DC system flow results

[Michael W. Clark]

Hi all,
I know Jim's link went to an S-type Pitot tube, it will work, but requires a much larger hole and also has a measurement factor associated with it. It is often used for environmental stack testing and very large ducts.

This is the option I would recommend http://www.dwyer-inst.com/PDF_files/122d.pdf. The 166/167-12 is very handy and will accommodate up to a 12" duct. You only need a 3/16" hole in the duct for insertion. The model with the shorter tip (1" I think) would probably be the easiest to use on smaller duct.

[Jim Neeley]

Jim,

What about just using an anemometer at the inlet and calculating flow based on airspeed and opening size? Do you think that would be reasonably close? The manometer I have is only a single port so it looks like I'm out of luck there.

Theoretically, yes. The difficulty is that, unlike blown air sucked air comes from all directions at once so air speed falls off at roughly 4 times pi times the distance squared. I wouldn't hazard how close it would be but I wouldn't wager much on it being really close.

As for my PDF link, although it's designed for tubes down to 4" diameter, as Michael says, a smaller one would be better. When I posted the PDF it wasn't to provide a product recommendation; just to show the methods.

Jim in Alaska

[Ole Anderson]

Well, I got an early Christmas present from my son, the one that got me a great price on all my ductwork. It was an anemometer. Not real high end, but it does the job. I just tested it on my 4" router table dust inlet. The diameter at the end is 4-1/8" so that gives me an open area of 0.0926 sq ft. The velocity measured was 6160 ft/min, so the flow rate was 570 cfm. (the meter is reading 616 with a x10 that you can barely read in the pic) Not bad for a 2 hp dust collector (Oneida Super Dust Gorilla). Here is to those that say a 2 hp just doesn't cut the mustard. Hmm, I wonder what the 3 hp would do? And this was with about 5 feet of 4" slinky hose, a 4" blast gate connected to about 10 feet of 5" duct with an increaser and one 90 and a 45 elbow before hitting the 7" main. While a pitot tube or a hot wire anemometer inserted in a small hole in the duct would be more accurate, I am quite happy with the results. As stated earlier, measuring the velocity on the suction end of a hose is problematic as air is coming from all directions. My rough assumption is that the slight loss of open pipe end area from the anemometer pickup balances the directional issue. The only other place I could easily use the device is on my bottom 5" pickup to the TS, but there I have a bellmouth so the directional flow issue is even more pronounced.

[David Kumm]

Ole, I've used that model and it matches with the UEI I PMed you about. You will find that when you move it around the opening it will vary about 500-1000 fpm but that is all. Check it at the blast gate before the flex to see what the flex takes off. My guess is that the velocity on a 5" might be close and then the cfm gets close to maxed out. I've found that before and after the blast gates changes things too. Blast gates to cost some flow. Your 7" main helps as well. Would be good info to post what you read at 5" and 6" ports to see where the CFM maxes out. Dave

[Rick Potter]

I would be curious to see how the ratings vary with the filter on and with it off, like I have. I've always been curious about how much, if any difference there is. Perhaps, next time you clean the filter, you could check it out for those of us whose heads are spinning with all those numbers.

Enquiring minds, with direct vents, want to know.

Rick Potter

[Michael W. Clark]

Ole, that's great. That is a pretty good velocity through the 4" duct. Also, you should have an even higher CFM when connected to the router table (I see you are using a quick connect fitting.) The losses for the open end duct you are measuring are higher than if this duct is stabbed into the side of the cabinet or connected to a hood with some type of taper. For example, if your duct velocity is 4000 FPM, the hood loss for an open end duct is about 1.93"wg, the loss for a port on the side of cabinet is 1.5"wg. You could check this by looking at your inlet SP reading with the hose connected or disconnected. You should also see a slight increase in fan motor amps as you would be pulling more CFM.

Basically, the design of the hood (or lack of hood) affect the system pressure losses, which affects where you are operating on the fan curve. We do this experiment with different hoods at the ventilation conferences and the results are usually quite noticable. This is another benefit to measuring the airflow in the duct with everything connected, but the anemometer can be accurate for inlet air velocities. If you were to measure the duct CFM with a pitot tube in the same configuration as above, you would probably see very little variation. Remember, most airflow readings are considered +/-10% accuracy unless you have a permanant flowmeter and associated insturmentation (and these are usually verified with a Pitot tube.)

Mike