CFD ANALYSIS FOR DESIGN OPTIMIZATION OF REVERSE FLOW TYPE CYCLONE SEPARATOR: PROBLEM DESCRIPTION(3)

 

Because the vertical component of the velocity in the cyclone is downwards everywhere outside the central core, the particles will rotate at a constant distance from the centre and move continuously downwards until they settle on the conical base of the plant. Continuous removal of the solids is desirable so that the particles do not get entrained again in the gas stream due to the relatively low pressures in the central core. Entrainment is reduced to a minimum if the separator has a deep conical base of small angle. The effect of the arrangement and size of the gas inlet and outlet has been investigated and it has been found that the inlet angle should be of the order of 180°, as indicated in the figure 2. Further the depth of the inlet pipe should be small and a square section is generally preferable to a circular one because a greater area is then obtained for a given depth. The outlet pipe should extend downwards well below the inlet in order to prevent short-circuiting.

Fig 1: Basic cyclone steps
Fig 2: Velocity profile of tracer along its spiral path
Fig 3: Efficiency Vs Particle size

The efficiency of the cyclone separator is greater for large than for small particles and it increases with the throughput until the point is reached where excessive turbulence is created. Figure 3 shows the efficiency of collection plotted against particle size for an experiment separator for which the theoretical “cut” occurs at about 10^m. It may be noted that an appreciable quantity of fine material is collected, largely as a result of agglomeration, and that some of the coarse material is lost with the result that a sharp cut is not obtained.

2.4 Cyclone Performance

From an engineering point of view, cyclone performance is measured by collection efficiency (The fraction of solids separated) and pressure drop.

Pressure drop: The pressure drop across a cyclone is an important parameter to the purchaser of such equipment. Increased pressure drop means greater costs for power to move exhaust gas through the control device. With cyclones, an increase in pressure drop usually means that there will be an improvement in collection efficiency (one exception to this is the use of pressure recovery devices attached to the exit tube; these reduce the pressure drop but do not adversely affect collection efficiency). For these reasons, there have been many attempts to predict pressure drops from design variables. The idea is that having such an equation, one could work backwards and optimize the design of new cyclones.

The pressure drop across a cyclone consists of a combination of local inertia-related losses and a frictional loss. The local losses include an expansion loss at the cyclone inlet and a contraction loss at the entrance of the vortex finder. The frictional loss includes a swirling loss due to the friction between the gas flow and the cyclone wall, and a friction loss of the gas flow in the outlet. In most cases, the contraction loss at the entrance of the vortex finder and the friction loss associated with the swirling motion of vortices are the major factors.

Collection Efficiency: A number of formulations have been developed for determining the fractional cyclone efficiency for a given size particle. Fractional efficiency is defined as the fraction of particles of a given size collected in the cyclone, compared to those of that size going into the cyclone.

Collection efficiency = [(inlet loading — outlet loading)/ (inlet loading)] x 100

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