6” or 7” ducting?

[Cabinet Monkey]

Believe what you want, nio skin off my nose. But that 8" port was put there by the same guys who claim it moves 2700+ CFM. Nuff said.

fred - you'd almost certainly be wrong


If you'd look a bit more closely at LT's marketing fluff you'd see they also state the real world , or connected airflow is 1600 cfm +/-.
So, what's more likely:  the guy in marketing coming up with the 2700cfm # or the fluid dynamic engineer that designed the cyclone with an 8" port ?

If you'd care to look at Cliff's V-3000 spex , you'll notice it has a 7" inlet - which is what their duct designers suggested he go with.  Pretty safe to assume they know he has tools with 4" (or smaller) dust ports.  If  6" were as good or better, don't you think they'd have mentioned that being that 6" pipe and fittings are much easier to source ?

I would suggest a 6" main trunk, and run 6" as far as it will go...to the point of modifying the tool's ports to take a 6" in every case possible.

This is not how good dust collection pipe networks are designed.  The main line is bigger than the branch lines, sub branches taper again if necessary.

With a 3 HP dc, 6" would be plenty anyway

Adequate or acceptable would probably be a better word choice.   Especially not knowing all the other variables in the equation.


Bottom line is that saying just use "x" inch duct and you'll be good is short sited.

[fredhargis]

Juss1, what ever you decide to do please post back and let us know how things worked out. BTW, you might find this by Wood Magazine to be of interest, maybe even useful.

[jteneyck]

The proper way to go about it is to look at the velocity required and actual cfm of the DC. For 3 hp I'm pretty sure 6" mains would be the correct size for most layouts.

Do not just go by the stated cfm of the unit. You need to look at the fan curve and see what it is at the pressure drop you expect at the machines. Also, flow will drop as your filters or bags load up.

John

[Juss1]

How do you calculate what the expected pressure drop will be at a machine for a given pipe diameter.  I have an Anemometer so I can measure the actual cfm at my DC.  Below are the specs for the current model of the PFlux.  I have an older model and am checking with Laguna but I think I read somewhere these values still hold true.

[Juss1]

Based on the table above and the parameters they used (8” flexible hose at 128” long) it seems like 8” provided the largest cfm and fastest velocity. Unless I’m missing something and didn’t read it properly?

[jteneyck]

The stated cfm is based on a standard test. The fan curve shows what the actual flow will be for different pipe diameters and static pressure. You calculate the expected pressure drop by looking up the values for the pipe and hose diameters you want to use, times the length of each. You add to that the pressure drop for every 45 and 90 deg elbow. It's not by guess by gosh. You just have to do the calculations. There are plenty of good online resources on how to do it.

John

[Mike in kc]

Industry sources suggest Sawdust, small wood blocks, and sand blast dust, should have a velocity of on the order of 4500 FPM, or 50MPH to remain suspended in the air flow.   An 8" duct moving 2700 CFM would have a velocity on the order of 90MPH or 8100 FPm, so suspension should certainly not be a problem.   It is important that minimum velocities be maintained to avoid system problems.  

[jteneyck]

That fan curve shows a maximum flow of 1600 cfm and suggests he'll have less than 1000 cfm at typical static pressure at the business end of a drop. An 8" duct is far too large to maintain even the minimum duct velocity, which I think is 3500 fpm.

John

[Tapper]

I redid my dust collection system last year. I have an older Penn State unit, 2 1/2 HP and runs on 220v; also upgraded to a Wynn filter and dispensed with the cloth bags. I decided to go with the Dust Right system from Rockler. All my tools are on wheels and I move them around as necessary for the job they perform. I installed a Dust Right connector on every tool.

I ran a 6" PVC trunk line in a ceiling joist bay that connects directly to the dust collector (15-20' long). I have two main 6" branches: one goes to a station for my Dewalt RAS and the other one goes to a station mounted on the wall; both are "necked down" to 4" flex at this point. At the wall station hangs the Dust Right hose with its handle (curled up and out of the way when not in use) that will reach any tool in the shop. Blast gates control the pressure. I was able to hook my jointer (which never moves) directly to the main trunk with its own blast gate.

For my purposes, this system has worked even better than I had hoped. I was not looking forward to attempting to run hard pipe to every stationary tool with its own blast gates; just too much work and not very flexible. The Dust Right system from Rockler was somewhat of an epiphany for me. Happened to have gotten an e-mail from them about it and decided to take a look - liked everything about it. I've also found their CS to be stellar.

I did not do any indepth calculations before choosing this system. I just did a little research and went with what I thought would work; luck helped. I'm sure this is not for everyone but if you're looking for flexibility in shop dust collection, this system is hard to beat IMO.

Good luck,

Doug

[Cabinet Monkey]

That fan curve shows a maximum flow of 1600 cfm and suggests he'll have less than 1000 cfm at typical static pressure at the business end of a drop. An 8" duct is far too large to maintain even the minimum duct velocity, which I think is 3500 fpm.

John

Certainly you're aware that the LT curve was derived using 8" duct, yes ?

And, that 6" duct has nearly twice the static pressure per foot of length of 8" ?   And 4" duct twice the s.p. of that ?

And, that the volume that DC can move doesn't really change between 8" or 6" or 4" pipe ?   
The velocity of the air moving in those pipes will change depending on size , but the collector's max flow is fixed by its fan, motor, and inlet.

8" ducting needs just over 1200 cfm to maintain a velocity of 3500fpm and you can see from Laguna's chart - that will get you in to the 7-8 inches of water column for static pressure.   Which is hefty.


You seem to be fixated on the notion that a duct network needs to have the same size pipe from collector to tool.   It does not. It can work, but so can a 10 to 20ft section of flex hose.  Should we just recommend juss1 get a piece of flex hose because that'd be the fastest , cheapest way to to get the max rated cfm from his machine ?