Hydro cyclone

Diagram of a hydro cyclone:1. the liquid-solid mixture enters,2. heavy solids leave,3. cleaned liquid leaves.

A hydro cyclone is a device to classify, separate or sort particles in a liquid suspension based the ratio of their centripetal force to fluid resistance. This ratio is high for dense (where separation by density is required) and coarse (where separation by size is required) particles, and low for light and fine particles. Hydro cyclones also find application in the separation of liquids of different densities.

A hydro cyclone will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining operating characteristics.

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Design

A hydro cyclone has two exits on the axis: the smaller on the bottom (underflow or reject) and a larger at the top (overflow or accept). The underflow is generally the denser or coarser fraction, while the overflow is the lighter or finer fraction.

Internally, centrifugal force is countered by the resistance of the liquid, with the effect that larger or denser particles are transported to the wall for eventual exit at the reject side with a limited amount of liquid, whilst the finer, or less dense particles, remain in the liquid and exit at the overflow side through a tube extending slightly into the body of the cyclone at the center [1].

Forward hydro cyclones remove particles that are denser than the surrounding fluid, while reverse hydro cyclones remove particles that are less dense than the surrounding fluid. In a reverse hydro cyclone the overflow is at the apex and the underflow at the base. There are also parallel-flow hydro cyclones where both the accept and reject are removed at the apex. Parallel-flow hydro cyclones remove particles that are lighter than the surround fluid.[2]

Hydro cyclones can be made of metal (mostly steel), ceramic or plastic (such as polyurethane, polypropylene, or other types). Metal or ceramic hydro cyclones are used for situations requiring more strength, or durability in terms of heat or pressure. When there is an occurrence of much abrasion (such as occurs with sand particles) polyurethane performs better than metals or ceramics. Metal lined with polyurethane is used in cases of combined abrasion and high pressure.

In a suspension of particles with the same density, a relatively sharp cut can be made. The size at which the particles separate is a function of cyclone diameter, exit dimensions, feed pressure and the relative characteristics of the particles and the liquid. Efficiency of separation is a function of the solids' concentration: the higher the concentration, the lower the efficiency of separation. There is also a significant difference in suspension density between the base exit (fines) and the apex exit, where there is little liquid flow.

If the size range of the particles is limited, but there are differences in density between types of particles, the denser particles will exit preferentially at the apex. The device is therefore a means of selective concentration of, for example, minerals.

This device is also related to the centrifuge; both of them are intended to separate heavies and lights in liquid by application of centrifugal force.

Uses

A hydro cyclone is most often used to separate "heavies" from a liquid mixture originating at a centrifugal pump or some other continuous source of pressurized liquid. A hydro cyclone is most likely to be the right choice for processes where "lights" are the greater part of the mixture and where the "heavies" settle fairly easily.

Generally, hydro cyclones are used in continuous flow systems so that the instantaneous liquid inflow to the hydro cyclone is equal to the total instantaneous outflow of "lights" plus "heavies". In cases where "heavies" are a very small part of the whole liquid, it is sometimes advantageous to accumulate them in the bottom of the hydro cyclone for batch wise removal.

Applications include:

In pulp and paper mills to remove sand, staples, plastic particles and other contaminants. In the drilling industry to separate sand from the expensive clay that is used for

lubrication during the drilling. In industry to separate oil from water or vice versa. In metal working to separate metal particles from cooling liquid. In potato processing plants to recover starch from waste water. In mineral processing hydro cyclones are used extensively both to classify particles for

recirculation in grinding circuits and to differentiate between the economic mineral and gangue.

Cyclonic separation

A cyclone separator

Cyclonic separation is a method of removing particulates from an air, gas or water stream, without the use of filters, through vortex separation. Rotational effects and gravity are used to separate mixtures of solids and fluids.

A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a spiral pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream and strike the outside wall, falling then to the bottom of the cyclone where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone the rotational radius of the stream is reduced, separating smaller and smaller particles. The cyclone geometry, together with flow rate, defines the cut point of the cyclone. This is the size of particle that will be removed from the stream with a 50% efficiency. Particles larger than the cut point will be removed with a greater efficiency, and smaller particles with a lower efficiency.

Airflow diagram for Aerodyne cyclone in horizontal position, an alternate design to minimize abrasion within the device

Airflow diagram for Aerodyne cyclone in standard vertical position

An alternative cyclone design uses a secondary air flow within the cyclone to keep the collected particles from striking the walls to protect them from abrasion. The primary air containing the particulate enters from the bottom of the cyclone and is forced into spiral rotation by a stationary spinner. The secondary air flow enters from the top of the cyclone and moves downward toward the bottom, intercepting the particulate from the primary air. The secondary air flow also allows the collector to be mounted horizontally because it pushes the particulate toward the collection area.

Large scale cyclones are used in sawmills to remove sawdust from extracted air. Cyclones are also used in oil refineries to separate oils and gases, and in the cement industry as components of kiln preheaters. Smaller cyclones are used to separate airborne particles for analysis. Some are small enough to be worn clipped to clothing and are used to separate respirable particles for later analysis.

Analogous devices for separating particles or solids from liquids are called hydro cyclones or hydro cyclones. These may be used to separate solid waste from water in wastewater and sewage treatment.

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Cyclone theory

Steady state

As the cyclone is essentially a two phase particle-fluid system, fluid mechanics and particle transport equations can be used to describe the behaviour of a cyclone. The air in a cyclone is initially introduced tangentially into the cyclone with an inlet velocity Vin. Assuming that the particle is spherical, a simple analysis to calculate critical separation particle sizes can be established.

Given that the fluid velocity is moving in a spiral the gas velocity can be broken into two component velocities, a tangential component, Vt, and a radial velocity component Vr. Assuming Stokes' law, the drag force on any particle in this inlet stream is therefore given by the following equation:

Fd = 6πrpμVr.

If one considers an isolated particle circling in the upper cylindrical component of the cyclone at a rotational radius of r from the cyclone's central axis, the particle is therefore subjected to centrifugal, drag and buoyant forces. The centrifugal component is given by:

The buoyant force component is obtained by the difference between the particle and fluid densities, ρp and ρf respectively:

The force balance can be created by summing the forces together

This rate is controlled by the diameter of the particle's orbit around the central axis of the cyclone. A particle in the cyclonic flow will move towards either the wall of the cyclone, or the central axis of the cyclone until the drag, buoyant and centrifugal forces are balanced. Assuming that the system has reached steady state, the particles will assume a characteristic radius

dependent upon the force balance. Heavier, denser particles will assume a solid flow at some larger radius than light particles. The steady state balance assumes that for all particles, the forces are equated, hence:

Fd + Fc + Fb = 0

Which expands to:

This can be expressed by rearranging the above in terms of the particle radius. The particle radius as a function of cyclonic radius, fluid density and fluid tangential and rotational velocities can then be found to be:

Experimentally it is found that the velocity component of rotational flow is proportional to r2[1], therefore:

This means that the established feed velocity controls the vortex rate inside the cyclone, and the velocity at an arbitrary radius is therefore:

Subsequently, given a value for Vt, possibly based upon the injection angle, and a cutoff radius, a characteristic particle filtering radius can be estimated, above which particles will be removed from the gas stream.

[edit] Alternate models

The above equations are relatively simple and provide a basic approximation to the behavior of a cyclone separator. These equations are, however, limited in many regards. For example, the geometry of the separator is not considered, the particles are assumed to achieve a steady state and the effect of the vortex inversion at the base of the cyclone is also ignored, all behaviors which are unlikely to be achieved in a cyclone at real operating conditions.

More complex differential equation based models exist, as many authors have studied the behaviour of cyclone separators [2]. Numerical modeling using computational fluid dynamics has also been used extensively in the study of cyclonic behaviour.

Hydro cyclone Frequently Asked Questions

What is a Hydro cyclone? What are the uses and limitations of Hydro cyclones?

See also our terminology/definitions page

What is a Hydro cyclone?

A hydro cyclone is a static device that applies centrifugal force to a liquid mixture so as to promote the separation of heavy and light components.

The hydro cyclone is a closed vessel designed to convert incoming liquid velocity into rotary motion. It does this by directing inflow tangentially near the top of a vertical cylinder. This spins the entire contents of the cylinder, creating centrifugal force in the liquid. Heavy components move outward toward the wall of the cylinder where they agglomerate and spiral down the wall to the outlet at the bottom of the vessel. Light components move toward the axis of the hydro cyclone where they move up toward the outlet at the top of the vessel.

Hydro cyclones are also related to centrifuges in that both are intended to separate heavies and lights by application of centrifugal force to liquids. The key difference is that hydro cyclones are passive separators capable of applying modest amounts of centrifugal force, whereas centrifuges are dynamic separators that are generally able to apply much more centrifugal force than hydro cyclones. Another key difference between hydro clones and centrifuges is cost. Centrifuges are expensive precision rotating machines that often need sophisticated control, whereas hydro cyclones have no moving parts and usually no controls at all so they are lower cost devices.

Hydro cyclones and centrifuges are complementary rather than competing devices. If gravity alone will settle a significant portion of your solids in a minute or two using a quick bottle test, you should investigate hydro cyclone separation. If settling takes much longer than this, then you may need a centrifuge or other separation method.

See the Hydro cyclone separation theory page for more information.

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What are the uses and limitations of Hydro cyclones?

A hydro cyclone is most often used to separate "heavies" from a liquid mixture originating at a centrifugal pump or some other continuous source of pressurized liquid. A hydrocyclone is most likely to be the right choice for processes where "lights" are the greater part of the mixture and where the "heavies" settle fairly easily.

Generally, hydro cyclones are used in continuous flow systems so that the instantaneous liquid inflow to the hydro cyclone is equal to the total instantaneous outflow of "lights" plus "heavies". In cases where "heavies" are a very small part of the whole liquid, it is sometimes advantageous to accumulate them in the bottom of the hydro cyclone for batch wise removal.

Chem Industrial's modular construction methods make it easy for us to provide accumulation configurations.

In some applications, Chem Industrial hydro cyclones are capable of sharp separations of "heavies" and "lights". In other situations, they are used for incremental enrichment, reducing the load on other, more costly separation equipment and improving the overall economics of operation.

Hydro cyclones are generally not recommended for removing long fibers from liquids.

Cyclones and Hydro cyclones

A cyclone is a commonly-used apparatus that makes use of gravity and centrifugal force to separate solid

particles from a gas stream. A typical cyclone is a cylindrical vessel with a tangential inlet and top and

bottom outlets. Cyclones are widely used in various industries because they are easy to build, inspect and

maintain.

Hydro cyclones are similar devices to cyclones where the operating fluid is a liquid rather then a gas.

Hydro cyclones operate under pressure. The feed, a mixture of possibly gases, liquids and solids enters

the hydro cyclone tangentially through the inlet which forces the mixture to spin inside the cyclone. This

spinning motion generates centrifugal forces which cause the gas to disengage quickly and exit through

the vortex finder. The liquid passes down into the conical section where the reduction in diameter

accelerates the fluid thus generating centrifugal forces strong enough to cause the solids to separate from

the liquid. The solids are forced towards the wall, because of density difference, and then travel down the

length of the conical section of the hydro cyclone in a spiral pattern towards the solids outlet, termed the

underflow. The gas and liquids migrate towards the center of the hydro cyclone where the flow reverses

and moves upwards towards the over-flow, through the vortex finder. Separated solids fall down under

gravity into the accumulator vessel situated beneath the hydro cyclone.

Because of the highly complex flows induced by the swirl, and the details of the cyclone geometry, even

the fluid flow is difficult to simulate accurately. For this application, the Second Moment Closure

turbulence models available in the ANSYS CFX software are very valuable for the correct prediction of

the fluid flow. The behavior of the particulates can then be simulated using either the Eulerian multiphase

model or the particle transport model.