Cyclone dust separators performance

Since separation performance, as measured by Stk or Stkinso, is virtually independent of Re it is not necessary to maintain a constant Re between the model and the protot3rpe when attempting cyclone scale-up. In most such laboratory studies, it is far preferable to use air rather than water. This also makes it easier to find a test dust, since many dusts give problems with solubilization or swelling or incomplete particle wetting, among other things, when dispersed in water. 

The intense velocity resulting from the aforementioned processional motion also can be expected to significantly impair separation performance as some fraction of the collected dust spiraling down the walls becomes abruptly re-entrained by the action of the precessing vortex. Furthermore, particles of dust in this region of the cyclone may attrite at a rate greater than what one might expect from considerations of inlet velocity or even the core spin velocity alone. 

A true multicyclone is not only a parallel arrangement of cyclones, but also one wherein the individual cyclones are housed within a common casing that constitutes the inlet chamber. Furthermore, the dust (or liquid) discharge and the gas outlet pipes also report to their own common outlet plenum and hopper, respectively. Multicyclone systems of the type shown in Figs. 16.2.2 and 16.2.3 are usually comprised of numerous small diameter cyclones (typically under 250 mm in diameter) and this normally leads to excellent separation performance relative to fewer, larger units handling the same total volumetric flow. 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

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. 

Many types of particulate collection devices are available commercially (see Table 53.2). Each operates on a different principle for accomplishing removal of particulates from the gas stream. Four basic types are common in drying systems (1) the drying vessel itself (in the case of vessel dryers), (2) cyclones, (3) bag filters, and (4) wet scrubbers. Electrostatic precipitators (ESPs) are not used widely in drying installations in spite of their low-pressure drop and high collection performance. The initial cost of purchase and construction is high. For this reason, the emphasis in Section 53.3 will be on the three most widely used devices, e.g., cyclones, fabric filters, and wet scrubbers. For a concise discussion of various types of solid-gas separation equipment and guidelines for selection of dust collectors,... 

For elutriator systems, mechanical screen separators, and cyclone separators, various devices to perform the classification and removal function, include eludriator systems, mechanical system separators, and cyclone separators. Once parts have been ground, there is often a need to remove dust or oversize slivers of material from the regrind prior to its reintroduction into the process. Removal of fines can be accomplished by means of elutriation or air scalper systems which are shown in Fig. 7.24. Mechanical screen separators, such as Fig. 7.25, have the ability to remove both fines and oversize particles in one step. 

An inert force commonly employed in separating solids particles from a gas stream is the centrifugal force. Cyclones and air classifiers represent techniques that make use centrifugal force as a means of performing separation. Air classification may be considered an analytical technique with some industrial applications, while cyclonic separation constitutes one of the most useful industrial operations to treat dust-laden air and gases in a number of processes and applications. 

The proportion of each particle size fraction which is precipitated in a cyclone of particular shape and rating is called the fractional dust collection efficiency and is plotted as a percentage against the particle size diameter (in microns). The curve thus obtained is called a separation curve. It is an important criterion for comparing different cyclones with one another. Table6 indicates, by way of example, the overall collecting performance of a cyclone for dust with known particle size distribution. 

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