DC ducts - 4" or 6"

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Greg G. wrote:

Testy over an urban myth? Me? I only like busting the balls of the myth tellers.
UA100
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Hi all,
I'm new to the group and don't want to get roasted here, but I have a tale to relate. Many moons ago I was cleaning out my shop after a long winter of woodworking projects, I use my compressor to blow the dust that had accumulated in the shop into the air then vacuumed up the dust that settled on the floor. I repeated this a few times until my shop was as dust free as the day I built it. It was not until later in the day that I learned the power of dust explosions. I had taken all the shop scraps and what not out to the burning barrel on the back of the property and was finishing up with yard waste when I remembered that the Shopvac needed to be emptied and the burning barrel seemed like a good repository for the contents. Now I will give you that a 45 gallon drum filled about 3/4 full of embers has slightly more energy than a static electricity spark, but the resulting 40 foot fire ball was enough to instill in me a healthy respect for the explosive quality of dust. I will for forever remember the wall of heat that over took me while I was admiring the way the dust had billowed up in the rising gasses of the burning barrel. It was over in milliseconds, left no permanent scars, but has left me with the attitude that I will not take any chances with dust explosions.
Just for fun I Googled dust explosions, perused the results to see if my mileage was not too different to others ( didn't want my first post to be attributed to a raving loony) and I happened upon an image that is a dead ringer for what is seared into my brain. (pun intended)
http://www.chemeng.ed.ac.uk/~emju49/SP2001/webpage/boom.jpg
I don't know how much of this is relevant to dust collectors, don't really care, but for my money if there is a chance of setting of something like the explosion I witnessed and it only takes a couple of dollars and a couple of hours then I'm in.
Howard H.

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duration
I want someone to tell me how they think that wrapping copper wire around the *outside* of an insulator is going to prevent sparks from occuring *inside* said insulated space. Since static charge builds up as a surface charge, I also want to be shown how having a discharge path on the inside of the collector will actually make the system safer regarding static discharge from the surface charge. One could postulate that having the wire on the inside is actually worse, because a potential from one side of the plastic surface can build up such that a discharge to the wire residing at some distance from the surface results. This can become even more of an issue if the wire becomes airborn, creating a gap for the discharge to take place.

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Mark & Juanita said:

To begin with, the wire goes on the inside, and is connected to all equipment and earth ground. And I believe the intention is to reduce potentials across varying sections of pipe and the metal equipment. As anyone familiar with science fair projects can attest, easily generated static charges can be on the order of 20,000 - 80,000 volts.
Again, I didn't design it, recommend it, or sell it. I am only reporting what is required by our commercial fire code.
Greg G.
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Greg G. wrote:

Should the ground go though a resistor to limit current?
-- Mark
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Mark Jerde said:

No, there is no current to speak of - it never has the opportunity to build up any potential as it is constantly leaked away to earth ground. It probably wouldn't hurt anything either, just more crap to possibly fail.
Greg G.
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Greg G. wrote:

Not necessarily doubting you, but can you point to more proof? Whenever I put a strap on my wrist to solder electronic components the ground had a large resistor in it. I confess to the (possibly irrational) fear of having a basically unlimited Columb/Sec path to ground in my shop.
-- Mark
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Mark Jerde said:

I have done electronic work for 35 years. How do you like soldering in those 82 pin SMDs? <g>
I believe the resistor in the strap (or the impregnated, high resistance strap itself) is to protect you in case you contact live wiring, not to protect against unlimited amounts of static charge. It also lessens the discomfort of discharging any static buildup in your body from shuffling across the carpet. I don't use them myself, as I simply discharge myself to the equipment and ground before working and don't squirm about while I work. Never had a failure. I have a friend, however, that we like to call Mr. Lightning. He can *look* at a PC board and it zaps. <g> I keep telling him it's his cheap shoes, but I don't think he believes me.
As to the unlimited path to ground, all of your equipment is grounded, do you fear it as well?
I can't give you any PHD references on this one, sorry. And I'm to tired to Search one out.
Greg G.
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<snip>

IIRC from my days with NCR, that large resistor was to protect you from the rare possibility you *might* get hold of some 120VAC. Theoretically you could grab a live lead with the other hand and not even feel it with the large resistor in the static ground lead.(I got tired of the dam wrist strap and soldered the snap to my watch band)
Nahmie
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On Mon, 22 Dec 2003 23:40:41 -0500, Greg G. wrote:

from http://mywebpages.comcast.net/rodec/woodworking/articles/DC_myths.html#inside
The inside wire or grounded screws: Many people use a bare grounded wire inside the ducts. This may provide some amount of leakage current, but the primary effect is to provide a shorter hop to ground than if no wire is added. Since the wire has a very small radius, the field due the charge drawn into it by the static charge is large, making discharge occur at smaller charge densities than if the wire was not there. The discharge will be a brush type discharge which will not ignite the dust. This limits the amount of charge that can build up, which in turn limits the strength of the field outside the pipe where your unsuspecting finger is. Basically, this reduces the chance that your finger will become the shortest path to ground! By limiting the charge build up, this also reduces the chance of a propagating brush discharge. Grounded screws poking through the duct wall provide much the same protection. For a single wire in a 4 inch duct, the maximum discharge distance is 4 inches. The maximum discharge distance for screws placed 4 inches apart along the pipe is 4 inches across the pipe and 2 inches sideways, for a total distance of just under 4.5 inches, so the protection is about the same, and may even be better as screws have sharp points that increase the strength of the electric field leading to discharges at smaller charge densities. Because this type of grounding does not have the potential to cause a propagating brush discharge, I think this is safer than the external ground wire. Using very short screws will not cause the jamming up of shavings that often occurs with the internal ground wire.
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snipped-for-privacy@cox.net said:

Does this mean I get a gold star? <g>
Greg G.
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On Tue, 23 Dec 2003 01:43:13 -0500, Greg G. wrote:

no star. to paraphrase the quoted link... attempting to ground plastic dust collection pipe is a waste of time. 'cause, it ain't gonna 'esplode no how no way. wrapping wire around the outside actually increases the discharge energy inside the pipe or some such, but it's still several orders of magnitude too little energy to 'esplode. putting wires inside the pipe causes the sawdust and shavings to hang up and clogs the system.
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snipped-for-privacy@cox.net said:

Your pretty broad interpretation, perhaps. My technical information was pretty much in line with what you posted from your "mywebpage" source - I didn't bother to read the link. And who made this guy the God of DC's, anyway?

It didn't say that. It said the probability was minute.

I never said anything about wrapping wire on the outside of pipe.

I don't see any mention of that in that paragraph... And properly done, not very likely.
And you totally dismiss the static discharge issue. I personally don't like to get zapped by PCV pipe discharge, nor do I like to see clods of sawdust hanging from the ducts due to static buildup. These factors alone are worth the $3 - to ME.
I won't bother to disseminate any more, it ain't worth the effort. Two people can read one man's opinion, and come up with two more of their own. Ain't this country great?!
Opinions are just that - we'll have to just agree to disagree.
Greg G.
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Greg G. wrote:

sigh...
UA100
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<snip>

So Greg, just how often do you hug your PVC pipe?
--
Regards,

Rick

(Remove the HIGH SPOTS for e-mail)
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Rick Chamberlain wrote:

Makes me wonder what he's like when he sees a frozen pipe.
UA100
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People spend hundreds for electrostatic filters in the shop. You get yours free with the PVC.
1) Don't urinate on electric fences when barefoot. 2) Don't lick the pump handle at 20 below. 3) Don't touch your PVC (or your spouse after sliding down the couch to her).
Results are, in all cases, predictable.

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A rational voice rang out from the void, and 6tester said:

Sort of my point. Any remotely combustible product finely atomized within an oxygen environment will burn at an accelerated rate - and when constrained by the walls of a DC system, *could* result in an explosion. Whether big or small doesn't matter - fire is fire. Apollo Astronauts aren't *supposed* to be char-broiled in their capsule on the launch pad, but it happened.

Nice fireball! Reminds me of my youth & home-made rocket experiments.

Potential for nice fireballs in your house, no less. I, too, tend to err on the side of caution. $3 and 20 minutes? Count me in as well.
Opinions are like as%holes - we've all got one. Oh well, made for a mildly entertaining hour, anyway. <g>
Greg G.
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even metal will burn if atomized sufficiently.
dave
Greg wrote:

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Bay Area Dave said:

Yep, and those magnesium plane parts, hmm...
Personally, I like to throw various metal powders into the fireplace to see the pretty colors... <g> SWMBO likes it too.
Greg G.
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