Chapter-2 Screen Analysis

7/18/2019 Chapter-2 Screen Analysis

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MECHANICAL OPERATION

(CEO-I) – 3320013Semester - 03

Kartik R !esai

Head, CHED, NGPP



Chapter II: Screen Analysis




Need of screen analysis:

• A method for nding the particle-size distribution

of any loose, owing, conglomerate material bymeasuring the percentage of particles that passthrough a series of standard screens with holesof various sizes.



Ideal and actual screen:

• Ideal screen = which sharplyseparates the feed miture in such away that smallest particles inoverow is !ust larger than thelargest particle in the underow.

• Actual screen = which don"t yield asharp separation, the screen analysisof overow and underow are similar.




Ideal screen:

1) Eciency of the screenis 1.

2) Not found in practice.

3) Perfect separation ofparticles are otained.

!) Separation is ased oncut dia"eter of

particle.


Actual screen:

1) Eciency of thescreen is less than 1.

2) Practically a#ailale.

3) Perfect separation isnot otained.

!) Separation is asedon si$e % shape ofparticle.



&ypes of screen analysis:

#$ %umulative analysis

&$ 'i(erential analysis




Application of screen analysis:

)radation is usually specied for each engineeringapplication it is used for. *or eample, foundationsmight only call for coarse aggregates, and thereforean open gradation is needed.

)radation is a primary concern in pavement midesign. %oncrete could call for both coarse and neparticles and a dense graded aggregate would beneeded. Asphalt design also calls for a dense graded

aggregate.)radation also applies to subgrades in paving, whichis the material that a road is paved on. )radation, inthis case, depends on the type of road +i.e. highway,

rural, suburban$ that is being paved.




Capacity and e'ecti#eness ofscreen:

• %apacity = is the mass of materialthat can be fed per unit time to unitarea of the screen.

• (ectiveness = measure of thesuccess of the screen in closelyseparating undersize and oversize

material.




• 'erivation of formula for overalle(ectiveness of screen

• = +* / 0$ +' / *$ ' +# / 0$

+' / 0$& +# / *$ *

1here

* = mass fraction of material A in feed

'= mass fraction of material A inoverow

0 = mass fraction of material A in

underow




• 2ypes of screen

• 2he screens can be classied into followingtypes

– )rizzlies

– 2rommels

– 3echanically sha4ing screens

–

5ibrating 6creens – 7scillating screens

– 8eciprocating screens




(ri$$lies:

• 2hese consist of a set of parallel bars held apart by spacers at somepredetermined opening.

• 0ars are fre9uently made of manganese steel to reduce wear.• A grizzly is widely used before a primary crusher in roc4- or ore-

crushing plants to remove the nes before the ore or roc4 enters thecrusher.

•

:t can be a stationary set of bars or a vibrating screen.

• Stationary grizzlies: – 2hese are the simplest of all separating devices and the least

epensive to install and maintain. – 2hey are normally limited to the scalping or rough screening of dry

material at ;.;< m +& in$ and coarser and are not satisfactory for

moist and stic4y material. – 2he slope, or angle with the horizontal, will vary between &; and

<;. – 6tationary grizzlies re9uire no power and little maintenance. – :t is di>cult to change the opening between the bars, and the

separation may not be su>ciently complete.




• Flat grizzlies:

– 2hese, in which the parallel bars are in ahorizontal plane, are used on tops of ore and coalbins and under unloading trestles.

– 2his type of grizzly is used to retain occasional

pieces too large for the following plant e9uipment. – 2hese lumps must then be bro4en up or removed

manually.

• Vibrating grizzlies:

–

2hese are simply bar grizzlies mounted oneccentrics so that the entire assembly is given abac4-and-forth movement or a positive circlethrow.




• Revolving Screens :• 8evolving screens are also called &ro""el screens,• 2hey consist of a cylindrical frame surrounded by wire

cloth or perforated plate, open at both ends, andinclined at a slight angle.

• 2he material to be screened is delivered at the upperend, and the oversize is discharged at the lower end.

• 2he desired product falls through the wire clothopenings. 2he screens revolve at relatively low speedsof #< to &; r?min.

• 2heir capacity is not great, and e>ciency is relativelylow.




echanical Sha*in+ Screens:

• 2hese screens consist of a rectangular frame which holds wirecloth or perforated plate and is slightly inclined and suspendedby loose rods or cables or supported from a base frame byeible at springs.

• 2he frame is driven with a reciprocating motion. 2he materialto be screened is fed at the upper end and is advanced by the

forward stro4e of the screen while the ner particles passthrough the openings.• :n many screening operations such devices have given way to

vibrating screens.• 6ha4ing screens, such as the mechanical-conveyor type made

by may be used for both screening and conveying.• 2he advantages of this type are low headroom and low power• re9uirement.• 2he disadvantages are the high cost of maintenance of the

screen and the supporting structure owing to vibration and lowcapacity compared with inclined high-speed vibrating screens.




• ,iratin+ Screens:

• 2hese screens are used when large capacity and high

e>ciency are desired. 2he capacity, especially in thener sizes, is so much greater than that of any of theother screens that they have practically replaced allother types when e>ciency of the screen is animportant factor.

• Advantages include accuracy of sizing, increased

capacity per unit area, low maintenance cost per ton ofmaterial handled, and less installation space andweight.

• 2he vibrating screens basically divided into two mainclasses +#$ mechanically vibrated screens and +&$ electrically

vibrated screens. :n electrically or electromagneticvibrating screens, the screen itself is vibrated, and inmechanically vibrated screen, the whole assembly isvibrated.

lectromagnetic vibrating screen 3echanically vibrating screen




• -scillatin+ Screens:

• 2hese screens are characterized by low speedoscillations @< to oscillations per second +B;; toC;; r?min$D in a plane essentially parallel to thescreen cloth.

•

6creens in this group are usually used from ;.;#Bm +#?& in$ to E; mesh. 6ome light free-owingmaterials, however, can be separated at &;; toB;; mesh.

• 6il4 cloths are often used.




• eciprocatin+ Screens:

• An eccentric under the screen supplies oscillation, ranging fromgyratory @about ;.;<-m +&-in$ diameterD at the feed end toreciprocating motion at the discharge end.

• *re9uency is F to #; oscillations per second +<;; to E;; r?min$, andsince the screen is inclined about <, a secondary high-amplitudenormal vibration of about ;.;;&< m +#G#; in$ is also set up.

• 2hey are not designed for handling heavy tonnages of materialsli4e roc4 or gravel.




(yratory Screens and iddles:

• 2hese are boli4e machines, either round or s9uare,with a series of screen cloths nested atop oneanother.

• 7scillation, supplied by eccentrics or counterweights,

is in a circular or near circular orbit.• 3achines of this type are operated continuously.

• 2he size ranges from ;.E to #.< m +&C to E; in$.

• Gyratory Riddles screens are driven in anoscillating path by a motor attached to the support

shaft of the screen.• 2he gyratory riddle is the least epensive screen on

the mar4et and is intended normally for batchscreening.


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Chapter-2 Screen Analysis