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The Way Forward for Pneumatic Conveying

January 14, 2013

Is pneumatic conveying in the doldrums or is it just taking time to consolidate its position? The final three decades of the last century saw a considerable amount of competition between companies through invention and development. A major innovation was that of low-velocity, dense-phase conveying systems, for materials that previously had to be conveyed in dilute phase and hence at high velocity. This helped to broaden the application and acceptability of pneumatic conveying for abrasive and friable materials, apart from the general benefits of reducing power requirements, and hence operating costs.
    The systems developed took many forms, from adding air at discrete points along the length of the pipeline in order to artificially create air retention, to providing a small bore pipeline with no additional air supply. This secondary pipe was either internal to the main conveying line or external to it, with access points along the length in order to artificially create permeability and so form plugs. The pulsing of the air supply was another innovation to convey materials continuously in short discreet plugs. A particular development was the use of high-pressure blow tanks for the conveying of materials. Their use in the cement and power generation industries led to the rapid demise of screw pump feeders in these industries. This also led to the development of a new generation of rotary valves capable of high-pressure operation.
    There is still a lot of secrecy associated with conveying data for bulk particulate materials, but it is perhaps not surprising since the building, operation, and maintenance of large-scale test facilities is a major overhead for companies to support. Manufacturing companies, therefore, are not going to publish test data. System user companies, however, are just as reluctant to publish data on the operation of their systems for fear of passing on valuable information to their competitors. It is not unusual for a research institution to be approached by a user company to undertake consultancy work, but neither the name of the company requesting the work nor the name of the material to be tested should be revealed, and a 25-year secrecy agreement on any publication has to be agreed.
    It is not surprising, therefore, that the technology has not advanced as quickly as other engineering topics. This has not been helped by the fact that there are still very few academic institutions in the world that undertake research work on pneumatic conveying. Even in academic institutions that have reasonably large-scale test facilities, however, there is an increasing trend away from such ‘hands on’ test work, for with the increasing power and capability of computers, particle modeling and simulation tends to be the preferred option for research study.
    There are significantly fewer systems manufacturing companies today following a gradual consolidation of companies through mergers and take-overs, and the number of companies exhibiting at trade shows has been in steady decline. This has possibly taken the edge off the need to develop new systems and components at the present time. Short courses on pneumatic conveying, however, are widely available now, being offered at many trade shows and by various associations. Attendance at such courses is often needed for the professional development requirements of the major institutions registering engineers.
    As a consequence engineers in companies that use pneumatic conveying systems are at least more conversant with their capabilities now, and appreciate that material testing is generally required for a new material and that the conveying capability of different grades of the same material can vary widely. Both fly ash and alumina, for example, can be conveyed with a conveying air velocity below 1000 ft/min quite reliably, but a course grade of the same material will require a minimum conveying air velocity of 3000 ft/min, otherwise the pipeline is likely to block. They are equally aware of the differences between dilute- and dense-phase conveying, and that materials that have good permeability will convey quite naturally at low velocity in plug flow and that materials that have good air retention will similarly convey at low velocity but in a fluidized sliding bed mode of flow.
    All the foundations for the subject have been laid and it is now time for the next major push forward. But once again it seems to be up to the industry itself.
    An independent consultant on pneumatic conveying, Dr. David Mills lectures at Glasgow Caledonian University and acts as a referee on PhD programs for universities around the world. Dr. Mills has a PhD in pneumatic conveying from Thames Polytechnic, London.