In order to transport a solids in dilute phase (coarse solids like pellets or powder), the conveying air must reach a threshold value called the mininum air conveying velocity. If the air velocity is below this value, the solids will ultimately settled at the bottom of the pipe and block the pneumatic transport line.
The pick-up speed is the conveying air velocity at the point of introduction of the solids in the conveying pipe. In order to be able to transport the solids, the pick-up speed must be higher than the minimum conveying air velocity. The minimum acceptable pick-up speed is said to be min 20% higher than the minimum air conveying velocity.
The minimum air conveying velocity is here assimilated either to the saltation velocity or to the choking velocity, in most of the cases with a complex layout having horizontal and vertical pipe sections, the minimum air conveying velocity will be the saltation velocity (saltation velocity is always higher than the choking velocity) ome correlations exist to estimate this value for a given bulk solids, but the most reliable remains to perform trials to establish this velocity.
The pick-up speed can be calculated by calculating the air speed in the pipe at the point of entry of the solids (most of the time at the beginning of the line).
Equation 1 : calculation of the pick-up air velocity in a conveying pipe of diameter Din
With :- upickup = pick-up air velocity in conveying pipe of diameter D at the point of entry of the solids (m/s)
Reversely, defining the minimum air conveying velocity and thus the pick-up speed can be used to calculate the required air flowrate and then specify a new system.
Note the following : for the calculation in dilute phase, the volume occupied by the solids is neglected. The pick-up speed is only valid at the point of entry of the material, further down the line, the air velocity increases.
Every conveying line is unique and will have its specific air velocity, however, some orders of magnitudes can be given for both dilute and dense phase.
Table 1 : Minimum air conveying
velocity for commons bulk solids
|Material||Minimum air conveying velocity||Minimum pick-up velocity for reliable transfer in dilute phase|
|Very fine Material
||at least 20% higher|
||at least 20% higher|
|Coarse material||16 m/s and above||at least 20% higher|
Note that the actual velocity of the product conveyed is lower than the conveying air velocity.