Heat Exchanger Design Guide: A Practical Guide for Planning, Selecting and Designing of Shell and Tube Exchangers PDF
Preview Heat Exchanger Design Guide: A Practical Guide for Planning, Selecting and Designing of Shell and Tube Exchangers
HEAT EXCHANGER DESIGN GUIDE A Practical Guide for Planning, Selecting and Designing of Shell and Tube Exchangers M. NITSCHE AND R.O. GBADAMOSI Heat Exchanger Condenser Reboiler With numerous practical Examples Amsterdam(cid:129)Boston(cid:129)Heidelberg(cid:129)London(cid:129)NewYork(cid:129)Oxford Paris(cid:129)SanDiego(cid:129)SanFrancisco(cid:129)Singapore(cid:129)Sydney(cid:129)Tokyo ButterworthHeinemannisanimprintofElsevier ButterworthHeinemannisanimprintofElsevier TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UK 225WymanStreet,Waltham,MA02451,USA Copyright©2016ElsevierInc.Allrightsreserved. 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ISBN:978-0-12-803764-5 BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ForinformationonallButterworthHeinemannpublications visitourwebsiteathttp://store.elsevier.com/ Publisher:JoeHayton AcquisitionEditor:FionaGeraghty EditorialProjectManager:CariOwen ProductionProjectManager:SusanLi Designer:VictoriaPearson TypesetbyTNQBooksandJournals www.tnq.co.in PrintedandboundintheUnitedStatesofAmerica FOREWORD Dear Reader, This book is not an academic treatise but rather a book for solving daily practical problems easily and for the illustration of essential influencing variables in the design of heat exchangers, condensers, or evaporators. All calculations are explained with examples that I am using in my seminars since several years. Inthisbook,youwillbeshownhowtoproceedinthedesignofaheatexchangerin the daily practice, how to determine the effective temperature difference for the heat transfer, and how to calculate the heat transfer coefficient using simple equations. The most important influence parameters for the heat transfer coefficient are intro- duced. Different calculation models are compared. It is shown how to calculate the required dew point and bubble point lines for mixtures. Fromthewiderangeofpublishedcalculationmethods,Ihavechosenthemodelsthat can simply be calculated using the hand calculator and deliver sure results. I refer to the modelswhichIhavechosenfromseveralexistingliteraturesasNitschemethodsbecause I recommend these. During the time from 1966 to 2007, I designed, planned, and built several chemical plants: distillation plants with evaporators, condensers, and heat exchangers, for fatty alcohols, fatty acids, nitrochlorobenzenes, amine and hydrocarbons and tar oils; storage tanks and vessels with filling stations for tank truck, rail tank car, and barrels; stirred tankplantsforreactionswithdecanters,centrifuges,andfilters;plantsforexhaustairpu- rificationandgasolinerecovery,methanolandethylacetateetc.;stripperforofsourwater purification or for methyl isobutyl ketone recovery. Since 1980, I report in seminars about piping, heat exchangers, and chemical plants aboutmypracticalexperiencesintheDesignandPlanning.Atthelatest,Irealizedduring plantstartupwhatIwronglycalculatedorwhatIwronglyplaned.Thisbookshouldhelp you to minimize the mistakes in the design of heat exchangers. Hamburg Dr Manfred Nitsche ix CHAPTER 1 Heat Exchanger Design Contents 1.1 ProcedureinHeatExchangerDesign 2 1.2 InformationaboutHeatExchangers 12 1.2.1 TubePattern 12 1.2.2 BypassandLeakageStreams 13 1.2.3 Baffles 14 1.2.4 TechnicalRemarks 15 1.2.5 SelectionofaShellandTubeExchanger 16 1.2.5.1 WhichHeatExchangerTypesCanBeCleaned? 17 Nomenclature 18 References 19 In heat exchanger design the required heat exchanger area A (m2) is determined for a certain heat load Q (W) at a given temperature gradient Dt ((cid:1)C). (cid:1) (cid:3) Q A ¼ m2 U (cid:3)Dt The overall heat transfer coefficient, U, is calculated as follows: 1 1 1 s ¼ þ þ þf þf U a a l i o i o f ¼inner fouling factor (m2K/W) i f ¼outer fouling factor (m2K/W) o U¼overall heat transfer coefficient (W/m2K) s¼tube wall thickness (m) l¼thermal conductivity of the tube material (W/mK) a ¼inner heat transfer coefficient in the tubes (W/m2K) i a ¼outer heat transfer coefficient on the shell side (W/m2K) o Figure 1.1 shows which overall heat resistances have to be overcome and how the temperature profile in a heat exchanger looks like. Theoverallheattransfercoefficientsandthetemperatureprofilewillbecalculatedin Chapter 6. Referencevaluesforheattransfercoefficientsandoverallheattransfercoefficientsare listed in Table 1.1. HeatExchangerDesignGuide ©2016ElsevierInc. http://dx.doi.org/10.1016/B978-0-12-803764-5.00001-8 Allrightsreserved. 1 2 HeatExchangerDesignGuide Figure1.1 Heattransferresistancesandtemperatureprofile. 1.1 PROCEDURE IN HEAT EXCHANGER DESIGN In order to calculate the convective heat transfer coefficients, the Reynolds number is needed. Theheattransfercoefficients,a,aredependentontheReynoldsnumber,Re,hence the flow velocity, w, on the tube and shell side, respectively. Tube side: afw0.8 Shell side:afw0.6 Therefore the cross-sectional areas must be known in order to determine the flow velocitiesandtheReynoldsnumbers.Foranexistingheatexchanger,thisisnotaprob- lemifadrawingisavailable.Inthecaseofthedesignofanewheatexchanger,theflow cross sections are not known. So, initially an estimation of the required area has to be done and then an appropriate equipment has to be selected. For the selection, the following criteria should be applied: (cid:129) Theflowvelocityonbothsidesshouldbeintheorderof0.5e1m/sforliquidsandin the range of 15e20m/s for gases. (cid:129) The required heat exchanger area should be achieved with tube length of 3e6m. In Figure 1.3 the flow chart for the heat exchanger design is provided [1]. In the following the procedure of heat exchanger design is explained in some more detail: 1. Determine flow rates, temperatures, and the fluid property data 2. Determination of the heat loads on tube and shell side Shellside: Q ¼ M (cid:3)c (cid:3)ðT (cid:4)T ÞðWÞ Sreq S S 1 2 Tubeside: Q ¼ M (cid:3)c (cid:3)ðt (cid:4)t ÞðWÞ Treq T T 2 1 HeatExchangerDesign 3 Table1.1 Referencevaluesforheattransfercoefficients,a Naturalconvection a(W/m2K) Gases at atmospheric pressure 4e6 Oil(viscosity¼100mm2/s) 10e20 Water 250e500 Hydrocarbons, lowviscosity 170e300 Condensation Steam 5000e10000 Organic solvents 1000e3000 Light oils 1000e1500 Heavy oils (vacuum) 100e300 Vaporization Water 4000e10000 Organic solvents 1000e2500 Light oils 700e1400 Flowingmedia Atmosphericgases 40e200 Gases under pressure 150e300 Organic solvents 300e1000 Water 2500e4000 Guidingvaluesforoverallheattransfercoefficients,U Condensation U(W/m2K) Water Water 1000e2000 Organic solvent 600e1000 Organic solventþinert gases 100e500 Heavy hydrocarbons 50e200 Evaporation Steam Water 2000e4000 Organic solvent 500e1000 Light oils 250e800 Heavy oils 120e400 Flowingmedia Steam Water 1500e4000 Organic solvents 600e1000 Gases 30e250 Water Water 1000e2000 Organic solvents 250e800 Gases 15e300 Organic solvents Organic solvents 100e300 4 Table1.2 GeometricdataofheatexchangersaccordingtoDIN28,184,part1,for25(cid:3)2tubeswith32mmtriangularpitch H e Typ DN Z Da B n AE AR AS fw VR VM at nr. e e (mm) (mm) e (mm2) (mm2) (m2/m) (m3/h) (m3/h) Ex e ch a n 1 150 2 168 30 14 1770 2425 1.1 0.251 8.73 6.37 g e r 2 200 2 219 40 26 2288 4503 2 0.366 16.21 8.24 D e 3 250 2 273 50 44 5520 7620 3.5 0.259 27.43 19.87 sig n 4 300 2 324 60 66 5088 11,430 5.2 0.375 41.15 18.32 G u 5 350 2 355 70 76 6230 13,162 6 0.397 47.38 22.43 id e 6 350 4 355 70 68 6230 5888 5.3 0.381 21.20 22.43 7 400 2 406 80 106 11,072 18,357 8.3 0.319 66.09 39.86 8 400 4 406 80 88 9072 7620 6.9 0.382 27.43 32.66 9 500 2 508 100 180 14,600 31,172 14.1 0.360 112.22 52.56 10 500 4 508 100 164 12,100 14,201 12.9 0.430 51.12 43.56 11 600 2 600 120 258 19,560 44,681 20.3 0.389 160.85 70.42 12 600 8 600 120 232 22,560 10,044 18.2 0.348 36.16 81.22 13 700 2 700 140 364 22,260 63,038 28.6 0.456 226.94 80.14 14 700 8 700 140 324 25,760 14,028 25.4 0.395 50.50 92.74 15 800 2 800 160 484 29,440 83,819 38 0.454 301.75 105.98 16 800 8 800 160 432 37,440 18,703 33.9 0.367 67.33 134.78 17 900 2 900 180 622 41,400 107,718 48.9 0.407 387.79 149.04 18 900 8 900 180 556 41,400 24,072 43.7 0.416 86.66 149.04 19 1000 2 1000 200 776 46,000 134,388 61 0.452 483.80 165.60 20 1000 8 1000 200 712 56,000 30,826 55.9 0.373 110.97 201.60 21 1100 2 1100 220 934 55,220 161,750 73.4 0.460 582.30 198.79 22 1100 8 1100 220 860 60,720 37,234 67.5 0.420 134.05 218.59 23 1200 2 1200 240 1124 72,240 194,655 88.3 0.416 700.76 260.06 24 1200 8 1200 240 1048 78,240 45,373 82.3 0.390 163.34 281.66 DN¼Nominalshelldiameter;Z¼numberoftubepasses;Da¼shelldiameter(mm);B¼bafflespacing(mm);n¼numberoftubes;AE¼shell-sideflowcrosssection (mm2);AR¼tube-sideflowcrosssection(mm2);AS¼Heatexchangerareapermtubelength(m2/m);VR¼Requiredflowratefor1m/sflowvelocityonthetube side(m3/h);VM¼Requiredflowratefor1m/sflowvelocityattheshellside(m3/h). HeatExchangerDesign 5 Figure1.2 Heatexchangerforconvectiveheattransfer. For condensers and evaporators, the condensation and the vaporization enthalpies need to be considered. 3. Calculation of the corrected effective temperature difference (CMTD) for the heat load First, the logarithmic mean temperature difference (LMTD) is determined for ideal countercurrent flow. Most heat exchangers have multiple passes in order to increase flow velocity in the tubes. For example, the heat exchanger in Figure 1.2 has two passes. The medium on thetubesideflowsforwardandbackward.Inonetubepassthefluidflowscocurrent to the flow on the shell side. In the other pass the flow is in countercurrent to the shell-side flow. Ideal countercurrent flow does not occur. The driving temperature gradient is worse. Therefore for heat exchangers with multiple passes, due to the nonideal countercurrent flow, the temperature efficiency factor, F, must be calculated. F should be >0.75! Using the temperature efficiency factor, F, the CMTD is determined. CMTD ¼ F (cid:3)LMTD: The calculation of the effective temperature difference is shown in Chapter 2. 6 HeatExchangerDesignGuide Figure1.3 Procedureflowchartforthethermaldesignofaheatexchanger[1]. With nonlinear condensation or evaporation curves, the average weighted temperature difference must be determined. Zones with approximate linear range of the condensation temperature are established for which the CMTDs are determined. Finally, the weighted average of the effective temperature differences in the zones is determined. 4. Estimation of the required heat exchanger area For the calculated heat load and the available effective temperature difference, the required heat exchanger area is estimated using the estimated overall heat transfer coefficient, U, from Table 1.1: (cid:1) (cid:3) Q A ¼ req m2 U (cid:3)CMTD 5. SelectionofanappropriateequipmentfromTable1.2fortherequiredheatexchange area,A,fromcolumn,AS,withtheareaoftheheatexchangerpermlength(m2/m) HeatExchangerDesign 7 6. Determination of the flow velocity using the columns, VR and VM, in Table 1.2 The volumetric flows on the tube and shell side are listed in columns, VR and VM, which are required for a flow velocity of 1m/s. Example 1: Selection of an appropriate heat exchanger for a required area A¼55m2 Tube-sideflowrateV ¼40m3/h,shell-sideflowrateV ¼80m3/h tube shell Selected:Type12with18.2m2/mtubelengthand232tubesin8passes,DN600 Tubelength¼4m,Heatexchangerarea¼4(cid:3)18.2¼72.8m2(32%excess) VR¼36.16m3/hfor1m/sandVM¼81.22m3/hfor1m/sfromTable1.2 Determinationoftheflowvelocity,w,onthetubeside: t V 40 w ¼ tube(cid:3)1 ¼ (cid:3)1 ¼ 1:1 m=s t VR 36:16 Determinationoftheflowvelocity,w ,ontheshellside: sh V 80 w ¼ shell(cid:3)1 ¼ (cid:3)1 ¼ 0:985 m=s sh VM 81:22 Primaryconditionforagoodconvectiveheattransferisanadequatelyhighflowvelocity. ThatiswhybothcolumnsforVRandVMinTable1.2areimportant. Figures1.4and1.5showthattheheattransfercoefficientsincreasewithrisingflowvelocities. On the shell side, the baffle spacing, B, can be shortened, for instance, if the flow velocity shall be increasedinordertoachieveabetterheattransfercoefficient. Sincetheflowsonwhichthedesignisbasedareknowntheflowvelocitiescanbeeasilydetermined withVRandVM(seeExample1). 7. Calculation of the convective heat transfer coefficients on the tube and shell side The Reynolds number can be calculated once the flow velocity is determined. With convective heat transfer, the heat transfer coefficient is dependent on the Reynolds number, Re, and the Prandtl number, Pr (see Chapter 3). Const(cid:3)Rem(cid:3)Pr0:33(cid:3)l w(cid:3)d n(cid:3)c(cid:3)r(cid:3)3600 a ¼ Re ¼ Pr ¼ d n l n¼kinematic viscosity (mm2/s) d¼tube diameter (m) w¼flow velocity (m/s) l¼heat conductivity (W/mK) r¼density (kg/m3) c¼specific heat (Wh/kgK)
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