Dust collection flex tubing, what's good?

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Ummm...yes, you can ground an insulator. Many people working in dry climates have been zap'ed from the generated static, and they've used wire or other methods to ground it out. Try using a comb or glass rod to generate static, then discharge one end.. You'll find there's little if any static charge left on the rest of it. Static charges do not propagate with the same mechanism as an electric current in a conductor. For static buildup on pvc you can: 1- Move it out of reach. 2- Wrap a wire (very small; current is low) around and ground it. 3- Spray lightly with conductive paint and ground one end. GerryG

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Well, some believe that the PVC can accumulate static electricity, then discharge (by means of a little electrical arc) and ignite the dust, thereby causing your own rendition of a grain silo explosion.
These are probably the same people who turn off their cell phones when the fill up their cars with gas to prevent the deaded "cell-phone gas station explosion"; because... well... gosh... even though we've never tracked down a documented/verified case of it happening, it *must* happen a lot because everyone talks about it, right?
Ugh. That's it! I'm submitting this one to MythBusters (http://dsc.discovery.com/fansites/mythbusters/mythbusters.html ).
To answer your question, the May 2003 issue of American Woodworker (page 54) has a section on setting up DC in your shop. The guy suggests using steel instead of PVC for a variety of reasons. *One* of the reasons is static buildup... but even then, the guy doesn't say that there will be an explosion. He merely talks about the zap you can get *yourself*. The other reasons are that steel comes in larger diameters than the 4" where PVC generally stops at.
Strangely, he didn't mention how great steel looks when you accidentally bump a tool into and dent it... or how yummy it feels when you slice your hand on the sharp steel ends... or how the sheetmetal screws protruding into the tube help the airflow!
He does point out though that, at large diameters, the price difference between PVC and steel starts to become a non-factor in the decision process.
Issue 13 of ShopNotes has an article about building your own cyclone and collector. The seem to use all steel.
I recall reading some article about a week ago where the author went into 4 more-likely fire hazards in your DC system than static electricity. One of them was hitting a nail (or other metal) which can cause red-hot sparks to go wafting through your DC system to the collection bag. There were a couple of others. If I track down that article, I'll come back and post it here.
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On 12 Jul 2004 21:50:11 -0700, snipped-for-privacy@emenaker.com (Joe Emenaker) wrote:
snip post about how pvc isn't a static explosion hazard, but metal duct is better anyway.

wouldn't metal duct (or more likely the sheet metal screws) present a lot more opportunities for bits of hard stuff like chips of metal or pebbles from the shop floor to strike sparks? of course the big one is still gonna be the impeller blades themselves....
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After the zinc is gone, perhaps.
One more reason for dropping the shavings before stuffing dust through the impeller.
Question, isn't duct tape good enough, that folks use sheet metal screws?
(Joe Emenaker) wrote:>

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FWIW, I highly recommend metal tape versus duct tape - at least from the standpoints of being a much better seal and the tape not delaminating from the adhesive layer after some time. The metal tape is more expensive. But a regular HVAC duct with metal tape at the seams and the joints, if you want, is darn "airtight" in this context, IMO. The metal tape won't handle bumps and such as well, so in some places you could use duct tape over the metal tape. -- Igor
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I... (Joe Emenaker) wrote:

I found it!
Fine Woodworking's "Tools & Shops" issue Winter 2001/2002.
On page 48, Rod Cole (a MIT professor who's office is next an MIT prof who happens to be an expert in the physics of lightning), wrote an article called "PVC Pipe Dangers Debunked". He makes reference to an even more exhaustive report on the web. I searched Google for "Rod Cole PVC" and found this:
http://mywebpages.comcast.net/rodec/woodworking/articles/DC_myths.html
Enjoy!
- Joe
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On the show " Ultimate Workshop" on the DIY network they said that the flex hose slows down the air current thus robbing power from your DC. Puff

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Yeah. I'm expecting a Delta 1.5hp DC for my birthday next month, and so I'm doing a lot of reading on how to "duct-up" the shop. Almost all of the articles seem to be in agreement on the following principles:
o 4" is good. 5" is gooder. 6" is gooderer. o Try to avoid any hose/tubing that's not smooth inside. o Try to avoid tight turns (ie, use 45-degree bends instead of 90-degree). o For the same reason to avoid tight turns, avoid right-angle "T" joints and opt for the "fork-in-the-road" style "Y"s.
I'm taking so much of this as religion that my current dillema is this. I'm also planning on building the home-made cyclone in ShopNotes, but its outlet is at the top and the DC that it feeds into is going to have its inlet either waist-high or at the floor. So, I'm debating either raising the DC onto a platform or modifying the cyclone design so that I can get a more-or-less straight shot across from the cyclone exhaust to the DC intake.
- Joe
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On 15 Jul 2004 15:19:48 -0700, snipped-for-privacy@emenaker.com (Joe Emenaker) wrote:

This comment is not to you so much as to a number of people here who say that bigger is better with ducts. I can see that as a general proposition, but at some point, relative to the size of the fan, won't velocity suffer in a significant way? And, doesn't velocity have a role to play in an effective DC system? After all, we are not just talking about air -- also talking about moving solid waste. Or, as is sometimes the case, am I missing something here? -- Igor
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igor said:

You are correct. When velocity drops too low, you cannot maintain the material in suspension - it just builds up in the pipes instead of flowing to the DC/pre-collector.
Greg G.
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you're not missing something. the bigger the duct the bigger the collector you need to have sucking on it. if the duct is oversize/collector is undersize sawdust will settle out in the duct.
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You are actually right on target. Most of the various schemes I've found for plumbing a single user home shop seem to optimize around a 1 1/2 to 2 HP collector and five inch pipe. Four inch pipe is too restrictive, six inch pipe slows the velocity too much. So I tend to get frustrated with all the woodworking suppliers that stock only four inch pipe, hose and fittings.
So until I can find a big enough crowbar for my wallet, I am still dragging around the forty feet of four inch flex that came with my Penn State collector.
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snipped-for-privacy@worldnet.att.net says...

But if your running a one man shop, and running only one machine at a time, does it really matter if the main runs are 4" or 5"? I've got a delta 1.5 HP DC in my garage shop. I use 4" PVC sewer and drain pipe for the main runs and flex pipe to the machines. The delta has 2 4" mains coming off it so I ran 2 runs on each side of the shop. I don't have any problems with airflow even to the farthest machine, which is a planer. The PVC was cheap and easy to work. I also didn't bother to ground it although is does develop some static when I run the planer or jointer. But I look at it as a dust filter. The suspended dust in the shop sticks to the pipe. I just vacuum it once in a while.
Just my 2cents.
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I was thinking the same thing myself. In one of the downloadable reviews of dust collectors that are available at WoodStore, they mention that you need a certain number of feet-per-second of air movement in order to keep the sawdust suspended. They use that as justification for why you need "xyz" amount of cubic feet per minute from your DC.
The first question that ran through my mind was "Why don't you just decrease the diameter of your hose?". If you just went from 4" to 3", the velocity of the air inside the hose (provided that your CFM didn't suffer too much because of it) would go up by 60%!!!
Of course, there's also the other extreme. You don't want to hook up half-inch pipe as your ducting, either, because the CFM will suffer so much at that point that, even the small diameter probably won't help the linear velocity of the air (and you'd have the new problem of not drawing enough CFM from the *tool* itself ...).
Where the magic cross-over point is depends upon the CFM of your DC, how long and curvy your ducting is, and how resistant to airflow the inside surface of your ducting is.
- Joe
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Joe Emenaker wrote:

But you'd also get more clogs. 3" is getting down to the shop-vac hose territory.

--
--John
Reply to jclarke at ae tee tee global dot net
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Not an engineer, but imagine the optimum transport pipe is probably ~5". Force/unit area calcs show 6" less than half the four.
wrote:

proposition,
suffer
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wrote:

Perhaps someone can point out the error of my thinking on this subject...
The system can only flow as much as the smallest port in the factory design. Take my Jet 1.5hp for example, what I'm getting at is that the port and hose from the blower housing to the bag hanging ring is, I believe, 5" diameter. To my thinking whatever size of the system outside of the factory setup is limited by this 5" - in other words, one can't fully draw 6" of main trunk air before the blower through a 5" hose after the blower - therefore the appropriate size of the main trunk should be no larger than 5" - or whatever the size of the smallest port in the manufactured assembly.
Wadya think?
--
Owen Lowe and his Fly-by-Night Copper Company

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Has to do with velocity. I'm sure others can explain it better. Let's say your blow has a true capacity of 1000 cfm. It will take in and blow out 1000 cubic feet every minute. If there is no duct at the entrance it will suck air from any place in the room it can. As you get closer to the blower you will feel the air moving. The more you restrict the opening, you will feel the air moving faster. The blower sucks in the air and puts it into a smaller outlet space and thus adds more force to the air and it moves faster on the way out. Ducting allows you to concentrate and "aim" the point of suction.
The blower can move 1000 cfm, but your compressor can make only 4 or 6 cfm, but when you glow that amount of air through a small nozzle, it feels like a lot more pressure than the outside of a fan or blower moving much more air. If the blower did not have enough velocity, the dust would just fall on the other side and not get moved into the bag. A window fan of the same capacity with not ducting will move little dust by comparison because it h as the cfm capacity, but at a much lower velocity. Ed
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Italian fellow name of Bernoulli, I believe, has some good words to say on the subject.
Consider the original force per unit area I mentioned. That's where the term PSI comes in. You can haul more air through a larger pipe, but the pressure drops, because you're not capable of real compression through the open sides of the impeller. This means that what's being carried along with the air will also drop. Reverse is also pretty true. Take your 4" hose, as I often do, and use a standard shop-vac adapter to 2", and notice you can pick up pencils, chunks of scrap, and even the bolt you dropped, and were looking for. Don't be frustrated and think you'll have to rummage through the cyclone, those things are just upstream of the adapter, if they made it that far, where there is no longer enough force/unit to carry them into the bin. I rely on this when looking for dropped objects in my shop.
As mentioned, the "standard" unit now moves 1200/CFM at (some PSI) or in reality, at some vacuum, measured in feet of water, inches/millimeters of mercury or furlongs per fortnight. Now since the old 650 CFM @ 8 types were the standard which spawned the 4" hose, I'm speculating that a 5" hose may be best for the 1200, because the impellers are still pretty leaky, if you read the mfrs specs. A 6" hose, as mentioned, would be 2 1/3 or so times the area of a 4, negating the additional chip-carrying power.
Oh yes, don't ask about 2" hose and 2" sanding discs for the lathe. Makes me veeery angry.

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This is, well, to be blunt, kind of gobbletygook. Well, the conclusions are more or less valid (big pipe => high flow, low speed), but the physical explanation is not correct.
In short, larger pipes can pass more air because the wall friction per unit length of pipe is less (because the airspeed is lower). Less friction means the pressure losses in the pipe are less and the impeller has less head to work against. Since it's working against less head, it can pull more air.
The lower airspeed leads to less friction (on the dust particles) and less turbulence, which allows dust to settle out.
(Of course, there are a few caveats involved above, but for practical purposes, this is the basic principle).
For those who care, there are many references to explain fluid flow in pipes and DC's; I think the FAQ has some decent references. I wrote a primer once, and if I ever get some web space again, I'll gladly post it.
By the way, Bernoulli wasn't Italian. He was Swiss. And Bernoulli's principles aren't really valid in this context (duct flow) because the viscous forces are too large.
Greg
George wrote:

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