International Journal of Engineering & Science Research DESIGN AND CFD ANALYSIS OF U TUBE HEAT EXCHANGER P.B. Borade* 1, K.V.Mali 2 1 P.G. Student, Mechanical Department, Sinhgad College of Engineering, Vadgaon (Bk.), Pune. 2 Assistant Professor, Mechanical Department, Sinhgad College of Engineering, Vadgone, ABSTRACT Pune. In present study, comparative analysis made on Thermal design and CFD analysis of U shaped heat exchanger tube. U shaped tube is used in kettle reboiler type shell and tube heat exchanger. Chlorosilanes and R22 fluid flows across the tube and shell respectively. Heat transfer and pressure drop for tube side flow were calculated using Kern method. Tube side Reynold number calculated for given mass flow rates. The tube side pressure drop was determined by CFD analysis. The calculated pressure drop of tube side was compared with CFD analysis. Keywords: U shaped heat exchanger tube, Pressure drop, CFD analysis. 1. INTRODUCTION Shell and tube heat exchangers are known as the work-horse of the chemical process industry and represent the most widely used vehicle for transfer of heat in industrial applications. In essence, a shell and tube exchanger is a pressure vessel with many tubes inside of it. One process fluids flows through the tubes of the exchanger while the other fluid flows outside of the tubes within the shell. Shell and tube heat exchangers have the ability to transfer large amounts of heat in relatively low cost, serviceable designs. They can provide large amounts of effective heat transfer surface while minimizing the requirements of floor space, liquid volume and weight[4]. The TEMA K shell, also termed a Kettle Reboiler, is specified when the shell side stream will undergo vaporization. The liquid level of a K shell design should just cover the tube bundle, which fills the smaller diameter end of the shell. This liquid level is controlled by the liquid flowing over a weir at the far end of the entrance nozzle. The expanded shell area serves to facilitate vapor disengagement for boiling liquid in the bottom of the shell. To insure against excessive liquid carry-though with the vapor stream, a separate vessel as described above is specified. Liquid carry-through can also be minimized by installing a mesh demister at the vapor exit nozzle. U-bundles are typically used with K shell designs. K shells are expensive for high pressure vaporization due to shell diameter and the required wall thickness. *Corresponding Author www.ijesr.org 1486
Nomenclature: A o Outer surface area of coiled tube, n No. of tubes (m 2 ) f constant,(-) k thermal conductivity, (W/m 2 0 C) G t Tube side mass flow rate,(kg/s) Nu Nusselt number,(-) R curvature radius,(mm) Re Reynolds number,(-) t thickness,(mm) U 0 overall heat transfer coefficient, (W/m 2 C) d i Inner diameter of U tube,(mm) ū fluid velocity in x-direction,( m/s) d 0 Outer diameter of U tube,(mm) H averaged convective heat transfer coefficient, (W/m 2 0 C) L Stretch length of coiled d h shell-side hydraulic diameter, (m) tube,(mm) ρ density,(kg/m 3 ) g Acceleration due to gravity,(m/s 2 ) FCV Flow control valve,(-) Q Heat transfer,(watt) µ Viscosity, (cp) total Total Cp specific heat (kj/kg 0 C) T1 temperature difference at outlet, ( 0 C) T Temperature, ( 0 C) T1 temperature difference at outlet, ( 0 C) LM TD Log mean temperature Difference,( 0 c) Subscripts i inside condition O outside condition Kettle Reboiler used when a high turndown or a high quality vapors is required. They are also used when large heat transfer surfaces are needed. Kettle reboilers are expensive due to the shell design but are able to handle large differences in temperature between the fluids due to their U-tube design [2]. This is the cheapest of all removable bundle designs, but is generally slightly more expensive than a fixed tubesheet design at low pressures. However, it permits unlimited thermal expansion, allows the bundle to be removed to clean the outside of the tubes, has the tightest bundle to shell clearances and is the simplest design [4]. 2. GEOMETRY OF U SHAPED HEAT EXCHANGER TUBE A Figure 1 gives the schematic of a U shaped heat exchanger tube. The tube has an inner diameter d i = 13.512 mm, outer diameter d o = 19.05 mm, length of straight tube L = 2000 mm, radius of curvature for 180 0 bend R = 215 mm, total length of tube = 4675 mm. While the working fluid Chlorosilanes. The geometry was created using CATIA V5 R18 design software. The shell and tube heat exchanger is single-phase heat exchanging system in which a hot chlorosilanes flow inside the tube-side is cooled by a R22 flowing in the shell-side. Copyright 2012 Published by IJESR. All rights reserved 1487
Fig. 1: Geometry of U Shaped Heat Exchanger Tube Operating Parameter and Dimension Table1: Various parameters of shell and tube heat exchangers Shell side Tube side Inlet Outlet Inlet Outlet R22 Chlorosilanes Total fluid quantity 654 11000 Liquid,(Kg/hr) 438 0 11000 11000 Vapor,(Kg/hr) 216 654 Inlet Outlet Inlet Outlet Liquid Vapor Vapor Liquid Liquid Density,(Kg/m 3 ) 1452 4.38 4.37 1401 1421 Viscosity,(cP) 0.242 0.011 0.011 0.493 0.569 Specific heat,( Kj/Kg 0 K) 1.08 0.62 0.62 0.82 0.8 Thermal conductivity,( W/m 0 K) 0.131 0.007 0.007 0.139 0.143 Temperature,( 0 C) -43.9-42.7-29 -39 Pressure,(Pa) 7845.32 97882.97 Table 2: Characteristic dimensions of U shaped heat exchanger tube Dimensional Parameters U shaped heat exchanger tube Tube id (d i ), mm 13.512 Tube od (d o ), mm 19.05 Thickness t, mm 2.769 Pitch, mm 23.81 Radius of curvature, mm 215 Triangular Pitch 30 0 Straight Tube length, mm 2000 2.1 Material and Method The U tube heat exchanger was constructed from SA 213 304 tubing. Chlorosilanes is the tube side fluid whose molecular formula is H 3 ClSi and molar Mass: 66.56 g mol -1. Thermal design was done by Kern s Method. In thermal design heat transfer rates and pressure drop were determined. A CFD methodology has been used to investigate the pressure and temperature variation across the U Tube. In this investigation, CFD package ANSYS Fluent v12.1 (double precision, 3D version) was used. Copyright 2012 Published by IJESR. All rights reserved 1488
2.2 Data Collection and analysis In present investigation work the heat transfer rates and pressure drop were determined. The heat flowing from tube side hot water to shell side cold water. The operating parameter is given in table 1. Tube Side Heat transfer = ( ) (1) Shell Side Heat Transfer = ( ) (2) The physical properties of taken on average temperature =( + )/2 (3) The heat transfer coefficient was calculated with, = (4) LMTD is the log mean temperature difference, based on the inlet temperature difference, and outlet temperature difference, = ( ) ( / ) (5) The tube side pressure drop is the sum of the pressure drop through the tubes, the pressure drop through the channels and pressure drop through the 180 0 bend, P T = [ + ρ ρ + ρ ] (6) Reynold Number for Tube Side Flow Re = µ (7) Table 3: Result of Thermal Design Tube side Heat Transfer, kw 29 LMTD, o C 6.75 Reynold number 3772.4 Pressure drop, Pa 272.93 2.3 Meshing The mesh was created using meshing module of the ICEM and ANSYS FLUENT package. The Hexa mesh is created by first making a blocking. Approach toward blocking is by Bottom up topology creation. In this approach blocking is built up like laying bricks. Copyright 2012 Published by IJESR. All rights reserved 1489
Fig 2: Mesh across straight Tube At the starting block is created across the straight tube, and then it extruded and associated through the remaining curve and straight tube. A blocking breaks down a geometry into large brick-shapes and structures the direction of grid lines by the arrangement of the blocks. An O grid is a series of blocks created in one step which arranges grid lines into an O shape or a wrapping nature Figure 2 and 3 shows Mesh model across the U Tube. O-grid is created in the tube. The mesh is composed of 2317128 nodes and 2528580 elements. 2.4 Boundary condition Fig 3: Mesh across Tube Bend In this analysis pressure drop and temperature drop across the tube were determine. The steady state, pressure based type solver with an absolute velocity formulation was used. As the reynold no. greater than 2300, turbulent case was used. Energy, momentum and turbulence equation were solved. Boundary at the Inlet given as velocity inlet with velocity 0.1 m/s and at outlet as pressure outlet with approximate pressure 97670 Pa. In the turbulent regimes, the Realizable k-ε turbulence model was used. Accurately predicts the spreading rate of both planar and round tube. For turbulent internal flow, the intensity and hydraulic diameter D specification method was used. The direction of the flow was defined normal to the boundary. Momentum, energy, turbulence kinetic energy and turbulence dissipation rate equations were discredited using second order upwind scheme. Pressure was discredited using linear scheme. The turbulent intensity was estimated for each case based on the formula I = 0.16(Re)-1/8 and was set at 3% from calculations. Copyright 2012 Published by IJESR. All rights reserved 1490
2.5 Solutions Steady state simulations were carried out by solving mass, momentum and energy conservation equations, which are expressed as: ρ ρū ρ + ρū 0 (8) Where, ρūū ρg P τ (9) ū ρe P K T τ ū (10) µ ū ūt ūt (11) Turbulent flow was modeled using realizable version of k-ε model. The k and ε equations are as given below ρ ρ a µ G G ρε Y S (12) Convergence criterion used was 10-5 for continuity, velocities, k, and ε. Convergence criterion for energy balance was 10-7. Under relaxation factors was 0.3 for pressure, 1.0 for density, 0.7 for momentum, 1.0 for body force, 0.8 for turbulent kinetic energy and turbulence dissipation rate and 1.0 for energy. 3. RESULT AND DISCUSSION The Result of Pressure drop and Temperature drop across the tube got by Post processing. While Post processing is done by CFX (CFD POST 12) software. As the Reynold no. greater than 2300, turbulent case was used. In the turbulent regimes, the Realizable k-ε turbulence model was used. 3.1 Result of Pressure drop Pressure drop across U shaped heat exchanger tube for liquid Chlorosilanes is shown in fig 4. Pressure at the inlet is vary from 97882.969 Pa to 97881.635 Pa. pressure drop across the tube inlet is 1.34 Pa. Fig 4: Pressure variation across tube inlet Copyright 2012 Published by IJESR. All rights reserved 1491
Figure 5 shows pressure variation across the tube length. Pressure at the inlet is 97882.969 Pa and pressure at the outlet is 97674 Pa. According to thermal design, pressure drop across the tube is 272.93 Pa. Fig 5: Pressure variation across the Tube Length Table 4 shows that pressure across the straight tube and bend varies. Pressure drop in straight tube from inlet to bend is 89.96 Pa while pressure drop across the bend is 33 Pa. Pressure drop from bend to straight tube is 86 Pa. Table 4: Result of Pressure variation across the length Pressure drop From inlet to bend Pa 89.96 Pressure drop across the bend Pa 33 Pressure drop From bend to outlet Pa 86 Total Pressure drop across the tube Pa 208.96 Figure 6 draws the graph of Pressure Versus Tube length. The graph shows Pressure variation across the tube length. 97900 97850 Pressure 97800 97750 97700 97650 0.00 0.57 1.22 1.78 2.28 2.80 3.36 3.92 4.48 Length of Tube Fig. 6: Variation of Pressure with Length of Tube Copyright 2012 Published by IJESR. All rights reserved 1492
3.2 Result of Temperature drop Figure 7 shows temperature variation across the tube. Temperature at the inlet is 244 K and at the outlet 234.993 K. Hence the temperature drop across the tube is 9.007 K. Required outlet temperature is 234 K. 4. CONCLUSION Fig 7: Temperature variation across the Tube Length An experimental study of U shaped heat exchanger tube was performed by using CFD analysis and thermal design with the help of Kern method. The pressure drop for across the straight tube and bend was checked. Heat transfer across the heat exchanger is maintained. Table 5: Comparison of Pressure variation across the length Pressure drop by thermal design Pressure drop by CFD analysis Pressure drop From inlet to bend Pa 115.67 89.96 Pressure drop across the bend Pa 53 33 Pressure drop From bend to outlet Pa 104.26 86 Total Pressure drop across the tube Pa 272.93 208.96 CFD Analysis gives pressure drop and temperature drop across the tube. Comparison between thermal design and CFD analysis shows that less pressure drop occurs across the tube while Temperature at inlet and outlet is approximately maintained. Acknowledgements Prof. V. N. Kapatkar & Mr. Umesh S. Ubarhande for their valuable input and encouragement, currently supports this work. Copyright 2012 Published by IJESR. All rights reserved 1493
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