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Dust extraction air flow

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My dust extraction is by Camvac-4” down to 2-1/2” at the lathe. It’s only used for sanding as the centrifugal force of the shavings means it only catches a small amount. At the lathe end of the semi rigid flexible pipe is a 6” x 3” mouth.

In another life I play with LBCs and carburettored engines. It’s accepted that nicely rounded inlet ports improves breathing.

Would it improve the dust collection if I machined a circular “mouth” with rounded edges? My instinct tells me that trying to cover more of the work with the 6” width is not helping.

Or am I fussing over something that won’t make much difference?!!0460FAAF-5890-42D5-961D-3BC4BEE0A6FF.jpeg
 
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In another life I play with LBCs and carburettored engines. It’s accepted that nicely rounded inlet ports improves breathing.
Ayup!
Would it improve the dust collection if I machined a circular “mouth” with rounded edges? My instinct tells me that trying to cover more of the work with the 6” width is not helping.
Your instinct is correct, a nice bellmouth inlet would work far better than your current one. I turned my own , using a piece of 4" PVC drain cap , (has slats to it , can cut them away for better air flow) which was then later epoxied into a ring of scrap maple 10-1/2" diameter turned a parabolic curvature from the outer edge to the inner edge that blended smoothly into the I.D. of the PVC - I'd have turned a larger diameter than that even but my lathe is too small to allow that.

Or, those bellmouth inlets can also be purchased premade in PVC to fit your dust collection pipe sizes...
 
Joined
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I believe you are absolutely on the right track. Here is one of the articles I found that helped me understand the how's-n-whys, altho you probably already understand the principal because of the carb stuff. http://profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf
I wanted to spin one but I think Daves example will probably be a more expedient route...
I would try to get your 4" hose all the way to the lathe.
 
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I also bought the big gulf for the end of my vacuum hose which is 2.5 inch dia.. Found that a lot of the dust didn't get taken away. I think the gulf dimensions in depth are too great and you lose a lot of the suction. I removed it and made a stand with just a regular adapter with a plumbing flange (toilet) and found it was better. Ended up taking it off and trying just a plain adapter at end of hose which was the 4 inch kind. Worked a lot better I think because it was that much closer to the work as far a maximum air flow. Still trying to find a solution, but I am going to try the bell mouth option and see if its better.
 
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Well, this is an old video, but pretty much the more a piece is enclosed, the more dust you can get, and with this, I get some dust on my hands, but nothing on my glasses or in my nose.

View: https://www.youtube.com/watch?v=AZsVc7qVx7A


I need to do a 'new and improved' version of this so that I can keep it on the lathe when I turn. Probably a wall mount option as well which would probably mean having the hood, but with options for an end panel to put on for keeping all dust out of the air if that makes any sense.... I have seen several variations. The sheet stock plastic can be bought in 5 by 10 foot sheets. 1/4 inch is pretty thick and is more difficult to bend. 1/8 inch might be a bit thin. Maybe even a club buy and that would have more than enough for several dust hoods. Even a 5 gallon plastic bucket would work.

robo hippy
 
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Several years ago Fine Woodworking tested dust collectors, and as an addition, tested the difference a bell mouth made over a rectangular mouth in efficiency. I’m running on memory here, so no numbers to support my remembrance, but they did find a significant difference in the performance between the two. You can certainly turn your own, or just google bell mouth velocity stack. I found a 4” that fits well and increases the air flow and area that it pulls from substantially (at least that’s my perception. Much more efficient flow. In the photo the rectangular intake is sitting on the stand. I do still use it when turning finials or short spindles that it fits…
BEE73088-E7F1-4670-AE0D-A9FB5EE36AC3.jpeg
 
Joined
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Depends on what you have that hood hooked up to. If it's hooked up to a shop dust collector, you are choke it down too far by reducing the opening. You are already choking it down going to 2 1/2. You need speed and volume to keep dust moving. I find shop vacs to be not be the best for real dust collection.
 
Joined
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Thanks to all. My problem is space-the lathe is midi sized on a bench against the wall. The Camvac is behind me (with the exhaust outlet through a hole in the rear wall (it’s a shed!). The 2.5” suction pipe goes under the lathe bed (rests on the surface of the bench)and bends back 180*. It’s held in place by a piece of elastic and two magnets. The suction pipe is “poseable?” so supports its own weight and shape.

Flexible pipe has ridges which impede air flow. So do the bends and the box on the end doesn’t help. I accept that ideally I need the 4” at the point of dust entry. Clearly the bell mouth would help. I think I need to combine all this as hope each small gain will contribute to better extraction.
 
Joined
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This thread motivated me to do something about the rectangular collector on my set up. It has only a 4” outlet, so a reducer was needed. First I glued up a blank from plywood scrap. The dust collector that I use with the lathe is a Harbor Freight unit modified with a Thein separator and a Wynn filter. The unit is 2 hp with a 5” inlet. I used metal 5” HVAC duct to attach to the 5” inlet hose on the DC. What an amazing difference! Turning your own bell mouth is a most worthwhile project.
 

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Joined
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I'm not sure about the bell mouth because my rectangular hood works great. However, I would suggest that, rather than adapting the 4" hose to a 2 1/2" hood, you look for a hood with a 4" connection. Sucking the dust from sanding into the dust collector is all about air movement. While the air most likely moves at a higher rate with the 2 1/2" opening, there would be a higher volume of air movement with a 4" connection. I believe that higher volume of air movement would improve the collection of fine dust from sanding.

I know Glenn Lucas uses round hoods on his machines and I believe in his knowledge and experience,

Last...I run my dust collector 100% of the time while turning. While you don't catch the shavings, you will collect any dust that is generated from turing and there is at least some; probably less if you turn green wood.
 
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I have 4" collector pipe, mostly smooth wall metal and a bit of flex plastic. I just google searched '4" bell mouth velocity stack' and right away I saw items used in this thread, plastic and metal versions for pretty cheap. Great ideas, thanks. And timely since I'm doing a minor bit of shop remodeling.

Edit- I just bought one of these from Amazon for $22, it will be here tomorrow. Surfing around (Amazon and Google), I did see these is smaller sizes, and up to 5" as well.
Autohaux 4" bellmouth velocity adaptor
 
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Another possibility is to cut a small opening in a wall and install a small exhaust fan. I would think, if the area is smally enough a basic bathroom fan would work. I realize that this causes some issues of heat retention if you live in a seasonal climate. It might give you some relief in summer by moving the hot air out if that's a problem.

I don't know what you mean by "box" when describing your collector, but the 4" versus adapting down to 2 1/2" issue is a big one. The effectiveness of the "box" is reduced, probably significantly, by reducing the opening size of the connecction. I'm sure you have looked at other options to maintain the 4" opening besides running the hose under the lathe, like running the 4" hose above, or around the side, etc.

Maybe using PVC pipe instead would increase the air volume some (or round metal heating ducts type ducting), but I think that might only be a marginal improvement.

I'm certainly no expert, but it's likely that the biggest issue is that you're just not pulling enough volume of air to pull the dust into the collector.

So, in the end, you're right, trying anything that will increase the volume of air moved by your collector will improve the situation, at least marginally.

I would, however, run it all the time, not just when sanding, as I said, unless you are turning green wood only. Turning dry wood still generates dust, not just shavings.
 
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Have you tried with 4" right to the lathe? I can't imagine that stepping down to 2 1/2" is helping things.

What are LBC's? I'm thinking opposite of small block Chevy is big block, not large block.
 
Joined
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Have you tried with 4" right to the lathe? I can't imagine that stepping down to 2 1/2" is helping things.

What are LBC's? I'm thinking opposite of small block Chevy is big block, not large block.
LBCs -Little British Cars! In this case MGB Full Service (Girl in MG).jpg


4” to the lathe would be good- not possible but it’s only the last 18” that is 2-1/2” and flexi. Airborne dust filter runs whenever I’m turning.
 
Joined
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Curious why you can do 4" the whole way... But if that is true, can you split to 2x 2.5" inlets and have two bell hoods?
I struggled with dust till I found a Craigslist add for a 1.5hp w/ Oneida cyclone for about the cost of a tank of gas. I run 5" to a big gulp hood and on an mdf backed sanding disc I watch the cyclone of dust get black holed into the hood
 

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I've been biting my tongue here reading this thread.

Bellmouths definitely improve fine dust capture, BUT there is a lot more to efficient DE....

First thing to do is buy and use a particle counter that provides PM2.5μ/m² readings. Then come back and tell us about any improvements in dust levels you have achieved in your workshop with some precise readings. An economy model like this one is all you need...


Someone mentioned Glenn Lucas and, yes, he does know what he is doing.... have a look at the size of the 6" ducting he is using. I wouldn't use anything less at the lathe.

The bellmouth itself is just the last small efficiency tweak, but if everything else is underdone than the bellmouth might as well just whistle.

It doesn't need to be very fancy...

Bell mouth fitting on dust hose.jpg

Apologies for sounding like a grumpy old man, which I am today!
 
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#1 Dust Collection starts at the Dust Collector= both rpm and fan diameter. This should exit with at least a 6 hose (8 if you can). This will be called the trunk line. This can be reduced along the way to 6 or 5.
#2 Pipe preferred is smooth inside PVC or metal. Plan on gates and wyes. Never never branch off the main with a 90 degree angle. Use gradual bends. ie two 45s to get a 90.
#3 The closer to lathe you get the larger duct the more airflow you will have . Remember higher volume thru larger bore is best to carry dust with the idea to suspend dust in AIRFLOW.
#4 you can use corrugated or ribbed when close to destination but reduce the use of ribbed pipe as much as you can. The ribbs disrupt the airflow and therefor slow it down.
#5 use gates to cut the airflow off at stations not in use.
#6 I use my hand to tell is airflow is slowed but there are meters that do this much better than my hand.

Items that will slow AIR FLOW
A.Clogged filters==clean them on a regular basis
B. Clogged ducts. Sometimes just a tap will loosen it.
C. Seal all leaks with duct tape and tight clamps. You can do a flame test or a smoke test for leaks.
D .You can run ducts on the floor but is a trip hazard and IMHO in the ceiling is best with drops for each tool
There must be more to tell but for now this is it.
 
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#1 Dust Collection starts at the Dust Collector= both rpm and fan diameter. This should exit with at least a 6 hose (8 if you can). This will be called the trunk line. This can be reduced along the way to 6 or 5.
#2 Pipe preferred is smooth inside PVC or metal. Plan on gates and wyes. Never never branch off the main with a 90 degree angle. Use gradual bends. ie two 45s to get a 90.
#3 The closer to lathe you get the larger duct the more airflow you will have . Remember higher volume thru larger bore is best to carry dust with the idea to suspend dust in AIRFLOW.
#4 you can use corrugated or ribbed when close to destination but reduce the use of ribbed pipe as much as you can. The ribbs disrupt the airflow and therefor slow it down.
#5 use gates to cut the airflow off at stations not in use.
#6 I use my hand to tell is airflow is slowed but there are meters that do this much better than my hand.

Items that will slow AIR FLOW
A.Clogged filters==clean them on a regular basis
B. Clogged ducts. Sometimes just a tap will loosen it.
C. Seal all leaks with duct tape and tight clamps. You can do a flame test or a smoke test for leaks.
D .You can run ducts on the floor but is a trip hazard and IMHO in the ceiling is best with drops for each tool
There must be more to tell but for now this is it.

All very sound advice there Gerald.

The reason that blast gates need to be fitted as close as possible to the main trunk run is that a branch line that is closed at the workstation has 'passive' air in it between the blast gate and the junction that will build up harmonics (compression/decompression) resulting in turbulence at the junction that reduces airflow. Pipe organs and blowing across an open bottle are other examples of how the harmonics are generated. I have used a pulley arrangement to operate the gates from the workstation, but if you have the financial means auto blast gates are a nice way to go...


There are also YouTube videos on how to make your own.

However, if your airflow is below the minimum required for fine dust extraction (4000 fpm in 6"ducting) you would be better spending your money on improving that than on fancy blast gate.

As a rule of thumb efficient dust extraction requires a fan unit with at least a 15" impeller with a 4 to 5hp motor running through at least 6" ducting and, if not venting directly outside, a filter assembly with adequate surface area. With a lot of tweaking you can get away with a bit less (but not the 6"ducting) but you won't get that with most of the off-the-shelf offerings. Clearview is one that I know of that meets the minimum requirement that is available in Nth America or you can build your own using the Bill Pentz' cyclone design, as I did.
 
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All very sound advice there Gerald.

The reason that blast gates need to be fitted as close as possible to the main trunk run is that a branch line that is closed at the workstation has 'passive' air in it between the blast gate and the junction that will build up harmonics (compression/decompression) resulting in turbulence at the junction that reduces airflow. Pipe organs and blowing across an open bottle are other examples of how the harmonics are generated. I have used a pulley arrangement to operate the gates from the workstation, but if you have the financial means auto blast gates are a nice way to go...


There are also YouTube videos on how to make your own.

However, if your airflow is below the minimum required for fine dust extraction (4000 fpm in 6"ducting) you would be better spending your money on improving that than on fancy blast gate.

As a rule of thumb efficient dust extraction requires a fan unit with at least a 15" impeller with a 4 to 5hp motor running through at least 6" ducting and, if not venting directly outside, a filter assembly with adequate surface area. With a lot of tweaking you can get away with a bit less (but not the 6"ducting) but you won't get that with most of the off-the-shelf offerings. Clearview is one that I know of that meets the minimum requirement that is available in Nth America or you can build your own using the Bill Pentz' cyclone design, as I did.
5hp and 6" is magnificent. I hope there are other assumptions that lead to those numbers. I am only at 1.5hp on 6" running through a 50" tall pleated stack and I lose sandpaper sheets to the cyclone if I set them on the ways too close to the inlet... I think I would fear losing a skew chisel to the inlet at 5hp.
 
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5hp and 6" is magnificent. I hope there are other assumptions that lead to those numbers. I am only at 1.5hp on 6" running through a 50" tall pleated stack and I lose sandpaper sheets to the cyclone if I set them on the ways too close to the inlet... I think I would fear losing a skew chisel to the inlet at 5hp.
I put 1/2” mesh over my 8” input (6” piping to 3hp Oneida cyclone. The mesh has saved me many times from losing sandpaper etc
 

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5hp and 6" is magnificent. I hope there are other assumptions that lead to those numbers.

Michael

My assumptions are all based on Bill Pentz' Research Work, which is on his website, plus a little bit from the fifteen years of my own experience from building and testing cyclones and DE installations. Bill's website isn't an easy read, but if you find any incorrect assumptions there please share those with us.

Here are two excerpts that are relevant from Bill's website...

Q: I've been looking for a blower for ages and still can't find a 1.5 to 2 HP motor blower combination anywhere. What should I do?

A: Why would you want one of these? You really need a 3 hp blower for a dust collector or 5 hp blower for a cyclone.


Q: I want a bigger impeller for my 1.5 HP Jet dust collector blower that I am using to power a Wood Magazine design cyclone. I understand that the upgrade will help quite a bit.

A: Frankly, this is one of those questions where you are wasting my time and yours. I am a retired engineering university instructor, inventor, and pretty senior engineer who dislikes giving out my time or advice to those who are doing foolish things that I know are unhealthy for them and their families. The best you are going to get from this gravity based cyclone is good "chip collection" and the cost to get that could be high. The reason your cyclone works poorly is you used a standard dust collector blower to power your cyclone. A cyclone adds about 4" of resistance which kills the airflow from a standard blower. To compensate you are right, you need a larger diameter impeller and you can use the same motor up to a point. [snip] You can order that bigger impeller from Jet directly. I did so myself for my own early system. The result still left me with a cyclone that separated so poorly I spent more time cleaning my fine filters than doing woodworking. Moreover, the result only moved on average about 700 CFM. All of the expert firms who guarantee customer air quality say we need a full 1000 CFM at our stationary tools to get good fine dust collection. To get that you need a 5 hp motor coupled to a 15" or 16" diameter impeller. I share plans on my blower page on how to build the housings for these sized impellers. You can make your cyclone work with one of these 5 hp blowers, but the only way I would recommend you use this cyclone and cyclones of similar design is to vent them directly outside as they just don't separate well enough to work with fine filters.
 
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My dust collection system is a bit more "Spartan" than most:

1707425167616.png

Old shop vac connected to a plastic downspout extension pipe, connected to 3D cardboard trapezoid collector. Sealed with duct tape at all seams, secured to my peg board a horizontal 1 x 2 with a clamp Testing it during sanding, and the amount of dust is considerably less than when i didnt use it for the first 2 woods bats I made. Total cost: ~$5.00 for new roll of duct tape.
 
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There are 'floor vents' that are similar to your box Greg. The hose comes in from the end. Cardboard works great, and I have made a few, just to test things out...

robo hippy
 
Joined
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There are 'floor vents' that are similar to your box Greg. The hose comes in from the end. Cardboard works great, and I have made a few, just to test things out...

robo hippy

I make everything out of cardboard to test ideas out and those prototypes don't get replaced unless they get refined or until they fall apart... :~}

Collector tray position.jpg Dusthood prototype 1.jpg
 
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Following suggestions I have been looking at rearranging things to have a 4” pipe at the work for sanding (as opposed to 2-1/2”). The Camvac has a 4” inlet, 2 (individually switched) motors each with a 2” outlet. I generally use a single motor and have the outlet piped through the back wall. (Partly for noise and partly so dust exits the building). Does the outlet size affect the inlet suction? Obviously if I use both motors it has greatly improved suction. But I’d need another hole in the wall!

Or put another way am I wasting my time have a 4” inlet with only a 2” outlet?
 
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