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  1. #1
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    Default Not quite dust extraction......rathermore hot air extraction....

    Soon I will be installing ducting for my 2HP extractor (generic). It will have to be 6" pipe because the Hammer thicky runs on 5" flexy and there is no 5" drainage pipe available.

    Last summer was the first I had using the DE and I found it necessary to walk the 5" flexy under the gable every 45 minutes or so to remove the hot air (even with aircon the hot layer starts at the shoulders and just gets worse towards the scalp ).

    So, I'm thinking that when I run the ducting for the various machines I'll also run a pipe down the length of the gable with holes drilled into it, and gated. That will make the process far superior.

    The question is: what diameter pipe to run along the gable? 100mm? 60mm? 50mm? I intend to drill smaller holes where the gable run meets the trunk (in the middle of the gable run), and expand the hole sizes as I get towards the end. I will probably be forced to use 100mm pipe running out of the trunk to join up to the gable run, so 100mm will be the choke point. Some quick calculations show:
    150mm dia has Xsectional area of 177 cm²
    100mm dia has Xsectional area of 79 cm²

    A series of 8 holes either side of the middle (so 16 holes altogether) in the following diameters will yield a Xsectional area of 82cm² (which is compatible with the 100mm choke point):
    Hole dia/mm AREA cm²
    10 0.79
    12 1.13
    15 1.77
    20 3.14
    25 4.91
    30 7.07
    35 9.63
    40 12.57

    That would place the holes about 30cm apart on a 5m run.

    Current thinking is to have the holes on the top of the gable run pipe to reach the maximum temp air - should pull up the layer underneath as the hottest air is removed.

    So, is that the best way to go or are there other ideas please?
    Regards, FenceFurniture

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  3. #2
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    A comparison of cross sectional areas is not the way to do it .

    If you look at this diagram
    DRAFT: FAQ - Dust Extraction (Theory)-flowratesx-jpg
    You can will that even though a 6" diameter duct is 2.25 greater in cross sectional area it takes three 4" ducts to equal the flow of one 6" duct
    Now look at the 4" relative to the 2" duct. The 4" duct is 4 times the area of the 2" duct but it takes around 6.2 2" ducts to equal the flow of a 4" duct and ~19 2" ducts to equal one 6" duct
    The smaller the duct the even greater that difference becomes because the wall effect becomes even more critical.

    Now here is a telling graph.
    The Flow = 10 x Ductsize2.69 is the equation that will allow you to calculate flow at any duct size.

    The black line shows flow rates at 10" of WC SP in CFM (refer LHS vertical scale).
    The red line shows the number of ducts or holes needed to equal the flow of a 4" duct (refer to RHS vertical scale)

    What you will see is that if you use 1/2" holes you will need ~240 (gulp) of them to equal the flow in one 4" duct
    If you use 1/4" holes you will need ~1800 of them
    This is why choked machines are useless at air flow and drilling a few small holes in a throat plate or around a router bit is virtually a complete waste of time.

    Not quite dust extraction......rathermore hot air extraction....-ductsizevflowrates-jpg

    Keep in mind that this is theory.
    If the holes are not smooth and have furry edges the numbers could be even higher.

    EDIT: Actually it's not even theory, it's an empirical approximation for air in a smooth pipe.
    The theory for fluid flow through orifii is that flow is proportional to "hole radius" to the 4th power so it's even worse than what I show in the graph.
    Also why even partially blocked blood vessels are so nasty.
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    Thanks very much Bob. Jeez that first graph looks like a Groucho Marx suit!

    The inlet on the Dusty is 5" (which happily fitted the odd sized 5" hose for the Hammer). That's why I have to use a 6" trunk, rather than a 4", as that will just choke the whole system down.

    So, a couple of Qs (I'll have to review this tomorrow when I'm not so tired and hungry):
    Are you saying that the hot air run should be 6" for maximum effect? Of course I could still get by with the substantially cheaper 4" for the hot air part, and just run it for longer (I got prices on 6" parts today - COUGH!).
    The holes will have to obviously be much larger than I had previously specified (and I'll try to work out some more appropriate sizes tomorrow), but am I right in thinking that they will need to be increased in diameter as they get further away from the middle to approximately even intake along the length?
    Regards, FenceFurniture

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    "Are you saying that the hot air run should be 6" for maximum effect? Of course I could still get by with the substantially cheaper 4" for the hot air part, and just run it for longer (I got prices on 6" parts today - COUGH!)."
    Yep - it will evac the hot air faster than a bad curry and a couple of pints of stale beer.

    What about a couple of whirly bird extractors.
    The hotter it gets the more effective they are.

  6. #5
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    Quote Originally Posted by BobL View Post
    What about a could of whirly bird extractors.
    The hotter it gets the more effective they are.
    The structure would require too much modification I think. In any case, I'm thinking that it would be useful to run this to get rid of fine dust in the air (dusty is outside).
    Regards, FenceFurniture

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    Wouldn't a ventilation fan or two exhausting outside be more efficient, quieter and cheaper?

    Pete

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    Pete, it probably would be marginally cheaper to put whirlybirds in, but a length of 4" pipe wouldn't be too bad for cost, and is very quick to install without having to put large holes in a very old rusty roof that is quite fragile. I also rather like the look of it the way it is. In fact when it comes time to put a new lid on the barn I'm thinking that I may even lay the old rusty lid on top of the new one for aesthetics.

    This pic is about 3 years old:



    However, I'm interested in Bob's thoughts on my idea about the ducting being useful for on-demand extraction of invisible dust after doing some sanding or whatever. I actually only just thought of that benefit as I typed it last night, but it is definitely worth considering (given that the extractor is outside). It wouldn't get all of it but would certainly reduce the levels I should think.
    Regards, FenceFurniture

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    Great looking shed FF it reminds me of some of the structures at MontSalvat up near Eltham

    I can understand why you don't want to touch the roof, but what is the structure like around the ends of the gables. Whether you run a duct or not, a decent vent at each of the gable ends will make a big difference

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    Yeah, I love it Bob. We actually use it just to sit in and yak quite a bit. As it was when I moved in:



    And the getting rid of the dust idea by running the extractor? What do you think?
    Regards, FenceFurniture

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    Quote Originally Posted by FenceFurniture View Post
    Yeah, I love it Bob. We actually use it just to sit in and yack quite a bit. As it was when I moved in:
    And the getting rid of the dust idea by running the extractor? What do you think?
    Sure - the more air you can move out the more dust that goes with it.
    I was just looking at ways to maintain your ambiance with the requirement for hissing ducting and fans running.
    I still thing that opening the gables up is the way to go for the heat.
    It looks like they were open to begin with.

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    ...at least the gutter has good fall on it.
    Where it goes from there


  13. #12
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    Quote Originally Posted by wolften View Post
    ...at least the gutter has good fall on it.
    Where it goes from there
    Onto the garden of course!

    Quote Originally Posted by BobL View Post
    Sure - the more air you can move out the more dust that goes with it.
    I was just looking at ways to maintain your ambiance with the requirement for hissing ducting and fans running.
    I still thing that opening the gables up is the way to go for the heat.
    It looks like they were open to begin with.
    Yeah, could be a combo of both - quick(ish) dust removal on demand, and vents for constant heat removal. I closed them up with 6mm perspex (and the windows). The front end now has a tarp continuing the gable (and it gets VERY hot under that, so a vent there may not do too much). Certainly one at the back will be good.

    A bit of thinking required. Insulating the roof will help outtasight of course! That will come when I put the new lid on.

    As for the ambience - yes indeed - but there is plenty of noise plenty of the time. The Dusty is just a low hum (being outside) and a constant low noise is pretty easy to handle - even for someone with my ridiculously low tolerance to irritating irregular sounds/noises.

    I'll try and work out some hole diameters in a while and post them to see what you think.
    Regards, FenceFurniture

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    Some current pics, to give a better idea of the gable:



    It just occurred to me (as theses things do) that the tarp at the front won't prevent a vent from working . You can see the Dusty behind the vertical tarp.

    Gable to the door:



    and the other way:

    Regards, FenceFurniture

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    I like the rustic look of your place but something like it here would only be comfortable for a few weeks a year. I was thinking along the lines of a shuttered fan at the ends of the shop, with a little bit of "distress painting" it would fit in with your decor, inside and out. If you want to have a duct that would match your wood ceiling, how about a wooden box sectioned one? Whatever boards you could find would be right and a few knot holes along with loose joints and splits would let the heated air in. It could be tapered along the length too if you wanted, either in width or height.

    Some of the cardboard tubes that carpet and awning materials are rolled around would be in keeping with your cardboard accent panels too. Oops. Never mind. I took anotherr look and realized they were plywood.

    Pete

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    Heh, heh, now you're getting the hang of it Pete. Some good thoughts there that I might just take you up on! For the vents at the gable ends I can just cut the tops off the triangular 6mm perspex (2 at each end) and hinge them back on - fold down when hot.

    No plywood there - you were closer when you said cardboard - much of the wall lining is MDF for drilling convenience. The ceiling is whatever was lying around, including fence palings etc.
    Regards, FenceFurniture

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