Should WW Forums have a position on dust hazards and practices?

[mic-d]

And probably I should put this here since it got lost before in some other thread.
If you put your extractor outside or vent it outside, have a look at where the fresh replacement air might be coming from. Bob's talked before about separating the dirty exhaust from entry points back into the shed like doors and windows, but there is an additional one to check. this is what I said in another post before:


... Not a problem in a tin shed or unlined structure. My workshop is a brick garage with timber beams on piers supporting a truss roof with a asbestos cement sheet ceiling. Unfortunately there is a (ventilation) gap between brick wall and timber beam which is open to the ceiling space. Keeping the garage doors and windows closed to prevent ingress of the exhaust dust created enough negative pressure to suck goodness knows what from in the ceiling space into the workshop. I first twigged something was up when a bit of insulation puffed into the room. I'm currently sealing off all off...

[Gunnaduit]

The work that led to 6" port/duct size was done long ago (the references are in the dust control handbook).

Mic, that's fine. What it all needs though, is to establish an independent source of advice that can corroborate BP. You've almost given it: "The dust control handbook". Can you remember the author or publisher or something of the book you're thinking of? Is it any of the ones listed below? Or another one?

I'd be very pleased to know.

DUST CONTROL HANDBOOK FOR MINERALS PROCESSING. A mining research contract report. FEBRUARY 1987. Martin Marietta Corporation/Bureau of Mines, U.S. Department of the Interior (See especially Chapter 3, Dust Collection System).
Dust Control Handbook for Minerals Processing

Dust Control Handbook (Pollution Technology Review nr.161) by V. Mody and R. Jakhete 1988, published by Noyes Data Corporation. [The content of this book seems to be similar or identical to the online book linked above]. Also, viewable online at
Scribd

Cecala, et. al. 2012. Dust Control Handbook for Industrial Minerals Mining and Processing. Report of Investigations 9689. National Institute for Occupational Safety and Health.
http://www.cdc.gov/niosh/mining/pubs/pdfs/2012-112.pdf

I guess you are not referring to any of these:

Is it "Woodshop dust control" by Sandor Nagyszalancsy, 2nd ed. 2002 by Taunton Press? (But which appears to give wrong advice on duct sizing, according to BP)
Amazon

"Controlling Dust In The Workshop" by Rick Peters, published by Sterling in 2000
(I haven't seen it)
Amazon

Workshop Dust Control: Install a Safe, Clean System for Your Home Woodshop (Tool Smarts), by American Woodworker Magazine, 2010, published by Fox Chapel Publishing (Paperback book, 136 p.) (I haven't seen it)
Amazon

"Designing your air handling dust system" by Air Handling Systems, Inc. (3-page pdf leaflet) (but which would be wrong duct sizing advice according to BP):
http://www.woodweb.com/knowledge_bas...sign_guide.pdf

[mic-d]

Hi Gunna, it was either of those first two references, I think references within were to Andersen and Dalla Valle...

[BobL]

*just for Bob*
If a system has stall points or eddies where the dust will fall out it is always going to fall out isn't it? If a small pile of dust settles out it will grow and be moulded by the dynamics of the flow in that section until it reaches an equilibrium with the 4000fpm boundary and then it shouldn't grow any more. The velocity in that area of constriction would increase too and help to limit the amount of buildup. If it continues to grow because flow is reduced by constriction (and if it's not because of a fundamental flaw in the duct design) then you've done something wrong with the dust extractor choice, not because the duct is too small haven't you? I have 4" to 6" to 8" duct. I reckon that around some of those transitions there are small build ups of dust, but they've never grown to block the duct.

I agree that its the interplay with between the power of the DC and size of the ducting. I also used the word "blocked" when I should have said something like "substantially blocked" or blocked sufficiently to compromise performance.

Your assumption is that flow (cfm) will scale linearly with speed (fpm) which for borderline DC/duct combinations is not the case.

If a 6" duct is used on a small e.g. 600 cfm DC then the air speed will be ~3000 fpm.
Lets say that 2000 fpm keeps the dust moving but when the system stalls all the dust will settle and not much of it will be resuspended. Lets say that eventually the duct cross section becomes equivalent to that of a 5" pipe. If flow scaled with speed the the air speed of the 5" pipe would now be 4400 fpm but it doesn't so instead of 600 cfm it will now might be 500 cfm which means an air speed of 3700 fm. This will continue until as you say some sort of equilibrium is achieved resulting in a partially blocked system. Whatever the case it won't have the same CFM as it originally had. This why 6" pipe is a waste of time on a small DC.

I have seen meter long lengths of 4" ducting 1/3rd full of dust on a 1HP system normally rated at ~450 cfm that was only pulling ~250 cfm. Since this is on average 4300 fpm inside the duct one may well ask why it did not clear. One thing I noticed was that the dust consisted of crusty layers. What I suspect contributed to this was stalling and then self binding of the dust surface due to something wet or maybe even slightly sticky like paint? This is another reason why clearing the dust from ducts is important.

The other thing to watch out for is metal dust. It gets caught up in sawdust and carried into ducting but then as the sawdust gets shaken around the metal dust gravitates to the lowest section of ducting - then even 4000 fpm will not pick it up because at the boundary the air speed will not be anywhere near that.

[mic-d]

gotcha thanks.

[BobL]

I remember BP advancing a similar argument but couldn't find it till now. I have highlighted a couple of key phrases

Does a 4" connection at the machine negate the benefit of the 6" duct going right to that machine? Yes, it kills the dust collection performance. At typical airspeeds and pressures for dust collection, air is virtually incompressible. Air can speedup some to get around a short obstruction, but just like a water valve, closing down the opening greatly restricts flow. The standard 4" connections on our larger hobbyist machines kill the CFM below what we need to collect the fine dust. We pretty much have to replace all the 4" ports on our larger machines if we are going to collect the fine dust at the source.

The other part of your question is what is the impact on airflow when using a 4" drop attached to a 6" line? My engineer friends at Dwyer Instruments that build most of the air measurement meters say roughly 10 diameters of pipe will both stabilize the airflow and set that airflow to about the same duct speed as your main. Most air engineers that are just interested in getting sawdust build systems targeted to get an airspeed of 4000 FPM in the main. That 4000 FPM when pulled through more than about 40” of 4” diameter duct will end up with a total air volume of 350 CFM. That is plenty for good chip collection at most small shop stationary machines, but far short of the 1000 CFM I recommend for good fine dust collection. The bad news is that roughly 350 CFM ends up with our main only having an total airflow of about 1782 FPM. That is way short of the minimum 2800 FPM needed to keep a horizontal main clear. The result is the main ends up building up first the larger chips then finer dust. It will continue to build up this dust until the duct is sufficiently restricted that the airflow is again fast enough to keep the remaining area clear.

So, putting that 4” diameter down drop on 6” diameter is bad news. These ducting piles are a serious fire hazard. When airflow is restored from open more area these piles go slamming down the ducting. This slamming ruins blower impellers, blower motors, knocks the ducting joints loose and destroys filters. Worse, when these piles break loose they create one of the few times that small shops end up with a potentially explosive dust to air ratio. The slightest spark say from a nail hitting a steel blower housing is enough to cause a serious problem. Plus, it is unpleasant to listen to this dust rumble around in the ducts.

We can easily do the math to understand why that 4” diameter is just too small to be used with a 6” duct unless we open up more airflow downstream. From the area formula A=r*r*Pi we know the area of our circles more formally known as the cross sectional area of a piece of duct. For 4” this is 2*2*Pi =4*Pi and for 6” ducting this is 3*3*Pi=9*Pi. The difference is the 6” duct has 125% more area and the 4” duct only has 4/9ths the area of a 6”. Not only will this cause piles in the horizontal runs, if we have any vertical runs there is a good chance that we will develop plugging because we need at least 3800 FPM going up to keep the heavier stuff moving. That 1782 FPM is less than half of the 3800 FPM minimum we need to keep the dust from plugging in the vertical runs.

I know of four solutions and there may be more, but I only recommend the first:
Use all 6” duct right to your tools. For tools that have two ports like a table saw, use a 5” duct to the larger cabinet port and a 4” to the smaller blade guard port. Cutting into my precious tools to put on big enough ports was one of the toughest things I had to do, but the result made a huge difference. Some tools have internal ducting that requires use of two 4” ports. This will work but not nearly as well as a 5” and a 4”. It is all a matter of trying to keep the areas as close to the down drop pipe as possible.
Go with overhead ducting so you don't have to pull dust up at the end of the run by the blower. This works only if the difference between main and down drop is minimal. That more than double 4” to 6” area is just not going to work.
Stay with all 6" and just use a tapered wye on the end with the extra pipe going to a dust hood to help with upper collection. People do this all the time and it sort of works. I tried it and measured the airflow. That 6” to 4” taper cost me about a third of my total airflow.
Build at the end of your main as far from the dust collection system as possible a weighted door that opens a little whenever the pressure in the line becomes too large. This will keep up the main airflow ample to ensure no plugging but you kill the total airflow needed for fine dust collection and often don’t end up even with good “chip collection”. I had to do this with my airfoil impeller as it will stall if I let the pressure get too high.

[Geekgirl]

Ok so I have read all this tread, took close to 2 hours (including reading BP's one page summary).

Right now I am outside in my carport which has two solid walls (north and south) and both East and West Fully open, so I get great air flow as evident by my van covered in dust when I use the table saw and table router.

Eventually I am going to own a house and build a monster office / workshop, It will probably end up with two extraction systems for two different tasks, one will be my electronics engineering lab where I need to remove fumes from soldering, the other will be the workshop area where it will be wood dust. As my machines are not stationary (I use triton table saw and router table, with the need to be packed away and unpacked what is the solution ?

I agree with large diameter duct, no flex, large blower to keep the air flow as high as possible. If the machinery was bolted down , having a small dust making area (eg saw table router table) with enclosed bases no problems, but say a lathe where I could have 1000mm length working and 300mm height working, what would be the best ? a small hood that I move as I work down the item being turned ?

I guess I like many others have had no idea until I read this thread, I have been in some brand new high schools, seen the impressive dust collection systems they have (a massive room ~8m high and 20m2 housed a dust collector / chip seperator this thing was huge) but the connection from the mobile pendant heads was 100mm flexable aircon metal duct (not the foil but the suff that is about 6 times the wall thickness) all the machines were connected buy the same size I think the main line was about 200 - 250mm diameter, going up to ~600mm, with what I have read here, this system is useless due to all the narrow drops between the main lines and the tools ?

TIA,

Kat.

[BobL]

Right now I am outside in my carport which has two solid walls (north and south) and both East and West Fully open, so I get great air flow as evident by my van covered in dust when I use the table saw and table router.

Don't be fooled into thinking that working outside is any sort of long term solution. I lost my sense of smell for 6 weeks working outside with MDF.

Eventually I am going to own a house and build a monster office / workshop, It will probably end up with two extraction systems for two different tasks, one will be my electronics engineering lab where I need to remove fumes from soldering, the other will be the workshop area where it will be wood dust.

This is what I have in my shed. A fume hood (~400 cfm extraction) where I can weld, open containers of solvent and acid and spray paint and not get gassed in the process. Then at the other end I have my WW gear with ducted DC (~1250 cfm).

As my machines are not stationary (I use triton table saw and router table, with the need to be packed away and unpacked what is the solution ?

One solution is to build an even bigger shed so that you can fix machinery in place. I've worked with fixed and movable machinery and while movable saves space it wastes a lot of time. Fixed also means you can use under floor ducting (I only have one of these to my TS). About the only other solution is to use drop down ducting which is a PITA as it just gets in the way. It is possible to use flex or flexi jointed ducting hanging from the ceiling but that restricts flow more than necessary. If I was building my she all over again I would install all under floor ducting about 300m m from the side of the shed with a port every 2-3 m.

I agree with large diameter duct, no flex, large blower to keep the air flow as high as possible. If the machinery was bolted down , having a small dust making area (eg saw table router table) with enclosed bases no problems, but say a lathe where I could have 1000mm length working and 300mm height working, what would be the best ? a small hood that I move as I work down the item being turned ?

Large full lathe length hoods are useful but they usually lack the airspeed required to grab enough dust a smaller hood or scoop that can be moved is better. This is what I will be doing with my lathe.

I guess I like many others have had no idea until I read this thread, I have been in some brand new high schools, seen the impressive dust collection systems they have (a massive room ~8m high and 20m2 housed a dust collector / chip seperator this thing was huge) but the connection from the mobile pendant heads was 100mm flexable aircon metal duct (not the foil but the suff that is about 6 times the wall thickness) all the machines were connected buy the same size I think the main line was about 200 - 250mm diameter, going up to ~600mm, with what I have read here, this system is useless due to all the narrow drops between the main lines and the tools ?

All that schools and industry have to meet is the industry standard of 5 mg of dust per m^3 of air for softwoods and 1 mg/m^3 for hardwoods. This can be done using the big systems they use however they totally ignore fine dust. This will change over then next few years as the OSH nazis get their teeth into more and more of this stuff.

[Chris Parks]

Dust hoods to be effective have to be close the source, in some cases so close as to be impractical. I supplied the DE for a dedicated sanding room and the hood in that had two 150mm ducts and a third ventilating the room, the size of which was kept to a minimum. The customer was very happy with the result. As an aside I think down draught tables need even more flow and could nearly be termed useless in most cases. At least with the overhead hood the dust us tending to rise towards it.

As for outdoors, it is better than working in a dust trap of a shed if they are the only two alternatives available.