Metal or PVC ducts?


andrew-in-austin

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For Nordfab type stuff you could get it from Grizzly or work with a local / regional dealer. The dealer I used was slightly cheaper and did a design on the computer based on my inputs of size, drops, locations etc. We went back and forth a few times, his initial design was a good 40% more expensive than the eventual compromise. 

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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…

 

I don't have a cyclone, I have several smaller collectors that come on when the tool connected to them is turned on. Knowing they weren't going to pick up the fines, I added a hanging dust filter that turns over the room air several times an hour (don't remember exactly how nany times/hr). I know the filter collects a lot of dust, but I don't know how it would compare to a well engineered cyclone system. It seems to be a reasonable in between solution however. My shop is 36' x 60'

 

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I bought 26 gauge snap lock pipe from my local sheet metal supplier. Bought wyes online, bought the self clearing blast gates from Lee valley, a few rolls of metal HVAC tape, and some self tapping metal screws. I'm running a Clearview with 10 drops, and I'm getting 1000-1400 CFM at each drop. With all the gates closed the pipe doesn't crush or anything

 

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Thanks for all of the great info.  Quick side question without hopefully hijacking the thread: is it better to vent outside if possible?  I'm trying to figure out my cyclone/dust collector and it sounds like many on the forum use some form of clearview or harbor freight as the two steps?

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19 minutes ago, Nemmy said:

Thanks for all of the great info.  Quick side question without hopefully hijacking the thread: is it better to vent outside if possible?  I'm trying to figure out my cyclone/dust collector and it sounds like many on the forum use some form of clearview or harbor freight as the two steps?

Venting outside is fundamentally better if possible...you get the fines out of the shop completely guaranteed.  You don't have to deal with filters, and you don't lose efficiency because of filters.   It's just better.    I do think the usual dichotomy of the forum exists here where some are going to buy the HF one and modify/extend/frankenstein it into a workable system.   If you're spending more, it's really hard to overlook the ClearVue IMO...the price point is really good especially if you're going to vent outside and can forego filters...and it's obviously a good proven system.

 

1 hour ago, Tom Cancelleri said:

I bought 26 gauge snap lock pipe from my local sheet metal supplier. Bought wyes online, bought the self clearing blast gates from Lee valley, a few rolls of metal HVAC tape, and some self tapping metal screws. I'm running a Clearview with 10 drops, and I'm getting 1000-1400 CFM at each drop. With all the gates closed the pipe doesn't crush or anything

I assume CV1800 Tom?   Do you run 6" to everything or 6" main and 4-6" drops based on the tool?  I'm glad to hear you have no problems with the 26ga.  I need to dig into @estesbubba shop thread and see what gauge spiral pipe he used...I rather like the look of those and dealing with all the snaplock seems a pain.

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I assume CV1800 Tom?   Do you run 6" to everything or 6" main and 4-6" drops based on the tool?  I'm glad to hear you have no problems with the 26ga.  I need to dig into @estesbubba shop thread and see what gauge spiral pipe he used...I rather like the look of those and dealing with all the snaplock seems a pain.

 

It is the cv1800. I used 6 inch drops to the tool then reduced to 4 with a tapered reducer which then goes to a short piece of flex hose. Spiral pipe looks a bit nicer, and might be a bit less work, but honestly the cost difference just wasn't worth it. I'm not in my dream shop so going with nordfab or spiral was worth it at this point.

 

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4 minutes ago, Tom Cancelleri said:

Spiral pipe looks a bit nicer, and might be a bit less work, but honestly the cost difference just wasn't worth it. I'm not in my dream shop so going with nordfab or spiral was worth it at this point.

Yea true...I'm always comparing quotes back to just buying PVC to stay in touch with reality.   Spiral prices are all over the place though both online and locally...they're really making it up as they go lol.

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On 3/1/2015 at 5:52 AM, Trip said:

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 really, really avoid these discussion most of the time but, there's some assumed knowledge of data here and I'm curious.  Is PVC not capable of the velocity due to diameter? The fittings? I've missed something (nothing new there).

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26 minutes ago, gee-dub said:

I really, really avoid these discussion most of the time but, there's some assumed knowledge of data here and I'm curious.  Is PVC not capable of the velocity due to diameter? The fittings? I've missed something (nothing new there).

I think it's the limited availability of diameters and fittings...with metal you have more diameters available...and you have a wider variety of fittings available to handle almost any scenario as necessary.  With PVC you often end up going with a simpler suboptimal design or using a ton of fittings which causes its own problems.

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2 hours ago, gee-dub said:

I really, really avoid these discussion most of the time but, there's some assumed knowledge of data here and I'm curious.  Is PVC not capable of the velocity due to diameter? The fittings? I've missed something (nothing new there).

I would say it's the lack of long sweep elbows that is the biggest downside to using PVC.

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2 hours ago, drzaius said:

I would say it's the lack of long sweep elbows that is the biggest downside to using PVC.

 

3 hours ago, Vyrolan said:

I think it's the limited availability of diameters and fittings...with metal you have more diameters available...and you have a wider variety of fittings available to handle almost any scenario as necessary.  With PVC you often end up going with a simpler suboptimal design or using a ton of fittings which causes its own problems.

Both good points, thanks.  I was assuming an apples to apples comparison and was confused how steel was just plain slippery-er :D.The 6" PVC long sweeps at $50 or so apiece are not much more than a pair of 45's at today's prices. 

I have been glad I over-bought a few times in the last 8 or 9 years.  When I bought, fittings from Ace Online were cheaper to my door than at the same supplier right up the street where I got 6" x 10' sticks for $11 each. That was then . . . this is now.

 

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I am fairly used to seeing box heaters and custom swept lengths of bell end conduit and pipe. I have tried it and there is a temp that is ideal. Buy some cheap lengths at 2" or so and experiment before trying bending a 6" section badly. Ross is right though, it is a good option. 

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We've custum bent up to 12" PVC conduit at work, using an oil bath heater. The pipe was filled with sand to keep it from collapsing. Carus is correct, to much heat, and the pipe can fall in on itself.


For smaller diameters, I've warmed it with a paint remover heat gun, quite successfully.
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PVC actually has the lowest friction coefficient of all the different types of ducts people run. PVC is really easy to seal as well. The downside of PVC is a couple of things, it weighs more, less available types of fittings and when you can find them you're going to pay a lot for it comparatively. As dust travels through PVC it will create a lot of static, and while dust isn't gonna explode, those PVC is gonna need to be vacuumed/wiped off pretty often as all the dust in the air is gonna cling to it leaving a nice static clung layer of dust on your pipes. 

The one downside of the CV1800. I need to ground my cyclone itself, as that dust spins through the cyclone it is like standing near a giant balloon that was being rubbed with a wool blanket. You feel all the hair on your body being pulled towards it if you're within 3 feet.

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On 5/17/2017 at 0:40 PM, Tom Cancelleri said:

 As dust travels through PVC it will create a lot of static, and while dust isn't gonna explode, those PVC is gonna need to be vacuumed/wiped off pretty often as all the dust in the air is gonna cling to it leaving a nice static clung layer of dust on your pipes.

This can be alleviated by running a grounding wire along the length of the pipe, making sure to attach it to the PVC every few feet, duct tape will work.   Not necessary at all, but if you find your ductwork isn't grounded well in some areas, this will help reduce an unpleasant (at worst) zap.  I learned this trick while in a sandblasting shop.  I got zapped a couple times and took the time to ground our duct work. No more zaps. 

 

Speaking of venting outside....  what do you guys that do this use to cover your vent?  What size screen would you use to keep bugs out, yet still let the small stuff escape?

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On 5/11/2017 at 3:41 PM, drzaius said:

Beware of the HD duct. It's usually 28 or 30Ga with snap lock seams. They can collapse if you leave all the blast gates closed & need lots of solid support. I used 26Ga spiral duct & long sweep elbows & it's plenty strong.

I couldn't justify the Nordfab because of cost & I can't imagine it being much easier to install the spiral duct. The hardest part of the installation was doing the layout & getting all the supports perfectly lined up (I was excessively anal about that).

Here's my installation, not entirely complete at the time of photo. the cyclone is on the other side of the back wall.

SW.jpg.42ca7f5d127d5f0641d757d0db86eea6.jpg 

Now there is the best way I've seen to compromise with the wife about still using the shop as a garage.  Don't make your shop portable or smaller, just get a smaller car.  Now that's smart. *rimshot* 

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7 hours ago, Marmotjr said:

Speaking of venting outside....  what do you guys that do this use to cover your vent?  What size screen would you use to keep bugs out, yet still let the small stuff escape?

I don't own a DC so this is just a general answer. Screens can be wide as they keep birds out. Gravity damper flaps do better at keeping bugs and drafts out. 

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On 5/17/2017 at 11:40 AM, Tom Cancelleri said:

all the dust in the air is gonna cling to it leaving a nice static clung layer of dust on your pipes.

Truth.  And not just to the pipes but wherever it's attached to the walls/ceilings as well.  My duct is a rich nutmeg color :lol: and the ceiling has a nice swath of dust about two feet wide on each side of the pipe. LOL  Jimmy crack corn.  Don't care enough to ground it.

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8 hours ago, Marmotjr said:

Speaking of venting outside....  what do you guys that do this use to cover your vent?  What size screen would you use to keep bugs out, yet still let the small stuff escape?

I used a big hooded dryer vent with a flap.  I had the siding guys put it on while they were repairing the tree damage (it was special order to match the siding...all they had in stock at HD was white).  For years all I had was a...how you say?...hole? LOL  A robin put a nest in there one spring.  Pick up Long Ranger, press button, bye bye birdie. :D

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