The design of dust extraction systems is not a very exact science. Much of the information contained in the Industrial Ventilation Handbook is only intended to be a general guide.

The pressure loss in the ducting system is not very sensitive to the amount of dust in the system. The pressure loss is proportional to the density of the fluid. At 100 - 200 gm/m3 this does not make a significant difference.

The main concern is the settling characteristics of the dust particles carried. In still air they follow Stokes Law - but settling rates vary with particle size, density, shape and so on. In most cases this is just too complicated - so the Handbook just gives an average figure that works most of the time.

As you have noted it is possible to run at far lower duct velocities. This introduces two dangers

1 - Over time the ducts may get blocked - particularly in the smaller sizes. We vacuum the main ducts every so often to reduce the internal dust buildup.

2 - With a dirty duct system, when you change to clean bags, the bags will become dirty within hours. Where you would expect to be running at 100 Pa clean, you quickly run up to 500 Pa or more. We had a case where the bags were clogged within 2 days of clean.

To avoid these problems it is better to run your ducts with a velocity high enough to scour the ducts clean. If you run with dirty ducts you are exposed to higher maintenance costs and addditional down time. ■

The Dust Control Handbook gives this advice:

Minimum Design Velocity fpm

Very fine, light dusts ----------------- 2,000

Fine, dry dusts and powders ------- 3,000

Average industrial dusts ------------ 3,500

Coarse dusts -------------------------- 4,000 - 4,500

Heavy or moist dust loadings ------ 4,500 and up

The minimum transport velocity indicated is for guidance only. The selected design velocity should include a safety factor to allow for the following

1 - dust buildup

2 - duct damage

3 - corrosion of ductwork

4 - duct leakage

The risk is that the light particles will travel through OK. The heavier ones will drop out, particularly at elbows, and at the bottom of vertical ducts. So you should look at your maximum particle sizes.

As you already have the plant running you are free to experiment with the actual gasses and particles involved. ■

Mall,

A duct system should never have any dust build-up if it is properly designed. Don't mess about. Design for 4,000 fpm and follow the guidelines of ACGIH to minimise wear. No problem.

Michael Reid. ■

Mall,

Another thing. Higher velocity = smaller ducts = lower cost.

Michael Reid. ■

Air flow requirement

I want to know if someone can guive me some guidelines about the flow of air that is necesari to dedusting belt conveyors, aeroslides, silos, bucket elevators, etc. Or where I can find this information.

Best regards ■

Go to the manual mentioned above; "Industrial Ventilation" published by the American Conference of Governmental Industrial Hygienists.

Michael Reid. ■