Fan Max Airflow from Motor Power
The most airflow a fan motor can move against a total static pressure.
Example
You enter
- Motor / brake power (hp) 1.936
- Power basis brake
- Total static pressure (in. w.c.) 2
- Fan static efficiency (0-1) 0.65
- Drive/belt efficiency (0-1) 1
You get
- Max airflow 3999
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeCFM_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 or tableCFM_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).
Source and editionAMCA / ASHRAE fan-power relation BHP = (CFM * SP) / (6356 * eta) (public); standard motor HP sizes per NEMA MG 1, by name.
Free accessFree principles in published HVAC texts; the 6356 constant is a pure unit derivation.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namespower_hp, power_basis, tsp_inwc, eta_fan, eta_drive, max_cfm
- Power ceiling only the delivered airflow is set by the system-curve/fan-curve intersection; this is the max the motor can supportfan-system interaction
- Total static pressure external + internal static at the duty point, not just externalAMCA fan-rating convention