- Email: [email protected]
Experimentation and modelling of an innovative geothermal desalination unit
DESALINATION Desalination 125 (1999) 147-153
ELSEVIER
vww.elsevier.com/locate/desal
Experimentation and modelling of an innovative geothermal desalination unit Karim Bourouni a*, Jean Claude Deronzier b, Lounes Tadrist a aLaboratoire de l'Institut Universitaire des Syst~mes Thermiques lndustriels (IUSTI), U. M. R. 65.95 Universit~ de Provence- Technopole de Chdteau Gombert, 5, Rue Enrico Fermi 13453 Marseille cedex 13, France Tel. +33 (4) 91 10 68 63, Fax +33 (4) 91 10 69 69, E-mail: [email protected] bLaboratoires du G R E T H - CEA Centre de Grenoble, 17, Rue des Martyrs - 38054 Grenoble cedex, France
Abstract
In this study, heat transfer of air-water-vapour mixtures in a desalination plant, using the aero-evapocondensation process is studied experimentally. The present process consists in a falling film evaporator and a condenser. The prototype is designed to work at low temperatures (60°C-90°C) using a geothermal energy. Two experimental pilots were developed. These latter, installed respectively in France and the south of Tunisia were supplied by fuel and geothermal energy. The experimental results highlighted a critical film flow rate, characterising the phenomenon of film breakdown. At this value, a maximum amount of evaporated water was obtained. Experimental results were compared with those derived from the model developed by Bourouni et al. [1 ] in previous investigation. From this comparison it can be learnt that the model is well able to predict the heat and mass transfer in the evaporator. This will help to optimise this component of the aero-evapo-condensation units. Keywords: Geothermal energy; Desalination; Evaporator; Horizontal tubes; Falling film; Evaporation; Heat transfer
*Corresponding author Presented at the Conference on Desalination and the Environment, Las Palmas, November 9-12, 1999. European Desalination Society and the International Water Services Association. 0011-9164199/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S0011 - 9 1 6 4 ( 9 9 ) 0 0 1 3 3 - 2
148
K. Bourouni et al. / Desalination 125 (1999) 147-153
1. Introduction The common methods of abstracting fresh water from salt water such as abstracting by distillation, reverse osmosis and electrolysis are intensive energy techniques. For this reason these desalination techniques are competitive only for large-scale production (thousands of cubic meter per day), Speigler [2]. However, in some circumstances, the desalination needs do not exceed a few cubic meters per day. It is the case of some arid and semiarid regions of south Mediterranean countries that have important resources of brackish water, which is sometimes deep, at a high temperature and unfortunately salt. In arid areas, generally in very hard conditions, it is necessary, to uphold the lives of human groups whose presence is linked to a specific economic activity (extraction of petrol for example). In semi-arid areas a minimum of economic activities, such as agriculture, must be maintained. For these reasons, there is constant research in this field. In fact, many new desalination methods are investigated, especially for small units, with a production lower than 10 m3/d. A desalination prototype, with a capacity of a few cubic meters per day, allowing the use of geothermal energy and reducing the capital cost, was patented by the firm Caldor-Marseilles in 1994. The present prototype includes two crossflow heat exchangers, a horizontal falling-film evaporator and a horizontal falling-film condenser. The two exchangers are made of polypropylene and operate by the humidificationdehumidification of air. The prototype was designed to use low temperature energy and in particular geothermal energy. The brackish water intended for desalination is at a temperature between 75°C and 90°C, which is less than the maximum operational temperature of the polypropylene, limited to 100°C. Bourouni et al. [3] presented an experimental investigation of the desalination plant func-
tioning by aero-evapo-condensation. The data exhibited show an influence of the air and hot water temperatures on the unit performances. These investigations show a linear increase in the amount of evaporated water with the air and hot water inlet temperatures. A critical Reynolds number corresponding to a maximum quantity of evaporated water was determined. Some flaws, especially in the liquid distribution system, were highlighted. In order to optimise the plant's performances, some modifications were proposed (system of water distribution, airflow, etc.). From these proposals, a second optimised pilot was achieved. The experiments of Bourouni et al. [3] are carried out in a laboratory in Marseilles. These conditions are different from the environment in which the plant is destined to operate. This investigation does not allow the possible interactions between the unit and its real environment to be determined (problems of scaling, sandstorms, temperature, etc.). For this reason we decided to carry out an experimental investigation of the optimised pilot in the south of Tunisia. For this purpose a geothermal spring with a water temperature of about 70°C is used. The present study analyses the performance of the desalination unit installed in the south of Tunisia. The process used in the unit is based on aero-evapo-condensation. The influence of different thermal and hydrodynamic parameters on water evaporation is investigated. The experimental results are compared to the data of Bourouni et al. [3] in order to confirm the improvement of the process and to the numerical predictions of Bourouni et al [1] to validate their model. 2. Process equipment 2.1. Description and operation
The prototype is presented in Fig. 1. It includes an evaporator (l) and a condenser (2). Each heat exchanger consists of circular plastic tubes and an insulated envelope. Heat recovery
K. Bourouni et al. /Desalination 125 (1999) 147-153
in a low-temperature process requires a large exchange surface. The two exchangers are 2 m long with a rectangular section (1.2 m in length and 0.8 m in width), and they include 865 m of tubes. Two tanks (3), under the evaporator and (3') under the condenser, contain salt and distilled water. The two exchangers are linked by a pipe (4) of a diameter of 0.2 m, allowing the circulation of humid air from the evaporator to the condenser. The airflow is maintained by a blower (5). Two pumps (6) and (6') permit the circulation of salt and distilled water. in order to avoid an excess of salt water concentration, the tank (3) level is held constant
149
by a continuous supply of salt water. The salt concentration is controlled by a purge sluice. Furthermore, the level in tank (3') is held constant by a distilled water levy. The cooled hot brackish water moves down the tubes. Its temperature at the entrance is about 70°C. The cooling air moves up in the space between the tubes. The cold salt water in the tank (3), at ambient temperature, is sucked up by the pump (6) to the condenser (2). In this exchanger the water moves up inside the tubes. At the condenser outlet, salt water is preheated to 50°C. The liquid is introduced through the water distribution system (7) into the top of the
Humid
Air
water
t____ Preheated salt water
(1) Evaporator
(2) Condenser
Cell i (5)
Ambiant
Cold water
water
distilled wate_.__..Lr
-..------
(3) ~Thermocouple
~
(3') Rotameter
(~) Hygrometer
0
Gas velocity rotameter
Fig. 1. Functioning principles and instrumentationof the desalination unit: (1) evaporator, (2) condenser, (3) and (3') tanks, (4) pipe, (5) blower, (6) and (6') pumps, (7) distributor. dint=20 ram; dext=25.6mm; s=l.3 de,a;B=1.2 dext
150
K. Bourouni et al. / Desalination 125 (1999) 147-153
evaporator and falls from tube to tube. The feed water distribution system is a slot placed at the top of a 25 mm diameter tube. The liquid film flows and evaporates on the outside surfaces of the tubes. The vapour is carried by the airflow to the condenser. At the top of the condenser, the air is hot and humid. In the condenser (2), the humid air moves down through the space between the tubes. On contact with the cold tube walls, there is film condensation coupled with latent heat restitution to the salt water circulating inside the tubes. Finally, the distilled water is recovered in the tank (3'). The characteristics of the film flowing around the horizontal tubes are visualised during the tests through two Plexiglas windows, placed on each exchanger. The humid air velocity is adjusted using a frequency controller linked to the ventilator. Two flow adjusting valves, placed upstream from the evaporator and the distributor, allow the control of the hot water and liquid film flow rates. 1.2. Instrumentation
Eight thermocouples are placed at different locations to measure the inlet and outlet temperatures of the hot water, humid air and stream water (Fig. 1). A bowl with an adjustable flow rate is placed at the bottom of the evaporator. The stream water temperature at the exchanger outlet is sensed with a thermocouple placed on the bowl. The irrigation flow rate is measured by a rotameter located upstream from the distributor. A second rotameter is used to measure the hot water flow rate. The air velocity is measured by a gas velocity rotameter inserted between the evaporator and the condenser. A hygrometer placed on the pipe joining the two exchangers allows the air humidity to be measured at the entrance of the evaporator. These probes are connected to a data acquisition system.
The test procedure was defined from previous experiments, (Bourouni et al. [3]). This procedure was repeated for all the experiments. It consists in running irrigation for several hours to ensure perfect moistening of the tubes. Next, the air is pumped through the exchangers. The hot brackish water is injected when different fluid temperatures present slightly stable values. The steady state is reached after several operating hours (2-4 h). This state is mainmined for two hours. The temperatures and fluid flow rates are recorded every ten seconds. The time between two humidity measurements is about two minutes. The height of the water in the tank (3') is measured every ten minutes.
3. Experimental results and comparison with numerical predictions The instrumentation developed to analyse heat and mass in the plant allows only the investigation of global parameters (inlet and outlet temperatures, pressures, humidity and total amount of evaporated water). 16 (10"3 Kg/s)
14
~ : Unit I; ~ : Unit 2; ~
: Model
_
[
1000
1200
12
rhe
0 8 6 4
200
400
600
Liquid film Reynolds number
800
4F Re r /.t
Fig. 2. Influence of air inlet velocity on the total amount of evaporated water. Ti. hl=62°C; Ti.f=34°C; Tin g=27°C; Rein hj=14000; Remg =18000; Xins=17g/Kg
K. Bourouni et al. / Desalination 125 (1999) 147-153
In the following, typical experimental results and those obtained from simulations (Bourouni et al. [1]) are presented. The basic parameter in this problem is the prediction of the amount of evaporated water they. For a given operational condition, the variation in this parameter versus liquid film flow rate is presented. The influence of the liquid film flow rate on evaporative exchanger performance is plotted in Fig. 2. In the same figure, experimental results corresponding to both desalination units and numerical predictions are reported. 3.1. Unit 1 experimented in Tunisia
The experimental results obtained show a continuous increase in the amount of evaporated water when the liquid film flow rate increases. This increase is more pronounced for a low liquid film Reynolds number. For a high liquid film Reynolds number rhev increases slightly, when Re/increases. A good agreement between numerical predictions and experimental data is highlighted; however a significant gap between the results is observed for low liquid film flow conditions. This gap reflects the onset of film breakdown on the lowermost rows of tubes at low liquid film flow rates. Film breakdown occurs when the liquid flow rate is too low to sustain a continuous liquid film on the tube surface, Mitrovic [4] and Ganic and Roppo [5]. Moreover, the model supposes perfect wetting over all the tubes. The steeper increase in the evaporative exchanger performances when the film flow rate increases, observed for low flow rate conditions, is due to a better wetting of the lower part of the tube bundle. This leads to an increase in the exchange surface and thus in the heat flux transmitted to the liquid film. 3.2. Unit 2: experimented in France, Bourouni et al. [3]
For a low Reynolds numbers (Ref < 630), the same behaviour observed with the first plant
151
is highlighted. For a film Reynolds number greater than 630 experimental data show a decrease in ~lev when Rein.f increases, while numerical results show a slight increase. Two effects could be considered in order to explain this discrepancy. The first one is related to the inlet film flow conditions. The visual observation of the water flow downstream from the distributor tube shows no more continuous irrigation of the tubes at values of Reynolds number Reinf close to 630. This phenomenon induces some dissymetrical effects of the water flow. At the top of the evaporator, tubes are observed to be non-uniformly wetted with sprayed water. On the other hand, the flow between successive tubes changes. The liquid sheet that joins the tubes at low inlet flow rates breaks down into a series of discrete droplets. These two effects, not included in the analytic approach, reduce heat and mass transfer inside the exchanger and so lead to some decrease in the evaporator's performances. This behaviour was not observed in the first unit. This is due to the difference in the water distribution system. In fact, the water distribution system used in the unit experimented in Tunisia is a slot, while a row of holes placed at the bottom of a tube inserted above the tube bundle is used in the plant studied in France. These results highlight the importance of the water distribution system in horizontal-tube, falling-film evaporators. The results reported by Fletcher et al. [6] show that evaporative liquid film heat transfer depends on the water distribution system at the top of the horizontal tube. Fletcher et al. [6] used two kinds of distributor, a "perforated-plate" and a "thin slot" water distribution system. The experimental data obtained show that the second system is more accurate and appropriate. Previous results show a good agreement between experimental data and results predicted by the model. This agreement supports the validity of the analysis developed here.
152 4.
K. Bourouni et al. / Desalination 125 (1999) 147-153
Conclusion
In this paper, an experimental investigation on a desalination plant functioning by aeroevapo-condensation is presented. This work was carried out on a geothermal desalination plant installed in the south o f Tunisia. The experimental results are compared to data of Bourouni et al. [3] obtained from an investigation carried out in a laboratory. Both experimental results are compared to the prediction results obtained from the model developed by Bourouni et al. [1]. Concerning the plant experimented with a geothermal source, a good agreement between experimental data and numerical prediction was observed. For the plant studied in a laboratory, a good agreement between numerical and experimental results is observed for a liquid film Reynolds number between 400 and 630. Nevertheless, for a liquid film Reynolds number lower than 400 both results present the same trend. However, numerical results are higher than experimental data. This discrepancy was explained by an imperfect wetting of the lower part of the tube bundle at low liquid film flow rates, while numerical results are based on the total exchange area. Until now it has not been possible to determine the area actually wetted. All calculation methods are based on the complete surface of the heat exchanger. For a film liquid Reynolds number higher than 630 numerical results present a slight increase in evaporative performance when Rein.f increases while an opposite trend is observed for experimental data. This is due to sudden modification in the liquid film distribution at the top of the exchanger. The water distribution system used by Bourouni et al. [3] does not allow continuous irrigation of the tubes for high Reynolds number. A liquid pulverisation takes place in the higher part of the exchanger. This phenomenon induces a loss of exchange surface, hence a decrease in evaporative and thermal exchanger performances.
5.
Symbols
- - Cell height, m - - Tube diameter, m __ Exchanger diameter, m - - Mass flux, Kg/s - - Tube spacing - - Local temperature, °C
B d Dexch
rh s T _
T~n + Tou~
T -
--
2
Average temperature, °C
- - Absolute humidity, Kg/Kg
X
Non dimensions number
dint U hl
Hot liquid Reynolds number
R e h t - vh~ R e e f _ 4F
/If
- - Liquid film Reynolds number
dextUg
Air Reynolds number
Rein.g - - vg Greek
H F
- - Dynamic viscosity, Kg/m s - - Mass flow rate o f film per unit length on one side of tube, Kg/(m s)]
Subscripts
c ev ext
f g int hl
v w
---
------
---
Critical Evaporated External Liquid film Gas Internal Hot liquid Vapour Tubes wall
References
[1] K. Bourouni, R. Martin, L. Tadrist and H. Tadrist, Desalination, 114 (1997) 111.
K. Bourouni et al. /Desalination 125 (1999) 147-153
[2] [3] [4]
K.S. Spiegler, Principles of desalination. Academic Press, New York, 1966. K. Bourouni, L. Tadrist and R. Martin, Desalination, 116 (1998) 165. J. Mitrovic, Influence of the spacing and flow rate on heat transfer from a horizontal tube to a falling liquid film., Proceedings of 8th Int. Heat
[5] [6]
153
Transfer Conf., San Francisco, USA, 1986, pp. 1949-1956. E.N. Ganic and M.N. Roppo, ASME J.Heat Transfer, 102 (1980) 342. L.S. Fletcher, V. Semas and L.S. Galowin, ASME 73-HT~12 (1973)265.
ELSEVIER
vww.elsevier.com/locate/desal
Experimentation and modelling of an innovative geothermal desalination unit Karim Bourouni a*, Jean Claude Deronzier b, Lounes Tadrist a aLaboratoire de l'Institut Universitaire des Syst~mes Thermiques lndustriels (IUSTI), U. M. R. 65.95 Universit~ de Provence- Technopole de Chdteau Gombert, 5, Rue Enrico Fermi 13453 Marseille cedex 13, France Tel. +33 (4) 91 10 68 63, Fax +33 (4) 91 10 69 69, E-mail: [email protected] bLaboratoires du G R E T H - CEA Centre de Grenoble, 17, Rue des Martyrs - 38054 Grenoble cedex, France
Abstract
In this study, heat transfer of air-water-vapour mixtures in a desalination plant, using the aero-evapocondensation process is studied experimentally. The present process consists in a falling film evaporator and a condenser. The prototype is designed to work at low temperatures (60°C-90°C) using a geothermal energy. Two experimental pilots were developed. These latter, installed respectively in France and the south of Tunisia were supplied by fuel and geothermal energy. The experimental results highlighted a critical film flow rate, characterising the phenomenon of film breakdown. At this value, a maximum amount of evaporated water was obtained. Experimental results were compared with those derived from the model developed by Bourouni et al. [1 ] in previous investigation. From this comparison it can be learnt that the model is well able to predict the heat and mass transfer in the evaporator. This will help to optimise this component of the aero-evapo-condensation units. Keywords: Geothermal energy; Desalination; Evaporator; Horizontal tubes; Falling film; Evaporation; Heat transfer
*Corresponding author Presented at the Conference on Desalination and the Environment, Las Palmas, November 9-12, 1999. European Desalination Society and the International Water Services Association. 0011-9164199/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S0011 - 9 1 6 4 ( 9 9 ) 0 0 1 3 3 - 2
148
K. Bourouni et al. / Desalination 125 (1999) 147-153
1. Introduction The common methods of abstracting fresh water from salt water such as abstracting by distillation, reverse osmosis and electrolysis are intensive energy techniques. For this reason these desalination techniques are competitive only for large-scale production (thousands of cubic meter per day), Speigler [2]. However, in some circumstances, the desalination needs do not exceed a few cubic meters per day. It is the case of some arid and semiarid regions of south Mediterranean countries that have important resources of brackish water, which is sometimes deep, at a high temperature and unfortunately salt. In arid areas, generally in very hard conditions, it is necessary, to uphold the lives of human groups whose presence is linked to a specific economic activity (extraction of petrol for example). In semi-arid areas a minimum of economic activities, such as agriculture, must be maintained. For these reasons, there is constant research in this field. In fact, many new desalination methods are investigated, especially for small units, with a production lower than 10 m3/d. A desalination prototype, with a capacity of a few cubic meters per day, allowing the use of geothermal energy and reducing the capital cost, was patented by the firm Caldor-Marseilles in 1994. The present prototype includes two crossflow heat exchangers, a horizontal falling-film evaporator and a horizontal falling-film condenser. The two exchangers are made of polypropylene and operate by the humidificationdehumidification of air. The prototype was designed to use low temperature energy and in particular geothermal energy. The brackish water intended for desalination is at a temperature between 75°C and 90°C, which is less than the maximum operational temperature of the polypropylene, limited to 100°C. Bourouni et al. [3] presented an experimental investigation of the desalination plant func-
tioning by aero-evapo-condensation. The data exhibited show an influence of the air and hot water temperatures on the unit performances. These investigations show a linear increase in the amount of evaporated water with the air and hot water inlet temperatures. A critical Reynolds number corresponding to a maximum quantity of evaporated water was determined. Some flaws, especially in the liquid distribution system, were highlighted. In order to optimise the plant's performances, some modifications were proposed (system of water distribution, airflow, etc.). From these proposals, a second optimised pilot was achieved. The experiments of Bourouni et al. [3] are carried out in a laboratory in Marseilles. These conditions are different from the environment in which the plant is destined to operate. This investigation does not allow the possible interactions between the unit and its real environment to be determined (problems of scaling, sandstorms, temperature, etc.). For this reason we decided to carry out an experimental investigation of the optimised pilot in the south of Tunisia. For this purpose a geothermal spring with a water temperature of about 70°C is used. The present study analyses the performance of the desalination unit installed in the south of Tunisia. The process used in the unit is based on aero-evapo-condensation. The influence of different thermal and hydrodynamic parameters on water evaporation is investigated. The experimental results are compared to the data of Bourouni et al. [3] in order to confirm the improvement of the process and to the numerical predictions of Bourouni et al [1] to validate their model. 2. Process equipment 2.1. Description and operation
The prototype is presented in Fig. 1. It includes an evaporator (l) and a condenser (2). Each heat exchanger consists of circular plastic tubes and an insulated envelope. Heat recovery
K. Bourouni et al. /Desalination 125 (1999) 147-153
in a low-temperature process requires a large exchange surface. The two exchangers are 2 m long with a rectangular section (1.2 m in length and 0.8 m in width), and they include 865 m of tubes. Two tanks (3), under the evaporator and (3') under the condenser, contain salt and distilled water. The two exchangers are linked by a pipe (4) of a diameter of 0.2 m, allowing the circulation of humid air from the evaporator to the condenser. The airflow is maintained by a blower (5). Two pumps (6) and (6') permit the circulation of salt and distilled water. in order to avoid an excess of salt water concentration, the tank (3) level is held constant
149
by a continuous supply of salt water. The salt concentration is controlled by a purge sluice. Furthermore, the level in tank (3') is held constant by a distilled water levy. The cooled hot brackish water moves down the tubes. Its temperature at the entrance is about 70°C. The cooling air moves up in the space between the tubes. The cold salt water in the tank (3), at ambient temperature, is sucked up by the pump (6) to the condenser (2). In this exchanger the water moves up inside the tubes. At the condenser outlet, salt water is preheated to 50°C. The liquid is introduced through the water distribution system (7) into the top of the
Humid
Air
water
t____ Preheated salt water
(1) Evaporator
(2) Condenser
Cell i (5)
Ambiant
Cold water
water
distilled wate_.__..Lr
-..------
(3) ~Thermocouple
~
(3') Rotameter
(~) Hygrometer
0
Gas velocity rotameter
Fig. 1. Functioning principles and instrumentationof the desalination unit: (1) evaporator, (2) condenser, (3) and (3') tanks, (4) pipe, (5) blower, (6) and (6') pumps, (7) distributor. dint=20 ram; dext=25.6mm; s=l.3 de,a;B=1.2 dext
150
K. Bourouni et al. / Desalination 125 (1999) 147-153
evaporator and falls from tube to tube. The feed water distribution system is a slot placed at the top of a 25 mm diameter tube. The liquid film flows and evaporates on the outside surfaces of the tubes. The vapour is carried by the airflow to the condenser. At the top of the condenser, the air is hot and humid. In the condenser (2), the humid air moves down through the space between the tubes. On contact with the cold tube walls, there is film condensation coupled with latent heat restitution to the salt water circulating inside the tubes. Finally, the distilled water is recovered in the tank (3'). The characteristics of the film flowing around the horizontal tubes are visualised during the tests through two Plexiglas windows, placed on each exchanger. The humid air velocity is adjusted using a frequency controller linked to the ventilator. Two flow adjusting valves, placed upstream from the evaporator and the distributor, allow the control of the hot water and liquid film flow rates. 1.2. Instrumentation
Eight thermocouples are placed at different locations to measure the inlet and outlet temperatures of the hot water, humid air and stream water (Fig. 1). A bowl with an adjustable flow rate is placed at the bottom of the evaporator. The stream water temperature at the exchanger outlet is sensed with a thermocouple placed on the bowl. The irrigation flow rate is measured by a rotameter located upstream from the distributor. A second rotameter is used to measure the hot water flow rate. The air velocity is measured by a gas velocity rotameter inserted between the evaporator and the condenser. A hygrometer placed on the pipe joining the two exchangers allows the air humidity to be measured at the entrance of the evaporator. These probes are connected to a data acquisition system.
The test procedure was defined from previous experiments, (Bourouni et al. [3]). This procedure was repeated for all the experiments. It consists in running irrigation for several hours to ensure perfect moistening of the tubes. Next, the air is pumped through the exchangers. The hot brackish water is injected when different fluid temperatures present slightly stable values. The steady state is reached after several operating hours (2-4 h). This state is mainmined for two hours. The temperatures and fluid flow rates are recorded every ten seconds. The time between two humidity measurements is about two minutes. The height of the water in the tank (3') is measured every ten minutes.
3. Experimental results and comparison with numerical predictions The instrumentation developed to analyse heat and mass in the plant allows only the investigation of global parameters (inlet and outlet temperatures, pressures, humidity and total amount of evaporated water). 16 (10"3 Kg/s)
14
~ : Unit I; ~ : Unit 2; ~
: Model
_
[
1000
1200
12
rhe
0 8 6 4
200
400
600
Liquid film Reynolds number
800
4F Re r /.t
Fig. 2. Influence of air inlet velocity on the total amount of evaporated water. Ti. hl=62°C; Ti.f=34°C; Tin g=27°C; Rein hj=14000; Remg =18000; Xins=17g/Kg
K. Bourouni et al. / Desalination 125 (1999) 147-153
In the following, typical experimental results and those obtained from simulations (Bourouni et al. [1]) are presented. The basic parameter in this problem is the prediction of the amount of evaporated water they. For a given operational condition, the variation in this parameter versus liquid film flow rate is presented. The influence of the liquid film flow rate on evaporative exchanger performance is plotted in Fig. 2. In the same figure, experimental results corresponding to both desalination units and numerical predictions are reported. 3.1. Unit 1 experimented in Tunisia
The experimental results obtained show a continuous increase in the amount of evaporated water when the liquid film flow rate increases. This increase is more pronounced for a low liquid film Reynolds number. For a high liquid film Reynolds number rhev increases slightly, when Re/increases. A good agreement between numerical predictions and experimental data is highlighted; however a significant gap between the results is observed for low liquid film flow conditions. This gap reflects the onset of film breakdown on the lowermost rows of tubes at low liquid film flow rates. Film breakdown occurs when the liquid flow rate is too low to sustain a continuous liquid film on the tube surface, Mitrovic [4] and Ganic and Roppo [5]. Moreover, the model supposes perfect wetting over all the tubes. The steeper increase in the evaporative exchanger performances when the film flow rate increases, observed for low flow rate conditions, is due to a better wetting of the lower part of the tube bundle. This leads to an increase in the exchange surface and thus in the heat flux transmitted to the liquid film. 3.2. Unit 2: experimented in France, Bourouni et al. [3]
For a low Reynolds numbers (Ref < 630), the same behaviour observed with the first plant
151
is highlighted. For a film Reynolds number greater than 630 experimental data show a decrease in ~lev when Rein.f increases, while numerical results show a slight increase. Two effects could be considered in order to explain this discrepancy. The first one is related to the inlet film flow conditions. The visual observation of the water flow downstream from the distributor tube shows no more continuous irrigation of the tubes at values of Reynolds number Reinf close to 630. This phenomenon induces some dissymetrical effects of the water flow. At the top of the evaporator, tubes are observed to be non-uniformly wetted with sprayed water. On the other hand, the flow between successive tubes changes. The liquid sheet that joins the tubes at low inlet flow rates breaks down into a series of discrete droplets. These two effects, not included in the analytic approach, reduce heat and mass transfer inside the exchanger and so lead to some decrease in the evaporator's performances. This behaviour was not observed in the first unit. This is due to the difference in the water distribution system. In fact, the water distribution system used in the unit experimented in Tunisia is a slot, while a row of holes placed at the bottom of a tube inserted above the tube bundle is used in the plant studied in France. These results highlight the importance of the water distribution system in horizontal-tube, falling-film evaporators. The results reported by Fletcher et al. [6] show that evaporative liquid film heat transfer depends on the water distribution system at the top of the horizontal tube. Fletcher et al. [6] used two kinds of distributor, a "perforated-plate" and a "thin slot" water distribution system. The experimental data obtained show that the second system is more accurate and appropriate. Previous results show a good agreement between experimental data and results predicted by the model. This agreement supports the validity of the analysis developed here.
152 4.
K. Bourouni et al. / Desalination 125 (1999) 147-153
Conclusion
In this paper, an experimental investigation on a desalination plant functioning by aeroevapo-condensation is presented. This work was carried out on a geothermal desalination plant installed in the south o f Tunisia. The experimental results are compared to data of Bourouni et al. [3] obtained from an investigation carried out in a laboratory. Both experimental results are compared to the prediction results obtained from the model developed by Bourouni et al. [1]. Concerning the plant experimented with a geothermal source, a good agreement between experimental data and numerical prediction was observed. For the plant studied in a laboratory, a good agreement between numerical and experimental results is observed for a liquid film Reynolds number between 400 and 630. Nevertheless, for a liquid film Reynolds number lower than 400 both results present the same trend. However, numerical results are higher than experimental data. This discrepancy was explained by an imperfect wetting of the lower part of the tube bundle at low liquid film flow rates, while numerical results are based on the total exchange area. Until now it has not been possible to determine the area actually wetted. All calculation methods are based on the complete surface of the heat exchanger. For a film liquid Reynolds number higher than 630 numerical results present a slight increase in evaporative performance when Rein.f increases while an opposite trend is observed for experimental data. This is due to sudden modification in the liquid film distribution at the top of the exchanger. The water distribution system used by Bourouni et al. [3] does not allow continuous irrigation of the tubes for high Reynolds number. A liquid pulverisation takes place in the higher part of the exchanger. This phenomenon induces a loss of exchange surface, hence a decrease in evaporative and thermal exchanger performances.
5.
Symbols
- - Cell height, m - - Tube diameter, m __ Exchanger diameter, m - - Mass flux, Kg/s - - Tube spacing - - Local temperature, °C
B d Dexch
rh s T _
T~n + Tou~
T -
--
2
Average temperature, °C
- - Absolute humidity, Kg/Kg
X
Non dimensions number
dint U hl
Hot liquid Reynolds number
R e h t - vh~ R e e f _ 4F
/If
- - Liquid film Reynolds number
dextUg
Air Reynolds number
Rein.g - - vg Greek
H F
- - Dynamic viscosity, Kg/m s - - Mass flow rate o f film per unit length on one side of tube, Kg/(m s)]
Subscripts
c ev ext
f g int hl
v w
---
------
---
Critical Evaporated External Liquid film Gas Internal Hot liquid Vapour Tubes wall
References
[1] K. Bourouni, R. Martin, L. Tadrist and H. Tadrist, Desalination, 114 (1997) 111.
K. Bourouni et al. /Desalination 125 (1999) 147-153
[2] [3] [4]
K.S. Spiegler, Principles of desalination. Academic Press, New York, 1966. K. Bourouni, L. Tadrist and R. Martin, Desalination, 116 (1998) 165. J. Mitrovic, Influence of the spacing and flow rate on heat transfer from a horizontal tube to a falling liquid film., Proceedings of 8th Int. Heat
[5] [6]
153
Transfer Conf., San Francisco, USA, 1986, pp. 1949-1956. E.N. Ganic and M.N. Roppo, ASME J.Heat Transfer, 102 (1980) 342. L.S. Fletcher, V. Semas and L.S. Galowin, ASME 73-HT~12 (1973)265.