Saltation Velocity
Do you have a question, a remark ? Please contact us at admin@powderprocess.net| Section summary |
|---|
| 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 :
ρg = Gas density (kg/m3)
USALT = Saltation velocity (m/s)
d = Particle diameter (m)
D = Pipe diameter (m)
g in S.I. units (9.81 m/s²)
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.
Interactive Rizk Saltation Velocity Sizer
FOR PRELIMINARY TRANSPORT SIZING ONLY — ALWAYS CONSULT A REPUTABLE CONVEYING SUPPLIER1. Process Transport Properties
2. Sizing Calculations
| Exponent 'a' (1100*d + 2.5): | - |
| Exponent 'b' (1440*d + 1.96): | - |
| Solids Loading Ratio (Φ): | - kg/kg |
3. Design Outputs
- m/s
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.
Download Dilute Phase Sizing Excel Spreadsheet
Get the professional Rhodes dilute phase pressure drop template offline. Customize piping geometry, calculate solid-gas velocity parameters, and check system fan pressure curves.
Frequently Asked Questions (FAQ)
What is the saltation velocity in pneumatic conveying?
The saltation velocity is the minimum linear gas velocity required inside horizontal pipes to keep bulk solid particles suspended in the gas stream. If the local gas velocity falls below this threshold, gravitational forces dominate over aerodynamic lift, causing particles to drop out of the stream, accumulate along the bottom of the pipe, and eventually cause line blockages.
How do you calculate saltation velocity using Rizk's correlation?
Rizk's correlation calculates saltation velocity by equating the solids loading ratio to Froude numbers raised to empirical exponents that depend on mean particle size. Because the equation has velocity terms on both the left (solids loading) and right (Froude) sides, it is traditionally solved implicitly or rearranged analytically as: $U_{SALT} = [ C_{const} \cdot 10^b \cdot (g \cdot D)^{a/2} ] ^ {1 / (a + 1)}$.
What is the difference between saltation velocity and choking velocity?
Saltation velocity is the horizontal deposition limit where solids drop out of air suspension, whereas choking velocity is the vertical transport limit where solids collapse into a dense phase slug. Because horizontal drag forces require more gas energy to keep particles suspended against gravity, the saltation velocity is consistently higher than the choking velocity for any given piping system.
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