Cooling Tower Fan Motor Power Calculation

How to calculate cooling-tower fan motor power — simple, correct, and with an example

Core idea (one line)

 Step-by-step method & formulas

1. Measure (or specify):

* Airflow (Q) (use m³/s; if you have CFM, convert below).

* Fan static pressure rise (\Delta p) (Pa; if you have inches H₂O, convert below).

* Fan efficiency (\eta_{fan}) (decimal, e.g. 0.6–0.8 typical).

* Drive efficiency (\eta_{drive}) (belt/gearbox; e.g. 0.95–0.99).

* Motor efficiency (\eta_{motor}) (from motor nameplate or standards; e.g. 0.90–0.97).

2. Convert units:

3. Compute air power (shaft delivered to air)

P_air =Q (m³ /s ) x △p (Pa) (watts)

4. Account for efficiencies to get motor input power:

(watts). Convert to kW by dividing by 1000 or to hp by dividing by 745.7.

5. Choose a motor size: round up to the next standard motor rating and include a safety/service factor (often 1.15–1.25) depending on starting conditions and service.

Handy combined formula (from CFM and inH₂O)

If you prefer direct use of CFM and inches H₂O:

(derived from the exact conversions above).

Worked example

Motor selection: round up and add service factor — choose a 15 hp motor (or 11–15 kW class) depending on available standard sizes, starting torque and duty.

Practical considerations / gotchas

* Use static pressure (not total pressure) for fan power calculations. Measure both inlet and outlet static to get the net.

* Fan efficiency matters a lot — older/dirty fans have lower η, which increases required motor power.

* Drive losses: belts, gearboxes, or VFD losses must be included. For a VFD, consider its efficiency and harmonics.

* Altitude & temperature change air density — if significantly different from standard conditions, correct pressure or density.

* Transient/starting torque: if frequent starts or locked-rotor torque high, size motor for starting or use VFD soft-start.

* Safety/service factor: typical service factor 1.15–1.25; follow motor supplier/manufacturer guidance and applicable codes.

* If you don’t know fan η, use a conservative 0.55–0.65 for older axial fans, 0.65–0.80 for well-designed modern fans.