Thread: Dust collection article in Australian Woodsmith issue 96

Originally Posted by doug3030

. . . .My opinoion is that the article should have differentiated between collection with a dust extractor and collecting with a gravity bag, and advised that when using a dust extractor you must ensure that the gaps left open in the saw cabinet need to be at least of the same cross section as the collection pipe. Letting more air in around the cabinet will maximise the overall collection of visible and invisible dust.

I reckon it should be "at least twice that of the cross section as the collection pipe". The reason for this is because most openings are not as regular as a circular inlet especially rectangular narrow gaps and slits which are much poorer performers if their simple cross sectional areas are used as a criteria.

I agree Bob, I said at LEAST the same but in my mind as I wrote it I was thinking at least but probably a whole lot more. I have been thinking of the feasibility of opening up more airflow through my table saw, rather than reducing it, so as to maximise dust capture.

Bob, I hope we have gained enough mutual respect for each other over the past few weeks for you to take this as it is intended, but a week ago you criticised me for posting a "gut feeling" and now you are using terms like "I reckon" and "at least" without hard data to back it up. Without our "gut feelings" we dont have a start point for experiment and research. I hope you understand what I mean here and it is meant to be a compliment to you on the way you have changed the way you are contributing to the forum with your research.

Cheers

Doug

Hi Doug,
I haven't read the article so can't directly make a comment, however in general I would agree with the guts of what you have said above.


Pete

Glad you raised this. I read that same article and had the exact same thoughts.

Trav

Originally Posted by doug3030

Bob, I hope we have gained enough mutual respect for each other over the past few weeks for you to take this as it is intended, but a week ago you criticised me for posting a "gut feeling" and now you are using terms like "I reckon" and "at least" without hard data to back it up. Without our "gut feelings" we dont have a start point for experiment and research. I hope you understand what I mean here and it is meant to be a compliment to you on the way you have changed the way you are contributing to the forum with your research.

You are right I don't have any hard or specific data on DC restrictions to back up my statement and my expression suggests I am guessing. However, what I said was based on the well known Poiseuille's law, which I have used many times both theoretically and practically so if anything it's a "well informed gut feel". The other piece of critical info is that Poiseuille's law applies to laminar flow while the flow through/around most DC openings will almost certainly be turbulent making the situation even worse. If the openings are small/convoluted enough then a factor two may not even be enough. I could go into Poiseuille's law in more detail but I'll leave that to reader to follow up.

Originally Posted by BobL

I could go into Poiseuille's law in more detail but I'll leave that to reader to follow up.

Hi Bob,

Thanks for the link to the Poiseuille's law article. I think we are delving into physics well beyond the comprehension of most of us on the forum. Unless Sheldon and his mates from Big Bang Theory take up woodworking I doubt that many will get past the first equation.

My understanding is that Poiseuille's law is one of those laws of physics that works within a very narrow range of parameters. Correct me if I am wrong but it is only fully applicable to a scenario where a viscous liquid is forced under pressure through a pipe and only if it is a laminar flow. While some of its principals can apply in other situations including gasses, its reliability is, well, a bit dodgy if it is not on its own firm home-ground.

It is used to predict the affects of the flow rate if the pipe changes diameter or is constricted in some way; if the pressure is increased or decreased or the viscosity changes.

Gasses can be compressed, liquids cannot, at least not to any degree that matters in the circumstances under discussion. This is one of the reasons that it is difficult to apply Poiseuille's law to dust extraction. Another though lesser reason is that Poiseuille's law applies to forcing the fluid through under pressure, not drawing it through the pipe by vacuum as in a dust collector scenario.

Poiseuille's law's major use, surprisingly is in medical research where it is used to analyse blood flow through the organs and the effects of constricted blood vessels, and to a lesser extent (gasses, not liquid) to breathing difficulties when air passages are subjected to swelling.

Bob, while I agree that what you said above is correct and relevant, I think you are drawing a long bow by linking your conclusions to Poiseuille's law.

When dealing with gasses it is probably a lot safer to rely on Bernoulli's Principal and Boyles Law. Navier-Stokes Equations or even Newton's Second Law may even be more applicable to dust collection than Poiseuille's law.

Doug

Originally Posted by doug3030

Bob, while I agree that what you said above is correct and relevant, I think you are drawing a long bow by linking your conclusions to Poiseuille's law.

When dealing with gasses it is probably a lot safer to rely on Bernoulli's Principal and Boyles Law. Navier-Stokes Equations or even Newton's Second Law may even be more applicable to dust collection than Poiseuille's law.
:

From what you write it is pretty clear you don't appreciate the pressure domain DCs operate under. The pressures differences generated in DCs are so low (eg < 30 mb and in most cases <10 mb) that gasses can be treated as non compressible (which is why it is usable in breathing analysis). This is why airflow is largely conserved as cross sectional areas change within the same duct. I have personally measured pressure differences in duct lines to confirm this is the case.

Poiseuille's law is just the beginning of analysing the problem, adding turbulence and other factors doesn't reduce the cross section area ratios required, it just adds to it.

Hi Doug
I only rarely see or read Australian Woodsmith, so will take the liberty of commenting without reading the article

Originally Posted by doug3030

The first thing the unidentified author suggests is to plug up or block off any openings into the body of the saw as much as possible. He makes no diferentiation as to whether this advice applies to when you use a dusty or ghe gravity collection bag, aslthough he does say that this is at least in part to prevent dust escaping from these openings.

This sounds as though it's been lifted from an article that was in Fine Woodworking Issue 145. The FWW author recommended closing off the back of a contractor's saw (the type that has the motor hanging out the rear) using ply or MDF. Openings were left to allow for slewing of the motor and belt when the blade is tilted. I've just looked at the article again and these openings represent around 20% of the area at the rear of the saw. see attached image
Without doing any calcs or measurements, (cue for BobL to chime in) a 65mm wide slot around 300mm long should allow a reasonable quanty of air to enter and obtain a reasonable velocity through the cabinet to the dust extractor port.

In principle, dust is spread around the shed via the air flow generated by the cutting device -- in this case the teeth on the blade.
effective dust collection requires negative pressure in the vicinity of the cutting tool and within the saw cabinet
If the saw cabinet can be kept at negative pressure, the major air flow will be into the cabinet through any openings and out of the cabinet via the extractor
the challenge is designing the size of the net cabinet openings so that the estractor works effectively.

as to the size of openings
smaller openings might be expected to result in greater negative pressure within the cabinet, which coupled with appropiate placement of the openings could induce higher air speeds within the cabinet assisting with collectioon efficiency -- more dust is air borne to the collector port, less dust drops to the base of the cabinet.

my opinion?
placing the extractor port high in the cabinet and blocking off most of the potential air intake should result in less dust escaping the saw and entering the shed the air.
There is probably a design that directs a high velocity air flow past that part of the blade within the saw cabinet (you might need a compressor to generate the required air flow) that disrupts the laminar flow around the blade diverting the dust in that laminar flow to the extractor port and the dusty.


The gravity method?
a well designed system of baffles within the cabinet, combined with a negative pressure gradient, would allow the heavier particles to settle within the cabinet (ideally into a removable box to make cleaning easier) and the finer stuff to be sucked out via the dusty.
The challenge will always be capturing and/or controlling the dust that is generated at each saw tooth and either flung around at the moment of creation or dragged into and then out of the cabinet by the laminar flow around the blade.
Again, I suspect the key is achieving a respectable negative pressure within the blade guard, which in turn will relate to how close the blade guard is to the surface being cut -- I was taught this should be around 10mm

Originally Posted by BobL

From what you write it is pretty clear you don't appreciate the pressure domain DCs operate under. The pressures differences generated in DCs are so low (eg < 30 mb and in most cases <10 mb) that gasses can be treated as non compressible (which is why it is usable in breathing analysis)...

Bob, As I said above, Poiseuille's law is about fluids being FORCED through a pipe. In the scenario we are discussing, the air is being SUCKED through the pipe. While the gas may not be compressed while being forced through at that pressure, it is being de-compressed, at least to some measureable extent, when it is being sucked through, or it would not move at all, would it?

Poiseuille's law by definition applies to VISCOUS fluid, as viscousity is one of the variables in the equation. To quote your earlier post:

" Poiseuille's law applies to laminar flow while the flow through/around most DC openings will almost certainly be turbulent making the situation even worse."

"ALMOST CERTAINLY"? Bob? another one of those "gut feelings" with no data?

Bob, i agree with you. it almost certainly would, but here you are using the terms you have berated others for using and synonyms therefore.

And just another point, Poiseuille's law was intended to predict the flow of fluid through a pipe, not a device connected to the end of that pipe, such as the much larger cavity presented by a tablesaw, bandsaw, sander or whatever.

As I said earlier, it is one of the laws of physics that only applies to a very narrow set of circumstances.

Originally Posted by ian

Hi Doug
my opinion?
placing the extractor port high in the cabinet and blocking off most of the potential air intake should result in less dust escaping the saw and entering the shed the air.

I agree but the standard contractor saw cavity is so big that a large enough air flow cannot be generated with a hobby level DC to sweep out the cavity. As a result sawdust will accumulate in the bottom (especially corners) of the cavity and spill out through the gaps used to adjust the blade. I have observed this on my saw so I made a inverted pyramidal hopper and moved the duct to the middle of the base and this keeps the cavity much clearer, and even clearer still with a 6" duct and 3 HP DC.

There is probably a design that directs a high velocity air flow past that part of the blade within the saw cabinet (you might need a compressor to generate the required air flow) that disrupts the laminar flow around the blade diverting the dust in that laminar flow to the extractor port and the dusty.

This is potentially a really interesting idea and one I am interested in trying. Rather than running the compressor continuously it might make sense to trigger the air stream only while the blade is cutting.

Added a picture of the FWW Issue #145 contractor saw dust back plate to my post (#9) above

Originally Posted by BobL

... but the standard contractor saw cavity is so big that a large enough air flow cannot be generated with a hobby level DC to sweep out the cavity. As a result sawdust will accumulate in the bottom (especially corners) of the cavity and spill out through the gaps used to adjust the blade. I have observed this on my saw so I made a inverted pyramidal hopper and moved the duct to the middle of the base and this keeps the cavity much clearer, and even clearer still with a 6" duct and 3 HP DC.

I suppose there's two ways of dealing with the "heavier" dust inside the saw
allow it to accumulate in a removable box at the base of the saw -- akin to the drum yoiu install under a cyclone -- or to suck it out as it's generated and then capture it in the dusty's bags.

I'd be interested if you have any info as to which is teh most effective.

Originally Posted by doug3030

Bob, As I said above, Poiseuille's law is about fluids being FORCED through a pipe. In the scenario we are discussing, the air is being SUCKED through the pipe. While the gas may not be compressed while being forced through at that pressure, it is being de-compressed, at least to some measureable extent, when it is being sucked through, or it would not move at all, would it?

It does not matter whether the air is pushed or pulled? Poiseuille's law just operates on a pressure differential. At a molecular level fluids are not sucked but move from areas of higher pressure to lower pressure. In the case of a DC the atmosphere pushes the air into the inlet. In the case or breath analysis the atmosphere pushes the air into any low pressure regions created by lungs.

Once again, the air in a DC duct is under the same pressure from one end to the other - I have measured this and is why Poiseuille's law is applicable inside some DC ducting.

Poiseuille's law by definition applies to VISCOUS fluid, as viscousity is one of the variables in the equation.

Air has sufficient viscosity to enable it to be used in Poiseuille calculations - otherwise it would not be used in beathing analysis.

To quote your earlier post:

" Poiseuille's law applies to laminar flow while the flow through/around most DC openings will almost certainly be turbulent making the situation even worse."

"ALMOST CERTAINLY"? Bob? another one of those "gut feelings" with no data?

Bob, i agree with you. it almost certainly would, but here you are using the terms you have berated others for using and synonyms therefore.

OK I believe have now done enough measurements around inlets to state that it is so.

And just another point, Poiseuille's law was intended to predict the flow of fluid through a pipe, not a device connected to the end of that pipe, such as the much larger cavity presented by a tablesaw, bandsaw, sander or whatever.

The key word here is "intended" and this I do agree with, but it does not mean it cannot be used to help shed light on this phenomenon.

Doug, I have shown you some of what I measure and some of my direct experience about predicting and measuring air flow so how about you tell us about some of your direct experience (not reading from books or websites) about analysing/measuring air flows, for example, have you ever predicted and measured air pressures and flows in a duct?

Originally Posted by BobL

Doug, I have shown you some of what I measure and some of my direct experience about predicting and measuring air flow so how about you tell us about some of your direct experience (not reading from books or websites) about analysing/measuring air flows, for example, have you ever predicted and measured air pressures and flows in a duct?

Hi Bob,

I am not going to get into a game of "you show me yours and I will show you mine".

Unlike you, my employer does not have $10,000+ flow meters for me to take home and play with on the weekends. But that does not mean I do not predit airflow while I am designing the dust collection on one of my machines nor does it mean that I do not in some way measure the flow once it is assembled to make sure it is effective.

I will actually explain how I go about this, firstly because it may help others to work out better ways and secondly because some of the members may also be able to help me improve my methods as well.

Lets use the example of my belt and disk sander which is a work in progress in my shed as we speak. I have attached a photo of the machine. I have a 2hp dusty with a nominal flow of 1200cfm

My first step was to work out where the dust needed to be collected from. Obviously from the 2" dust port on the end of the machine, and also from the cavity beneath the machine because there are vent holes into there from the bottom of the sanding belt and also to create a flow into the machine to try to encourage as much invisible dust as possible into the collection system.

Second step is to design the collection system/ducting to achieve the best collection. SO I look for readily available plumbing or dust collection fittings and come up with a way of assembling them to maximise collection. I also look at custom making fittings if it appears to be worth the extra effort and cost.

Third step is to cut the pipes and assemble and test the system i have designed based on the predictions I have made. The attached picture shows what I came up with as the first design to try, a four inch pipe from under the cavity of the machine with a 2" branch pipe to the manufacturer's collection port. I was hoping that the air flow restrictions of the vents and other openings in the machine cavity would provide enough choke on the 4" pipe that the 2" pipe would provide enough suction to work on the manufacturer's collection port.

To test the machine, first of all I went around the machine holding strips of tissue paper to see how much suction was created through any of the inlets. I use the paper strips because the angle they are deflected gives a better indication of the strength of the flow rather than just putting your hand near the opening to feel the draft. It may not be very accurate or give a reading you cluld plot on a graph, but I got a lot of change out of $10,000 when I bought it. There is a pic of my HIgh-Tech testing equipment below too.

The other test is to obviously use the machine and just see how effective it is.

In this case, I was not getting sufficient extraction through the 2" branch to be effective, so I started blocking of vents in the cavity to see if that would help. It did not work with all the machine cavity vents and openings effectively blocked off (yes its the high pressure/low volume - low pressure/high volume thing).

The next stage of course is to redesign to correct the design faults and rebuild. In this case I am going to remove the 2" branch completely and retain the 4" main line from the machine cavity, so now instead of wanting to restrict the flow into the body cavity I will be looking at opening it up to collect more air and therefore more invisible dust.

The 2" branch pipe on the manufacturer's dust port will be replaced by a connection to the shop-vac, which was what it was made for in the first place. I was trying to avoid using it because it does not (yet) vent outside of the shed.

Once I have made the modifications I will repeat the testing stage and modify again and retest again and so on until I am satisfied with the result.

So yes, Bob, I do predict, and I do analyse and I do measure, within the resources available to me of course.

I welcome comments from anyone on the procedures I outlined above. Hopefully it will help someone and If someone can show me a better way I would be delighted to learn.

Back to the shed now.

Doug