Fan Law
Why does a 10% RPM increase not just give 10% more horsepower?
Because fan power scales with the cube of speed. A 10% RPM increase raises power by about 1.1^3, or 33.1%, not 10%. This is why motor overload protection must be re-evaluated before raising fan speed.
Why do dust-collection systems often care more about pressure than flow?
Long duct runs, small branch lines, flex hose, and dirty filters create resistance that must be overcome to maintain static pressure. Without adequate static pressure, capture velocity at the machine hood falls and chips or dust escape.
When are the fan laws most reliable?
They are most reliable when the fan remains geometrically similar, air density is roughly constant, and the operating point is extrapolated from a known performance curve. Changing wheel type, blade angle, or inlet geometry invalidates the proportional relationships.
The fan laws (fan affinity laws) govern how geometrically similar fans perform when speed, impeller diameter, or air density changes. Airflow varies linearly with RPM, static pressure varies with RPM squared, and power varies with RPM cubed. These relationships predict CFM, pressure, and motor load in CNC dust collection and ventilation systems.
In a CNC machining or millwork shop, the fan laws are the reference for predicting what happens when a VFD setpoint changes on a dust collector, when an impeller is resized, or when ductwork is modified. The flow follows speed directly, so dropping from 3600 to 3000 RPM reduces airflow by about 17 percent. Static pressure drops by roughly 31 percent, and power demand drops by about 42 percent. That steep power curve is why slowing a fan can cut electrical costs dramatically, but also why a 10 percent speed increase can overload a motor that was already near its nameplate rating. Technicians use these relationships to estimate whether a collector still produces enough capture velocity at the machine hood after filters load up, to compare fan curves against system resistance, and to verify that a retrofit wheel or motor will not surge, stall, or draw excessive amperage.
Dirty Duct Assumption: Assuming the fan laws hold after filters load or blast gates close. System resistance shifts, so actual CFM falls far below the simple RPM-based estimate, starving machine pickup.
Quiet Overload: Increasing RPM only slightly while overlooking that power rises with the cube of speed. A small speed gain can push motor amperage past nameplate and trip the overload relay.
Broken Similarity: Applying the fan laws after changing wheel geometry, blade angle, or inlet shape. Those modifications break geometric similarity, making the proportional estimates inaccurate for real retrofits.