Review of chapters
The layout of this second edition follows the style of the original guide. The work is divided into three main parts:
• Systems and components
• System design
• System operation
A number of chapters are presented in each part and these are numbered continuously, as referenced below. Two appendixes are included. One is used to present information on material characterization, specifically for pneumatic conveying, and additional conveying data is presented in the second.
Systems and components
This section of the book presents an introduction to all the systems and components that comprise a pneumatic conveying system. This provides both an introduction to the subject of pneumatic conveying and background to the selection of systems and components for a given duty.
Review of pneumatic conveying systems
A review is given of all the various types of pneumatic conveying system that are cur- rently employed and available. This includes:
• Open and closed systems
• Positive pressure and vacuum conveying systems
• Fixed and mobile systems
• Conventional and innovatory systems
• Batch and continuously operating systems
• Pipeline and channel flow systems
Comparisons between the different types of system are given in order to help in the selection process. The influence of the properties of conveyed materials is incorporated into this review. Such properties include abrasive, friable, hygroscopic, toxic, explosive and cohesive. The suitability for multiple product conveying and multiple distance conveying is also examined.
Pipeline feeding devices
A review is given of all the commercially available devices that are used for feeding materials into pneumatic conveying system pipelines, and that meet the requirements of all the different types of conveying system considered in the previous chapter. This includes:
• Rotary valves and the many derivatives
• Screw feeders and the various types available
• Venturi feeders
• Gate lock valve feeders
• Blow tank devices and the multitude of arrangements and configurations
• Vacuum and suction nozzles
• Trickle valves
Chapter 3 concentrates on feeding devices for low pressure and vacuum conveying systems and Chapter 4 considers high pressure systems. Issues such as feed rate cap- ability, control, problems of air leakage, and suitability for different types of conveyed materials are discussed.
Pipelines and valves
Both pipeline bends and valves represent major problems in pneumatic conveying and are probably responsible for the majority of operating problems with regard to pneu- matic conveying systems, particularly when abrasive materials have to be handled. As a consequence there have been many developments with regard to both bends and valves that have resulted specifically from pneumatic conveying.
The blower, compressor or exhauster is at the heart of the pneumatic conveying sys- tem. It is essential that the correct type of machine is selected and that it is correctly specified, particularly in terms of free air delivered. A wide variety of machines are considered, from fans and blowers to compressors, together with their operating char- acteristics. Most of the power required for a pneumatic conveying system is that for the compressor and much of this goes into increasing the temperature of the air. Both of these features are considered in detail. The possible benefits of cooling air and the provision of oil free air are also considered.
Gas–solids separation devices
This is a particular area of the system in which health and safety issues impact. Dis- engagement of coarse particles can be achieved by using a gravity-settling chamber. With finer materials a cyclone may be suitable. For dust and very fine materials a fab- ric filter is probably most appropriate. The methods and associated equipment are reviewed and their applications, limitations and control discussed.
System selection considerations
The selection of a pneumatic conveying system for a particular application involves consideration of numerous parameters associated with the conveyed material, the con- veying conditions and the conveying system. The primary aim is usually for a material to be conveyed at a specified flow rate over a given distance. For illustration purposes extremes of material type are considered. The conveying requirements can usually be met by a wide combination of pipeline bores and conveying line pressure drops. Power consumption, and hence system-operating costs, are factors that can be used in the decision-making process but problems of material and system compatibility have to be taken into account. The inter-relating effects of all these parameters are considered.
This group of chapters is concerned with the design of pneumatic conveying systems. The first two chapters are devoted to the considerations of air alone, but it is here that the basic modelling for pneumatic conveying begins. Materials are then added to the pipeline and the influence of the materials is considered and compared. Scaling parameters and design procedures are then introduced and these are reinforced with two case studies. Some first approximation design methods are presented to allow feasibility studies and system checks to be undertaken quickly, and the possibilities of multiple- material and multiple distance conveying are considered.
Air flow rate evaluation
Air is compressible with respect to both pressure and temperature, and air movers are generally specified in terms of ‘free air conditions’. The correct specification of an air mover in terms of volumetric flow rate is essential in terms of achieving the correct conveying air velocity. The derivation of all the models necessary is given and the results are displayed graphically. The equations are presented in terms of both volu- metric flow rate and conveying air velocity, so that they can be used for the design of future systems, as well as the checking of existing systems. In addition to the influence of pressure and temperature, stepped pipelines, pipeline purging and plant elevation are also considered.
Air only relationships
The reference point for any pneumatic conveying system is the performance of the empty pipeline, and so equations are developed that will allow the air only pressure drop to be evaluated for any pipeline system. Bends and other pipeline features are considered for both positive pressure and vacuum conveying systems. Models and methods for air flow rate control are also included.
Conveying characteristics for a material provide a valuable aid to system design. They provide the design data in terms of air flow rate and air supply pressure for a given material flow rate and quantify the effect of pipeline bore and conveying distance. In addition the conveying characteristics identify the minimum conveying conditions and provide the means to determine power requirements, thus enabling comparisons to be made for different conveying systems. Conveying characteristics are presented for representative materials and, in addition to total pipelines, data is also presented for individual sections of pipeline, as well as bends.
In this chapter the conveying characteristics of a much wider range of materials are presented to illustrate the full influence that different materials can have on conveying capability and performance. High and low pressure and dilute and dense phase con- veying are considered for a broad range of materials.
Material property influences
A goal in pneumatic conveying is to make it possible to design a pneumatic convey- ing system without the need for carrying out full-scale conveying tests with a material. The results of a study into correlations between material properties obtained from bench scale tests and material conveying characteristics obtained from full-scale pneumatic conveying trials are given. Correlations were sought as to whether a material will con- vey in dense phase and what type of pressure drop/material flow rate characteristic is to be expected. The work is extended by investigating the influence that conveying itself might have on the subsequent conveying performance of a material.
Pipeline scaling parameters
It is generally not practical to replicate a plant pipeline for the purposes of undertak- ing tests in order to design a conveying system. Over the years, however, with the accumulation of practical experience and specific research programmes, scaling parameters have been developed for the purpose. These will take account of the dif- ferences between a test facility pipeline and a plant pipeline with respect to lengths of horizontal and vertical pipeline, number and geometry of bends, and pipeline bore. In addition to these parameters, pipeline material and pipeline steps are also considered.
Logic diagrams are presented for pneumatic conveying system design based on both mathematical models and test data. They are presented for the purpose of checking the capability of an existing system, as well as for the design of a new system. Some of the available equations and bench scale test correlations are evaluated and the more useful relationships are included to show how they can be used in conjunction with the logic diagrams.
Two case studies are presented. One is for a fine material that is capable of dense phase conveying, in sliding bed flow, in Chapter 16. The other is for a coarse material that is only capable of dilute phase conveying, in Chapter 17. The scaling process is illus- trated, by way of example in each case, and for the fine material an investigation into the unstable region in sliding bed flow is also presented.
First approximation design methods
Very often a first approximation solution is all that is required. This may be for system design purposes, particularly if a feasibility study is being carried out, or to provide a quick check on the performance of an existing system. An approximate value of power required is often required so that the operating cost of such a system can be estimated in terms of pence per tonne. Two such methods are included, one of which can be used for dense phase conveying systems in addition to dilute phase.
Multiple use systems
In many industries more than one material is required to be conveyed by the same sys- tem. Different materials, however, can have very different conveying characteristics. Some have very different air requirements as well as different flow rate capabilities. There are also many systems that require material to be conveyed over a range of dis- tances. Conveying distance, however, has a marked effect on material flow rate and can influence air flow rate in certain situations. These various conveying situations are considered and a variety of solutions are presented.
This group of chapters is concerned with the operation of pneumatic conveying sys- tems. Pipeline blockages, do unfortunately occur, but mainly due to poor design and maintenance and so this topic is given particular consideration. Means of improving the performance of an existing system are considered, which may be to reduce power requirements or to increase material flow rate. Many problems relate to the properties of the conveyed material, and not least of these are abrasive and friable materials and so one chapter is devoted to erosive wear and another to particle degradation. Moisture and condensation is similarly considered, as well as the issues relating to health and safety.
Troubleshooting and material flow problems
Due to the complexities of system design, a lack of reliable design data, and a poor understanding of compressible flow, many pneumatic conveying systems pose numer- ous problems on commissioning. Pipeline blockage and conveying systems not capable of achieving the desired material flow rate are common problems. A detailed analysis of all possible causes is given and a checklist is provided for quick reference.
Optimizing and up-rating of existing systems
In some cases, if a system is over designed, it may be possible to optimize the conveying parameters and either reduce the power requirement for the system or increase the con- veying capability. Very often an increase in conveying performance is required for an existing system and so the procedures for reviewing the possibility are explained in detail. The procedures are given for both positive pressure and vacuum conveying systems.
Potential users are often reluctant to install a pneumatic conveying system because they anticipate operating problems. Pneumatic conveyors can experience problems but the situation has been improved by the introduction of new types of conveyor and by the modification of existing systems, based on a better understanding of the mechanisms of conveying. This often results in a choice of solutions to a particular problem. The most common problems affecting pneumatic conveyors are examined, such as static electri- city and material deposition. Some practical solutions to these problems are presented.
Many materials that have to be conveyed are very abrasive, such as silica sand, alu- mina, cement and fly ash. As a consequence the conveying pipeline, bends and various components that are exposed to impact by the gas–solids flows have to be specified such that the problem is minimized to an acceptable level. It is not uncommon for steel bends installed in a pipeline conveying an abrasive material to fail in a matter of hours. The mechanics of the erosive wear process is explained, and a review of possible pre- ventative measures that can be taken, and alternative components or materials that can be used, is given.
Many materials that have to be conveyed are friable and so particles are liable to be broken when they impact against retaining surfaces, such as bends in the pipeline. It is for this reason that pneumatic conveying systems are not generally used for this type of material. There are numerous means by which the problem can be reduced, however, relating to conveying conditions, bend geometry and materials of construc- tion and so a detailed review of these is given.
Moisture and condensation
As the temperature of air reduces the capacity for air to support moisture reduces and condensation is likely to occur. The same situation occurs with an increase in pressure. Air is the prime mover in pneumatic conveying systems and changes in both tempera- ture and pressure are very common. The modelling of air with respect to moisture is presented to illustrate the nature of the problem and to provide guidance on the poten- tial magnitude of the problem, and for the sizing of air drying plant and equipment should this be required.
Health and safety
Most dusts pose a potential health problem, and many materials that have to be con- veyed are potentially toxic. Pneumatic conveying is often chosen for hazardous mater- ials because the system provides a theoretically totally enclosed environment for their transport. It is also considered that the majority of conveyed materials are potentially explosive, and this certainly applies to most food products, fuels, chemicals and metal powders. In this chapter a detailed review of precautions and modifications to plant and components is given. The nature of the problems is explained and information on appropriate measurable properties of dust clouds is provided.