Fans and energy
Use impeller diameter and shaft RPM to calculate tip speed, or solve for diameter or speed.
Calculate the peripheral velocity at a fan impeller's outside diameter, or solve the same relation for shaft RPM or impeller diameter.
Tip speed is the distance a blade tip travels per unit time. One revolution covers the circumference defined by the impeller outside diameter, so diameter and actual shaft RPM together determine peripheral velocity. The calculation describes the rotating tip, not the air moving through the duct, and does not establish airflow or pressure rise.
The same geometry can be inverted to find RPM or diameter for a supplied target speed. That inverse result is mathematically consistent with the target but does not show that a suitable fan exists or can operate at that speed. Duct diameter, pulley diameter and motor RPM refer to different parts of the system and should not replace the impeller and fan-shaft inputs.
Tip speed [FPM] = π × impeller outside diameter [in] × fan shaft RPM / 12; RPM = 12 × target FPM / (π × diameter [in]); diameter [in] = 12 × target FPM / (π × RPM).
The FPM, ft/s and m/s outputs describe the same peripheral velocity. Use the resolved RPM or diameter as a geometry result and compare it separately with the selected fan’s documented limits.
Inputs: Constructed fan: 12 in impeller outside diameter at 1,750 fan shaft RPM.
Calculation: Tip speed = π × 12 / 12 × 1,750 = 5,497.787 FPM; divide by 60 for ft/s and multiply ft/s by 0.3048 for m/s.
Result: 5,497.787 FPM, 91.630 ft/s and 27.929 m/s. The result does not determine an allowed or safe fan speed.
Inputs: Constructed 12 in impeller and a user-selected target tip speed of 4,000 FPM.
Calculation: RPM = 12 × 4,000 / (π × 12) = 1,273.2395 RPM.
Result: 1,273.240 RPM mathematically gives 4,000 FPM at the selected diameter; verify fan, drive and project limits separately.
Inputs: Constructed 3,000 fan shaft RPM and a user-selected target tip speed of 6,000 FPM.
Calculation: Diameter = 12 × 6,000 / (π × 3,000) = 7.639437 in.
Result: 7.639 in impeller outside diameter mathematically corresponds to 6,000 FPM at 3,000 RPM; no product availability or rating is inferred.
Use the outside circle swept by the rotating blade tips. The casing or duct diameter can include clearance and is not automatically the rotating impeller diameter.
The target is user-supplied geometry input. Mechanical limits, drive capability and fan performance must come from the actual equipment evidence; this tool supplies no universal allowable threshold.