Thread: 2hp dust extractor mod - build

Ev
I think turning bell mouths would work, but if I was going to use them on machines as well as the impeller intake, then I will need quite a few. Happy to pilot one if you are keen to make one!

As an alternative, a bit of Google stalking showed up this pot at Bunnings
http://www.bunnings.com.au/eden-22cm...c-pot_p2941386

There are other options around the same size and with a similar shape. I think I might take a trip to Bunnings and try a few pots. A quick trip to the bandsaw should remove the base, then they should be simple to install.

Trav

Originally Posted by Trav

It's not a bell mouth, nor is it cheap, but what about something like this?

https://www.masters.com.au/product/9...teel-150-200mm
=

That adapter will in practice be not much better than using a naked duct opening.
Let's look at that adapter in detail and divide the space outside and inside into 5 volumes A - E

Although it is too short to really be effective ramped adapters are best suited at getting air from B to D or vice-versa.

But before air can go right through it also has to transit from A to B, or the other way, E to D and its at these points where things get turbulent.
At the D/E or AB boundary the baked open duct will draw too much air from the sides and behind the duct which is not helpful because you will normally want to grab the dust from in front of the duct opening. The air streams being dragged in from the sides and around the back has to do performs 180º to 270+º turns and crashes into each other air causing turbulence and slowing down flow.
It matters naught how short the lengths of

More importantly for dust collection the bell mouth will provide for a greater air speed (hence greater collection) from in further in front of the duct than a bare open duct.

If you can't make a bell mouth then the best thing to buy is one of these.
I think they are called floor drain inlets, the 150 mm versions are also not cheap ($8) for what they are but they will be better than the Masters adapter.




150mmflange.jpg


A square or rectangular piece of MDF with a decent round over will be even better because the curvature on the edge of the floor drains is too severe.

A bell mouth hood on the end of a long taper (5 or preferably 10X duct diameters long) would be even better but now we are getting int +1% improvement territory.
The Bell mouth is only in the + 5% category for flow but MORE IMPORTANTLY it is in the +100% for air directionally and greater air speed directly in front of the duct.

What great info.

So, if we were using 6" duct, we'd want a 9" bell bottom. This would imply a 1.5" round over radius.

Should this round over on the bell be circular in itself? Make it like the inside of a toroid? Is this smooth circular shape ideal, or should it be a little more obtuse?

Good stuff!

Trav, I'm watching your thread as its something I'm keen on. I'm restricted with the 4" at the moment, a big shed rearrange has just been done and was thinking some 6" would be cool (but heinously expensive!)... You are a perfect guinea pig

Turning up toroids are easy, so this is sort of the same... I might need a jig or two, but you know I love jigs . As for making a few, if we get the impeller one done, we can then crank out more as needed. I'm thinking of roughly making a segmented one out of pine and making it exactly the same way as I'd do a throat on a pot or amphora.

It gives me an excuse to use the new wedgies from the USA (segeasy.com) that just came in

It would be a gnarly inlet, that's for sure!

If you dont have the facility to make a bell mouth on a lathe, or with a router, rounding over the edges with a rasp and then some sand paper doesnt take too long in MDF. Just be mindful of the required radius.

Originally Posted by Trav

Thanks bob. Will look at bell mouths to see what I can do. Can anyone recommend a simple place to buy something that would work as a bell mouth? I've seen a few pot plants around with that profile, but I'm not sure of the size.

I used a large soup bowl. The end of the pipe was heated in cooking oil and then pressed over the base of the bowl to form a bell mouth.

You can leave it like that if you want a bell mouth, or you can pull the pipe off the bowl and press it on a flat surface to form a flange.

Dead simple and very fast. You can make your own straight connectors the same way. Heat one end and press another pipe into the heated end. No end of thermo-forming possibilities exist.

Originally Posted by John Samuel

I used a large soup bowl. The end of the pipe was heated in cooking oil and then pressed over the base of the bowl to form a bell mouth.
You can leave it like that if you want a bell mouth, or you can pull the pipe off the bowl and press it on a flat surface to form a flange.
Dead simple and very fast. You can make your own straight connectors the same way. Heat one end and press another pipe into the heated end. No end of thermo-forming possibilities exist.

The reference to a Bell mouth is not so much to the overall shape of a bell, i.e. a flared shape like a bowl or a flower pot, but more to the "rolled over edge" of the bell mouth itself - that is why it refers specifically to the "mouth"

This is he shape - note that only the entrance is shaped like a bell - the remainder can be completely straight sided.


Bell mouths were developed to improve air flow into carburettors and also for acoustic speaker operation.

For those that have not read about this before, instead of reinventing the wheel here have a look in this thread.
https://www.woodworkforums.com/f200/air-speeds-duct-openings-blades-nb-invisible-dust-discussed-157526

There is a lot of dross and unrelated material to wade through but it's all there.
The bottom line is shown in post #89 where I present experimental data showing the improvement made by a true bell mouth.

Originally Posted by Evanism

What great info.

Originally Posted by Evanism

So, if we were using 6" duct, we'd want a 9" bell bottom. This would imply a 1.5" round over radius.

Yes - the radius of curvature of the roll over should be 1/4 of the duct diameter

Should this round over on the bell be circular in itself? Make it like the inside of a toroid? Is this smooth circular shape ideal, or should it be a little more obtuse?

There has been a lot of testing to determine the optimum shape of a bell mouth.
The best profiles are more or less rounded followed by a little flaring of the duct (there is a maths formula for it but it gets very technical) but the improvement in flow compared to a circular profile is in the 1% range.
Circular profiles are often used because they are easy to make

This might help.

None of the "A's" are bell mouthed, while all of the "B's" are bell mouthed

"A's" are really adapters but if all if the "A"'s are used as hoods then their order of effectiveness (least to most) would be 2A, 5A, 1A &4A finally 3A

In terms of the Bell mouthed hoods (B's) the order would be 2B&3B, and then, depending on dust generation source profile, 1& 4B and finally 5B

5B is the most effective for point sources of dust as it will collect air from further out in front of its mouth than either 1 or 4B
1 & 4B would be more useful for diffuse sources of dust like a sander

Remember the total amount of air collected is limited by the duct diameter so having wider and wider flared ends does not increase the air flow but it will slow down the air collection speed..

All the shape of the port and the bell mouth does is eases the air flow into the duct up to a maximum amount.

Originally Posted by John Samuel

I used a large soup bowl. The end of the pipe was heated in cooking oil and then pressed over the base of the bowl to form a bell mouth.

You can leave it like that if you want a bell mouth, or you can pull the pipe off the bowl and press it on a flat surface to form a flange.

Dead simple and very fast. You can make your own straight connectors the same way. Heat one end and press another pipe into the heated end. No end of thermo-forming possibilities exist.

John - This sounds like a great way to do it. My wife is out tonight, so perhaps I should crank up the stove...

In all seriousness, I think this might be a winner. I read on the bill pentz site that you can stretch pipe by around 33% which might be enough to move from a 6" pipe to 8" plus a round over/flange. I think I'll give this a go on the weekend to see if I can make up a version that might fit.

Ev - do you feel like some cooking practice this weekend? If you were feeling creative, you could mock up a mould that we could form the pipe over? I will buy a couple of litres of cooking oil and source a suitable pot. If we can get the system working and a suitable design, then we can make up a stack for machines pretty quickly.

Trav

Originally Posted by BobL

This might help.

None of the "A's" are bell mouthed, while all of the "B's" are bell mouthed

"A's" are really adapters but if all if the "A"'s are used as hoods then their order of effectiveness (least to most) would be 2A, 5A, 1A &4A finally 3A

In terms of the Bell mouthed hoods (B's) the order would be 2B&3B, and then, depending on dust generation source profile, 1& 4B and finally 5B

5B is the most effective for point sources of dust as it will collect air from further out in front of its mouth than either 1 or 4B
1 & 4B would be more useful for diffuse sources of dust like a sander

Remember the total amount of air collected is limited by the duct diameter so having wider and wider flared ends does not increase the air flow but it will slow down the air collection speed..

All the shape of the port and the bell mouth does is eases the air flow into the duct up to a maximum amount.

Bob - this is excellent. I'm beginning to understand what's needed and what may be possible for me.

Best case scenario, I think we could achieve 2a although the length of the bell mouth would be limited by the depth of the hot oil pot we can use. I'm keen to give this a go, especially if Ev can turn me a mould to use.

Thanks for your ongoing advice. I'd probably be running my stock 2hp dusty inside the shed if it wasn't for your advice and that of others in this place.

Trav

Originally Posted by Trav

Bob - this is excellent. I'm beginning to understand what's needed and what may be possible for me.

Maybe not - none of the A's are bell mouth hoods, and, as a hood 2A is the worst possible of all the A's
If it is also the worst when used as an adapter

Sorry, 2b. It's been a long day. Plus my iPad version means I can't see the images when replying.

☺️

2B is along with 3B the worst of the bell mouths.
A bowl shape is not as good as a flower pot for either an adapter or a hood.
If I get some time I will draw the turbulence diagram.

Sorry, now I'm confused. Didn't you suggest that

Originally Posted by BobL

In terms of the Bell mouthed hoods (B's) the order would be 2B&3B, and then, depending on dust generation source profile, 1& 4B and finally 5B

A turbulence diagram would be helpful if you get time.

Thanks.

Trav

Originally Posted by Trav

Ev - do you feel like some cooking practice this weekend? If you were feeling creative, you could mock up a mould that we could form the pipe over? I will buy a couple of litres of cooking oil and source a suitable pot. If we can get the system working and a suitable design, then we can make up a stack for machines pretty quickly.

Trav

Now, how could this POSSIBLY go wrong, you, Evan and a pot full of boiling oil............ ?

fletty