Cyclone applications

In most cyclone applications in industrial ventilation, the particle concentration in the cyclone is very low and it may be assumed that the particles have very little chance of colliding with each other in the main body of the cyclone. Hence, the fate of the particles, whether they are collected or they penetrate the cyclone, may be determined by tracking the motions of individual, isolated particles suspended in the fluid flow. 

The rotary lock, star or feeder valve and the screw conveying discharge device, shown in Figs. 11.1.1a and 11.1.1b and 11.1.2, are two devices often used to seal against high differential pressures existing between the cyclone and the underflow. These devices are very suitable for some cyclone applications such as the flour mill application shown in Fig. 11.1.3. However, rotary valves do require motors that render them unsuitable for use inside of vessels... 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

The criteria for application and performance characteristics of blowdown drums, cyclone separators, and quench tanks are discussed as follows. 

Electric cyclones, which utilize an electrode in the center of the cyclone to establish an electric field within the cyclone body. This device is more efficient than the standard cyclone. It is probably more applicable to mists and droplets than to dry particulates, due to possible fire or explosion hazards with combustible dusts. 

Settling chambers, low-pressure-drop cyclones, and dynamic precipitators are among the most common and important devices applied in end-of-pipe control applications for dust and particulate matter. 

Cyclones are designed for marry applications. Cyclones themselves are generally not adequate to meet stringent air pollution regulations, but they serve an important purpose as precleaners for more expensive final control devices such as fabric... 

Air Flow Typical gas flow rates for a single cyclone unit are 0.5 to 12 standard cubic meters per second (smVsec) (1,060 to 25,400 standard cubic feet per minute (scfm)). Flows at the high end of this range and higher (up to approximately 50 smVsec or 106,000 scfm) use multiple cyclones in parallel (Cooper, 1994). There are single cyclone units employed for specialized applications which have flow rates of up to approximately 30 smVsec (63,500 scfm) and as low as 0.0005 smVsec (1.1 scfm) (Wark, 1981 Andriola, 1999). 

A variety of forced-circulation evaporator designs are available, with and without elutriating legs or cyclones. Different inlets and outlets are used on the vapor body, tailored for the particular application. For some applications, such as super-... 

Fabric filters can be more costly to operate and maintain than electrostatic precipitators, cyclones, and scrubbers however, fabric filters are more practicable for filtration of specific dusts. For example fabric systems are the typical control method for toxic dusts from insecticide manufacturing processes, salt fumes from heat treating, metallic fumes from metallurgical processes, and other applications. Any other control method may not be as efficient, nor economically feasible for such applications. 

The termination of the cone section is the apex orifice. The critical dimension is the inside diameter at the discharge point. The size of this orifice is determined by the application involved and must be large enough to permit the solids that have been classified to underflow to exit the cyclone without plugging. The normal minimum orifice size would be 10% of the cyclone diameter and can be as large as 35%. Below the apex is normally a splash skirt to help contain the underflow slurry in the case of a hydroclone. 

In determining the proper size and number of cyclones required for a given application, two main objectives must be considered. The first is the classification or separation that is required, and the second is the volume of feed slurry to be handled. In the case of hydroclones, before determining whether these objectives can be achieved, it is necessary to establish a base condition as follows Feed liquid - water at 20 C. Feed solids - spherical particles of 2.65 specific gravity Feed concentration - less than 1 % solids by volume Pressure drop - 69 kPa (10 psi) Cyclone geometry - "standard cyclone" as described above. 

Figure 52 also shows that the actual recovery curve does not decrease below a certain level. This indicates that a certain amount of material is always recovered to the underflow and bypasses classification. If a comparison is made between the minimum recovery level of solids to the liquid that is recovered, they are found to be equal. Therefore it is assumed that a percent of all size fractions reports directly to the underflow as bypassed solids in equal proportion to the liquid split. Then each size fraction of the actual recovery curve is adjusted by an amount equal to the liquid recovery to produce the "corrected recovery" curve shown in Figure 52. As the Djoc point changes from one application to another, the recovery curves shift, along the horizontal axis. In order to determine a single graph which represents the corrected recovery curve, the particle size of each size fraction is divided by the Dj value and a "reduced recovery" curve can be plotted, as shown in Figure 53. Studies reported by Arterburn have shown that this curve remains constant over a wide range of cyclone diameters and operating conditions when applied to a slurry...

Another important objective which must be considered is to provide adequate cyclone capacity for the application. The volume of feed slurry that a given cyclone can handle is related to the pressure drop across the cyclone. The relationship between flow rate and pressure drop for several different sizes of standard cyclones is shown in Figure 56. As shown, the flow rate increases as the pressure drop increases. In order to utilize this graph, the pressure drop used for calculating the separation is used to determine the flow rate for the cyclone diameter which was... 

A final note is with regard to flat bottom cyclones which have gained acceptance in recent years. The installation of a flat bottom in place of the conical section will coarsen the separation by more than twofold. Additionally, the sharpness of the recovery curve will decrease significantly. As such, flat bottom cyclones should be restricted to those applications in which coarse separations are required. The flat bottom cyclone does produce a very clean underflow but at the expense of a large amount of misplace coarse solids in the overflow. An illustration of a flat bottom cyclone is shown in Figure 58. The reader should refer to the reference section of this chapter for citations that provide more in-depth coverage of this equipment, as well as design case studies and example. 

The cast grids are made into battery anode and cathode plates by the application of a lead oxide paste of 70 percent lead oxide (PbO) and 30 percent metallic lead. Lead ingots are tumbled in a ball mill with airproducing lead oxide and fine lead dust (referred to as leady oxide ). Leady oxide particulates are entrained in the mill exhaust air, which is treated sequentially by a cyclone separator and fabric filter. The used fabric filter bags are shipped to a RCRA-permitled commercially operated ha2ardous waste landfill located in Colorado. The leady oxide production process does not produce wastewater. 

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