Module 1 · Hydrocyclone Fundamentals
02
Lesson 2 of 5

Process Flow & Separation Principles

The double vortex is the whole trick. One spiral moves down and out; a second, faster spiral forms inside it and moves up and out. Every particle's fate depends on which one wins.

2.1 From straight line to spiral

Slurry enters the cyclone tangentially — aimed at the wall, not the centre — under pressure typically between 40 and 150 kPa. Because the vessel is round and the inlet is offset, that straight jet has nowhere to go but around. It is forced into a tight, high-velocity spiral that hugs the cylindrical wall.

As the spiralling slurry works its way down into the narrowing cone, conservation of angular momentum takes over: the same effect that makes a spinning ice skater speed up as they pull their arms in. The rotational velocity increases sharply as the flow is squeezed toward the apex.

2.2 Two vortices, one vessel

Not all of the slurry can leave through the narrow apex — most of the water and fine solids have to go somewhere else. Near the apex, the outer spiral reverses direction and travels back up the centre of the cyclone as a second, inner vortex, exiting through the vortex finder at the top. This inner vortex spins even faster than the outer one.

Outer vortex: slower, descending, carries coarse and dense particles toward the apex (underflow).

Inner vortex: faster, ascending, carries fine and light particles up through the vortex finder (overflow).

Where the two vortices meet, in the low-pressure zone at the very centre, atmospheric air is often drawn in through the apex and vortex finder, forming a thin, visible air core running the full length of the unit. Its presence is actually a sign that the cyclone is operating correctly.

2.3 The forces deciding each particle's path

Every particle inside the cyclone is a tug-of-war between two forces acting in opposite directions:

  • CENTRIFUGAL FORCEPushes particles outward toward the wall — scales with particle mass, so it favours coarse, dense particles
  • DRAG FORCECarries particles inward with the fluid toward the vortex finder — scales with particle surface area, so it favours fine, light particles

A particle heavy enough for centrifugal force to win gets flung to the wall, loses velocity to friction, and slides down the cone into the underflow. A particle light enough for drag to win stays entrained in the fluid and is swept up the inner vortex into the overflow.

2.4 The cut point — d50

Because the balance of forces changes continuously with particle size, there is no sharp knife-edge between "goes up" and "goes down." Instead, engineers describe cyclone performance using the d50 — the particle size that has exactly a 50% chance of reporting to either stream. Particles much coarser than the d50 almost all report to underflow; particles much finer almost all report to overflow.

d50 ≈ f ( cyclone diameter, inlet pressure, feed density, cone angle, vortex finder diameter )

Operators tune the cut point by adjusting feed pressure, feed density, or by swapping the vortex finder and apex — not by touching any moving mechanism, since there isn't one.

Key Takeaways
  1. Separation happens through two nested vortices — an outer descending spiral and an inner ascending spiral — created by tangential entry into a converging cone.
  2. A visible air core along the central axis is normal and indicates stable operation.
  3. Particle fate is decided by the balance of centrifugal force (favours coarse/dense → underflow) against drag force (favours fine/light → overflow).
  4. The d50 is the standard measure of separation size, and it can be shifted by pressure, feed density, or geometry — with no moving parts to adjust.