Energy and Buildings 50(2012)204–211Contents lists available at SciVerse ScienceDirectEnergy andBuildingsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e n b u i ldExperimental determination of fouling factor on plate heat exchangers in district heating systemSrbislav B.Geni´c a ,∗,Branislav M.Ja´cimovi´c a ,Dragan Mandi´c b ,Dragan Petrovi´c aa Faculty of Mechanical Engineering,University of Belgrade,Kraljice Marije 16,11000Belgrade,Serbia bJKP Beogradske elektrane,Savski nasip 11,1070Belgrade,Serbiaa r t i c l ei n f oArticle history:Received 5March 2012Received in revised form 14March 2012Accepted 17March 2012Keywords:District heatingDomestic hot water Plate heat exchanger Fouling factora b s t r a c tThis paper presents the results of experimental research on 8plate heat exchangers.Their purpose is to heat water for radiator heating system and domestic hot water.Measurements were carried out in 4substations in district heating system in Belgrade (Serbia).The main achievement of this research was that the fouling factors were determined experimentally,and the found values can be used further on in design of plate heat exchangers in district heating systems.©2012Elsevier B.V.All rights reserved.1.IntroductionDomestic hot water (DHW)system is a part of district heating system in Belgrade (Serbia).JKP Beogradske elektrane (municipal company for district heating in Belgrade)continuously improves the system by incorporating prefabricated compact substations which serve two purposes:heating of water for radiator heating system and heating of DHW.Basically there are two ways of preparing hot water [1],and both have been applied in Belgrade:•the “old”system that included the accumulation tank in which hot water is stored.•the “new”system with instantaneous hot water production.The last option is of particular interest in this paper.DHW heaters,accompanied by the tap water filters,are the main part of this sys-tem.Nowadays,in Belgrade DHW system,practically only magnetic filters and plate heat exchangers are used,since their performances and prices are significantly favorable than performances and prices of tubular heat exchangers in this field of application.The total number of substations with DHW is 413with serve more than 34,000individual consumers.Overall heat duty for com-plete DHW system in Belgrade is 65.2MW,with nominal heat duties of individual substations in range 4.5–1070kW.∗Corresponding author.Tel.:+381113302360;fax:+381113370364.E-mail address:sgenic@mas.bg.ac.rs (S.B.Geni´c).This paper will describe the influence of water velocity on foul-ing factor in plate heat exchangers,based on measurements on 4district heating substations (DHS)in the Sector of the Heat Plant Zemun.2.Plate heat exchanger foulingDuring the operation of a heat exchanger,its heat duty reduces due to a phenomenon called fouling:the deposit of undesirable dirt on the heat exchange surfaces increases the resistance to heat transfer.Heat exchanger fouling is a very significant problem in heat exchanger exploitation,because still there is no reliable theoretical method for predicting the fouling phenomena and all relevant information are obtained by monitoring the changes in heat exchanger performance during the exploitation period [2].Based on these measured data it is possible,to a certain extent,to predict the behavior of apparatuses of similar design,at approx-imately the same exploitation conditions (identical working fluids,pressures and temperatures,velocities,etc.).Also,it is possible to predict the shut off period for cleaning of the heat exchanger.In general,working regime (fluid flow rate,temperature,pressure and concentration of components)as well as the actual plate geometry (shape of corrugations –angle,amplitude and wavelength)affects the formation of deposits [3–5].The overall heat transfer coefficient for plain wall is defined as1k =1˛1+R 1+ıpl pl+R 2+1˛2(1)where0378-7788/$–see front matter ©2012Elsevier B.V.All rights reserved.doi:10.1016/j.enbuild.2012.03.039S.B.Geni´c et al./Energy and Buildings50(2012)204–211205NomenclatureA channelflow area(flow area between plates),m2b meanflow channel gap(distance between plates),mCF cleanliness factorc p specific heat capacity at constant pressure,J/(kg K)CR correlation ratioDHW domestic hot waterDHS district heating substationsDM design margink overall heat transfer coefficient,W/(m2K)LMTD mean logarithmic temperature difference,◦CL length,m˙m massflow rate,kg/sN numberp plate pitch,m˙Q heat duty,WR fouling factor(resistance),m2K/WRMSD root-mean-square deviationRW radiator waterS heat transfer surface,m2t temperature,◦Cufluid velocity,m/sW plate width,m˛heat transfer coefficient,m2K/Wıthickness,mst unsteadiness of working regime(dispersion of heatduty)p pressure drop due to friction,Pathermal conductivity,W/(m K)(wall)shear stress,PaSubscripts1hotfluid2coldfluidav averagec clean(unfouled)in inletm meanout outletpl one platetot total•˛1and˛2are the heat transfer coefficients for hot and coldfluidsrespectively in W/(m2K);•R1and R2are the fouling resistances in m2K/W for hot and coldfluids respectively;•ıpl(m)and pl(W/(m K))are the thickness and thermal conduc-tivity of the plate.Total fouling factor(for plain wall)is defined asR tot=R1+R2=1k −1k c(2)where k c is the overall heat transfer coefficient for clean(unfouled) surfaces.In addition,there are other ways for expressing the fouling phe-nomena:•cleanliness factor[6]CF=kk c=11+R tot·k(3)Table1Recommended fouling resistances for plate and frame heat exchangers.Water R,m2K/kWDemineralized or distilled water0.009River water0.045Ocean or coastal sea water0.025–0.045Treated cooling tower water0.035–0.045Soft water0.018Hard water0.045•design margin[7]or heat transfer surface area margin(k-valuemargin)DM=k ck−1=R tot·k c=1CF−1(4)Regardless on the way the fouling resistance is expressed generallyspeaking it depends on heat transfer surface material,flow rates orvelocities and fouling affinity of thefluids.The usual approach to heat exchanger design is to include thefouling factors in(1).Fouling factors can be found in the open lit-erature[6,8–12],and selected values for water are presented inTable1.Other open literature sources claim the following:•most manufacturers of plate and frame heat exchangers recom-mend that the excess surface should not exceed25%of the heattransfer surface area calculated for the clean duty[2];•plate and frame heat exchangers fouling factors are typically1/10of TEMA values[13];•safe and proven fouling factors for plate heat exchanger designare10%excess surface area,also expressed as10%safety margin,equivalent to approximately(but not exactly)a cleanliness factorof90%[14];•“for a plate heat exchanger in a water/water duty a margin of0÷15%depending on water quality is normally enough”[7].In past few decades,wall shear stress became a significant parame-ter in fouling factor analysis,since it is a measure of thefluid stressalong the face of the corrugated plate.For example in[15]it is saidthat“Shear stress is the metric to evaluate fouling tendency,ratherthan absolute velocity.”Wall shear stress is defined as follows[14]= p·b2·L(5)where• p,Pa,is the pressure drop due to friction;•b,m,is the distance between plates;•L,m,is the effective plate length.The following recommendations about the influence of shearstress on fouling factor are found in the open literature:•greater shear stress provides the greater“resistance”to the depo-sition of particles[16]and this is the main reason that plate heatexchangers are characterized with smaller fouling factors thantubular heat exchangers;•according to[17]it is not recommended to size the heatexchanger with the shear stress below50Pa,and if there is asignificant risk of fouling it is recommended that the shear stressshould be increased to at least100Pa;•in[18]the experiment was conducted on heat exchanger with themixture of water and crushed ice and authors came to the con-clusion that the smooth operation of apparatus occurred when206S.B.Geni´c et al./Energy and Buildings 50(2012)204–211Fig.1.Process flow diagram of DHS.Table 2Codification of DHS and heat exchangers (type—number of plates).Address of DHS (street and number)DHS codeDHW heaterRW heaterBaˇc ka no.9A A-113C-222Zagorska no.12B A-89C-104Zagorska no.18–46C A-87C-152Prvomajska no.33DB-113C-200Table 3Design data of heat exchanger for DHW system.DHS codeABCDDHW heaterA-113A-89A-87B-113Number of plates,N pl 1138987113Number of active plates 1118785111Plate thickness,ıpl ,mm 0.60.6One plate surface,S pl ,m 20.040.0940Mean flow channel gap,b ,mm 0.96 1.0Plate width,W ,mm119243Flow area between plates,A pl ,m 20.0001140.000243Heat transfer surface,S ,m 24.443.487.9210.43>50Pa.The regimes with lesser values of were characterized with ice choking.3.Measurements and resultsThe objective of this research was to obtain accurate results on fouling resistances in plate heat exchangers for DHW system in Bel-grade.Substations discussed in this paper serve for two purposes:heating of water for radiator heating system as well as heating of DHW.Substations are located in a heating area of the Heat Plant Zemun in Belgrade (Serbia).A process flow diagram of DHS is pre-sented in Fig.1.Each substation was equipped with necessary control systems (valves,flow limiters,etc.),two heat exchangersTable 4Design data of heat exchanger for RW system.DHS codeABCD RW heaterC-222C-104C-152C-200Number of plates,N pl 222104200152Number of active plates 220102198150Plate thickness,ıpl ,mm 0.6One plate surface,S pl ,m 20.109Mean flow channel gap,b ,mm 2.44Plate width,W ,mm243Flow area between plates,A pl ,m 20.000593Heat transfer surface,S ,m 223.9811.121.516.35(basic data is given in Tables 2–4)and magnetic filters on the DHW pipeline.In all tested DHW heaters cold fluid was tap water,all appa-ratuses had counter-current flow,and each one of them was thermally insulated.Our laboratory holds the Accreditation Certificate for the follow-ing standards:•BS EN 305:1997Heat Exchangers:Definitions Of Performance Of Heat Exchangers And The General Test Procedure For Establishing Performance Of All Heat Exchangers;•BS EN 306:1997Heat Exchangers:Methods Of Measuring The Parameters Necessary For Establishing The Performance;•BS EN 307:1999Heat Exchangers:Guidelines For Preparing Installation,Operating And Maintenance Instructions Required To Maintain The Performance Of Each Type Of Heat Exchanger;so our measurements were performed according to proce-dures from these standards.For each heat exchanger the following parameters were measured:•˙m1,kg/s,mass flow rate of hot fluid;•t 1in ,◦C,inlet temperature of hot fluid;•t 1out ,◦C,outlet temperature of hot fluid;•˙m2,kg/s,mass flow rate of cold fluid;•t 2in ,◦C,inlet temperature of cold fluid;•t 2out ,◦C,outlet temperature of cold fluid.Measurements of water flow rate were performed with ultrasonic flow meters with the relative uncertainty of ±3%.Temperatures were measured using four platinum resistance thermometers (PT 100)with the maximal uncertainty of ±0.1◦C.Three heat duties of the heat exchanger could be determined from each set of test data:•heat duty calculated with measured values for the hot fluid˙Q1=˙m 1·c p 1·(t 1in −t 1out )(6)•heat duty calculated with measured values for the cold fluid˙Q2=˙m 2·c p 2·(t 2out −t 2in )(7)•mean value of heat duty˙Qm =˙Q 1+˙Q 22(8)where c p 1,J/(kg K)and c p 2,J/(kg K)are specific heat capacities of hot and cold fluids,respectively.S.B.Geni´c et al./Energy and Buildings 50(2012)204–211207The unsteadiness (dispersion of heat duty)of each workingregime is defined byst =(˙Q1−˙Q m )2+(˙Q 2−˙Q m )2˙Qm (9)In engineering practice it is commonly assumed that the disper-sion of 3–7%or even 10%in the heat balance is acceptable [19,20].Hereby,similarly to [21],further analysis of the fouling resistance was done only for the working regimes with st <10%.Since all the heat exchangers operated with the counter-current flow,the mean logarithmic temperature difference isLMTD =(t 1in −t 2out )−(t 1out −t 2in )lnt 1in −t 2out t 1out −t 2in(10)and the overall heat transfer coefficient isk =˙Q m S ·LMTD(11)The overall fouling thermal resistance (R tot )was calculated using (2).The overall heat transfer coefficient for the heat exchanger with clean (unfouled)heat transfer surfaces k c was calculated for each regime using the manufacturer’s design software.The experimental work was divided in two parts.First set of measurements was conducted shortly after the substation was con-nected to the heating system (about 5to 15days).These results are indicated using subscript Y 0.Second set of measurements was done a year later (subscript Y 1).During this one year period substations worked in the normal regime dictated by the heat plant,including a half year pause for RW heaters during the summer period.Table 5presents the results of measurements for RW heaters and Table 6presents the results of measurements for DHW heat exchangers.Fluid velocities u 1and u 2are based on flow area between plates,and refer to hot and cold fluids,respectively.4.DiscussionFor further discussion it is useful to calculate average values based on Tables 5and 6.Tables 7and 8contain the average values of u 1,av ,u 2,av ,R tot,av ,CF av ,and DM av .4.1.Fouling of radiator water heatersAnalyzing the data presented in Tables 5and 7it can be noticed that the both fluid velocities were very consistent during the test.Also,in one year period fouling resistances of 4heat exchang-ers did not change significantly (the increase R tot is close to the order of magnitude of measurement uncertainty)and ranged from R tot =0.082to 0.113m 2K/kW,i.e.R tot =0.096m 2K/kW with maxi-mal deviation of ±17%.This means that in heat exchanger design process the fouling resistance value of R tot =0.096m 2K/kW can be adopted as a statistically proven one.Average design margin changes over the greater range DM av =0.17–0.33and average cleanliness factor over a lesser range CF av =0.75–0.86,so it can be concluded that despite the conformity of usage of just one parameter like in case of DM or CF ,it is more rec-ommendable to use fouling resistance R in heat exchanger design,because it gives a more realistic physical insight into the fouling process.In further analysis of the fouling resistance it must be taken into account that the radiator heating system is filled with water from the magisterial pipelines.That means that both hot and cold water has approximately the same fouling characteristics,so it can be writtenR 1=R 2=R tot 2(14)Fig.2.DHW heaters—fouling factor vs.hot fluid velocity.and the fouling resistances R 1=R 2=0.048m 2K/kW can be recom-mended for each fluid.Two more conclusions are of interest:•measured fouling resistances R 1=R 2=0.048m 2K/kW are a little greater than the greatest recommended value from the Table 1that is 0.045m 2K/kW;•variation of fouling resistances was within the measurement uncertainties,so the asymptotic fouling model can be applied in this case.Similar conclusion was derived for shell-and-tube exchangers (with straight and helical tubes)in a district heating system in [3,22,23].4.2.Fouling of domestic hot water heatersAccording to Tables 6and 8it can be noticed that the water velocities (flow rates)for 3heat exchangers were almost con-stant during a one year period.For exchanger A-113(substation A)average hot water velocity dropped from 0.346m/s to 0.214m/s,while the cold water velocity increased from 0.183m/s to 0.267m/s.In this substation the smallest increase of the fouling factor was detected over a one year period,while in other 3substations the increase was very significant.Besides the fluid velocities (i.e.flow rates),other working conditions (temperatures,pressures and con-centrations)varied in pretty small ranges.An important conclusion from these data is that the fluid velocities had shown the signifi-cant influence on the fouling factor.This observation corresponds to very similar conclusions from [24,5,25,26].Figs.2and 3show the averaged data based on Table 8.In this specific case,it can be expected that the hot fluid fouling factor must have the same value as in case of radiator water heaters R 1=0.048m 2K/kW,so the fouling resistance for DHW (cold fluid)after a one year period isR 2,Y 1=R tot,Y 1−0.048After the calculation of values R 2,Y 1(data are given in Table 9)we managed to correlate with cold fluid velocity byR 2,Y 1=0.51×10−3·exp(−8.50·u 2)(15)but also to cold fluid wall shear stress ( 2,Pa)in the following form R 2,Y 1=0.4×10−3+0.28×10−3·exp(−0.060· 2)(16)Wall shear stress for cold fluid (DHW)is calculated for each working regime using the heat exchanger manufacturer’s design software.208S.B.Geni´c et al./Energy and Buildings50(2012)204–211Table5RW heaters—results of measurements.DHS code Heat exchanger Year u1(m/s)u2(m/s)k(W/(m2K))k c(W/(m2K))R tot(m2K/kW)CF DMA C-22200.0460.150173220940.09980.8270.2090.0460.149168820780.11120.8120.2310.0460.149185620800.05800.8920.1210.0460.148184220680.05930.8910.123A C-22210.0460.150175820620.08390.8530.1730.0460.150176620610.08110.8570.1670.0460.150163420630.12730.7920.2630.0460.150163420630.12730.7920.2630.0460.150163420630.12730.7920.2630.0440.150160620340.13100.7900.2670.0450.150178520480.07190.8720.1470.0450.150192420580.03380.9350.070B C-10400.0550.198254631200.07230.8160.2250.0560.199265831320.05690.8490.1780.0550.199250331500.08210.7950.2580.0550.198248031520.08600.7870.2710.0560.199251631450.07950.8000.2500.0540.199242831050.08980.7820.2790.0540.199229131060.11450.7380.356B C-10410.0510.198234931000.10310.7580.3200.0510.198230831070.11140.7430.3460.0500.198226930890.11700.7350.3610.0490.198219330460.12770.7200.3890.0480.198223030420.11970.7330.3640.0450.197245629440.06750.8340.199C C-20000.0440.165160520780.14180.7720.2950.0450.165192220910.04210.9190.0880.0450.165197220880.02820.9440.0590.0460.165173821210.10390.8190.2200.0450.165166521060.12580.7910.2650.0450.165172021080.10700.8160.2260.0440.165189520900.04920.9070.1030.0450.165180521040.07870.8580.166C C-20010.0400.165172120250.08720.8500.1770.0400.165161420270.12620.7960.2560.0400.165164220300.11640.8090.2360.0400.165163620230.11690.8090.2370.0410.165164620340.11590.8090.236D C-15200.0440.182163220810.13220.7840.2750.0440.182162020680.13370.7830.2770.0440.182162820770.13280.7840.2760.0440.182194320850.03510.9320.0730.0450.182194820900.03490.9320.0730.0440.182178020740.07960.8580.165Fig.3.DHW heaters—fouling factor vs.coldfluid velocity.Statistical parameters(correlation ratio–CR and root-mean-square deviation–RMSD)were used for quality check of the presented correlation(16)and it can be concluded that they are quite good:CR=0.965and RMSD=16.04%.Correlation(16)is graph-ically presented in Fig.4.Minimal measured value R2,Y1is∼0.05m2K/kW and it cor-responds to shear stress value of∼50Pa.This is the lowest recommended value of shear stress in[17,18].The conclusion is that the correlation(16)is limited with these values:50Pa for shear stress and0.05m2K/kW for fouling resistance.Similar experimentally gathered results were published in [24,26]and elsewhere but they are obtained in the laborato-ries.This time the dependency of the fouling factor on shear stress is proven to be useful in an industrial scale experi-ment.Maximal measured values of R2,Y1are between0.25and 0.30m2K/kW,and they are obtained at very small DHW veloci-ties∼0.08m/s.These R2,Y1are very similar to the fouling factors obtained on shell-and-tube heat exchangers[22,23],due to small shear stress.Having on mind previously described behavior of DHW heat exchanger,we assume that measured data and the correlation(16)S.B.Geni´c et al./Energy and Buildings50(2012)204–211209 Table6DHW heaters—results of measurements.DHS code Heat exchanger Year u1(m/s)u2(m/s)k(W/(m2K))k c(W/(m2K))R tot(m2K/kW)CF DMA A-11300.3470.174439362700.06810.7010.4270.3490.188449464230.06680.7000.4290.3450.181429763720.07580.6740.4830.3450.183409464000.08800.6400.5630.3440.188411964340.08740.6400.562A A-11310.2160.244364763470.11660.5750.7400.2150.275375465140.11290.5760.7350.2130.274421165220.08410.6460.5490.2140.267401564840.09480.6190.6150.2140.275389365250.10360.5970.676B A-8900.1690.169435254260.04550.8020.2470.1670.180412654930.06030.7510.3310.1710.185369655460.09030.6660.5010.1690.178453854380.03650.8340.1980.1690.180456654570.03580.8370.1950.1710.165401454050.06410.7430.3470.1670.167426553980.04920.7900.266B A-8910.1560.173306053980.14150.5670.7640.1600.179294654790.15690.5380.8600.1520.169253953040.20530.479 1.0890.1480.177250753180.21080.471 1.1210.1050.169248847320.19060.5260.902C A-8700.0990.102289038380.08550.7530.3280.0880.088282435420.07180.7970.2540.0660.080226531770.12670.7130.4030.0660.0912********.06920.8160.226C A-8710.0880.112201940930.25100.493 1.0270.0890.109202340760.24900.496 1.0150.0950.107210741320.23260.5100.9610.0950.107193941230.27320.470 1.1260.0930.105190840780.27890.468 1.1370.0910.108211040920.22960.5160.939D B-11300.0660.0711********.22620.7130.4020.0630.0691********.22660.7160.3960.0630.064119616990.24750.7040.4210.0630.067122917280.23500.7110.406D B-11310.0610.0731********.32110.6400.5620.0610.072115017440.29620.6590.5170.0600.0771********.31670.6410.5600.0610.0731********.30430.6520.5340.0600.080114717860.31190.6420.5570.0660.0851********.29860.6420.5580.0650.0811********.33130.6220.609Table7Averaged values for RW heat exchangers.DHS code Heat exchanger Year u1,av(m/s)u2,av(m/s)R tot,av(m2K/kW)CF av DM avA C-22200.0460.1490.0820.860.17A C-22210.0460.1500.0980.840.20B C-10400.0550.1990.0830.800.26B C-10410.0490.1980.1080.750.33C C-20000.0450.1650.0850.850.18C C-20010.0400.1650.1130.810.23D C-15200.0440.1820.0910.850.19 Table8Averaged values for DHW heat exchangers.DHS code Heat exchanger Year u1,av(m/s)u2,av(m/s)R tot,av(m2K/kW)CF av DM avA A-11300.3460.1830.0770.670.49A A-11310.2140.2670.1020.600.66B A-8900.1690.1750.0550.770.30B A-8910.1440.1730.1810.520.95C A-8700.0800.0900.0880.770.30C A-8710.0920.1080.2520.49 1.03D B-11300.0640.0680.2340.710.41D B-11310.0620.0770.3110.640.56210S.B.Geni´c et al./Energy and Buildings50(2012)204–211Table9Fouling factor vs.wall shear stress for DHW.DHS code Heat exchanger Year u2(m/s)R tot,Y1(m2K/kW)R2,Y1=R tot,Y1−0.048(m2K/kW)2(Pa)A A-11310.2440.11660.068643.520.2750.11290.064954.360.2740.08410.036154.000.2670.09480.046851.460.2750.10360.055654.36B A-8910.1730.14150.093522.960.1790.15690.108924.460.1690.20530.157321.980.1770.21080.162823.950.1690.19060.142621.98C A-8710.1120.25100.203010.230.1090.24900.20109.720.1070.23260.18469.390.1070.27320.22529.390.1050.27890.23099.070.1080.22960.18169.56D B-11310.0730.32110.2731 2.910.0720.29620.2482 2.830.0770.31670.2687 3.220.0730.30430.2563 2.910.0800.31190.2639 3.470.0850.29860.2506 3.900.0810.33130.28333.55Fig.4.DHW heaters—fouling factor for DHW vs.shear stress.have to be included in the sophisticated new models in thefield of district heating,like[27]or[28].Just a slight look back on CF and DM data from Tables6and8is enough to reinforce the impression that they cannot be recommend for the heat exchangers design and analysis.5.ConclusionsThis paper presents the results of the research on8plate heat exchangers in district heating system in Belgrade(Sector of the Heat Plant Zemun).The primary factor of interest was the fouling factor for radiator water heaters and for domestic hot water heaters.Heat exchange parameters were tested soon after the substations started to work and again after one year of service.It was found that the fouling factor for radiator water heaters is changing in narrow range,so the value0.048m2K/kW can be used with significant confidence.In case of domestic hot water heaters it was noticed that the fouling factor strongly depends on the water velocity,or in other words on the coldfluid wall shear stress.The following correlation was found to be a good one after the one year exploitation period R DHW,Y1=0.4×10−3+0.28×10−3·exp(−0.06· DHW)and it should be used having in mind that the minimal measured value is∼0.05m2K/kW that is equivalent to shear stress∼50Pa.Shear stress value of50Pa was proven to be a lower limit for design of plate heat exchangers that are working withfluids of high fouling tendency.Practical domain of presented research is obvious;obtained fouling factors can be used for design of plate heat exchangers in district heating system,or for estimation of the period of heat exchanger cleaning.AcknowledgementWe thank the Ministry of Science and Technological Develop-ment of Serbia for a partial support to this study through the Project of Energy Efficiency.References[1]I.Bajsi´c,M.Bobiˇc,Modeling and experimental validation of a hot water supplysubstation,Energy and 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