Pump Specific Speed and Impeller Type
The index that says which impeller a duty wants: Ns = N sqrt(Q) / H^(3/4) (rpm, gpm at BEP, ft per stage) classifies the wheel - radial below ~2,000 (high head, low flow, the building-service norm), mixed flow ~2,000-4,500, axial above ~4,500. An 1,750 rpm pump at 500 gpm and 100 ft runs Ns 1,237 (radial); drop the head to 25 ft and Ns jumps to 3,500 (mixed flow) - the head-to-flow ratio, not the size, sets the geometry. US dimensional form; the suction specific speed Nss is separate. An engineering aid; the manufacturer's curves govern.
Formula and source
Ns = N sqrt(Q) / H^(3/4) (N rpm, Q gpm at BEP, H ft per stage); class = radial < 2,000, mixed <= 4,500, else axial.
The US pump specific speed Ns = N sqrt(Q) / H^(3/4) and the customary impeller-type bands, as compiled in the Hydraulic Institute and pump-handbook references, by name.
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