Saltation Velocity
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| 1. Definition of
saltation velocity |
| 2. Calculation of saltation velocity |
| 3. Usual values for common materials |
1. Definition of saltation velocity
In a pneumatic conveying system, the saltation velocity is the air conveying velocity below which the solids being conveyed starts to settle at the bottom of horizontal pipes. In order to be able to convey in dilute phase a bulk solids such as a powder or granulates, the minimum air conveying velocity in all part of the line must be HIGHER than the saltation velocity.
2. Calculation of saltation velocity
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 saltation velocity calculation formula below is due to Rizk, it is not very precise, +/-54% in average, but may be useful :
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, equation of Rizk for saltation velocity calculation 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.
3. Usual values of saltation velocities for common materials
Few literature sources are giving references of saltation velocities for materials. One can actually refer to pick-up velocities, higher than the saltation velocities, but which can give a 1st idea.
SourcePrinciples 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