You can control the dust without any dust suppression system if you control firstly the material flow and then the induced air flow across the material surface. Flow control is critical as you must minimise the air displacement through any impact or direction change of the material, induced air flow is logical. To do this you need to look at the geometry of what you have and flow characteristics of the material. Once you have minimised the generation of dust at the transfer point you then need to look at methods that can be used to slow the residual air flow mass within the transfer enclosure. These are the general principles, applying them correctly requires specialised knowledge or a lot of trail and error.

To give you an example, we reduced the dust levels in a belt to belt transfer handling alumina (very dusty, very hard to control the flow) that had a dust extraction system from 2500ppm to 80ppm and as part of this we removed the dust extractor as it was increasing the induced air flow across the material face.

The problems you talk about as John has said are common and a huge industry issue. To solve the issue requires a much bigger commitment to quality design.

Col Benjamin

Gulf Conveyor Systems Pty Ltd ■

This is a different kind of problem Colin. ■

I agree Larry, it is different from normal transfers but you can apply the same principles successfully. It all gets down to the geometry of the transfer and how you can manipulate the flow within it.

All the best

Col Benjamin ■

Colin, it is not a transfer flow control problem. The crusher is acting like a fan. The negative pressure created at the inlet and positive pressure at the outlet far exceed gravity flow and dust control. The outlet pressure and velocity of its gas stream needs to be abated. The cannot be easily accomplished by increasing the plenum.

The most successful means is to feed back the positive pressure into the negative side, which in effect cancels most of the aggressive gas velocity exiting the crusher beyond the feedback ducting. ■

Hi Larry,

I understand what you are saying and agree with the principles you are stating but it will still get down to the geometry of the transfer and the points I raised. I have seen some extremely clever ways to address the pressure relativity issues but whether you can use them gets down to the configuration of the collecting chamber or transfer as they all get down to flow control. Pressurizing requires sealing, it is much easier to work through means to equalise the pressure as once you do this you do not have an ongoing maintenance problem trying to maintain seals.

Have not seen you for a while, how have you been by the way

Col B ■

The best way to depressurise the discharge of the crusher is to provide an expansion chamber at the discharge and vent this chamber to a pneumatic dust control sytem with an air volume slightly larger then the displacement air being "pumped" by the crusher it self. Same time you may also duct the chamber to the inlet of the unit. But an expansion chamber and power venting is the key.

Should you have specific information of the system or drawings please e-mail to me at hsi@wcoil.com we may halp you with the design.

Unsal Yonak,

Vice President

Hawthorne Systems Inc.

312 W.Main Str

Beaverdam Ohio 45808 USA

phone: 419-643-5861

fax: 419-643-4762

e-mail: hsi@wcoil.com ■

Unal,

Please tell us how you control the exit gas velocity and pressure once your expansion chamber is filled and pressurized with the fan action.

First impression tells us the exit gas velocity and entrained dust will not change at the exit chute aperture with or without the expansion chamber unless you apply a powered dust collector.

Venting to a dust collection system will help but at a substantial cost and maintenance penalty.

Coal impact crushers have a recycle chute to equalize the input and output pressures and then decant the remaining dust within an appropriate skrt/chute system. ■

Greetings from the soon to be frozen "Eastern Wilderness" @ 1140 feet above mean sea level.

Urwithdilup, we still need more information- feed rate tonnage, screen sizes, impact crusher settings/speed/relating to final product size, but I will offer my suggestions below:

Under the assumption that you are crushing bituminous coal that has been screened and you are crushing oversize coal only:

You have five available options.

(1) slowing down the feed rate to the screener if possible by reducing flow to the screener and there by reducing generation of fines. this may be as simple as reducing the feed rate to the crusher with a chute adjustment or as complicated as installing a different gearbox to reduce the speed of the delivery belt-assuming you are using a conveyor to feed your screener.

(a) adding an addtional screen to your screener if possible to imrove quality of product and reduce oversize material entering crusher.

(b) If you have the ability to slow down the rotation of the impact crusher with a variable frequency drive you can do that. we had to do that with a spokane crusher that was rotating to fast for the application. If that is not an option changing the pulley size of the crusher rotor with a larger pulley is the easiest way-if allowed by the maufacturer.

(c) adding more suppression water to the downstream flow under the crusher-more water nozzles.

(d) spraying the precrushed coal at the point of entry into the crusher with a water mist.

(e) reducing the feed tonnage entering the crusher. the simplest first experiment/algebra problem if it is an option with you system.

George, john, lyn, larry if I missed something my humble apologies.

leon z ■

Have you studied creating a feedback circuit to cut the pressure differential between input and output of crusher? The crusher is acting like a fan. It sucks air in and pumps it out. Place a connectiion from the crusher output plenum to the input plenum.

This will drop the downstream gas velocity, beyond the feedback loop, whereby you might be able to then baffle the flow stream sufficient to allow decanting of the coal particles. Building of the circuit may require a CFD analysis to give duck sizing and downstream receiving skirtboard plenum details.

Engineering needs to be done with the details, but the concept should get you going. ■

We had the same problem with a gyratory crusher running at 2000 rpm connected to a dust extraction system.

Everytime we started up the crusher it acted as a fan, upsetting the balance of airflow to the ducting system. ■