Help choose Ducting size - explain fan curves to me

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  • Help choose Ducting size - explain fan curves to me

    Ok so I'm trying to find the optimal duct size. Both for main line and branch lines. So I'm trying to wrap my head around fan curves. Below is the performance chart for my dust collector. Also I used the table on the link to determine the equivalent length for each elbow at a given diameter. Am I reading this correct. Is the sp loss per foot 0.167 at 8" , 0.221 at 7" , 0.358 at 6"? If that's the case I'm having trouble seeing any benefit in downsizing the duct at any portion even given the extra length you get at the elbows.
    Attached Files
    I reject your reality and substitute my own.

  • #2
    Sorry, it’s been 30 yrs since I had to read that chart.
    Considering the lack of duct sizes I think that trying to properly calculate home shop duct size is just an exercise. If you fit your duct length for the max CFM and pressure loss and come up short of your equipment and have to run flex an extra foot or so you haven’t accomplished anything. When I installed my ducts I installed 6”pvc as close to the equipment as possible and used as short of a piece of flex as possible. Not much help on your question.


    • #3
      To use that Laguna curve, you need to know the total pressure loss of your tools and the ductwork. Find that number on the X (horizontal) axis of the plot and go straight up until it hits the curve to read the CFMs your dust collector should be flowing against that much resistance. I too have a Laguna but the 1.5 HP HEPA filter model... not quite as beefy as your unit. It's rated at 900 CFM. I have a small anemometer with all sorts of built-in conversions so I measured the CFM myself I got 850 CFM at the inlet with a short 6 inch diameter feed pipe. So Laguna's measurements aren't overly optimistic like so many others (HF especially). The way Laguna makes that chart is to use a straight inlet pipe going to the dust collector, with airflow and pressure measurements inside the pipe... and then blocking the far end with various sized openings to simulate dust collection ports on machines. Laguna's data says sticking a 4 inch opening at the very end of the straight pipe creates 9.7 inches of water pressure loss and cuts flow rate to 938 CFM in the piping and dust collector. I don't see how you got your .167, .221, and .358 numbers. How did you solve for them?

      I had a different source of "pressure drop per foot" and "equivalent length of straight pipe for 90 bends" and whatnot... and my measurements on my completed system did not jive with those theoretical numbers. In fact, my ductwork showed less overall CFM restrictions than the theory predicted and that included a few 90 degree bends that are tighter radius than recommended. My sources said there is about 0.070 inches of water static pressure loss per foot in 4 inch pipe, and 0.045 in 6 inch pipe if the flow velocity is 4000 feet per minute. 45 degree elbows equate to 3 times the elbow's length for 4 inch ducts, and 6 times for 6 inch ducts. 90 degree elbows equate to 6 times the pipe length for 4 inch ducts, and 12 times for 6 inch ducts. Thus, a 90 degree elbow that measures 1.5 feet along its centerline, 6 inches in diameter, is equivalent to 1.5*12 = 18 feet of straight pipe. 18 feet of 6 inch duct has a loss of 0.045 per foot = 6*0.045 = 0.27 inches of water static pressure loss. Doing that same calculation for 4 inch diameter parts results in 1.5*3 = 4.5 feet equivalent length for the elbow, 4.5*0.07 loss per foot = 0.315 static pressure loss. In reality, the 4 inch elbow would cause even more restriction, relative to the 6 inch elbow, because the flow velocity would probably be higher. Some items need scaling factors (aka "K factors") on the theory to jive with my measurements: I found 45 degree "flue wye" (flue wye is an HVAC Y fitting designed for combining flue/vent gasses from two gas burning appliances into one chimney - it has the crimped ends facing the right direction for dust collectors compared to normal HVAC wyes) have about 115% of the theoretical resistance for example. My HVAC based 90 and 45 elbows came out close to theory even though my elbows are tighter than 1.5Radius.

      The biggest restrictions in my system, by far, are the 4 inch flex hoses connecting the hard pipe ductwork to the tools (a couple feet each so I can move the tool on its mobile cart a little bit) and the tool inlets themselves. Most of my tools have 4 inch dust collection ports... but they introduce as much resistance as the rest of the hard piping plus flex hose combined. 2.5 inch ports on some tools? Massive choke points.

      The problem with computing pipe sizes and pressure losses is the fact that pressure loss in a pipe is a function of the air velocity within the pipe. So you end up with a "circular equation" that needs iterative methods to solve. That's why the "Friction Loss per 100 Feet of Duct" chart in your linked PDF file has CFM on the X axis... that's just another way of specifying velocity in the pipes.

      I have two 6 inch mains feeding my Laguna; one runs up and then over, along the ceiling, to 4 inch hard pipe drops that end in blast gates and 4 inch flex hose. The other snakes around a bit, via several elbows, to run underneath wall mounted cabinets for tools mounted on roll-around carts. That main uses 6 inch to 4 inch reducers just before the blast gates and flex hose. The total pipe length (actual length, not equivalent length) are 20 to 31 feet at the various tools. It works well enough. I have around 400 to 500 CFM at my 4 inch tool ports. A few tools have 2.5 inch ports which really chokes the system, the net CFMs end up in the 100 to 150 CFM range. Those tools cause the air velocities in the ductwork to fall below the recommended 3500 feet/minute minimum velocity in the horizontal trunk lines and 4000 feet/minute in vertical drops. That does let some bigger sawdust bits fall out... I heard them getting picked up again when I opened a 4 inch port when using another tool. Drilling a few extra vent holes in the 4 inch to 2.5 inch adapters eliminated those issues. Those tools work better with shop vacuum suction levels... dust collectors depend on rapid air movement with far less suction.

      In theory, my 1.5HP dust collector should have 5 inch mains feeding it, not 6 inch. 5 inch HVAC piping is more expensive and I could not find 5 inch flue wyes at all. Oneida dust collector piping was 3 times the cost; Nordfab was 12 times the cost, and Air Handling Systems was about 4 times as expensive. I did a quick test with standard 30 gauge HVAC pipes with my dust collector and had no issues with it trying to collapse; Oneida and others warn against using HVAC ductwork as it may not be strong enough. 30 gauge held up to my 1.5 HP system but I went with 26 gauge for a little extra strength at only a small cost penalty. 30 gauge HVAC may not withstand 4+ HP collectors... I did need two full rolls of metal foil tape though to seal the HVAC ductwork... that was a nuisance.

      One thing about Oneida and other online ductwork recommendations is they tend to be oriented towards pro shops where more than one tool will be in use at a time. That's why you see them using large mains that neck down to smaller and smaller mains after each major tool drop. For shops using one tool at a time there really is no need for significant differences in main versus drop size other than piping convenience.

      Last edited by mpc; 09-20-2022, 01:47 PM.


      • #4
        I think that everyone should watch our choices of flex hose, any flex hose that the length is expandable should be looked at to make sure it is smooth inside rather than accordion style. Hose that is not smooth inside should be replaced.


        • #5
          Thanks for responses guys. Really appreciate it. MPC I will read your post more thoroughly when I get home from work today but to answer 1 of your questions I got those numbers from converting laguna's sp values from inch/H20 to feet/H20. I assumed that was needed otherwise the sp loss would not be a fraction and when you multiplied sp times total run for one tool you would get a number literally off the chart (fan curve chart). So for example 2 in/h20 = .167 ft/h20. Again I'm walking into this pretty blind and may (ok probably) made a bad assumption. So any help is appreciated. Thanks again
          I reject your reality and substitute my own.


          • #6
            Ah, unit conversions. I should have figured that out myself. I used to be quite good at recognizing "that's a factor of 12" or "that's a factor of pi" differences in numbers when I worked. Anyway, for dust collection ductwork, the basic process is to draw out your entire plan with dimensions. Use the "equivalent to xx feet of straight duct" for elbows and wyes and add up the total duct length in feet. Determine the static pressure losses from the charts based on CFMs. Since your DC is rated at up to 1624 CFM but you know you'll get less than that once connected to a system you need to guess at the "actual" CFMs. Pick a low value (assuming a worst-case ductowrk setup) such as 400 CFM at first. Most large woodworking tools specify around 400 CFM as well. See what the total SP losses are. Then look at that SP loss on the Laguna diagram and see if the DC's CFMs are above 400 CFM or not. If not... the ductwork is long/restrictive and even your powerful DC may not be enough. If the Laguna chart says the CFMs are way above 400 CFM, repeat the calculations at 800 CFM. Play high-low until you get convergence. Once you have the system CFMs, convert them into airflow velocities in the various diameter pipes:
            airflow velocity in feet/minute = CFMs divided by ( 3.14 * pipe diameter in inches * pipe diameter in inches divided by 4.0 and then divided by 144).

            Ideally the velocities are >4000 feet/minute in vertical runs and >3500 feet/minute in horizontal runs or in the main trunk lines; some places suggest >4500 ft/min. Thanks to restrictions at some tool dust ports themselves, my CFMs sometimes get low enough that my trunk velocities are in the 2040 ft/min ballpark. That seems to work though; stuff isn't collecting in the 4 or 6 inch ductwork. I did some Excel calcs comparing flow velocities in 4 inch and larger pipe sizes for various CFMs, including trying to see "if I have 4000 ft/min in the 4 inch drop duct, what diameter trunk line gives me 3500 ft/min?" Answer: 4.28 inches. Yeah, right. Like I'm going to find 4.28 inch diameter piping anywhere! To maintain 3500 ft/min in a 6 inch trunk you'd have to draw 7875 ft/min through a 4 inch duct. Likely not going to happen either. So widely differing main vs. drop diameters is probably not a great idea without other ways for air to get into the trunk line. 4 inch piping (or 4 inch tool ports) will choke your DC unless you either a) leave multiple blast gates open at the same time or b) add air inlet vent holes at the far end of the trunk line.

            My setup works for me. Given the lengths of runs in my setup, and the fact my DC flows 50% of your setup (and is about half the HP too), unless you have horribly long duct runs or horrible elbows/wyes, I don't think you'll have issues with 6 or 8 inch mains. Most of the DC book & Internet authors preach doom-and-gloom at any restrictions or losses in the system... as though any loss in efficiency is catastrophic. If the system does its job, what does it matter if the airflow could be 5 or 10% better? On my setup, the tools with smaller 2.5 inch dust ports cause all of the problems.

            What material do you plan to use - metal HVAC piping, PVC or other plastic, or designed-for-DC piping such as Oneida and Nordfab? I've seen all of the horror stories about static electricity buildup if plastic is used and how it could lead to explosions of fine dust in the DC bin... is that real or not? I don't know. Or the claims that homeowner's insurance policies will be null-and-void with plastic ducting... I went with metal ductwork as it seemed easier to deal with compared to piping - and it's easily changed compared to glued together plastic pipes. I didn't particularly look forward to trying to fasten plastic pipes with screws either... drilling that many pilot holes and whatnot. The PVC-to-ductwork adapters (for those that don't know about these - they convert the PVC radius to the 4 inch ports used by blast gates and tool ports) aren't inexpensive either. PVC looked like it would cost at least as much as 26 gauge HVAC stuff so I went with HVAC stuff ordered from Home Depot and shipped to my house for free. I already had metal cutting snips so that wasn't a new cost. The only tool I needed to buy was the one that crimps the ends of HVAC pieces. I made a wood jig to bend metal straps into "J" shapes for hangers; the straps came from HD's plumbing section. Inexpensive.

            Another way to "design" things is to look at your tools, and any you might purchase, and see their CFM requirements. A simple chart converts that CFM into an appropriate duct diameter. So pick your highest CFM tool and find the appropriate duct diameter. See Nordfab simple sizing instructions for an example. They describe a system where multiple tools may be ON at once - no blast gates - so their ductwork is based on summing the CFMs from everything and pipes get larger and larger behind each wye branch. With blast gates you need only to look at the most CFM hungry machine. For typical home woodworking shops, this process will say 6 inches is probably plenty big enough. Another guideline I read is that the duct trunk line should match the dust collector's inlet diameter... which is another reason I went with 6 inch trunk lines rather than 4 or 5 inches. My 4 inch drops limit the flow velocity in the 6 inch trunks though. As I said before though it hasn't been a problem.



            • capncarl
              capncarl commented
              Editing a comment
              I think that CFM is like horsepower on a tablesaw, nobody ever complains about having too much!

          • #7
            Originally posted by mpc View Post
            I've seen all of the horror stories about static electricity buildup if plastic is used and how it could lead to explosions of fine dust in the DC bin... is that real or not? I don't know. Or the claims that homeowner's insurance policies will be null-and-void with plastic ducting...
            I used 6" ASTM D2729 sewer and drain pipe. It worked great. I painted the outside of the pipe and the fittings metallic silver (I like the way that looks). To address static, I used aluminum foil HVAC tape on each side, inside and out. A machine screw near each end on each tape connects the tape electrically. A stranded copper wire bridges the fittings to ensure there is continuity the whole way. I have had no static issues. I don't glue the fittings, just use the HVAC tape to seal (but given the low pressure, I'm doubtful there is any for leakage that amounts to something). The friction fit of the PVC is really good.

            I've been very happy with the system. (It serves my table saw, sliding miter, workbench, band saw, scroll saw, drill press, planer, jointer, router table). The fittings were long-turn DWV.

            I don't understand the drilling concern, plastic is simple to drill. At the time I installed it, it seemed like it was easily the most cost-effective approach.

            For blast gates, I used Clear Vue Cyclones Self-Cleaning Blast Gates. Very happy with them as well.


            • #8
              For peoples who don’t think that static in PVC is a thing…. It can be. At my old work we handled a lot of flour, most bring transfered by stainless duct. Someone decided that maintenance would install a 4”pvc pipe dust collector to catch any dust that escaped around a transfer point and floor sweep pick up. Airborne dry dust inhibits a lot of static, and these pvc pipes had enough static to knock you to your knee’s. You could not even walk close to the pvc without getting a shock like a taser! This could possibly lead to an explosion due to the nature of flour dust.

              The 6” PVC duct in my system that has copper wire inside the pipe. The only static I’ve noticed was making the hair on my arms standing up, no shock. I can’t imagine how this could ever cause an explosion.