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  1. #1
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    May 2011
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    Default Total Tools 3hp Dust Extractor

    I have a generic 3hp (FM 300S) twin bag dust extractor (like this http://res0.graysonline.com/handlers...50142812000000) that I am planning to link to a ducted system for my woodworking shed. I'm hoping that some experienced forum members can help me with some suggestions/advice on the best way to go to maximise the efficiency and convenience of use of the system.

    My plans include:
    - 6" PVC duct approx. 6 metres total length. The blower has a 6" inlet that I've removed the front restriction section from.
    - probably 3 6" branches from the main duct fitted with 6" flex hose to table saw, jointer/thicknesser and bandsaw with ability to move to router table/other equipment.
    - all branches to be fitted with blast gates in close proximity to the main duct.
    - the extractor will be located in an adjacent shed, so outside the woodworking shed.

    I have wondered about constructing a Thein style separator to use in the system but am concerned about its potential negative effect on airflow - any thoughts on this?

    In the absence of a separator would there be any point in building a baffle to go above the inlet to the filter bags?

    Does anybody have practical experience with optimising one of these dust extractors? I was surprised to see that the blower fan is only 12" in diameter. There seems to be space for a 14" one in the volute housing and I would expect the motor would have enough power to drive a larger fan. That being said the inlet (with the outer restriction removed) and the rectangular outlet appear to be fairly well balanced (177 sq cm to 178 sq cm) so maybe a bigger fan wouldn't move too much more air anyway.

    Any advice or assistance would be greatly appreciated.

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  3. #2
    Join Date
    Feb 2006
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    Quote Originally Posted by aldav View Post
    I have wondered about constructing a Thein style separator to use in the system but am concerned about its potential negative effect on airflow - any thoughts on this?
    While it will keep the bags from filling up and maybe the filters cleaner for a bit longer it will also immediately rob about 25% of the flow. Remember its the flow that moves the fine dust out of the shed and you'll need every bit of flow you can get. I always reckon it's better to keep the filters clean and maximise the air flow.

    In the absence of a separator would there be any point in building a baffle to go above the inlet to the filter bags?
    If you are hoping to deflect dust away from the filters it will not be effective. The fine dust that blocks filters rides around like a gas and will just move around a baffle.

    Does anybody have practical experience with optimising one of these dust extractors?
    This design has been around for many years but I have not seen any attempt to improve its flow. The S shaped rectangular cross section connector between the impeller and the bag housing is probably too tricky to deal with.

    I was surprised to see that the blower fan is only 12" in diameter. There seems to be space for a 14" one in the volute housing and I would expect the motor would have enough power to drive a larger fan.
    Using a bigger impeller may result in a much greater current being drawn by the motor and it may significantly reduce the motors short (let alone long) term longevity. I would not recommend that without some serious testing.
    It would be pretty soul destroying to put a big effort into fitting a larger impeller and then have to throttle the DC intake down just to keep it alive.

    My welding booth fan is like that. Unrestricted it will move ~2600 cfm but draws so much current at this flow rate that it gets VERY hot and trips out. it was never designed to run unrestricted but to push air through a HEPA filter so I have to run it on <50% speed to stop if from tripping out. I can crank it up to full speed for a couple of minutes to really clean out the shed - if it dies it's not a big deal as it cost me nothing.

    That being said the inlet (with the outer restriction removed) and the rectangular outlet appear to be fairly well balanced (177 sq cm to 178 sq cm) so maybe a bigger fan wouldn't move too much more air anyway.
    177 cm2 is equivalent to 6" ducting in cross sectional area but being rectangular it will be a little more restrictive than 6" round ducting. If the motor could cope, a 14" impeller will theoretically generate a couple of extra inches of water column additional pressure which translates to about 20% more flow in a 6" duct - in practice it won't quite be that because of what your say. Much greater flows of around 60% would be obtained by going to an 8" main but that would definitely end up frying the motor. This is why Clearvue and other industrial level blowers that need to move that much air use 4 and 5HP motors.

    if your motor was 3 phase I would recommend a VFD for a 20% boost in flow (trade off being more noise) soft start and current limiting capability.

    Once you have moved the DC outside and are using 6" ducting the next most critical aspect of most DC systems with regards to improving flow are the connections to and the air flow inside machines. Only when these have been attended to should you consider doing anything about beefing up the DC.

  4. #3
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    Quote Originally Posted by aldav View Post
    Does anybody have practical experience with optimising one of these dust extractors?

    Just a thought, with that S type duct where the two halves separate there could well be a fair bit of turbulence, a possible improvement might be to take a central divider from the middle point of the split down into the S duct a distance, say 9 to 12", the divider is only sheet metal .6 to .9mm thick, it theoretically halves the flow before the split and thus should be less turbulent, I say theoretical because we have no idea of the actual distribution of flow across the ducting as it leaves the volute housing.

    I was surprised to see that the blower fan is only 12" in diameter. There seems to be space for a 14" one in the volute housing and I would expect the motor would have enough power to drive a larger fan. That being said the inlet (with the outer restriction removed) and the rectangular outlet appear to be fairly well balanced (177 sq cm to 178 sq cm) so maybe a bigger fan wouldn't move too much more air anyway.

    Pressure that the fan developes is a function of diameter, so if a given dust extraction system (ducting, hoses, shrouds etc) has high restriction (long lengths of duct, small diameter 4", long pieces of hose etc) it will require a larger diameter fan to move a given quantity of air when compared to a different system with less restriction, so if you build your system with low restriction a small diameter fan will provide more flow than if you had a high restriction system, to give an indication whether a larger diameter fan on your system would burn out your motor you would need to test the current drawn with the 12" fan and make a judgement on how close to full load the current draw is and whether you have capacity to fit a 14", I have a 12" dia fan 3kw (4hp) motor, I know I am not at full load so I reckon I could go to 14" with not too much drama maybe 15" but I have 8" main and twin 4" hose (currently upgrading to 6")from the big dust makers so low a restriction system so I reckon so 14 will most likely do.
    It sounds like you are along the right path with what you are doing


    Any advice or assistance would be greatly appreciated.

    Pete

  5. #4
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    May 2011
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    Thanks for the advice guys. I suspected that I would be fairly restricted as to any improvements I could make.

    With your comments in mind I think the thing to do is install the system and see how it performs as BobL has suggested. I expect the 6" ducting will move enough air for my machines in my relatively small shed (about 4m x 4.5m - just as well all my machines are mobile ) and if the system proves effective in removing the fine dust I really should not be looking to do anything further. However I will certainly keep in mind your suggestion, Pete, re. splitting the airflow in the single outlet duct before the split, it should be doable. Having read your posts BobL and others the size of the ports on my machines is a priority. There is a bit of work to be performed in that area.

    Your replies are certainly appreciated and your opinions valued. At least I now feel comfortable going ahead with what I have planned with some confidence. From what I have read on the Forums dust extraction strikes me as one of those 'dark arts' that we get dragged off into when all we really want to do is make a few boxes or a bit of furniture!

    Thanks guys.

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