Screening Principles

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

Screening Principles I

ERNST R. WOLFF Bloomingfon, 111.

S

CREENIKG operations are performed in a large number of

chemical manufacturing and refining processes. Like all other industrial operations, screening, too, does not achieve perfection. Persons not frequently concerned with classifying problems easily overlook this fact. At first glance it would seem that on a screen with, say, circular openings of 0.1-inch radius, all spherical particles with radii smaller than 0.1 inch pass the screen, while larger spheres are retained.

+

50 80 5 60 a

40

5-

g 20 -t

0

-

MIN. Figure 1. Amount of Material Passed through Screens as Function of Sifting Time (3)

3.

I

No. 1 3 cloth, 0.0042 inch No. 16 cloth, 0.0034 inch No. 1 8 cloth, 0.0031 inch

To complete the sifting of a measured amount of particles, a certain length of time is required in order to give every undersize particle a chance to come into contact with a screen opening and to pass it ( 3 ) . On intermittent test equipment, loading of the sieve drops a t the same time, which enhances the chances of remaining particles to come into contact with the screen. Figure 1 illustrates the sifting times required by wheat flour to clear three screen sizes on an intermittent test screen. Theoretically, a perfect separation on the screens is achieved when all the curves have reached a horizontal course. A great many experiments have shown that a relatively long sifting period is necessary to reach even the approximation to the horizontal that is considered sufficient for practical purposes. In practical applications with continuous flow production equipment, the screening time cannot be controlled within exact limits. Means of propulsion can be selected to convey the stock a t a known average speed from which the average length of time the stock is in contact with the screen area can be calculated. However, because of their individual size, shape, and weight variations, individual particles remain on the screen longer or shorter times than the average. Since new stock enters continuously on production machinery, the loading of the screens does not decrease with added sifting time, and the curves, consequently, rise less steeply than those of Figure 1. In this discussion it has been assumed that ball-shaped particles are separated. Csually, particles are not spherical, nor are mesh openings circular. T o cite an extreme example, particles the shape of a toothpick may pass a 3/32-inchscreen opening, 1778

1. Dynamic locking is due to the motion of the screen and is shown in Figure 5C. 2 . Pitch on an inclined scrccn n i t h horizontal motion reduces the actual mesh opening - to a smaller effec.tive opening-, e = a X cos p (Figure 2). 3. Clogged and partly blocked openings affect sizing. 4. Heavy loading may squeeze ?lightly oversize elastics particles through the screen. 5, Xa&facturing toleranccs of the screen itself are important -

IO 20 30 40 50 60

1. 2.

or they may be retained by 23lj,-inch openings, depending on the w.ay in which the material landa on the screen. A screening process for stocks of such shapes does not accoinplish a great tlc:tl; it requires a size dimension of 1.238 inches with a tolerancc of 1 1 . 1 3 8 inches. While this example may be exaggerated, st,ocks with small irregular particles of fibrous, flaky, or cryst d i n e character are fiometimes screened. I t is d l to realize the limitations of such an operation. Particles and screen opcninga are oonirnonly proportioned more nearly as s h o m in Figure 2 . Material of noncircular section can easily be retained on a noncircular mesh, although the stock is actually undersize because the same particlrs are passed if they enter the mesh opening in a different position. The following additional €actors influence the sizing of products :

A S L I W T L Y UNDERSIZE RECTANGULAR PARTICLE MAY EITHER PASS A RECTANGULAR MESH OPENING OR BE RETAINED BY I T

INFLUENCE OF PITCH ON SCREEN OPENING

Figure 2.

Factors Affecting Capacity and Efficiency of Screens

Most of these factors tend t o retain undersize particles on t,op of the screen. Squeezing of the particles allows a portion oi the oversize t,o pass the mesh. I t is often helpful to combinc the factors that add 01’ dcduct from the free opening of the screen, and speak of an “cffcctive” mesh opening in a d d i h n t o the actual mesh opening (6). The effeciive opening must be determined erperinientally for each case. Figure 2 illust,rates clearly that,, while little trouble is encountered separating particles that are a great deal ovorsixe or a great deal undersize, part’icles very nearly the effective mesh

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 46, No. 9

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT If a grinding method could be devised in which the largest proportion of the material is ground to the desired granulation, and the peak of the granulation c w e could be maintained consistently a t the same particle size, screening could be eliminated from most processes. Unfortunately, it is not possible to check grinding in such a manner. In efficient modern pulverization a peak is reached a t the granulation for which the equipment is set, but additional finer product is also created when the stock is ground. This accounts for the more gradual descent of the granulation curve and for the additional hump, indicating fines, a t the right end of the curve. A knowledge of the granulation curve for the product to be screened is a prerequisite for intelligent planning of the screen areas and setting of tolerance specifications.

size are difficult to separate. They require the longest screening time. The curve for screen Yo. 18 in Figure 1 demonstrates this point. Graphical Analysis I s Aid to Quality Control in Screening Processes

The screening of difficult grains can be reduced to a minimum by means of a graphical analysis. In Figure 3 8 percentages of material on consecutive screens of the Tyler series are plotted. This material comes from a roll grinder; its granulation pattern is typical of many ground stocks. The two vertical lines represent “mean” meshes before and after grinding. Mean meshes are calculated by considering mesh opening steps as lever arms. These are multiplied by the percentage on the respective screen; the resultant is the mean mesh of the product. The difference between the mean mesh of the original stock and the mean mesh of the product of reduction is the reduction ratio-4.96 sieves or about 6 : l in this example. The steeper the grade of the curve where a screen separation is made, the easier is the screening operation. Steep grades offer a smaller proportion of difficult grains than sections where the curve runs more nearly horizontal. Where the screen openings are determined beforehand, the grinding equipment should be so adjusted that a granulation curve refiults that offers a steep course a t the screen openings where separations are required. The granulation curve (Figure 3 B ) indicates graphically the quantities of material handled by each screen. It permits planning of the screen areaa provided for the various separations. If it is assumed that a step of one screen in the Tyler scale can handle about 10% of the material, about four times as much screening area IS required between screens 6 and 10 as, for instance, betwern screens 14 and 28 The importance of maintaining control over the incoming fepd for efficient screening is also illustrated by this chart. Suppose that the grinding curve becomes coarser by one Tyler screen, without any change in its contours, as shown by the dotted line in Figure 3B. This could occur with a change in the feed to the grinder, with increasing humidity, or with wear of the grinding elements. The screens that were selected for the original setup no longer offer sufficient surface for satisfactory sifting, nor do the steps occur on steeply rising sections of the new curve. Therefore, the quality of the feed to the screens either has t o remain under constant control, or sufficient extra screening capacity has to be provided to allow for variations expected in the process. This chart also illustrates the principal reason for screening.

25

0

Functions of Friction and Inertia Are Fundamentals of Sifting

If a coin is placed on top of a sheet of paper and the sheet is moved slowly, the coin stays on the same spot on the paper and moves with it. However, if the paper is pulled rapidly, the coin does not move with it; the coin remains on the table while the paper is pulled away. The critical speed a t which motion begins-that is, when the body stands still while the support moves-is different for different materials. Smooth objects on even surfaces start to move a t lower speeds than rough particles. This is caused by the opposing roles played by friction and inertia. According to Galileo’s law, inertia tends to preserve the motion bodies have a t the start of a process. Friction binds bodies together; it tends to enforce the motion of one body to the other with which it is frictionally connected. In screening, stock with no horizontal speed drops on a moving supporting sieve. The material should not ride on the mesh; slippage between product and screen is necessary for a sifting

Figure 4. M

Horizontal Force versus Friction

=

Mass F = Friction force C or P = Horizontal force

r

I

I

September 1954

I

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-vI

I

I

,

I

I

I

I

I

I

1

0 1


1779

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

Figure

5. Effect

of Surface Speeds on Bolting through Screens

operation. Figure 4 shows a body of mass 114 moving at fipeed h. Force C or P , which acts on the body, must exceed the friction force, F , trying to hold it to the screen surface. The magnitude of friction between stock and mesh is influenced by a coefficient of friction. These coefficients have been determined A. Floor mounted B. Suspended gyratory C. Screen with D. Eccentrically driven gyrutory sifter sifter vertical rotary reciprocating screen oxperimentaIly. For flour mill stocks and motion (eccentrimany products of similar granulation the cally actuated macoefficient ranges roughly from 0.5 for chines also avaiiable) coarser to 0.6 for more powdery products. The variation is due to the €act that the different products vary in shapc, size, and specific gravity. Not only do thesc products have various coefficients of diding friction, but the case is further aggravated by the fact that some products roll to a certain degree a t some speeds E Weight actuated F. SemireciprocatG. Electromagnetic H. Electromagnetic rvhile others merely slide. The factors reciprocating screen ing sifter vibrating screen screen, vibration normal to screen of sliding and rolling friction are surface widely separated-the resistance for rolling friction is often one tenth of the Figure 6. Types of Gyrating and Reciprocating Screening Machines value for sliding friction for the same material. Differences in the coarseness of meshes introduce another factor Iocitj- of the particles. These are conditions actually prevailing of variation; the Bame stock has different coefficients on different) in comriiercial equipment. The shaded circles indicate the meshes. To ensure a reliable screening motion of all stocks on threads of the screen at one instant and the unshaded circles the sieve in spite of these variables, commercial screening equipthe same t,hreads a fraction of a sccond later. The repulsing acment applies a factor of safety (often 100%) to the critical speed tion of points 2 and 3 still occur. In addition, even particles a t which sifting just begins. a t point 1 are in danger of bcing r u t off by the rapidly moving Screen Velocity. To overcome friction it is only necessary to mesh before they have a chance to clear the center linc of the cxceed a certain minimum speed. Tests have been run to see if threads. This dynamic locking of openings explains why the results justify very high gyratory speeds, possibly 400 r.p.m. a t clothings of moving srrcens must he coarser t,han shtionary 2-inch radius of gyration ( 2 ) . These esperiments resulted in very screens t,o obtain identical results. If the speed of thc sifter is low capacity values. It was concluded that other factors besides extremely fast, it can possibly stop all sifting: the screens in this friction and inertia contribute to the performance of a screen. case act, like solid supports. These reflections show tha In A , B, and C of Figure 5 are shov-n schematic sections sprcd is an important factor in both t,he fiixing characteristirs and through a screen cloth. The cloth is stat,ionary in Figure ;A. capac*ity of scrwning machines. As a particle is dropped vertically, it passes the mesh in one of the three viays shown. When the mesh moves very slowly, industrial Screening Machines Employ slower than the falling particle (Figure 5 R ) , the particle may still Various Principles of Operation pass a t point 1. At point, 2 it is repulsed from the bolting surFor the various industrial applications, numerous typw of face by the oncoming thread. The particle a t point 3 starts to screening machines are used, employing several different prinroll over the top of the thread. Before it has time to drop past' ciples of operation. Some of the most widely used basic types the center of the screen, the next thread of the fahric catches the are shown schematically in Figure 6. This survey is confined ;)art,icle and throws it up, much as the particle at point 2 is t,o screens with horizontal motion, illustrated in A, B, D-G of thrown. A screen a t this velocity would consequently hold Figure 6. Screens with vertical motion ( C and Hof Figure (i), arc ;lpprosiniately two thirds of the stock in suspension. This two not discussed in detail because their principal field of application thirds would not be in constant contact with the bolting surface, is classificat,ion of coal, minerals, and other coarse materials. t,hereby reducing the capacity of the Screen per unit of area. They are not well suited to stocks of finer granulation found in At position C it is assumed that the sieve velocity is still the processing industries. I n addition, a comprehensivc study greater; its horizontal speed is greater than the downward ve-

1780

INDUSTRIAL AND ENGINEERING 'CHEMISTRY

Vol. 46, No. 9

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ( 6 ) of these screens is available. In addition to the models shown in Figure 6 there are others that are less frequently encountered, such 8s screens actuated by tapping with hammers or latchet cams. Confronted with the various dcsigns and makes of machines, the user often finds it difficult to decide which type is best suited lor his particular job. If he looks for impartial information he is likely to be disappointed; few comprehensive data have been published on screening mchines. rZlmost no broad summarization of performance or teet data is available; it seems that d o signers of equipment often check just a fen- experimental models and collect scattered empirical information, rather than study the broad principles involved and make these available to the users of the equipment. It is hoped that this explanat'ion of some of the basic 1an.a of screening will help in the selection and operation of equipment. Development of Mechanical Screening. Possibly the most ividespread use of screening is made in the grain processing industries. Many phases in the history of sifting are therefore related to events in milling technology. The development oi' mechanical screening in flour mills started with reels~lorvlyrotating cylindrical or hexagonal screens u-ith a slightly pitched axis. Stock to be separated was fed at the upper end, undersiee material passed t,he screen, and oversize disc>hargedat the loner end. Later, centrifugal reels with larger capacities camc into use. Metal bcat,ers operating inside the cylindrical screen forced the undersize material through the mesh. At the same time, pulsating screens n w t ' used in the grain cleaning department. When adoption of roller mills and the gradual reduction system process increased the number of screening operations required in milling, the gyratory sifter was developed which al1on.s many fiquare feet of bolting cloth to be stacked in layers in compact machines of high capacity. It is claimed that the first sifter wm built in 1888 by a Hungarian designer named Haggenmacher. The first sifters used in America were imported from Hungary; they had a radius of gyration of about 2 inches and speeds between 170 and 190 r.p.m. Thousands of gyratory machines subsequently manufactured in America as well as in Europe and applied in a variety of processes, had substantially the same combination of throv and speed. .ibout IS years ago, Thomas Cecka, then superintendent of a largc Buffalo mill, had to install additional flour dresiiing capacity in limited space. He experimented with various sifting motions and found that a 1-inch radius iind rotation a t 300 r.p.m. resulted in higher capacity per unit of screen surface for flour on silk bolting cloth. This development started discussion of sifting principles with the result that both conventional motion (I70 to 1'30 r.p.m. at 2-inch radiufi)and high speed sifters (270 to 300 r.p.m. at 1-inch radius) are now furnished by a number of American manufacturers.

which can be solved for n, the speed at which sifting starts, 1-

n = 30d:

If p = 0.6 and r = 2 inches, n = 103 r.p.m. A eonventiond speed sifter running a t 190 r.p.m., therefore, has a safety factor of 85%. For r = 1, a critical speed of 146 r.p.m. and a safety factor of 103% in a sifter running at a speed of 300 r.p.m. are found. A graphical representation of diff erently composed gyratory sifting motions can be developcd by use of Equation 4. Figure 7 include,! a curve for the critical speeds as a function of the circle of gyration for materials with cocfficient = 0.6. It also shows a curve for the same friction coefficient rvith a 90% safety factor added. B curve developed from the motions of commercially manufactured sifters is also indicated. This latter curve appear? to be almost a straight line. St one time this fact misled some designers into a false theory which could not be substantiated by tests or experience (8). Screen Velocity. A short calculation demonstrates t,he magnitude of the locking effect. at actual screening speeds. Silk bolting cloth No. 9XX is a screen t,hrough which powdery materials pass. The schematic scale section shown in Figure 5 0 indicates the basic dimensions of this cloth. Actually, the threads are not circular in cross section but have an irregular shape, since they are made up of two intertwisted silk strands. The time needed for one particle of stock to fall from the upper edge to the center of the thread is

0

9 8

d ,

6'

E 6

tE 2

4

3 2

D 2C

60

8c

lo0

120 110

I60 180 2CO 220 2u) 261 R.P.Y.

280

JW 120 344

At critical speed, when sifting ?tarts, the centrifugal force, C, must be equal or larger than the friction force F (Figure 4). I~xpressedin mathematical tcrme, C 2 F . The equation for cmtrifugal force is

Columb's ionnula for friction is

Introducing these expressions

September 1954

420

Figure 7. Critical Speeds of Gyratory Sifters The movement of the thread is expressed by d = ut

Advantages of Gyratory Sifters Are Outlined

bo 380 Ki

(6)

In the case of a conventional speed sifter

d = 101 X 0.0038 = 0.342 cm. The screen moves 0.342 cm. during the time required by the particle to drop by its own weight to the center line, or 23.3 times as much as the free opening of t,he mesh. In case of a high speed sifter with a screen velocity of 80 cm. per second, t>he screen moves 0.27 em. or 18.4 times the mesh opening of 0.0147 em. On a No. 2OW tinned mill screen, used to separate coarser products and shown in Figure 5E, the time required for particle2 of stock to fall freely from the top to the center of the mesh is 0.0061 second. In a convenlional speed sifter, operating with a surface speed of 101 em. per second, the mesh moves 0.67 tin. during the drop, 7.34 times the mesh opening. With high speed gyration, the mesh moves 0.488 cm. during the same period, or 5.38 times the free opening of this screen. Of course, less locking can be expected on this wider opening.


1781

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Particles actually pass the screen because the time needed by them to traverse the radius of the thread is shorter than the above values. Two variable factors contribute to speed up the drop-the fact that particles are passing the upper edge of the screen with an initial speed as they drop from above, and pressure from other particles on top of the ones in contact with the meen tends to hold parts in the screen openings and prevent them from jumping off the screen; loaded scieens have higher capacities than bare sifting areas. The values for velocities on the Purface of the screen were derived by applying the basic equation

By using the constants for convent,ional speed sifters, a velocity of 101 cm. per second results. In the case of the high speed sifter, a velocity of 80 cni. per second is calculated. Screen velocities on so-called high speed machines are actually lower than those of the conventional speed. Preliminary reflections indicated that low velocities alloJ7 the Etock to pass the sieve more rapidly. In this respect, the high speed motion constitutes an improvement of 27% over the earlier construction. In the author's opinion it is the combination of improved data on both the centrifugal force and velocity factors that account for the better performance of high speed sifters on fine stocks. There are secondary advantages connected with high speed sifting equipment. Counterweight,s are employed to offset forces created by spinning parts. Their weight is calculated by the formula

W (counterweights) =

W (sifter) X T (sifter) T (counterweights)

(8)

In other wordu, the amount of offsetting weight required is proportional to the radius of gyration, and the radius of gyration of high speed machines is lesc than that of conventional speed equipment. The iesulting smaller counterweight can be placed in a compact epace directly opposed to the centrifugal resultant of the gyrating structure and operates more effectively than the heavier, more bulky aeights of machines with larger circlea of gyration. Varying Aeceleration and Velocity of Reciprocating Screens Minimizes locking

The basic functions of friction and inertia apply to reciprocating screens as well as to gyratory sift,ers. In the case of reciprocating machines the field is not liniited to only two predominant combinations of speed and throw. Instead, amplitudes range from over 1 to less than l/,b inch, and frequenoiw range between 150 and 1800 cycles per minute. This vide range precludes any simple comparison of only two actions. In most cases, reciprocating motion is produced by transforming rotary drive motion into forward and return strokes. Since at the terminal points of the strobes the screen reverses its direction it can easily be concluded that there is no uniforni motion or velocity in these machines. The screen stands still twice during each cycle-when the velocity in one direction decreases to zero and mot,ion in the opposite direction starts. Friction. On a horizontal yeciprocating screen the friction force holding stock on the screening surface must be overcome, a t each pulsation of the screen, by the force created in accelerating and decelerating the mass of the particles. The force of acceleration is P = p

If the crank radius,

1782

I',

11' ~

(9)

the length of connecting rod, I , and the

crank angle, sion is

a,are

considered, the a,cceleration, 'p, in this expres-

p =

p

;

)

(cos a i -cos 2a

Actually, since the length of the connecting rod of comniercial equipment is usually much greater than the crank radius, the simplified formula p =

02

; cos a

(lob)

can be used. This equation applies correctly t o the pulsating mechanism of Figure 6E and in close approsiniation to the mechanisms using crank drives. The conditions of equilibrium demand that the moving force of acceleration equals the holding friction force

P Z F

Sifting stark on a level reciprocating screen when the espri:sJion,

_r exceeds the value of the coefficient of friction. Further in0'

crease of the acceleration beyond the critical point increases the margin of safety of the sifting operation.

Table 1.

Characteristics of Reciprocating Motion of Sifters

Crank Angle (e), Degrees

Velocity

Acceleration

Cm'%ec.

Cm.lseo.2

(.4

? ! I

Shaking Shoe, 600 K.P 11. a n d //winch Crank Radius 0 1 4 0 - 1900 10 5.19 1.87 1870 20 10.2 1 78 -1780 30 14.9 - 1650 1.65 19.2 40 1460 1 46 50 22.9 -1220 1.22 25.9 0.95 60 -950 6;10 0 65 70 28.0 80 29.4 0.33 -330 90 0 29.9 0 0.33 100 29.4 330 0.65 28.0 650 110 0.95 28.9 120 950 1220 1 22 22.9 130 1460 I 46 19.2 140 150 1 66 1650 14.9 1780 1.78 10.2 160 5,19 1870 1.87 170 1 $1 180 0 1900

-

Sifter (Figiire 6 F ) , 180 R.P.M. and Il;r-Inch Crank Radius 0 -1140 1.14 0 1.12 10 4 - 1120 10 20.5 - 1070 1.07 20 30 29.9 -988 0.!488 40 - 873 0,873 38 5 50 45 6 -733 0.733 0.570 60 51.8 570 56.2 0.390 70 -390 58 8 0.198 -198 80 0 0 90 59.8 0.198 100 58.8 198 0.390 110 56 2 390 51.8 0.570 120 570 45.8 733 0.733 130 38.5 873 0.873 140 29.9 150 988 0.988 20,5 160 1070 1.07 1.12 170 10 4 1120 1140 I 14 180 0

-

Kith an inclined horizontal motion there are two critical speeds. On the downgrade the critical speed is increased, mid on the upgrade stroke it is reduced, as compared t,o the hori5ontd screen; the average for both is the same as for a level screen. The result of these inodifieations is that the stock is carried downgrade when the actual speed approaches the downgrade critical speed. Material slides on the upward stroke. Most of the sift-

INDUSTRIAL A N D ENGINEERING CHEMISTRY

Vol. 46, No. 9

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT ing occurs on the upstroke, most of the conveying on the downstroke. Screen Velocity. The magnitude of screen velocities a t varying crank angles during each stroke is determined by the expression h = rw sin

(12)

CY

Low screen velocities promote easy passing of the stock in pulsating devices, just as they do on gyrating sifters. Data for two typical reciprocating machines are listed in Table I. The sifter design shown in Figure 6F is widely used in industry for a great variety of grading jobs since it combines a gyrating motion a t the head end with pulsation a t the tail. The motion is imparted a t the top by a pin rotating on a ll/a-inch radius a t 180 r.p.m. The approximate length of the screen, and the distance from the eccentric drive to the sleds, is 100 inches. At the head, where gyratory motion prevails, the speed of the screen equals 59.8 cm. per second and the critical gyratory speed is 130 r.p.m. Adjacent to the sleds a t the lower end, reciprocation with the characteristics listed in Table I is encountered.

openings to become filled with lodged material-also increases with small btrokes, which sets practical limits of minimum strokes for each material and mesh. Consequently, in selecting screens it is advantageous to select the combination of smallest throw with fastest speed that still allows the screen to operate essentially unblocked.

A

c m b r i w OF CWEYING

Q

FLIGHTS IN A GYRATORY SIFTER

PATH CF STOX OV PITCHED SCREEV I N A GYRATORY SIFTER

C

PROGRESSION OF STOCK 0% A PITCHED RECIF%XATlNG SCREEN

Figure 9. Conveying Patterns

\

.25 .30

!

Figure 8.

'

/

'\

~

\.->'

Relation of Crank Angle and Velocity and Acceleration of Screen Table I

The advantageous low screen velocities a t crank angles from 0' to 60" and 120' to 180' coincide with the highest and, there-

fore, best values for acceleration (Table I and Figure 8). When the maximum relative motion between stock and sieve takes place, screen motion is slow and stock can pass through easily. Close to the 90" point there is no relative motion between sieve and stock, but agitation results as the direction of the acceleration reverses itself. This helps to distribute the stock, and, by turning particles around, also maintains a conveying motion. The velocities reverse their direction a t 0" and 180", an action which helps to keep the screen surfaces clean. Particles first thrown against one side wall of a screen opening and lodged or wedged there often free themselves and drop through a t the reversal of the stroke. The locking effect illustrated in Figure 5C is largely avoided. Even when maximum velocities are high, the speed drops to zero twice during each cycle to free the openings for passage of material. Ofhet Facfors Influence Performance of Screening Equipment

Sieve Cleaners. These observations lead to the conclusion that fast speeds with small throws generally give best sifting performance on all types of screens. Unfortunately, the tendency of screens to plug, clog, block, or blind-that is, for the screen September 1954

In many cases it is advisable to experiment with severaI different types of sieve cleaners to determine which of the maay methods of keeping screens open is best suited for the process. Screen cleaners have a marked effect on the capacity and quality of any sifting operation and careful choice is mandatory for efficient sifting. Since there are perhaps more types of cleaning devices in use than types of screening machines, a detailed analysis cannot be attempted in this article. Conveying Motion. Horizontal sieves, operating a t higher than critical speeds, are screens only-Le., a particle placed on one point of the screen remains near that point indefinitely, if it is oversize, with the screen moving below it. I n production flow, processed stock has to be removed from the surface to make room for more oncoming stock. To accomplish this, commercial screens also need some of the features of a conveyor; a transporting motion must be created in the machine. In gyratory sifters this can be done by use of conveyor flights or pitched sieves. I n pulsating equipment, pitched sieves are generally used. Figure 9 illustrates the operation of the two propulsion methods. Only slight pitches can be used on screens with horizontal motion if sifting capacity is to be maintained. As the pitch increases, contact between stock and sieve is reduced. Where a very steep slope is required to handle the material, a reciprocating screen with an action normal to the screening surface gives the best results; the pulsations, rather than gravity action, control the passing of the stock in this design. Propulsion created by pitching reciprocating screens and pane is similar to the functioning of trough and vibratory conveyors. In Figure 96, stock is held to the surface by friction during the leftward stroke but remains in suspension during the return stroke to the right, when the supporting surface retracts from the stock. Flow Conditions. The flow diagram of a process involving screening can often be arranged in several ways. When large capacities are required, splitting the product into two or more parallel operations may make the process more efficient. Even for small streams, separation of the process into two steps of different character, in series, may help. For instance, when there is a sizable proportion of coarse oversize and fine undersize, elimination of this bulk by a scalping operation is recommended, possibly


1783

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT on a reciprocating screen. -4more accurate sizing operation for a smaller quantity or material can complete the job. Size of Units. Results obt'aincd from machines of different sizes but of similar construction arc not always comparable. Small scale machines seldom duplicate the conditions of full size production equipment. For this reason, many manufacturers of screens ask for test sample large enough to run through full size machines before they make recommendations. The experimental gyratory sifter used for some of thc tests described in this paper is shown in Figure 10. Summary

The constant characteristics ol their a,ction make gyratory sifters suitable for high capacity, accurate granulation screening of dry, free-flowing materials of rather uniform shape and weight. Large capacities and quiet, vilrationless and dustless operation cari bc achieved with the compact sifters designed for this service.

roughness. The act,ion of these screens turns oblong particles -e.g., kernels of ivheat-so that the long axis of the grain is parallel to the direction of flow. This makes it possible t,o use ohlorlg perforations on reciprocating screens. Equipment with gyratory action a t the head and pulsating 1110tion a t the tail combines some of the features of both types of motion, The gyration a t the top is relied upon to spread t,he stock over the surface. The gyrat,ory speed ~ h o u l dhe chosen safely above thc critical speed of the bulk of the material in orcic,~, t,o separate a large proportion in the head section. Gyratory sifters are generally well balanced. They a n > , therefore, preferred for use in existing buildings where no foi from the screen cari be transmitted to the struct,ure. Of tht. horizontally pulsating designs, the design of Figure 6E' is alw well balanced, but most other constructions transmi t some vibration to the building. Because of the many factors involved, it is impossible to evolvean equation, or draw a chart, to determine the capacities and t h c b degree of effect'iveness of screening equipment. Instead, a, car('ful and logical analpis of the entire operation is required i n every instance. Nomenclature

3 2 = Mass, grams C = centrifugal force, grams T = radius, cm.

F' IT*

= friction force, grsms =

weight, grams

I'

= force of accPlcraiioii, gra

01

=

crank angle, d

= coefficient of friction 'p = acceleration, cm. isec.l I = length of connecting rodl cni. n = velocity of rotation, r.p.m. i = time, see. s = distance, cm. v = vertical velocity, em. /seta d = distance, cm. n = nctual mesh opening, cin. e = cJTective mesh opening, cm

p

Figure 10.

Experimental Gyratory Sifter

Fixture at upper left records circles o f gyration

The constantly varying velocities and accelerations of recipiocating action guarantee plenty of agitation and resistance to plugging. Thilc they a n less compact than sifters, the screens of these reciprocating machines have the advantage of being easily accessilslc-they can be inspected during operation. Pitch and the related stock motion can be adjusted in most pulsating screens; in a number of constructions amplit,udes are also adjustable to take care of stock, loading, and operating conditions. When the tolerances of separation are slightly more liberal, this type of screen is well suited t o grade less free-flowing materiak that require x positive st,irring and conveying action, wet and moist stocks, and particles heterogeneous in weight, size, and surface

References (1) (2) (3) (4) (5) (6)

Coghill, IT. H., Eiag. M z h i a g J . . 126, 934 (1928). Marsh, A. M., Alli8-Chalmcrs I n d . Rei!. (1940).

Pfister, Northtoest. M i l l e r (trans. from Die Muehle) (1929). Porter, J. B., Katl. Research Council Can., Itept., 22, 1929. Rockwood, N . C., Rock P ~ o d u c t s .1944, 1845. W.S. Tyler Co., Cleveland, "Profitable Use of Testing Sirl-es." 1949. ( 7 ) Wolff, E. R., Sorthzcest. X i U e v (1940). RECEIVED for review October 7 , 1963.

ACCEFTEDhlay 28, I!).?

L

END OF ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT SECTION

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Vol. 46, N o . 9

Screening Principles

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