DC Collection Theory / Question ....

[Tony Falotico]

Okay guys, read this through before responding ……………………

Back when I was doing land development work, when designing a municipal water supply system you did your best to avoid dead ends. Whenever possible, you loop your system back to the supply main (or secondary feeder line), this way; there are multiple paths from the supply to the demand. The more loops back to the supply, the better. It allowed you to maintain a higher flow rate with less pressure loss.

Okay, will this theory work with a dust collection system? My shop is 20’ x 28’. I’m thinking of one loop around the perimeter, feeding direct to a Jet 1100 DC. This way, I’ll always have two paths back to the collector, increasing volume significantly. This is contingent on the machine hook-up being large enough not to be a restriction on the system.

Those of you who have the Jet DC (or similar) try this. On the in feed with two 4” openings, cap one and watch the airflow in the collection bag. Then uncap the second one and see the difference.

Okay, now it’s your turn to let me know what I’m missing here. I’m sure if it is a good idea, it would have been adopted long ago. But be gentle on me.

[John Miliunas]

Tony, I’m not a DC collection specialist, nor have I ever designed water delivery systems. Take this with a grain of salt for what I consider to be just logic on my part. (That and a WAG!) For DC, the one “instant” problem I see is that you want to keep the airflow as smooth as possible. Hence, avoid 90* angles, nice smooth wye’s, limit use of flex hose, etc. That said, what happens when the perimeter system hits the halfway point? In other words, the wye’s would be switching direction. I think this would make for an extremely *inefficient* means of transporting the materials within. I can certainly see where in a supply line scenario (going OUT to the branches) this might be preferred, but in this case, those branches are doing the reverse.

Secondly, even if what you’re describing did work, it seems to me that you’d be buying more duct than would be necessary to do the job from a single main trunk, branching out to the individual work stations.

Then there’s the issue of cfm from your main unit. Is it really capable of pulling that much volume over an extended run? Lastly, I think you’d be well served to ditch as much of the 4” as you can, go to, at least, 5” for your main trunk and only drop down in diameter at the last possible point before entering you collection point. Again, just my opinion on all the above and you know what opinions are worth! :rolleyes: :cool:

[Ken Garlock]

Hi Tony. One thing you must remember is that you are dealing with a compressible gas, whereas with water, as you know, does not compress worth a hoot. The incompressible attribute is why you need a loop-back in your water system, IMO. With a dust collector system, the loop back is implicit, you pull air in through the ductwork, and expel it through the filter. The expelled air will take up the void left by the air you sucked into the duct thus completing the loop.(Actually there is no void, but it think it makes the point.)

There are many things to consider when building a dust collector system, here are a couple that come to mind.

1) Build one that will suck the chrome off a trailer hitch.

2) Do not believe the people who tell you that 400 CFM is sufficient air flow. 0ver 800 CFM at the tool is a better figure to design for in order to assure the dust keeps moving in the duct work. Use at least 6" dust everywhere.

3) It is the under 2 micron dust that is the most dangerous to your lungs. Your body can naturally trap dust over 2 microns.

4) Do not read Bill Pentz website, STUDY IT. Bill has spent several years researching the dust collection problem due to extreme lung problems coupled with a desire to do woodworking.

5) Do not believe manufacturers CFM figures, with the exception of the Oneida people. From everything I have heard, Oneida gives real world numbers.

6) See #1 above.

Bill Pentz dust collection site (http://cnets.net/~eclectic/woodworking/cyclone/index.cfm)

Oneida Air Systems (http://www.oneida-air.com)

These are a few of the points I have picked up over the past couple years from reading this and other sites. I do not represent myself as an expert by any means as I learn something about dust weekly.

They say to buy quality and spend your money once. A paraphrase might be build your dust collection right the first time, and not need to do it over again. :)

Good Luck on your collection system....

[Jim Becker]

One further comment...a perimeter duct design is about the least efficient design you can put in; it increases the duct length significantly which reduces performance. On the Oneida site that Ken links to, there is an excellent free document about duct work design which shows some very clear illustrations about routing...what works and what works not so good.

[Tony Falotico]

With a dust collector system, the loop back is implicit, you pull air in through the ductwork, and expel it through the filter. The expelled air will take up the void left by the air you sucked into the duct thus completing the loop

Ken and Jim, I'm not sure you understood what I was getting at. The loop would be connected to TWO intake ports on the DC, thereby giving two routes for the air to go from the intake point to the DC. Hopefully this schematic will explain it better. It still seems to me that it will significantly increase volume.

John, your points are well taken, large radius sweeps are always better.

[Jim Becker]

Tony, two things occur to me. One, two 4" ducts are too large for your current DC when combined together and two, the effective distance things need to travel is much, much more than it needs to be. One of the basic rules for dust collection is to keep your duct as short as possible. Each extra foot robs you of CFM...and CFM is what DC system depend upon to move material. The more air you can move at a given velocity, the more material will be carried along with it. Your current DC will likely perform best with a 6" main duct that takes the most expedient route across your shop with drops in appropriate locations for the tools. It will look like a "tree" when you are done. Avoid perimeter designs if at all possible.

[Joe Breid]

Tony,
I have also used loop systems on water and compressed air piping. The loop provides a means to serve several locations and still keep the pressure loss in the system at a minimum. The loop also lowers the velocity of water so that if a valve is closed rapidly there is less of a chance of water hammer. Your design does have the potential to increase airflow if it produces a smaller pressure drop than a single pipe would.

A dust collection system has another variable that water and compressed air systems do not. The air movement in a dust system is used to move product (wood chips) from the machine to the collector (not to just move the air from one place to another). To do this efficiently the velocity in the ductwork must be above what is called "transport velocity".

If the airflow is below transport velocity the product being moved will settle to the bottom of the pipe and eventually can cause a plug. If there were two paths one would begin to plug which would further reduce the velocity in that pipe and would cause more sawdust to drop out.

Where I work we pump food particles (beans) with water and would not consider running in a loop system. We have experienced plugging in the pipes when a pump becomes worn and the water velocity drops below the transport velocity.

I hope this all makes sense and helps.

Joe

[Ian Barley]

At first sight a loop appears attractive and superficially should provide benefits in ease of layout - but - the first and most obvious reason not to do it is because the big boys don't and if they could save time and money in this way they would.

I think that the simplification you have used in your diagram confuses the thoughts. Consider your the "Dust Source" duct. At the point at which air is moving into this duct it is travelling at a speed - for the sake of easy maths lets say 10 units/sec. If the cross section of the pipe is a unit of 4 then the air is moving at a rate 400 units/sec. When it hits the bifurcation it suddenly has to fill twice the cross section. There is no more air volume available so speed will decrease to half - 5 - 200 units/sec in each pipe section. (The next bit is still trying to make sense of itself in my head so bear with me). Beacuse the airflow is diverting in two different directions I think you have to consider that what will happen is that the total amount of air moving is 200 units one way (-200) and 200 units the other way (+200) which means that at some point the amount of air being moved is 0 units. This is bad.

The problem with comparing extraction ducting to systems like compressed air and water is that you are concentrating on different outcomes. If I turn on a faucet and draw a gallon of water it doesn't really matter to me if 90% of the water arrives from one leg of the loop and 10% arrives from the other. Whatever the speed of flow, I get my gallon of water . In an extraction system the outcome we are looking for IS the speed of flow and without it the system doesn't work.

[Tony Falotico]

Thank you all very much for your input, you have convinced me, I will put this theory to rest. I kinda figured if it was such a good idea you all would have adopted it a long time ago.

Thanks for your patience in the explanations. You all bring up good points I didn't consider.

Guess I need to go looking for a source of 6" pipe. The cost of pipe wasn't too bad, the quotes I got on fittings was pretty high, $35 for a 6" Wye :eek: .

[John Miliunas]

Guess I need to go looking for a source of 6" pipe. The cost of pipe wasn't too bad, the quotes I got on fittings was pretty high, $35 for a 6" Wye :eek: .

Tony, when I was putting my DC system together, I got *most* of my fittings here: http://www.kencraftcompany.com/24_gauge_sheet_metal_wyes.htm

Not affiliated with them in any way, shape or form, but good products at a great price point. :cool:

[Ken Garlock]

Last thoughts Tony. In dust collection you never want to use a 90 degree Tee. It raises havoc with the air flow. A sanitary wye or a pants-leg wye are the preferred fittings.

Lastly, don't feel "put upon" by our discussions of your idea. Often things that seem correct in one discipline just don't carry across into another, even though it looks like they should :) Life would be so much easier if they did....

Best of luck on your project!

[Tony Falotico]

Lastly, don't feel "put upon" by our discussions of your idea. Often things that seem correct in one discipline just don't carry across into another, even though it looks like they should :) Life would be so much easier if they did....

Best of luck on your project!

The absolute beauty of Sawmill Creek and it's inhabitants is that we can have great discussions, we can disagree, we can correct each other, we can throw out crazy ideas for discussion, and we all walk away feeling good about it.

That's why I keep coming back. Thanks for being straight forward and frank with your answers, when my thinking strays I greatly appreciate being gently pushed back on track!

[Chris Padilla]

Good for you, Tony. In the OT section, I have a thread going about the NEC and rewiring my garage. I went in with certain ideas and now I see I may need to rethink what I was doing much to my chargrin but I absolutely appreciate the extra knowledge/advice...even if it wasn't what I wanted to hear.