metal ducting vs. plastic

[getoverit]

I came in possession of a pretty good 1.5 hp dust collector system, but it didnt include any ducting. I see in a lot of shop pics in the gallery that most people use metal dust collector chutes, but some use PVC pipe instead.

Can anybody tell me which is better or if there is about the same differences?

[Modat22]

I helped a guy install some dust duct a few months ago and he used 3 different types of pipe in his.
Metal (smooth wall)
PVC schedule 40
and clear vinyl stretch type pipe.

The metal pipe was lighter and slightly harder to install where the PVC was heaver and easier to install. Only problem we had with the PVC was the necessity to run a ground wire in the pipe to cancel out the static.

The flex ducting was just a fit and clamp type but had the ground wire already inside it. Since then he told me he dosn't care for the flex duct much, he said it clogs easier then the rigid pipes. I suspect it clogs easier due to more extreme bends in the pipe (which is why he used flex to begin with)

Hope this helps.
Sam

[metalspinner]

I went with pvc because it is easier to get the pieces I needed.  I used a couple of self tapping sheet metal screws at each joint to hold it together tight.  I also wrapped each joint with duct tape as a seal.  The plastic blast gates I have are junk.  They clog all the time and don't shut all the way.  The metal blast gates are probably better.  A remote control is a really nice convenience - if you don't lose it!

[scsmith42]

GOI - the biggest advantage to metal is the grounding.  If you use plastic, be sure to run a bare ground wire through it - otherwise it can explode and/or catch fire.

Scott

[Den Socling]

That's due to static charge if dry dust rubs by PVC.

[Larry]

Ya might wanta read this bout the PVC verus metal before making a decision.  Lot of other smart guys have argued the point both ways.

http://www.woodcentral.com/cgi-bin/readarticle.pl?dir=shop&file=articles_221.shtml

I have my own opinion but will withhold it...I will tell ya fact...bigger is better, sweeps are better than 45's, and 45's are better than 90's.  There are a few calculators around that you can use to figure out your system...just can't remember where.

[getoverit]

Thanks for the link Larry! Interesting reading indeed.

[scsmith42]

Larry - that's a GREAT LINK!  Thanks for sharing. 

I love learing from issues and facts such as this, as opposed to rhetoric.

Scott

[firecord]

I agree with the bonding and grounding, completely. IF IT IS DONE RIGHT!
If you are insured, I would follow NFPA.   If you have a fire that starts in the duct work they could say you didn't build it to code, and not pay

NFPA
8.2.2.2.1 General Requirements.

8.2.2.2.1.1 Ductwork shall be metallic.

Exception:  Flexible ducting shall be permitted for final machine connection in a length not exceeding the minimum required for machine operation.

8.2.2.2.1.2* Nonconductive ducts such as PVC pipes shall not be permitted.

8.2.2.2.1.3 Unless equipped with drainage, horizontal ductwork shall be capable of supporting the weight of the duct half-filled with material. Where sprinkler protection is provided in the duct, horizontal ductwork shall be capable of supporting the weight of the system plus the weight of the duct half-filled with water or material being conveyed, whichever has the higher density.

8.2.2.2.1.4 Ductwork shall be protected from corrosion.

8.2.2.2.1.5 The capacity of the system shall be calculated on the basis of all hoods and other openings connected to the system being open.

8.2.2.2.1.6* Dampers, gates, or orifice plates provided for the specific purpose of balancing the airflow in the system shall be fastened to prevent inadvertent movement.

8.2.2.2.1.7 Ducts with a circular cross section shall be used.

Exception:  Transition to a noncircular cross section of equal area shall be permitted where ducts connect to other equipment or where external obstructions necessitate a noncircular cross section.

8.2.2.2.1.8 Ductwork shall be bonded and grounded in accordance with  7.9.2(1).

8.2.2.2.2* Ducts with a Fire Hazard. Ducts conveying dry material released by equipment having a high frequency of generated sparks shall be designed and constructed in accordance with one of the following:

(1)      Equipped with a listed spark detection and extinguishing system installed downstream from the last material entry point and upstream of any collection equipment 

(2)*      Equipped with a listed spark detection system actuating a high-speed abort gate, provided the abort gate can operate fast enough to intercept and divert burning embers to atmosphere before they can enter any collection or storage equipment 

(3)      If conveying material to locations representing minimal exposure to personnel and the public at large, equipped without spark detection and extinguishing systems but subject to a risk analysis acceptable to the authority having jurisdiction 



  Do you collect the dust in a bin or just discharge it on the ground?  If you collect it in a bin, I would add a sprinkler head inside.  A 165deg. ordinary hazard head would do, you could pipe it with PVC.  As long as you have between 60 and 75 psi water pressure it should eaisly supress a fire in something as large as a 10X10 holding bin.  I can send you one if you want it.  Saw dust makes an extremely hot fire starter, you could quickly loose your whole operation.   

Ps: I am a industrial fire chief according to my pay check

[getoverit]

Actually, I wont be using a collection bin at all. My workshop is a small one and not anywhere near the size of an industrial shop. The discharge from the blower will go directly into the chicken pen and the chickens are excellent workers for spreading out the sawdust to keep it from piling up :D

I was really wondering if there were more clogging up issues with one type of duct over the other. I can see where plastic is slicker and should clog less than metal, but PVC may also be more prone to sweating and cause the dust particles to stick to it because of being moist. I may be completely backwards on this thinking though, and metal may be the one to sweat instead?

To me, PVC is the way to go since it requires no special tools to install it and making changes to it for adding drops for new equipment in the future would be a LOT easier. The high cost of metal is also a factor to consider.

Thanks for these opinions folks !
Please keep them coming!

[metalspinner]

I've considered venting mine outside as well, but running it for long periods of time might suck all my nice cool air out of the shop.  I haven't noticed any moisture problems in my PVC ducts.  This collecter is 1.5hp as well.

[Den Socling]

Unless you have a fire code problem, I would guess that PVC is fine for the home shop because quantity and velocity of wood sliding over plastic is relatively low. In another life, I had a 24" trunk that took the dust from a DET, shavings from a 9-head Weinig, shavings from a 7-head Weinig and then very fine dust from a wide belt sander. IMO, it would be nutty to use PVC in a situation like this.

[DanG]

I guess if you could afford those 2 Weinigs, the cost of the metal ducting wouldn't be much of an issue,eh? :D :D

[Den Socling]

Actually, we couldn't afford the Weinigs. I was just the plant engineer and the place ended up belly up.  :( However, I left the mess with ideas about vacuum drying wood.  ;D

[jgoodhart]

The plastic blast gates work real good if ya trim the closed end off so the sawdust pushes out instead of getting stuck in the slide ;D

[low_48]

If you're just blowing it outside I would not worry about grounding at all. If I was blowing it into a bag, I would not worry about plastic either. Use the drain pipe because it will have one end that is sized to slide over a straight and will save you another fitting. The idea that a spark will discharge inside the pipe and ignite the dust? I can't imagine it is at all possible. Wait a minute, I'm going to send that into MythBusters 8). Also if you send that into the chickens, and it explodes like many will tell you, we can have a chicken roast this fall instead of the piggy roast. :D Also, don't use the bigger is better with the pipe size. You must maintain velocity to keep the dust and chips moving. Too big of pipe will let the heavy material fall out and plug. One 90 degree elbow is the same as 10' of straight pipe for resistance. Flexible tubing is a real killer. A foot of that stuff is also like 10' of straight. Here's a link to help with the design.
http://www.airhand.com/designing.asp

[Faron]

I have PVC in our current shop.  I have wire wound around the outside , and a wire in the inside.  I have noticed  dust collects on the outside, increasing as you get farther away from the wire.  I can assure you from experience that PVC can discharge enough shock to stop your average enraged elephant  under the right conditions.  I have no doubt a fire or explosion can result given the right dust conditions. 

The new shop is going to have metal ducts.  I saw a potential problem the other day I will have to address.  Currently we blow sawdust from the sawmill into a wooden container.  The new shop will go in the end of that building, and I intended to blow dust from the shop into that container.  I was using the shaper on some hickory the other day, and suddenly sawdust hanging on the edge of the fence ignited, pullin sparks into the blower and out the pipes.  Currently that shop just goes out a tube and onto an outside pile, so it was easy to check out.  I am going to have to come up with a metal container, and maybe a fire supression setup when the new shop is up and going.

[OneWithWood]

Here is another site worth checking out

http://billpentz.com//woodworking/cyclone/index.cfm

My shop is ducted with 4", 6" PVC and flexible smooth interior PVC hose.  It exhausts through the wall into the back of a dump truck.  I did not ground the pipe and I have not experienced any static build up.  The system is hooked up to a 10 ' table saw, 12" sliding compound miter saw, 8" jointer, 18" planer/moulder and an LT40 Woodmizer mill.  I used metal blast gates that are push thru so they do not clog.  The blower is a 5hp unit.

[jack]

Hey,
Great thread.  Awhile back while grinding the TREX plastic lumber (can i say that? Oxymoronic me thinks) We were grinding the reject stuff for the plant,  and they were changing all the 90 degree sweeps out.  They wear out too fast,  All that plastic 1/4 inch minus blowing thru the miles of ducts.  Well, they changed the sweep 90s to  "T"s.    Yea,  "T"s, they sed that the material packs up in the end of the T and it doesnt wear out the fitting.  The material becomes the wear surface.  NOW why didnt i think of that?
Well for the production shop, it might be something to think about.  I know im gonna set my two 5hp suckers that way.

just a thot.
Jack