Dear mall,

If the duct system can only catch cement dust (<10 micron), then even 10 m/sec is more than high enough.

However, higher velocities in dust lines do not harm at all, as long as there is no possibility that cement enters the duct.

Therefore, before cement dust enters a ducting, there must be a region, where the air velocity is very low (< 0.75 m/sec). Any cement cannot pass this area and will fall out. The dust remains in the airflow

It is all balancing the local air velocities in relation to the suspension velocity of the cement and the cement dust.

Using the variation in suspension velocity is also a method of separating particle sizes.

Have a nice day

Teus ■

You can go to a lot of trouble measuring the particle size, deciding an appropriate settling velocity for each fraction but finally, you will end up using a figure which transports the heaviest/largest particles, even though the finest fraction can be transported at very low figures. This velocity will generally be about 22 m/s for cement dust (or any dust for that matter) and will ensure that there will be no settling in the ducts.

Tues is correct to say that there needs to be a settling zone in the area before the extraction hood so that there will not be too much product entrained.

For practical economic reasons selecting 20 m/s rather than say, 15 m/s means smaller duct sizes. There is a balance between higher pressure losses with smaller ducting and more power consumption and increased maintenance through abrasive wear.

For cement dust, it is better to make bends from harder steel than normal structural grades and to make bends (where most wear happens) of longer radius than standard (say 2.5 x R).

Michael Reid. ■

Dear mall,

The air velocity in a duct is not related to the dust load (10 g/m3 or 300 g/m3 equaling a SLR of 0.0083 and 0.25)

The air velocity is related to the suspension velocity of the particles.

Michael explained more practical considerations for choosing the air velocity and also indicated the character of the dust conveying as a pneumatic conveying system with a very low SLR. (pressure drop, velocity, settling of particles, etc.)

Have a nice day

Teus ■

In dust collection systems, dust loading has no direct influence on the design of the ducting, except that heavy dust loads will cause abrasive wear and will influence the wall thickness, bend radius etc. There needs to be no allowance for material friction, only for air friction.

Pneumatic conveying systems however, must take into account material losses as well as air losses, because of the much higher S/L ratio, as explained by Teus.

Michael Reid. ■