Why 6" ducting?

[SpiderDave]

I kinda have ideas from what I've read.   But I was hoping you guys n' gals could help me to understand this better before I go put my system all together and wonder if it could've been better if I took the time needed.  I'm not expecting perfect, I just want to find that balance of performance + budget = not wanting to re-do it anytime soon because it fell short of needs or expectations.  

I've noticed how common it is that people with 3HP dust collectors that have an 8" inlet/outlets like myself, will reduce the piping or ducting mains to 6".  I'm not knockin' it at all, it's what I'm planning on doing too.  Mainly because that's what the cyclone has on it, not much choice.  But why do they make it that way for a machine that typically has 8" in/outs?

My first thought's on duct reduction was the lower cost of course, or availability in parts.  Or that most cyclones I see that fit the 3-5 HP units have around 6" piping anyway.   Is there a functional purpose, financial, or both?  I have a lot of ideas and reasons to why this is, but that's all they are.  So, I am curious to hear from a real person than more mind numbing random internet searching - why it's so common to go from 8" to 6" ducting?  Won't an 8" pipe keep dust suspended too? Or does it focus the speed better?

One the subject of keeping the ducting larger and as a related or extended question that mostly points towards people like me that only hook up one (or two) tools at a time on our system:  If it's better to keep an 8",... or even 6" ducting, then why go down to 4" at the tools?  I know most come with a 4" (or 2") port, and some even have two of them.  But why not use a blow by or bypass style connector to keep the flow from being restricted on the larger hose sizes to the machines being used?  Maybe it's too much monitoring?  Or to have a second hose that has a hood to help catches what escapes the first connection and also helps maintain the extra air flow in the mains that the single 4" end couldn't keep up with?  I've found this subject come up on searches, but I don't remember ever finding anything definitive on it. Any help to offer before I start putting my DC System together?

[Cian]

Here's a good start:
https://www.oneida-air.com/blog/how-to-s...t-diameter

I'm traveling at the moment, but I'm sure others will chime in too.

[SpiderDave]

Here's a good start:
https://www.oneida-air.com/blog/how-to-s...t-diameter

I'm traveling at the moment, but I'm sure others will chime in too.

Thanks, I've read that one over a few times.  It made a great argument why not to use 4" mains on a fairly straight forward set up.  Their design isn't too far from my own actually.  That article is one of the reasons I have so many questions now  Unfortunately the rest was somewhat lost on me - too close to the trees to see the forest perhaps.  I've shown those graphs to 2 people that are pretty intelligent and right off the bat they couldn't make sense of half of what they were displaying without a complete understanding of those charts first.  Maybe if they'd spent more time making sense of it, they might've understood it more clearly.  I understand what I read, for the most part.  But applying it in lamens terms escapes me.  I'm sure by the end I'll be going ohhhhh, okay now I get it, duh.  I'm just not there yet.  Individually, I understand static pressure, friction, air speed, FPM and CFM's etc and most if not all of the things they talk about in that article.  But collectively or to apply it all, I've got a lot more to learn in using it or putting it all together as a whole.  Cian, hope your enjoying yourself and have a safe trip.  Thanks for chiming in!

[Cian]

Hey Dave -

In summary, what I got from that link, other articles, as well as what other more knowledgable folks have posted here over the years is that you need a certain level of CFMs to be able to effectively extract sawdust from the source machines as well as be able to keep the debris in suspension over the total run of your piping.  6" is a happy medium for hobby shops where 4" becomes too restrictive for most 2-3hp machines to pull.  Sure, one can say that velocity will be higher with 4" and you may be able to get away with it for shorter runs, but the friction stacks up dramatically over a longer run and will effectively overcome your velocity.  But I'll let smarter folks explain the science, and I'll just offfer my main layman's reasoning for why "I" went with 6 inch:

1.  MATH. I calculated the inches of water drop per linear foot of 6" run and then also substracted the loss imposed by each of the fittings.  Even with 6" pipe, I was slightly below where Oneida recommends I needed to be at my farthest tool which is a 15" planer that is over 30' away with turns in between.  4" tubing wasnt even remotely close IIRC.  However, gosh be darn, that cyclone pulls pretty much all the chips from that puppy all the way at that far end(!), so either my math was off (most likely) or my cyclone is underrated.  And I figured if it can pull the chips/dust from that major chipmaker all the way at end of that long run, then it will be fine for everything else in between.  
2.  BECAUSE ONEIDA SAID SO. I called Oneida and spoke with their ductwork design folks and described what I wanted to do with my total run lengths with the machine I had in mind.  We went over my layout and calculations, and they didn't hesitate agreeing with my 6" assessment.
3.  COST. #2 was relieving because (arguably most importantly) lightweight 6" PVC S&D piping and fittings are commonly available at the local Borg.  7" or 8" PVC and fittings would have been a special order and MUCH costlier.

I went with reducers to 4" gates for connections to the machines themselves, because 4" gates were all that Rockler and Woodcraft had locally at the time.  Plus I didnt want to start cutting sheet metal off of my machines(!) to use 6" gates. 
 I was concerned at first, but my 4" flex runs are relatively short.  

Ultimately, the most subjective display of my system's effectiveness is a 22" drum sander at about the 24' mark, and I cannot see nor smell any wood dust at all when using that machine.  There is no settling of sawdust on the floors either, but I do also run an air scrubber.

I also have a floor sweep about midway in the piping as shown below.  With the cyclone running and that gate open, I can just use a broom to literally fling any debris and even small sized cutoffs on the floor toward the sweep and it sucks up everything.  I can hear the chunks of wood travel up the 8' vertical wall of piping to the ceiling, do a 90 degree turn, run across the ceiling for an 8' run, do another effective 90 degree turn, then 12' run into the cyclone body itself, and I can hear the large heavier chunks bounce very quickly all the way through.  

So, yeah 6" works, but that may be half the equation.  You'll also need to have a dust collector that can do it justice.



Hope this helps.

[Alan S]

When sizing the ducts, one consideration is that you want lower friction so more air will move.  You got that.  It's the part that makes 6" duct superior to 4" in many cases.  

If I understand, you are asking why not increase the duct still further?  The other consideration is that as you increase duct diameter and slow down the airspeed, at some point the airspeed is too slow to carry the dust and chunks of wood, which will settle into the duct and may eventually clog it.  At first glance this seems unlikely, as accumulation of stuff in the duct will narrow it and increase the airspeed so you might think it would clear itself out enough to keep working.  But it can clog because the crud accumulates unevenly, and too much in one spot is hard to get moving.

That and the increased price of duct make a particular size of duct preferable for a given amount of airflow, frequently 6" for a hobbyist shop.  As Cian shows, it's enough.  Note that the need for high airspeed to carry dust is not present between cyclone and blower or between blower and filters, so these can be larger to reduce friction.  (Assuming the usual pull-through setup.)

[fredhargis]

One thing I want to mention: just because your tools have a 4" (or 2" port doesn't mean you have to use them. I've taken everything I can to 6"....easy to do on the jointer, not so much on my old drum sander where I had to make a complete replacement top. Many of the newer table saws are very difficult to upsize as well with the internal blade shroud plumbing. But you seem to assume that because the DC has an 8" inlet you have to use it, but do not make the same assumption on the tool outlets.

[BloomingtonMike]

One thing I want to mention: just because your tools have a 4" (or 2" port doesn't mean you have to use them. I've taken everything I can to 6"....easy to do on the jointer, not so much on my old drum sander where I had to make a complete replacement top. Many of the newer table saws are very difficult to upsize as well with the internal blade shroud plumbing. But you seem to assume that because the DC has an 8" inlet you have to use it, but do not make the same assumption on the tool outlets.

My ICS sawstop was that way with 4" hose internal and internal shroud. I wonder how many people would mod their sawstop? I actually bolted on a flange that encloses that port and opens up the cabinet as well. I have an 8" drop to the saw though so I had some room to work. Positive I could have done the same with 6" though with less cabinet opening up.

[BloomingtonMike]

IMO, many folks are choosing 6" because its what they can get in PVC at their home center on the cheap(ish) or cheaply at hvac places. For many collectors and layouts it is NOT the ideal solution and folks are not running the numbers on the amount CFM needed to keep chips in flight and the static pressure they need to overcome in their layout. It is easy and cheap. Simpler just not to do math and research and not pay for a proper setup.

You spend some time designing your layout, looking at your tools and their chip and dust particle requirements, your layout, and the machine you have you will know what it will cost you. Many folks that say I am just a hobby shop I don't need x size mainline or drops are just guessing. Their shop, whatever makes them happy.

You do it right though according to science and physics, and its crazy nice.

AirHandling has a great doc on sizing your duct work correctly.

[BloomingtonMike]

http://airhand.com/wp-content/uploads/20..._Guide.pdf
Air Handling doc

Link from this page:
https://airhand.com/designing

[SpiderDave]

Thanks guys, 6" seems the way to go. I agree I don't have to use it because it's there, but couldn't help but wonder that since it is, why not, yuh know? :-) Sounds like it could backfire since my blower's most likely not up to the challenge. When I read about guys keeping an additional gate open or adding a dust hood to allow for more flow and less restriction at the inlet end of the line. So, a bypass or compensated type of connector so you came to mind so you wouldn't need to change 4" connection ports to hook up to them. That's certainly no argument, just explaining where it all came from - you probably already figured that out. I did check into the price difference between 6" & 8" pipe earlier on, jeez, talk about sticker shock! Unless there where major benefits, I don't think I would've been patient enough to wait for it to be shipped special order either. I have seen quite a bit of straight pieces on C.List, but not sure it's worth running around after the pieces even if I did want it. As always, I'm grateful for all the help and education you all take time out for.