THERMAL ANALYSIS OF SHELL AND TUBE HEAT EXCHANGER

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 5, May 2017, pp , Article ID: IJMET_08_05_066 Available online at aeme.com/ijmet/issues.asp?jtype=ijmet&vtyp pe=8&itype=5 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed THERMAL ANALYSIS OF SHELL AND TUBE HEAT EXCHANGER N Santhisree, M Prashanthkumar, G Priyanka Mechanical Engineering, IARE, Hyderabad, India. ABSTRACT Heat exchangers are devices that facilitate the heat exchange between two fluids that are at different temperatures while keeping them from mixing with each other. They differ from mixing chambers in that they do not allow the two fluids involved to mix. The most common type that is used in industrial applications is shell and tube heat exchanger. It containn a more number of tubes packed in a shell with their axes parallel to that of the shell. One fluid of it flows through the tube and the other fluid flows outside the tube through the shell and causes exchange of heat between the fluids. To enhance heat transfer and to maintain uniform spacing between the tubes baffles are placed in the shell to force the fluid to flow across the shell. In this present study thermal analysis is carried out in Ansys fluent 15.0.The heat exchanger is designed using CATIA V5. Key words: Baffles, Effectiveness, Heat Transfer, heat exchanger, temperature difference, Overall heat transfer coefficient. Cite this Article: N Santhisree, M Prashanthkumar, G Priyanka Thermal Analysis of Shell and Tube Heat Exchanger. International Journal of Mechanical Engineering and Technology, 8(5), 2017, pp IType=5 1. INTRODUCTION One of the most common type of heat exchanger is shell and tube heat exchanger. These are used for higher pressure applications. It consists of a shell a large pressuree vessel with a more number of tubes inside it. The heat exchange takes place between the two fluids as one fluid runs through the bundle of tubes and another fluid flows over the tubes i.e. through the shell. The performance and efficiency is depending upon the amount of heat transfer. Heat exchangers are basically classified as direct contact and indirect contact. Shell and tube exchanger is an indirect contact type heat exchanger basically consists of tube bundles, shell, front header, rear header and baffles. Enhancement of heat transfer takes place by using active and passive methods. Active method include techniques like surface vibration injection electrostatic fields etc. Whereas passive methods includes inserts, coiled or twisted tubes[1], extended surfaces, baffles etc. like mechanical modifications Baffles are provided to increase the turbulence of the shell fluid and to direct the flow of fluid normal to the tubes. The space editor@iaeme.com

2 Thermal Analysis of Shell and Tube Heat Exchanger between the baffles as expressed as percentage of segment height to inside diameter of shell. Segmented shell and tube heat exchanger improves heat transfer by creating turbulence. By increasing intensity of turbulence level flow resistance can be increased and also increases high pressure drop [3]. Which leads to increase in consumption of power. This is a major problem. Therefore always it is desirable to select a heat exchanger with more turbulence, high heat transfer coefficient, and low pressure drop as well as less fouling. In the present analysis 25%cut is considered.in general it may vary from 15 to 45%.Any heat exchanger design is based on its effectiveness and cost. The basic flow distribution of shell and tube heat exchanger is shown below. Figure 1 Schematic representation of shell and tube heat exchanger 2. LITERATURE REVIEW 1. Paresh Patel and Amitesh Paul Paresh Patel and Amitesh Paulhad performed thermal analysis of tubular type heat exchanger using ANSYS and CFD analysis has been carried out for different materials like steel, copper and aluminium and on the basis of results obtained they have described which material gives best heat transfer rates [1]. 2. Vindhya Vasiny Prasad Dubey, Raj Rajat Piyush Shanker Verma, had investigated that the performance of a shell and tube heat exchanger depends on various factors affect the performance of the heat exchanger and the effectiveness obtained by the formulas depicts the cumulative effect of all the factors over the performance of the heat exchanger [2]. 3. Huadong Li and Volker Kottke et al. proposed a model to investigate local heat transfer and pressure drop for different baffle spacing in the shell and tube heat exchangers with segmental baffles. He analysed that for same Reynolds number, the pressure drop and average heat transfer are increased by increase in baffle spacing [3] editor@iaeme.com

3 N Santhisree, M Prashanthkumar, G Priyanka 4. B. Jayachandriah and K. Rajasekharmade an attempt to design a shell and tube heat exchanger with copper and brass as tube material and steel 1008 as shell material. From their results they have analysed the shell and tube heat exchanger by varying tube materials and declared that they are highly efficient when they are used for liquid to liquid applications [4]. 5. S. Noie Baghban, M. Moghiman and E. Salehiperformed thermal analysis of shell side flow of shell and tube heat exchanger with different baffle spacing s and different baffle cut. A shell-and-tube heat exchanger of gas-liquid chemical reactor system has been used in the experimental method. The experimental and numerical result shows good agreement [5]. 6. Usman Ur Rehmanperformed a CFD analysis of shell and tube heat exchanger with respect to heat transfer coefficient and pressure drop. He found that in parallel flow the outlet temperature of fluid outside tubes is more than the outlet temperature of tube side fluid and compared experimental results with numerical values. With the comparing it is found that the design has to be modified.[6] 3. MATHEMATICAL MODELLING By using Kerns method preliminary design of shell and tube heat exchangers is done. It provide conservative results. The steps of mathematical design is as follows. Consider energy balance to find out unknown values of temperatures. Consider some input parameters like inlet temperature of hot fluid and cold fluid and velocity flow rates. The fluid properties at particular temperatures etc. The energy balance equation is given as Q = h Ch (h1-h2) = Cc (2-1) Calculate area of tube and shell side. Mathematical modelling involves calculating unknown temperatures by using input parameters. The detailed procedure is given below. 3.1 Input Data Inlet temperature of hot fluid Th1=90 0 C Inlet temperature of cold fluid Tc1=7 0 C Specific heat of hot fluid Cp h =4.205 KJ/Kg K Specific heat of cold fluid Cp c =4.190 KJ/Kg K Density of hot fluid ρ h =967.5Kg/m editor@iaeme.com

4 Thermal Analysis of Shell and Tube Heat Exchanger Density of cold fluid ρ c =1001.3Kg/m 3 Thermal conductivity of water K t = 0.61W/m K Thermal Conductivity of Copper K s = 386W/m K Tube side fluid velocity v h = 0.1m/s Shell side fluid velocity v c = 0.5m/s µh = dynamic viscosity of hot fluid = 0.315*10-3 Ns/m 2 µc = dynamic viscosity of cold fluid = *10-3 Ns/m 2 Tube side mass flow rate mh=ρ h A t v h = kg/s per tube Shell side mass flow rate mc= ρ c A s v c =0.6167kg/s Reynolds Number: For tube side: Ret = 4mh / πdiµ h = For Shell side: Res = 4mc / πdiµ c = Nusselt number: Nut= 0.023*Re 0.8 *Pr 0.4 = Heat transfer Coefficient in tube and shell ht= Nut*(Kt/dt) = w/m 2 k hs = (0.36*ks/De)* Res 0.55 *Pr 0.33 = w/m 2 k Overall Heat Transfer Coefficient U U=1/ (1/hi*(ro/ri) + 1/ho + ((ro/k)*ln(ro/ri))) = Using NTU Method: C h = m h Cp h = *4.205 = Kw/ 0 C Cc = m c Cp c = *4.190 = Kw/ 0 C Heat capacity Ratio R=C min /C max = 2.583/2.858 = Number of Transfer Units NTU = UAs/Cmin = * /2.583=0.930 Using R and NTU values, from graphs Effectiveness = 0.46 But = Q actual / Q maximum editor@iaeme.com

5 = m h Cp h (Th1-Th2) / Cmin (Th1- Tc1) 0.46= *4.205*(90-Th2) / 2.583*(90-7) Th2 = C Qactual = *4.205*( ) = KW N Santhisree, M Prashanthkumar, G Priyanka But heat lost by hot fluid=heat gain by cold fluid Number of tubes Which gives Tc2= C Area As = n*π*(do 2 /4) n = A/ π* (do 2 /4) n = So No of tubes = DESIGN OF SHELL AND TUBE HEAT EXCHANGER By using the data obtained the shell and tube heat exchanger components have been designed using CATIA V5. The simulated Shell and Tube Heat exchanger has 4 baffles with 25% cut in the shell side direction with total number of tubes 22. The whole computation domain is bounded by the inner side of the shell and everything in the shell contained in the domain. The inlet and outlet of the domain are connected with the corresponding tubes. To simplify simulation, some basic assumption are made. 1. The shell side fluid is constant thermal properties 2. The fluid flow and heat transfer processes are turbulent and in steady state 3. The leak flows between tube and baffle and that between baffles and shell are neglected 4. The natural convection induced by the fluid density variation is neglected 5. The tube wall temperature kept constant in the whole shell side 6. The heat exchanger is well insulated hence the heat loss to the environment is totally neglected. 4.1 Design The components that are designed for shell and tube heat exchanger are Tubes Tube sheets Shell Baffles Tube side channels nozzles editor@iaeme.com

6 Thermal Analysis of Shell and Tube Heat Exchanger Figure 3 Tube sheets Figure 2 Tubes Figure 4 Shell Figure 5 Baffles Fig Figure 6 Tube side Channels and Nozzles The model is designed according to TEMA (Tubular Exchanger Manufacturers Association) Figure 7 Shell and Tube Heat Exchanger Assembly IJMET/index.asp 601 editor@iaeme.com

7 4.2 Mesh Generation A fine mesh is generated with nodes. N Santhisree, M Prashanthkumar, G Priyanka Figure 8 Mesh generation 4.3 Thermal Simulation Simulation is carried out as pressure based under steady state condition.. Liquid water as fluid in tubes, copper [1] as tube material and steel as shell material. A shell and tube heat exchanger has one inlet and outlet for tube and shell. Viscous k- model has been implemented for general CFD codes. This is considered as standard industry model. 4.4 Boundary Conditions The inlets were defined as velocity inlets and outlets were defined as pressure outlets, inlet velocity profile assumed,, slip condition assigned to all surfaces, gauge pressure assigned to the outlet nozzle, heat flux boundary condition assigned to the shell outer wall (excluding the baffle shell interfaces), assuming that the shell is perfectly insulated. The surrounding air temperature was kept 27 0 C Table 1 Boundary conditions applied while doing analysis Quantity used in CFD Tube inlet temperature shell inlet temperature Tube side velocity shell side velocity Gauge pressure velocity profile slip heat flux condition/value 90 0 C 7 0 C 0.1m/s 0.5m/s zero Pascal s uniform velocity No slip 0 w/m 2 k 4.5 Solution The meshed component is kept for run to calculate the output parameters till the convergence is reached by giving more number of iterations with step size. The convergence criteria were set to 10-4 for the three velocity components and continuity, 10-7 for energy and 10-4 for turbulent kinetic energy and dissipation energy editor@iaeme.com

8 Thermal Analysis of Shell and Tube Heat Exchanger 5. RESULTS AND DISCUSSIONS Various contours were plotted and different parameters were calculated such as weighted average of total temperatures at out let and inner wall, total wall flux, pressure drop across the hot fluid inlet and outlet to calculate pumping power. 5.1 Variation of Temperature The temperature contour plots across the cross section of heat exchanger with and without baffles Figure 9 Temperature Variations in tubes without Baffles Figure 10 Temperature Variations in tubes with Baffles Figure 11 Cold Temperature variation in shell with baffles editor@iaeme.com

9 5.2 Variation of Velocity N Santhisree, M Prashanthkumar, G Priyanka Figure 12 Variation of Velocity in Tubes without baffles Figure 13 Variation of Velocity with baffles 5.3 Variation of Pressure Figure 14 Variation of velocity in shell with baffles Figure 15 Variation of Pressure without baffles editor@iaeme.com

10 Thermal Analysis of Shell and Tube Heat Exchanger Figure 16 Variation of pressure with baffles Figure 17 Variation of Pressure in shell with baffles 5.4 Discussions The outlet temperature of tube and shell side fluids have been analysed with baffles and without baffles. The effectiveness is also calculated. Heat transfer and flow distribution is discussed in detail and the results obtained are tabulated as below. Table 2 Results obtained in simulation Tube side outlet temp( 0 C) Shell side outlet temp( 0 C) Velocity drop(m/s) Pressure drop(k Pa) Heat Transfer Rate(KW) WITH BAFFLES WITHOUT BAFFLES CONCLUSIONS The analysis is carried out by using water as fluid and results are compared with baffles and without baffles. The model predicts the heat transfer and pressure drop with an error of 25%. Thus the model can be improved. The outlet temperatures of hot and cold fluids are 67 0 C and 27 0 C respectively with baffles of 25% cut and 70 0 C and 40 0 C respectively. 7. FUTURE SCOPE The analysis is carried out by using water as fluid and results are compared with baffles and without baffles. The analysis can also be carried out by using different working fluids like Nanofluids editor@iaeme.com

11 N Santhisree, M Prashanthkumar, G Priyanka The tubes shell and baffle can also made with different materials like galvanized steel, aluminium etc. and the results can be compared. The effective performance of shell and tube heat exchanger is increased by adding fins, inserts, coils etc. REFERENCES [1] Paresh Patel, Amitesh Paul Thermal Analysis Of Tubular Heat Exchanger By Using Ansys, International Journal of Engineering Research & Technology (IJERT); ISSN: Vol. 1 Issue 8, October [2] Vindhya Vasiny Prasad Dubey, Raj Rajat Verma, Piyush Shanker Verma, A.K.Srivastava Performance Analysis of Shell & Tube Type Heat Exchanger under the Effect of Varied Operating Conditions IOSR Journal of Mechanical and Civil Engineering Volume 11, Issue 3 Ver. VI (May-Jun. 2014), PP [3] Huadong Li, Volker Kottke, Effect Of Baffle Spacing On Pressure Drop And Local Heat Transfer In Shell-And-Tube Heat Exchangers For Staggered Tube Arrangement, International Journal of Heat Mass Transfer, Elsevier Science, Germany, [4] B.Jayachandriah1, K. Rajasekhar Thermal Analysis of Tubular Heat Exchangers Using ANSYS International Journal of Engineering Research Volume No.3 Issue No: Special 1, pp March [5] E. Salehi, S. Noie Baghban and M. Moghiman, Thermal analysis of shell-side flow of shell-and tube heat exchanger using experimental and theoretical methods, International Journal of Engineering, Vol. No. 13, pp , February [6] Uttam Roy and Mrinmoy Majumder. Estimation and Analysis of Cycle Efficiency for Shell and Tube Heat Exchanger by Genetic Algorithm. International Journal of Mechanical Engineering and Technology, 8(2), 2017, pp [7] Sunil Jamra, Pravin Kumar Singh and Pankaj Dube. Experimental Analysis of Heat Transfer Enhancement in Circular Double Tube Heat Exchanger using Inserts. International Journal of Mechanical Engineering and Technology, 3(3), 2012, pp [8] S. Bhanuteja and D.Azad Thermal Performance and Flow Analysis of Nanofluids in A Shell and Tube Heat Exchanger. International Journal of Mechanical Engineering and Technology, 4(5), 2013, pp [9] Usman Ur Rehman, Heat Transfer Optimization of Shell-And-Tube Heat Exchanger through CFD Studies, Chalmers University of Technology, [10] LearnCAX.ORG editor@iaeme.com