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Heat Exchanger Dimensioning

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Heat Exchanger Dimensioning LAPPEENRANTA UNIVERSITY OF TECHNOLOGY Faculty of Technology LUT Energy HEAT EXCHANGER DIMENSIONING Jussi Saari 1 TABLE OF CONTENTS 1 INTRODUCTION......................................................................................................................7 2 GENERAL BACKGROUND.....................................................................................................8 2.1 BASIC ISSUES OF HEAT EXCHANGER DESIGN ...............................................................................8 2.2 HEAT EXCHANGER DESIGN PROCESS...........................................................................................9 3 TYPES OF HEAT EXCHANGERS.........................................................................................12 3.1 FLOW ARRANGEMENTS ...........................................................................................................12 3.1.1 Basic single-pass arrangements..................................................................................12 3.1.2 Multipass arrangements..............................................................................................14 3.2 COMMON TYPES OF PHYSICAL CONSTRUCTION .........................................................................16 3.2.1 Tubular –double-pipe ................................................................................................17 3.2.2 Tubular –shell and tube .............................................................................................17 3.2.3 Plate heat exchangers.................................................................................................21 3.2.4 Spiral heat exchanger.................................................................................................25 3.2.5 Compact heat exchangers ...........................................................................................25 3.3 SELECTION OF APPROPRIATE HEAT EXCHANGER TYPE...............................................................27 3.3.1 Fluid pressures and temperatures ...............................................................................27 3.3.2 Fouling.......................................................................................................................28 3.3.3 Material choices.........................................................................................................29 3.3.4 Cost............................................................................................................................30 4 METHODS OF HEAT EXCHANGER CALCULATION ......................................................31 4.1 QUICK ESTIMATES ..................................................................................................................34 4.2 LOGARITHMIC MEAN TEMPERATURE DIFFERENCE (LMTD) METHOD.........................................35 4.2.1 Definition of LMTD ....................................................................................................35 4.2.2 Complex flow arrangements........................................................................................38 4.2.3 Special Cases .............................................................................................................41 4.2.4 Summary ....................................................................................................................41 4.3 e-NTU -METHOD ....................................................................................................................42 4.3.1 Dimensionless parameters: e, C* and NTU.................................................................42 4.3.2 Effectiveness –NTU relationships...............................................................................44 4.3.3 Summary ....................................................................................................................50 4.4 P-NTU -METHOD....................................................................................................................53 5 OVERALL HEAT TRANSFER COEFFICIENT ...................................................................55 5.1 DEFINITION ............................................................................................................................55 5.2 U-VALUE OF EXTENDED-SURFACE HEAT EXCHANGERS .............................................................57 6 HEAT TRANSFER AND PRESSURE DROP CORRELATIONS.........................................60 6.1 A BRIEF INTRODUCTION ON CONVECTION HEAT TRANSFER .......................................................60 6.1.1 Dimensionless parameters ..........................................................................................61 6.1.2 Analogy of friction and heat transfer: practical consequences .....................................63 6.2 SOLVING CONVECTION HEAT TRANSFER COEFFICIENT FROM EMPIRICAL CORRELATIONS ............64 6.3 INTERNAL FLOW IN A PIPE .......................................................................................................66 6.3.1 Fully developed turbulent flow....................................................................................68 6.3.2 Fully developed laminar flow......................................................................................70 6.3.3 Entry length................................................................................................................71 6.4 FLOW ACROSS TUBE BUNDLES ................................................................................................72 6.4.1 Plain tubes .................................................................................................................73 6.4.2 Finned tube banks.......................................................................................................75 6.4.3 Shell-side flow in a shell-and-tube exchanger..............................................................77 6.5 PLATE HEAT EXCHANGER SURFACES........................................................................................80 6.5.1 Gasketed plate heat exchangers ..................................................................................80 2 6.5.2 Spiral heat exchangers................................................................................................81 6.6 COMPACT HEAT EXCHANGER SURFACES ..................................................................................81 7 PRESSURE DROP...................................................................................................................83 7.1 PRESSURE DROP CALCULATION –GENERAL PRINCIPLE .............................................................84 7.1.1 Core entrance pressure drop.......................................................................................84 7.1.2 Core pressure drop.....................................................................................................85 7.1.3 Core exit pressure drop ..............................................................................................87 7.1.4 Total pressure drop ....................................................................................................87 7.2 PRESSURE DROP IN SPECIFIC TYPES OF HEAT EXCHANGER .........................................................88 7.2.1 Tubular heat exchangers, outside flow ........................................................................88 7.2.2 Gasketed plate heat exchangers ..................................................................................92 8 FOULING.................................................................................................................................94 8.1 FOULING PROCESS ..................................................................................................................94 8.1.1 Rate of fouling............................................................................................................95 8.1.2 Effects of operating parameters on fouling..................................................................96 8.2 ACCOUNTING FOR THE EFFECTS OF FOULING ............................................................................97 3 NOMENCLATURE SYMBOLS Roman A area [ m2 ] B width [ m ] c specific heat [ J / kgK ] CC& heat capacity rate [ W / K ] C* ratio of heat capacity rates in e-NTU method. [-] Cf Fanning friction factor (coefficient of friction) [ - ] d diameter [ m ] dh hydraulic diameter [ m ] e surface roughness [ m ] f Darcy (Moody) friction factor [ - ] F correction factor in logarithmic mean temperature difference method [-] G 1. mass velocity [ kg / m2s ] 2. conductance [ W / K ] h convection heat transfer coefficient [ W / m2K ] 2/3 jH Colburn j-factor, St Pr [ - ] k thermal conductivity [ W / mK ] K unit resistance [ - ] L length [ m ] m mass [ kg ] m& mass flow rate [ kg / s ] NTU Number of Transfer Units, (dimensionless conductance), [ - ] Nu Nusselt number [ - ] p pressure [ Pa ] P 1. temperature effectiveness [ - ] 2. power [ W ] Pr Prandtl number [ - ] q heat transfer rate [ W ] q” heat flux [ W / m2 ] 3 qV volume flow rate [ m / s ] r radius [ m ] R 1. (thermal) resistance [ K / W ] 2. ratio of heat capacity rates in logarithmic mean temperature difference method or P-NTU method. [-] R” (thermal) resistance per area [ m2K / W ] 2 1. R”tc contact resistance per area [ m K / W ] 2 2. R”f fouling resistance per area [ m K / W ] Re Reynolds number [ - ] s 1. wall thickness [ m ] 2. fin spacing [ m ] S spacing [m] St Stanton number [ - ] t thickness [ m ] 4 T temperature [ K ] U overall heat transfer coefficient [ W / m2K ] v
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