Catalog fan performance curves are measured with long, straight ductwork that guarantees a completely uniform velocity profile. In industrial facilities constrained by space, contractors frequently bolt a sharp 90° duct elbow or abrupt transition directly onto the fan inlet flange. The asymmetric swirl chokes half the impeller wheel, causing severe flow separation. The fan delivers 20% to 35% less CFM than certified in the catalog, but the motor continues drawing near-rated power. The only remedy is inserting turning vanes or redesigning ductwork with at least 3 to 5 straight equivalent duct diameters.

Induced draft (ID) fans on boilers, kilns, and thermal oxidizers are engineered for flue gases at 150°C to 300°C (gas density $ ho approx 0.60 - 0.80 ext{ kg/m}^3$). During commissioning or winter morning startups, the process is cold ($0^circ ext{C}$ to $15^circ ext{C}$), meaning air density is nearly double ($ ho approx 1.25 ext{ kg/m}^3$). Because brake horsepower scales linearly with density, the cold air demands 60% to 90% higher motor shaft torque. If operators start the fan with the inlet damper wide open, the electric motor draws massive locked-rotor current, tripping main breakers or burning stator windings.

Forward-curved (squirrel cage) centrifugal fans have an overloading horsepower curve: as duct static resistance drops, airflow increases and power climbs steeply. If an access door is opened, a duct filter is removed, or a ductwork damper is misaligned, the fan moves toward free delivery. The motor operates severely beyond its service factor, overheating windings within 30 minutes. Critical industrial applications must specify backward-inclined, backward-curved, or airfoil impellers whose power curve peaks at the design point.

When an existing facility requires a 25% increase in ventilation airflow, maintenance managers frequently dial up the VFD frequency from 50 Hz to 62.5 Hz (a 1.25× speed multiplier). While flow increases by 1.25×, the fan affinity power law dictates that power escalates as $(1.25)^3 = 1.953 imes$—**nearly doubling the motor power demand!** The existing electric motor and electrical supply cabling overheat instantaneously. VFD speed increases must always be pre-checked against cubic motor power ratings.

Throttling a centrifugal fan below 40% of its rated flow pushes the operating point onto the positive slope of its static pressure characteristic curve. The flow separates from the blade suction surfaces, forming localized rotating stall cells. This generates cyclic pressure waves that cause duct walls to oil-can with violent booming vibrations, rapidly fatiguing flex connectors, loosening flange bolts, and shaking ceiling hangers loose. Fans must never be continuously operated in the unstable stall zone.