Improving the efficiency of natural draft cooling towers

Energy Conversion and Management 47 (2006) 1086–1100 www.elsevier.com/locate/enconman Improving the efficiency of natural draft cooling towers J. Smrekar, J. Oman *, B. Širok Faculty of Mechanical Engineering, Aškerčeva 6, SI-1000 Ljubljana, Slovenia Received 24 January 2005; accepted 28 July 2005 Available online 12 September 2005 Abstract This study shows how the efficiency of a natural draft cooling tower can be improved by optimising the heat transfer along the cooling tower (CT) packing using a suitable water distribution across the plane area of the cooling tower. On the basis of cooling air measurements, it is possible to distribute the water in such a way that it approaches the optimal local water/air mass flow ratio and ensures the homogeneity of the heat transfer and a reduction of entropy generation, thus minimising the amount of exergy lost. The velocity and temperature fields of the air flow were measured with the aid of a remote control mobile robot unit that was developed to enable measurements at an arbitrary point above the spray zone over the entire plane area of the cooling tower. The topological structures of the moist air velocity profiles and the temperature profiles above the spray zone were used as input data for calculation of the local entropy generation in the tower. On the basis of the measured boundary conditions, a numerical analysis of the influence of the water distribution across the cooling towerÕs plane area on entropy generation and exergy destruction in the cooling tower was conducted. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Cooling tower; Entropy generation; Exergy destruction; Cooling tower efficiency; Heat transfer * Corresponding author. Tel.: +386 1 4771 303; fax: +386 1 2518 567. E-mail address: janez.oman@fs.uni-lj.si (J. Oman). 0196-8904/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2005.07.012 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 Nomenclature A CT h L m_ S_ E_ Q_ s t, T P r w v W x Greek e g q / condition of air cooling tower specific enthalpy, kJ/kg height of CT packing, m mass flow rate, kg/s entropy flow rate, kW/K exergy flow rate, kW heat flux, kW specific entropy, kJ/kg K temperature, °C, K power on generator, MW water/air mass flow ratio velocity of moist air in cooling tower, m/s velocity of ambient air in vicinity of cooling tower, m/s condition of water humidity ratio of moist air letters effectiveness efficiency of cooling tower density of moist air, kg/m3 relative humidity Subscripts 0 condition of ambient 1 inlet 2 outlet A, B, C, D measurement points of air condition in vicinity of cooling tower a dry air gen generation i ith segment in cooling tower L local m wet bulb temperature o outlet from cooling tower R heat rejection opt optimal w water z condition of air in vicinity of cooling tower 1087 1088 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1. Introduction The cooling system is one of the most important parts of a power plant. Its task is to extract as little heat as possible from the thermodynamic cycle to the environment, thereby ensuring improved efficiency of the power plant. Estimating the operating efficiency of a cooling system usually involves the coefficient Q_ R =P [1]. A reduction of this coefficient means that more energy from the fuel was successfully transformed into useful work, which means that less heat was lost to the environment. Being able to reduce the coefficient Q_ R =P depends on the integral and differential characteristics of the circulating water system in the plant. It is important to remember that because we are dealing with large energetic flows in power plants, small improvements to the cooling system can mean large fuel savings and a reduction in the amount of pollution produced by the exhaust gases. In this study, the main element of the cooling system is a natural draft cooling tower, which, in physical terms, is a relatively simple device. Although the basic parts of the tower can easily be described [6] and understood, the processes of heat and mass transfer are very complex. In natural draft cooling towers, a process of counter flow heat transfer, which the water is cooled by air, takes place. Between the water and the air, a boundary layer is established, which is considered to be saturated air at the same temperature as the water. In the heat transfer process more than two-thirds of the heat is transferred by evaporation, with the rest being transferred by convection. A mathematical description of the variables that are changing along the cooling tower (CT) packing and in the spray zone is very complicated. It could be described by a system of ordinary differential equations [10,14]. However, an analysis of the heat and mass transfer in cooling towers can also be described empirically with the NTU method, providing the construction characteristics of the tower are known [11–13,16,17]. Good operating conditions of the cooling tower mean that there is a homogeneous heat transfer across the entire plane area of the tower. Homogeneity is shown by a uniform temperature field of the air above the spray zone across the plane area. Possible anomalies can be due to the construction characteristics of the water distribution system, the impassability of the CT packing and nozzles, or the influence of the surrounding air on the inflowing air velocity to the cooling tower. Usually, these anomalies are not known a priori, but they can normally be determined with a suitable measurement system. In order to do this, a variety of diagnostic methods to determine the effectiveness of the heat transfer [4,5] that take into account the cooling towerÕs operational irregularities were developed. The temperature and the velocity field of the air are closely connected as was explained in Ref. [2] with an analogy to the convective transfer of mass, heat and momentum. An improved cooling tower performance is the result of an optimum mass flow rate of cooling water with respect to the power plantÕs operating conditions [9]. For this kind of operation, pumps with a variable speed, which is unusual for todayÕs cooling systems with large water mass flow rates, are required. Besides eliminating local anomalies in the temperature and velocity fields, it is also possible to improve conditions with a proper distribution of water across the cooling towerÕs plane area. It is this distribution of water that is analysed here. The result is a reduction of entropy generation and an improvement in the cooling towerÕs efficiency, which also results in an increased efficiency of the power plant. The analysis considers a constant total mass flow rate of cooling water, which J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1089 is optimally distributed according to the air flow conditions over which we have no influence with natural draft cooling towers. 2. Experimental section 2.1. Description of the measurements and the measuring equipment used in the cooling tower The basis of the presented measuring system is a mobile robot unit that moves gradually over the entire measuring plane. The sensors are mounted on the robot in such a way that the system fulfils all of todayÕs standards. A vane anemometer, designed for operation in 100% humidity, and a Pt-100 temperature sensor are used. The details of the equipment and its calibration are described elsewhere [7]. The duration of a measurement at a single point, depending on the chosen magnitude of the observation scale (quasi-steady state condition), is 15–30 s. After the measurement is performed, the servo positioning system moves to a new pre-chosen measuring point. The following integral parameters are measured simultaneously according to the DIN 1947 standard [3]: the inlet and outlet temperatures of the cooling water, the cooling water flow rate, which is measured with an ultrasonic flow meter, and the output power of the thermo-energetic system. The air velocity and the temperature in the cooling towerÕs measuring plane are measured continuously with stationary sensors. As the velocity and temperature measurements are not performed simultaneously, the stationary velocity meter, together with the integral parameters, serve as correction elements for these measurements. Other parameters that cannot be measured by the mobile unit (the parameters in and below the CT packing) were also acquired with a stationary reference point, which is represented as a vertical segment in Fig. 2. Besides the mobile unit and the vertical segment, the measuring system also included devices for data acquisition and analysis. The position of the mobile unit was determined by measuring the radial distance and the angle from the reference point in the measurement plane. The radial distance and angle were then converted to x, y coordinates. The distance was measured using a linear cable extension transducer with a 50 m range and a typical uncertainty of less than 0.1 m. The angle was measured by a device tracking the position of the cable and rotating the cable extension transducer. An angular resolution of about 0.15° was achieved. The results of the measurements and their analysis are presented in a report [15]. 2.2. Measurements of the ambient air parameters and the exit air mapping results from the cooling tower Using the results of the velocity and temperature measurements from the measuring plane of the cooling tower, three-dimensional topological structure diagrams of the velocity and temperature data were obtained. These results are shown in Fig. 1 and provide a direct means of evaluating the extent to which the falling hot water droplets and the cooling air mix in the cooling tower. Because the air mappings completely characterize the water/air interface across the plane area of the cooling tower, problems with the water distribution and the fill system can be identified in the mappings and successfully solved. 1090 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 Fig. 1. Topological structure of the velocity and temperature exit air profile. All measurements were performed at a single operating regime, with the power stationÕs block output power of 350 MW and a cooling water volume flow rate of 180,000 m3/h. The following ambient air parameters were measured simultaneously: the ambient air velocity at four locations (vA, vB, vC, vD), the air temperature in the vicinity of the cooling tower (tz) and the air density in the vicinity of the cooling tower (q). In Table 1, the average values of the various parameters for the duration of the analysis are presented. It is clear that the ambient parameters did not change significantly and, therefore, did not influence the measurement results inside the cooling tower. Fig. 1 shows the velocity and temperature structures of the exit air acquired with the robot mobile unit. From the left diagram of Fig. 1, one can recognize the local non-homogeneities in the velocity field, which can be caused by different factors, such as constructional properties (obstacles) and the local fill system impassability. The non-homogeneous temperature field presented in the right diagram of Fig. 1, is closely connected with the non-homogeneous velocity field [8]. Good operation of a cooling tower is defined by homogeneous conditions for heat and mass transfer across the plane area of the tower. The result is a uniform temperature field of the exit air above the spray zone across the plane area. In general, a non-optimal operating condition is reflected in a deviation of the actual operating point from the expected nominal value. Table 1 Ambient air parameters Quadrant 1 2 3 4 vA (m/s) vB (m/s) vC (m/s) vD (m/s) tz (°C) q (kg/m3) 1.8 2.2 1.6 2.2 9.8 1.23 2.2 1.9 2.2 2 10.2 1.22 2.1 2.7 1.8 2.1 9.5 1.23 2.3 2.4 1.9 1.8 10 1.23 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1091 2.3. Measurements of the temperatures and mass flow rates on a vertical segment of the cooling tower To determine the basic heat and mass transport properties of the cooling tower, measurements were made of the aerodynamic and thermodynamic characteristics of the air and water flows in a selected reference vertical segment of the cooling tower, shown in Fig. 2. Measurements on the vertical segment served as a measurement correction element for the non-simultaneous measurements made with the robot mobile unit. The vertical segment was also used to determine the characteristic function of the analysed cooling tower [4,5] and for a study of the variation of the mass flow rate of water on the effectiveness of the heat transfer. From this, it is possible to determine the local effectiveness of the cooling tower simply from the local air mapping measurements. A reference vertical segment of the cooling tower was selected in a region of the cooling tower where the geometrical properties are homogeneous. The vertical segment in Fig. 2 consists of the CT packing located at the bottom, a spray element in the central part and a drift eliminator in the upper part of the segment. The following parameters were measured for the experimental system: the inlet moist air temperature ta1, the outlet temperature of the saturated air ta2, the inlet water temperature tw1, the outlet water temperature tw2, the mass flow rate of the water m_ w and the velocity of the moist air wa. The uncertainty of the measurements made with the Pt-100 thermometers was estimated to be less then 0.25 °C. The air velocity measurements were performed using a pre-calibrated vane anemometer. The same type of vane anemometer was used for the measurements made with the mobile robot unit. The sampling frequency was 1 Hz, and the total acquisition time was 700 s. The inlet air humidity was determined on the basis of dry and wet bulb thermometers. Fig. 2. The selected vertical segment of the cooling tower. 1092 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 All the measurements in the cooling tower were performed according to the DIN 1947 standard [3]. 3. The influence of the water/air mass flow ratio on the effectiveness of the heat transfer The improvement in the heat transfer can be explained by Fig. 3, which shows the situation in the CT packing, where the counter flow heat exchange from the water to the air is taking place. More than two-thirds [6] of the transferred heat is rejected from the water by evaporation with the remainder being transferred by convection. Fig. 3 shows the increase in the airÕs relative humidity along the CT packing as a result of the evaporative cooling of the water. Air enters at the bottom side of the CT packing and becomes saturated at some height; in the diagram, this happens at section P–P. Above this section, the air would be saturated if the temperature were constant. However, above this section, the process of evaporation is actually still occurring because of the increasing air temperature and, thus, the decreasing relative humidity of the air. How big a portion is transferred by evaporation depends on the position of section P–P, which means the water/air mass flow ratio. The physical phenomena in the CT packing can be explained using an example of the measured values in the vertical segment, which are presented in Fig. 4. In the vertical segment, air enters in condition A1 with a relative humidity of 0.82 and a temperature of 10 °C, and saturated air leaves in condition A2 at 31 °C. The mass flow rate of the water is 4.7 kg/s with an inlet temperature of 33.9 °C and an outlet temperature of 24.2 °C, as shown in the right diagram in Fig. 3 with points W1 and W2, respectively. The mass flow rate of the water was then reduced to 4.2 kg/s. The inlet temperature of the water and the air remained unchanged, while the outlet water temperature Fig. 3. Condition of the air through the CT packing and the Mollier h–x diagram, which shows the condition of the air through a vertical segment for two different mass flow rates of water. J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1093 Fig. 4. Measured temperatures and velocities of the air and water in a vertical segment for two different mass flow rates of water. decreased to 21.9 °C. At the same time, the outlet temperature of the saturated air did not change significantly. This can be easily explained with the equations for energy and mass balance: m_ a dh ¼ m_ w dhw þ m_ a dxhw . ð1Þ While the mass flow rate of the water was reduced, the mass flow rate of the air remained approximately the same, which means more cooling air per unit of water mass and, consequently, a lower outlet water temperature. The cause is the higher proportion of air and, thus, a greater difference in the partial pressures of the water vapour between the boundary layer and the air flow along the CT packing. Under normal operating conditions, the section P–P is at the middle height of the CT packing. This situation suits a process in the CT packing where the water flow is relatively high compared to the air flow, leading to a high water/air mass flow ratio. In this case, the entering air quickly approaches saturation, and the rest of the CT packing does not cause such intensive evaporation, which increases the proportion of the convective heat transfer. This means that it has not reached the optimal moistening of the CT packing. The analysed example is presented by the full curve between points A1 and A2 in the Mollier h–x diagram in Fig. 3. We can conclude that this is a less desirable process of heat transfer in cooling towers. In the second case, when the mass flow rate of water was decreased, and the air flow rate remained approximately the same, we obtained a better water/air mass flow ratio, the consequence 1094 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 of which was a drop in the temperature of the outlet water. Thus, section P–P in Fig. 3 moved up the CT packing. This situation suits a process where the water flow is relatively small in comparison to the air flow, which means a lower water/air mass flow ratio. Because of this smaller water/ air mass flow ratio, the air is in a nearly saturated state in the upper part of the CT packing, which means intensive water cooling by evaporation, although convective heat transfer is also present. In this way, the height of the packing is better used. In the Mollier h–x diagram in Fig. 3, this example suits the broken line between A1 and A2, which intersects the saturation line later (at a higher section of the CT packing height) than the full line. In this case, the water is cooled to 21.9 °C (state W 02 ), which is 2.3 °C less than in the normal operating regime, where the air is near saturation at a lower height in the CT packing. This water/air mass flow ratio is more suitable from the heat transfer point of view. The positions of the lines in the Mollier h–x diagram in Fig. 3 were calculated from Refs. [11,17]. Because the task of the cooling tower is to cool the water as much as possible, the consequence of an improved heat transfer effectiveness is a bigger drop in the temperature of the output water, which is shown in Figs. 4 and 5. The effectiveness of the heat transfer on the water side is calculated using the following equation: e¼ hw1
hw2 ; hw1
hwm ð2Þ where hwm is the specific enthalpy of the water evaluated from the wet bulb temperature of the entering air, which is the minimum temperature to which the water can be cooled. The effectiveness of the heat transfer is improved by about 8% with a 0.51 kg/s lower mass flow rate of water, which is equivalent to 10.8%. Fig. 5. Heat transfer effectiveness of vertical segment by decreasing the water mass flow rate. J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1095 3.1. Improved heat transfer by proper distribution of water across the plane area of the cooling tower The analysed cooling tower had the same type of nozzles across the plane area of the cooling tower. The water distribution system with flow channels is exposed to atmospheric pressure, which means that with the correct operation of the nozzles and channels, the water distribution across the plane area would be uniform. On the basis of the air–velocity and air–temperature diagrams in Fig. 1, we can determine the position and the level of entropy generation. At the edge of the analysed plane area, there is a relatively extensive region of high air velocities and low air temperatures. These represent the areas with a high entropy generation. The rounded areas in the velocity and temperature fields, shown in Fig. 1, that deviate from the expected values are called areas of anomalies and should, in any case, be additionally investigated [4]. If the ambient differentiations and operational irregularities of the water distribution system, the CT packing and the nozzles are not taken into account, we can focus on improving the operation of the cooling tower. This kind of improvement involves determining the optimal water/air mass flow ratio on a local basis across the plane area of the cooling tower. From the left diagram of Fig. 1, it is clear that there are three relatively distinctive areas with different air velocities. It is also clear that because of the specific hyperbolic shape of the cooling tower, the air velocity decreases from the edge of the tower to the interior. Because of the uniform inlet water temperatures and uniform mass flow rates of water across the plane area and because of the different air mass flow rates, the outlet water temperatures are different, which results in increased entropy generation in the cooling tower. With a constant water/air mass flow ratio, the same amount of air per unit of water is ensured, which results in the same outlet water temperatures across the plane area of the cooling tower and smaller entropy generation. To achieve a constant water/air mass flow ratio there has to be a variable mass flow rate of water that is adapted to the mass flow rate of the air across the plane area. Because of a lower total entropy generation, a lower outlet water temperature is achieved and, thus, a better overall cooling tower efficiency is ensured. A suitable distribution of water relative to the air flow can be achieved by regulation of the water distribution system or by different sizes of nozzles across the plane area of the cooling tower. 3.2. Determination of the optimal water/air mass flow ratio on a local basis for a cooling tower For a cooling tower, it is possible to determine the optimal water/air mass flow ratio on a local basis, which, for fixed operational and ambient conditions, achieves the best heat transfer effectiveness. The optimal ratio can be calculated from measurements of the local air velocities, to which the total amount of water has to be adapted so that the water/air mass flow ratio will be minimised. At the same time, a more constant water/air ratio across the cooling towerÕs plane area is achieved by a suitable water distribution, which ensures the homogeneity of the heat transfer and gives the best operating conditions. The sum of the local measured mass flow rates of air is equivalent to the total mass flow rate of air through the cooling tower: X m_ a;i ¼ m_ a . ð3Þ i 1096 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 The portion of the mass flow rate of the air on a local basis is m_ a;i ki ¼ . ð4Þ m_ a The total amount of cooling water has to be distributed across the plane area of the cooling tower relative to the mass flow rate of the air. Thus, the local mass flow rate of water has to be proportional to the local mass flow rate of air: m_ w;i ¼ k i  m_ w ; ð5Þ where m_ w is the known total mass flow rate of water. From the local air flow and a suitable water flow, calculated by Eq. (5), the optimal water/air mass flow ratio is obtained, which is constant along the entire plane area of the cooling tower: ropt;i ¼ m_ w;i ¼ const. m_ a;i ð6Þ Because of the changing ambient conditions, the power plant load and the resulting conditions in the cooling tower, it is reasonable to calculate the average value of the optimal water/air mass flow ratio for a given cooling tower. For the analysed example, this ratio is equal to 1.54 for a 180,000 m3/h flow rate of cooling water. 4. Entropy generation and exergy destruction with the optimal water/air mass flow ratio Fig. 6 shows two diagrams, the first of which represents the water/air mass flow ratio acquired from measured values, while the second diagram represents the entropy generation across the plane area of the cooling tower, calculated with the equation: X X S_ gen;i ¼ m_  s. ð7Þ m_  s
out in In Fig. 6, a significant non-uniform distribution of the water/air mass flow ratio can be seen, which has an unfortunate influence on the homogeneity of the heat transfer and the overall efficiency of the cooling tower. The scheme in Fig. 3 can be used to explain the reduction in entropy generation shown in Fig. 7, where we distributed the same amount of water with respect to the mass flow rate of air. Fig. 6 shows that the water/air mass flow ratio is very low on the edge of the cooling tower, which means that we have large amounts of air compared to the amounts of water. It could happen that in some regions, the air does not become saturated along the fill system, which represents unused potential for evaporation and, thus, less transferred heat. In the interior of the cooling tower, we can see the opposite state, shown in Fig. 6, where the water/air mass flow ratio is too high. In this case, the condition of the air approaches saturation in the lower part of the CT packing, while the upper part of the CT packing is not so effectively used for the process of evaporation. Both examples show non-optimal usage, depending on the air flow distribution across the plane area. Consequently, there are large non-homogeneities, which contribute to the generation of entropy. The analysis showed that we should distribute water from the cooling towerÕs interior, where we have more than enough water relative to the air flow, to the cooling towerÕs exterior. We have J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1097 Fig. 6. Measured relations between water/air mass flow ratio and entropy generation across the plane area of the cooling tower. Fig. 7. Entropy generation from an optimal water/air mass flow ratio across the plane area of the cooling tower. obtained the best possible results for the given operating conditions when we have the optimal distribution of water across the plane area of the cooling tower. Consequently, we had less entropy generation on a local basis, shown in Fig. 7. Table 2 shows a comparison of the results for a numerical calculation of the optimal water distribution and the uniform water distribution. 1098 J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 Table 2 Comparison of the integral parameters from a uniform water distribution across the plane area of the cooling tower with the optimal water distribution S_ gen (kW/K) E_ lost (kW) tw,o (°C) g (%) Uniform water distribution Optimal water distribution 101.4 28,696 24 38.9 60.6 17,150 22.6 44.4 The calculations showed that the results of the water distribution according to air flow are significantly better. The sum of the local entropy generations gives the total entropy generation of the cooling tower, which is 101.4 kW/K. In this case, the measured outlet water temperature from the tower is 24.0 °C, and the cooling towerÕs efficiency, from Eq. (2), is 38.9%, where the values are related to the inlet and outlet water temperatures of the cooling tower, respectively. From the total entropy generation of the cooling tower, the lost exergy can be calculated using the Gouy– Stodola theorem: E_ lost ¼ T 0  S_ gen . ð8Þ In the case of the measured values, the exergy lost is 28,696 kW. Fig. 7 shows numerical results when the water/air mass flow ratio is constant across the entire plane area of the cooling tower, and for the given boundary conditions, it is 1.54. In a numerical calculation of the water and air outlet parameters, as a result of the water distribution, the same boundary conditions as with the measured values were used, i.e., the same inlet water temperature, the same inlet air conditions and the same air velocity profile across the cooling tower plane area. With the last assumption, we included the anomalies in the air flow that are a consequence of the constructional irregularities. This assumption makes it possible to see the difference in the quality of the operation of the cooling tower before and after the water distribution with the same constructional conditions. The numerical analyses for the cooling tower were based on algorithms from Ref. [13]. The thermodynamic properties of the water and the moist air were calculated from Refs. [18,19]. Fig. 7 shows better results in comparison with Fig. 6. A suitable water distribution in terms of the air flow conditions results in a homogeneous heat transfer across the plane area of the cooling tower, which gives a lower and more uniform entropy generation. In spite of this, a two times larger entropy generation can be seen on the edge of the cooling tower, which is a consequence of the larger mass flow rates of water and air. The entropy generation and lost exergy in this case are 60.6 kW/K and 17,150 kW, respectively, which is 40.2% less than in Fig. 6, where there is a uniform water distribution. As a result of the lower entropy generation, we obtained a lower output water temperature from the cooling tower, which, in the case of the numerical analysis, is 22.6 °C, i.e., 1.4 °C less than with a uniform water distribution. The consequence of the water temperature drop is a 5.5% higher cooling tower efficiency of 44.4%. Because of the unchanged total amount of water, this is the result of an improved heat transfer, a lower outlet water temperature from the cooling tower and, thus, also the outlet water temperature from the condenser, which dictates the pressure in the condenser. J. Smrekar et al. / Energy Conversion and Management 47 (2006) 1086–1100 1099 5. Conclusion With the aid of a robotized method, measurements of the temperature and velocity fields in a cooling tower were performed for the given power plant parameters, cooling tower constructional characteristics and ambient air velocity conditions in the vicinity of the cooling tower. The last two parameters influence the homogeneity of the heat transfer, from which we can see the anomalies in the cooling towerÕs operation. Homogeneity in the heat transfer could not only be achieved with fault free construction characteristics but also with a proper distribution of water across the plane area of the cooling tower. In this study, we have analysed the water distribution across the plane area of the cooling tower. We have adjusted the amount of water to suit the air flow conditions, which cannot be influenced with natural draft cooling towers. We have found that it is possible to determine the optimal water/air mass flow ratio for a cooling tower, which has to be as small and as constant as possible across the entire plane area of the cooling tower. In this way, the optimal moistening of the CT packing is ensured, which results in a more effective heat transfer. With a constant water/ air mass flow ratio, a constant local water outlet temperature is obtained, which decreases the entropy generation and the exergy lost from the cooling tower. The result is a lower outlet water temperature from the cooling tower and, thus, from the condenser, which results in greater efficiency of the power plant. References [1] El-Wakil MM. Power plant technology. 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