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Duct sizes for dust collection?

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Forum topic by WoodNSawdust posted 05-11-2015 05:14 PM 935 views 0 times favorited 11 replies Add to Favorites Watch
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WoodNSawdust

1417 posts in 641 days


05-11-2015 05:14 PM

Topic tags/keywords: dust collection hose size question

This morning I started the annual cleaning of the Table Saw. As I was removing the home made dust collection cover and the short metal hose that runs to where I can plug in the 4” hose I wondered if it would make sense that when I replace the dust collection cover I should take the time to make the hole and 36” hose be 6” in preparation for the future project of converting to 6” hose.

A friend of mine used to work on oil rigs and he would always say that it did not matter what size pipe used but what mattered was the smallest (diameter) fitting or piece of pipe used. So, in Dust Collection terms even if I ran a 6” pipe right up to the tool, if I had a 4” port on the tool the entire dust collector would behave as if the entire run of pipe was 4”.

I never could understand Mark’s explanation of this. Can anyone explain why this is? Of it this not true why / how / ???

The quick question would be weither I should take the time and expense to convert this short section of the Dust Collection system to 6” or leave it at 4”

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


11 replies so far

View AZWoody's profile

AZWoody

693 posts in 688 days


#1 posted 05-11-2015 07:54 PM

Think of a garden hose.

Say it’s 3/4 of an inch inside diameter. As you open the valve, you are not getting full flow until the valve is open all the way. Half open, you’re only getting partial flow. The valve is the smaller opening in a length of 3/4 hose in the front, and 3/4 pipe in the back. So the smallest point in the system, which is the valve, will restrict your flow.

If you want to set it up, for future possibilities of using 6”, modify your ports at the machine to 6” and then reduce it to the 4” hose you have now.

View bigblockyeti's profile

bigblockyeti

3668 posts in 1185 days


#2 posted 05-11-2015 08:12 PM

Where ever your oil rig friend got his degree from, someone owes him some money back. The flow of air with suspended dust is not homogenous and the air must sustain different velocities for different sized dust or chips that need to stay suspended in the air to make it to the collector instead of causing a clog. A 6” hose on a 4” port would allow a high velocity (assuming everything else in the system is designed and working correctly) at the port and lower velocity in the 6” hose. This works much better if the size is not reduced until running horizontally to the collector instead of requiring the collector to maintain adequate air velocity in a vertical run with only a 4” port supplying the air.

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patron

13537 posts in 2805 days


#3 posted 05-11-2015 08:19 PM

as i understand this
air will compress under pressure
but not thru suction
(it can flow faster if the suction is strong enough)
i have run 6” trunks to close to all the tools
(in case i ever need to add a bigger one someday)
then take off from the trunk
with whatever the tool allows
this brings the main trunk closest to each tool
so the ‘choked’ runs aren’t so long

shop vac’s run at a higher speed
(hence more noise)
so can have a smaller hose
but a small hose on a system vac trunk line
do more volume
but just choke down with a smaller hose
(like a 2 1/2” or smaller shop vac size
and barely suck at all)

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

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WoodNSawdust

1417 posts in 641 days


#4 posted 05-24-2015 09:42 PM



Think of a garden hose.

Say it s 3/4 of an inch inside diameter. As you open the valve, you are not getting full flow until the valve is open all the way. Half open, you re only getting partial flow. The valve is the smaller opening in a length of 3/4 hose in the front, and 3/4 pipe in the back. So the smallest point in the system, which is the valve, will restrict your flow.

If you want to set it up, for future possibilities of using 6”, modify your ports at the machine to 6” and then reduce it to the 4” hose you have now.

- AZWoody

So, even if I run a 6 inch pipe directly to the tool, if the tool only has a 4 inch port I will get the same performance as if I had done the total run in 4 inch?

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

View rwe2156's profile

rwe2156

2194 posts in 945 days


#5 posted 05-24-2015 09:54 PM

When I set up my system I talked to tech guy at Penn State Ind.
I asked him about this and he said it won’t make a practical difference to change the port at the machine.

I run 6” main duct and reduced to 4” flex keeping it as close the machine as possible, never more than 4’.

Works just fine.

My advice: don’t obsess about this!!

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

View Fred Hargis's profile (online now)

Fred Hargis

3941 posts in 1957 days


#6 posted 05-25-2015 11:47 AM

Let me offer some measurements I just did on my system (magnehelic and pitot tube). I have all 6” ducting, in most cases all the way to the tool. But for my table saw has some internal DC parts that wouldn’t allow me to enlarge it to 6”. So, with 6” duct open all the way, my DC pulls about 1150 CFM (5 HP, 14 1/4” impeller). When I choke that down to 4” I pull about 550 CFM, which is actually quite a bit of air to pull through 4”...it usually tops out at 500. As patron says, air won’t compress under suction so I think your friend was essentially correct. That doesn’t mean you shouldn’t go to 6”...just convert all the tools to a larger port, whatever that takes.

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

View JeffP's profile

JeffP

573 posts in 856 days


#7 posted 05-25-2015 11:53 AM

Inexperienced newbie with an engineering degree and nearly 30 years of experience, weighing in on this issue…

(steps into flame retardant suit)

I won’t bore you with another analogy comparing this system of hoses and ports to some other system that carries other “fluids” with different goals and different parameters.

Instead, let me invite you (however briefly) into the scary world of engineering…in a world where not everyone is an engineer. Happily, some folks on this planet have other things to think about than laminar flow and particle suspension…you know, important things like annoying the crap out of engineers.

Here is a real-world scenario that plays itself out thousands of times a day in any venue involving a mix of engineers and non-engineers:

Pointy-haired-boss: Hey Dilbert, how are you coming on the solution to problem A that I gave you to work on yesterday.

Dilbert: I figured out a solution to that yesterday while you were about half-way through your explanation of the “problem”.

Pointy-haired-boss: That’s excellent, we’ll get the marketing department spun up on coming up with a name for your solution…and we have to gear up the legal department to handle any potential fallout.

Dilbert: There won’t be any fallout.

Boss: How do you know?

Dilbert: Because my solution was to take a nap and see if you had forgotten about the problem by the time I woke up. Besides, I was more interested in problem B.

Boss: Why are you wasting your time on problem B when I clearly explained that I wanted you to work on problem A?

Dilbert: Because problem A is not actually a problem…and problem B may make it necessary for me to find a new job next week.

Boss: Why?

Dilbert: Because problem B has about a 90 percent probability of putting this company out of business in the next few weeks, and problem A will have no impact on the business.

Boss: But the experts in Marketing have already conferred with the Lawyers and determined that problem B is unsolvable. That is why we have the PR department readying a response to the investors…

Dilbert: I solved problem B while I was taking a dump this morning.

Boss: You should have been working on problem A this morning while you were taking a dump…

...

Dilbert: please describe the problem you were having with your 4” hose…

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View bondogaposis's profile

bondogaposis

4030 posts in 1815 days


#8 posted 05-25-2015 01:10 PM

It is a bottleneck, literally. Think of a bottle, it doesn’t matter what the diameter of the bottle is, but the diameter of the opening that determines how fast you can empty the bottle.

-- Bondo Gaposis

View AandCstyle's profile

AandCstyle

2571 posts in 1721 days


#9 posted 05-26-2015 01:12 AM

Wood, there is a difference between gasses (air) and liquids (oil). Gasses will compress when constricted while liquids won’t (to any appreciable amount). However, to answer your question, I spoke with an Oneida technician when I needed to enhance my dust collection and he suggested running 6” tubing (I used S&D PVC) as close to the machine’s duct port as practical, then dropping to 5” flex tubing and finally a 5”-4” metal reducer to the machine’s dust port. The 5” flexible tubing creates much less turbulence (resistance) than 4” (approximately 50% greater cross sectional area) and the air compresses through the short 4” section so the flow is still adequate to move the dust satisfactorily. At least that is the way I understood his recommendation. I just tore my tubing down today and there was very little dust in the tubing and that made me a happy camper. :) HTH

-- Art

View AZWoody's profile

AZWoody

693 posts in 688 days


#10 posted 05-26-2015 02:58 AM

One thing to remember, if you run 6” and reduce it to 4” and have a dust collector designed to run with the 6” ducting, you will be starving your impeller and not get the air velocity you will need in the main trunk.

To alleviate this, you can have a gate at the end of the line that you can open partially to get the desired air flow or, split your 6” into 2 4” lines. One to the table saw port and the other to an overhead blade guard. Then, you’ll have a much better capacity to pick up the fine dust that gets in the air.

Also. having a 6” trunk without an impeller and motor designed to move that volume of air will result in poor collection.

View Fred Hargis's profile (online now)

Fred Hargis

3941 posts in 1957 days


#11 posted 05-26-2015 11:19 AM


Also. having a 6” trunk without an impeller and motor designed to move that volume of air will result in poor collection.

- AZWoody

Key point, this only works when you have enough DC to make it work.

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

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