Metal or PVC ducts?


andrew-in-austin
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I am preparing to add some ducting for dust collection, but I am having some trouble deciding between metal and pvc.  I would like to do metal, so I do not have to worry about grounding pipes (to avoid static shock).  The issue I have is the cost.   It's looking like metal will be at least triple the cost.  Is this what others have seen, or is there something I am missing?

 

For those who used pvc, did you bother trying to ground the pipes?

 

Thanks, 

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I built my system about 10yrs ago with pvc, I've never had any issues or regrets. I did ground mine, many say it's not necessary but I'd rather not have to always wonder. One thing I would suggest, either make your own blast gates or buy the metal ones. I had nothing but problems with the cheaper plastic ones, they would get crud built up and wouldn't close all the way. Hope this helps.

God bless

Nate

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Mine is PVC, not grounded.  Airborne dust sticks to the pipes like crazy, but I've had very few and very minor shocks, surprisingly.  My shop hasn't burned down and I'm still alive and breathing.  Metal duct looks awesome...if you can afford it, go for it.  If not, PVC is fine.  Ground it if it helps you sleep.  I'm not gonna tell you you shouldn't, but I know of exactly zero confirmed cases of a shop going up in flames because of an ungrounded PVC DC system.  You're much more vulnerable to a spark flying off the bandsaw into your dust bin, or a space heater melting down, or a pile of oily rags combusting.  IMO.

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For anyone that has added grounding to PVC... Did you just daisy-chain a series of wires along each segment or did you also wrap the wire around the piping? What did you do at each connector, junction, blast gate, etc?

Maybe a few pictures would be the clearest way to answer the questions.

Also, is the grounding wire entirely on the outside or has anyone installed grounding on the inside of the PVC piping?

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The heart of a shop’s DC setup is the ductwork. Hanging high-quality well-sealed ductwork designed by a professional is more important than the DC itself. This is the key concept of shop-DC that most don’t fully realize (because DC Mfg’s don’t go out of their way to tell you). It’s so important that I’ll repeat it: ducting is the heart of a shop's DC and the cyclone/bag/whatever just connects to it.

 

I know PVC –vs- Metal ductwork borders on religious dogma, but it is what it is, so don’t shoot the messenger… If you cut through all the BS on ductwork, the real issue is velocity... First and last, it's velocity... You need a certain velocity to capture the fine dust in the 0.3u to 0.5u range (the 'fines') and move it down the system. To get the 'fines' you need to maintain around 3500FPM over the length of the main and around 4000FPM on each branch. This is the ugly little secret at the heart of the 6" PVC ductwork universe -- it's not that metal is 'better' than PVC or you'll get a fire with PVC or you'll get too much static buildup with PVC or any of that nonsense ---- it's that it’s near impossible to deploy a constant-velocity design with PVC. That's the real issue at the heart of the PVC/Metal debate (for those who stayed-awake during physics lectures on Bernoulli’s Law and fluid dynamics)...

 

You can deploy PVC ductwork and your shop will look clean – but you’ll still have to wear a respirator because you won’t be capturing any sub-0.5u dust -- the dangerious dust that doesn't make your shop look dusty, but plays Hell with your lungs…

 

You can largely mitigate the issues inherent in PVC ductwork by over-provisioning the cyclone… If you have a small shop, you can use 6” PVC to the drop if you attach a mongo cyclone (5HP+ high-pressure unit) and operate one tool at a time… In many cases, this setup will work if the runs aren’t too long… Further, if you’re an infrequent hobbyist (one project a year), then it's probably not an issue --- you probably won't breathe enough sub-0.5u dust to make much difference...

 

 

Anyway, good luck…

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 ---- it's that it’s near impossible to deploy a constant-velocity design with PVC. That's the real issue at the heart of the PVC/Metal debate (for those who stayed-awake during physics lectures on Bernoulli’s Law and fluid dynamics)...

 

I wasn't awake during that lecture.  Assuming all else is equal in two otherwise identical systems...power, length of runs, radii of turns, diameter of pipe...what is it about PVC as a material that so drastically reduces velocity?  Don't spend your morning explaining it...just a sentence or two will suffice. :)

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==>what is it about PVC as a material that so drastically reduces velocity?

Nothing – Nothing at all... Well, as wtn observes, the joinery doesn’t help.... But that's minor in the scheme of things...

 

Perhaps a better explanation is required – especially because I’ve seen several WTO members deploy 6” metal ductwork to the drop and equally loose the inability to collect sub-0.5u dust… So they spent the money, and got zero of the benefit – other than it looks cool… We're talking 6" dogma, not metal -vs- PVC... Although the two are related...

 

There are some good guides available that explain this stuff much better than I can… Oneida’s Ductwork Design Guide, Air Systems has a good Design Guide and so does Air Handling Systems... So I’ll try a brief explination with the understanding that it’s oversimplified and a bit wrong -- so don't shoot the messenger:

 

The size of particles captured and/or transported by the DC depends on several factors --- velocity of the column being a key parameter… As particles move along the pipe, there is resistance (there is a great deal more resistance generated in elbows, joints, etc – but we’ll just stick with straight pipe to keep things simple). Over distance, resistance accumulates… With accumulating resistance, there is loss... See Wood Magazine diagram...

 

 

You need to maintain approx 3500-3800 FPM in horizontal runs and 4000-4200FPM in vertical runs to collect and transport sub-0.5u particles (fluid dynamics)… As loss accumulates, transport resistance increases, column velocity decreases, etc... An easy way to favorably modify the characteristics of the column to counter resistance loss is to change the diameter of the pipe (our friend Bernoulli). So you may start with a 4” inlet at the tool, and then move to a 5” riser, then a 5-5-6 Wye as the riser joins the branch, then travel along a 6" branch pipe, then to a 6-6-7 Wye as the branch joins the trunk, then a 7” section of trunk... then through a 7x8x7 Wye as the 7" trunk transitions to an 8" trunk, then travels along an 8" trunk.... you get then idea... until you get to the inlet DC inlet flange… See the Oneida-air design diagram below. Note the changes in diameter as you move along the pipe... It’s these diameter changes that keep the velocity above the 0.5u particle inflection point… It's also why a 3HP cyclone can collect sub-0.5u dust from any two tools running simultaneously (as long as the wide-belt sander isn't one of them) and any three blast gates open (as long as the wide-belt sander isn't one of them)...

 

 

 

What's important here: Metal/PVC doesn’t matter – it’s the ability to introduce diameter changes and long-radius / gradual transitions that are important… So, if you want to deploy a PVC system with five different diameters (four through eight inch), custom Wyes in 4-4-6, 5-6-6, 5-6-7, and 6-6-8, then you can deploy a constant-velocity PVC ductwork design – or you can go metal for a lot less money… :)

 

Hope that helps a bit... The free guides available from the pros are really a lot better -- I've had to simplify stuff to the point where it's kind of wrong -- the pro guides are 15+ pages and do a much better job... Also, they deal with ductwork and dust collection, not really selling a particular DC setup...

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Got it, makes sense.  It's more about the availability and cost of materials for a physics-correct setup than it is the material itself.  Which is interesting info and good info to have should I endeavor a real system.  But as I stand now with my little 2HP rice-burner...I'd be no better off swapping my 4" PVC for 4" metal aside from cool factor.  Perhaps an upgrade to a 6" main would be beneficial and worth considering...I always worried that 6" would be too much for a little 2HP unit.  Think I'll keep my mediocre system as-is until I upgrade to beast.  Thanks for the lesson...

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==>I'd be no better off swapping my 4" PVC for 4" metal aside from cool factor.
Largely no difference at all… Although ‘cool’ does count.. :)

 

 

The difference comes in the availability of long-radius transitions, wye-joints, etc… These do add-up… If you play with an on-line SP calculator, you see that a single elbow has about the same loss as 10’ of straight pipe (give or take)… So if you start pulling elbows, flex hose, etc and substituting long-radius transitions, wyes for elbows you may greatly increase the collecting capability of your existing setup…  BTW: the biggest SP killer is flex hose… Try to keep it to 4’ on each drop…

 

 

The heart of a shop’s DC is the ductwork – the cyclone/bag/etc just connected to it… Improving the ductwork can pay big dividends…

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Thanks for all of these comments.  I have been going back and forth on what to do probably 20-30 times!  I have also done some research, and what TripleH has stated keeps coming up.  So, I have decided to go with metal ducts, and even go for the clamp together variety (Nordfab, GorillaDuct, etc).  The two biggest reasons I am going to go for the more expensive metal ductwork are (1) reducing my time putting this together.  I made the realization that I should reduce my time doing things I like less (plumbing) so it does not steal a lot of time from the things I like to do (woodworking).  (2) The nordfab sytle ductwork gives me, in my opinion, the most flexibility to reconfigure.  PVC can do this as well to some extent, but I have crossed PVC off the list for a couple reasons (I'll get to in a second).  

 

I am going to start this system very small: dust collector->ductwork across garage ceiling->single drop.  This seems like an incredibly small system, and some people might opt to just use a rolling dust collector next to the machine.  I have tried that, and I hated it!  The damn dust collector was always in the way!  Drove me crazy.  So I will embark on what probably is the smallest duct solution ever :)

 

I do plan to expand this eventually, but I want to take this one step at a time, and I only want to invest in what I really need, while keeping the quality and effectiveness very high.

 

As for PVC, I came really close to going this route.  I was at Home Depot last night and almost pulled the trigger.  And I want to be clear, I appreciate the suggestions from the members here recommending that.  I don't think it is "wrong" in any way to use it.  I just decided that (1) I don't want to deal with the spark nuisance, and more importantly, I don't want to be limited to pipe sizes in 2" increments.  I already have 1 machine (planer) which has a 5" port, and soon I will have another (SawStop) which in effect will use a 5" port (It has a 4", but I want the DC drop to provide both under and over duct collection).  The ability to use 4,5,6,78" ducting IMO is too important for me.

 

BTW, I have an anemometer, and I plan on using it for testing all of this.  I want to hit that 4200 FPM for current and future machines.  I'll report back eventually and let you know what worked/didn't work.  Thanks again for all of your help! 

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Awesome thread! I actually have 3 drawings on my desk for my forthcoming new shop with DC. Just read the Air Handling Systems design guide, great recommendation HHH. Two questions:

 

1) My DC - Jet 2HP - has a one 6" intake. Looks like I'll be borderline on going to a 8" main line. Would it be recommended that I go 8" then drop down to 6" at the intake or just stay with 6" for the main trunk?

 

2) I've read a bit about High Pressure/Low Volume (aka dust extractor/shopvac) vs LP/HV (aka dust collector). How important is it to have both? While I have a blank slate I want to get it right. Anyone know of a good resource or thread on this? I've just seen it mentioned vs finding a good educational article.

 

-jason

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1) My DC - Jet 2HP - has a one 6" intake. Looks like I'll be borderline on going to a 8" main line. Would it be recommended that I go 8" then drop down to 6" at the intake or just stay with 6" for the main trunk?

 

-jason

 

It's my understanding that if you have a 6" inlet to your DC, you shouldn't go any bigger than 6" for your main since it will be choked back down to 6" anyway and it will kill efficiency.  Diameter reduction should only be done in the direction moving toward the tool, not the other way around.  Of course every time I think I understand DC, another wrench is thrown into the mix.

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For those who just don't want to foot the bill for metal ductwork, eliminating as many joints as possible from the PVC run may help. Although a bit awkward at 6" diameter, PVC can easily be heated and bent to a sweeping radius. Using gentle transitions for changes in direction should have a positive effect on air flow.

If you try this, be sure to fill the pipe with sand, sawdust or something that can flex with the bend, but resists compression, so the softened pipe doesn't collapse when you bend it.

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you can nearly match a metal sweeping radius by using a sweeping PVC radius and you can get even closer by using 2 sweeping 45's

 

when running biggish sewer lines (6-8") we always use 2-45's in place of a 90. The supplier where I got this pic, has these up to 6" cheaper than a metal one.

post-16388-0-07908700-1425304276_thumb.j

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Yeah Mike, my fines blow right out of the shop...no filters.  I'm sure it helps efficiency but I've never hooked any meters up to the system.  After I installed it I was happy to find that it picked up the vast majority of my mess so I moved on mentally.  I've made a number of tweaks to improve collection, but most of them have been insignificant aside from modifications to the tools themselves...I find that the poor engineering on most of the machines' DC ports are the biggest problem and probably the least considered.  You can have a hurricane force DC, but if it's pulling a foot away from the dust source and pulling open air all around it...no good.  The table saw and bandsaw are both terribly engineered for DC.

 

One day I'll upgrade to a 5HP unit and redo all the duct.  Meantime it's farthest from my mind.  Too much wood to mangle to waste my time on it.

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Mike - how would you put that in context?  Is that a lot or a little?  How much extra power do you need to make up for 1 - 1.5 SP loss?

 

You don't, to try and put a number on it is not possible unless you set up a manifold with a manometer. All filters hurt a dust collector   just as they do spray booths and other devices. You need a large filter bank if you don't want to see any losses caused by the filters but even at that it takes a manometer to monitor the filter bank. 

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