Dust Collection System Component Sizing

A long conversation on sizing ducts, blast gates, et cetera to match a small cyclone dust collector. January 28, 2009

I've just purchased a 3hp cyclone collector. It is rated at 2100CFM and 10" Static Water Pressure. It has an 8" inlet and an 8" outlet (to a bag). It is a relatively inexpensive (compared to others) unit made by Craftex.

I will be using it in my own shop with only one machine running at a time. The largest machine I have is a 24" Wadkin thickness planer, so I was thinking things need to be sized for that. But I also have the following:

Delta RC31 12" table saw
36" Wilson Bandsaw
24" dual drum sander
12" disk, 48 belt sander
12" jointer
48" Record Lathe
16" Drill Press
14" 'Chop Saw'

From some reading on the net, I was considering going with an 8" main and then 6" drops to each machine. However, my knowledge about this stuff is sketchy at best. Any advice would be much appreciated.

Forum Responses

(Dust Collection and Safety Equipment Forum)
From contributor J:
I had to plan out ducting for my small shop (2hp Oneida) a couple of years back. Static pressure is a way of measuring the resistance created by moving air through a given duct path at a certain speed, so it's not very helpful to say that your DC is rated at '10" static pressure.' It will be able to move a certain number of CFM at a given static pressure, but the CFMs will change as the static pressure changes. Longer runs of duct, elbows, wyes, blast gates and hoods all increase static pressure and decrease cfms.

From contributor J:
It's been awhile since I went through it, but I remember 3HP as being well within the scale of machines he works with. One area where that spreadsheet will be less helpful for you is in addressing having more than one machine running/blast gate open at once. I did wish it was a bit more sophisticated.

Most likely, those 2100 cfm / 10" of static pressure numbers are describing the very upper and lower limits of your machine's performance curve, meaning that your unit will move 2100 cfm when running wide open with no ductwork attached to it, essentially zero static pressure, and becomes essentially worthless if a duct run is so long and convoluted as to produce 10" of static pressure. That would be fairly similar to other 3HP performance curves I've seen.

One reason things will get a bit more complicated than just using 6" drops to each machine is that some of those machines may have dust ports substantially smaller than 6". A bottleneck anywhere in a duct run reduces the volume of air that your DC can possibly pull through that run. Less volume through a given diameter of pipe means slower airspeed. If the airspeed drops too low then chips don't move along towards the collector.

For example, if the dust port for your chop saw were a dinky little 2" connection on the back of the blade guard, that 2" bottleneck would dramatically increase static pressure for that run. Your DC may only be able to move 400 cfm through such a restricted pipe.

Now, 400 cfm through the 2" flex hose between the saw and the 6" drop would be a blistering 18,334 feet per minute; very possibly enough to collapse the flex hose. Even if it doesn't do that, however, the air will still only be moving at 400 cfm in the 6" drop. 400 cfm in a 6" tube is only 2,037 fpm, which is not enough to keep wood dust aloft in a horizontal run, let alone enough to pick it up vertically. The chips would quickly pile up at the bottom of the 6" drop and clog there.

It took me a while to wrap my head around these sorts of calculations, because the numbers are so dynamically interrelated. It certainly helped me appreciate why so many dust collection systems in shops where I've worked didn't do a great job. Happily, the system I ultimately put together works very well. Unless I forget to empty the bin and chips back up into the cyclone, it never clogs.

From contributor K:
I would be most concerned with your drum sander. It takes a lot of air movement and very good filter system to deal with the health effects of sanding dust.
Some rules of thumb for DC:

1. Minimum duct size is the inlet opening of the impellor, or larger if feasible.
2. Maintain this duct size as far as possible even if you need to customize the machine connection (optional).
3. Install blast gates and use them.
4. Use a good quality cartridge filter, the bag will not be effective at removing the fine dust.

From the original questioner:
I do not really understand the limitations in flow rates that Im reading about. When I think about a river flowing and how that works, it makes sense to me. However, even though air is like a fluid, it seems to operate differently. For example, if a river is flowing and there is a sudden narrowing of the banks (i.e., a restriction), then the current speeds up though the restriction and carries the same volume of water through the restriction. When the river opens back up again, the same volume is present, but the flow is slower again. So, why does this same concept not apply to DC? If the air moves through a restriction (i.e., a small inlet) and then into a 6 duct, why doesnt the same volume of air get pulled through the small inlet at just a higher velocity and then into the 6 duct with the same volume and a lower velocity (ideally the 4000FPM design rate)?

I have a drawing Ive done on CAD of the machine layouts and duct runs. Ive kept the main branch as 8 ducting. Then, I drop down to 6 (or 5) duct to each machine.

Contributor K - are you saying that I should keep the 8 duct to each machine and just have a fitting to mate with the machine? And yes, I was planning for blast gates at each machine.

Im fully aware that it isnt the best unit out there. But, it cost around $800 and everything else Ive looked at is anywhere from four to eight times this amount. So, the almighty dollar has won out on this purchase. I just picked up a bunch of spiral duct for $20 at the scrap dealer. I ended up with the following pieces:
37 of 5 duct
25 of 6 duct
16 of 4 duct
Unfortunately no 8 duct.

Looking at the static calculator, I should be able to use all of this in the drops to specific machines with appropriate flow rates for the ducts. If what Im interpreting what contributor K is saying, then most of this is redundant. However, Ive never seen an installation that did not step down in duct size to specific machines.

What are the thoughts on plastic flex pipe to mate the last foot or so to the machine? Is flex steel pipe better? Ive even seen some stuff that looks like canvas.

From contributor K:
Stay with the large duct as far as economically practicable. But lets not get silly about it, when you get a good deal on large diameter metal ducting use it, but at the closest point to the machine.

Think of moving air the same as any other form of work, place a restriction on it and it takes more energy to get the work done. If you try to build six cabinets in the same time that you normally build four you have to work harder, you need to add energy. In the case of the river, when it hits a restriction the water moves faster, energy is added in the form of an elevation change from the high end of the restriction to where the river broadens out again. With a DC system the energy level is maintained constant by the rating on the electric motor, so as you increase the restriction of the air flow, you move less air (do less work).

The other key point in the ducting is radius of elbows and cross connections. Large radius elbows are good, "Y" connections are good, sharp radius and "T" are bad. Stay away from the canvas type flex, the smoother the inside of the duct the better, less friction = lower restriction. Give some thought to where you locate your blast gates. You may be able to save yourself a lot of wasted steps. They do not have to be at the machine.

From contributor J:
I agree with contributor K on the vast majority of his advice. The narrowing river analogy doesn't work for a variety of reasons, most obviously because a river isn't an enclosed pipe, so it's fighting gravity rather than compression.

If the air moves through a restriction (i.e., a small inlet) and then into a 6 duct, why doesnt the same volume of air get pulled through the small inlet at just a higher velocity and then into the 6 duct with the same volume and a lower velocity (ideally the 4000FPM design rate is perfectly apt, and that is pretty much what happens). The problem is that, when the restriction is very much smaller than the main line, the DC is nowhere near powerful enough to force enough air through the restriction to maintain sufficient velocity in the unrestricted part. Back at the river, picture an alluvial fan; the flow slows dramatically and the sediment drops out.

There are ways to address this problem; you just need to know to do it. To continue the chop saw example I started above, with a tiny 2" port on the saw itself, if you were to bring a 6" drop down to the tool, and just above the tool put in a 6" X 5" x 2" wye, with a 2" flex-hose branch going to the little integral dust port and the 5" branch to an open-sided plywood catch box behind the blade, that would allow a sufficient volume of air to keep the velocity functional all the way through.

I do think, though, that to say "maintain this largest duct size as far as possible" is overstating the case; there are definitely situations where the duct can be inappropriately large. If you had your 8" pipe coming out of the DC go into an 8x8x8 wye and then two 8" branches, and you had two blast gates open simultaneously, one on each branch, then the airspeed could easily be too slow in those 8" branches even if it were fast enough in both the drops and the main trunk. Likewise, bringing 8" pipe all the way to the machine and then necking it down to 4" or 5" where it connects, even if you were only running one machine at a time, would just produce clogs.

From the original questioner:
Yes, Ive read much about large radius elbows and 30 or 45 degree wye connections and solid point on the canvas flex connections. Ive read that having wire wound flex ducts are preferred for the grounding. I havent found any wire wound locally (yet), but there are many places with the plastic hose with a nylon (Im guessing) winding around it. What are your thoughts on these choices?

Due to the vertical drops to the machines, I think the only viable locations for the blast gates would be right at the machine. And, in the case of the 12 jointer, I would need to put one at the base of the machine and another in the line that would go over to the table saw as they will share the same 6 duct. There will be a floor sweep (with a door) sharing this run also. 99% of the time, I will run only one machine at a time.