What is the difference in air speed measurements of CFM vs. LFM?
Designers often need air speed measurements to calculate thermal derating and power dissipation for their DC/DC converters and for their overall systems. There are two basic units of measure: CFM (cubic feet/minute) is a measurement of volume, LFM (linear feet/minute) is a measurement of velocity. Fan manufacturers use CFM because they rate their fans according to the quantity of air they can move. Velocity (speed) is more meaningful to heat removal at the board level. This is what most DC/DC converter manufacturers will specify when calculating thermal derating curves and other performance specifications. To convert CFM measurements to LFM, use the following equation: LFM = CFM/AREA LFM = Linear feet per minute of airflow CFM = Cubic feet per minute of air volume AREA = the area of the opening in square feet. For example, let’s assume you are blowing air through a 6″ x 6″ opening across the top of a DC/DC converter with a 100CFM, unobstructed fan. LFM = 100/ 0.25 sq feet or about 400L
