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Saltation Velocity

Do you have a question, a remark ? Please contact the author at powder.process@protonmail.com


Section summary
1. Definition
2. Calculation
3. Usual values for common materials


1. Definition

In a pneumatic conveying system, the air conveying velocity below which the solids being conveyed starts to settle at the bottom of horizontal pipe is the saltation velocity. In order to be able to convey in dilute phase a bulk solids, the minimum air conveying velocity in all part of the line must be HIGHER than the saltation velocity.

2. Calculation

The saltation 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 Rizk, it is not very precise, +/-54% in average, but may be useful :

Saltation Velocity in pneumatic conveying line - Rizk correlation
Equation 1 : calculation of the saltation velocity by Rizk correlation

With :
Ms = Solids mass flowrate (kg/s)
ρg = Gas density
(kg/m3)
USALT
= Saltation velocity
(m/s)
d = Particle diameter
(m)
D = Pipe diameter
(m)
g in S.I. units

From recent works, this correlation looks to be best suited for fine powders, below 200 microns particles diameter.

Note : The saltation velocity, which happens in horizontal lines, should not be confused with the choking velocity which happens in vertical lines. However, 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
Principles of Powder Technology, M.J. Rhodes, 1990, page 147
On the prediction of pickup and saltation velocities in pneumatic conveying, Gomes and Amarante Mesquita, Brazilian Journal of Chemical Engineering, 2014