by A Terry — This article is limited to air classifiers that are also called, traditionally but less appropriately, air separators. In such equipment, classification in
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Although there are a great many devices that use air to separate cles by size, they are all based on a few general principles. Here is a down on the way that they operate, together with a description of the principal types of fiers available. AIR CLASSIFIERS Ivan V. Klumpar and Fred N. Currier. Sturtevant. Inc. Terry A. Ring. Massachusetts Institute of Technology C lassification is the separation of a particulate material into a coarse and a fine fraction; the separation is usually by size, but may also be by other particle properties such as density. Depending on the equipment used, classification might also be affected by particle shape, electric, magnetic and surface properties, and other factors. For example, density is insignificant when using screens or sieves, but is a majo1· factor in air classifie1·s where fluid-drag forces are involved, as will be discussed later. Classification should be distinguished from solid-fluid separation, although the two operations overlap and the terms are often used interchangeably. For example, cyclones are considered separation equipment even though a sfine fraction is entrained in the outlet gas and might be recovered in another separation step -as in a bag filter downstream. (Although cyclones are very efficient particle-fluid separators in a medium size-range, they have a low classification efficiency. This efficiency will be defined late1·.) This article is limited to air classifiers that are also called, traditionally but less appropriately, air separators. In such equipment, classification in the medium to submicrometer particle range -1,000-0.1 µ.m -is effected in a stream of gas, using a combination of any of the following forces: gravity, drag, centrifugal and collision. Classification can be done in any gas, but ai1· is used in the overwhelming majority of cases. However, othe1· gases are geous under special conditions, e.g., nitrogen or flue gases if the solid material is highly explosive. Other classifier devices, such as grates and screens that operate in the large to medium particle-size range (500 mm down to 0.1 mm), will not be considered in this article. Early classifiers Classification equipment evolved from two sources, the simple expansion chamber and the Mumford and Moodie Separator. In the former, coarser particles drop out of an air stream as its velocity is decreased upon expanding to a larger space. Baffles, vanes or other directional and impact devices were later incorporated in the expansion chamber to change the air direction and provide collision surfaces to knock out coarser particles. The Mumford-Moodie Separator, patented in 1885, is similar to the Sturtevant Whirlwind (Fig. 9). Solids are fed into a rising air stream, using a rotating distributor plate that imparts a centrifugal force. Coarser particles drop into an inner cone; the fines are swept upward by the action of an internal fan, separated from ai1· between vanes in the expansion section of the outer cone, and collected at its bottom. The air is recirculated up toward the distributor. Unlike the Mumford-Moodie machine, the Whil’lwind enhances separation by an additional rejection device (called either selector blades or a secondary-, auxiliary-or counter-fan) that knocks out most of the remaining coarse particles. Thus, the Whirlwind incorporates almost all the features used (in modified form) in the later air classifiers. Based on a paper presented at the May 1984 meeting of the Belgian F.nginccring Soc. in Brussels. 3, 1986 77

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Modern equipment types Equipment classification Air classifie1·s can be conveniently grouped by the following critel’ia (see the table on the opposite page): ŁForces that act on the particles. These are gravity, the drag force of air, the centrifugal force exerted either by an air vortex or a mechanical part such as a rotating distl’ibutor plate, and collision force, effective when a particle hits a solid surface. Collision force becomes an imp01·tant classification factor in machines equipped with a rejection device that is part of the rotor. The rejector preferentially knocks out coarser particles entrained by the air. The presence of a rotor is indicated by a positive entry in Column 2 of the table . The table specifically lists only centl’ifugal forces (Columns 3, 4); the other forces are always present. However, some forces may be insignificant in certain cases. Examples are collision force in expansion chambers, (Item 1) and gravity in fiers having a hol’izontal rotor axis (Item 19). Ł Relative velocity and direction of the air and the particles controlled by the solid feed system, air vector and position of the rotor if present (Columns 5, 6 and 2, respectively). If particles enter the classifier with the ail·, the coarse1· ones will separate as the drag force is overcome by gravity (and possibly centrifugal and collision forces). But, if the solids are fed separately, fines will be swept away by the air, since the drag force prevails (if the feed is properly distl’ibuted). Ł Directional devices, such as vanes, cones, or zigzag plates, that change flow patterns of the air or particle-air mixture and provide collision surfaces (Column 7). ŁLocation of the fan and the fines-collection device umns 8,9). Although these do not directly affect the tion process, they are important control and design factors. For example, the inside collection of fines in the expansion section of the Whirlwind (Fig. 9) is less efficient than the r————–1 I I I I I I I I I I Air in fan Exhaust–Fines plus air Fines collector Air out Fines ‘ Figure 1 -Classifier System with outside fines collector (Sturtevant Superfine Air Separator) 78 CHEmCAL BNGINEERING/llARCH 3, 19&> outside fines separation in the cyclone dust-collector that is part of the external air-recycle loop (Figs. 1,2). Independent ail’flow control is difficult if the fan is mounted on the same shaft as the feeder plate and selection blades, as in the Whil’lwind. Separate shafts (possibly concenh·ic, Fig. 12) can overcome the problem, but an outside fan (Figs. 2,11) simplifies design and allows precise conh’ol of the air flowrate. Individual equipment types will be briefly discussed below with reference to Figs. 1 through 19. Published drawings of some classifiers are difficult to read because of the complex internal design, but Figs. 1-19 are conceptual sketches ·that emphasize important features rather than showing neering details. Principal parts, such as distributors, shafts, and the flat surfaces of vanes and blades, are drawn in contours, whereas minor items, such as spokes, walls and partitions, are indicated by single lines only. Furthermore, ·some parts are left out to show the passage available for ah· or particles, e.g. spokes, fan details, hub walls and other rotor elements on the left side of symmetrical sections. Air flow inside the equipment is indicated by heavy arrows. Classifiers without a rotor In expansion chambers, coarser particles drop out as the linear air velocity decreases (when. the air-solid mixture expands from a duct into a wider space). In the grit separator and zigzag classifier (Figs. 3,4), separation is enhanced by tortuous passages and collision surfaces placed across the particle trajectories. The grit separator is designed for rating small amounts of fines from the bulk of coarse material. Linear air velocity is controlled by moving the buoy, or “definer cone” up or down. In the rotary drum classifier (Fig. 5), the rotational movement stirs the solid mass to facilitate disengagement of coarse pat’ticles from Intake n air \1 Exhaust air Makeup air Feed l Coarse Outlet air Fines plus air Fines Figure 2 -Classifier system with outside fan and fines collector -broken tines indicate optional air-recycle subsystem (Sturtevant SD)

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Table -Air classifiers: devices for separating particulate material into a coarse and a fine fraction, using a gaseous entraining medium Inside or outsldea Rotor axls No. (If any) Plane of centrlfugal force generated by Solld feed system Prlnclpal ary di· Fines col· Fan lector Example of classifier (separator) Maximum 1 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 None None None None None None None None Vert. Vert. Vert. Vert.e Vert. Vert. Vert. Vert. Vert. Vert. Horiz.g Air Feeder 3 4 None None None None Vert. Vert. Horiz. Horiz. None None None None Horiz. Horiz. Horiz. Horiz. Horiz. Horiz. Vert. None None None None None None None None Horiz. Horiz Horiz. Horiz. None None None Horiz. Horiz. Horiz. None 5 Air Air Chute Belt Air Air Air Air Rotor Rotor Rotor Platee Air Air Chute Rotor Rotor Rotor Chute alr rectlonal direction device 6 7 8 9 10 Various None Out Vert. Buoy<: Out Vert. Zigzag Out plates Horiz. None Out Vert. Incl. Out Vert. lncl.C Out Horiz. PortsC Out Horiz. Vert. pipe Out Vert. None In Out Expansion chambers Out Sturtevant Grit Out Alpine Zigzag Out Iowa Mfg. Rotary Drum Out General Electric Buell Out Hukki Centrifugal Out Hardinge Double Cone In Cyclonesd In Sturtevant Whirlwind Vert. Vert. None None In Out Sturtevant Superfine Vert. Vert. Horiz. Horiz. Horiz. Horiz. Horiz. Vert. Out Out Humboldt Wedag None lne None Out Vert. vanes Out None None In In Vert. vanes Out Conesf Out None In Cyclone Air In Polysius Turbo Out Donaldson Majac Out Onoda O'SEPA Out Donaldson Acucut In Alpine Micropiex MPV Out Bauer Centri-Sonic Out Sturtevant SD Out Afpine Micropiex MP Notes: Vert. = vertical; Horiz. = horizontal; Incl. = inclined; NA = not available; capaclty,b Maximum Fig. t/h energy No. Ref. 11 12 13 14 Plant 500 100 50 Plant Plant Plant Plant 2,700 5 1,500 Plant 6 1,400 0.01 13 10 1,400 1.6 130 m3/s NA 16 m3/s 460 m3/I NA NA 24 m3/s 600kW 1 3 2 4 3 5 4 6 5 7 6 8 7 8 9 9 75 kW 10 9 1,000 kW 11 10 NA NA 800 kW 6kW 30 kW 56 kW 500 kW 19 kW 12 11 13 12 14 13 15 14 16 3 17 15 18 16 19 3 a. Classifier systems with outside fan and fines collector are shown in Fig. 2; those with inside fan and outside fines collector are shown in Fig. 1. b. If precise capacity is not known, or dependent on specific model, "Plant" denotes up to approximately 100 tons/h. c. Adjustable d. Because of low classification efficiency, cyclones are not considered to be classifiers. e. Fan is independent of rotor with feeder and selector blades. f. Alternatively horizontal vanes or none. g. Gravity not used in classification. Fines plus air Fines plus air ¢=i Coarse Air Figure 3 -Diagram of the Sturtevant grit separator Figure 4 -Alpine Zigzag cassifier employs tortuous passages CHEmCAL ENGINEERING/MARCH 3, 79 PAGE - 4 ============ EnGlnEEAlnG FEATURE fine ones that are then entrained by the ait· and dropped out upon expansion in the fines collector. The gravitational-inertial and centrifugal classifiet·s (Figs. 6,7) use the centrifugal force generated by the rotational flow of air that is directed by wall cmvatures and vanes. The Hukki machine has adjustable vanes and a horizontal airoutlet vil'tually perpendicular to the plane of rotation. In the double cone classifier (Fig. 8), air develops a rotational flow as it passes through adjustable peripheral ports in the upper part of the inner cone. Unlike the grit separator's buoy, this Air ¢:::J Fines collector J Fines Figure 5 -Iowa Manufacturing Co.'s rotary drum classifier c::::::> Secondary air Figure 6 -General Electric Buell gravitational-inertial classifier 80 CHDllCAL El\GIHERING/)IARCH 3. 19S6 device’s buoy serves as an air lock for the coarse material, that flows down after being separated in the inner cone. Updraft and sidedraft classifiers The classifiers of Figs. 10-12 are modifications of the wind (Fig. 9). We call them updraft machines because the principal aitŁ direction in the critical separation zone is vertically upward. The central part of the rotor is a hollow hub suspended on a vertical shaft. (Only selected elements of the hub are indicated in the figures.) ‘l\vo plates, an upper and a Plan view I I Section Feed plus air I I \/ v t Fines = 0 I I I I I Adjustable vanes J Coarse Figure 7 -Hukki centrifugal classifier uses rotating airflow Buoy—plus air Port –.Coarse material passage t Feed plus air Figure 8-Hardinge double cone classifier also uses rotating airflow

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lower, are mounted on the hub; the upper plate provides support for the selector blades. The Whirlwind and fine air separators have a fan mounted on the upper part of the hub; the Turbo Separator has a separate fan shaft. The cyclone air separator has an outside fan (see Fig. 2). The material, fed via a top or side chute, passes along the shaft and through ports in the hub wall to the lower tor plate, which spreads the feed into the ascending air. The coarse particles either drop directly or are rejected by the selecto1· blades into the inner cone; fines are carl’ied over to \;eed /-Shaft /Fan blade / / Selector blade, \ \ ‘ -·Damper Port———·Distributor Air/ –·Expansion section Coarse Finest Figure 9 -Sturtevant Whirlwind air separaior \;eed Distributor -Coarse , , ,-Fan blade Fines plus air outlet (to cyclone) —Hub wall ¢=::i Air (from cyclone) Figure 10 -Sturtevant Superfine separator-a modified Whirlwind the vessel’s top. The upper plate redistributes any rejected particles toward the shaft. In the Whirlwind and Turbo classifiers, fines are then swept down the annular space along the wall of the external cone, and are collected at the bottom. Air, separated from the fines in the expansion section and between the vanes, is returned to the rotor. In the Superfine (Fig. 1) and cyclone (Fig. 2) classifiers, fines are separated in outside collectors. The Majac Air Classifier (Fig. 13) operates on the same principle as the cyclone classifier (Fig. 11) except that feed ls introduced in a stream Feed\ Selector blade–Distributor–,-” /Shaft Coarse ¢:=i Air (from fan} Figure 11 -Humboldt Wedag cyclone air classifier Outer shaft-_ ——Inner shaft Selector, blade -, Port–Expansion'” section Coarse _,·Fan blade Feed Figure 12-Polysius Turbo separator has separate fan shaft 3, 1986 81

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EnGlnEEAlnG FEATURE I I I I I I I —Rotating plates.!–Coarse ,-Shaft / Feed plus air .. Figure 13-Donaldson Majac air classifier is similar to cyclone type Section Annular space,, Volute// Plan view Blade / Shaft J Coarse Secondary air ‘Spoke F::dplus air inlet ‘ Feed plus air \ ‘‘-. u -d D Disk u 0 0 10-1 u-cl:o—d –1-1 T ‘ \ Sphere u -*qi _.. Ellipsoid 1 : 1.a’·..-· Reynolds number, NRe Figure 23 -Capture efficiency by inertial impaction on an isolated cyclinder (Taken from Ref. 22) centrifugal forces, and will be directed into the rotor. ciently small particles will pass through the pins without contact and out through the fine-particle chamber with the bulk of the airflow. (Fine particles are separated from the airflow by an external cyclone.) The intermediate-sized cles impact the pins and are redirected into the particle chamber. Collision For collision to occur, a particle must be aerodynamically captured by the rotating pin. There are p1·imarily three ways for this to occur (Fig. 22): (a) direct interception, (b) inertial deposition, and (c) electrostatic precipitation [22]. static precipitation and othe1· mechanisms of capture such as diffusional deposition and thermal precipitation will not be discussed in this section, as they affect only the smallest particles that might remain stuck to the pins until being knocked off by large colliding particles. The efficiency of capture, E1;, is given by the ratio of the cross-sectional area of the fluid stream from which all the pa1·ticles are removed to the cross-sectional area (projected in the flow direction) of the pin. Each capture mechanism has its specific cy relationship. For direct interception, the capture efficicy [19] is given by: Ek= (1 + clldpi11)-(l + dldpi11)-1 (6) For other geometries, other captme-efficiency expressions will apply. A discussion of flat plate geometry is given by Rajhans [20], and one for spheres is given by Ottavio and Goren [21]. For inertial collision, the capture efficiency is a function of the Stokes number, as shown in Fig. 23 where the Stokes number is given [ 19] by: Ns1 = (Vf:i11pcf2)/(l8µdpin) (7) Where vgin is the angular velocity of the pin. Other tries are discussed in Refs. 19-21. The total capture efficiecy for both mechanisms is simply the sum of the efficiency from all the active mechanisms. For air classification using 86 CHEmCAL. EXGINEElllNG/MARCH 3, 0.6 iii 0.6 ·o !i: Q) 0.4 E. 766 KB – 16 Pages