Thread: Air speeds in and around duct openings and blades (NB Invisible dust discussed)

Originally Posted by BobL

I think your idea of venting through the box is a good once as long as the seal was reasonable.

Vacuum cleaners are extremely filthy things. Their air speeds are so high they can actually mince fine dust into even finer dust and then just sitting there they represent a big reservoir of fine dust. We have done tests on vacuum cleaners and just small hour by hour atmospheric pressure changes mean they breathe fine dust into the air. I would prefer to run the vacuum cleaner box air tight and just open up another vent/port on the DC line so the DC doesn't choke. But like you say some judiciously placed flaps could be rigged on the box to open when the DC came on and fell shut when the DC went off. Oh yeah and a bung on the nozzle would also be needed.

So what I suggested about removing as much filtration from the vacuum cleaner as possible would allow more of the dust to go out of the box and be vented outside by the dusty would be a good idea as it would reduce the amount of fine dust trapped in filters etc, leaving the vacuum cleaner less dust to vent out into the shed as the atmospheric pressure changes throughout the day, and if the baffles close under gravity when the dusty is off, presumably the dust will be contained in the box until the dusty kicks in again and the baffles open and that dust goes out the side of the shed.

Of course the other advantage of air flowing through the box containing the vacuum is that it would cool the vacuum cleaner, ort at least not create a situation where it could overheat.

But seriously, how much of an impact would the small amount of dust coming out of a vacuum cleaner under those circumstances have on the air quality in the average workshop? I wold have thought that more bad things would blow in every time you opened the door. Putting a bung in the end of the nozzle sounds a bit extreme too. Do you have any data on how much benefit there is in utilising these measures?

I generally take the vacuum cleaner outside and clean all the filters etc with the compressor (i wear a dust mask while doing this of course) So just how dangerous is the vacuum cleaner sitting in the corner of the garage?

Doug

Originally Posted by doug3030

I generally take the vacuum cleaner outside and clean all the filters etc with the compressor (i wear a dust mask while doing this of course) So just how dangerous it the vacuum cleaner sitting in the corner of the garage?

If you took it outside and cleaned it every time it was used it should be ok but otherwise read on.

See this photo.

This much dust (a whole 0.27 g) represents enough dust to send the air in 4, 6 x 4 x 2.7 m sheds above the standard OHS limit. BTW the OHS limit says nothing about particle size which is somewhat odd since it is the invisibles that do the damage.
I'd guess 99% of empty vacuum cleaners used in woodwork sheds contain at least 100 times that much wood dust and the unemptied ones probably contain more than 10000 times. The changes in atmospheric air pressure alone are enough to constantly pump out the OHS limit on wood dust from these vacuum cleaners and DCs that sit inside a shed. The cheaper ones are horrible grotty pieces of gear that IMHO should be banned from use in sheds. As soon as even the cheap (even empty) ones are turned on they immediately dump at least this much wood dust into the air. This is why venting them via DC is potentially a way of making them safer.

Now I know that most woodworkers have sheds with probably 10000x more wood dust (that's 2.7kg) on their floor, shelves and all over their equipment, but that doesn't make it right or safe. It just shows how far behind the 8-ball most back yarders are. And I know that my grandmother lived to be 90 (96 actually) and she smoked for much of her adult life but that also doesn't make smoking right either.

How far above the OHS limit should back yarders consider safe? 10 x , 100 x , 1000 x ?
If it is a straight exposure thing then a back yarder that spends one day a week in the shed should think about a 5 x figure. But how many even know what the OHS figure is? 5x is still not much wood dust dust and it's much less than one thinks. I like the way Bill Pentz puts it - "its about the amount of dust remaining on a dust jacket after it has been shaken out". Yet I know turners especially that sit covered in a fog of sawdust day after day with a pissy little dust mask on - these guys are just asking for problems later in life.

Like I said, the OHS limit says nothing about particle size as the OHS people haven't even got a decent handle on this yet. IMHO the invisible dust problem for woodworkers is really only taking off as only up until recently did the average person had the funds to buy machinery and powertools (and the time to use them as well) that make a lot of invisible dust. I think this problem might even turn into an asbestosis like time bomb for us in the future.

Anyway I don't wish to start another argument - we know each others position on this matter. I want to reassure you I am not an OHS Nazi. I have worked in dangerous environments (chemicals, radioactive materials and high voltage gear) for most of my working life and fought the OHS people at work time and time again over bureaucracy and mindless rules and regs, many of which do little for real OHS, and is one of several reasons why I am retiring early.

Originally Posted by BobL

If you took it outside and cleaned it every time it was used it should be ok but otherwise read on.

See this photo.

This much dust (a whole 0.27 g) represents enough dust to send the air in 4, 6 x 4 x 2.7 m sheds above the standard OHS limit. BTW the OHS limit says nothing about particle size which is somewhat odd since it is the invisibles that do the damage.

Bob, Are you seriously suggesting that a vacuum cleaner sitting idle in the corner of the shed is going to put out the amount of dust you have pictured in the bottle cap due to daily air pressure fluctuations, particularly if you forget to put a bung in the nozzle? ok its only supposed to be a quarter of a gram and the photo shows VISIBLE dust. that weight would represent a hell of a large amount of INVISIBLE dust. Without any guidelines about particle size in the OHS regulations neither the regulations or your post really have any meaning at all, do they?

Bob, I do not wish to start another argument either. We went behind the bike shed and had our fight, you gave me a blood nose and I gave you a black eye, then came in and turned the cold hose on both of us, we shook hands and sought some common ground and have been working well on meaningful debate ever since. thats the way its always worked and as I see it the forum is better off because we both expressed ourselves. But seriously I cant see any vacuum cleaner leaking that much dust back into the system unless you stick the compressor into it inside the shed.

Doug

Originally Posted by doug3030

Bob, Are you seriously suggesting that a vacuum cleaner sitting idle in the corner of the shed is going to put out the amount of dust you have pictured in the bottle cap due to daily air pressure fluctuations, particularly if you forget to put a bung in the nozzle? ok its only supposed to be a quarter of a gram and the photo shows VISIBLE dust. that weight would represent a hell of a large amount of INVISIBLE dust.

It only needs 1/4 of that amount or 1/16 of a g to send it to the OHS limit.

I agree the vacuum cleaner would have to be pretty dirty to even do the 1/16 g. I must have got my numbers screwed up somewhere between particles per cubic metre and grams per cubic metre - I have to check some charts at work when we did those vacuum cleaner tests.

Anyway I need to get back to your 4" duct air speed mapping data. Will post that soon.

Following along from Doug's suggestion here is the 4" ducting speed map.



A couple of points of explanation are needed.

Firstly it's not really a 4" opening, it's 112 mm, as this is the size of the opening in the 150 - 100 mm reducer I had on hand and I forgot to stick in a bit of 4" PVC to act as a proper reduced. I don't think it will change the results that much.

Next I have stuck a 150 - 100 mm reducer on a 3.5 m long 150 mm diam duct. If 4" ducting was used all the way back to a DC this would reduce the air flow which in turn will reduce the air speed a little more.

If turbulence/friction was not an issue one would expect the the reduction in duct area from 6 to 4" to increase the air speeds by the inverse of the cross sectional area ratio (ie 3^2/2^2 or a factor of 2.25). However it is pretty clear from this graph that the measured air speed for all points mapped for the 4" duct are always lower than the 6".

Close to the duct the air speeds for the 4" duct are quite respectable but as soon as the distance is more than 100 mm from the duct the 4" duct only generates about half the air speeds of the 6" duct. Another way to look at it is the 6" ducting at 90º is about as effective as the 4" is at 45º.

Now consider the fact that the cross sectional area is 2.25 smaller and one can see why 4" ducting is not very effective in scavenging invisible dust.

Bottom line, if you use 4" ducting you have to get your ports really close to the dust source to grab invisibles.

The other thing I did was map the air speeds around a moving BS blade. The numbers on the figure are in m/s and are ~5 mm from the moving blade and above the table

The air speed across the table is quite symmetrical so the numbers shown for the line out across the table from the blade apply all the way around the table. This explains why the dust spreads out across the table from the blade as it does. The dilemma about where to locate a scavenging duct still applies. It's almost like one needs an air gutter all the way around the table.

Originally Posted by BobL

..... I want to reassure you I am not an OHS Nazi. I have worked in dangerous environments (chemicals, radioactive materials and high voltage gear) for most of my working life and fought the OHS people at work time and time again over bureaucracy and mindless rules and regs, many of which do little for real OHS, and is one of several reasons why I am retiring early.

Bob

I am a member of several forums, wood related, motorcycles and hifi and I truly appreciate the benefit of having smart experienced people like yourself around to guide us. If you retire we will learn more (if a couple get peeved on the way through that journey who cares).

I look forward to more posts

Best wishes

Peter

I would look at using at least a flanged duct rather than just a plain hose end and even better would be to fashion it into a bell end, much more efficient.* And for the lathe and perhaps BS explore the practicality of a push/pull system.

*Dust Control Handbook for Industrial Minerals Mining and Processing, Andrew B. Cecala, Andrew D. O’Brien, Joseph Schall, Jay F. Colinet, William R. Fox, Robert J. Franta, Jerry Joy, Wm. Randolph Reed, Patrick W. Reeser, John R. Rounds, Mark J. Schultz, p13, p18.

Originally Posted by mic-d

I would look at using at least a flanged duct rather than just a plain hose end and even better would be to fashion it into a bell end, much more efficient.* And for the lathe and perhaps BS explore the practicality of a push/pull system.

*Dust Control Handbook for Industrial Minerals Mining and Processing, Andrew B. Cecala, Andrew D. O’Brien, Joseph Schall, Jay F. Colinet, William R. Fox, Robert J. Franta, Jerry Joy, Wm. Randolph Reed, Patrick W. Reeser, John R. Rounds, Mark J. Schultz, p13, p18.

Thanks for the lead Mic - I was going to follow what I have done up with a few measurements using the standard shrouds/hoods available with DC kits but seeing as none offer a bell shape it sounds like another pot plant pot hunt is coming up. One problem with too much length of bell, duct and also a duct bend behind a lathe is that often lathes are placed up against walls however it might be possible to grab stuff from above although that could interfere with operator vision and for those that want it, reduced bigger chip collection. I suspect that using hoods will be a trade off between being able to get the work into higher air speed space, and increasing the efficiency of collection from a wider volume. Air speed profiles with distance and total volume collection will be one way to assess this.

What do you mean by a push-pull system. If you mean something like a fan blowing towards a duct, I wonder is that might stir up the dust rather than help collect it.

Originally Posted by The Hun

Bob

I am a member of several forums, wood related, motorcycles and hifi and I truly appreciate the benefit of having smart experienced people like yourself around to guide us. If you retire we will learn more (if a couple get peeved on the way through that journey who cares).

I look forward to more posts

Best wishes

Peter

Cheers Peter, but feel free to challenge anything I say or ask questions.

When I retire I plan to borrow a particle detector from work and make some measurements of dust emitted by various machines and processed that might be of value to back yarders. I'll see how I go.

Originally Posted by BobL

Thanks for the lead Mic - I was going to follow what I have done up with a few measurements using the standard shrouds/hoods available with DC kits but seeing as none offer a bell shape it sounds like another pot plant pot hunt is coming up. One problem with too much length of bell, duct and also a duct bend behind a lathe is that often lathes are placed up against walls however it might be possible to grab stuff from above although that could interfere with operator vision and for those that want it, reduced bigger chip collection. I suspect that using hoods will be a trade off between being able to get the work into higher air speed space, and increasing the efficiency of collection from a wider volume. Air speed profiles with distance and total volume collection will be one way to assess this.

What do you mean by a push-pull system. If you mean something like a fan blowing towards a duct, I wonder is that might stir up the dust rather than help collect it.

I don't think the flange needs to be anything more than 2-4" extra in diameter than the duct, ie a 2" border around the end. And the bell end I think can be quite compact, much the same size as flange but a slightly more conical shape with curved sides - have a look in the manual.

The push part of the push/pull would not be a fan but a jet of air directed through the dust production site and into the duct. It would be a tight jet as in one generated by a jet from a compressor for instance. The jet entrain more dust laden air as it passes through the dust production site and assists in capture (p13 of manual)

Originally Posted by mic-d

I don't think the flange needs to be anything more than 2-4" extra in diameter than the duct, ie a 2" border around the end. And the bell end I think can be quite compact, much the same size as flange but a slightly more conical shape with curved sides - have a look in the manual.

I had a look - it looks like a very useful reference. The curved flange looks easy enough to knock up.

The push part of the push/pull would not be a fan but a jet of air directed through the dust production site and into the duct. It would be a tight jet as in one generated by a jet from a compressor for instance. The jet entrain more dust laden air as it passes through the dust production site and assists in capture (p13 of manual)

It looks like the push pull system is designed to collect dust from a more or less unobstructed volume. This would apply to say the upper half of a shed filled with dust. However, I would still be concerned about using it in an obstructed space. If an air jet is aimed at object producing dust, the jet air speed at object must be less than the suck at the object otherwise the air jet could deflect off the object and scatter the dust. This would be feasible to set up for a static situation but somehow I can't see an operator checking air speeds every time they change workpiece etc. On something like a lathe a air jet mechanism might also get in the way of an operator and I'm not sure if it would be worth the noise of the jet and cost of having a compressor running while turning.

Where I think the push pull method with an air jet is worth exploring is in venting a shed. This is essentially what a fan at opposite side of a shed to a DC inlet does but a jet would be much more efficient as you say it entrains dust within the jet. I can envisage this being done on an occasional basis where large volumes of dust are generated eg after ripping a large amount of dry hardwood. ie Open up a DC duct on one wall and an air jet on the opposite wall and collect dust for 5 minutes or so.

Another place for air jets is inside horizontal ducting just before the rise of vertical ducting as this is where duct can accumulated if a DC stalls. A burst of air at this point can then fluff the settled dust up enough for the DC to carry the dust away.

I will add that to the experiment list for when I retire.

Interesting concept but I'm with you Bob, the practical applications are likely to be limited. While it might work in theory I tend to think it could do more harm than good if an object deflects it. But worth playing with to satisfy curiosity.

Walking through Bunnings this evening and I ran across some 4" floor ducting flanges in the DVW fittings section. The fittings are not unlike the bell mouth inlets that Mic refers to in his post earlier in this thread.

This is what they are supposed to look like in X-section


So I set up a test to compare air flow with and without the flange.
Here you can see what it looks like


The 100 mm ducting fits inside the flange so it leaves a ridge so I stuck the 100 mm ducting into a lathe and turned an (not very symmetrical) internal taper on the leading edge.

I measured the air speed for
1) the smooth blunt end of the 100 ducting
2) the internally tapered leading edge of the ducting
3) The internally tapered duct plus the flange

And here are the results - and they quite revealing.

Just tapering the internal leading edge of the duct adds an average of 6% more air speed across the 300 mm distance measured while the Bell mouth flange adds about 20%!

So overall well worth the effort.

Now I gotta build/find me one for the 6" duct opening.

BobL,

there appears to be an inconsistency here. Hopefully you can explain it.

Look back at youur post #21 in this thread, At 50mm from the opening the airflow from the 4" is 9 and the 6" is 11.

In the latest post I saw the 4" with the bellmouth at 13 and thought initially that this is therefore better than the 6" in post #21, but then I looked again and the standard 4" has suddenly jumped to 11.5, when it was 9 in post #21.

The baseline reading (unmodified 4" pipe 50mm from windspeed measurement) is not consistent over the two experiments which makes me ask "what was different?"

However the latest set of readings do give a great deal of hope to the 4" pipe gang, since if the 20% gain in windspeed is correct then the bellmouth will assist greatly to collect at least some more of the invisible dust.

Doug