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  • 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 66 CFD ANALYSIS OF ECONOMIZER TO OPTIMIZE HEAT TRANSFER K. Obual Reddy1 , M. Srikesh2 , M. Kranthi Kumar3 , V. Santhosh Kumar4 1 Asst Prof, Department of Mechanical Engineering, GITAM UNIVERSITY HYDERABAD INDIA 2, 3, 4 UG Student, Department of Mechanical Engineering, GITAM UNIVERSITY HYDERABAD INDIA ABSTRACT Economizer is the best mechanical component which is used for trapping the heat of flue gasses. Economizers can be best applied in electricity generating power plants. Best way of utilisation of flue gas heat is by transferring it to the boiler feed water. Hence economizers are designed in such a way that feed water is allowed to pass the economizer through pipes. Economizer shell is completely filled with the flue gasses. There is a cross flow heat exchange process takes place between boiler feed water and flue gas. Boiler feed water is heated up to its boiling temperature by consuming the heat from flue gas. Hence as the boiler feed water is already preheated the amount of fuel burnt to generate steam is reduced in this way in boilers. This paper explains about one of the ways to increase the heat transfer rate of economiser. Heat transfer is always enhanced by increasing the surface area of heat transfer between heat exchanging bodies. Hence by providing addition area between the flue gas and boiler feed water we can increase the heat transfer. Additional area is provided with the help of fins which are considered as extended surfaces. A CFD (Computational Fluid Dynamics) analysis is carried with two cases with and without fins. Analysis of two cases is carried out to determine, how much value of heat transfer has enhanced with fins. Keywords: CFD (Computational Fluid Dynamics), Economiser, K- Ԑ Model. NOMENCLATURE ρ = density of fluid u = initial velocity p = pressure f = body force Q = vector variable INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 3, March (2014), pp. 66-76 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2014): 7.5377 (Calculated by GISI) www.jifactor.com IJMET © I A E M E
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 67 F = vector fluxes V = volume of control volume A = area of control volume Ri = equation residual Wi = weight vector Ve = volume of element. k = turbulence kinetic energy Ԑ = dissipation rate ω = specific dissipation rate Gk = generation of turbulence kinetic energy due to mean velocity gradients uj = jth Cartesian component of the instantaneous velocity τij = viscous stress tensor fi = the body force h = static enthalpy Gb = generation of turbulent kinetic energy that arises due to buoyancy YM = represents the fluctuating dilation Sε, Sk = source terms defined by the user. C1ε, C2ε, Cµ = constants that have been determined experimentally σk, σε = turbulent Prandtl numbers for the turbulent kinetic energy, its dissipation rate. S = mean rate-of-strain tensor. 1. INTRODUCTION Economizer is a mechanical component which helps in pre heating of water which is feed into boiler. It is a part of steam generating unit which intended to reduce energy consumption of boilers in particular. Generally in power plants water which is introduced into boilers is called as boiler feed water. This water is converted into steam after absorbing the heat which is produced from combustion of fossil fuel. Combustion of fossil fuels gives flue gases as by products. These gases have lower level of thermal energy within them which is trapped. Economizer carries a vital role in trapping this heat and transferring it to the boiler feed water. Hence economizer acts as an external heat transfer device where heat transfer takes place from flue gases to the boiler feed water. This action preheats the boiler feed water; thereby reducing the amount of fuel required to convert the boiler feed water into steam. Economizer is a tubular structure with main objective of obtaining lower cost efficient and maximum heat transfer rate with the boiler feed water. This action of economizer helps in economic production of power in power plants. It reduces the operating costs of power plant also. Heat is considered to be a form of energy. So as the saying goes energy is neither created nor destroyed, we should not allow the heat to go out as waste in the flue gases. One of the ways to conserve the heat energy which is going out as waste in the exhaust gasses is to extract the heat and transfer it to a boiler feed water. One of the important components of heat transfer system in power plants is the economizer. Economizer is widely used equipment in various industries such as power generation, petroleum refining, chemicals paper, refrigeration and also in HVAC (Heating Ventilation and Air Conditioning). According to a market survey conducted in Europe, economizers are used in about 42% of the power plants. It also used for the heat exchange process in Nuclear industry as well. Hence there arises a need for analysis of Economizer. 1.1. Fins extended surface In order to optimize the heat transfer rate and increase it there are several ways in achieving it. One of the easiest and simple ways of doing it is to increase the area of heat transfer between two
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 68 heat exchanging bodies. Hence this paper explains about the analysis which is carried out on an economizer which has fins attached to its tubes. As these fins are considered as extended surface areas, hence there is increase in heat transfer between the boiler feed water and flue gas. Orientation of the fins surface with respect to the direction of flue gas, type of fins shapes, dimensions of the fins and the material used as fin all play a significant role in the heat transfer between two bodies. 2. LITERATURE SURVEY Krunal P. Mudafale & Hemant S. Farkade[1] work on “CFD analysis of economizer in a tengential fired boiler”. In this a simulation of the economizer zone, which allows for the condition of the shell-side flow and tube-side and tube-wall, thermal fields, and of the shell-tube heat- exchange. As per their observation maximum number of cause of failure in economizer unit is due to flue gas erosion. The past failure details revels that erosion is more in U-bend areas of Economizer Unit because of increase in flue gas velocity near these bends. But it is observed that the velocity of flue gases surprisingly increases near the lower bends as compared to upper ones. The model is solved using conventional CFD techniques by STAR- CCM+ software. The Computational Fluid Dynamics (CFD) approach is utilised for the creation of a three-dimensional model of the economizer coil. With equilibrium assumption applied for description of the system chemistry. The flue gas temperature, pressure and velocity field of fluid flow within an economizer tube using the actual boundary conditions have been analyzed using CFD tool. Such as the ability to quickly analyse a variety of design options without modifying the object and the availability of significantly more data to interpret the results. A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve[2] work on “To optimize economizer design for better performance” This paper focuses on optimisation of economiser design with finned & bare tube economiser. The aim of this work is to develop methodology which finds optimisation of economiser design.CFD analysis is used to compare the new economiser design with traditional strategies. A brief historical overview on economiser design & optimisation is given along with the main advantage of the authors proposed method is also discussed. A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve [3] work on “Optimization of Economizer Design for the Enhancement of Heat Transfer Coefficient”. This paper presents an approach for the optimization of economiser design. The aim of this work is to develop methodology which finds optimization of economiser design. CFD analysis is used to compare the new economizer design with traditional strategies. The most economical solution of this problem seems to distribute gas flow uniformly at inlet of economiser by using vanes. In the present work commercial software Fluent is used for the 3D simulation using its inbuilt K- Ԑ Reliable model. Optimization of economiser is done for effective heat transfer with reducing number of tubes required. In the present work commercial software Fluent is used for the 3D simulation using its inbuilt K- Ԑ Reliable model. Optimization of economiser is done for effective heat transfer with reducing number of tubes required. TSUNG-FENG WU[4] work on “failure analysis for economizer tube of the waste heat”. This paper is about failure analysis of the leakage of the economizer tube of the waste heat boiler in the energy factory. The results show that although the material and mechanical properties of the failed tube, were inferior to those of the new one, most of them were still satisfactory to the criterion requirement it is clear that the crack initiated in the outer surface and propagated toward the inner surface of the tube and the crack was identified to be rectangular in shape. Deendayal Yadav, Dr. G. V. Parishwad, P. R. Dhamangaonkar*, Dr. S. R. Kajale, Dr. M. R. Nandgaonkar, Dr. S. N. Sapali.[5] work on “effect of arreasters on erosion in economizer zone and its analysis”. The authors in this paper have attempted to suggest a probable solution for reduction of erosion in economiser zone and its analysis using CFD tool. In this paper the authors have submitted the findings of analysis of finned tube economizer with Arresters at different inclinations. A steady
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 69 3D CFD tool is used for analysis and flow of the flue gases over the coils has been observed. The effect of provision of arresters on the surface temperature, the flue gas temperature, pressure and velocity field of fluid flow within an economizer tube using the actual boundary conditions have been analyzed using CFD tool. The optimum dimensions of arrester and feasible inclination is recommended as a result of the study. The authors have analyzed the performance and tried to comment on this issue too. A.N. Aziz, P. Siregar, Y.Y. Nazaruddin, and Y. Bindar [6] work on “Improving the Performance of Temperature Model of Economizer Using Bond Graph and Genetic Algorithm”. The mathematical model of economizer, based on physical laws is derived using bond graph methodology. Pseudo multi-energy bond graph, which employs mass flow-rate and enthalpy flow- rate as flow variables, as well as pressure and temperature as effort variables, is used in achieving that. Overall heat transfer coefficient of economizer is obtained by using logarithmic temperature mean difference between flue gas and boiler feed water. A modification of overall heat transfer coefficient in the form of parameterized polynomial is also done by using the help of genetic algorithm technique. A step simulation of the model at maximum, continuous, and minimum boiler operating condition demonstrates, that model’s performance has been improved. 3. COMPUTATIONAL FLUID DYNAMICS Computational Fluid Dynamics CFD is one of the best suited software for the analysis of flow patterns which involve the dynamic parameters in the flow. It is very stable robust and accurate in providing the required outputs. Generally experiments on moving fluid particles are not feasible. Dynamics study on moving particles involves many complex calculations and large variables. But their analysis can be easily carried with the help of CFD. 3.1. Working of CFD CFD is based on Finite Volume Method. The area in the flow pattern for the analysis is first modelled in any of the modelling software. In terms of CFD this is called as Computational domain. This computational domain is discretized into many small elements of finite volume. This process of dividing the domain is termed as meshing. Hence here each volume is considered to be a control volume. The control volume have certain properties like mass, momentum, energy, turbulence quantities, and mixture fractions, species concentrations and material properties. Based on the flow problem we pick up the control volume properties to be analysed and solve the flow problem. The theoretical flow in control volume is represented physically with the help of numerically solvable partial differential equations. These equations govern the flow of fluid in the computational domain. These equations also undergo the discretization process for the flow analysis. There are three equations which are applicable commonly to all fluid dynamics problems are the conservation of mass, momentum and the energy equations. Equations when represented in differential form: Continuity equation: డఘ డ௧ + ‫׏‬ (ߩ ‫)ݑ‬ = 0 (1) Momentum equation ߩሺ ௗ௩ ௗ௧ + ‫.ݒ‬ ߘv) = - ‫׏‬p ൅ ‫׏‬ T ൅ f (2)
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 70 Energy equation: ࣔ ࢚ࣔ (ρh) + ࣔ ࣔ࢞࢐ (ρujh) = - ࣔࢗ࢐ ࣔ࢞࢐ + ࣔρ ࢚ࣔ + uj ࣔࡼ ࣔ࢞࢐ + τij ࢛ࣔ࢏ ࣔ࢞࢏࢐ (3) Above equations along with many other equations depending on the type and problem of flow are considered as PDE’s Partial Differential Equations. These equations when subjected to discretization give out a set of algebraic equations. These algebraic equations contain terms that have been used for specifications of that particular flow and also these equations contain our required output parameters. After solving these equations, the solution of these algebraic equations is found with the CFD program called as FLUENT. This process of solving the flow problem involves generation of output at each element nodes for accurate and precise results. Analysis in CFD involves repeated and sequential solving of algebraic equations. The main purpose for this repeated iterations is to improve the quality of solution constantly after each and every iterations. The process of iterations is continued until convincing values of the global residuals are obtained. Global residuals are difference between the values of output parameters obtained in the current and previous solution which are averaged over the entire computational domain. After continuous iterations a state of “convergence” is obtained where the residuals have decrease by 4-5 orders of magnitude. One of the most important points to be considered when working with CFD software is that the quality of its output is completely dependent on the quality of its input. Hence when best quality inputs are given to the solver best quality outputs are obtained for the analysis. 3.2. K- Ԑ turbulence model This model is most reliable and robust very frequently used CFD platform. This model of analysis has some special from others models like K- omega, RANS, Large Eddy simulations etc. In the K- Ԑ model K represents the kinetic energy and Ԑ represents the turbulence. Both of the parameters are determined with the help of certain following equations ࣔ ࢚ࣔ (ρk) + ࣔ ࣔ࢞࢏ (ρkui) = ࣔ ࣔ࢞࢐ {[µ+ ೟ ఙೖ ] ࣔ࢑ ࣔ࢞࢐ } + Gk + Gb – ρε - Ym + Sk (4) ࣔ ࢚ࣔ (ρε) + ࣔ ࣔ࢞࢏ (ρεui) = ࣔ ࣔ࢞࢐ {[µ+ ౪ ఙε ] ࣔε ࣔ࢞࢐ } + C1ε ε ௄ (Gk+C3εGb) – C2ερ Єଶ ௄ + St (5) µt = ρCµ ௄ଶ ε (6) Gk = -ρui’uj’ ࢛ࣔ࢐ ࣔ࢞࢏ (7) S = √2SijSij (8) Gb = βgi ୲ ௉௥௧ ࣔࢀ ࣔ࢞࢏ (9) Ym = 2 ρЄMt 2 (10) All the above terms have been defined in the nomenclature which is specified before the starting of paper.
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 71 4. ANALYSIS OF ECONOMIZER Analysis of heat exchanger to optimize the heat transfer takes place in two cases. First case is plain tube economizer and second case is finned tube economizer. In order to provide accurate results, the input boundary condition parameters are obtained from a real time coal fired power plant. The table below represents the values which are given to fluent solver Table 1: (Temperature constrains) Flue gas inlet temperature into economizer 508°C Flue gas outlet temperature into economizer 330°C Feed water inlet temperature into economizer 254°C Feed water outlet temperature into economizer 312°C The geometric specifications of the computational domain are not in accordance with the real time economizer design because of lack of proper computer capacity, but the domain is scaled as per the power plant dimensions. And the table represents them Table 2: (Geometrical specifications) Specifications Dimension Length (Shell) 60 centimetre Width (Shell) 30 centimetres Height (Shell) 30 centimetre Diameter of tubes 0.2 centimetre Material Steel and carbon Number of tube assemblies 1 Some other specifications considered in the analysis are Table 3: (Flue gas specifications) Mass flow rate 17.6 Kg/s Specific heat 1.12 kj/kg k Thermal conductivity 0.00046 kw/mk Density 1.337 kg/m3 viscosity 0.101 kg/mh Table 4: (Feed Water Specifications) Mass flow rate 6.5 Kg/s Specific heat 0.42 kj/kg k Thermal conductivity 1 kcal/m-hr-°c density 913 kg/m3
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 72 4.1. Geometric model and meshing Modelling of the computational domain is carried out on GAMBIT (Geometry and mesh building intelligent kit). In order to get best accurate results the domain is discretized into small elements. The shape of the elements is Tet/Hybrid which mainly includes Tetrahedron along with some other shaped elements. Case 1 meshed model Figure 1: Plain tube economizer with mesh In the modeling of case 2, only one finned tube economizer pipe is considered. From the analysis of case 1 it is seen that temperature profile of all the water tubes are same and uniform. Hence analysis of one finned tube gives information about all the remaining tubes in the economizer shell. Case 2 meshed economizer tube Figure 2: Finned tube economizer model
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 73 4.2. Conduction and convection in finned tube economizer Convection in the analysis is the heat transfer between the flue gas and the surface area of economizer tube. Conduction is the heat transfer within the thickness of the water tube. Convection in the case 2 increases because of increase the heat transfer surface area between the flue gas and water tube. This increase is due to external fins which are provided on the water tube. Conduction in the case 2 decreases because of the increase in thickness due to fins. Figure 3: Economizer water tube with and without fins In the case 2 conduction, there is increase in the thickness of tube because of fins. When we are considering the conduction formula the thickness of tube material is inversely proportional to the thermal conductivity. So as the thickness increases when the fins are considered conduction in case 2 decreases. 5. RESULTS These results are obtained after detailed analysis of economizer with two cases. Case 1 Figure 4: Heat transfer along the economizer water tube (Static Temperature)
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 74 Figure 5: Heat transfer along the economizer water tube (Velocity Vector) Figure 6: Heat transfer along the economizer shell tube (Velocity Vector) Figure 7: three dimensional graph showing temperature profile of shell and water tube
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 75 Case 2 Figure 8: temperature profile of water tube in case 2 6. CONCLUSION Hence from the analysis it is found that heat transfer rate has been enhanced by providing the fins in case 2. As they are considered as extended heat transfer surface areas there is significant increase in heat transfer rate. Other technical explanation for the increase in heat transfer is, by creating fins in the case 2, uniform flow of flue gas in the economizer is disturbed. Where as in case 1 there is no obstruction in the motion of flue gas. Because of fins there is disturbance in its flow, which creates turbulence into the motion of flue gas. This turbulence energy is transferred to the fins in the water tube. Due to this more amount of heat is transferred to the fins. Hence more amount of heat is transferred in the second case than the first case. And this heat energy is transferred to water flowing in the tube. So in this way we experience increased heat transfer because of the external fins which act as increased heat transfer area and also due to generation of turbulence.
  • 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 3, March (2014), pp. 66-76, © IAEME 76 REFERENCES [1] Krunal P. Mudafale & Hemant S. Farkade CFD analysis of economizer in a tengential fired boiler, International Journal of Mechanical and Industrial Engineering (IJMIE) ISSN No. 2231 –6477, Vol-2, Iss-4, 2012. [2] A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve to optimize economizer design for better performance, New aspects of fluid mechanics, heat transfer and environment isbn: 978-960- 474-215-8. [3] A.D.Patil, P.R.Baviskar, M.J.Sable, S.B.Barve Optimization of Economiser Design for the Enhancement of Heat Transfer Coefficient, International Journal of Applied Research In Mechanical Engineering (IJARME), ISSN: 2231 –5950 Volume-1, Issue-2, 2011. [4] TSUNG-FENG WU Failure Analysis for the Economizer Tube of the Waste Heat Boiler. [5] Deendayal Yadav, Dr. G. V. Parishwad, P. R. Dhamangaonkar*, Dr. S. R. Kajale, Dr. M. R. Nandgaonkar, Dr. S. N. Sapali Effect of Arresters on Erosion in Economizer Zone and its Analysis, AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010. [6] A.N. Aziz, P. Siregar, Y.Y. Nazaruddin, and Y. Bindar Improving the Performance of Temperature Model of Economizer Using Bond Graph and Genetic Algorithm, International Journal of Engineering & Technology IJET-IJENS Vol: 12 No: 01. [7] N.S. Venkatesh Kumar and Prof. K. Hema Chandra Reddy, “CFD Analysis of Wind Driven Natural Cross Ventilation for a Generic Isolated Building”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 5, 2013, pp. 200 - 207, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [8] Sanjay Kumar Patel and Dr. A.C. Tiwari, “Performance Analysis of Supercritical Boiler”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 422 - 430, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. [9] Sunil Jamra, Pravin Kumar Singh and Pankaj Dubey, “Experimental Analysis of Heat Transfer Enhancement in Circular Double Tube Heat Exchanger using Inserts”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 306 - 314, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.