Hi Ham,

I can understand the problems you are having. We had similar problems on our plant.

The calculation of pressure losses in a ducting system is very simple. Any Ventilation Handbook will give you the charts and formulae to use. I prepared a detailed spreadsheet calculation for our dust collection system [see spreadsheet on www.cospire.com / Engineering Tips / Bulk Gases. The note on Dust Collection Duct Calculation does quote some literature, but any ventilation design literature is also helpful. It's not magic - it is basic principles of airflow. The key is to keep duct velocities above particle settling velocity of say 20 m/s].

With an existing system you can measure static and dynamic pressures, and hence deduce velocities and flow rates. Use these to produce a detailed spreadsheet which accurately reflects your current system. From there you can make any changes on a What-If basis, and see the effects without having to spend any capital.

The dents and deformations have been caused by your maintenance department in trying to break down internal blockages to keep production going. We installed compressed air jets at problem bends to allow us to purge the dust blockages without having to shut down the plant for manual cleanout - not a very elegant solution, but very pragmatic when you have a production line to run.

Regards - John.rz

http://www.cospire / Engineering Tips / Bulk Gases

http://www.cospire.com/koviewer.aspx?id=13638 ■

Thanks Ham,

There is always a problem at the wet-to-dry interface. We dropped our dust/powder down a gravity pipeline into an aggitated mixer. The bulk density of the dust might be 400 kg/m3 - so it would float on top. Every so often the duct would block, and back up with dust several meters to the rotary valve. The only solution was a shut-down, and a manual cleanout. I think the moisture might have developped a collar of dust at the interface. Ultimately the collar was large enough to cause bridging [it may not be an actual bridge. But the restriction causes enough backpressure, together with the powder already in the void above the liquor surface, to stop flowing].

The key seems to be - how to get the powder to mix with the liquor. The same thing happens when you make a cup of coffee and pour the powder on top of the water. It is better to pour the water on top of the powder. That mixes quicker. Options are

1 - Change the mixer paddle to create more vortex action

2 - Spray liquor onto the powder

3 - Provide more volume above the liquor surface to allow a longer settling time

4 - Change the powder duct from steel to stainless, or PE to reduce the wall friction

We did not solve the problem. The cost of modification did not seem warranted, so the operators still have to shut-down and clean out manually every month or so.

Regards - John.Rz

www.cospire.com /Browse /Engineering Tips ■

Hi Ham,

It depends on the nature of the powder buildup.

1 - Dry powder - We did not use vibrajets. All we did was direct a jet of air at the section where dust was prone to settle [anything to aggitate the dust and cause it to again be engaged in the active air stream]. It was not a continuous thing. Our operators turned on the air jets when they felt there was a significant buildup.

Dust tends to build up in areas where there is a lower local velocity - such as on the inside of a bend. The bend accelerates the air stream, but parts of the cross-section experience eddies at low velocities.

2 - If your dust tends to cake you have a bigger problem. Lumps of cake are too heavy to be carried by an airstream.

3 - Knockers are just another form of vibration of the duct walls. Try some simple experiments. That would give you more information to guide your decision.

All the best - John.Rz ■

Hi Ham,

I have some suggestion for you.

As Mr John advise, ducting velocity should be 18-20 m/s, below you can get some problem as clogging up. You can improve performance of current ducting by some ways:

+ After operation time of mixer, let extraction system run 15-20 mn to suct all dust stay inside ducting system.

+ Close some unuse suction points by damper to increase ducting velocity.

+ Change to use smoother ducting, such as straight duct instead spiral duct.

Best regards.

The vagrant. ■

Hi Ham,

I have some suggestion for you.

As Mr John advise, ducting velocity should be 18-20 m/s, below you can get some problem as clogging up. You can improve performance of current ducting by some ways:

+ After operation time of mixer, let extraction system run 15-20 mn to suct all dust stay inside ducting system.

+ Close some unuse suction points by damper to increase ducting velocity.

+ Change to use smoother ducting, such as straight duct instead spiral duct.

Best regards.

The vagrant. ■

Ham,

Knockers or vibrators are often installed on pipes with out success due to the difficulty of mounting a standard vibrator on a pipe at the exact location of the flow restriction.

We can engineer custom fitted saddle mount vibrators (knockers) for pipes and chutes in any diameter and adjusted to fit the exact application. We deal with dust collection applications very often.

BENEFITS:

1. Rugged construction of the bracket provides reinforcement of the pipe.

2. Maximum surface area contact to transmit the vibration energy directly through the pipe and into the material.

3. A clamping assembly that can easily be loosened and moved to changing flow restrictions.

4. A repetitive short time duration impulse vibrator for effective vibration and efficient vibration energy transmission to the chute interface.

See pictures here:

http://www.navco.us/pipe.html

Here are some other baghouse/dust collector vibrator applications:

http://www.navco.us/baghouse-solutio...-vibrator.html ■

Here is picture of the custom made saddle mount vibrator for pipes and chutes by NAVCO.

Attachments

pipe vib 2 on 3d pipe (JPG)

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Moisture is the cause of your problem and will be very difficult to overcome. Mechanical vibrators and hammers will only wreck the ducting.

Fit clean-out doors (flush fitting to the inside of the duct) at points upstream and downstream of the blockage and clean the ducting regularly.

Michael Reid. ■