Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S53 IMPACT OF THE COLD END OPERATING CONDITIONS ONENERGY EFFICIENCY OF THE STEAM POWER PLANTS by Mirjana, S. LAKOVI -, Mladen M. STOJILJKOVI , Slobodan V. LAKOVI , Velimir P. STEFANOVI , and Dejan D. MITROVIFaculty of Mechanical Engineering, University of Ni, Ni, Serbia Original scientific paper UDC: 621.311.22:551.52/.57 DOI: 10.2298/TSCI100415066L TheconventionalsteampowerplantworkingundertheRankineCycleandthe steamcondenserasaheatsinkandthesteamboilerasaheatsourcehavethe same importance for the power plant operating process. Energy efficiency of the coalfiredpowerplantstronglydependsonitsturbine-condensersystem operation mode. Forthegiventhermalpowerplantconfiguration,coolingwatertemperature or/andflowratechangegeneratealterationsinthecondenserpressure.Those changeshavegreatinfluenceontheenergyefficiencyoftheplant.Thispaper focusesontheinfluenceofthecoolingwatertemperatureandflowrateonthe condenser performance, and thus on the specific heat rate of the coal fired plant anditsenergyefficiency.Referenceplantisworkingunderturbine-followmode with an open cycle cooling system. Analysis is done using thermodynamic theory, inordertodefineheatloaddependenceonthecoolingwatertemperatureand flowrate.Havingthesecorrelations,forgivencoolingwatertemperatureitis possibletodetermineoptimalflowrateofthecoolingwaterinordertoachieve an optimal condensing pressure, and thus, optimal energy efficiency of the plant. Obtainedresultscould beusedas usefulguidelinesinimprovingexisting power plants performances and also in design of the new power plants. Key words: cold end, cooling water, condensing pressure, energy efficiency Introduction Theneedforelectricalenergywillcertainlycontinuetogrow,andithasbecome imperative to lower the cost of electricity and enhance the operational economy of the turbine unit.FortheconventionalsteampowerplantworkingundertheRankineCycle,thesteam condenser as a heat sink and the steam boiler as a heat source have the same importance for the power plant operating process. - corresponding author; e-mail: lmirjana@masfak.ni.ac.rs Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S54THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 Thecoldendsystemofthesteamturbineiscomposedofthelaststagegroupthat includesthelowpressurecylinder,thecondenser,thecirculationwatersupplysystem,air extract system and, in case of closed-cycle cooling system, includes cooling tower. It can be dividedintotwosubsystemsthecondensatesystemandthecirculationwatersystem.The condenser is the key heat exchange system of the cold end.Thecondenseroperatingconditionsareofthegreatinfluenceonthemaximum generatedpowerandtheheatratevalue.Inthesametime,theoperatingconditionsofthe cooling water system determine the operating conditions of the condenser. Forcoolingitscondenser,steampowerplantsusebasicallytwotypesofcooling systems:open-cycleandclosedcycle[1].Open-cycleoronce-throughcoolingsystems withdraw large amounts ofcirculating water directly from and discharge directly tostreams, lakesorreservoirsthroughsubmergeddiffuserstructuresorsurfaceoutfalls.Anopen-cycle system depends on the adequate cool ambient water to support the generation at full capacity. Aclosed-cyclecoolingsystemtransferswasteheatfromcirculatingwatertoairdrawn throughcoolingtowers.Conventionalwetcoolingtowersdependonevaporatingheat exchangeandrequireacontinuoussourceoffreshwatertoreplaceevaporationlosses.The abilityofcoolingtowerstoprovidecoldwatertosteamcondensersofathermoelectricunit decreaseswithincreasingairtemperaturesand,incaseofwetcoolingtower,increasing humidity.Heatrateandgeneratedpoweroutputofaturbogeneratorunitstronglydependon the condenser pressure. The influence of the condensing pressure is similar in units operating in turbine-follow mode and for the units working in the boiler-follow mode [2]. In the turbine-followmodetheturbinegovernorissettocontrolthegeneratorload.Steamboilercontrol system adjusts the fuel firing rate and other parameters so as to maintain the parameters of the steamattheturbinethrottleascloseaspossibletotheirdesignvalues.Intheboiler-follow mode, the fuel-firing rate is set at a fixed value. In this case, the turbine governor adjusts the throttlevalve(andthereforegeneratedpower)soastomaintainthesteampressureatthe throttle valve inlet at its design value.Ontheturbogeneratorunitscontrolledinturbine-followmode,ariseinthe condenserpressurewillcauseariseintheenthalpyattheendpoint.Thiswillresultinthe reductioningeneratedpower.However,turbinegovernorwillincreasethethrottleand exhaust flows in order to set the generated power at the designed level. Steam boiler control system will increase the fuel firing rate, and specific heat rate of the turbo generator unit will increase due to fuel firing rate increasing. Otherwise, on the fall in the condenser pressure, the reversewilloccur,butonlyifitdoesnotfallbelowthepointwheretheexhaustannulus become choked. In the boiler-followmode, a rise in condenser pressure will cause a drop in powerforthesamenetheatinputtothesystem,whilethereversewilloccurwhenthe pressure falls.Thepressureinthesurfacesteamcondenserwilldependoncondenserdesign,an amount of latent heat to be removed, cooling water temperature and flow rate, maintenance of thecondenserandairremovalsystem.Atanygiventimetheseoperatingconditionswill determine the relationship between the heat rate and the power output. The designed characteristics of the condenser have a significant impact, but it is very expensivetoreplacethemwhentheplantisoperating.Still,withtheconstantscienceand technologydevelopment,todayitispossibletoimprovethedesignfornewplants,for Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S55 example,byusingthethin-walltitaniumtubingofthecondenser[3].Itisalsopossibleto makesignificantimprovementsintheexistingoperatingplants,forexample,byusingthe modularcondenserreplacement,withnewcondenser waterboxesandtitaniumtubebundles [4].Proper maintenance of the condenser is necessary for its proper operation. Constant cleaningofthecondensertubesisnecessarytomaintainadegreeoffoulinginacceptable level. Also, it is necessary to prevent any air leakage into the system and to assure constant air removal.Allowingnon-condensinggasestoaccumulateincreasesthermalresistanceonthe shell-side,andthustheoverallheattransfercoefficientofthetubes.Theproperventing equipment has to remove any non-condensing gas from the system.If the venting equipment is not adequate, the pressure in the condenser will rise, and efficiencyoftheturbinewillbereduced[5].Inthiscase,ventingequipmentislimitingthe vacuuminthecondenser.Iftheventingequipmentisproperlysized,vacuumwillbesetby temperatureofthecoolingwaterandheattransferrate.Moreefficientventingsystemsare also developed in order to reduce problems caused by non-condensing gases [6].Thispaperfocuses ontheinfluenceofthecoolingwatertemperatureandflowrate on the condenser performance, and thus on the power output, heat rate and energy efficiency of the power plant. The thermodynamic model of reference of the chosen power plant Thissimulationmodelprovidesamodularstructuresothataplantmodelquickly canbeadaptedtorepresentvariouspowerplantconfigurations.Figure1showstheprocess flow diagram of the plant model set up for power plant "Kostolac B". Figure 1. Thermodynamic plant model Kostolac B Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S56THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 In thismodel emphasis has been put on modeling the turbine and the cold end. For theanalysisoftheimpactofthecoldend,theboilerperformancewasconsideredless important. Therefore, the boiler has not been modeled in detail. Table 1. 18 K 348 turbine and condenser properties 18 K 348 turbine properties Nominal output348.5 MW Number of revolutions3000 rev per min Fresh steam pressure17.9 MPa Fresh steam temperature537 C Fresh steam flow rate277.78 kg/s Pressure of steam leaving HPT4.73 MPa Temperature of steam leaving HPT338C Pressure of steam in front of MPT4.24 MPa Temperature of steam in front of MPT537 C Pressure of steam leaving LPT0.6 MPa Temperature of steam leaving LPT265 C Outlet pressure0.0042 MPa Number of extraction ports7 18 K 348 turbine condenser properties Technical dataHeat transfer surface8468 m2 Number of tubes17 550 Tube diameter x length 24 x 6400 mm Hotwell volume31 m3 Operating data at 100% turbine loadSteam flow rate183.1 kg/s Pressure0.044 bar Cooling water flow rate13 m3/s Cooling water velocity1.95 m/s Cooling water temperature12 C Turbine18K348ofthisplantisfour-cylindrical,reaction-axialflowwith overheatingbetweenstages.Itconsistsofthehighpressureturbine(HPT),themedium pressureturbine(MPT)andtwolowpressureturbines(LPT).Thesteamturbinesetis Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S57 equippedwithsevenextractionports.Theplanthasfourlow-pressurefeedwaterheaters (LPH4, LPH5, LPH6, and LPH7). Heating up the boiler (SB) feed water to the final stage at the input of the boiler is done by two high pressure regenerative heaters (HPH1, HPH2).Thesedevicesaresuppliedwithheatingsteamfromtheextractionports(Ep1,Ep2...). Removing the non-condensable gases from the steam cycle is being realized by the deaerator (DA3).Exhauststeamfromthelowpressureturbineiscompletelycondensedinthesurface condenser(CN).Thetechnicalcharacteristicsforthisturbineandthetechnicaldataforthis condenser are given in the Table 1 [7]. For cooling its condenser, the Kostolac B power plant uses cooling water from the Danube. The temperature of this cooling water varies from around 4 C in winter up to 28 C insummer.Atthismoment,flowrateofthecoolingwatercannotbecontrolledinawider range.Themathematicalmodeloftheproposedthermodynamicproblemformulationis createdasatypeofsteadystatesimulation.Flowsheetingformulation(givenallinput information,determinesalloutputinformation)wasdevelopedbyapplyingtheconservation lawsformassandenergybalance[8,9].Thisformulationforeachcomponent,subsystems and the whole system is presented by following mass balance eq. (1) and energy balance eq. (2) equations: ( , ) ( , )0,j jj in i j j out i jm m i Ie e = e (1) ( , ) ( , )0,j j j j i ij in i j j out i jm h m h W Q i Ie e + = e (2) { } ,... , , , , ,i iHPH LPH CN HPT MPT LPT i e For calculating the heat transfer coefficient Bermans method was used [10]:

( )22

1410, 42 1,13500 1 35 [ / ]1000xw zWK t W m KDo|| uu| |(= |( |( \ .(3) Coefficient x is calculated as following eq. (4): ( )10.12 1 0.15wx t | = + (4) heat transfer surface fouling rate [], W water velocity in tubes [ms-1], D1

tube inner diameter [m], z factor which takes into account number of water passes through the condenser, eq. (5), [], factor which takes into account influence of the condenser steam load, eq. (6.1), eq. (6.2), [], tw1 cooling water inlet temperature, Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S58THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 121 110 35wzt zu | |= + |\ .(5) z number of water passages through the condenser. For steam load between nominal value dknom and boundary value dkbound : ( )10, 09 0, 012 , 1bound nomk kd t dou = = (6.1) 2bound k kk kbound boundk kd dd dd dou| |< = |\ .(6.2) For once-through cooling and pure water, the fouling rate of the heat transfer surface is = 0.80-0.85. For circulating cooling and chemicallytreated water = 0.75-0.80, and for dirty water with mineral and organic sediments forming on the tubes, = 0.65-0.75.Cooling water velocity can be written in the following form 214wt wmWD n t = (7) Afterthisreplacementtheheattransfercoefficientcanbecalculatedfromthe following equation: ( )1[0.12 (1 0,15 )]22 1,1 9 410, 42 4, 4 23500 1 35 1 1 [ / ]1000 10 35wtw wwt wm t ZK t W m KD n|||t + (| |( | |= + (||(\ .( \ . (8) Considering the following relations: Energy balance formula eq. (9) ( ) ( )2 1 c w w w w c s cQ m c t t m h h = = (9) Logarithmic mean temperature difference eq.(10) ( )( )( )1 221lnw wmc wc wt ttt tt tA =(10) Peclet equation eq.(11)

c mQ KA t = A(11) Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S59 Where: .Q condenser heat transfer rate [kW], wm cooling water flow rate [kg s-1], cm steam flow rate through condenser [kg s-1], ct condensing temperature [C], 2 wt cooling water outlet temperature [C], K heat transfer coefficient [kWm-2K-1]. The heat transfer rate of the condenser, depending on cooling water temperature and flow rate, condensing temperature (i. e. condensing pressure) and steam flow rate through the condenser, can be calculated from eqs. (12, 13) as follows: ( )11 ew wAKm cc w w c wQ m c t t | |= | |\ .(12) ( )11 ew wAKm cw w c wcs cm c t tmh h | | | |\ .=(13) Plant power output can be calculated from eq. (2). Thespecificheatrateisoneofthemainthermodynamicparametersforthebest evaluationoftheenergyefficiencyofthesteampowerplant.Thespecificheatratecanbe calculated as: ( ).1 fwSB iSB SB SBm i iQ B HqN P P q q q = = = (14) Where: q specific heat rate [kJkW1s1], SBQ steam boiler heat transfer rate [kW], P power output [kW], B fuel mass flow rate [kgs-1], Hi lower calorific value of the fuel [kJkg-1], h1 enthalpy of the steam leaving steam boiler [kJkg1K1],hfw feed water enthalpy [kJkg1K1], m boiler steam load [kgs-1], SBq steam boiler efficiency []. Overall energy efficiency of the power plant can be calculated from eq. (14): Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S60THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 iPBHq = (15) NumericalintegrationwasdoneusingMicrosoftExcelprogrammingplatformand VisualBasicsforApplications.Thesoftwareincludesthewater/steamthermodynamic propertiessimulator(settledintheExcelAdd-incomponent),basedonIAPWSIndustrial Formulation1997(IAPWSIF97).ThisformulationisproposedbytheInternational AssociationforthePropertiesofWaterandSteam.Thevalidityfieldextendsoverto temperatures between 0-800C, for pressures up to 100 MPa, [11]. Hereinafter certain of the obtained results are given and discussed. Cooling water parameters influence on the condenser performance Condenser heat transfer rate strongly depends on condensing pressure, cooling water flow rate and temperature. In an ideal situation, when the venting system properly removes air fromthesteamcondenser,theachievablecondensingpressureisdeterminedbytemperature of the cooling water, as it is mentioned above.For the steam power plant with once-through coolingsystem,coolingwatertemperatureisdeterminedbynaturalwatersource(i. e.river) temperature. This means that cooling water temperatureis changing with weather conditions in particular region, and cannot be changed in order to achieve better condenser performances (i.e.highervacuuminthecondenser).Still,coolingwatertemperaturedirectlyaffects condenser performances. Suitable parameter for on-line control is cooling water flow rate, and itcanbevariedinawiderange,withappropriatecirculationpumps.Duringplantoperation the objective is to operate at the optimum cooling water flow rate, which depends on cooling watertemperatureandpowerdemand[12].Inthatmanner,coolingwatertemperatureand flowrateareconsideredvariableparametersinthesimulationoftheplantoperating conditions. Condensing pressure and condenser heat transfer rateWithcoolingwatertemperaturerise,themeantemperaturedifferenceinthe condenserdecreases,andcondenserheattransferrateforthesamecondensingpressurewill alsodecrease,asitisshowninfigure2.Onthisfigure,aninterestingdetailcanbeseen:at coolingwatertemperaturenearlydesigned,condenserheatcapacityhasitsmaximum,and lower temperature will not lead to significant increase of the heat transfer rate.It means that this particular condenser is designed at its maximum heat transfer rate andwithincreasedcoolingwatertemperatureitcannotachieverequiredvalue.Inthisway, the question of the valid designed parameters is opened. Increasing of cooling water flow rate will increase the condenser heat transfer rate for the given cooling water temperature.Condensingpressuredependenceoncoolingwatertemperatureisobtainedforthe given water flow rate and steam load of the condenser. Steam load is considered constant, in order to obtain a clear illustration of this dependence, as it is shown in figure 3. It is obvious that with cooling water increasing, pressure in the condenser will also increase. Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S61 Figure 4 shows the correlation between condenser heat transfer rate and condensing pressure.Inthisfigure,eachcurveshowsthechangeofcondensingpressureataconstant coolingwatertemperature,fordifferentvaluesofcoolingwaterflowrate.Thiscorrelation wasusedalsoformodelperformanceverificationsincethesamedependenceisgivenby manufacturer of this condenser, obtained from plant management [13]. Figure 2. Condenser heat transfer rate due to cooling water temperature and flow rate change 01002003004005006007004 6 8 10 12 14 16 18 20 22 24 26 28 30Cooling water temperature[oC]Condenser heat transfer rate [MW]10000110001200013000140001500016000170001800019000Cooling wayer flow rate[kg/s]pk=0.044 ba r Figure 3. Condensing pressure change due to cooling water temperature change 00,020,040,060,080,10,122 6 10 14 18 22 26Coolimg water temperature [oC]Condensing pressure [bar] . Cooling wayer flow rate [kgs1] Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S62THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 Figure 4. Condenser characteristic curves Steam load of the condenser Withcoolingwatertemperatureincreasing,inordertomaintaindesignedheat transferrateofthecondenser,condensingpressurewillincrease.Asthisplantisworking underturbine-followmode,theturbinegovernorwillincreasethethrottleandexhaustflows inordertosetthegeneratedpoweratthedesignedlevel,butwithincreasingofheatrate, whichwillbediscussedinsection4ofthispaper.Figure5showsthesteamloadofthe condenser dependence on condensing pressure, at a constant cooling water temperature as for different values of cooling water flow rate. Figure 5. Steam load of the condenser for cooling water temperature 12 C Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S63 Condenser operating conditions influence on the plant performances As it can be seen in figure 5, with increasing of condensing pressure and flow rate of thecoolingwater,steamflowthroughthecondenser,andthusthroughthelow-pressure turbineisincreasing.Thiswillincreasenetpoweroutput.Thisincreasingofthenetpower output, however, is correlated to the increasing of the heat rate.Specific heat rate Specificheatratechangeduetocondensingpressurechangeisshowninfigure6. Thoseresultsareidenticaltoexperimentalresultsforsimilarplantgiveninliterature[14]. With decreasing of the pressure in the condenser, specific heat rate decreases. With pressure decreasingtobellowthepointwheretheexhaustannulusbecomeschoked,excessive condensatesub-coolingwillresult,tendingtoreducetheimprovementinheatrateresulting fromthelowercondensingpressure.Condensatesub-coolingisdefinedasthesaturation temperature corresponding to the pressure in the condenser minus the condensate temperature in the hot well [2]. This is evident in figure 6 especially for 337 MW curve. Condensate sub-coolingisundesirablesincemoreheathasbeenremovedfromthecyclewithoutgenerating any additional power, but still has to be replaced by adding fuel. Generated power [MW]-4-2024681012140,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,1Condensing pressure[bar]Change of specific heat rate [%] .150200250300337 Figure 6. Specific heat rate change due to condensing pressure and generated power change Having the condensing pressure change due to the cooling water temperature change (figure 3) and dependence given in figure 6, the change of the specific heat rate dependence on the cooling water temperature is obtained, and it is shown in figure 7. In order to present thisdependencemoreclearly,itwaschosentokeeptheplantpowerproductionconstant. Power consumption of the cooling water pumps was not considered.Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S64THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 -25-20-15-10-505101520252 4 6 8 10 12 14 16 18 20 22 24 26 28Cooling water temperature [oC]Change of specific heat rate [%] . Figure 7. Specific heat rate due to cooling water temperature change Energy efficiency Using the known assumption, from the literature [14], that with increasing pressure inthecondenserof1kPa,efficiencydecreasesto1.0-1.5%andconsideringthatinthis particularcasethereductionis1.2%, dependenceoftheenergyefficiency(generatedpower divided by an amount of energy consumed) in the function of the cooling watertemperature rise is obtained and is shown in figure 8. 30313233343536378 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29Cooling water temperature [oC]Energy efficiency [%] . Figure 8. Energy efficiency of the plant due to cooling water temperature change Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66S65 Cooling water temperature rise causes the reduction of energy efficiency and power of steam power plants (in Germany the production of electricity in summer months due to the increase of cooling water temperature is decreased by 18%). The problem is more severely set forplantswithclosed-cyclecoolingsystemincomparisontotheplantwithanopencycle coolingsystem.Drybulbtemperature,thewetbulbtemperature,theatmosphericpressure, flowrateofthecirculatingwater,thecharacteristicofcoolingtowerfill,theresistance coefficientofthepartsinatower,andtheworkingconditionofawaterdistributionsystem andsoon,canaffecttheoutletwatertemperatureofthecoolingtower[15].Itmeansthat coolingwatertemperatureischanginginamuchwiderrangeinonedayincomparisonto once-through cooling system. Conclusions Steam power plant strongly depends on its cold end operating conditions, where the condenser is the key of the heat exchange system. On the other hand, operating conditions of thecoolingwatersystemdeterminecondenseroperatingconditions.Thepressureinthe surfacesteamcondenserwilldependoncondenserdesign,theamountoflatentheattobe removed,coolingwatertemperatureandflowrate,maintenanceofthecondenserandair removalsystem.Atanygiventimethecombinedstatusoftheseoperatingconditionswill determine the relationship between heat rate and power output.In this paper, influence of cooling water temperature and flow rate is considered for powerplantKostolacBwithonce-throughcoolingsystem.Thethermodynamicmodelof the plant is described, together with the mathematical model. Numerical integration was done byusingtheMicrosoftExcelprogrammingplatformandVisualBasicsforApplications, including the water/steam thermodynamic properties simulator based on IAPWSIF97. As the resultofcalculation,setofdifferentdependenciesisobtainedanddescribedherein:the condenser heat transfer rate and pressure in the condenser are given for variable cooling water temperatureandflowrate,thespecificheatratechangeduetothechangeofcondensing pressure,andthespecificheatratechangeduetothecoolingwatertemperaturechange. Finally, the energy efficiency for the reference plant is given as the function of the change in condensing pressure.Havingthosedependences,itisclearlythattheincreasingincoolingwater temperature,inevitableinthesummer,willleadtothedecreaseofenergyefficiencyofthe powerplantforthegivenpowerproduction.Theincreasingofcoolingwaterflowrateto maintain the same heat transfer rate at higher vacuum in the condenser, and thus, to increase energy efficiency of the plant, is a good way in optimizing plant operation. Nomenclature A heat transfer surface, [m2] B fuel mass flow rate [kgs-1] cw water specific heat, [kJkg1K1] D1 tube inner diameter [m] Hi lower calorific value of the fuel [kJkg-1] hfw feed water enthalpy [kJkg1K1] hc, hs steam and condensate enthalpy [kJkg1K-1] h1 enthalpy of the steam leaving steamboiler [kJkg1K1] K heat transfer coefficient [kWm2K1] sm steam flow rate through condenser[kgs-1]wm cooling water flow rate [kgs-1] Lakovi, M. S., et.al.: Impact of the Cold End Operating Conditions on Energy Efficiency S66THERMAL SCIENCE: Year 2010, Vol. 14, Suppl., pp. S53-S66 nt number of condenser tubes P power output [kW]Q specific heat rate [kJkW1s1] cQ condenser heat transfer rate [kW] SBQ steam boiler heat transfer rate [kW] tc condensing temperature [C] tw1 cooling water inlet temperature [C] tw2 cooling water outlet temperature [C] W water velocity in tubes [ms-1] z number of water passages through thecondenser Greak letters heat transfer surface fouling rateSBq steam boiler efficiency [] q overall energy efficiency of the power plant [] d factor which takes into account influenceof the condenser steam load [] z factor which takes into account number of water passes through the condenser [] w water density [kgm-3] References [1]Mihajlov J., Thermal Power Plants, Design and Building, (in Serbian), Tehnika knjiga, Zagreb, 1965 [2]Harpster,J.W.,Putman,R.E.,TheEconomicEffectsofCondenserBackpressureonHeatRate, Condensate Sub-Cooling and Feedwater Dissolved Oxygen,Proceedings on CD, of 2000 International Joint Power Generation Conference, Miami Beach, Fla., USA, 2000 [3]Schumerth, D., Thin-Wall Titanium Condenser Tubing: Explore the Opportunities, Proceedings on CD, of 2000 International Joint Power Generation Conference, Miami Beach, Fla., USA, 2000 [4]Edgell, D., Davidian, A., Modular Condenser Replacement at ANOA Solves Operating Problems and ImprovesPerformance,ProceedingsonCD,oftheInternationalJointPowerGenerationConference, Vol. 34, Book No. G011101999, Miami Beach, Fla., USA, pp. 479-482 [5]Trela,M.,etal.,MonitoringofairContentinaMixtureRemovedfromCondensersinApplicationto SteamTurbineDiagnostics,ProceedingsonCD,of2000InternationalJointPowerGeneration Conference, Miami Beach, Fla., USA, 2000, IJPGC200015002 [6]Kubik, W.J., Spencer, E., Improved Steam Condenser Gas Removal System in The American Society of Mechanical Engineers, http://www.grahammfg.com/downloads/36.pdf[7]Mitrovi,D,ivkovi,D.,Lakovi,M.,EnergyandExergyAnalysisofa348.5MWSteamPower Plant, Energy Sources, 32 (2010), 11, pp. 1016-1027 [8]Bejan,A.,Tsatsaronis,G.,Moran,M.,ThermalDesignandOptimization,JohnWiley&Sons,Inc., New York, USA, 1996 [9]Incropera, F., De Witt, D., Fundamentals of Heat and Mass Transfer, John Wiley & Sons, Inc., New York, USA, 1985 [10]Berman, L.D., Tumanov A., Heat Transfer with Condensation of Moving Vapour on a Horizontal Tube, Teploenergetika, 9 (1962), 10, pp. 77-83 [11]FonescaJr,J.,Schneider, P.,ComparativeAnalysisoftheIAPWSIF97Formulation Performancefor Thermodynamic Properties of Water on a Rankine Cycle, Engenharia Termica, 51 (2004) 5, pp. 52-55 [12]Kromhout J., Goudappel E., Pechtl P., Economic Optimization of a 540 MWel Coal Fired Power Plant Using Thermodynamic Simulation, VGB Powertech, 81 (2001), 11, pp. 43-46 [13]Swierczewskiego,K.,OperatingRegulations,Condenser18K348TurboGeneratorUnit(inSerbian) 8503686D, Zamech, 1983 [14]Truhnii, A. D, Losev, S. M, Stationary Steam Turbines, Energoizdat, Moscow, 1981 [15]Zhao, B., Liu, L., Zhang W., Optimization of Cold End System of Steam Turbine, Frontiers of Energy and Power Engineering in China, 2 (2008), 3, pp. 348-353 Paper submitted: April 15, 2010 Paper revised: July 20, 2010 Paper accepted: August 30, 2010