I do not think the situation is quite as simple as is put forward. First, powders that fall into the class of Geldart group C are cohesive and difficult to fluidize because injected gas tends to form channels or cracks. Air passing through materials in groups B and D form bubbles and therefore have an irregular composition, tending to spout in group D. As for other fluidized conditions, they may be in different stages of quiescent, bubbling or boiling beds that are all of very variable states, so that the resistance to deformation does not have a unique value. Likewise, particle size and density state are not the only factor that influences internal shear strength, as particle shape, internal friction value and even mass density, are also likely to affect the apparent ‘viscosity’ of a dilated bed.

Solid volume fractions are similarly indeterminate without closely fixing the conditions of bulk formation, particularly with fine fractions. For background on particle packing structures I suggest that you review ‘They packing of particles’ by Grey and perhaps see ‘The diffusion of air through beds of low surface area powders’ by Allen and Stanley-Woods, Powder Technology, 3 (1969/70) 227 – 234.

It may be possible to advise further if the purpose of the requirement were known. Alternatively, for secific materials and given, stable fluidized conditions, measuements could presumably be taken. ■

Lyn,

I've just completed the Pneumatic Conveying module of the Masters Degree on Bulk Solids Handling from the University of Newcastle. I however now need some knowledge on airslide technology to complete my project. We need to re-design a fly ash plant on one of our power stations due to increased ash production as a result of decreasing coal qualities.

Can you perhaps direct me to literature or papers where I can gain some knowledge on the above subject.

Also some info on airlifts will be welcome, although I can probably use conventional pneumatic conveying theory there.

Regards

Henk ■

Airslides and air lifts are a little outside my field, but there is plenty of literature on fluidisation and elutriation that should be relevant. Air slides are in comon use in the cement industry, so I would expect to find some references on that application. Generally, it is only necessary to negate the bulk strength so that the material will deform under minor stresses of slope inclination for an air slide to work. There is therefore a trade off between the angle of slope and the amount of air needed and it would be an interesting excercise to optimise a speccific installation.

Having studied at Newcastle, you may find Mark Jones a mine of information on pneumatic conveying type information. ■

If you would like to send me your postal address, I would be able to send you a copy of a paper written by The Wolfson Centre some years ago entitled "Airslides versus pneumatic conveyors: a comparison of energy consumption" authors: Ashenden SJ, Pittman AN & Bradley MSA. This paper has some pointers on airslide design that could be of interest to you.

Regards

Richard Farnish

The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, Chatham Maritime, Kent ME4 4TB

www.bulksolids.com

e-mail: r.j.farnish@gre.ac.uk ■