# 4" or 6" feeder pipe for DC?



## 250knots (Jun 30, 2010)

Hi all,
I am about to put together a small shop from scratch. It will be with limited space and limited resources due to the geographical area I am in. I will rout my dust collection pipes mainly under the floor and I only have a choice between 4" and 6" pipes available.

My question to the LJ community is whether I should use 4" or 6" pipes for the main feeder? Not a difficult question exept for the fact that my Dust Collector is 5". My understanding is that the aim is to keep the main branch as large as possible and then reduce the size towards the individual tools for optimum air flow. But how about doing all that but having to reduce the pipe from 6" to 5" just before the dust and air enters into the DC? Will I loose the benefit of using a 6" pipe if I compress the air just before the DC? Should I stick to 4" throughout instead?

All the pipes and fittings are widely available and design wise it won't make any difference. I just want some advice on how a can optimize the airflow into my DC.

Appreciate any thoughts.

Regards,


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## b2rtch (Jan 20, 2010)

This question has been asked repeatedly and there are two schools which seems to never agree; one which says that you need maximum velocity to carry the wood chips and so use 4.00" and the other one which say that you need minimum pressure drop and so use 6.00". 
It depends how big a dust collector you have. 
If you have small shop you most likely have a small dust collector with a limited capacity ( like mine a 2HP Harbor Freight ) and for that I recommend 4.00" because you wan to keep the velocity of the air stream as high as possible.
If you have a small dust collector like mine you can kill two birds with one stone by replacing the bag it came with, with cartridge filter. 
Not only the filter filters the air much better and stop much smaller particles, which is very good for your lungs,than the bag does but in addition it increases the air flow because it creates less flow restriction than the bag. 
You can find tons of posts about this subject on LJs, just make search for dust collector or Wynn Filter.
Also you will find endless discussion about using PVC piping or not. 
My opinion is that PVC piping is jut fine for a small shop.


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## crank49 (Apr 7, 2010)

This has been discussed at great length. Both statements of b2rtch are correct. You want velocity to keep chips and dust from falling out of suspension in the air stream and you also want the lowest pressure drop* practical*. The key word is *practical.* Also, since you say you are going under the floor with your ducts, a place where you may have a hard time taking pipe apart to remove a blockage, I would say you want to keep the velocity up. Perfect would be to keep the main trunk line the same size as the fan inlet, but the next best thing is a little smaller in your case.

You didn't say what your collector is so I can't be more specific about length of pipe runs or number of runs, or number of pipes open at the same time except to say that generally the square inch, cross section area of open ducts should never be more than the cross section area of the fan inlet. You don't have to get into geometry or formulas to figure this out. Just square the diameter for a reference number. i.e. 2" = 4, 3" = 9, 4" = 16, 5" = 25, 6" = 36 What you want is duct openings that total 25 because you said your fan inlet is 5". You can see that a 3" and a 4" duct will have reference numbers of 9 and 16 respectively, and added together, 9 + 16 = 25. This means the 5" fan inlet could effectively handle the flow from 4" and a 3" ducts, both open at the same time, without any loss of volume or pressure. I have a Harbor Freight 2 hp collector and I use a 4" and a 2 1/2" on it, both open all the time, and it works great.

In all cases, you want to keep the number of elbows and length of flex duct to a minimum. Use large sweep ells if you can, or even back to back 45 degree ells, to make 90 degree turns. Consider what will happen if an offcut about 1/4" x 3/4" x 12" long gets in the duct (can happen on a table saw easily). How's that going to get around an elbow? It won't. So it will hang, and catch everthing that follows it till the duct is plugged. You have to allow for dis-assembly at places where this may happen, or provide a "cleanout wye" just in front of the elbow where you can reach in and remove the obstruction.


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## NBeener (Sep 16, 2009)

I like to answer this one in a very particular way. If it doesn't help you, the hope is … it will help somebody else.

There are "shoot from the hip" methods to determining DC system design, and there is the "science-based" approach.

I think it's worth the time to evaluate the scientific way. It's fairly quick and fairly simple.

Sites like this one lay it out pretty clearly.

Good luck !


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## crank49 (Apr 7, 2010)

I hope the advise I gave didn't come across as "shooting from the hip". I was trying to offer good advise based on 38 years experience as a foundry engineer who has designd and built hundreds of dust collection systems including three major woodworking pattern shops. I know there are sites out there that offer scientific knowledge. I also know, from experience, that most peoples eyes glaze over when they try to interpret that knowledge. The methods I offered were some simple shortcuts to acheive good results without having to read pages and pages of design criteria.

Sorry if I came across as less than professional.


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## NBeener (Sep 16, 2009)

*crank49*:

I genuinely didn't even read your response, so … posting what I posted, AFTER your post … wasn't very smart.

Apologies. You consistently give EXCELLENT advice about dust collection.

I meant for my statement to be a GENERAL statement


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## DIYaholic (Jan 28, 2011)

I'm in the process of setting up my basement shop. Like NBeener, I prefer the "scientific" approach, however I would be paralyzed like a "dear caught in headlights" doing all the research and math. So, I will be "winging it", (of course utilizing the plethora of information and knowledge found here @ LJ's) when I install my 2HP HF DC, just so I can have good DC and get to actually make some saw dust. I fiqure that after using the shop for a while I will find a need to rearrange the shop for work flow efficiency. Once a "final" shop layout is determined (right, like it will ever be finalized), I will do a more "scientific" approach to running ducting. In the meantime, watch out, I'll be "shooting from the hip" and my aim ain't too good!


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## Pop (Aug 6, 2007)

There are a couple of books on this subject. "Dust Collection Basics" published by Woodstock International and "Woodshop Dust Control" by Sandor Nagyszalanczy. I much prefer the Woodstock book. It's a grand total of 57 pages and gets right to the point. The approach is much like the article (a good one) NBeener gave us. The other book is 202 pages that IMPO goes overboard in dust control. Sandor is even opposed to sweeping the shop for fear of raising dust.


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## 250knots (Jun 30, 2010)

Thanks everyone for your advice.
What an outstanding source for information this LJ website is!

I have sort of gotten my previous conclusions confirmed with the great replies I have received. One thing that I forgot to mention was the fact that my DC port is 5" and my jointer/planer that will be positioned nearest to the DC does also have a 5" port. All the rest of my machines require only 4" or 2.5". Thus the reason for my desire to keep the initial piping wider than the 4". I agree that it makes sence to keep the speed up with a 4" pipe as mentioned In some of your replies. I will stick to 4" for those machines.

Now I only wonder if it is a bad idea to use 6" in between the 5" DC and the 5" jointer/planer? The pipe area from the machine would transport the chips initially through a 5" port in the machine, expand (slow down) through the 6" pipe, and finally compress (accelerate) through 5" into the DC. The pipe/hose distance between the two units would only be about 2 meters (6ft). (including a couple of 45 deg bends.)

Alternatively I would have to do the opposite by compressing the air and accelerate it from the machine straight into a 4" pipe and then slow it down with a widening pipe into the DC.

Probably some of the easiest advice given above is that things might not have to be to complicated and academical. Maybe it is just a matter of some Nike-logic - Just do it!

It would have been nice if 5" dimensions would have been available where I live…

Again thanks for your advice and support.

Per


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## NBeener (Sep 16, 2009)

Your air flow, and-accordingly-your dust collection, will be limited by the smallest opening.

So … if 6" is what's indicated to pull all the chips and fines from a given machine, then THAT machine won't get opimal extraction with a 5" port.

But … you'll get most of it.

And sometimes that's enough ;-)


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## b2rtch (Jan 20, 2010)

'Now I only wonder if it is a bad idea to use 6" in between the 5" DC and the 5" jointer/planer? " 
Yes it would be a bad idea as the diameter increases the pressure and velocity drops. As the Velocity drops then you might have some saw dust and chips setting there.
You want to keep the diameter as constant as possible.
Each time the diameter changes the air flow dynamics change also and this is not good.


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## Pop (Aug 6, 2007)

Air speed is increased by going from a large pipe to a smaller pipe. Air speed is reduced by going from a small pipe to a larger pipe. This is the venturi effect. I know all the books tell you to start small and increase pipe size as you head for the dust collector. At some point you will get a slow enough air flow to let chips settle out in your pipe. IMPO I think this effect would require a drastic change in pipe size. Something like a 2-1/2 in. pipe into a 6 or 8 in Pipe. Just food for thought.

I myself have designed my system with 4 in. pipe & hose everywhere. The exception to this is when a machine uses a dust port like 1-1/2 or 2-1/2 in. Then I change to 4 in. as soon as practical as near to the machine dust port as I can.

Pop


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