CFM and RPM—Know what you're Looking For

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Cubic feet per minute, more commonly known as CFM is calculated by the following formula:  air speed (feet per minute) X area (square feet)=CFM. 

This formula and the measurement of CFM is a way of helping you know what fan is right for your home.  Not everyone is going to take a look at CFM, but for those who do it is a helpful tool.  In simpler words than that of the formula, it is the amount of air a fan moves.   The amount of air depends on some other factors as well, such as the diameter and shape of the blades, speed at which the blades turn (revolutions per minute or rpm), horsepower (hp) of the motor, and overall fan design.  These combined factors establish the air moving capacity of a fan.  Fan capacity is measured in terms of the cubic feet, and again, this is how CFM (cubic feet per minute is determined.)

A standard measurement of airflow indicates how many cubic feet of air passes by a stationary point in one minute.  The higher the number, the more air is being forced through the system.  The volumetric flow rate of a liquid or gas in cubic feet per minute equals the CFM, and one CFM equals approximately 2 liters per second. 

Here is an example of CFM calculations.  For the example, picture a room 10 feet by 12 feet with an eight foot ceiling, equaling 960 cubic feet of air in the room.  The minimum requirements say that this air should be changed 15 times an hour.  To cool this room a fan must have the ability to move 14,400 cubic feet of air an hour.  Since there are 60 minutes in an hour, we must divide the 14,400 by 60 to get our CFM.  (14,400 divided by 60 equals 240 CFM).

CFM and RPM are the two most important things to look for in a fan, so that you are guaranteed correct and effective operation.  If you only know the RPM, and not the CFM, or vice versa, you should feel confident in your fan purchase.  As long as you know one of the calculations, you are ensured of a well-working fan.  However, if you are not satisfied with these calculations, this is not the only criteria one can use for evaluating fan performance.  One of the main qualifications, second to rpm and CFM measurement is noise level or decibel rating, followed by the next qualification of vibration.  Look for fan noise levels rated in sones or decibels.  Check these if the CFM or RPM still leave you unsettled about your fan choice.

Some more detailed information:

Fan manufacturers base their measurements on a standard measurement with clean, dry air at a density of 0.075 pounds mass per cubic foot, barometric pressure at sea level of 29.92 inches of mercury, and a temperature of 70°F.  These standard measurements are used to determine SCFM:  Standard Cubic Feet Per Minute. 

SCFM=mass flow rate of air through fan in standard cubic feet per minute 

A volume of air will not be affected in a given system because a fan will move the same amount of air regardless of the temperature.  If, for example, a centrifugal fan moves 1,000 CFM at 70?F it will also move 1,000 CFM at 200?F. The air volume delivered by the centrifugal fan is not affected by any temperature changes.  This brings us to ACFM:  Actual Cubic Feet Per Minute.  This refers to the actual volume flow at ambient conditions.

ACFM=volume flow rate of air through fan in actual cubic feet per minute

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