IJIRST International Journal for Innovative Research in Science & Technology Volume 1 Issue 11 April 2015 ISSN (online): 2349-6010 Analysis of Evaporative Cooler and Tube in Tube Heat Exchanger in Intercooling of Gas Turbine G Rahul Krishna Saintgits College of Engineering, Kottayam, Kerala, India Adharsh S Saintgits College of Engineering Kottayam, Kerala, India Akhil George Kurian Saintgits College of Engineering Kottayam, Kerala, India Aswin Zachariah Saintgits College of Engineering Kottayam, Kerala, India Bibin Varkey Assistant Professor Saintgits College of Engineering Kottayam, Kerala, India Abstract In this project a study is conducted by using an evaporative cooler and a tube in tube heat exchanger for intercooling. In an evaporative cooler the water absorbs heat from the stream of the flowing air in order to change its phase and then evaporate, thus providing a net cooling effect to the incoming stream of compressed air. A typical tube in tube heat exchanger may be used to reduce the temperature of the flowing stream of gases. In our study we calculated the reduction in work done by the compressor for a particular flow rate of air through the system for both evaporative cooler and tube in tube heat exchanger. Calculations were made for different flow rates of cooling water and corresponding graphs were plotted. Keywords: Compressor, Turbine, Evaporative, cooler, Heat Exchanger I. INTRODUCTION A major portion of the power developed by the gas turbine is utilized by the compressor. It can be reduced by compressing air in two stages with an intercooler between the two. First of all the air is compressed in a low pressure compressor as a result the pressure and temperature of air is increased. Now the air is passed through an intercooler which reduces the temperature of the compressed air to its original temperature, but keeping the pressure constant. After that the compressed air is once again compressed in the high pressure compressor. Now the compressed air is passed through the heating chamber and then through the turbine. In this project we are going to analyses the evaporative cooler and counter flow tube in tube heat exchanger as an intercooler. Evaporative coolers are used with gas turbines to increase the density of the combustion air, thereby increasing power output. The air density increase is accomplished by evaporating water into the inlet air, which decreases its temperature and correspondingly increases its density. The water vapor passes through the turbine, causing a negligible increase in fuel consumption. In an evaporative cooler, cold water is sprayed to the incoming hot gases. The water absorbs some of the heat from the stream of the flowing gases in order to change its phase and then evaporate, thus providing a net cooling effect to the incoming stream of gases. It is very important to understand the velocity at which water is sprayed to the flowing gases. Knowing the velocity at which one gets the minimum air temp at outlet one can design the equipment by calculating the water mass flow rate. A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. A typical tube in tube counter flow heat exchanger is used to reduce the temperature of air stream from low pressure compressor. Water may be flown in the outer tube and air stream in the inner tube would lead the air to get cooled and a net increase in the density of air entering the high pressure compressor. The main objective is to investigate the process of intercooling using evaporative cooler and tube in tube heat exchanger in gas turbines. The objectives involve to design an evaporative cooler and tube in tube counter flow heat exchanger for intercooling and to find the outlet temperatures. It also includes the work required for compression at various flow rates of cooling water. All rights reserved by www.ijirst.org 355
II. ANALYTICAL METHOD Fig. 1: Schematic Representation of a Gas Turbine with Intercooler First of all the atmospheric air enters the low pressure compressor at 300K (T 1 ) and is compressed isentropically. Assuming the compression ratio of the low pressure compressor as 10 the outlet temperature T 2 is found using the relation, = ( (Equation 1) Now using the outlet temperature of the low pressure compressor the outlet temperature T 3 of the intercooler is found by the analysis of intercooler. The cooling occurs at constant pressure. Analysis is done for evaporative cooler and counter flow tube in tube heat exchanger using Ansys Fluent 14.Thus we obtained the outlet temperature of the intercoolers. Using the outlet temperature of the intercooler the outlet temperature of the high pressure compressor T 4 is found using the relation. (Equation 2) Finally the work done by the compressor is calculated using the equation = [ ( - ) + ( - )] (Equation 3) III. MODELING THE EVAPORATIVE COOLER Fig. 2: Solid works model of evaporative cooler The Evaporative cooler was modeled using the CAD software. The diameter of the air passage and water spray inlet are 10cm and 1 mm respectively, length of the air passage is 20 cm. The model was imported to the Design Modeler module of Ansys Fluent. The fluid and solid domains were defined and the named surfaces were created. The model was opened in the Meshing module of Ansys and the model was meshed with default settings. The mesh obtained can be found out that the Model was meshed with hexahedral elements. This can be found from the fact that the shape of the mesh is triangular in shape. The concentration of smaller elements near the water inlet tubes tells that the program has identified the salient points and has made the necessary mesh correction. The mesh elements size is 37357m and nodes number is 193567. Fluent uses for CFD analysis. The defined material is a mixture of Water liquid and Air. The cell zone condition is then selected using this mixture. Boundary Condition Air inlet Velocity inlet Temperature 581 K Velocity 1m/s Percentage of water 0% All rights reserved by www.ijirst.org 356
Air Outlet Pressure outlet Backflow Temperature 581 K Gauge Pressure 0 Pa Percentage of Water 100% Water inlet Velocity inlet Temperature 300K Percentage of water 0.01% Velocity 15m/s,50m/s,70m/s,100m/s, 150m/s, 200m/s and 250m/s IV. MODELING THE COUNTER FLOW TUBE IN TUBE HEAT EXCHANGER The counter flow tube in tube heat exchanger was modeled using the CAD software. The dimensions of the equipment are, air Stream Diameter is 10cm, Air stream length is 20cm and Water Annulus diameter is 12cm. Fig. 3: Solid Works model of counter flow tube in tube heat exchanger As per the dimensions the model was created. The model was imported to the Design Modeler module of Ansys Fluent. The fluid and solid domains were defined and the named surfaces were created. The model was opened in the Meshing module of Ansys and meshed with default settings. The mesh obtained tetrahedral elements. This can be found from the fact that the shape of the mesh is rectangular in shape. The concentration of smaller elements near the water inlet tubes conveys the salient points are identified and has made the necessary mesh correction, the obtained number of mesh elements is 2976 and number of nodes is 2563. We read the previously created mesh file in Fluent. We define the material as a mixture of liquid water and air. The cell zone condition is then selected using this mixture. The inner tube is set as air and the outer tube is set as water. The boundary condition we set has been tabulated below. Table-2: boundary condition of evaporative cooler Boundary Conditions Air inlet Velocity inlet Temperature 581 K Velocity 1m/s Percentage of water 0 % Air Outlet Pressure outlet Backflow Temperature 581 K Gauge Pressure 0 pa Percentage of Water 100% Water inlet Velocity inlet Temperature 300K Percentage of water 0.01% V. RESULT The analysis of both evaporative cooler and counter flow tube in tube heat exchanger, were done on Ansys Fluent. The temperature contour for various flow rate of water was obtained from the analysis. Using this the work done by the compressor All rights reserved by www.ijirst.org 357
was calculated for both evaporative cooler and counter flow tube in tube heat exchanger. The results obtained are mentioned below for both the case. A. Results of Evaporative Cooler: The temperature contours are taken in the CFD Post to get the outlet temperature of the evaporative cooler. The mass flow rate of air stream is kept constant and different temperature contours are obtained by varying the inlet velocities of the cooling water in evaporative cooler. The inlet velocities of cooling water for which the temperature contours were obtained are 15m/s, 50m/s, 70m/s, 100m/s, 150m/s, 200m/s and 250m/s Fig. 4: Temperature Contour for Water Velocity 15m/s Fig. 5: Temperature Contour for Water Velocity 50m/S Fig. 6: Temperature Contour for Water Velocity 250m/S All rights reserved by www.ijirst.org 358
B. Tabulated Results of Evaporative Cooler: Analysis of Evaporative Cooler and Tube in Tube Heat Exchanger in Intercooling of Gas Turbine Table-3: Results of evaporative cooler Water velocity m/s (K) (K) Work done (KJ) 0 581 1365.35 1069.61 15 533 1252.55 1004.55 50 514 1207.9 978.79 70 511 1200.85 974.73 100 499 1172.65 958.47 150 491 1153.85 947.62 200 481 1130.35 934.07 250 469 1102.15 917.81 The table 3 where T 1 is constant throughout the process of 300K and T2 is maintained at temperature of 581K,the table comprises of the temperatures at inlet and outlet of low pressure and high pressure compressors with variation of velocity at cooling water inlet. Table also shows work required by the compressor while using the evaporative cooler at different velocities of cooling water. C. Performance Curves for Evaporative Cooler: Fig. 7: Outlet Temperature of Evaporative Cooler Vs Velocity of Water Figure 7 shows the variation of temperature at evaporative cooler outlet with variation in cooling water inlet velocity. The graph shows that the evaporative cooler outlet temperature decreases with increase in cooling water inlet velocity. Fig. 8: Work Done Vs Velocity of Water All rights reserved by www.ijirst.org 359
Figure 8 shows the variation of work done by the compressor with variation in cooling water inlet velocity in case of the evaporative cooler. From the graph it can be inferred that work done by the compressor reduces with increase in cooling water inlet velocity D. Results of Counter Flow Tube in Tube Heat Exchanger: The temperature contours are taken in the CFD Post to get the outlet temperature of the tube in tube counter flow heat exchanger. The mass flow rate of air stream is kept constant and different temperature contours are obtained by varying the inlet velocities of the cooling water in the outer tube. The inlet velocities of cooling water for which the temperature contours were obtained are 0.01m/s, 0.1m/s, 0.5m/s, 1m/s, 10m/s, 15m/s, 20m/s, 25m/s and 150m/s. Fig. 9: Temperature Contour for Water Velocity 0.01m/S Fig. 10: Temperature Contour for Water Velocity 0.1m/S Fig. 11: Temperature Contour for Water Velocity 150m/S All rights reserved by www.ijirst.org 360
E. Tabulated Results of Counter Flow Tube in Tube Heat Exchanger: Table-4: results of counter flow tube in tube heat exchanger Water velocity Work done m/s (K) (K) (KJ) 150 453.8 1066.43 897.20452 25 453.8 1066.43 897.20452 20 452.5 1063.375 895.4425 15 446.1 1048.335 886.76794 10 439 1031.65 877.1446 1 434.5 1021.075 871.0453 0.5 434.2 1020.37 870.63868 0.1 433.7 1019.195 869.96098 0.01 433.7 1019.195 869.96098 The table 4,in which T 1 is same through out of 300K, and T 2 follows a constant temperature of 581K, the gives the temperatures at inlet and outlet of low pressure and high pressure compressors with variation of velocity at cooling water inlet. Table also shows work required by the compressor while using a tube in tube counter flow heat exchanger at different velocities of cooling water. F. Performance Curves for Counter Flow Tube in Tube Heat Exchanger: Fig. 12: Outlet Temperature of Evaporative Cooler Vs Velocity of Water Figure 12 shows the variation of temperature at the outlet of tube in tube counter flow heat exchanger with variation in cooling water inlet velocity. The graph shows that the evaporative cooler outlet temperature decreases with increase in cooling water inlet velocity. Fig. 13: Work Done Vs Velocity of Water All rights reserved by www.ijirst.org 361
Figure 13 shows the variation of work done by the compressor with variation in cooling water inlet velocity in case of the tube in tube counter flow heat exchanger. From the graph it can is clear that work done by the compressor increases with increase in cooling water inlet velocity. VI. CONCLUSION A major portion of the power developed by the gas turbine is utilized by the compressor. It can be reduced by compressing air in two stages with an intercooler between the two compressors. This method has been proved to increase the efficiency the entire gas power cycle. Evaporative cooler and tube in tube heat exchanger is analyzed in the project. Calculations and results show that the use of an intercooler to the power circuit decreases the work required for compression in power generation process. Calculations were made for different flow rates of the cooling water keeping the mass flow rate of air stream constant. It was seen from the graph that the work required for compression decreases with increase in mass flow rate in the evaporative cooler whereas work required for compression decreases with increase in mass flow rate in case of tube in tube heat exchanger. VII. ACKNOWLEDGMENT The authors would like to acknowledge the support of Mechanical Engineering Department of Saintgits College of Engineering for conducting the present investigation. VIII. REFFERENCE [1] Ali Marzouk, Abdalla Hanafi, G. F., and Klein, S. A., Thermo Economic Analysis of Inlet Air Cooling In Gas Turbine Plants,Journal of Power Technologies, Vol. 93(2), pp. 90 99, 2013. [2] R S Johnson, The Theory and Operation of Evaporative Coolers for Industrial Gas Turbine Installations, Journal of Heat Transfer, Vol. 1, pp.1 9, 1988. [3] Gregory F Nellis, John M, Effectiveness NTU Relation for a Counter Flow Heat Exchanger subjected to an external heat transfer, Journal of Heat Transfer, Vol. 127, 2005. All rights reserved by www.ijirst.org 362