In a pneumatic conveying system, the air conveying velocity below which the solids being conveyed starts to settle at the bottom of vertical pipe, leading either to a blockage either to dense phase conveying, is the choking velocity. In order to be able to convey in dilute phase a bulk solids, the minimum air conveying velocity in vertical part of the line must be HIGHER than the choking velocity.
The choking velocity is better defined through trials in a pilot plant. However, in case one tries to make a 1st design assessment or to try troubleshooting an existing line, correlations are available.
The one below is due to Punwani and al :
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Note : The choking velocity, which happens in vertical lines, should not be confused with the saltation velocity which happens in horizontal lines. The saltation velocity is higher than the choking velocity, thus designing the system to run above the saltation velocity will allow to avoid crossing the choking velocity in vertical pipes.Source