# DC system design question



## mr.ambiguity (Oct 25, 2017)

Hi guys, got my hands on a early 2000's "reliant nn831". 3hp system, what little info I can find on it claims:

Model Number: NN-831
Motor Specifications: 3HP/3450 RPM/1Ph/60Hz/18A/60 Deg C/220V (prewired)
Inlet Diameter: 6"
air speed: 2300 CFM
Bag Volume: 10.8 cu.ft

I am going to be re-designing/mounting the motor/etc to mirror the wall attached units with a cyclone and (most likely) an exterior vent for the exhaust. After reading tons of Bill Pentz stuff about DC, I'm kinda leaning toward venting outside and sucking up the heat/cooling loss in the garage. I'l likely have many more questions, but to start:


1-I plan on running 6" piping, with 6" drops to the tools. HOWEVER, I'm trying to understand, isn't the required 6" to 4" reduction one needs to plug into most all tools (ie, tablesaw (or worst, 2.5" bandsaw/planer) immediately reducing the cfm capacity of the system? Doesn't this bottleneck AT the tool basically eliminate the benefit of using 6" piping in the rest of the system? I'm buying the Grizzly g0833p, which has a 4" DC port...


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## J.C. (Jan 20, 2012)

Generally, you don't want to oversize your drops. If you run too big of a pipe to a machine and then choke it off with the smaller port on the machine, you could be setting yourself up for clogged pipes because there won't be enough airflow to clean out your larger diameter pipe.


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## TomCT2 (May 16, 2014)

CFM = cubic feet per minute
CFM is not "air speed" and it's rather more than a little bit important to understand the differences if you're going to build out anything more than a very basic and very simple system.

the basics are here:
http://billpentz.com/woodworking/cyclone/ducting.cfm#layout_n_ducting_design

if the blower capacity is 2300 CFM, using six inch ducting will produce at most 2930 FPM air speed
i.e. 2300 CFM divided by (Pi x R^2 = 3.1416 x .5 x.5 =) 0.785 sq ft area

that's fairly marginal when compared to Penz's discussion on ducting size and where size reductions are needed and how all that affects friction losses / static pressure loss / etc.

bottom line: be careful with six inch ducting orientation - and make provisions to 'shut off' unused collection ports.


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## Carl10 (Feb 3, 2017)

I agree with Tom in that you need to understand that there is a difference between CFM and FPM, however I don't understand the example. 

There are several aspects that will help with your decision. First and most important is that the rated CFM on a single stage DC is unattainable in the real world. Most people believe that their DC hose will provide 2300CFM. When testing right at the blower (without the restrictions of the bags, hoses separator housing, etc) you would be lucky to get 1600-1800CFM. But to really know what your machine is capable of producing you should test it with a length of pipe attached. That pipe size will restrict your flow. The smaller diameter the lower your CFM. So a 6" pipe has Pi x R^2 = 3.1416 x 3" x3" = 28.27 sq-in (.196 sq-ft) area vs. a 4" pipe with 12.56 sq-in (.087 sq-ft). You should notice that the area isn't 50% more (4 to 6") as most would think but is over 100% larger. So in a 6" run you need to maintain 4000FPM to avoid clogs of various dust/chip sizes. Testing right at your machine you probably have well over 5000FPM in a 6" duct (this is based on what others with similar units have found). However the more duct you run the air through the more resistance you add to the flow lowering what your system can produce. Reducing the pipe size or adding filtration will also reduce the flow. 

So to answer your question...it depends. With high velocities on short runs dropping to 4" pipes will still provide enough velocity in the 6" to maintain proper flow without clogging. The more restrictions you add (dirt filter bags, separators [trashcan, cyclone, etc], long piping, flex piping) the more resistance you add dragging down flow, resulting in lower cfm. This relates to your 4" branch vs 6-4" tool connection as the longer the 4" pipe is the more resistance you add. In real world testing, the short neck down from the tool to the 6" pipe produced the least amount of flow reduction. It is like in traffic, a 6-4" reduction is like a tow truck on the side of the road. Traffic slows down a lot from 70 in the right lane and some in the middle lane and less in the left lane. After the bottleneck you only get back to 60, still good but not 70. Compare that to a long 4" run or a construction zone that forces traffic to one lane at 35. After the construction you may only get back to 45 and that may not be fast enough.

So before you jump to the neck down solution at the tool you need to make sure you don't have too many restrictions. I know that is vague but it depends on your setup. One guy described his setup with a 1200FM DC and 50 ft of 4" duct. He was measuring 200CFM at the end and was getting clogs.

The best solution is to modify or make larger connections at the tool to get the most flow. Necking down to only one 2.5" hose is too restrictive for a DC and should use a shop vac.

Hope that helps, let us know what you do.

Carl


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## TomCT2 (May 16, 2014)

Carl - my bad . . . 

keeping the units consistent - 
if you divide CFM ( = ft^3/min ) by the area ( Pi * R^2 = ft^2) you get ft/min - or "speed" or "flow"
but the area has to be calculated in square feet, not square inches
which is the Pi x .5 x .5 thing - i.e. 6 inches = .5 ft

. . . which is all okay except for my brain booboo / error that 6 inches is the diameter, not the radius (ack!) so it's
Pi x .25 ft x .25 ft = 0.19625 sq ft
alternately
Pi x 3 inches x 3 inches = 28.26 square inches divided by 144 square inches per square ft = 0.19625 sq ft

then: 2300 CFM / 0.1925 ft^2 = 11719.xx fpm

which is indeed much more adequate for a longer 6 inch install.


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## mr.ambiguity (Oct 25, 2017)

Thanks for ya'll input. I've been getting ready to modify the 'ole reliant, mount it high on the wall and exhaust outside. I'm going to use a cyclone below it. I'm hoping by not using bags, the effect of using a cyclone will be minimized. 

My longest lay of 6 inch would be about 25 ft, with only one real 90 deg "turn". (which i will soften with 2x45's).

I picked up a Grizzly G0833p, which comes with a 4" port. Internally, it runs a 4" hose from that port to the shroud below the blade, and a 2" line up and over the saw to the blade shroud. I'm going to try to run 6" hose all the way to the ports of all my tools, as (i think) the general consensus seems to be. (I'm mathematically moronic, so reading all that purdy numerology made my eyes cross and ears smoke).


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## Carl10 (Feb 3, 2017)

I was wondering what that smell was?!! (-:

Sounds like you will have no problems with your plan and equipment.

Carl


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