Bulk materials handling is by far the largest industrial activity, with over 16 billion tons of common products being handled annually, usually many times from source to use, (1), most of which is undertaken by mechanical equipment. This massive scale and enormous variety of products inevitably raises many potential operating problems that the industry has to address.
Belt conveyors are the work horse of large-scale/long-distance handling, with screw type equipment widely used in plant and process operations. Belt conveyors are continuously undertaking longer transport distances and developing techniques for accommodating undulating terrain and curved routes, with pipe and cable conveyors achieving better containment and extended run lengths. As the product is carried by a moving surface, the main interactions of bulk material properties with belt conveyors are the stability of repose conditions, particularly on inclined units, and any tendency for the material to stick to the belt. By contrast, mechanical conveyors that promote the movement of bulk material by means of a sliding, (screws), or pushing action, (scraper, wire, and disc, etc.), the physical properties of the bulk material is a crucial factor. The work of Jenike on hopper design developed a wide understanding of the deformation characteristics of loose solids and the importance of ‘wall friction’, ‘critical state’, and ‘active and static stresses’, in relation to how a bulk material will behave in given circumstances. Test equipment used for measuring shear strength and wall friction that were developed for hopper design are also valuable tools for evaluating the handling properties of bulk material. The recent introduction of the Brookfield tester, developed under guidance from the Wolfson Centre of the University of Greenwich for securing these values, make the procurement of these measurements more economically attainable.
Conveying is intrinsically linked to feeding, as the rate of transfer must be regulated to within the conveying capacity. Belt conveyors sometimes include a feed section and screws are widely used for this purpose, although the subsequent conveying distance is much more constrained. In these applications, the feeding mechanism essentially involves the geometry of the feed hopper and the mechanics of the extracting mechanism as an integral entity. Great strides have been made in hopper design for securing reliable flow through hopper outlets by exploiting the combination of plane and mass flow and corresponding developments in securing progressive extraction that is a pre-requisite for mass flow. A recent innovation even secures progressive extraction with reversing screws running in either forward or reverse direction. (2). Another development in a mature form of conveying is the ‘static screw’ elevator developed by Robert Olds. The casing of this remarkable machine rotates around a stationary screw, to convey material more gently, more evenly, and at greater capacity than conventional elevators, and also has a much lower pickup height.
The range of products that are conveyed also continues to expand and change. Biomass and waste are increasingly employed as a fuel, either directly or for pyrolysis systems. Large scale installations as at Drax power stations, the largest in Europe, face challenges in handling delicately to avoid degradation, potential dust explosions, and recovering from huge stocks at controlled rates with limits on storage time to avoid spontaneous combustion.
At the other extreme, feeding and conveying of such material as plastic film, shredded wood, oily rags, paper selvage, and graded municipal waste defy current scientific design methodology and depend heavily on the practical experience of the designer and fundamental principles as, apart from measurement of wall friction and bulk density, conventional test equipment cannot accommodate the nature of the material for shear testing. In-plant conveying is often custom-made, to suit specific discharge, receiving points and transfer condition and the construction can include provision for containing differential pressures and noxious gasses. Conveyors also are used for process operations, such as heating, cooling, blanching, and various reaction duties. There remains a massive need for industrial education in the basics of bulk technology, but the industry offers an interesting and satisfying career for those lucky enough to be involved.
Lyn Bates, M.D of Ajax Equipment, has served on many committees in the UK, Europe, and the U.S., and has published numerous papers at seminars, conferences, and for professional bodies. Bates is secretary of the British Materials Handling Board.
1. ‘The Global status of bulk materials handling.’ British Materials Handling Report. 2013.
2. ‘A technique for improved the characteristics of a reversing feeder’, Bates. L. IBMHC 2013 Conference. University of Newcastle. Australia.
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