Jurnal Teknologi. Design Verification of Heat Exchanger for Ballast Water Treatment. Full paper

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1 SW FROM SYSTEMS/ BALLAST TANKS SW FROM SYSTEMS/ BALLAST TANKS Jurnal Teknlgi Full paper Design Verificatin f Heat Exchanger fr Ballast Water Treatment Raj Balaji a*, Omar Yaakb b, Faizul Amri Adnan b, Kh K. K. b a Malaysian Maritime Academy (ALAM), Windw delivery 205, Masjid Tanah Pst Office, 78300, Melaka, Malaysia b Marine Technlgy Centre (MTC), Universiti Teknlgi Malaysia, 830 UTM Jhr Bahru, Jhr, Malaysia *Crrespnding authr: rajbalaji@alam.edu.my Article histry Received :25 July 203 Received in revised frm : August 203 Accepted : December 203 Graphical abstract EXH.GASES FROM ENGINE MICRO FILTRATION MODULES BALLAST WATER HEATER SUPPLEMENTARY TREATMENT UNIT TREATED WATER TO TANKS EXH. GASES TO ATMOSPHERE Abstract Using waste heat frm ship s engines is ne f the methds cnsidered fr heat treatment f ballast water. Fr such a system harvesting the engine exhaust heat, a heat exchanger will be vital. Design ptimisatin f a heater emplying exhaust gases f the engine as utility fluid and ballast sea water as the prcess fluid was achieved using Lagrangian methds, keeping the annual cst as the bjective functin. Csts fr installatin, maintenance as als csts fr the utility and prcess fluids were cnsidered. Heat balance data, specific fuel cnsumptin values frm a typical peratinal ship and current fuel csts were cnsidered fr the design. The thermdynamic and gemetric designs were wrked ut using cmputer based sftware fr cmparing the designs. Csts were als cmputed using a different apprach fr all the designs. Since the amunt f heat transferred was specified and the applicatin was limited t a single prcess, direct cst methd was used fr the cmputatin. The bjective functin values btained frm Lagrangian equatins were cmpared with the values btained frm direct cst cmputatins. Frm the ptimal designs, chice was justified based n annual cst, ptimum exit temperature f shell side fluid and ptimum mass flw f tube side fluid.. Key wrds: Ballast water treatment; waste heat recvery; heat exchanger; ptimizatin; csts 204 Penerbit UTM Press. All rights reserved..0 INTRODUCTION Internatinal Cnventin fr the Cntrl and Management f Ships' Ballast Water and Sediments (BWM Cnventin) will cme int frce ne year after the full ratificatin. As f 30 th September 203, 38 cuntries ttalling 30.38% f glbal tnnage have agreed while the requirement fr full ratificatin is 30 cuntries and 35% f wrld tnnage. Subsequent t the ratificatin, ships have t cmply with stricter perfrmance standards. This is pssible nly by treatment f the ballast water replacing the current practice f ballast water exchange (BWE). Many ballast water treatment (BWT) systems are at cmmercial readiness after apprvals but nne f the systems are efficient enugh t meet the stricter standards prpsed by US Administratin 2. Research n BWT systems cntinues and heat treatment is ne f the physical methds which have been prbed int. Balaji and Yaakb 3 had shwn heat availability n bard thugh bserving that heat treatment alne may nt suffice fr treating large quantities f ballast water. Thugh issues remain with heat treatment, the waste heat ptential n bard makes it an attractive ptin. Mst f available BWT systems are designed as a cmbinatin f tw t five methds 4,5. A ship bard heat resurce based system harvesting heat frm engine exhaust gases in cmbinatin with anther methd culd be a viable treatment system. Figure shws a simple layut f such a BWT cmbinatin system 6. Fr this heat-reliant BWT system, a welldesigned heat exchanger is essential. EXH.GASES FROM ENGINE MICRO FILTRATION MODULES 2.0 METHODOLOGY BALLAST WATER HEATER SUPPLEMENTARY TREATMENT UNIT TREATED WATER TO TANKS Figure Ballast water treatment system EXH. GASES TO ATMOSPHERE Mdels have been prpsed fr ptimising heat exchanger designs t enhance waste heat recveries 7. Heat exchanger design selectin based n genetic algrithms 8 and multi bjective ptimisatin 9 etc. have been prpsed but these methds are suitable fr prcessing plants invlving a netwrk f heat exchangers and ther cmpnents. Since marine heat exchangers 66:2 (204) eissn

2 62 Raj Balaji et al. / Jurnal Teknlgi (Sciences & Engineering) 66:2 (204), 6 65 are mstly singular, simpler ptimisatin techniques with engineer defined parameters and cnstraints can be emplyed. 2. Optimisatin f Heat Exchanger The heat duty fr the heat exchanger was fixed cnsidering a recvery f 0% frm the input energy. A single pass, shell and tube heat exchanger having a cunter flw pattern with baffles was designed. The fluids were assumed t underg n phase change. Other assumptins included steady state peratin, cnstant specific heats fr the fluids, cnstant ver all heat transfer cefficient and negligible heat lsses 0. The heat duty, inlet temperature f the shell side cld fluid and the tube diameters were treated as knwn variables. 2.. Basic Equatins fr Optimisatin The bjective functin f annual cst can be written as an additin f annual variable csts, csts incurred fr utility fluid and the pwer lsses n the tube and shell circuits,2. C T = A K F C A + m u H y C u + A E i H y C i + A E H y C () C T is the ttal annual variable cst including peratinal csts. A (m 2 ) is the area fr heat transfer and K F is the factr applied fr cmputing fixed charges including maintenance (20% f the installatin charges, C A ). m u (kg/s) is the mass flw f utility fluid, H y dentes the hurs f heat exchanger peratin (taken as 2000 frm vessel data) and C u is the cst f utility fluid. E i and E are the pwer lsses incurred per unit area n the inside and utside f the tubes whereas C i and C are the csts t pump the fluids. The relatinship fr the thermal design is based n the enthalpy rate equatins fr single phase fluids where j = i, denting each f the fluids inside the tubes and utside respectively and m j representing the mass flw 3. q = q j = m j Δh j = (m c p ) j ΔT j = (m c p ) j T j,i T j, (2) Fr heat balance f ht and cld streams, the heat absrbed will be the prduct f the mass flws, specific heats and the temperature differences. Q = m c C pc (t 2 t ) = m h C ph (T T 2 ) (3) Where m c and m h (kg/s) are the mass flw f cld (sea water) and ht fluid (exhaust gas), and C ph and C pc (J/kg K) are the specific heats f ht (exhaust gas) and cld fluid (sea water). The inlet and utlet temperatures ( C) f the shell side fluid (sea water) are t and t 2. The inlet and utlet temperatures ( C) f tube side fluid are T and T 2. The mass flw f the fluids can be btained frm, m u = m u = Q C ph (Δt 2 Δt +t t 2 ) Q C pc (Δt Δt 2 +T T 2 ) (4a) (4b) Where Δt = T 2 t and Δt 2 = T t 2 are the respective temperature differences between fluids in the cunter flw pattern at entry and exits. Equatins (4a) and (4b) represent exhaust gases and sea water respectively. The fundamental equatin fr heat transfer in the heat exchanger is, Q = UA T lm (5) U (W/m 2 K) is the Overall Heat Transfer Cefficient. A represents the surface area and T lm ( C) is the Lgarithmic Mean Temperature Difference (LMTD). The ptimum verall heat transfer cefficient can be calculated frm, U = h + h i D D i + R f + R fi (6) Where h i and h (W/m 2 K) are the Heat transfer cefficients fr the inside and utside f the tubes respectively, D i and D are the inside and utside diameters f the tube and R fi and R f (m 2 K/W) are the fuling resistances n the tube and shell sides. The verall heat transfer cefficient equatin is further simplified by cmbining the fuling factrs. R dw (m 2 K/W) is the cmbined fuling resistance (tubes, scale & dirt). U = ( D D i h i + h + R dw ) (7) The LMTD is calculated frm, T lm = F (T t 2 ) (T 2 t ) ln[ T t2 T2 t ] (8) The heat duty is then written as, Q = F U A (Δt 2 Δt ) ln(δt 2 Δt ) The subscript represents the ptimum value. A crrectin factr F is applied fr cunter current heat exchangers depending n the number f tube and shell passes f the prcess fluids. Equatin (9) can be rewritten as, U A = F(Δt 2 Δt ) Q ln(δt 2 Δt ) Substituting fr U frm equatin (7), F(Δt 2 Δt ) (9) (0) = ( D + + R Q ln(δt 2 Δt ) A D i h i h dw ) () The pwer lsses inside and utside f the tubes, E i and E are represented as fllws. ψ i and ψ are the dimensinal factrs fr estimating pwer lsses in the tube and shell circuits. E i = ψ i h i (2) E = ψ h 4.75 (3) These derivatins were inserted in Equatin () apprpriately after substituting Equatin (4) fr m u in (). The bjective functin equatin may be written fr exhaust gases r sea water. Thus the bjective functin equatins were structured n fur variables t 2, A, h i and h f which nly three can be

3 63 Raj Balaji et al. / Jurnal Teknlgi (Sciences & Engineering) 66:2 (204), 6 65 independent. If three f the variables, say A, h i and h are knwn, the temperature difference t 2 can be fund Cst Cmputatin using Lagrangian Multiplier With the use f Lagrangian multipliers, ptimal candidate pints may be btained where the prblem is equality cnstrained 4. With Equatins (), (2) and (3), the bjective functin can be expressed as an uncnstrained prblem with λ, the Lagrangian multiplier. Substituting Equatins (4a), (4b), (), (2) and (3), the Equatin () can be expressed in the uncnstrained frm with the Lagrangian multiplier. Equatins (4a) and (4b) represent exhaust gases and sea water respectively. C pu is the specific heat f utility fluid (exhaust gas). C T = A K F C A + Q H y C u C pu (Δt 2 Δt +t t 2 ) + A ψ i h i H y C i + A ψ h4.75 H y C + λ [ F( t 2 t ) ( D + + R Q ln( t 2 t ) A D i h i h dw )] C T = A K F C A + Q H y C u C pu ( t t 2 + T T 2 ) + A ψ i h i H y C i + (4a) A ψ h4.75 H y C + λ [ F( t 2 t ) ( D + + R Q ln( t 2 t ) A D i h i h dw )] (4b) The btained expressins are differentiable with respect t the fur chsen variables resulting in fllwing simultaneus equatins. Equatins (8a) and (8b) represent exhaust gases and sea water respectively. Slving the equatins and eliminating λ, the ptimum values can be btained. The variables fr ptimum values are subscripted as pt. 2.5 = A h pt ψ i h i pt H y C i + i 3.75 = 4.75 A h pt ψ h pt H y C + λd A pt D i h i pt λd A pt D i h pt 2 = 0 (5) 4.75 = K A F C A + ψ i h i pt H y C i + ψ h pt H y C + λ A2 ( D + pt D i h i pt 2 = 0 (6) h pt + R dw ) = 0 (7) ptimum value fr the temperature difference at the warm end t 2 pt is btained frm the fllwing equatin. F U pt H Y C u c pu (K F C A + E i pt H Y C i + E pt H Y C ) = ( + 2 T T 2 ) (ln t 2 pt t + ) (9) t t 2 pt t t 2 pt The bjective functin values were cmputed fr eight cases using the abve equatins. Fur ptimal design cases were develped with varius cst cnsideratins and fr each design, ptimum utlet temperature fr sea water was calculated. Three mre cases were cmputed by keeping the utlet temperature f sea water cnstant but fr similar cst cnsideratins. One extra design was develped using sftware where Bell-Delaware appraches were emplyed with n cst cnsideratins Cmputatin f Direct Csts The ttal annual csts will be an additin f capital cst C CA, csts incurred in energy cnsumptin C E, and the perating csts C s. Csts were cmputed fr tw cnsideratins, ne applying an interest rate n capital and a payback perid and the ther withut cnsidering bth 5. C tt = C CA + C E + C s (20) Capital csts were cmputed by assuming tw different values fr energy csts. The first energy cst was btained fr ship bard generatin f unit pwer (US$0.2/kWh) and the secnd ne based n average lcal (ashre) cst (US$0.06/kWh). Capital Csts assuming these tw energy csts were cmputed fr tw cases. In ne case, csts fr sea water pump (fr prcess fluid) were cnsidered and in anther the pump csts were neglected. The cst f turbcharger was neglected as the cst wuld have been included with the main engine csts. Standard values fr all reference csts and indexes were btained frm relevant handbks 0,5 as als frm the Chemical Engineering Plant Cst Index (CEPCI) published by Chemical Engineering. The capital cst was calculated frm the fllwing. C CA = ( n + z 2 ) (I EX + I P tch + I Ppump ) = λ F = ( ) + ( F( t t 2 ) t 2 Q ln( t 2 t ) Q t 2 ln( t 2 t ) 2) + C u H y Q = 0 C pu ( t t 2 +t t 2 ) 2 (8a) a(i EX + I P tch + I Ppump ) (2) I EX = I EX ref ( A EX ) A EX ref m EX (22) λ F = ( ) + ( F( t 2 t ) t 2 Q ln( t 2 t ) Q t 2 ln( t 2 t ) 2) + C u H y Q = 0 C pu ( t t 2 +t t 2 ) 2 (8b) Frm equatins (4a) and (4b) it can be seen that the mass flw f the utility fluid m u depends n the temperature difference at the warm end t 2, while the ther values are fixed. The I P tch/pump = I P tch/pump ref ( L m P P tch/pump ) (23) L P tch/pump ref Where a the payback cefficient is equal t 0.25 and s is a cefficient based n intensity f maintenance. Assuming a medium intensity fr cleaning and maintenance, a value f was btained frm the Cst Index tables. The number f years f cst recvery perid n was taken as 0 and the interest rate z was assumed t be 5%.

4 64 Raj Balaji et al. / Jurnal Teknlgi (Sciences & Engineering) 66:2 (204), 6 65 I EX, I P tch and I Ppump are the purchase csts f the heat exchanger, turbcharger and sea water pump cmputed frm the reference csts I EX ref and I P tch/pump ref btained frm the industry indexes. Fr the reference csts, the pwer (kw) referred t was dented by L P tch/pump ref. Fr the pump, the reference pwer was taken as 00 kw. The values f expnents m EX and m P were 0.59 and 0.67 as btained frm VDI Atlas 5. The heat exchanger area and reference area are represented by A EX and A EX ref. The energy csts and perating expenses were calculated as fllws. C E = C EL H y ( M ex gas p tube + M pump p shell ) + C ρ eg η tch ρ sw η T + C M (24) pump C s = si EX (25) The mass flw f exhaust gas M ex gas was taken as 4.67 kg/s and sea water M pump as kg/s. The densities f the exhaust gas ρ eg and sea water ρ sw were kg/m 3 and 07 kg/m 3 respectively. These values and the pressure drps p shell and p tube (Pa) were taken frm the heat exchanger design parameters. The sea water pump efficiency η pump was taken as 0.7 and the turbcharger pump efficiencyη tch as unity. The additinal energy csts fr increasing temperature C T and cst f supplies C M were nt cnsidered as the heat exchanger was independent and nt part f a netwrk. The bjective functin values and direct cst values were cmpared and variatins bserved. The least values f bjective functins and the least variatin frm direct cst values were cnsidered fr selecting the area f the heat exchanger. The final design was determined by applying tw ther factrs f mass flw and utlet temperature f sea water which are significant fr the ballast water treatment. 3.0 RESULTS AND DISCUSSION The assumed and derived csts used fr calculatins are tabulated in Table and the cst summary in Table 2 shws the cst variatins (in brackets). All designs were thermdynamically feasible. Fr treatment f ballast water, a minimum temperature f 55 C was kept as the target. Case was excluded because the ptimum utlet temperature was much belw the targeted temperature. Case 5 was used nly as a reference fr the gemetrical values. Case 2 had the highest bjective functin value fllwed by Case 3 and hence bth were nt cnsidered, thugh ther ptimal values were tangible. Amngst the rest f the cases, Case 8 had the least value f bjective functin, even lwer than the direct csts. The next tw cases with lw bjective functin values were Case 7 and Case 4. The bjective functin value (Lagrangian) varied by 0.03% fr Case 4 and 0.07% fr Case 7, when cmpared with direct cst values. But the btained value f sea water utlet temperature (ptimum value) was highest (>80 C) fr Case 4. This established the scpe fr higher mass flws and temperatures which are crucial fr treating large vlumes f ballast water 3. S, Case 4 with least variatin as als the highest ptimum sea water temperature was identified as the ptimum design and all ther gemetric values were calculated fr this design. 4.0 CONCLUSION Aviable heat exchanger design emplying simple ptimisatin techniques has been verified. Since cst was the chsen bjective, anther apprach was emplyed t verify if the bjective functin values were clser and s the ptimisatin culd be validated. Fr further validatins, a heat exchanger mdel based n the chsen design has t be erected. The cnditins assumed in the design have t be simulated t btain the calculated temperatures. The species mrtalities at these temperatures have t be assessed which will prve the effectiveness f the treatment methd. The scpe f these discussins has been limited t cst cmputatins nly. The thermdynamic and gemetric values used in develping the design and all equatins related t heat exchanger design are nt prjected. Acknwledgement The paper is part f the research funded by Ministry f Science, Technlgy and Innvatin (MOSTI), Malaysia. References [] Last accessed 09 Nv LT. [2] EPA Reprt. 20. Efficacy f Ballast water Treatment Systems: a Reprt by EPA Science Advisry Bard, (US Envirnmental Prtectin Agency-Scientific Advisry Bard Reprt). [3] R. Balaji, O. Yaakb An Analysis f Shipbard Waste Heat Availability fr Ballast Water Treatment. Jurnal f Marine Engineering and Technlgy. (2): [4] Llyds Register Reprt. 20. Ballast Water Treatment Technlgy Current Status. 4 th Ed. (June 20) [5] R. Balaji, O. Yaakb. 20. Emerging Ballast Water Treatment Technlgies: A Review. Jurnal f Sustainability Science and Management. (): [6] R. Balaji, O. Yaakb Envisaging a Ballast Water Treatment System frm Shipbard Waste Heat. Prceedings f the Internatinal Cnference n Maritime Technlgy June 202, Harbin, China. [7] M. S. Söylemez On the Therm Ecnmical Optimisatin f Heat Pipe Heat Exchanger HPHE fr Waste Heat Recvery. Energy Cnversin and Management. 44: [8] S. Rajasekharan, T. Kannadasan Optimisatin f Shell and Tube Heat Exchangers Using Mdified Genetic Algrithm. Internatinal Jurnal f Cntrl and Autmatin. 0(4). [9] S. Sanaye, H. Hajabdllahi Multi-bjective Optimisatin f Shell and Tube Heat Exchangers. Applied Thermal Engineering. 30: [0] D. W. Green, R. H. Perry Perry s Chemical Engineers Handbk. 8 th Ed. Sectin. New Yrk: The McGraw Hill Cmpanies. [] T. F. Edgar, D. M. Himmelblau, L. S. Lasdn Optimisatin f Chemical Prcesses. 2 nd Ed. New Yrk: The McGraw Hill Cmpanies [2] M.S. Peters, K.D. Timmerhaus, R.E. West Plant Design and Ecnmics fr Chemical Engineers. 5 th Ed. New Yrk: The McGraw Hill Cmpanies. [3] R. K. Shah, D. P. Sekulić Fundamentals f heat exchanger design. Chapter 2. New Yrk: Jhn Wiley & Sns [4] A. Ravindran, K. M. Ragsdell, G. V. Reklaitis Engineering Optimisatin. 2nd Ed. New Jersey: Jhn Wiley & Sns. [5] VDI Heat Atlas nd Ed. Berlin: Springer-Verlag.

5 65 Raj Balaji et al. / Jurnal Teknlgi (Sciences & Engineering) 66:2 (204), 6 65 Table Csts derived fr design and verificatin Csts (in US$) Cst f purchase C pur 23/m 2 Cst f installatin C A 4.45/m 2 Cst f utility fluid, exhaust gas C u 0.04/kg Cst t pump exhaust gas C i 0.2/kWh Cst t pump sea water C 0.2/kWh Cst f energy ashre C EL 0.06/kWh Reference cst I EX ref 250/m 2 Table 2 Summary f cst cmputatin (All csts in US$) Area (m 2 ) Annual Cst I Ppump, I Ppump, C EL = 0.06 C EL = 0.06 I Ppump, I Ppump, C EL = 0.06 C EL = 0.06 t 2 Optimum Case C u = 0 C i = 0.2 C = 0.2 Interest rate 5% payback 0 years N Interest & payback (-0.57) (-0.43) (-0.33) (-0.20) (-.3) (-0.25) (-.08) (-0.02) Case C u = 0.04 C i = 0.2 C = 0.2 (-0.3) (-0.04) (0.02) (0.) (-0.58) (0.09) (-0.43) (0.24) Case 3 C u = 0.04 C i = 0.2 C = (-0.2) (-0.04) (0.03) (0.) (-0.58) (0.0) (-0.43) (0.25) Case C u = 0.04 C i = 0 C = 0.2 (-0.45) (-0.34) (-0.26) (-0.5) (-.02) (-0.6) (-0.83) (0.03) t 2 fixed Case n.a N csts; Sftware Case 6 C u = 0.04 C i = 0.2 C = 0.2 Case 7 C u = 0 C i = 0.2 C = 0 Case 8 C u = 0 C i = 0 C = (-0.8) (-0.09) (-0.0) (0.08) (-0.68) (0.06) (-0.52) (0.22) (-0.40) (-0.29) (-0.2) (-0.0) (-.0) (-0.2) (-0.8) (0.07) (-0.92) (-0.77) (-0.66) (-0.50) (-.75) (-0.54) (-.48) (-0.27)