Air Velocities and Duct Sizes for Main and Branch Runs

      Large dust collection systems require complex airflow calculations. What do all the numbers mean? April 30, 2006

I’m trying to learn a little bit about wood dust and chip collection systems. has a CFM table of pipe diameters as a function of branch and main chip velocities, and they give an example explaining how to utilize it. They also sell standard 45 degree tees on taper (one pipe diameter plus a second diameter adds to a third diameter for correct flow).

From their chart, a main (horizontal pipe to collector) 6” dust collection pipe should draw 700 Cubic Feet per Minute to maintain a dust collection chip velocity of 3,500 Feet Per Minute, and a branch (vertical or from machine dust collection pipe) 6” dust collection pipe should draw 785 CFM to maintain a 4,000 FPM. Also, a 4” pipe should draw 300 CFM in the main at 3,500 FPM and 350 CFM in a branch at 4,000 FPM.

At this point, the example on the website states “A 4” branch will be run until it joins with the 4” branch from the Lathe. At this point your main starts and you need to increase the pipe to handle the combined CFM (350+350 = 700). Using the CFM Chart 1 look up 700 CFM under the appropriate velocity (3500 FPM in the main for wood dust), then look at the corresponding diameter (6"). Run 6" pipe in the main from the Lathe until the branch of the Radial Saw joins the main... Here again you need to increase your main to handle the total CFM (700+550=1250 CFM). Using the chart again you will see that 1250 CFM is slightly more than volume for 8” diameter. Drop back to 8” diameter so as not to go below transport velocity. Run the 8" duct in your main from the Radial Saw to your Dust Collector.”

After the chart and example, I thought I’d learned how to basically apply this stuff. The worst case scenario of a 6” and 4” wye is 785 CFM (6” at 4,000 FPM) + 350 CFM (4” at 4,000 FPM) = 1,135 CFM. The best is 700 CFM (6” at 3,500 FPM) + 300 CFM (4” at 3,500 FPM) = 1,000 CFM. Now, they only sell stock 7” x 6” x 4” and 8” x 6” x 4” 45 degree tees on taper in their catalog.

The way I interpreted their information, 1,135 CFM was less than the required 1,200 CFM of an 8” pipe to maintain 4,000 FPM and needed to be down sized to a 7” to maintain velocity. 1,000 CFM was less than the required 1,100 CFM of a 7” pipe to maintain 3,500 FPM and needed to be down sized to a 6”. Under this interpretation, I could understand offering standard 6” and 7” x 6” x 4” wyes. However, they offer standard 7” and 8” x 6” x 4” wyes. Since they are the pros, and sell 7” and 8” x 6” x 4” wyes, I must be making errors in my calculations. Can you please tell me where I’m going wrong?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From Curt Corum, forum technical advisor:
Super job interpreting the data. Please don't be thrown for a loop when you see a listing of 45 degree tee on tapers, wyes, etc. from a blowpipe manufacturer. Most of them manufacture a variation of over 200,000 fittings and typically show a limited spectrum, but do note that other sizes are available.

Common junction fittings are the lateral tee, which has a straight main body and a 45 degree branch and could be made from 3" on 4" to 36" on 40" and all sizes in between; and the 45 degree tee on taper, which has a tapered body and a 45 degree branch, and is also made in hundreds of different sizes. The branch of this fitting would not typically be larger than the small end of the tapered body, due to the fact that you might lose the physical area to fit the branch. If this occurs as a result of design, you would use a lateral tee and back end it with a reducer. If the branch is only one inch larger than the small end of the taper, it normally can be made. Example, a 6"x4"x5". Branch is 5" and can just fit on a 6" to 4" taper. Looks funny, but sometimes is required.

The Y-Branch is the third common junction, otherwise known as a pants y. You can either get this with all openings same size or legs which are smaller than the main opening. Example, 6"x6"x6" or 6"x4"x4". If you need a 6"x5"x4", you would order a 6"x5"x5" and reduce one of the legs. If the fitting you are looking for is correct according to design, I can assure you that it is available. Also, in keeping up transport velocity, we might get into the 4,500 FPM area, but that is okay - the resistance will be factored into the design equations. It is important not to size a pipe that is below transport, because the dust will not convey properly.

From Curt Corum, forum technical advisor:
I forgot to mention something else. A listing of fittings could pertain to applications other than sawdust. Different materials being conveyed require different velocities.

From Curt Corum, forum technical advisor:
I promise, this is it. After reviewing your post again... Maybe some of the confusion was due to the different velocities in the main versus the branch. Design criteria calls for a faster velocity in the branch and slower in the main. It has been explained to me that the higher branch velocity creates a better capture velocity at the end of the branch and carry to the main. I guess it's like traffic entering a thruway. You get on the angled entrance ramp, step on the gas, crank up the ramp and blast out ahead of the cars coming behind you. Then, once you get in with the traffic, you slow back to the speed limit and everyone moves along fine. This analogy does not relate to the interstates in Florida or California. Those guys are definitely conveying heavy, moist material.

From the original questioner:
That’s a lot of great information, and I appreciate you taking the time to explain it. I now understand that a 6” x 6” x 4” would be found under lateral T's instead of reducing. I also was only viewing the site from the standpoint of wood dust collection and hadn’t considered additional velocities for other applications allowing the manufacturer to sell items I wouldn’t really use. Finally, the thruway example was priceless. I was convinced “branch” and “main” meant overcoming gravity in vertical and horizontal runs.

The thruway example also caused me to wonder about something else. Picture an industrial system with a large horizontal main, some medium horizontal offshoots from the main and small branches from the medium offshoots to machines. What is the correct terminology for the “medium horizontal offshoot” in this system (main, sub-main, horizontal branch, branch, etc.)? Does the transport velocity of this medium section need to match the main, branch or something else?

From Curt Corum, forum technical advisor:
Let's take one example, a gang rip saw with 3, 6" ports on top. Each port would require 785 cfm. They would be combined together right above the machine with a 3 way Y fitting to 10" diameter. The ports are all under simultaneous suction with a velocity of 4,000 fpm and would maintain same velocity in the 10" all the way to the main pipe. Let's call this scenario a branch divvied up at the machine. An example of a sub main would be individual main pipes with branches. This gets a little tricky and harder to balance than one common main pipe. I like to keep the velocity of the branches and sub mains at 4,000 fpm till they join together to create the main pipe.

Let's say that we have a 10 hp collector that is moving around 3,000 cfm. A 12" diameter pipe comes through the wall at the center of the building. Shop owner says, I don't want a main pipe down the center, I want two mains down each wall. Dilemma, we can either take the whole nut of air and run a 12" main down each wall or split the air in half and run an 8" main down each wall. In either case, they would step up in size coming toward the collector till they reached max size, 12" or 8". They would be considered sub mains.

Now, 4 guys work in the shop all day. If we send the whole nut down each wall, they will only be able to work each side of the shop together. If two guys go to the other side and open gates, in both cases, they won't have enough cfm for transport in the sub mains. Thus, dust will drop out in the sub mains. Too large pipes and not enough air. Consequently, if we split the air in half, that means 2 guys will have to work on one side and 2 guys on the other. If 4 guys work machines on one wall in this case, it will be very difficult to now get the whole nut down the 8" sub main. Thus, there won't be enough air for the 4 guys on one wall in the split scenario. If the shop guys have a full understanding of the sub main scenario they are presented with, they can probably work it out. Moral of the story, sub mains are hard to balance for transport and not typically recommended.

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