Tenologia/Tenology EVALUATION OF ALTERNATIVE ONFIGURATIONS OF A WATER-OIL HEAT EXHANGER SYSTEM A. L. V. Gonçalves a, and A. S. Franiso b Universidade Federal Fluense (UFF) Esola de Engenaria Industrial Metalúrgia de Volta Redonda Avenida dos trabaladores, 420 Bairro Vila Santa eília Volta Redonda, RJ, Brasil EP 27255-125 a alvg.velloso@gmail.om b afraniso@metal.eeimvr.uff.br ABSTRAT Te suess in performane of equipment at industrial failities is assoiated wit its apaity of normal operation. Wen some equipment is subjet to maintenane at large ost, te operation is uninteresting or even impratiable. Te eat exanger is an equipment widely employed, wi performs transfer of eat between two fluids at different temperatures, separated by a solid wall. In tis work, we evaluate alternative onfigurations of a system of sell-and-tube eat exangers, aig to improve te performane of tis system in eonomial point of view, and to mitigate te maintenane ost. We use te effetiveness metod to obtain outlet temperature values, and ompare su results wit values measured in situ. We onlude tat alternative onfigurations are feasible in order to improve te termal performane of te system of eat exangers, reduing installation ost. NOMENLATURE Reeived: April 13, 2016 Revised: July 11, 2016 Aepted: September 12, 2016 Keywords: eat exanger, effetiveness metod, termal performane INTRODUTION A eat transfer area, m 2 termal apaity of te ot fluid, J/(kg.K) termal apaity of te old fluid, J/(kg.K) imum termal apaity, J/(kg.K) imum termal apaity, J/(kg.K) N number of eat transfer units (NUT) atual eat transfer rate, J/s largest possible eat transfer rate, J/s T inlet temperature of te ot fluid, K,1 T,1 inlet temperature of te old fluid, K U overall eat transfer oeffiient, W/(m 2.K) Greek symbols Subsripts parameter effetiveness old fluid ot fluid imum imum 1 inlet 2 outlet In tis work, te effetiveness metod is applied for evaluating alternative onfigurations of a system of eat exangers, aig to improve its termal performane. Sine te start of operation, su system is subjeted to inlay penomena, wi ause loss of termal performane of te system (Tonin, 2003). We suggest four alternative onfigurations for te system in order to improve te termal performane of an existing onfiguration. We apply te effetiveness metod to obtain outlet temperatures of te existing onfiguration. Te metod is validated by omparisons of its results wit data in situ. Next, we obtain te outlet temperatures of te four alternative onfigurations, applying te validated metod. Te implementation of te four alternative onfigurations as provided improvements in termal performane of te system. However, just two of te alternative onfigurations ave been eonomially feasible relative to te installation ost for arrying out te modifiations in te system. THE SYSTEM OF HEAT EXHANGERS In some emial plant in wi is neessary to disard oil at low temperatures, a ooling proess is arried out troug a system of eat exangers tat takes ooling water from a river. Te system onsists of a set of ten sell-and-tube eat exangers, lassified as BES type in ompliane wit te ode ASME (2007). Engenaria Térmia (Termal Engineering), Vol. 15 No. 2 Deember 2016 p. 03-07 3
Tenologia/Tenology Gonçalves and Franiso. Evaluation of Alternative Te oil gets into te system of eat exangers at temperature about 112 and omes out at temperature about 36. In ea eat exanger, te oil flows in four passes on te tube side, and proeeds to te next eat exangers in series, as sown in Fig.1. Te water flows on te sell side, entering in ea pair of eat exangers in parallel. 1st Pair 2nd Pair Figure 1. Te existing onfiguration of te system of eat exangers. For te existing onfiguration of te system of eat exangers, strutural and proess data are available, su as geometri dimensions of eat exanger, inlet temperatures, inlet pressures, oil and water properties. Table 1 sows te geometri dimensions of one eat exanger. Table 1. Sizes in ea eat exanger. Desription Value Spaing between tube 0.006045 m Tube lengt 5.77 m Lengt of te tube bundle 5.77 m Outside diameter of te sell 1.275 m Outside diameter of te tubes 0.025705 m Internal diameter of te sell 1.25 m Internal diameter of te sell 0.024705 m Spaing between te baffles 0.3 m Tikness of te tubular wall 0.00165 m Number de baffles 16 Number of passes on te sell 1 Number of passes in te tubes 4 Number of tubes 998 Tubular step 0.03175 m Outer radius of te tube 0.0128525 m Inner radius of te tube 0.0123525 m METHODOLOGY OF ALULUS Effetiveness Metod 3rd Pair 4t Pair 5t Pair In termal analysis of eat exangers tere are two important proedures to be onsidered: te alulation of te termal apaity and te dimensioning of te eat exanger. Aording to Ozisik (2007), te assessment of te termal apaity onsists of te deteration of te eat transfer rate, te outlet temperatures and te pressure drops; wile te dimensioning onsists of te deteration of te sizes of sell and tubes needed to aieve te operational requirements. Wen te inlet temperatures are known and te output temperatures an be speified or detered immediately by te expressions of te energy balane, te metod of logaritmi mean temperature differene an be used. On te oter side, wen only te inlet temperatures and te flow rates of te fluids are known, te overall eat transfer oeffiient as to be estimated. Aording to Holman (1983), te termal analysis an be easily performed by te use of te effetiveness metod. In Kreit (2003) te effetiveness is defined as follows: (1) were is te atual eat transfer rate, and is te largest possible eat transfer rate. Tus, te effetiveness for eat exangers an be detered by: AU 1 e (2) were A is te eat transfer area, U is te overall eat transfer oeffiient, of te ot fluid, old fluid, is te termal apaity is te termal apaity of te is te imum termal apaity, is te imum termal apaity, and is a parameter defined by: 1 1 (3) For onveniene, in pratial appliations a dimensionless parameter, te number of eat transfer units (NUT), is defined by: AU N (4) Tus, aording to Ozisik (2003), te equation (2) an be rewritten for eat exangers in parallel streams as follows: N (1 1 ) 1 e (5) (1 1 ) Similar expressions an be find and te NUT relationsips an be developed in eat exangers aving oter arrangements, su as 4 Engenaria Térmia (Termal Engineering), Vol. 15 No. 2 Deember 2016 p. 03-07
Tenologia/Tenology Gonçalves and Franiso. Evaluation of Alternative ounter urrent, ross urrents and multiple passes. Table 2 sows te relationsips of te effetiveness for sell-and-tube eat exangers wit multiple passes. Table 2. Relations of te effetiveness for eat exangers sell and tube (Holman, 1983). Flow onfiguration One pass in te sell and 2, 4 and 6 passes in te tubes All exangers, = 0 Relationsip 1 1 2 (1 ) 2 N 2 1 e 1 2 21 (1 ) 1, 2 N (1 ) 2 1 e 1 e N Te NUT relationsips an easily be used for solving termal alulation and dimensioning problems. Te eat transfer rate an be alulated as follows: ( T T 1) (6) were T, 1 is te inlet temperature of te ot fluid and T, 1 is te inlet temperature of te old fluid. Tus, te outlet temperatures are obtained from te following equations: and,1, T, 2 T, 1 (7) T 2 T, 1, (8) Proedure for alulating outlet temperatures For alulating outlet temperatures, we establis an iterative proedure in wi material properties (visosity, termal ondutivity and density) are initially onsidered at inlet temperature for te fluids. Despite te overall eat transfer oeffiient an vary due to anges in material properties and flow onditions (Inropera and Witt, 2008), in many appliations su variation is not signifiant so tat is reasonable to work wit average values. By applying te effetiveness metod, we alulate te outlet temperatures. Ten atual material properties are onsidered at average fluid temperatures of inlet and outlet sides. Based on te atual material properties, te alulation of outlet temperatures is performed again. Tis iterative proedure is arried out until onvergene on te outlet temperatures is verified. As te system of eat exangers is onstruted by eat exangers in series, te outlet temperatures of previous eat exanger are te inlet temperatures of posterior eat exangers. RESULTS AND DISUSSION Validation of results for te existing onfiguration In Table 3 is presented outlet temperatures for te oil at te end of ea pair of eat exangers, for te existing onfiguration. Results from te effetiveness metod and te data measured in situ an be ompared in order to validate te metod. Table 3. Outlet temperatures for te oil. OUTLET TEMPERATURES (º) Exangers alulated Measured 1st pair 84.036 77 84 2nd pair 63.953 62 66 3rd pair 50.683 55 59 4t pair 41.932 37 41 5t pair 36.243 36 40 Te outlet temperature alulated by using te effetiveness metod mat te temperature range measured in situ. An exeption for te tird pair is observed (te error is about 8%). Te results for te existing onfiguration using te effetiveness metod an be onsidered valid in relation to te data in situ. Tus, te metod an be applied for evaluating te performane of alternative onfigurations of te system. Alternative onfigurations of te system For evaluating te performane of te system of eat exangers, four alternative onfigurations will remain all geometri dimensions and omponents of ea eat exanger, just modifying operational onditions and flow looping. In te first onfiguration, te water enters in ea pair of eat exangers in parallel. Te water gets into bot eat exangers at 26, as illustrated by te piture on te rigt side of te Fig. 2. Existing onfiguration Figure 2. Proposed water looping. In te seond onfiguration, te modifiation is made in mass flows. Te water and oil mass flows Proposed onfiguration Engenaria Térmia (Termal Engineering), Vol. 15 No. 2 Deember 2016 p. 03-07 5
Tenologia/Tenology Gonçalves and Franiso. Evaluation of Alternative are inreased of 50% (te values are sowed in te Table 4). All oter parameters remain as te existing onfiguration. Te mass flows affet strongly te termal performane of te system. Table 4. Inreasing mass flows in te onfiguration. Mass Flows (kg/s) Existing System 50% inreased in te sell 83.34 125.01 in te tubes 47.41 70.71 Tis onfiguration is proposed due to te mass flows diretly affet te onvetive eat transfer oeffiients in bot sell and tubes, tereby influening te effiieny of te system of eat exangers. Te 50% inrease in te mass flows is proposed sine small inreases get a better performane of eat exangers. Terefore, we expet a signifiant improvement in te performane wit te inrease of mass flows. Te tird onfiguration is a ombination of te previous two onfigurations. Tat is, water at 26 enters in ea pair of eat exangers in parallel, as illustrated on te rigt of te Fig. 2, and te mass flows of te fluids are inreased of 50%, as sown in Table 4. Te fourt onfiguration onsists of anging te diretion of te water flow. On te ontrary of te existing onfiguration, te water first gets into te even eat exangers, and next gets into te odd eat exangers, as illustrated in te Fig. 3. Tis modifiation permits tat average temperatures of te water approaes to te average temperatures of te oil, improving te eat transfer in ounter flow eat exangers. Existing onfiguration Figure 3. Proposed water diretion. Installation osts for alternative onfigurations In tis setion, we present te installation osts for te alternative onfigurations based on urrent values referened to te market. Tese values may be subjeted to anges from time to time. For implementing te first onfiguration are needed strutural modifiations, basially onerning te tubes. Te seond onfiguration requires te replaement of water and oil pumps, wat means iger material osts. Te tird onfiguration brings Proposed onfiguration togeter te previous osts. Te fourt onfiguration is te simple and eapest one. Te installation osts for te alternative onfigurations an be seen in te Table 5. Table 5. Te installation osts for te onfigurations. Performane of te onfigurations A urve of outlet temperatures of te oil is presented in te Fig. 4. Te goal of te system of eat exangers is to aieve outlet temperatures for te oil less tan 36, aording to te operational ondition of te emial plant. Te first and seond onfigurations ave te outlet temperatures less tan 36 at end of te 4t pair of eat exangers, wi means tat te last pair an be taken away or operated in stand-by. Tis represents a great eonomy in terms of maintenane ost. urrently, maintenane in te existing onfiguration is arried out at six-mont time interval, for ea pair of eat exangers. Despite of all alternative onfigurations ave sueeded in general, anoter parameters must be investigated. For example, te inlay proess ause trapping in tubes, and onsequently inreases te maintenane osts. So te system must be prevented against tis proess. Aording to Fiorentin (2004), te inlay proess an arise from emial reations. An inrease in temperatures ontributes to te inrease in reation rate, and in te inlay proess. In te first onfiguration, te water enters in ea eat exanger in parallel providing low temperatures in tubes, and dereasing te inlay proess. In te fourt onfiguration, te inlay proess dereases sine te lowest temperatures are observed in te ounter rossing flow. In te seond and tird onfigurations, te inreasing mass flows provide an inrease in flow veloities, and onsequently a derease te inlay proess. ONLUSIONS Hand labor ost (R$) Material Total (R$) 1st alternative 47,051.20 36,029.55 83,080.75 2sd alternative 43,820.48 235,000.00 278,820.48 3rd alternative 90,870.68 271,029.55 361,900.23 4t alternative 11,762.80 12,009.85 23,772.65 All te alternative onfigurations ave presented improvements in termal performane of te system of eat exangers, reduing te outlet temperatures for te oil. However, te seond and tird onfigurations require ig implementing osts. Te fourt onfiguration as te lowest implementing osts, but also te lowest improvement in termal performane. 6 Engenaria Térmia (Termal Engineering), Vol. 15 No. 2 Deember 2016 p. 03-07
Outlet Temperatures (º) Tenologia/Tenology Gonçalves and Franiso. Evaluation of Alternative 110.0. 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 Existing Alternative 1 Alternative 2 Alternative 3 Alternative 4 Objetive Tonin, P.., 2003, Metodologia para Monitoramento do Desempeno Térmio de Redes de Troadores de alor, Master Tesis, entro Federal de Eduação Tenológia do Paraná, uritiba, PR. (in 20.0 1 st 2 nd 3 rd 4 t 5 t 6 t 7 t 8 t 9 t 10 t Exangers Figure 4. Outlet temperatures of te oil in ea eat exanger. Te first onfiguration is te best one, sine it as sowed low installation ost ombining wit reasonable termal performane. Te implementation of tis alternative onfiguration ould provide a derease in te inlay proess, and allows te system to be operated wit one pair of eat exangers in stand-by, saving te maintenane osts. Aording to Madi (2005), supervising te performane of eat exangers is a good way of quantifying te osts related to inlay proess and identifies te appropriate period of leaning. Follow tat, te first onfiguration an provide a great advantage regarding to te maintenane, witout ompromising prodution. REFERENES ASME, 2007, Rules for onstrution of Pressure Vessels, setion VIII, Div. 1. Fiorentin, L. D., 2004, Estudos em Modelos Matemátios para Previsão de Inrustação por oque em Tubulações, Master Tesis, entro Federal de Eduação Tenológia do Paraná, uritiba, PR. (in Holman, J. P., 1983, Transferênia de alor, MGraw-Hill. (in Kreit, F., and Bon, M. S., 2003, Prinípios de Transferênia de alor, Pioneira Tomson Learning. (in Inropera, F. P., and Witt, D. P., 2008, Fundamentos de Transferênia de alor e de Massa, Livros Ténios e ientífios Editora S.A. (in Madi, M., 2005, Otimização do Período de Limpeza de Troadores de alor Utilizados no Refino de Petróleo, Master Tesis, entro Federal de Eduação Tenológia do Paraná, uritiba, PR. (in Oliveira, G. A., 2010, Dimensionamento de um Troador de alor Tipo aso e Tubos, Undergraduate Tesis, Universidade Federal de Uberlândia, Uberlândia, MG. (in Ozisik, M. N., 2007, Transferênia de alor: um Texto Básio, Editora Guanabara. (in Engenaria Térmia (Termal Engineering), Vol. 15 No. 2 Deember 2016 p. 03-07 7