### Dust Settling Velocity

The theoretical settling velocity is of no practical use in actual situations. You may very well be able to suspend a 40 micron particle of iron ore at 0.36 m/s but if you want a functioning de-dusting system you will design the ducting for 20 to 22 m/s and end up with economicaly sized ducts with minimal potential for blockage and minimal abrasive wear.

Michael Reid ■

### Dust Settling Velocity

Dear Reid,

As mentioned in the Design of Injdustrial Ventilation hand Book by ACGIH that minimum velocity to be maintained in the duct is the particle settling velocity. So thats why i have calculated the theoretical settling velocity to cross-check with the recommended duct velocities in ACGIH but i end up with very low veloity of 0.36 sec for iron ore dust.

Thanks,

Pavan. ■

### Settling Velocity

In practial terms, if you are handling iron ore dust in a typical de-dusting system, the maximum particle size will be much larger than 40 microns. Every change in direction or enlargement in the ducting will cause a drop in velocity. There is a marked velocity profile across the duct, even in ideal conditions (again, see ACGIH illustrations).

Do not try to design a system from first principles but follow the guidelines. They are well proven from long practical experience.

Michael Reid. ■

### Settling Velocity

Dear Reid,

Thanks for your suggestions. I want to bring to your notice that in an existing dedusting system we have maintained the duct velocity as per ACGIH guidelines but still we have faced dust settling problems in these ducts. Please suggest.

Thanks,

Pavan. ■

### Re: Dust Settling Velocity

Dear Pavan,

The velocity profile in the ducts has to be turbulent, whereby a minimum of dead zones occur.

That is the reason that the average air velocity needs to be higher than the suspension velocity of the particles.

To create an air velocity along the duct wall (thin boundary layer), the average velocity needs to be higher.

As Re=(density*Velocity*D)/(viscosity), small ducts require higher velocities, to reach the necessary Re number.

The wall velocity for a round duct is:

wall velocity = (-0.0165 + 0.0369 * ln(Re)) * average velocity

Dust settling problems are therefore related to the design, in which dead velocity zones must have been prevented. (Just as in pneumatic conveying)

Have a nice day

Teus ■

Teus

### Wall Velocity

Dear Teus,

Thank you very much for your reply. I have gone through all your replies on the thread "Pneumatic transport with different grain size distribution"

and got some idea. In the "Pneumatic transport" thread you have mentioned the minimum wall velocity to avoid sedimentation so simillarly i need your suggestion on the minimum wall velocity & Re number to avoid settling of irone ore dust in Dust Extraction Systems.

Thanks,

Pavan.

The velocity profile in the ducts has to be turbulent, whereby a minimum of dead zones occur.

That is the reason that the average air velocity needs to be higher than the suspension velocity of the particles.

To create an air velocity along the duct wall (thin boundary layer), the average velocity needs to be higher.

As Re=(density*Velocity*D)/(viscosity), small ducts require higher velocities, to reach the necessary Re number.

The wall velocity for a round duct is:

wall velocity = (-0.0165 + 0.0369 * ln(Re)) * average velocity

Dust settling problems are therefore related to the design, in which dead velocity zones must have been prevented. (Just as in pneumatic conveying)

Have a nice day

Teus

■

### Settling Velocity

Do you understand that the system airflows need to be "balanced" using a damper in each branch? This is so that the design velocity is maintained in all parts of the system. Blockages will inevitably happen if the dampers are not adjusted properly.

Michael Reid. ■

### Re: Dust Settling Velocity

Hi Tues, may i know this wall velocity Reynolds number that you are using, is it pipe reynolds number or particle reynolds number ? ■

### Re: Dust Settling Velocity

**The wall velocity for a round duct is**

The Re-number for a pipe is used here.

Have a nice day

Teus ■

Teus

### Re: Dust Settling Velocity

Hi Tues,

I tried calculating for the duct velocity using, wall velocity = (-0.0165 + 0.0369 * ln(Re)) * average velocity

In this case the wall velocity will be the one calculated Pavan to be 0.366 m/s.

Assuming pipe diameter of 200 mm, a duct velocity of 2 m/s will have a wall velocity of 0.72 m/s which is still 1.5 times higher ( taken from your quote in Pneumatic Transport with different grain size ) than the suspension velocity ( far from the 20 m/s suggested by ACGIH ). Your kind assistance is deeply appreciated, thanks in advance

PS : May i know from what book do you made this " wall velocity = (-0.0165 + 0.0369 * ln(Re)) * average velocity " formula ? ■

### Re: Dust Settling Velocity

Dear SeventhEro,

Calculating your example:

Re=(density*Velocity*D)/(viscosity)

Re = 1.12*2*0.2/(2.10^-5) = 22400

ln(Re) = 10.01

wall velocity = (-0.0165 + 0.0369 * ln(Re)) * average velocity

wall velocity = (-0.0165 + 0.0369 *10.01) * 2 =0.353 m/sec

The wall velocity in a pneumatic conveying pipe (or dust duct) needs to stay above the particle local suspension velocity (for small particles approx. 1.5 to 2 times).

In this example, the average velocity/wall velocity = 2/0.353 = 5.66 times.

Hence the average velocity in this case should be 5.66 * 1.5 = 8.49 times.

However, then the calculation has to be repeated for the higher average velocity, as this higher average velocity influences the Reynolds number and thereby the wall velocity. (Iteration)

The formula is a regression formula based on velocity profile graphs and must be considered as a general approximation. Influence of pipe roughness and presence of material neglected.

Have a nice day ■

Teus

## Dust Settling Velocity

Dear All,

I am having two doubts as described below:

1. Determination of Settling Velocity

I had tried to calculate the settling velocity of iron ore dust by using stokes law (i.e Drag force= gravitational force-buoyancy force) as below:

Density of particle = 2100 kg/m3

Density of fluid = 1.12 kg/m3

Highest Particle Size = 40 microns

Dynamic viscosity of fluid = 2*power(10,-5) kg/(m.s)

Finally i got settlig velocity of iron ore dust as 0.366 m/s, which is very low because we generally consider duct velocity of 20-22m/s. So please suggest me where i went wrong.

2. Settling Velocity as per stokes law

In a horizontal duct drag force will be parallel to centerline of duct whereas gravitational force will be in the downward direction and buoyancy force will be in upward direction. so whther can i use stokes law here. Pelase suggest me.

Thanks,

Pavan. ■