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Thread: Serial Cyclones, is this just a daft idea?

  1. #1

    Serial Cyclones, is this just a daft idea?

    Dear Creekers,

    I just had the crazy idea of connecting multiple cyclones in a serial chain.
    It would be essentially something like Dirt->DD->DD->Vac.

    Unlike the Dyson (parallel cyclones all at once), I'm not sure if a second stage would help with capturing more fine dust. Has anyone tried this?

    Would this just be a stupid idea?

    -Matt

  2. #2
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    In a refinery, they use rough cut cyclones ahead of finish cyclones in series inside a fluid catalytic converter unit. The rough cut cyclones were about twice the size of the finish cyclones, and not the same size. That was done to drop the majority of the catalyst with less velocity and damage to the catalyst, and the finish then had higher velocity and greater separation. Something like that would work to drop out the larger dust/chip particles and then a smaller, and therefore higher velocity, drop out the finer dust.

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    It's an interesting question, and need a lot more knowledge of the parameters determining cyclone performance than I have. Suspect Thomas is more or less correct. Two the same in series seem likely to produce only a small improvement - if the first wasn't 100% effective at retaining say fine dust approaching it's limit, the second might catch some more. If against that the first worked very effectively right down to the limit of its capability then there would be very little left for the second to do.

    Other factors are that (a) both would have to be able to handle the total airflow which might rule out chasing efficiencies through scale effects the way Dydson probably do (i.e. could make it difficult to use two very different cyclone layouts), and (b) would double the pressure drop which could easily be a problem on a low pressure system like those we use...

    Against that it's not a daft idea, it could well be that there are ways to use serial cyclones to improve separation. It's possibly not as simple as just bunging any two cyclones one after the other though...

    ian

    PS a quick dig suggests that it's done on occasion, including when the incoming airfow is heavily loaded with solids. This book deals with the theory but it's pretty heavy duty fluids: http://tiny.cc/43jlcw
    Last edited by ian maybury; 04-11-2012 at 6:48 AM.

  4. #4
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    Both Thomas and Ian are correct. You get better collection efficiency at the expense of pressure drop. The pressure drop from each cylone in series will add. If the cyclones are in parrallel, the pressure drop is equal to the pressure drop across one cyclone (assuming they are the same).

    The way the improved efficiency works for a series installation, assuming both cyclones are the same, is that the fractional efficiency of each cyclone is the same. For example, assume the DD is 95% efficient on 10 micron. The first cyclone will get 95% of the 10 micron particles, and pass 5% through to the second cyclone. The second cyclone will get 95% of the 5%, this gives you an overall collection efficiency of 99.75% on 10 micron for two series cyclones. If the cyclones are different, then the fractional efficiency of each cyclone has to be taken into account.

    These are typically used when you are trying to recover valuable product, such as the refinery and petrochem example Thomas mentioned. Since you are using an afterfilter (shop-vac), the improved efficiency may not be worth the cost, extra space required, and extra pressure drop. It is not a bad idea, it is a strategy employed in many industrial processes.

    Mike

  5. #5




    Last edited by Bob Faris; 04-11-2012 at 2:12 PM.

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    Don't keep us in suspense. How well does it perform/what have you noticed Bob? Does the second cyclone actually drop anything much out?

    ian

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    Don't keep us in suspense. How well does it perform/what have you noticed Bob? Does the second cyclone actually drop anything much out?
    Yes, Inquiring minds want to know!!

    It may be the camera angle, but the second cyclone is more transparent, maybe cleaner??

    Nice setup Bob.

    Mike
    Last edited by Michael W. Clark; 04-11-2012 at 5:20 PM.

  8. #8
    Exactly! Wait, it's on?!

  9. #9
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    One of the major points in the refinery cyclones was the system operated at 1250F + and the catalyst was very abrasive in addition to expensive.

    Bob - I notice that you have the standard hose coming off the output of the Oneida Dust Deputys. Oneida has the odd size elbow available on their unit for Festool that fits the Dust Deputy and takes a lot of the strain on the hose out. I don't know why they don't offer it an an option for the lower price unit, but they do sell. I bought 6 because of the shippping and then sold the extras to some of my local friends.

  10. #10
    Quote Originally Posted by ian maybury View Post
    Don't keep us in suspense. How well does it perform/what have you noticed Bob? Does the second cyclone actually drop anything much out?

    ian
    Sorry guys, it's not mine. I should have given proper credit in my reply. The picture is from a thread in another forum I read occasionally. Here's a link to the third page of the thread. The picture and a couple of comments about how well the setup operates are near the bottom.

    http://www.buildyourtools.com/phpBB3...t=703&start=20
    Last edited by Bob Faris; 04-11-2012 at 11:27 PM.

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    Thanks Bob, it's always good to get info from whatever source. He says there's a little very fine dust that ends up in the second cyclone. Which is a measure I guess of how much carry over you get past the first cyclone - these things are not 100% efficient. There's probably of course some even finer dust that cyclones can't stop at all.

    We've talked before of how little there is by way of published data on cyclone retention performance - especially since the fine fraction of the stuff that's a health risk comprises only a very small % by weight or volume of the total dust produced in most operations. This sort of discussion really focuses the issue for me in that it begs some questions as to (a) how much (at best) gets carried over, (b) by how is this amount influenced by the installation specifics (CFM, airspeed, turbulence, dust loading, dust particle size etc), and (c) how long do cartridge filters last given that (b) is likely to make this a moveable feast.

    ian
    Last edited by ian maybury; 04-12-2012 at 6:29 AM.

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    Thomas and Ian are correct. Therefore the performance will decrease. The shop vac remains the same in both cases. Adding a second cyclone adds pressure drop to the system. More pressure drop decreases the total air flow. Reduced air flow results in decreased collection of fine dust particles. Because the larger wood particles are still collected, the difference will probably not be noticed.

  13. #13
    The owner of the serial DD's says there is still some stuff that makes it to his shop vac's filter.

    In my own case, I found it better to just use a bag in my shop vac, and catch all the super-fines that my separator doesn't get.

    In fact, I recently emptied the drywall bag I had in my vac (after three 25-gallon fillings of my separator). I found I had 19.9 ounces of the finest, talc-like dust in the bag. I measured with a measuring cup and found almost 8 cups, about 1/2-gallon. 1/2-gallon is .67% of 75-gallons, so the separator got 99.33% on the last three drums.

    I had been generating a tremendous amount of super-fines, sort of on purpose. To prepare rough lumber, I had been jointing one face and one edge of boards, resawing to rough thickness, and then drum-sanding to final thickness. That is a ton of drum sanding, LOL. So I guarantee you that I generated gallons of fine sanding dust.

    I was doing it this way for two reasons: (1) A good test of the separator's capabilities. (2) I didn't have a lunchbox thickness planer. I had the Inca jointer/planer, but it was a PAIN to put the thicknessing head on.

    Going forward, I now have a thickness planer, and I've sold the drum sander. So I'll be generating far less super-fine matter. It will be interesting to see what is in the shop vac's bag next time around.

    Going forward, I'm going to start measuring the weight of the dust in the drum each time, so I can get a separation rate by weight AND volume. I also have to come up with a way to measure volume that is more consistent. Right now I spoon the dust into the measuring cup, making sure it doesn't get packed-down. I could have easily packed that 8-cups into four, it compresses real easily. I think I need a large plastic pitcher or something with a lid that I can shake when I'm done, and which is graduated.
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    Last edited by Phil Thien; 04-12-2012 at 9:30 AM.

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    Wow, Phil, weighing your sawdust. I don't know what to say.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  15. #15
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    Phil, that sounds like an excellent test and speaks well for your separator. If I were you, I would provide the efficiency by weight. Volume is going to depend on how well the material packs as you noticed with the wood flour. You may know this, but what you are providing with the volume measurements is dependant on the bulk density (very different from particle density or specific gravity, not the same). The bulk density is going to vary depending on wood species and type of dust or shavings. The company I work for provides industrial cyclones, and we always provide efficiencies on a by weight basis.

    Mike

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