Conveying speed / air velocity

Section summary
1. Definition
2. Calculation
3. Usual values for dilute phase and lean phase pneumatic transport

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
Piping
Pick-up velocity
Conveying speed / velocity
Air volumetric and mass flowrate
Pipe Equivalent Length
Solids velocity in pipe
Bends
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

The conveying air velocity is the speed at which air circulates in the conveying pipe of a pneumatic conveying line. Air velocity is changing all along the conveying line, thus one must always refer to a reference position (beginning or end of the line) when calculating the air velocity. The air velocity is sometimes called conveying speed.

2. Calculation

The air velocity in the pipes is calculated by dividing the air volumetric flowrate (m3/s) by the passing section of the pipe (m2).

Calculation of air velocity in pneumatic transport pipes

Equation 1 : calculation of the air velocity in a conveying pipe of diameter D

With :

- uair = air velocity in conveying pipe of diameter D (m/s)
- Qair = air volumetric flow rate (m3/s)
- D = pipe diameter (m)

Note the following : the air volumetric flowrate is changing with the pressure and temperature, along the conveying line pressure is decreasing thus air expands, which makes the volumetric flowrate higher and thus the conveying air velocity higher (at constant D). To counter this phenomena, it is possible to change the diameter of the pipe, thus D may not be constant all along a same pipe.

3. Usual values for dilute and dense phase conveying

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 : Reference end of line velocity for dilute and dense phase conveying

Type of conveying Pressure Vacuum
Dilute phase
20-25 m/s 20-30 m/s
Dense phase
4-8 m/s
5-10 m/s

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




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