Cyclonic separation

Uses

Cyclone separators are found in all types of power and industrial applications, including pulp and paper plants, cement plants, steel mills, petroleum coke plants, metallurgical plants, saw mills and other kinds of facilities that process dust.

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. Cyclones are increasingly used in the household, as the core technology in bagless types of portable vacuum cleaners and central vacuum cleaners. Cyclones are also used in industrial and professional kitchen ventilation for separating the grease from the exhaust air in extraction hoods. 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.

Similar separators are used in the oil refining industry (e.g. for Fluid catalytic cracking) to achieve fast separation of the catalyst particles from the reacting gases and vapors.

Cyclone fans are widely utilized in industrial air handling and ventilation systems due to their ability to generate high centrifugal force. Their primary application is in dust collection systems, where they separate and recover medium-to-large particulate matter—such as sawdust, cement dust, and agricultural grains—in woodworking, construction, and milling facilities. Additionally, cyclone technology is integrated into industrial roof ventilators for heat extraction and in household vacuum cleaners to separate debris from airflow, preventing filter clogs and maintaining consistent suction power.

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

Types

The most common types of centrifugal, or inertial, collectors in use today are:

Single-cyclone separators

Single-cyclone separators create a dual vortex to separate coarse from fine dust. The main vortex spirals downward and carries most of the coarser dust particles. The inner vortex, created near the bottom of the cyclone, spirals upward and carries finer dust particles.

Multiple-cyclone separators

Multiple-cyclone separators consist of a number of small-diameter cyclones, operating in parallel and having a common gas inlet and outlet, as shown in the figure, and operate on the same principle as single cyclone separators—creating an outer downward vortex and an ascending inner vortex.

Multiple-cyclone separators remove more dust than single cyclone separators because the individual cyclones have a greater length and smaller diameter. The longer length provides longer residence time while the smaller diameter creates greater centrifugal force. These two factors result in better separation of dust particulates. The pressure drop of multiple-cyclone separators collectors is higher than that of single-cyclone separators, requiring more energy to clean the same amount of air. A single-chamber cyclone separator of the same volume is more economical, but doesn't remove as much dust.

Secondary-air-flow separators

This type of cyclone uses a secondary air flow, injected into the cyclone to accomplish several things. The secondary air flow increases the speed of the cyclonic action making the separator more efficient; it intercepts the particulate before it reaches the interior walls of the unit; and it forces the separated particulate toward the collection area. The secondary air flow protects the separator from particulate abrasion and allows the separator to be installed horizontally because gravity is not depended upon to move the separated particulate downward.

Cyclone theory

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 ${\displaystyle V_{in}}$. Assuming that the particle is spherical, a simple analysis to calculate critical separation particle sizes can be established.

If one considers an isolated particle circling in the upper cylindrical component of the cyclone at a rotational radius of ${\displaystyle r}$ from the cyclone's central axis, the particle is therefore subjected to drag, centrifugal, and buoyant forces. Given that the fluid velocity is moving in a spiral the gas velocity can be broken into two component velocities: a tangential component, ${\displaystyle V_t}$, and an outward radial velocity component ${\displaystyle V_r}$. Assuming Stokes' law, the drag force in the outward radial direction that is opposing the outward velocity on any particle in the inlet stream is:

${\displaystyle F_d=-6\pi r_p\mu V_r.}$

Using ${\displaystyle {\rho }_p}$ as the particle's density, the centrifugal component in the outward radial direction is:

${\displaystyle F_c=m\frac{V_t^2}{r}}$

${\displaystyle =\frac{4}{3}\pi {\rho }_p{r}_p^3\frac{V_t^2}{r}.}$

The buoyant force component is in the inward radial direction. It is in the opposite direction to the particle's centrifugal force because it is on a volume of fluid that is missing compared to the surrounding fluid. Using ${\displaystyle {\rho }_f}$ for the density of the fluid, the buoyant force is:

${\displaystyle F_b=-V_p{\rho }_f\frac{V_t^2}{r}}$

${\displaystyle =-\frac{4\pi r_p^3}{3}\frac{V_t^2}{r}{\rho }_f.}$

In this case, ${\displaystyle V_p}$ is equal to the volume of the particle (as opposed to the velocity). Determining the outward radial motion of each particle is found by setting Newton's second law of motion equal to the sum of these forces:

${\displaystyle m\frac{d{V}_r}{dt}=F_d+F_c+F_b}$

To simplify this, we can assume the particle under consideration has reached "terminal velocity", i.e., that its acceleration ${\displaystyle \frac{d{V}_r}{dt}}$ is zero. This occurs when the radial velocity has caused enough drag force to counter the centrifugal and buoyancy forces. This simplification changes our equation to:

${\displaystyle F_d+F_c+F_b=0}$

Which expands to:

${\displaystyle -6\pi r_p\mu V_r+\frac{4}{3}\pi r_p^3\frac{V_t^2}{r}{\rho }_p-\frac{4}{3}\pi r_p^3\frac{V_t^2}{r}{\rho }_f=0}$

Solving for ${\displaystyle V_r}$ we have

${\displaystyle V_r=\frac{2}{9}\frac{r_p^2}{\mu }\frac{V_t^2}{r}({\rho }_p-{\rho }_f)}$.

Notice that if the density of the fluid is greater than the density of the particle, the motion is (-), toward the center of rotation and if the particle is denser than the fluid, the motion is (+), away from the center. In most cases, this solution is used as guidance in designing a separator, while actual performance is evaluated and modified empirically.

In non-equilibrium conditions when radial acceleration is not zero, the general equation from above must be solved. Rearranging terms we obtain

${\displaystyle \frac{d{V}_r}{dt}+\frac{9}{2}\frac{\mu }{{\rho }_p{r}_p^2}{V}_r-\left(1-\frac{{\rho }_f}{{\rho }_p}\right)\frac{V_t^2}{r}=0}$

Since ${\displaystyle V_r}$ is distance per time, this is a 2nd-order differential equation of the form ${\displaystyle x^{″}+c_1{x}^{\prime }+c_2=0}$.

Experimentally it is found that the velocity component of rotational flow is proportional to ${\displaystyle r^2}$, therefore:

${\displaystyle V_t\propto r^2.}$

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

${\displaystyle U_r=U_{in}\frac{r}{R_{in}}.}$

Subsequently, given a value for ${\displaystyle V_t}$, 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.

Alternative models

The above equations are 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 behaviours which are unlikely to be achieved in a cyclone at real operating conditions.

More complete models exist, as many authors have studied the behaviour of cyclone separators. Simplified models allowing a quick calculation of the cyclone, with some limitations, have been developed for common applications in process industries. Numerical modelling using computational fluid dynamics has also been used extensively in the study of cyclonic behaviour. A major limitation of any fluid mechanics model for cyclone separators is the inability to predict the agglomeration of fine particles with larger particles, which has a great impact on cyclone collection efficiency.