How To Calculate Cooling Tower Fan Capacity

Here is a clear, practical way to calculate the cooling-tower *fan* capacity (airflow) starting from the heat to be rejected. 

Here shows the exact formulas, a worked example, and how to estimate fan power.

 1) Inputs you need

* Water flow through tower, (G) (GPM) or mass flow (  ṁ_w) (kg/s)

* Water temperature drop across tower (range) (ΔT_w ) (°F or °C)

* Ambient wet-bulb (for detailed design) — not required for the simple method below

* Estimated air temperature rise through the tower, ( ΔT_air ) (°F). Typical design values are ~10–30°F but vary by tower type and load.

* Fan static pressure  S_p  (in H₂O) and fan efficiency (nfan ) (for fan power)

2) Step A — find heat rejected (Q)

3) Step B — estimate required air volumetric flow (V) (CFM)

4) Step C — fan brake horsepower (approx.)

5) Worked example

6) Important caveats & next steps

* The simple method above assumes a guessed (ΔT_air) (air temperature rise through the tower). In reality, tower performance is governed by wet-bulb temperature, airflow, tower airflow effectiveness (Merkel number), and psychrometry.

* For accurate sizing you usually:

  * Use tower manufacturer performance curves (preferred), or

  * Perform a Merkel analysis (enthalpy vs. mass transfer), or

  * Use a psychrometric enthalpy balance (Q = ṁ_air(h_hot - h_in)with known inlet wet-bulb and outlet conditions.

* Fan static pressure must include losses: inlet/outlet screens, louvers, drift eliminators, transition ducting, guard, and velocity pressure. If you underestimate SP the chosen fan will be underpowered.

* Use the manufacturer’s recommended correction factors for altitude and temperature.

 7) Quick checklist to produce a proper calculation

* Water flow (GPM) and water range ΔT (°F) — required

* Ambient wet-bulb temp (for accuracy)

* Expected air temperature rise ΔT_air or enthalpy change (or use psychrometric data)

* Total system static pressure (in H₂O)

* Fan efficiency (or motor & gearbox efficiencies)