Re: How can I roughly figure the CFM in a fan?

Mike;
A rough calculation of the CFM of a fan can be accomplished by using an air 
speed indicator to determine the velocity of the air coming out of the fan 
and then multiplying it by the area of the fan face.  If the fan face is 
square (like a box fan) take several air speed reading over the full area 
of the fan and average them together to get an average velocity.

If you don't have an air speed indicator available, a VERY rough 
calculation of air speed can be made using a large piece of cardboard and a 
ping-pong ball.  Place the cardboard in front of the center of the fan and 
parallel to the air flow.  Make two marks on the cardboard about 2 to 4 
feet apart.  Place the ping-pong on the cardboard in front of the fan and 
use a stopwatch to determine the amount of time it takes the ball to travel 
the distance between the two marks you made.  If you have a large fan you 
may need to make the distance between the two marks longer in order to time 
the event.  Repeat the timing at least five times and average them 
together.  Then divide the time by the number of feet between the two marks 
and multiply by 60 to get the number of feet per minute.  This number is 
then multiplied by the area (in feet) of the fan to get CFM.  This method 
will underestimate the CFM by about 20% - so you should take this into 
consideration by multiplying your calculated CFM by 1.2.  This should put 
you in the ball park.

Thanks for you questions and let me know how your experiment turns out.

Mike C.