EFFECTS ON HEAT TRANSFER RATE FOR SHELL SIDE IN TEMA E-TYPE SHELL AND TUBE HEAT EXCHANGERS DUE TO VARIATION IN THE BAFFLE CUT PERCENTAGE USING CFD SOFTWARE Devanand D Chillal Research Scholar, Department of Mechanical, Basaveshwar Engineering College, Bagalkot, India Uday C Kapale Professor, Department of Mechanical, SGBIT Belagavi, India ABSTRACT An attempt has been made to study the effects of baffle percentage cut on rate of heat transfer on shell side liquid flow in a shell and tube heat exchanger in this work. The Segmental baffles with different percentage of cut ranging from 15% to 40% have been considered for this purpose. A computational fluid dynamics approach using a commercially available software has been adopted to find the results for above cases mentioned. For this study, a shell and tube heat exchanger having single tube pass and single shell pass with parallel flow arrangement is considered. E- type of heat exchanger with staggered tube arrangement is designed using CATIA, a modeling software and the investigation is carried numerically by using Fluent, a commercially available CFD package. The performance is investigated for E- type shell and tube heat exchanger by varying the baffle cut percentage for a given mass flow rates of both the incompressible fluids. The output results from the CFD simulation, the shell side outlet temperatures, the compartmental temperature distribution for the geometry are found. Key Words : Baffle cut, Shell and Tube heat exchangers, Rate of heat transfer, Computational Fluid Dynamics 1. Introduction Heat exchangers are equipments used to exchange heat duty in wide variety of industries. They are available in various classes like tube type, plate type and extended surface type. Shell and tube heat exchangers are tube type of heat exchangers which are available in varied sizes with different heat duty and are the most versatile and commonly used heat exchangers. They find applications in power plants, process industries, nuclear power stations and conventional industries where these may be used as steam condensers, generators in pressurized water reactors, as feed water heaters to boilers, and even for air-conditioning and refrigerating systems. The most popular and versatile are Shell and tube heat exchangers since they may be used to provide large ratios of heat transfer area against volume and its weight. They are available fabricated to industry standards for achieving higher heat transfer coefficients, and do have compact features. The ease of construction and maintenance also is a key feature for its popularity. These www.apjor.com Impact Factor ( GIF) 5.42 Page 39
exchangers can be easily cleaned. Their robustness and shape make these exchangers suitable for high pressure requirement applications in industries. As per TEMA specifications these are available as E,F,K, type and so on Shell and tube heat exchangers are constructed using a bundle of round tubes with different tube arrangement and tube pitch. The bundle is mounted in to a large cylindrical shell using end plates. The axis of the tube axis is made parallel to shell axis. One fluid flows into the tube bundle and the other into the shell. The shell side fluid flow is complex nature and may be both parallel and across to the tubes. This is achieved using different numbers of baffle plates arrangement. The performance of exchanger is involved with various parameters like inter baffle spacing, baffle inclination, baffle cut, tube bundle arrangements, tube pitch, shell side fluid flow volume, tube side fluid flow volume, flow velocity,fluid viscosity etc. The fluid flow in the tube bundle is simple and easy to realize, however the fluid flow on the shell side fluid is of complex nature involving localised pressure and temperature variations with baffle arrangements. The Cad model is initially modeled using CAD software for a given configurations like tube diameter, tube numbers, bundle arrangement, tube thickness, shell length, shell diameter, shell thickness baffle numbers, baffle thickness, baffle spacing, baffle cut etc. A basic CAD model with 11 baffles of segmental type with fixed spacing of 75 mm is developed using the CAD software and modified to six model configurations having baffle cut percentages of 15, 20 25, 30, 35, and 40 are developed. Each model is separately meshed and analysed. For analysis initially the complete model of Shell and tube heat exchanger comprising of tube bundle, baffles, shell and tube end nozzles is exported to a meshing package and meshed. Here the model is meshed by volume meshing using Hypermesh and optimum mesh density may be achieved by repetitive process. The meshed model from the meshing software is used to solve for heat duty cases with the use of temperature boundary conditions defined for inlet of both shell and tube side fluid, discharge rates, fluid properties and pressure boundary conditions. For this purpose a commercially available CFD software Fluent is used and analyzed. The results from the solution are used in studying the heat transfer rates for the shell side flow. Some of the research works have been published in the last two decades.some of the related works carried out on the E-type shell and tube heat exchanger may be summarized as below. The shell side flow is considered as complex and divided into various zones, where pressure drops and heat transfers are evaluated, a study carried out by Uday kapale and Satish Chand[1]. The heat transfer rate distribution with in tube bundle cross sectional area has been evaluated by Li and Kottke[2], the effect leakage stream existing between baffle and shell has been studied by Roetzel and Lee[3],[4] and is related to heat transfer rate. Evaluation of heat transfer coefficients in compartmental and zonal regions has been studied by Jenkins, Gay and Nibber[5]. The heat transfer rates for various baffle inclination angles in a spherical baffled heat exchanger has been evaluated Jian-Fei-Zhang[6],[7],[8]. The effects of baffle cut and baffle spacing on the performance of a shell and tube heat exchanger has been undertaken by E. Ozden and I. Tari[9].The effects of crude oil fouling on Shell and tube heat exchanger has been carried out Clark and Nicolas[10] 2.MODELING OF HEAT EXCHANGER AND SOLUTION. The computational modeling involves three stages of formulation and solution of the problem. The pre-processing using a modeling and meshing software, then solving using the CFD tool and post-processing of the results. 2.1. Geometry Modeling : Using CATIA E-type shell and tube heat exchanger is modeled. The tube bundle is inline arrangement. The details of the geometry is given in Table-1. Table-1 Exchanger design data 1 Shell Diameter inner 250mm 2 Shell Diameter outer 258mm 3 Shell length inside 1000mm 4 Tube diameter inner 22.452mm 5 Tube diameter outer 25mm 6 Tube length 1000mm www.apjor.com Impact Factor ( GIF) 5.42 Page 40
7 End plate thickness 6.403mm 8 End plates distance outer surface 1000mm 9 Baffle thickness 5mm 10 No of tubes 30 Nos 11 Tube pitch triangular 60 0 32.24mm 12 Shell/Tube nozzle diameter 80mm 13 Shell nozzle center distance 870mm 14 End tube head inner distance 17.5mm 15 No of baffles 11 Nos 16 Baffle spacing 75 mm Odd number baffles will have shell nozzle on same side of shell. For baffle cut, baffles of 11 numbers have been fixed in the shell at distance of 75 mm and the six models of different baffle cut combinations have been developed. These models are neshed using meshing package. These meshed models are analysed for heat transfer rate using a CFD package. Table-2 Properties of tube material Specific heat Conductivity Density J/kg 0 K (W/m 0 K) (kg/m 3 ) 871 202.4 2719 Figure 1 Heat exchanger model with 11 baffles www.apjor.com Impact Factor ( GIF) 5.42 Page 41
Figure 2 Tube meshing 2.2. Mesh Generation The three-dimensional model is meshed using free mesh of 3D tetrahedral elements. The figure shows a simplified model for shell and tube heat exchanger for the purpose of reducing grid size and computational effort. 2.3 Load and Boundary conditions defined for CFD solution. The working fluid of the shell is oil and tube side is water with the following flow conditions given in table 3 used into analysis. Table-3 Shell side fluid flow specifications Tube side fluid flow specifications Flow rate(kg/s) 0.05 Flow rate(kg/s) 0.08 Inlet Temperature (K) 353 Inlet Temperature (K) 300 Inlet Pressure (Pa) 9.34E+15 Inlet Pressure (Pa) 8.29E+14 The properties assumed for both fluids in the heat exchanger that is shell side oil and tube side water is given in table 4. Table -4 Shell side fluid properties Tube side fluid properties Heat capacity(j/kg C) 1996 Heat capacity(j/kg C) 4182 Conductivity(W/m C) 0.1264 Conductivity(W/m C) 0.5984 Density( kg/m 3 ) 869.6 Density( kg/m 3 ) 998.2 Viscosity(Kg/m-s) 0.01616 Viscosity(Kg/m-s) 0.001002 www.apjor.com Impact Factor ( GIF) 5.42 Page 42
Zone Name Type Momentum Table-5 Thermal condition Material Name Baffle Wall Stationary wall, No Slip Coupled Steel Shell Wall Stationary wall, No Slip Heat flux Steel Shell Wall Wall Stationary wall, No Slip Coupled Steel Shell Fluid Fluid Oil(Vg 46) Shell Inlet Mass Flow Inlet Shell Outlet Pressure Outlet Gauge pressure = 0 Pa Shell Inlet Wall Wall Stationary wall, No Slip Heat flux Steel Tube Inlet Mass Flow Inlet Water Tube Outlet Pressure Outlet Gauge pressure = 0 Pa Tube Inlet wall Wall Stationary wall, No Slip Heat flux Aluminum Tubes Wall Stationary wall, No Slip Coupled Aluminum The table 5 shows the boundary conditions used in CFD solution. 3. Results and Discussion The following tables show output results for the baffle cut combinations using CFD technique. The results are listed below, heat load and output temperatures for hot and cold fluids. Table 6.1 Baffle Cut 15% Baffle Cut 20% Oil Water Oil Water Mass flow rate kg/s 0.05 0.08 0.05 0.08 Heat Transfer Watt 309.5-310.5 222.9-223 Temp In Deg K 353 300 353 300 Temp Out Deg K 350 301 345.5 300.66 Table 6.2 Baffle Cut 25% Baffle Cut 30% Oil Water Oil Water Mass flow rate kg/s 0.05 0.08 0.05 0.08 Heat Transfer Watt 204.1-203.9 203.34-203.38 Temp In Deg K 353 300 353 300 Temp Out Deg K 346.77 300.61 345.65 300.60 www.apjor.com Impact Factor ( GIF) 5.42 Page 43
Table 6.3 Baffle Cut 35% Baffle Cut 40% Oil Water Oil Water Mass flow rate kg/s 0.05 0.08 0.05 0.08 Heat Transfer Watt 192.4-192.45 190.23-190.3 Temp In Deg K 353 300 353 300 Temp Out Deg K 345.7 300.57 346 300.56 Figure 3 temperature contours for 15 % baffle cut Figure 4 temperature contours for 20 % baffle cut www.apjor.com Impact Factor ( GIF) 5.42 Page 44
Temperature in kelvin Heat Transfer in Watts International Journal of World Research, Vol: I Issue XXXVII, January 2017 Print ISSN: 2347-937X Figure 5 temperature contours for 35 % baffle cut 350 300 250 200 150 100 50 0 15% 20% 25% 30% 35% 40% Figure 6 Variation of heat transfer rate against baffle cut percentage 354 352 350 348 346 344 15% 20% 25% 30% 35% 40% 342 Exchanger length Figure 7 Variation of temperature along the length of exchanger www.apjor.com Impact Factor ( GIF) 5.42 Page 45
4. Conclusion The tables 6.1, 6.2 and 6.3 show heat transfer rates for exchanger for different percentages of baffle cut. The Figure 6 also shows the same in chart form. It is seen that the variation in heat transfer rates for 20 to 40 % baffle cut is min. The variation in rate of heat transfer for 25% and 30% is minimum. Similar to this, the variation in rate of heat transfer for 35% and 40% is minimum. The temperature variation through the compartment for successive compartments is gradual. The variation in compartment temperatures for a combination of 25% through 40% is found to be minimum. References 1. [1]. Uday C Kapale and Satish Chand, Modeling for shell-side pressure drop for liquid flow in shell-and-tube heat exchanger, International journal of heat and mass transfer, Feb 2006, vol 49, issue 3-4, pp 601-610. 2. Huadong Li and Volker Kottke, Effect of baffle spacing on pressure drop and local heat transfer in shell and tube heat exchanger for staggered tube arrangement, International journal of heat and mass transfer, 1998, Vol 41, No 10, pp 1303-1311 3. Wilfred Roetzel and Deiying W Lee, Effect of baffle/shell leakage flow on heat transfer in shell and tube heat exchangers, Experimental thermal and fluid science, 1994;Vol 8, pp 10-20. 4. Wilfred Roetzel and D Lee, Experimental investigation of leakage in shell and tube heat exchangers with segmental baffles, International journal of heat and mass transfer, 1993, Vol 36, No 15, pp 3765-3771. 5. J D Jenkins, B Gay and S P S Nibber, Shell side heat transfer coefficients in cylindrical heat exchangers variation along the exchanger length, I-The no leakage case,, International communication on heat and mass transfer, 1991, Vol 18, pp 1-10. 6. Jian-Fei Zhang, Bin Li, Wen-Jiang Huang, Yong-Gang Lei, Ya-Ling He, Wen-Quan Tao, Experimental performance comparison of shell-side heat transfer for shell-and-tube heat exchangers with middle-overlapped helical baffles and segmental baffles, Chemical Engineering Science, 2009 Vol 64, pp 1643-1653. 7. Jian-Fei Zhang, Ya-Ling He, Wen-Quan Tao, 3D numerical simulation on shell-and-tube heat exchangers with middleoverlapped helical baffles and continuous baffles Part II: Simulation results of periodic model and comparison between continuous and non continuous helical baffles, International Journal of Heat and Mass Transfer, 2009, Vol 52,pp 5381 5389. 8. Jian-Fei Zhang, Ya-Ling He, Wen-Quan Tao, 3D numerical simulation on shell-and-tube heat exchangers with middleoverlapped helical baffles and continuous baffles Part I: Numerical model and results of whole heat exchanger with middleoverlapped helical baffles, International Journal of Heat and Mass Transfer, 2009, Vol 52, pp 5371 5380. 9. Ender Ozden and Ilker Tari, Shell side CFD analysis of a small shell-and-tube heat exchanger, Energy Conversion and Management, 2010, Vol 51, pp No 1004 1014. 10. R H Clarke and F Nicolas, CFD investigation of maldistribution effects on crude oil fouling in shell and tube exchangers,, Cal Gavin ltd, Alcester, UK. 11. Dogan Eryener, Thermo-economic optimization of baffle spacing for shell and tube heat exchangers, Energy Conversion and Management, 2006, 47, pp 1478 1489. 12. M Saffar-Avval and E Damangir, A general correlation for determining optimum baffle spacing for all types of heat exchangers, International journal of heat and mass transfer, 1995, Vol 38, No 13, pp 2501-2506. 13. T Tinker and N Y Buffalo, Shell side characteristics of shell and tube heat exchangers, a simplified rating system for commercial heat exchangers, Trans ASME,, Jan 1958, pp 36-52. 14. Sadik Kakac and Hongtan Liu, Heat Exchangers, Selection, Rating and Design, CRC Press, New York. www.apjor.com Impact Factor ( GIF) 5.42 Page 46