PVC and Dust Collection a way to find out without blowing up your shop

[Jason Beam]

I just don't understand why you guys are so defensive about doing some real homework on the leading cause of woodshop destruction.

Now this myth is the leading cause of woodshop destruction?

I think you're using "fire" to support your argument when we're not talking about any and all fires. We're talking specifically about wood dust explosions caused by the discharge of static electricity built up on PVC dust collection duct work. Every word in that sentence is significant and to cast your insults with such a broad brush isn't going to get you any credibility.

Show me ONE. ONE documented woodworking shop explosion that was proven caused by static discharge built up on PVC dust collection duct work. Just one. You make the statements, back 'em up.

I don't disagree that fire prevention is key. But don't think for a second that you're safe just because you trailed a wire along your PVC pipe. You're safe because it's not a risk to begin with. If it were a legitimate risk, the wire wouldn't do much (if any) good. You'd have to completely coat the surface of the PVC pipe with conductive material to be sure to capture any errant static buildup. At that point, you might as well go with a conductive material to begin with, no?

To accuse any of us of advocating unsafe and harmful practices is purely a lack of comprehension. When you don't even appear to understand the concepts we're discussing, maybe you shouldn't be so quick to speak up.

We're simply saying that the risk isn't there and not to put so much stock in the popular "solution" without knowing the physics behind it.

[Dan Friedrichs]

I think everyone agrees that it's possible, it's just horribly improbable. You'd be better off worrying about a rabid skunk walking in your shop door and biting you in the leg.

One thing I particularly like about SMC is that many members here are very, very well educated people who are venerable experts in their fields. Several electrical engineers here have repeatedly explained that wrapping a wire around an insulator will remove very little of the static charge built up on it. Basic physics supports this.

[Edwood Ferrari]

I found this link a while back when I was planning my workshop as I too read all the worries of using PVC ducts. I am an electrical engineer so I understand electricity. This article put all my worries to rest

http://home.comcast.net/~rodec/woodw.../DC_myths.html

Ed

[Frank Hagan]

Bill Pentz has a link on his site to this page: http://home.comcast.net/~rodec/woodw.../DC_myths.html

Shorter Link Here

The author, Rod Cole, has a PhD in Mathamatics and a BS in Physics, and works at "MIT Lincoln Laboratory heading some research projects".

He mentions that the three areas of concern for fires are the dust bin, the dust bag, the ducts ... in that order. According to Uniform Building Code, Uniform Mechanical Code, and the National Fire Protection Association which (according to OSHA, 1997) all require that dust bins, dust bags, and cyclones be located outdoors. Nearly every homeowner shop in the country is in violation of these standards. The qualifier is that the standards seem to apply to systems delivering 1500 CFM or greater (in actual use, not on the marketing literature).

The article has references at the end, and seems well researched and explained to me. He does include non-scientific accounts of fire captains that cannot find any reference to dust duct fires in residential shops, but also searched for any reference himself.

I searched Google, and found plenty of references to shop fires, but none saying the ducting was involved. A couple said the fires were started by electrical shorts. If you really want to be safe, don't use electricity.

In the absence of scientific evidence, or even one instance of the problem happening, I would be comfortable with PVC ducting.

[Chris Padilla]

On the static spark not being able to ignite an explosion question, it is temperature not amperage that is the problem. Because you can't "gound" an insulator (whatever that means), the voltage necessary for the break down in the air dielectric is higher. Hence, the ignition temperature is far exceeded in a static charge in dry air when initiated from an "ungrounded" insulator. What that means is that it is the ignition temperature that matters not how many electrons move. The drier the dielectric air, the higher the temperature.

Okay, bear with me here: Amperage is the flow of charge...which is in coulombs per second. When charge flows, it implies a potential difference from one point to another point...known as voltage. During a static discharge, you have a potential difference because there is a flow of charge (kind of a chicken or the egg thing but I digress...) and thus power can be calculated: P=iV. Power is in watts which is in Joules per second. Joules are a unit of energy. So, we have energy dissipation (or absorption) occuring over a period of time. This energy dissipation is what heats the air along the path of discharge. This is basically what lightning does only that has a lot more energy. It is the HEAT generated by this energy dissipation that "stings" one's finger when one is shocked. Yes, you are actually being slightly burned when you are shocked...or burned a lot when struck by lightning.

As to the breakdown of an air dielectric, it is only dependent upon what is in the air and the electric field across it. In other words, there needs to be another potential difference to allow current flow. That is fine...we may well have it with the plastic having a localized pool of charge built up from wood dust movin past it. But here is the trick, in order to have a discharge, there must be a potential difference...where is it? When your body rubs across a wool carpet, you can develop charge. You touch a doorknob...or rather...you come extremely close to touching the doorknob and you set of a discharge. Why? The doorknob is likely at a ground potential...and your finger is not (it could be at a higher or lower potential...only matters that it is different). I question where the potential difference is in a plastic pipe full of wood dust. WE only notice it when WE touch the pipe because WE are at a different potential. And even then, the discharge is OUTSIDE the pipe and not in it. What INSIDE a pipe full of moving wood dust causes a discharge? Wouldn't you think the wood dust and pipe to all be at the same potential?

I just don't understand why you guys are so defensive about doing some real homework on the leading cause of woodshop destruction. At this point I am wondering to what degree some of you are trying to sell unlisted products to home owners. Calling a markert "home owner" actually increases your responsibility both legally and finacially. You may want to hit LegalZoom or some other cheap source and get some idea how deep a hole you are in.

I apologize if we sound defensive but every once in a while someone like yourself comes along and starts informing us how we are going to more or less blow our shops up along with outselves. The plain truth is that there is no such evidence and many here have already pointed it out to you.

No one here is selling anything. No one here is making any money off of suggesting PVC as an alternative to metal for dust collection ducting. What you need to understand here is that most of us are HOBBYIST woodworkers and we simply don't generate enough wood dust for it to cause any sort of explosion from some mythical static discharge that has been shown time and again to not even be remotely powerful enough to ignite...assuming it goes off in the first place.

If any of these discussions on woodshop fire confuse you or you can't tell who has got it right, CALL YOUR INSURANCE AGENT. Please get some help. People really will help you out and it is cheap.

At this point, Mark, I question your motives and what it is you are trying to do. What do you do for a living? I am an electrical engineer (master's degree if it matters to you) and a hobbyist woodworker with a shop built cyclone dust collector using 6" PVC strung all over my garage working splendidly for about 4 years now. At the beginning, I got shocked from my PVC ducting...after 6 months of using the system, no more shocks.

[Steve knight]

my clearview has not blown up yet and I can make a lot of dust surfacing mdf on my cnc router. so there

[Rob Russell]

Folks,

Some of the rhetoric in this thread has crossed the line beyond what is acceptable at SMC. Personal and/or derogatory comments will not be tolerated.

Obviously, "DC explosions" is a topic that can degenerate into a "flame fest". Please keep your posts FACTUAL, not based on opinion.

Thanks,

Rob

[Chris Padilla]

I didn't have my table of Dielectric Strength in front of me last night when I wrote my mini novel above.

The dielectric strength of air is about 3 x 10^6 V/m (Volts per meter) or if we want to adjust the field to something more in line with the size of a 6" plastic pipe: 7.6 x 10^4 V/in (Volts per inch) or 76,000 V/in.

Now fill that air with something like wood dust and what happens. I would surmise that the dielectric breakdown of wood dust mixed with air (and at what ratio...no clue) would be higher than pure air since air is your basic dielectric (relative permittivity or dielectric constant of 1).

Wood is listed on my chart and while a dielectric constant is given (range of 2.5 - 8.0), the column with dielectric strength is blank.

[Nick Abbott]

After about 25 years of testing I can confidently say no worries here.

[Chris Padilla]

That is dedicated testing, Nick! Can you ellaborate a little bit?

[Frank Hagan]

I didn't have my table of Dielectric Strength in front of me last night when I wrote my mini novel above.

The dielectric strength of air is about 3 x 10^6 V/m (Volts per meter) or if we want to adjust the field to something more in line with the size of a 6" plastic pipe: 7.6 x 10^4 V/in (Volts per inch) or 76,000 V/in.

Now fill that air with something like wood dust and what happens. I would surmise that the dielectric breakdown of wood dust mixed with air (and at what ratio...no clue) would be higher than pure air since air is your basic dielectric (relative permittivity or dielectric constant of 1).

Wood is listed on my chart and while a dielectric constant is given (range of 2.5 - 8.0), the column with dielectric strength is blank.

The link I gave above has a section on just how hard it is for researchers to get wood dust to explode in a lab setting. The author concludes it would probably never happen in a home shop setting, except perhaps in the dust bin or filter bag.

[Ken Fitzgerald]

Frank,

I'd spent a couple of hour looking for that article here. It's been posted before.

What I like about it is..it points out the significant difference between "possibility" and "probability".

Excellent study IMHO.

Thanks again for posting the link. I have it saved on my home computer now for future reference.

This subject ....urban legend...internet rumor comes up way too often.

[Frank Hagan]

Frank,

I'd spent a couple of hour looking for that article here. It's been posted before.

What I like about it is..it points out the significant difference between "possibility" and "probability".

Excellent study IMHO.

Thanks again for posting the link. I have it saved on my home computer now for future reference.

This subject ....urban legend...internet rumor comes up way too often.

I think he did a good job of analysis, and also did well with making it easy to understand. Now if we could get him to work for the IRS ...

[Chip Lindley]

Zactly! Dan! I can count the static dust explosions caused in home shops on NO fingers, assuming I cut both hands off in tablesaw accidents!! LOL

[Angie Orfanedes]

It seems to me that all we need to do is enclose our PVC ducting inside of metal duct and then ground the metal duct......