Crush­ers and screens in in­dus­trial plants

DEMM Engineering & Manufacturing - - MOTORS & DRIVES - By AMIN ALMASI

IN­TRO­DUC­TION

A crusher is a ma­chine used to re­duce large size solids into smaller sizes; crush­ers are widely used in in­dus­trial and man­u­fac­tur­ing plants to re­duce the size of raw ma­te­ri­als or other solids for pro­cess­ing, man­u­fac­tur­ing, pro­duc­tion, etc. Crush­ers are of­fered in many dif­fer­ent mod­els and de­signs; in a type of crush­ers, the ma­chine holds ma­te­ri­als be­tween two par­al­lel or tan­gent sur­faces, and ap­plies force to bring the sur­faces to­gether or to move ma­te­ri­als to­ward low gap sec­tions to crush the trapped ma­te­ri­als. Dif­fer­ent crush­ers such as jaw crush­ers or cone crush­ers work on this ba­sis; al­though there are other ma­chines work­ing on sim­i­lar prin­ci­ples.

Im­pact crush­ers in­volve the use of im­pact rather than pres­sure to crush ma­te­ri­als. The ma­te­rial is usu­ally con­tained within a cage

(or sim­i­lar), with open­ings on the bot­tom, end, or side. De­sired size of ma­te­ri­als will be dis­charged and other ma­te­ri­als would be held for fur­ther crush­ing. There are again many dif­fer­ent types of im­pact crush­ers. In in­dus­trial, man­u­fac­tur­ing and sim­i­lar in­dus­tries, im­pact crush­ers are more pop­u­lar there­fore the fo­cus of this ar­ti­cle would be on im­pact crush­ers par­tic­u­larly ring ham­mer crush­ers.

Vi­brat­ing screens have been the most im­por­tant screen­ing ma­chines. They are used to sep­a­rate feeds con­tain­ing solid and crushed ma­te­ri­als down to ap­prox­i­mately 0.3 mm in size, and are ap­pli­ca­ble to a wide range of ma­te­ri­als from per­fectly wet­ted to dried. Vi­brat­ing screens are also dis­cussed as an im­por­tant parts of over­all size re­duc­tion fa­cil­i­ties.

GEN­ERAL NOTES ON CRUSH­ERS

There have been many drive ar­range­ment and con­fig­u­ra­tions pro­posed for crush­ers such as VSD elec­tric mo­tor sys­tems, wide ranges of me­chan­i­cal vari­able speed sys­tems, me­chan­i­cal cou­plings, etc. Fluid cou­plings have been widely used par­tic­u­larly for medium size crush­ers.

A crusher should be ca­pa­ble of de­liv­er­ing the nor­mal rated out­put even when han­dling ma­te­ri­als some­how dif­fer­ent than rated ma­te­ri­als such as ma­te­ri­als wet­ter or sticker than nor­mal, high­est mois­ture con­tent than the spec­i­fied ones, dif­fer­ent sizes than rated sizes, even slightly dif­fer­ent com­po­si­tions, and so on. This is of­ten a great chal­lenge as a crusher should be pro­vided to tol­er­ate all these pos­si­ble changes. No clog­ging or build­ing-up of ma­te­ri­als on the crush­ing ele­ments should de­velop. Pro­vi­sions should be kept such that the gap be­tween dif­fer­ent ele­ments can be ad­justed to take care of de­mand of out­put size vari­a­tions or other op­er­a­tional changes. Uni­form crush­ing is an­other im­por­tant re­quire­ment which should as­sured by proper de­sign and op­er­a­tion of a crusher.

Re­quired power of a crusher is re­lated to many dif­fer­ent fac­tors such as ma­te­rial prop­er­ties, feed­ing lumpi­ness, dis­charg­ing gran­u­lar­ity, crusher speed, etc. It is gen­er­ally dif­fi­cult to com­pute re­quired power of a crusher. Most likely em­pir­i­cal and ex­per­i­men­tal for­mula are used for the power siz­ing. In other words, most of­ten, em­pir­i­cal for­mula or spe­cific power con­sump­tion meth­ods are utilised to com­pute power and con­se­quently other im­por­tant fea­tures such as ro­ta­tional in­er­tia, mo­ment of fly­wheel, de­sign de­tails, etc. There have al­ways been con­cerns about power cal­cu­la­tions for crush­ers. Usu­ally dif­fer­ent cal­cu­la­tion meth­ods should be used to dou­ble check the rated power and other fea­tures; em­pir­i­cal co­ef­fi­cients used in such for­mu­las are im­por­tant and they should prefer­ably be se­lected from up­per range of of­fered val­ues to pro­vide some mar­gins. Oth­er­wise cal­cu­la­tion meth­ods come to un­der­es­ti­mated power val­ues. For many crush­ers, rated power has been suf­fi­cient on pa­per; but prac­ti­cally it has been mar­ginal or un­der­es­ti­mated. As the re­sult, these crush­ers can­not pro­vide rated ca­pac­i­ties or eas­ily get over­loaded. For in­stance, for many crush­ers over­load co­ef­fi­cient is considered be­tween 1.3 - 1.4 whereas over­load fac­tors should of­ten be around or above 1.8. Ob­vi­ously power is un­der­es­ti­mated with such low fac­tors and the ex­pected ca­pac­ity and per­for­mance can­not be achieved by such an un­der­es­ti­mated power rat­ing and un­der­sized driver. Also siz­ing of im­por­tant parts such as fly­wheel, bear­ings, etc should be prop­erly checked and ver­i­fied. Mon­i­tor­ing and the on­line check of per­for­mance are im­por­tant for many crush­ers; dif­fer­ent mon­i­tor­ing sensors and ele­ments such as vi­bra­tion mon­i­tor sys­tems, tem­per­a­ture de­tec­tors mounted on bear­ings, etc, should be pro­vided.

RING HAM­MER CRUSH­ERS

Ring ham­mer crush­ers break solids by im­pact­ing them with ham­mers that are fixed on a spin­ning ro­tor. These crush­ers are used tra­di­tion­ally for rel­a­tively soft ma­te­ri­als such as lime­stone, phos­phate, gyp­sum, shales, coals, etc. How­ever, im­prove­ments in their de­signs and met­al­lurgy have made them suit­able for a wide range of ma­te­ri­als and ap­pli­ca­tions. Nowa­days, ring ham­mer crush­ers are used in many ap­pli­ca­tions. By com­par­i­son with other types of crush­ers, ring ham­mer crush­ers have many ad­van­tages such as high per­for­mance, less noise, less dust, low power ra­tio, etc.

Ring ham­mer crush­ers mainly make use of the im­pact ef­fect to crush ma­te­ri­als. When the ma­te­ri­als en­ter the crusher, they are crashed by the high speed­ing ring ham­mer. The crushed ma­te­ri­als gain en­ergy from the ring ham­mer, rush to the crush­ing board at high speed, and crashed at sec­ond time. At the same time, the ma­te­ri­als bump with each other, and are re­peat­edly cracked. Ma­te­ri­als smaller than the grate bar gap are dis­charged, while larger ones on the grate bar are im­pacted, grinded, squeezed again to be crashed by the ham­mer, and at last the ma­te­ri­als are ex­truded by the ham­mer from the gap to ob­tain prod­ucts with the re­quired size. Ring ham­mer crush­ers have usu­ally been of­fered in two types: re­versible and ir­re­versible. Re­versible ham­mer crusher is with re­versible ro­tor, gen­er­ally used for fine crush­ing; ir­re­versible ham­mer crusher is with ir­re­versible ro­tor, gen­er­ally used for medium crush­ing.

Vi­bra­tion level should be kept at min­i­mum for both nor­mal and ab­nor­mal/emer­gency sit­u­a­tions. The crusher de­tails and char­ac­ter­is­tics should pro­mote this low vi­bra­tion; for in­stance, a crusher with its drive, supporting struc­ture and foun­da­tions should be de­signed ad­e­quately to take care of any un­bal­ance forces aris­ing out due to op­er­a­tion of the crusher even with max­i­mum two bro­ken ham­mers. There are many emer­gency cases like above-men­tioned which should be considered. An­other ex­am­ple, as the crusher ham­mers start wear­ing, im­bal­ance of ro­tor takes place and vi­bra­tion in­crease. Hence, ad­justable bal­anc­ing weights should be pro­vided on both sides of the ro­tor to help achiev­ing proper bal­ance. As a very rough in­di­ca­tion, vi­bra­tion lim­its of a ring ham­mer crusher are of­ten spec­i­fied some­where be­tween 70 and 140 mi­crons when all ham­mers are in in­tact con­di­tion.

Ease of main­te­nance is a key re­quire­ment; all ham­mers should be toothed ham­mer weigh­ing

max­i­mum 25 or 30 Kg. Max­i­mum ac­ces­si­bil­ity should be pro­vided for rou­tine in­spec­tion. On the other hand, re­li­a­bil­ity is a ma­jor con­sid­er­a­tion. The crusher ro­tor should usu­ally be over­sized to min­i­mize de­flec­tion and to pro­vide ex­cess safety fac­tor. The crusher hous­ings are of suit­able grade of steel with rugged de­sign for rigid­ity dur­ing op­er­a­tion and ease of ac­cess for main­te­nance. The crusher frame should be pro­vided with quick open­ing, large in­spec­tion doors fit­ted with dust tight seals. It is fit­ted with wear re­sis­tant thick lin­ers from a very suit­able ma­te­rial. The en­tire inside sur­face of crusher com­ing in con­tact with ma­te­ri­als should be pro­vided with abra­sion re­sis­tant lin­ers most of­ten from suit­able grade of ma­te­ri­als such as al­loy steels, etc. For ring ham­mers, the gap be­tween cage bars and the ham­mers should be ad­justable to com­pen­sate for the nor­mal wear.

VI­BRAT­ING SCREENS

Vi­brat­ing screens usu­ally op­er­ates at an in­clined an­gle, tra­di­tion­ally vary­ing be­tween 0° to 25° and can go up to a max­i­mum of 40°. Above this an­gle is pos­si­ble and was used, nut not rec­om­mended. Flat vi­brat­ing screens or those with small an­gles (such as be­low 5°) are used in special ap­pli­ca­tions; sim­i­lar is true for in­clined an­gle above 35°. Vi­brat­ing screens have be­come more stan­dard­ised and widely adopted in ma­te­rial clas­si­fi­ca­tion pro­cesses; they al­low ef­fi­cient cuts and fine sep­a­ra­tions. For many plants, they are im­por­tant parts of size re­duc­tion and crush­ing sys­tems.

Com­mon types for screen­ing decks are ei­ther sin­gle or dou­ble deck. Al­though triple decks (or even more) have been of­fered by some man­u­fac­tur­ers and used in special ap­pli­ca­tions. The screen­ing per­for­mance is af­fected sig­nif­i­cantly by var­i­ous fac­tors such as equip­ment ca­pac­ity, an­gle of in­cli­na­tion, de­tails of in­duced vi­bra­tion and oth­ers. The per­for­mance of a screen can be mea­sured by screen­ing ef­fi­ciency and flux. Flux is de­fined as the amount of a de­sired com­po­nent (un­der­size ma­te­rial) that has car­ried over the screen­ing me­dia from the feed per time per unit area. Screen­ing ef­fi­ciency is ex­pressed as the ra­tio of the amount of ma­te­rial that ac­tu­ally passes through the aper­ture, di­vided by the amount in the feed that the­o­ret­i­cally should pass. Screen­ing ef­fi­ciency is com­monly used to judge and eval­u­ate the per­for­mance of a vi­brat­ing screen.

The screen­ing ca­pac­ity is al­most di­rectly pro­por­tional to screen width. Ef­fi­ciency is linked to length; this means that by in­creas­ing the length, there will be ad­di­tional chances for pas­sage, and this will usu­ally lead to in­crease in ef­fi­ciency. As an in­di­ca­tion, screen length should be around two to three times the width. How­ever, cer­tain special sit­u­a­tions such as re­stricted space may re­quire a dif­fer­ent ra­tio. An­gle of in­cli­na­tion can be se­lected based on the de­sired per­for­mance. In­creas­ing the slope of a screen will ef­fec­tively re­duce the aper­ture by the co­sine of the an­gle of in­cli­na­tion and also the ma­te­ri­als move across the screen faster which leads to more rapid strat­i­fi­ca­tion. How­ever, the per­for­mance tends to de­crease af­ter a cer­tain point, say above 25° or 30°, de­pend­ing on ap­pli­ca­tion. Since the slope of the deck is too high for the feed ma­te­ri­als and most par­ti­cles will re­main on the over­sized stream in­stead of pass­ing through the aper­ture, thus, it re­sults in lower ef­fi­ciency. There­fore, an­gles for screens are of­ten lim­ited to 25° or 30°; most of screens in com­mon in­dus­trial ap­pli­ca­tions have an­gles be­tween 15° and 25°.

In a vi­brat­ing screen, par­ti­cles are in­tro­duced to the gaps in the screens re­peat­edly. The vi­bra­tion fre­quency of the screen should be high enough so that it pre­vents the par­ti­cles from block­ing the aper­tures and the max­i­mum height of the par­ti­cle tra­jec­tory should oc­cur when the screen sur­face is at its low­est point. At low fre­quency, screen­ing ef­fi­ciency might be high but blind­ing is se­vere. Blind­ing will de­crease as fre­quency in­creases but the par­ti­cles will have dif­fi­culty go­ing through the aper­tures. Sim­i­larly, an op­ti­mum value is avail­able for vi­bra­tion am­pli­tude. There­fore, there is an op­ti­mum set of fre­quency and am­pli­tude of vi­bra­tion for each vi­brat­ing screen. More­over, block­ing and blind­ing are ma­jor is­sues in vi­brat­ing screens; there­fore, all pa­ram­e­ters and fac­tors re­lated to them should be dealt with great care.

The se­lec­tion of the screen type and de­tails will be based on the ma­te­ri­als feed­ing to it. There­fore, each screen is spe­cially- sized and de­signed equip­ment for each spe­cific ap­pli­ca­tion. There are stan­dard frames and mod­els of vi­brat­ing screens, but ide­ally de­tails of each vi­brat­ing screen should be tai­lored for each ap­pli­ca­tion. If the screen is not suit­able for the ma­te­rial fed to it, there would be se­ri­ous is­sues and prob­lems; for in­stance, the ma­te­ri­als will blind the aper­tures and reg­u­lar main­te­nance will be re­quired.

There have usu­ally been con­cerns on the ca­pac­ity and sizes of vi­brat­ing screens; as many screens have been un­der­sized and they can­not achieve the claimed screen­ing ef­fi­cien­cies, ca­pac­i­ties and per­for­mances. It is of­ten use­ful to ask man­u­fac­turer to pro­vide cal­cu­la­tions and ap­pro­pri­ate sim­u­la­tions or op­er­at­ing ref­er­ences to show that the se­lected model, sizes and de­tails are sat­is­fac­tory for han­dling of the in­tended ap­pli­ca­tion and ca­pac­ity.

CASE STUDY – RING HAM­MER CRUSHER

This case study is for a 745-rpm ring ham­mer crusher to crush 700 t/h raw ma­te­ri­als in an in­dus­trial plant. Ro­tor di­am­e­ter and ro­tor work­ing length are 1.2 m and 1.3 m, re­spec­tively. The re­quired power of crusher is cal­cu­lated by two dif­fer­ent meth­ods to make sure on the suf­fi­ciency of de­sign and power rat­ing of the elec­tric mo­tor driver. Us­ing spe­cific power con­sump­tion method, very sim­ple lin­ear em­pir­i­cal for­mula: P=K Q

Where:

P: Power (kW); Q: Crusher Ca­pac­ity (t/h); K: Em­pir­i­cal Spe­cific Power Con­sump­tion.

Based on pre­vi­ous ex­pe­ri­ences, the spe­cific power con­sump­tion for ring-ham­mer type crusher for this ser­vice would be K= 0.35~0.50; se­lected fac­tor is K= 0.48. Rated power “P” is cal­cu­lated as 336 kW.

By us­ing an­other em­pir­i­cal for­mula for the ring ham­mer crusher for this ap­pli­ca­tion:

P = C ×D2×L×Na×K

Where:

P: Power (kW); C: em­pir­i­cal co­ef­fi­cient, 0.1 0.15; D: ro­tor di­am­e­ter (m); L: ro­tor work­ing length (m); Na: speed (r/min); K: over­load co­ef­fi­cient, for this case 1.8

P= 0.14×D2×L×Na

×K= 0.14×1.22×1.3×745×1.8=351 kW

The larger value, 351 kW, is considered as the brake power for siz­ing of other com­po­nents; elec­tric mo­tor power is cal­cu­lated as 450 kW.

CASE STUDY – VI­BRAT­ING SCREEN

The case study is for an 1800 t/ h vi­brat­ing screen for a raw ma­te­rial han­dling and crush­ing sys­tem for an in­dus­trial plant. This vi­brat­ing screen was of the dou­ble deck type and used for sep­a­rat­ing the raw ma­te­ri­als of 100 mm and 35 mm. The screened ma­te­ri­als, larger than 100 mm, are dis­charged to hop­pers and sent back to up­stream crusher (for large sizes – above 100 mm), be­tween 35 mm and 100 mm are dis­charged to a down­stream im­pact crusher (to crush ma­te­ri­als to 35 mm), and smaller than 35 mm are dis­charged di­rectly to the con­veyor to be trans­ferred to the pro­cess­ing unit.

The rated ca­pac­ity of the vi­brat­ing screen was 1800 t/ h. The ini­tially se­lected vi­brat­ing screen was a 30° an­gle 7 m × 2.4 m screen­ing area; it was claimed that such a screen can han­dle 1800 t/ h of ma­te­ri­als with screen­ing ef­fi­ciency of 85 per­cent. Ques­tions were raised by en­gi­neers and con­sul­tants on this. Con­cerns were raised on claimed pa­ram­e­ters and per­for­mances; for ex­am­ple, screen­ing ef­fi­ciency. It was proved that con­sid­er­ing 30° an­gle, sizes and other de­tails for this vi­brat­ing screen, screen­ing ef­fi­ciency above 75 per­cent could not be achieved. On this ba­sis, the screen se­lec­tion and de­sign was mod­i­fied to larger and bet­ter one; the mod­i­fied vi­brat­ing screen was a 25° an­gle

8 m × 2.7 m screen­ing area.

Amin Almasi is a lead me­chan­i­cal en­gi­neer in Aus­tralia. He is char­tered pro­fes­sional en­gi­neer of En­gi­neers Aus­tralia (MIEAust CPEng – Me­chan­i­cal) and IMechE (CEng MIMechE) in ad­di­tion to a M.Sc. and B.Sc. in me­chan­i­cal en­gi­neer­ing and RPEQ (Reg­is­tered Pro

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