Dedusting pressures/velocities

Posted in: , on 4. Sep. 2003 - 10:03

We are redesigning a filter ducts to improve the effectiveness of the dust extraction from a system comprising a conveyor belt and a crusher.Material handled are shale and pozzolana.What I want to know are the recommended speeds/pressures for dust extraction in the ducts. I'll equally be happy if someone recommends good literature that covers this subject at length from the basic concepts.

fan power 13.6kw,total pressure 290dapa,capacity 230m3/min

we need to limit dust in clean air to <50mg/Nm3

Richard Davies - Dustex, USA
(not verified)

Duct Velocities

Posted on 4. Sep. 2003 - 05:25

Suggestion you received earlier is a good one. The Industrial Ventilation handbook is like our industry bible.

Based on the information you provided, I suspect you are looking at maintaining a duct velocity for the shale and pozzolana dusts of about 1380 M/min (4500 fpm).

Ducts would then be sized at somewhere between 45 and 50 cm in diameter.

Hope this helps.

Dedusting Pressures/Velocities

Posted on 5. Sep. 2003 - 12:43


Be careful not to ecxeed 23 m/s (4500 fpm) or abrasive wear will be a problem. Use 2.5 mm to 3 mm th duct material. Bends two gauges heavier.

Good Luck,

Michael Reid.

Duct Sizes/Velocities

Posted on 9. Sep. 2003 - 11:10

Thank Messrs Reid and Davies for your input,most literature i've read never mentioned the length of the duct from dust source to filter as a factor in the the determination of the duct diameter? is the duct length a parameter of no significance?

when the upper limit for the velocity is 23m/s,how low can one go in the velocity?

Dedusting Pressures/Velocities

Posted on 10. Sep. 2003 - 03:34


As I said, "Industrial Ventilation" will tell you all you need to know, (except perhaps max. and min. velocities!). Follow the ducting design procedures. The "equivalent foot" method is probably easiest.

Start with selecting exhaust quantities, example: Refer to Specific Operations, Fig. VS-50-20. A conveyor transfer needs 500 cfm per foot of belt width for speeds above 200 fpm, at the head enclosure and on the following conveyor. Add 50% for dry dusty materials. Don't forget additional exhaust at "A".

Size the duct according to velocity. Heavy materials like mineral dust need more than 20 m/s. You can convey these dusts at lower velocities but.... More than 23 m/s will result in too much wear at bends and junctions. Duct length only affects the friction losses, diameter determines velocity.

Work out the diameter and length of duct run, add the equivalent length of bends and branches. Calculate the losses in inches of water per 100 ft. (or other units if you prefer). Use the Darcy Formula at the bottom of Fig. 5.21a. That is easier than trying to use the chart. Add the entry loss for the hood (Fig. 5-15). Add 0.25" w.g for a balancing damper.

This will give you losses up to the dust collector inlet. Other losses depend on the type of collector, the duct between the collector and fan, the discharge stack, silencer etc.

Michael Reid.