The Potential Influence of Particle Size on Pneumatic Conveying System Performance

Pneumatic conveying is potentially one of the most aggressive means of transporting materials. Only in low-velocity, dense phase conveying systems can the conveying of the material be described as being ‘gentle.’ In dilute phase suspension flow most of the damage probably occurs with the high-velocity impact of the material against the pipeline bends. Degradation will cause a change in particle size and there is a tendency for ‘fines’ to be generated. Particle size distribution has the effect of reducing the permeability of a material and of increasing the air retention. Many materials are available in a number of different ‘grades,’ such as alumina, soda ash, sugar, and fly ash. Different grades of the same material will have different particle size distributions and this is fundamental to the conveying capability of a material in a pneumatic conveying system.
    A fine grade of alumina, for example, with a mean particle size of about 40 micron will convey quite reliably with a minimum conveying air velocity of about 1000 ft/min in a conventional conveying system. If the mean particle size is about 120 µm, however, the pipeline will block if the conveying air velocity falls below about 2500 ft/min. The name of a material, therefore, is not sufficient in identifying the capability of the material for pneumatic conveying. These values of particle size and velocity apply equally to both sugar and fly ash.
    In conveying granulated sugar with a mean particle size of about 500 µm in a 165-ft-long pipeline of two-in. bore and containing nine bends the pipeline would almost instantly block the if the air velocity fell below about 3200 ft/min. After re-circulating the material half a dozen times the minimum velocity dropped to 3000 ft/min, and after re-circulating 50 times it had dropped to 1600 ft/min. As far as pneumatic conveying was concerned it became a totally different material.
    Not only is there a significant reduction in conveying air velocity with reduction in mean particle size, there is generally a marked increase in the conveying rate for the material with exactly the same conveying line pressure drop. With fly ash, for example, the author has recorded a 100% increase in material flow rate from a reduction in mean particle size from 110 to 75 µm. Once the mean particle size is down to about 40 µm, however, the capability of the material will change altogether, generally being capable of low-velocity, dense phase conveying. At this point there is a further improvement in conveying rate but the bonus is that the conveying line inlet air velocity drops potentially to below 1000 ft/min, which means that significantly less air is required for conveying. With the combined effect of an increase in material flow rate and a decrease in air flow rate, the power requirements for conveying reduce dramatically.
    With fly ash, however, the reverse situation appears to be occurring at the present time. The quality of coal available is declining, but clearly has to be used in existing boiler plants at power generating stations, and as a consequence there is a significant increase in the particle size of the ash produced. The existing ash handling plant will invariably have been designed specifically for a fine grade of ash and so is no longer capable of the duty. Because of the importance of reliable power generation, however, a safety margin on conveying capability of 100% is generally incorporated in the plant design. As a consequence of the significant increase in particle size, however, this is insufficient and the power generating companies are desperately seeking solutions.
    An independent consultant on pneumatic conveying, Dr. Mills lectures at Glasgow Caledonian University and acts as a referee on PhD programs for universities around the world. He maintains an association with The Indian Institute of Technology in New Delhi, where he has lectured since 1980 and has helped to set up extensive pneumatic conveying laboratory facilities. Dr. Mills is currently a visiting professor at the University of Newcastle in Australia and is a worldwide consultant. He also acts as an expert witness in litigation cases. He has been a member of the Institution of Mechanical Engineers in the UK since 1968 and has written more than 200 articles for technical journals and conferences. He has been associated with the Powder and Bulk Solids conferences since the first one in 1976. Dr. Mills has a PhD in pneumatic conveying from Thames Polytechnic, London.
    
References
Mills, D. (2004). Pneumatic Conveying Design Guide - 2nd Edition. ISBN 0 7506 5471 6. Elsevier Butterworth-Heinemann.
Mills, D. Jones, M.G., and Agarwal, V. K. (2004). Handbook of Pneumatic Conveying Engineering. ISBN 0 8247 4790 9. Taylor and Francis (Marcel Dekker).
Mills, D., and Agarwal, V.K. (2009). Pneumatic Conveying Systems: Design, Selection and Troubleshooting with Particular Reference to Pulverised Fuel Ash - 2nd Edition. ISBN 978-3-8343-3127-4. Vogel Buchverlag.