6" inlet design with a small diameter separator?

[sploo]

I've got a Fox F50-843 dust extractor: impeller based, 3hp, claimed around 3900 cubic meters per hour (2295 CFM).

I've been using just the impeller/motor unit and expelling air and dust into a trash can inside a small enclosure outside my workshop. It works pretty well, but occasionally "puffs" some dust out of a vent that's near the top of enclosure.

I'm wondering about adding a Thien separator to try to reduce the amount of dust expelled in the exit air. The restrictions/limitations I'm working around are:

• A rectangular inlet to the separator (as the exhaust on the extractor is rectangular, with a cross sectional area similar to a 6" diameter pipe)
• The inlet would probably have to come in from the top, rather than the side
• A tight space into which to fit a can/drum/bin - probably only 40cm (16") diameter

I'm assuming that having a rectangular inlet (with a suitable elbow) wouldn't be a major problem, and that I should go with an appropriately wide circular pipe for the exhaust. Would having such a small diameter separator for a relatively large inlet cripple the air flow?

I may be able to work out a design with the rectangular inlet coming in from the side if that'd make a significant difference to the efficiency. I've seen some good top hat designs, and that looks ideal for a rectangular side inlet - though it would be a push rather than pull system (i.e. the extractor would push air and dust into the separator). Would such a design (top hat + push + 6" rectangular inlet + 16" diameter separator) be realistic?

[tomservo]

Well, it's been a while with no replies but I'll give it a go.. I think 16" is too small for that much flow - if you run the math, with 16" OAD, that's 15.5" with a 1/4" wall, minus 6" for the outlet, probably more like 6.5" to account for whatever it's made of, and you only have about 9" left, so that's 4.5"of width for the incoming air. I'd bet the pressure loss will be huge, but with 3HP behind it, you may do all right.

[dabullseye]

i would not want a push system cuz stuff is still hitting the impeller. the reason i went to the tophat was i once sucked up a baby food bottle full of sanding sealer mix. it shattered when it hit the impeller. since then a few scotchbrite pads and a few pieces of sandpaper have been sucked up but they stayed in my tophat or fell into trash can below tophat. now imagine if a chunk of wood gets sucked up and gets wedged in impeller housing.

[sploo]

Well, it's been a while with no replies but I'll give it a go.. I think 16" is too small for that much flow - if you run the math, with 16" OAD, that's 15.5" with a 1/4" wall, minus 6" for the outlet, probably more like 6.5" to account for whatever it's made of, and you only have about 9" left, so that's 4.5"of width for the incoming air. I'd bet the pressure loss will be huge, but with 3HP behind it, you may do all right.

Thanks. I'd guessed it might not work well.

The problem is that if I go to the effort of ripping out some existing infrastructure to get more space I would then have a chance of fitting in a Pentz cyclone design. I'm assuming that might be worth the difference in effort.

i would not want a push system cuz stuff is still hitting the impeller. the reason i went to the tophat was i once sucked up a baby food bottle full of sanding sealer mix. it shattered when it hit the impeller. since then a few scotchbrite pads and a few pieces of sandpaper have been sucked up but they stayed in my tophat or fell into trash can below tophat. now imagine if a chunk of wood gets sucked up and gets wedged in impeller housing.

The motor+impeller unit I'm using is from a dust extractor that pushed air+dust into standard bags/filters so has a "material handling" impeller (i.e. one that can take a beating). It does actually have a guard/grill at the front of the housing to catch anything substantial. I.e. I'm fairly confident it'd be OK used as a push system as that's kinda what I'm doing now (just with no separator at the end).

[tomservo]

I'd say that if you have room for a cyclone, just do that, the pressure loss is much lower. Have you seen the newer oneida cyclones? they look like great value to me.

In other news, my separator is done enough for some testing.. I made the fiberglass inlet and the spray foam attachment is drying now. I'm hoping to get a felt bag for the outlet from kijiji tomorrow.

[phil (admin)]

I'd say that if you have room for a cyclone, just do that, the pressure loss is much lower. Have you seen the newer oneida cyclones? they look like great value to me.

In other news, my separator is done enough for some testing.. I made the fiberglass inlet and the spray foam attachment is drying now. I'm hoping to get a felt bag for the outlet from kijiji tomorrow.

Do we know pressure loss is much less?

The pressure loss should be somewhat proportional to the D of the separator.  Some of those newer cyclones are pretty tiny.

Mind you, I'm not saying you're wrong and I'm not trying to limit discussion, this is a free and wide-open forum.  And someone may have posted evidence that the pressure drop is lower somewhere (even here).

Just asking.

[sploo]

Do we know pressure loss is much less?

The pressure loss should be somewhat proportional to the D of the separator.  Some of those newer cyclones are pretty tiny.

Mind you, I'm not saying you're wrong and I'm not trying to limit discussion, this is a free and wide-open forum.  And someone may have posted evidence that the pressure drop is lower somewhere (even here).

Just asking.

Based on the info at Bill Pentz's site it looks at though smaller diameter cyclones do suffer from significant pressure loss; quoting from his site "Instead of building my recommended 18" diameter cyclone, those with 3 hp motors should make 20" diameter cyclones and those with 2 and even 1.5 hp motors should use 22" diameter cyclones"

He does note that a 6" cyclone to his design will work well, but with a shop vac, which of course is a totally different proposition to an impeller based extractor in terms of suction (lots more) and air flow (a lot less).

With a 3hp motor it looks like I'd be pushing my luck with a 16" cyclone too, so I may well need to go for the harder route of making more space.

I would be interested in pressure loss data though, as I'm less worried about separation efficiency (venting outside, so mostly interested in just separating the visible dust - some smaller particles making it through don't worry me). So, I'd prefer low pressure loss and average separation to high pressure loss and really good separation.

[phil (admin)]

I would be interested in pressure loss data though, as I'm less worried about separation efficiency (venting outside, so mostly interested in just separating the visible dust - some smaller particles making it through don't worry me). So, I'd prefer low pressure loss and average separation to high pressure loss and really good separation.

I'm including a pic of my "inline separator."  It has about the lowest pressure drop you can achieve.

When a vacuum is pulled on a pipe, the air moves fastest through the center, slower at the edges.  This means the pipe itself is acting like a separator.  I can watch this with my clear pipe network, I see all the debris swirling around the outside and trying to stay out of the way of the faster moving air in the center of the pipe.

So in the pic I'm attaching, the gadget's left side is attached to my (or your) ductwork.  On the right side is a smaller pipe to which suction is attached (I use a shop vac but this can be scaled-up to larger blowers).  This extends through a sealed cap and past the end of the Wye.  This smaller suction pipe is the vortex finder.

So now what happens is that the suction puts the dust in the pipe into a spin.  Once it arrives at the vortex finder, it can't reverse direction fast enough and passes the finder and continues into the Wye, where it is directed down into a discharge.

One downside to this method is that you have to size it so the largest expected bit of debris can fit between the inside, and outside pipes.

One other downside would be lower separation rates.  It works way better than anyone to which I've demonstrated it would believe, though.

There are numerous advantages, though.  Takes up almost no space and limited ceiling height isn't a problem.  Very little pressure drop.  Not too difficult to build even in larger sizes.

I need more time to experiment with it.

[sploo]

I'm including a pic of my "inline separator."  It has about the lowest pressure drop you can achieve.

Very clever. For a low pressure system such as an impeller based extractor, would the smaller pipe not kill the airflow though? I understand that low pressure air behaves more like water - i.e. incompressible - so forcing it through a smaller gap doesn't really work (unlike with a high pressure shop vac, which works fine).

[tomservo]

I need to clean up my garage so I can make some proper sawdust, all I have right now is an enormous pile of hardboard sawdust made with a 120 tooth melamine blade. It's all I have right now and I hardly think it's a fair test to put a pile of super fines in front of it, but the result was quite a big puff of dust out the blower, no idea how much got separated but most of it, certainly.   I also don't have a hose for the thing either.

[phil (admin)]

I'm including a pic of my "inline separator."  It has about the lowest pressure drop you can achieve.

Very clever. For a low pressure system such as an impeller based extractor, would the smaller pipe not kill the airflow though? I understand that low pressure air behaves more like water - i.e. incompressible - so forcing it through a smaller gap doesn't really work (unlike with a high pressure shop vac, which works fine).

You're correct.  The key would be a vortex finder that matches the mains, and an outer pipe that is upsized for a length.  So if your mains are 6", the outer pipe would be 10" for at least some length.  I don't know what that length is, you obviously don't want settling before the wye.

Obviously need a lot more testing.

[sploo]

I need to clean up my garage so I can make some proper sawdust, all I have right now is an enormous pile of hardboard sawdust made with a 120 tooth melamine blade. It's all I have right now and I hardly think it's a fair test to put a pile of super fines in front of it, but the result was quite a big puff of dust out the blower, no idea how much got separated but most of it, certainly.   I also don't have a hose for the thing either.

Bear in mind that, with pretty much any kind of separator (cyclone, Thien baffle etc.) if you bury a hose into a big pile of dust you're likely to overload the system and get some dust out of it. That seems to be the case of pretty much any one I've seen tested anyway. However, in normal conditions (e.g. a stream of dust from cutting or routing) they tend to work pretty well.

The killer is sometimes planer shavings, as the volume of material they that can produce can cause issues.

You're correct.  The key would be a vortex finder that matches the mains, and an outer pipe that is upsized for a length.  So if your mains are 6", the outer pipe would be 10" for at least some length.  I don't know what that length is, you obviously don't want settling before the wye.

Obviously need a lot more testing.

My ducting is rectangular, but now I'm just being an awkward troll  ;)

It sounds to me like you've got some really good ideas but are doing a lot of empirical testing. Would it be worth trying to find a friendly fluid flow expert with access to the relevant simulation software? It occurs to me that you could probably move things forward quite quickly with visualisation of the actual flows. I'm making significant assumptions that you don't already have that, given I don't know your background, I'll admit.

[phil (admin)]

It sounds to me like you've got some really good ideas but are doing a lot of empirical testing. Would it be worth trying to find a friendly fluid flow expert with access to the relevant simulation software? It occurs to me that you could probably move things forward quite quickly with visualisation of the actual flows. I'm making significant assumptions that you don't already have that, given I don't know your background, I'll admit.

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

[BernardNaish]

One such did some work that he presented here. When it was pointed out that the assumptions he had made did not match real life he just disappeared!

This was a pity as it looked as though that would have been a fabulous resource, saving a hugh amount of time and perhaps the only way to optimise these designs.

[phil (admin)]

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.