Minimum air conveying velocity

Pick-up speed

Section summary
1. Definition
2. Calculation
3. Usual values for common materials

Pneumatic Transport
Types of pneumatic transport
Conveying phases
Dilute Phase transport
Dense Phase transport
Air mover
Roots Blower
After Cooler
Airlock Rotary Valve
Product inlet / Injector
Pick-up velocity
Conveying speed / velocity
Air volumetric and mass flowrate
Pipe Equivalent Length
Solids velocity in pipe
Solids Breakage
Pressure drop
Pipe Diameter or Bore
Design methods
Pipe Blockage
Powder Build-up
Selecting dilute or dense phase
Horizontal Conveying
Vertical Conveying

1. Definition

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 10% to 20% higher than the minimum air conveying velocity.

2. Calculation

Minimum air conveying velocity

Some correlations exist to estimate this value for a given bulk solids, but the most reliable remains to perform trials to establish this velocity.

Pick-up speed

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).

Air pickup velocity calculation

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)
- Qairin = air volumetric flow rate at the inlet of the solids (m3/s)
- Din = pipe diameter at the inlet of solids (m)

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.

3. Usual values for common solids

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
Very fine Material
10-12 m/s
10-20% higher
Fine material
14-16 m/s
10-20% higher
Coarse material 16 m/s and above 10-20% higher

Note that the actual velocity of the product conveyed is lower than the conveying air velocity.

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