6" inlet design with a small diameter separator?

[sploo]

LOL, there is a shortage of friendly experts with access to simulation software.

Indeed!

Phil - returning to my original question about a separator with a 6" inlet, are there any best practice guidelines: i.e. for a 6" would it be better to go for a top entry vs side entry? Roughly what diameter of separator would be advisable? What depth/height should I use between the top plate and the baffle? How far below the top plate should the exhaust pipe protrude?

I'm not looking for a detailed drawing (though that would be great), I was more just hoping there might be some info/evidence on the lines of "18" dia, side entry more efficient, 8" gap between plates, 4" protrusion of exhaust pipe etc".

I think the problem is that CFD is time consuming, too, and there are errors involved.  You can find studies comparing CFD predictions to real-world (line wind-tunnel) tests.  Pretty interesting stuff.  Can take months of work to get close, and the CFD results improve because of feedback learned from empirical tests.

I suppose if CFD sims were accurate, outfits like Oneida and ClearVue would be beating one another over the head with it.

Yea, it's not really an accessible subject. Even if there were budget software tools available for the layman I suspect that few would be able to use them well enough to replicate reality.

[phil (admin)]

Side inlet has the least pressure drop, so that is superior.  With side inlet, distance between bottom of lid and top of baffle only has to be large enough to accommodate the height of the inlet.

The tighter the radius, the greater the separation, but also the greater the pressure drop.  I'd go with as large a D as your drum will allow.

[sploo]

Side inlet has the least pressure drop, so that is superior.  With side inlet, distance between bottom of lid and top of baffle only has to be large enough to accommodate the height of the inlet.

The tighter the radius, the greater the separation, but also the greater the pressure drop.  I'd go with as large a D as your drum will allow.

Thanks Phil. How far down from the top should the exhaust pipe protrude (as a percentage of the lid-to-baffle gap)?

I have a fairly standard UK size trash can to hand, but I think it's only about 17" diameter at the opening. I assume that, within reason, once the dust has dropped through the slot in the baffle, what happens below isn't a big deal? I.e. if I made an 18 or 20" separator and had what was effectively a stubby cone underneath the baffle (simply to interface the larger separator to a smaller trash can) that would be OK?

[phil (admin)]

No, the cone wouldn't work, you can't really put a larger separator on top of a small drum.

The outlet tube should expend roughly 1/2 to 1D of the tube, so a 6" tube should extend 3" to 6" into the separator.  Exception is if you're doing a 2x high separator, in which case the inlet should be held to the top and outlet tube should extend past the bottom of the inlet by 1/2 to 1D of the tube.

[sploo]

No, the cone wouldn't work, you can't really put a larger separator on top of a small drum.

The outlet tube should expend roughly 1/2 to 1D of the tube, so a 6" tube should extend 3" to 6" into the separator.  Exception is if you're doing a 2x high separator, in which case the inlet should be held to the top and outlet tube should extend past the bottom of the inlet by 1/2 to 1D of the tube.

Thanks Phil. I'll see if I can get a larger diameter can/drum then; I guess it would also make the build simpler. Whilst I'm struggling for height with a cyclone, that's obviously not an issue with your separator, so is there benefit to going for a 2x high design? I can see it might be useful in that you've got material hugging the outside wall long before it's dropped enough to get near the opening of the outlet tube, so might be beneficial to separation.

[phil (admin)]

No, the cone wouldn't work, you can't really put a larger separator on top of a small drum.

The outlet tube should expend roughly 1/2 to 1D of the tube, so a 6" tube should extend 3" to 6" into the separator.  Exception is if you're doing a 2x high separator, in which case the inlet should be held to the top and outlet tube should extend past the bottom of the inlet by 1/2 to 1D of the tube.

Thanks Phil. I'll see if I can get a larger diameter can/drum then; I guess it would also make the build simpler. Whilst I'm struggling for height with a cyclone, that's obviously not an issue with your separator, so is there benefit to going for a 2x high design? I can see it might be useful in that you've got material hugging the outside wall long before it's dropped enough to get near the opening of the outlet tube, so might be beneficial to separation.

The 2x high design has increase separation a small amount and reduced pressure drop a small amount as well.

[sploo]

The 2x high design has increase separation a small amount and reduced pressure drop a small amount as well.

Thanks.

That gives me an idea (which may well have already been done)... Pentz's cyclones have the inlet protruding into the cylinder of the cyclone, and a spiral ramp to help direct air/dust down. With a 2x height separator it strikes me that you'd have the potential to do the same, as air/dust that makes it all the way around should then be below the level of the inlet (so you're not fouling the path of the air/dust, which you would with a 1x height design with a protruding inlet).

I guess the key is also the width of the slot in the baffle, and the placement of the slot vs the position of the side inlet. Are there any best practices for that? (e.g. 20" separator => 1" wide slot, 2x height => slot opening at 60 degrees after the inlet)

[phil (admin)]

I haven't tried a helical ramp, the Pentz version really seems like a complete knock-off of the Donaldson-Torit and I've studied their approach a little.

I think we like to imagine that the helical baffle "trains" the debris to the outside wall where it is forced lower until the air reverses and the debris drops.

The reality is, though, that having that helical baffle means we've now basically made a winding sort of rectangular duct.  And if you've ever watched sims of airflow through a rectangular duct, you know there is a lot of weird turbulence within them, I'm not really sure how much they force debris to the outside wall.  Maybe the much heavier stuff but the much heavier stuff isn't the problem anyway.

Don't let me dissuade you, by all means give it a shot.

[sploo]

I haven't tried a helical ramp, the Pentz version really seems like a complete knock-off of the Donaldson-Torit and I've studied their approach a little.

I think we like to imagine that the helical baffle "trains" the debris to the outside wall where it is forced lower until the air reverses and the debris drops.

The reality is, though, that having that helical baffle means we've now basically made a winding sort of rectangular duct.  And if you've ever watched sims of airflow through a rectangular duct, you know there is a lot of weird turbulence within them, I'm not really sure how much they force debris to the outside wall.  Maybe the much heavier stuff but the much heavier stuff isn't the problem anyway.

Don't let me dissuade you, by all means give it a shot.

I think the idea was that it, also with the inlet being angled down slightly, encourages the dust to spiral down (i.e. more than just gravity) and so less of it ends up getting pulled into the outlet. Easy to say, time consuming to test, and probably very hard to perfect (assuming it's actually a benefit at all).

Reading through the "Here's the plans" thread, I see that a 1.25" wide slot in the baffle was mentioned for a 20" diameter separator, as well as some thoughts that a smooth surface on the baffle helps. I was wondering if a thin sheet for the baffle would also be beneficial - i.e. perhaps a 1/4" thick sheet of smooth plastic, rather than a 3/4" sheet of MDF or ply?

[BernardNaish]

It has been noted many times that the baffle needs to be as thin as possible. Frequently 1/4" oil tempered hardboard has been used.

[sploo]

It has been noted many times that the baffle needs to be as thin as possible. Frequently 1/4" oil tempered hardboard has been used.

Thanks. The reason I asked was that I've seen a lot of builds using 3/4" sheet material for both the lid and baffle (more so than a thin baffle).

So... I'd have to make a "push" system due to the space I have, and the exhaust from my dust collector is roughly 7" tall x 5" wide (allowing for losses due to the rectangular shape, I understand it's roughly the equivalent of a 6" round pipe). This would form the inlet for the Thien separator. Assuming I can obtain a ~20" diameter barrel, I'd be heading for something on the lines of:

• 20" overall diameter
• 14" tall (2x height of the 7" tall inlet)
• Side inlet mounted just under the top lid (called a "Top hat" design?)
• Inlet maybe protruding into the separator like a Pentz cyclone
• 6" round outlet pipe, protruding ~10.5" down from the underside of the lid (inlet height + 1/2 of inlet height)
• 1/4" thick material for the baffle - preferably something very smooth
• 1.25" wide slot in the baffle

Does that sound sensible, and is there anything else I'm missing (such as the angle/positioning of the baffle slot vs the inlet location)?

I can vent outside so I don't need to run the outlet to filters. Does it matter then how far the outlet tube extends above the top of the lid? Could I just have a hole in the lid that I round over, or would it be better to have some amount of pipe protruding?

[BernardNaish]

sploo,

Perhaps you have not seen this:

http://www.jpthien.com/smf/index.php?topic=563.msg3034#msg3034

You could leave the outlet of your fan rectangular and attach it to the inlet rectangle of the Top Hat as in retired2's design. If I have understood you correctly this means:

     20" overall diameter Thats fine
    14" tall (2x height of the 7" tall inlet) 1 1/2 x would do because gains not huge but may stop hang up of big shavings
    Side inlet mounted just under the top lid (called a "Top hat" design?)  your fan outlet tight to the top
    Inlet maybe protruding into the separator like a Pentz cyclone.  No. It will cause turbulence and degrade separation
    6" round outlet pipe, protruding ~10.5" down from the underside of the lid (inlet height + 1/2 of inlet height)  OK
    1/4" thick material for the baffle - preferably something very smooth               OK
    1.25" wide slot in the baffle         OK

I hope this helps and if so I would love to hear how well it performs......or otherwise.

[sploo]

sploo,

Perhaps you have not seen this:

http://www.jpthien.com/smf/index.php?topic=563.msg3034#msg3034

You could leave the outlet of your fan rectangular and attach it to the inlet rectangle of the Top Hat as in retired2's design. If I have understood you correctly this means:

     20" overall diameter Thats fine
    14" tall (2x height of the 7" tall inlet) 1 1/2 x would do because gains not huge but may stop hang up of big shavings
    Side inlet mounted just under the top lid (called a "Top hat" design?)  your fan outlet tight to the top
    Inlet maybe protruding into the separator like a Pentz cyclone.  No. It will cause turbulence and degrade separation
    6" round outlet pipe, protruding ~10.5" down from the underside of the lid (inlet height + 1/2 of inlet height)  OK
    1/4" thick material for the baffle - preferably something very smooth               OK
    1.25" wide slot in the baffle         OK

I hope this helps and if so I would love to hear how well it performs......or otherwise.

That's a great looking build. I had flicked through it a while back when I was still considering cyclone vs Thien (and top inlet vs side) so other than being impressed I probably hadn't taken in enough of the fine details. I'm not surprised he had problems with plexiglass* though - polycarbonate or PETG would be much preferred.

By "Side inlet mounted just under the top lid" I was either thinking of tight to the top, or angled down a bit. Certainly it'll be easier to build it tight to the top, and as it's already rectangular there's no need for a round-to-rectangle transition, so that makes life a bit easier.

For supporting the thin baffle, I was thinking that you could have a rod protruding from the centre of the underside of the baffle (maybe 12" long), and another rod coming down from the middle of the solid part of the baffle (near the inside of the drum/trashcan) and then join them with a horizontal piece. Whilst it'd get buried in debris when the can gets full, it would keep the support well out of the way of the air flow above the baffle.

Otherwise, I was looking at his support (reply #6 http://www.jpthien.com/smf/index.php?topic=563.msg3034#msg3034, after the text "I had a circular piece of MDF waste, so I decided to make a stiffener plate for the baffle"). Would it make sense to extend that to the side wall (right hand side of the picture), but still stepped back from the baffle slot? I assume that some small amount of disruption to the air flow in that area on the underside of the baffle wouldn't be too detrimental, and may even reduce scrubbing.

* Also known as "acrylic", or "bl**dy h*ll the bl**dy stuff has bl**dy cracked again"

[BernardNaish]

sploo, acryllic was called Plexiglass by one manufacturer.

I believe that retired2 came to the conclusion that the threaded rod he used to support the baffle was probably not needed. I do not like it being in the airstream, only because some long shavings might hang up on it. Your idea of a sort of scaffolding sounds OTT. Glueing a thicker piece of material to the underside while making sure there is at least 2" of clearance will certainly make it certain that it does not vibrate.

I do not think that angling the inlet duct downwards would improve performance very much and it just makes construction more complex.

Let us know how you get on.

[sploo]

sploo, acryllic was called Plexiglass by one manufacturer.

I believe that retired2 came to the conclusion that the threaded rod he used to support the baffle was probably not needed. I do not like it being in the airstream, only because some long shavings might hang up on it. Your idea of a sort of scaffolding sounds OTT. Glueing a thicker piece of material to the underside while making sure there is at least 2" of clearance will certainly make it certain that it does not vibrate.

I do not think that angling the inlet duct downwards would improve performance very much and it just makes construction more complex.

Let us know how you get on.

Yep - plexy is just a brand name, like Vivek or Makrolon. The "glass" bit is indeed appropriate though  ;)

I worried that the supporting rod above the baffle face would catch shavings too; hence my thought of putting it underneath, but a supporting thicker sheet underneath would be much simpler.

I'm currently hunting for a suitably sized drum or trashcan, which is strangely proving to be slightly harder than I anticipated, but I may have a lead.