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Thread: Bell mouth hoods
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30th May 2019, 11:20 PM #1SENIOR MEMBER
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Bell mouth hoods
https://youtu.be/DX2HBYQp4YM
Interesting demo of the effectiveness of the bmh.
Russ
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30th May 2019 11:20 PM # ADSGoogle Adsense Advertisement
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30th May 2019, 11:35 PM #2.
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Thank Russ.
Apart from the significant pressure loss descriptions I can't see much happening in the video itself.
I am surprised to see they did not use a 2" radius for an 8" duct as it supposedly would give a coefficient of 0.01 - 0.02.
Maybe it's too hard to make as it requires spreading the sheet metal out a long way.
Fortunately that is not a problem with PVC BMHs.
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31st May 2019, 03:33 PM #3
Here's something interesting!
I have a 150mm bendy hose and a 150mm BMH.
It has a plumbing collar that slips over both. As it has a small divider section half way inside for the PVC pipes to have a positive stop, I know that half the pipe and half the BMH are in.
What is happening, plug on a BMH is the BMH is popping off! Its almost like the suction is so great inside the rim of the BMH that some sort of negative feedback/vortex is pushing it away!
How cool is that!
Without using a huge elastic band to hold it in place.... off it pops!
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31st May 2019, 04:47 PM #4.
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Ive seen that effect on other hoods but not as much as with BMHs. I usually just add a Tek screw to hold it in place.
This shows the pressure in front of the BMH is significantly lower than the pressure behind it. However, the rim of the BMH restricts the high P reaching "out back", reaching the low P "out front " so it tried to pull the whole BMH off the duct.
Ive seen that effect on other hoods but not as much as with BMHs. I usually just add a Tek screw to hold it in place.
No other hood generates as much pressure drop out front compared to the back which is why they work so well.
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31st May 2019, 05:02 PM #5
An excellent description of what is occurring.
I went and checked it with some sawdust. It is as you say - the negative pressure in front is very intense.
I'm impressed!
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31st May 2019, 06:42 PM #6.
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What is interesting is the projection of the negative pressure field out in front of the hood.
Compared to a simple open duct with a nominal air speed X m/s at a distanced Y m in front of the hood, a BMH will have 2X m/s at the same distance (ie Ym) in front of the hood. This makes it ideal for lathes where you don't want the hood interfering with the rotating workpiece.
Maybe this is a time to reinsert one of my fave vids.
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16th October 2019, 01:18 AM #7New Member
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I want to turn a BMH for 6" pipe on the lathe. I would like to cut out a template that I could use to compare the shape to.
What is the ideal BMH shape?
Steve
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16th October 2019, 03:43 AM #8GOLD MEMBER
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Steve the first sticky at the top of the section has the information you want in post #5. It is also scattered throughout the forum as the topic comes up every now and then.
DRAFT: FAQ - Dust Extraction (Practical Aspects)
If you would like to buy one that is pretty close a speaker port does the trick.
Precision Port PSP-6OFN Flare for 6" Port Tube
Pete
And BobL shows how to make them from PVC in #37 in this jam packed thread.
Improving machine cabinet dust ports
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16th October 2019, 05:04 AM #9New Member
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I found this flare from a scholarly paper that is supposed to be optimized, can't remember the source now, I think it is eliptical. I scaled it up to 6" pipe diameter. It is much larger than the simple 1/4 dia. BMH
Untitled.png
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16th October 2019, 05:07 AM #10New Member
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I don't know if it would actually make a substantial difference from the 1/4 dia. though
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16th October 2019, 08:51 AM #11
The difference is UNBELIEVABLE.
Absolutely worth doing.
BobL has gone over this with his usual scientific rigour.
The optimal radius is simple - 50% of the diameter. If it is a 4 inch (100mm) pipe the radius of the flare is 2 inches (50mm).
There is, literally, no better shape for inducting air.
edit - this is the Link QC was referring to: Improving machine cabinet dust ports
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16th October 2019, 09:17 AM #12.
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There have been quite a few investigations into the optimum shape which in the papers I have read is supposed to be a long trombone shape but that is impractical to use in a workshop or in the situations such as in the backs of speaker cabinets..
Other shapes like the 1/4R come quite close (within 1%) to optimum.
None of the hoods I make are perfect but these are the ones I have tested which shows that any flare is better than no flare so I no longer fuss too much about getting them exactly 1/4R.
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