Determining blower CFM 3

[element94]

How can I determine the output CFM of a blower? I know the horsepower of the motor, outlet size and fan diameter. What else do I need and how do I figure out the volume flow rate?

Thanks.

 

[TheTick]

If the blower is installed, you check flow with a pitot tube at various points across the duct. This is called a "traverse" in air balancing parlance.

 

[reidh]

Element 94,

Remember when checking flow velocities in a duct, the air velocity distribution is not uniform throughout the duct. This holds especially true when there are bends or reductions in your duct near your measurement point.


-Reidh

 

[MintJulep]

Do you have the blower's fan curve and actual motor HP (not name plate HP)?

 

[Twoballcane]

Did you try calling the vendor and ask? =)

Tobalcane
"If you avoid failure, you also avoid success."

 

[Twoballcane]

Well ask for the CFM of the blower...

Tobalcane
"If you avoid failure, you also avoid success."

 

[element94]

No, the plate and all indentification is gone. I know nothing. I have a pitot tube, I'll try that. Thanks.

 

[dik]

TheTick: if you divide the duct x-section into 5 parts and then take readings in 1 quarter, ie 9 measurements, would this be a good indication or can you break the duct x-section into 3 parts

 

[Twoballcane]

I would think that you take your measurment in the center because that is where your highest velocity would be. It will be zero at the walls...

Tobalcane
"If you avoid failure, you also avoid success."

 

[MintJulep]

This site provides decent direction on how to perform a proper duct traverse with a pitot tube:

http://www.flowmeterdirectory.com/flowmeter_artc/flowmeter_artc_02111201.html

For best results you need to be in a good straight section of duct, at least 8 equivalent diameters downstream. Doing a traverse directly at the fan discharge is useless.

 

[dik]

Thanks, Mint... excellent link. More information than I really needed, but great info nonetheless.

Dik

 

[TheTick]

There are standards published by NEBB (National Environmental Balancing Bureau) as on how ducts of various sizes should be divided for a pitot tube traverse. Round ducts are treated differently than square. You may be able to get a professional air balancer to do this for you. Contact any large HVAC or balancing firm.

In a traverse, the duct is divided into sections small enough to be sure that enough data points are taken to ensure a representative sampling. For round ducts, samples are usually taken 90 degrees apart, and distances calculated so each position represents an annular region of equal area.