Performance Optimization of Air Cooled Heat Exchanger Applying Analytical Approach

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1 e-issn Volume 2 Issue 6, June 2016 pp Scientific Journal Impact Factor : Performance Optimization of Air Cooled Heat Exchanger Applying Analytical Approach Aamir Beg 1, Deepti Kushwaha 2, Shubham Shrivastava 3 1 Assistant Professor of Mechanical Engineering Department, UIT RGPV, Bhopal 2,3 Assistant Professor of Automobile Engineering Department, UIT RGPV, Bhopal Abstract Thermal optimization and analysis of an air cooled heat exchanger has been carried out analyzing the effect of fin geometric parameters (fin radial clearance, fin diameter and fin height) on the overall performance of heat exchanger. The Analytical study is performed taking into consideration the input variables to evaluate the various performance output parameters. The steel tube with L base wrapped fin of aluminum is taken for the analytical analysis. Varying fin height of 9.53, 9.78, 10.03, and mm is taken into consideration along with varying pitch in mm (3.18, 6.35, 9.53, and mm) of tube. The analysis shows that increasing the fin height result in the decrease in overall heat transfer coefficient for outside air. On the same side increasing the fin radial clearance account in the increase in overall heat transfer coefficient, as this will result in increase in the tube bundle size. For the effective analysis of fin, hot air is passed over the fins to heat up the cold fluid inside the tubes; this made air side analysis more effectively. Key Words Ambient Air, optimization, Tube Bundle, fin height, Fin pitch, and overall heat transfer coefficient 1.1 Thermal design and optimization I. INTRODUCTION Air cooled Heat Exchanger are used in petrochemical and oil and gas refineries to utilize the atmospheric air to cool the hydrocarbon, process and utility fluids by way of indirect heat transfer from hot process fluid (inside the tube) to be cooled by ambient air being circulated by means of forces / induced draft fan. Fins are attached to the periphery of tubes to increase the heat transfer area. These heat exchangers are usually designed, inspected and tested as per API 661 standard. A high efficient, well designed heat exchanger can aid to save energy, material and process operating expenditure, in order to obtain an best possible heat exchanger design, many factors need to be well thought-out such as overall heat transfer coefficient, total heat transfer area and desired outlet temperature requirements etc. Amongst these performance parameter air side heat transfer coefficient matters most significantly as the thermal resistant is typically on the air side. Studies are done to analyze analytically the performance parameters to enhance air side heat transfer, using optimization approach to adopt effective fin geometric parameters at the same side administrating the optimum heat transfer area. II. LITERATURE REVIEW Alinia Kashani et. al [[1]] (2013) studied Thermodynamic modeling and optimal design of an aircooled heat exchanger (ACHE) unit. In this study, entu method and mathematical relations are applied to estimate the outlet temperatures of fluids and pressure drops in tube and air All rights Reserved 355

2 A disjunctive mathematical model for the optimal design of air cooled heat exchangers has been presented by Juan I. Manassaldia, Nicolás J. Scennaa, Sergio F. Mussatia 2013 [[2]]. In these mathematical model parameters such as number of passes, type of the finned tube, number of tube rows, fins per unit length, number of tube per row, mean fin thickness and the type of the flow regime are associated with the seven discrete decisions. Disjunctions, Boolean variables and logical propositions are use for modeling for each discrete decision. Experimental design on the elements of the fin-and-tube heat exchanger has been studies by Ching- Yen Ho et. al 2009 [[3]]. Three different fins (plate fin, wavy fin, and compounded fin) are considered in the experiment, which were investigated in a wind tunnel. The plane fins as simple and easy to manufacture are simple in construction and cost effective as compared with other fin types are examined. The heat transfer coefficient, the pressure drop of the air side, the Colburn factor (j), and fanning friction factor (f) against air velocity (1 3 m/s) and Reynolds number ( ) have been discussed. CC Wang (2000) [[4]] presented the air-side side performance of fin and tube heat exchangers with various fin geometry including the data reduction method and correlations. Wang et al. (2000) focused on data reduction method to evaluate the air-side heat transfer and fluid flow characteristics of fin-and-tube heat exchangers in their literature. They proposed to calculate the air side heat transfer coefficient from overall thermal resistance equation, and to solve the equation they suggested to compute UA value from ε-ntu relationship equation, tube-side heat transfer coefficient from Gnielinski (1976) [Error! Reference source not found.] semi-empirical correlation for smooth tube, and air-side overall surface efficiency from Schmidt(1949) equation. They mentioned that water velocity should be maintained high to keep the water-side thermal resistance less than 15% of the overall thermal resistance for obtaining accurate air-side data. They recommended as :1) the energy balance in the air-side and tube-side should be less than 5%, 2) the temperature drop in the tube-side should be higher than 2% for better accuracy 3) selection of proper ε-ntu relationship equation for calculating UA value 4) suggested equation for determining friction factor negating the entrance & exit losses. III. PROBLEM AREAS The design of an ACHE is more intricate than for a Shell and Tube Heat Exchanger, as there are numerous more components and variables. In our analysis we concern our approach to optimize fin geometry parameter with other associated parameters like fin pitch, fin height, tube pitch to optimize the overall unit. Fins correspond to the major element in transferring the heat into the ambient air and contribute in the overall principal cost of the unit. For this reason, tube wrapped with L Base fins with variable fin height, fin pitch, fin radial distance and other operating parameters are analyze. In addition, validation of the employed code indicates a possible set-up that can be adopted for analyzing the heat exchange from these tubes with air. A comprehensive analysis has undoubtedly revealed the potential or possibilities of improvement in heat transfer in ACHE. Finally, a planned experiment is also described for further evaluation of this system. IV. THERMAL ANALYSIS Most Optimization problems fall into one of the following categories: a. Improving the overall heat transfer coefficient. b. Minimizing the heat transfer area. c. Minimizing the All rights Reserved 356

3 4.1 Design Data Requirements The following data should be determined and made available for design: a. The required process fluid temperature. b. The minimum design air temperature. c. All alternative process conditions, including reduced flow (turndown) operations. d. The design wind velocity and the prevailing wind direction. e. The available plot area, f. Pressure drop requirement. 4.2 Thermal Design Calculations In our study of analysis of performance parameters of ACHE, the ACHE like any other type of heat exchanger must assure the general heat transfer equation Analysis data taken Air Inlet temperature: 505 Kelvin Air outlet temperature: 455 Kelvin Water Inlet temperature: 405 Kelvin Water outlet temperature: 445 Kelvin Air mass flow rate: kg/hr Water mass flow rate: kg/hr Tube length: 1.22 m The heat transfer area is been calculated as below Heat transfer Area A = Q/ (U* LMTD) The overall heat transfer coefficient is defined as the sum of heat transfer coefficient of outside air and inside fluid. Optimization of ACHE aims to reduce the overall expenditure of the heat transfer unit and overall expenditure depends upon the material used. For this reason, optimization should be such as that utmost heat transfer accomplishment in minimum heat transfer area. The important aspect in deigning the heat exchanger is to consider the capital cost, operating cost and the other performance parameters. The heat balance principle phenomenon is acquired to ascertain the total heat content in hot fluid. The total heat transfer rate between hot and cloud fluid, over the complete length of the air cooled heat exchanger is given content in the fluid is used as given below. Q = U x A x LMTD The mean temperature difference LMTD, is define as above is called as Logarithmic Mean Temperature Difference. The value for LMTD is defined as the difference of temperature in cross flow arrangement. θ1 = Th 1 -Tc 2 θ2 =Th 2 -Tc 1 Projected perimeter of fin surface is defined as the sum of all the external distances in the plan view of a transverse finned All rights Reserved 357

4 P = (2* H fin *2*N fin ) + 2*(L tube T fin ) The equivalent diameter of the finned tube is required to estimate the flow area of the tube bundle, the flow area is the area remaining area after removing the constrained area by finned tubes. De = 2(AF + AB)/ (π x Projected Perimeter) Mass flow per frontal area is defined as the total mass flow of outside air through the duct to the flow area of duct. G = Mass flow / Flow area V. RESULTS AND PERFORMANCE ANALYSIS The analysis of thermal design of tube bundle assembly of air cooled heat exchanger emphasis on the optimization of performance parameters. The analysis focuses on fin profile parameters which show relationship with air cooled heat exchanger performance parameters. The fins constitute the extended surface for heat to transfer from hot fluid to ambient air. The phenomena of conduction and convective heat transfer constitute the heat to transfer from hot to cold fluid. Our study shows the characteristics behavior of fin geometry and associated parameters with over all air cooled heat exchanger operation. Fig 5.1 Reynolds number Vs Fin Height Combination of variable fin height and tube radial clearance clearly shows that the relationship of fin height with fin side heat transfer coefficient for outside air shows a behavior a gradual decrease behavior with increase in fin height up to 10.4 mm fin height. Fig 5.2 Heat transfer coefficient for outside air vs Fin All rights Reserved 358

5 The overall heat transfer coefficient is also known as the heat transfer coefficient which is based on the outside air flow and inside fluid flow. The contribution to overall heat transfer is very important and it should be of higher value for better heat transfer. The radial clearance R (Tube pitch Fin outside diameter), is the radial clearance space for the outside air to travel between the adjacent tubes. The fin outside diameter governs the radial clearance and affects the overall heat transfer coefficient for outside air. Fig57.3 Overall heat transfer coefficient vs. Fin Radial Clearance The overall heat transfer coefficient shows a gradual increase with fin radial clearance. At radial clearance of 0.05 mm a low value of 547 W/m2C is observed which increase progressively 565 W/m2C with 16 mm, with further increase in fin radial clearance the increase in overall heat transfer coefficient is observed. VI. CONCLUSIONS The optimization parameters of maximum heat transfer require the maximum value of heat transfer coefficient so that maximum heat transfer to air takes place. Hence increasing the fin radial clearance will contribute increase in flow area, which will account lower fan power requirement and better process parameters. Hence Fin radial clearance of 11 mm and fin height 9.78 mm is considered as optimal for the analysis. REFERENCES [1] Amir Hesam Alinia Kashani, Alireza Maddahi, Hassan Hajabdollahi, Thermal-economic optimization of an air-cooled heat exchanger unit. Applied Thermal Engineering 2013 [2] Juan I. Manassaldia, Nicolás J. Scennaa, Sergio F. Mussatia, Optimization mathematical model for the detailed design of air cooled heat exchangers [3] Mao-Yu Wen and Ching-Yen Ho, Heat-transfer enhancement in fin-and-tube heat exchanger with improved fin design, Appl. Therm. Eng [4] Wang, C.C., Recent progress on the air-side performance of fin-and-tube heat exchangers. I. J. Heat Exchangers 1, [5] Kern, D. Q., Process Heat Transfer, McGraw-Hill, New All rights Reserved 359