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1987 | Buch

The nature of bulk solids Kapitel lesen Erstes Kapitel lesen

Bulk Solids Handling

An Introduction to the Practice and Technology

verfasst von: C. R. Woodcock, DipTech, MSc, PhD, CEng, MIMechE, J. S. Mason, BSc, PhD, CEng, FIMechE, FIMarE, MIMinE

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

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Über dieses Buch

An understanding ofthe properties and the handling characteristics of liquids and gases has long been regarded as an essential requirement for most practising engineers. It is therefore not surprising that, over the years, there has been a regular appearance of books dealing with the fundamentals of fluid mechanics, fluid flow, hydraulics and related topics. What is surprising is that there has been no parallel development of the related discipline of Bulk Solids Handling, despite its increasing importance in modern industry across the world. It is only very recently that a structured approach to the teaching, and learning, of the subject has begun to evolve. A reason for the slow emergence of Bulk Solids Handling as an accepted topic of study in academic courses on mechanical, agricultural, chemical, mining and civil engineering is perhaps that the practice is so often taken for granted. Certainly the variety of materials being handled in bulk is almost endless, ranging in size from fine dust to rocks, in value from refuse to gold, and in temperature from deep-frozen peas to near-molten metal.

Inhaltsverzeichnis

Frontmatter

Characterization, Flow and Storage

1. The nature of bulk solids
Abstract
A bulk solid consists essentially of many particles or granules of different sizes (and possibly different chemical compositions and densities) randomly grouped together to form of a bulk. The ‘nature’ of such a material—that is, its appearance, its ‘feel’, the way it behaves in various circumstances, and so on— is thus dependent upon many factors, but principally upon the size, shape and density of the constituent particles.
C. R. Woodcock, J. S. Mason
2. Gravity flow of bulk solids
Abstract
A good understanding of the nature of bulk solids flow is an essential prerequisite to the design of virtually any system involving the storage or handling of such materials. Observation of a bulk material discharging from a hopper or flowing under gravity along a steeply inclined channel will immediately suggest similarities to the behaviour of liquids. Whilst there are certainly some similarities between the flow characteristics of bulk solids and liquids, the analogy is one that it is unwise to pursue. In general it is more appropriate to model a bulk solid as a plastic solid than as a fluid continuum.
C. R. Woodcock, J. S. Mason
3. Dynamics of fluid/solids systems
Abstract
In modern industry there is an increasing number of situations where particulate and granular materials are handled in bulk, and there is a greater awareness than ever before of the importance of safety and efficiency in processing and handling such materials. Designers and plant operators, perhaps schooled in traditional fluid mechanics involving only liquids and gases, thus have a considerable task in understanding and predicting the unfamiliar flow characteristics of bulk solids.
C. R. Woodcock, J. S. Mason
4. The design of storage bins and hoppers
Abstract
The storage bin, silo or hopper is one of the most important items of equipment in any bulk solids handling installation, since a poorly flowing hopper can have repercussions extending throughout the plant. All too often hoppers are ‘squeezed in’ after the remainder of the system has been designed, and this can result in various flow problems, such as those described generally as ‘arching’ or ‘rat-holing’ (section 2.3.4). Obviously, if this occurs, even the most sophisticated and expensive equipment downstream of the hopper will be unlikely to perform effectively because of the erratic supply of material. Part of the problem is often a lack of appreciation by designers and operators that, for a system to operate satisfactorily, bulk solid must flow from the hopper when required and in a predictable manner. Thus, as with any other part of the handling system, gravity-flow storage hoppers should be designed or selected to handle the actual product under consideration.
C. R. Woodcock, J. S. Mason
5. Dust control
Abstract
One of the main problems arising from almost any process involving the handling of bulk particulate materials is the generation and release of dust. Designers and operators of bulk handling systems have recently become increasingly aware of the hazards associated with the release of airborne dust in significant quantities. Thus, whereas the principal incentive for the control of dust emissions used to be an economic one (i.e. the more valuable the product, the more trouble would be taken to ensure its total recovery) there are now the additional factors of environment, health and safety to be given the most serious consideration. Since the Health and Safety at Work Act (1974) became law, the avoidance of excessive environmental pollution has become of prime importance, and where the product concerned is potentially dangerous (e.g. toxic or explosive) extreme measures must be taken to prevent its escape from the plant in which it is being handled.
C. R. Woodcock, J. S. Mason
6. Explosion hazards
Abstract
Many bulk solids, when dispersed in air to form a cloud or suspension and ignited, rapidly propagate a flame through the suspension, with a subsequent sudden increase of pressure as a result of the release of heat and gaseous products from the burning dust. This is commonly called a ‘dust explosion’, in contrast to a ‘fire’ which would be said to occur if the burning dust were in a pile or layer. In fact, dust will generally smoulder or burn with a flame: some, especially plastics, tend to melt or flame or give off noxious vapours which are readily detected, but others may glow and smoulder, remaining an undetected hazard which could persist for days. Although only a minority of dust fires actually result in an explosion, the potential danger is a very real one. Typical examples would be the explosions of airborne dust following the sudden disturbance of a smouldering layer during cleaning or the collapse of a burning pile of material.
C. R. Woodcock, J. S. Mason

Mechanical Handling

7. Belt conveyors
Abstract
The belt conveyor is one of the commonest means of transportation for bulk solids and is capable of carrying a greater diversity of products at higher rates and over longer distances than any other kind of continuously-operating mechanical conveyor.
C. R. Woodcock, J. S. Mason
8. Bucket elevators
Abstract
In the preceding chapter on belt conveyors, brief mention was made of adaptations to the basic flat- or troughed-belt to enable it to operate on steep inclines. For example, whereas a conventional belt conveyor would generally be limited to a slope of about 20°, texturing the surface of the rubber belt to incorporate moulded ribs or nubs will allow conveying up an incline of some 60–70°, or even more, depending upon the nature of the bulk solid being carried. Taking this idea further, the rubber belt could be fitted with sidewalls and curved or sloping transverse slats so that it is capable of lifting the particulate or granular material vertically. The conveyor then approaches the design concept of the well-known bucket elevator.
C. R. Woodcock, J. S. Mason
9. Chain and flight conveyors
Abstract
In addition to the very familiar belt conveyor and the scarcely less familiar bucket elevator, there are a number of alternative mechanical techniques that are commonly used to carry, drag or scrape bulk solids from one location to another. It is not particularly easy to place these various techniques into distinct categories, and the division of this part of the book into separate chapters and sections, while not being entirely arbitrary, should be regarded as a matter of convenience rather than as a serious attempt at classification of bulk handling systems. In some cases there is an almost continuous gradation of design from one type of conveyor to another, so that the placing of an artificial ‘boundary’ between the two types becomes somewhat subjective. For example, if an apron conveyor is fitted with deep pans and operated on a steep incline it becomes a bucket elevator, and if a bucket elevator is fitted with shallow bottomless buckets and enclosed in a casing it becomes an en-masse conveyor.
C. R. Woodcock, J. S. Mason
10. Screw conveying
Abstract
The modern screw conveyor is essentially a development of the well-known Archimedean screw which was conceived some 2000 years ago as a means of raising water for irrigation. Applications of this device were naturally very limited until relatively recent times, and its evolution has consequently been slow. A fundamental feature of the original pattern of Archimedean screw which distinguishes it from other types of screw conveyor is that the helical screw (or ‘flight’) is attached to the inner surface of the cylindrical casing and rotates with it (Figure 10.1). It will be noted from the diagram that the Archimedean screw operates effectively as a positive displacement elevator, the angle at which it will work successfully depending upon the diameter of the casing and the pitch of the screw.
C. R. Woodcock, J. S. Mason
11. Vibratory conveyors
Abstract
Vibratory conveyors are commonly used in industry to carry a wide variety of particulate and granular materials. Although the majority of engineers involved in bulk materials handling will be aware of vibratory conveying as a useful technique, few have the necessary understanding of this method to be able to design or select a system with confidence. However, there is little doubt that vibratory conveyors have some useful advantages, and an insight into their mode of operation and into the parameters governing their performance should enable the system designer to ensure that his choice of conveyor is the most efficient and the most reliable.
C. R. Woodcock, J. S. Mason

Pneumatic and Hydraulic Transport

12. Basic pneumatic conveying systems
Abstract
The entrainment of solid particles in a high-velocity flow of air is a well known phenomenon, with examples ranging from sandstorms to domestic vacuum cleaners, and it is therefore not surprising that it should be the basis of an essentially simple and reliable method for the controlled conveying of bulk solids. Pneumatic conveying, as the method is called, may be formally defined as the transportation of dry bulk particulate or granular materials through a pipeline by a stream of gas. Whilst the gas concerned would normally be air, other gases are occasionally used, such as nitrogen in situations where there is a fire or explosion risk.
C. R. Woodcock, J. S. Mason
13. Components of pneumatic conveying systems
Abstract
In Chapter 12 the various types of pneumatic conveying system were discussed, and attention is now turned to the components that go to make up these different systems. In addition to the conveying line itself, which would normally be of steel but may be alternatively of aluminium, plastic, glass or rubber, the essential components of a pneumatic conveying system are the air mover (for example, fan, blower, compressor or vacuum pump), the feeding device, and the gas/solids disengaging device. In order to design and construct a reliable and economic pneumatic conveyor it is essential to have a good awareness of the different types of air-mover, feeder and disengaging unit that are available, their advantages and disadvantages, and the criteria for their selection. In this chapter the various designs of each of these components commonly used in practical pneumatic conveying installations are described and illustrated.
C. R. Woodcock, J. S. Mason
14. Pneumatic conveyor design
Abstract
In the two preceding chapters discussion has been restricted to general details of the arrangements of pneumatic conveying systems and their components. It is necessary now to give some more positive pointers to successful pneumatic conveyor design.
C. R. Woodcock, J. S. Mason
15. Air-assisted gravity conveying
Abstract
The three preceding chapters have been concerned primarily with pneumatic conveying by pipeline, and consideration will now be given to a variation on this technique in which the particulate bulk solid is made to flow along a channel inclined at a shallow angle. Pneumatic conveying has several advantages over other methods of transporting bulk solids, but it suffers from two drawbacks. Firstly, the power consumption is quite high; and secondly, especially when conveying in dilute phase, the solids velocity is relatively high and may cause problems as a result of particle degradation and erosive wear of the pipeline and system components. Both of these difficulties may be minimized by conveying in dense phase, that is, with a higher ratio of solids to air, so that the quantity of air used is smaller and the conveying velocity is lower.
C. R. Woodcock, J. S. Mason
16. Hydraulic conveying
Abstract
Hydraulic conveying of bulk solids, or ‘slurry transport’, involves the conveyance of solid particles in suspension in a moving liquid. Although the majority of commercially viable slurry pipelines have been constructed to carry mineral particles in water, almost any combination of solids and liquids could be possible provided, obviously, that the solid material is not dissolved or affected in some other unacceptable manner by the carrying liquid. High-tonnage, long-distance transportation of coal, iron, copper, phosphate, limestone and various other minerals in hydraulic pipelines is now a well-established commercial alternative to other modes of bulk solids transport such as lorries, railway trains and barges.
C. R. Woodcock, J. S. Mason
17. Capsule transport
Abstract
In the preceding chapters (12–16) of this book, various aspects of the transportation of bulk particulate and granular materials in pipelines have been discussed. The concept was that if the bulk solid were to be fed continuously into a gas or a liquid flowing steadily along a pipeline, the particles would be conveyed by the fluid to the outlet end where they could be disengaged from the carrier fluid in a suitable separation unit. An alternative approach to the pipeline transportation of bulk solids, especially in cases where, for some reason, it is undesirable for the conveyed material to come into contact with the carrier fluid, is to enclose the bulk solid in cylindrical or spherical capsules, of diameter only slightly less than that of the pipeline, and then use the gas or liquid to propel these capsules from one end of the pipeline to the other.
C. R. Woodcock, J. S. Mason
Backmatter
Metadaten
Titel
Bulk Solids Handling
verfasst von
C. R. Woodcock, DipTech, MSc, PhD, CEng, MIMechE
J. S. Mason, BSc, PhD, CEng, FIMechE, FIMarE, MIMinE
Copyright-Jahr
1987
Verlag
Springer Netherlands
Electronic ISBN
978-94-009-2635-6
Print ISBN
978-94-010-7689-0
DOI
https://doi.org/10.1007/978-94-009-2635-6