# Is large Pipe always best?



## Psydoc (Feb 20, 2012)

I have the Oneida V-2000 with a 7" intake. I am going to use PVC piping, but chose 6" instead of 7". Am I understanding correctly when I hear that the larger the pipe (within limits) the better it is for dust collection? I am thinking of making 6" connections to my table saw, Miter saw and router table. Would it be more effective to use 4" hose on any of these?
I look forward to the feedback.


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## hwebb99 (Nov 27, 2012)

Yes, within the limits. A 18" culvert won't yield best performance from a 1500 cfm collector.


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## Psydoc (Feb 20, 2012)

*Size of pipe*

There must be a formula to help determine when a particular size pipe would become ineffective and when a specific size becomes ideal (I would think). Do you, or anyone reading this for that matter, have any knowledge regarding this?

Thanks in advance


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## FrankC (Aug 24, 2012)

A pretty good indicator is the size of inlet on the unit, the engineers have done their work.
Going larger will not really be an advantage, reducing it can be done from what is indicated at the tool into the line.


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## NickB (Sep 24, 2013)

Psydoc said:


> There must be a formula to help determine when a particular size pipe would become ineffective and when a specific size becomes ideal (I would think). Do you, or anyone reading this for that matter, have any knowledge regarding this?
> 
> Thanks in advance


Large pipes decrease velocity, which is why I think a lot of people believe that you can "negatively oversize" a duct. In reality, any time you create a choke point, you increase velocity anyway. If you run a 24" duct up to your small jointer and step down to a 4" fitting, you're going to get the air speed of a 4" pipe, but without the friction loss of a run of 4" pipe.

In HVAC, they avoid excessively large ducts because you get a lot of unconditioned air pushing through the registers before you get the nice warm or cool air. In a negative pressure system, it;s seen as a delay in suction while the system has to make up the static pressure across the entire system. This is true whether you have one really big duct or many small branches (volume being the same).

Aside from the delay, you really can't have "too big" of a line. The machines step down the duct size at the hookup, so decreasing the size of your duct run just creates additional friction, thereby actually decreasing air speed and flow. One caveat is that if you're using a duct with seams, larger ducts = larger seams = more leakage. 

The formulas, calculators and explanations for all of this are available on engineering toolbox - Sizing Circular Ducts


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## jacko9 (Dec 29, 2012)

Without getting into the flow velocity discussion, I can add that increasing my planer duct from a 4" opening to a 8" opening on a custom made hood for my Powermatic Model 100 planer with my Oneida V-3000 system eliminated the dust and chips that were still coming out with the board on the out feed side. My planer is located within 8' of my V-system so a larger opening and 6" pipe worked the best for me.


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## J.C. (Jan 20, 2012)

Larger pipe isn't always better. As mentioned, larger pipe decreases velocity. You need a certain about of velocity to carry your chips/saw dust. If you run an 8" pipe and choke it to a 4" port, there's a could chance you won't have the velocity to carry the chips through the 8" pipe. At least that's how it was explained to me by a dust collecting manufacturer...


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## jdonhowe (Jul 25, 2014)

> There must be a formula to help determine when a particular size pipe would become ineffective and when a specific size becomes ideal (I would think)


Okay, here's the formula: FPM=CFM/Area. You need a minimum airspeed (FPM) of about 2800 FPM to keep most sawdust and chips moving in a horizontal run. For typical dust/chips, you need 4000 FPM for a vertical duct (you're fighting gravity); with large chips you might even need 4500 FPM.

So, let's say your DC can move 1000 CFM. The area of a 7" duct is ~38.5 in^2, or 0.27 ft^2. The airspeed is 1000/0.27= 3700 FPM, which is fine for horizontal runs, but might be a bit low for vertical pick up. That's why you often need to have a short run with a smaller pipe for dust pickup, to increase the CFM in that section. Once the air in that smaller pipe enters a larger pipe, its speed will slow down according to the difference in pipe size (using the formula).

The hard part is determining how much each part of your DC system affects air flow (CFM). I'm not aware of any formula which is able to figure out all the factors- duct size, length, smoothness, transitions, curvature, etc., all of which affect air turbulence and resistance to air flow (not to mention filter resistance)- to calculate the specific impact of individual components on performance. So, the best us mere mortals can do is estimate an expected airflow, and use large ducts where possible (as long as adequate air speed is maintained for good dust movement), and go to smaller ducts only when needed to increase the airspeed for good dust pickup. All of this has to be modified by other factors, such as cost and availability of ducts, shop layout, size and accessibility of individual tool ports, etc.


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