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CYCLONES AND INERTIAL SEPARATORS
Inertial separators are widely used for the collection of medium-sized and coarse particles. Their relatively simple construction and absence of moving parts means that the capital and maintenance costs are lower than the other control devices available in the particulate control industry. However, the efficiency is not as high and thus inertial separators are usually used as precleaners upstream of the other control devices to reduce the dust loading and to remove larger, abrasive particles.
The general principle of inertial separation is that the particulate-laden gas is forced to change direction. As the gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream (Air Pollution Engineering Manual, 2000).
Areas of Application
Cyclones and centrifugal collectors are utilized in various industries such as chemical, coal mining and handling, combustion fly ash, metal melting, metal working, metal mining, rock products, plastics and wood products. Common uses of cyclones and inertial separators are the collection of grinding, crushing, conveying, machining, mixing, sanding, blending and materials handling dust and for particle collection.
Cyclones are the most common type of inertial separators. Cyclone separators are gas devices that employ a centrifugal force generated by a spinning gas stream to separate the particulate matter, which could be solid or liquid, from the carrier gas. The separator unit may be a single large chamber, a number of small tubular chambers in parallel or series, or a dynamic unit similar to a blower. Units in parallel provide increased volumetric capacity while units in series provide increased removal efficiency. Cyclone separators can be classified as vane-axial or involute. The only difference between these two is the method of introducing the gas into the cylindrical shell in order to impart sufficient spinning motion. In the simple dry cyclone separator, shown in Figure 1, the circular motion is attained by a tangential gas inlet. The rectangular inlet passage has its inner wall tangent to the cylinder and the inlet is designed to blend gradually with the cylinder over a 180-degree involute. Figure 2 shows a vane-axial cyclone. In this case, the cyclonic motion is imparted to the axially descending dirty gas by a ring of vanes. In either case, the operation depends upon the inertia of the particles to move in a straight line even as the direction of the gas stream is changed. The centrifugal force due to a high rate of spin flings the dust particles to the outer walls of the cylinder and the cone. The movement of the particles across the gas stream can be seen in Figure 3. The particles then slide down the walls and into the storage hopper. The cleaned gas reverses its downward spiral and forms a smaller ascending spiral. A vortex finder tube that extends downward into the cylinder aids in directing the inner vortex out of the device.
The cyclone separator is usually employed for removing particles 10 μm in size and larger. However, conventional cyclones seldom remove particles with an efficiency greater than 90 percent unless the particle size is 25 μm or larger. High efficiency cyclones are available and are effective with particle sizes down to 5 μm. A high-volume design sacrifices efficiency for high rates of collection. It might be used as a precleaner to
Figure 1: Involute Cyclone Separator Figure 2: Vane Axial Separator
Figure 3: Movement of particles across the gas streamlines
remove the larger particles before the gas passes through another piece of collection equipment. Cyclones can be optimized for high collection efficiencies by using small diameters, long cylinders and high inlet velocities.
Factors Affecting Collection Efficiency
For cyclones to have good collection efficiency, proper installation procedures are of primary importance. The cyclone collector must be airtight in order to eliminate reentrainment of the particles back into the gas stream. Therefore, while installing equipment such as access doors, inlet and outlet plenums and dust disposal features these areas must be completely sealed. Any leakage in the cyclone collector can cause a 25 percent or more loss in the collection efficiency.
Erosion and Fouling
Erosion and Fouling of cyclones are problems that seriously affect the cyclone collection efficiency and are encountered during the operation and maintenance activities.
Erosion in cyclones is caused by the striking or rubbing of dust particles on the inside wall of the cyclone. Erosion increases with high dust loadings, high inlet velocities, high particle specific gravity values and the strike angle (Air Pollution Engineering Manual, 2000). The area of the cylindrical shell opposite to the inlet may experience excessive wear if the gas contains large dust particles. Welded seams in the cyclone design are also areas that tend to be susceptible to erosion because of surface irregularities. Choosing the proper cyclone diameter size can control erosion. Further, using thicker material in the cone area and abrasion resistant removable wear plates (linings) at the strike zone are good design options that help in controlling erosion.
Fouling of a cyclone collector occurs on account of the plugging of the dust outlet or dust buildup on the cyclone walls. Plugging of the dust outlet occurs by large pieces of material becoming lodged in the outlet thereby forming an obstruction about which small particles can build up. These conditions can lead to reentrainment of the dust into the gas stream. For large-diameter cyclones, an axial cleanout opening with a bolted cover plate in the top of the outlet pipe can be provided so that a rod can be inserted to clear a blockage. Material buildup on the cyclone walls is a function of the dust. Soft, fine dust has a tendency to build up on the cyclone walls. Particles below 3 µm in diameter possess inherently greater cohesive and adhesive forces. Condensation of moisture on the cyclone walls also contributes to the accumulation of material on the walls. Wall smoothness can help to reduce the amount of material buildup. Electropolishing of walls has been a successful method in minimizing buildup.
Collection efficiency is a strong function of particle size and increases with increasing particle size. Also, the efficiency is greater for particles with higher densities than for
Figure 4: Collection Efficiency as a function of particle size for different types of
lower densities. Figure 4 shows the variation of the cyclone collection efficiency with different particle sizes for different types of cyclones.
Representative overall cyclone efficiencies are presented in the table shown below.
Table 1: Cyclone Collection Efficiencies for varying sizes of the particles
(Stern, et al., 1955)
Physical properties of the gas can also have some effect on the collection efficiency of a cyclone. Increasing the gas temperature decreases its density and increases its viscosity. The direct effect on efficiency by changes in the gas density is so much smaller than the density of the particles. If the viscosity of the gas that carries the dust particles to the cyclone increases then the collection efficiency will decrease with all the other factors remaining constant.