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Thread: Longer exhaust run or suction run?

  1. #1
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    Longer exhaust run or suction run?

    So, I have to reroute my dust collection because right now where it used to dump outside is now inside a new addition. I can move the dust collector closer to the new exit and have a longer suction run or I can keep it where it's at and have about a 15 foot exhaust run. Which one would be more efficient or is there no difference?

  2. #2
    Can you leave it where it is and run the exhaust straight up and out the roof?

  3. #3
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    Quote Originally Posted by keith micinski View Post
    So, I have to reroute my dust collection because right now where it used to dump outside is now inside a new addition. I can move the dust collector closer to the new exit and have a longer suction run or I can keep it where it's at and have about a 15 foot exhaust run. Which one would be more efficient or is there no difference?
    I read once that the length of the exhaust run didn't make much difference as long as the duct cross-sectional area didn't restrict the flow. This might mean that duct should be larger than the intake. Perhaps an expert will chime in or maybe research it on the Pentz site.

    Making the input run longer could decrease the efficiency some depending on the duct size, length, additional turns. Whether this matters might depend on the dust collector - for example for one that is a bit underpowered. I'd probably redo the design calculations from scratch with the new duct length.

  4. #4
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    I have a harbor freight 2 horse dust collector with a 5" outlet. I was worried about having a longer exhaust length would potentially start leaving chips behind causing a build up over time.

  5. #5
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    On the intake side you are supposed to have at least 5' of straight intake with no turns so there is smooth airflow into the cyclone. I have never read it is that important on the output side.
    Bill D.

  6. #6
    Maybe someone more expert can comment, but I'd imagine the static pressure losses are the same whether it's suction or exhaust, so I'm not sure it matters. However, it's much easier to duct the pressurized exhaust (ie - you can buy 10" or 12" HVAC ducting for very cheap - certainly cheaper than flexible duct collection hose), and if you upsize it like that, the losses should be minimal.

    As you note, Keith, you could get problems with chips settling (although that's true on the suction side, as well). But I wonder if that's not a "self-solving problem" - ie, any settling chips reduce the duct volume, increasing the velocity, and allowing the faster air to scrub up anything that settled. Any chance you could give the exhaust duct a bump every so often to shake out anything that's settled?

  7. #7
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    Up size the exhaust duct to 6 or 7", avoid bends as much as possible or at least use two 45's instead of a 90, and you should be fine. I up sized mine and used an 8' piece of insulated HVAC duct for part of my exhaust duct and it significantly reduced the exhaust noise outside with no noticeable loss of efficiency. The insulated duct is flexible, so I made a shelf for it to keep it as straight as possible.

    Charley

  8. #8
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    There should be no chips/dust after the cyclone so upsizing the exhaust duct with insulated flex ac will help quiet things down for less cost then intake plumbing. Since there is very little dust after the cyclone the flex hose should be fine. Might give it a shake with a broom every year or two when it is running.
    Bill D.

  9. #9
    Quote Originally Posted by Charles Lent View Post
    Up size the exhaust duct to 6 or 7", avoid bends as much as possible or at least use two 45's instead of a 90, and you should be fine. I up sized mine and used an 8' piece of insulated HVAC duct for part of my exhaust duct and it significantly reduced the exhaust noise outside with no noticeable loss of efficiency. The insulated duct is flexible, so I made a shelf for it to keep it as straight as possible.

    Charley
    Reading through, this is pretty much what I was going to say, with 1-2 added points.

    You can leave the DC where it is and upsize the exhaust 1-2” or you can move the DC and upsize the intake pipe from where the DC used to be to its new location by 1-2”. The only problem with upsizing before the DC is generally you want more rigid pipe. After the DC you can usually use simple ductwork.

    Many people don’t realize what static pressure does.
    My simple explanation is:
    Try to blow through half inch Pex that is 10 foot long then,
    Try to blow through half inch Pex that is a 50 foot long then,
    Try to blow through Half inch Pex that is 10 foot long attached to 1.5 inch PVC 40 foot long.

    You’ll definitely notice the difference between all three. But there won’t be a big difference between one and three.

  10. #10
    Matt, that's a great explanation.

    Bill, I don't think the OP has a cyclone. Just a blower and exhausting outside.

  11. #11
    Quote Originally Posted by Dan Friedrichs View Post
    Matt, that's a great explanation.

    Bill, I don't think the OP has a cyclone. Just a blower and exhausting outside.
    Thanks but I did forget to add when you upsize before the DC you want to try to make sure that your air speed in that upsized pipe is above 60 miles an hour to keep chip flow. If it doesn’t have at least 50 miles an hour, you start to have a slight natural linger of the finer dust sitting in the pipe.

    This exposes a possible dangerous problem. If you have a fine or coarse sawdust hanging out in a pipe, if you hit a nail and there’s enough spark or heat, this can transmit to igniting or a sinder, with a good flow of air. Just imagine the outcome.

  12. #12
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    I do not have a cyclone I just exhaust to the outside. I will leave it where it’s at and make the exhaust 6” for the 12 feet to get out the wall.

  13. #13
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    Quote Originally Posted by Dan Friedrichs View Post
    Matt, that's a great explanation.

    Bill, I don't think the OP has a cyclone. Just a blower and exhausting outside.
    Dan may be correct. I assumed he was not pumping in cold air in Indiana winters.

  14. #14
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    As Bill mentioned, if you can improve the fan inlet condition it will help. Other than that, it makes little difference.

  15. #15
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    I ended up mounting it about ten feet from the wall it exists and taking the exhaust side and bumping it up to 6". I had some insulated 6" flex left over and hooked it up thinking it might be a temporary solution but it works so well I don't see any need to change it.

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