Patent Publication Number: US-2015068886-A1

Title: Water distilling apparatus using saturated air currents and methods for maximising the performance thereof

Description:
The invention relates to both improved water distillation apparatuses of the type using streams of forced saturated damp hot air and recovery of latent heat of condensation, and methods to maximize the performance thereof. Applications of these distillation apparatuses include desalination of seawater and, more generally, demineralization of any clear water, especially for producing ultra-pure water. The invention also relates to the concentration of various industrial aqueous solutions, to be transformed (food industry) or destroyed (polluted effluent waste) and the separation of certain volatile liquids from their aqueous solvent (ethanols, acids, bases . . . ). 
     Two water distillation apparatuses, using streams of forced saturated damp hot air with recovery of latent heat of condensation are described and commented on (1) in FIG. 5 of a French patent application of 1975, published under No. 2,281,896, and (2) a U.S. Pat. No. 3,860,492 of the same year. 
     In the water distillation apparatus of this French patent application,
         three distillation stages are arranged one on top of the other;   each distillation stage includes a pair of chambers, one for evaporation and the other for condensation, an upper portions of these chambers communicating by a duct;   the ducts of the two lower distillation stages include manual means for adjusting their port cross-sectional area;   the three evaporation chambers are filled with wettable artificial nuts loosely packed;   the three chambers include three condensing coils connected in series;   means are connected to the apparatus to circulate a stream of water at a low initial temperature from bottom to top in the three condensing coils of the condensation chambers;   a water heater is connected to an upper portion of the condensing coils to bring the exiting water to a high temperature of 95° C.;   means for distributing and spreading are installed above an upper evaporation chamber to spread this water at high temperature over an upper portion of this chamber;   a fan is installed at a lower portion of a lower evaporation chamber, to force saturated damp hot air flow rates to flow from the bottom to the top in all three evaporation chambers and from top to bottom in the three condensation chambers, in order to cause air streams circulating up of the evaporation chambers to acquire predetermined temperatures;   means are provided for collecting the distilled water and the concentrated aqueous solution, respectively installed at a lower portion of a lower condensation and evaporation chambers.       

     This water distillation apparatus has been arranged to form these three distillation stages in order to improve heat exchange between streams of hot water and hot air or air and water circulating in opposite directions, taking place in the evaporation chamber and the condensation chamber of the apparatus. Such a solution (not justified in the document) is indeed necessary because, depending on the temperature, the heat capacity Cp E  of one kilogram of water is constant (4.18 kJ/K), while the apparent heat capacity Cp A  of one kilogram of dry air at standard pressure, incorporated into saturated hot damp air, increases with temperature and exhibits a hyperbolic branch which tends to infinity at 100° C. With three stages of distillation, the pair of curves representing the enthalpy fluxes of rising and descending air is transformed into two sequences of three festoons. In these conditions, we greatly minimize the variation between, on the one hand, these two series of festoons and on the other hand, the two parallel straight lines representative of the enthalpy flux of the water flowing upwardly and downwardly. The total heat exchange thereby achieved is much higher than it would have been with a single distillation stage and the amount of distilled water produced is, under these conditions, greatly increased. But the performance of such water distillation apparatus is nevertheless far from maximal. 
     Indeed, in this water distillation apparatus, with a stream of air at initial temperature T A0 =20° C. and a stream of water at a temperature T low  of T E0 =20° C. and T high  of T E2 =95° C., we have, at an upper portion of the three stages of distillation of ranks 1 to 3, for air circulating in an open loop: T A1 =55° C., T A2 =75° C., T A3 =90° C., and 35° C. at the lower portion of the condensation column. For water circulating in open loop, at the lower portion of the three evaporation chambers we have, from top to the base of the evaporation column: T E2 , 85° C., 65° C. and T E3 =40° C. At an upper portion of the three condensation chambers we have, for the water, from the base to an upper portion of the condensation column: T E0 , 45° C., 65° C. and T E1 =75° C. Under these conditions, the coefficient of performance (CoP) of the apparatus (the ratio between the thermal energy used in distillation and the thermal energy supplied by the water heater), CoP=(T E2 −T E0 )/(T E2 −T E1 )=3.75, which is an average value, acceptable when the energy used is cheap. But the question is, is this calculated coefficient of performance (CoP) possible with the apparatus described? 
     To look into this, we will look more closely at the temperatures announced for the air and water. And we will see that these temperatures (clearly not experimental) are not suitable for producing average performance and a fortiori, high-performance, both from a technical and an economic point of view. This firstly is the case for the air temperatures at an upper portion of the evaporation chambers and at the lower portion of the condensation chambers of the three distillation stages. 
     Initially, what is involved is choosing the two intermediate air temperatures T A1  and T A2  at an upper portion of the evaporation chambers of rank 1 and 2. This choice is usually made starting out from extreme air temperatures: a low temperature T A0  imposed by the ambient air and a high temperature value T A3  relatively close to T E2 , the high temperature of the water distributed at an upper portion of an upper evaporation chamber. As a result of the three distillation stages provided in the distillation apparatus, the curve which it is desired to represent the corrected enthalpy fluxes of the ascending saturated hot damp air streams in the three evaporation chambers is an ideal sequence of three segments of a straight line of slopes A=ΔH/ΔT, with ΔH and ΔT the increases in enthalpy flux and air temperature in each of the evaporation chambers. In fact, the actual curve representing the corrected fluxes is constituted by three festoons which depart from a straight line and which meet angularly at two points. The maximum difference between these two curves, actual and optimal, is the degree of deflection ΔT F  of each festoon. For each festoon, the value in degrees of the deflection ΔT F  is the quotient ΔH*/A where ΔH* is the difference in enthalpy flux between the middle of the ideal line and the point on the curve for air enthalpy flux at the mean temperature of the chamber. In the case of extreme temperatures of saturated damp hot air in an upper portion and lower portion of the evaporation chambers, announced in the patent concerned, namely  30  (approximately), 55, 75 and 90° C., the values of enthalpy flux for 1 kg/s of dry air, carried away by the stream of saturated damp hot air ascending in each evaporation chamber are given by the table for specific enthalpy H of the saturated damp hot air. For significant temperatures of the damp air (middle and ends of the straight line segments), we have H 90 =3887 kW; H 82.5 =1864, H 75 =1088; H 65 =600; H 55 =353; H 42.5 =179 and H 30 =100 kW. For the lower festoon, we have ΔH 1 *=½ (H 55 +H 30 )−H 42.5 =48 kW and A 1 =(H 55 −H 30 )/25=10 kW/K, so that ΔT F1 =4.8° C. For the central festoon we have ΔH 2 *=120 kW, A 2 =37 kW/K and ΔT F2 =3.2° C. For the upper festoon we have ΔH 3 *=623 kW, A 3 =187 kW/K and ΔT F3 =3.3° C. All these values are unacceptable, for the reason that they are too high. But as the deflection ΔT F1  of the lower festoon is the most significant, it is, therefore, the only one to be taken into account for determining (T E2 −T E1 ). Here, it represents a quarter of the temperature increase required of the water heater, which leads to a corresponding reduction in coefficient of performance (CoP) of the apparatus. By way of conclusion of these initial comments, we see that for water distillation apparatus to be efficient, the number of distillation stages and the choice of air temperatures at an upper portion of the evaporation chambers cannot be determined in an arbitrary fashion even if this does seem to be in line with common sense. 
     As regards temperatures at the lower portion of condensation chambers, there is no reason for the temperature of the air stream deviated by the two communication paths between the evaporation and condensation chambers of ranks 1 and 2, and the temperatures of the descending air streams in condensation chambers of rank 2 and 3 to be the same. Indeed, as the architecture of the two chambers of a distillation stage is different one from the other, then a priori, their respective heat exchange coefficients are also different. The inequality of these temperatures causes a decrease in coefficient of performance (CoP), if it is not corrected. 
     In addition, as the shape and dimensions of the cross-sections of the ducts discharging into the condensation chambers, nor the shape of the cross-section of these chambers are not specified, the exact conditions under which air streams deviated by these ducts enter these chambers and mix with the air streams descending from chambers of higher rank is not known. Whatever the case may be, streams of air which actually come into close contact with the coils are, a priori, in very small minority compared to those which do not. This is at the origin, locally, of air/water thermal couplings which are unbalanced and very different one from the other. Overall, the saturated damp hot air is very poorly cooled and vapor is very badly condensed in the condensation chambers equipped with condensing coils. For this to be otherwise, it is necessary that the surface area per unit volume S/V for heat exchange at these coils be relatively high in the condensation chamber. As such a ratio for efficient heat air/water exchange is S/V&gt;150 m 2 /m 3 , we can see that this is inconceivable when using conventional zigzag or spiral coils, for which S/V&lt;10 m 2 /m 3 . The situation is better when multiple coils or sections of coils are assembled in parallel, with upstream and downstream manifolds. This is something which the cited document does not suggest. In any case, the thread-like structure of conventional coils often leads to unacceptable pressure loss in the water stream. In addition, their cost is often prohibitive since, being a priori of metal, their metal must be insensitive to the corrosive action of seawater The conclusion of these discussions is simple: conventional metal coils are completely unsuitable to fulfil the double function assigned to them in high performance (coefficient of performance (CoP&gt;4) water distillation apparatus, namely: efficiently heat up an ascending seawater stream using a descending stream of saturated damp hot air and condensing the maximum amount of water vapor carried by the air stream under satisfactory economic conditions for the relevant markets. 
     Moreover, packing wettable artificial nuts into the evaporation column leads to significant pressure losses in the air stream blown in by the fan installed at a lower portion of this column. This leads to a requirement, for the air stream, for relatively high pressures and high local velocities, which inevitably leads to the presence of droplets of brine in air streams entering into the condensation column. In the case of water distillation apparatus treating standard seawater, this leads to the presence of a 1 to 2 g salt content per liter of distilled water produced, making consumption thereof unpleasant. In the case of water distillation apparatus treating polluted industrial water, discharge of which is prohibited, the situation is worse: the distilled water remains polluted and discharge thereof is prohibited. 
     To complete the discussion of the problems posed by water distillation apparatus disclosed in the cited document, we can still note that the temperature T E2 =95° C. provided for the hot water to be distributed at an upper portion of the evaporation column, is a temperature which is too high, in the long run dangerous because it accelerates the precipitation of certain salts dissolved in the water and, as a result, leads to significant limescale deposits within the passages taken, causing disruption and, in the long-term, these finish by blocking the operation of the machine. 
     The water distillation apparatus according to U.S. Pat. No. 3,860,492 is designed to be an automatic system for concentrating industrial water. It includes a first distillation stage, materialized in a lower half of the two columns, and an indeterminate number of stages arranged thereabove. The wettable components of the evaporation column consist of suspended netting and the hollow heat exchange components of the condensation column are condenser coils. The majority of the above negative remarks apply. In addition, arrow of the festoon of the air enthalpy curve in this first stage is, a priori, notably greater than that in the case discussed above. Higher performance distillation apparatus using eight stages is additionally envisaged, without further details. 
     To conclude the commentary on these two documents, we can affirm that one major condition to insure the process involved does exhibit high energy performance is that the difference in temperature between the damp air and the water be as small as possible throughout the length of the evaporation and condensation columns. The fact of splitting up each column into several stages, where the damp air flow rates are different and the water flow rate is practically constant already makes possible an overall reduction in the difference in temperature between the damp air and the water. The two documents cited do propose such a solution. The French patent does not give an explanation. The US Patent gives a partial explanation starting from the curve showing the evolution of saturated damp air enthalpy as a function of temperature. But neither of these two patents deals with a way of minimizing the difference in temperature between the damp air and the water within each one of the stages. 
     Accompanying this major condition there is another condition which is just as important: thermal exchange coefficients which notably result from thermal exchanges in dry air, the mechanism for distributing water vapor into the air and the surface area per unit volume (S/V) of the components of the distillation units, must be as high as possible, in order to be able to sufficiently reduce this difference in temperature between the damp air and the water. But this is all the more difficult when we consider that obtaining good mastery of heat exchanges in saturated damp air is something which is not at all obvious. 
     The first subject matter of the invention concerns various improved water distillation apparatuses, using forced saturated damp hot air streams with recovery of the latent heat of condensation, including several distillation units adapted to operate under optimal conditions. 
     The second subject matter of the invention concerns such improved water distillation apparatuses in which, through construction, these optimal conditions are, within the condensation chambers, established by the use of components having a very high thermal exchange coefficient. 
     The third subject matter of the invention concerns such improved water distillation apparatus in which these optimal conditions, in both chambers of each distillation unit, are established by differences in temperature between the damp air and the water which are as low as possible. 
     The fourth subject matter of the invention is the realization of such improved water distillation apparatus having a high and/or variable capacity for daily production of distilled water, constituted by modular distillation units which are easy to construct, install and operate. 
     A fifth subject matter of the invention concerns methods for maximizing the performance of these various water distillation apparatuses. 
     A sixth subject matter of the invention concerns the construction of such improved water distillation apparatuses which are suitable for demineralizing water, notably desalinated seawater, concentrating industrial waters and separating volatile liquids from their aqueous solvents, with a high energy yield. 
     In accordance with an enlarged definition of the water distillation apparatus disclosed in the French patent commented on above, the present distillation apparatus is of the type in which:
         a number N of distillation units is provided, the unit of rank 1 being the least hot and the one of rank N being the hottest;   each of these N distillation units includes a pair of vertical chambers, one for the evaporation of water and the other for vapor condensation, communicating at an upper portion;   each of the N evaporation chambers is occupied by a set of wettable or hydrophilic components;   each of the N condensation chambers is occupied by a hollow heat exchanger device, these N devices being connected in series;   means are connected to the apparatus to provide a water stream, at a mass flow rate Q E0 , at a low initial temperature T E0 , to cause the water stream to flow upwardly in the N hollow heat exchange devices;   a water heater is connected to the outlet of the N hollow heat exchange devices, to bring the stream of water exiting therefrom at a temperature T E1  up to a high temperature value T E2  lower than 100° C.,   means are installed in the apparatus to spread the hot water at a temperature T E2 , at an upper portion of the components of an evaporation chamber of rank N and to cause it to trickle down the components of evaporation chambers of lower rank;   means are installed in the apparatus for forcing N adjusted saturated damp hot air flow rates to circulate upwardly in the N evaporation chambers and downwardly in the N condensation chambers so as to respectively bring the N streams of air, circulating at an upper portion of the evaporation chambers, up to predetermined temperatures T A1  to T AN ;   means for collecting distilled water flow rates are installed at a lower portion of the condensation chamber of rank 1;   means for collecting flow rates of concentrated aqueous solution produced are installed at a lower portion of the evaporation chamber of rank 1.       

     According to the invention, the improved water distillation apparatus of the broad type defined above is characterized in that:
         the N heat exchange devices have cross-sections and heights substantially identical to those of the N condensation chambers;   the N heat exchange devices are groups of hollow polymer plates assembled so as to be separated at a constant pitch, each group being provided with upstream and downstream manifolds;   in the N condensation chambers, the hollow plates are installed vertically.       

     According to the invention, such water distillation apparatus is further characterized in that it includes N perforated trays adapted to distribute total flow rates of air, respectively entering into the N condensation chambers, into substantially equal partial air flow rates penetrating into the spaces between the hollow plates of these chambers. 
     According to the invention, such water distillation apparatus is more precisely characterized in that in each of the N distillation units:
         communication between the evaporation and condensation chambers includes a horizontal elongated rectangular window;   the cross-sections of the condensation chamber, perforated tray and hollow plates are all rectangular;   the perforated tray is installed without significant edge portion leakage and has rows of holes, surmounting all the hollow plates;   said holes are oblong and are formed at the same pitch as that at which the plates are assembled, their width being substantially equal to that of the spaces between the plates.       

     These simple provisions are both novel and non-obvious, since they provide practical solutions to concrete non-obvious problems, considering they were overlooked and/or not expressed until now. Firstly, the choice of condensation chambers having a rectangular cross-section is not the only one possible but, clearly, the most rational choice considering that the heat exchange device is constituted by an assembly of rectangular hollow plates, which are the only ones which are currently available on the market. With condensation chambers which have a non-rectangular cross-section, the arrangement of the hollow plates will be done on the basis of the specific geometry or geometries, which the manufacturer has given these plates. 
     With a horizontal rectangular entrance window, according to the invention, the stream of saturated damp hot air which penetrates into the condensation chamber of each one of the N distillation units, does this in the form of uniform horizontal and parallel veins of air which have overall, an elongated rectangular cross-section (40×10 cm, for example). Under these conditions, in the distillation unit of rank N, such veins of air can correctly spread out above the heat exchange device which is occupying the whole cross-section of the condensation chamber (40×30 cm, for example). In the other units, the two flow rates, the horizontal one and vertical one, which enter into the condensation chamber at the same temperature (below it will be seen why and how this is so) but the mixing thereof sets up disordered movements and excess pressures. 
     The perforated tray, which is installed without notable lateral leakages, corrects the potential action of these disordered excess pressures, by forcing the total air flow, entry into each condensation chamber, to be redistributed into substantially equal partial flow rates, which penetrate into the spaces separating the vertical hollow plates of the heat exchange device. 
     Using condensation chambers having a rectangular cross-section and rectangular hollow plates which are assembled at a constant pitch, the perforations of the perforated tray are rows of oblong holes, having the same pitch as these plates. Within each condensation chamber of rank 1 to N, equality of the partial flow rates of air circulating between the plates is a result of the substantial drop in pressure Δρ n= ½ρ n v n   2 , which is created during their passage through the holes (ρ n  being the density of the saturated damp hot air entering into the chamber of rank n, and v n  is the velocity at which it passes through the holes). As a consequence, the surface areas, both total and individual, of the oblong holes of the perforated distribution and spreading plate of each condensation chamber can be determined further to a simple calculation, done using the total flow rate of incoming air, which itself is calculated, as will be seen below. Additionally, it is possible to replace each row of holes by one or a plurality of slots of an appropriate calculated width, but the distribution of the partial air flows circulating between the hollow plates is in this case less uniform and, inside the condensation chamber, the heat exchange performed is less effective, but generally satisfactory. 
     Thanks to the arrangements according to the invention, effective heat exchange is performed between, on the one hand, the descending (3 to 6 mm thick) layers of saturated damp hot air, sweeping both faces (30×40 cm for example) of the (4 to 6 mm thick), thin-walled (&lt;1 mm) hollow plates of polymer material and, secondly, the narrow (2 to 4 mm) thicknesses of water which is ascending inside these plates. This is achieved despite the relatively high thermal resistivity of polymer materials. The coefficients of heat exchange and conductance between these air and water streams are significant since they are spread over two relatively large faces of this plurality of thin-walled hollow plates, assembled close to each other, occupying the whole volume of a condensation chamber. This is possible since the surface area per unit volume of the heat exchanger using separated, assembled hollow plates can reach S/V=250 m 2 /m 3 , when plate pitch is 8 mm. Thermal conductance here is several tens of times greater than that produced between an air stream which is passing through a condensation chamber occupied by a conventional metal coil, whether this be in zigzag or helical, and the water conveyed therein. 
     Further, if we do replace these conventional coils of 1975 by their current up-to-date versions, in other words bundles of polymer material tubes, having upstream and downstream manifolds, the surface area per unit volume of such a bundle, (depending on the manufacturer, 15&lt;S/V&lt;110 m 2 /m 3 ), remains on average three times less than that of a group of assembled hollow plates. If one were to replace the hollow plates by such bundles of tubes, this would lead, on average, to tripling the thicknesses of air and water, and consequently substantially diminishing thermal conductance per unit volume. A further significant and non-obvious comparative advantage of heat exchangers using hollow plates, installed in the condensation chambers of water distillation apparatus will be discussed below, in relation with the temperatures and flow rates of the saturated damp hot air streams inside these chambers. 
     In addition to these initial advantages specific to hollow plates of polymer material, there is the further advantage provided by the perforated tray according to the invention, in other words the division of the total air flow rate of incoming air in each condensation chamber into substantially equal partial flow rates that circulate between the hollow plates. When it is desired to maximize performance of the apparatus, it is essential to ensure that these partial flow rates are the same. In effect, simple calculation shows that a moderate disequilibrium (&lt;20%) between these partial air flow rates would result in a division by up to two of the efficiency of heat exchange achieved, and consequently of the performance of the apparatus. And this all the more so when we consider when we are seeking a target coefficient of performance which is even higher. As a consequence, thanks to these initial provisions according to the invention, in water distillation apparatus improved in this way, the effective heat exchange achieved inside the condensation chamber of each distillation unit leads to a maximum condensation of vapor and a maximum heating up of the water. 
     Further, experience has shown that with oblong holes in the perforated tray, having a width which is substantially equal to the distance between two hollow plates (&gt;3 mm), we eliminate the risk of these holes becoming blocked by droplets of condensed water. Additionally, each partial air flow rate exiting from a hole is efficaciously distributed between two plates, when there is a distance of less than about 10 cm separating two rows of holes of a perforated tray. 
     According to the invention, a method for maximizing the performance of these water distillation apparatuses according to the invention is characterized in that, depending on the conditions of use of the apparatus, it includes the following preliminary steps:
         select an appropriate value for the mass flow rate Q E0  of the water to be treated and a high temperature value T E2  between 45 and 90° C. for the water to be distributed at the upper portion of evaporation chamber of rank N, then take the initial low air temperature T A0  and water temperature T E0 ;   select an apparatus having a number N of distillation stages which is at least 3 and a maximum of 6, determined as a direct function of the values of the differences (T E2 −T E0 ) or (T AN −T A0 ) between the extreme temperatures of the air or water, the number N=4 being suitable for all cases;   select N approximate optimal predetermined temperatures T A1  to T AN , which the air streams at an upper portion of the N evaporation chambers should adopt whereby the departures from a straight line of the N festoons of the enthalpy curves for saturated hot damp air rising in these N evaporation chambers, have satisfactory reduced amplitudes, and which are more or less equal;   calculate the N approximated mass flow rates of dry air Q A1  to Q AN  and then the N approximated volume flow rates of saturated damp hot air Q S1  to QS/V, respectively circulating in the N distillation units of the apparatus.       

     Through the choice of a temperature T E2 &lt;90° C. for the hot water to be distributed and spread, scaling of components of the apparatus is minimized. The choice of the value T E2 , between 90 and 45° C., depends on the primary heat source available (solar, thermal engines, hot effluent, burners . . . ) and, where applicable, the temperature of boiling of the volatile liquid (ethanol, acid, base) dissolved in the water that is to be separated from the solvent. 
     To implement the various arrangements of the method according to the invention, we first make use of the table for enthalpy of saturated damp hot air. We saw earlier that the deflection of a festoon which departs from a straight line in a particular evaporation chamber is the difference in degrees, which corresponds to the maximum difference in watts between the curve representing the actual value of the enthalpy flux of the saturated damp hot air, and the deflection it would have if the curve was a straight line segment. Appropriate software makes it possible to draw up a digital chart for the amplitudes of these N festoons which depart from a straight line for every pair of extreme temperatures (T A0 , T AN ) used for the air stream rising in the evaporation column. In a four-distillation-stage distillation apparatus, with an upper water temperature value of T E2 =85° C. and extreme air temperatures of T A4 =83° C. and T A0 =33° C., the approximated optimum temperature of the air at an upper portion of the evaporation chambers of rank 1 to 3 are substantially: 47.5°, 61° and 73° C. In this case, the four increments ΔT A1  to ΔT A4  between the extreme temperatures of the air in the evaporation chambers are, from top to bottom, 10, 12, 13.5 and 14.5° C., while the departures from a straight line of the festoons are substantially equal to 1.2° C. (to which there should be added an imposed 0.5° C. deviation due to the salinity of the water). If we increase the number of distillation stages, departure from a straight line or deflection of the festoons decreases, but below about 0.5° C. for these deflections, increasing the number N is of little value. In an improved water distillation apparatus according to the invention, the optimal number of distillation stages may range from N=3 to N=6, as a function of the difference between the extreme temperatures of the air (T AN −T A0 ), ranging from 35 to 65° C. In practice, the number N=4 is suitable in all cases, for technical and economic reasons. 
     Within the four evaporation chambers, the temperatures T A1  to T A4  as well as the increments ΔT A1  to ΔT A4  being established, the heat capacities Cp A1  to Cp A4  of the saturated damp hot air, at the average temperatures T m1  to T m4  in these chambers are known. Enthalpy fluxes exchanged in an evaporation chamber of rank n are: ΔH Aη =Q An . Cp An . ΔT An  and ΔH En =Q EN . Cp En ΔT En . As increments ΔT An  and ΔT En  are substantially equal and Q En  only slightly differs from Q E0 , the approximated mass flow rate of dry air in a stage of rank n is substantially Q An =Q E0 . C pE /Cp An . Tables giving the density of saturated damp hot air make it possible to know the approximate volume flow rates Q S1  to Q S4  of this hot air, at average temperatures T m1  to T m4  in the four evaporation chambers. From these values, it is easy to calculate, for each condensation chamber, total and individual surface areas of the holes in the perforated tray and the pressure drop to be generated by the presence of the distribution plate. 
     For example, with Q E0 =100 g/s and in the evaporation chamber of rank 1, T m1 =41° C., Cp A1 =8.74 kW/kg·K, Q A1 =47.8 g/s, we get Q S1 =43.9 dm 3 /s. In the evaporation chamber of rank 4, with T m4 =79° C. and Cp A4 =102.6 kW/kg·K, we get Q A4 =4.1 g/s and Q S4 =4.92 dm 3 /s. These two very different flow rates indicate that the air flows in the condensation chambers of ranks 1-4 have progressively shifted from a turbulent regime to a laminar regime. This leads to very different surface areas for the holes in the distribution plates of ranks 1 to 4. Moreover, this leads to an unobvious additional justification for the exclusive use of heat exchangers having hollow plates in the water distillation apparatus according to the invention. 
     Indeed, in the case of dry air, thermal exchange coefficients depend on:
         the velocity of the air (exchanges increased with speed);   a characteristic dimensions of the exchanger (the smaller this dimension is, the greater the exchanges): in the case of an assembly of hollow plates, it is the thickness of the air channel between two plates; for a bundle of tubes, it is the diameter of the tubes; and, in general terms, channel thickness is considerably smaller than the diameter of the tubes;   the physical characteristics of the air (viscosity and thermal conductivity) which depend slightly on temperature.       

     In the case of hollow plates, below a certain airspeed, flow changes from one regime to another (from turbulent, it becomes laminar) and the coefficient of exchange no longer depends on velocity but remains relatively high since the characteristic dimensions is small. This is not the case when we are dealing with bundles of tubes, where the transition from one flow regime to another is less sharp and the characteristic dimension is larger than in the case of plates. 
     With saturated damp air, operating under an evaporation or condensation regime, the apparent thermal exchange coefficient depends on the exchange coefficient for dry air defined above but, additionally, the diffusion mechanism which intervenes in the evaporation or condensation process considerably amplifies this exchange coefficient, and the coefficient of amplification is substantially proportional to the rate of increase in saturated vapor pressure as a function of temperature. As a consequence, in the hottest condensation chamber of the distillation apparatus (rank N), where the equivalent flow rate of dry air (and consequently its speed) is small, a heat exchanger using hollow plates will have significantly better performance than one using a bundle of tubes; and this will be increasingly true as the surface area per unit volume S/V increases. The same will apply more or less for the condensation chambers which are cooler of the distillation apparatus. 
     According to the invention, water distillation apparatus of the type in which:
         the N distillation units are located one on top of the other, forming two evaporation and condensation columns;   and a fan is installed at a lower portion of the evaporation column;       

     is characterized in that:
         the N evaporation chambers include thin planar supports, having wettable or hydrophilic surfaces installed vertically at a constant pitch;   the N communication paths between pairs of chambers are N horizontal rows of vertical slots, with the same pitch as the preceding one, formed in a partition separating the two chambers:   fan control is preset and the respective cross sections of passage of the rows of slots are fixed,   the setting of the fan control and the cross-sections of the slots have been established in accordance with design specifications arising from final manual adjustment of the first piece of apparatus of a series of identical water distillation apparatuses.       

     In an alternative embodiment, in water distillation apparatus of the type in which:
         the N distillation units are located one on top of the other, forming two evaporation and condensation columns;   the N evaporation chambers are filled with artificial nuts which are wettable, loosely packed,   a fan is installed at a lower portion of the evaporation column;       

     it is characterized in that:
         the N communication paths between the pairs of chambers are ducts of rectangular elongate section, having upstream a perforated horizontal area, adapted to retain said nuts and to allow air to pass, and having downstream, said horizontal elongated rectangular window;   in each of the N ducts, there is installed a droplet separator, arranged immediately downstream of the perforated area followed by a partition with a horizontally elongated rectangular slot;   control of the fan is preset and the respective widths of the N horizontal slots are fixed,   the setting of this fan control and the widths of the slots have been established in accordance with design specifications, resulting from final manual adjustments of the first piece of apparatus of a series of identical distillation apparatuses.       

     According to the invention, a method to maximize performance of the first of a series of one or the other of these two distillation apparatuses includes the following additional steps:
         adjust the fan speed to cause the volumetric flow rate of saturated damp hot air Q S1  previously calculated to circulate in closed or open loop in the distillation stage of rank 1, in order to raise the temperature of this stream of air at an upper portion of the evaporation chamber of this stage, to the optimal approximate predetermined value T A1 ;   successively adjust the adjustable cross-sections of the communication paths established between the evaporation and condensation chambers of the distillation stages of ranks 1 to (N−1), to respectively bring the temperatures of the air streams at an upper portion of the evaporation chambers of distillation stages of ranks 2 to N, to the predetermined approximate optimum values T A2  to T AN ;   maximize the temperature T E1  of the water leaving the condensation column, in order to determine constructional specifications for the fan and for these (N−1) port cross-sectional areas, (a) by making respectively equal the temperatures T A1  to T A(N-1)  of the saturated damp hot air streams, entering the evaporation chambers of distillation stages of ranks 2 to N, and the temperatures T A1*  to T A(N-1)*  of the saturated hot damp air streams leaving the condensation chambers of these same distillation stages, by means of slight correction of the previously adjusted port cross-sectional area of the (N−1) communication paths; and (b) by readjusting the air flow rate previously produced by the fan, in particular by a small correction of a frequency of the supply voltage of the motor, when it is of the synchronous type.       

     The implementation of the method according to the invention can begin as soon as the number N of stages of distillation and the N temperatures T A1  to T AN  have been selected, as soon as the hot water has been distributed and spread for a certain amount of time (&gt;1 hour, in view of the thermal inertia of the apparatus), the fan is causing the air to circulate in an open or closed loop, and the partition openings are half closed. This first adjustment is terminated as soon as the intended temperature T A1  is reached. The mass flow rate of dry air Q A1 , included in the flow rate of air supplied by the fan, is now, just like T A1 , at an approximated optimal value. When circulation is taking place in a closed loop, the temperature T E0  of the cold water and the initial state of the port cross-sectional areas of the openings in the partition determine the initial low-temperature T A0  of the air. In this case, T A1  is a provisional predetermined temperature, obtained at the end of this first adjustment step for the flow rates of the air streams circulating in the evaporation chambers. 
     The second step in the method according to the invention employs means for adjusting cross-sections for air passage. We start with the lowest opening in order to adjust the temperature T A2  of the air stream in the upper portion of evaporation chamber of rank 2. Once this adjustment has been done, we do the same for the evaporation chambers of increasing rank from 3 to N. The operation is restarted at least one second time in order to correct variations in the temperatures T A1  to T AN  inevitably brought about by the successive adjustments, since the mass flow rates of dry air Q A1  to Q AN  and their temperatures T A1  to T A1  are independent. In the case of closed-loop air circulation, the low temperature T A0  of the air being blown in at the lower portion of the evaporation column becomes progressively closer to T E0 , the initial water temperature. 
     The third step in the method according to the invention has the aim of respectively bringing to equal temperatures, the temperatures T A1  to T A(N-1)  and T A1*  to T A(N-1)*  of the layers of air circulating at the lower portion of the evaporation and condensation chambers of the (N−1) highest superposed distillation stages. When the air temperature, at the lower portion of a condensation chamber of a given rank is different to that of the lower portion of the evaporation chamber of the same rank, it is not known whether the mass flow rate of dry air concerned, Q A1  to Q AN  at that point is a bit too high, or too low. In practice, we start with the highest stage and increase or reduce the flow rate of air circulating in the evaporation chamber of rank N, by slightly and carefully increasing or decreasing the port cross-sectional area of the communication path between the chambers concerned. It is necessary to wait a short while to see whether this action has resulted in the two temperatures concerned coming closer together. If this is the case, one continues up until the point where they are equal. Otherwise, one does the opposite. If this is not sufficient to obtain the desired equality, the best adjustment is noted and this is supplemented by adjusting the port cross-sectional area of the communication path delimiting the upper portion of the distillation stage of rank (N−1) up until this equality of temperatures is found. One then proceeds in the same fashion for this distillation stage of rank (N−1), adjusting the port cross-sectional area of the opening delimiting the stage of rank (N−2) and then returning to the two openings of higher rank, in order to maintain temperature equality at the bottom portion of the evaporation and condensation chambers of the distillation stages of ranks (N−1) and N. The procedure is renewed for all the stages of lower rank, up to the stage of rank 2. Next, it is a priori necessary to adjust the air flow rate produced by the fan in order to maximize T E1 , the water temperature at the outlet from the condensation column. 
     With air circulating in a closed loop, we will have finally minimized as much as possible the temperature of the air T A0 * in the lower portion of the condensation column by causing it to approach T E0 , the temperature of the water entering therein and, at the same time, we will have maximized as much as possible the temperatures T AN  of the air and T E1  of the water at the top portion of this column, by bringing both of them close to the temperature T E2  of the water being distributed and spread. The air temperatures at the lower and upper portion of the distillation stages would then have optimal set point values T A0C  to T ANC , slightly different to the initial theoretical optimal temperatures T A1  to T AN , which correspond to the values adopted for the three terms Q E0 , T E2  and T E0  of one particular piece of distillation apparatus. In the N stages, the representation f(T) of enthalpy fluxes of mass flow rates of water comprise two parallel straight lines having a slope Cp E , separated by a number of degrees (T E2 −T E1 ), and the representation f(T) of enthalpy fluxes of the air comprise, between these two straight lines, one single line formed festoons having successive extremities T A0C  to T ANC , the mean slope of this line being slightly greater than that of these two straight lines. The relations between the extreme temperatures of the water and air are: (T E1 −T E0 )=(T E2 −T E3 )&lt;(T ANC −T A0C ). 
     To finish, we can note that, in view of the high thermal inertia of the various components of the apparatus, all the operations for implementing the method for maximizing performance of the first piece of distillation apparatus of a series can take several hours, this number of hours being itself directly proportional to the number N of stages. 
     Under these conditions, by carrying out the method according to the invention, experience has shown that with water distillation apparatus having four distillation stages, constructed in accordance with the invention, it is possible to obtain a coefficient of performance (CoP)&gt;6, for a daily flow rate of distilled water produced which is equal to 3 to 5 times the total volume of the evaporation and condensation chambers of the apparatus. 
     According to the invention, the first model of a second type of improved water distillation apparatus having the general architecture of the new water distillation apparatus defined above and having N distillation units placed one above the other and forming two columns, one for evaporation and the other for condensation; 
     is characterized in that
         these N distillation stages are separated from each other by (N−1) horizontal partitions;   two communication paths, one at an upper portion and one at a lower portion, are established between the vertical evaporation and condensation chambers of each distillation stage;   N variable-speed fans, notably achieved through the use of synchronous motors external of the condensation chambers have their blades installed in lower communication paths of the N distillation stages in order to produce N independent air streams respectively circulating in these N stages;   the sets of hollow components of the condensation chambers of these N distillation stages are connected together by tubes passing through these (N−1) horizontal partitions;   at an upper portion and a lower portion of the hydrophilic or wettable components of the evaporation chambers of the N distillation stages there are respectively installed means for distributing and spreading and for collecting the water circulating in the evaporation column, the means for distributing and spreading of evaporation chambers of ranks (N−1) to 1 being respectively fed by the collection means of the chambers of ranks N to 2.       

     According to the invention, the second model of this second type of improved water distillation apparatus having the general architecture of the new water distillation apparatus defined above, is characterized in that:
         the N distillation units are juxtaposed;   two communication paths, one at an upper portion and one at a lower portion, are established between the vertical evaporation and condensation chambers of each distillation unit;   N fans having adjustable flow rates, notably through the use of synchronous motors external to the condensation chambers have their blades installed in lower communication paths of the N distillation units to produce N independent air streams respectively circulating in these N units;   the N hollow heat exchange devices of these N condensation chambers are interconnected by (N−1) thermally-insulated conduits;   the N sets of wettable or hydrophilic components of the N evaporation chambers include at an upper portion and lower portion of their respective ends, means for distributing and spreading and means for collecting water to be distilled;   (N−1) pumps and (N−1) thermally-insulated conduits are installed between the N evaporation chambers, for causing the water collected by the collection means of the distillation units of ranks N to 2, to be discharged into the means for distributing and spreading of the units of ranks (N−1) to 1.       

     According to the invention, a method to maximize performance of the first piece of apparatus of a series of one or the other of the two improved water distillation apparatuses of the second type, is characterized in that it includes the following additional steps:
         adjust successively the N flow rates of air respectively produced by the N fans in order to generate at an upper portion of the N distillation units approximated predetermined optimal temperatures T A1  to T AN , corresponding to the selected values Q E0  and T E2 ;   then, correct slightly these N flow rates to maximize T E1 , the temperature of the water leaving the heat exchanger of rank N, and display optimal setpoint temperatures T A1C  to T A4C ;   read the final control settings for the N fans in order to notably make therefrom the specifications for series-production distillation apparatuses.       

     With these latter provisions, one can construct two particularly useful novel water distillation apparatuses adapted to treat water using fixed inlet parameters: mass flow rate Q E0  and high temperature value T E2 . Indeed, the N distillation units are substantially identical, except, however, that the rows of holes of the perforated trays are different from one apparatus to another and the N fan controls are set differently. Such water distillation apparatus employing juxtaposed distillation units is of considerable interest when daily production of distilled water is large (&gt;10 m 3 /day). Indeed, it does make it possible to build distillation units which are identical, of medium height, easier to handle than multi-stage towers having the same number of distillation units. 
     In the case where both parameters Q E0  and T E2  are variable (solar energy), an interesting situation also exists. Indeed, as noted above, for any group of input parameters Q E0 , T E2 , T E0  and T A0 , as soon as temperatures T A1  to T AN  have been set, it is easy to calculate Q A1  to Q AN  as well as Q S1  to Q SN  and thus the operating conditions of the fans. When using fans with synchronous motors, one thus determines the frequencies F 1  to F N  of their supply voltages. This implementation of the method according to the invention, in order to maximize the performance of water distillation apparatus having N juxtaposed units, is particularly easy, using a computer and establishing correspondences between the frequencies F 1  to F N  and setpoint temperatures T A1C  to T ANC  to be obtained, as soon as the four values Q E0 , T E2 , T E0  and T A0  are input. To do this, an experimental database is set up, by performing at least three operations to get optimum settings for each of the four input parameters Q E0 , T E2 , T E0  and T A0 . Then we develop software that associates these data to the N optimal set point temperatures T A1C  to T ANC  and to the N frequencies F 1  to F N  of motor supply voltages. These frequencies will be determined by computer, from the values chosen for Q E0 , T E2 , T E0  and T A0 , and then manually or automatically adjusted. 
     In order to be able to properly dimension the evaporation and condensation chambers of the N distillation units of the different types of water distillation apparatus discussed above, according to the desired flow rate of distilled water from the apparatus to be constructed, it is necessary to determine their thermal conductance CT (watts per degree) and/or their thermal resistance RT (degrees per watt). To do this, they are calculated. This is done as a function of the respective components and architectures contemplated for construction of the two columns of the distillation apparatus. One then calculates the thermal conductance of an elementary slice of the evaporation and condensation chambers. These results are then integrated over the height of each of the two chambers of each one of the N distillation stages, in order to finally draw up charts for thermal conductance and resistances of the two columns. The accuracy of these calculations is estimated to be 10%. Having chosen the structures of the two columns, the next step is to respectively provide appropriate thermal resistances to the two chambers of the N distillation stages. This is done in accordance with equal enthalpy fluxes ΔH E  and ΔH A  to be exchanged in the two chambers of a given distillation stage. From one stage to another, these fluxes are, or are not, equal. The same applies to the thermal resistances of the two chambers of a distillation unit, in line with practical considerations imposed by the geometry of the apparatus. 
    
    
     
       The characteristics and advantages of the invention will become clearer from the following description, made with reference to the accompanying drawings in which: 
         FIG. 1  is a diagram of water distillation apparatus with four distillation stages, notably for desalinating seawater; 
         FIG. 2  is a diagram of similar water distillation apparatus for concentrating polluted process water; 
         FIG. 3  shows the means for adjusting the effective cross-section of opening of a partition; 
         FIG. 4  shows curves for enthalpies of air and water circulating through water distillation apparatus according to the invention; 
         FIG. 5  shows a distribution plate for the air in the condensation chambers; 
         FIG. 6  is a diagram of water distillation apparatus with four juxtaposed distillation units according to the invention. 
         FIG. 1  shows schematically the first piece of apparatus of a series of seawater distillation apparatuses according to the invention. It consists of a tower  10 , having insulating walls  12 , 280 cm high, which includes two columns, one being evaporator column  14  and condensation column  16  being the other, separated by a partition  18  in polymer material. The set of components of evaporator column  14  is constituted by a juxtaposition of forty plates  20  in polymer, 2 mm thick, 240 cm high and 20 cm wide, arranged at a pitch of 15 mm. The total evaporation surface of the column  14  is 38.4 m 2  and its volume is 0.29 m 3 . The plates  20  are coated with a hydrophilic flocking, 0.5 mm thick, and they are juxtaposed vertically, the distance between the plates being 12 mm. The condensation column  16  is 240 cm high and it includes, connected in series by 5 cm tubes, six heat exchangers  22   1-6 , 35 cm high and 20 wide, formed by juxtaposition at a constant pitch (10 mm), of forty rectangular hollow plates  23 , installed vertically. These hollow plates  23  are blowmolded and have thin embossed walls (&lt;1 mm) in polymer and an inner thickness of 3 mm, the distance between plates being 5 mm. The connection heads of these assembled hollow plates are welded to form the upstream and downstream manifolds of the heat exchanger so formed. These hollow plates and these heat exchangers are described in International Patent Application publication WO2011/145065 A1, filed by the applicant. Above each heat exchanger there is installed a distribution plate with a row of holes  25   14 , adapted to equalize the partial air flows circulating between the hollow plates. A particular embodiment of such a plate is shown in  FIG. 5 . The total condensation surface of the column  16  is 29 m 2  and the total chamber volume is 0.19 m 3 . 
     
    
    
     The partition  18  is 240 cm high and includes two wide openings: one  24  at its top and the other  26  at its base. The partition  18  further includes three identical intermediate openings  28 ,  30  and  32 , and each of these openings is a horizontal row of vertical slots, 10 cm high and 2 mm wide, having the same pitch (15 mm) as plates  20  of evaporation column  14 . Such a row of slots is shown in  FIG. 3 . These three rows of slots have an effective cross-section which can be adjusted, thanks to the use of covers which are also shown in  FIG. 3 . The bottom of opening  32  is located 2 m from the floor of the column, and the bottoms of openings  30  and  28 , at 1.6 m and 0.8 m therefrom. From this, the heights of the four distillation stages of tower  10  can be deduced. The two stages at the top of tower  10  each include a heat exchanger  22   6 ,  22   5  and the two bottom ones, two exchangers  22   4 - 22   3  and  22   2 - 22   1  each. Polymer conduits, which connect the manifolds of these heat exchangers  22   1-6  face the three openings  28 ,  30 ,  32  of the partition  18 . The respective thermal resistances of the evaporation and condensation chambers of the four distillation stages, thus arranged in the tower  10  were calculated as described above. 
     The opening  26 , arranged at the lower portion of the partition  18 , is equipped with a fan  34  for drawing in air from the bottom of the condensation column  16  and propelling it upward in evaporation column  14  (arrow  36 ). The electric motor of the fan  34  is for example of the synchronous type and is powered by a variable frequency voltage. The graph giving the maximum flow rate and pressure of the air supplied by the fan  34 , as a function of supply frequency, is available. The upstream manifold of heat exchanger  22   1  of condensation column  16  is connected by a polymeric conduit  38  to a storage tank providing it with seawater to be distilled, at an appropriate mass flow rate Q Eo . This seawater constitutes the cold source of the apparatus. The downstream manifold of heat exchanger  22   6  of condensation column  16  is connected by a conduit  39  in polymer to the inlet of water heater  40 , equipped with heating means, constituted by a heat exchanger  42 , similar to exchangers  22   1-6 . This exchanger  42  is supplied by two upstream and downstream pipes  44 - 46 , in which a heat transfer fluid circulates, supplied by an external primary hot heat source (not shown). The water heater  40  has an outlet weir  48 , from which hot seawater  50  trickles, entering a basin-like member  52 . This basin-like member  52  has its bottom perforated with forty pairs of collared holes, corresponding to forty distribution troughs (not shown) covering an upper portions of the forty plates  20  of evaporation column  14 . At the base of condensation column  16 , there is installed a container  54  for collecting the distilled water  56 ; this container  54  is provided with walls in the form of a widely-opening V, and an evacuation conduit  58 . Evaporation column  12  and tower  10  include, at their common base, a container  60  for collecting the concentrated seawater, this container  60  being provided with an evacuation conduit  62 . Arrows  64 - 66 - 68  represent the ascending air streams in the evaporation column, and the arrows  70 - 72 - 74  show the air diverted through openings of the partition  18 . The arrows  76 - 78 - 80  represent air streams descending down condensation column  16  and arrow  82 , the stream entering the column  16 , through opening  24  arranged at an upper portion of the partition  18 . 
     In this first piece of apparatus  10  of a series of identical distillation apparatuses several thermocouples measuring the temperature T A1  to T A4  and T A1*  to T A4*  defined above, are installed at significant locations of the distillation apparatus; they are represented by black dots with a white center and are connected to a digital conversion circuit and display, not shown. Two thermocouples, respectively installed in hollow metal inserts sealingly fitted into holes through the wall of the inlet conduit  38  and outlet conduit  39  of condensation column  16  measure T E0  and T E1  and a third thermocouple in water heater  40  measures T E2 . Five thermocouples, respectively located in front of upper opening  24 , lower opening  26 , and intermediate openings  28 ,  30 ,  32  of the partition  18 , measure T A0  and T A4 . Three more thermocouples, located respectively at the lower portion of heat exchangers  22   3 ,  22   5 ,  22   6  and adjacent the outer wall of the condensation column  16 , measure T A1* , T A2* , and T A3* . Three thermocouples, respectively fitted into metal inserts pass through the wall of the conduits connecting the heat exchangers  22   2 - 22   3 ,  22   4 - 22   5  and  22   5 - 22   6 , measuring at intermediate openings  28 ,  30 ,  32 , the temperatures of the water ascending in the condensation column. 
       FIG. 2  is a simplified representation, partially schematic, of the first piece of apparatus  100  of a series of identical water distillation apparatuses having four-stages according to the invention, adapted to concentrate industrial water (polluted effluent or food industry aqueous solutions). In order to produce a highly concentrated liquid, this distillation apparatus operates with water circulating in a closed circuit and air circulating in open circuit. The ambient air constitutes the cold source of the apparatus. According to  FIG. 2 , the water distillation apparatus  100  is enclosed in a cabinet which is 240 cm high, 100 cm wide and 50 cm deep and has an insulating outer wall  102 , of polymer material foam, 10 cm thick. The distillation apparatus  100  has two columns, one being evaporation column  104  and the other condensation column  106 , separated by a partition  108 , 15 cm thick and 220 cm high. The evaporation column  104  has a square section of 25 cm side and includes an oblique bottom  110 , perforated with openings of 1 cm diameter, staggered, with a distance separating them of 2 cm, a 3 cm strip at the bottom not being perforated. The outer face of bottom  110  is smooth and its inner face includes a 5 mm high collar around each opening. On the bottom  110  wettable artificial nuts are loosely stacked up to a level  112 , near an upper portion of the partition  108 . These nuts may be 12 mm long ceramic or plastic NOVALOX® saddles. Beneath oblique perforated bottom  110 , there is a chamber  114  having a circular opening occupied by a fan  116 , having adjustable air-flow, capable of producing an excess pressure of several hundred Pascals. In a lower imperforate strip of oblique bottom  110 , the vertical portion of a conduit  118  for evacuation of concentrated aqueous solution discharges. 
     Condensation column  106  includes four heat exchangers  120   1-4 , having hollow rectangular plates  121  and upstream-downstream manifolds, similar to the exchangers  22   1-6  of  FIG. 1 . These exchangers  120   14  are connected in series, with intervals of 5 cm between their manifolds, and above each heat exchanger there is installed a distribution plate having rows of holes  123   1-4 , similar to the distribution plate  25   14  in  FIG. 1  and the distribution plate  190  in  FIG. 5 . Underneath heat exchanger  120   1  there is provided a chamber  122  having a window  124 , opening to the outside, and a concave bottom  126 , with an evacuation conduit  128 . The downstream manifold of heat exchanger  120   4  is connected, via a pipe  130 , to the inlet of water heater  132 , which is constantly filled with polluted water. In this water heater  132 , there is fixedly mounted a heat exchanger  134 , identical to heat exchangers  120   1-4 . This heat exchanger  134  is provided with an upstream pipe  136  and a downstream pipe  138 , connected to the outlet and to the inlet of an external heat source, which supplies the heat exchanger  134  with hot fluid at an appropriate temperature and flow rate. Water heater  132  is provided with a weir  140 , by which hot water flows into a spreading basin-like member  142 , resting on the top  112  of the stack of wettable artificial nuts filling the evaporation column  104 . The bottom of basin-like member  142  is perforated with numerous holes having upstanding overflow collars, arranged in staggered fashion, these holes being 3 mm in diameter and 2 cm apart. 
     The partition  108  is of expanded polymer. The lowest portion of this partition  108  does not include any fittings and the remaining part thereof is occupied by four independent transit chambers  144   1-4 , provided with elongated rectangular inlets, formed by rows of vertical slots, shown in  FIG. 3 . These slots have a width which is smaller than any dimension of the nuts packing evaporation column  104 . The height of a lower edge portion of the row of slots upstream of the transit chamber  144   1  is 65 cm and those of a lower edge portions of the other rows of slots  144   2-4 , are, respectively, 105, 145 and 185 cm. These heights are the heights of the ceilings of the evaporation chambers placed one on top of the other of column  104  of distillation apparatus  100 . 
     As air streams, deviated by the inlets of the transit chambers  144   1-4 , usually carry drops of industrial water, which is concentrated to a greater or lesser degree, these inlets include droplet separators, formed by baffles  145   1-4  and deflectors  146   1-4 , which are hook-shaped, and co-operate to trap water droplets and bring them back into evaporation column  104 . Beyond these baffles, lines are arranged leading to rotary valves  148   1-4  then to the intervals  150   1-4  which separate heat exchangers  120   1-4  and water heaters  132 . Inside rotary valve  148   1-4 , rotating cylinders, with diametrically opposed longitudinal openings (black in the figure) are equipped with manual control means, not shown. Rotary valve  148   4  is opened to the maximum and the pressure drop created by chamber  144   4  is negligible. 
     The conduit  118  for evacuating concentrated contaminated water produced by the distillation apparatus  100  is connected to a device  119  for natural cooling of the water, which discharges above a storage tank  152 . This tank  152  has two outlet pipes, one of which  154 , is connected to the inlet of a pump  156 , and the other of which  158 , is provided with a solenoid valve  160 , for discharging this concentrated water to a industrial storage cistern. To close the loop of water circulation in the distillation apparatus  100 , a pipe  162  connects the outlet of the pump  156  to the upstream manifold of heat exchanger  120   1  of condensation column  106 . The industrial apparatus which is producing the polluted water to be concentrated is connected to the distillation apparatus  100  through a conduit  164  discharging above storage tank  152 , this conduit being provided with a solenoid valve  166 . Like the distillation apparatus  10  in  FIG. 1 , thermocouples which are not shown are installed at different points in distillation apparatus  100  in order to measure significant temperatures of air and water circulating therein. 
     When, in distillation apparatus  100  according to  FIG. 2 , industrial water circulates in a closed circuit and air in open or closed circuit to perform concentration of dissolved solids in the water, it is necessary to use a device for cooling  119 , preferably natural cooling, to lower the maximum initial temperature T E3  of the concentrate to be discharged into the storage tank  152  and to make thereof one, or the, cold source of the apparatus. If this cooling is not sufficient for bringing the concentrate down to a temperature T E0  lower than the temperature T A0  of the air stream entering the evaporation column or a temperature T A0*  of the outgoing air stream from the condensation column, the coefficient of performance (CoP) of the distillation apparatus will be directly affected. The more the recycled concentrated liquid is cooled, the smaller will be the deterioration in CoP. Maximum natural cooling of this lukewarm concentrate can be obtained when it is discharged at an upper portion of a set of vertical supports, having hydrophilic or wettable surfaces, then cooled during its descent, by natural convection or forced convection of ambient air. When the dew point of the ambient air is sufficiently low, the final temperature T E0  of the liquid concentrate to be recycled is, as shown in  FIG. 4 , less than T A0 , the initial low temperature of the air injected at the foot of the evaporation column. 
       FIG. 3  shows a portion of a horizontal row  170  of vertical slots  172 , formed in the partition  18  of the distillation apparatus  10  of  FIG. 1  and at the inlet of a transit chamber  144   1-4  of the partition  108  of the distillation apparatus  100  of  FIG. 2 . This  FIG. 3  also shows the sliding cover  174  disposed in front of each row of slots in the partition  18  of the distillation apparatus  10 . These slots  172  are 10 cm high, 2 mm wide and have a pitch of 15 mm. The cover  174  includes a row of slots having a width identical to the width of the slots of the row  170  and, preferably, a height which decreases with the rank of the relevant distillation stage. This cover  174  is integral with a threaded shaft  176 , rotatably mounted in a fixed nut, not shown. 
       FIG. 4  shows the curves H E =f(T) for the enthalpy flux of the water circulating in open circuit, and the curves H A =f(T) for the air circulating in closed circuit, in a distillation apparatus  10  in which all adjustments have been made correctly. The straight lines  180  and  182  respectively represent enthalpy fluxes, increasing and decreasing, of the streams of rising cold water and descending hot water. These straight lines are parallel and their slope is Cp E . The temperatures at the ends of these two lines  180 - 182  are respectively T E0 , T E1 , T E2 , and T E3 . Curve  184  represents both the enthalpy flux of streams of rising and falling saturated damp hot air circulating in distillation apparatus  10 . Curve  184  has four festoons which depart from a straight line,  188   1-4 , corresponding to the four stages of distillation of apparatus  10 . The temperatures at the ends of these festoons  188   1-4  are respectively T A0c , T A1C , T A2c , T A3C  and T A4C . Between successive sections of the festoons  188   1-4  and the two rising and falling straight lines  180  and, respectively,  182 , the horizontal distances between them are representative of the various apparent thermal resistances of the corresponding sections of evaporation column  14  and condensation column  16  of the distillation apparatus. 
       FIG. 5  shows a perforated tray  190 , made of polymer material, surmounting one of the condensation chambers of the water distillation apparatus according to the invention. The heat exchanger of this chamber is formed by the juxtaposition and the putting in parallel of two identical devices having 24 hollow plates, 20 cm wide assembled on a 10 mm pitch with 5 mm spacings. Perforated tray  190  is 40 cm long, 33 cm wide and 3 mm thick; it includes four rows  192   1-4 , each with 25 holes separated by 10 cm intervals. Each row has 23 oblong holes, such as oblong hole  193 , these being 6 mm wide and 12 mm long, with at the end two round holes, 6 mm in diameter. The oblong holes  193  are arranged above spaces separating the hollow plates and the round holes arranged above the spaces between the end plates and the walls of the condensation chamber. The perforated tray  190  has three edge portions  194   1-3 , 30 mm thick and 50 mm high, to seal the air duct between two condensation chambers located one on top of the other, and two apertures  196   1-2  to enable sealed passage of the two tubes providing connection between the two heat exchangers of each condensation chamber. 
     In discussing operation of the first piece of apparatus  10  or  100  of one or the other of the two series-production distillation apparatuses concerned, it is considered that distillation apparatus  100  is, initially, virtually simplified and arranged to operate in the same way as distillation apparatus  10 . The fundamental data are the mass flow rate Q E0  of water that is spread and distributed and water temperatures T E0 , T E2  and air temperature T A0 . For example, Q E o=100 g/s and T E2 =85° C., for both pieces of apparatus. As distillation apparatuses  10  and  100  both operate with closed loop air streams and open loop water streams, then we have T E0 =20° C. and then T A0 =27° C. and T A4 =83° C. From these data, we can carry out the adjustment procedure described in detail above for the fan and the cross-sections of the openings in the partition. We start by determining the temperatures of the air streams we need to establish at the rows of slots  28 ,  30 ,  32  of partition  18  of the distillation apparatus  10  and the similar rows of partition  108  of the distillation apparatus  100 . With extreme temperatures of 27 and 83° C., we will have successively (approximately) 45, 61, and 73° C. and local steps of substantially 18, 16, 12 and 10° C. Next, we spread and distribute hot water, for a fairly long period to bring the components of each piece of apparatus to a suitable temperature, and then turn on the fan  34  or  116 . As evaporation column  16  is provided with vertical plates, having wettable or hydrophilic surfaces juxtaposed on a constant pitch and column  104  having wettable artificial nuts loosely packed, the passages offered to air streams and the pressure drops therein are very different. Under these conditions, the same thing applies to the excess pressure of the air to be produced by the fans: from 100 to 200 Pascals for the fan  34  of the distillation apparatus  10 , and from 300 to 500 Pascals for fan  116  of distillation apparatus  100 . Indeed, in the distillation apparatus  10 , the ascending air streams have uniform velocities and are unlikely to be carrying water droplets, which enables the rows of vertical slots  24 ,  26 ,  28 ,  32  to directly open into the condensation column  16 . However, the same thing does not apply in the distillation apparatus  100 , since here the air streams can have significant local velocities carrying along droplets of water and brine, notably in the row of slots arranged upstream of the entrance to transit chamber  141   1  of the distillation stage of rank 1 These carried-along drops of water and brine necessitate the presence of droplet separators  145   1-4 - 146   1-4 , that trap and return them to the evaporation chambers. 
     The flow rates of the fans  34  or  116  are adjusted to temporarily bring the temperature T A1  of the air passing through the row of slots of rank 1 of partitions  18  or  108  of distillation apparatuses  10  and  100  to 45° C. Then, using the covers  174  or rotary valves  148   1-3  we adjust the port cross-sectional area of the openings  18  of the partition concerned of the distillation apparatus  10  or those of the partition  108  of the distillation apparatus  100  to have temperatures T A2 =61° C. and then T A3 =73° C., at the tops of the evaporation chambers of ranks 2 and 3. Then we re-adjust the settings of covers  174  or rotary valves  148  of rank n, so that the temperature T An  at the entrance to the evaporation chamber of rank n becomes equal to the temperature T An*  at the outlet of the condensation chamber of the same rank. These temperatures then constitute optimal setpoints T A1C , T A2C  and T A3C , which only differ from the initial temperatures by a difference &lt;1 5° C. After this, we slightly correct the speed of the fan to maximize the temperature T E1  of the water at the outlet of the condensation column and to bring it as close as possible to T E2 , up to T E1 =0.75° C. for example. 
     Having done this, container  54  or  126  receives an amount of 40 liters per hour of distilled water and with the values given above for T E0 , T E1  and T E2 , the coefficient of performance (CoP)=6.5. The thermal power supplied to the apparatus by the water heater is P ch =3.8 kW and that used in distillation P Dist =24.7 kW, whereby the apparent overall thermal conductance of the evaporation and condensation columns of the distillation apparatus  10  or  100  is given by CT=P Dist /(T E2 −T E1 )=2470 W/K. 
     Then, the values of the settings of the controls of fans  34  and  116  are noted as are the settings for the port cross-sectional area of the communication paths between chambers of the distillation units and these values are used as constructional specifications for series production apparatus. This will be frequency for the synchronous motors of the fans and values, accurate to one-tenth of a millimeter, for the four rows of vertical slots of the apparatus  10  and the horizontal slots of the four partitions replacing the valves which have been adjusted of apparatus  100 . As for the dimensions of the holes in distribution plates  25   14  and  123   14 , these are calculated for each condensation chamber, as a function of approximated volumetric flow rates of saturated damp hot air Q S1  to Q S4  circulating in these four chambers. 
     The thus constructed water distillation apparatuses are designed to operate with inlet parameters Q E0  and T E2  which are constant. As long as this is so, the coefficient of performance (CoP) of the apparatus is at a maximum, but when for any reason the value of one and/or the other of these parameters deviates somewhat from its initial value, the overall performance of the distillation apparatus is moderately lowered while nevertheless remaining satisfactory. 
     In the case of a simplified distillation apparatus  10  or distillation apparatus  100 , both treating seawater or brackish water, notably fossil, the distilled water is collected and converted into drinking water, by adding appropriate mineral salts, while seawater with a low salt concentration is directly discharged into the sea. 
     In the case of the distillation apparatus  100  described, operating to produce a concentrate of industrial water, the air stream circulates in an open or a closed loop and the water stream in a closed loop. The concentrate, pre-cooled in device  119  (which becomes the sole cold source when the air circulates in a closed loop) is collected in tank  152  and is recycled until its concentration is sufficient. To do this, the contaminated water, becoming progressively concentrated in the tank  152 , is drawn in by the pump  156  and injected into the manifold upstream of heat exchanger  120   1  of condensation column  106 . When this concentration is sufficient, pump  156  is stopped, the solenoid valve  160  for emptying the tank  152  is operated and a given volume of process water, highly concentrated (5-10 times), is stored in a remote cistern, awaiting collection. At the end of this operation, a fresh volume of industrial water is poured into the storage tank  152  by actuating the solenoid valve  166  for an appropriate period of time. Following this, pump  156  is restarted. The cycle of these operations is determined by trial, depending on the efficiency of the apparatus; solenoid valves  160  and  166  and pump  156  are controlled cyclically, by a programmed controller. The cost of getting rid of a concentrate of polluted water is reduced in correspondence with the degree of concentration achieved. For its part, the distilled water can be collected for local use or rejected into nature. Making use of the same supplementary subassemblies, a distillation apparatus  10 , having an evaporation column fitted with evaporation plates with wettable surfaces can be converted into apparatus to produce a concentrate of industrial waste water. And, vice versa, distillation apparatus  100  can be used to produce distilled water and highly concentrated brine. 
       FIG. 6  shows at A a diagram of water distillation apparatus  200  formed by four juxtaposed distillation units in front view,  202 ,  204 ,  206  and  208 , similar to the distillation stages of  FIGS. 1-2 , having respectively ranks of 4, 3, 2 and 1. At B,  FIG. 6  gives a schematic view of the distillation unit  208 , seen in section C-C. 
     In each of the evaporation chambers of these units, there are arranged vertically, at a constant pitch of 15 mm, one-hundred non-metallic plates  210  having wettable surfaces as a result of the presence of suitable reliefs, 6 mm thick, 80 cm wide and 125 high cm. The total surface area for exchange of the plates is 200 m 2  and the volume of an evaporation chamber is 1.5 m 3 . In each of the condensation chambers, the heat exchanger  212  is of the same type as those in  FIGS. 1 and 2  (hollow blow molded plates with upstream and downstream welded manifolds). Each heat exchanger  212  comprises one hundred and fifty hollow vertical plates  213 , 80 cm wide and 125 cm high, assembled on a pitch of 10 mm. The total surface area for heat exchange of the plates is 300 m 2  and the volume of each condensation chamber is 1.5 m 3 . The upstream coupling  214  of the heat exchanger  212  of the coolest unit  208  is connected to a connection point  216  for supplying seawater at ambient temperature (20° C.) and at an appropriate excess pressure. The downstream coupling  218  of this heat exchanger  212  is connected to a heat-insulated conduit  220  which leads to the upstream coupling of the heat exchanger of distillation unit  206 . The same applies to the units  204  and  202  in which the thermally-insulated conduits  222  and  224  connect the downstream coupling of the first of these to the upstream coupling of the heat exchanger of the second one of these. The downstream connection of the heat exchanger of the hottest unit  202  is connected to the inlet of a water heater  226  the outlet of which is connected to a basin-like member  228  feeding the individual spreading and distribution troughs (not shown) for the wettable plates  210 . The floor of the evaporation chamber of unit  202  is arranged so as to form a container  230  for collecting seawater, having increased salt content, which is running from its wettable plates  210 . In this container  230  there is installed a pump  232  connected by a thermally-insulated conduit  234  to the basin-like member  236  of the distributing and spreading device of unit  206  which includes a collection container  238  and a pump  240 . This pump  240  is connected by a thermally-insulated conduit  242  to the container  244  of the distributing device of the unit  206 , which includes a collection bin  246  and a pump  248 . This pump  248  is connected by a thermally-insulated conduit  250  to the basin-like member  252  of the distributing device of unit  208 , which includes a collection bin  254  provided with an evacuation conduit  256 . Just like the preceding ones, the basin-like member  252 , shown at B of  FIG. 5 , is provided with one hundred pairs of overflow weirs  253   a - b , feeding the distributing and spreading troughs of the one hundred plates  210 . On  FIG. 5B , above the condensation chambers there are arranged distribution plates having rows of holes  257 , similar to the distribution plates  25  and  123  of  FIGS. 1 and 2 . In a lower opening formed in a partition  258  separating the evaporation and condensation chambers of unit  208 , fan blades  260  are mounted, connected by a shaft  262  to the synchronous motor for a fan. This motor together with its variable-frequency power supply unit are represented by block  264 , external of the condensation chamber. This block includes either a manual control provided with a dial  266 , or automatic control driven by computer. The four units  202 ,  204 ,  206  and  200  are fitted with similar internal fan blades  260   1-4  and external motor units  264   1-4 . The power of these motors and/or the pitch of the fan blades are adapted to the operating conditions of each unit. The arrows  265  and  267  show the direction of closed-circuit circulation of the air currents produced by the fans. The distilled water is collected in a bin  268  installed at the foot of each condensation chamber. All these bins are connected to an evacuation pipe  270 . Temperature continuity between the juxtaposed distillation units is assured by the rising or descending stream of water passing from one unit to the other. 
     This distillation apparatus having four juxtaposed distillation units is well suited to large quantities of daily production of distilled water. With a power for water heater  226  of 120 kW and a mass flow rate Q E o of 4.5 kg/s of water to be distilled, daily production is around 30 m 3 /day for the distillation apparatus  200 . In addition the latter is notably well suited to situations in which the amount of thermal energy available is variable notably over the course of one day, thereby imposing mass water flow rates which more or less follow the same trends. In this case, a computer with the appropriate software defined above, supplies the adjustments for the motor blocks, once the parameters Q E0 , T E2 , T E0  and T A0  have been entered. These settings are then made in two possible ways: (1) read on the computer screen values for the setpoints of the dials associated with the controls for these four frequencies, and manually display these values on the dials or (2) program the computer to directly set the control means to these four frequencies. 
     The modifications to be introduced into the distillation apparatuses  10  and  100  using distillation units placed one above the other shown in  FIGS. 1 and 2  in order to transform them into pieces of apparatus in which the currents of air in the distillation stages are independent of each other are obvious from the very definitions of these pieces of apparatus. Between four stages, which are more or less identical to the one of rank 1 on these two figures, these will be horizontal partitions through which pipes connecting heat exchangers one to the other pass, supplemented, in each stage, by means for collecting and for spreading and distributing the water to be distilled, identical to those in  FIG. 6 . There is little point in providing a diagram illustrating the water distillation apparatus according to  FIG. 1  or  2  fitted with such everyday supplementary equipment. Further, it can be useful to combine these two embodiments and to constitute groups formed of two distillation units placed one above the other, and then to juxtapose these groups. This is suitable for medium-height (&lt;120 cm) distillation units that can be easily stacked without causing any particular problems in handling. 
     The invention is not limited to the embodiments described with reference to  FIGS. 1 ,  2  and  6 . 
     Indeed, the components of the evaporation and condensation chambers can be different from those presented (plates having hydrophilic or wettable faces, wettable nuts in ceramic or polymer material). In all cases, these components will be non-metallic in order to avoid problems of corrosion and too high cost. Regarding the evaporation chambers, these can be any thin planar support, notably formed by a stretched woven hydrophilic sheet, or yet again, artificial nuts of baked clay. Regarding the condensation chambers, one can use heat exchangers which are formed by commercially-available alveolar plates provided with primary manifolds, connected to secondary upstream and downstream manifolds. In distillation apparatus  200 , using juxtaposed distillation units, the evaporation chambers can be filled with wettable artificial nuts and communications between the pair of chambers of each unit can include droplet separators. In addition to standard distillation apparatus which is manufactured pre-set, apparatus which is identical to a first one of each series, including the majority of the measurement and adjusting means can obviously be put on the market in order to satisfy individual customers.