10
Calculating Volume Flow Rate
Volume flow rates through airways or apertures
may be calculated if the cross-sectional area of
the airstream and its average velocity are
known. To arrive at volume flow rate, the cross-
sectional area of the airstream is multiplied by
the average airstream velocity, using the same
units of linear measurement throughout the
calculation.
EXAMPLE:
The dimensions of a rectangular opening to a
kitchen exhaust hood is 36 x 72 in. The average
air velocity is 138 ft/min (fpm). Using the
formula for determining the area of a rectangle,
you would obtain an area of 18 square feet.
36 x 72 in. = 2592 in.
2
2592 / 144 = 18 ft
2
Inputting the area and the average velocity into
the volume flow formula, you will obtain 2484
ft
3
/min (cfm).
Q = AV
(area x velocity)
Q = 18 ft
2
x 138 ft/min (fpm)
Q = 2484 ft
3
/min (cfm)