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Thread: Need help deciding on dust collection strategy

  1. #1

    Need help deciding on dust collection strategy

    I know this topic has come up a lot and I read most of the threads on here on the topic. Still, I'm undecided. My shop is 600sf (20x30). I'm a hobby woodworker and have the. HF dust collector in one corner. I intend to upgrade the impeller on the HF unit to the Rikon part.

    The tools I have right now are an old Delta unisaw, an 8" Delta joiner, a radial arm saw, a 13' thickness planer, an old Delta/RW lathe and a small drill press station. I ran 4" PVC under the concrete from one side to the center for use by the tables. I have put a drop ceiling in with troffer lights and will be installing batt insulation and ceiling tiles so the space will be constantly heated and cooled.

    What I am undecided on is to use either 5" metal duct for my main trunk lines or use 6" metal/pvc. My local hardware shop has 6" pvc drain pipe. I have both 5" and 6" metal duct. My longest horizontal run is about 25ft.

    My two approach ares:

    a) 5" mains using wyes that I fabricate myself (already done done) or order (sheet metal kid) with 4/5" drops too tools. The reasons to do this is because it likely will have higher air speed.

    b) 6" mains using metal duct or pvc. 6" parts are more readily available. If I decide to go with 6" then I would enlarge the inlet on the HF unit to 6". The main I can think of doing this is so I can upgrade my DC to a 3HP unit later on and not have to dig into my ceiling to replace the 5" pipe to 6". I know 5" pipe can work with 3HP unit as well but 6 would be better. I prefer to use PVC simply because it's easier to work with but metal is ok too.

    I just don't know if the HF unit with the upgraded impeller can keep the air speed up high enough in the 6" duct. I see people on Youtube that say yes, but just not sure.

    Whatever I decide I will be running my mains above my drop ceiling so I really don't want to be digging into my ceiling for any "cleanup" work. I will be either building or buying a cyclone.

    Any advice welcome!
    Last edited by Reggie Burnett; 05-17-2020 at 11:32 PM.

  2. #2
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    Seems to me if the smaller collector does not like 6" pipe it will still be fine for horizontal runs. Use 4 or 5" for the drops to keep the dust moving. If it drops out on the horizontal runs who cares? It will sit on the bottom inch and effectively be a 5" horizontal run until you upgrade to a bigger dc. Then it magically becomes a 6" pipe with no added expense. it might be a job idea to brush it out before you turn on the new dc. Use a chimney brush to do it.
    It is probably easier and cheaper to do it in 6" pipe, at least for the horizontals.
    Bil lD

  3. #3
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    Using 5" for drops is a sound plan, IMHO, based on my own experience over the last 20 years. My shop is almost the same size as yours, although my DC is more capable. If you upgrade the impeller and put a 6" flange on the HF, I suspect you will be fine with a 6" main run if you make your duct network efficient. (one trunk, not around the perimeter, etc.) As you note, that may be better for any potential system upgrade later. I personally prefer metal duct and with 5" drops, you'll need to use metal anyway. Plastic tends to only be available in even number inch sizes.
    --

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

  4. #4
    I use a HF with 5 inch metal duct as far as possible. I use a super dust deputy and discharge outside. I have a horizontal run of 5 inch for my table saw and noticed about 1/4 inch of dust in the bottom when I put in a wye for the overarm collection. I was surprised because it also sucks small offcuts up off the floor and miter saw and I hear them get to the cyclone. You will probably have more dust in horizontal runs of 6 inch but I don't see it as a real issue. I get fair dust collection with two gates open, I suspect that would be more true with 6 inch.

  5. #5
    Quote Originally Posted by Jim Becker View Post
    Using 5" for drops is a sound plan, IMHO, based on my own experience over the last 20 years. My shop is almost the same size as yours, although my DC is more capable. If you upgrade the impeller and put a 6" flange on the HF, I suspect you will be fine with a 6" main run if you make your duct network efficient. (one trunk, not around the perimeter, etc.) As you note, that may be better for any potential system upgrade later. I personally prefer metal duct and with 5" drops, you'll need to use metal anyway. Plastic tends to only be available in even number inch sizes.
    Thanks for taking the time to respond. I'm going to use plywood and hvac start collar to make a 6" intake for the HF DC and then take some air readings at 10, 20, and 30 ft of duct. When I'm convinced that it can pull that then I'll commit to the 6" trunks.

    I'm going to post my shop design in an image to see if you guys think my dust collection main line runs make sense.

    Thanks!

  6. #6
    Quote Originally Posted by Jim Dwight View Post
    I use a HF with 5 inch metal duct as far as possible. I use a super dust deputy and discharge outside. I have a horizontal run of 5 inch for my table saw and noticed about 1/4 inch of dust in the bottom when I put in a wye for the overarm collection. I was surprised because it also sucks small offcuts up off the floor and miter saw and I hear them get to the cyclone. You will probably have more dust in horizontal runs of 6 inch but I don't see it as a real issue. I get fair dust collection with two gates open, I suspect that would be more true with 6 inch.
    Thanks for your reply. I like the idea of discharging outside but my space has a 18k btu heat pump on it and the negative pressure might be a problem. Guess I'll just discharge into a filter.

  7. #7
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    Quote Originally Posted by Reggie Burnett View Post
    I'm going to use plywood and hvac start collar to make a 6" intake for the HF DC and then take some air readings at 10, 20, and 30 ft of duct. When I'm convinced that it can pull that then I'll commit to the 6" trunks.
    I've already done that testing. What would you like to know?

    Test Duct.jpg

    Versus_Original.jpg
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  8. #8
    Quote Originally Posted by David L Morse View Post
    I've already done that testing. What would you like to know?

    Test Duct.jpg

    Versus_Original.jpg
    David,
    Have you measured the pressure drop and flow across your tools to estimate the entrance loss coefficient for the tool? I just have played with my set up learning how to get good data with the Testo manometer and hot wire anemometer. That is the goal.

    Are you measuring flow with a Pitot tube? The hot wire is really not suited for less than 6” pipe. Max velocity for it is 6000 FPM.

    Thomas

  9. #9
    Quote Originally Posted by David L Morse View Post
    I've already done that testing. What would you like to know?

    Test Duct.jpg

    Versus_Original.jpg
    What was your airspeed at different lengths away from the DC? Are you using 6" mains with your upgraded HF unit? In your opinion can it handle it decently? I know it won't be great but "good enough" for awhile until I can upgrade to a better DC?

    and thanks!

  10. #10
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    Quote Originally Posted by Thomas Wilson View Post
    David,
    Have you measured the pressure drop and flow across your tools to estimate the entrance loss coefficient for the tool? I just have played with my set up learning how to get good data with the Testo manometer and hot wire anemometer. That is the goal.

    Are you measuring flow with a Pitot tube? The hot wire is really not suited for less than 6 pipe. Max velocity for it is 6000 FPM.

    Thomas
    Yes, the entrance loss is critical. It was the need for more pressure at the hoods that prompted me to do the Rikon transplant. I have both the Testo hot wire anemometer and manometer. The hot wire has more issues in a 6" duct than just the maximum velocity rating. It's useful for checking temperature however. I did all of my testing with a pitot tube.

    Quote Originally Posted by Reggie Burnett View Post
    What was your airspeed at different lengths away from the DC? Are you using 6" mains with your upgraded HF unit? In your opinion can it handle it decently? I know it won't be great but "good enough" for awhile until I can upgrade to a better DC?

    and thanks!
    Six inch duct has so little loss at these flow rates that you can almost ignore it. At 600CFM you lose about 1% for each ten feet of duct that's added. As pointed out by Thomas, it's the entrance loss that you need to be concerned with. That's what will determine your velocity, not the length of duct.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  11. #11
    Quote Originally Posted by David L Morse View Post
    Six inch duct has so little loss at these flow rates that you can almost ignore it. At 600CFM you lose about 1% for each ten feet of duct that's added. As pointed out by Thomas, it's the entrance loss that you need to be concerned with. That's what will determine your velocity, not the length of duct.
    Can you better explain to me what you mean by "entrance loss"?

  12. #12
    Quote Originally Posted by Reggie Burnett View Post
    Can you better explain to me what you mean by "entrance loss"?
    Reggie,

    Flow in a duct loses energy as it goes down the pipe. Friction against the wall is one type of loss. Fittings like elbows, wyes, and diffusers (area change) causes a slightly different type of pressure loss. The loss is more from swirls in the flow that dissipate energy by the fluid rubbing against itself. The entrance at the tool is in that second kind of loss. The flow loss at the entrance is geometry dependent so odd ball shapes like the shroud on a planer or table saw are large and hard to estimate. You can be sure that every tool is going to lose 1 to 3 inches of water (commonly used unit of measure for pressure in ductwork). It is just hard to guess how big it is. That is what I was asking David. It seems we could provide a service to people asking questions about how to design a dust collection system by measuring the loss coefficient on our equipment.

    TW

  13. #13
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    Quote Originally Posted by Reggie Burnett View Post
    Can you better explain to me what you mean by "entrance loss"?
    Outside the tool you are trying to collect dust from the air velocity is near zero. Inside the pipe it has a significant velocity and thus kinetic energy. To get that kinetic energy you need to spend some potential energy. The potential energy is expressed as "static pressure". So, there's a cost in static pressure just to get the air moving. It's not really a loss at that point, just a pressure drop. In addition, the shape of the inlet causes an unrecoverable energy loss.

    The kinetic energy is usually expressed as "velocity pressure". Bernoulli tells us how those are related. The unrecoverable loss is a multiple of the velocity pressure and that multiple is called the "loss coefficient".

    Entrance_1.jpg

    So let's say you have a 6" pipe with 600CFM flowing through it. The velocity pressure is about 0.58" water column. If it's a plain pipe entrance, as in the illustration on the left, above, the loss coefficient is 0.8. So total static pressure loss at the inlet is 0.58" + 0.8x.058" = 1.04". If the pipe is 30' long it has a frictional loss of about 0.6".

    Velocity pressure is proportional to the square of CFM and inversely proportional the square of area.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  14. #14
    Quote Originally Posted by David L Morse View Post
    Six inch duct has so little loss at these flow rates that you can almost ignore it. At 600CFM you lose about 1% for each ten feet of duct that's added. As pointed out by Thomas, it's the entrance loss that you need to be concerned with. That's what will determine your velocity, not the length of duct.
    What do you think about the minimum flow velocity at that flow? Does it work on your system? At 600 CFM, the velocity in the 6" duct is just over 3000 FPM. The recommended minimum flow velocity in mains is usually quoted as 3500 FPM which is about 700 CFM for 6 inch pipe. This is to keep dust from settling out. Reducing 6" down to a 4" port on the tool takes a lot of suction. To get 700 CFM through a 4" tool port, I calculate 3.3 inches H2O using an entrance loss coefficient of .8. Sucking air into a bare pipe has a textbook loss coefficient of 0.8 so that is a representative number. It is much easier to maintain minimum velocity or better with 5" mains. Pipe losses are still not huge. The difference in flow between a 5" and 6" main is 30 to 40 CFM. It seems 5" main might be the sweet spot for a system that only operates single tool with a 4" port at a time.

    On the other hand the 3500 FPM guidance may be conservative. I do not see any dust lingering in my 6" flex hose that I am using right now. Yet, I have not measured flow velocity greater than 3100 FPM in a 6" duct coming from any tool. The router table is the lowest flow. It measures only 1800 FPM, yet the dust moves through the system without accumulating. I am still processing this information and looking for others thoughts.

    The fact is that not many people have tried to either calculate or measure the flow in a system. It is a shame. We are buying hugely expensive dust collection systems and it is a shot in the dark whether we are buying too much, too little, or that happy Goldilocks system that is just right. So, I am trying to measure and calculate. I am playing with my pipe flow calculator program today and thinking about my fixed duct design. In the shop right now, I am operating with a portable DC and moving it from machine to machine. I need to work on the pipe entrance to the dust collector where I measure flow. It is not straight. The flex hose is unsupported and just goes down to the floor.

  15. #15
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    This is an interesting thread. I will be tackling some of these issues at some point hopefully sooner than later. I appreciate the efforts being put into providing input. I have read a great deal of material on here and elsewhere about DC systems and it seems there is always something to glean from these posts. I bought one of the last GO440 units Grizzly had and will be reporting on that install when the time comes (although likely without any calculations or theory ). Keep up the good work!
    Regards,

    Kris

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