The air balance technician should have these Square Areas in feet formulas memorized.
square/rectangle = h x w
Circle = pi r 2
Oval =r1 r2
Now you can take any one of these formulas and divide it by 144 to get the cubic feet.
When air balancing Pulley Speeds are very important. If you don’t have a tachometer of some kind to take the actual measurement, you can calculate the speeed in the following way:
Speed of the motor pulley x diameter of the motor pulley divided by the diameter of the fan pulley;
Motor Speed= 1750
Motor Pulley = 2.75″
Fan Pulley = 7″
1750 x 2.75 = 4812.5
4812.5 / 7 = 687.5
If you need to increase the speed of the fan just calculate the percentage of increase and either increase the motor pulley size of decrease the fan pulley size