Performance Analysis of Shell And Tube Heat Exchanger

Transcription

1 Performance Analysis of Shell And Tube Heat Exchanger G.V.N.Santhosh 1, Y.V.RamanaMurty 2, S.SwethaRadha 3 1 M.Tech Scholar, Dept. of Mechanical Engineering, B.V.C. Engineering College, Odalarevu 2 Associate Professor, Dept. of Mechanical Engineering, B.V.C. Engineering College, Odalarevu 3 Assistant Professor,Dept. of Mechanical Engineering, Pragati Engineering College, Surampalem ABSTRACT As we know that a shell and tube heat exchanger is designed where high pressures and high pressure differences between the s relative to the environment are applied. These exchangers are generally built of a bundle of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. There is considerable flexibility in the design because the core geometry can be varied easily by changing the tube diameter, length and arrangement. One flows inside the tubes, and other flows across and along the tubes. In this project, the hot will be cooled using seawater with the help of shell and tube heat exchanger. A characteristic of heat exchanger design is the procedure of specifying a design heat transfer area and pressure drops and checking whether the assumed design satisfies all requirement or not. The purpose of this project is how to design the heat exchanger which is the majority type of liquid-to-liquid heat exchanger. General design considerations and design procedure are also illustrated in this project. KEYWORDS: Shell and Tube Heat Exchanger, Liquid-to-Liquid Heat Exchanger. I.INTRODUCTION A heat exchanger is a heat transfer device that is used for transfer of internal thermal energy between two or more s available at different temperatures. In most heat exchangers, the s are separated by a heat transfer surface and ideally they do not mix. Heat exchangers are used in process, power, petroleum, transportation, airconditioning, refrigeration, cryogenic, heat recovery, alternate fuels and other industries. The relation was formulated by Newton and is called Newton s law of cooling, which is given by Q h*a*dt Where h is the heat transfer coefficient [W/m 2 K], where s conductive/convective properties and the flow state comes in the picture, A is the heat transfer area [m 2 ], and T is the temperature difference [K]. Fig.1: The basic heat transfer mechanism II SHELL AND TUBE HEAT EXCHANGER Shell and tube heat exchangers are used primarily for liquid-to-liquid and liquid-to-phase change heat transfer applications. They are used for gas-to-liquid and gas-to-gas heat transfer applications primarily when the operating temperature and pressure is very high or fouling is a severe problem on at least one side and no other types of exchangers would work. A variety of different internal constructions are used in shell and tube exchangers depending on the desired heat transfer and pressure drop performance and the methods employed to reduce thermal stresses, to prevent leakages, to provide for ease of cleaning, to contain operating pressures and temperatures, to control corrosion, to accommodate highly asymmetric flows, and so on. Page 42

2 Fig.2: The basic Shell and Tube Heat Exchanger III Basic Components of Shell and Tube Heat Exchanger Shell and tube heat exchanger is generally built of a bundle of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. The major components of this exchanger are tubes(or tube bundle), shell, frontend head, rear-end head, baffles and tubesheets. TEMA Standards Shell and tube heat exchangers are classified and constructed in accordance with the widely used TEMA(Tubular ExchangerManufacturers Association) standards. TEMA has developed a notation system to designate major types of combination, the first letter indicating the front-end head type, the second the shell type, and the third the rear-end head type. Some common shell and tube exchangers are AES, BEM, AEP, CFU, AKT, and AJW. It should be emphasized that there are other special types of shell and tube exchangers commercially available that are different from those of above. Classification Based on TEMA Construction: There three basic classification based on TEMA based on their end connection and shell type. a. BEM b. CFU c. AES Fig 3: Construction Parts and Connections IV Objectives of the study The main objective of the present work is to fabricate a shell and tube heat exchanger and calculate the parameters under balanced conditions. 1. Design and fabrication of the test rig for STHE. 2. To determine the thermal performance parameters like overall heat transfer coefficient, Page 43

3 effectiveness and pressure drop through hot testing under balanced flow condition. 3. To compare the experimentally obtained values of effectiveness, overall heat transfer coefficient with the values that are obtained from various correlations. Basic Design Procedure Heat exchanger must satisfy the Heat transfer requirements (design or process needs) Allowable pressure drop (pumping capacity and cost) Steps in designing a heat exchanger can be listed as: Identify the problem Select an heat exchanger type Calculate/Select initial design parameters Rate the initial design Calculate thermal performance and pressure drops for shell and tube side. Evaluate the design. Is performance and cost acceptable? terminal temperatures, and flow rates in a heat exchanger, the basic equations used for analysis are the energy conservation and heat transfer rate equations. The energy conservation equation for an exchanger having an arbitrary flow arrangement is,,,, and the heat transfer rate equation is To use this equation, it is necessary to determine the heat transfer coefficient and the temperature difference. For a double pipe heat exchanger the required average temperature difference is the log mean temperature difference (LMTD). Unfortunately, the flow patterns in shell and tube exchangers are such that the LMTD by itself is no longer adequate. It must first be adjusted by means of a correction factor. The second parameter that must be calculated for a typical process design is the pressure drop in the s moving through the exchanger. Correcting the LMTD The maximum driving force for heat transfer is always the log mean temperature difference(lmtd) when two streams are in countercurrent flow.the true mean temperature difference of such flow arrangements will differ from the logarithmic mean temperature difference by a certain factor dependent on the flow pattern and the terminal temperatures. This factor is usually designated as the log mean temperature difference correction factor, F. the factor F may be defined as the ratio of the true mean temperature difference(mtd) to the logarithmic mean temperature difference. The heat transfer rate equation incorporating F is given by Fig.4: Basic Design Procedure V EXPERIMENTAL SETUP AND PROCEDURE Shell and Tube Heat Exchangers: Calculations In order to develop relationships between the heat transfer rate q, surface area A, The correction factor charts are available from many sources. They are based on two parameters: Page 44

4 International Journal of Mechanical Engineering and Computer Applications, These parameters are cross-referenced on the appropriate chart to find the F factor. F factor curves drop off rapidly below 0.8. Consequently, if the design is indicating an F less than 0.8, we probably need to redesign (add tube passes, increase temperature differences, etc.) to get a better approximation of counter-current flow and thus higher F values. Tube Side Heat Transfer Coefficients The tube side heat transfer coefficients are easy to determine, since the Seider- Tate equation (or equivalent) applies. Thestream flow among the tubesis to incorrectly divide which the most common slipup is made at this stage. allthe flow rates and velocities for the number of tubes and tube passes are to be adjusted. If an exchanger has 200 tubes in 2 passes, the total flow will be moving through 100 tubes at a time; if there are 4 passes, it will go through 50 tubes. Tube Side Pressure Drop Using the same pipe flow factors developed in the mechanics the tube side pressure drop is calculated. The isothermal friction factor can be obtained from the Moody/Stanton charts or an appropriate correlation. This friction factor must be corrected for the effect of temperature on viscosity. The results can be obtained using the Seider-Tate viscosity correction:. This can then be used with the mechanical energy balance to get the pressure drop: 4 2 Shell Side Heat Transfer Coefficients This requires calculation of several values -notably G b, the mass velocity of the shell side if it was all moving parallel to the tubes, and G C, the mass velocity if all the was moving across the tubes. The equations are 1 Where, Ds the shell inside diameter f B the fraction of the shell cross-section that makes up the baffle window N bt the number of tubes in the baffle window (usually approximated by f * b N tubes) P B the baffle pitch (spacing) p t the tube pitch, d o the tube outside diameter which then are used to find the shell side coefficient: h 0.2 with properties based on the shell side bulk temperature. The flow is outside the tubes, so the wall temperature correction is based on the outside wall temperature. Shell Side Pressure Drop The calculation of shell side pressure drop is significantly more complicatedas the shell side flow path is considerably more complex. For our purposes, we will use a correlation presented by Kern. It has the advantage of mirroring the tube side calculation. The shell side equivalent diameter, D eq is given by 4 4 (0.86 ) (we may recognize the 0.86 in the triangular layout expression as the sine of a 60 degree angle.) This shell side equivalent diameter is combined with the cross flow mass velocity to obtain a Reynolds number which can be taken to an appropriate chart and used to get a friction factor. Note that the chart provides a dimensional friction factor (unlike the dimensionless values used for pipe flow). The friction factor has to be transformed to a pressure drop, a count of how many times the crosses the tube bundle is needed. It crosses. Page 45

5 between the baffles, so the cross will be one more than the number of baffles, N B. The number of baffles can be determined using the baffle spacing: + 1 h The pressure drop is then determined using the equivalent diameter, cross flow velocity, friction factor, number of crosses, and properties: ( + 1) 2 EXPERIMENTAL SETUP The physical setup of this project consists of 1. Hot Fluid Inlet with tube 2. Cooling chamber 3. Sea water inlet 4. Pump 4. PUMP:- 1. HOT FLUID INLET WITH TUBE:- The tubes are made up of mild steel materials. The is supplied from the reservoir. r. The inlet is subdivided into the number of tubes (total is 10 number) with in the cooling chamber. 2. COOLING CHAMBER:- The cooling chamber is consisting of the number of tubes and the one sea water inlet and sea water outlet. The hot is converted into the cold with the help of surrounding sea water and this cold is come out through the outlet provided. 3. SEA WATER INLET:- The sea water is used to cooling the hot into the cold. This sea water is pumped into the hot chamber with the help of pump. MOTOR WITH IMPELLER: The single phase induction motor is used. The impeller is connected to the motor shaft. Impeller casing consist of suction and a delivery output. DESIGN CONSIDERATIONS Baffle Design Definitions: To prevent failure of tubes due to flow-induced vibration and to enable a desirable velocity to be maintained for the shell side the bafflesare used to support tubes. There are two typesof baffles: 1. Plate and 2.Rod. Plate baffles may be single-segmental, double- segmental, or triple-segmental: Rod Baffles: Shell side cross flow area as is given by:. Where: a s Shell side cross flow area D Shell Inside diameter C Clearance between tubes B Baffle spacing P T Tube pitch. Page 46

6 Minimum spacing (pitch) of baffles normally should not be closer than 1/5 of shell diameter (ID) or 2 inches whichever is greater. Maximum spacing (pitch) spacing does not normally exceed the shell diameter. Tube support plate spacing determined by mechanical considerations, e.g. strength and vibration. Maximum spacing is given by: 74 Most failures occur when unsupported tube length is greater than 80% due the designer is trying to limit the shell side pressure drop. Baffle cuts: They 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. Kern shell side pressure drop correlations are based on 25% cut which is standard for liquid on shell side. Baffle clearances: The outer tube limit (OTL) is the diameter created by encircling the outermost tubes in a tubelayout. The actual OTL is usually 1.5 times the design pressure. It is used during a hydrostatic test that detects leaks at any joint on the heat exchanger. Heat Exchanger Bundles: Tube bundles are also known as tube stacks are designed for applications according to customer requirements, including direct replacements for existing units. Bundle diameter D b can be estimated using constants shown:. K N Tube Diameters: The most common sizes used are Ø3/4" and Ø1". Use the smallest diameter for greater heat transfer area with a minimum of Ø3/4" tube due to cleaning considerations and vibration. For shorter tube lengths say < 4ft can be used Ø1/2" tubes. Tube Quantity and Length: Select the quantity of tubes per side pass to give optimum velocity. For liquids 3-5 ft/s ( m/s) can be used. Gas velocities are commonly used ft/s (15-30 m/s). If the velocity cannot be achieved in a single pass consider increasing the number of passes. The tube length is determined by heat transfer required to process and pressure drop constraints. To meet the design pressure drop constraints may require an increase in the number of tubes and/or a reduction in tubelength. Long tube lengths with few tubes may carry shell side distribution problems. Tube Arrangement: Triangular pattern provides a more robust tube sheet construction. Square pattern simplifies cleaning and has a lower shell side pressure drop. Tube pitch is defined as: P T tube pitch d o tube outside diameter C clearance + Where: d o Tube Outside Diameter. N t Number of tubes K 1 &n see table below: Triangular Pitch 1.25 No. of Passes K N No. of Passes Square Pitch Typical dimensional arrangements are shown below, all dimensions in inches. Tube Triangular Square Pitch Diameter Pitch 5/8 (16mm) 7/8 (22mm) 25/32 (20mm) 3/4"(19mm) 1 (25mm) 15/16 (25mm) 1 (25mm) 1 ¼ (32mm) 1 ¼ (32mm) 1 ¼ (32mm) 1 9/16 (39mm) 1 9/16 (39mm) 1 ½ (38mm) 1 7/8 (47mm) 1 7/8 (47mm) Page 47

7 Note: For shell 12 square pitch in. The table above uses minimum pitch 1.25 times tube diameter i.e. clearance of 0.25 times tube diameter, the smallest pitch in triangular 30º layout for turbulent or laminar flow in clean service. For 90º or 45º layout allow 6.4 mm clearance for tube for ease of cleaning. Corrosion Fouling: Foulingis deposit formation, encrustation, deposition, scaling, scale formation, or sludge formation inside heat exchanger tubes. However if economics determine that some corrosion is acceptableand no data is available from past experience an allowance of 1/16 in (1.59 mm)is commonly applied. Calculations: The main aim of present work is to calculate the performance parameters like, effectiveness, overall heat transfer coefficient of the shell and tube heat exchanger. Table: Experimentally observed data. Volume tric flow rate of cold (m 3 /h) Volumet ric flow rate of hot (m 3 /h) Cold inlet Fluid temperatures ( o C) Cold outlet Hot inlet Hot outlet Overall heat transfer coefficient Overall Heat transfer coefficient Vs Volumetric flow rate of cold Volumetric flow rate of cold U(exp) U(sim) VI CONCLUSION This project work provided an opportunity and experience, to use our limited knowledge. It is a good solution to bridge the gates between institution and industries. Through this project a shell and Tube heat exchanger is developed which is working satisfactorily under standard conditions and which is helped to know how to achieve low cost semiautomation application. VII Scope for Future Work: Present tests are conducted at room temperatures and in future we can perform the experiment at low temperatures in order to check the performance of the present heat exchanger for various applications. VIII References: 1. Brodkey, R.S. and H.C. Hershey, Transport Phenomena: A Unified Approach, McGraw-Hill, 1988, pp Kern, D.Q., Process Heat Transfer, McGraw- Hill, 1950, pp , Levenspiel, O., Engineering Flow and Heat Exchange, Revised Edition, Plenum Press, 1998, pp McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (5 th Edition), McGraw-Hill, 1993, pp , McCabe, W.L., J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering (6 th Edition), McGraw-Hill, 2001, pp Mehra, D.K., "Shell-and-Tube Heat Exchangers", Chemical Engineering, July 25, 1983, pp Standards of Tubular Exchangers Manufacturer's Association, 6th Edition, 1978, pp. 24, 26, 144, Sadikkakac, Heat Exchangers Selection, Rating and Thermal Design, Ramesh K shah and Dusan P. Sekulic, Fundamental of heat exchanger design, Rochester Institute of Technology, Rochester New York, G.N. Xie, Q.W. Wang, M. Zeng, L.Q. Luo, Heat transfer analysis for shell and tube heat exchanger with experimental data by artificial neural networks approach, Applied Thermal Engineering 27 (2007) Page 48

8 11. B.V. Babu, S.A. Munawarb, Differential evolutionstrategies for optimal design of shell and tube heat exchanger, Chemical Engineering Science 62 (2007) José M. Ponce-Ortega,Medardo Serna- González, Arturo Jiménez-Gutiérrez, Use of genetic algorithms for the optimal design of shell and tube heat exchanger, Applied Thermal Engineering 29 (2009) M. Fesanghary, E. Damangir, I. Soleimani, Design optimization of shell and tube heat exchanger using global sensitivity analysis and harmony search algorithm, Applied Thermal Engineering 29 (2009) JiangfengGuo, Lin Cheng, Mingtian Xu, Optimization design of shell and tube heat exchanger by entropy generation minimization and genetic algorithm, Applied Thermal Engineering 29 (2009) SepehrSanaye, HassanHajabdollahi, Multiobjective optimization of shell and tube heat exchanger, Applied Thermal Engineering 30 (2010) V.K. Patel, R.V. Rao, Design optimization of shell and tube heat exchanger using particle swarm optimization technique, Applied Thermal Engineering 30 (2010) Heat Exchanger Design Handbook, by KuppanThulukkanam. 18. Fundamentals of Heat Exchanger Design, by Ramesh K. Shah, Dusan P. Sekulic G.V.N.Santhoshis pursuing M.Tech with the specialization inthermal Engineering in BVC Engineering College, Odalarevu. He received the B.Tech degree in Mechanical Engineering from PragatiEngg College, Surampalem in Y.V.RamanaMurty working as Associate Professor in the Department of Mechanical Engineering, BVC Engineering College,Odalarevu. Page 49