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Powder/Bulk Solids

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Pneumatic Conveying System Design

David Mills

Anything can be pneumatically conveyed. In disaster movies, large trucks are lifted off the ground and carried away in hurricanes and tornadoes. In industrial applications, conveyed objects need to be reliably transferred from one point to another; thus, a pipeline is used for the purpose. Materials conveyed through a pipeline can be blown (positive-pressure pneumatic conveying) or sucked (vacuum or negative-pressure conveying).

Powders and bulk materials can be conveyed using several different flow methods. First, there is the division into dilute- and dense-phase conveying. Dilute-phase conveying is essentially suspension flow, requiring relatively high values of conveying air velocity, 3000 ft/min being a typical minimum value for granular materials. Virtually any material can be conveyed in dilute phase if it can be fed reliably and at a controlled rate into a pipeline.

In dense-phase conveying, or nonsuspension flow, two flow modes are possible. For fine powdered materials that exhibit good air retention, such as cement and flour, material flow can be in the form of a sliding fluidized bed. For essentially monosized materials such as polyethylene pellets, which have good permeability, conveying takes the form of material plugs separated by air gaps. Both of these flow types are likely to occur with conveying-line inlet air velocities below 2500 ft/min. Some materials can be conveyed at much lower air velocities down to 750 ft/min.

Gas-solid flow is a highly complex phenomenon and is not amenable to easy theoretical analysis. In the common vernacular, I often tell my students that pneumatic conveying is nothing like rocket science; it is far more difficult than that. Rumor has it that when Einstein was looking for something to do, he considered pneumatic conveying but gave it up as too difficult.

Nowadays, one would think that a pneumatic conveying system can be designed only by using a computer program. A client looking for a pneumatic conveying system would probably expect a manufacturer to use such a program. Is this the case? And if a computer program is used, what degree of accuracy might be expected?

Many manufacturing companies that serve a wide range of industries advertise the vast number of different materials they can convey because they know that different materials can behave very differently in a pneumatic conveying system pipeline. Most reputable manufacturers have facilities to test clients’ materials. Testing services are generally offered free of charge, and clients are invited to witness conveying trials to see that their materials can be conveyed reliably.

It is most unlikely that a test facility’s piping geometry will match that of a manufacturing plant under construction. Pipeline bore, horizontal and vertical conveying distances, and the number and geometry of bends will differ. Nonetheless, the use of appropriate scaling parameters can adjust for such differences. When a material is tested, the computer program may not have to take particle properties into account, but that program could not be used for another material—or even a different grade of the same material—with any degree of reliability. Slight differences in particle properties can change the conveying capability of a material quite remarkably.

Industry is probably responsible for most developments in the area of pneumatic conveying. Experimental test facilities possess a wealth of valuable information. However, industry is reluctant to publish information on pneumatic conveying because of its commercial value. This reluctance applies to systems manufacturing companies as well as users of pneumatic conveying systems.

Very few universities conduct research into pneumatic conveying. Individuals who do undertake such research discover that it is becoming increasingly difficult to finance such work because funding bodies do not recognize the need for basic fundamental research in this area. Topics relating to nanoparticles and discrete element modeling are considered technologically more relevant. I anticipate, therefore, that pneumatic conveying technology will continue to advance at a very slow pace.

With a degree in mechanical engineering, David Mills has worked as a nuclear engineer and thermodynamicist. He began a PhD program in pneumatic conveying in 1973 and has been working in the area ever since—first at the University of Greenwich in London and then as professor of bulk solids handling at Glasgow Caledonian University in Scotland. Since 1997, Mills has been an independent consultant. He has written more than 200 articles and has received the 2007 UK Institution of Mechanical Engineers Solids Handling Award in recognition of his professional excellence in the area of bulk solids handling technology.