wye, oh wye, oh wye is there dust unto dust

[Bruce Seidner]

I am hopeful of understanding the dynamics of fabricating a hood or vent in or around my work benches. I am not worried about my own dust so much. Scattered in a meaningful mountain outcrop or interned in our church columbarium, it will make little difference to me in the long run.

It is the dust generated at the level of the bench and the general atmospherics of a closed shop that has me concerned.

I had a friend who did a high dollar "geurmet" kitchen that showed off an indoor charcoal grill with a down draft vent. You could see the smoke coming off those steaks being drawn up and down magically into that vent like a recalcitrant Genie being pulled back into it's urn.

I have been trying to think about the dynamics of air in terms of the dynamics of water. It is easier to relate to water.

If I had a 6' pvc pipe that was 2" diameter and cut a slit .25" wide, the whole length, I can easily visualize the results of capping one end and pumping water into it. At some point, sooner than later, there would be a steady flow of water, equally distributed out of the slit in a cascade of water that did not betray some privilege end to end. It would be a solid wall of water that flowed out of the slit

As compelling an image as this is, I have no confidence that any of these expectations or dynamics would be realized if the same pvc pipe with the same or larger slit were used as input from a work bench with vacuum supplied to the tube.

I have seen pipes that taper from large end to small input that seen to address the dynamics of air in a vacuum.

So, rather than a down draft table or a direct vent positioned close to the work piece, how would one think about a strip of "vent" along the side of a work bench that pulled enough air off the surface to make a difference?

If this can't be achieved with a single pipe I had the thought of multiple 1" vents and a matrix of wyes that allowed one vent per 6" of bench top. I am a patient chap and can see fabricating a myriad of pvc wyes that allow this configuration.

I do believe that the main offender is dust at the source. But looking at these whole room air filters I see a bunch of dust being thrown around with some fractional quantity captured by the filter. As I reconfigure my work benches I wanted to install the equivalent of a down draft vent but level with the bench and back into the wall.

Ideas or questions of my sanity are equally welcome.

[Don Alexander]

huh? oh my aching head :D

[Mike Cruz]

Alan, James, Steve? ;) Your cue... The floor...is yours....

[Alan Schaffter]

Bruce just threw his line in the water- CHOMP!

The problem with small holes sucking if you will is the same problem a shopvac has as compared to a DC and the same problem you have with 4" ducting vs 6" ducting- small holes just suck when it comes to sucking- they aren't worth a darn. Too bad, if they were it would be much easier and cheaper to build commercial spray booths. You just can't get the CFM needed from small holes.

In order to get the same suction as one 6" DC port you would need more than 36 1" diam. holes -6" ID pipe has as cross section of 28.2 sq. in. while a 1" diam hole has a cross of only .785" sq in. The reason I say more than 36 (28/.785) is the resistance of each hole and all those little wyes will be horrible and so will the CFM.

Here is a good analogy, ooooh analogies again. Exhale cigarette smoke through a soda straw- easy and relatively quick to make a decent sized cloud (pi r sqrd to 4/3 pi r cubed). Now try to suck the smoke back up with the same straw (4/3 pi r cubed back into pi r squared) you can even use multiple attempts- impossible! That is what you are trying to do with small holes. There is just no substitute for large duct/ports and max CFM.

You would be better served by a window box fan perched at the end of the bench, a line of them across the back of your bench (cheap at WallyWord), or just wearing a good mask. Do you do the kind of work at your bench that generates a lot of fine dust? If it is a hand power tool, have you connected a shopvac to it?

[James Phillips]

Alan is correct. The problem with suction is it really only exist right at the slit in your arguement. Move even an inch or two away and you have virtually no air flow. You see 6" pipe so often because with the right motor it give the right balance of air volume and velocity, but it does not have the suction to get the dust into the stream. The dust movement at the tool combine with gravity (a well designed tool) will get the dust in the stream and all the collector does is move it.

Hope this helps

[Joe Leigh]

A question then, based on the logical explanations of both Alan and James.
Some here have recommended drilling a series of holes in their table saw inserts to help pick up some of the dust thrown off of the blade above the table. Is there any benefit to this? Can someone expect any reasonable improvement from increased air flow through the tablesaw dust collection fitting?

[Bruce Seidner]

It is one thing to read about this stuff and quite another to be part of a conversation. Thank you all for the conversation. I get it.

Since I have had to reconfigure my whole shop, having taken it down to the block walls in a subteranian corner of a garage/dirt crawl space I have been force to think outside the box a bit. Not that I have much of a box to think in. So this is very helpfull and ties together what I have read in the dust control book by Nagyszalanczy

[Dave Gaul]

I thought this was going to be about wye vs. delta transformers!

[Kent A Bathurst]

A question then, based on the logical explanations of both Alan and James.
Some here have recommended drilling a series of holes in their table saw inserts to help pick up some of the dust thrown off of the blade above the table. Is there any benefit to this? Can someone expect any reasonable improvement from increased air flow through the tablesaw dust collection fitting?

Joe - As I understand it, the point is this - the TS blade in a ZCI is very nearly a "point source" - only 1/8" wide. If you have a collection hole very close to the point source, you have a different critter than a manifold/plenum stretched across the workbench. The blade is throwing dust away from the table surface, and the holes have airflow going in the opposite direction, right at the source. The example I have read about is to put holes connected to the slot in the ZCI, at the front and back ends of the slot - but there have been other schemes described here. I think you have to first get the total DC air flow under control, though - seal up all the holes, gaps under the table, etc, so that you are managing the entry points/paths.

[Rod Sheridan]

A question then, based on the logical explanations of both Alan and James.
Some here have recommended drilling a series of holes in their table saw inserts to help pick up some of the dust thrown off of the blade above the table. Is there any benefit to this? Can someone expect any reasonable improvement from increased air flow through the tablesaw dust collection fitting?

As the workpiece would be covering the holes, they would be completely ineffective. In addition the sawdust is being ejected from the top of the blade, and is moving at saw tip velocity. You need lots of airflow and a barrier such as an overhead guard to capture that.

To collect the above the table dust, you need an overhead guard with dust collection. I had an Excalibur guard, it worked great, although now you need 400 or 500CFM in the cabinet and 200 to 300CFM in the overhead guard.

That's twice as much airflow as the far too low 400CFM figure you often see quoted.

Regards, Rod.

[Callan Campbell]

So Bruce, what is the main type of work that you DO at your workbench? Sanding, hand work like drilling, chopping or sawing? If it's a fair amount of sanding, you're better off getting a sander that works well with a vacuum unit and capture the dust while it's being created, at the source. Not all sanders enable good dust collection, even with a vacuum attached, so research what's out there and decide on a budget if this is your main concern of dust created in your shop. I have a basement shop, with just 2 VERY small windows that are centered in glass block replacements, which were installed to take the place of the much larger original wood windows that probably used to be there. So, I don't have the luxury of moving air out of the shop in a great amount since my "exits" are so small. I feel your pain, we all do, and caring about DC and its set-up now, while you're sounding like you're in the planning stages is a great thing to do for yourself and others around the shop.:)
If it's hand work that creates chips and curls and sawdust, with little to no sanding involved, you'll attack the problem in a different way. Let us know so we can still help, and question your sanity.;)

[Chris Nolin]

Bruce, I enjoyed reading your post as much as I liked thinking about your engineering problem. I can add nothing more on the technical side of this discussion. The real experts have already weighed in, and well. Just wanted to compliment your prose and encourage more in that interesting style. Cheers!

[Rod Sheridan]

I thought this was going to be about wye vs. delta transformers!

Funny, I thought someone had bought a large enough saw to need wye/delta start!

regards, Rod.

[Alan Schaffter]

As the workpiece would be covering the holes, they would be completely ineffective. In addition the sawdust is being ejected from the top of the blade, and is moving at saw tip velocity. You need lots of airflow and a barrier such as an overhead guard to capture that.

To collect the above the table dust, you need an overhead guard with dust collection. I had an Excalibur guard, it worked great, although now you need 400 or 500CFM in the cabinet and 200 to 300CFM in the overhead guard.

That's twice as much airflow as the far too low 400CFM figure you often see quoted.

Regards, Rod.

Rod, you are exactly correct.

Also, unfortunately the blade tips AND the gullets can spew a lot of dust really fast. How fast? Hmmm, at the tangential velocity of a 10" blade rotating nominally with a 1:1 pulley ratio at 3450 RPM. The diameter is 10" so that makes the circumference (pi X D) equal to 31.4". So we have 3450 RPM X 31.4 IPR = 108330 IPM which, when divided by 12 equals 9027.5 FPM. That is a little more than TWICE the typical target speed you would like to see your dust moving in a DC of 4000 FPM- the chips are out-running the DC air!

Bottom line, there is VERY LITTLE chance of catching this high velocity dust from small holes near the blade (see the straw analogy in my earlier post). But, all you need to do is stop the motion of the dust long enough so a reasonable source of CFM can collect it- hence the over-blade dust pick-up shroud.

I don't like the hard-sided plexiglass "pickups" (you'll notice I am not using the term "blade guard") because they open up a large gap at the sides especially when riding up and over 2" or thicker wood. Unless you have good suction dust can be thrown out the sides too. So here is what I did. It is just a prototype. I cannibalized a broom for the bristles but have since found a source for finer, more supple ones at McMaster Carr. The bristles provide a physical barrier and stop the dust yet allow take-up air to enter the shroud. It works well though needs a few other refinements.

http://www.ncwoodworker.net/pp/data/1403/medium/PB050012.JPG

http://www.ncwoodworker.net/pp/data/1403/medium/PB050010.JPG

http://www.ncwoodworker.net/pp/data/1403/medium/PB050013.JPG

[Joe Leigh]

Bottom line, there is VERY LITTLE chance of catching this high velocity dust from small holes near the blade (see the straw analogy in my earlier post). But, all you need to do is stop the motion of the dust long enough so a reasonable source of CFM can collect it- hence the over-blade dust pick-up shroud


I thought perhaps the plexiglass guard could stop and trap most of the particles so they can be picked up by the holes in the throat plate.

[Alan Schaffter]

Due to low CFM, once further than 1/2" or so from the holes the dust won't be captured. The dust may not spray out at you, but you will still be left with a pile of it on top of the board and around the guard. And remember, and VERY IMPORTANTLY, most of the time boards being cut will cover most of the holes!

[Bruce Seidner]

Let us know so we can still help, and question your sanity.;)

I have been assembling what I will need to do the heavy lifting in the shop and have a cyclone, 5hp 14" blower, and a local source for 6" S&D PVC.

I noticed the apparent popularity of shop air filters but it is counter intuitive to hang them from the ceiling where they look like they would "stir the pot" in a shop as much as they would filter the air.

Remembering the Jen-Air down draft cook top that did such a great job with the smoke off an indoor grill it seemed to make sense to have an overall "ambient" shop filter closer to the source at the "cook top" bench of a wood shop.

So, to be more clear, I am not thinking about primary dust collection, but of secondary collection and filtering. I have low ceilings in addition to my native ambivalence about these ceiling hung shop filters.

Clearly I have learned that small ports or a long slit of a port would be useless for even auxiliary dust collection at a source like a tool or even hand sanding. This is the job of a primary collection port.

But I am still thinking it makes sense to have the secondary ambient filter inspiration close to the bench and distributed, filtered, and expired in a way that was difused and would not be a hanging box from the ceiling.

[Alan Schaffter]

I have one but rarely use it.

There are some facts about air cleaners most people don't know, didn't realize, or just ignore, so read on. First, if they didn't aleady know, everyone here now knows the goal is to collect as much dust as you can, especially the fine and dangerous .5 - 10 micron dust, at the source , hence good CFM and good hoods, shrouds, ports, etc. at each machine.

All but the best air cleaners, may in fact be WORSE than having no air cleaner at all. What!!! What you talkin' bout Willis? You got some splaining to do, Lucy!

Stay with me here. Since most dust is generated at or near waist height. And unless your shop air is circulated by some mechanical means (fans, etc.), the dust will generally stay at waist level and below as it settles. So what does an air cleaner do? It circulates air. Hmmm, ok, but isn't that ok since it filters out the baddies? Only if ALL dust laden air is sucked through its filter, and then only if the filter is good down to .5 microns!!!

The claims by air filter manufacturers that they filter the air in a certain sized shop X times every hour are misleading. First, it is probably an inflated figure like many in this business, but if it is valid, it is only so when the filters are new/clean. But the biggie here is, that figure doesn't mean every liter of air passes through the filter X times in an hour. It only means that "a volume of air equivalent to the air volume of your shop" passes through the cleaner X times every hour. It could be that some liters of shop air pass through 30X times while some, not at all!!! Just look how close the intake is to the exhaust of a typical air cleaner- less than two feet away. So, as a reasonable person, do you think dust laden air billowing around your disk sander way over yonder will be sucked up as often as the air that just left the cleaner, IF AT ALL?

As to filters- typical furnace filters are barely good enough to stop small birds. Seriously, unless you get the EXPENSIVE HEPA filters and use a multi-stage filter stack, you are just circulating and stirring up dusty air. AND since the fine dust can stay airborne by itself for 30 min. or longer, you are now helping it to stay airborne much, much longer, you are giving yourself many more opportunities to breathe it! The ugly truth, air cleaner manufacturers don't tell you.

Frankly I use mine help keep the shop clean when I'm not there and don't even consider that it will protect my health. I have my air cleaner on a timer. I turn it on when I leave my shop and it turns itself off 1 - 2 hours later.


That's my story and I'm sticking to it!

[Bruce Seidner]

Alan has just fully explained the spirit of my concerns and the basis of my thinking about a long slit tube that wraps the wall edge of my corner bench rather than a ceiling box that I would constantly bang my head on and which would do marginal good for my lungs.

I was thinking that this slit pipe would be powered by it's own fan or blower into a 1micron filter bag. I really liked the folks at American Fabric Filter Co. who were very kind and flexible and sold me 90sq feet of fabric for the filter bag I am constructing for my cyclone/blower. I plan to tap them again for this room filter. My thinking is that I will get a wide but shallow input of air that will be pushed through another long filter tube in my crawl space. This air will find its way back to the main shop area at its own leisurely pace. I am trying to play a bad hand well by exploiting this adjoining crawl space to do things that would be unreasonable with a conventional floor plan.

I just picked up this SKY EAGLE TOOL KING USA 3/4 HP / RPM 3405 / 3105 / 2600 / AMP 7 / 6 / 5.5 / 3 SPEEDS blower for $30 and it is wild. It is made to power those huge fabric inflatable sky dancer thingie's and I reckon if it can keep Santa afloat it can be put to better use filling a filter bag with shop air. It seemed like a good idea at the time. I'll see if it actually works. But it was cheaper than the squirrel cage fans I was finding and it is a veritable hurricane when you turn it on. I think this is what we would call off label use in medicine. But I want to avoid the problems that I saw but could little explain in conventional room air filters.