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APPENDIX C-AIRFLOW VOLUME CALCULATIONS
To calculate cubic feet per minute (CFM) from a measured air velocity (FPM),
you need the calculated cross-sectional area of the air flow stream:
Volume Flow (CFM) = Velocity (FPM) X Area (sq ft).
In a rectangular duct this cross sectional area equals the Width times the Height.
T
H
j_
--r -w---.j
W x H=A (cross-sectional area)
In a circular duct this cross section area equals the radius squared times
1t
(3.14).
Rx Rx 3.14=A (cross-sectional area)
To convert an area calculated in square inches to an area calculated in square
feet (which is required for the Volume Flow equation above) divide by 144:
(area in sq in.)/144 = (area in sq ft.).
Example: The air duct is rectangular, the width is 24 in. and the height is
12 in. The air velocity reading through the duct is 450 FPM.
Calculate the Volume Flow.
Step 1: Cross-sectional area= 24 in. x 12 in. = 288 sq in.
Step 2: 288 sq in /144 = 2 sq ft.
Step 3: Volume flow= Air Velocity x Area, therefore,
Volume flow rate= 450 FPM x 2 sq ft.= 900 CFM.
Rev 1.8, 1 0-Jun-2008
Page 11 of24