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  1. #1
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    Default cheap bellmouths

    I think I found a source of cheap bellmouths:

    Orange PVC electrical & comms sweep bends of all sizes and types

    I bought a 125mm and a 150mm for $40 delivered to Melbourne. They are made from HD conduit which is very thick and strong. If connecting them to flex you need to order the ones with a socket end. The flex will slide right in. I ordered socket ends, but they sent me the wrong ones. They also have them with a straight end. These are called a 'spigot' on the website.

    I found that the 125mm bellmouth can be connected to 100mm flexi by using a 100-125 adaptor (see pic). The adaptors are $10 at Carbatec.

    I had a play with both bellmouths using an incense stick. TBH I didn't see much, if any, difference in the flow patterns with and without the bellmouth. Even when I held the incense well behind the open end of the flexi the airflow would pull the smoke forward and into the open end. I was quite surprised at how far this effect occurred. So I'm open minded about how effective these bellmouths are. They certainly look impressive though! Some pics for your enjoyment.
    Attached Images Attached Images

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  3. #2
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    Default

    And the 150mm bellmouth on the 125mm flexi. As you can see, the edges of the bellmouth are a bit rough in places. I cleaned them up later with a file and sandpaper.
    Attached Images Attached Images

  4. #3
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    FWIW a 150 mm BMH should be ~225 mm across the widest part.
    The other issue with those BMHs is that they don't have a truly smooth transition from the curved to straight part of the duct.
    Check out the lip indicated by the blue arrows.
    OrangePVCBMH.JPG

    See how much smoother the curve is in this one
    IMG_1880.jpg
    If I am careful I can stretch the ones I make to ~230 mm but beyond that its a crap shoot and some start to split.

    However these things are second order and like I think I have said before, I would not expect much improvement in flow using a 6" BMH when using a 5" flexy and a small DC.

    BUT . . . . . .
    The biggest restriction to flow in your setup is likely to be the turbulence caused by the step step adapter between the flex and the back of the BMH. I'd go so far as to suggest that turbulence is likely to cancel out any benefit of a BMH and possibly even make the flow less than a naked duct opening.

    To get around this I would require using an adapter with a smooth conical transition. Ideally this needs to be about 5 duct diameters long (5 x 125 = 625 mm) which then turns the BMH into a trombone shape which is not always possible poking out from a WW machine. Sorry I should have thought of that before sending you off to find a 150 mm BMH to fit to the 5" flex. Of course the transition does not need to be exactly 625 mm long, it can be longer, or shorter, but at some point the shorter it isit will effect the BMH performance. I maybe start by making one say about 300 mm long or maybe consider using the 125 mm hood.

    When BMHs are working correctly a more definitive test using an incense stick would be to move the incense stick away from directly in front of the hood. At a certain distance from the hood or naked duct the smoke will rise up faster than it is drawn away by the hood. A working BMH should attract smoke from further away than a naked duct.

  5. #4
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    Quote Originally Posted by BobL View Post
    FWIW a 150 mm BMH should be ~225 mm across the widest part.
    OK, but 210mm is pretty close.

    Quote Originally Posted by BobL View Post
    The other issue with those BMHs is that they don't have a truly smooth transition from the curved to straight part of the duct.
    Agreed, they aren't perfectly smooth, but they are a lot smoother than a naked pipe end.

    Quote Originally Posted by BobL View Post
    ...I would not expect much improvement in flow using a 6" BMH when using a 5" flexy and a small DC.
    What's your reasoning behind that belief? If a BMH improves airflow into a naked pipe end I would expect that to be independent of flow rate. Are you saying the improvement is dependent on flow rate? If you have less than ideal flow in your system, shouldn't you be chasing any improvement you can get?

    Quote Originally Posted by BobL View Post
    The biggest restriction to flow in your setup is likely to be the turbulence caused by the step step adapter between the flex and the back of the BMH. I'd go so far as to suggest that turbulence is likely to cancel out any benefit of a BMH and possibly even make the flow less than a naked duct opening.
    Just to clarify, the pictures comparing the smoke are for the 150mm bellmouth on the 125 flexi vs the naked 125mm flexi. There was no adaptor in that setup. Just a bit of foam tape to take up the small difference in the flexi OD and bellmouth ID.

    Quote Originally Posted by BobL View Post
    When BMHs are working correctly a more definitive test using an incense stick would be to move the incense stick away from directly in front of the hood. At a certain distance from the hood or naked duct the smoke will rise up faster than it is drawn away by the hood. A working BMH should attract smoke from further away than a naked duct.
    OK, I'll try that test and see what I get. Have you performed this test yourself to compare your BMHs and the naked pipe?

  6. #5
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    Quote Originally Posted by jack620 View Post
    OK, but 210mm is pretty close.
    Yes its close and as it turns out the outer most edge has the least effect but I was just pointing out the simplest ideal shape for WW machinery is (ideal is a trumpet shoe)

    Agreed, they aren't perfectly smooth, but they are a lot smoother than a naked pipe end.
    Sure but if you're good money paying for them I would expect to get something closer.

    What's your reasoning behind that belief? If a BMH improves airflow into a naked pipe end I would expect that to be independent of flow rate. Are you saying the improvement is dependent on flow rate? If you have less than ideal flow in your system, shouldn't you be chasing any improvement you can get?
    I agree any improvement is better than nothing but sometimes its really hard to see any improvement
    I've done quite a few measurements on BMHs and found the improvement in flow only really starts to shows when the flow is close to maximum for that size duct

    The lower the pressure (between the inside and outside of the duct) the lower the turbulence is at the opening so a BMH makes little difference to the flow

    As the pressure drop increases the greater the flow but this then generates more turbulence in front of and immediately inside the duct opening which restricts some of that increase in flow. This will get worse and worse (as witnessed by the noise made) as the pressure difference increases with eventually not much improvement in flow with added pressure diff. Adding a BMH helps reduce turbulence so that increasing pressure allows more flow.

    At low flow it makes very little or no difference - if the naked duct is screaming like a banshee a BMH will make a significant difference.

    Just to clarify, the pictures comparing the smoke are for the 150mm bellmouth on the 125 flexi vs the naked 125mm flexi. There was no adaptor in that setup. Just a bit of foam tape to take up the small difference in the flexi OD and bellmouth ID.
    it doesn't matter, if there is any sort of a step, even a few mm then this will produce some back up effect and spill air off that step into the incoming air in the middle of the duct and cause more turbulence.

    OK, I'll try that test and see what I get. Have you performed this test yourself to compare your BMHs and the naked pipe?
    Better still I have done it with an air flow meter.

    In the graph below the two sets of markers to compare are the two 154 mm (Blue diamond and red square) done at nominal 900CFM, and the 102mm (green diamond and purple circle) performed at a nominal 425 CFM
    The speeds are directly measure out in front of and way from the openings
    Note the air speeds rae similar but of course the opening of the 154mm duct is much larger so more air is flowing into this diameter duct.

    For the 102mm ducting, the BM consistently has higher air speed all the way out from the end of the opening.
    For the 154 mm close to the opening, the naked duct has the higher air speed but further out the BMH has the higher air speed.
    If I were to crank up the fan the 154mm profile would become similar to the 102mm result.

    This demonstrates the BMH draws more air from further out in front of the hood.
    This makes sense because the naked hood draws more air from the behind the hood. as there is no rim on the hood


    Screen Shot 2020-02-21 at 6.01.14 pm.png

  7. #6
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    There are plenty of posts in the forum discussing the recommended profile but to summarize, the radius of curvature of the lip should be 1/2 the radius of the duct.

    Since my son upgraded his 3D printer I have been threatening to get him to print one. Just remember it needs a smooth finish. While you are at it maybe print a 6-5” adapter? Maybe 200mm will be enough. The level inverts are even shorter but the usually are used the other way.
    Attached Images Attached Images

  8. #7
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    I’m really sorry for editing over your Previous post. I’m doing this on my very small mobile and it’s too easy to hit the wrong buttons. I get heaps of weird auto correct spelling probs and am going bling trying to see what’s going on.

  9. #8
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    The energy loss at an entrance actually happens just inside the entrance where a vena contracta forms. A good diagram from this:

    cheap bellmouths-entrance_vena_contracta-jpg

    Air has momentum. When it hits a sharp corner it overshoots and forms its own constriction, just like you would do in your car trying to take a sharp corner too fast. The BMH you show still has a sharp corner, just not quite 90 degrees. There will still be overshoot and a pressure robbing vena contracta.

    Entrance_vena_contracta.jpg
    Dave

  10. #9
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    Some real nice pics in that link Dave will have to take a closer look when I get back to the laptop. Currently working of this tiny phone so can’t see much.

  11. #10
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    Quote Originally Posted by qwertyu View Post
    There are plenty of posts in the forum discussing the recommended profile but to summarize, the radius of curvature of the lip should be 1/2 the radius of the duct.

    Since my son upgraded his 3D printer I have been threatening to get him to print one. Just remember it needs a smooth finish. While you are at it maybe print a 6-5” adapter? Maybe 200mm will be enough. The level inverts are even shorter but the usually are used the other way.

    Just so I understand correctly - in my diagram that would mean the circle is 37.5mm for a 150mm duct (duct radius is 75mm)

    Isnt it better to just use the BMH directly onto 6" pipe rather then print a 6-5" adapter?

    In terms of smoothness, I think the easiest way to get a smooth wall from a printed object is to paint on some epoxy.

  12. #11
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    Quote Originally Posted by qwertyu View Post
    Just so I understand correctly - in my diagram that would mean the circle is 37.5mm for a 150mm duct (duct radius is 75mm)
    yep
    Isnt it better to just use the BMH directly onto 6" pipe rather then print a 6-5" adapter?
    sure but the op has 5” ducting/flex
    In terms of smoothness, I think the easiest way to get a smooth wall from a printed object is to paint on some epoxy.
    thanks

  13. #12
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    So I went and performed the smoke test as per Bob's recommendation and I'm happy to report a noticeable difference in the distance the smoke is pulled from the intakes. My setup was crude. I just mounted the 5" flexi so it was protruding horizontally beyond the top of my table saw as I wanted to avoid interference from the floor. I held the incense stick so the smoking bit was at centre height of the duct. Then I drew the incense away horizontally until the smoke was no longer being pulled into the duct. A tape measure in the other hand gave me the distance. I repeated the test a few times and found the naked duct would stop pulling the smoke at about 650mm. With the 6" bellmouth attached that increased to about 750mm. So around 100mm of improvement. Not bad.

    The people who make the bellmouths are sending me some slip joints so I'll be able to perform the test with the 5" bellmouth on the 5" duct.

    In relation to the less than perfect curve- it's worth noting these bellmouths are designed to be cast into concrete pits to protect cable being pulled into the conduit. They aren't designed for improving airflow. There's enough thickness in the pipe that you could probably mount them in a lathe and smooth out the offending bump. I doubt the improvement would be worth the effort though.

  14. #13
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    Quote Originally Posted by jack620 View Post
    So I went and performed the smoke test as per Bob's recommendation and I'm happy to report a noticeable difference in the distance the smoke is pulled from the intakes. My setup was crude. I just mounted the 5" flexi so it was protruding horizontally beyond the top of my table saw as I wanted to avoid interference from the floor.
    The keeping it away from the floor or any other obstruction is good point.
    For anyone else doing this test, it the DC vents outside the shed watch out for any preferred air movement cause by air moving towards the duct opening from any nearby openings in the wall of the shed (ie door and window) which could help drag the smoke from further away. This preferential flow is caused by the inside of the shed becoming negatively pressurised by the DC. If your DC vents outside it would be better to do this test in a quiet corner of the shed.

    Provided those two things are attended to this test could be come a basic air flow performance test provided testers use the same size duct and opening profiles.
    Eg naked PVC ducts, naked Flexies, BMH.
    However it won't allow flow performance to be checked between using and not using a BMH (see below) but providing 'Jack" uses the exact same duct opening as "Jill", it will enable Jill to compare her flow with Jack's.

    I held the incense stick so the smoking bit was at centre height of the duct. Then I drew the incense away horizontally until the smoke was no longer being pulled into the duct. A tape measure in the other hand gave me the distance. I repeated the test a few times and found the naked duct would stop pulling the smoke at about 650mm. With the 6" bellmouth attached that increased to about 750mm. So around 100mm of improvement. Not bad.
    Just heads up that this test may not mean much in terms of improvement in actual air flow. What that test shows is that more air is being collected from the front of the duct opening. It could even be less flow but just more from the front. Only an air flow test using an air flow meter inside a test duct downstream from the duct opening will truly measure the flow.

    Here is a video of the same test on a solder fume extractor with a BMH.

    This fan only has a 250W motor and it's air flow is restricted by a very small robot vac HEPA filter but it can still draw fumes from about 250 mm way.
    Details and a WIP of this extractor are here
    Solder fume extraction.

  15. #14
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    Good thread.

    I was wondering if these bellmouths could be further refined by heating and forcing them onto a properly shaped mandrel like Bob does when making one from scratch. They have already got a head start.

    Pete

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    Quote Originally Posted by QC Inspector View Post
    Good thread.

    I was wondering if these bellmouths could be further refined by heating and forcing them onto a properly shaped mandrel like Bob does when making one from scratch. They have already got a head start.

    Pete
    I think they may be too thick to do this unless perhaps a press is used. With the standard thickness PVC ducting hoods the tail stock on my lathe is close to its limit in providing the required pressure. There is a temptation to crank up the hot air gun temp but the the risk of developing rucks is much greater.

    The problem is getting an even enough heat which is why turning them on a lathe while heating is useful. The other way is heating them in oil (water does not get hot enough) - I've tried oil, it is decidedly messy and dangerous which is why I went for the lathe method.

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