The cubic feet per minute (CFM) of a gas describes its volumetric flow rate through a pipe or vent. The volumetric flow is a good measure of how much gas goes through the system, but it isn't the clearest way of picturing how quickly it moves. To picture this speed, calculate the linear speed, which simply describes the linear distance that the gas travels in terms of miles per hour.
Divide the flow rate in cubic feet per minute by the duct's cross-sectional area. If, for instance, 2,000 cubic feet flow through a duct with a cross-sectional area of 4 square feet each minute: 2,000 / 4 = 500 feet per minute.
Multiply this answer by 60, the number of minutes in an hour: 500 x 60 = 30,000 feet per hour.
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Divide the answer by 5,280, which is the number of feet in a mile: 30,000 / 5,280 = 5.68. This is the air's speed in miles per hour.