Choking Velocity
Why do pneumatic transport choke and how to avoid it ?
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| Section summary |
|---|
| 1. Definition |
| 2. Calculation of
the choking velocity |
1. Definition
What is the choking velocity of a conveying line ?
In a pneumatic conveying system, the air conveying velocity below which the solids being conveyed starts to settle at the bottom of vertical pipes, leading either to a blockage, either to dense phase conveying, is the choking velocity.
How to prevent pneumatic conveying lines choking ?
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.
- Figure 1 : pipe blockage due to air velocity below the choking velocity
2. Calculation of choking velocity
How to calculate the choking velocity of a material ?
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 [1]:
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
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Both equations must be solved simulateously by trial and error to calculation ϵCH and UCH
Note : What is the difference in between choking velocity and saltation velocity ?
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.
Sources and other references[1] Principles of Powder Technology, M.J. Rhodes, 1990, page 148