OPTIMIZATION OF SHELL AND TUBE HEAT EXCHANGER

Transcription

1 OPTIMIZATION OF SHELL AND TUBE HEAT EXCHANGER Abhishek Arya 1, Dangar Sunilbhai Dhanjibhai 2 (Assistant professor Department of mechanical engineering, SCE Bhopal) (M.tech scholar department of Mechanical engineering, SCE Bhopal) ABSTRACT A heat exchanger is a device, which transfer internal thermal energy between two or 1. INTRODUCTION Heat Exchanger: more fluids at different temperature. Without this Aheat exchanger is a device, which essential piece of equipment most industrial transfer internal thermal energy between process would be impossible. Heat exchangers are two or more fluids at different widely used in refrigeration air conditioning, and chemical plants. They can be employed in various uses, for instance, to effectively transmit heat from one fluid to the other. Shell-and-tube heat exchangers (STHXs) are widely applied in various industrial fields such as petroleum refining, power temperature. Without this essential piece of equipment most industrial process would be impossible. Heat exchangers are widely used in refrigeration air conditioning, and chemical plants. They can be generation and chemical process, etc. Tremendous employed in various uses, for instance, to efforts have been made to improve the effectively transmit heat from one fluid to the performances on the tube side. In this project experimental performance is done on the fixed designed STHX and calculate the heat transfer coefficient and effectiveness. Validation is to be carried out using which gives the result comparison with that of experimental result. Here other. Shell-and-tube heat exchangers (STHXs) are widely applied in various industrial fields such as petroleum refining, power generation and chemical process, etc. Tremendous efforts have been made to improve the performances on the tube side. For the shell side, the velocity and temperature fields flow parameters are not varied but size and are relatively complicated and the thermal number of tubes are varied and best efficient model is selected as Optimized value. 3 different number of tubes are used with same shell size remaining same. 40 tubes, 32 tubes and 36 tubes were tried. It's been observed for same input temperatures and mass flow rates for three different models one with 36 tubes, 32 tubes model &other with 40 tubes, the temperature variation in 36 tubes is more and also requires less tubes compared to 40 tube model. so it is more effective than tubes model. hydraulic performance depends on the baffle elements to a great extent. Shell-and-tube heat exchangers are fabricated with round tubes mounted in cylindrical shells with their axes coaxial with the shell axis. The differences between the many variations of this basic type of heat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and shell. These are types of heat exchangers with the outer area surrounding the tubes called shell side and the inside of the tubes are called tube side baffles are usually installed to increase the 27

2 convective coefficient of the shell side fluid by inducing turbulence. 1.1Classification of Shell-and-Tube Heat Exchangers Shell-and-tube heat exchangers can be classified based on construction, or on service. Both classifications are discussed in the paragraphs below. tube sheet is offset by the additional costs incurred for the bending of the tubes and somewhat larger shell diameter (due to the minimum U-bend radius), making the cost of a U-tube heat exchanger comparable to that of the fixed tube sheet heat exchanger Classification based on construction: Fixed Tube sheet: A fixed tube sheet heat exchanger (Figure 1) has straight tubes that are secured at both ends to tube sheets welded to the shell. The principal advantage of fixed tube sheet construction is its low cost because of its simple construction. In fact, the fixed tube sheet is the least expensive construction type, as long as no expansion joint is required. Other advantages are that the tubes can be cleaned mechanically after removal of the channel cover or bonnet, and that leakage of shell-side fluid is minimized since there are no flanged joint. A disadvantage of this design is that the since the bundle is fixed to the shell and cannot be removed, the outside of the tubes cannot be cleaned mechanically. Thus, its application is limited to clean services on the shell-side. Figure 1.2 U-Tube Heat Exchanger [B1] The advantage of a U-tube heat exchanger is that because one end is free, the bundle can expand or contract in response to stress differentials. In addition, the outsides of tubes can be cleaned as the tube bundle can be removed. Floating Head: The floating head heat exchanger is the most versatile type of shell-and-tube heat exchanger, and also is the costliest. In this design, one tube sheet is fixed relative to the shell, and the other is free to float within the shell. Figure 1.1 Fixed Tube sheet Heat Exchanger [B1] U Tube Shell and heat Exchanger: U Tube As the name implies, the tubes of a U-tube heat exchanger (Figure 2.3) are bent in the shape of a U. There is only one tube sheet in a U-tube heat exchanger. However, the lower cost for a single 28 Figure 1.3 Pull-through Floating Head Heat Exchanger (TEMA S) [B1] The TEMA S design is the most common configuration in the chemical process industries. The floating head cover is secured against the

3 floating tube sheet by bolting it to an ingenious split backing ring. This floating head closure is located beyond the end of the shell and is contained by a shell cover of a larger diameter Classification based on service: Basically, a service may be single phase (such as cooling or heating of a liquid or gas) or two phase (such as condensing or vaporizing). Since there are two sides to shell-and-tube heat exchangers, this can lead to several combinations of services. Broadly services can be classified as follows: Single-phase (both shell-side and tube-side) Condensing (one side condensing and the other single-phase) Vaporizing (one side vaporizing and the other single-phase) Condensing/ vaporizing (one side condensing and the other vaporizing) The following nomenclature is normally used: i. Heat Exchanger: Both sides single phase and process streams (as opposed to utility) ii. Cooler: One stream a process fluid and the other cooling water or air iii. Heater: One stream a process fluid and the other a hot utility such as steam or hot oil iv. Condenser: One stream a condensing vapour and the other cooling water or air v. Chiller: One stream a process fluid being condensed at sub-atmospheric temperature and the other a boiling refrigerant or process stream. vi. Reboilers: One stream a bottoms stream from a distillation column and the other a hot utility (steam or hot oil) or a process stream General TEMA exchanger classes-r, C and B. There are three basic categories of shell and tube type heat exchanger in heat exchanger in TEMAclass R, class C, and class B. The difference I class is 29 the degree of severity of service the heat exchanger will encounter. Descriptions of the three classes are as follow: i ii iii Class R includes heat exchanger specified for the most sever service in the petrochemical processing industry. Safety and durability are required for exchangers designed for such rigorous conditions. Class C includes heat exchanged for the generally moderate services and requirements. Economy and overall compactness are the two essential features of this class. Class B are exchangers specified for general process service. Maximum economy and compactness are the main criteria of design According to process requirement: Process requirements dedicate the type of design used. Figure shows some of the type of the major construction. The standard TEMA classification of exchanger is use the shell identification and number with the exchanger designation type. 1.2 Introduction to Heat Exchanger Components 1. Shell 16. Tubes (U-type) 2. Shell cover 17. Tie rods and spacers 3. Shell flange 18. Transverse (or cross) baffles (channel end) or support plates 4. Shell flange (cover 19. Longitudinal baffles end) 5. Shell nozzle or 20. Impingement baffles branch 6. Floating tube 21. Floating head support sheet 7. Floating head 22. Pass partition cover 8. Floating head 23. Vent connection flange 9. Floating head 24. Drain connection gland 10. Floating head 25. Instrument connection backing ring 11. Stationary tube 26. Expansion bellows

4 sheet 12. Channel or 27. Support saddles stationary head 13. Channel cover 28. Lifting lugs 14. Channel nozzle or 29. Weir branch 15. Tube (straight) 30. Liquid level connection tube pitch, tube layout, number of tube passes, type of heat exchanger and design pressure. 1.3 Basic Component of Shell and Tube Heat Exchanger Components: Shell: Shell is the container for the shell fluid and the tube bundle is placed inside the shell. Shell diameter should be selected in such a way to give a close fit of the tube bundle. Tube: Tube OD of ¾ and 1 are very common to design a compact heat exchanger. The most efficient condition for heat transfer is to have the maximum number of tubes in the shell to increase turbulence. The tube thickness should be enough to withstand the internal pressure along with the adequate corrosion allowance Tube pitch, tube-layout and tube-count Tube pitch is the shortest centre to centre distance between the adjacent tubes. The tubes are generally placed in square or triangular patterns (pitch) as shown in the Figure 2.8.The widely used tube layouts are illustrated in Table 1.1 Table 1.1 Common tube layouts Tube OD, in Pitch type Tube pitch, in ¾ Square ¼ ¾ Triangular 15/16 ¾ - 1 The number of tubes that can be accommodated in a given shell ID is called tube count. The tube count depends on the factors like shell ID, OD of tube, Figure 1.7 Heat exchanger tube-layouts [B1] 2. LITERATURE REVIEW Vindhya Vasiny Prasad Dubey, Raj Rajat Verma:[1]- This paper is concerned with the study of shell & tube type heat exchangers along with its applications and also refers to several scholars who have given the contribution in this regard. Moreover the constructional details, design methods and the reasons for the wide acceptance of shell and tube type heat exchangers has been described in details inside the paper. M. M. El-Fawal, A. A. Fahmy and B. M. Taher:[2]- In this paper a computer program for economical design of shell and tube heat exchanger using specified pressure drop is established to minimize the cost of the equipment. The design procedure depends on using the acceptable pressure drops in order to minimize the thermal surface area for a certain service, involving discrete decision variables. Also the proposed method takes into account several geometric and operational constraints typically recommended by design codes, and provides global optimum solutions as opposed to local optimum solutions that are typically obtained with many other optimization methods. 30 M.Serna and A.Jimenez:[3]-They have presented a compact formulation to relate the shell-side pressure drop with the exchanger area and the film coefficient based on the full Bell Delaware method. In addition to the derivation of the shell side compact expression, they have developed a compact pressure drop equation for the tube-side stream, which accounts for both straight pressure drops and return losses. They have shown how the compact formulations can be used within an efficient design algorithm. They have found a satisfactory performance of the proposed

5 algorithms over the entire geometry range of single phase, shell and tube heat exchangers. Andre L.H. Costa, Eduardo M. Queiroz:[4]-Studied that techniques were employed according to distinct problem formulations in relation to: (i) heat transfer area or total annualized costs, (ii) constraints: heat transfer and fluid flow equations, pressure drop and velocity bound; and (iii) decision variable: selection of different search variables and its characterization as integer or continuous. This paper approaches the optimization of the design of shell and tube heat exchangers. The formulation of the problem seeks the minimization of the thermal surfaces of the equipment, for certain minimum excess area and maximum pressure drops, considering discrete decision variables. Important additional constraints, usually ignored in previous optimization schemes, are included in order to approximate the solution to the design practice. G.N. Xie, Q.W. Wang, M. Zeng, L.Q. Luo:[5]- carried out an experimental system for investigation on performance of shell-and-tube heat exchangers, and limited experimental data is obtained. The ANN is applied to predict temperature differences and heat transfer rate for heat exchangers. BP algorithm is used to train and test the network. It is shown that the predicted results are close to experimental data by ANN approach. Comparison with correlation for prediction heat transfer rate shows ANN is superior to correlation, indicating that ANN technique is a suitable tool for use in the prediction of heat transfer rates than empirical correlations. It is recommended that ANNs can be applied to simulate thermal systems, especially for engineers to model the complicated heat exchangers in engineering applications. B.V. Babu, S.A. Munawarb:[6]- in the present study for the first time DE, an improved version of genetic algorithms (GAs), has been successfully applied with different strategies for 1,61,280 design configurations using Bell s method to find the heat transfer area. In the application of DE, 9680 combinations of the key parameters are considered. For comparison, GAs are also applied for the same case study with 1080 combinations of its parameters. For this optimal design problem, it is found that DE, an exceptionally simple evolution strategy, is significantly faster compared to GA and yields the global optimum for a wide range of the key parameters. Resat Selbas, Onder Kızılkan, Marcus Reppich:[7]- Applied genetic algorithms (GA) for the optimal design of shell-and-tube heat exchanger by varying the design variables: outer tube diameter, tube layout, number of tube passes, outer shell diameter, baffle spacing and baffle cut. From this study it was concluded that the combinatorial algorithms such as GA provide significant improvement in the optimal designs compared to the traditional designs. GA application for determining the global minimum heat exchanger cost is significantly faster and has an advantage over other methods in obtaining multiple solutions of same quality. Zahid H. Ayub:[8]-A new chart method is presented to calculate single-phase shell side heat transfer coefficient in a typical TEMA style single segmental shell and tube heat exchanger. A case study of rating water-to-water exchanger is shown to indicate the result from this method with the more established procedures and software available in the market. The results show that this new method is reliable and comparable to the most widely known HTRI software. Yusuf Ali Kara, Ozbilen Guraras:[9]-Prepared a computer based design model for preliminary design of shell and tube heat exchangers with single phase fluid flow both on shell and tube side. The program determines the overall dimensions of the shell, the tube bundle, and optimum heat transfer surface area required to meet the specified heat transfer duty by calculating minimum or allowable shell side pressure drop. He concluded that circulating cold fluid in shell-side has some advantages on hot fluid as shell stream since the former causes lower shell-side pressure drop and requires smaller heat transfer area than the latter and thus it is better to put the stream with lower mass flow rate on the shell side because of the baffled space. Su Thet Mon Than, Khin Aung Lin, Mi Sandar Mon:[10]- In this paper data is evaluated for heat transfer area and pressure drop and checking whether the assumed design satisfies all requirement or not. The primary aim of this design is to obtain a high heat transfer rate without exceeding the allowable pressure drop. 3. PROBLEM IDENTIFICATION Problem definition: 31

6 The most commonly used baffle is the segmental baffle, which forces the shell-side fluid to go through in a zigzag manner. But there are three major drawbacks in the conventional shell-andtube heat exchangers with segmental baffles (STHXSB): (1) it causes a large shell-side pressure drop; (2) it results in a dead zone in each compartment between two adjacent segmental baffles, leading to an increase of fouling resistance; (3) the dramatic zigzag flow pattern also causes high risk of vibration failure on tube bundle. Thus, higher pumping power is often needed to offset the higher pressure drop under the same thermal load. Therefore, it is essential to develop a new type of STHXs with improved baffles and reduce pressure drop while maintaining and even increasing shell side heat transfer performance. Helical baffle heat exchangers have shown very effective performance especially for the cases in which the heat transfer coefficient in shell side is controlled; or less pressure drop and less fouling are expected. It can also be very effective, where heat exchangers are predicted to be faced with vibration condition. From experimental performance, validation is to be carried out. One design problem is taken from the SAL STEEL, KANDLA as a case study and design optimization of Heat exchanger and validate the result with the software data. Limitations: 1) Thermal and mechanical design should be consider as basic aspects. 2) Design of shell and tube heat exchanger has to be done in accordance to ASME and TEMA standards. 3) The materials and size for the shell, tubes and other components are to be selected as available in Indian market. Rating and sizing are the main problems related with the STHX. For Rating: 32 Experimental performance is done on the fixed designed STHX and calculate the heat transfer coefficient and effectiveness. Validation is to be carried out which gives the result comparison with that of experimental result. For the effective and satisfactory solution of the given problem, a systematic step by step procedure is required to be followed as under: Calculation of heat transfer co efficient and effectiveness of the experimental data. 1) Validation of CFD code with experiment data. 2) Analytical calculation of mechanical design data. 3) CFD analysis of shell and tube heat exchanger for different number of tubes and optimising result for greater effectiveness. For sizing problem: Design problem from a company has been taken as a case study. For that purpose following steps are required: 1. Thermal design and mechanical design is calculated analytically. 2. Selection of the tube diameter, and other data according from the different standard. 3. Mass flow rates and inlet temperatures are kept constant, no of tubes are to be varied for effective heat transfer. 4. Calculation of the no of tubes required is done by using CFD. 4. METHODOLOGY Heat exchanger thermal design problems may be categorized primary as rating and sizing problems. The fluid outlet temperatures, total heat transfer capability, and pressure drops on each side of the heat exchanger are then determined in the rating problem. In contrast, in the sizing problem, the core lengths, surface areas and core dimensions are to be determined. Inputs to the sizing problems are surface geometries (including their heat transfer and pressure drop characteristics), fluid flow rates inlet and outlet fluid temperatures, fouling factors,

7 and pressure drop on each side. The sizing problem is also sometimes referred to as the design problem Heat Exchanger design methodology for optimum design is illustrated in fig-4.1, which include various quantitative steps involved in arriving at the optimum heat exchanger design. These steps include thermal and hydraulic design, mechanical design, evaluation procedure and costing. Now mechanical design is done to ensure the mechanical integrity of the exchanger under steady and transient operating conditions. to stabilize from several minutes to several hours for each point in order to obtain good equilibrium conditions and Constant. If room air is used care should be taken to avoid temperature irregularities arising from the opening and closing of door and windows A proper selection of the material and method of bonding fins thickness. A proper selection of the material and method of bonding fins to plates or tubes is made depending upon the operating temperatures, pressure and fluids. A proper selection of headers, tanks, manifolds, nozzles or pipes is made to ensure uniform flow distribution through the exchanger passages. Several option solutions may be available when the thermal and mechanical designs are completed. The designer then considers the evaluation procedure (evaluation criteria and tread-off factors.) and cost estimating to arrive at an optimum solution. 4.1 Testing of the performance: Different types of tests are employed for the heat exchangers. For the evaluation of the heat transfer, pressure drop, temperature distribution and velocity by using small models to acceptance test of large full scale units. The selection and delineation of the test to be run, the design of the test set up, the conduct of the tests. 4.2 Heat transfer based performance test: a Heat Balances:- In the analysis of the heat transfer test data is the heat balance obtained by comparing the heat given up by the hot fluids the heat absorbed by the cold fluid. The difference between these two quantities can be compared with the estimated heat losses. b Temperature Stabilization:- For heat transfer performance tests, the test procedure should be worked out concurrently with the design of the test rig to facilitate the conduct of the test and the reduction of the test data. Depending on the size of unit, the heat capacity of various components m the test setup, and heat losses, it is usually necessary Correlation of data It always important to correlated the experimentally evaluated performance of the heat exchanger with the analytically. This is desirable partly to check both the design and testing techniques and partly to provide the engineer to with a better background for future work of a similar nature. In reducing test data, first step in deciding how we organise and reduce the data for finding out to which the physical properties of the fluid changes with the temperature covered in the test Flow Test: Tests of this sort may be carried out with simple models, since no provisions for heat addition or extraction need be employed. In these tests the only requirement is that the model be geometrically similar and that the Reynolds number be in the range of interest. Thus the tests may be carried out with water or air rather than with fluids that would be difficult to handle. Air is especially suited to tests of this sort, because the models can be inexpensive in construction and small leaks will not make a mess. 4.4 Structural tests:

8 Structural tests on heat exchanger components such as pressure vessels or head can be carried out using any of a variety of techniques. Probably the most common approach is to build a fractional-or full-scale model of the structural element in question and determine the stress distribution with strain gauges. These can be applied in gauge lengths of as little as 6 mm. In one instance some 1300 strain gauges were installed on a 1/5-scale model of a complex pressure vessel; the cost of the test amounted to approximately 3% of the cost of the completed vessel, but it increased enormously the degree of confidence that could be placed in the design 4.5 Leak Test The problem related to the heat exchanger is to find out the location of the leakage and try to detect it. For that purpose various testing methods are applied. I. Soap Bubble Test: II. Rate of pressure rise: III. Helium Leak Test 5. EXPERIMENTAL SET UP 5.1 Experimental Set up and Specification data of Shell and tube heat exchanger: Table 5.1Specification data Tube Material: Stainless steel Tube side pass number: 1 Tube number: 24 Tube Pitch: 16 mm Tube inner diameter: 4.5 mm Shell inner diameter:116 mm Baffle No: 4 Baffle Spacing: 300mm Tube side fluid: warm water Tube Arrangement: Triangular Tube Effective Length: 750 mm Tube type: smooth Tube outer diameter: 6.35 mm Shell side Fluid: cool water Baffle Type: 25% cut Baffle geometry Angle: 90 0 Figure 5.1 Dimension of the experimental set up Table 5.2OBSERVATIONS SR.no Hot water Cold water INLET OUTLET INLET OUTLET Parallel 42ᵒC 32ᵒC 24ᵒC 28ᵒC Counter 53ᵒC 39ᵒC 26ᵒC 35ᵒC Specifications: Specific heat water = kj/kg k Inside area of tube = AA ii = ππ = mm 2 Outside Area of tube = AA 0 = ππ = mm 2 Density of Water = 1000 kg/m 3 34 For Parallel Flow: 1. Hot mass flow rate of the hot fluid m h = 1/t h = 1/43 = Lit/sec 2. Heat transfer rate at hot side Q h = m h c p T h = (42 32) = 1.34 kj/sec 3. Mass flow rate of cold fluid m c =1/t c = 1/26 = Lit/sec 4. Heat transfer rate at cold side Q c = m c c p t h =

9 m International Journal of Advanced Technology for Science & Engineering Research. Volume 1, Issue 2, = kj/sec T llll = T ii T h log T ii Th ii Where, T ii = T hi -T ci = = 18ᵒC T h = T ho - T co = = 4ᵒC.For Parallel Condition 5. Heat Transfer Co efficient i) Inside Heat Transfer Co efficient QQ h 1.34 UU h = = AA ii T llll = kw/mm 2ᵒC ii) Outside Heat Transfer Co efficient Q c U c = = A o T lm = kw /m 2ᵒC 6. Effectiveness of heat Exchanger, εε = (Thi Tho ) (Thi Tci). Because m h < R c εε = = REFERENCES 1. Vindhya Vasiny Prasad Dubey1, Raj Rajat Verma Shell & Tube Type Heat Exchangers: An Overview vol.2 issue ISSN (ONLINE): M. M. El-Fawal, A. A. Fahmy and B. M. Taher, Modelling of Economical Design of Shell and tube heat exchanger Using Specified Pressure Drop, 28 (2010) Journal of American Science. 3. M.Serna and A.Jimenez, A compact formulation of the Bell Delaware method for Heat Exchanger design and optimization, Chemical Engineering Research and Design, 83(A5) (2009): Andre L.H. Costa, Eduardo M. Queiroz, Design optimization of shell-and-tube heat exchangers, Applied Thermal Engineering 28 (2008) 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) B.V. Babu, S.A. Munawarb, Differential evolution strategies for optimal design of shell and tube heat exchanger, Chemical Engineering Science 62 (2007) Resat Selbas, Onder Kızılkan, Marcus Reppich, A new design approach for shell and tube heat exchanger using genetic algorithms from economic point of view, Chemical Engineering and Processing 45 (2006) Zahid H. Ayub, A new chart method for evaluating singlephase shell side heat transfer coefficient in a single segmental Shell and tube heat exchanger, Applied Thermal Engineering 25 (2005) Yusuf Ali Kara, Ozbilen Guraras, A computer program for designing of Shell and tube heat exchanger, Applied Thermal Engineering 24(2004) Sadik kakac, Heat Exchangers Selection, Rating and Thermal Design,