Thread: calculate cfm in rectangular duct

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calculate cfm in rectangular duct

Can anyone explain me I want to calculate the cfm in a rectangular duct. what is the procedures and what steps I should follow.

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traverse the duct with test holes

link didn't work

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area of duct in inches divided by 144 equals square feet. find velocity in feet per minute and multiply to get cfm. or get the right tools.

4. You need to take probably about 20 samples across the duct to finds the flow pressure. Static press-Total press=flow pressure. You can then calculate with your area.

http://www.dwyer-inst.com.au/Product...troduction.cfm

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Quick, but not Cheap

Flow Hood. Hold it over the opening and read the scale. Problem solved.

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Originally Posted by hedtedjr
Flow Hood. Hold it over the opening and read the scale. Problem solved.
Flow hood is the best.... cover and read, cover and read. What was the old fashion way? traverse with a tip that would pick up velocity and somehow it would cancel static to get only velocity and then you get the average of the sum. A few more steps and you have CFM.

I would like to try a flow hood against an anemometer or a u tube to see how close the readings would be.

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Spend a few bucks and hire a professional with a flow hood(that has been calibrated recently).

SG

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A traverse is best and most accurate. But just to get a ball park, you can measure velocity in a couple of places across the duct, take the average, and multiply by the square footage of the duct. Hot wire anemometers are pretty cheap.

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