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Thread: DC duct sizing

  1. #16
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    Quote Originally Posted by Kevin Cackler View Post
    I knew that 4" stuff sounded off. The Airhand.com guide says that if your machine has a 4" port, then the manufacturer has determined that a 4" branch will support the machine.
    What it really means is that the machinery manufacturer has just done what they've done "forever" despite more attention to what's required to fully evacuate dust and chips from machines in recent years. The tools have 4" ports because they have always had 4" ports, in other words. It sounds like Air Handling's guide hasn't changed at all, either. Note, I'm not saying that a 4" port isn't appropriate for a specific machine; I'm only saying that it's not universal as it might appear to be since so many machines just happen to have 4" ports "just because". Consider that many folks have "kicked things up a notch" on table saws by up-sizing the port, having a hood around the blade and simultaneously support over-arm dust collection from that same larger hookup. The results are usually compelling.

    I agree with Chris that feeding multiple machines at the same time requires you to insure that the drop supporting multiple machines is adequately sized to handle the air flow required for the number of open ports at one time. So if you have two 4" ports that share a drop, the drop, itself, must be sized to adequately support the air flow that will allow the presumable 400-500 CFM max that can flow through a 4" port times the number of ports. And, of course, the blower on the collector needs to be adequate for the task, too. A 6" drop servicing two 4" ports "likely" will work ok (without doing the math). Designing duct for a shop that has multiple simultaneous machines/workers is a lot more complicated than designing a duct work system for a one-person operation!
    --

    The most expensive tool is the one you buy "cheaply" and often...

  2. #17
    Knowing that the branch needs to be upsized to support potentially more than one 4" port does answer one of my primary concerns. Laying everything out was the hard part. Resizing things is easy in comparison!

    I appreciate everyone's input. I'm going to go resize things on my own and report back here so I can find out what else I did wrong, lol

    Thanks guys

  3. #18
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    If you keep in mind that it's all about "volume", it will be easier for you to visualize what needs to be done even before any of the math gets tackled. The volume of two machine ports is what it is and the drop that supports them simultaniously has to accommodate that fact.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  4. #19
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    Kevin,

    In addition to what the air handling guide provides you can use the attached chart to help with your calculations. So if you have 3 4" machines with each requiring 300-400 cfm running simultaneously, then you will need 900-1200 cfm from that feed duct or a 6-8" duct depending on your velocity. If you want/need even more CFM then the ducting requirements just goes up and a 4" wouldn't cut it. As Glenn mentioned depending on how much is being fed simultaneously will make a difference. I would guess only feeding one machine at a time with three running at minimum cfm levels would be worst case. The path of least resistance would be through the other two non feeding/running machines ducts, increasing their flow and reducing the machine duct with material (not what you want). Increasing above minimum flow to 500-800 cfm just increases the feed duct size.

    Also with the type of blower you are using the specified flow rate is usually exaggerated a great deal. Since you are not using the filter bags that helps but the numbers are well beyond real world. I would guess your blower throughput is 25-40% less than what Grizzly states.

    Duct data chart.JPG
    Hope this helps. Let Us know what you do.

    Carl

  5. #20
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    That's a handy chart, Carl! Thanks for sharing it. I'm going to post it as a sticky "up top" for folks to reference in the future.
    Last edited by Jim Becker; 09-12-2018 at 2:25 PM.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  6. #21
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    I don't suppose there is a chart like that for flex hose? I know it is about triple and varies according to stiffness and smoothness of the hose but a base line would be handy.

  7. #22
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    The challenge with flex for such a chart is that different "makes and models" of flex hose have different characteristics relative to drag, etc. It would have to be a compromise if someone could come up with it.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  8. #23
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    Just as a comment...Grizzly shows 5 hp and 15" impeller and 3961 cfm. Neither the Oneida or ClearVue with similar specify anywhere near that cfm.

  9. #24
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    Quote Originally Posted by Larry Frank View Post
    Just as a comment...Grizzly shows 5 hp and 15" impeller and 3961 cfm. Neither the Oneida or ClearVue with similar specify anywhere near that cfm.
    I don't find that surprising...
    --

    The most expensive tool is the one you buy "cheaply" and often...

  10. #25
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    Quote Originally Posted by Chris Parks View Post
    A machine manufacturer has determined what is best..........I can hear the laughter form here. No machine manufacturer to my knowledge has even the vaguest idea of what good dust control is.
    Hi, I have a saw/shaper and a jointer/planer.

    Both came with airflow requirements for the machines, and a certificate of conformance for dust collection performance.

    There are many manufacturers who actually design in dust collection................Regards, Rod.

    P.S. That said, I agree with you in general, if you have a machine with a 4" port, it's most likely that the port is too small and the manufacturer didn't desin in dust control.

  11. #26
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    Quote Originally Posted by Larry Frank View Post
    Just as a comment...Grizzly shows 5 hp and 15" impeller and 3961 cfm. Neither the Oneida or ClearVue with similar specify anywhere near that cfm.
    Of course they don't, with good reason............Rod.

  12. Quote Originally Posted by Larry Frank View Post
    Just as a comment...Grizzly shows 5 hp and 15" impeller and 3961 cfm. Neither the Oneida or ClearVue with similar specify anywhere near that cfm.
    Dead on. My Clearvue with 16" impeller, pulling just shy of 20 Amps at 240 V, gets a genuine, measured, 1000cfm at my table saw and a little less at other machines using 6" ducting and 6" machine ports. I expect that without any ducting / restriction it may possibly get around 1500cfm. I wouldn't expect the Grizzly with a 15" impeller to do any better so planning on a total of approximately 1000cfm with decent duct layout would be appropriate. Spread over 2-4 machines at once that isn't a great deal. I run 1 machine at a time only and find the dust collection very good, but I wouldn't want to run 2 simultaneously.

    Also, running multiple machines simultaneously really requires some thought regarding tuning the restriction of each ducting run and machine restriction to balance the flow rates appropriate for each machine - path of least resistance and all that.

    If the OP uses only 4" ports and ducting to the machines but with a larger main branch, he will also need to ensure that several 4" ports are always open to maintain 4000fpm velocity in the main branch - which will of course mean that even if only using one machine at a time, the dust collection at that machine may not be great.

    I'm not saying there is not a work-able solution, just that it requires careful calculation and planning - and then using the system appropriately once installed.

    Cheers,

    Dom

  13. #28
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    Quote Originally Posted by Kevin Cackler View Post
    Due to ceiling height restrictions (Only 12') I would like to keep things in the wall-ceiling corners as much as possible.
    Any and all help is very much appreciated!
    The problem with putting the duct in the wall/ceiling corner is that the wyes will point down the wall. That can result in chips/offcuts falling down the unused downstream wyes as debris passes the opening on the bottom of the duct. Best if you can locate the duct just far enough away from the wall to align the wye opening horizontal then add a long radii sweep to take the pipe down the wall. Man, I wish I had 12' instead of under 8' ceiling height.
    NOW you tell me...

  14. #29
    Never even thought of that, Ole! Very good point, though. We will definitely move things out a bit. Thank you

  15. #30
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    Look inside the machine to see how small the opening is at the point of collection. This is the true port size for your machine. Also see the post in the stickies above for good info on dust collection duct design. Sadly the real port inside the machine is the limiter for nearly all of us. The two machines where this is an exception for me are my Powermatic planer and Powermatic dual drum sander. Both take a 6" duct at the machine and do not have a smaller port inside the machine.

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