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

Hi Graeme, no one is denying the existence of invisible dust, the naysayers are just looking for balance and a practical system for capturing it. There is no one on this forum that would turn down a free 100% collection system and a new shed to put it in if they were being given away, but back in the real world we buy what is practical in a physical size that can be afforded.

Not wanting to side track this interesting thread, you raise a valid and interesting point… by your own admission these two things are contradictory to each other, and where a contradiction exists so does a compromise. Based on your own finding you would actually need two different systems running parallel to each other to collect both sawdust and invisible dust effectively… Or perhaps the answer is in a dual system, one large pipe, one smaller pipe???

Hi Graeme,

Thanks for your comments and feedback.
Your summary is indeed a very good one.

The reason I am focussing on measuring air speed is
a) Air speed is far easier to measure than volume capture rates. Higher overall air speed for a small specific volume (e.g. a 1 cm^2 cylinder between a workpiece and an inlet) is generally going to translate into a higher volume capture rate for that specific volume. Of course this is not the case over the whole volume between the workpiece and the inlet - see discussion below
b) Invisibles track with air current directions. If there is no (or very low speed) air current the invisibles like odours will rapidly diffuse from higher to lower concentrations. When I measure the air speed I try to measure the maximum air speed so this in effect tells me the direction and thus indirectly where the invisibles will move to. So far I am mainly measuring close to uncluttered inlets and of course I observe the direction is towards the inlet but things may start to get unpredictable as workpieces, blades and operators get in the way.

As Doug has said, at 50 mm from the front of the duct the air speed for the 4" may indeed be 20% higher than for the 6", but that lower air speed for the 6" extends over an area 225% greater than for the 4" thus the total volume of air collected per unit time will be significantly greater for the 6" than the 4".

I could do some radial mapping of air speeds and then used the integration approach as used for determination of the air flow in a duct for my DC but I think one could obtain an estimate of the relative difference in volume capture rates as follows.
Multiplying the 6" air speeds by 2.25 and then divide the result by the 4" air speed.

Thus the air capture at 50 mm and zero degrees becomes 11 *2.25/13 = 1.9 so the 6" pipe captures about double what the 4" pipe. This is of course only a rough estimate. I really need to whip up a bell profile inlet for the 6" pipe to see what it can really do.

Bob what about a modified version of mic-d’s push – pull, a pull – pull.
Here’s the idea… One central large diameter pipe with two smaller diameter pipes either side of it.
The smaller pipe at either side have a higher air speed than the central pipe which has higher volume.
The two outside pipes would create a pseudo pull on the slower moving air steam of the centre pipe.
If all three pipes were put closely together in a common hood, would this not capture more invisible dust?

The two outer black arrows would acts as scavenger feeds for the main orange arrow pipe.

Thanks, Bob

We are very much on the same wavelength. I keep wondering - what's he going to do next? why did he do it that way? and so on.

Have you thought about the possibility of making the invisible dust visible? Or a proxy, such as a smoke generator?

Fair Winds

Graeme

Unless air is forcibly injected into an inlet the air speeds must be the same for all three flows at the junction and thus at inlet and the flow rate will simply apportion itself according to same air speed x cross sectional area.

Originally Posted by GraemeCook

Thanks, Bob

We are very much on the same wavelength. I keep wondering - what's he going to do next? why did he do it that way? and so on.

SWMBO says I have a head like a can-o-worms.

Have you thought about the possibility of making the invisible dust visible? Or a proxy, such as a smoke generator?

One can hire or buy a smoke generators that output known particle size distributions to test filters and systems but these are expensive and not really useful for simulating actual WW dust generation processes. The other way to look at invisible dust in in humid environments whereby it acts as a nucleating site but this is incompatible with wood and metal working machinery.

Originally Posted by BobL

Unless air is forcibly injected into an inlet the air speeds must be the same for all three flows at the junction and thus at inlet and the flow rate will simply apportion itself according to same air speed x cross sectional area.

The junction was for illustration purposes only... I was thinking more along the lines of using two smaller cyclones and one larger cyclone that was completely isolated up to and including the point where they draw their air from the DC.

Imagine having two straws in your mouth, one smaller diameter and one larger diameter, using the same amount of vacuum present in your mouth, wouldn't one move at high speed / low volume and the other at low speed high volume, or is this only possible with two different vacuum sources?

Originally Posted by HeadScratcher

The junction was for illustration purposes only... I was thinking more along the lines of using two smaller cyclones and one larger cyclone that was completely isolated up to and including the point where they draw their air from the DC.

Imagine having two straws in your mouth, one smaller diameter and one larger diameter, using the same amount of vacuum present in your mouth, wouldn't one move at high speed / low volume and the other at low speed high volume, or is this only possible with two different vacuum sources?

I agree they have the same pressure difference BUT the pressure differences are not enough to significantly compress the air in the pipes so the air already in the duct ahead of the junction is what is determines the air speed and it prevents the air moving any faster in the narrower pipe.

I just (tried to ) measured the air speeds in both arms of a 150 with 100 mm 45º side junction. The air speed is too high for my hot wire anemometer and it is very hard to measure with the pitot tube because of turbulence and significant variability across the pipe (a 500 mm piece of straight pipe should be inserted in line so the air flow stabilises). Taking all this into account if anything the max air speed in the 150 mm diam pipe was about 10% higher ???

Originally Posted by BobL

SWMBO says I have a head like a can-o-worms.

Mine's not quite so polite.

One can hire or buy a smoke generators that output known particle size distributions to test filters and systems but these are expensive and not really useful for simulating actual WW dust generation processes. The other way to look at invisible dust in in humid environments whereby it acts as a nucleating site but this is incompatible with wood and metal working machinery.

I wasn't thinking of anything quite so elaborate, or so measurably correct. Possibly just a smoking candle or a smoldering rag.... For the invisibles, it is the capture zone and the capture range that is key.

Fair Winds

Graeme

Originally Posted by GraemeCook

Mine's not quite so polite.

I wasn't thinking of anything quite so elaborate, or so measurably correct. Possibly just a smoking candle or a smoldering rag.... For the invisibles, it is the capture zone and the capture range that is key.

I don't know about you but I would get very nervous about holding such things near my DC. The DC pulls a fair bit of air and I could envisage it easily sparking a smouldering rag into a flaming torch.

However, I do have another idea - drops of water into liquid nitrogen!

Chris (Interwood) - could be time for some more videos!

Originally Posted by BobL

I agree they have the same pressure difference BUT the pressure differences are not enough to significantly compress the air in the pipes so the air already in the duct ahead of the junction is what is determines the air speed and it prevents the air moving any faster in the narrower pipe.

I just (tried to ) measured the air speeds in both arms of a 150 with 100 mm 45º side junction. The air speed is too high for my hot wire anemometer and it is very hard to measure with the pitot tube because of turbulence and significant variability across the pipe (a 500 mm piece of straight pipe should be inserted in line so the air flow stabilises). Taking all this into account if anything the max air speed in the 150 mm diam pipe was about 10% higher ???

Hi Bob there wouldn't be a junction, it would just be three into three like this. Obviously wouldn't be in a straight line like this, but it is easier to represent the idea.

Interwood came around this evening and we did some air speed mapping around a couple of different hoods.



and


The results will take a bit of analysis - sorry you will have to wait for these.
We also observed some interesting effects with these hoods and hopefully we will have a video to show you what we found.

The other thing I wanted to show was this ridge inside the 150 to 100 mm reducer.

This seemingly insignificant ridge seems to explain the difference between the a air speeds measured in the first and second experiments reported for the 100 mm pipe.

Originally Posted by HeadScratcher

Hi Bob there wouldn't be a junction, it would just be three into three like this. Obviously wouldn't be in a straight line like this, but it is easier to represent the idea.

The overall dust collection performance of a DC in that configuration would depend on the capacity of the impeller/motor.

If the impeller/motor is already working to close capacity pulling the air through the bigger duct, then adding two smaller ones will just redistribute the original flow rate across all 3. There will probably be a greater air speed in the narrower pipes but it will be at the expense of losing considerable speed and flow in the original bigger duct.

If the Impeller/motor had spare capacity then with all three ducts it would pull more air in total although it would still ultimately distribute its load across all 3 ducts. Once again there will probably be a greater air speed in the narrower pipes but it will still be at the expense of losing some speed in the original bigger duct.

Multiple inlet impellers need to be carefully designed to work properly. Most hobby level machines generate >2 times the FPM required but instead are hobbled by a lack of CFM. The single best improvement that can be made to most hobby level DCs is to replace the multiple 100 mm inlet intakes on these DCs and go for one 150 mm duct and then connect into this using junctions. Besides installing ducting for 1 connection to each machine is difficult enough without going to 3.

For me, the use of the term 'invisible' is something that is hard for me to understand how could anyone see 'invisible dust' when it is in terms of woodworking? A better word, for me to understand, would be micrometer dust.
I have a lot of small or 'micrometer' dust that collects in my shop and have to clean the shelves every week or it becomes a big dust problem.
How can I see 'invisible' dust?
How can you measure 'invisible' dust?
Just trying to understand the thread.
srichard44