HEAT TRANSFER OPERATIONSCONDUCTION: The transfer of heat from one molecule to an adjacent molecule h!le the"art!cles rema!ns !n f!#ed "os!t!ons relat!$e to each other !s called conduct!onE#am"le:%& If a "!ece of "!"e has a hot flu!d on the !ns!de and cold flu!d outs!de' heat !s transferredthrou(h the "!"e )* conduct!on+& The rate of flo of heat !s "ro"ort!onal to the chan(e !n tem"erature throu(h the sol!dand heat transfer area of the sol!d and !n$ersel* "ro"ort!onal to the th!c,ness of the sol!dThe "ro"ort!onal!t* constant -,. !s ,non as thermal conduct!$!t* of the sol!d& /T re"resents the tem"erature d!fference )eteen the cold and hot surface of the sol!d&0 re"resents the th!c,ness of the sol!d !n the d!rect!on of heat of flo&12 3A/T40Conduct!on of heat occurs )* the e#c!tat!on of adjacent molecules here sol!d moleculesha$e l!ttle or no mo$ement& Conduct!on !s thus !s the "r!mar* mechan!sm !n sol!ds and ma*)e an !m"ortant com"onent mechan!sm !n some l!5u!ds at lo flo rates&CON6ECTION:Con$ect!on !s that mechan!sm here heat ener(* !s transferred )* the "h*s!cal mo$ementof molecules from "lace to "alce&12 ha/TH2 f!lm coeff!c!ent 7 "ro"ort!onal!t* constant8Rate of flo of heat !s "ro"ort!onal to the tem"erature d!fference )eteen the hot and coldl!5u!ds heat transfer area&RADIATION:The transfer of heat from a source to a rece!$er )* rad!ant ener(* !s rad!at!on&Rad!at!on !s the "rocess here )* a )od* em!ts heat a$es that ma* )e a)sor)s' reflectedor transm!tted throu(h a cold )od*&The sun transfers ener(* to the earth )* rad!at!on&SENSI90E HEAT:Sens!)le heat !s the ener(* re5u!red to chan(e the tem"erature of a flu!d !thout chan(!n("hase&0ATENT HEAT:The ener(* re5u!red to affect the chan(e of a flu!d as !n )o!l!n( or condens!n(&STEA: HEATERS;hen the steam flo demand !s (reater 7h!(h8 the )ac, "ressure !s also h!(h' lea$!n( theloest dr!$!n( force 7"ressure dro"s8 for the control $al$e& H!(h flo and lo "ressure dro"s results !n lar(e $al$e s! t+8 & Str!ctl* s"ea,!n( the a)o$e e5uat!on !ll onl* a""l* hen%& hen there !s no chan(e !n the s"ec!f!c heat+& the o$erall heat transfer coeff!c!ent !s constant&F& there are no heat losses In des!(n th!s cond!t!on s can )e assumed to )e sat!sf!ed "ro$!d!n( the tem"& chan(e!n each flu!d stream !s not lar(e& In most shell and tu)e e#chan(ers the flo!ll )e a m!#ture of co current'countercurrent and cross flo& The usual "ract!ce !n the des!(n of shell and tu)e heat e#chan(er !s to est!mate the -true tem"& d!fference. from the 0:TD )* a""l*!n( a correct!on factor to allo furtherde"arture from true countercurrent flo&/Tm 2 Ft /Tm;here /Tm2 true tem"& d!fference 7the mean tem"& d!fference for use !n the des!(ne5uat!on8Ft 2 the tem"& correct!on factor& The correct!on factor !s a funct!on of %& the shell and tu)e flu!d tem"& and+& the num)er tu)e and shell "assers& It !s normall* correlated as a funct!on of to d!mens!onless tem"& rat!os&R 2 7T% > T+84 7t+ >t%8S 2 7t+ >t%8 4 7T% > T+8R !s e5ual to the shell s!de flu!d flo rate t!mes the flu!d mean s"ec!f!c heat' d!$!ded)* the tu)e s!de flu!d flo rate t!mes the tu)e s!de flu!d s"ec!f!c heat&S !s a measure of the tem"& eff!c!enc* of the e#chan(er& The follo!n( assum"t!ons are made !n the der!$at!on of the tem"& Correct!on factorFt & !n add!t!on to those made for the calculat!on of 0:TD&%& e5uall* heat transfer areas!n each "ass+& a constant o$erall heat transfer coeff!c!ent !n each "ass&F& the tem"& of the shell s!de flu!d !n an* "ass !s constant across an* cross sect!on&D& the reason of lea,a(e of flu!d )eteen shell "assers& Th!s cond!t!on !ll not )e str!ctl* sat!sf!ed !n "ract!cal heat e#chan(ersL the Ft $alueso)ta!ned !n the cur$es !ll (!$e an est!mate of the true mean tem"& d!fference' that!s suff!c!entl* accurate for most des!(n& The shell s!de lea,a(e and )*"ass streams !ll effect the mean tem"& D!fference)ut are not normall* ta,en !n to account hen est!mat!n( the correct!on factor& The $alue of Ft!ll )e closed to 7%8 hen the term!nal tem"& d!fference are lar(e')ut !ll a""rec!a)l* reduce the 0:TD hen the tem"& of the shell and tu)e flu!dsa""roach each other' !t !ll fall drast!call* hen there !s tem"& cross& A tem"& cross!ll occur !f the outlet tem"& of the cold stream !s (reater than the !nlet tem"& of thehot stream' here the Ftcur$e !s near $ert!cal $alues can not )e read accuratel*'and th!s !ll !ntroduce a cons!dera)le uncerta!nt* !n to the des!(n& Aneconom!ce#chan(er des!(ncannot normall*)each!e$ed!f thecorrect!onfactorFtfalls)elo&H@& !nth!sc!rcumstancesanalternat!$et*"eof e#chan(ershould )e cons!dered h!ch (!$es a closer a""roach to true counter current flo&The use of to or more shells !n ser!es' are mult!"le shells!de "assers !ll(!$eclosure a""roach to true counter current flo and should )e cons!dered here thetem"& cross l!,el* to occur& ;heresens!)leandlatent heat !stransferred' !t !ll )enecessar*tod!$!dethetem"& "rof!le!ntosect!onsandcalculatethemeantem"& d!fferencefor eachsect!on&Shell and tube heat exchangers general design considerations :Flu!d allocat!on: ;here no "hase chan(e occurs the follo!n( factor !ll determ!ne the allocat!onof flu!d streams to shell and tu)es& Corros!on: the more corros!$e flu!ds should )e allocated to the tu)e s!des& Th!s!ll reduce the cost of e#"ens!$e allo*s are clad com"onent& Foul!n(: that flu!d that has a (reater tendenc* to foulto heat transfer surfaceshould )e "laced !n the tu)es& Th!s !ll(!$e )etter control o$er the des!(ns offlu!d $eloc!t*' and the h!(her alloa)le $eloc!t* !n the tu)es' !ll reduce foul!n(&Also the tu)es !ll )e eas!er to )e clean& Flu!dtem"&: !f thetem"& !sh!(henou(htore5u!retheuseof s"ec!al allo*s'"lac!n( the h!(h tem"& flu!ds !n the tu)es !llreduce o$erall cost& At moderatetem"' "lac!n( the hotter flu!d !n the tu)e s!de !llreduce the shells!de tem"&'and hence the need for la((!n( to reduce heat loss' or for safet* reasons& O"erat!n( "ressures: the h!(h "ressure stream should )e allocated to tu)e s!de&H!(h "ressure tu)e !ll )e chea"er than the h!(h "ressure shell& Pressure dro": for the same "ressure dro"' h!(her heat transfer coeff!c!ent !ll)e o)ta!ned !n the tu)e s!de than the tu)e s!de' and the flu!d !th the loestalloa)le "ressure dro" should )e allocated to the tu)e s!des& 6!scos!t*: (enerall* h!(her heat transfer coeff!c!ent !ll )e o)ta!ned )* allocat!n(the more $!scous mater!al to the shell s!de' "ro$!d!n( the flo !s tur)ulent' thecr!t!cal Re*nolds no& for tur)ulent flo !s !n the re(!on of of +CC& !f tur)ulent flocannot )e ach!e$ed !n the shell !t !s )etter to "lace the flu!ds !n the tu)es' as thetu)e s!de transfer coeff!c!ent can )e "red!cted !th more certa!nt*& Stream flo rate: allocat!n( the flu!d !th loest flo rate to the tu)e s!de !llnormall* (!$e the most econom!cal des!(n&Shell and tube fluid velocitiesHigh velocities will give high transfer co-efficient but also high pressure drop. The velocity must behigh enough to prevent any suspended solids settling , but not so highas to cause erosion. High velocities reduce fouling Plastic inserts are sometimes used to reduce erosion at the tube inlet.Liquids : Tube side : Process Fluids 1 to !trs. Per "ec. !a# : $ !trs. Per "ec. %f re&uired to reduce fouling. 'ater : 1.( to .( !trs. Per "ec.Shell Side : ).* to 1.) !+" ,apors : For vapors the velocity used is depends on the operating pressure and fluid density, the lowervalues given below will apply to high molecular weights materials.. ,acuum :() to -) !+". .tmospheric Pressure : 1) to *) !+". High Pressure : ( to 1) !+".Stream Temperature : The closure the temp. approached used /the difference between the outlettemp. of one stream and theinlet temp. of other streams0.The larger will be the heat transfer area re&uired for the given duty. The optimum value will depend on the application and can only be determined by ma1ing an economicanalysis of alternative design. .s a generalguideline the greater temp. difference should be at least ) 2eg. 3. and the least temp.difference ( to - 2ec. 3. for cooling by usingcooling waterand * to ( 2eg. 3. by using the 4efrigeratedbrains. The ma#. temp. rise in di-circulated cooling water is limited to around *) 2eg. 3. 3are should be ta1en toensure that the cooling media temp. are 1ept well above the free5ing point of the process materials. 'hen the heat e#change is better in process fluids for heat recovery and optimum approach temp. willnormally not be lower than ) 2eg. 3. Pressure Drop : %n many applications the pressure drop available to drive the fluid through the e#changer will be set by theprocess conditions and available pressure drop willvary from a few mille barsin avacuum service toseveral bars in pressure systems. Liquids : ,iscosity : 6 1 mns+m. ,*( 1n+m. 1 to 1) mns+m- ( to -) 1n+m.Gases & Vapors : High vacuum : ).$ to ).7 1n+m !edium vacuum : ).1 to # absolute pressure.+ 1 tobar- ).(# "ystem 8auge Pressure. .bove 1) bar 9 ).1# "ystem 8auge Pressure. 'hen a high pressure drop is utili5ed- care must be ta1en to ensure that the resulting high fluid velocitydoes not cause erosion or flow induced tube vibration.Fluid Physical Properties : Fluid physical properties are re&uired for heat e#changer design are : 2ensity. ,iscosity. Thermal 3onductivity. Temp. :nthalpy co-relation /specific and latent heats0. The physical properties derived are evaluated from the co-relation at the mean stream temp. This is satisfactory when the temp. changes small, but can cause a significant error when the change intemp. is large. %f the variation in physical proper is too large for simple methods to be used, it will be necessary to dividethe temp. enthalpy profile in to sections and evaluate the heat transfer co-efficient and area re&uired foreach section.Tube Side eat Transfer !o"efficient & Pressure Drop : Turbulent flow 9 Heat transfer data for turbulent flow inside conduced of uniform cross section are usuallyco-related by ;u < c 4e a Pr b /=+=w0c'here ;u < ;usselt ;o. / hide+1f0.4e < 4eynolds ;umber / Prude+=0 < /8e 2e+=0Pr < Prandle number /3P=+>f0.Hi < %nside 3o-efficient'+m 2ec. 3..2e