There is no simple answer for arriving at the solution of your problem. Start by looking at "Industrial Ventilation- A Manual of Recommended Practice" published by The American Conference of Governmental Industrial Hygienists.

Talk to experienced dust extraction contractors for a quicker answer.

Michael Reid. ■

Hi Salim,

The Industrial Ventilation Manual will give you a couple of pictures and some rule-of-thumb formulae of unknown origin.

The problem is control of air flow. With a discharge nozzle it is possible to estimate what the air flow jet will look like, and the throw of the jet.

Try doing that with a vacuum cleaner and you soon learn that you have to be real close to the dust for the dust collection nozzle to work. Once you get one or two diameters away from the exhaust duct you have practically lost all control of the air flow.

In an open dump hopper situation you have to move massive amounts of air to ensure reasonable capture velocities. In most cases that just isn't feasible.

A more rational approach is to look at the natural flow of the dust plume when dumping. Can you predict where it would naturally want to go? How can you protect your dust extraction vacuum cleaner being swamped by wind? Even a gentle breeze will throw your Industrial Ventilation formulae out the window.

There is no easy answer. Even the dust consultants shy away from advising on this type of application because there are so many variables outside their control.

A similar problem occurs with screen covers. The cover extends over a large flat area. Usually one central nozzle is provided. Once you get one or two diameters away from the suction nozzle you have lost control of the air. If you over-suck you start picking up fines from the screen deck, which is not what a dust collector wants to see.

The Industrial Ventilation Manual will give you a few diagrams, again with formulae of unknown origin and reliability. Screen covers vary a great deal in detail. The best hope is to seal the covers as well as practical, and rely on developping a fair negative pressure inside the enclosure. This is affected by what is happening at the feed and discharge chutes - air flow from other equipment sources. In addition an estimate has to be made of the amount of air leaking in through gaps in the covers. The negative pressure under the cover will not be uniform. It too is governed by the law of least resistance.

What type of crusher are you using? A high speed crusher can act as a fan, creating its own internal air pressure, which again may be just enough to affect dust extraction air flow.

That is why experience is so essential in this business. And that is why different people will give you different advice.

Regards - John.Rz ■

Originally Posted by Michael Reid

There is no simple answer for arriving at the solution of your problem. Start by looking at "Industrial Ventilation- A Manual of Recommended Practice" published by The American Conference of Governmental Industrial Hygienists.

Talk to experienced dust extraction contractors for a quicker answer.

Michael Reid.

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