Measuring CFM

[Richard Spain]

I have seen a number of posts where the writer states a CFM value that is other than the manufacturers rating: typically lower. I have no doubt that there is a reduced value under certain conditions but how is value derived?

I am interested in testing my DC using different configurations and I would like to know what is used to measure the CFM. Are there any home grown methods for testing versus specialty equipment?

As always you help is greatly appreciated.

Richard

[Tom Veatch]

Getting a "true" CFM value requires some fairly precise lab equipment. In the most basic terms, the CFM (cubic feet per minute) flowing through a duct is the product of the flow velocity and the cross sectional area of the duct. The problem is the flow velocity isn't the same at all points in the cross section. Typically, due to viscous effects, the velocity at the center of the duct will be the maximum, and the velocity decreases as you move from the duct center toward the duct wall and approaches zero at the wall.

So to get the true volumetric flow rate you have to sample the velocity at a number of points across the duct (preferably all at the same time) in a manner that disturbs the flow as little as possible.

You can get a "reasonable" approximation of the volumetric flow rate using a small hand held anemometer (smaller the better) held at various points, duct center, near one wall, halfway between center and wall, etc., and averaging the values, to get a flow velocity. That velocity (converted to feet per minute) multiplied by the area of the duct, measured in square feet, will give you an approximation of the CFM. It won't be "laboratory accurate", but it will give you some "talking numbers" for not very much money.

[Phil Thien]

+1 what Tom said.

[chet jamio]

You can measure the centerline flow rate (will be the max flow rate) and multiply that by 83%. That will give you the average flow rate. Multiply the average flow rate by the duct area and you get cfm.

[Cliff Holmes]

You can measure the centerline flow rate (will be the max flow rate) and multiply that by 83%. That will give you the average flow rate.

Do you have a source on that?

[chet jamio]

My fluids dynamics book.

The value of 83% works well for 2"-15" ductwork, 2000-9000 fpm, and any possible temperature of a shop.

The equations cover everything from lava flowing down a hill to air compressor lines at 300mph. The typical DC system is a very narrow slice of the overall fluid dynamics universe. Consequently, a single value of 83% covers any home or small shop.

[Tom Veatch]

You can measure the centerline flow rate (will be the max flow rate) and multiply that by 83%. That will give you the average flow rate. Multiply the average flow rate by the duct area and you get cfm.

The concept is valid. The 83% factor can vary depending on the shape and aspect ratio of the duct (round, oval, rectangular) as well as the condition of the duct walls (slick or rough).

But, 83% should be good to go for what we're talking about here. My original point was that a single measurement at the duct centerline cannot be taken to be the bulk velocity of the flow.