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  1. #1
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    Default Ducting - do I care if dust drops in horizontal 225mm PVC pipes

    Hi all,

    I am still in the long process of putting together my dust extraction system. I will have 225mm PVC running horizontal from the cyclone prior to splitting into 150mm pipes down to the machines. The theory (as I understand it) is that I should have a second 150mm open (or equivalent) so that the air-speed in the 225mm pipe is enough to keep the dust suspended. From all of my reading, it would have been better if we had 200mm PVC but that is the way it goes. So I am thinking (and trying to increase my understanding)........
    If I only run with a single 150mm opening (connected back to my 225mm) I would expect to get at least some of the dust dropping into the 225mm pipe as it won't have the air-speed to keep it all suspended. However, I think having a bit of dust permanently in the 225mm pipe will decrease its area / volume so this could have a benefit.
    The reason I am thiinking of the above is so I can run my DC at 50hz instead of 60hz (at least sometimes on less noisy machines) which will be a lot quieter. With a 4hp motor I would still expect to get the 1250cfm through my 150mm pipes.

    Does it make sense to do what I am thinking? I am pretty sure I will have missed something.....I have even tried to think of a way of 'filling in' some of the 225mm PVC (eg: glue some foam on the inside) but that seems painful.

    thanks

    Mick

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  3. #2
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    Default

    The fall out is not a YAY or NAY sort of thing
    The sort of sawdust that falls out depends on what it is.
    Heavier chips will fall out first.
    Really fine dust stays suspended easily and it's only if the system stalls then virtually all the sawdust will fall out.
    Large volume curly shavings fall out easily but are also resuspended easily as they catch more air.
    What happens is the dust will build up until the X section of the pipe will decrease so the air speed increases enough to hold the dust in suspension dust.
    At some point or other some sort of equilibrium will be achieved.

    The sawdust won't settle evenly and settled sawdust (especially if it crusts over) does represent a rougher surface so the ducting will have some frictional losses compare to smooth walled pipe.
    I wouldn't go so far as to fill out the pipe with anything but inserting an inspection port at a convenient point so you can look into the ducting and if necessary poking a compressor hose up into and along the duct to fluff up the settled dust is probably worth the hassle.

    On really big systems the amount of settled sawdust can represent a significant amount of weight so don't skimp on the ducting supports.

  4. #3
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    Default

    BobL,
    thanks very much for the reply. That increases my understanding. I think some inspection points would be very worthwhile on the 225mm PVC regardless of what I do, especially as even having 2 x 150mm open ports is not quite as big an area as the single 225mm.
    cheers
    Mick

  5. #4
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    Can you add a 250mm gate at the far end of the pipe? If you can, you could open that gate up at the end of the day and allow the dc to clear the pipe of any build up from that day. The gate could double as an inspection port.
    Brad
    Brad, HardingPens.ca

  6. #5
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    Quote Originally Posted by HardingPens View Post
    Can you add a 250mm gate at the far end of the pipe? If you can, you could open that gate up at the end of the day and allow the dc to clear the pipe of any build up from that day. The gate could double as an inspection port.
    Brad
    That would work.
    There is also a very simple gate that can be made that automatically opens when the system approaches stalling.
    It's basically a large rotating valve that is lightly weighted to keep it closed but allows it to rotate open under a certain rise in the pressure generated by the DC.

  7. #6
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    Quote Originally Posted by HardingPens View Post
    Can you add a 250mm gate at the far end of the pipe? If you can, you could open that gate up at the end of the day and allow the dc to clear the pipe of any build up from that day. The gate could double as an inspection port.
    Brad
    Brad (and / or BobL),
    still plenty for me to learn
    - by 'open the gate' do you mean reduce the area of the pipe (via the blast gate) that the air is flowing through so there is an increase in air speed? I also assume I would need to be 'blocking off' the top of the pipe in doing this so any settled dust is picked up? My lazy thought on this (assuming I want to clear the 'droppings') was to increase the machine to 60hz and have two ports open. This would act as a bit of an air scrubber at the end of the day as well.
    - for the gate to double as an inspection port, I assume this means remove the gate and look down the pipe? I was thinking I was going to duct tape the joins in the gate so inspecting this way would be no different to pulling apart some other pipes (eg: where I start my 150mm). Have I missed something here? Perhaps I don't need to duct tape the gates but these would seem another source for air leaks.

    cheers and thanks

    Mick

  8. #7
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    Quote Originally Posted by mick59wests View Post
    - by 'open the gate' do you mean reduce the area of the pipe (via the blast gate) that the air is flowing through so there is an increase in air speed? I also assume I would need to be 'blocking off' the top of the pipe in doing this so any settled dust is picked up? My lazy thought on this (assuming I want to clear the 'droppings') was to increase the machine to 60hz and have two ports open. This would act as a bit of an air scrubber at the end of the day as well.
    He means (I think) just have a full size (225mm) junction in the main/trunk that just has a 225 mm blast gate on the end of it and you open this up and then DC will then have access to a large volume of air with minimal back pressure.


    for the gate to double as an inspection port, I assume this means remove the gate and look down the pipe? I was thinking I was going to duct tape the joins in the gate so inspecting this way would be no different to pulling apart some other pipes (eg: where I start my 150mm). Have I missed something here? Perhaps I don't need to duct tape the gates but these would seem another source for air leaks.
    Like this
    Ducting - do I care if dust drops in horizontal 225mm PVC pipes-dcsystem-jpg
    Attached Images Attached Images

  9. #8
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    Quote Originally Posted by BobL View Post
    He means (I think) just have a full size (225mm) junction in the main/trunk that just has a 225 mm blast gate on the end of it and you open this up and then DC will then have access to a large volume of air with minimal back pressure.




    Like this
    Ducting - do I care if dust drops in horizontal 225mm PVC pipes-dcsystem-jpg

    BobL,

    thanks once again. That makes sense - then everything is only using the 225mm (and with luck without any bends or drops) as long as I close off all of the other gates. I'll look at how to incorporate this.

    cheers

    Mick

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