Dust collection question

I still have my fancy new 2HP Grizzly Cyclone set up and ready to go, but with no ductwork installed. I borrowed the book "Controlling Dust in the Workshop" by Rick Peters from a buddy hoping that it would answer a few of my simple questions, but so far all it's done is confuse me.
For the most part, my machines all have 4" dust ports, whereas the collector has a 7" input port. My "understanding" (based on *what* I'm not sure) was that I should run 7" ducting from the collector and defer the reduction to smaller diameters in a gradual fashion, perhaps based on my branching needs and/or some magic formula I thought the book would reveal. However, it instead seems to imply that I could (should?) reduce from 7" down to 4" right at the collector and run nothing but 4" ductwork the whole way. Is that logical? Which approach is "correct"?
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Works for me. I only run one machine at a time and my runs are not all that long.
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On Thu, 05 Mar 2009 21:29:45 -0600, Steve Turner

You are touching on the balancing act between two mutually exclusive requirements. First, the requirement to keep the ducting as large as possible to reduce the pressure loss in the ducts and the flow rate (CFM high) and, second, the requirement to keep the velocity in the duct high enough to keep the dust and chips entrained in the airflow. Large ducts = lower pressure losses and lower velocities. Small ducts = higher pressure losses and higher velocities. So you're looking for a balance that keeps the velocity high enough at the same time it keeps the pressure losses to an acceptable level.
Ok, that's the background. Everything from this point, either from me, or anyone else not intimately familiar with your layout is a SWAG. But, conventional wisdom holds that the drops can be smaller than the main trunk so my SWAG is that you should maintain the trunk at the larger diameter, 6" or 7", whichever matches best with the availability of duct pipes and fittings, and transition to 4" diameter at the drops. Your DC should be able to maintain enough airflow in the 6" or 7" ducts to keep them swept clean.
Tom Veatch Wichita, KS USA
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Tom Veatch wrote:

Awesome description Tom, thanks!
I thought a little bit more about this last night and I got to wondering about the simplest case. Suppose I had a single "dust generator" with a 4" port some distance away (say 32 feet) from the collector. Would the absolute ideal situation be to construct a custom "cone shaped" duct that increased in size from 4" to 7" along the entire length? Assuming "yes", would it be a simple linear increase or an exponential one? In other words, if I was to attempt to replicate this (assumed) ideal situation with one section each of 4", 5", 6", and 7" ducting (and three reducers), would all pieces be of equal length or otherwise?
(feeling rather anal this morning)
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In my feeble opinion, price of pipe will heavily decide what size.
Most people use the 4" because (a) Cheap (b) readily available
It sounds great to recommend 7" pipe until you price out what pipe costs, and then find out what each "drop" is going to cost.
The other major problem is availability, which you will find is almost zero at most local spots.
Try to google up some 7" PVC pipe
Here are a few examples of other sizes.
http://www.plumbingsupply.com/pvc.html#45
http://www.charlottepipe.com/cpf_PriceSheets/ProductsView.aspx?Select1g0&Level=Current&lng=english
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Pat Barber wrote:

http://www.charlottepipe.com/cpf_PriceSheets/ProductsView.aspx?Select1g0&Level=Current&lng=english
No plans to use PVC...
I think galvanized HVAC will do me just fine and it isn't all that hard to find in the 4", 5", 6", and 7" sizes, but the "laterals" are where I'm having trouble. It's hard to find a good variety of size combinations at a reasonable price. I think I can meet my needs with the various pieces I've found at the Borg, but most literature I've read suggests using ducting that's 26 gauge or thicker and the sample pieces I brought home from the Borg the other night are 28 gauge...
Luckily, I have about 30 feet of 4" galvanized and stainless *spiral* at my disposal (which my buddy has leftover from his shop) so it's mostly only the larger sizes that I'll need to worry about.
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Buy the real thing...
http://www.ductincorporated.com/index.html
http://store.oneida-air.com/ductwork.aspx
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Pat Barber wrote:

I'd love to... if I was made of money.
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On Fri, 06 Mar 2009 09:17:49 -0600, Steve Turner

You're really proposing a 4" to 7" transition that's 32' long. Your maximum airflow, minimum pressure drop condition would be to maintain the 7" diameter duct as far as you could, stepping down to the 4" diameter as close to the "dust generator" as possible. A long, slow tapered transition from the 4" to 7" diameter would help reduce turbulence and pressure losses at the transition but, I don't believe you'd get any measurable benefit out of making the transition 32' long as opposed to a couple of feet.
Tom Veatch Wichita, KS USA
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Think of the pressure in the pipe - in the pickup and transport area you have small pipe - higher velocity air - causing turbulence and transport. When it expands from 4 to 7 the pressure drops and the material flows not slams into the machine. If the material went to fast, the far side might be beat constantly and fail.
A local pickup area might be a thin slit for high velocity or a small circle...
Being a hose from an input gate that reaches other parts of the saw or machine.
Suck wood and metal out of machines, blowing them clean might fling material hard into joints. Might jam machinery.
Martin
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On Thu, 05 Mar 2009 21:29:45 -0600, Steve Turner

You may not need it with a 2HP unit, but you will get slightly better results running a main 7" line with 4" side branches. Using all 4" pipe makes installation a little easier, and probably fine unless you are producing a lot of dust. Power varies with length, connections, elbows, turbulance, DC size, and piping size.
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