Thread: Focused Dust Collection

It doesn’t surprise me. You already probably have so much turbulence inside that pipe that adding the bmh makes little difference. Like I said ina previous post you’ll be better off using a 150mm diameter duct.

I am very confused as earlier in the thread you stated-

"A bell mouth hood (BMH) increases the fine dust collection because they are very efficient increasing the flow by as much as 20% for a 100 mm...."

Nothing was mentioned about turbulence issues but just the increase in air flow. I tried it like you suggested.

I guess the answer is that a BMH sometimes can increase flow if certain other conditions are OK.

If I put a BMH port on a machine, what do I need to do to avoid turbulence issues. I see people attaching flex tube to a BMH port and considering what you have written turbulence may negate any potential improvements. What does one have to do to avoid such turbulence?

Re turbulence re-read post number 6.
re: rest of your post: am tied up ATM will get back to you when I can.

So, how long does my 4" pipe need to be to reduce turbulence so I can see the 20% improvement. Or what would someone need to do to be able to obtain the benefits of a BMH. It appears that it is not good enough to just use the BMH and that you cannot have turbulent flow.

How about some practical guidelines for the ones who just want improvement from the BMH. I guess that I am asking to avoid the technical flow diagrams and just a how do I do this.

I have read a number of technical articles on BMH and understand how they work. I have no doubt that they work. I would like to do an experiment to demonstrate the effect of a BMH to myself and see the improvement.

Maple take a piece of sheet metal or heavy cardboard and hold it across the middle of the opening like so, -----O and sprinkle fine sawdust from above onto the sheet. A flour sifter would be perfect for this if you can find one. Do it with the straight pipe and again with the bell mouth. Compare the difference in the patterns of dust on the sheet. That should give you a visual of what happens in front with naked and bell mouth openings.

Would you post a link to the place you got the hood from? Do they have 6" or 8" ones?

Thanks
Pete

I have no doubt that a BMH changes the flow patterns as I completely understand that. My questions has nothing to do with that. I am asking about the 20% improvement in cfm that was mentioned and how do I achieve that.

I bought the BMH from Parts Express: Speakers, Amplifiers, Audio Parts and Solutions and search for speaker tubes.

[QUOTE=Maple71;2115640]So, how long does my 4" pipe need to be to reduce turbulence so I can see the 20% improvement. Or what would someone need to do to be able to obtain the benefits of a BMH. It appears that it is not good enough to just use the BMH and that you cannot have turbulent flow.

Back in post number 6 I clarified the results from my BMH experiments - note the section in bold

Improvements in flow from using a BMH are ~flow dependent.
The 20% flow improvement is for ~400 cfm in a 100 mm duct.
For lower flow rates the improvements scale proportion to air speed changes, ie at 200 cfm only a 10% improvement is obtained.
I haven't tested above about 400 cfm as this what Aussies typically get using their 50Hz motors (you should get at ~20% more as your mains power operates at 60Hz).
I'm not sure what will happen at higher flows.
Higher flows might improve a big more than 20% but it depends where the turbulence limitations are, if it's inside the ducting a BMH may not make much or any difference.

The flow in 100mm DC ducting systems is almost always turbulent and it doesn't matter how long the duct is, even at 200 CFM in a 100 mm duct there is some turbulent, but above 400 CFM it really gets turbulent. the giveaway is the howling noise from the rushing air. If flex is used the flow will be turbulent down at even lower speeds.

A naked DC duct opening will always have some turbulence in front and just immediately inside the opening, and then more turbulence all the way down the duct. If turbulence inside the duct is much greater than the turbulence at the opening then adding a BMH won't improve flow by much.

The only way to then improve the flow is to use larger ducting.

How about some practical guidelines for the ones who just want improvement from the BMH. I guess that I am asking to avoid the technical flow diagrams and just a how do I do this. I have read a number of technical articles on BMH and understand how they work. I have no doubt that they work. I would like to do an experiment to demonstrate the effect of a BMH to myself and see the improvement.

My detailed posts on air flow testing are all on the forum and I'm not about to retype them here.

RE

If I put a BMH port on a machine, what do I need to do to avoid turbulence issues. I see people attaching flex tube to a BMH port and considering what you have written turbulence may negate any potential improvements. What does one have to do to avoid such turbulence?

The number one thing you can do to reduce turbulence is to use larger ducting. That's the main reason myself and others push the use of a minimum of 150mm ducting on most machinery. If you use multiple machines at the same time then trunk ducting should be commensurately larger eg 200mm.

In terms of improving air flow from cabinets this thread has a lengthy discussion on this topic.
The 3 key parameters are always; powerful DC, large ducting, and opened up machine ports and cabinets.

Flex tube is indeed flow restricting and the floppier the flex the worse it appears to be. Ideally this means not using flex unless it is necessary such as enabling the movement of a machine or dust port, and when it is used it length should be kept to a minimum. A couple of

Very interesting. I thought that the flow in DC ducting was always turbulent.

Thanks for the link Maple but they don't go over 4" and neither do the ones on eBay and Amazon. They start at about 8 times as much plus shipping unless you left off a zero in the $2 cost. No biggy anyway since to get 6"+ sizes I'll have to make them by heating and flaring them like Bob showed in one of his threads.

Pete

Originally Posted by dmorse

Very interesting. I thought that the flow in DC ducting was always turbulent.

It is but the degree of turbulence varies approx inversely with duct diameter.

Originally Posted by QC Inspector

Thanks for the link Maple but they don't go over 4" and neither do the ones on eBay and Amazon. They start at about 8 times as much plus shipping unless you left off a zero in the $2 cost. No biggy anyway since to get 6"+ sizes I'll have to make them by heating and flaring them like Bob showed in one of his threads.

Pete

There are two 4" ports that I bought. Parts 260-403 for $2.89 and 260-411 for $2.99. Shipping is a flat $6.95. I bought them and that is what I paid. I do not know where you got the 8 times as much.

No insult intended if you took it that way. You didn't notice that I live in the Great White North.

One of each fitting comes to $5.88+$19.25shipping=$25.13US which in our currency now is $33.03Can not counting the bank charges of a few more cents on the dollar more to exchange it and maybe tax and brokerage if Canada Customs decide to stop and charge that too. They do give me the shipping options of UPS for $25.08 or USPS Priority for $40.48 if I am in a real hurry. Now the cost would come down spread out over a bunch but I can only think of one machine that I would consider 4" on and that is a drill press. That's why I was hoping they had 6" & 8" to sell.

The 8 times as much came from eBay and Amazon rates for similar products. I never looked at Aliexpress to see what it would cost from China directly.

We get dinged hard for living here but the Aussies understand as they get hit harder for items mailed from the US.

It was an option worth exploring though.
Pete

I hope you noticed where I live and I quoted prices for me.

Okay you win.

Pete