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

yes that make perfect sense... In a vacuum / pressure situation with a single source it is always going to take the path of least resistance so that completely nullifies my earlier theory of different sized cyclones to create air speed differential.

This thread is possibly one of the most innovative ever on the woodwork forums. 77 posts long at the moment and covering a wider range of topics than the OP would have thought possible (I think)
It started out as measurements of airspeed at different distances and angles from a dust collector nozzle.
It now is discussing a number of topics, starting out with mentioning the original topic again:
- measurements of airspeed at different distances and angles from a dust collector nozzle,
- the effects of a workpiece in the airflow of the collection nozzle,
- Bernoulli's principal and its effect on the airflow around stationary and rotating workpieces,
- The difference between the effects of rotation on visible dust as opposed to invisible dust,
- The effects of multiple inlets into an impeller, different daiameters,
- Differences in Push as opposed to Pull systems of collection,
- shaping of collection nozzles to maximise collection of dust from the dirtiest areas so that maximum efficiency of capacity is achieved, including the effect of flanges and bellends on the collection effort,
- Dust emanating from dust collecting devices as a result in regular daily air pressure readings and ways to minimise the effect on the clean working environment,
- turbulence and its effect on changing daimeters of ducting,
- Airflow through shrouds and hoods
- definitions of dust, invisible dust, visible dust. microparticles, etc,
That is just topics that have come up in this thread that I believe have not been satisfactorily adressed elsewhere in this forum and have mainl arisen as a result of this thread.

Bearing in mind that the forum is formulating a FAQ on dust extraction, both theory and practical, I am concerned that with all these major topics being discussed in the one thread things are getting mixed up and a lot of valuable contributions are not being made as the thread changes tack.
This is a lot of diverse information and there are so many ground-breaking ideas coming out, it would be a shame to lose this info and possibly lose valuable contributions by not dividing this thread up into bite-sized chunks under the above very relevant topics. I know that it would not be an easy task to ask of a moderator and I dont know how well i could do it myself but i think its a task well worth considering, and I volunteer to help out with it if it is deemed a worthwhile project.
There are a lot of great contributions here and I dont want to see them wasted.

Doug

Doug, try as you may you won't convince me on a gut feel. I spent my whole working life working with hard data and arguing the 15th decimal places with like minded nerds. have seen too many gut feels come a cropper.

Originally Posted by BobL

Nonetheless Mic's bell mouth is well worth the effort. I think the flange I have is just the beginning and a more refined bell mouth design is likely to improve things a little more.

This thread has jumped way ahead of where I was upto a few nights ago so I haven't got to the end of the thread but a couple of things come to mind here, a clear example of a bell mouth on an inlet is the air inlet for fuel injection/carb for race car engines, just look at Jack Brabham's cars from back in his day.
Further to this inlet discussion (for a square end) the bell shape tends to form inside the pipe therefore reducing the effective diameter of the pipe so if a bell shape is moved outside the pipe (a bell end) we obtain closer to full airflow inside the pipe as this directs airflow into the pipe with a smooth transition, Bob might be able to find a pic/ref, I tried but


Pete

Originally Posted by pjt

Bob might be able to find a pic/ref, I tried but

Put "carburetor velocity stacks" into a google image search and you will see a few.

"velocity stack" in wikipedia also shows what Pete is talking about.

I was going to post an idea that I had, but the plastic speaker ports seemed like a better idea… What popped into my head was a metal spinning shop I visited about 7 or 8 years ago while I was looking at building a custom intake manifold for my 300rwkw 2 litre car.
Once you reach the point of crazy horsepower the standard intake manifold will no longer do the job. Looking to have some custom velocity stacks made I was directed to a metal spinner south of Sydney, but I can’t remember their name. I went to their shop and was blown away with the way metal can be shaped, but as I say the plastic speaker ports seemed like a better option since plastic can be glued to plastic. If you are looking for the ultimate in custom collection hoods then a metal spinner would be the way to go… http://metalspinningworkshop.com/

Originally Posted by HeadScratcher

. . . I went to their shop and was blown away with the way metal can be shaped, but as I say the plastic speaker ports seemed like a better option since plastic can be glued to plastic.

Yeah metal spinning is fun stuff.

I have already started to turn up a bell shaped inlet port from 3 x 16 mm layers of MDF for the 150 mm ducting. The port is 250 mm wide with a edge radius of ~30 mm.
It would be nice to use PVC or something similar but a piece of PVC rod that size is not cheap and it would be pretty wasteful to turn most of it into shavings. The other alternative is a mold and a heat gun which would make sense if I had to make several but at this stage I only have one naked port like this in my shed so an MDF based model should be OK.

Turning MDF has never been my idea of fun because of the dust it generates but with the 150 mm duct just mm away from the workpiece it makes it a lot easier with much of the curlies and dust generated being sucked straight into the inlet. With so much flow in the vicinity of the work I am also more comfortable about using puffs of compressed air to sweep the dust out of the workpiece.

Sorry no pics so far and tonight and Saturday I am busy so it will be sunday before I can make any more progress with this.

The thing that did my head in the most with metal spinning is that they are not making these shapes out of a billet block like you are currently doing with the MDF.
They spin those weird and wonderful shapes out of a small pill. I found it hard to get my head around how they did it even after looking it up and seeing photos.
I guess it is like people who can see six moves of a Rubiks cube ahead and say that square is going to be here… Or the chess player that can say in five moves you’re going to be in checkmate…

If you turn an internal mold to the correct shape out of something less noxious, you could use this product to prototype various shapes ad nauseam. I've had a go of it and it's fun stuff.

Originally Posted by BobL

Yeah metal spinning is fun stuff.

I have already started to turn up a bell shaped inlet port from 3 x 16 mm layers of MDF for the 150 mm ducting. The port is 250 mm wide with a edge radius of ~30 mm.
It would be nice to use PVC or something similar but a piece of PVC rod that size is not cheap and it would be pretty wasteful to turn most of it into shavings. The other alternative is a mold and a heat gun which would make sense if I had to make several but at this stage I only have one naked port like this in my shed so an MDF based model should be OK.

Turning MDF has never been my idea of fun because of the dust it generates but with the 150 mm duct just mm away from the workpiece it makes it a lot easier with much of the curlies and dust generated being sucked straight into the inlet. With so much flow in the vicinity of the work I am also more comfortable about using puffs of compressed air to sweep the dust out of the workpiece.

Sorry no pics so far and tonight and Saturday I am busy so it will be sunday before I can make any more progress with this.

Originally Posted by BobL

Put "carburetor velocity stacks" into a google image search and you will see a few.

"velocity stack" in wikipedia also shows what Pete is talking about.

Yep, that's it, I was going down the air intake path but got lost


Pete

Originally Posted by mic-d

If you turn an internal mold to the correct shape out of something less noxious, you could use this product to prototype various shapes ad nauseam. I've had a go of it and it's fun stuff.

An out of left field alternative is 3D printing. I'm getting a small one for my birthday. We have a bigger one at work and it is set up in a lab where I spend 6 hours a week. Today I was watching it print out hinges and door handles - very entertaining interesting

I mocked up a Bell mouth (BM) for the 150 mm ducting. Instead of being attached to 150 mm ducting I'm attaching it direct to a 45º elbow which has an opening of 160 mm so comparisons are perhaps not so straightforward.

Here is what it looks like. Its made from 3 x 16 mm thick pieces of MDF glued together. The BM edge radius is 32 mm Unfortunately I made the BM about 0.5 mm too large so it's not the best transition from the BM to the ducting and there is a small ( 0.25 mm) stepped taper at the end of the BM.





The air speed data is surprising.

Instead of the air speed at 50 mm from the inlet for the BM going up, like it did for the 100 mm ducting(black arrow) , it went down (red arrow)?

Fortunately after that the speed profile is the flattest of all the hoods/BMs/Inlets. Combined with the fact that it is 160 mm in diam this means it will have the greatest air flow over the widest volume.

This will have to do for now while I attend to a number of backlogged jobs but I will get back to it some time and try to make another BM.

I've done a bit more reading and looked at the results of the hoods experiment Interwood and I did last week (sorry still getting that together) and it seems like the 100 mm bell mouth I used is acting more like a small hood than a BM. On an efficient BM the air speed close to the front of the centre of a BM inlet should be less than that for a plain ducting inlet. This is because the air speed will be smoother and spread more uniformly across the wider entry than the plain ducting which will have significant turbulence near the edges hence compressing the majority of the air flow closer to the centre line of the duct and the smaller the diameter of the ducting the bigger this effect.