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Selecting Tube Inserts for Shell- and-Tube Heat Exchangers Group 4: Daniel Ehlers David Ehlig Erik Maki Darren Finney Tasnim Mohamed

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Nomenclature A = correlation constant for static mixer heat- transfer equation (Eq. 3) B = exponent for static mixer heat-transfer equation (Eq. 3) C p = specific heat, J/kg-K D = inside tube diameter, m D e = equivalent inside tube diameter for turbulent flow heat transfer, m D h = inside hydraulic tube diameter, m D h1 = inside hydraulic tube diameter with Insert 1, m D h2 = inside hydraulic tube diameter with Insert 2, m G = mass velocity of fluid, kg/s-m2 G z = Graetz number (Eq. 1) h core = heat-transfer coefficient with core insert, W/m2-K h tube = heat-transfer coefficient without insert, W/m2-K h 1 = heat-transfer coefficient with Insert 1, W/m2-K h 2 = heat-transfer coefficient with Insert 2, W/m2-K k = thermal conductivity, W/m-K L = fluid flow length inside tube from entrance to first boundary layer interruption, m L 1 = interrupted flow length with Insert 1, m L 2 = interrupted flow length with Insert 2, m N fa = net free area inside tube with or without insert, m2 Nu = Nusselt number (Eqs. 2 and 3) Pr = Prandtl number = Cpμ/k Re = Reynolds number = ρvDh/μ v = velocity of the fluid, m/s Greek Letters μ = fluid viscosity, N-s/m2 μ w = fluid viscosity at the inside tube wall temperature, N-s/m2 ρ = fluid density, kg/m3 2

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Introduction/Methodology Shell-and-tube heat exchangers are a class of heat exchanger designs. They are the most common type of heat exchangers in oil refineries and other large chemical processes. Steps to specifying a shell-and-tube heat exchanger: Select a shell design Determine most effective baffle arrangement Focus on tube-side design Figure 1: Cross-sectional diagram of a U-tube Heat Exchanger. Arrows show fluid flow pathways on both shell and tube sides. 3

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Laminar Tubeside Flow Patterns Velocity Profile Temperature profile Velocity Flow Pattern Velocity is lowest at the walls and greatest at the center An inviscid region forms in the center of the pipe Temperature Flow Pattern An isothermal region forms at the center of the pipe Fluids with high thermal conductivity form short thermal entry lengths Fluids with low thermal conductivity form long thermal entry lengths R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, Figure 2 Figure 3 4

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Single-phase Heat Transfer Inserts Heat transfer inserts improve heat exchanger efficiency by: Disturbing the inviscid, isothermal separation region which increases thermal energy transfer inside the tube Increasing heat exchanger life-span and dependability Reducing energy usage and maintenance expenditures Reducing general emissions Student generated figure TUBE INSERT Isothermal Separation Thermal Mixing Figure 4 5

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Types of Single-Phase Heat Transfer Inserts [1] [2] R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, Static Mixers Boundary Layer Interrupters Displaced-Flow Insert Swirl-Flow Insert Figure 5 Figure 6 Figure 7 Figure 8 6

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Determining Optimal Tube Inserts function chooseinsert(Re,Pr,Dh,L) %function determines which tube insert is optimal %Re = Reynolds number %Pr = Prandtl number %Dh = tube's hydraulic diameter (m) %L = fluid flow length from the tube's entrance to the first boundary layer %interruption (m) Graetz=Re*Pr*(Dh/L) if Graetz<=20 'insufficient energy in the flow for augmentation' End if Graetz 20 'Laminarization Present - USE STATIC MIXER INSERT' end if Re 200 'USE BOUNDARY LAYER INTERRUPTION INSERT' elseif (Re > 200) & (Re 200 'USE EITHER BOUNDARY LAYER OR SWIRL FLOW INSERTS' elseif (Re>1000) & (Re 200 'USE SWIRL FLOW INSERT' elseif Graetz>200 & Re>2300 'USE DISPLACED FLOW INSERT' end >> chooseinsert(2500,12,6,5) Graetz = ans = USE DISPLACED FLOW INSERT >> chooseinsert(500,12,10,.5) Graetz = ans = USE EITHER BOUNDARY LAYER OR SWIRL FLOW INSERTS >> chooseinsert(500,.1,1,1) Graetz = 50 ans = Laminarization Present - USE STATIC MIXER INSERT Examples 7

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Laminarized Flow Graetz Number: Laminarized flow - occurs when the thickness of the laminar boundary layer becomes equal to the dimension of the flow channel and there is no free flow stream beyond the boundary layer A useful dimensionless number used to estimate the onset of the laminarized regime Laminarization occurs for viscous liquid flow at Graetz numbers less than about Sieder-Tate Equation for Laminar Flow: 8

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Numerical Differentiation of Gz 9

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LGz LGz Figure 9Figure 10 10

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Numerical Differentiation of Gz 11

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Nusselt Number Plot for Laminar Flow Scenario A) MATLAB Function Creation (Nugraph.m): function Nugraph(Pr,Dh,L,Visc,wVisc) % Input: % Pr = Prandtl number % Dh = Hydraulic Diameter (m) % L = Fluid flow length from tube’s entrance to the first boundary layer interruption (m) % Visc = Fluid Viscocity (n=s/m^2) % wVisc = Fluid viscocity at the inner tube wall’s temperature (N-s/m^2) % Output: Nusselt Number plot over Laminar Reynolds Numbers Re = linspace(0,2300,5000); Nu = 1.75*(Re*Pr*Dh/L).^0.33.*(Visc/wVisc)^0.1 4; plot(Re,Nu); hold on xlabel('Re - Laminar Reynolds Number (0 to 2400)'); ylabel('Nu - Nusselt Number'); title('Nusselt Number Values vs. Laminar Reynolds Numbers'); xlim([0 2400]); grid on; hold off end 12

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Nusselt Number Plot for Laminar Flow Scenario B) MATLAB Input: >> Nugraph(2.0409,.05,5,.001,.001) C) MATLAB Output: Figure 10: Nusselt Number vs. Laminar Reynolds Flow for a given flow scenario. 13

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Static Mixing Insert Laminar Region Static Mixing InsertWell Mixed Region Static mixers are motionless inserts which accelerate the inline mixing by disturbing the flow layers. Commonly used for cooling highly viscous polymers The only mixers which can operate in the laminarized flow region Can increase heat transfer efficiency by six fold There are various types and designs consisting of plates, baffles, helical elements all positioned to direct flow and increase turbulence Figure 11: depicts the fluid mixing performed by a static mixing insert 14

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Boundary-Layer Interruption This insert is preferred for very high Graetz numbers (typically with Reynolds numbers between 1-1,000). This allows the boundary-layer of the fluid to be easily reduced and thinned to its minimum thickness. How effective the reduction/thinning is determined by how high the interrupts are and the spacing between them. This method is usually used to augment the flow of oils that are laminar in nature. R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, Figure 12: Typical interruption layers placed in a laminar flow tube 15

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Boundary-Layer Interruption Cont. The interrupt cannot allow the boundary thicker than the interrupt can handle. The heating process will be rendered ineffective if this happens. The more symmetrical an interrupt is the more likely it will be effective in transferring heating to a fluid. [1] Journal of Heat Transfer: [2] Journal of Heat Transfer: Figure 13: Symmetrical interruption panel and the general flow patterns it creates [1]. Figure 14: Heat flux redistribution for interrupts [2]. 16

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Heat Transfer Increase 17

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Taylor Series Expansion 18

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Zero- to Fourth Order Zero Order: First Order: Second Order: Third Order: Fourth Order 19

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Analysis It seems that the value is oscillating between the value of about 0.75 and 0.63, roughly, with its value slowly approaching the true value. It is probable with more iterations the true value would have been found. Figure 15: Taylor Series value and Relative and True Error values (student generated). Figure 16: Plots of the Relative and True Error (student generated) Order Value EaEa EtEt N/A

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Swirl Flow The swirl-flow augment is most effective with the higher laminar flow rates, which typically has a Reynolds number from ,000. The helical design produces a higher velocity within the tubes. This velocity is related to the flow angle of the insert. As well, this design creates a rotational and centripetal flow that further increases the mixing and turbulence within. –Inducing turbulence at lower Reynolds numbers is what causes successful and effective heat transfer. Figure 18: Illustrated flow through twisted tapes (tubes) Figure 17: Piping with uniform flow that provides no “dead spots” for heating 21

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Displaced Flow Cylindrical Rod Insert Inserts like the one shown above are the simplest types of displaced flow inserts they are supported in the center of the tube and extend the entire length of the tube. Displaced Flow inserts increase heat transfer by lowering the net fee area (Nfa) inside the tube, which creates higher velocities along the tube wall heat transfer surface. R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, 2012 Figure 19 22

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Displaced Flow cont. Equivalent Diameter, D e –The equivalent diameter is used to calculate the new heat transfer coefficient by the equation h core is the new heat transfer coefficient with the insert h tube is the heat transfer coefficient with no insert For fluids with similar viscosities to water, heat transfer can be increased by over 2.5 times –The value increased is dependent on the pressure drop that occurs over the tube 23

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Displaced Flow Rate The flow rate with a displacing insert as shown above can be calculated using the Hagen-Poiseuille equation: Where Figure 20 24

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Flow Regime Overlap Flow Regime overlap: Usually more than one type of insert can be used to improve heat transfer. –The exception being static mixers Some inserts like the one above are designed to take advantage of more than one kind of flow augmentation. –The insert above utilizes both displaced and swirl flow thus further enhancing the heat transfer beyond the value of either insert alone. Wire wrapped core insert R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, 2012 Figure 21 25

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Two-Phase Flow Inserts What is two-phase flow? –Flow in which two phases exist (i.e. gas and liquid flow) Types of two-phase flow regimes: Figure 22 26

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Two-Phase Flow Inserts What are two-phase flow inserts? Devices placed in a tube containing two phases Types of two-phase flow inserts: Static Mixers Boundary-Layer Interruption Devices Displaced-Flow Mechanisms How do these inserts help? These inserts increase turbulence and enhance mixing Figure 23: Enhanced tubes for augmentation of heat transfer. (a) Corrugated or spirally indented tube with internal protuberances. (b) Integral external fins. (c) Integral internal fins. (d) Deep spirally fluted tube. (e) Static mixer insert. (f) Wire-wound insert. 27

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Two-Phase Flow Inserts Static Mixers and Boundary-Layer Interruption Devices Improve heat transfer by a full order of magnitude Improper design results in high pressure drops Figure 24: Simulation of flow division and radial mixing in a static mixer Figure 25: Luer Connection Flow Interrupter This design can be used to automatically purge air from a pressurized fluid delivery system where gravity cannot be used. us/resources/Documents/techpubs pdf 28

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Two-Phase Flow Inserts Displaced-Flow Mechanisms –These inserts enhance heat transfer only as much as the resulting increase in velocity Flow disruption caused by a wire matrix turbulator (A) Laminar flow conditions (B) Turbulence caused by tube inserts Figure 26 29

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Practical Considerations when using Tube Inserts Pressure drop –Tube inserts can significantly increase pressure drop of plain-tube system conditions –Most inserts designed to produce same pressure drop as experienced by a longer, plain tube Figure 27: Pressure drop evolution with vapor quality for G plain = 75 kg/m 2 s and T sat = 5 °C. 30

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Practical Considerations when using Tube Inserts Upset conditions Inserts are attached to faces of tube sheets to allow for maintenance Attachment can be designed to withstand pressure drop if upset conditions are known For example, inserts can be found embedded in a downstream pump if upset conditions not accounted for. Sedimentation occurs when particles (e.g. dirt, sand or rust) in the solution settle and deposit on the heat transfer surface. Like scale, these deposits may be difficult to remove mechanically depending on their nature. Figure 28 31

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Practical Considerations when using Tube Inserts Transient operation –Flow stops and cools to ambient temperature –Start-up pressure drop can reach 100 times normal operating conditions –Heat tube-side fluid to operating temperature before reaching desired flow rate to prevent problems. stress-point-or-temperature-stress-point/ Figure 29: Thermal and temperature stress on heat exchanger 32

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Practical Considerations when using Tube Inserts Materials compatibility Insert material must be compatible with tube material and fluid Carbon steel inserts in a water service often “weld” themselves to the tube wall. Sometimes scrapping of the entire tube bundle is required. Stainless steel and other corrosion- resistant metallurgies is often the best way to avoid this problem Figure 30: Stainless Steel- Threaded tube inserts serve as end plugs in tubing inserts-to-be-fitted-tube-end html 33

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Practical Considerations when using Tube Inserts Fluid condition –Be aware of tube-side fluid conditions –For example, fluid in laminar flow should be relatively free of particulates to prevent tube plugging An interrupter can act as a particulate dam in laminar flow Swirl flow may not produce enough turbulence to carry particulates through each helical rotation Figure 31: Particle Trajectories in a Laminar Static Mixer 34

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Practical Considerations when using Tube Inserts Anticipated fouling Evaluate the extent and types of fouling expected Determine if removing insert for maintenance is possible Figure 32: Heat exchanger in a steam power plant, fouled by macro fouling 35

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Typical Application Process stream preheated using waste heat Maximum energy recovery involves a temperature cross Outlet temperature of cold stream higher than inlet temperature of hot stream Heat exchanger must be either a single counter-flow or multiple shells in series Five alternate tube-side designs were compared and are summarized in Figure 35 on the next slide.. Figure 34: Flow pattern and temperature profile in exchanger showing cross flow Figure 33 36

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Figure 35 Figure 35: Effects of tube inserts on heat transfer [1]. [1] R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep,

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Conclusions Through the use of many different numerical methods, such as Taylor series expansion, centered finite differences, and Richardson extrapolation, we are able to determine several parameters involving pipe flow which allow us to better understand the nature of the flow as well as analyze and compare the effectiveness of different heat transfer inserts. Further Research: –Further research may be performed regarding Flow regime overlap in order to determine a maximum effectiveness of different combinations of heat transfer inserts with regards to heat exchange. –Further research in static mixing may also be performed in order to maximize the effectiveness of the mixing and minimizing the inhibition of the flow through the tube. –Both of these studies would involve using a variety of numerical methods such as the ones used earlier in order to obtain the desired results from the data. 38

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Further Work Suggestions Fluid flow simulations of various inserts Graphical model depicting efficiency of insert Figure 37: A rendering of a set of fluid flow lines from a simulation of a shell and tube heat exchanger. Figure 36: Geometries of peripherally-cut twisted tapes (PTs) and typical twisted tape (TT) 39

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Further Work Suggestions content/uploads/2011/06/temp_micro.png Algorithm to determine best design for each type of tube-insert Figure 38: Fluid flow velocity profile over valve connections Figure 39: Fluid flow velocity profiles over insert Figure 40: Velocity and temperature profiles over tube-inserts 40

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References R. L. Shilling, “Selecting Tube Inserts for Shell-and-Tube Heat Exchangers,” CEP Magazine, pp , Sep, stress-point/ stress-point/ html html

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