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DIY dust collection pipe/tube options

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Forum topic by William Shelley posted 03-16-2017 07:43 PM 815 views 0 times favorited 9 replies Add to Favorites Watch
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William Shelley

479 posts in 1308 days


03-16-2017 07:43 PM

I’m planning out dust collection for my new workshop and coming up on some obstacles that I think everyone runs into at some point. The local big box stores only sell schedule 40 PVC, not schedule 20, and only 4” in 10-ft lengths. As far as metal ducting goes, at least home depot only sells the thinner wall stuff that I’ve heard can collapse if all the gates are closed at once. The spiraled metal ducting is not sold at any of my local home improvement stores and I am assuming it’s quite expensive at specialty stores.

Is there anything wrong with building a rectangular main trunk out of cheap 1/8 or 3/16” plywood? Or perhaps the white melamine-coated stuff to reduce friction. My idea is to basically build a box that is about 12” x 12”, and ripping 2×2 furring strips into triangular strips, to install in the corners (so the corners are not sharp 90’s). I’d make my own wyes that would simply attach to a rectangular cut-away section on the main trunk.

My shop is going to be 25 feet long and only about 14 feet wide, with the dust collector at one end. I currently have a HF 2HP dust collector but may upgrade to a bigger one. As far as tools that need dust collection… I have a table saw, bandsaw, miter saw station, drill press area, and my new 16” planer/jointer. The planer/jointer is the main concern, as taking a heavy pass on a 16” wide board through the planer is going to stuff a LOT of chips into the system really fast.

Am I overthinking this? Should I just hold out for finding a score of some cheap schedule 20 pipe on craigslist or something?

There’s also Matthias Wandel’s solution which is to build, for each tool, a custom dust-collector / chip separator out of wood and salvaged motors…

-- Woodworking from an engineer's perspective


9 replies so far

View Fred Hargis's profile

Fred Hargis

4762 posts in 2332 days


#1 posted 03-16-2017 09:37 PM

Well, Stumpy Nubs built some ductwork out of wood and it seemed to work fine. Check his website, I’m sure that story is in there somewhere. Personally, I’d keep looking for thinwall PVC….or perhaps go ahead and do metal. You are correct, the box stores normally don’t have it (if you have Menard’s nearby they have 3034, which works well and is relatively inexpensive. But check farm supply stores and maybe lumber yards that sell S&D pipe, or lastly if you have a plumbing supply they may have, or will get, some. I’ve never seen any on CL, but anything is possible on that site.

-- Our village hasn't lost it's idiot, he was elected to congress.

View TungOil's profile

TungOil

747 posts in 333 days


#2 posted 03-16-2017 10:02 PM

rectangular duct will be prone to collapse under vacuum and the corners will be dead spots (even with the strips) that will collect chips.

look here: https://www.airhand.com

they sell inexpensive “hobby shop” ductwork which is lighter gauge. Boxes of 5’ lengths are not that expensive to ship and they have all the fittings you will need to connect the system.

regarding a new DC- I suggest that you ddesign the ductwork before you select a unit, not the other way around. The correct way to design an AH system is to figure out the piping first, then size the DC unit to meet the demand.

Good luck!

-- The optimist says "the glass is half full". The pessimist says "the glass is half empty". The engineer says "the glass is twice as big as it needs to be"

View AZWoody's profile

AZWoody

1138 posts in 1062 days


#3 posted 03-16-2017 10:18 PM

for S&D forget the big box stores and call the plumbing supply stores. They will be able to get you any size you need in 20’ sticks and all the fittings to match.

View Bumpy's profile

Bumpy

33 posts in 320 days


#4 posted 03-16-2017 11:27 PM

Look at the post from Saturday- similar question with some good answers.

FYI, I have just under 100’ of 6” duct on a 3hp cyclone, never an issue with collapsing duct.

View JBrow's profile

JBrow

1274 posts in 759 days


#5 posted 03-16-2017 11:39 PM

William Shelley,

I find that dust collection is a complex and confusing subject. Given the fact that you can pour a lot of time and money into centralizing your dust collection, I do not believe you are overthinking this project.

My suspicion is that round pipe facilitates air movement better than square or rectangular pipe since turbulence could be less of a problem with round piping. But that is a pure guess and I could well be wrong. I looked through the three books on which I relied when designing my system but all were silent regarding duct geometry and duct work material. They seemed to contemplate round metal or PVC piping. If someone knowledgeable of duct work geometry does not respond, it may be worth looking into.

I was a little confused by your plan for constructing wyes. I think there would be better performance if the wyes enter the main at a 45 degree angle rather than at 90 degrees. Gently turning 90s would likewise provide better performance than sharp 90s.

From all I have read, the main dust collection trunk line should be sized to maintain 4000 feet per minute (fpm) of air flow to keep dust and chips from settling out of the air stream. Duct work that has an interior cross sectional area of 144 square inches, such as the 12” x 12” ducts you propose would require 4000 cubic feet per minute (cfm) of air to maintain 4000 fpm. I therefore think 12” x 12” duct work is too large. For example, Oneida’s 10 hp dust collector would be too small to operate with your proposed duct size.

By contrast, a duct with a cross sectional area of 50 square inches requires a dust collector to move air at 1390 cfm at the machine end of the main trunk. A cross sectional area of 28 square inches would require 780 cfm to maintain an air velocity of 4000 fpm. A cross sectional area of 13 square inches would need 350 cfm to maintain 4000 fpm.

The formula I am using is…

CFM = 4000 fpm X Pipe Cross Sectional Area (in square inches) / 144 square inches

Air velocity in drops is best kept at or above 3500 fpm.

I see the advantage of a dedicated dust collector for each machine is that the short run from the collector to the tool would have minimal adverse impact on air flow and could provide good collection. The impact of pipe size, material, elbows, and overall length are best considered when designing a central collection system and should be of minimal concern in the one collector/one machine design. The disadvantages are that there are more dust collectors to empty and otherwise maintain. The cost of even inexpensive dust collectors, especially when equipped with fine filtration, could approach or exceed the central dust collection system. Lastly, there is the problem of carving out enough floor space for the tool and nearby collector.

View JBrow's profile

JBrow

1274 posts in 759 days


#6 posted 03-17-2017 12:39 AM

William Shelley,

My apologies. I posted before checking. The main trunk feet per minute (fpm) standard is 3500 fpm; not 4000 fpm. 4000 fpm is for branch lines and drops. Therefore, if you are interested, my values can be re-calculated by the formula I posted but substituting 3500 fpm for the 4000 fpm in the my original reply.

View William Shelley's profile

William Shelley

479 posts in 1308 days


#7 posted 03-17-2017 08:52 PM

You guys are fantastic. I totally forgot about the minimum velocity needed to keep particles suspended.

I know I should have sized the DC after getting the system put together. But in the real world, I had already bought the DC for my basement workshop, and use it with a 25ft flex hose connected to which ever tool I’m currently using. The ducting will be for my new workshop (not in the basement), where dust will be collected properly.

That being said, I’ve read that the true CFM of the HF 2HP unit I have is about 500cfm with the stock bag or 800cfm with a Wynn cannister filter. I’ll probably upgrade to a cannister (or a pair), within the next 6 months.

A 6” trunk line would give me a 28.27in2 cross section, or about an ideal velocity for a DC that can suck 650-750CFM, based on JBrow’s math.

-- Woodworking from an engineer's perspective

View Redoak49's profile (online now)

Redoak49

2904 posts in 1827 days


#8 posted 03-17-2017 10:02 PM

Just as a point of data, Wood Magazine in testing dust collectors looked at the effect of using a canister filter.

They test the Jet 1-1/2 hp dust collector with a 30 micron bag and compared it to using a 2 micron canister. They found a 40-80 cfm improvement.

I wish I could provide the link. You can find the pdf by searching Google for Oneida Air dust collector tests. The article will provide the information and comparisons. My point is that it is unlikely that you will go from 500 cfm to 800 cfm by going from the stock bag to a canister filter. Perhaps, someone has good test data to disprove the Wood Magazine testing.

View William Shelley's profile

William Shelley

479 posts in 1308 days


#9 posted 03-17-2017 10:08 PM



Just as a point of data, Wood Magazine in testing dust collectors looked at the effect of using a canister filter.

They test the Jet 1-1/2 hp dust collector with a 30 micron bag and compared it to using a 2 micron canister. They found a 40-80 cfm improvement.

- Redoak49

Was that a 40-80 cfm improvement with the 2 micron canister?

Interesting, but it’s an apples to oranges comparison. I’d be more interested in seeing how a 5 micron bag compares against a 2-5 micron canister.

I’m also going to be looking more at buying a 0.5 micron canister filter. I was thinking I might want to buy two of them so that there’s less resistance, the cost of one extra canister might be offset by not having to move up to a 3hp or 5hp DC unit.

-- Woodworking from an engineer's perspective

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