Specific speed: picking the shape
One number that tells you whether a job needs a radial, mixed-flow or axial impeller.
⏱ 6 min
- Everyday comparisonStep 1 of 5
The right shoes
Sprinting spikes, hiking boots and swimming flippers are all “footwear”, but each is shaped for its job. Put flippers on for a mountain hike and you won’t get far.
Impellers are the same. The job — how much flow, how much head, at what speed — decides the shape.
- ExplainedStep 2 of 5
One number for the job
Engineers squeeze the job into a single number called : nq = n × √Q ÷ H^0.75 (n in RPM, Q in m³/s, H in m).
Lots of flow and little head give a high nq. Little flow and lots of head give a low nq.
- Try itStep 3 of 5
Choose a job, see the shape
Set a speed, a flow and a head. The pointer shows the specific speed, and the side cut morphs into the impeller shape that suits it best.
Challenge: Find a job that needs an axial impeller. Then keep the flow and raise the head — watch the impeller turn radial.
- ExplainedStep 4 of 5
The three zones
Below about 40: radial — narrow, large-diameter impellers for high head. About 40–160: mixed flow. Above about 160: axial, like a propeller in a pipe.
This is the same rule of thumb you met in Module 3 — now with a number behind it.
- The impeller connectionStep 5 of 5
When the number is too low
Very low nq means very narrow channels and tiny flow for a big wheel. Friction then eats a large share of the energy, so efficiency suffers.
The fix: split the head across several stages (a pump), or run faster. Each stage then does less head and gets a healthier specific speed.
📌 Remember this
Specific speed nq = n·√Q / H^0.75 turns a job into one number. Low nq → radial, middle → mixed flow, high → axial. Too low? Use more stages or more speed.