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Thread: Why 4/5 inch dust collector piping?

  1. #1

    Why 4/5 inch dust collector piping?

    Hi All,
    I've had dust collection on the brain for awhile, thinking of ways to pick one up used and modify it into a 2 stage one for my small shop. Most of these I find are 1-2hp, usually about 1.5. In thinking of how to get the most efficiency out of the motor I've always had the question as to why we tend to run 4-5inch piping for dust collection systems? Wouldn't thinner piping equate to better suction on the end of the run? I get a lot of tools use 4 inch fittings, but expansion fittings are easy to adapt and it would still seem to me that you'd get more power with a thinner pipe line. And that doesn't explain the 5 inch piping to me. Obviously 4 and 5 are the standard out there so what am I missing in all of this?

    Thanks all.

  2. #2
    Join Date
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    If by thinner you mean 2-3" piping then no it doesn't work better. Dust collectors very basically work on large airflow, low suction pressure. Vacuums work on high suction, low air flow. There fore you want the largest pipe the dust collector can handle that MAINTAINS THE AIR SPEED IN THE PIPE REQUIRED SO THAT THE DUST/CHIPS DON'T SETTLE OUT ON BOTTOM OF PIPE.
    There are people on here who have extensive knowledge of this, have done testing and written some very involved posts on this. I am not in that category, I am in the learning category. I need to modify my system, just haven't figured out exactly what to do. So I am looking forward to the answers to this question, also
    Ron

  3. #3
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    Actually 6" and above are more common with the folks that I talk to often. For systems larger than most home shops or out-buildings 8", 10" and larger mains are reduced in steps based on length of run and a whole lot of other variables that I am not prepared to deal with. I run 6" all the way to the tool whenever possible. I reduce as close to the tool as I can. I reduce from 6" to 7" at the DC inlet. As Ron says, DC's are DC's and vacuums are vacuums. For spoil collection generally a vac can do the job. For dust collection you need large volumes of air (much more than fits in a smaller duct) moving at speed to carry things away.
    Last edited by glenn bradley; 06-27-2020 at 2:38 PM.
    "What kind of chump do you take me for?"
    "First class."

  4. #4
    As noted, the problem with smaller pipe is that you loose a lot of that suction to friction in smaller pipes. You only get so much volume of air moved and static pressure from a given sized dust collector. Reducing the pipe size will increase the speed of the air moving, but the cost of that speed increase is loss of volume due to friction. Think of the difference between using a drinking straw vs a coffee straw.

    A 1.5HP dust collector doesn't have a whole lot of volume, and typically not a lot of static pressure at that volume, so you tend to be limited to around 4" or maybe 5" pipe and short runs. You could go smaller, but it will be at the ultimate cost of volume and static pressure. For some tools this doesn't matter, and for some it does. I have 2 1/2" hose going to my router table and belt sander, despite having a 3 HP cyclone. It is more than adquate for the router table, and almost adequate for the sander (the sander has 2 1/2" dust ports). 2 1/2" would not be close to good enough for my 15" planer, and likely not enough for my jointer or table saw.

  5. #5
    Yes, what everyone else said.... The old Bill Pentz website is site is still up which contains a lot of good explanations of the principles of dust collection. As others have said, a shop vacuum produces a small flow rate and very high static pressures. This "feels" like a strong vacuum because it is easier to feel the high static pressure (the suction) than to appreciate the flow rate (cubic feet per minute), which is very small compared to a large dust collector. This kind of vacuum is suitable for small handheld power equipment like small sanders, domino joiner, biscuit joiner, etc... which have small 1-2" vacuum ports. Now take a planer, jointer, table saw, or other large stationary tool. These machines create a large amount of wood chips and fine dust. To collect both large visible chips and tiny dust particles which you may not see but you do breathe in, a very large volume of air movement is required on these big tools. Enter the shop dust collector, which may have filters, cyclone, or both. These machines move an extremely large volume of air, anywhere from say 400-600CFM to 2000CFM and more for large units with 3-5HP motors and up. In order for these machines move such large volumes, they do so with less static pressure (the feeling of suction) than the smaller shop vacs. That is why such large main lines are needed (often 6" in small to medium size wood shops) - the vacuum cannot pull such large volumes against high static pressures that would be generated pulling that volume of air through smaller lines. Flow is severely reduced with undersized vacuum tubing on large dust collectors, and performance suffers.

  6. #6
    Join Date
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    The answer is actually pretty simple...Dust Collectors do not work primarily by "suction". DCs work by moving large volumes of air at a given velocity (measured in cubic feet per minute, aka, CFM) which in turn moves the dust and materials. The negative pressure associated with "vacuum" is quite low with a dust collector, unlike with something like a shop vac. More air fits in the larger duct and more air means more material can be moved at a given velocity with an appropriately sized blower.

    One thing related to this...most mass market DCs have bogus specifications for CFM. The better systems provide actual fan curves that will tell you more about what real-world performance will be.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  7. #7
    Larger pipe means more airflow with a DC. They are VERY sensitive to back pressure. You need something like 500 CFM to effectively remove the dust from a table saw (depending some on how it is designed). If you hook a 2.5 inch hose to a 1.5hp DC, you will not get 500 CFM at a tool.

    Turns also inhibit airflow and flexible hose is much worse than straight pipe. Metal is a little better than plastic pipe in reducing air resistance. It takes more tools than I have to measure the air resistance in my piping and more knowledge than I have to calculate it. So I used 5 inch metal to try and get the necessary airflow with a "2hp" HF DC pulling through a super dust deputy. It seems to be working pretty well. But it was a guess.

  8. #8
    Join Date
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    There is also an upper bound to useful duct diameter for a given collector.

    If the diameter is too large for a given CFM collector, the velocity of air (linear feet per minute, not volumetric cubic feet per minute) will drop to the point that the airflow will no longer keep the dust and chips suspended in the airflow. When this happens, you get piles of dust/chips in the ductwork, which if they break loose all at once, can damage the ducting, separator, impeller or even motor.

    Bill Pentz's site discusses this and provides data for minimum acceptable air velocity in the ducting. If I recall correctly, for a ~500 CFM collector, 5" duct diameter is about the upper bound. Keep in mind that many dust collector providers ridiculously inflate their CFM ratings.

    You can also look at the port sizes that reputable dust collector manufacturers, such as Oneida, provide on their collectors, as a guide for the maximum appropriate duct diameter for a given CFM collector.

    So the sweet spot is to have the duct sized such that the air velocity is high enough to carry the dust and chips, while low enough to keep airflow friction losses low and maximize CFM.

    -- Andy - Arlington TX

  9. #9
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    It is very hard to find odd size pipes for dc use. So most people use 4 or 6 even if 5" is better. The only 5" PVC I have seen is for electrical conduit use. It is cheaper to buy 6" then to special order 5" and all the fittings.
    Bil lD

  10. #10
    5" duct is more easily found in metal pipe, either HVAC or spiral. I have some in my system.

  11. #11
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    Quote Originally Posted by Bill Dufour View Post
    It is very hard to find odd size pipes for dc use. So most people use 4 or 6 even if 5" is better. The only 5" PVC I have seen is for electrical conduit use. It is cheaper to buy 6" then to special order 5" and all the fittings.
    Bil lD
    Not a problem with metal duct...
    --

    The most expensive tool is the one you buy "cheaply" and often...

  12. #12
    Quote Originally Posted by Dolfo Picanco View Post
    Hi All,
    I've had dust collection on the brain for awhile, thinking of ways to pick one up used and modify it into a 2 stage one for my small shop. Most of these I find are 1-2hp, usually about 1.5. In thinking of how to get the most efficiency out of the motor I've always had the question as to why we tend to run 4-5inch piping for dust collection systems? Wouldn't thinner piping equate to better suction on the end of the run? I get a lot of tools use 4 inch fittings, but expansion fittings are easy to adapt and it would still seem to me that you'd get more power with a thinner pipe line. And that doesn't explain the 5 inch piping to me. Obviously 4 and 5 are the standard out there so what am I missing in all of this?

    Thanks all.

    Can you drink a milkshake faster with a coffee straw or with a giant straw?

  13. #13
    Quote Originally Posted by Bert McMahan View Post
    Can you drink a milkshake faster with a coffee straw or with a giant straw?

    Great analogy.
    Dennis

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