Dust Collector duct size / cfm?

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Forum topic by WoodNSawdust posted 01-23-2016 02:22 PM 5510 views 2 times favorited 19 replies Add to Favorites Watch
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1417 posts in 2142 days

01-23-2016 02:22 PM

Topic tags/keywords: duct collection duct sizes cfm question

The Internet is a great resource and I don’t know how I would live without it, but there is a lot of contradictory information some of which might even be inaccurate.

I am looking at a new Dust Collector for my shop. I am looking at 3HP units which have an 8” inlet. Some sites say you should keep the largest duct size possible as long as possible others say you should decrease the size to 6” to increase CFM another suggested it did not matter (an 8” hose pulled to a sander with a 2” port will not perform any better than a 2” hose).

I am confused (which is normal for me)!

What I am planning:
a 3hp (Jet or Laguna) cyclone as I don’t have a tall ceiling where it will be located.
3 duct runs:

1. directly next to dc to handle my sanders (Performax 16/32, Flatmaster v-drum 24”, Rigid spindle/belt sander, Delta 1”x42” belt, plus hand sanders) using 10’ of hose. All of which have 2.5” or smaller ports.

2. straight line run to pickup the router table, router mounted in table saw wing, and the table saw. All three currently have 4” ports.

3. up to the ceiling and across 20’ to the other side to pickup the 17” bandsaw, drill press, and chop saw.

Either run 2 or 3 (probably 2) will end with a 10-20 foot flexible hose to connect to the mobile joiner and planer.

So, do I want to plan on 8” ducts, Ys, blast gates, etc. or should I step each run down to 6”?

Thanks in advance

-- "I love it when a plan comes together" John "Hannibal" Smith

19 replies so far

View bbasiaga's profile


1255 posts in 2961 days

#1 posted 01-23-2016 03:11 PM

The only way to know for sure is to get the fan curve from the manufacturer (how much CFM it moves for a given pressure drop/resistance in the system) and then calculate the resistance for your runs of pipe, matching the pipe size to the flow you want. Since the fan curves aren’t ususally made available, you’ve got to eyeball it.

The ‘largest duct size’ theory you say above is based on the fact that the tool connection and internals are often times the highest resistance to flow in the system. Large ducts mean the fan isn’t working hard just to get the flow through the duct, so it has more ability to pull through the hard parts of the machine. In general, this is a good theory. The down side is if there is not enough resistance in the system you can over-amp your fan and damage it (usually the motor). But you can correct that by adding some blast gates to add some resistance if necessary.

Decreasing one duct size will not increase CFM, it will in all likelihood reduce it due to higher resistance in the pipe. Centrifugal fans like these always pull more when it is easier to pull. What the smaller pipe does increase is velocity. Velocity is determined by the ratio of flow to the cross sectional area of the pipe. So a 6” pipe has less than half the cross sectional area of an 8”, meaning for the same flow the velocity will be roughly twice as high. Remember though you wont have the same flow, so you give some of that back. Velocity is important so you don’t let the dust settle in your pipe.

The 2” hose to a 2” machine is not exactly true either for the reasons stated above. You’re adding resistance in the piping so there will be less work done at the machine. Note that if you are using a high static low flow device like a shop vac, the answer changes on this one.

For your situation, you might be OK with 6” for all the close runs. I would still consider 8” for the long run. That’s just a guess.

Hope that at least gives you something to think about.


-- Part of engineering is to know when to put your calculator down and pick up your tools.

View Fred Hargis's profile

Fred Hargis

6717 posts in 3459 days

#2 posted 01-23-2016 03:41 PM

The only comment I’ll have is about the puny factory ports on the tools. They are way less than adequate abd if at all possible enlarge them…as much as possible. That may take replacing the hood with a shop built one (did that on my drum sander), cutting larger holes into the tool’s cabinet, or just replacing the existing port with a flange. To be sure with some of them it just can’t be done, for those use a good shop vac which can deal with a 2.5” port.

-- Our village hasn't lost it's idiot, he was elected to congress.

View BobAnderton's profile


312 posts in 3756 days

#3 posted 01-23-2016 11:22 PM

My 2 cents, and I’m no expert, is that the duct size strategy depends on whether you’re going to have multiple gates open at the same time, like in a production shop, or if you’re going to only have one gate open at a time. If you’re going to have multiple gates open at different tools at the same time then your branches that go down to the tools will be smaller than your main run. If you’re going to only have one source open at a time then ideally you’ll stay as large as possible all the way to the tool port. You want to maximize the CFM pulled at the tool while maintaining sufficient linear speed (like 4000 linear ft/min) in the horizontal runs to prevent clogs forming.

-- Bob Anderton - Austin, TX - Nova 3000 lathe, Alaskan Mark III mill, Husqavarna Saw

View OSU55's profile


2709 posts in 2955 days

#4 posted 01-24-2016 12:18 AM

The laws of physics dictate that any reduction in duct size will reduce CFM – whether it’s a relevant reduction in a particular case is a separate discussion. For a 2-1/2” end connection, an 8” duct will have less flow resistance than a 6” duct, and flow more CFM. However, velocity may be dropped enough to allow dust/chips to settle out. A 6” circle has 56% of the area of an 8” circle. 4000 FPM seems to be the minimum velocity to carry the chips/dust to the collector. An excellent point has been made regarding the # of blast gates to be open at any given time. More gates need a larger trunk duct so that flow resistance is reduced. Once that question is answered as to your proposed layout (which machines open), you will be able to calculate where a particular duct size is needed, using the min FPM, estimated pressure drop, and the flow curves for the DC’s.

View WoodNSawdust's profile


1417 posts in 2142 days

#5 posted 01-24-2016 02:50 AM

I only plan on using a single machine, besides the dc, at a time. If it would improve the system I can leave one or more extra gates open.

-- "I love it when a plan comes together" John "Hannibal" Smith

View Jim Finn's profile

Jim Finn

2866 posts in 3888 days

#6 posted 01-24-2016 03:04 AM

You may find that the duct will cost as much as the dust collector. I have a two HP dust collector and I ran 6” to each machine. To keep the length of run short I ran the duct on the floor along the wall. No up and back down duct.

-- No PHD just a DD214 Lubbock Texas

View WoodNSawdust's profile


1417 posts in 2142 days

#7 posted 01-24-2016 02:40 PM

Thanks for all of the replies.

Jim: One of the runs will be along the wall just below window level (for the table saw, router, and hose). The cross shop run for the bandsaw to be has to be overhead as I don’t want to trip on a pipe running diagonally across the shop floor, not to mention trying to roll a heavy machine over the duct. The other alternative would be a horizontal run along three walls of the shop probably twice the distance, so would up, over, and down with a shorter run be better or worse than the longer run around the shop?

Fred: you are correct that machines seem to come with a 4” port. I was surprised that even the larger stationary planers only had a 4” port. You mentioned a different hood for your drum sander do you have pictures? I have not figured out how to do this particular mod yet.

For the 17” bandsaw I have two 4” ports (one just below the table and the other at the bottom) I was planning on running 6” flexible duct to the saw and they use a Y to adapt down to two 4” ports. Anyone see any problems with this?

-- "I love it when a plan comes together" John "Hannibal" Smith

View Fred Hargis's profile

Fred Hargis

6717 posts in 3459 days

#8 posted 01-24-2016 02:53 PM

I do, as it turn out. When you look at this please realize I’m very much a “function over form” guy, so I was less concerned about how ugly it is, versus how well it works (fantastic). But this gave me a 6” connection and I didn’t have to cut up the factory hood. That’s actually a 2 piece fixture, the little frame on top of the sander is the first, the hood itself is the second.

-- Our village hasn't lost it's idiot, he was elected to congress.

View WoodNSawdust's profile


1417 posts in 2142 days

#9 posted 01-24-2016 05:56 PM

Thanks for the picture it will help with my design.

-- "I love it when a plan comes together" John "Hannibal" Smith

View Jim Finn's profile

Jim Finn

2866 posts in 3888 days

#10 posted 01-24-2016 07:15 PM

Which ever duct run is shortest and with the less bends is the best. I arranged my equipment along two walls just to avoid a long duct run.

-- No PHD just a DD214 Lubbock Texas

View JBrow's profile


1368 posts in 1886 days

#11 posted 01-25-2016 02:56 AM


I many respects, I am simply restating what Brian and others have already said. I hope I am amplifying these remarks rather than injecting confusion.

Determining the optimum design in your shop requires a three step process. The design process begins with determining the dust collection requirements of each machine. This gives you the diameter of the piping in the branches. One table I looked at suggests the table saw with a dado blade needs 650 Cubic Feet per Minute (CFM) of air flow. This appears to be your largest requirement, but not knowing your shop, I could be wrong. To keep this air moving at 4000 fpm, a 6” diameter pipe is required (diameter = 12 X Square Root [ (4 X CFM) / (4000 fpm X 3.14) ], rounded up if the dust collector can move the volume of air required with 6” duct work – 785 CFM; rounding down increases Static Pressure while increasing air velocity). The other tools you mentioned require somewhere in range of 500 to 600 cubic feet per minute, with the exception of the router table which is lower.

The second step in the design process is determining static pressures. Static pressure is the resistance to air flow. Long runs of smaller diameter pipe that make many turns and include long lengths of Flex Hose will increase static pressure and reduce the CFM. Generally bad are small pipes, long runs, lots of turns, and long lengths of flex hose. The path of the duct is needed before estimating Static Pressure.

Estimating Static Pressure for each branch is tedious but important to get the optimum performance. It requires assigning static pressures associated with the length of pipe in the branch. Each fitting is assigned a static pressure (SP) in terms of equivalent feet of straight pipe. For example, a branch of 6” pipe moving air at 4000 fpm that has a total of 15’ of straight pipe (SP = .68) + a pair of 90 degree elbows at radii of 1.5 x the diameter of the pipe (equivalent to SP of 12’ of straight pipe = .54 for each Elbow) and a pair of 5’ lengths of 4” flex hose ( SP = 1.05 each) results in Total SP = .68 + .54 + .54 + 1.05 +1.05 = 3.86 inches of water in a column. To that add 1” for the filter and the static pressure from any bends made in the flex hose to arrive at the Total Static Pressure in the Branch.

I believe Bill Pentz offers a Static Pressure calculator on his web site. However, I used procedures and Static Pressure tables outlined in “Woodshop Dust Control” by Sandor Nagyszalanczy and “Dust Collection Basics” by Woodstock International to calculate mine. These are two extremely helpful books for dust collection in general. Both books are easy reads, available from Amazon, and inexpensive.

The last step, after you have sized your branches and calculated Static Pressure for each branch, is to check the Fan Performance Curve for any dust collector you are considering. The Fan Performance Curve will show whether the dust collector can move the volume of air required at the Static Pressures of your branches. In the example, a 6” branch with a Static Pressure of 4.86”, the Fan Curve should show that the dust collector will move 785 CFM at 4.86” of SP. I know Penn State, Oneida, Grizzly, and Clearvue all provide Fan Performance curves, which can be hard to find on their internet sites; often in the downloaded manuals. I am not sure whether Laguna or Jet offers Fan Curves for their products.

The duct size going into the dust collector should be sized to slow air down from the 4000 fpm in the branches while maintaining a minimum velocity of 3500 fpm. I am not sure why air should be slowed (perhaps to reduce static pressure), but all sources I reviewed make this statement. Base on a calculation I made, an increase from the largest branch sized pipe by 1” will slow the air velocity from 4000 fpm while keeping the velocity above 3500 fpm. Air moving slower than 3500 fpm can result in dust settling out of the air stream.

I assume you are looking for a cyclone dust collector, but you mentioned Jet so I am not sure. A 2-stage dust collector will remove dust from the air stream and thereby prolong filter performance. A clogged filter will increase static pressure and reduce air flow.

I happened across an Oneida product line (Premium SMART Dust Collection Systems – portable 2hp and 3hp), before I purchased Clearvue’s dust collector, that may interest you. It is not too tall and claims to offer variable dust collection for smaller port sizes like those you intend to use. I am not sure about the magic Oneida claims, but may be worth a look. It offers a number of nifty features priced about the same at the Laguna 3 hp cyclone system. If interested, the link is:{D55CD13D-5CAA-4DD1-8646-157B8B3BDF44}}

Lastly, in my setup, I ran 6” duct work throughout. I work alone so dust collection occurs only at one machine at a time. At machines with a single 4” dust port, I added a second 4” dust port using a 4” bi-metal hole saw. These were the table saw, surface planer, and spindle shaper (all with metal cabinets) and router table. A pair of 4” flex hoses connects the dual dust ports on the machines to a true wye that transitions to the 6” duct work. I used a true 6” x 4” x 4” wye fitting rather than a wye fitting with a 45 degree leg entering from the side. The true wye transitions both 4” legs directly to 6”. Otherwise a 6” to 4” reduction occurs somewhere in the branch.

I purchased the true wyes from Grizzly, who calls it 6” X 4” D/C ADAPTER, which is ABS, and cost about $6. I covered the 4” holes cut in the equipment with 4” ANGLE DUST PORT also ABS from Grizzly for $2.50.

View Robert's profile


4287 posts in 2447 days

#12 posted 01-25-2016 11:55 AM

Either run 2 or 3 (probably 2) will end with a 10-20 foot flexible hose to connect to the mobile joiner and planer.
You’ll get tons of advice/opinions. DC systems will work even if designed wrong, they just won’t work as well.

IMO, this is way too much flex. Keep it to last few feet to machine and if that’s not necessary pipe directly to machine. I agree with poster re: port sizes, but I don’t have an issue with my jointer or planer with 4” ports.

Oh, there are big diffs in quality of flex. Make sure you get the good stuff. I think Penn State Industries is a good source for fittings.

I notice Laguna has this 3HP model on clearance for $400 off.

After designing/building/revamping 2 DC systems, in retrospect I should have paid someone to design it for me.
Penn State Industries and Oneida have design options they apply the fee to the collector when you buy it. I think their units should be on your list.

Well worth it IMO.

-- Everything is a prototype thats why its one of a kind!!

View Redoak49's profile (online now)


5018 posts in 2954 days

#13 posted 01-25-2016 12:28 PM

A very good thread and cuts to what I think is key to dust collection.

The key is the dust ports on each machine being not large enough. Clearing we need mfg to provide options for 6” ports. I do not really like the idea of cutting into an expensive table saw. On my planers it would be difficult to add a 6”port. The same goes for a drum sander.

So if I have a 3 hp cyclone and use 6” duct and then taper it to 4” at the machine port, how much, if any, improvement is that over just 4” ducting?

View Severian's profile


7 posts in 1818 days

#14 posted 01-26-2016 01:08 AM

A few months ago I was looking into DC solutions and spent so much time researching that I became sort of numb… Some people recommend that you cut 6” ports into all of your tools, but that made me nervous.

Ultimately I decided to just chill out and build a system. I chose the ClearVue Cyclone. It was expensive, and was something of a DIY project to install, but I really love it. I used 6” sewer & drain pipe (the heavier green stuff, which was readily available and not difficult to work with) and followed the basic advice on no 90 degree angles, limiting flex runs, etc. I didn’t change any tool ports; for instance my Dewalt planer and Sawstop table saw only have 4” ports, and I didn’t Frankenstein them to 6” ports despite the advice some folks gave. (For those with a Sawstop, adding the Sawstop overarm guard collector is a massive improvement over using just the standard port, IMHO. I also find that the Incra Wonderfence really helps with router table DC.)

The result? I’m very, very happy with the system. Other than the troublesome mitre saw, my tools create almost no dust in the shop, either chips or fine dust. I just planed a bunch of 5/4 oak today and you can hardly tell, the shop is so clean. I’m glad I didn’t succumb to paralysis by analysis and just built my system.


View WoodNSawdust's profile


1417 posts in 2142 days

#15 posted 01-26-2016 01:19 AM

... spent so much time researching that I became sort of numb… I m glad I didn t succumb to paralysis by analysis and just built my system.


- Severian

Guilty of both. I read and study so much I can’t remember what I am doing and become gridlocked.

-- "I love it when a plan comes together" John "Hannibal" Smith

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