Page 1 of 2 12 LastLast
Results 1 to 15 of 18

Thread: Computing CFM

  1. #1
    Join Date
    Mar 2014
    Location
    Jacksonville, Fl
    Posts
    41

    Computing CFM

    Ok I am pretty much done setting up my dust collection system and wanted to try and take some CFM measurements for my system. I watched alot of videos and was also able to borrow an anemometer over the weekend and did some readings. The anemometer even had a setting for CFM, However the problem I have now is that that the anemometer in CFM mode gave me a reading in thousands not the hundreds.

    My system is completely made up of 4" PVC with minimum flex hose lengths (all less than 5').

    So what do I need to do with this number to get my true CFM reading?

  2. #2
    Join Date
    Mar 2006
    Location
    SoCal
    Posts
    19,980
    These threads have a way of taking off into the twilight zone. At the airspeeds a home shop might normally see, a 4" pipe will move somewhat less than 400 CFM. 395 CFM at 4500fpm is a number I recall but, I am sure folks will start chiming in with more math than you can ever use ;-)
    Who knows what stands in front of,
    our lives; I fashion my future on films in space.

  3. #3
    Join Date
    Jan 2007
    Location
    Michiana
    Posts
    1,587
    V x A = CFM.

    Measure the ID of the 4" PVC. (D x D X .7854)/144 is area in Sq Feet. FPM Velocity x Area = CFM.
    Sharp solves all manner of problems.

  4. #4
    Join Date
    Aug 2015
    Location
    N. Texas
    Posts
    1,518
    Quote Originally Posted by Larry Blighton View Post
    ... The anemometer even had a setting for CFM, ...
    So what do I need to do with this number to get my true CFM reading?
    The anemometer can't calculate CFM unless it has the area of the 'duct' (or other measurement area) entered into its configuration/calibration settings. Or, maybe it is designed for only one duct..?? (No idea of its specs.)

    Quote Originally Posted by Rob Luter View Post
    V x A = CFM.

    Measure the ID of the 4" PVC. (D x D X .7854)/144 is area in Sq Feet. FPM Velocity x Area = CFM.
    ^+1 - - Skip 'direct to CFM' and use Rob's method. Make sure you measure in FPM, so units match.

    See other threads regarding uncertainty introduced by fan anemometers; more better good to use hot wire.

    Best of all is dPress from pitot tube set ~2/3 off center of duct (position makes sure you get decent average of velocity profile across duct's diameter. Or carefully transit the pitot across the diameter, recording values to build a real velocity profile, then integrate the area, calcu...zzzssscckkKK!...
    ....And now return control of this thread to Glenn. Hope you have enjoyed this brief transit of ... The Twilight Zone.

    Engineers: Ask 'em the time; they tell you how to build a clock!
    Last edited by Malcolm McLeod; 01-27-2020 at 11:08 AM.
    Molann an obair an saor.

    If Heaven ain't alot like Texas, I don't wanna go. - Hank Jr.

  5. #5
    Join Date
    Sep 2006
    Location
    Deep South
    Posts
    3,918
    If the anemometer you are using is a little fan type, then your measurements are probably so far off as to be unusable. When you put the device in the air stream, it partially blocks the air flow. Since the area is constricted, the air velocity near the fan blades could either increase or decrease by a huge margin, depending on how close to the pipe you hold the meter. To make usable measurements, you need what is called a hot wire anemometer and it needs to be inserted into the air stream through a small hole somewhere upstream of the end.

  6. #6
    Join Date
    Dec 2004
    Location
    Boston, MA
    Posts
    873
    I have a couple of cheap anemometers that measure mph. I use both to get an average reading, then use an online calculator to convert to cfm. I then write the mph/cfm readings on the port so I can come back in the future and take the same measurement to see how my filter is doing or whether I have a restriction. I'm more interested in the relative reading then whether its supper accurate. It appears to be in the ballpark of what I'd expect given my DC and ducting. This is a link to an online air flow calculator. Enter your flow rate in what ever unit you choose and duct size, hit calc. https://www.engineering.com/calculators/airflow.htm

    I dont do this often because I have a manometer hooked to my DC so I can monitor the filter performance and like others have mentioned its not easy to get accurate flow rates with cheap anemometers. Its still an interesting exercise.

  7. #7
    Join Date
    Nov 2009
    Location
    Peoria, IL
    Posts
    1,530
    Curious about your goal with the calculation? Do you intend to tear down the system and change it if the CFM calculation is low? Time to calculate CFM is in the design stage.

  8. #8
    Retired engineer here, yes CFM = velocity x area but the velocity in a pipe is not constant over the full area. The velocity near the walls of the pipe are less than the average (due to friction) and the velocity in the center is more than the average. To get the true CFM you need to measure the velocity at several locations across the diameter (called a traverse) and calculate the average, there are standards as to how many measurements and at what location and how far away from any bends or obstructions upstream or downstream.

  9. #9
    Join Date
    Jan 2007
    Location
    Michiana
    Posts
    1,587
    Engineer looking forward to retirement here . In a past life I worked in commercial and industrial ventilation. I've conducted a number of multi point pitot traverses. In this instance it would be like measuring a 2 x 4 with a micrometer. An anemometer will be close enough. As an alternative you could measure the pressure drop across the opening with a U -Tube or an inclined manometer. and use the entry coefficient to calculate airflow. This would also be a close approximation. One of the most accurate methods is to compare fan RPM and motor amp draw with an accurate fan curve, but then you need to know RH, Temp, and Barometric pressure to convert from Standard Air. That's one of the challenges of trying to measure a compressible fluid. It's always a close approximation. Air is hateful stuff that way.
    Sharp solves all manner of problems.

  10. #10
    Agree with Rob, my airflow measurement experience is more at the industrial level with duct dimensions measured in feet not inches. Multi point traverses in a 4" pipe would be difficult and not worth it. Getting back to the OP, not sure why your anemometer readings are so far off. Accurate readings are difficult to get and probably of limited value. A better approach may be to get a Dylos or other air quality measurement device and monitor your shop air quality with the system in operation.

  11. #11
    Join Date
    Jun 2015
    Location
    Sacramento, CA
    Posts
    1,911
    Keep an eye out on eBay for one of these (LINK). Sometimes they can be found for very cheap.
    If at first you don't succeed, redefine success!

  12. #12
    Join Date
    Nov 2007
    Location
    NW Indiana
    Posts
    2,443
    A fan anemometer at the end of a dust is just not accurate. The flow at the end of a duct is turbulent and there are many issues of using a fan anemometer. People do it all the time but the numbers are not accurate. Getting good numbers takes a test duct and doing a proper traverse with a pitot tube or hot wire anemometer.

  13. #13
    Join Date
    May 2005
    Location
    Highland MI
    Posts
    3,856
    Blog Entries
    11
    Ah, the fan anemometer haters have surfaced...again, please define accurate.
    NOW you tell me...

  14. #14
    Join Date
    Nov 2007
    Location
    NW Indiana
    Posts
    2,443
    Not a hater but just realistic. If you think they are fine, please go ahead and use them. Personally, I used a hot wire to test my system.

    I did some tests with a fan anemometer holding it at various distances from the end of a 4" hose . I got velocity from 3000 to almost 7000 fpm by varying the distance by 3/4" . If it was not perfectly centered the number also had huge variation on the order of a couple thousand fpm.

    The flow at the end of a pipe is turbulent and almost impossible to measure.

    This is why I do not trust the fan anemometer. If such a variation is accurate enough for you...OK. Most changes people make to their systems only result in changes of 10% or so. Trying to measure and evaluate such changes given the variation with a fan anemometer is not reasonable.

    You can also read the opinion of an expert on the Aussie forum. He goes by BobL on the sub forum Dust Extraction.

  15. #15
    Join Date
    Aug 2015
    Location
    N. Texas
    Posts
    1,518
    Quote Originally Posted by Ole Anderson View Post
    Ah, the fan anemometer haters have surfaced...again, please define accurate.
    Not a hater, or a user. I think the fan type is fine for SWAG at wind speed on a rifle range. It beats a wet finger.

    Industrial accurate: don't even talk to me with a sensor that is less than +/- 2% of span.
    Home shop accurate: +/- 10% on air flow would be OK. (linear measurements I try to be a 'little' closer to 0.25% - that's approx +/- 1/32" in 12").

    I'll almost leave 'repeatability' out of this, but whether using a +/- 2% accurate thermocouple or a +/- 10% fan anemometer, I want <0.5% repeatable.

    Placed in the same duct 2-3 times, will the fan anemometer be accurate? Or repeatable? ..again, not a user. Just curious.
    Last edited by Malcolm McLeod; 01-28-2020 at 8:18 PM. Reason: clarity
    Molann an obair an saor.

    If Heaven ain't alot like Texas, I don't wanna go. - Hank Jr.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •