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PROPERTIES OF COAL :

1)Swelling index

2) Grindability

3) Weatherability

4) Sulpher content

5) Heating value

6) !h !o"tening te#perature$

1$ Swelling Index%Swelling index i! a &ualitative evaluation #ethod' u!ed

to deter#ine the extent o" ca(ing o" a coal$ "ree burning coal ha! a high

value o" !welling index' which indicate! that it !lightly expand! in volu#e

during co#bu!tion$hi! !welling index i! o" 1* i#portance "or pulveri!edcoal$

2$Grindability :+t i! one the i#portant propertie! in !electing the coal$hi!

property i! #ea!ure by !tandard grindability index$he grindability index i!

inver!ely proportional to the power re&uired to grind the coal to a !peci"ied

particle !i,e "or burning$

3$Weatherability :+t i! a #ea!ure o" how well coal can be !tored "or long

ti#e with out cru#bling to piece!$Generally #odern power plant! !tore! coal!upply o" -* day! in large'$ rape,oidal pile$.ver !ince cru#bling o" coal

due to cli#atic condition #ay re!ult in !#all particle! o" coal which can be

carried away by wind or rain$

4$Sulphur Cntent :Sulphur in coal i! co#bu!tible and generate! heat by

it! oxidation "or#ing S/2 i! very e!!ential during co#bu!tion$+t i! a #a0or

!ource o" air pollution$+t! re#oval i! very e!!ential$

5$!eating "alue :hi! property i! o" "unda#ental i#portance$+t i! the heattran!"erred when the product! o" co#plete co#bu!tion o" !a#ple o" coal are

cooled to the initial te#perature o" air and "uel$+t i! generally deter#ined by

o#b calori#eter$

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6$ A#h S$tening Te%perature :he a!h !o"tening te#perature i! the

te#perature at which the a!h !o"ten! and beco#e! pla!tic$hi! te#perature

i! !lightly le!! than #elting te#perature o" a!h$hi! $S$$ i! an i#portant

"actor in de!igning a !tea# generator$

or a "urnace that would di!charge a!h in !olid "or#' a high $S$$ i!

re&uired otherwi!e clic(ne!! would be "or#ed$+t i! di""icult to re#ove the!e

clic(ne!! re!ulting ine""icient co#bu!tion$

&EC!A'ICAL FIRI'G (STO)ERS*

echanical !to(er! are co##only u!ed to "eed !olid "uel! into the "urnace in

#ediu# and large !i,e power plant!$

he variou! advantage! o" !to(er "iring are a! "ollow! %

i) arge &uantitie! o" "uel can be "ed into the "urnace$ hu! greater

co#bu!tion capacity i! achieved$

ii) oorer grade! o" "uel can be burnt ea!ily$

iii) Sto(er !ave labour o" handling a!h and are !el"7cleaning$

iv) y u!ing !to(er! better "urnace condition! can be #aintained by "eeding

coal at a uni"or# rate$

v) Sto(er! !ave coal and increa!e the e""iciency o" coal "iring$ he #aindi!advantage! o" !to(er! are their #ore co!t! o" operation and repairing

re!ulting "ro# high "urnace te#perature!$

Prin+iple# $ St,er#- he wor(ing o" variou! type! o" !to(er! i! ba!ed on

the "ollowing two principle!%

.- O"er$eed Prin+iple- ccording to thi! principle ig$) the pri#ary air

enter! the grate "ro# the botto#$

he air while #oving through the grate opening! get! heated up andair while #oving through the grate opening! get! heated up and the grate i!

cooled$

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Fig- Sto(er!

he hot air that #ove! through a layer o" a!h and pic(! up additional

energy$ he air then pa!!e! through a layer o" incande!cent co(e where

oxygen react! with co(e to "or#78*2 and water vapour! acco#panying the

air react with incande!cent co(e to "or# 8/2' 8/ and "ree H2$ he ga!e!

leaving the !ur"ace o" "uel bed contain volatile #atter o" raw "uel and ga!e!

li(e 8/2' 8/' H2' 92 and H2/$ hen additional air (nown a! !econdary air i!

!upplied to burn the co#bu!tible ga!e!$ he co#bu!tion ga!e! entering the

boiler con!i!t o" 92' 8/2' /2 and H2/ and al!o 8/ i" the co#bu!tion i! not

co#plete$

/- 0nder$eed Prin+iple- ig$!how! under"eed principle$ +n under"eed

principle air entering through the hole! in the grate co#e! in contact with

the raw coal green coal)$

Fig- :nder"eed rinciple$

hen it pa!!e! through the incande!cent co(e where reaction! !i#ilar to

over"eed !y!te# ta(e place$ he ga!e! produced then pa!!e! through a

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layer o" a!h$ he !econdary air i! !upplied to burn the co#bu!tible ga!e!$

:nder"eed principle i! !uitable "or burning the !e#i7bitu#inou! and

bitu#inou! coal!$

Type# $ St,er#- he variou! type! o" !to(er! are a! "ollow!%

Fig- ;ariou! ype! o" Sto(er!$

8harging o" "uel into the "urnace i! #echani,ed by #ean! o" !to(er! o"

variou! type!$ hey are in!talled above the "ire door! underneath the

bun(er! which !upply the "uel$ he bun(er! receive the "uel "ro# a

conveyor$

i) Chain Grate St,er- 8hain grate !to(er and traveling grate !to(er di""er

only in grate con!truction$ chain grate !to(er ig$) con!i!t! o" an endle!!

chain which "or#! a !upport "or the "uel bed$

Fig- 8hain Grate Sto(er$

he chain travel! over two !proc(et wheel!' one at the "ront and one at the

rear o" "urnace$ he traveling chain receive! coal at it! "ront end through a

Under

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hopper and carrie! it into the "urnace$ he a!h i! tipped "ro# the rear end o"

chain$ he !peed o" grate chain) can be ad0u!ted to !uit the "iring condition$

he air re&uired "or co#bu!tion enter! through the air inlet! !ituated

below the grate$ Sto(er! are u!ed "or burning non7co(ing "ree burning high

volatile high a!h coal!$ lthough initial co!t o" thi! !to(er i! high but

operation and #aintenance co!t i! low$

he traveling grate !to(er al!o u!e! an endle!! chain but di""er! in that

it carrie! !#all grate bar! which actually !upport the "uel "ed$ +t i! u!ed to

burn lignite' very !#all !i,e! o" anthracite! co(e bree,e etc$

he !to(er! are !uitable "or low rating! becau!e the "uel #u!t be burnt

be"ore it reache! the rear o" the "urnace$ With "orced draught' rate o"

co#bu!tion i! nearly 3* to 5* lb o" coal per !&uare "oot o" grate area per

hour' "or bitu#inou! 2* to 35 pound! per !&uare "oot per hour "or

anthracite$

ii) Spreader St,er- !preader !to(er i! !hown in ig$ +n thi! !to(er the

coal "ro# the hopper i! "ed on to a "eeder which #ea!ure! the coal in

accordance to the re&uire#ent!$ eeder i! a rotating dru# "itted with blade!$

eeder! can be reciprocating ra#!' endle!! belt!' !piral wor#! etc$

Fig- Spreader Sto(er$

ro# the "eeder the coal drop! on to !preader di!tributor which !pread

the coal over the "urnace$ he !preader !y!te# !hould di!tribute the coal

evenly over the entire grate area$ he !preader !peed depend! on the !i,e

o" coal$

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Ad"antage#

he variou! advantage! o" !preader !to(er are a! "ollow! %

1$ +t! operation co!t i! low$

2$ wide variety o" coal can be burnt ea!ily by thi! !to(er$

3$ thin "uel bed on the grate i! help"ul in #eeting the "luctuating load!$

4$ !h under the "ire i! cooled by the inco#ing air and thi! #ini#i,e!

clin(ering$

5$ he "uel burn! rapidly and there i! little co(ing with co(ing "uel!$

1i#ad"antage#

1$ he !preader doe! not wor( !ati!"actorily with varying !i,e o" coal$

2$ +n thi! !to(er the coal burn! in !u!pen!ion and due to thi! "ly a!h i!

di!charged with "lue ga!e! which re&uire! an e""icient du!t collecting

e&uip#ent$

iii) &ulti2retrt St,er- #ulti7retort !to(er i! !hown in ig$ he coal

"alling "ro# the hopper i! pu!hed "orward during the inward !tro(e o" !to(er

ra#$ he di!tributing ra#! pu!her!) then !lowly #ove the entire coal bed

down the length o" !to(er$ he length o" !tro(e o" pu!her! can be varied a!

de!ired$ he !lope o" !tro(e help! in #oving the "uel bed and thi! "uel bed#ove#ent (eep! it !lightly agitated to brea( up clin(er "or#ation$ he

pri#ary air enter! the "uel bed "ro# #ain wind box !ituated below the

!to(er$ artly burnt coal #ove! on to the exten!ion grate$ thinner "uel bed

on the exten!ion grate re&uire! lower air pre!!ure under it$ he air entering

"ro# the #ain wind box into the exten!ion grate wind box i! regulated by an

air da#per$

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Fig- ulti7retort Sto(er$

! !u""icient a#ount o" coal alway! re#ain! on the grate' thi! !to(er

can be u!ed under large boiler! upto 5**'*** lb per hr capacity) to obtain

high rate! o" co#bu!tion$

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Ad"antage#

i) he !y!te# i! !i#ple and cheaper than the central !y!te#$

ii) here i! direct control o" co#bu!tion "ro# the pulveri!ing #ill$

iii) 8oal tran!portation !y!te# i! !i#ple$

3in r Central Sy#te%- +t i! !hown in ig$ 8ru!hed coal "ro# the raw coal

bun(er i! "ed by gravity to a dryer where hot air i! pa!!ed through the coal

to dry it$ he dryer #ay u!e wa!te "lue ga!e!' preheated air or bleeder

!tea# a! drying agent$ he dry coal i! then tran!"erred to the pulveri!ing

#ill$ he pulveri!ed coal obtained i! tran!"erred to the pulveri!ed coal

bun(er bin)$ he tran!porting air i! !eparated "ro# the coal in the cyclone

!eparator$ he pri#ary air i! #ixed with the coal at the "eeder and the

#ixture i! !upplied to the burner

Fig-in or 8entral Sy!te#$

Ad"antage#

l$ he pulveri!ing #ill grind! the coal at a !teady rate irre!pective o" boiler

"eed$

2$ here i! alway! !o#e coal in re!erve$ hu! any occa!ional brea(down in

the coal !upply will not e""ect the coal "eed to the burner$

3$ or a given boiler capacity pulveri!ing #ill o" !#all capacity will be

re&uired a! co#pared to unit !y!te#$

1i#ad"antage#

1$ he initial co!t o" the !y!te# i! high$

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2$ 8oal tran!portation !y!te# i! &uite co#plicated$

3$ he !y!te# re&uire! #ore !pace$

o a large extent the per"or#ance o" pulveri!ed "uel !y!te# depend!

upon the #ill per"or#ance$

he pulveri!ed #ill !hould !ati!"y the "ollowing re&uire#ent!%

1$ +t !hould deliver the rated tonnage o" coal

2$ ulveri!ed coal produced by it !hould be o" !ati!"actory "inene!! over a wide

range o" capacitie!$

3$ +t !hould be &uiet in operation$

4$ +t! power con!u#ption !hould be low$

5$ aintenance co!t o" the #ill !hould be low$

ig$!how! the e&uip#ent! "or unit and central !y!te# o" pulveri!ed coal

handling plant$

Fig- .&uip#ent! "or 8entral and :nit Sy!te#$

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P0L4ERISE1 COAL 30R'ERS

urner! are u!ed to burn the pulveri!ed coal$ he #ain di""erence between

the variou! burner! lie! in the rapidity o" air7coal #ixing i.e., turbulence$ or

bitu#inou! coal! the turbulent type o" burner i! u!ed wherea! "or low

volatile coal! the burner! with long "la#e !hould be u!ed$ pulveri!ed coal

burner !hould !ati!"y the "ollowing re&uire#ent!%

i) +t !hould #ix the coal and pri#ary air thoroughly and !hould bring thi!

#ixture be"ore it enter! the "urnace in contact with additional air (nown a!

!econdary air to create !u""icient turbulence$

ii) +t !hould deliver and air to the "urnace in right proportion! and !hould

#aintain !table ignition

o" coal air #ixture and control "la#e !hape and travel in the "urnace$ he

"la#e !hape i! controlled by the !econdary air vane! and other control

ad0u!t#ent! incorporated into the burner$ Secondary air i" !upplied in too

#uch &uantity #ay cool the #ixture and prevent it! heating to ignition

te#perature$

iii) 8oal air #ixture !hould #ove away "ro# the burner at a rate e&ual to"la#e "ront travel in order to avoid "la!h bac( into the burner$

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Fig- ulveri!ed 8oal urner Sy!te#$

he variou! type! o" burner! are a! "ollow! %

.- Lng Fla%e 3urner (02Fla%e 3urner*- +n thi! burner air and coal

#ixture travel! a con!iderable di!tance thu! providing !u""icient ti#e "or

co#plete co#bu!tion =ig$a)>$

/- Shrt Fla%e 3urner (Turbulent 3urner*- +t i! !hown in ig$b)$ he

burner i! "itted in the "urnace will and the "la#e enter! the "urnace

hori,ontally$

5- Tangential 3urner- tangential burner i! !hown in ig$ 4$2-c)$ +n thi!

!y!te# one burner i! "itted attach corner o" the "urnace$ he inclination o"

the burner i! !o #ade that the "la#e produced are tangential to an

i#aginary circle at the centre$

6- Cy+lne 3urner- +t i! !hown in ig$d)$ hi! burner u!e! cru!hed coal

intend o" pulveri,ed coal$

Fig- ;ariou! ype! o" urner!

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Fig- ulveri!ed 8oal7"ired oiler$

+t! advantage! are a! "ollow! %

i) +t !ave! the co!t o" pulveri!ation becau!e o" a cru!her need! le!! power

than a pulveri!er$

ii) roble# o" "ly a!h i! reduced$ !h produced i! in the #olten "or# and

due to inclination o" "urnace it "low! to an appropriate di!po!al !y!te#$

ig$ !how! a pulveri!ed coal7"ired boiler$

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C7CLO'E F0R'ACES

8yclone7"urnace "iring' developed in the 1-4*!' repre!ent! the #o!t

!igni"icant !tep in coal "iring !ince the introduction o" pulveri,ed7coal "iring in

the 1-2*!$ +t i! now widely u!ed to burn poorer grade! o" coal that contain a

high a!h content with a #ini#u# o" 6 percent to a! high a! 25 percent' and

a high volatile #atter' #ore than 15 percent' to obtain the nece!!ary high

rate! o" co#bu!tion$ wide range o" #oi!ture i! allowable with pre7drying$

/ne li#itation i! that a!h !hould not contain a high !ul"ur content or a high

e2/3? 8a/ @ g/) ratio$ Such a coal ha! a tendency to "or# high a!h7

"u!ion te#perature #aterial! !uch a! iron and iron !ul"ide in the !lag' which

negate! the #ain advantage o" cyclone "iring$

he #ain advantage i! the re#oval o" #uch o" the a!h' about 6*

percent' ao #olten !lag that i! collected on the cyclone wall! by centri"ugal

action and drained o"" the botto# to a !lag7di!integrating tan( below$ hu!

only 4* percent a!h leave' with the "lue ga!e!' co#pared with about A*

percent "or pulveri,ed7coal "iring$ thi! #aterially reduce! ero!ion and "ouling

o" !tea#7generator !ur"ace! a! well a! the !i,e o" du!t7re#oval precipitator!

or bag hou!e! at !tea#7generator exit$ /ther advantage! are that onlycru!hed coal i! u!ed and no pulveri,ation e&uip#ent i! needed and that the

boiler !i,e i! reduced$8yclone7"urnace "iring u!e! a range o" coal !i,e!

averaging -5 percent pa!!ing a 47#e!h !creen$

he di!advantage! are higher "orced7dra"t "an pre!!ure! and there"ore

higher power re&uire#ent!'the inability to u!e the coal! #entioned above'

and the "or#ation o" relatively #ore oxide! o" nitrogen'9/2 which are air

pollutant!' in the co#bu!tion proce!!$he cyclone i! e!!entially a water7cooled hori,ontal cylinder ig$) located out!ide the #ain boiler "urnace' in

which the cru!hed coal i! "ed and "ired with very high rate! o" heat relea!e$

8o#bu!tion o" the coal i! co#pleted be"ore the re!ulting hot ga!e! enter the

boiler "urnace$ he cru!hed coal i! "ed into the cyclone burner at le"t along

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with pri#ary air' which i! about 2* percent o" co#bu!tion or !econdary air$

he pri#ary air enter! the burner tangentially' thu! i#parting a centri"ugal

#otion to the coal$ he !econdary air i! al!o ad#itted tangentially at the top

o" the cyclone at high !peed' i#parting "urther centri"ugal #otion$ !#all

&uantity o" air' called tertiary air' i! ad#itted at the center$he whirling

#otion o" air and coal re!ult! in large heat7relea!e7rate volu#etric den!itie!'

between 45*'*** and A**'*** tuBh$"t) about 4C** to A3** (WB#3)' and

high co#bu!tion te#perature!' #ore than 3***D 165*D8)$ he!e high

te#perature! #elt the a!h into a li&uid !lag that cover! the !ur"ace o" the

cyclone and eventually drain! through the !lag7tap opening to a !lag tan( at

the botto# o" the boiler "urnace' where it i! !olidi"ied and bro(en "or

re#oval$ he !lag layer that "or#! on the wall! o" the cyclone provide!

in!ulation again!t too #uch heat lo!! through the wall! and contribute! to

the e""iciency o" cyclone "iring$ he high te#perature! al!o explain the large

production o" 9/' in the ga!eou! co#bu!tion product!$ he!e ga!e! leave

the cyclone through the throat at right and enter the #ain boiler "urnace$

hu! co#bu!tion ta(e! place in the relatively !#all cyclone' and the #ain

boiler "urnace ha! the !ole "unction o" heat tran!"er "ro# the ga!e! to thewater7tube wall!$ 8yclone "urnace! are al!o !uitable "or "uel7oil and ga!eou!7

"uel "iring$

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+nitial ignition i! done by !#all retractable oil or ga! burner! in the

!econdary air port!$ i(e pulveri,ed7coal !y!te#!' cyclone "iring !y!te#! can

be o" the bin' or !torage$ or direct7"iring type!' though the bin type i! #ore

widely u!ed' e!pecially "or #o!t bitu#inou! coal!' than in the ca!e o"

pulveri,ed coal$ he cyclone !y!te# u!e! one7wall' or oppo!ed7wall' "iring'

the latter being pre"erred "or large !tea# generator! he !i,e and nu#ber o"

cyclone! per boiler depend upon the boiler !i,e and the de!ired load

re!pon!e becau!e the u!ual load range "or good per"or#ance o" any one

cyclone i! "ro# 5* to 1** percent o" it! rated capacity$ 8yclone! vary in !i,e

"ro# 6 to 1* "v in dia#eter with heat input! between 16* to 425 #illion

tuBh about 4C'*** to 125'/aE (W)' re!pectively $

he cyclone co#ponent re&uiring the #o!t #aintenance i! the burner'

which i! !ub0ected to ero!ion by the high velocity o" the coal$ .ro!ion i!

#ini#i,ed by the u!? o" tung!ten carbide and other ero!ion7re!i!tant

#aterial! "or the burner liner!' which are u!ually replaced once a year or !o$

S&O)E A'1 10ST RE&O4AL

+n coal "ed "urnace! the product! o" co#bu!tion contain particle! o" !olid

#atter "loating in !u!pen!ion$ hi! #ay be !#o(e or du!t$ he production o"!#o(e indicate! that co#bu!tion condition! are "aulty and a#ount o" !#o(e

produced can be reduced by i#proving the "urnace de!ign$

+n !preader !to(er! and pulveri!ed coal "ired "urnace! the coal i! burnt

in !u!pen!ion and due to thi! du!t in the "or# o" "ly a!h i! produced$ he

!i,e o" du!t particle! i! de!ignated in #icron! 1 F *$**1 ##)$

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direction o" "low =ig$b)> o" "lue ga!e! cau!e! the heavier particle! o" !ettle

out$ So#eti#e ba""le! are provided a! !hown in ig$c) to !eparate the

heavier particle!$

echanical du!t collector! #ay be wet type or dry type$ Wet type du!t

collector! called !crubber! #a(e u!e o" water !pray! to wa!h the du!t "ro#

"lue ga!e!$

Fig- echanical

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he principal characteri!tic! o" an a!h collector i! the degree o"

collection$

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range "ro# very "ine to very coar!e !i,e depending on the !ource$ article!

colour varie! "ro# light tan to grey to blac($ an colour indicate! pre!ence o"

ion oxide while dar( !hade! indicate pre!ence o" unburnt carbon$ ly a!h

particle! !i,e varie! between 1 #icron l F) to 3** F$ ly a!h concentration

in "lue ga!e! depend! upon #ainly the "ollowing "actor! %

i) 8oal co#po!ition$

ii) oiler de!ign and capacity$

ercentage o" a!h in coal directly contribute! to "ly a!h e#i!!ion while boiler

de!ign and operation deter#ine the percentage retained in the "urnace a!

botto# a!h and "ly a!h carried away by "lue ga!$ ly a!h concentration

widely varie! around 2*7-* gB##3 depending on coal and boiler de!ign$ ly

a!h particle !i,e di!tribution depend! pri#arily on the type o" boiler !uch a!

pulveri,ed coal "ired boiler typically produce! coar!er particle! then cyclone

type boiler!$ .lectro!tatic precipitator .S) i! &uite co##only u!ed "or

re#oval o" "ly a!h "ro# "lue ga!e!$

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COOLI'G TOWERS

8ooling ower! re#ove heat "ro# the water di!charged "ro# the conden!er

!o that the water can be di!charged to the river or recirculated and reu!ed$

cooling tower extract! heat "ro# water by evaporation$ +n an

evaporative cooling tower' a !#all portion o" the water being cooled i!

allowed to evaporate into a #oving air !trea# to provide !igni"icant cooling

to the re!t o" that water !trea#$

8ooling ower! are co##only u!ed to provide lower than a#bient

water te#perature! and are #ore co!t e""ective and energy e""icient than

#o!t other alternative!$ he !#alle!t cooling tower! are !tructured "or only

a "ew litre! o" water per #inute while the large!t cooling tower! #ay handle

upward! o" thou!and! o" litre! per #inute$ he pipe! are obviou!ly #uch

larger to acco##odate thi! #uch water in the larger tower! and can range

up to 12 inche! in dia#eter$

WOR)I'G OF COOLI'G TOWERS: When water i! reu!ed in the proce!!' it

i! pu#ped to the top o" the cooling tower and will then "low down through

pla!tic or wood !hell!' #uch li(e a honeyco#b "ound in a beeJ! ne!t$ he

water will e#it heat a! it i! downward "lowing which #ixe! with the above

air "low' which in turn cool! the water$ art o" thi! water will al!o evaporate'

cau!ing it to lo!e even #ore heat$

T7PES OF COOLI'G TOWERS:

/ne way to di!tingui!h between cooling tower! i! how the air and water

interact'

pen +ling twer# r +l#ed +ling twer#-

/pen cooling tower!' al!o called direct cooling tower!' allow the water

to co#e into contact with out!ide air$ +" cooled water i! returned "ro# the

cooling tower to be u!ed again' !o#e water #u!t be added to replace the

water that ha! been lo!t$ ollutant! are able to enter into the water u!ed in

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the!e proce!!e! and #u!t be "iltered out$ nother #ethod o" co#bating the

exce!! #ineral! and pollutant! i! !o#e #ean! o" a di!!olved !olid control'

!uch a! a blow down$ With thi!' a !#all percentage o" the "low i! drained o""

to aid in the re#oval o" the!e conta#inant!$ hi! i! "airly e""ective' but not

a! e""icient a! "iltration$

8lo!ed loop or clo!ed circuit) cooling tower !y!te#!' al!o called

indirect cooling tower !y!te#!' do not allow the water to co#e into contact

with any out!ide !ub!tance' there"ore (eeping the water #ore pure due to

the lac( o" "oreign particle! introduced$

nother cla!!i"ication o" cooling tower! i! #ade between $ield

a##e%bled twer#and $a+try a##e%bled twer#$ ield a!!e#bled

tower! are !hipped in piece! and a!!e#bled on !ite by a highly &uali"ied and

certi"ied in!tallation tea#$ actory a!!e#bled tower! typically only re&uire

the "an #otor to be #ounted$

'atural 1ra$t Twer#

9atural dra"t tower! are typically about 12* # high' depending on the

di""erential pre!!ure between the cold out!ide air and the hot hu#id air on

the in!ide o" the tower a! the driving "orce$ 9o "an! are u!ed$

Whether the natural or #echanical dra"t tower! are u!ed depend! on

cli#atic and operating re&uire#ent condition!$

he green "low path! !how how the war# water leave! the plant

proper' i! pu#ped to the natural dra"t cooling tower and i! di!tributed$ he

cooled water' including #a(eup "ro# the la(e to account "or evaporation

lo!!e! to the at#o!phere' i! returned to the conden!er$

&e+hani+al 1ra$t

echanical dra"t tower! u!e! "an! one or #ore) to #ove large &uantitie! o"

air through the tower$ hey are two di""erent cla!!e! %

a) orced dra"t cooling tower!

b) +nduced dra"t cooling tower!

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he air "low in either cla!! #ay be cro!! "low or counter "low with re!pect to

the "alling water$ 8ro!! "low indicate! that the air"low i! hori,ontal in the

"illed portion o" the tower while counter "low #ean! the air "low i! in the

oppo!ite direction o" the "alling water$

he counter "low tower occupie! le!! "loor !pace than a cro!! "low

tower but i! taller "or a given capacity$ he principle advantage! o" the cro!!

"low tower are the low pre!!ure drop in relation to it! capacity and lower "an

power re&uire#ent leading to lower energy co!t!$

ll #echanical tower! #u!t be located !o that the di!charge air

di""u!e! "reely without recirculation through the tower' and !o that air

inta(e! are not re!tricted$ 8ooling tower! !hould be located a! near a!

po!!ible to the re"rigeration !y!te#! they !erve' but !hould never be located

below the# !o a! to allow the conden!er water to drain out o" the !y!te#

through the tower ba!in when the !y!te# i! !hut down$

Forced Draft

he "orced dra"t tower' ha! the "an' ba!in' and piping located within the

tower !tructure$ +n thi! #odel' the "an i! located at the ba!e$ here are no

louvered exterior wall!$ +n!tead' the !tructural !teel or wood "ra#ing i!covered with paneling #ade o" alu#inu#' galvani,ed !teel' or a!be!to!

ce#ent board!$

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he induced dra"t tower !how in the "ollowing picture ha! one or #ore "an!'

located at the top o" the tower' that draw air upward! again!t the downward

"low o" water pa!!ing around the wooden dec(ing or pac(ing$ Since the

air"low i! counter to the water "low' the coole!t water at the botto# i! in

contact with the drie!t air while the war#e!t water at the top i! in contact

with the #oi!t air' re!ulting in increa!ed heat tran!"er e""iciency$

ig$+nduced dra"t

!ybrid 1ra$t

hey are e&uipped with #echanical dra"t "an! to aug#ent air"low$

8on!e&uently' they are al!o re"erred to a! "an7a!!i!ted natural dra"t tower!$

he intent o" their de!ign i! to #ini#i,e the power re&uired "or the air

#ove#ent' but to do !o with the lea!t po!!ible !tac( co!t i#pact$ roperly

de!igned "an! #ay need to be operated only during period! o" high a#bient

and pea( load!$

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ig$ Hybrid dra"t

!AR1'ESS OF WATER

Hardne!! o" a water !a#ple i! a #ea!ure o" it! capacity to precipitate !oap'

i$e$ to prevent the lathering o" !oap$ Hardne!! i! cau!ed by #ultivalent#etallic cation! li(e 8a2@' and g2@ ion!$ t !uper!aturated condition!' the

hardne!! cation! will react with anion! in the water to "or# a !olid

precipitate$ he principal hardne!!7cau!ing cation! are the divalent calciu#'

#agne!iu#' !trontiu#' "errou! ion!' and #aganou! ion!$ he!e ion! react

with !oap which i! !odiu# !alt o" higher "atty acid! and "or# in!oluble !cu#!

precipitate!) o" calciu# or #agne!iu# !oap!$ typical divalent cation

reaction i! a! "ollow! %2 81C H35 8//9a @ 8a8l281CH358//)2 8a @ 2 9a8l

Sodiu# !tearate$

athering doe! not occur until all o" the hardne!! ion! are precipitated' at

which point the water ha! been K!o"tenedJ by the !oap$ he polyvalent ion!

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occur in water o"ten in co#plex "or#! and not a! "ree ion!$ ! a re!ult' they

cannot precipitate !oap$ he #o!t i#portant anion! with which #ultivalent

#etallic cation! a!!ociated are bicarbonate carbonate)' !ulphate' chloride'

nitrate' and !ilicate$

Sur+e#

he #ultivalent #etallic ion! in water are derived largely "ro# contact with

the !oil and roc( "or#ation$ he ability to di!!olve i! gained in the !oil where

carbon dioxide i! relea!ed by bacterial action$ hi! carbon dioxide "or#!

carbonic acid to reduce the pH value$ :nder the low pH condition!' ba!ic

#aterial! are di!!olved in it$

Type# $ !ardne##

Hardne!! i! cla!!i"ied in two way! %

i) With re!pect to the #ultivalent #etallic ion' and

ii) With re!pect to the anion! a!!ociated with #ultivalent #etallic ion!$

!ardne## ba#ed n &etalli+ In

8alciu# and #agne!iu# are the #o!t abundant ion! in natural water which

contribute the greate!t portion o" the hardne!! occurring in natural water!$hu!' it i! a!!u#ed that the hardne!! due to the!e two #etallic ion! i!

con!idered a! total hardness$ 8alciu# hardne!! i! cau!ed by 8a2@ and i!

deter#ined a! the a#ount o" 8a2@ re&uired to !o"tening the li#e7!oda a!h$

Si#ilarly' #agne!iu# hardne!! i! due to g2@ ion! and i! deter#ined a! the

a#ount o" g2@ re&uired "or !o"tening li#e7!oda a!h$ +" #agne!iu# hardne!!

i! deter#ined' calciu# hardne!! i! obtained by !ubtracting #agne!iu#

hardne!! "ro# total hardne!!' a! "ollow! %otal hardne!! L #agne!iu# hardne!! calciu# hardne!!

!ardne## ba#ed n Anin#

hi! hardne!! i! cla!!i"ied a! carbonate hardness and non-carbonate

hardness' depending upon the anion with which it a!!ociate!$ When the

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hardne!! ion! are a!!ociated with 8/32M and H8/3

M ion! in water' the

hardne!! i! called carbonate hardne!!' otherwi!e' it i! called non7carbonate

hardne!!$ he carbonate hardne!! i! che#ically e&uivalent to the carbonate

plu! bicarbonate al(alinity pre!ent in water$ hu!'

When al(alinity N total hardne!!

8arbonate hardne!! in #g L1) al(alinity in #g L1)$

When al(alinity O total hardne!!

8arbonate hardne!! in #g L1) total hardne!! in #g L1)

he carbonate hardne!! i! al!o (nown a! temporary hardness which can be

re#oved by boiling$

he non7carbonate hardne!!' (nown a! permanent hardness' can be

deter#ined a! %

9on7carbonate hardne!! total hardne!! L carbonate hardne!!

he per#anent hardne!! cannot be re#oved by boiling$ he non7carbonate

hardne!! cation! are a!!ociated with chloride' nitrate and !ulphate anion!$

P#eud !ardne##

he #etallic cation which i! not a hardne!!7cau!ing cation' but exhibit!

hardne!! when pre!ent in high concentration i! (nown a! pseudo-hardness$or exa#ple' !odiu# #etal ion which i! pre!ent in very high concentration in

!ea and brac(i!h water' exhibit! hardne!!$

Re%"al $ !ardne##

a) Re%"al $ Carbnate r Te%prary !ardne##

8arbonate hardne!! i! !en!itive to heat and precipitate readily at high

te#perature!$ hu!' it i! re#oved a! precipitate! a! %

he other #ethod! o" re#oval are a! "ollow! %

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he carbonate hardne!! o" calciu# #ay be re#oved che#ically by u!ing

li#e 8a/)' cau!tic !oda 9a/H) and !oda a!h 9a28/3) a! depicted below$

8arbonate hardne!! o" #agne!iu# #ay be re#oved che#ically by u!ing

li#e "ollowed by cau!tic !oda a! "ollow! %

b) Re%"al $ 'n2+arbnate r Per%anent !ardne##

9on7carbonate calciu# hardne!! calciu# with non7carbonate anion!) can be

conveniently re#oved by u!ing !oda a!h 9a28/3)$ hu!'

8a8l2 @ 9a28/38a8/3 @ 2 9a8l

he non7carbonate #agne!iu# hardne!! #agne!iu# with non7carbonate

anion!) can be re#oved by u!ing cau!tic !oda or a co#bination o" li#e and

!od! a!h$ he repre!entative che#ical reaction! are %

Order of Removal

he order o" re#oval depend! on the !olubility o" the precipitate! li(e 8a8/ 3

and g/H)2$ Since g/H)2 i! #ore in!oluble than 8a8/3' the reactionproducing g/H)2 !hould ta(e precedence$

c) Split Treat%ent

Water with a high concentration o" #agne!iu# i! !o"tened by !plit

treat#ent$ +n thi! treat#ent' the raw water i! !plit into two !trea#!' the

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Si#ilarly' the anion exchange resins are !tyrene divinyl ben,ene or a#ine

"or#aldehyde copoly#er!' which contain &uaternary a##oniu# group! a!

an integral part o" the re!in #atrix$ he M /H group! pre!ent in the!e re!in!

can enter into the exchange reaction with anion! in water$

3OILER CORROSIO' A'1 ITS PRE4E'TIO'

8orro!ion i! a proce!! in which the #etal ato#! leave their location on the

!ur"ace and !tabili,e in the "or# o" ion! in !olution$ 8orro!ion in an a&ueou!!y!te# occur! due to the interaction between the !ur"ace o" the #aterial!

and water body under condition! o" che#ical !tre!!' i$e$ acidic or al(aline$

Water i! a polar #olecule and the polari!ibility o" the water #olecule! on

contact with the iron !ur"ace lead! to the wea(ening o" the / L H bond$

=e H2/)5 /H>@i! the pri#ary corro!ion product which give! !econdary

hydroly!i! product' a! e /H)2$ he other !econdary product! are e/$/H'

e/' e3/4' e2/3 etc$ ll the!e !econdary product! are pH dependent$

3iler Crr#in

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oiler corro!ion i! the deterioration o" the boiler #aterial by direct che#ical

or electroche#ical attac( o" it! a&ueou!) environ#ent$ +n high pre!!ure

boiler!' the "ailure o" boiler tube! #ade up o" carbon !teel i! due to internal

corro!ion and external Phot corro!ionQ$ he external corro!ion i!

#etallurgical in nature' there"ore' we !hall di!cu!! about the internal

corro!ion$

Cau#e#

he internal boiler corro!ion i! cau!ed due to the "ollowing i#portant

rea!on! %

i)

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(ii*3y &e+hani+al 1eaeratin : he deaeration i! done by a vacuu# pu#p

to re#ove all the "ree ga!e! /2) a! it provide! high te#perature' low

pre!!ure and large !ur"ace area

Dissolved CO/

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Re%"al $ A+id#

y adding acid neutrali,ed al(ali to the boiler water$

Cau#ti+ E%brittle%ent

+t i! a type o" boiler corro!ion cau!ed by u!ing highly al(aline water in the

boiler$ +n high pre!!ure boiler!' when 9a28/3 hydroly!e! with water re!ulting

in !odiu# hydroxide and carbon dioxide' the 9a/H #a(e! the boiler water

cau!tic$

hi! 9a/H containing water "low! into the #inute hair crac(!' u!ually "ound

on the inner wall! o" the boiler by capillary action$

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Pri%ing and Fa%ing

+n the !tea# "or#ation proce!!' !o#e drop! o" water carried along with

!tea# i! called pri#ing$ ri#ing i! cau!ed #ainly due to the "ollowing "actor!

i) rapid boiling o" water

ii) the pre!ence o" !u!pended !olid!

iii) high level o" water in boiler

iv) the growth o" algae

v) !udden change o" !tea# production rate

vi) i#proper boiler de!ign$

ri#ing can be #ini#i,ed by the "ollowing "actor! %

i) by "itting #echanical !tea# puri"ier$

ii) #ini#i,ing the a#ount o" di!!olved and !u!pended i#puritie!$

iii) avoiding rapid change in !tea#ing rate$

iv) #aintaining low water level!$

Fa%ing

oa#ing i! the "or#ation o" continuou! and per#anent "oa# or bubble! in

the boiler water which do not brea( ea!ily and are carried over !tea#

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cau!ing pri#ing$ hu!' pri#ing and "oa#ing occur together which i!

ob0ectionable$

he #ain proble#! cau!ed by "oa#ing are %

i) +" the bubble! do not brea(' they #ay be carried along the !tea# to

!upper heater and turbine blade! where the water evaporate! and the

di!!olved !alt! get depo!ited$

ii) ctual water level in!ide the boiler cannot be 0udged properly$

iii) he!e decrea!e the li"e ti#e and e""iciency o" the #achinery$

oa#ing can be #ini#i,ed by the "ollowing way! %

i) dding anti7"oa#ing agent! li(e ca!ter oil' polya#ide! etc$

ii) e#oving oil content "ro# boiler7"eed water by adding alu# and !oda

a!h$ iii) y blow down operation$

WATER TREAT&E'T

Water treat#ent generally con!ider! the puri"ication o" raw water "or

drin(ing purpo!e!$ he drin(ing water !hould be colourle!!' odourle!!' "ree

"ro# !u!pended i#puritie!' ger#!' bacteria and other di!!olved i#puritie!

which are in0uriou! to health$ +t !hould be "ree "ro# ob0ectionable di!!olved

ga!e!' turbidity' #ineral!$ he pH value o" the drin(ing water !hould be

within the range o" 6$5 to A$ here"ore' to #a(e pure drin(ing water' !o#e

i#portant !tage! o" puri"ication o" raw water are %

a) retreat#ent

b) Sedi#entation

c) iltration

d)

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he raw water i! !tored "or !o#e period be"ore the actual treat#ent !tart! in

order to i#prove the &uality o" water by natural !edi#entation o" !ilt and

inactivation o" certain pathogenic organi!#!$

ii) Prechlorination

he !torage water !ub0ected to prechlorination to re#ove algal cell! and

bacteria$ rechlorination i! !uitable to low !ilt load water and it reduce! the

colour and !li#e "or#ation$ o !o#e extent' it oxidi!e! the iron and

#angane!e which are precipitated out$

iii)eration

he raw water i! "ir!t !aturated with oxygen by bubbling co#pre!!ed air$

he "ree ga!e! li(e 8/2' H2S i" pre!ent' will be re#oved$ he proce!! o"

aeration help! in re#oving ta!te! and odour! cau!ed by algal !ecretion in

the "or# o" volatile oil!$ ew e2@ and n2@ will al!o be oxidi,ed and

precipitated out$

iv) Coagulation and !locculation

he aerated water which #ay contain !#all particle! in the colloidal range'

can be re#oved by pro#oting their agglo#eration by "locculation in the

pre!ence o" coagulant!$ he "ollowing are the #o!t co##only u!edcoagulant! a) alu#' b) !odiu# alu#inate' coppera! or "errou! !ulphate!$

coagulant i! a che#ical which react! with water to "or# a bul(y precipitate

called floc$ he agglo#erated particle! called floccules are latter !eparated

"ro# water by "iltration$ he reaction! are

he overall reaction i!

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he "loccule! are !eparated by !edi#entation and "iltration$

b) Sedi%entatin

he water with "loccule! are allowed to !ettle down "or !o#e period in a

!edi#entation ba!in in which "loccule particle! !ettled down at the botto#

due to the "orce o" gravity$

he clear !upernatant water i! then pu#ped out$ he !etteling rate depend!

on the #a!! and den!ity o" the particle!$

c) Filtratin

he partly clari"ied water "ro# the !edi#entation ba!in i! then pa!!ed to the

!tand gravity "ilter! to re#ove the !#all "loc particle! and #icroorgani!#!

!till re#ained in water a"ter !edi#entation$

ll the !u!pended #atter are "iltered out in the !and bed$ he!e "ilter! are o"

two type! %

i) apid !and "ilter! iltering rate i! #ore and i! rapidly exhau!ted)' and

ii) Slow !and "ilter! ate i! !low but durable)$

Generally' !low !and "ilter! are u!ed in the ab!ence o" coagulation and

!edi#entation$

(d* 1i#in$e+tin

he "inal !and "iltrate i! very clean but #ay not be "ree "ro#

#icroorgani!#!$ here"ore' di!in"ection o" that "iltered water i! &uite

e!!ential which i! obtained by i) chlorination' and ii) o,oni!ation$(i* Chlorination

8hlorine i! a power"ul di!in"ectant a! it produce! H/8l' /8l' 8hlora#ine!$

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he hypochlorou! acid i! an un!table co#pound and &uic(ly deco#po!e! by

relea!ing na!cent oxygen$ he pre!ence o" a##onia in water lead! to the

"or#ation o" chlora#ine! a! "ollow! %

he che#ical !pecie! H/8l' /8l are "ree chlorine re!idual! wherea!

chlora#ine! are co#bined re!idual! and re#ain in water body "or a longer

period which help to (ill the #icroorgani!#! even in a di!tribution !y!te#$

he bacterial action o" chlorine i! #ainly becau!e o" the toxic nature o" there!idual! and the oxidation o" bio#olecule! by the na!cent oxygen$ he

do!e! o" chlorine are decided depending upon the pH' 8/2' te#perature and

the ti#e o" contact with water$

i&uid chlorine i! #o!t e""ective and the apparatu! u!ed "or thi! purpo!e i!

called Chlorinator$

Ad"antage#

i) +t i! e""ective$ii) +t i! very #uch !table and doe! not deteriorate on (eeping$

iii) +t can be u!ed at low a! well a! high te#perature!$

iv) +t introduce! no !alt i#puritie! in the treated water$

v) +t i! a very power"ul di!in"ectant$

1i#ad"antage#

i) .xce!! o" chlorine produce! unplea!ant ta!te and odour$ +t al!o produce!

irritation on #u!cu! #e#brane$ +t !hould be u!ed within the range o" *$1 to*$2 pp#$

ii) +t i! le!! e""ective in al(aline condition!$

(ii* Ozonisation

/,one behave! in a !i#ilar #anner to chlorine by relea!ing na!cent oxygen$

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i) +t re#ove! ionic' nonionic' colloidal and high #olecular weight

co#pound!$ ii) he li"e ti#e o" #e#brane i! &uite high$

iii) he #e#brane! can be replaced very ea!ily

Ele+trdialy#i#

+t i! another #e#brane technology which i! u!ed "or the concentration o"

ion!$ +n thi! #ethod the ion! are pulled out o" the brac(i!h water' under an

electric "ield$

Si%ple Ele+trdialy#i#

+t con!i!t! o" three cha#ber! and electrode! cathodic and anode) are u!ed

in the !ide cha#ber!$ he brac(i!h water i! "ed in all the cha#ber!$ ro#

the central co#part#ent' the ion! are pulled out "ro# !alt water by pa!!ing

direct current through the pla!tic #e#brane pair$ Since the cell #e#brane!

with "ixed charge negative or po!itive)' whereby the #e#brane! beco#e

i#per#eable to either cation! or anion!$ here"ore' in brac(i!h water 9a@

!tart! #oving toward! cathode while the 8lM ion! toward! the anode through

the #e#brane$ he rate o" pulling out o" the ion! "ro# the !alt water

depend! on the !alinity o" water$

Ele+trdialy#i# Cell u#ing In Sele+ti"e &e%brane#+on !elective #e#brane! are generally e#ployed "or e""icient !eparation$

hi! type o" electrodialy!i! cell con!i!t! o" !everal pair! o" pla!tic

#e#brane!$ or !ati!"actory electrodialy!i!' "luorocarbon #e#brane

#aterial! !uch a! P9a"ionQ are pre"erred$ n ion !elective #e#brane i!

per#eable to one (ind o" ion! only' i$e$ per#eable to either cation or anion$

cation !elective #e#brane! are poly!tyrene poly#er! with !ulphonic acid

group! M S/3M

)$ ecau!e o" the pre!ence o" "ixed negative charge on the#e#brane' it i! per#eable only to the cation!$ Si#ilarly' &uarternary

a##oniu# group 9H4@) i! u!ed a! anion !elective #e#brane$

Method

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he cation and anion !elective #e#brane! are (ept alternatively !o that

#any cha#ber! can be #ade through which brac(i!h water pa!!e! $y the

pa!!age o" direct current to the electrode!' the cation !elective #e#brane

repel! negative charged ion!' but allow! the @ve ion! to enter' while the

rever!e i! true "or the anion !elective #e#brane$ here"ore' water in one

cha#ber o" the cell i! de!alinated water' while the !alt water concentration

increa!ed in the next cha#ber$ hu!' we get alternative !trea#! o" pure

water and concentrated brine !olution$ Hence' thi! #ethod enhance! the

e""icient !eparation$

he i#portant u!e! o" the electrodialy!i! are in treat#ent o" #etal

plating wa!te!' battery #anu"acturer' gla!! etching and de!alination o"

e""luent!$

p! 4AL0E A'1 WATER TREAT&E'T

The Cn+ept $ p!

Soren!on 1-*-) propo!ed to expre!! the hydrogen7ion activity in ter#! o"

negative logarith#! o" hydrogen7ion activity and de!ignated the!e value! a!

pH$

When the !olution i! very dilute'

Si#ilarly' the activity o" /HM ion! expre!!ed in ter#! o" p/H$

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hu!' a! pH increa!e!' p/H #u!t decrea!e and vice7ver!a$ +n pure water'

which i! neutral ' i$e$

the pH o" water i! C$ Hence' the neutral !olution ha! a pH C$ ny !olution

having pH le!! than C will be acidic and a !olution having pH #ore than C

will be al(aline$ he pH !cale range! "ro# * to 14' with pH C at

25C/HH1*88MM@o8 repre!enting ab!olute neutrality$

he value o" "w change! with change in te#perature' hence' the pH o"neutrality change! with te#perature$ +t i! C$5 at *o8 and 6$5 at 6*o8$

pH doe! not #ea!ure total acidity or total al(alinity$ he

value! can never be reduced to ,ero' no #atter how al(aline or acidic'

re!pectively a !olution beco#e!$ he!e are only the co#parative value!

depending on the degree o" ioni,ation$ or exa#ple' 9B1* !olution! o"

!ulphuric acid and acetic acid which have !a#e neutrali,ing value? the pH o"

9B1* !ulphuric acid i! approxi#ately 1 becau!e o" it! high degree o"

ioni,ation' and the pH o" acetic acid i! about 3 becau!e o" it! low degree o"

ioni,ation$ pH i! an i#portant "actor which #u!t be con!idered in control o"

the water &uality$

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