Six factors affecting hydrocyclone performance

By Edited by Kevin Yanik| August 7, 2023

Hydrocyclones, also simply called “cyclones,” separate particles according to size and/or density using centrifugal force to accelerate the settling rate of solid particles.

They are cylindrical-conical shaped devices with one entrance and two exits, consisting of a feed box, feed inlet, vortex finder, optional feed box extension, cone sections and a spigot.

Cyclones are fed under pressure, causing the slurry to swirl around inside the cylindrical feed box. The swirling motion produces a vortex and an air core generated along the cyclone center line. Coarse, heavy material is pulled toward the outer shell of the cyclone, where it makes its way in a helical motion downward toward the underflow discharge at the bottom of the cyclone. The fine, light material is pulled upward in a helical motion toward the overflow discharge at the top of the cyclone.

Different applications have different requirements for cyclone performance specifications. Depending on the application, performance can be measured by the particles that end up in the underflow or the particles that end up in the overflow.

For some applications, like those making C-33 concrete sand, hydrocyclone performance is measured by the cut point, where the underflow is more important than the overflow. For other applications, such as minerals or hard rock, the separation efficiency is more of an indicator of cyclone performance where the overflow – not the underflow – is more important.

In all instances, cyclone performance can be affected by six factors: size, flow rate, inlet area, vortex finder diameter, underflow diameter and length of the cyclone. Let’s look at how and why these factors affect cyclone performance.

1. Size

The size of a hydrocyclone plays a key role in its performance.

Inside the cyclone, each particle migrates to a position where the centrifugal force is equal to the drag force. If the centrifugal force is higher than the drag force, the particle has a higher chance of exiting out the underflow. If the centrifugal force is less than the drag force, the particle has a higher chance of exiting out the overflow.

Think of it as the forces being in a tug of war, where the centrifugal and drag forces are at opposing ends of a rope and the particle is the flag in the middle. Team Centrifugal Force is trying to pull the particle flag toward the shell of the cyclone, while Team Drag Force is trying to pull the particle flag toward the air core at the center.

Separation occurs when the two forces are equal for a particle size and the particle gets to decide which way it wants to go: toward the shell and out the underflow or toward the air core and out the overflow. There’s a 50-50 chance it will go one way or the other. This is called the D50 or cut point.

The size of the cyclone affects the amount of centrifugal force that is used for separation. The smaller the cyclone radius, the more the centrifugal force influences a particle – and the stronger that force is in the tug of war. The particles are pulled more toward the shell to create a smaller cut.

The larger the cyclone radius, the less centrifugal force is available to win the tug of war. So, the cut size is larger.

For primary sand production and desliming, larger cyclones are recommended. Fines recovery applications typically employ smaller cyclones and, potentially, multiple units to obtain the desired level of fines.

2. Flow rate

A pump provides the pressure necessary to affect separation in a cyclone. Photo: McLanahan

Flow rate is another factor that plays a role in hydrocyclone performance. This deals with the pressure of the material being fed to the cyclone.

A low feed pressure results in a coarser cut, while a high feed pressure results in finer separation. Pressure can be decreased by decreasing the flow rate, or it can be increased by increasing the flow rate.

When optimizing the performance of a hydrocyclone – be it a coarser cut or a finer cut – look to the pump that’s feeding the cyclone. The pump provides the pressure necessary to affect separation in the cyclone. Changing the speed of the pump changes the pressure and flow rate.