Checking CFM Of A Dust Collector

[Mike Chalmers]

I have a fan style anemometer and if used to compare losses in percentages I think it works just fine if it is held the same way for each measurement.
A simple homemade static pressure meter is also very good to see how adding to the system adds airflow resistance.
If nothing but absolute and accurate airflow numbers are required there's really no way around spending the money to get the equipment and spending the time to learn how to make the measurements.

+ a Gazillion for this. If you want to be a purist, you better have deep pockets. Providing Relative information that allows one to guage the effectiveness of additions, modifications, changes, need not be an expensive exercise. Boils down to ROI, Return on Investment. Is it really worth the expense? According to the experts and purists, we are never going to get it "right" anyway.

[Larry Frank]

The way I calculate given the video, you are about 2200 fpm on a cross section of about .60 square ft. or about 1300 cfm. That is very high for a 4" port. What dust collector are you running - hp and impeller size?

I can not get that with a 5 hp motor and 15" impeller.

[Larry Frank]

I do not want to be a purist but want numbers that I can believe. A fan anemometer provides some highly variable numbers depending on the placement of it from the mouth of the duct. I did an experiment where I used a fan anemometer on a 4" duct and mounted it securely at several distances from the mouth of the duct. The results I obtained are shown in the graph. It is extremely difficult to hand hold one in the same place every time that you do a test. Any differences in how you hold it will result in fairly large differences in air speed.

Graph Fan Data.jpg

I think that the best, cheapest, and most accurate way to determine performance is by measuring the static pressure. You can either build your own water-tube one or buy a digital one. You can see any changes to your system by looking at differences in pressure. You can also look at a curve like those that Wood Magazine did to estimate the flow from the pressure. The graph below is from Wood Magazine testing 1.5 and 2 hp dust collectors.

Wood182 CFM.jpg

[John K Jordan]

...
I think that the best, cheapest, and most accurate way to determine performance is by measuring the static pressure. You can either build your own water-tube one or buy a digital one.

Larry,

Your advice on these topics has always been excellent. Thanks!

Can you recommend a digital meter? I see this inexpensive differential l meter on Amazon. Does it look useful?
https://www.amazon.com/d/Manometers/...tube+manometer

Any recommendations if someone wanted to buy a water tube? How about this one?
https://www.amazon.com/Yellow-Jacket...tube+manometer

JKJ

[Larry Frank]

I have a UEi EM151 which just measures static pressure and not available anymore. The one you picked can measure differential pressure and with the right probes can be used to measure flow also.

The water manometer you referenced will work or build your own. There are lots of instructions on YouTube. I built my own.

obe64q8.jpg

[Matt Mattingley]

The way I calculate given the video, you are about 2200 fpm on a cross section of about .60 square ft. or about 1300 cfm. That is very high for a 4" port. What dust collector are you running - hp and impeller size?

I can not get that with a 5 hp motor and 15" impeller.

Yeah, Larry I was thinking it was around 1322 CFM. I never thought that was crazy high. I’m running just a homemade BP cyclone and just a homemade impeller that is 17 1/4 inches I did get a little inventive building it. The impeller, I built as a two stage. I have no clue if this is improving the performance. I did a whole thread on it. 5hp. Motor.
https://sawmillcreek.org/showthread....ild&highlight=

With the system completely closed it is pulling 19 3/4” WC. One day soon I want to complete a full fan curve chart with amperage draw.
2CA03153-0B99-4E5C-B4C6-B872C6CA6E2D.jpg 795F3D17-DE9E-4C70-8FFB-835E7B07A1DF.jpg C1FD7CED-B526-4DDA-B1AD-ED4C2B4445E9.jpg
No clue why the pictures are showing up sideways.

[Larry Frank]

That is a very large impeller and it would be interesting to see a performance curve. It takes a lot of effort to get the data. I posted a couple of threads that showed how I did mine and used a hot wire anemometer with appropriate did positions across the duct. I did a lot of repeat measurements.

I just do not trust the fan type anemometer to get anywhere near accurate results. With the effort you put in to build your own system, I think it would be worthwhile to get accurate measurements.

[Matt Mattingley]

That is a very large impeller and it would be interesting to see a performance curve. It takes a lot of effort to get the data. I posted a couple of threads that showed how I did mine and used a hot wire anemometer with appropriate did positions across the duct. I did a lot of repeat measurements.

I just do not trust the fan type anemometer to get anywhere near accurate results. With the effort you put in to build your own system, I think it would be worthwhile to get accurate measurements.

Why do you not trust the fan type Aneometer? I understand how it can increase pressure due to its design. But sometimes I like to keep things simple. A simple fan does a very accurate measurement if you’re not closing off the complete duct work. In my case I increased the ductwork to 10.5 inches and interrupted some of the airflow with a 3 inch dia. fan testing device. I do think the impact pressure would be very minimal. I think the static pressure with the added 8 foot pipe could be a balance factor.

My fan impeller design might be the interesting factor. The blower was built very closely to Bill Pentz’s design except for the fan impeller and height of the blower and impeller. I used his 16 inch blower design. And I jammed a 17 1/4 inch blower in there with (I think) 3/8 clearance to what I think is called fulcrum. I think Bill had it set at 1”. I believe I only gave a quarter inch clearance top and bottom from the impeller to the blower walls. Don’t get me wrong, I’ve measured The decibel sound reading (not professionally just using an iPhone app) but… It comes in way higher than any advertised purchased unit. I had a 1.5 horse power somewhat portable dc unit with maybe a 13 inch impeller. Sometimes I found my Rigid shop vac or my 1980s double blower garbage pail DC unit better. When I was done building this DC unit I tested almost 112 A on start up, 19 A at full flow through the 6 inch main duct and about 12 A with all blast gates closed.

When I did my testing with no ductwork and the 40 in.³ intake I was pulling 23 A on my 23 amp motor. When I reduced the intake to 6 inch diameter I found 19 apps to draw on the motor. I restricted it by 12 in.³. If I knew then what I know now, I probably would’ve run 7 inch duct. But, I’m not planning to going to go backwards unless I arequire a machine that needs tons of CFM. I have tested somewhat reluctantly using a junkie test metre on my 6 inch main, and I was pulling over 10,000 ft./m. It was only rated for 6000 ft./m.

My future dream is to add a overhead filter duct with replaceable filters, always pulling maybe 100 to 200 CFM.

[Bill Dufour]

Do folks really care about the flow rates or just relative differences with new configurations. Seems to me a propeller type is cheapest and accurate enough with good repeatability? Please correct me if I am wrong.
BTW what kind of mph speeds do dc's run?
Bill D

[Matt Mattingley]

Do folks really care about the flow rates or just relative differences with new configurations. Seems to me a propeller type is cheapest and accurate enough with good repeatability? Please correct me if I am wrong.
BTW what kind of mph speeds do dc's run?
Bill D

in the main trunk, 50-60ish mph keeps main trunk clear. Branches should be sized for each machine on cfm on machine requirements. You can’t pull more cfm than than than the main trunk, always less due to static pressures.

[Larry Frank]

It all depends on what you want to know. A fan type anemometer is certainly cheap. As far as accurate and repeatable, not for me but maybe for you.

I originally felt like some who thought it was great. I did a bunch of testing with one and found it highly variable depending on how you hold it. I posted my results of testing varying the distance of the fan anemometer to the end of a duct earlier in the thread. The results are dependent not only on the distance but the angle and where you hold it. In addition, the air causes it to move around.

If you want to get even more details, go over to the Aussie Woodwork Forums in the Dust Extraction sub forum. BobL goes into great detail about all things of measuring cfm in dust collection.

I will say one last time that the easiest way to estimate performance is to use a manometer to measure pressure and then use a published performance curve to estimate cfm. I built my own performance curve for my system and when I want to know cfm, I use a digital manometer to get pressure and then read a curve.

I would love to see some data that shows using a fan anemometer for measuring dust collector performance is accurate. I am not going to discuss this any further as I have done enough testing of one to know how inaccurate they are. However, please measure your system any way that makes you happy.

[Peter Christensen]

Here is a paper that talks about fan type anemometers. It was posted on another forum and while not specific to dust collection it does point out the problems with them.

https://www.cdc.gov/niosh/mining/use...dfs/ri9061.pdf

[Larry Frank]

There is a current discussion on the Aussie forum on the vane anemometer that is worth reading.

Woodwork Forums sub Dust Extraction

It points out that measuring at the mouth of a duct has issues due to the turbulence generated at the mouth.

[David Kumm]

Impeller diameter, blade design and housing design are all critical in delivering cfm under pressure. Look at the Cincinnati fan tables for cfm at pressure per hp for various fan sizes and designs. The table for New York and Chicago fan are very similar. I think the op would spend money better saving for a larger motor and impeller. A cyclone adds 2-4" SP to a system and while a 2 hp cyclone may turn out to be adequate, I'm betting when you measure it will just quantify the number behind a marginal at best system. Good numbers are good to know ( I'm a numbers guy ) but I've also worked with systems from 2-7.5 hp and radial and BC impellers of various diameters and I use the numbers more to show improvement at higher pressures rather than on an absolute basis. After experimenting with a bunch of systems, I consider a 3 hp 14" impeller to be the minimum cyclone with 15" and 5 hp a more safe bet. If I'm limited to 2 hp or maybe even 3 hp, I'll generally stay with a bagger with oversize singed poly bags as I'd rather have more cfm and most 14" BC impellers are low pressure and suffer from the preseparator pressure cost. Diameter is huge when dealing with a BC impeller. Not so much with a radial. Dave

[Doug Walls]

I think the op would spend money better saving for a larger motor and impeller.

I've thought about that, But the blowers design looks to be closely matched to the impeller.
The current impeller is 12-1/4" & there's only about 3/4" clearance at the closet point.

The last two pics have a black marker line that was drawn with the impeller placed in position.

Doug