Effect of fouling on thermal and hydraulic parameter of Shell and Tube Heat exchanger

Transcription

1 Eect o ouling on thermal and hydraulic arameter o Shell and Tube Heat exchanger Rahul Kr. Gautam, Kumar 1 Nirmal S. Parmar, Biin G. Vyas 3 1 MSc. Process Engineering (ursuing), Czech Technical University, Prague, Czech Reublic PhD Fellow, Czech Technical University, Prague, Czech Reublic 3 Senior Research Fellow, Sardar Patel Renewable Energy Research Institute (SPRERI), Gujarat, India Abstract: Fouling is inevitable henomenon in shell and tube heat exchanger. Even regular water has considered as working luid in shell and tube heat exchanger, it has signiicant imact on thermal and hydronic erormance due to ouling roerties. Amount o layer deosition has direct imact on heat transer and ressure dro, as it aect on hysical dimension o shell and tubes o heat exchanger. The heat transer surace ouls during oeration, resulting in increased thermal resistance and oten an increase in the ressure dro and uming ower. It is required to understand, how mass low rate can aect on deosition rate o ouling layer inside the tube. In this resent aer, the study has been conducted to evaluate otimal oerating condition that lead to lower ouling deosition rate inside the heat exchanger while keeing in mind thermal and hydraulic requirements. Keyword: Fouling; Heat Transer coeicient; Pressure dro; Puming Power; Eect o ouling. 1. Introduction Heat Exchanger is device, which exchange the low o thermal energy between two or more luids by convection mainly at dierent temeratures. The main roblem caused by ouling is related to the act that the accumulation o ouling deosits has an imortant eect on the thermal and hydraulic erormance o the aected equiment. Heat exchangers are used in a wide variety o alication. This includes ower engineering, waste heat recovery, manuacturing industry, air-conditioning, rerigeration and sace alications. Shell and tube heat exchangers are most versatile tye o heat exchanger; they are used in rocess industries, in conventional and nuclear ower station as condenser, in steam generators in ressurized water reactor ower lants, in eed water heaters and in some air conditioning rerigeration systems. Shell and tube heat exchanger rovide relatively large ratio o heat transer area to volume and weight and they can be easily cleaned. Shell and tube heat exchanger oer great lexibility to meet almost any service requirement. Shell and tube heat exchanger can be designed or high ressure relative to the environment and high ressure dierence between the luid streams.. Literature review A detailed study o shell and tube heat exchanger with various mass low rate and ouling actor has been rovided in reerences. Thanhtrung Dang et al. [1] gives idea about theoretical idea o shell and tube heat exchanger heat transer rate and ouling thickness with two cases, investigated in this study, when the thickness o substrate o the heat exchanger increases, its actual heat transer rate decreases. However the accumulation o deosits rate describe by Rima Harcheon et al [] which eect the heat transer surace and inally leads to a reduction o the energy transer eectiveness. Figure 1: the deosition thickness [] The irst art (A) corresonds to the initiation hase. This eriod deends largely on the deosit tye varies ew minutes at a ew weeks, or examle in the air-conditioning systems. The second art (B) corresonds to the increase in this deosit resulting rom a cometition between the mechanisms rom deosit and wrenching. The ouling rate decreases gradually while wrenching increases, inally, leading that to a balance in (C) and a height ouling held Corresonding author: RahulKumar.Gautam@s.cvut.cz

2 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering constant. J.W. Suitor et al. [3] gives idea about theoretical develoments by using kern method concluded that ouling resistance increases indeinitely with time. The deosition term is a comlex unction o velocity, water chemistry, the wall, scale surace and bulk luid temerature. P.J. Frayer et al. [4] demonstrated the design o ouling resistance in which ulsate low system may be eective in many cases, but that antiouling exchangers should not be designed without understanding the ouling mechanism. D. Gulley [5] Studied that, ressure dro is a big hel in analyzing erormance roblems. They also rovide a rough check o low rates. Liljana Markovska et al. [6] Studied that, increased luid velocities result in larger heat transer coeicients and, consequently, less heat-transer area and exchanger cost or a given rate o heat transer. On the other hand, the increased luid velocities cause an increase in ressure dro and greater uming ower cost. Kevin M. Lunsord [7] resented some methods or increasing shell and tube heat exchanger erormance. The method considers whether the exchanger is erorming correctly to begin with, excess ressure dro caacity in existing exchanger, the re-evolution o ouling actor and their eect on exchanger calculations. K.C.Leong and K.C.Toh [8] describes user riendly comuter sotware(htri and HTFS) develoed or the thermal and hydraulic design o shell and tube heat exchanger based on oen literature Bell Delaware method. The use o this sotware will bridge the ga between engineering ractice and teaching o shell and tube heat exchanger design. A rating, sizing and simulation can be erormed in this comuter sotware such as HTRI & HTFS. For estimation o otimal oeration condition o shell and tube heat exchanger these tye o otimization sotware are very useul. D.I.Wilson et al. [9] roosed a semi-emirical aroach to quantiy the eect o low velocity on tube-side ouling in crude oils at high temeratures which ilot lant studies indicated that: (i) Fouling rates increased with increasing temerature initially interreted as ilm temerature, elsewhere as wall/deosit temerature. (ii) Fouling rates decreased with increasing low velocity (igure 1) By using the ouling model and the concet o the ouling threshold ouling rate in heat exchanger can be redicted. Figure : The Ebert-Panchal model [9] John M. Nesta and Christoher A. Bennett [10] showed the ouling layer is a conductive resistance to heat transer that must be accounted or in the design heat transer coeicient. Also, Fouling thickness and thermal conductivity both contribute to the resistance. Reduced cross-sectional low area also increases ressure dro in the ouled region. M.S. Abd-Elhady et al. [11] Studied the inluence o low direction with resect to gravity on articulate ouling o heat exchangers is investigated exerimentally to determine the otimal low direction to minimize ouling. Four orientations o low have been investigated, horizontal low, uward low, downward low and a low under an angle o 45. E.M.Ishiyama et al. [1].resented an analysis o the thermal and hydraulic imacts o ouling. For ouling rate laws that incororate the threshold ouling concet, characteristic times or ouling o an exchanger were identiied, based on hydraulic or thermal limitations. D.H. Lister and F.C. Cussac [13] showed the ormulation mechanism or the eect o the bubbles on deosition in water under boiling condition. Paer describes the main mechanisms occurring during boiling and to determine how bubbles inluence the deosition o iron oxide articles. Deosition, removal and consolidation o the deosit are included in the model. The relation between heat transer, bubbles ormation near wall and ouling amount is studied. Su Thet Mon Than et al. [14] evaluated design data or heat transer area and ressure dro and checking whether the assumed design satisies all requirement or not. The rimary aim o this design is to obtain a high heat transer rate without exceeding the allowable ressure dro. Arturo Reyes Leon et al. [15] develoed relationshi between heat transerred and energy loss or turbulent low and also identiied the advantages o having the aroriate exchanger with working conditions, environmental conditions and economical asects. In

3 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering addition to thermal design, mechanical design o heat exchanger is also art o it. Rajiv Mukherjee [16] develoed correlation or otimal condition o shell and tube heat exchanger. The otimized thermal design can be done by sohisticated comuter sotware however a good understanding o the underlying rinciles o exchanger design is needed to use this sotware eectively..1 Pressure dro in STHE From the literature review we had carried out that Pressure dro is one o the imortant actors which aect the erormance o STHE. Two methods have been adoted or calculating the Pressure dro inside the STHE based on name o their ormulators 1. Kern ; and. Bell Delaware. In this research we have adoted the ormulation roosed by Kern.. Preliminary calculation A selected shell and tube heat exchanger must satisy the rocess requirements with the allowable ressure dros until the next scheduled cleaning o lant. The methodology to evaluate thermal arameters is exlained with suitable assumtions. The ollowing are the major assumtions made or the ressure dro analysis; 1. Flow is steady and isothermal, and luid roerties are indeendents o time.. Fluid density is deendent on the local temerature only or is treated as constant. 3. The ressure at a oint in the luid is indeendent o direction. 4. Body orce is caused only by gravity. 5. There are no energy sink or sources along streamline; low stream mechanical energy dissiation is idealized as zero. 6. The riction actor is considered as constant with assage low length. Heat transer or the size o heat transer exchanger can be obtained rom equation, For the single tube ass, urely countercurrent heat exchanger, F= For reliminary design shell with any even number o tube side asses, F may be estimated as 0.9 Heat load can be estimated rom the heat balance as: Q = (mc ) c (T c T c1 ) = (mc ) h (T h T h1 ) (3) I one stream changes hases: Q = mh g (4) LMTD (Log Mean Temerature Dierence Method) calculation: I three temeratures are known, the ourth one can be ound rom the heat balance, T lm ( Th 1 Tc ) ( Th Tc 1) ( T T ) h1 c ln ( Th Tc 1 ) (5) Heat transer area can be calculated rom equation (1). Number o tubes o diameter (d o ), shell diameter (D s ) to accommodate the number o tubes (N t ), with given tube length (L) can be estimated, A N L (6) o e t One can ind the shell diameter (D s ), which would contain the right number o tubes (N t ), o diameter (d t ). Q = U o A o T m (1) The overall heat transer coeicient U o based on the O.D. o tubes can be estimated rom the estimated values o individual heat transer coeicients, the wall and ouling resistance and the overall surace eiciency using equation () 1 A 1 R o i Ro 1 AR o w U A h h o i i i i o o o () Figure 3: Square and Triangular Pitch Tube Layout [17] The total number o tubes can be redicted in air aroximation as unction o the shell diameter by taking the shell circle and dividing it by the rojected area o the tube layout (ig 3) ertaining to a single tube A 1.

4 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering N t Ds ( CTP) 4 A i (7) LN Gi P t 4 d (1) i Where CTP is the tube count calculation constant that accounts or the incomlete coverage o the shell diameter by the tubes. Based on ixed tube sheet the ollowing values are suggested: One tube ass: CTP = 0.93 Two tube ass: CTP = 0.90 Three tube ass: CTP = 0.85 A 1 = (CL) (P T ) (8) Where CL is the tube layout constant: CL = 1.0 or 90 and 45 CL = 0.87 or 30 and 60 Equation (7) can be written as: N t CTP D CL (P ) s r do Where P R is the Tube Pitch Ratio (P R = P T /d o ). (9) The shell diameter in terms o main construction diameter can be obtained as rom equations (6) and (9), CL Ao(P r ) d o Ds CTP L.3 Tube side ressure dro 1/ (10) The tube side ressure dro can be calculated by knowing the number o tube asses (N ) and length (L) oh heat exchanger, The ressure dro or the tube side luid is given by equation LN m Pt 4 (11) d i The change o direction in the asses introduction in the asses introduction an additional ressure dro due to sudden exansions and contractions that the tube luid exeriences during a return that is accounted or allowing our velocity head er ass m Pt 4N (13) The total ressure dro o the side becomes: LN Pt 4 4N di m.4 Puming ower and ressure dro (14) The luid uming ower is roortional to the ressure dro in the luid across a heat exchanger, the equation can give by: P mp (15) Where is the um or an eiciency. ( = 0.80 to 0.85) The cost in terms o increased luid riction requires an inut o uming work greater than the realized beneit o increased heat transer. For gases and low density luids and also or very high viscosity luids, ressure dro is always o equal imortance to the heat transer rate and it has a strong inluence on the design o heat exchangers. Pressure dro o shell and tube as well as uming ower can be calculated by these theories. Fouling actor, the eect o ouling on ressure dro and overall heat transer coeicient estimation are exlained in next chater..5 Eect o ouling on heat transer Heat transer rate can be given by, Q = U c A o T M (16)

5 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering Where, U is based on outside heat transer surace area o the exchanger. Fouling coeicient (U ) can be related to the clean surace overall heat transer coeicient (U c )as: The ouling layer decreases the inner diameter and roughens the surace, thus causing an increase in the ressure dro given by equation (0). Pressure dro under ouled and clean condition can be related as: 1 1 Rt (17) U U c P d u c Pc cd uc (4) The heat transer under ouling conditions (Q ) can be exressed as: Q = U A T M (18) Overall heat transer coeicient based on outer surace area under ouled conditions can be obtained by adding the inside and outside thermal resistances. U 1 d o Ao ln A d o Ao i 1 R i R o Ah A kl h i i i o.6 Eect o ouling on ressure dro (19) There is a inite thickness o ouled surace; causes change in geometry due to ouling aects the low ield and the ressure dro. Fouling layer roughness o surace decreases the Inner Diameter and increases the Outer Diameter o the tubes. L P4 dl u m (0) The receding dimensionless grou involving has been deined as the Fanning riction actor : By assuming that the mass low rate (m=u m A) under clean and ouled conditions is the same, equation (4) can be modiied as: P d c Pc c d (5) The ouling actor can be related to the ouling thermal conductivity k and the ouling thickness t as: R R t (For a lane wall) (6) k d c dc ln d k (For a cylindrical tube wall) (7) Inner diameter under ouled conditions, d, can be obtained by rearranging equation (7): d kr dcex dc (8) L u 4 dl m (1) Re or 3x10 4 < Re <10 6 () Re or 4x10 3 < Re <10 5 (3) And the ouling thickness, t, is exressed as: t kr 0.5tc 1 ex dc (9) These theories show the correlation between ressure dro clean and ouled condition. Aects o ouling on heat transer is also discussed which will helul in calculation o overall heat transer coeicient.

6 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering These undamental logics are going to use in otimization rocess. 3. Results and Discussion The result in orm o grah has been generated rom the data roduced by various iterations in HTRI exchanger suit 5.0 and ormulating excel sheet to veriy with manual calculations. As an alication ASE TEMA tye water cooling liquid-to-liquid shell and tube heat exchanger has been considered. The data has been granted by CCPL Pvt. Ltd. or the research urose. The irst set o grah reresents inluence on velocity o luid in tube under dierent ouled condition and relative ressure dro. Figure 5: ouling actor vs ressure dro at1.03 m/s velocity Figure 4: ouling actor vs ressure dro at 0.9 m/s velocity Figure 6: ouling actor vs ressure dro at 1.07 m/s velocity

7 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering Figure 7: ouling actor vs ressure dro at 1.11 m/s velocity Figure 9: Fouling Factor vs U o Figure 8: ouling actor vs ressure dro at 1.14 m/s velocity The grahs clearly show that, or the same ouling condition, at low velocity ressure dro has high result. This result could lead us to the concet o ouling accumulation would more signiicant at lower velocity. Moreover, theses results also gives the idea about the negotiating between higher velocity and resective heat transer under certain ouled condition. Results will be helul to adjust otimal oerating condition o shell and tube heat exchanger in ouled conditions. Figure 10: Fouling Factor vs U The grahs reresent the eect on heat transer because o ouling inside the tubes and luid velocity; overall heat transer condition and overall ouled heat transer condition. The signiicant dro in heat transer can be observed because o increase in ouling actor. Moreover, change in velocity is also aecting on heat transer under ouled condition. With increase in velocity heat transer also increases, but the dierence in increase o heat transer in very less than change in overall heat transer.

8 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering Figure 11: Fouling actor vs ower consumtion Figure 13: Fouling actor vs Fouling layer thickness These two grahs show the increase in ouling thickness in tube inner side with resect mass low rate and dierent ouling actor o water. The last grah clear give the idea that, at low velocity the ouling deosition is high with resect higher mass low rate. In other words, or water, it is ossible to eliminate ouling at somewhat level only with regulating the mass low rate in tube side. Nevertheless, change in mass low rate deinitely going to aect on the heat transer through the surace o the tubes. Conclusion: Figure 1: Mass low rate vs Power consumtion The grahs shows the ower consumtion or tubes in dierent ouled and mass low rate conditions. The minor increase in ressure dro has been noted due to ouling. This minor change could be because o luid is water in tube side. This research work highlights an idea about how the luid ouling roerties, although it is water, have signiicant imact on thermal and hydraulic arameters o the shell and tube heat exchanger tube. The irst art o research (igure 4 to 8) is ocused on how mass low rate aects on deosition rate o ouling. As the velocity increase under same ouling condition the rate o deosition decrease resectively or considered shell and tube heat exchanger. It has been also observed rom the results (igure 14) that at low velocity in tube, a deosition rate is high and develo a thick layer o ouling. However, this henomena lead to lower heat transer and higher ressure dro in heat exchanger. The most signiicant oint to be noted that (igure 9 & 10), minor change in ouling layer thickness is directly aecting on heat transer comare to ressure dro or given heat exchanger. (igure 1 and 13) In nutshell, it would be very advantageous to minimize ouling henomena just with aroriate oerating conditions; this could

9 Student s conerence 017 Czech Technical University in Prague Faculty o Mechanical Engineering be achieved by adjusting mass low rate inside the tube. Reerences: [1] Thanhtrung Dang and Jyh-tong Teng, Eect o the Substrate Thickness o Counter-low Microchannel Heat Exchangers on the Heat Transer Behaviors 010 International Symosium on Comuter, Communication, Control and Automation [] J. W. Suitor, W. J. Marner and R. B. Ritter, The History and Status o Research in Fouling o Heat Exchangers in Cooling Water Service 16 th National Heat Transer Conerence, august 8, [] P. J. Fryer, S. E. Bradley, T. A. Graiin and D. I. Wilson, The Design o Fouling Resistant Heat Exchangers, Deartment o Chemical Engineering, Cambridge. June [3] D. Gulley, Troubleshooting Shell and Tube Heat Exchanger, Tulsa Science Foundation, [4] Liljana Markovska, Vera Mesko, alekanadar Grizo and Radmila Kiijanova, Otimal Design o Shell and Tube Heat Exchanger, Bulletian o the chemist and technology o Macedonia, 1996, 15, [5] Kevin M. Lunsord, Increasing Heat Exchanger Perormance, Bryan research and Engineering inc., March [6] K.C.Leong and K.C.Toh, shell and tube heat exchanger design sotware or educational alications, int.j.engng.ed., 1998,3,17-34 [7] D.I.Wilson, G.T.Polly and S.J.Pugh, Ten Years o Ebert, Panchal And The Threshold Fouling Concet, Deartment o Chemical engineering, 005. [8] John M. Nesta and Christoher A. Bennett, Reduce ouling in shell-and-tube heat exchangers, Heat Transer Research Inc [9] M.S. Abd-Elhady, C.C.M. Rindt, and A.A. van Steenhoven, Otimization o Flow Direction to Minimize Particulate Fouling o Heat Exchangers, Deartment o Chemical Engineering [10] E.M.Ishiyama, W.R.Paterson and D.I.Wilson, The Eect O Fouling On Heat Transer, Pressure Dro And Throughut In Reinery Preheat Trains: Otimisation O Cleaning Schedules, Deartment o Chemical Engineering [11] D.H. Lister and F.C. Cussac, Modelling O Particulate Fouling On Heat Exchanger Suraces: Inluence O Bubbles On Iron Oxide Deosition. 7th International Conerence on Heat Exchanger Fouling and Cleaning. July 1-6, 007. [1] Himanshu Joshi, Fred Hoeve and Eric van der Zijden, Alying Technology Advancements to Imrove Heat Exchanger Economic and Environmental Perormance in Reinery Crude Preheat Trains, 7th International Conerence on Heat Exchanger Fouling and Cleaning. July 1-6, 007. [13] Andre L.H.Costa and Edurado M. Queiroz, Design and Otimization o shell and tube heat exchanger, Alied Thermal Engineering. 19 November, 007 [14] Su Thet Mon Than, Khin Aung Lin, Mi Sadar Mon, Heat Exchanger Design, world academy o science, engineering and technology, 008. [15] Arturo Reyes Leon, Miguel Toledo Velazquez, Pedro Quinto-Diez, Florencio sanchez sliva and Juna Abugaber-Francis, The Design O Heat Exchanger, Panchuca de Soto Hidalgo, 011. [16] Rajeev Mukharji, Eective design o shell and tube heat exchanger. American Institute o Chemical Engeenering [17] Nirmal Parmar and Adil Khan, Design validation o shell and tube heat exchanger by HTRI exchanger sotware 5.0, Indian journal o technical education IJTE 01