CYCLONE DESIGN EXAMPLES

With consideration to the domestic industrial practitioner, the following illustrative examples utilize conventionally recognizable units as viscosity in centipoise, density in lbs/ft3, velocity in ft/sec, vessel dimensions in inches, and particle diameter in microns. 

Example A. Suppose it was desired to design a cyclone to collect all particles larger than 97 microns and release no more than 60 lbs/hr from a 6328 ACFM gas stream bearing 80 grains of solids per cubic foot of gas. The particle size distribution and density of the solids is given in Fig. 18 the gas has a density of 0.1 lbs/ft3 and a viscosity of 0.02 centipoise. The pressure drop across the cyclone is not to exceed 10 H20. Note from Fig. 1 of the text that 100% collection efficiency of 97 micron particles would require a Dth of 9.7 microns which would be collected at an average 50% 

The overall collection efficiency shown in Table 1 is determined from Fig. 2 of the text for the feed distribution given in Fig. 18. From Fig. 4 of the text at 80 grains/cu.ft, EL = 98.7%, therefore  

The anticipated pressure drop is calculable from the relationships in Fig. 21. Assuming the cyclone is located within, or attached externally to, the shell of an 8 diameter fluidized bed reactor operating at a superficial gas velocity of 2 fi/sec, then  

Subscripts G ss Gas p Particle (Cyclone Exit cyclone Barrel) or (Cyclone Inlet Vessel] Area Ratios  

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Table 10.12. Calculated Performance of Cyclone Design (Example 10.4)...

Figure 8.8 Collection efficiency curve for the cyclone design in Example 8.1.

Figure 24. Cyclone design for Example C (one of two parallel units).

Figures 1.3.3 a, b show two commercially available cyclones designed for light industrial use. An example of a much larger scale cyclone installation is presented in Fig. 1.3.4. This is a good example of a complete system— including cyclone, blower, rotary airlock valves and ducting—all supplied by the same manufacturer. Fig. 1.3.5 illustrates a huge spent catalyst regenerator cyclone system typical of today s modern FCCU installations. Such cyclones are used to capture and return the catalyst entrained off the vessel s fluidized bed. Fig. 1.3.6 illustrates where these and other cyclones are typically used in a commercial FCCU refinery process. The left- and right-hand frames in...

Cyclones may be designed to effectively remove virtually any size particulate from a gas stream. Several worked examples of this are presented herein. The barriers to cyclone usage for small particle collection are largely those of economics. Small cyclones are routinely used for particulate as small as. 5 micron with 90% removal efficiency. Unfortunately, these small cyclones are not an attractive economical choice for many industrial applications. Conversely though, cyclones are now able to satisfy environmental and process requirements on particulate that is much finer than is commonly believed. With the advances in cyclone design that have begun in the late 20 century cyclones are commonly used for emission control and product recovery on particulates with average particle sizes below 10 microns. 

Rarely does the technical community have the opportunity to learn from the accumulated efforts of a first-rate academic author of impeccable qualifications teamed with an also-academically qualified and broadly experienced practitioner, and where both individuals are natural teachers. Such an opportunity is available in this book. Comprehensive methods are presented, coupled with real-life examples gathered from the published literature and from the authors own experiences in cyclone research and application. Those charged with responsibility for cyclone design or trouble-shooting will eagerly study and absorb this book s teachings—from theoretical basis to worked example problems. 

Current designs for venturi scrubbers generally use the vertical downflow of gas through the venturi contactor and incorporate three features (I) a wet-approach or flooded-waU entry sec tion, to avoid dust buildup at a wet-dry pmction (2) an adjustable throat for the venturi (or orifice), to provide for adjustment of the pressure drop and (3) a flooded elbow located below the venturi and ahead of the entrainment separator, to reduce wear by abrasive particles. The venturi throat is sometimes fitted with a refractoiy fining to resist abrasion by dust particles. The entrainment separator is commonly, but not invariably, of the cyclone type. An example of the standard form of venturi scrubber is shown in Fig. 17-48. The wet-approach entiy section has made practical the recirculation of slurries. Various forms of adjustable throats, which may be under manual or automatic control. 

Combined Cyclonic and Fabric Filtration There are a number of arrangements where separate cyclones and fabric filters are combined into a single design to enhance gas cleaning. The side figure provides an example. 

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. 

An alternative method of using the scaling factor, that does not require redrawing the performance curve, is used in Example 10.4. The cyclone should be designed to give an inlet velocity of between 9 and 27 m/s (30 to 90 ft/s) the optimum inlet velocity has been found to be 15 m/s (50 ft/s). 

These equations can serve as a guide for estimating performance but cannot be expected to provide precise predicted behavior. However, they can be used effectively to scale experimental results for similar designs of different sizes operating under various conditions. For example, two cyclones of a given design should have the same efficiency when the value of Nst is the same for both. That is, if a given cyclone has a known efficiency for particles of diameter d, a similar cyclone will have the same efficiency for particles of diameter d2, where... 

Since neither design in Figs. 22 or 23 will satisfy the specified pressure drop limitation, the only alternative lies in reducing inlet width without increasing inlet velocity, which requires multiple cyclones operating in parallel as illustrated in the design of Example C. 

This still exceeds the design specification of 10" H20 and could have been anticipated, since to meet this pressure drop criterion, Vi from Example A must not exceed about 75 ft/sec which would require 6 cyclones in parallel as opposed to the 2 in this Example C. 

Assume the designer does not desire to use 6 cyclones in parallel, but must still meet all the specifications in Example B (i.e., cannot accept 15.35" H20 pressure drop) but is willing to accept 4 cyclones in parallel. Could 4 cyclones suffice (by a reduction in inlet velocity compensated in performance by an increase in exit gas velocity) ... 

Particulate Scrubbers Wet collectors, or scrubbers, form a class of devices in which a liquid (usually water) is used to assist or accomplish the collection of dusts or mists. Such devices have been in use for well over 100 years, and innumerable designs have been or are offered commercially or constructed by users. Wet-film collectors logically form a separate subcategory of devices. They comprise inertial collectors in which a film of liquid flows over the interior surfaces, preventing reentrainment of dust particles and flushing away the deposited dust. Wetted-wall cyclones are an example [Stairmand, Tram. lmt. Chem. Eng., 29,356 (1951)]. Wet-film collectors have not been studied systematically but can probably be expected to perform much as do equivalent dry inertial collectors, except for the benefit of reduced reentrainment. 

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