Dust Collection Ductwork

[Jim Becker]

When i was inquiring about duct size i was warned away from using the larger pipe because of loosing pressure (or something to that effect, I can't remember the exact words). My system is a 1200 cfm and it pulls everything quite well. Is there any truth to what I was told? (I might have to get some 6in pvc)

Unfortunately, no, Keith. You're system is not going to provide it's best REAL CFM performance with the smaller duct. For a typical mass-market single stage "1200 CFM" DC, that would be about 600 CFM, give or take. 4" duct maximally can handle about 300-350 CFM tops. Duct work should be optimized to the blower of your system as well as to the individual tools.

[Steve Sawyer]

When i was inquiring about duct size i was warned away from using the larger pipe because of loosing pressure (or something to that effect, I can't remember the exact words). My system is a 1200 cfm and it pulls everything quite well. Is there any truth to what I was told? (I might have to get some 6in pvc)

I believe that the pipe diameter needs to be matched to the static pressure and CFM rating of your system, Keith.

Bill Pentz's famous website has tons of information about this. I seem to recall that he recommends maintaining a linear velocity of 800 FPM to keep chips and dust airborne all the way to the collector. If you have a blower capable of moving the air much faster than that, a too-small pipe will restrict the air flow, and could cause the that air velocity to fall below the 800 FPM rate, leading to inefficient dust collection at your machines and possible duct clogs. At the very least you're wasting a lot of power that could be better applied to sucking up chips and dust.

On the other hand, using a pipe that is too large (e.g. 6" on my little 650 CFM unit) could also cause a problem because I can't move a large enough volume of air through that 6" pipe to maintain the 800 linear FPM. In this case (and I'm just speculating here) a 4" duct might actually work better than a 6" duct.

The only way to know for sure is to a) get accurate air-velocity readings at the impeller, b) get accurate static pressure (vacuum) readings at the impeller and c) do the math to determine how small a cross-section of pipe you can use and still keep the air velocity at an acceptable value.

An alternative is to experiment - find a device to measure air velocity, attach various size pipes to your DC and see how much your ducting is slowing down the air flow.

Or, do like the rest of us - ask a lot of questions, (ignoring the folks that seem to think they're the only ones who aren't complete idiots), make your best guess, pay your money and take your chances.

[Chris Padilla]

A nice box of stuff from McMaster-Carr I got a year and a half ago.

Oh, and my BP/CE cyclone build I did a little while ago:

http://www.sawmillcreek.org/showthread.php?t=9933

[Mike Monroe]

Any more details? I am on the blast gates myself....

Details? Sure, two were Folgers and one was a Hills Bros.

Seriously though... I found plans for PVC based blast gates on Wood Central, Badger Pond, or maybe even here (I can’t remember). I used the plans as a starting point and built my gates out of ¾ ply, ¼ hardboard, and coffee cans. The blast gates are kind of like a sandwich where the ¾ ply is the bread and the ¼ hardboard is the meat. The ¼ hardboard is cut into a rectangle with a hole in one half and handles on the ends. The hardboard slides back and forth between the ¾ ply with ¼ spacers along the edges. This design is self cleaning. I also made catches/locks out of some scrap dowel. I drilled a hole in one corner of the gate in the open and closed position. With the dowel catches/locks I can lock the gate open or closed. They work great and weren’t all that hard to build. I’ve made 6, 5, and 3 inch blast gates. You don’t need coffee cans, you can use regular HVAC duct, but coffee cans are seamless and easy to hacksaw in half. I’ll try to post a pic of one with dimensions. You’ll need a pair of duct crimpers, some caulk and small screws or nails to make these blast gates.

-Mike

[Rod Sheridan]

Sure there is...put a window in the top of the bin using 1/4" Lexan, caulk (to seal it) and screws. I've had one in mine for years. Of course, you still have to actually REMEMBER to look.

I had to laugh Jim, my Oneida cyclone has a length of clear flex from the bottom of the cyclone to the dust bin.

Last Sunday I was working in the shop and looked over at the flex, I didn't see anything swirling around, for good reason. I had overfilled the drum and was halfway up the cyclone! Yes you have to remeber to check.

Regards, Rod.

[Steven Wilson]

When i was inquiring about duct size i was warned away from using the larger pipe because of loosing pressure (or something to that effect, I can't remember the exact words). My system is a 1200 cfm and it pulls everything quite well. Is there any truth to what I was told? (I might have to get some 6in pvc)

By going with larger duct size you can loose velocity in your duct work and the chips/dust will fall out of suspension. The parameters you are trying to control are velocity (move the air fast enough to move the dust and chips), amount of flow (CFM), and resistance to flow (usually expressed as Static Pressure). What you use to control those variables are your Dust Collector (how much suck power you have to work with), duct size, duct length, and fittings (wye's, elboe's, etc and their physical properties). All pipe and fittings will restrict flow. The restrictions to flow will reduce the CFM available at the tool. All else being the same, smaller duct work restricts flow more than larger duct work. To overcome the effects of the smaller duct work you end up needing a very large increase in impellor size and horse power. On the other hand as you increase duct size the CFM will improve but you may end up with material droping out as your velocity drops. Bill Pentz has a nice discussion on the duct size vs. dust collector size and what seems to work the best in the introduction of his Ducting section; 5" pipe <= 1100CFM rated blower, 6" pipe - 1100CFM-1800CFM blower, 6" and larger pipe for larger systems. Instead of concerning yourself with maintaining velocity you would be better off (dust collection wise) trying to maximize CFM at the tool and only downsize pipe if you experience problems moving material.

[Brad Peterson]

Thanks Mike.

Brad

[Jim Becker]

Last Sunday I was working in the shop and looked over at the flex, I didn't see anything swirling around, for good reason. I had overfilled the drum and was halfway up the cyclone! Yes you have to remeber to check.

Better pull your filter, too, and check it. Once you get up into the cyclone, you'll get blow-by and that will pack the filter in a very nasty way. Unpleasant (and outdoor) task to un-pack it! DAMHIKT!!!

[Jim Becker]

(how much suck power you have to work with),

Just to clarify this...we generally don't refer to "suction" when working with dust collectors because they work at very low pressures...0-11" of water static pressure, or so...unlike a vacuum cleaner that sometimes gets to high pressures approaching 100" of water SP. Dust collectors work primarily by moving large amounts of air at a given velocity (measured in CFM at a particular static pressure) which you point out nicely. Maximizing that air flow without falling below the drop-out velocity is the key to maximizing system performance.

[Ken Lutes]

I used original furnace metal duct. 6 in for the main line and 4 in drops. I have have been using it for about three years now without any problems. But like most have said there are several types of material to use and I am sure they alll work fine.