Transcription

1 Journal o Amerian Siene, 22;8(3 Predition o Thikness and Fouling Rate in Plate Heat Exhanger o MTR Reator A.A. Fahmy, M.M. EL Fawal 2 and B.M. Taher 3 Reators Department, Nulear Researh Center, AEA, Cairo, Egypt 2 National Center or Nulear Saety and Radiation Control, Atomi Energy Authority 3 Dept. o Engineering Physis and Mathematis, Faulty o Engineering, Cairo niversity, Egypt Mohamed_elawal@hotmail.om Abstrat: Heat exhangers are important and essential omponents in nulear reators and power plants. In this ontext, studying the perormane o heat exhanger under normal and abnormal operational onditions is o great importane relevant to the eonomi and operational saety in power plants. Fouling and sale ormation in heat exhangers ould have serious impats on the operating onditions o the nulear reators. This study aims at the simulation o ouling rystallization proess in plate-type heat exhanger in MTR reator by developing an Engineering Equation Solver (EES Program. The inding o this work would enable us to evaluate the thikness and ouling rate in plate-type heat exhanger in MTR reator. The rystallization ouling o alium sulphate (CaSO 4 in plate heat exhanger was also investigated. Also, the eet o luid veloity on ouling resistane and the rate o deposit thikness were studied in eah o primary and seondary iruits. [A.A. Fahmy, M.M. EL Fawal, and B.M. Taher Predition o Thikness and Fouling Rate in Plate Heat Exhanger o MTR Reator. Journal o Amerian Siene 22; 8(3: ].(ISSN: Keywords: Fouling; Heat exhanger; Fouling Resistane; Calium Sulphate.. Introdution Plate type heat exhangers (PHEs are one o the most eiient types o heat transer equipment. This equipment is muh more ompat, requires muh less materials or prodution, muh smaller ootprint, than onventional shell and tube unit. PHEs have a number o advantages, suh as ompatness, low total ost, less ouling, lexibility in hanging the heat transer area, aessibility and energy saving. PHE onsists o a set o orrugated heat transer plates lamped together. In multi pass PHE plates are arranged in suh way, that they are orming groups o parallel hannels. Suh group is orresponding to one pass and the stream is going onsequently through the passes, as shown in Fig. (, whih presents the main omponents o a gasket plate heat exhanger. Nowadays, PHEs are used worldwide in all industries suh as petroleum industry, ood industry, pharmaeutial, nulear reators and material industry by replaing shell and tube heat exhangers. PHE is a type o heat exhanger that uses metal plates to transer heat between two luids. This has a major advantage over a onventional heat exhanger in that the luids are exposed to a muh larger surae area beause the luids spread out over the plates. This ailitates the transer o heat, and greatly inreases the speed o the temperature hange. It is not as ommon to see plate heat exhangers beause they need well-sealed gaskets to prevent the luids rom esaping, although modern manuaturing proesses have made them easible. Plate-type heat exhangers also have higher heat transer eiieny; this eature enables smaller platetype heat exhangers to have the same heat transer apability as larger shell and tube devies. However, the slow progress made in developing a dependable method o sealing the gaskets loated between the plates limits its use or high-pressure appliations. Thereore, plate-type heat exhangers work only or low-pressure systems. Improvements in the seal tehnology do allow or the replaement o older shell and tube exhangers or more eiient plate models. The advantages o plate heat exhangers are: higher thermal ondutivity, higher auray o temperature ontrol, lower o ontat time, lower onstrution area, and easier or leaning than shell and tube heaters []. However, disadvantage o plate heat exhangers is ouling ormed by CaCO 3 at high operating temperature that aused by low pattern in plate heat exhanger. Fouling is deined as the aumulation o unwanted materials on the surae o heat exhanger. It has a lot o problems in design and operation suh as the low thermal ondutivity due to ouling layer and, the redution o ross-setional area as deposition ours, that auses an inrease in pressure drop. Partiulate ouling has a major problem when sea water and under ground water that ontain high amount o alium ion were used [2-4]. Epstein N. [2] lassiies ouling into six types:. Partiulate ouling, 2. Reation ouling, 3. Corrosion ouling, 377

2 Journal o Amerian Siene, 22;8(3 4. Preipitation ouling, 5. Biologial ouling and 6. Solidiiation ouling. Fig. : Main omponents o gasket plate and rame heat exhanger. Fouling aumulation on heat transer impedes the heat transer and inreases the resistane to luid low, resulting in higher pressure drop. Industrial heat exhangers rarely operate with non ouling luids. The growth o deposits auses the thermo-hydrauli perormane o heat exhanger to degrade with time. Fouling aets the energy onsumption o industrial proess, and it an also deide the amount o extra material required to provide extra heat transer suraes employed in heat exhanger to ompensate or the eets o ouling. In addition, where the heat lux is high, ouling an lead to loal hot spots and ultimately it may result in mehanial ailure o the heat transer suraes. So designers o heat exhanger have to take into aount ouling eet and selet ouling oeiient in the urrent heat exhanger design. Sine the ouling on heat transer surae an signiiantly deteriorate the perormane o any heat exhanger, it should be orretly predited in alulation o required surae area at the design stage. This is even more important or PHEs with enhaned heat transer whih as a rule have muh high ilm heat transer oeiients than onventional shell and tube units. Analysis o data available in literature has shown that thermal resistane o Fouling in PHEs is up to ten times lower than reommended by Tubular Exhanger Manuaturers Assoiation (TEMA or shell and tubes working at same onditions as shown in Table [ and 5]. Table : Liquid-Side Fouling Resistanes or PHEs vs. TEMA Values [] Proess Fluid R, (m2 K/kW PHEs TEMA Cooling tower water Steam (oil bearing.9.8 Seawater River water Lube oil Organi solvents Sot water Perormane data As mentioned above, ouling has many eets on the heat exhanger perormane. It dereases the exhanger thermal apaity and inreases the pressure drop through the exhanger as shown in Fig. (2 [6]. From the igure, it is lear that the total resistane to heat transer is dereased during the irst stages o ouling due to the surae roughness resulting rom initial deposition. Ater that and with deposits building up, the thermal resistane returns to inrease again. 378

3 Journal o Amerian Siene, 22;8(3 resistane and the rate o deposit thikness were also studied in eah o primary and seondary iruits. Fig. 2: Fouling eets on exhanger perormane [6] In order to model and predit the industrial proesses ouling problems it is irst neessary to understand what is happening and what are the auses and eets o ouling. To ahieve this, it is neessary to areully examine and evaluate all the data and operating onditions at various plants in order to understand what the variables whih are eetive on ouling and what are the mehanisms o suh phenomena. The possibility o whether the ouling material is a part o the eed to the system or it is a produt o reation / aggregation / loulation in the system must be lariied. The role o various operating onditions suh as pressures, temperatures, ompositions, low rates, et. and their variations in the system on ouling must be understood and quantiied. Only with appropriate modeling onsidering all the possible driving ores and mehanisms o ouling one may be able to predit the nature o ouling in eah ase and develop mitigation tehniques to ombat that. Beause ouling resistane varies with operation onditions and time, it is needed to know the ouling trends o heat transer suraes to determine easible periodi leaning o the heat exhanger. Design engineers beome more interested in theoretial study o predition o the ouling proess in ertain onditions [7]. So some simply and pratial models have emerged to estimate the eet o ouling on heat exhanger [8-2]. Most o the existing models only give the initial ouling rate. Clear guidelines and orrelations to predit the exat value o ouling in PHE on designing stage are absent. So the aim o this work was to simulate the rystallization ouling proess in plate heat exhanger in MTR reator by developing an Engineering Equation Solver (EES Program. Also the ouling in PHEs working with water as both heating and ooling media was investigated. The rystallization ouling o alium sulphate (CaSO 4 in plate heat exhanger and the eet o luid veloity on ouling. 2. Fouling Modeling Fouling is usually onsidered to be the net result o two simultaneous proesses: a deposition proess and a removal proess. The net rate o ouling an be expressed in terms o the rate o deposition o ouling mass ( m d and the rate o removal ( m r rom the heat transer surae. Thus, dm dt m d - m r ( One o the earliest models o ouling was that by Kern and Seaton []. In this model, it was assumed that ( m d remained onstant with time (t but ( m r was proportional to ( m and thereore inreased with time to approah ( m d asymptotially. m d m (2 Then integration o Eq. ( rom the initial ondition m = at t = gives t m m ( e (3 Where is the asymptoti value and. m The time onstant t represents the average residene time or an element o ouling material at the heating surae. Equation (3 an be expressed in terms o ouling resistane R at time (t in terms o the asymptoti value R by: t R R ( e (4 Where, R is the ouling resistane (m 2. C/W. R is the asymptoti ouling resistane. (m 2. C/W. t is the duration time o deposit. t is the time onstant, (hr and is given by. t = / k 2 λ (5 t 379

4 Journal o Amerian Siene, 22;8(3 Where, k 2 is the removal onstant. is the luid shear stress exerted on the deposit surae. is the strength or toughness o the deposit layer. The ouling resistane is not usually measured diretly, but must be determined rom the degradation o the overall heat transer oeiient. The ouling ator R ould be expressed as: R or R x ( (6 (7 C From the deposition removal model, whih was irst presented by Kern and Seaton [], the overall heat transer oeiient o the ouled surae, may be given as: Then, C (8 R C t R t e (9 The rate o heat transer in a heat exhanger under lean and ouled onditions is given by the q A Tm A Tm ( Thereore, A A ( Combining Eqs. (5 and (7, then A A R (2 Another important ator whih is related to the ouling resistane is the leanliness ator, CF whih is given as: CF R (3 Watkinson [2] reported the eet o luid veloity on the asymptoti ouling resistane in the ase o alium sulphate saling as ollows: R v. D (4 Where, v is the luid veloity. Sand deposition rom water may be alulated as ollows:.5 R (5.2 v Program has been omposed using the above equations to alulate the deposit weight and the variation o deposit thikness and ouling resistane with time or both primary and seondary ooling iruits o MTR reator. Also the variation o luid shear stress with luid veloity or primary and seondary iruits is also investigated. The eet o time and veloity on asymptoti ouling resistane has been also investigated. The obtained results are shown below. 3. Results and Disussion A new heat mass transer model was developed to predit the ouling proess o alium arbonate on heat transer surae based on Kern - Seaton model. The present model took into aount the eet o operating parameters like, time, onentration and luid veloity on the sale ormation o alium sulphate. Also the eet o onentration on alium sulphate deposition has been studied. The obtained results show that the deposition weight o alium sulphate inreases with the inrease o onentration due to the inrease o luid partiles stikiness probability with low surae as shown in Fig. (3. Figs. (4 and 5 illustrate the variation o deposit thikness with time or seondary and primary iruits o the heat exhanger, it is lear rom these igures that ouling is a time dependent proess; hene the deposit thikness inreases with inreasing time due to the inrease o deposit weight o the oulant. Also the alulated results show that ouling resistane o primary side inreases with time as shown in Fig. (6. This ould be attributed to the aumulation o the suspended partiles on the 38

5 Journal o Amerian Siene, 22;8(3 lowing surae. Hene the thikness o the oulant inreases, so its resistane inreases. Figs. (7 and 8 show that the luid (water shear stress inreases with inreasing o luid veloity in the primary and seondary sides. As the ouling deposit builds up, the ross-setional area or low dereases, thus ausing an inrease in the average veloity o the luid or a onstant mass low rate and inreasing the shear stress. Higher shear stress promotes dislodging o deposits rom surae. No doubt that, the luid veloity is an important parameter in the sale ormation proess. It an either enhane the diusion o the oulant speies towards the heating surae or aelerate the removal o sale deposit rom it. The variation o the asymptoti ouling resistane o alium sulphate with time or dierent lowing veloities was investigated and shown in Fig. (9. It was ound that oulants suspended in the proess luids will deposit in low veloity regions hene, at low veloity the oulants suspended in the luid tend to stik with heat transer area, so the ouling thikness inrease with dereasing luid veloity, hene inreasing the ouling resistane. The asymptoti ouling resistane or primary and seondary iruits was studied with respet to the veloity variation. It was ound that the inrease o low veloity tends to inrease the thermal perormane o the exhanger and derease the ouling rate. The asymptoti ouling resistane value dereases with the inrease o veloity till about.2 m 2. C /W at.8 m/s or primary side and till about.2 m 2. C /W at 6 m/s or seondary side as shown in Figs. ( and respetively. Deposit Thikness, (m,225,2,75,5,25,,75,5,25 Deposit Thikness at primary side Time,(hr Fig. 5: Variation o deposit thikness with time or primary iruit R, ( m 2. C/ W Fouling resistane at primary side 2, 4, 6, 8,, 2, Tim e, (hr Fig. 6: Variation o ouling resistane or primary side with time. shear stress, (N/m2 6,5 6 5,5 5 4,5 4 Shear stress or pimary side.5 Deposit Thikness at seondary side 3,5 Deposit Thikness, (m ,2,4,6,8 2 Veloity, (m/s Fig. 7: Variation o luid shear stress with lowing veloity or primary side. 2, 4, 6, 8,, 2, Time,(hr Fig. 3: Variation o deposit weight with onentration. Deposit weight,(g/l Primary side and alium sulphate onentration,(ppm Fig. 4: Variation o deposit thikness with time or seondary iruit. shear stress, (N/m Shear stress or seondary side Veloity, (m/s Fig. 8: Variation o luid shear stress with lowing veloity or seondary side. 38

6 Journal o Amerian Siene, 22;8(3 R*, ( m2. C/ W R*, ( m2. C/ W R, ( m 2. C/ W,45,4,35,3,25, R,(m 2. C/W.2. 2, 4, 6, 8,, 2, Calium Sulphate v =.4 (m/s v =.8 (m/s v = 2 (m/s Model Re ( Time, (hr Fig. 9 : Variation o asymptoti ouling resistane with time at dierent lowing veloity.,5,2,4,6,8 2,6,4,2,,8,6,4,2 Veloity, (m/s Time, (hr Calium Sulphate veloity in the primary hannel Fig. : Variation o asymptoti ouling resistane with lowing veloity or primary side. Sand deposition rom seondary water side Veloity, (m/s veloity in the seondary hannel Fig. : Variation o asymptoti ouling resistane with lowing veloity or seondary side Comparisons o the model with experimental data The variation o the asymptoti ouling resistane with time alulated using the urrent model was ompared with the available experimental data o a typial two pass heat exhanger given by Hasson [6] as shown in Fig. (2. This exhanger had 66 tubes o 9.5 mm outside diameter and 6. m. The omparison shows that both alulated and experiment results have the same trend with a deviation whih ould be attributed to the dierene between the exhanger type, the operating ondition and also exhanger materials, so the new model ould give a good predition o the ouling proess. Fig. 2: Comparisons o obtained results o present model and experimental data o re [] 4. Conlusion A new heat mass transer model was developed to predit the ouling proess o alium arbonate on heat transer surae based on Kern- Seaton model. The eet o operating parameters like, time, onentration and luid veloity on the sale ormation o alium sulphate has been taken into onsideration in the present model. Also onentration eet on alium sulphate deposition has been studied. Comparative results o the model with experimental data showed that the present model, has the same trend and in good agreements with results investigated by others, so the new model ould be redible to predit the ouling proess. Also rom the obtained results we ould onluded that the sale ormation in water is time dependant and has an asymptoti approah.. The veloity o low an redue eetively the sale deposit due to the strong eet on the removal mehanism o the sale layer. And with inreasing o water veloity in the seondary side the asymptoti value o sand deposition dereases. Nomenlature: A Heat transer surae area (m 2 A C Heat transer surae area or lean ondition (m 2 A Heat transer surae area or ouled ondition (m 2 m d Deposition (kg/m 2.s m r Removal rate (kg/m 2.s m Aumulate mass (kg/m 2 R Fouling resistane (m 2. C/W R Asymptoti ouling resistane (m 2. C/W t Time (s t Time onstant (s C Overall heat transer oeiient in lean ondition (W/m 2. K Overall heat transer oeiient in ouled ondition (W/m 2. K v luid low veloity (m/s D hydrauli diameter (m x ouling layer thikness (m q Heat lux (W/m 2 Tm Logarithmi mean temperature dierene ( C MTR Material Testing Reator Greek letters: β Fator (/hr λ Thermal ondutivity o ouling layer (W/m.K 382

7 Journal o Amerian Siene, 22;8(3 τ Fluid shear stress (N/m 2 Ψ Deposition strength ator Subsripts: lean ondition. d deposition. ouled ondition. r removal. Corresponding author M.M. EL Fawal National Center or Nulear Saety and Radiation Control, Atomi Energy Authority Mohamed_elawal@hotmail.om Reerenes: ( S,M,Zubair and R.K.Shah, "Fouling in Plate-and Frame Heat Exhangers and Cleaning Strategies",in Compat Heat Exhangers and Enhanement Tehnology or the Proess Industries, (R. K. Shah, A. Deakin, H.Honda and T. M. Rudy, eds., Begell House, NewYork, pp , (2. (2 S.H.Lee and J. G. Knudsen, Saling Charateristis o Cooling Tower Water," ASHRAE Trans. 85 (Part. pp , (989. (3 A.P. Watkinston, "Water Quality Eet on Fouling From Hard Waters" ASHRAE Trans, 85 (Part, pp.28-32, (989. (4 A. Cooper, J. W. Suitor, and J. D.sher," Cooling Water Fouling in Plate Heat Exhanger"Heat Transer Eng. Vol. (3, pp. 5-55, (98. (5 TEMA, "Standard o the Tubular Exhanger Manuaturers Assoiation", 8th ed.,tubular Exhanger Manuaturers Assoiation, New York, (999. (6 M.. M. Awad, Fouling o Heat Transer Suraes Heat Transer-Theoretial Analysis, Experimental Investigations and Industrial Systems. (7 N.Epstein, "Fouling:Tehnial Aspet", In Fouling o Heat Transer Equipmen", eds. Somersales, E.F.C., and Knudesen, J. G., Hemisphere Publishing Corp., New York, pp. 3-53, (98. (8 T. R. Bott., "Fouling o Heat Exhangers", Elsevier Siene & Tehnology Books, (995. (9 W.T.Kim, Ch.Bai, and Y.I.Cho, "A Study o CaCo3 Fouling with A Mirosopi Imaging Tehnique", International Journal o Heat and Mass Transer, 45, , (22. ( S.H.Chan, K.F.Ghassemi, "Analytial Modeling o Calium Carbonate Deposition or Laminar Falling Films and Turbulent Flow in Annuli: Part I Formulation and Single Speies Model", J.Heat Trans. 3, (99. ( D.Q.Kern and R.E.Seaton,"A theoretial Analysis o Thermal Surae Fouling",Chem. Eng. Prog., vol. 55, no. 6, pp. 7 73, (959. (2 A.p.Watkinson, A.p.,"Proess Heat Transer: Some Pratial Problems",Can. J. Chem. Eng., 58, pp , (98. (3 S.M.Zubair and R.K.Shah, "Fouling in Plate and Frame Heat Exhangers and Cleaning Strategies",in Compat Heat Exhangers and Enhanement Tehnology or the Proess Proess Industries, (R. K. Shah, A. Deakin, H. Honda and T. M. Rudy, eds., Begell House, New York, pp , (2. (4 Awad, M. M,et al,"eet o Flow Veloity on the Surae Fouling", Mansoura Engineering Journal (MEJ, Vol.32, No, pp M27- M37, Marh, (27. (5 N.Andritsos, "Calium Carbonate Saling in a Plate Heat Exhanger in the Presene o Partiles" International Journal o Heat and Mass Transer, 46, ,(23. (6 D.Hasson, et al "Mehanism o Calium Carbonate Sale Deposition on Heat Transer Suraes",Ind Eng. Chem. Fund.7 ( (7 B.Bansal, H.M.Muller-Steiinhagen and X.D.Chen, X. D., " Perormane o Plate Heat Exhangers During Calium Sulphate Fouling Investigation with an In line Filter", Chemial Engineering and Proessing, vol. 39, pp.57-59, (2. (8 G.T.Polley, G.G.Garia, Proedure or Applying Fouling Models to Predit Overall Fouling Rates in Industrial Heat Exhanger International Conerene on Heat Exhanger Fouling and Cleaning VIII, Shladening, Austria, pp , June 4-9 (29. (9 A.J. Karabelas et al, 997, Liquid Side Fouling o Heat Exhangers. An Integrated R& D Approah or Conventional and Novel Designs. Applied Thermal Engineering, Vol.7, Nos.8-, pp (997. (2 A.L.Gogenko,etal "Aounting or Fouling in Plate Heat Exhanger Design",www. nt.ntnu.no/users/skoge/prost/pr. 2/2/22 383