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Choking Velocity

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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 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.

2. Calculation

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  :

Choking Velocity in pneumatic conveying line - Rizk correlation
Equation 1 : calculation of the choking velocity

With :
Gs = Mass flux of Solids (kg/m2/s)
ϵCH = the voidage of the pipe at the choking velocity UCH = (1 - volumetric concentration of solids)
ρs = Particle density (kg/m3)
ρg = Gas density (kg/m3)
UCH
= Choking velocity
(m/s)
D = Pipe diameter (m)
All in S.I. units

Both equations must be solved simulateously by trial and error to calculation ϵCH  and UCH

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
Principles of Powder Technology, M.J. Rhodes, 1990, page 148