Dust collection and ducting....Help set me free!

Okay, so I am opening a chapter of this subject again for debate. I would like to get some mech eng boffins to weigh in here on this subject with some science.

I saw the latest post on Stu's bench and rather than hijack his thread on the bench I decided best to start one of my own.

Stu went and put a 4 inch connection onto his saw and then feeds that ultimately into a 6 inch pipe.

Here is my question relating to this.

From what i have read and researched the 4 inch diameter duct whilst supplied by all the manufacturers limits the cfm flow of air to around say 500 cfm. As I understand it the larger diameter 6 inch increases this permissible max airflow.

What i do not understand fully is what happens when 4 meets 6. Does the flow of material (dust) stall at that junction due to inability of the size of the prior pipe 4" to deliver enough air to feed the 6" main.

I would love someone that trullly understands this and can supply the math behind it to weigh in here.

I need the math to understand and believe all this.

Something does not seem right here to me from what i have read.

The next thing is what size should the port be to have the best dust removal without build up in the pipe? Regardless of what the manufacturer has fitted.

I have read from the famours Bill Pentz site that 4 inch is inadequate on a TS when one wants to deal with fine dust and not just "chip collection"

I have read enough on this subject in terms of conflicting opinions that I am now totally confused and only the math will set me free.

I also buy the issue that there is no perfect fine dust collection system, that you need to capture the dust at source, and that a huge %tage of the fine dust created at machines gets into the air directly from the active area where cutting, planning etc takes place.

What i percieve is that rather than actual vacuum (high velocity cfm) for the purpose of fine dust collection one is looking at volume of air movement and frequency or rate of flow (turnover). Hence the 4 inch port limits that compared to 6 inch port.

But then one has to have a fan on the other end that can cause that volume of air movement. Which in Stu's case is capable of the movement.

So weigh in all you pros. I know form pictures i have seen of some of your shops (the pros that is) that some of you dont seem to take the fine dust issue to heart enough in my opinion.

Looking forward to the responses here.

Just in case someone reads this post with an error of parallax again, this is nothing to do with critizing Stu's config. I am trying to learn and understand and fundamentally temper my own nuttyness towards this subject. My most expensive investment and part of the reason for my own dedicated shop was dust treatment and control.

Fine dust collection has to do with the flow of the air, not the vacuum.

The higher the flow, the better the fine dust collection.

You need a large impeller and a strong motor to get the full benefit from a 6" duct, I have both and I have been very please with the performance of my set up.

I have had to make a compromise in hooking up the DC for under my back bench, I actually bought all the 6" stuff and tried to make it fit, but it was just not going to happen.

On the dust dropping out of the air flow, I did not see that happen when I had the SawStop, which has a 4" connection, hooked up to a 6" hose, via an adapter, no dust drop out at all, my pipes and such are all nice and clean.

I know that a lot of the big time commercial set up will run 10" main ducts, to 6" sub ducts and then either 6" to the machine, or even down to 4" to the machine, depending on the machine. I would imagine that if there is a lot of dust dropping out of the airstream, they would not do it like this.

Cheers!

I'm not the guru you're looking for, and I've forgotton all the math I learned in High School (didn't have to have any for my degree in college) BUT: If I read what you wrote correctly, you are asking what happens to the air flow when the 4" pipe dumps into the 6" pipe. Does the CFM actually drop particles out of the stream because of the flow speed drops when it hits the larger pipe. I would say, yes this would/could happen. To keep this from happening, there needs to be some air flowing in the 6" pipe from a source further away from the DC than where the 4" dumps into the pipe. That would keep the air flow up in the 6" pipe so that the heavier particles don't drop out. Say, a blast gate partially open, so that when the 4" pipe dumps in, the flow in the 6" pipe is close to it's max capability. Does this make sense? Again, no proof to back it up, just my common sense kicking in. Jim.

edit. Stu's post makes sense too. I never understood the step down in the duct work of a DC, except in big commercial installations, and it is stepping down as more pipes go off to machines so that when 2 6" pipes that are collecting at the same time come together, you need more pipe to handle the flow and keep it up. Otherwise it would slow down in the feeding pipes and create problems there. Just like a HVAC handler steps down as it goes along it's path because there are outlets letting part of the flow out of the pipe to cool/heat the room. DCs just work backwards because the flow is backwards. Jim.

Rob Keeble said:

What i do not understand fully is what happens when 4 meets 6. ...<snip> ... I need the math to understand and believe all this.

Click to expand...

I'm also not a mech eng, but the math is not that complicated. When 4 meets 6 the airspeed (FPM) is more than halved, because the cross section of the 6" duct is a tad more than double that of the 4" duct. I say the math is not that complicated because even though the cross sectional area of the duct is calculated with the formula (pi x r^2), when comparing 2 ducts (pi) can be factored out of the equation. Thus the ratio of a 4" to a 6" duct is 4:9, and if the FPM in the 6" duct is 3500, the 4" is at ... 7875 FPM. BTW, from Bill's static calculator, you'll lose about .27" of static pressure for each foot of 4" duct @ 7875 FPM.

When working with round duct I normally keep these factors in my head: 2(1), 3(2.25), 4(4), 5(6.25), 6(9), 7(12.25), 8(16) ... 18(81). My longest duct run (7") goes to a 6" jointer and a 13x20" bandsaw. There is a 7>4-4-4 transition after the blast gate. One 4" goes to the jointer, 2 to the BS, and all are open when I use that run so as not to starve the 7".

Rob Keeble said:

The next thing is what size should the port be to have the best dust removal without build up in the pipe? Regardless of what the manufacturer has fitted.

Click to expand...

It depends on what the machine is spewing. In general, finer dust requires HVLP which isn't conducive to picking up chunks of 2x4.

I'm not a mechanical engineer but, I did stay in a Holiday Inn Express last night so I'll go ahead and start talking anyway even though you asked for qualified responses . . .

My TS has about 7 ft of 4" flex that connects to a 6x6x4 wye to transition it into my 6" PVC main which transitions to a 7" inlet at the 2HP cyclone with a 14.5" impeller. I don't have any dust left laying in the hose which is clear and easy to check. The bottom of the TS stays pretty clean also so there's my unscientific real-world input ;-)

Although I am not a mech engineer, here goes;
There are 2 things that have been mentioned, air speed as in FPM, and air volume in cfm(cubic feet per minute).

If there is only one air inlet and a 6" duct is connected to a 4" duct then the air volume is the same in each section. The air speed at any point in the pipe
is equal to the air volume (cfm) divided by the cross-sectional area. This will mean that because the cross sectional surface area of the 4" section is smaller than the 6", the air speed is greater in the 4" section.

The down side of the 4" section is that it will increase the air resistance of the pipe thus reducing the volume. The amount of volume reduction will depend on the length of the 4" section.

This is sorta an answer to your question.

I too felt like it was a waste to have a 6" reduce to a 4".

So I feed 2 4" feeds into a 6" main. I think this makes sense. It does seem to work. I did this on my bandsaw also.

Bartee scores again. I didn't even think about that; I have a 4" lower and a 4" upper for my TS, 4" lower and 2.5" upper for the RT, 4" lower and 4" plus a 2.5" for the upper on the BS. I guess this explains why things stay pretty clean even though there are 6" to 4" transitions at my tools ;-)