The Capability of Bulk Particulate Materials for Pneumatic Conveying

For research studies fly ash is an ideal material as it is readily available in a wide range of particle sizes. Research has shown that such materials with a mean particle size greater than about 70 microns can only be conveyed in dilute phase, suspension flow, with a minimum conveying air velocity of about 2400 ft/min and above for larger-sized particles, whereas the finer grades can generally be conveyed in dense phase flow with a minimum conveying air velocity of about 600 ft/min.
    
Significant differences have also been shown in terms of conveying capability for dilute phase conveying. The mass flow rate of the fly ash having a mean particle size of about 50 µm was 100% greater than that for fly ash having a mean particle size of about 100 µm. The 50 µm appeared to be an optimum maximum for the flow rate of the fly ash, for with further decrease in particle size the flow rate started to decrease (Ref 1).
    
The ‘grade’ of a material, such as ‘fine’ or ‘coarse’, therefore can be a significant parameter. Many materials are available in a range of grades and so it must be recognized that conveying parameters for the reliable flow of each may be significantly different. Stories abound about companies installing a pneumatic conveying system for a given material, but then find a cheaper source for the material. When the conveying system is commissioned, however, it is not capable of conveying the new material at the required rate, if at all.
    
In another case (Ref 1), a company provided two different pneumatic conveying systems manufacturing companies with a sample of soda ash to be conveyed. Engineers from the soda ash manufacturing company went to witness tests and provided each of them with a fresh batch of the material for the purpose. In both cases, however, the pipeline clogged and neither company could convey the fresh material. After much subsequent research, it was found that the fresh material degraded in a way that rapidly changed its conveying characteristics and both the air flow rate and air supply pressure were incorrect for the fresh material.
    
An approximate means of determining the conveying potential of a material, in terms of low velocity, dense phase flow, is to half fill a glass jar with the material. Fit a lid and invert the jar a few times to aerate the material. Then remove the lid and drop a ball bearing into the jar. If the ball bearing runs straight through the material to the bottom, it is probably a candidate for low-velocity dense phase conveying. If it comes to rest on the surface, it clearly has little air retention capability and so is unlikely to be a candidate for low-velocity, dense phase conveying.
    
Another class of materials that are ideal candidates for pneumatic conveying are naturally occurring grains and seeds from agricultural products, and those ‘chemical materials’ that are produced in the form of small pellets for the convenience of handling, storing, and subsequent processing, such as nylon and polyethylene. These materials are also generally capable of being conveyed in dense phase and hence at low velocity. In this case, the property relevant to pneumatic conveying is permeability, rather than air retention, and consequently the mode of conveying is very different. At conveying air velocities below the transition from dilute phase at about 3000 ft/min, these materials will naturally form plugs and will convey reliably with conveying air velocities down to 600 ft/min. For a given conveying line pressure drop, however, the mass flow rate reduces with decrease in velocity below 3000 ft/min in most cases, but the materials will convey reliably and problems of particle degradation will be significantly reduced.
    
David Mills undertook a PhD in Pneumatic Conveying in 1973 and was appointed Professor of Bulk Solids Handling at Glasgow Caledonian University in 1988. Since 1997 he has been operating as an independent consultant. He has published over 250 articles for conference presentation and technical journals.

(Ref 1) Pneumatic Conveying Design Guide. 3rd Edition 2016, David Mills. Butterworth-Heinemann.

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