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6" dust collection vs. 4"

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Blog entry by ocwoodworker posted 09-13-2011 at 11:38 PM 2535 reads 0 times favorited 6 comments Add to Favorites Watch

This month in Wood Magazine there is an excellent article about sizing the right DC to the right shop. And they make the comment to take off the (2)-4” Y-port and connecting straight to the 6” opening with a 6” main line.

Here is my question… How is a 6” better than a 4”?

If you only have one port open at a time it should stay fairly consistent in velocity all the way to the impeller.
The way I see it…. if you increase the size of the pipe, the velocity (even the though the CFM stays the same) goes down causing the heavy debris to settle in the pipe. What is to be gained by increasing the pipe size? For argument sake if that holds true why not have an 8” pipe?

Love to hear LJ’s response.

Thanx in adavance

Kevin

-- I'd like to believe Murphy's Law haunts my woodshop, because if it's Karma it would mean I had something to do with it. - K.R.



6 comments so far

View MedicKen's profile

MedicKen

1599 posts in 2099 days


#1 posted 09-13-2011 at 11:46 PM

6” will flow over twice the amount of air that 4” will. turbulence will be lower in 6”. Take a look at Workshop Dust Control by Sandor Nagyzslancy, there is a lot of good info in the book.

-- My job is to give my kids things to discuss with their therapist....medic20447@gmail.com

View Manitario's profile

Manitario

2309 posts in 1520 days


#2 posted 09-14-2011 at 12:05 AM

this is one of the topics that comes up on LJ’s again and again. I encourage you to do some searches as you’ll read some of the great responses others have made to this question in the past. As well, a quick google search for Bill Pentz will bring you to a site that has a great in depth discussion of this.
Briefly; “dust collection” means different things to different people, but typically we want a) good chip collection eg. from a jointer and b) good fine dust capture. Chip collection is more dependant on the air velocity in the pipe, and the fine dust collection is mostly dependant on moving a lot of air, ie. CFM. Without going into the math behind this, both air velocity and CFM are related to the diameter of the pipe, but differently. A smaller diameter pipe will increase the air velocity to a point (similar to putting a thumb over a garden hose), however a smaller diameter pipe increases the amount of friction between the pipe and the air; this is the “static pressure” component that you may read about. The increase in static pressure that happens when switching from a 6” pipe to a 4” pipe is quite dramatic, most DC’s are not able to handle this and still keep the CFM at a level where it is able to capture the dangerous fine dust. Conversely, unless you have long runs of ducting the air velocity in a 6” duct compared to a 4” duct is not usually affected to the point where it cannot collect chips.

-- Sometimes the creative process requires foul language. -- Charles Neil

View rustfever's profile

rustfever

621 posts in 1947 days


#3 posted 09-14-2011 at 05:40 AM

Many of the tools we have in our woodshop need more air velocity & CFM than is provided by a 4” system. However, most tool manufactureres only give us a 4” port. It is important, when designing a dust collection system to understand these needs of each piece of equipment.

-- Rustfever, Central California

View Cosmicsniper's profile

Cosmicsniper

2199 posts in 1796 days


#4 posted 09-14-2011 at 03:19 PM

Kevin:

You need both velocity AND air mass to move your dust. Velocity without air mass is like trying to drain an air mattress with a straw..it moves air fast, but can’t grab a whole lot of it. Air mass without velocity might enter the duct, but it probably won’t get too far.

Pentz says that you need about ~750 CFM (air mass) and ~4000 fpm (velocity) to grab the fine dust from the source and then successfully move it io the DC. A 4” duct naturally carries less air mass. In a typical 1 to 2 hp DC, it maxes out at around ~400 to 500 CFM…and whereas you might expect a velocity increase to compensate for that (the same amount of power will move air faster through smaller ducts), the smaller tube does come with a relative hit to velocity because of the increase of air resistance…the amount of surface drag is greater when compared to the small volume of air in a 4” duct as compared to larger ducts.

But even so, a compensation in velocity doesn’t overcome the loss of air mass linearly. If you do lose half the air mass (as with a 4” duct), then you’ll need a lot more than twice the velocity for adequate compensation. Increasing horsepower on the DC helps, especially to yield greater velocities in overcoming the drag, but there’s just a natural performance decrease with 4” duct that cannot be overcome with reasonable horsepower increases…IIRC, you need a 10hp DC or more in order to use 4” ducting at the same degree of effectiveness as a 2hp DC with 6” ducting.

So, air mass (CFMs) are the important measure…and you need velocity to assure the ducts don’t clog up. With 6” duct, you pretty much are guaranteed enough CFM at the machine for the fine dust AND enough remaining velocity to move the chips to the DC.

-- jay, www.allaboutastro.com

View ocwoodworker's profile

ocwoodworker

204 posts in 1641 days


#5 posted 09-17-2011 at 06:18 PM

So then my follow up question would be…
If a tool has a four inch DC connection, is there any benefit to using a 6”? I have a TS and a jointer with a 4” connection.

Thanx for the reply BTW!

-- I'd like to believe Murphy's Law haunts my woodshop, because if it's Karma it would mean I had something to do with it. - K.R.

View Grandpa's profile

Grandpa

3107 posts in 1312 days


#6 posted 09-19-2011 at 11:13 AM

Friction is your enemy when using tubing. Once upon a time I had to replumb my water supply from my meter to my house. I used 1” pipe. The old pipe was 1/2” for all the homes on the block. There was a 5/8’ fitting on the water meter. When I turned on the water in my yard The flow was much better than it had been earlier even though i had a 5/8 fitting ahead of my 1” pipe. I was never sorry I used the larger pipe. the same application could be used for your duct collection. You have a 4 inch at the collector and you would like to use 4 inch main trunk lines in teh shop. Sure it will work. The 6 inch will probasbly work better. The only thing that can happen is if you install a pipe that is too large and the flow actually slows and large chips fall out of the flow and just lay in the pipe.

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