Cfm

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I need to find the CFM of air in a ___" diameter pipe. I have a slack tube manometer left over from a previous life, if that will help. Can someone point me in the right direction.

The process I am working on requires 4000CFM (?). Changes to the duct a few years ago has allowed more air to flow, consequently putting an overload on the fan motor. I need to determine the CFM as it is now in order to suggest pulley changes at the fan.

I don't need the accuracy some of you put into VD, but just a basic understanding of the process.
 
The way i do for the fans which we use for cooling is to check windspeed using a digital anemometer at 10 points equally distributed at duct cross section and multiply it by the area of the duct.

For example if the average (of 10 points) windspeed is say 12ft/sec and the cross sectional area of duct is say 30 square feet, then your cfm is

12 * 30 * 60 = 21600 cfm

Cheers.
 
Ok,...

The cross sectional area is pi X d X d
----------
4

3.14 X 2 X 2
------------ = 3.14 sq ft.
4

Now the next thing is to ensure one end of the manometer tube reaches the centre of the duct facing the flow and the other end on the wall of the pipe.

Measure the difference in water column (in inches)

Now Velocity = 4005 X Square root of water column

CFM = Area X V

In your case if you (say) get your water column as 3 inches then...

CFM = 3.14 X 4005 X sqrt of 3 = 21781 cfm.

Cheers.
 
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