THEORETICAL AND EXPERIMENTAL ANALYSIS OF THE EVAPORATIVE TOWERS COOLING SYSTEM OF A COAL-FIRED POWER PLANT

Transcription

1 THEORETICAL AND EXPERIMENTAL ANALYSIS OF THE EVAPORATIVE TOWERS COOLING SYSTEM OF A COAL-FIRED POWER PLANT Francesco Asdrubali*, Giorgio Baldinelli* * Università degli Studi di Perugia - Dipartimento di Ingegneria Industriale, Via G. Duranti, 67 Perugia ABSTRACT The paper presents a theoretical and experimental analysis of the cooling system of a 150 MW coal-fired power plant located in central Italy, where ten evaporative towers cool down the plant using water taken from a river. An updated research on the state-of-the-art evaporative tower cooling system has been carried out to show the theoretical analysis of the tower heat and mass balance, and taking into account the sensible and latent heat exchanged during the processes which occur inside these towers. A long-term statistical data analysis has been carried out concerning the operation of the cooling towers; this information was correlated to the corresponding values of the environmental conditions of the site, and used to evaluate the heat exchanged by the towers at various conditions. The whole analysis permitted to evaluate the optimal conditions as far as the operation of the towers is concerned and to suggest an improvement of the plant. Finally, since plant re-powering has become a quite common issue today, the evaluation of the cooling system operation was conducted under the hypothesis of an increase in the plant nominal power of about 10%. 1. INTRODUCTION The water supply exploitation in the cooling systems of the energy conversion plants represents one of the most remarkable aspects from the point of view of the environmental effect on the area surrounding thermal power stations. Climate changes in the last decades together with an increase of human activities have given as a result a lack in water supply and any effort aimed at reducing its consumption is of great interest. A theoretical and experimental study has been carried out on the cooling systems with evaporative towers in a thermal power station using steam cycles, coal-powered and located in central Italy. In most thermal power stations, the cooling system is a closed circuit one: once the heat necessary to condense the steam has been removed, the condensation water is cooled down by being in touch with the air in appropriate devices, called evaporative towers (fig. 1). Mechanical draft cooling towers are installed in the examined plant: the air enters the tower through a fan or several fans and passes through the packing onto which the water is sprayed (fig. 2). Figure 2: forced draft cooling tower [2]. Figure 1: cooling with a closed circuit condenser [1]. The water flows through the packing, in contact with the flow of air and is collected in a cold water basin at the bottom of the tower. Entrainment of fine water drops occurs and these are collected using the drift eliminator, which is in the form of a wire mesh above the sprays. The warm, moist air leaves from the top of the tower. Towers of this kind give good thermodynamic efficiency and the relatively high air

2 velocity out of the tower gives fewer tendencies to fog formation in the vicinity. These towers are also easy to maintain. Against these advantages must be set the disadvantages of the power requirement for the fan and also the problem of the noise. This choice increases the costs related to the installation and the operation, but in this way the location of the plant does not need the nearby water basins, with huge capacity, to guarantee water supplies to the towers. The examined plant consists of two independent groups. Each group has a power of 75 MW, which covers the basic electrical need and work at their full capacity and continuously during the whole year, except the periods in which maintenance services are necessary. The analysis has been carried out taking into consideration the whole cycle cooling system, from the condenser to the towers, up to the adduction conducts of the reintegration water taken from a small nearby stream [3, 4]. 2. HEAT AND MASS BALANCE Figure 3 shows the ideal transformation of fluids in the cooling circuit: the hot water to be cooled down, formed by the recirculation water outlet from the bottom of the tower, together with the quantity of reintegrated water, is sprayed on the upper part of the tower towards the filling material. q = L C p,w (T 3 T 2 ) + L 1 C p,w (T 2 T 1 ) (3) Combining both equations and adopting the symbology shown in figure 3, the following flow rate of reintegration is obtained: L 1 = ( h2 h1 ) C p,wt1 ( X X ) 2 1 q In the evaporative towers, the flow of cold air is used to cool the hot water coming down the structure. It is possible that, before it is expelled, the water reaches a temperature lower than the temperature of the dry bulb of the air it meets, without it ever being, however, lower than the wet bulb temperature of the inlet air. In the higher sections of the tower, the hot water comes into contact with a flow of air which is preheated, and which is, in any case, colder than the water inlet. It is important to note how, in this section, the partial pressure of the steam outside the liquid is higher than that of the steam in the air flow outlet and, at the same time, the temperature of the water is higher than that of the discharged air. Both can be considered potential factors that tend to reduce the water temperature through the double physical mechanism of evaporation and of the sensible heat transfer; both these potential factors operate simultaneously and adiabatically during air saturation. When the water moves towards the lower levels of the tower it tends to reach a temperature close to, or little below, the dry bulb temperature of the air. In these conditions, the transfer of sensible and latent heat takes place in opposite directions; the air, in fact, will tend to saturation by taking heat from the liquid flow, while the difference in temperature, that guides the transfer of sensible heat, favours the passage from air (hotter) to water (colder). In an adiabatic balance condition between air inlet and water, inside an evaporative tower, the temperature of the atmospheric air humid bulb represents the limit below which the temperature of the discharge water cannot go. The heat flux per unit area exchanged inside a tower can be divided into two portions [6]: heat transferred by evaporation and by convection: (4) q t = q = q d + q c (5) The heat transferred by evaporation can be expressed in terms of the average latent heat of the evaporated water: Figure 3: transformations in a cooling tower with crosscurrent fluids. In thermodynamics, the proportions referred to the transversal section area unit of the tower as well as the quantity of inlet air [5] are considered. If Q is the heat transfer rate that must be released in the condenser, we can define it as: q = Q / A (1) the heat transfer rate per unit area introduced in the tower through the hot water; we can, thus, write the thermal balance of the system on the side of air and on the side of water: q + L 1 C p,w T 1 = G (h 2 h 1 ) (2) q d = L 1 r ave (6) Combining this equation with Eq. (3) the following is obtained: q q LC ( T T ) + L C ( T T ) L r c p,w p,w ave = (7) d L1rave which gives the parameter to evidence the reciprocal weight of the two forms of heat transfer. The part of sensible heat transferred from water to air is: dq c = h (T w T a ) a dv (8) where a represents the area in contact with water per unit volume of the tower (film and little drop surfaces) and dv is the tower differential volume. The value of a cannot be

3 determined directly: the film surface depends on its thickness, while the surface of the drops is related to the water and drop generator adduction mechanism and to the distribution of the drops already formed. By introducing the global mass transfer coefficient K and, designating C p,w,med as the average value of the specific heat, the heat altogether exchanged is obtained [6]: C and A-D represent, respectively, the initial and final driving forces. h dq = KadV ( h h) + C ( T T ) 1 (9) sat p,w,ave w a kc p,w,ave dq can be expressed in terms of the total reduction of the water enthalpic contents (or of air flow total enthalpy increase). The contribution of G air entering the tower can be considered constant while the inlet water flow is not constant due to evaporation losses. In the case of nominal operation, the evaporation losses are of about 2% of the flow of the circulating water, therefore a reasonable estimation of the various parameters can be done also by assuming constant L as a first approximation. With this assumption, the Eq. (9) is reduced to: L C p,w dt = G dh = K (h sat h) a dv (10) This equation (10) represents a starting point for analysing the tower system, both for the performance evaluation and for the development of an initial project point of view. The combination of the equations (2) and (3), with the approximation of the absence of evaporation, can be written in the following way: G (h 2 h 1 ) = L C p,w (T 3 T 2 ) (11) The reference approach is defined as the difference between the temperature of the cold water outlet from the tower and the wet bulb temperature of the atmospheric air entering the tower. Once the wet bulb temperature and the cooling range (the temperature difference between the water at inlet and outlet of the tower) have been established, approach fixes all the operative temperatures of the tower fluids. The transformations that take place in the exchange system of crosscurrent fluids represented in fig. 3 are described in the enthalpic chart in fig. 4. The curve that goes through A-B represents the saturation curve; points A and B fix the temperatures of water outlet (T o ) and inlet (T i ). The segment A-B gives the enthalpy of the saturated air at different temperatures included in the cooling range of the refrigerating fluid. The segment C-D (operative curve) gives the value of the air enthalpy in relation to the water temperature it meets in crosscurrent. The wet bulb temperature of each point on C-D can be calculated from the temperature at the intersection point of the horizontal line going through the point meeting the saturation curve. The segment E-F represents the enthalpy of the air flow in relation to the real temperature of air. If it happens that the line E-F intersects the curve A-B there is the possibility of fog formation. The area between the balance curve (or saturation curve) and the operative curve represents the potential that determines the total thermic exchange. Supposing some of the process conditions should be modified in order to move the line C-D towards the bottom, a larger area between the two curves would be obtained. From the physics point of view, the explanation of this increase is to be found in the lower value of the transfer unit and, therefore, a value lower than the height of the tower. Lines B- Figure 4: table of the working parameters essential for the functioning of a cooling tower with crosscurrent fluids. 3. THE EXAMINED COAL-FIRED POWER PLANT A study has been done regarding the cooling system of a thermal power station working with steam cycle and coal powered (fig. 5). Figure 5: cooling towers of the examined thermal station. The examined plant is made of two independent groups. Each group has a power of 75 MW which covers the basic electrical need and work at their full capacity and continuously during the whole year.

4 The cooling system is made with two condensers, each of which has exchange chambers with a volume of about 3 m 3, where the water of the power cycle circulates. The chambers are crossed by cooling pipes inside of which circulates the water that is afterwards sent to the evaporative towers. In the examined power station, the towers are made of 10 cells placed in 5 series which are structurally the same. The hot water arriving from the condensers is taken to the higher part of the towers, at a height of about 9 m. The contact surface between the two fluids must be maximised in order to allow the thermal exchange. To meet this requirement, a series of numerous pulverisers move from the adduction channels, sending the water towards the filling plastic material, made up of dispersion layers (drop separator). This component maximises the contact surface by fractioning the water flow and by the splashes that are caused by the fall of a layer to the one below. It also assumes the importance of avoiding the water dragging upwards pushed by the air flow rate which is, in turn, boosted by an electroventilator. The final result is to reduce the water flow into a drizzle (fig. 6). to calculate the titer X of the real steam entering the condenser. The thermal balance in the condenser can be therefore explained as: Q cond = h X L (12) lv cond Once the quantity of heat that must be transferred has been calculated, it has been possible to do the direct analysis of the cooling circuit. In the management of the station, and therefore also of the data acquisition, the temperature of the inlet and outlet cooling water of the condenser are monitored. All the rest of the information (in particular the water flow rate sent to the towers) has been worked out by the Authors from the knowledge regarding energy balance. The station has five control units for reading atmospheric conditions placed in the area where the plant is. One of these is placed right inside the perimeter of the station, at a few meters from the towers: the parameters collected from this survey centre (table 1) have been considered valid due to the correlation between the weather climatic data and the functioning of the cooling chambers. All the working parameters of the station given by the manager of the plant have been compared by the Authors with the meteorological data of the days taken into consideration in order to classify the information on how the towers work at different weather conditions and, in particular, to evidence the differences with the information supplied by the constructor. Table 1: meteorological data of an average day. Figure 6: lower part of the tower: collection basin. 4. DATA ANALYSIS The examined station has been working for 40 years. Therefore, it has been possible to do an identification using the historical information supplied by the company, together with the information supplied by the constructor for each component. In order to describe correctly how the plant works, according to the various environmental conditions to which it is subjected, some significant reference days have been chosen and specifically one day in each month of the whole year, during which the station has worked without being neither stopped nor restarted. The first step in the analysis of the station has been the evaluation of the thermal power removed by the condenser. The process and the transformations that take place in the thermal cycle and particularly at the last stage of the turbine and of the condenser have been considered. By knowing the characteristics of the condensate (pressure, temperature and flow rate at outlet saturation conditions), pressure and temperature of the steam entering the turbine and the expansion performance inside the turbine it has been possible Hours [mm] [m/s] [%] [kpa] [ C] [kpa] [gr/kg] [kj/kg] Rain Wind speed R.H. Pressure Temp Satured pressure Specific humidity Associated enthalpy 1 0 0,1 97 0, ,425 2, ,1 95 0, ,425 2, ,2 95 0, ,394 2, ,1 93 0, ,394 2, ,1 93 0, ,394 2, ,1 92 0, ,365 2, ,1 94 0, ,365 2, ,1 96 0, ,365 2, ,1 96 0, ,425 2, ,1 98 0, ,573 3, ,3 95 0, ,713 4, ,2 77 0, ,881 4, ,6 63 0, ,881 3, ,0 62 0, ,012 4, ,7 63 0, ,012 4, ,5 65 0, ,012 4, ,5 72 0, ,822 3, ,1 86 0, ,663 3, ,1 95 0, ,573 3, , , ,533 3, , , ,494 3, ,1 99 0, ,494 3, ,1 97 0, ,494 3, ,1 96 0, ,459 2, MIN 0 0,1 62 0, ,365 2, MAX 0 1, , ,012 4, AVE 0 0,2 88 0, ,590 3, RESULTS The ten cooling towers have been designed to work all together. When the meteorological conditions are favorable, the control system of the plant starts fewer towers while assuring the cooling supply of the condenser. In this case towers work outside the nominal conditions; for this reason a thermofluidodynamic analysis of the towers functioning during the whole year has been carried out and then a comparison with the information supplied by the constructor has been carefully investigated. Firstly the quantity of heat

5 released by each tower per unit time has been calculated assuming equal distribution of the heat quantity. Table 2 reports the trend month after month of the cooling supply for each group of the power generation, together with the number (on the average) of the working towers and the power released by each operating tower. Table 2: power values processed during the whole year. [MW] [MW] numbe [MW] month Q group 1 Q group 2 of towers Q 1 123,0 2,5 50,3 3 38,9 3,2 12, ,8 123,7 7,5 33, ,4 124,6 8,0 30, ,4 121,2 9,0 27, ,3 126,3 10,0 24, ,2 123,8 10,0 24, ,6 120,8 8,7 28, ,1 118,2 8,3 28, ,2 118,5 8,0 29, ,7 122,3 6,5 37,8 Heat released by each tower [kw] Time [months] nominal power calculated power Figure 7: heat released by each tower: nominal and calculated value. Figure 7 reports the trend of the power processed on each tower, during the year, together with the nominal value. The graph shows that the power released month after month is quite always higher than the nominal value, the month of March was excluded, when however the functioning group worked at a reduced capacity. January represents the extreme case, where one group only was in function at nominal capacity and the double of the power indicated by the constructor was produced on the towers; on February the plant was off. There are two possible causes of this phenomenon: on one hand, the towers performance can be noticeably changed with the time if the components considered as obsolete are substituted by more efficient ones (the information supplied by the constructor are referred to the first installation dating back to 1968); on the other hand, the winter time atmospheric conditions, more favorable than those ones listed by the constructor of the tower, allow to release more heat. In summer (especially in July and August) the performance of the cooling system is more similar to the nominal one regarding the number of functioning towers (10) as well as the amount of the released heat. The second parameter to be considered is the rate of water evolving in each tower. Also in this case, the working extreme conditions are recorded in January, when the low temperatures of the inlet air and the service of one group only allow using few cells (2 or 3 cells work the charge of one group). Otherwise, with the exception of March, the evolving charges are noticeably higher than the nominal ones (25% average increases) (fig. 8). Flow rate [m^3/h] Time [months] nominal power calculated power Figure 8: comparison between the processed flow rates (the nominal and the calculated ones). In order to check the towers performance, the most relevant parameters are: the rate of evaporated water (fundamental to calculate the necessary water to keep the water basin level) and the sharing out between the sensible and the latent rate of the exchanged heat (useful to foresee the highest potential performances of the towers) [7]. Figure 9 shows the daily trends of the evaporated water for the most critical month (July). flow rate [m^3/h] Time [hours] Figure 9: daily trend of the rate of the evaporated water in July. Figure 10 shows the flow rate of the evaporated water on one year scale (in percent) vs. the overall flow rate of water flowing through the towers. Evaporated water [%] 0,9% 0,8% 0,7% 0,6% 0,5% 0,4% 0,3% 0,2% 0,1% 0,0% Time [months] Figure 10: trend of the evaporated water on one year scale (%).

6 Figure 11 shows the two ways of exchanging heat in the evaporative towers on one year scale (%). Heat transfer rate [%] 100% 80% 60% 40% 20% 0% Time [months] Sensible Latent Figure 11: heat transfer rate in the evaporative towers on one year scale (%). In summer most of the heat is exchanged by evaporation. Some authors [7, 8] report a distribution of the exchanged heat in the evaporative towers: 98% by evaporation and 2% by sensible heat; these values are referred to July and August. However the conditions of temperature and the pressure of the atmospheric air surrounding the towers are not able to absorb the overall quantity of humidity led by the outlet air [8, 9]. The condensing water quantity forms a plume of vapor, sometimes visible to the naked eye, whose dimensions can be observed even on a psychrometric diagram. The points representing the air status above the saturation curve mean a condition of oversaturation, where the exceeding water contained in the air flow tends to condense immediately after having left the tower, making visible the plume in the atmosphere. If we draw on the diagram a straight line from the point representing the air inlet and the outlet one, the wider is the area limited by this line and the saturation curve (upwards) and the thicker and more visible is the plume (fig. 12). 6. PLANT OPTIMIZATION The cooling liquid flows from the condensers to the towers by only a pipe. If one group is only in function, only a half of the nominal flow rate is worked. Moreover, the flow rates entering each tower change depending on the number of the towers in function. The instability of the flow rates causes a hydraulic unbalance in the circuit, an increase of the charge losses and a bigger absorption of energy in the pump. It has to be remarked that the shifting from the nominal conditions of the water flowing to the nozzles does not assure a sufficient dispersion of the drops in the crosscurrent air flow; as a consequence, an increase of the water flow rates waste occurs because of the dragging and the exchange efficiency decreases due to the reduction of the liquid-gaseous exchange surface. The circuits of the two condensers should be separated by assigning a certain number of towers to each one; so the plant with reduced loads could be managed in a more flexible way, especially if one group only is in operation (for instance depending on some possible stops due to routine and extra-routine maintenance). In the coldest months, when the t for the air is high, the exchange conditions in each tower are so favorable that the work of a limited number of cells is enough (2 or 3 in January). As a consequence, however, each tower works very high quantities of water (very higher than the standard ones), giving rise to the above mentioned disadvantages. The system optimization can also be achieved by introducing varying speed fans into the towers. At present they are regulated by an on/off device, so the inlet air flowing into the tower is practically constant. If the fans are regulated at different speed (for example 2 or 3), it is possible to keep into operation all the towers (maintenance excluded), independently of the external conditions, by reducing the heat released from each tower, at the aim of sending constant quantities of water into the ducts and the nozzles. It should make easier the regulation of the plant, but at the same time it could raise more problems in maintenance, so that an economic analysis becomes an indispensable condition. winter summer Figure 12: conditions of oversaturation in summer and in winter. The figure shows that winter is the best period for this event. Although in summer the content of specific humidity of the outlet air is very high, the phenomenon takes place because the temperature of the external air is not much lower than the outlet air one, having similar capacity of water absorption. In winter the difference between the temperature of the outlet air and the temperature of the exterior air is very high and the environment is not able to locally absorb the quantity of water evaporating in the tower. 7. PLANT RE-POWERING The opportunity of increasing the rate of 10% of the power produced by the plant, focusing on the impact on the cooling system, has been taken into consideration. Assuming that the steam turbine group can make this increase, it is necessary to check the changed conditions of the heat exchange in the condenser in order to reach the evaporative towers and the re-powering system. If the other working parameters of the plant remain unchanged, even the heat to be released in the condenser increases in the rate of 10%. Given that it happens in the steam side of the condenser, it has to be stated the effect on the cooling water side. The equation regulating the exchange in the condenser is: Q = e L C p, w ΔT (13) The cooling fluid is the same, so the thermophysical properties (e, r, C p,w ) are invariable. Even the value of the exchange efficiency of the condenser can be considered as invariable, because its structural features and the fluid exchanging heat are the same of those ones in the present configuration of the plant. The parameters that can change in order to counterbalance the increase of the power to be

7 released are the flow rate (L) and the temperature difference (T). The comparative analysis of the T data recorded in the months when the plant functioned at full power and in the month when the plant functioned partially shows a substantial invariance of this parameter. Consequently it can be assumed that, even in the case of increase of the produced power, the temperature difference in the condenser remain unchanged. On the basis of the equation (13) the only parameter that can increase is the flow rate of the cooling fluid (G); as being a linear term in the equation, it can be assumed that the plant power increase gives rise to an increase of the same percentage of the circulating water charge inside the cooling circuit. In winter, when a limited number of cells is in function, the power increase could be easily released by starting a new tower. The crucial period is represented by those months when all the ten cells are necessary for the plant functioning. In these conditions the quantity of sensible exchanged heat tends to decrease gradually while the external temperatures increase. In presence of particular environmental conditions the air temperatures can reach values that can not assure the presence of a driving force related to the sensible heat and a decrease of the air temperature can take place. On the other hand, the high values of the air temperature assure a higher potential of latent heat, thanks to the non-linear increase of the specific humidity in saturation with temperature (fig. 13). in the day with the worst climate conditions. July hour Tin [ C] R. H. in [%] S. H. in [gr/kg] Tout [ C] R. H. out [%] S. H. out [gr/kg] % 7, % 20, % 7, % 20, % 6, % 19, % 6, % 19, % 5, % 18, % 5, % 18, % 5, % 18, % 5, % 18, % 5, % 18, % 5, % 18,58 August hour Tin [ C] R. H. in [%] S. H. in [gr/kg] Tout [ C] R. H. out [%] S. H. out [gr/kg] % 5, % 18, % 9, % 22, % 5, % 18, % 4, % 17, % 2, % 15, % 4, % 17, % 4, % 17, % 3, % 16, % 2, % 15, % 1, % 14,60 The hottest months represent the most crucial period: the highest percentage of heat exchange by air saturation implies a higher evaporated charge; moreover in summer the level of water in the water basin is at minimum. Figure 14 shows the trend of the total percentages of the evaporated water in the day with the worst climate conditions. S. H. [gr/kg] H = f(t) Temperature [ C] Evaporated water [m^3/h] Time [hours] Figure 13: absolute humidity in function of the temperature in conditions of saturation. Taking into account the meteoclimatic data recorded in the last years, referring to the worst external conditions, the value of the specific humidity of the air coming from the towers was calculated assuming that 100% of the power had to be released by evaporation only and the inlet air temperature had to be equal to the outlet air one (table 3). The highlighted column shows that, even in the worst conditions of the climate, it is possible to obtain acceptable thermodynamic parameters of the outlet air; therefore, on the basis of available data, the present dimensioning of the cooling system appears fit for releasing the supposed power increase. The change of the flow rate evolving in the cooling system implies fluidodynamic conditions different than the nominal ones. It is therefore necessary to evaluate the increase of the water lost by evaporation during the exchange inside the towers and to correlate it with the water stored in the water basin and in the circuit where water is treated and driven up to the plant [10]. Table 3: calculation of the specific humidity of the outlet air 10% increase nominal power Figure 14: trend of the total percentages of evaporates water in the day with the worst climate conditions regarding the nominal power and the 10% increased production. The straight line represents the maximum value obtained with nominal power conditions. In the same environmental conditions, while the produced power increases in the rate of 10% (and the evaporate percentage increases at the same rate), the maximum value of thr evaporated water (435 m 3 /h) is obtained; it is however much less than the capacity assuring the minimum vital outflow of the water stream from where the withdrawal is performed (900 m 3 /h). 8. CONCLUSIONS Water is becoming a more and more precious resource and every effort has to be done to minimize its consumption. To this extent, large-size thermal plants are potentially big consumers of water taken from rivers to cool down the condensing vapour. The paper presents a theoretical and experimental analysis of the cooling system of a 150 MW

8 coal-fed power plant with ten evaporative towers, located in central Italy. First of all, a theoretical analysis of the tower heat and mass balance, taking into account sensible and latent heat exchanged during the processes which occur inside these towers, has been carried out. A long-term statistical data analysis has been carried out concerning the operation of the cooling towers, such as water inlet and outlet temperatures and cooling circuit water flows and considering one day per each month during several years. This information was correlated to the corresponding values of the environmental conditions of the site such as air temperature and relative humidity, and used to evaluate heat exchanged by the towers at various conditions. The variability of these atmospheric parameters, in fact, strongly influence the operation of the towers, specially regarding water losses by dragging and by evaporation. The results showed that nominal water consumption was reached only in the worst conditions (hottest summer days). In the winter, the water losses are highly reduced reaching in the coldest periods even to a half of the nominal values due to the higher quote of sensible heat as well as the lower relative humidity of the surrounding environment. Besides, the study evidences that, in all working conditions, the potential cooling capacity of the towers is far from its complete exploitation. The whole analysis permitted to evaluate the optimal conditions as far as the operation of the towers is concerned and to suggest an improvement of the plant. This improvement consists of the optimisation of the water flow rate in the ducts between the condenser and the towers and of the introduction of a partial load in winter. Finally, since plant re-powering has become quite a common issue today, the evaluation of the cooling system operation results has allowed to take into consideration the hypothesis of an increase in the plant nominal power of about 10%. The study of this case has given an opportunity for an exhaustive treatment of evaporative cooling tower systems, both from a theoretical and from an operational point of view. 9. ACKNOWLEDGEMENT The Authors wish to thank Dr. Manuele Battisti, who helped during the data recovery and analysis. 10. LIST OF SYMBOLS V tower volume [m 3 ] X vapour titer of condensing steam [-] Subscripts 1 reintegration 2 recirculation 3 tower inlet a air ave average c convection cond condenser d diffusion in inlet lv liquid-vapour out outlet p constant pressure sat saturation t tower w water wb wet bulb 11. REFERENCES 1. Sfera - Gruppo Enel, Le torri di raffreddamento, (GC 3 Specialty Chemicals Inc.). 3. ASHRAE, Fundamentals, SI Edition, F. Cotana, Experimental determination of the water vapour mass transfer under various environmental test conditions, International Journal of Heat and Technology, vol. 18, n. 1, pp , ETS, May-June Bird, W.E. Stewart and E.N. Lightfoot, Transport phenomena, J. Wiley & Sons, New York, Kern, Process heat transfer, McGraw-Hill, New York, Book Co., N.P.Cheremisinoff, P.N. Cheremisinoff, Cooling towers selection, design and practice, Ann Arbor Science Publishers, Second Edition, S. Strauss, Guide to evaluate cooling tower performance, Power, J. Lichtestein, Performance and selection of mechanical draft cooling towers, Transactions of the ASME, V. R. Pludek, Design and corrosion control, John Wiley & Sons, a contact area per unit volume [1/m] A condenser area C specific heat [J/(kg K)] e condenser efficiency [-] G air mass flow rate per unit area [kg/(s m 2 )] h specific enthalpy [J/kg] k water thermal conductivity [W/(m 2 K)] K global mass transfer coefficient [kg /(s m 2 )] L water mass flow rate per unit area [kg/(s m 2 )] q heat transfer rate per unit area [W/m 2 ] Q heat transfer rate [W] r evaporation latent heat [J/kg] R.H. relative humidity [-] S.H. specific humidity [gr/kg] T, t temperature [K]