Radial, axial and mixed flow
Out the side, straight through, or at a slant — the first big choice in any impeller.
⏱ 6 min
- Everyday comparisonStep 1 of 6
Salad spinner or desk fan?
A salad spinner throws water out to the sides. A desk fan pushes air straight ahead, along its shaft.
Impellers do both — and everything in between. Which way the fluid leaves is the first thing that tells you what an impeller is good at.
- Try itStep 2 of 6
Cut the pump in half
This is a side cut through the pump, with the shaft lying flat. Water comes in from the left. The orange zone is the spinning impeller.
Switch between the three flow paths and follow the water.
Challenge: In which design does the water travel furthest away from the shaft? That one gives the most pressure.
- ExplainedStep 3 of 6
Radial: pressure champion
In a impeller the water turns 90° and goes straight outward. It travels a long way from the centre, so the vanes speed it up a lot.
Result: high pressure, modest flow. Used for pushing water up tall buildings, through long pipes, or into boilers.
- ExplainedStep 4 of 6
Axial: flow champion
In an impeller the water goes straight through, like air through a fan. It hardly moves outward at all, so it gains little pressure.
Result: enormous flow, low pressure. Used to drain flooded land, move water along canals, and circulate cooling water in power plants.
- ExplainedStep 5 of 6
Mixed: the in-between
A impeller sends the water out at a slant, like a cone. It gives more pressure than axial and more flow than radial.
You find it in irrigation pumps, large deep-well pumps and some waterjet boat drives.
- The impeller connectionStep 6 of 6
Shape follows the job
Tall and thin job (lots of pressure, little water)? Go radial. Short and wide job (little pressure, huge amounts of water)? Go axial.
In Module 6 you will learn a single number, “specific speed”, that tells engineers which shape to pick. For now, the rule of thumb is enough.
📌 Remember this
Radial impellers throw fluid outward: high pressure, less flow. Axial impellers push it straight through: huge flow, low pressure. Mixed flow sits in between.