How to use Fan Affinity Laws& Sizing Calculator
- Enter Baseline airflow and adjust the options as needed.
- Run the tool and inspect its results, evidence and limitations.
- Copy or download the result, then verify it against the actual website, source data or business records.
Try a concrete example
At baseline 1450 RPM with 2000 CFM, 1.5 in.wg, 1.2 kW: accelerating to 1750 RPM (ratio 1.207) increases airflow to 2414 CFM, pressure to 2.18 in.wg, and power to 2.11 kW (+75.8%).
How to interpret results
Calculates linear airflow, quadratic static pressure, and cubic brake horsepower scaling based on turbomachinery affinity laws with motor overload warnings.
Frequently asked questions
What are Fan Affinity Laws?
Fan affinity laws are empirical turbomachinery relationships predicting airflow, pressure, and power changes across RPM, impeller geometry, or fluid density.
Why does a 20% speed increase require over 70% more motor power?
Power scales with the cube of speed (P2/P1 = (N2/N1)³). A 1.2x speed multiplier requires 1.2³ = 1.728x power (+72.8%). Increasing speed without verifying motor nameplate capacity risks motor burnout.
What effect does impeller diameter scaling have on performance?
At constant RPM, airflow scales with diameter cubed (Q ∝ D³), pressure with diameter squared (SP ∝ D²), and power with diameter to the fifth power (P ∝ D⁵). Trimming or enlarging impellers causes massive aerodynamic shifts.
When should the air density ratio be adjusted?
Standard conditions assume sea level at 20°C (1.204 kg/m³). High altitudes or hot boiler exhaust reduce air density; while CFM remains constant, static pressure and power decrease proportionally with density.
Are there discrepancies between measured field values and affinity law calculations?
Affinity laws assume turbulent flow and kinematic similarity. Duct resistance shifts, stall/surge regions, or belt slippage can cause 3% to 8% variances in field measurements.