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Thread: Dust Collection Duct Size Question

  1. Dust Collection Duct Size Question

    I have a Kufo UFO-101 dust collector. It is rated at 2HP and 1224 CFM. The inlet is 5". I plan to either purchase a cyclone (probably Oneida's dust deputy) or make a trash can separator and covert it to two stage. I'm considering venting directly outside with no filter. My largest question/concern is the size of pipe. I've done lots of research and it appears 4" pipe just isn't that adequate. My shop is 30x40, but I'll probably only use about half of that for my woodshop portion, so let's call it 30x20. My plan is either to use 6" PVC or 5" metal HVAC pipe. I've seen on various forums where people run 6" PVC with a 5" inlet on the DC. But Oneida says this isn't a good idea. I bought the DC I have now on Craigslist with my table saw - it was basically "thrown in", so I don't have that much money invested. The point is I might upgrade in the future if I need a better unit as I add tools. Sorry for the long windedness - I guess my question is - is there anything wrong with running 6" PVC pipe if I have a DC with a 5" inlet? Or would I be better off sticking to the 5" metal? Thanks for any comments.

  2. #2
    The challenge you will have is the speed of air movement when you move up to a 6" duct. If the speed falls too low, the system is not able to keep the dust in suspension along the duct. The common speed I see is to maintain 4,000 FPM (feet per minute). Claimed CFM is only a part of the story and is often overstated. With any system, as one increases the duct diameter, FPM will go down even though CFM may be retained due to the larger diameter.

    If you've not explored the information Bill Pentz has published on his website about ducting, this would be an excellent place to spend some time:
    http://www.billpentz.com/woodworking...ne/ducting.php

  3. #3
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    Actually CFM will increase as you increase duct size, all else being equal. Increasing duct size results in less pressure loss so you operate farther to the right on the fan performance curve.

    The CFM is determined by the tool requirements. Duct size is based on minimum conveying velocity for the CFM required.

  4. #4
    Yes, CFM will go up, but FPM (velocity) will go down. It's a balance one has to manage in order to keep the dust in suspension. This may be why Oneida is not supporting increasing the duct size from 5" to 6" on this dust collector.

  5. #5
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    Quote Originally Posted by Rush Paul View Post
    Yes, CFM will go up, but FPM (velocity) will go down. It's a balance one has to manage in order to keep the dust in suspension. This may be why Oneida is not supporting increasing the duct size from 5" to 6" on this dust collector.
    Yes, I completely agree between 5” bs 6” in this application. I have also seen where others have posted that a duct supplier had increased their duct size in part of the system. I assume it was to lower losses which would happen, but the lower velocity (even though CFM increased some) could be a potential problem.

    If you were talking 4” vs 5”, you may have such a low flow due to high velocity and pressure loss for the given fan that you don’t collect the dust at the hood. This is what happened to the gentleman in the link you posted. Except, he lost flow due to filters plugging prematurely, according to his account of the events.
    Last edited by Michael W. Clark; 04-21-2021 at 10:18 PM.

  6. So here is the question and response from Oneida (edited slightly for brevity):

    Hi. I'm designing my dust collection system for my shop. I have a Kufo UFO 101 that Iplan to turn into a 2 stage system. I'm considering purchasing your cyclone aspart of this system. I've read that 6" ducting is much better than4", but my question: the inlet on the Kufo unit is 5" . PVC is notreally an option at this size. So should I use 6" PVC pipe that reducesdown to the 5" inlet. Or should I use 5" hvac metal pipe? Or should Ijust end up using 4" pipe. I'd like to go bigger and use the 6" PVC,but I don't know if that makes sense with the reduction to 5" at the DC. Anyhelp would be appreciated. Thanks

    Response:
    ...you will either want to utilize metal HVAC 5” ducting off the 5” inlet of the SDD as this will maximize the potential of the system or you could reduce down to 4” from the inlet if you are wanting to use PVC. .............. If you are really wanting to use 6” lines and see the potential for a better dust collector in the future, you could go with the 6” SDD XL but this is really not going to work as well and would not be recommended for long runs as your dust collector is only 2HP and it would not be able to handle longer runs due to this. You would need at least a minimum of 850 CFM to use this SDD. The 6” SDD XL is really meant for 3HP-5HP dust collectors, so if you see yourself getting a DC that falls in that range and would needs 6” lines then this option may be the way to go otherwise I would stick to the 5” SDD.

    My followup:
    If I don’t have any resistance from an air filter, and I keep my lines fairly short, would my 2HP unit do an OK job using the 6” pipe? ......................So, with no air filter and short runs, would the 6” PVC ducting do at least as good as 5” ducting? In other words, would reducing the main line down from 6 to 5 at the inlet hurt anything?

    Response:
    Performance wise the 5” SDD with 5” metal ducting is going to be your best bet, as it will out perform the 6” SDD with the 6” main run do to your system having the 5” inlet and lower HP. I cannot guarantee the performance with the 6” SDD with your dust collector since it is not one of ours. You may want to reach out to the manufacturer of your system to see if they believe it can handle the 6” runs and going to the 6” outlet on the SDD but it will run just fine with the 5” SDD.


    OK, so there was a bit of CYA going on there, which I totally understand. They have to be careful about recommending anything, especially if it isn't their products and also some DIY thrown in. But I still am trying to find the best answer to this question: If I run 6" PVC pipe, then reduce down with a reducer to 5" to go into the DC or a cyclone unit (SDD), is that going to hurt anything? I'd probably opt for the 5" cyclone and reduce from 6 to 5 before that. At this point, I'm ruling out 4". Based on the expert stuff I've read, 4" sucks and isn't worth the time and effort. But rather than spend $500 on a new unit, I'd like to use what I have at least for a while. I looked at HD last night and they have 5" metal duct, but not much selection for Wyes or other fittings. So I'm inclined to go with the 6" PVC. Keeping in mind that (1)my runs will be short, (2) I will limit bends and turns and step downs, and (3) there won't be any resistance from a filter as I'll be venting outside. Just want to make sure I'm not missing anything or that this decision isn't asinine. Again thanks for any input.

    So here's the setup as I envision it:

    Vented outside <------> 2HP Dust Collector <-----> 5" Cyclone <------> 5 <> 6 reducuer <-------> 6" pipe

  7. #7
    Everything the Oneida folks told you sounds right to me. I agree with your desire to use the 6" PVC as that will serve you well if you increase the capacity of your impeller and motor and step up to a cyclone designed for 6" ducts. But anytime you keep a 5" reduction in the system, that reduction point will limit air flow. So the 5" cyclone will always be a limiting factor in your system even though a much more powerful motor and larger impeller can overcome the limitations to a modest extent.

    The most likely worst case of starting out with the 6" duct is that you will have a decrease in air velocity sufficient to cause the dust to fall out of suspension and settle along the bottoms of the ducts. Keeping everything as short and direct as you describe will help a great deal. And, you might want to design for an easy access point to check for dust build up. In any event, you won't know unless you try it.

  8. Thanks. I guess I didn't think of that. If I step up in the future to a 6" DC, then I would probably need to upgrade the cyclone from a 5" to a 6" to take full advantage. The other option is I could build a DIY cyclone and just upgrade that in the future if I need to. That would be simple and cheaper.

  9. #9
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    When I first installed my Clear Vue cyclone, I really couldn’t afford the 5hp motor designed to turn the 14” impeller. I bought a 2hp table saw motor off ebay. I had to add a reducer to the 6” PVC pipe to keep the motor from drawing to many amps. I reduced it to 5”. It did well and ran for 20 years. I replaced it with a 5 HP last year and removed the restricter. I have more airflow now but I never had problems with stoppages with the 2hp. I say go for it.
    Charlie Jones

  10. Awesome. Thanks for the encouragement. Can you buy just the cyclone body from Clear Vue? I may be missing it, but on their website it looks like they only have packages with motor included. Bill Pentz has instructions on how to make one on his site, but I'm not much of a metal worker.

  11. #11
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    My theory is 6" for the horizontals is fine. If the velocity is too low dust will settle to the bottom of the pipe until the velocity increases enough. An inch of dust on the bottom is not enough weight to worry about.
    6" pvc is much cheaper and easier to find then 5" anything.
    Bill D.

  12. #12
    Quote Originally Posted by ChristopherShannon Wilson View Post
    Can you buy just the cyclone body from Clear Vue? I may be missing it, but on their website it looks like they only have packages with motor included.
    Clear Vue used to sell the cyclone body only as an option. You'd have to check with them as to whether they're still willing to do that.

  13. #13
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    Your reduction from 6" to 5" at the cyclone is of little concern from a performance perspective. The primary concern is the cyclone size. The SDD with 5" connections is designed for a lower flowrate than the SDD with 6" connections. What Oneida is telling you is that with 2HP, you will get better separation with the 5" SDD instead of the 6" SDD. This is because you don't have enough fan to pull enough air for the 6" SDD to have full collection efficiency. If the cyclone is oversized too much, it is not as efficient as one more properly sized.

    Whether your system duct is 6" or 5", the cyclone doesn't care, but your fan does. The 6" is fine as long as there are no major issues with material buildup in the duct. Weight is probably not the concern with small ducts like this and light dry wood dust, its when you cut something metal by accident and get a spark that gets collected at the tool but drops out in the duct with dry sawdust in it.

    Filter pressure drop is much less than cyclone pressure drop when the filters are in proper working order. Taking the filters off and adding a cyclone does NOT result in more flow when compared to having the filters only. However, your new flow will be steady (assuming no duct plugging) and you don't have to worry about cleaning the filters.
    Last edited by Michael W. Clark; 04-22-2021 at 7:33 PM. Reason: Added clarification

  14. Gents,
    Thanks so much for all your feedback. I bought the 5" cyclone today from woodcraft and I'm going to begin putting the system together. I'm also considering replacing my impeller. I've seen this a few times on youtube and it looks doable. The Rikon impeller is about $120 but the consensus is that it makes a big difference. My DC is 240V on a dedicated 20 amp circuit, so I think I'll be ok here. I also priced some 6" PVC today at about $4/linear foot, and a 45 degree fitting was $13. This isn't going to be cheap but should be a good system when I'm done. I'll update with some pics as I put it all together. Thanks again for all the advice. Much appreciated!

  15. #15
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    Heating ductwork and PVC are not good choices for dust collection. The heating ducts have unsmooth surfaces and poor connections. It's not critical if it leaks a bit, has high friction, doesn't transfer debris well etc. Sure you can seal it up but it still is inefficient. And PVC fittings are not efficient either. They are designed for water at low velocities.

    You would have better system performance with proper 5" duct and fittings.

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