Cyclonic separation: is a method of removing particulatesfrom an air, gas or liquid stream, without the use of filters, through vortexseparation. Rotationaleffects and gravityare used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream. A high speed rotating (air)flow is established within a cylindrical or conical container called acyclone. Air flows in a helical 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, then falling 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, thus separating smaller and smaller particles. The cyclone geometry, together with flow rate, defines thecut pointof 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 . 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. 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 offrom the cyclones central axis, the particle is therefore subjected to drag, centrifugal, and buoyantforces. Given that the fluid velocity is moving in a spiral the gas velocity can be broken into two component velocities: a tangential component,, and an outward radial velocity component. 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: Usingas the particles density, the centrifugal component in the outward radial direction is: The buoyant force component is in the inward radial direction. It is in the opposite direction to the particles centrifugal force because it is on a volume of fluid that is missing compared to the surrounding fluid. Usingfor the density of the fluid, the buoyant force is: Determining the outward radial motion of each particle is found by setting Newtons second law of motion equal to the sum of these forces: To simplify this, we can assume the particle under consideration has reached terminal velocity, i.e., that its accelerationis 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: Which expands to: Solving forwe have . 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 Sinceis distance per time, this is a 2nd order differential equation of the form. Experimentally it is found that the velocity component of rotational flow is proportional to, [ 2 ]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, 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. A
Posted on: Mon, 04 Aug 2014 17:27:10 +0000
Trending Topics
Recently Viewed Topics
© 2015