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Downdraft Exhausts

Downdraft Exhausts

Downdraft Exhausts

The Problem:
I have an application where i have designed a perforated dial (rotating very less than 10rpm)so that we can pull an exhaust through the perforated holes (5/16" diameter).  Our plenum cross section at the duct inlet was designed to equal to the 6" duct area and the plenum spirals around (donut shape) while tapering lower in cross section until it reaches a seal off wall.   The material we are exhausting is powder/dust.   The transport velocity in the duct is 4500fpm. Dial diameter is 36inches and the calculated holes (perforated) is 1,920 holes.  The dial thickness is 1/2" thick and the holes are straight holes with no leading edges (sharp edges).  I have only one 6 inch duct exhaust connected to the plenum.   The plenum exhaust is going in the same direction as the rotating dial.

The Issue:
The downdraft exhaust works well for the first 90 degrees of the plenum but it looses vacuum (exhaust) thereafter.

The Question:
Is it possible to design the plenum to have more uniform air exhaust 360 degrees and if so, how would you go about doing it?  I have thought about decreasing the taper in order to decrease the xsection of the plenum as it goes on around in an effort to increase the velocity - is my thinking correct here or are the pressure loses so great that i would not be able to overcome the static pressure?  I would like to try to solve this problem with the plenum design and not have to add an additional exhaust duck to the plenum.


RE: Downdraft Exhausts

If I understand the problem correctly, your plenum cross-sectional area is smaller than the total hole area. For effective distribution across slots or holes in a plenum, the total slot area must be about half the cross-sectional area of your plenum. So, either you need a much larger plenum, or you need way fewer holes.

RE: Downdraft Exhausts

Thanks for the response inventaguy!  Very much appreciated!

What i have is a circular plenum where the air exhaust is perpendicular (transverse) to the holes.  I have attached a picture of the problem. I hope this helps.   

I assume that slot area also is the same as hole area meaning that you treat holes like they are slots?

Can the pressure drop through the holes since there are so many holes cause enough of a resistance to where it dead heads your plenum?

RE: Downdraft Exhausts

Sounds like you have proven the answer to your question.

What is the static pressure in the 6" dia. duct?  See page 27 of the following link for some insight into pressure drop.  


It would be interesting to take a pitot traverse at different radial positions of your plenum to see what velocity is, you should be able to back into a pressure drop number from that.

RE: Downdraft Exhausts

Once again, thank you!

I took some pilot transverse readings this afternoon. Please find the attached excel spreadsheet.  As you can see, i definitely have some pressure drop that is affecting the flow.  WHat is the best way to back into the pressure drop by knowing that (equation)?

RE: Downdraft Exhausts

Can't say that I know how to relate your data to the sketch posted earlier.  Please provide better explanation of the locations.

RE: Downdraft Exhausts

Look at the 3rd reply -there's a link that shows the problem more in details.  Hopefully this would be enough.  I also posted the pitot data (link in reply 4)

RE: Downdraft Exhausts

Can't correlate the sketch to the data.  What do the positions represent? i.e. point 1= what point on your sketch, etc.

RE: Downdraft Exhausts

Oops, my bad  I see your point.  There are a total of 1,920 holes and it's a 36" diameter dial.  There are a total of 120 rows of holes (radially).  Therefore, i took pitot measurements by going every other row.  Therefore, the holes represent in essence degrees.

360degrees/120 holes = 3 degrees per hole so each hole is really degree where hole 1 would really be 3 degrees and so forth.  

Sorry for the confusion :)

RE: Downdraft Exhausts

You're getting warmer.  State where point 1 is with relation to some point on the sketch.  Is hole 1 at the blanked-off end or the duct, etc.  A sketch would really help to nail it down.

RE: Downdraft Exhausts

I appreciate your wish to maintain a carrying velocity in the plenum, however in most cases this is impractical. Typically I design my maximum plenum velocity for 1000 fpm and my slot velocity for 2000 fpm , or a minimum of 2x the plenum velocity. At 4500 fpm in a 6" duct you have nearly 900 cfm at your disposal which is quite a bit.

Of course the problem then is that your dust particles will drop to the bottom of the plenum, so you need a means to easily clean out the plenum, or slope the bottom of the plenum to allow gravity to move the dust into some kind of container which needs to be periodically dumped out.

To avoid this you could once again use gravity inside the plenum and install the exhaust duct at the lowpoint of the plenum. You would have to get creative here and you may not have the space necessary to do this.

You will still want to maintain the 4500 fpm duct velocity however so you don't have dust dropping out inside the ductwork.

If you don't want to scrap the plenum and disk at this point you may want to experiment by blocking off 3/4 of the holes in your disk with tape. This will improve your distribution but will lower your total airflow. I don't know how much static pressure your exhaust fan can provide.

I hope this helps.

RE: Downdraft Exhausts

Your sketch shows a nice solution, if there is room for it.

Peter Ott

RE: Downdraft Exhausts


Hole 1 is at the blanked-off of the plenum.  Thanks so much for the sketch. I originally designed the system to have that type of plenum (see attached original calculations) but we could not do that due to room constraints (we have drives in the way so we could not let gravity help.


Thanks also for the replies and insightful information.  You and DVD have been really great to bounce ideas off of and it has greatly helped me. This is my first plenum design so i have and still am learning a lot.

This past weekend, i closed off 1,680 of 1,920 holes; therefore, leaving only 240 holes.  I noticed that the plenum velocity (air distribution) got a lot better - almost centerlined (see the attach spreadsheet also showing this data (it's actually pretty interesting - see the tab labeled "holes plugged" - you can compare this to the other tabs "with bleed on and with bleed off".

THANKS again guys!

RE: Downdraft Exhausts


It was my pleasure to help. Good luck with your project!

Peter Ott

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