DC ductwork question

[Russ Ambrose]

just purchased an oneida cyclone. understanding that the best cyclone can be hamstrung by poorly planned ductwork, i was curious to see what sort of ductwork plan they would recommend. just got their preliminary recommendations and i'm not sure i understand their plan.

rather then running 6 in (or larger) ductwork through the shop with 6 in drops (and 6-to-4 reducers where necessary), they are recommending 5 in drops (and 5-to-4 reducers). i thought that when setting up DC ductwork you want to (among other things) use the largest possible ductwork. assuming this is so, why would you want to use 5 in drops instead of 6 in drops? this is a 3 hp unit with a 15 in impeller, so it's not like it can't pull sufficient air through 6 in pipe.

i'm certainly NOT saying they're wrong. they know more about this stuff than i will ever know. i'm just not sure i understand the basis for their recommendation. i want as little SP in this system as possible and my understanding is that as the size of the pipe decreases the SP increases. i'd appreciate any insight that y'all can offer.

also, does anybody know of good places to get DC ductwork? before buying from oneida i want to make sure i can't get the stuff elsewhere for a better price. i have a feeling i can, but just don't know where to look.
many thanks.

[Chris Padilla]

Russ,

I'm no expert but everything I've read tells me take the 6" as far as possible before reducing (if necessary) so that is what I did.

For ductwork, are you going metal or PVC? I went PVC.

[Russ Ambrose]

thanks, Chris. i'm open to either metal or PVC at this point. dumb question, but where do you get 6 in PVC? haven't really looked for any, but don't recall ever seeing any at HD or Lowe's. assuming both are safe and equally effective, it'll probably come down to price and ability to re-route down the road. i went with the 3 hp because my shop size will likely double in the next 2-3 years (and i only want to buy a DC once). so, i'll eventually need to re-route some/much of the initial ductwork.

[Steven Wilson]

There are a number of ways you can go. With a 3HP Oneida you could be served well with an 8" main (going into your cyclone, IIRC it's 8") and then run 7" branch circuits. From there you can run 6" or 5" drops to meet your machine needs. One 7" branch can support (2) 5" drops (say saw cabinet and overhead guard, or shaper hood and cabinet). If your machines have single 4" dust ports and you're not planning on modifying them to 5" or 6" then what Oneida is suggesting is fine as those 4" ports will be the limiting factor. To get the most out of system you've purchased you need to look into your machine hoods and upgrade them where appropriate (i.e. a 20" planer needs a 6" port whereas a 4" port on a 13" lunchbox planer is sufficient). Oneida's designs tend to be rather conservative and always err on having enough velocity to keep a maximum load suspended. They'll sacrifice CFM for velocity. I would switch it around and favor CFM over velocity and risk having some material fall out periodically.

[Mike Goetzke]

I have seen 6" PVC advertised at Menard's, but, I think it's special order. Also, I learned here that bigger isn't always better on DC ducting. You need a certain CFM to keep the wood dust airborn - so 5" drops was probably suggested assuming you will/could be using more that one tool at a time.


Mike

[Chris Padilla]

Russ, do some searching here as this has been pretty thoroughly gone through but, for 6" PVC and associated fittings, you are going to want to look for S&D (sewer & drain) ASTM 2729. Now I know that sounds kinda strange and scary but it is the most common S&D pipe out there. If your local borgs don't have it, try an irrigation warehouse.

For quick look online, go to http://www.mcmaster.com and in their search window, type: drain waste vent pvc pipe

You will be confronted by a page of various fittings and pipe. It will provide some prices to start working on.

One disadvantage with PVC pipe, it is only available in 4, 6, and 8" sizes (the most common you'll want in DC)...no in-between like you can get with metal. I didn't find that too big a deal but you may.

[Ken Fitzgerald]

Russ,

I paid Oneida to design mine. It's 6" to the vertical than reduced to 5" after the 90s for the downdrop.

[John Keeton]

Chris, is the S&D (sewer & drain) ASTM 2729 the same as "thin wall" PVC? That is what I was going to use, but don't want to mess up here. The thin wall was much cheaper than Sch. 40.

[James Williams 007]

The pvc you want is SDR 35 I have found most of it at Lowes but they don't carry it at every store and to special order it is unreal because the price goes up x3. Just hunt for it and get creative with the connections. To transition off my dads Onieda from 7 to 6 we used a rubber gasket type transition meant for iron to pvc transitions it had a screw clamp on either end and fit perfectly over the 7 in inlet.

[Larry Fox]

I went with 6" PVC for my system and I got all the pipe and fittings from US Supply which was - by far - the cheapest that I was able to find. I was seeing quotes from other places of like $30+ for a 6x6x6 Y. I believe US Supply charged me $13.

To give you a pricing data point to work with, my order was;

80' of 6" S&D pipe
8 - 6x6x6 Y's
21 - 45-degree fittings
3 - couplers

for a total of just around $425 (+/- $10). It aint cheap by any measure but it does do a nice job and I am much better equiped to work with PVC than metal so that helped determine my choice as well.

[Russ Ambrose]

thanks for all the great info. with respect to Steven W's post above, let me see if i understand this.

there are two components to dust collection, air speed (FPM) and air volume (CFM). these two are related and generally as one increases the other decreases (generally speaking).

i need a certain minimum air volume (CFM) to collect the dust and then a certain minimum air speed (FPM) to then move the collected dust.

seems to me that if i don't have enough CFM to collect the dust in the first place, it doesn't matter if i move it at the speed of light. i can't move what i never collected.

we get dust collectors to move the fine dust that is dangerous to our health. if i understand what i've read, it takes lower air speed (FPM) to move "fine dust" as opposed to chunks/chips (please correct me on this if i'm wrong).

so, if i'm more concerned with collecting/moving "fine dust" (and am willing to accept some amount of chips/chunks not getting removed or "falling out"), then i want to increase the CFM of the system so long as the FPM remains high enough to move the fines and "most" of the other stuff....whereas oneida appears to be focusing on keeping the FPM high enough to move everything and is willing to sacrifice CFM (collection) to get there.

assuming i'm understanding this correctly, i'm not sure i agree with their approach and agree with Steven W.

am i even close to understanding this?

[Chris Padilla]

ASTM 2729 or SDR 35 will work fine. They both have the same outside diameter but differ in wall thickness. I *believe* that 2729 is THINNER in wall thickness compared to 35. Both have much thinner wall thickness and are much cheaper than Sch. 40 -- don't use Sch. 40, very pricey and not necessary.

Whatever you get, be sure to only use S&D fittings for these as you see the prices of Sch. 40 fittings are crazy high at 6" sizes...like the pipe.

Use two 45-degree fittings to make a long-radius 90-degree fitting as it is cheaper.

A 6x6x6 wye at McMaster is currently $15.40. I got my 2729 pipe from a local irrigation warehouse for about $22 per 10 foot piece and all my fittings from McMaster. My pipe came with a "coupler" (bell-shape) built-in to the pipe but I found that piece ended up mostly being cut off and not used.

[Larry Fox]

My pipe came with a "coupler" (bell-shape) built-in to the pipe but I found that piece ended up mostly being cut off and not used.

Mine did too and, like Chris, most of mine ended up getting cut off (I think I used one). I bought actual couplers because I had not planned my run's to a high degree of detail and got them "just in case". Never ended up using them though and they will likely go back.

[Reed Wells]

Russ, I just finished running 6" PVC in my shop. If you want to save a lot of money on fittings make them yourself. I made eight wyes, one with three 4" branches. The following link is for a guy who wrote a program, free to use, online that will print a template for all the cutts you have to make. It was very easy and I must say, kind of fun. The guy who wrote it has a shop and must be over the top when it comes to smarts! And that is a compliment in the highest order. If I can remember I will take some photos tomorrow.

http://harderwoods.com/pipe.html

[Terry Achey]

Russ,

CFM and FPM do have a relationship, but not quite as you described. Increasing or decreasing either one does not necassarily mean that the other responds in opposite. In otherwords, the CFM (volume of air) can remain the same in iether a 5" or a 6" duct under certain conditions. However, in that example the FPM (velocity) would be greater in the 5" duct.

The other significant factor in the calculation is the static pressure losses that occur based upon factors such as distance, fittings, directional changes, piping reductions, air leaks, etc.

Larger ducting isn't necassarily better. I would stick with manufacturers suggested sizing or have your layout (drawing) calculated by a knowledgable source.

Terry