Size reduction equipment characteristics

Characteristics of the main common types of size reduction equipment are listed in Table 12.3, including size of feed, size of product, capacity, power consumption, and average reduction ratio. Coarse comminuters perform with reduction ratios less than 10, fine ones with ratios of 100 or more. From very large to very fine may require several operations in series, as in the flowsketch of Figure 12.4(b), where three stages of crushing and two of classification are shown. 

The ability of comminuting mills to scale-up from lab to production scale is one of the most important characteristics size reduction equipment must offer. Scale-up in conical screen mills is achieved through the maintaining of the tip velocity of the tooling as applied in the pilot scale (Fig. 17). 

The most important factors that determine the selection of the size-reduction equipment are the mechanical characteristics (shear strength, ductility, etc.) of the feed material, as well as the size distribution of feed and comminuted product. From the aforementioned analysis, it is clear that the mechanical characteristics determine the acting force for size reduction and, consequently, the selection of the proper equipment. The size distribution of the feed stream and product determines the type of the corresponding equipment as well as the dimensions of feed and discharge openings. 

Figure 4-2 is an application chart for size-reduction equipment, showing classification of types and characteristics of several examples in each type. Figure 6-25 lists prices for typical size-reduction equipment. 

In practice, the process regime will often be less transparent than suggested by Table 1.4. As an example, a process may neither be diffusion nor reaction-rate limited, rather some intermediate regime may prevail. In addition, solid heat transfer, entrance flow or axial dispersion effects, which were neglected in the present study, may be superposed. In the analysis presented here only the leading-order effects were taken into account. As a result, the dependence of the characteristic quantities listed in Table 1.5 on the channel diameter will be more complex. For a detailed study of such more complex scenarios, computational fluid dynamics, to be discussed in Section 2.3, offers powerful tools and methods. However, the present analysis serves the purpose to differentiate the potential inherent in decreasing the characteristic dimensions of process equipment and to identify some cornerstones to be considered when attempting process intensification via size reduction. 

The range of applications of size reduction is very wide. It includes the preliminary breakup of large masses into pieces that can be handled in a process and the grinding of smaller particles into fine powders. The size and design of equipment reflect this situation. Broadly, size reduction apparatus can be divided into those types which depend on mechanical crushing and those which use impact to fracture the solids. Table 3-13 shows some of the common types of equipment with their principal characteristics. Major hazards are those associated with the machinery and with the material being processed. 

Design methods for calciners In indirect heat calciners, heat transfer is primarily by radiation from the cylinder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam reduction, or heat for other process steps, the thermal efficiency can e increased considerably. The limiting factors in heat transmission lie in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 W/(m2 K) in low-temperature operations to 85 W/(m2 K) in high-temperature processes. 

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