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

Originally Posted by BobL

Placing this piece in the way actually increased the air speed around it.
I think the workpiece blocks the air so like a plane wing the air has to go around it resulting in higher air speeds all around the workpiece. It is interesting to see the air speed is highest underneath the workpiece compared front and side - this is consistent with the rotational direction of the workpiece.

I found this somewhat unexpected.

Hi Bob,

That is a fantastic piece of research. I had never considered what the effect would be on Bernoulli's Principal if the object in the airstream was round and rotated, and yet thats what woodturners deal with on a daily basis if they are employing dust extraction. I am not surprised that the airspeed is increasing as that is what Bernoulli's Principal is all about but I never thought about if rotating the object would have any significant effect.

What would be great is if you could you could reduce your inlet to 100mm and repeat all the readings so that we can see how much more effective 6" is as opposed to 4".

Doug

Originally Posted by doug3030

what would be great is if you could you could reduce your inlet to 100mm and repeat all the readings so that we can see how much more effective 6" is as opposed to 4"

ok.

Thanks BobL. looking forward to seeing a comparison table posted. By the way what HP is the dusty you are using for the test?

Doug

It's a 3HP twin bag
My DC set up is described here.

Interesting read and raises a few questions… While the object is rotating, like on a lathe the air path would be fairly consistent relative to the cutting tool (I could however see it flinging some visible and invisible dust in a 360 degree pattern) but what about when you’re cutting a circle using a router, and the object is stationary and the router is spinning while being rotated 360 degrees. My limited experience so far is that it tends to disperse the dust fairly evenly around the full 360 degrees. How do you go for collecting visible /invisible dust when you have the router between the duct and the dust?

This was what prompted me to redesign my DE to have three separate inlet pipes. I was figuring that placement of the dust pickup might be just as important, if possibly not more important than the equipment collecting it. As you well know a router is a dirty machine capable of throwing dust in all directions. I am working on the premise that if I collect dust with pickup points every 120 degrees it may just compensate a tiny bit for the lack of grunt of a bigger DC.

This theory appears to be backed up by your last chart that shows once you get further away than about 150mm from the duct, the air speed drops dramatically. Which would severly hinder the pickup of invisible dust...


Edit:
Also curious to know if you have ever played with multiple inlets before i.e. three pipes entering the propellor area?

Originally Posted by HeadScratcher

nteresting read and raises a few questions… While the object is rotating, like on a lathe the air path would be fairly consistent relative to the cutting tool (I could however see it flinging some visible and invisible dust in a 360 degree pattern)

Sure, the visible dust will spin off as it has enough mass and therefore momentum, but the invisible dust is rapidly slowed by collisions with other gaseous components and will therefore just end up riding the major air currents. If there are no major air currents being spun off (and my measurements indicate there are none) the invisible will not "spin off" but be carried away by air currents made by a DC. This is why I am focussing on measuring air speeds in the first place.

but what about when you’re cutting a circle using a router, and the object is stationary and the router is spinning while being rotated 360 degrees. My limited experience so far is that it tends to disperse the dust fairly evenly around the full 360 degrees. How do you go for collecting visible /invisible dust when you have the router between the duct and the dust?
This was what prompted me to redesign my DE to have three separate inlet pipes. I was figuring that placement of the dust pickup might be just as important, if possibly not more important than the equipment collecting it. As you well know a router is a dirty machine capable of throwing dust in all directions. I am working on the premise that if I collect dust with pickup points every 120 degrees it may just compensate a tiny bit for the lack of grunt of a bigger DC.

This is an example of where high velocity dust collection has to be placed on the tool itself and what invisible stuff spins off into the shed has then to be efficiently scavenged by a high volume DC. I think it is awkward enough having to have to trail just one vacuum line to a power tool without having to trail 3. It will be interesting to measure the air currents in the vicinity of a spinning router bit and I will add it to the list. I suspect there won't actually be that much as the rotation is so high and the bit is usually small so it will be spinning in its own spinning boundary layer.

On a router table the work tends to mask the bit on one side anyway and the other side is usually where the fence is so a pick there will cover the 180º on that side of things. The other major dust ejection area is not even in the plane of the rotation but at right angles to the bit so a collection point is needed there. This is why some routers are located in fully enclosed cupboards with DC ports in them. To cover these two pick ups I have two 4" ducts headed for my router table.

Also curious to know if you have ever played with multiple inlets before i.e. three pipes entering the propellor area?

Sorry I haven't looked at that.

I was thinking more along the lines of free hand router use rather than using a router table. But you just gave me an idea worth considering…. What about a cabinet similar to a sand blasting cabinet, surely that would have to be a far more efficient method of visible / invisible dust control for a regular router user? Probably wouldn’t need to go to the extent of those cumbersome heavy duty rubber gloves, as you are not sending shot everywhere, but I’m betting you could suck a ton of dust…

I don’t have the optional vacuum attachment for my router but what size hose could you practically hook up to it? How efficient would that be compared to catching the dust externally?

Originally Posted by HeadScratcher

I was thinking more along the lines of free hand router use rather than using a router table. But you just gave me an idea worth considering…. What about a cabinet similar to a sand blasting cabinet, surely that would have to be a far more efficient method of visible / invisible dust control for a regular router user? Probably wouldn’t need to go to the extent of those cumbersome heavy duty rubber gloves, as you are not sending shot everywhere, but I’m betting you could suck a ton of dust…

This is what existing router cabinets already do.

I don’t have the optional vacuum attachment for my router but what size hose could you practically hook up to it? How efficient would that be compared to catching the dust externally?

About 1000% at capturing all dust, but most vacuum cleaners fail the important OHS test in they then let invisibles straight thru their filters back into the shed. The high quality cleaners with real HEPA filters are a dream to use with tools like routers and especially sanders. They do let a few big chips escape but they capture a very high proportion of the fine visible dust and by extension I guess the same for invisibles.

Some hand power tools with internal fans like small belt sanders can be adequately vented even using a duct direct from a DC as long as the tools internal fan has a lower flow rate than the DC is capable of through the duct. I use a 50 mm flexy between my 100mm Makita belt sander and a DC duct and also have a 6" duct open at the same time to scavenge any escaped dust. I have no idea how effective this is for invisibles so I vent the whole shed for 10/15 minutes after I finish the sanding job.

A round-about but simple and very effective way to get around the cheap HEPA-less vacuum cleaner would be to build an air tight box that holds the vacuum cleaner and vent the box to the outside using a higher volume DC. There may in fact be some smart cookie out there that has already done this but I have not see one - If my old vacuum cleaner was working I would probably think about making one for it.

Didn't know there was such a thing as a routing cabinet... another good idea I am too late for.

I was only calling it a vacuum attachment because of the smaller size not necessarily for use with a vacuum. What I was asking was how effective would the on board attachment be compared to catching it externally with three bigger hoses nearby? from your answer I am seeing that although the on-board attachment is much smaller, because it is closer to the source it will outweigh the size limitation.

Not wanting to get off topic I have knocked up a comparison shot to show a single 150mm outlet vs three 100mm (105mm OD) outlets. From measurements I have already done, an imaginary circle that would be required to cover all three inlets would have a rough diameter of about 240mm vs the 150mm for a single inlet. When I was looking at the inlet the other day the end of the blades finish roughly where the 150mm hole starts.

You mentioned before that invisible dust was more prone to acting like a gas than a solid. Hypothetically speaking if the 100mm pipe was partially feeding into the curved blade, I would be inclined to think this would move the invisible dust more effectively, rather than having cavitation in the centre, more like gas would.

Originally Posted by HeadScratcher

I was only calling it a vacuum attachment because of the smaller size not necessarily for use with a vacuum. What I was asking was how effective would the on board attachment be compared to catching it externally with three bigger hoses nearby? from your answer I am seeing that although the on-board attachment is much smaller, because it is closer to the source it will outweigh the size limitation.

Correct.

Not wanting to get off topic I have knocked up a comparison shot to show a single 150mm outlet vs three 100mm (105mm OD) outlets. From measurements I have already done, an imaginary circle that would be required to cover all three inlets would have a rough diameter of about 240mm vs the 150mm for a single inlet. When I was looking at the inlet the other day the end of the blades finish roughly where the 150mm hole starts.

You mentioned before that invisible dust was more prone to acting like a gas than a solid. Hypothetically speaking if the 100mm pipe was partially feeding into the curved blade, I would be inclined to think this would move the invisible dust more effectively, rather than having cavitation in the centre, more like gas would.

I'm not sure what you are asking here. If it is would be better to have 3 pipe joining up before or at the entrance to the impeller then I am not sure.

I'm talking about joing the three pipes into the front plate, not 3 into 1 before the front plate, unlike the adaptor that comes with it, as that would serve little benefit.

Figuring bacause of the larger imaginary diameter that 3 pipes would take up, I'm wondering if it would it help the invisible dust any.

Here is another pic to try better illustrate.

Originally Posted by BobL

A round-about but simple and very effective way to get around the cheap HEPA-less vacuum cleaner would be to build an air tight box that holds the vacuum cleaner and vent the box to the outside using a higher volume DC.

A similar project is actually on my list! But I was not going to make it an airtight box because it would be choking the dusty. I was thinking of increasing the airflow over the vacuum cleaner with maybe a baffle to ensure that there is a positive air flow through the dusty with some kind of obstruction to the invisible dust coming out of the box.

There would of course be a chip collector in this system between the tool and the vacuum cleaner and as much filtration as possible could be removed from the vacuum cleaner itself to increase its air-flow.

I dont have a graphics program lilke HeadScratcher's or I would draw a picture. Hopefully I have described my plan adequately.

Bob, I am curious as to why you thought it should be an airtight box? Am I on the right track here?

Doug

Originally Posted by doug3030

A similar project is actually on my list! But I was not going to make it an airtight box because it would be choking the dusty. I was thinking of increasing the airflow over the vacuum cleaner with maybe a baffle to ensure that there is a positive air flow through the dusty with some kind of obstruction to the invisible dust coming out of the box.

There would of course be a chip collector in this system between the tool and the vacuum cleaner and as much filtration as possible could be removed from the vacuum cleaner itself to increase its air-flow.

I dont have a graphics program lilke HeadScratcher's or I would draw a picture. Hopefully I have described my plan adequately.

Bob, I am curious as to why you thought it should be an airtight box? Am I on the right track here?

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.

Originally Posted by doug3030

I dont have a graphics program lilke HeadScratcher's or I would draw a picture. Hopefully I have described my plan adequately.

Now you do... Trimble SketchUp