable to other types of controlled feeding systems (such as belts), but space limitations preclude presenting this information in a single article.
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