Shop dust sytems: Is there STILL debate over steel vs. plastic?

[Mark Burnette]

I haven't been tuned in to the DC threads for quite some time but I'm curious about this post in the Wood magazine thread:


I wonder if WOOD will talk about the "need" to "ground" a DC system (metal or PVC duct). There is a can o' worms not opened up yet on this thread.

I built my system 20 years ago around a 3HP Cincinnati 2-stage DC and used 4 & 3 inch pvc ducts with home-made blast gates. Back then there were dire warnings in the magazines & the still-new internet that my shop would explode, but evidence of the danger was always lacking. After all these years is this STILL a debate?

[Alan Schaffter]

I haven't been tuned in to the DC threads for quite some time but I'm curious about this post in the Wood magazine thread:


I built my system 20 years ago around a 3HP Cincinnati 2-stage DC and used 4 & 3 inch pvc ducts with home-made blast gates. Back then there were dire warnings in the magazines & the still-new internet that my shop would explode, but evidence of the danger was always lacking. After all these years is this STILL a debate?

No debate. Danger debunked. A few uninformed newbies still occasionally ask.

By the way, you need to upgrade to 6" ducting- 4" too small, 3" WAY too small for a 3 hp blower if your impeller is over 12" in diam.

[Ole Anderson]

Exploding dust due to static discharge in hobby shops has been pretty much debunked, but static shocks from ungrounded PVC pipe or flex hose in dry climates/shops is a concern for some. This the the most recent thread on the subject: http://www.sawmillcreek.org/showthread.php?197879-Metal-or-plastic

[paul cottingham]

I have drawn some awesome arcs off my flex leading to my planer. That is the only place it has happened. As for explosions, the Mythbusters tried to create one with static and wood flour. They couldn't do it, so I'm not worried.

[Lee Schierer]

Exploding dust due to static discharge in hobby shops has been pretty much debunked, but static shocks from ungrounded PVC pipe or flex hose in dry climates/shops is a concern for some. This the the most recent thread on the subject: http://www.sawmillcreek.org/showthread.php?197879-Metal-or-plastic

I used to get some static on my 4" pvc dust duct but it went away with use. Now it won't even raise the hair on your arm if you get close to the duct.

[Ken Fitzgerald]

I installed mine with metal. I do draw arcs from the connecting flex. Now that I have a cochlear implant, I am supposed to minimize any contact with static electricity as it could ruin the cochlear implant, necessitating explantation, and re-implantation or just remain deaf.

[Glenn Kotnik]

The static explosion theory doesn't worry me but with PVC the weight can get astronomical if you go larger than 4". The price goes up to match. I know you'll hear that the 28 and 30ga. galvanized duct you buy in the big box stores is too flimsey, but I've had it hanging from my ceiling hooked to a monster cyclone and not one problem. If you stick with 4" you're probably strangling your system. What do you consider the advantage of PVC to be?

[Ken Fitzgerald]

Glenn......Go to this thread http://www.sawmillcreek.org/showthread.php?79131-And-the-fun-begins-yet-another-DC-project!/page2 and scroll down to post #43. Our own John Keeton accidentally got some 30 gauge.....

[Alan Schaffter]

The static explosion theory doesn't worry me but with PVC the weight can get astronomical if you go larger than 4". The price goes up to match. I know you'll hear that the 28 and 30ga. galvanized duct you buy in the big box stores is too flimsey, but I've had it hanging from my ceiling hooked to a monster cyclone and not one problem. If you stick with 4" you're probably strangling your system. What do you consider the advantage of PVC to be?

You must be confusing thick-walled heavy Sched 40 PVC standard plumbing pipe available at big box stores etc. with the thin walled ASTM 2729 S&D available at landscape and irrigation pipe suppliers, etc. ASTM 2729 is the right PVC to use. It is much cheaper and lighter than Sched 40. Even with the prices of plastics going way up, unless you get your metal fittings at a discount locally, you will pay much less for PVC fittings which have a smoother inside wall than metal fittings available from the usual suspects like Oneida, etc.

[Paul Wunder]

+1 to what Alan said

[Curt Harms]

I found PVC easier to work with, just need woodworking tools. I'm sure that's a personal choice though.

[John Coloccia]

re: grounding and static shocks
You can just wrap a wire around the pvc duct and ground that. Everything will discharge to the wire instead of you. I would imagine you can use foil tape, too. You don't have to go nuts covering the entire pipe with tape and wire. Just vaguely wrap it around the pipe.

[Mark Burnette]

re: grounding and static shocks
You can just wrap a wire around the pvc duct and ground that. Everything will discharge to the wire instead of you. I would imagine you can use foil tape, too. You don't have to go nuts covering the entire pipe with tape and wire. Just vaguely wrap it around the pipe.
That's exactly what I did. 20 years ago the current vogue was if you just HAD to ignore the warnings & use PVC, you should run a bare ground wire INside the pipe. Tried that for a day--long planer shavings totally jammed the pipes by getting wrapped around the wire. Who dreams up this crap anyways? So then I wrapped the wire (cheap copper-plated antenna wire) around the outside. It works--as evidenced by the lack of any dust near the wire.

Before I did the ground wire on my first system which was in a basement I could feel my hair raise every time I walked underneath a pipe. After grounding that no longer happened and I no longer got static zaps.

OK so it appears WE all know better but do the magazines & DC mfrs. still scare people away from PVC?

[Mark Burnette]

ASTM 2729 is the right PVC to use. It is much cheaper and lighter than Sched 40.
Agreed. The only problem I ran into was the poor choice of fittings available for S&D pipe. These wyes for example

aren't made for S&D http://home.comcast.net/~mburnette/Shoppipes.jpg but you can get them in sch.30 & there are bushings to adapt that to S&D. Only at REAL plumbing or landscape supply places though--not at the Borg.

[Matt Meiser]

What do you consider the advantage of PVC to be?

- Ease of working with the materials
- Readily available (if you know where to look)
- Easier to seal due to fewer seams vs snap-lock pipe and the HVAC type elbows. Spiral pipe and purpose-made fittings are similar obviously.
- Very little mechanical fastening needed, easier to rework for shop layout changes
- Water-tight if buried under a slab

Biggest disadvantage is the availability of specialty fittings and the common misconception that the heavy sewer pipe is what you want.

Where I usually get shocked is my drum sander which has a plastic hood. That, and I've gotten shocked vacuuming out inside the cabinet on my old TS using a long flex hose. Come to think of it, same hose and I think it has a wire inside so maybe I should try connecting that wire to ground--might solve all my static problems.

[John Coloccia]

I'm all 6" PVC, and I would do that over sheet metal any day of the week. That said, if I had to do it again, I would spend the $$$ on laser welded with all the fancy fittings. Talk about easy to work with!

[Ole Anderson]

I have a steel duct system, but was getting a few shocks off the flex until I grounded the wire in the flex to the steel duct.

One of the advantages to steel is that it is available in one inch increments, unlike PVC which you have to choose 4", 6" or 8". Often a 3", 5" or 7" pipe is the best size to carry the volume of air you need, keep the losses to a minumum, yet maintain the optimum velocity for moving wood chips without clogging.

[Chris Padilla]

Ha! Well I'm happy to see my attempt to rile things didn't go for naught! :)

I have 6" 2729 all-around and got all my fittings from McMaster-Carr. One look at the price of Sch40 fittings will have you running to metal so stick with the DWV or S&D fittings. Also, to keep the SP low (static pressure), use 2 45* fittings to make up a 90* fitting if possible. 2 45s are much cheaper than a long-radius 90 and if you have room for a long-radius 90, then you have room for 2 45s.

A local irrigation warehouse is where I got my 10' lengths of 6" and it was around $20 for that. It has gone up a lot since but I don't recall the price now.

I used metal blast gates from Lee Valley as well as their "offset" binding clamps (worm screw clamps) to secure the 6" flex. In order to fit the plastic, flex, blast gates, and what-have-you, it was necessary to incorporate some 6" metal duct as well. 6" metal duct seems to fit pretty well inside the 6" 2729.

As to the static shock, yes, it is there for the plastic. Any time air moves through/around plastic or an insulator, it can sweep charged particles on areas of the plastic that can collect in fairly large quantities. One thing I've found is that NEW, CLEAN plastic tends to collect more charge than plastic that has had time for dust settle on it. Now that my plastic is basically dirty, it no longer shocks me. In fact, the collection of charges on the plastic serves to attract dust which then prevents more charge from accumulating. Finally, one cannot "ground" an insulator. You aren't really grounding the insulator so much as you are providing a path to ground for random accumulations of charge on the SURFACE of the insulator. The flow of charge (known as current) specifically relates to the movement of free electrons in a material. Insulators have very very little free electrons to move within their structure/chemical make-up. That is why they are insulators and not conductors.

[glenn bradley]

Shop dust sytems: Is there STILL debate over steel vs. plastic?

There is now :D. Once upon a time PVC was a lot less cost than metal and so was a logical choice. The 10' sticks of 6" ASTM-2729 I bought were just over $10 each. Last time I priced them they were about $24 each and the local warehouse had closed so, add a 100 mile round trip to the mix. PVC is light, easy to change around and airtight with just a friction fit so I am pretty happy with it. I'd be happier if it were still dirt cheap.

[David Kumm]

I'm with Ole and of the opinion that the size limitations of plastic are the bigger deal. 6" has it's place, 4" seldom does. For most collectors 3 hp and 14" and above the 6" doesn't fully utilize the available cfm and restricts the impeller and as Ole proved a little 7" if done correctly can juice a 2 hp unit. I've always found the odd sizes are more useful 5", 7", and in my case 9". For a 5 hp collector 8" or 7" are far more useful. Dave