Has anyone measured MPH/FPM on their 4" ducts?

[cgallery]

The thread on the agreed-upon duct size for the HF dust collector (consensus was 4") got me to thinking.

Bill Pentz says:

The amount of air moved like water depends upon pressure and the pipe size. When moved at the up to 12” of pressure typical for hobbyist blowers, maximum air volume in a 3” diameter pipe is about 250 CFM, in a 4” pipe about 400 CFM and about 600 CFM in a 5” diameter pipe. It takes roughly a 5 hp motor turning a 16” diameter impeller to move the same amount of air through a 4” pipe that a 1.5 hp motor turning an 11” impeller can easily move through a 6” pipe. Picking pipe with the smoothest interior helps some, but not that much.

I'm wondering whether anyone has ever used an anemometer to measure the airspeed through their 4" ducts?

I've been playing with a small anemometer and doing just that on my Ridgid shop vac as well as my small DC blower that I'm using for experimentation on my cyclone lids. The #'s that Bill posts are pretty much what I'm seeing, but I wonder whether anyone else out there has tried this with their larger DC's.

[LCHIEN]

I've studied the problem some when I thought I could profile my DC system.
Its relatively simple to convert velocity to CFM if you know the cross-sectional area and you can assume that the air velocity is the same all the way across the pipe.

That's where the measurements fall apart.
the air velocity across a pipe is non linear because of boundary layer drag along the wall and the profile of this varies with the material and roughness and volume of air. Further more air in a pipe tends to have lots of eddies and vortexes so the air velocity measured at one point may be wildly off for a few fractions of an inch away. The recommendation for killing vortices is to have a straight length of very smooth pipe with no bends or changes of direction or diameter for about 40 diameters before and after the measurement. That means 160" or nearly 15 feet before and after or a 30 foot section of pipe for 4" ducting. Then the anemometer has to be smooth and offer no change in the flow pattern and no drag. A rotation cup anemometer like the weather stations use would be near useless for this because it blocks airflow on one side and upsets your pipe smooth flow. You have to use something like a hot wire anemometer which uses a piece of resistive wire that heats up from passing current. the reisistance of the wire varies with temperature so you can tell if the wire is being cooled by air flow...

I tried using pitot tubes for differential air pressure but it was very innacuate for the short piece of 4" ppe I tried. Needless to say, accurately measureung (an by accurate I mean +/-25 or 30&#37 is very difficult for the kinds of systems we have where a 30 foot run drastically changes the chatracteristics of our system.

Maybe someone else can do some magic but what I found from professional guides to measring airflow is that its pretty difficult to get an accurate measurement. Mostly the measurable setups bear no likeness to your actual system.

Now the thing I am thinking is that you take a professional setup with the 30-feet of pipe and accurate air flow measurements with either multiple Pitot tubes or hot wire anemometers so you get multiple measures at different diameters and integrate across the tube. Then you measure the amperage drain and plot amps versus airflow. So at any time by checking the amps you can measure effective airflow. But that's beyond my easy home capability.

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Yeah, I've been experimenting with tubes that are only up to 6-feet long and a (borrowed) hotwire anemometer that is only capable of up to 5000-FPM. I also have a Kestrel weather-vane type meter that is shockingly close to the Dwyer HW unit. Kestrel says their meters trace back to NIST and are accurate to +/- 2%. I have found that they parrot the Dwyer to within +/- 5% and are very repeatable.

The problem is I've had to *******ize many of my measurements because the Dwyer is only good for up to 5000-FPM and I don't want to hose it, so I've been using 6" PVC as my test tube, which kinda throws a wrench in the works in many ways.

But because the Kestrel is good for up to about 8000-FPM and seems to match the Dwyer, I've been using it on smaller ducts just to try to get some relative data.

I've found all the difficulties you highlight in measuring CFM through a pipe using both meters. But when I am being precise as I can, averaging readings over different areas of the pipe, I cannot get more than about 378-CFM through a 4" pipe. I figured that if any other readers here had any meters (perhaps someone here does HVAC work) they may have tried a similar experiment.

Nothing scientific, just sorta interesting (to me). OTOH, the more I read BP's site and compare it to manufacturer data, the more I'm convinced that manufacturers are using FM (F'ing Magic) gauges to test their DC's. The fact that my measurements come in pretty close to what he states as a maximum, and that both #'s are 1/2 to 1/3 the rated capacity of the blowers I'm testing, is pretty amazing.

[Uncle Cracker]

I'm thinkin' you two guys got way too much free time on your hands...

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I'm thinkin' you two guys got way too much free time on your hands...

There is a big difference between Loring and I. He pondered the problem and realized the difficulty before getting started. I got started and then realized the difficulty.

[Uncle Cracker]

You and me both... It's the difficulty that gets him started... It's the starting that I find most difficult...