Fan Max Airflow from Motor Power
The inverse of the fan-motor-bhp tile: the most airflow a fan motor can move against a total static pressure, CFM_max = 6356 x BHP x fan efficiency / TSP (a nameplate motor HP converts to brake HP with the drive efficiency). A 1.94 BHP fan at 65% against 2 in wc moves about 4,000 CFM; raise the static to 3 in and it falls to 2,670 CFM. Answers 'how much air can this motor push' instead of the power for one airflow. A power ceiling, not a guaranteed delivery (the fan/system curves set the real CFM). The fan curve and motor data govern.
Formula and source
CFM_max = 6356 * BHP * eta_fan / TSP; a motor (nameplate) HP converts to brake HP first with BHP = motor_hp * eta_drive. The inverse of BHP = (CFM * TSP) / (6356 * eta_fan).
AMCA / ASHRAE fan-power relation BHP = (CFM * SP) / (6356 * eta) (public); standard motor HP sizes per NEMA MG 1, by name.
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