Thermal Design Considerations

Contents Acknowledgments............................................................................................ i List of figure .................................................................................................. iv List of table..................................................................................................... v NOMENCLATURE ...................................................................................... vi Preface .......................................................................................................... vii Chapter 1: Introduction ................................................................................. 1 1.1. Heat exchanger......................................................................................................... 1 1.2. Classification of heat exchanger............................................................................... 1 1.2.1. Fixed tube sheet exchanger............................................................................................ 2 1.2.2. Removable tube bundle ................................................................................................. 3 1.2.2.1. U – Tube ........................................................................................................................ 3 1.2.2.2. Floating head ................................................................................................................. 4 Chapter 2: Thermal Design Considerations ................................................... 6 2.1. Shell.......................................................................................................................... 6 2.2. Tube.......................................................................................................................... 7 2.3. Tube pitch and tubelayout ....................................................................................... 8 2.4. Tube passes............................................................................................................... 9 2.5. Tube sheet............................................................................................................... 10 2.6. Baffles..................................................................................................................... 11 2.7. Fouling Considerations .......................................................................................... 13 2.8. Selection of fluids for tube and the shell side.......................................................... 15 Chapter 3: Thermal Design Process ............................................................. 16 Chapter 4: Design problem .......................................................................... 20 4.1. Problem statement .................................................................................................. 20 4.2. Fluid properties ...................................................................................................... 21 4.2.1. Crude oil ...................................................................................................................... 21 4.2.2. Kerosene ...................................................................................................................... 22 4.3. Assign fluid to shell and fluid to tube...................................................................... 24 4.4. Evaluate heat duty and outlet temperature of crude oil .......................................... 25 4.5. Estimate the overall heat transfer coefficient.......................................................... 27 iii 4.6. Calculate Logarithmic mean temperature different................................................ 30 4.7. Calculate heat transfer area.................................................................................... 32 4.8. Decide appropriate tubes......................................................................................... 33 4.9. Calculate tube – side heat transfer coefficient ........................................................ 35 4.10. Calculate shell diameter.......................................................................................... 37 4.11. Calculate shell – side heat transfer coefficient........................................................ 39 4.12. Overall heat transfer coefficient ............................................................................. 41 4.13. Pressure drop.......................................................................................................... 42 Conclusion .................................................................................................... 47 Reference ...................................................................................................... 48

i Acknowledgments I would like to thank to all of the teachers in the department petrochemical, oil and gas faculty, Hanoi University of Mining and Geology was dedicated teach me during the time I study and practice at school I am deeply extending my sincere appreciation to my instructor, Dr Vũ Văn Toàn, for his valuable advice, constant support, commitment, dedication, encouragement and precious guidance, creative suggestions and critical comments, and for his being everlasting enthusiastic from the beginning to the end of the seminar Without his urge, no doubt, this work would not have been possible at all And finally, I would like to thank my parents, friends who always there encouraging me during years as well as during I my graduation thesis ii Contents Acknowledgments i List of figure iv List of table v NOMENCLATURE vi Preface vii Chapter 1: Introduction 1.1 Heat exchanger 1.2 Classification of heat exchanger 1.2.1 Fixed tube sheet exchanger 1.2.2 Removable tube bundle 1.2.2.1 U – Tube 1.2.2.2 Floating head Chapter 2: Thermal Design Considerations 2.1 Shell 2.2 Tube 2.3 Tube pitch and tube-layout 2.4 Tube passes 2.5 Tube sheet 10 2.6 Baffles 11 2.7 Fouling Considerations 13 2.8 Selection of fluids for tube and the shell side 15 Chapter 3: Thermal Design Process 16 Chapter 4: Design problem 20 4.1 Problem statement 20 4.2 Fluid properties 21 4.2.1 Crude oil 21 4.2.2 Kerosene 22 4.3 Assign fluid to shell and fluid to tube 24 4.4 Evaluate heat duty and outlet temperature of crude oil 25 4.5 Estimate the overall heat transfer coefficient 27 iii 4.6 Calculate Logarithmic mean temperature different 30 4.7 Calculate heat transfer area 32 4.8 Decide appropriate tubes 33 4.9 Calculate tube – side heat transfer coefficient 35 4.10 Calculate shell diameter 37 4.11 Calculate shell – side heat transfer coefficient 39 4.12 Overall heat transfer coefficient 41 4.13 Pressure drop 42 Conclusion 47 Reference 48 iv List of figure Figure 1.1: Classification of shell and tube heat exchanger .2 Figure 1.2: Fixed tube sheet heat exchanger Figure 1.3: U - tube heat exchanger Figure 1.4: Floating head heat exchanger Figure 2.1: Shell of a heat exchanger Figure 2.2: Tube of a straight tube heat exchanger Figure 2.3: Heat exchanger tube layout Figure 2.4: Straight tube heat exchanger one and two pass tube side .9 Figure 2.5: Tube passes arrangement Figure 2.6: Stainless tube sheet .10 Figure 2.7: Different type of heat exchanger baffles 11 Figure 2.8: Baffle cut 12 Figure 2.9: Baffle cut and fouling in shell .12 Figure 2.10: The fouling in heat exchanger 13 Figure 4.1: Specific heat of hydrocarbon liquid .25 Figure 4.2: Overall heat transfer coefficient 28 Figure 4.3: Correction factor F 31 Figure 4.4: Tube side heat transfer factor 35 Figure 4.5: Shell bundle clearance .38 Figure 4.6: Shell side heat transfer factor 40 Figure 4.7: Tube side friction factor 42 Figure 4.8: Shell side friction factor 43 v List of table Table 2.1: Typical values of fouling coefficients and resistances 14 Table 2.2: Guidelines for placing the fluid in order of priority 15 Table 4.1: Typical overall heat transfer coefficient 27 Table 4.2: Constant for calculating shell diameter 37 Table 4.3: The common tube per pass 45 vi NOMENCLATURE Symbol q̇ mh mc Cp,h Cp,c Th,i Th,o Tc,i Tc,o U A ΔTm di Nt Np At Pt As Ds De Db Cb L ht hs Ua U0 u P k Re Pr Nu Description Total heat transfer Mass of hot fluid Mass of cold fluid Specific heat of hot fluid Specific heat of cold fluid Hot fluid temperature at inlet Hot fluid temperature at outlet Cold fluid temperature at inlet Cold fluid temperature at outlet Overall heat transfer coefficient Heat transfer area Logarithmic mean temperature different Outer diameter of tube Inner diameter of tube Number of tubes Number of tube pass Cross area of tube Tube pitch Shell side cross flow area Internal shell diameter Equivalent diameter Bundle diameter Clearance Length of shell Tube – side heat transfer coefficient Shell – side heat transfer coefficient Assumed over all heat transfer coefficient Calculated over all heat transfer coefficient Velocity Pressure Thermal conductivity Renoldous number Pranditle number Nussult number Unit W kg kg kJ/kg.oC kJ/kg.oC o C o C o C o C W/m2.oC m2 o C m m m2 m m2 m m m m m W/m2.oC W/m2.oC W/m2.oC W/m2.oC m/s Bar W/m.oC vii Preface Heat exchangers are systems of thermal engineering in which its applications are occurred in different industries Heat exchangers are the basic or heart of once organized plant since it transfers energy to the processing plant This paper describes about thermal design considerations and the thermal design of shell and tube heat exchangers Chapter 1: Introduction 1.1 Heat exchanger Heat exchanger is a device that is used to transfer thermal energy no mixing between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid For the heat transfer to occur two fluids must be at different temperatures and they must be thermal contact Heat exchange involves convection in each fluid and conduction through the separating wall Heat can flow only from hotter to cooler fluids However, they are not only used in heating applications but are also used in cooling applications, such as refrigerators and air conditioners 1.2 Classification of heat exchanger Many types of heat exchangers can be distinguished from on another based on the direction the liquids flow In such applications, the heat exchangers can be and be parallel-flow, cross-flow, or countercurrent In parallel-flow heat exchangers, both fluid involved move in the same direction, entering and exiting the exchanger side by side In cross-flow heat exchangers, the fluid paths run perpendicular to one another In countercurrent heat exchangers, the fluid paths flow in opposite directions, with each exiting where the other enters Countercurrent heat exchangers tend to be more effective than other types of exchangers Heat exchangers are also typically classified according to transfer processes, number of fluids, surface compactness, construction features and heat transfer mechanisms Amongst of all type of exchangers, shell and tube exchangers are most commonly used heat exchange equipment Figure 1.1: 1: Classification of shell and tube heat exchanger 1.2.1 Fixed tube sheet exchanger Mostly, it is used in high pressure and high temperature applications Fixed tube sheet heat exchangers are the one that are very much used in process chemical industries and refinery services, as there is absolutely no chance for intermixing of fluids This type of heat exchanger is employed where even slightest intermixing of fluids can’t be tolerated A fixed tube sheet heat exchanger has straight tubes that are secured at both ends to tube sheets welded to the shell The principal advantage of the fixed tube sheet construction is its low cost because of its simple construction In fact, the fixed tube sheet is the least expensive construction type The disadvantage of fixed tube sheet heat exchanger is shell pass cannot be cleaned with the mechanical method and can be cleaned with chemical method only Their maintenance process is difficult The Fixed tube -sheet heat exchanger changer is applicable to all services where the temperature difference between the shell and tube is small The temperature difference is slightly great but the pressure of shell pass is not high with media in the shell pass not easy to scale Figure 1.2: Fixed tube sheet heat exchanger 1.2.2 Removable tube bundle A fixed tube sheet heat exchanger is the cheapest because of the ease of fabrication This heat exchanger requires periodic cleaning, replacement of tubes etc and inside of the tubes can be easily cleaned by mechanical means (by forcing wire brush or worm) and cleaning of the tubes from outside require removal of the tubes bundles from the heat exchanger, in addition to the above cited difficulty many heat exchangers are provided with removable tubes bundles So as to make removal of the tube bundles possible and to allow for considerable expansion of the tubes, a removable tube bundle exchanger is used 1.2.2.1 U – Tube In the U-tube exchanger, a bundle of nested tubes, each bent in a series of concentrically tighter U-shapes, is attached to a single tube sheet Each tube is free to move relative to the shell, and relative to one another, so the design is ideal for situations that accommodate large differential temperatures between the shell side and the tube side fluids during service Such flexibility makes the U-tube exchanger ideal for applications that are prone to thermal shock or intermittent service The Utube bundle provides minimum clearance between the outer tube and the inside of the shell for any of the removable-tube-bundle constructions Clearances are of the same magnitude as for fixed-tube-sheet heat exchangers 34 Tube side velocity u = 75 = 1.59 m/s 785 ∗ 0.06 35 4.9 Calculate tube – side heat transfer coefficient Re = ρρu d 785 ∗ 1.59 ∗ 17.018 ∗ 10 = μ 1.3 ∗ 10 Pr = = 16340 μC 1.3 ∗ 10 ∗ 2.219 ∗ 10 = = 21.37 37 k 0.135 L 4880 = ≈ 287 d 17.018 By using the following figure, I look up the value of jh = 4*10-3 with Re = 16340 and L/din = 287 Figure 4.4: Tube side heat transfer factor Calculate Nusselt number Nu = j ∗ Re ∗ Pr μ μ Viscosity of water µw is 0.354 mPa.s Nu = j ∗ Re ∗ Pr μ μ = ∗ 10 = 217.6 ∗ 16340 ∗ 21.37 37 1.3 0.354 36 Tube – side heat transfer coefficient h = Nu k 217.6 ∗ 0.135 = = 1726 W/m d 17.018 ∗ 10 C 37 4.10 Calculate shell diameter An estimate of the bundle diameter Db can be obtained from equation N K D =d ∗ The constants for use in this equation, for triangular and square patterns, are given in the following table Table 4.2: Constant for calculating shell diameter For tube passes and triangular pitch layout tube, K1 = 0.175, n1 = 2.285, D = 25.4 ∗ 265 0.175 = 626 mm For tube pass and square pitch layout tube, K1 = 0.158, n1 = 2.263, D = 25.4 ∗ 265 0.158 = 675.69 mm Since bundle diameter of square pitch is greater than bundle diameter of triangular pitch the smallest diameter is better to use With this concept the corresponding shell clearance for Db = 0.626m and split ring floating head 38 Figure 4.5: Shell bundle clearance For a split ring floating head exchanger the typical shell clearance is 63mm, so the shell inside diameter is D = 626 + 63 = 689 mm 39 4.11 Calculate shell – side heat transfer coefficient As a first, I take the baffle spacing (Bs) = 0.5*Ds, say 344.5 mm The area for cross flow A = p −d p ∗D ∗B = 31.75 − 25.4 ∗ 689 ∗ 344.5 31.75 = 47472 mm ≈ 0.0475 m The shell equivalent diameter for a triangular pitch arrangement D = 1.1 1.1 ∗ (p − 0.917d ) = ∗ (31.75 − 0.917 ∗ 25.4 ) d 25.4 = 18.03 mm Volume flow rate on shell – side 60 = 0.084 m /s 715 Shell – side velocity u = 0.084 = 1.77 m/s 0.0475 Calculate the shell-side Reynolds number Re = u D ρ 1.77 ∗ 18.03 ∗ 10 = μ 0.21 ∗ 10 Pr = ∗ 715 = 108656 μC 0.21 ∗ 10 ∗ 2.558 ∗ 10 = = 4.1 k 0.131 Baffle cuts can vary between 15% and 45% and are expressed as ratio of segment opening height to shell inside diameter The upper limit ensures every pair of baffles will support each tube In this problem, I use segmental baffles with a 25% cut This should give a reasonable heat transfer coefficient without too large a pressure drop 40 By using the following figure, I look up the value of jh = 2*10-3 with Re = 108656 and baffle cut = 25% Figure 4.6: Shell side heat transfer factor Calculate Nusselt number Nu = j ∗ Re ∗ Pr μ μ = ∗ 10 0.21 0.354 ∗ 108656 ∗ 4.1 = 323 Shell – side heat transfer coefficient h = Nu k 323 ∗ 0.131 = = 2347 W/m D 18.03 ∗ 10 C 41 4.12 Overall heat transfer coefficient U= +R h d d ln d d + + +R d 2k h ht: tube – side heat transfer coefficient hs: shell – side heat transfer coefficient do: outside diameter of tube di: inside diameter of tube RC: fouling factor of Crude oil RK: fouling factor of Kerosene kw: thermal conductivity of the wall Both Crude oil and Kerosene are corrosive so I chosen stainless steel for either tube and shell Thermal conductivity of stainless steel is 55 Wm-1oC-1 U= 25.4 25.4 ∗ 10 ∗ ln 25.4 17.018 + 0.00054 + + + 0.000352 1726 17.018 ∗ 55 2347 = 393 W/m C This value is too high with initial assumed overall heat transfer coefficient so I need adjust some value in my design 42 4.13 Pressure drop 4.13.1 Tube – side ∆P = N 8j L d μ μ + 2.5 ρu Np: number of tube – side passes jf: friction factor ut: tube – side velocity L: length of one tube m = 0.25 for laminar flow, Re < 2100, = 0.14 for turbulent flow, Re > 2100 Figure 4.7: Tube side friction factor By using tube – side friction factor, I look up jf = 4.4*10-3 with Re = 16340 And in this problem, the tube flow is turbulent flow so the value of m equal 0.14 Pressure drop of tube – side 43 ∆P = ∗ 4.4 ∗ 10 4880 17.018 1.3 0.354 + 2.5 785 ∗ 1.59 = 43316 N/m = 0.43 atm The pressure drop is within the limit (0.43atm < 0.8atm) the design on tube – side is acceptable 4.13.2 Shell – side D ∆P = 8j D L ρu B μ μ jf: friction factor Bs: baffle spacing us: shell – side velocity Figure 4.8: Shell side friction factor By using tube – side friction factor, I look up jf = 3.5*10-2 with Re = 108656 and baffle cut = 25% 44 Pressure drop of shell – side ∆P = ∗ 3.5 ∗ 10 689 18.03 4880 715 ∗ 1.77 344.5 0.21 0.354 = 182635 N/m = 1.8 atm The pressure drop of shell exceeded permissible limit (1.8 > 1) so shell must be adjusted To decrease pressure drop in shell side, we can decrease the number of baffle or decrease velocity of shell side The first trial, to decrease the number of baffle, baffles spacing need to be increased To achieve pressure drop decrease 1.8 times, baffle spacing need increase about 1.8 times So I adjust Bs = 0.9Ds Recalculate from step “Calculate shell – side heat transfer coefficient”, I have result Bs = 620 mm Nu = 214 As = 0.0854 m hs = 1557 W/m2oC us = 0.98 m/s U = 362 W/m2oC Re = 55250 jf = 3.9*10-2 jh = 2.6*10-3 ΔPs = 1.1 atm (approximate the restriction) The second trial, to decrease the velocity of shell side, we can increase the number of tube By use the following table, I chose 338 tubes per pass and recalculate from “Decide appropriate tube” I have result Tube side Shell side ut = 1.24 m/s Ds = 760 mm Nu = 280 Re = 12770 Bs = 380 mm hs = 2024 W/m2oC Nu = 170 As = 0.0577 m jf = 3.6*10-2 ht = 1350 us = 1.46 m/s ΔPs = 1.27 atm jf = 4.5*10-3 Re = 89355 ΔPt = 0.27 atm jh = 2.1*10-3 45 U = 350 W/m2oC By using the following table and combine two trials, I decide to chose 338 tubes per pass and increase baffle spacing Bs = 0.7Ds for third trial I recalculate from “Decide appropriate tube” Table 4.3: The common tube per pass 46 I obtained the result: Tube side Shell side ut = 1.24 m/s Ds = 760 mm Nu = 237 Re = 12770 Bs = 530 mm hs = 1721 W/m2oC Nu = 170 As = 0.08 m jf = 3.8*10-2 ht = 1350 us = 1.04m/s ΔPs = 0.5 atm jf = 4.5*10-3 Re = 63825 ΔPt = 0.27 atm jh = 2.5*10-3 U = 340 W/m2oC After three revisions, I think the values of the equipment were consistent with design requirements The difference of overall heat transfer coefficient calculated and assumed equal 340 − 300 ∗ 100% = 13.33% 300 This difference is acceptable so I can summary some characteristic of heat exchanger Summary: The proposed design  Split ring, floating head, shell pass and tubes passes  338 carbon steel tubes, 4.88 m long, 25.4 mm o.d., 17.018 mm i.d., triangular pitch, pitch 31.75 mm  Shell i.d 760 mm, baffle spacing 530 mm, 25% cut  Overall coefficient estimated 300 W/m2 oC  Overall coefficient calculated 340 W/m2 oC  Pressure drops: 0.27 atm on tube side, 0.5 atm on shell side 47 Conclusion From knowledge gained through the process of graduation thesis with the subject: “Design a shell and tube heat exchanger using in petroleum industry”, I obtained the following results: Learn functions, classification for heat exchangers Learn the basis, the factors necessary for designing a shell and tube heat exchanger with single phase Calculate a particular problem to provide a complete thermal design 48 Reference Sadik Kakac Hongtan Liu, Heat Exchangers Selection, Rating, and Thermal Design second edition, 2002 by CRC Press LLC Donald Q Kern, Process Heat Transfer (1965), McGraw – Hill book Co Singapore for manufacture and export Standards of Tubular Exchanger Manufacturers Association Stephen Whitaker, Fundamental Principles of Heat Transfer (1983), Robert E Krieger Publishing Company http://www.hrs-heatexchangers.com/en/resources/how-to/design-a-tubularheat-exchanger.aspx http://web2.clarkson.edu/projects/subramanian/ch302/notes/designshelltube pdf http://www.hcheattransfer.com/shell_and_tube.html http://www.chemstations.com/content/documents/Technical_Articles/shell.p df https://www.youtube.com/watch?v=NherLU3IVPY 10 https://www.youtube.com/watch?v=QMg3vr7KgDA