Duct diameter formula please

[davestanton]

Hi guys. I have been reading Bill Pentz's site and have some questions. I am struggling with his formula of " FPM=CFM/duct size" He goes on to say, "a little math shows we need 6" diameter ducting to move 800 CFM at 4000 FPM".

Using his formula to get the 4000 fpm in a 4 inch duct, the CFM drops to 400.

I am about to purchase a 2 hp 1600 CFM machine, remove all the filters and fit a Dust deputy with a 5 inch inlet prior to the DC's impeller, then exhaust to the outside of the shop without filters. The total maximum run of duct would be 50 feet with 3 sweeping changes of direction. The duct is sewer and drainage PVC pipe and a maximum of 4 feet of flex hose at the machines connection point. The machine with the most chip would be a 6 inch jointer and maybe further down the track a 15 inch thicknesser. There will be a 16 inch drum sander as well. At the end of each run and also at the connection point to each machine will be a y junction that has a screw on rubber sealed cap that can be undone to allow an eel to be fed in to clear any blockage that may arise over time as well as another way of allowing more air into the pipe if there is a smaller, say 2.5 inch, hose used on a machine to lift the CFM level up.

I guess I am asking what speed I can expect in my duct given the info above and will it be sufficient?.

[dwacker]

My dust collection system was a bad experience. I paid for a computer designed duct diagram with all the cool flow numbers and full parts list. When it was said and done the pipe collapsed do to all the bad advice I found on the internet. I tore out hundreds of dollars worth of destroyed duct work and just winged it with pvc. In the long run it works and works well, I wouldn't get all caught up in the numbers.

Don

[Bram]

I also have a 2hp dust collector. i've read a lot from Bill Pentz's website, but I didn't care much about the cfm and those other stuff. I've just used 5" PVC drain pipe for my whole dust collection system.

I was worried if my dust collector would 'suck' enough. So the first time i turned my DC on, I've tested it by putting some sawdust in a plastic container and let the DC suck up the dust. The suction was so high that it almost tore my plastic container appart.

I've made a dust hood for my 16" planer and it sucks up 100% of the chips that the planer produces. Also the DC at the tablesaw is very good.

I don't recommend reducing your (let's say) 5" duct to a 2" duct, cuz that will reduce A LOT of suction. I've done that for my belt sander and miter saw. It catches only the tiny dust particles from my mitersaw, the rest get's on my mitersaw station, so after every cut 'session', i need to clean up my mitersaw station.

Good luck with your dust collection system.

[wilburpan]

I wouldn't worry about it too much. I really like Bill Pentz and his website, but it's a bit like drinking from the firehose.

Basically, you can executive summarize Bill's approach to dust collection like this:

1. Fine dust is bad for your lungs.

2. You have to move a lot of air to get the fine dust.

3. There are barriers to moving a lot of air. Basically, you can break those down to the resistance from the filter, and the resistance from your ducting.

4. Because the filter will get caked up with fine dust, which decreases air flow, you want to use a cyclone to get the fine dust to drop into a container before it gets to the filter. The cyclone will also add resistance and decrease air flow.

5. That's why you need a big honking motor and impeller to move the air needed to collect the fine dust, and overcome the resistance from the ducting and the cyclone.

There is another way to deal with this issue, which you already are doing. If you are pumping the dust to the outside, you don't have to worry so much about the filter caking or a cyclone. So right there your DC system is going to work better than you might think.

The second is that although a sufficient diameter of ducting is needed for good dust collection, you also can reduce resistance by keeping your runs short. I don't know how your shop is laid out, but if you can move your DC from machine to machine and hook it up with a short length of flex hose, that will also improve the efficiency of your DC system.

Finally, even with a huge cyclone, it's very hard to catch all the fine dust anyway. So use air cleaners. The gold standard is collecting all the dust at the source, but I have to assume that some will escape regardless of what you do.

By the way, jointers and planers tend to generate a lot of larger wood chips and particles, which makes a mess but isn't a health hazard. They don't generate fine dust very much. Tablesaws, bandsaws, and your sander will be much worse at generating the fine dust, and that's what you should concentrate your DC system on.

[davestanton]

To reply to comments and suggestions so far.

Don, I remember your post about the collapse and your wife saying the colours didn't match when you had replaced with pvc. Is your system a 4 inch or larger diameter and what power/diameter is your motor and impeller?

Bram, what is the longest run to the DC that you have? I understand that the further away from the DC you get, the less efficient the suction will be. As I mentioned I will be about 50 feet at the furthest point with 3 directional changes, Being that they are only gradual, they will still make a difference. Regarding reducing to 2.5 inch, I will have an adjustable valve fitted at the end of the run where any 2.5" points may be so that I can add to the volume of air being drawn.

Wilbur, I know we had bumped heads earlier but thank you for your input, much appreciated. I will be using a cyclone prior to the DC and venting outside. I will mount the DC on an external wall in a stationary position and run duct to each machine. I intend on 2 blast gates right at the entry to the cyclone to shut either of the 2 mains off and reduce the dead air in another 50 feet of run of duct. (there are 2 mains and the DC is going to be central as I only have one wall that is suitable to vent through.)

The plan is to remove the motor and impeller from the DC trolley, tip it sideways so the motor is pointing toward the ceiling and mount it directly over a cyclone and have a short, maybe 1 foot length of 6 inch pipe without any bends running through the wall from the exhaust. The inlet to the cyclone would have 2 four inch ports fashioned from pvc 4 inch fittings, a blast gate to both runs and also a blast gate at each machine. One of my main concerns was if a 2hp, 1600 cfm would have the balls to do the job of keeping the materials suspended in the air stream over the required distance.

[Bram]

My longest run is only 25 feet wich includes 5 feet of flexibel hose. My longest run also has 3 directional changes (2 elbows and 1 y). Each machine has it's own blastgate.

When i use my miter saw or my belt sander (which both have a 1 1/2" flexibel hose) I alwase put those 2 blast gates open so there would be enough air flow.

Oh BTW, it has no point of putting a blast gate at the beginning of a run and at each machine. If there is a blastgate at every machine, then that's enough because the dust collector makes the closed runs vacuum. You will not lose any suction if you only put a blast gate at every machine.

And i recommend to make your own blast gates. Those are so easy to make and are a lot better than those plastic ones that u can buy + homemades are cheaper .

You're saying that you're going to use 4" pipes wich I think is too small for a 2hp 1600cfm DC. Especially if you have long runs. as i mentioned earlyer i recommend to use atleast 5" pipes, it will not only 'suck' a lot more, it will also have bigger air flow. Compare it with this: through what straw will you suck up milk faster and easyer? a 1/16" straw or a 3/8" straw? --> something to think about.

I assume that you've made a couple of different layouts of your dust collection system? try moving the DC to another spot, maybe move a machine or put them in a different order? try putting the machines that needs the most dust collection as close as possible to the DC so the run from the DC to the machine is as short as possible.

[davestanton]

I am now considering 6 inch ducting which is over twice the price of 4 inch here in Australia. The advantage of course is that at any machine I run the duct to, I can change to two four inch ports or a four and two inch port with little effect on the mains.

Bram, did you make your gates from plywood or thick plastic sheet?

The layout of the shop is pretty much how I want to keep it, or should I say that the DC will be in one spot only and the shop will be laid out to suit the machines, not the DC. This is the reason I am asking a few questions at this early stage.

[Bram]

For my blast gates, I've used some scraps laying around in the shop. I've used 5/8" plywood where you've put in your piping and i've used 5/16" plywood for the sliding thingy which blocks off the air (you know what i mean). When you open the blastgate, you will have a gap the thickness and width of your sliding thingy, that's why i've attached a piece of rubber or any kind of material to the blast gate it's gap with tape so the tape will act as a hinge. when the blast gate get's opened, the rubber will get sucked against the gap, which blocks off any air from outside.

6" ducting would be a whole lot better than 4" ducting. Even if it's expensive, it's still cheaper than buying 4" ducting and then realise that it just doesn't suck enough so that you will eventually need to buy 6" pipes and stuff.

It is important to do a lot of research and asking a lot of questions, cuz i think you want to build your dust collection system only once. But you will need to decide if you want to rearrange your machines or putting your DC in another spot. I sure wanne help but it's kinda hard to tell sush things if i can't see the shop or get dimensions.

[davestanton]

You asked for it.

[Eric.]

Dave, the DC you're putting together is EXACTLY the one I'll be installing as soon as I'm done drywalling the shop. I have a 2HP DC and the big boy Dust Deputy sitting there waiting for me. Just waiting.

I am now considering 6 inch ducting

After tons of research and debates with myself, I decided that I'm going to run 4". My shop is a garage, and usually when I'm working in there I have the door open, so air is usually circulating. I'm also an idiot, so I'm not as concerned about the fine dust as I should be. I mostly just want my DC to pick up the bulk of the mess. So remember that increasing your duct size will increase your CFM and help collect the fine dust, but will decrease the air velocity that moves the big chips through your duct. With a 2HP DC, it's very possible you'll have fallout with 6" duct, especially in vertical runs.

Keep posting as you progress...I'm really interested in how your system works since it's basically identical to mine. I'm even venting straight outside like you are. I won't be able to get mine in for at least a couple months since I have to finish hanging this drywall and then I'll need to get caught up on the Roubo build...THEN it's DC time.

[davestanton]

Thanks for the heads up Eric. I am way off doing this. Today I laid bricks to act as a base for some internal walls that I will put up end of the week, then lay some floor joists over the existing concrete floor, build a timber wall and clad inside and out, probably with cement sheeting, lay some 17mm ply flooring and then move some of the crap into its new section. This will enable me to lay the rest of the floor joists, flooring, run power and clad the rest of the interior. Next is all the light fittings and then, only then will I buy and fit my DC into its permanent position.

After looking at the shop (roof and most external walls are all done) I am thinking I might do 2 six inch mains. These will have fall back to the big boy dust deputy. From these mains I will have 4 inch drops every 6 feet or so that are available for how the shop will be laid out now and also in the future. The other thing is that I will not glue the joints. Push fit and duct tape. I will have screw on end caps at the end of each main that I can push a flexible cleaning apparatus through if there is a blockage.

I am going to read a book that I saw about this stuff as well. This is too much fun and can't wait to get it all happening!

[wilburpan]

Wilbur, I know we had bumped heads earlier but thank you for your input, much appreciated.

We did? I honestly don't recall this. Anyway, bygones be bygones and all that.

The layout of the shop is pretty much how I want to keep it, or should I say that the DC will be in one spot only and the shop will be laid out to suit the machines, not the DC.

If you are taking your DC seriously, and from your questions you certainly are, I think that this is a bit of a misstep. If you are still in the layout process of your shop, take the DC into consideration as well. It's easy to decide that you want your bandsaw in a different spot. It's a much harder thing to realize that your DC layout needs to be optimized and you have to rework your ducting.

One thing to consider is to group your big dust generators (table saw, bandsaw, drum sander) together and put them close to your DC.

[Bram]

1 Option would be to hang a pipe from your dust collector (wich is in the south east corner of your shop) to the north west corner (of the yellow area of the shop) and put several 4" (6" prefered) drops to every machine, and then go through the wall to the area where you have your drill press and other tools. So if everything would be done well, you should have a hole to the right side of your workbench. Then you hang a 6" pipe from that hole in the wall (wich connects with the pipe that hangs in the main area) to the center of the north wall of that area with 1 drop against the wall, wich spilts up in 3, so you will have DC for your mitersaw, planer and drill press.

Unless you can put your planer in the main area (the yellow colored area) and use your shop vac for drill press and mitersaw dust collection. This will save you money on pipes and your longest run will be a lot shorter than the previous setup, wich results in better dust collection to your tools.