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CFM to MPH????

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#1 concept07



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Posted 20 August 2007 - 04:29 PM


this is a bit of a strange question but this is my last place to turn.

Ok High output fans or any fan really is measured by CFM (Cubic feet ,Minute)

Ok now what i am interested in is how to convert a fans CFM to actual WINDSPEED MPH

I am sure some variables would include the following.

Size of Area, Resistance, CFM, and possibly other things.

But is there a way to calculate this or is that something that is just impossible. and i would just have to build a wind tunnel and run a fan with X CFM coming out of it in a specific area and then measure windspeed.

i would hope not, i would like to think that is a way to mathmatically figure it out

thanks for anyone here who can help with this,

And what the basis of this question is for is wind tunnel testing.

THanks for your time and reading this post and hopefully and informative answer.

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#2 insane_alien



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Posted 20 August 2007 - 04:41 PM

velocity = volumetric flow rate/area

i assume you can do basic unit conversion. change cubic feet per minute into cubic miles per hour and area into square miles and plug them into the equation. numbers are going to be horrible. but thats the imperial system for you.
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#3 doG



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Posted 20 August 2007 - 05:05 PM

Divide the volume out by dividing the quantity of cubic feet by the total area to find how many feet per minute one cubic foot travels. Convert this to miles per hour.

Say you have a box fan 4 foot square that puts out 21120 CFM. 21120/4 means that in each square foot there are 5280 CFM or that each individual cubic foot travels 5280 feet per minute. 5280 * 60 is 316,800 feet per hour which is 60 MPH...

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#4 concept07



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Posted 21 August 2007 - 12:36 PM

i made a really fast (2 minute) php script so that you can review my work to determine if i am on the right track or if i am way off in my numbers
$cfm = 1900;
$length = 30;
$width = 10;
$height = 10;
$minute_seconds = 60;
$hour = 60;
$mile = 5280;

$squarefoot = $length * $width * $height;
echo "<br> total cfm = $cfm <br>";
echo " <br> square foot $squarefoot<br>";
$minute_air_speed = $squarefoot / $cfm; // per minute air speed
echo " <br> minute aire speed: $minute_air_speed <br>";

$feet_per_second = $minute_seconds / $minute_air_speed;
$mph = ($mile / ($minute_air_speed * $hour));

echo " <br> feet per second: $feet_per_second <br>";
echo " <br> MPH= $mph ";

the above script will produce this following output

total cfm = 1900

square foot 3000

minute aire speed: 1.5789473684211

feet per second: 38

MPH= 55.733333333333
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