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Thread: Calculating fan power - [REVOLVED]

  1. Jan 18th, 2004, 06:33 AM #1
    wossname
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    Calculating fan power - [REVOLVED]

    I have mounted a standard PC case fan blade onto a big motor. I currently have it spinning at about 11,000 revs.

    If the outer diameter of the blade is 120mm and the diameter where the blades meet the hub is 60mm this should give me a surface area of...

    (Pi * (60 * 60)) - (Pi * (30 * 30))

    = 8482mm^2

    I reckon the pitch of the blades is about 28 degrees.

    Is there a way to calculate roughly the volume of air that passes through the fan?

    PS Currently the fan easily lifts its own weight!
    Last edited by wossname; Jan 20th, 2004 at 02:22 PM.
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  2. Jan 19th, 2004, 02:44 AM #2
    DiGiTaIErRoR
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    Get some cardboard or other material.

    Build box the size of fan.

    Cut across one side of the box.

    Make a door to go in cut to stop flow of air (until the fan is at full RPM).

    Tape a bag to one side of the box.

    Tape other side to fan or...

    Fill bag will a quantifiable material. Possibly water. Or pellets of some sort.

    Either way, time how long it takes for the bag to fill with air from fan.

    And viola.
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  3. Jan 19th, 2004, 06:00 AM #3
    wossname
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    Lol, that project in itself would be larger than what I'm trying to achieve!
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  4. Jan 19th, 2004, 05:16 PM #4
    DiGiTaIErRoR
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    Originally posted by wossname
    Lol, that project in itself would be larger than what I'm trying to achieve!
    Well... if you want to know....
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  6. Jan 20th, 2004, 06:29 AM #5
    wossname
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    I was thinking more along the lines of some kind of algebraic formula.
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  7. Jan 20th, 2004, 12:09 PM #6
    TheManWhoCan
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    Without making some huge modelling assumptions that's pretty much impossible. Try asking a fluid mechanics professor...
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  8. Jan 20th, 2004, 01:49 PM #7
    DiGiTaIErRoR
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    I had a dream in which I solved this. But I cannot recall the math.

    It was probably bogus anyway.

    Google.com?
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  9. Jan 20th, 2004, 02:21 PM #8
    wossname
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    Resolved!

    558 cubic feet per minute +/- 70cfm
    (about 15 cubic metres per min)

    Dayummm!
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  11. Jan 20th, 2004, 08:12 PM #9
    NotLKH
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    lol.
    he said revolved.
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  12. Jan 21st, 2004, 06:46 AM #10
    wossname
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    I wondered if anyone would notice.
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  13. Jan 21st, 2004, 04:19 PM #11
    fundu
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    Very good that U got the solution. Now can U please let us know the method of solving this problem. We all will appreciate.
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  14. Jan 22nd, 2004, 05:10 AM #12
    wossname
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    My work colleague has a graphing calculator with a program on it that worked it out for me. You'd have to ask him!

    I'll see if I can get it off him.
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  15. Jan 23rd, 2004, 01:37 PM #13
    DiGiTaIErRoR
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    But I doubt the calculations take account for the curve of the fan blades. The curve makes a big difference. How well it can 'catch' the air then redirect it.

    The only accurate way to measure would be a scientific method, as I described above.
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  16. Jan 23rd, 2004, 03:47 PM #14
    wossname
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    He gave me a list of all the things he reckons it compensates for...

    RPM
    Blade cross section (curvature I suspect)
    Fan diameter
    Fan hub diameter
    Duct length and diam
    Dist from fan to duct entrance
    Dist from fan to duct exit
    Local ambient pressure (pascals)
    Local humidity (%)
    Blade angle (wrt. axis of rotation)
    Number of blades
    Temperature (???)

    And he reckons it takes about 2 minutes to run this program on his TI-82 graph calc.

    I'm not entirely convinced his results are accurate since my own observations have been generally incongruous with his results.

    Hmmm.
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  17. Jan 24th, 2004, 01:22 PM #15
    DiGiTaIErRoR
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    I'd think it'd have to be pushing more than 70 CFM for it to 'float'.

    How big is the motor?
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  18. Jan 26th, 2004, 08:17 AM #16
    wossname
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    Yeah I know. The 70cfm was just the margin of error I'd allowed for.

    Originally posted by wossname
    558 cubic feet per minute +/- 70cfm
    Motor spec:
    (nondescript type fom Maplin's)
    7.2v, 6 amp, 42watts. 19,000 rpm unloaded.
    Running from a 7.2v 1.8ah nicad race battery.
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