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Thread: HVAC duct for 10hp dust collector?

  1. There are lots of videos and articles "out there" on system design. There are computer models (free) that help you calculate the resistance within a given size pipe while factoring in things like elbows etc to come up with a reasonably close estimate of how many cfm you will experience at various drops. You need to factor both volume and velocity into your calculations.

    Another thing to keep in mind is "what machine is going where?" If the furthest drop in the system is for your stand-mounted scroll saw, you can get by with a lot lower cfm at that station than if, for instance, it was serving your planer (while cutting live oak).

    If all of your drops are eventually going to end up with a flex hose connected to a 4" port on a machine, then there's no point in having your blast gate any bigger than that port (installed in the smallest line in the run). The most resistance will be in the smallest diameter flex hose, and the design of those is important too. The "cheapies" are ribbed on both the inside and the outside (like dryer vent hose) and have the highest resistance and tendency to clog. The good ones are a lot smoother on the inside (and thereby heavier and more expensive).

    The prices of those blast gates rises rapidly as size increases. Probably the only time it makes sense to use bigger blast gate is if the "last thing on the branch" is a wye that has two pickup lines connected to the same tool (like some drum sanders, table saws, and some band saws do). Even then, you may find that two 4" blast gates are cheaper than a single 6" or 8" and can almost always be located in a more easily accessible location.

    Don't forget about grounding the system by bridging with bare copper wire anywhere you have flex hose between machine and piping or between two sections of piping. Static is a bad thing.

  2. for blast gates and hose try blastgateco.com, really good prices on blast gates

  3. #18
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    Dan, determining duct size has nothing to do with HP, it has to do with the system performance curve, and the layout of your shop and what your machines require.

    As for HVAC duct, it is meant to be at a positive, not negative pressure. It will not have the reinforcement to stay round, you"ll need to purchase the correct round duct for your application........Regards, Rod.

  4. #19
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    I just completed my spiral pipe installation two days ago. A jig saw with metal-cutting blades made quick work of cutting the pipe. In order to draw an accurate cut line all around the pipe, I got some steel strapping from the plumbing section of the borg and made ring a little bigger than my pipe. A wingnut and a 1/4-20 bolt allowed me to tighten and loosen the ring. Slide the ring down the pipe, tighten, trace the line with a sharpie, remove the ring, and cut.

    I mostly used metal tape to connect things but on drops I used rivets and silicone caulk. I highly recommend getting a pneumatic riveter for this task. I got one on Amazon for about $65 and it is WAY better than using a hand riveter. Kind of fun, actually.


  5. #20
    Quote Originally Posted by Brian W Evans View Post
    I just completed my spiral pipe installation two days ago. A jig saw with metal-cutting blades made quick work of cutting the pipe. In order to draw an accurate cut line all around the pipe, I got some steel strapping from the plumbing section of the borg and made ring a little bigger than my pipe. A wingnut and a 1/4-20 bolt allowed me to tighten and loosen the ring. Slide the ring down the pipe, tighten, trace the line with a sharpie, remove the ring, and cut.

    I mostly used metal tape to connect things but on drops I used rivets and silicone caulk. I highly recommend getting a pneumatic riveter for this task. I got one on Amazon for about $65 and it is WAY better than using a hand riveter. Kind of fun, actually.
    Thanks for the advice. Will pick up a pneumatic riveter. What kind of wyes did you use? The kind meant for spiral pipe or standard HVAC wyes? I plan to use spiral pipe for the trunk line and modified HVAC wyes for the drops. I looked on blastgateco.com and their prices on 12x12x6 wyes were almost $60+ each. I can get the HVAC 12x12x6 wyes for $32 per pair. So you didnt purchase any special couplers to connect the spiral pipe to the wyes? I could use female to female couplers for that but I do realize that performance will suffer if I have any crimped seams in the direction of air flow. I had my prior d/c system plumbed with all HVAC duct, elbows and wyes with no regard for crimped seams in the direction of air flow and I never noticed a drop in performance or ever had a clog although I do understand the inherent risk in doing so.

  6. #21
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    I used dust collection fittings from Air Handling Systems, which is local(ish) to me. None of my couplings, wyes, 45s, etc. have crimped ends. The prices you're quoting would make me balk, too, but for my size dust collector proper fittings were much more reasonable.

    I don't know if this is a possibility for a collector your size, but have you looked into PVC? I see sewer & drain pipe in pretty large sizes on construction sites sometimes.

    Best of luck.


  7. #22
    Do you guys think that I could get close to as good of performance with 8 trunk instead of 12? The intake on this d/c is 12 so I just assumed that is what the original manufacture intended. I know that I could measure the CFM and calculate the answer to this question but I am short on time and not very good at math. I am not trying to eek out every little bit of performance from this thing, I just want it to work well enough to keep the area that it is going in clean.

  8. #23
    Quote Originally Posted by Wes Grass View Post
    It can't collapse if it stays round. Yeah, I know ... duh ...

    A friend of mine built an airplane from carbon fiber tubing. To prevent buckling, they glued 'biscuits' laminated from balsa and foam inside at increments of 'X' x the diameter of the tube.

    You could make split rings of plywood, spaced along the outside at 3 or 4 foot intervals. You need something snazzy to hang it with anyway, right?

    And, 12" sounds kinda big for 10 hp.
    You know that is a great idea. I wondered if I could make some kind of structural ring or brace to mount along the 12 HVAC duct if I choose to go that way. What did you have in mind for this? Just some rings perhaps made out of 2-3 layers of 3/4 ply spaced along the trunk line with one in the center of every 5ft section of HVAC duct? The only reason that I want to use the HVAC duct is because;
    A. I already have a bunch of it.
    B. I have a buddy that works at a place that sells duct and he can get me the duct and fittings at a large discount.

  9. #24
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    Quote Originally Posted by Dan Coker View Post
    Do you guys think that I could get close to as good of performance with 8” trunk instead of 12”? The intake on this d/c is 12” so I just assumed that is what the original manufacture intended. I know that I could measure the CFM and calculate the answer to this question but I am short on time and not very good at math. I am not trying to eek out every little bit of performance from this thing, I just want it to work well enough to keep the area that it is going in clean.
    Unless you are running very large machinery, such as a big wide-belt, that has intense dust collection requirements, you will not be compromising performance enough to matter much if you immediately step down to 8" to make acquiring your duct work more affordable. IMHO...for what that's worth. Yes, it will not move as much air, but the demands of the tool at the other end is what matters. These large systems were designed to be able to extract from multiple tools concurrently...if that's not your situation, you have wiggle room in your duct design for sure.
    --

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

  10. #25
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    Apr 2006
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    Kansas City, MO
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    There is an underlying message that is not being heard. The system requirements are determined by the dust producer requirements. What tools do you have and what are their cfm requirements, how many are in use at one time, and what size duct at what point is needed to provide the needed CFM and air velocity.
    Chuck

  11. Go back and read the whole thread again, Dan. The number one issue is "how much at once?"

    Most shop tools have a 4" (or smaller pickup port). If you simply look at a cross section of the area of the various sizes of pipe, you'll see this:
    1) You can put 2.25 4" pipes into a 6" pipe. Any less than this (open) is going to decrease the cfm going through the 6" pipe while increasing the velocity (at least at the point of transition).
    2) Under the same scenario, you can put 4 of the 4" pipes into an 8" pipe
    3) You can put 6.25 of the 4" pipes into a 10" pipe
    4) You can put 9 into a 12" pipe

    Remember, the number of POTENTIAL sources of pickup is irrelevant (assuming that you close off the unused ports when the machines are idle). Unless you plan on having 9 ports open at once, there is absolutely nothing to be gained by going bigger.

    In the length of run you've described, you'll save a small amount of friction loss by using a larger diameter main line, but not enough to be of significance.

    Since my system is yet to be completed, I temporarily hooked up my Powermatic 15" helical head planer to two pieces of (smooth interior wall) corrugated flex hose that was stretched 30' across the floor. With a second 4" blast gate wide open (and closer to the dust collector), I had way more than adequate suction to vacuum up the chips from the planer. My dust collector is 3 hp and nominally 2850 cfm ("real" cfm being about 1650). The machine inlet is 8", immediately reduced to 6" for about 4'...and the rest is the 4" flex hose (which still has a lot more resistance than piping).

    What you are doing here is stressing over the relative performance of a 500 hp Ferrari vs a 675 hp car when all you want to do is drive to the grocery store.

  12. #27
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    It is worthwhile to take a look at the impeller. If the blades are straight, the impeller will pull less cfm at low pressure but outperform a curved blade fan at higher pressure. The straight blade is well suited to an 8" main, a curved blade might benefit from a 10" main but that will also mean you need to leave more gates open. The velocity of the main and the size of the cyclone will determine the proper airflow into the cyclone for best separation. Once you determine the impeller size and design, you can go to the Cincinnati fan tables and determine the cfm at various pressures. The Torit site will give you the cfm range of similar sized cyclones and from that info you can determine the proper main and the number of ports needed to be open for the system to perform well. Better to do some homework before buying pipe. Been there, done that. Larger diameter spiral can also be found in commercial HVAC " boneyard " of left over commercial jobs. Like Darcy, I've found local sources of new and used spiral in my town of 20K. Dave

  13. #28
    A sawzall with a metal blade works well to cut the spiral pipe. You start flat, and when you break through, cut perpendicular to the pipe and work your way around. Don't use the orbital setting if your sawzall has one; that is for wood only. Mark the cut with a sharpie the whole way around the pipe, as it is easy to go off course when cutting the pipe, especially with something as wide as 12". I found spiral pipe to be easier to cut and work with than HVAC pipe, actually.

  14. #29
    If you have an Ace Hardware close by, check with them. My local Ace has 26 gauge snaplock pipe and fittings for a reasonable price.

  15. #30
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    Wow, it has been 8 years now since my DC install using metal snap lock pipe. Here are a bunch of suggestions based on my experience: https://sawmillcreek.org/showthread....all&highlight=
    NOW you tell me...

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