The simplest device to use centrifugal force to achieve separation is the hydro cyclone. It not really a centrifuge: the centrifugal separation is produced by the motion of the slurry, induced by the tangential introduction of the feed material. Its principle of operation is based on the concept of the terminal settling velocity of a solid particle in a centrifugal field. The following picture describes the conditions in an operating hydro cyclone.

The feed enters tangentially into the cylindrical section of the hydro cyclone and follows a circulating path with a net inward flow of fluid from the outside to the vortex finder on the axis. The centrifugal field generated by the high circulating velocities creates an air core on the axis that usually extends on the spigot opening at the bottom of the conical section through the vortex finder to the overflow at the top. In order for this to occur the centrifugal force field must be several times larger than the gravitational one. Particles that experience this centrifugal field will tend to move outwards relative to the carrier fluid because of their relatively greater density. The larger, heavier particles will migrate rapidly to the outside walls of the cylindrical section and will then be forced to move downward to the inside of the conical wall. Small particles will, on the other hand, be dragged inward by the fluid as it moves towards the vortex finder. The solid separation occurs in the passage of the suspension along the barrel of the hydro cyclone, to form thickened slurry at the outer wall, which than leaves the hydrocyclone as a continuous stream from its discharge nozzle.

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