Abstract:
The inventive device is embodied in the form of a chamber-oven for diffusing vapour and saturated hot air which circulate in a closed circuit by natural convection. Said device is embodied in the form of a domestic-use solar energy collecting device provided with a greenhouse whose surface is equal to 1 m 2  and produces from 50 to 100 litres/day of distilled water. The device comprises a distillation unit arranged between two furnaces ( 59′, 79 ′) in a temperature-controlled container ( 48 ′). Said distillation unit comprises 100 flat thin hollow plates having a surface of 20 dm 2  by face and an active volume of 200 dm 3 . The fine and tensioned walls ( 54 ) of said plates are provided with a hydrophilic coating ( 60 ′) and internal ( 56 ′) and inter-plate ( 58 ′) spaces. The lower chimney ( 59 ′) comprises a greenhouse ( 118′, 119 ′) whose bottom is embodied in the form of an impermeable black layer provided with a thin hydrophilic carpet on the rear part thereof. Saturated hot air at a temperature of 80° C. enters inside ( 56 ′) hollow plates from bellow and exits from the top at a temperature of 50° C. A high chimney ( 79 ′) is provided with a monoblock heat exchanger ( 84 ′) which is transversed by a non-potable water to be distilled which, afterwards is spread warm (40° C.) over the hydrophilic coating ( 60 ′). During passage through the heat exchanger ( 84 ) the air is cooled to 30° C. and moved down by gravity to the inter-plate spaces ( 58 ′) and exits therefrom at a temperature of 78° C. The distilled water condensed in the plates and by the heat exchanger is collected and removed. Brine is received in the bottom of the inter-plate space and distributed along the thin hydrophilic carpet of the bottom ( 122 ′) of the greenhouse. An air current passes along said hot carpet is heated and saturated and enters the plates. The brine liquor finally flows in an air-preheating tank ( 63 ′) which is emptied each morning. The greenhouse can be substituted by a heating tube transversed by a heating fluid or associated with another steam-jet tube. The more powerful chamber-ovens can produce at least 200 m 3 /day of distilled water for collective consumption. Said invention can be used for salt removal from seawater, co-generating electricity and potable water and for producing food concentrates.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention, resulting from the collaboration of Jean-Paul DOMEN and Stéphane VIANNAY, relates to improvements to a prior invention of the first-named, relating to distillation methods and devices, described in an international PCT Patent Application, filed by the applicant and published on 20th Dec. 2001 under No. WO 01/96244 A1. This PCT Application describes a general multiple-effect distillation method, intended to separate substances in solution from their liquid solvent, as well as two methods and particular stills.  
         [0002]     These distillation methods and devices are chiefly intended for producing fresh water, easily transformable into potable water. They use low-temperature heat sources of various types (usual boiler, solar boiler or heat engine radiator) and treat most types of non-potable water, such as seawater, brackish groundwater or clear but polluted surface water. To this principal application are added those relating to the production of concentrates in various industries, in particular the food or chemical industries.  
         [0003]     According to the general distillation method, the subject of this prior invention,  
         [0004]     counter-current heat exchanges are carried out by a single heat-transfer fluid, liquid or gaseous, circulating in closed circuit along surfaces, hot S c  and cold surfaces S f  respectively, linked by significant thermal conductance;  
         [0005]     said surfaces S c  and S f  are faces of walls of thin hollow distillation-heat-exchange plates, installed in large numbers, vertical or inclined, in a thermally insulated treatment chamber, comprising narrow inter-plate spaces, of a more or less constant width, filled with a non-condensable gas, in particular with air at atmospheric pressure;  
         [0006]     the heat-transfer fluid circulates from the top downwards along the surfaces S c , passing from a high initial temperature T 1  to a final temperature T 3  below T 1 , then from the bottom upwards along the surfaces S f , passing from an initial temperature T 4 , below T 3 , to a final temperature T 2 , above T 4  and below T 1 ;  
         [0007]     over the top of the external faces of the walls of the hollow plates, inside which the heat-transfer fluid circulates from the top downwards, liquid to be distilled is poured, which spreads out and runs down slowly in fine layers along these external faces;  
         [0008]     under the action of the flow of heat-transfer fluid circulating from the top downwards along the surfaces S c , some of the liquid to be distilled poured over said external faces evaporates, whilst this flow cools down, passing from T 1  to T 3 , and the vapour produced diffuses in the non-condensable gas present in the inter-plate spaces;  
         [0009]     under the action of the flow of heat-transfer fluid circulating from the bottom upwards along the surfaces S f , the vapour diffused in the non-condensable gas condenses, whilst this flow heats up again, passing from T 4  to T 2 , under the effect of recovery of a significant part of the latent heat of condensation of the diffused vapour;  
         [0010]     a heat source is arranged between the hottest ends of the surfaces S c  and S f , in order to increase the temperature of the heat-transfer fluid from T 2  to T 1 ;  
         [0011]     a cold source is arranged between the least hot ends of these surfaces S c  and S f , in order to reduce the temperature of the heat-transfer fluid from T 3  to T 4 .  
         [0012]     According to a particular first vapour-diffusion distillation method, derived from this general method:  
         [0013]     the heat-transfer fluid is the liquid to be distilled;  
         [0014]     the thin hollow distillation heat-exchange plates are hot or cold and they are installed alternating in the heat-insulated treatment chamber, their respective walls constituting said surfaces S c  and S f ;  
         [0015]     liquid to be distilled is poured over the top of the external faces of the walls of the hot plates only;  
         [0016]     the heat-transfer liquid circulates from the top downwards inside the hot plates, it enters hot at temperature T 1  and it exits cooled down at temperature T 3 , having caused partial evaporation of the liquid to be distilled flowing over the external faces of these plates;  
         [0017]     at the outlet from the hot plates, the heat-transfer liquid undergoes an additional cooling down and passes to temperature T 4 ;  
         [0018]     then, this heat-transfer liquid at temperature T 4  enters inside the cold plates where it circulates from the bottom upwards, on the one hand, causing, on the external faces of the walls of these cold plates, a condensation of the vapour diffused through the layer of non-condensable gas in the inter-plate space and, on the other hand, recovering part of the condensation heat of this vapour in order to heat up again and finally it exits from the cold plates at temperature T 2 ;  
         [0019]     during these operations, the flows of heat pass through the walls of the hot and cold plates as well as the immobile layers of non-condensable gas which separate them;  
         [0020]     the distilled liquid runs down along the external faces of the walls of the cold plates whilst the concentrated liquid runs down along the external faces of the walls of the hot plates.  
         [0021]     According to a second particular distillation method derived from this general method:  
         [0022]     the heat-transfer fluid is said non-condensable gas, saturated with vapour of the liquid to be distilled;  
         [0023]     liquid to be distilled is poured over the top of the external faces of the walls of all the hollow distillation heat-exchange plates, these external faces constitute said cold surfaces S f , whilst the internal faces of the walls of these plates constitute said hot surfaces S c ;  
         [0024]     the flow of gas at temperature T 1  enters inside all the hollow plates where it circulates from the top downwards, whilst part of its vapour condenses on the internal faces of the walls of the plates, and flows of heat, resulting from a partial recovery of the condensation heat, pass through the walls of the plates in order to evaporate some of the liquid flowing over their external faces and, consequently, this flow of gas cools down and exits from the hollow plates at temperature T 3 ;  
         [0025]     at the outlet from these hollow plates, the flow of gas at temperature T 3  undergoes additional cooling and passes to temperature T 4 ;  
         [0026]     then this flow of gas at temperature T 4  enters the inter-plate spaces where it circulates from the bottom upwards, carrying along the vapour produced in these spaces and heating up again, and finally it exits from these spaces at temperature T 2 ;  
         [0027]     the distilled liquid runs down along the internal faces of the walls of the hollow plates whilst the concentrated liquid runs down along their external faces.  
         [0028]     For the implementation of these distillation methods, thin and hollow heat-exchange elements, made of polymer (in particular of polypropylene), are described in this PCT Application. These elements are thin, hollow, rectangular plates, of large dimensions (generally from 50 to 150 dm 2 ), with walls provided with a welded and/or glued hydrophilic or wettable coating. They are of two main types: (1) flexible panels, with very fine corrugated walls (0.15 mm in thickness), forming narrow ducts (15 mm), separated by parallel weld lines, and (2) rigid cellular panels, with fine plane walls (0.3 mm in thickness). These two types of thin hollow plates are carried by a flat rod, resting on uprights.  
         [0029]     These distillation methods, referenced hereafter by their author&#39;s initials JPD, are clearly differentiated from that implemented in the solar still, described in the European patent EP 1 312 404 A1, published on 21st May 2003 and referenced hereafter by its inventor&#39;s initials AVP. This AVP solar still comprises, in a treatment chamber, an evaporator and a condenser arranged vertically, and outside, a solar boiler, a radiator and a pump. The external wall of the evaporator is constantly dampened by seawater poured over its upper edge. Under the pump action, a heat-transfer liquid circulates in closed circuit from the bottom of the boiler upwards, from the top of the evaporator downwards, from the bottom of the radiator upwards and from the bottom of the condenser upwards, to finally arrive at the bottom of the boiler. The radiator is a unit for cooling down the heat-transfer liquid, subjected to the action of a flow of air. The seawater poured over the evaporator, heated by the heat-transfer liquid which exits from the boiler, evaporates in part and, consequently, the air which surrounds it heats up again and is saturated. By natural convection, this saturated hot air rises along the walls of the evaporator, begins to circulate in closed circuit in the treatment chamber and, during its passage, passes the zone occupied by the condenser from the top downwards. The heat-transfer liquid, which is cooled down while passing through the evaporator, undergoes an additional cooling down while passing through the radiator. As a consequence, it arrives at the bottom of the condenser at a temperature lower than that which it had when exiting from the evaporator. The particular structure given to the condenser (see  FIG. 3 ) results in the flow of saturated hot air and the flow of cooled down heat-transfer liquid circulating in crossed circuits. The vapour contained in the air condenses on the relatively cold walls of the condenser and the latent heat of condensation is recovered by the heat-transfer liquid, thus reducing the thermal energy required at the solar boiler. The distilled water and the salt water are recovered in tanks installed under the condenser and under the evaporator respectively.  
         [0030]     By comparison of the JPD and AVP distillation methods, described above, it is seen that, in spite of the common concepts, already set out in another JPD document, the publication WO 98/16474 A1, cited as prior art in the PCT Application referred to above, fundamental differences appear which clearly distinguish one from the other. In the JPD method, a single liquid or gaseous heat-transfer fluid is used and, in the AVP method, two fluids, liquid and gaseous respectively, are used simultaneously. Moreover, in the JPD method, the evaporator and the condenser are thin hollow plates, with fine walls, juxtaposed with narrow inter-plate spaces filled with air, in order to constitute heat-exchange surfaces, linked together by significant thermal conductance. In the AVP method, the evaporator and condenser units both have complex forms and, moreover, are different and relatively distant from each other. This scarcely makes it possible to establish a significant thermal conductance between them. When, in the JPD method, the heat-transfer fluid is a liquid, the thin hollow plates are alternately hot and cold and the hot and cold flows concerned circulate a very small distance from each other and in opposite directions (and not in crossed directions), whilst the thin layer of air which separates these plates remains immobile. This allows particularly efficient heat exchanges which are at the origin of the sought high productivity of distillation. When in the JPD method, the heat-transfer fluid is saturated hot air, the internal and external faces of the fine walls of the thin hollow plates constitute heat-exchange surfaces, linked by maximum thermal conductance, which, is evidently not to be found in the AVP method. Moreover, the JPD method can be the subject of mathematical modelling, really representative of the phenomena concerned, which alone makes it possible to understand and therefore to optimize these phenomena, since the elements to be taken into account have a simple geometry and well-defined arrangements in space. This scarcely appears to be possible with the elements of the AVP solar still, which have complex geometries and very weak and poorly-defined thermal conductance connections.  
         [0031]     In the publication WO 01/96244 A1 referred to above, on page 21, lines  2  to  7  it says: “The maximization of the performance coefficient of a distillation device the parameters of which are fixed ( . . . ), requires the difference in temperature between the flows of hot and less hot water exiting from the boiler and entering it, to be as small as possible, whilst the difference in temperatures between the top and bottom of the heat-exchange elements must by contrast, be as great as possible”.  
         [0032]     Such a statement is correct in certain cases, but as will be seen below, its generalization leads to conclusions which are simplistic and incomplete in certain cases and even false in other cases. By way of example, it will now be noted that the Performance Coefficient C OP  of a heat-exchange or distillation device, i.e. the ratio between the exchanged thermal power Q and the power P, provided by the boiler, also determines the cost price of the exchanged energy and/or of the distilled water, using two other parameters, namely, (1) the cost of the energy used, which is inversely proportional to the performance coefficient C OP , and (2) the amortization of the price of the device, which is itself proportional to the C OP , as will be demonstrated hereafter.  
         [0033]     In a standard counter-current heat exchanger, between two fluids with a constant heat capacity C p , the temperature of the heat-transfer fluid at the outlet from the boiler or at the inlet to the hot surfaces of the exchanger will hereafter be designated T 1 , the temperature of the fluid at the outlet from the cold surfaces T 2 , the temperature of the heat-transfer fluid at the outlet from the hot surfaces T 3 , and the temperature of the fluid at the inlet to the cold surfaces of the exchanger T 4 . And the difference in temperature which exists on both sides of the hot and cold surfaces concerned will be designated dT. Ignoring the heat losses from the exchanger, the two differences in temperature (T 3 −T 4 ) and (T 1 −T 2 ) are generally both equal to dT.  
         [0034]     It will be noted that such heat exchangers can only operate within a temperature range imposed by the behaviour of the materials used at high and low temperatures, and by the various change-of-state temperatures of the fluids concerned. Consequently, there is a maximum value imposed for the difference (T 1 −T 3 ). And it is for this maximum value that the exchanged power Q will also take on its maximum value.  
         [0035]     The exchanged power Q is expressed in two ways, depending on whether the heat-transfer fluid or the heat exchanger are considered. In the first case, this gives Q=C p .D.(T 1 −T 3 ), with C p  being the heat capacity at constant pressure of the heat-transfer fluid, (in the case of water, C p =4.19 joules, per gram and per degree), and D, the circulating mass flow rate. In the second case, we have the equation Q=k.V.dT, with k being the volume thermal conductance of a heat exchanger, and V the active volume of this exchanger. The volume thermal conductance k of a heat exchanger is defined as being the thermal power in Watts, transmitted through an exchanger with one cubic metre of active volume, in response to a difference in temperature of one Kelvin. The dimension of the term k is therefore W/m 3 .K.  
         [0036]     In the case of a standard counter-current heat exchanger, it is known that the performance coefficient C OP =(T 1 −T 3 )/dT. In the case of a vapour-diffusion still with a counter-current of water, the heat-transfer liquid circulates in closed circuit, this liquid enters the boiler at temperature T 2  and exits from it at T 1 , so that the power supplied by the boiler is P=Cp.D.dT, As regards the gross performance coefficient of the counter-current exchanger, constituted by this still, i.e. with the ratio Q/P, its value is also C OP =(T 1 −T 3 )/dT.  
         [0037]     If we now concern ourselves with the product of C OP  by Q, the exchanged thermal power, we find that the value of the quantity C OP .Q characterizes the practical performances of a heat exchanger, which is the more efficient, the larger this quantity. If, moreover, this same quantity is divided by the active volume V of the exchanger, it is then possible to compare two heat exchangers of different volumes and define their Intrinsic Efficiency Criterion, which is defined by the term C IE =C OP .Q/V=Q 2 /P.V=k.(T 1 −T 3 ).  
         [0038]     For a standard counter-current heat exchanger, the volume thermal conductance k of the exchanger and the heat capacity C p  of the liquids concerned have constant values, independent of the temperature T and of difference dT. Consequently, the term C IE  passes through a maximum when C OP  and Q are themselves maximum, i.e. for the extreme values, high and low respectively, imposed for T 1  and T 3 , according to the statement referred to above. But this is not at all the case for the distillation heat exchanges which occur in vapour-diffusion stills.  
         [0039]     In fact, in vapour-diffusion distillation heat exchangers, such as the stills according to the prior invention, the volume thermal conductance of the exchanger varies considerably within the theoretical temperature range that the device could explore, i.e. from 20 to 30° C., a range of the possible low limits of the cold source, constituted by the cold liquid to be distilled entering the still, up to a value at the most equal to the boiling temperature of this same liquid. This results from the quasi-exponential character of the partial vapour pressure, expressed as a function of temperature. Consequently, in the case of a vapour-diffusion still with counter-current of water, the Intrinsic Efficiency Criterion of this still C IE , does not show its maximum for the possible low limit of T 3 .  
         [0040]     In order to be able to make a first improvement to the distillation methods referred to above, in order to determine then obtain the optimum value of the temperature T 3 , at the outlet from hollow rectangular heat-exchange plates, which are flexible (crescent-shaped cells, corrugated wall) or rigid (rectangular cells, plane wall), described in the PCT Application referred to above, it is first necessary to establish the quantitative theory of the vapour-diffusion stills. This is in order to work out the fundamental physical laws which govern them. In order to do this, a systematic logic analysis will first be made of the operation of a vapour-diffusion still with counter-current of water.  
         [0041]     To this end, for such a still using seawater as heat-transfer fluid, the two types of parameters concerned will be defined, namely, those of construction (fixed on leaving the factory) and those of use, which determine their operation.  
         [0042]     The construction parameters are the following:  
         [0043]     (e), the average internal thickness of the hollow plates and of the water in these plates,  
         [0044]     (a), the average thickness of the layer of air between the plates,  
         [0045]     (2p), the pitch of plates of same kind, hot or cold,  
         [0046]     (h), the height of the plates,  
         [0047]     (N) the number, and (I) the width of the plates,  
         [0048]     (V), the active volume of the exchanger with V=p.N.l.h,  
         [0049]     the thickness, the thermal conductivity and the form (plane or corrugated) of the walls of the hollow plates.  
         [0050]     The use parameters are at the user&#39;s disposal and are the following:  
         [0051]     (D), the flow rate and (v), the velocity of the water in the hollow plates,  
         [0052]     (t), the transit time of the water in these plates, with t=h/v and D=V.e/2p.t,  
         [0053]     (dT), the difference in temperature between the fluids circulating in the hot and cold plates,  
         [0054]     (P), the thermal power of the boiler,  
         [0055]     (Q), the thermal power exchanged by distillation, expressed as m 3 /day of distilled water, each unit corresponding to approximately 27 kW;  
         [0056]     the range of temperature of use in the hot hollow plates, with T 1  at the inlet and T 3  at the outlet.  
         [0057]     Given the two expressions referred to above, which define the thermal power exchanged between the hot and cold surfaces concerned, namely that Q=C p .(T 1 −T 3 ).D, provided by the heat-transfer fluid and Q=k.V.dT, transmitted by the exchanger, the following equation is deduced: t.dT=Cp.(T 1 −T 3 ).e/k, which means that this term t.dT takes on a determined value as soon as T 1 , T 3  and k are themselves determined. It results from this that the thickness of water e and that of air a being fixed by the constructor, the values at the user&#39;s disposal, namely, the transit time (t) and the difference in temperatures dT vary in reverse directions, as soon as their product has a chosen determined value. Consequently, the term t.dT appears to be a composite variable, a function both of certain construction parameters, of the extreme temperatures T 1  and T 3  and of all the intermediate T values. Therefore t.dT must be considered as the determinant independent variable, to be taken into account for calculating the temperature of the heat-transfer fluid as it runs down along the whole length of the hot heat-exchange surfaces.  
         [0058]     On the basis of these fundamental physical equations which govern the operation of the vapour-diffusion stills with counter-current of water, according to said prior invention, it becomes possible to optimize this operation. In order to do this, a software package has been produced making it possible to model the transfers of mass and heat which are produced along the whole length of the hollow plates of these stills. In the first case studied, the heat-transfer liquid circulates from the top downwards in the hot plates and the interface between the heat-transfer liquids rising and running down is then the external face of the walls of the cold condensation plates. The plates are made of polymer (polypropylene in particular) and their thermal conductivity is 0.2 W/m.K. The calculation relates to the temperatures which appear from the top of the hot surfaces of these plates downwards, as a function of all the parameters concerned, namely the temperature T 1 , the construction and use parameters and the thermal conductances referred to above. This calculation is carried out step by step in order to work out the temperature curves of the portions of heat-transfer fluid, as a function of their height h, measured from the top of the hot hollow plates downwards, i.e. curves T=f (h) corresponding to the maximum natural value of T 1  and a possible minimum natural value (without artificial cooling) of T 3  for different values chosen from the construction parameters e and a.  
         [0059]     With seawater as heat-transfer fluid, at a constant heat capacity Cp, the curve of the temperature T=f (h) and that of C IE =g(h), will now be calculated, section by section, along the whole length of the hot surfaces, using the software concerned, for rigid cellular plates with plane faces, having a total wall and coating thickness of 0.5 mm, an internal thickness e=3 mm and a pitch p=8.5 mm. These three dimensions are the minimum values that it has been possible to give to the experimental prototypes, in order to ensure that the hot and cold plates can never touch each other.  
         [0060]     The curve A 1  shown in  FIG. 1  hereafter T=f(h) was calculated for plane hot plates, with T 1  approximately 100° C., a plate height h=100 cm, a plate pitch p=8.5 mm, with an internal thickness of water e=3 mm, an air-layer thickness a=5 mm and a total thickness of walls and hydrophilic coatings of 0.5 mm, a water-circulation velocity v=0.5 mm/s and a difference in temperature dT=5.5 K. The axis of the composite variable t.dT=dT.h/v has been drawn parallel to the h axis.  
         [0061]     For the value T=32° C., which corresponds to a height h=100 cm, the transit time t=2.000 s and the composite variable t.dT=11,000 K.s. For any intermediate value of T between 32 and 100° C., the corresponding value of t.dT is immediately deduced.  
         [0062]     In this same  FIG. 1  the curve B 1  is also drawn, which represents the variation in the Intrinsic Efficiency Criterion of distillation of the still C IE =C OP .QN=Q 2 /P.V, as a function of the composite variable t.dT (deduced from h), which is linked to V, the active volume, by the equation specified above V=p.N.l.h. This criterion C IE  is representative of the product of the C OP  by Q/V, the volume in cubic metres of distilled water per day and per cubic metre of active volume of still. In the present case, the range of the possible variations of this criterion is established from 0 to 18. The curve B 1  shows a maximum C IE =17.8 for t.dT=3000 K.s, and gives T 3 =68.5° C. when T 1 =100° C. According to  FIG. 1 , the ranges of optimum values of t.dT and T 3  are defined by the curve A 1  and that of C IE  by its maximum (C IE &gt;17), i.e. 1900&lt;t.dT&lt;4450 K.s and  58 &lt;T 3 &lt;78° C. These ranges of optimum values vary little when the e/p ratio remains constant, the maximum of C IE  itself being highest when the parameters e and p have their minimum values. Consequently, for any C IE &gt;17 and any particular value of the composite variable t.dT, situated within the optimum range which follows from the latter, the operation of a vapour-diffusion and heat-transfer-liquid still will be optimized as soon as one of the parameters t or dT has been chosen and T 3  deduced from this choice.  
         [0063]     As C OP =Q/P, this term is also inversely proportional to the cost of the energy used. As regards the ratio Q/N, this is inversely proportional to the active volume V and therefore to the number of plates installed in order to obtain a determined daily production Q. As Q/V can also be written=C IE /C OP , when the price of energy at the operating site is high (fossil fuels or electricity), a high value will be chosen for C OP . And, in the case where this energy is inexpensive (solar or co-generation from the cooling liquid or exhaust gas from heat engines), a lower C OP  will be chosen and therefore a limited investment (fewer heat-exchange plates). It will be noted that these variations in reverse directions make it possible to obtain maximum efficiency when the price of the energy is equal to the amortization of the investment relative to the total volume of distilled water, produced over the duration of this amortization.  
         [0064]     When flexible plates with corrugated walls, composed of parallel ducts, crescent-shaped in cross-section, are used, the two curves A 1  and B 1  in  FIG. 1  are a little different: the curve A 1  which represents T=f(h), has more or less the same shape but the curve B 1 , C IE =f (t.dT), has a clearly less pronounced maximum with a value of approximately 9 instead of 18. Comparable results are obtained for the two types of hollow plates (rigid and plane or flexible and corrugated), when the temperature T 1  is appreciably below the optimum temperature indicated above (100° C.), i.e. 85° C. for example.  
         [0065]     In a second case studied, the direction of circulation of the heat-transfer liquid taken into account in the preceding calculation (from the bottom upwards instead of from the top downwards, in the hot plates) was reversed whilst keeping the hollow hot and cold plates considered unchanged. The results obtained in this second case are scarcely different from those obtained in the first.  
         [0066]     It will be noted that in the case of the stills with counter-current of water, implementing these two distillation methods according to the invention, the interface, through which the transfers of heat take place between the two flows of water which circulate in reverse directions, is situated in the wall which separates the rising flow of water from the liquid trickling down, which is distilled water in the first case studied (circulation of the heat-transfer liquid from the top downwards in the hot plates) and salt water in the second case (circulation in the reverse direction).  
         [0067]     We shall now consider the vapour-diffusion distillation method with a heat-transfer gas (air), saturated with vapour. The construction and use parameters, referred to above for a distillation method with a counter-current of water, are again taken up in the present case. On the other hand, with the apparent heat capacity C p  of the air saturated with vapour increasing hugely as a function of temperature, the difference in temperature between the internal and external faces of the plates must vary in the reverse direction. Under these conditions, if it is simple, for a vapour-diffusion still with counter-current of water, to express the difference in thermal power applied between the heat-exchange surfaces, as a function of dT alone, since C p  is then constant, in the case of a vapour-diffusion still with counter-current of air, it becomes necessary to return to the thermal power applied between the internal and external faces of the hollow distillation plates.  
         [0068]     According to the invention, this difference in thermal power is expressed by the local difference in enthalpy flows (in Watts) between the flows of saturated air along the hot external faces S c  and cold internal faces S f  of these hollow plates. These enthalpy flows are defined at two opposite levels of the faces of a wall of plates, by H c =D.C pc .T c , for the face S c , and by H f =D.C pf .T f , for the face S f . A local difference in enthalpy flows is defined by dH=D.(C pc .T c −C pf .T f ). In these expressions, C pc  and C pf  are the apparent heat capacities of the flows at temperatures T c  and T f , existing at two opposite levels. The term C p =δH/D.δT, with on the one hand, δH and δT, conjugated elementary variations in enthalpy flows H and temperature T and, on the other hand, D, the flow rate of dry air and C p , the apparent heat capacity of the saturated hot air, at any temperature T, expressed in degrees Kelvin. In this regard, it will be recalled that the heat capacity C p  of the dry air has a constant value of 1000 joules per degree and per kilogram but that, on the other hand, the apparent heat capacity C p  of the hot air saturated with vapour is 740 kJ/K/kg of dry air, between 91 and 92° C., and only 16.4 kJ/K/kg of dry air, on average between 24 and 45° C.  
         [0069]     It will be noted that, in the case of a still with counter-current of air circulating opposite the natural convection, the interface through which the transfers of heat take place between the upward and downward flows is the free surface of the salt water. On the other hand, in the case of a still with a counter-current of air circulating by natural convection, this interface is the free surface of the distilled water.  
         [0070]     On the basis of these basic findings, relating to a vapour-diffusion still with counter-current of air, a second software package has been developed which makes it possible to calculate the temperature profiles along the internal and external walls of hollow plates with plane faces of polymer. Starting with the temperatures at the top of the plates, with a value t.dH/V=2400 kJ/m 3  and values T 1 =92° C. and T 2 =91° C., (which corresponds to a given local difference in enthalpy flows with a value dH 1 ), and adopting a dry air circulation velocity v 1  of 10 cm/s (which, by way of example, gives v=20 or 40 cm/s for humid air with 50% or 75% partial vapour pressure), the calculation was made for a temperature T 4 , at the bottom of the inter-plate spaces, compatible with that of the available natural cold sources (20 to 30° C., for example). The curve A 2  represents the function T=f 1 (h) along the hot internal faces of the hollow plates and the curve C 2  represents the function T=f 2 (h) along their cold external faces. As regards this curve C 2 , it will be noted that no curve C, appears in  FIG. 1 , since it could be deduced from the curve A 1 , by a simple constant shift dT (except for losses). On the other hand in  FIG. 2 , this curve C 2  is clearly differentiated from the curve A 2  since the increasing difference in degrees (° C.), which separate them from each other at each decreasing level h, expresses the constant local difference in enthalpy flows dH 1 , which corresponds to the values of T 1  and T 2 , expressed above. With T 1  reduced to 91.5° C. and T 2  maintained at 91° C., the curve C 2  would remain unchanged and the curve A 3  obtained would be situated at an almost equal distance from the curves A 2  and C 2  represented.  
         [0071]     With T 1  maintained at 92° C. and T 2  reduced to 90° C., the curve A 2  would remain unchanged and the curve C 3  obtained would be deduced from the curves A 2  and C 2  represented by a shift at each level, almost double that of these two curves represented. It will be noted that these comments apply without correction to pure water, which is the subject of a distillation, but not at all to salt water. In fact, for two types of seawater, with 35 or 70 grams of sodium chloride per litre, the boiling temperatures are 100.5° C. and 101° C. respectively. This means that for these types of water, during a distillation operation, a difference in temperature of approximately 0.5° C. or 1° C. is used up by the operation of desalination of these types of water and therefore neutralized for their distillation.  
         [0072]     As regards the curves A 1  in  FIG. 1  (counter-current of water) and A 2  in  FIG. 2  (counter-current of air), it will be noted that the curve A 2 , unlike A 1 , shows very significant concavity, directed towards the top of the plates. This means that the heat exchanges by vapour diffusion, in the case of a still with counter-current of air, are much greater towards the low temperatures than towards the high. Under these conditions, the maximum C IE  has been sought starting from the low temperatures. In this zone in fact, the flows of heat which pass through the walls of the hollow plates, between the opposite portions of the two fluids circulating in reverse directions, are much greater for the same difference in enthalpy flows, because of the greater difference in temperature to which this local difference in enthalpy flows corresponds.  
         [0073]     The curve B 2  in  FIG. 2  represents the evolution of C IE  as a function of the composite variable t.dH/V (in kilojoules per m 3 ) to be retained in the case of a still with a counter-current of air. This evolution is not very sensitive to the difference (T 1 −T 2 ) and therefore to the value of the local difference in enthalpy flows dH 1  referred to above, which served to calculate this curve B 2 . The maximum C IE  corresponds, on the curve A 2 , to a temperature T 1  of approximately 85° C., at the inlet to the hollow plates and, for the local difference dH 1  retained, the value of T 2  read on the curve C 2  is approximately 80° C., at the outlet from the inter-plate spaces. The value of T 1 , at the inlet to the hollow plates, is determined by the maximum temperature of the available heat source and the value of T 4  at the inlet to the inter-plate spaces, by the minimum temperature of the available natural cold source. In the case where this cold source is the liquid to be distilled entering at 25° C., it is possible, by means of an appropriate heat exchanger, to have a value T 4  of 30° C. In this case, with the conditions referred to above (dH 1  and v 1 ), we have T 3 =68° C. With a higher value T 4 , the value of T 3  increases and, in this case, the maximum C IE  is less high and it is obtained for a greater value t.dH/V. Similarly, for a local difference dH greater or smaller than the value dH 1  referred to above, if a value as low as possible (30° C., for example) is kept for T 4 , there will be a maximum C IE  varying in the reverse direction, i.e. a little less high or a little higher than previously, provided that the composite variable t.dH/V varies like dH.  
         [0074]     In  FIG. 2 , the Intrinsic Efficiency Coefficient of the C IE  still, defined by C OP .Q/V or Q 2 /P.V or also k.(T 1 −T 3 ), shows a maximum of 95 m 3  of fresh water per day and per active m 3  of still, for a value of the composite variable t.dH/V of 382 kilojoules/m 3 . The optimum C IE  value is greater than 84, which corresponds to 210&lt;t.dH/V&lt;740 kJ/m 3  and an optimum range 78&lt;T 1 &lt;91° C. In practice however, it is clear that any C IE  value greater than one-third, for example, of its maximum possible value (which is of the order of 100, for the hollow plates, with the characteristics defined hereafter, adopted for the calculation) is completely welcome. This means that as soon as T 4  has been able to take on a very low value (down to 10° C., for example, if it has been possible to cool down the liquid to be distilled entering, by economic natural means), all values of T 1  defined by the temperature range 74° C.&lt;T 1 &lt;91° C. and, in the case of the results of the study shown in  FIG. 2 , all the values of the composite variable t.dH/V which are defined by the range 100 kJ/m 3 &lt;t.dH/V&lt;1300 kJ/m 3 , make it possible to construct a vapour diffusion and heat-transfer gas still with high productivity.  
         [0075]     The preceding results were obtained for thin hollow plates with plane walls, which have an internal thickness e=2 mm, an identical inter-plate space, a wall and hydrophilic coating thickness of 0.6 mm and therefore a pitch p=5.2 mm. The ranges of optimum values defined above vary little when the e/p ratio remains more or less constant, the maximum C IE  being highest when the parameters e, a and p have their minimum values, imposed by practical considerations. These considerations are aimed at ensuring that the losses of charge in the inter-plate spaces are always acceptable, which limits to 2 mm the minimum internal thicknesses e and a of the hollow plates and of their inter-plate spaces. On the other hand, if this value of e is retained and a new model of hollow plates with particularly thin plane walls (0.15 mm total thickness of wall and hydrophilic coating) is created, results much better than those illustrated by the curves in  FIG. 2  are obtained. Such a hollow distillation-heat-exchange plate is described hereafter in  FIG. 13 . With this new model of plates, the maximum C IE  calculated is considerably increased (297 m 3 /day, per m 3  of active volume, instead of 95), if we take e=2 mm and p=4.5 mm. With an internal thickness e=3 mm, an identical inter-plate space and a pitch p=6, 5 mm, this maximum drops to  132 . As regards the curves A 2 -C 2  and the optimum (or simply efficient, because they provide completely satisfactory results) values of the temperatures T 1 , T 2 , T 3  and T 4 , they remain almost the same.  
         [0076]     If the direction of circulation of the heat-transfer gas taken into account in the preceding calculation is reversed, by causing this gas to circulate from the bottom upwards (and no longer from the top downwards) in the hollow plates and from the top downwards (and no longer from the bottom upwards) in the inter-plate spaces, more or less identical results are obtained.  
         [0077]     These results demonstrate the exceptional usefulness presented by the stills with vapour diffusion and counter-current of air and since, within the framework of the limits of the technology currently available, they can easily display CEs comprised between 30 and 100, whereas the stills with a counter-current of water display C IE s of 18 at the most.  
       SUMMARY OF THE INVENTION  
       [0078]     The first subject of the present invention relates to improvements and extensions, capable of being made to the general distillation method, with liquid or gaseous heat-transfer fluid and vapour diffusion in a non-condensable gas, described in said prior invention, which follow from the physical laws governing the functions of the stills implementing this general method.  
         [0079]     The second subject of the invention relates to two types of improvements, resulting from the physical laws in question, capable of being made to the particular vapour-diffusion distillation methods and devices, in which the heat-transfer fluid is the liquid to be distilled, and the direction of circulation of this liquid that described in said prior Application or the reverse direction.  
         [0080]     The third subject of the invention relates to two other types of improvements, resulting from the physical laws in question, capable of being made to the particular vapour-diffusion distillation methods and devices, in which the heat-transfer fluid is the non-condensable gas, saturated with vapour of the liquid to be distilled, and the direction of circulation of this gas, that described in said prior Application or the reverse direction.  
         [0081]     The fourth subject of the invention relates to vapour-diffusion stills, in which the simple heat exchangers used have a new, compact, low-cost architecture.  
         [0082]     The fifth subject of the invention relates to a distillation heat exchanger, comprising a monobloc active element, suited to the requirements of a vapour diffusion and heat-transfer-gas still.  
         [0083]     The sixth subject of the invention relates to means of completely safely connecting large thin hollow distillation-heat-exchange plates to their heat-transfer fluid inlet and outlet ducts.  
         [0084]     The seventh subject of the invention relates to means of pouring the liquid to be distilled over the external faces of the walls of distillation-heat-exchange hollow plates efficiently and completely safely.  
         [0085]     The eighth subject of the invention relates to new, thin and flexible hollow distillation plates with very fine plane walls, which can be used in vapour-diffusion and heat-transfer gas stills;  
         [0086]     The ninth subject of the invention relates to hot sources especially suited to the particular requirements of certain of the distillation devices referred to above.  
         [0087]     According to an improvement to said prior invention, a general multiple-effect distillation method, intended for separating substances in solution from their liquid solvent, in particular for producing fresh water or concentrates, in which:  
         [0088]     counter-current heat exchanges are carried out by a single liquid or gaseous heat-transfer fluid, circulating in closed circuit along surfaces, hot S c  and cold S f  respectively, linked by significant thermal conductance;  
         [0089]     said surfaces S c  and S f  are faces of walls of thin distillation-heat-exchange hollow plates, installed in large numbers, vertical or inclined, in a heat-insulated treatment chamber, comprising narrow inter-plate spaces, of more or less constant width, filled with a non-condensable gas, in particular air at atmospheric pressure;  
         [0090]     is characterized in that:  
         [0091]     the heat-transfer fluid circulates, in a first upward or downward direction, along the hot surfaces S c , passing from a high initial temperature T 1  to a final temperature T 3  below T 1  then, in a second direction opposite the first, along the cold surfaces S f , passing from an initial temperature T 4 , below T 3 , to a final temperature T 2 , above T 4  and below T 1 ;  
         [0092]     at the top of the external faces of the walls of the hollow distillation plates, inside which the heat-transfer fluid circulates in said first direction, liquid to be distilled is poured which spreads out and runs down slowly in fine layers along these external faces;  
         [0093]     under the action of the flow of heat-transfer fluid circulating in said first direction, some of the liquid to be distilled poured over said external faces evaporates, whilst this flow cools down, passing from T 1  to T 3 , and the vapour produced diffuses in the non-condensable gas present in the inter-plate spaces;  
         [0094]     under the action of the flow of heat-transfer fluid circulating in said second direction, the vapour diffused in the non-condensable gas condenses, whilst this flow heats up again, passing from T 4  to T 2 , under the effect of a virtually total recovery of the latent heat of condensation of the diffused vapour;  
         [0095]     a heat source is arranged between the hottest ends of the surfaces S c  and S f , in order to increase the temperature of the heat-transfer fluid from T 2  to T 1 ;  
         [0096]     a cold source is arranged between the least hot ends of these surfaces S c  and S f , in order to reduce the temperature of the heat-transfer fluid from T 3  to T 4 ;  
         [0097]     a more or less constant local difference dH in enthalpy flows is established between the surfaces S c  and S f , by giving appropriate amplitudes to the heat exchanges carried out between the flow of heat-transfer fluid and said hot and cold sources respectively;  
         [0098]     the optimum temperatures of the heat-transfer fluid T 1 , T 2  and T 3 , T 4 , at the ends of these same surfaces, are determined from the maximum Intrinsic Efficiency Criterion C IE =Q 2 /P.V of the installation, Q being the exchanged distillation thermal power, P being the thermal power provided by the heat source, and V the active volume of the installation.  
         [0099]     Thanks to these arrangements, the general distillation method, using a single liquid or gaseous heat-transfer fluid, described in said prior invention, is both extended and optimized. Firstly, two new types of stills are added to the two prior types, previously described. This is done by using identical or equivalent components and giving two possibilities instead of only one, to the direction of circulation of one or other of the two heat-transfer fluids provided. Then, by application of the conclusions of the mathematical modelling of the particular heat-exchange phenomena existing in the vapour-diffusion stills according to said prior invention, the relative and absolute thermal characteristics, namely the constant local difference in enthalpy flows between the surfaces S c  and S f  and the temperatures of the heat-transfer fluid at the inlets to and outlets from the thin hollow plates and/or of their spacings are fixed. Thanks to which, the efficiency of the distillation carried out can be understood, controlled and thus maximized.  
         [0100]     According to the invention, a first particular vapour-diffusion distillation method, in particular for producing fresh water, according to the improved general method defined above, in which:  
         [0101]     the heat-transfer fluid is the liquid to be distilled;  
         [0102]     the thin, hollow distillation-heat-exchange plates are hot or cold and they are installed alternating in the heat-insulated treatment chamber, the internal faces of their respective walls constituting said hot S c  and cold surfaces S f ;  
         [0103]     liquid to be distilled is poured over the external faces of the walls of the hot plates only;  
         [0104]     is characterized in that:  
         [0105]     the heat-transfer liquid circulates, in a first upward or downward direction, inside the hot plates, it enters very hot at temperature T 1  and it exits cooled down to the temperature T 3 , having caused a partial evaporation of the liquid to be distilled flowing over the external faces of the walls of these hot plates;  
         [0106]     at the outlet from these hot plates, the heat-transfer liquid at temperature T 3  is cooled down to temperature T 4 ;  
         [0107]     then, the heat-transfer liquid at temperature T 4  enters inside the cold hollow plates where it circulates in a second direction, opposite the first, causing, on the external faces of the walls of these cold plates, a condensation of the vapour diffused through the layer of non-condensable gas in the inter-plate space and recovering virtually all of the condensation heat from this vapour in order to be heated up again, and finally it exits from the cold plates at temperature T 2 ;  
         [0108]     during these operations, the flow of heat passes through the walls of the hot and cold hollow plates as well as the immobile layers of non-condensable gas which separate them;  
         [0109]     the distilled liquid runs down along the external faces of the walls of the cold plates whilst the concentrated liquid runs down along the external faces of the walls of the hot plates;  
         [0110]     the optimum temperature T 1  of the heat-transfer liquid, at the inlet to the hot hollow plates, is as little as possible below the boiling temperature of this liquid at atmospheric pressure;  
         [0111]     the optimum temperature T 3  of the heat-transfer liquid, at the outlet from the hot hollow plates, is relatively high and situated in a range which corresponds to a zone surrounding the maximum Intrinsic Efficiency Criterion C IE  of the installation;  
         [0112]     the differences in temperature (T 1 −T 1 ) and (T 3 −T 4 ) are small, with (T 1 −T 2 ) being slightly greater than (T 3 −T 4 ).  
         [0113]     According to additional characteristics of this vapour-diffusion and heat-transfer-liquid distillation method,  
         [0114]     the correspondence between the optimum range of the temperatures T 3  and the maximum C IE , is achieved by means of their respective relationships with a composite variable t.dT, in which t is the transit time of the heat-transfer liquid in the plates and dT the difference in temperature between the liquids circulating in the cold and hot hollow plates;  
         [0115]     the useful range of the temperature T 3  is the range from 58 to 78° C., when the liquid to be distilled is water;  
         [0116]     the optimum difference in temperature dT is established by an adjustment of the ratio between the heating power of the heat source and the mass flow rate D of circulating heat-transfer liquid;  
         [0117]     the optimum value chosen for dT is relatively high when the unit cost of the thermal energy easily available at the site of implementation of the method, is relatively low;  
         [0118]     the optimum transit time t of the heat-transfer fluid in the heat-exchange plates is established by adjustment of the mass flow rate D of the heat-transfer liquid circulating in a closed loop.  
         [0119]     Thanks to these arrangements, the distillation method with vapour diffusion and heat-transfer liquid becomes a really efficient method, using new stages which are particularly simple to implement, applying the conclusions of the mathematical modelling of the phenomena concerned. These stages consist of increasing in particular the temperature of the liquid to be distilled entering the installation, before mixing it with the liquid to be distilled circulating in a closed loop, by a simple heat exchange with the distilled and concentrated liquids, exiting from the installation at a high average temperature, of approximately T 3 . This value T 3  is particularly high (58 to 78° C.), applying said conclusions, because of the maximum temperature T 1  (100° C.) of the liquid exiting from the boiler and appropriate adjustment of the transit time t of the heat-transfer liquid in the hollow plates, in accordance with the value chosen for the difference dT in temperature between these plates.  
         [0120]     According to the present invention, this first particular vapour-diffusion distillation method, in which the heat-transfer liquid circulates, preferably by thermosiphon, from the top downwards inside the hot hollow plates and from the bottom upwards inside the cold hollow plates, is moreover characterized in that, according to a first set of arrangements:  
         [0121]     a heat exchange for heating is carried out between the flow d of liquid to be distilled entering the installation at temperature T L1  and the two flows of distilled and concentrated liquids leaving it, so as to take the temperature of this flow d to a relatively high optimum intermediate value T L2 ;  
         [0122]     mixing is carried out between this entering flow d, thus heated to the temperature T L2 , and the flow D of heat-transfer liquid exiting from the hot plates at temperature T 3 , the ratio d/D being adjusted so that the mixture produced has an optimum temperature T 4  at the inlet to the cold plates.  
         [0123]     According to the present invention, this first particular vapour-diffusion distillation method, in which the heat-transfer liquid circulates by thermosiphon, from the bottom upwards inside the hot hollow plates and from the top downwards inside the cold hollow plates, is characterized in that, according to a second set of arrangements, the flow d of liquid to be distilled entering at temperature T L1  is added to the flow D of heat-transfer liquid exiting at the temperature T 3  of the hot plates, the ratio d/D being adjusted so that the mixture produced is at an optimum temperature T 4  at the inlet to the cold plates, a flow d of liquid at temperature T 3  or T 4  being poured over the top of the external faces of the hot plates.  
         [0124]     Thanks to these last two arrangements according to the invention, a first and a second embodiment of the stills, with vapour diffusion and heat-transfer liquid in counter-current circulation in closed circuit, are possible, the first having however a C OP  greater than that of the second, which however remains useful, although the temperatures of the concentrated T 1  and distilled liquids T 2 , to be removed are high. This drawback can however be easily corrected if, by appropriate heat exchanges, this thermal energy is recovered in order to reheat the liquid to be distilled to be poured over the top of the hot plates.  
         [0125]     According to the invention, a second particular distillation method with vapour diffusion, in particular for producing fresh water, according to the improved general method defined above, in which:  
         [0126]     the heat-transfer fluid is said non-condensable gas, saturated with vapour of the liquid to be distilled;  
         [0127]     liquid to be distilled is poured over the top of the external faces of the walls of all the distillation-heat-exchange hollow plates, these external faces constituting said hot surfaces S c  whilst the internal faces of the walls of these plates constitute said cold surfaces S f ;  
         [0128]     is characterized in that:  
         [0129]     the flow of heat-transfer gas at temperature T 1  enters inside all the hollow distillation plates, where it circulates in a first upward or downward direction, whilst some of its vapour condenses on the internal faces of the walls of the plates, flows of heat, resulting from a partial recovery of the latent heat of condensation, pass through the walls of the plates in order to evaporate some of the liquid flowing over the external faces of these walls and, as a result, this flow of gas cools down and finally exits from the hollow plates at temperature T 3 ;  
         [0130]     at the outlet from these plates, this flow of heat-transfer gas at temperature T 3  is cooled down to temperature T 4  and the distilled liquid, condensed on this occasion, is recovered;  
         [0131]     then, this flow of heat-transfer gas, at temperature T 4 , enters the inter-plate spaces, where it circulates in a second direction, the reverse of the first, carrying away the vapour produced in these spaces and reheating it, and finally it exits from these spaces at temperature T 2 ;  
         [0132]     the distilled liquid, condensed on the internal faces of the walls of the hollow plates, runs down along these internal faces whilst the concentrated liquid runs down along the external faces of these walls;  
         [0133]     the optimum temperature T 1  of the flow of saturated heat-transfer gas, at the inlet to the hollow plates, is situated within a range which corresponds to a wide zone around the maximum Intrinsic Efficiency Criterion C IE  of the installation;  
         [0134]     the optimum temperature T 4  of the flow of heat-transfer gas, which enters the inter-plate spaces, has previously been made as close as possible to the minimum temperature of the available natural cold source present, by cooling down in an appropriate manner the flow of gas at temperature T 3  which exits from the hollow plates;  
         [0135]     the difference in temperature (T 1 −T 2 ) is small and the difference (T 3 −T 4 ), considerable.  
         [0136]     According to additional characteristics of this improved vapour-diffusion and heat-transfer-gas distillation method,  
         [0137]     the correspondence between the optimum range of the temperatures T 1  and the maximum C IE  zone is achieved by means of their respective relationships with a composite variable t.dH/V, in which t is the transit time in the plates, dH a more or less constant local difference in enthalpy flows between the internal and external walls of the plates, and V the active volume of the installation;  
         [0138]     the useful range of the temperature T 1  is approximately comprised between 74 and 91° C.;  
         [0139]     the optimum local difference in enthalpy flows dH, between two levels opposite the internal and external walls of the plates, is established by adjustment of the ratio between the heating power and the circulating mass flow of the heat-transfer gas;  
         [0140]     the optimum value of dH is relatively high when the cost of the thermal energy easily available at the site of use of the device, is relatively low;  
         [0141]     the optimum transit time t of the heat-transfer gas in the heat-exchange plates is established by adjustment of the mass flow D of this gas.  
         [0142]     Thanks to these arrangements, the temperature T 4  of the heat-transfer gas, injected at the inlet to the inter-plate spaces, (at the bottom of these spaces in a first case, or at the top in a second) is only slightly higher than the temperature of the liquid to be distilled entering the device (for example 25° C.) and well below the temperature T 3  of this same heat-transfer gas exiting from the hollow plates. Under these conditions, the local difference in enthalpy flows dH, between the flows of heat-transfer gas, with a variable temperature and heat capacity along the whole length of the internal and external faces of the hollow heat-exchange plates, can, over the whole height of these plates, remain more or less constant and equal (except for losses) to that imposed by the appropriate heat source, arranged between the outlet from the inter-plate spaces and the inlet to these same plates. In this regard, it will be noted that the differences in temperature between the flows of heat-transfer gas at the outlet from the hollow plates and at the inlet to the inter-plate spaces, are on the other hand very different. By way of example, there will be a difference (T 1 −T 2 )=5° C., with T 1 =85° C., at one end of the plates and (T 3 −T 4 )=38° C., with T 3 =68° C., at their other end.  
         [0143]     According to the invention, this second particular vapour-diffusion and heat-transfer-gas distillation method, is moreover characterized in that, according to a first set of arrangements,  
         [0144]     the flow of gas at temperature T 1  is introduced at the top of the hollow distillation plates and it exits at the bottom at temperature T 3 ;  
         [0145]     at the outlet from the hollow distillation plates, this flow of gas at temperature T 3  is subjected to a cooling-down heat exchange, ensured by a cold source at temperature T L1 , constituted by the entering flow of liquid to be distilled, in order that, given the respective mass and thermal characteristics of this flow of gas and of this flow of liquid, the temperature T 3  of the flow of gas is reduced to an optimum temperature T 4  and the temperature of the liquid taken to T L2 ;  
         [0146]     after this heat exchange, the liquid to be distilled at temperature T L2  is reheated by a heat source;  
         [0147]     the flow of gas at temperature T 4  is introduced at the bottom of the inter-plate spaces and it exits at the top at temperature T 2 ;  
         [0148]     the flow of gas circulates in closed circuit in the hollow distillation plates and in the inter-plate spaces, under the action of at least one means of propulsion;  
         [0149]     at the outlet from the inter-plate spaces, the flow of gas at temperature T 2  is reheated and saturated with vapour, by an appropriate physical contact with the liquid to be distilled reheated by the heat source, so as to take on an optimum or simply effective temperature T 1 ;  
         [0150]     after its physical contact with the flow of gas at temperature T 2 , the liquid to be distilled is poured, at a temperature of approximately T 1 , over the top of the external faces of the walls of the hollow plates, and it exits at the bottom, at a temperature of approximately T 4 ;  
         [0151]     the distilled liquid, condensed during said cooling-down heat exchange, and that condensed on the internal faces of the hollow plates, are collected, then removed and recovered;  
         [0152]     the concentrated liquid is collected at the bottom of the external faces of the walls of these plates, then it is removed and, if appropriate, recovered.  
         [0153]     According to the invention, this second particular vapour-diffusion and heat-transfer-gas distillation method is moreover characterized in that, according to a second set of arrangements,  
         [0154]     the flow of gas at temperature T 1  is introduced at the bottom of the hollow distillation plates and it exits at the top at temperature T 3 ;  
         [0155]     at the outlet from the hollow distillation plates, this flow of gas is subjected to a cooling-down heat exchange, ensured by a cold source at temperature T L1  constituted by the entering flow of liquid to be distilled, so that, given the mass and thermal characteristics of this flow of gas and of this flow of liquid, the temperature T 3  of the flow of gas is reduced to an optimum temperature T 4 ;  
         [0156]     after this heat exchange, liquid to be distilled is poured over the top of the external faces of the walls of the hollow plates, it runs down along these external faces and leaves them at a temperature of approximately T 2 ;  
         [0157]     the flow of gas, at temperature T 4 , is introduced at the top of the inter-plate spaces and it exits at the bottom at temperature T 2 ;  
         [0158]     at the outlet from the inter-plate spaces, the flow of gas at temperature T 2  is reheated and saturated with vapour, so as to take on an optimum or simply effective temperature T 1 ;  
         [0159]     the flow of gas at temperature T 1  is introduced at the bottom of the hollow plates and, at least by natural convection, it rises inside these plates, it then passes through a zone where it undergoes said cooling-down heat exchange then, at temperature T 4 , it enters and runs down by gravity in the inter-plate spaces;  
         [0160]     the distilled liquid, condensed during the cooling-down heat exchange and that condensed along the internal faces of the walls of the hollow plates are collected, then removed and recovered;  
         [0161]     exiting from the inter-plate spaces, the liquid to be distilled which has become concentrated is collected with a view to immediate or subsequent removal.  
         [0162]     According to a particular characteristic of the method thus defined, the concentrated liquid to be distilled which exits from the inter-plate spaces, is reheated by a heat source and, by an appropriate physical contact with this thus-reheated liquid, the flow of gas at temperature T 2  is reheated and saturated, in order to take on an optimum or simply effective temperature T 1 .  
         [0163]     According to another particular characteristic of the method thus defined,  
         [0164]     the distilled liquid circulates from the bottom upwards in vertical auxiliary hollow plates for heat recovery, separated by narrow inter-plate spaces;  
         [0165]     if appropriate, the same applies to the concentrated liquid collected;  
         [0166]     these auxiliary hollow plates are both thin, rigid and provided with hydrophilic or wettable external coatings;  
         [0167]     liquid to be distilled, preferably at a temperature as low as possible, is poured over the top of these coatings;  
         [0168]     some of the flow of gas at temperature T 4  circulates from the top downwards along these thus moistened coatings;  
         [0169]     the flow of saturated hot gas leaving these coatings is added to that which exits from the inter-plate spaces of the hollow distillation plates, then the mixture is reheated and saturated in order to take on an optimum or simply effective temperature T 1 ;  
         [0170]     the distilled and concentrated liquids leave at the top of these hollow heat-recovery plates with significantly reduced temperatures, then they are removed and at least one of them is recovered.  
         [0171]     By means of these last two chief sets of arrangements, two embodiments of the vapour-diffusion and heat-transfer-gas stills, with counter-current circulation in a closed circuit, in one direction or the other, are possible. In conclusion, commenting on the curves in  FIG. 2  above, they have numerous particularly useful advantages, as will be specified in detail hereafter. It will be noted straightaway that the temperatures of the distilled and concentrated liquids which exit from the heat-recovery unit and of the supply of additional vapour to the flow of heat-transfer gas, have relatively small differences compared with the temperature of the entering liquid to be distilled. This has the result of ensuring a high C OP  and C IE  for the distillation devices concerned.  
         [0172]     As regards the boiler, which can be used in the two forms of implementation of each of the two particular improved distillation methods according to the invention, it will be noted that it can take on the most diverse forms, either for heating the liquid to be distilled or for reheating and supersaturating the heat-transfer gas. In principle, if the primary form of heating, which consists of heating by a flame the base of a container in which the liquid to be distilled circulates, is used only as a last resort, it will be advantageously possible, as will be seen below, to use this heating means for reheating and supersaturating the flow of heat-transfer air. The same applies in general to electric heating, for economic reasons. In general, a boiler will be used, the heating chamber of which comprises one or more appropriate heating tubes, for example immersed in or sprinkled by the liquid to be distilled, through which an available heating fluid will pass. Such a heating fluid can be the primary cooling liquid of a heat engine, the exhaust gases of such an engine, the gases produced by a burner of liquid or gaseous fuel, or also a heating oil, heated during the day by a solar boiler with a cylindrical parabolic reflector, and stored at a high temperature (&gt;130° C.), for use day and night, in a heat-insulated reservoir, at atmospheric pressure. An appropriate solar boiler can be used during the day, for reheating and supersaturating the flow of heat-transfer air.  
         [0173]     Moreover, if, in stills with vapour diffusion and heat-transfer gas circulating in the hollow plates from the top downwards, it is ensured that the liquid to be distilled, whatever the type of boiler used, is heated to a temperature and a pressure higher than their standard boiling values (102° C. and 60 millibar overpressure, for example, for water), it becomes possible to do away with any mechanical means of propulsion of the heat-transfer gas and replace it with a simple calibrated vapour jet, correctly oriented. This technique produces a result equivalent to that provided by the natural convection which would be obtained with a heat-transfer gas circulating from the bottom upwards in the hollow plates. These techniques both have a considerable benefit for the reliability of stills having to operate outside an industrial environment. The same applies to the vapour-diffusion and heat-transfer-liquid stills, in which this liquid circulates by thermosiphon.  
         [0174]     For the implementation of these different particular distillation methods according to the present invention, it is necessary to use several heat-exchange devices, respectively suited to the particular functions which are assigned to them, namely: gas/liquid exchange or liquids/liquid exchange. For simple heat exchanges, without distillation, it is possible to use the heat exchangers available on the market, but their prices appear particularly high, if compared with that of all distillation heat exchangers, in the form of thin and flexible hollow plates, of polymer, described in the PCT Application relating to the prior invention. In the case of a vapour-diffusion still according to the present invention, this renders these exchangers on the market unusable from an economic point of view. As regards the simple heat exchanges provided according to the present invention, the flexible and thin hollow plates described in this PCT Application can ensure these, if they are adapted to their new functions. But, it would be desirable for another type of heat exchanger, better suited to its two types of use, (simple exchange or distillation) to be available under satisfactory technical and economic conditions.  
         [0175]     According to another invention of Jean-Paul DOMEN, which is the subject of the international PCT Patent Application entitled “Heat exchanger. Methods and means of producing this exchanger”, filed under No. Fr 03/03692, 12th December 2003, by “TECHNOLOGIES DE L&#39;ECHANGE THERMIQUE”, a compact counter-current heat exchanger, in particular for confined fluids, is described which provides particularly advantageous conditions of implementation for the third embodiment of the present invention. In fact, this new heat exchanger combines on the one hand, four significant technical characteristics, namely: high efficiency, optimum compactness, reduced weight and intrinsic stability and, on the other hand, an essential economic characteristic, which the heat exchangers currently available on the market do not have, namely, a low production cost. Such a heat exchanger is particularly well suited to the standard heat exchange requirement of the four embodiments of the present invention. Moreover, this new type of exchanger makes it possible, by means of an improvement according to the present invention, to design a new architecture for a vapour-diffusion and heat-transfer gas still according to the third embodiment of the invention. This multiplies the benefit and advantageously makes it possible to replace the large, flexible or rigid, rectangular heat-exchange plates described in the PCT Application referred to at the beginning of the present document.  
         [0176]     According to the PCT Application concerned, an elementary monobloc heat exchanger with high efficiency, limited space requirement, reduced weight, low production cost and, generally, intrinsic stability,  
         [0177]     is constituted by a single active part, in particular made of polymer, formed with neither assembly nor welding, by a stack of pairs of hollow, thin, elongated plates, connecting and globally symmetrical;  
         [0178]     the internal faces of the walls of each hollow plate, and similarly the external faces of the walls of two contiguous hollow plates, are at all points separated from each other by narrow, more or less constant spaces;  
         [0179]     these pairs of hollow plates constitute the elementary ducts of the active part, which ducts comprise elongated central parts the two ends of which are linked to each other, by two hollow couplings;  
         [0180]     each elementary duct of the active part possesses two main feed lines the axes of which are identical with the stacking axes of the end couplings;  
         [0181]     one of the ends of each line ends in a connection tube of the active part.  
         [0182]     This monobloc element of a heat exchanger can be used either as it is, when it has to be installed in the non-confined flow of a fluid to be reheated or cooled down again, or enclosed in a casing, when the two fluids concerned are confined. In both cases, the most efficient way of using such a heat exchanger is with counter-current operation.  
         [0183]     A method for producing such a monobloc heat exchanger comprises the following stages:  
         [0184]     producing in a mould, by blow moulding, a blank made from an appropriate material, constituted by a stack of bellows, which are biconvex overall, relatively deep with regard to the transversal dimension of the blank and comparable to those of an accordion, said bellows comprising elongated central parts, provided with end couplings, sides, crests and bases having respectively shapes adapted such that these sides have a much greater rigidity than those of the bases and of the crests, said stack being provided on its side with two connection tubes, centred on the stacking axes of said end couplings;  
         [0185]     the elements constituting this blank having appropriate temperatures, flexibilities and elasticities, applying to them an internal vacuum and/or external compression forces, parallel to the stacking axis of the bellows, until the compressed part thus produced becomes a stack of pairs of hollow plates, connected and globally symmetrical, with a more or less constant small internal thickness and spacing;  
         [0186]     leaving this part to cool down whilst keeping it in its compressed state;  
         [0187]     if necessary after this cooling down, surrounding this part with an element clamping it, in order to keep the gaps between the walls of the pairs of plates at their initial values.  
         [0188]     According to the present invention, this new counter-current heat exchanger for confined fluids is provided with an additional function, intended to allow good evaporation of the liquid to be distilled, in a vapour diffusion and heat-transfer gas still. In order to do this, the external wall of the blank of each active heat-exchange element used, is rendered hydrophilic or wettable, either by a hydrophilic coating, if appropriate preformed, in the case of a polymer, or by a chemical polishing treatment, in the case of glass. Such an improved blank can once again be produced by blow-moulding of a pasty sleeve, flattened in shape, produced by an extruder, then introduced into a mould suited to this purpose. In the case of a polymer, the internal walls of the mould will have been previously provided with said hydrophilic coating.  
         [0189]     By means of the latter arrangements, the problems of welding, with complex and relatively costly solutions, encountered during the production, installation and use of the large flexible or rigid, rectangular heat-exchange plates, described in the PCT Application concerned, no longer arise. In fact, the only welds to be provided if appropriate for the production of these different compact heat exchangers, used for the implementation of the distillation methods according to the present invention, are those for assembling the constituents of the casing of the active part, said welds being both few in number and relatively easy to produce. The service life of these new heat exchangers depends on that of the material used and, in the case of glass and of a polymer such as polypropylene, it is longer than the service life of the device. One of the additional advantages of this type of monobloc heat exchangers with elongated hollow blades is its extreme compactness. This makes it possible to install, in a given treatment-chamber volume, heat-exchange surfaces, which are in particular more extensive than those obtained with the hollow and flat elements, of large dimensions, described in the PCT Application (i.e. approximately 400 m 2  per cubic metre, instead of 120). Moreover, as the symmetrical pairs of hollow plates, which make up this compact heat exchanger, can in particular be completely safely brought closer together than large hollow plates (2.5 mm instead of 5 mm), the temperature gradient in the inter-plate spaces of the active element of such an exchanger is multiplied by a factor at least equal to two. Consequently, with compact heat exchangers, making it possible to carry out a distillation, the Intrinsic Efficiency Coefficient C IE  of the vapour-diffusion and heat-transfer-gas still which uses them, is multiplied by at least four for each construction. To this, it must be added that, in the case of an active element made of glass, the thermal conductivity of this material is 1.5 W/m.K, i.e. seven times more than that of the polymers. This appreciably increases the total thermal conductance to be taken into account and, in  FIG. 2 , takes the maximum C IE  to a value of 270 instead of 95. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0190]     The characteristics and advantages of the present invention will be shown more precisely by the following description of particular embodiments, given as non-limitative examples, with reference to the drawings hereafter in which:  
         [0191]      FIGS. 1 and 2  represent the curves commented on in the above preamble;  
         [0192]      FIG. 3  represents a diagram of a vapour-diffusion still, using the liquid to be distilled as heat-transfer fluid circulating inside hot hollow plates from the top downwards;  
         [0193]      FIG. 4  represents a diagram of a vapour-diffusion still, using the liquid to be distilled as heat-transfer fluid circulating inside hot hollow plates from the bottom upwards;  
         [0194]      FIG. 5  represents a diagram of a vapour-diffusion still, using large hollow plates for the distillation heat exchanges and a non-condensable gas, saturated with vapour of the liquid to be distilled, as heat-transfer fluid circulating from the top of these hollow plates downwards;  
         [0195]      FIG. 6  represents a diagram of a vapour-diffusion still, using flexible hollow plates for the distillation heat exchanges and a non-condensable gas, saturated with vapour of the liquid to be distilled, as heat-transfer fluid circulating from the bottom of these hollow plates upwards;  
         [0196]      FIG. 7  represents the arrangement in perspective of a set of three large thin and flexible, hollow plates, with corrugated walls, which can be used for distillation heat exchanges in a still according to the invention;  
         [0197]      FIG. 8  represents the feed device of six even or odd plates of a set of these large flexible hollow heat exchange plates according to the invention;  
         [0198]      FIG. 9  represents the means according to the invention for pouring the liquid to be distilled over the coating of the hot hollow plates of a vapour-diffusion and heat-transfer-liquid still;  
         [0199]      FIG. 10  represents the profile and top views of a monobloc distillation heat exchanger, with a low production cost, as well as cross-sections of this exchanger and of the blank from which the active element of this exchanger is produced;  
         [0200]      FIGS. 11-12  are of the representations in simplified perspective of an overall view and details of a still with vapour-diffusion and heat-transfer-gas still circulating from the top downwards inside rigid hollow plates, forming part of monobloc distillation heat exchangers;  
         [0201]      FIG. 13  represents a partial simplified perspective view of a still with vapour-diffusion and heat-transfer-gas still circulating inside flexible, plane, thin, hollow distillation plates from the bottom upwards. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0202]     According to the diagram in  FIG. 3 , which constitutes the first embodiment of a still according to the invention, two plates  10 ,  12  symbolically represent a vapour-diffusion distillation and heat-transfer-liquid unit, constituted by a set of large rigid cellular plates (50 to 150 dm 2 ), rectangular in shape, installed in the treatment chamber of a vapour-diffusion and heat-transfer-liquid still, according to the present invention. These hollow plates  10 ,  12  have a small internal thickness (2 to 3 mm for example) and are separated from each other by a narrow free space  14 , having a thickness of approximately 5 mm, filled with a non-condensable gas, in particular air at atmospheric pressure. The hollow plate  10  is referred to as hot since it is assigned to the evaporation of the liquid to be distilled and, to this end, it is provided with a hydrophilic or wettable coating  16 . The hollow plate  12  is referred to as cold since it is assigned to the condensation of the vapour diffused in the non-condensable gas. It comprises, preferably, an identical coating  15 . A boiler  18 , provided with a heat source  17  and a heating chamber  19 , situated a good distance below the top of the plates  10 ,  12 , is arranged between the high ends of these plates and connected to these ends by pipes  11  and  13  and coupling devices  11   a  and  13   a . This boiler  18  causes a heat-transfer liquid constituted by the liquid to be distilled to circulate in these hollow plates  10 ,  12 , in closed circuit and by thermosiphon. This boiler  18  is of any available type, in particular with a solar collector or burner. The circulation of the heat-transfer liquid takes place in the hot evaporation plate evaporation  10  from the top downwards and in the cold condensation plate  12  from the bottom upwards. The temperature of the liquid entering the plate  10  is T 1  and that of this same liquid, poured over the top of the coating  16 , by means of an appropriate device  11   c , quickly becomes slightly lower than T 1 , because of its rapid evaporation. During its passage through the hollow hot plate  10 , the heat-transfer liquid is cooled down whilst the liquid poured over the coating  16  evaporates  16  and its vapour diffuses in the non-condensable gas. The temperature of the heat-transfer liquid at the outlet from this plate  10  is T 3 . The liquid which exits from the hot plate  10 , through a coupling device  11   b  identical to  11   a , enters a mixer  20  which receives by gravity seawater to be distilled, originating from a counter-current heat exchanger  22 . This exchanger  22  is of the compact, low-cost type, which is described in detail hereafter. This exchanger  22  comprises two active exchange elements  24 ,  26  and a casing  28  enclosing them. These active elements are connected to the two collecting troughs  30 ,  32  of the salt water and of the distilled water which flows off the coating  16  of the evaporation plate  10  and the coating  15  of the condensation plate  12 . In the casing  28 , cold seawater circulates originating, through a flow control valve  34 , from a reservoir  36  arranged above the plates  10 ,  12 . At the outlet from the exchanger  22 , the fresh water and the salt water are discharged into drainage troughs  38 ,  40 . The temperature of the seawater in the reservoir is T L1  and that of the reheated liquid exiting from the exchanger  22 , to enter into the mixer  20 , is T L2 . At the outlet from the mixer  20 , the temperature of the seawater to be distilled is T 4 . In the still, the ratio D/d of the flow rates of the circulating D and entering liquids d is comprised between 8 and 12, as a function of the efficiency of the exchanger  22  and of the usual temperature of the flow entering. The seawater exiting from the mixer  20  enters the cold plates  12  through a coupling device  13   b , identical with to the device  13   a . The condensation of vapour on the external face of the plate  12  causes a progressive increase in the temperature of the circulating liquid, so that, at the outlet from the plate  12 , this liquid is at a temperature T 2 . The fresh water, condensed on the external face of the plate  12 , flows off at a temperature of approximately T 4 , and the salt water, at the bottom of the coating  16 , at a temperature of approximately T 3 .  
         [0203]     In order to assess the efficiency of such a vapour-diffusion still, implementing heat exchanges with a counter-current of water, two numerical examples will be carried out. By way of example, the compact heat exchanger  22  being outside the circuit, the cold seawater at 25° C. is directly mixed with the heat-transfer liquid exiting from the hot plates  10  at T 3 . Given the ratio, generally comprised between eight and ten, existing between the two flow rates D and d, the temperatures at the ends of the plates are, for example, the following: T 1 =99° C., T 2 =95° C. T 3 =68° C. and T 4 =64° C., with dT=4° C. and C OP =(T 1 −T 3 )/dT=8. But if the price of the energy at the site is high, it is necessary to increase the value of C OP  as well as possible, by reducing the value of dT. By way of example, if a gross C OP  of approximately 16 is desired, the value of dT=(T 1 −T 3 )/16. This result can be obtained without heat exchanger  22 , as in the preceding case, for a value T 3 =54° C. and dT=2.8° C., by adjusting the thermal power P of the boiler and the flow of the circulating liquid D. This new value of T 3  is outside the optimum range of the outlet temperatures of the hot plates. According to the curve B in  FIG. 1 , for a temperature T 3 =54° C., we have a C IE  value of 15.6 instead of 17.8 in the middle of the optimum range of T 3 , i.e. 12% less and therefore a daily production of 12% lower, for an unchanged C OP  and active volume of still. On the other hand, if the compact, low-cost heat exchanger  22  is utilized, in order to take the seawater to be distilled to a temperature of 45° C. and therefore the difference dT to 2° C. and T 3  maintained at 68° C., the value of C IE  remains 17.8. This improvement results in an increase in the price of the still equal to the price of the exchanger  22 . With a compact, low-cost heat exchanger, of the type described hereafter, this price is low, in contrast to the high price of the other useable heat exchangers available on the market, and the increases in the distillation C OP  and C IE  which result, for a still thus equipped, are perfectly justified from the economic point of view. It will be noted that the calculation demonstrates that any increase relative to the C IE  of distillation of a vapour-diffusion still allows a symmetrical relative reduction in the total heat-exchange surface utilized, without however modifying the distilled flow rate and the energy consumed. The economic consequence of such a reduction is the difference between the relatively high acquisition and amortization costs of the hollow heat-exchange plates saved, with a relatively short service life (less than five years), and the relatively low, similar costs of the compact heat exchanger used, which both benefit from a low construction cost and particularly long service life.  
         [0204]     Consequently, with a vapour-diffusion still with counter-current of water, according to said first embodiment, which uses large heat-exchange plates, of the type described in said prior invention, and which operates at optimum temperatures T 1  and T 3 , in accordance with the present invention, the use of a compact, low-cost heat exchanger is particularly advantageous. In fact, this type of exchanger makes it possible, for a reduced cost, to bring the cold seawater entering the still to a relatively high temperature which, after mixing, brings the seawater entering the cold plates to a higher optimum temperature. This optimum temperature is obtained by giving an appropriate C OP , by construction, to the exchanger used. This intermediate result for a still with a given active volume V, leads to an improved distillation efficiency, obtained under advantageous economic conditions as regards the daily volume of production of fresh water.  
         [0205]      FIG. 4  represents a diagram of a vapour-diffusion still, according to the second embodiment of the invention, in which the direction of circulation of the heat-transfer liquid in the hot plates is from the bottom upwards, the reverse of that in  FIG. 3 . Consequently, the components of the two distillation units in  FIGS. 3 and 4  are identical, and the diagram is more or less symmetrical to that in  FIG. 3 , their other components being identical or equivalent. They have been given all the same numerical references, with however an additional sign (′) for those in  FIG. 4 . This is in order to differentiate them from each other, the ways in which they are connected together being different. The inlet to the hot hollow plate  10 ′ is connected, by its bottom coupling  11 ′ a  and a duct  11 ′, to the outlet from the heating chamber  19 ′ of a boiler  18 ′ equipped with a heating tube  17 ′. The outlet from the hot plate  10 ′ is connected, by its top coupling  11 ′ b, to one of the inlets to a mixer  20 ′, the other inlet to which is connected to a reservoir  36 ′ containing the seawater to be distilled. The outlet from this mixer  20 ′ is connected to the inlet to the cold hollow plate  12 ′, by a duct  13 ′b. The outlet from this plate  12 ′ is connected, by its bottom coupling  13 ′a, to the inlet to the heating chamber  19 ′ of the boiler  18 ′. The salt water and the fresh water produced are removed by troughs  30 ′ and  32 ′.  
         [0206]     By means of these arrangements, the temperatures at the inlets and at the outlets of the hot  10 ′ (T 1 , T 3 ) and cold  12 ′ (T 4 ,T 2 ) plates are more or less identical to those which it is possible to have with the still according to  FIG. 3 .  
         [0207]     The same applies with regard to the operation of the distillation carried out. As regards the overall efficiency of this still according to  FIG. 4 , it is very clearly below that of the still according to  FIG. 3 , since the temperatures of the fresh water and of the salt water removed (approximately T 1 and T   2 ) are much higher than those (approximately T 3  and T 4 ) obtained in the case in  FIG. 3 . This type of still however remains a second advantageous possibility for implementation of one of the heat-transfer-liquid distillation methods according to the invention, since this drawback can be easily corrected. In fact, it is simple to considerably reduce the temperature of the distilled and condensed liquids to be removed, by means of a double heat exchanger (identical to that referenced  22  in  FIG. 3 ), circulation takes place in the reverse direction in which, in order to better reheat the liquid to be distilled to be poured over the hydrophilic coatings of the hollow plates, it is circulated in the reverse direction.  
         [0208]      FIG. 5  is a flow diagram of a first vapour-diffusion still using air, saturated with vapour of the liquid to be distilled, as heat-transfer fluid. It has the characteristic of causing the air to circulate inside hollow distillation plates from the top downwards. This device constitutes the third embodiment of a still according to the invention.  
         [0209]     According to this  FIG. 5 , the internal  50  and external faces  52  of one of the two walls of a large rectangular hollow distillation plate  54  respectively border its internal volume  56  and the free space  58  which separates two adjacent plates. This plate  54  symbolically represents a vapour-diffusion and heat-transfer-gas distillation unit, constituted by a large number N of flexible or rigid hollow distillation plates, separated by narrow inter-plate spaces. The external face  52  of the wall of the plate  54  comprises a hydrophilic coating  60 . In the vicinity of these first N hollow plates, a reduced number n of auxiliary hollow plates for preheating the liquid to be distilled are arranged. They are similar to the preceding (N) plates but without coating. These n auxiliary hollow plates are symbolically represented by a tube  66 , passed through by the liquid to be distilled, which occupies a space  67 , delimited by the internal faces of the walls  62 ,  64  of a casing  63 . The major part of the flow of hot heat-transfer air enters the top end  57  of the hollow plate  54  and a small part enters that  68  of the space  67 . By a passage  70 , the bottom of the space  67  is connected directly to the outlet from the inside  56  of the hollow plate  54 . The tube  66  is provided at the bottom with an inlet  72 , and at the top with an outlet  74 . A reservoir  76 , containing the liquid to be distilled (brackish water, for example), at temperature T L1 , is installed above the still and, by gravity, it feeds this still through a flow control valve  78  and a tube  77 . The liquid to be distilled is firstly introduced into an appropriate heat exchanger  80 , with counter-current operation. This exchanger  80  comprises, in a casing  82 , a monobloc active element  84 . The inlet to the active element  84  is connected to the tube  77  taking the non-potable water to be distilled and its outlet is connected, by another tube  86 , to the inlet to the casing  87  of a compact heat exchanger  88 , with counter-current operation. The inlet to the casing  82  of the heat exchanger  80  is passed through by the flows of air exiting from the N hollow distillation plates  54  and from the n auxiliary preheating hollow plates  66  and, to this end, this inlet is connected to their common outlet  90 . The outlet  81  from the casing  82  is connected upstream of the helix of a ventilator  92 , installed in the bottom part  94  of the inter-plate space  58 . The distilled water, condensed on the walls of the active element  84  of the heat exchanger  80 , accumulates at the bottom of its casing  82  and is removed by a duct  83 .  
         [0210]     Above the N hollow plates  54 , in  96  a long tray  98  is arranged, covered with a spongy mat  100 , (a thick layer of fabric, for example), provided with a base perforated by numerous holes connected to distribution ducts  102 , installed just above the coatings  60  of these N plates  54 . The duct  104  collecting the salt water, which flows off at the bottom of the coatings  60 , opens into a drainage trough  106 . The duct  108  collecting the thin film  110  of distilled water, which trickles down over the internal faces  50  of the walls of the N hollow plates  54 , is joined by the duct  112  collecting the distilled water, condensed on the external walls of the tube  66  symbolizing the n auxiliary preheating hollow plates, before being connected to the inlet to the monobloc active element  114  of the heat exchanger  88 . The outlet  115  from this element  114  as well as the outlet  83  of the casing  82  opens into a drainage trough  116  for distilled water. The casing  87  of the heat exchanger  88  is passed through by the liquid to be distilled, its outlet being connected to the inlet  72  of the tube  66 , representing the n plates for preheating this liquid. The outlet  74  from the tube  66  is connected to the inlet to the heating chamber  118  of a boiler  120 , provided with a heat source  122 . The heating chamber  118  possesses an outlet duct  124  which feeds a sprinkler  126 , installed lengthwise just above the spongy mat  100  covering the tray  98 . The maximum temperature of the brackish water to be distilled contained in the heating chamber  118  is below its boiling temperature.  
         [0211]     By means of these arrangements, the heat source  122 , for example adapted to supply brackish water with a maximum value of 95° C., for a given entering flow of this water, fixed once for all by an appropriate adjustment of the valve  78 , governs the whole operation of a vapour-diffusion and heat-transfer-gas still, in accordance with the new characteristics of the methods according to the present invention. The hot water, provided by the heating chamber  118  at a temperature of 95° C., falls as rain on the spongy mat  100 . Placed in the flow of heat-transfer air exiting at the top  96  of the inter-plate space  58 , at a temperature T 2  (80° C., for example), appreciably below that of this rain and of the water impregnating the fabric  100 , this water evaporates in part and cools down appreciably, to 87° C., for example. Through the outlet ducts  102 , this water is poured over the top of the hydrophilic coatings  60  of the N hollow distillation plates  54 . The flow of heat-transfer air, which has circulated through said rain and along the tray  98  and its spongy fabric  100  soaked with hot water, is reheated to T 1 =86° C. and, saturated with vapour, introduced inside the N hollow plates  54  and around the tube  66 . During its descent in these plates, the vapour carried along by this flow of air condenses on their internal faces, whilst this flow of air cools down, the brackish water which flows along the coating  60  evaporates in part and that which rises in the tube  66  heats up again. At the bottom of the N hollow distillation plates  54 , the temperature T 3  of the heat-transfer air is 68° C. and, at the bottom of the n auxiliary hollow plates for preheating the liquid to be distilled represented by the tube  66 , the temperature of this air is approximately 42° C. At the inlet to the casing  82  of the heat exchanger  80 , the temperature of the mixture is approximately 62° C.  
         [0212]     The liquid to be distilled enters the active element  84  of the heat exchanger  80 , at a temperature T L1  of 25° C. for example. There it circulates in counter-current with the heat-transfer air. With an exchanger  80 , with a high efficiency coefficient, during its passage through the element  84 , the liquid gains 5° C. whilst the flow of heat-transfer air, which has passed through the casing  82  loses 32° C. to return to a temperature T 4  of 30° C., upstream of the helix of the ventilator  92 , installed at the bottom of the inter-plate space  58 . In order to prevent the electric motor of the ventilator  92  from being damaged under the action of the saturated hot air, this motor is arranged outside. When it is rising in the inter-plate space  58 , the flow of heat-transfer air heats up again and arrives at the top  96  of this space at a temperature T 2  of 80° C. On exiting from the active element  84 , the brackish water is at a temperature T L2  of only 30° C., because of the very different respective calorific capacities and of the respective mass flow rates of the two fluids concerned. As regards the temperature T L3  of the brackish water exiting from the casing  87 , its value is approximately 50° C. The four temperatures T 1  to T 4  appear in  FIG. 2 : T 1 =86° C., T 2 =80° C., T 3  68° C. and T 4 =30° C. If the exchanger  80  had had a lower efficiency coefficient and/or if the temperature T L1  had been higher, the temperature T 4  could have been 40° C. instead of 30° C. and, in this case, the temperature T 3  which would have resulted would have been 72° C. instead of 68° C. The efficiency of the distillation then carried out would then have been reduced since the third expression of C IE  is k.(T 1 −T 3 ).  
         [0213]     The brackish water which flows off the coating  60  of the N hollow distillation plates  54  is at a temperature of approximately T 4  (30° C.), i.e. at a temperature close to that (25° C.) of the brackish water to be distilled. Consequently, it is removed directly by the duct  104  and the trough  106 . On the other hand, the distilled water at the inlet to the active element  114  of the counter-current heat exchanger  88  is at a temperature of approximately 62° C., the same as that of the heat-transfer air at the inlet to the casing  82  of the heat exchanger  80 . It is therefore completely justified to recover the thermal energy from this distilled water and ignore that carried away by the salt water. As the flow of distilled water at 62° C. circulating in the active element  114  of the heat exchanger  88  is weaker than that of the brackish water at T L2 =30° C. which passes through its casing  87 , the temperature T L3  of the brackish water which exits from it is only approximately 52° C. For its part, the brackish water which exits from the n auxiliary hollow plates (tube  66 ) is T L4 =75° C., i.e. 11° C. less than the temperature T 1  of saturated hot air at the inlet to the space  67 . The brackish water at 75° C. which enters the heating chamber  118  of the boiler  120  gains 20° C. there.  
         [0214]     The ratio between the total surface area of the N distillation plates  54  and that of the n auxiliary plates symbolized by the tube  66  is approximately six to ten and the heat exchangers  80  and  88  are, by construction, suited to the results sought. As has been indicated above, the optimum value of the composite variable t.dH/V is relatively high, when the boiler  120  is fed by free thermal energy (solar boiler or cooling water of a heat engine, for example).  
         [0215]      FIG. 6  is a flow diagram of a second vapour-diffusion still using air, saturated with vapour of the liquid to be distilled as heat-transfer fluid. This device has the characteristic of causing the flow of heat-transfer air to circulate inside the hollow plates from the bottom upwards, which is the reverse of that in  FIG. 5 .  
         [0216]     Consequently, the components of the two distillation units are identical and this diagram is more or less symmetrical to that in  FIG. 5 , several of their other components being identical or equivalent.  
         [0217]     All have been given the same numerical references, but with an additional sign (′) for those in  FIG. 6 . This is in order to differentiate them from each other, the ways in which they are connected together being different. This device constitutes the fourth embodiment of a still according to the invention.  
         [0218]     According to  FIG. 6 , in a heat-insulated treatment chamber  48 ′, represented by a frame of unbroken lines, a wall  54 ′ of a thin and flexible hollow plate, possessing an internal space  56 ′ and an inter-plate space  58 ′ between two contiguous two plates, is drawn. In order to simplify the drawing, these two spaces  56 ′ and  58 ′ are limited by the lines defining the chamber  48 ′. The mixture assembly symbolically represents a distillation unit with vapour-diffusion and heat-transfer-gas distillation unit circulating by natural convection. Each hollow plate comprises two walls  54 ′, two bare internal faces  50 ′ and two external faces  52 ′ provided with a hydrophilic coating  60 ′, as well as an inlet  57 ′, situated in its bottom part, and an outlet  55 ′, situated in its top part. The inlets  57 ′ of the hollow plates of said assembly are connected by a bottom vent  59 ′, of appropriate height, to a saturated hot air generator, described hereafter. The outlets  55 ′ of the hollow plates open into a large space  79 ′, of appropriate height, occupied by a monobloc active heat exchange element  84 ′ f . This space  79 ′ constitutes the top vent of the treatment chamber  48 ′ of the still. It extends beyond the active element  84 ′ through another wide space  81 ′ which ends above the inlets  94 ′ of the inter-plate spaces  58 ′ of said assembly. The outlet  96 ′ of the inter-plate space  58 ′ opens into a wide collecting space  83 ′.  
         [0219]     A reservoir  76 ′, containing for example seawater to be distilled, is installed at an appropriate distance above the treatment chamber  48 ′ to feed the monobloc active heat exchange element  84 ′ by gravity, through a tube  77 ′ and a valve  78 ′. The outlet from this element  84 ′ is connected by a tube  86 ′ to spouts  102 ′, arranged just above the top edges of the walls of the hollow plates  54 ′ of said assembly and of their hydrophilic coatings  60 ′. The salt water which runs down along the coatings  60 ′ ends in a single collecting trough  103 ′, connected by a tube  104 ′ to another trough  105 ′, intended to feed a particular solar boiler  120 ′ with salt water. This solar boiler  120 ′ is adapted to evaporate some of this salt water and to diffuse its vapour in a flow of air, in order to constitute said saturated hot air generator. To this end, the base of the heating chamber  118 ′ of this boiler  120 ′ is constituted by a black fleece  122 ′ made of composite material (for example, film made of polymer or oxidized metal foil with an isolated backing on the one hand, and cellulose or polymer non-woven, on the other), impermeable and unalterable on the black side and more or less hydrophilic on the other. This fleece  122 ′ is installed on a rigid grid and its black face, obliquely exposed to solar radiation (S) in accordance with the latitude of the site, is protected from the ambient air by a transparent wall  119 ′. This transforms this heating chamber  118 ′ into a solar collection unit. The upper edge of the fleece  122 ′, with hydrophilic coating, forms a free roof slope which dips into the feed trough  105 ′. The inlets  57 ′ of the hollow plates are arranged just above the tube  104 ′ and the upper edge of the heating fleece  122 ′ and its thin hydrophilic mat, constantly moistened by capillarity and gravity. A reservoir  63 ′, arranged under the heating fleece  122 ′, occupies a large part of the base of the treatment chamber  48 ′. Above the part upstream of this reservoir  63 ′ is installed an isolating block  65 ′ which, on the one hand, separates the outlets  96 ′ of the inter-plate spaces  58 ′ from the inlets  57 ′ to the insides  56 ′ of the hollow plates and, on the other hand, delimits a passage  99 ′ constituting the inlet to the bottom vent  59 ′ of the treatment chamber  48 ′ of the still. This reservoir  63 ′ is intended to collect the salt water which flows down from the hydrophilic coating of the fleece  122 ′. The reservoir  63 ′ comprises a drain tube  128 ′, provided with a valve  130 ′ arranged upstream of a salt water drainage trough  106 ′, both installed outside chamber  48 ′. The distilled water, which condenses into a film  110 ′ on the internal faces  50 ′ of the walls  54 ′ of the different hollow plates, is collected in a single trough  109 ′, itself connected by a tube  115 ′ to a drainage trough  116 ′, installed outside of the chamber  48 ′. As regards the distilled water, condensed on the external faces of the active heat exchange element  84 ′, it is collected in a trough  111 ′ connected by a duct  112 ′ which opens at the top of a vertical tube  113 ′, open to the outside air, ending in the drainage trough  116 ′.  
         [0220]     By means of these arrangements, in the circuit in closed loop thus formed, the black face of the fleece  122 ′, installed on the base of the solar collection unit  118 ′, absorbs the solar radiation (S), heats the salt water which impregnates the thin hydrophilic mat on its other face, evaporates some of its water and diffuses the vapour produced in the air which surrounds it. In this way, this air is thus progressively reheated and kept saturated and it becomes, by natural convection, a flow of saturated hot air which passes through the bottom vent  59 ′ then penetrates to the insides  56 ′ of the hollow plates, by their inlets  57 ′, and then it circulates from the bottom upwards in these vertical hollow plates then in the top vent  79 ′ and along the external faces of the monobloc active heat exchange element  84 ′. This heat exchange element  84 ′ is passed through by the flow of seawater to be distilled entering the still. Passing along the walls of this element  84 ′, the flow of air cools down, then it runs down by gravity in the space  81 ′, the inter-plate spaces  58 ′ and the passage  99 ′ then, subjected to the draught engendered by the heating fleece  122 ′, it sweeps the surface of the hot salt water contained in the reservoir  63 ′ and that of the constantly moistened hydrophilic face of the fleece  122 ′, which soaks in the feed trough  105 ′, thus looping the passage taken in closed circuit. The height which separates the lower edge of the heating fleece  122 ′ from the edge upstream of the heat exchange element  84 ′, must be relatively great. It is adjusted once for all by setting of the heights of the bottom and top vents  59 ′ and  79 ′. This is done in order that the speed v of rising circulation of the flow of heat-transfer air, to the inside  56 ′ of each hollow plate is sufficiently great (20 to 50 cm/s). Under these conditions, given the local difference in enthalpy flows per unit of active volume dH/V of the hollow plates, engendered by the boiler  120 ′ between the inlet  57 ′ of the insides  56 ′ of the hollow plates and the outlet  96 ′ of the inter-plate spaces  58 ′, a possible optimum value at of the transit time t of this flow in the hollow plates can be determined. This is done on the basis of the optimum range of the values of the composite variable t.dH/V arbitrarily delimited by the values of C IE  greater than 84 m 3 /day.m 3 . namely 200 to 740 kilojoules per cubic metre (see curve B 2  in  FIG. 2 ). The height of the distillation plates and those of the bottom and top vents are chosen at the same time, given the maximum value of the temperature T 1  (which must remain comprised within the range concerned of its optimum or simply effective values) of the flow of air circulating by natural convection, that the solar boiler  120 ′ can produce.  
         [0221]     At the level of the inlets  57 ′ of the hollow plates, the temperature T 1  of the flow of air is limited because of the solar boiler without reflector used, but this temperature remains in its optimum range, i.e. approximately between 70 and 80° C., at least when the sun is at its highest point. Passing through the space occupied by the active heat exchange element  84 ′ of, passed through by the seawater entering at a temperature of 25° C., the flow of air which exits from the hollow plates at a temperature T 3  of approximately 68° C. is cooled down and its temperature falls to an optimum value T 4  which is very low, namely approximately 30° C., when the efficiency of the monobloc active element  84 ′ is appropriate.  
         [0222]     The outlet tube  86 ′ of the active element  84 ′ of the heat exchanger  80 ′ feeds the spouts  102 ′ with seawater at a temperature of approximately 50° C. This tepid seawater thus poured over the coatings  60 ′ runs down slowly along the external faces  52 ′ of the walls  54 ′ of the hollow plates. Consequently, the water vapour carried along by the flow of saturated hot air, which rises inside  56 ′ the hollow plates, condenses on the internal faces  50 ′ of the walls of these plates and forms a thin film of distilled water  110 ′. While the seawater runs down in the coatings  60 ′, this seawater heats up again, under the action of the latent heat of condensation recovered through the walls  54 ′ of the hollow plates. Therefore, this water evaporates in part and the vapour produced diffuses in the flow of air cooled down, which runs down in the inter-plate spaces  58 ′, and thus progressively reheats this flow. At the outlet from these inter-plate spaces, the temperature of this flow of air reaches a value T 2  of approximately 78° C. As regards the salt water, collected at the bottom of the hydrophilic coatings  60 ′ of the walls  54 ′ of the hollow plates, its temperature is also approximately 78° C. This salt water collected by the trough  103 ′ is taken by the duct  104 ′, into the feed trough  105 ′, by capillarity and gravity, from the rear hydrophilic coating of the fleece  122 ′ to the front black face, installed at the base of the heating chamber  118 ′ of the particular solar boiler  120 ′. The maximum temperature of this heating fleece  122 ′ and of the salt water that its coating contains is with at the most 85° C. (such a solar boiler without reflector scarcely makes it possible to reach a higher temperature). A small part of the water of this salt water evaporates and the remainder flows slowly into the reservoir  63 ′, which thus gradually fills with slightly more concentrated salt water, the temperature of which is approximately 82° C., intended to be removed. The vapour thus produced at the surface of the hydrophilic coating of the heating fleece  122 ′ is carried along by the flow of air which has discharged from the inter-plate spaces  58 ′ then swept the surface of the hot salt water contained in the reservoir  63 ′ and, with approximately one degree thus gained, penetrates with a temperature of slightly more than 78° C., at the foot of the hot constantly moistened coating of the fleece  122 ′, along which it heats up again and is saturated once again.  
         [0223]     It will be noted that is possible to pour the seawater from the reservoir directly over the coating  60 ′ in place of the tepid seawater exiting from the heat exchanger  84 ′. In this case, this tepid water is directly removed. The temperatures T 4  and T 2  are slightly affected by this but the general operation and performances of the assembly are scarcely modified.  
         [0224]     The advantage of this vapour-diffusion still with a flow of heat-transfer air circulating in the hollow plates from the bottom upwards, is multiplied if compared to the still in  FIG. 5 , in which the flow of heat-transfer air circulates inside these plates from the top downwards. The first advantage resides in the fact that no means of propulsion (ventilator or vapour jet) is necessary to ensure the circulation of this flow of air, since this circulation is here engendered by natural convection. The second advantage results from the fact that the temperature of the heat source can be comprised between approximately 75° and 885° C. and however remain effective since capable of ensuring, at the inlet to the hollow plates, a temperature T 1  which is also optimum or simply effective. This has the direct consequence of adding a third advantage, namely to render a solar boiler without a reflector perfectly suitable for such a still. A fourth advantage resides in the total absence of moving parts operating permanently. This constitutes a particularly useful advantage (no need for any of the maintenance generally required by such parts) in all cases where this type of still is used in a non-industrial environment. A fifth advantage appears in the fact that a very considerable performance coefficient C OP  of the still can in principle be obtained, since the increase in the temperature of the salt water, supplied by the boiler, can be very small (&lt;2° C.). In the still according to this  FIG. 6 , the temperatures of the salt water and of the distilled water to be removed are high (approximately 82° C.), but it will be described hereafter, in comments on  FIG. 13 , how it is possible to recover this thermal energy in order to diffuse additional vapour into the flow of heat-transfer air and thus considerably increase the C OP  of the device. A sixth advantage originates from the considerable increase in the C IE , referred to above, which results from the very small wall and hydrophilic coating thickness of the new type of thin hollow plate, with very fine, flexible, plane walls, described hereafter in  FIG. 13 . The comments, which accompany this  FIG. 13 , relate to an actual embodiment of a vapour-diffusion still with heat-transfer gas, circulating in closed circuit by natural convection. They will confirm, by the low production cost of this new hollow distillation plate, the particularly great advantage of this last embodiment of the invention.  
         [0225]     It will be noted that with such a solar-boiler still, it is possible to produce fresh water after the end of the effective insolation of the installation site. This firstly requires a well heat-insulated reservoir  63 ′ which is moreover, deep enough to be able to contain at least all the salt water produced during one day&#39;s insolation. By keeping the drain valve  130 ′ closed after sunset and reducing (for example by half) the flow of seawater entering, by action on the valve  78 ′, the production of fresh water by this solar still on can be extended late into the night and thus increase the day&#39;s production by approximately 20%. This result is obtained by means of the additional reheating and saturation provided for the flow of air which sweeps the surface of the large mass of hot salt water contained in the reservoir at the end of the day and constantly re-fed with a salt water the temperature of which is slightly below its own. As the temperature of this salt water drops, the flow rate of distilled water will do the same until it ends up just dripping. The normal operation of the still will be resumed in the morning and will simply comprise emptying the reservoir by opening the valve  130 ′ provided for this purpose for a moment, and giving the flow of seawater entering its day-time value (which will in general depend on the maximum intensity of the solar radiation to be provided for the day). Under these conditions, the temperature of the salt water removed in the morning is relatively low and the overall C OP  as well as the overall C IE  of such a solar still are appreciably improved.  
         [0226]      FIG. 7  represents diagrammatically three large, flexible hollow plates, provided with their frame and their coupling washers.  FIG. 8  represents a longitudinal cross-section of one of the four feed devices of a large number ( 6 , in the drawing) of large hollow, rectangular plates, odd or even in number, ensuring heat exchanges in a still, according to the invention, which operates with a heat-transfer fluid liquid. As regards  FIG. 9 , it represents the device ensuring the distribution of the hot liquid to be distilled, over the hydrophilic coatings only of the plates assigned to the evaporation of this liquid, when the heat-transfer fluid is a liquid.  
         [0227]     According to  FIG. 7 , each flexible rectangular plate  140   1,2,3  which measures, for example, 120 cm in height and 100 cm in width is made from a thin sheet (in particular of polypropylene), provided with a welded hydrophilic coating (in particular, a cellulose non-woven, represented in dots), folded in two, the fold constituting the upper edge of each plate.  
         [0228]     When the plates  140   1,2,3  are of the flexible type, sets of parallel weld lines (up to 50) are formed, which define the internal ducts  140   1,2,3  of these plates, which are for example 15 to 20 mm in width and 80 cm in length. At the top and at the bottom of these sets of parallel ducts  141   1   142   1,2,3 , two oblique weld lines  141   1   144   1,2,3 , et  146   1,2,3 , inclined and parallel, are produced, which define a top common channel  148   1,2,3 , and a bottom common channel  163   1,2,3  respectively, both trapezoid in shape. The part of each plate  140   1,2,3  situated above the oblique line  144   1,2,3 , constitutes a sleeve  150   1,2,3 , the two ends of which are cut, in order to provide one large and one small cutout  152   1,2,3  and  153   1,2,3 . On both sides of the sets  142   1,2,3  of parallel lines, two weld lines  154   1,2,3  and  156   1,2,3 , are produced parallel to the preceding ones, which constitute the external edges of each plate  140   1,2,3 . These same lines  154 - 156 , in cooperation with the external line, extended by its two ends, which borders the first and the last duct of each plate, delimit two vertical sleeves  158   1,2,3  and  160   1,2,3 , approximately 4 cm in width, over the whole height of the elements. Such flexible plates possess corrugated walls.  
         [0229]     The two slopes of wall  162   1,2,3 , situated below the low oblique line  146   1,2,3  of each plate, are folded upwards in order to constitute, with the external wall of its common low channel  163   1,2,3 , two collecting liners for the liquids which have seeped into the hydrophilic coatings of the two walls of the plates  140   1,2,3 . A trough (not shown) is arranged under the bottom ends of the two collecting liners of each plate, so that, because of the opposite orientations of the collecting liners of two contiguous plates, one of the troughs will collect the liquid which flows off the odd-numbered cold plates and the other, that from the even-numbered hot plates.  
         [0230]     Each plate  140   1,2,3  has a semi-rigid frame which comprises two horizontal rods and two vertical strips, both made of steel, for example, or of a reinforced polymer with high mechanical strength. The rods have a U-shaped cross-section, the top one  164   1,2,3  an inverted U, for the suspension of the plate and the other bottom one  166   1,2,3  a U the right way up, to give it a longitudinal pressure and complete the frame. By way of example, the external thickness of these rods is 3 mm, their height 10 mm and their wall thickness 1 mm. The ends of these rods comprise, set back over their sides, two cusps (not shown). The openings of the inverted-U rods  164   1,2,3  are engaged on the ends of vertical strips  168   1,2,3  and  170   1,2,3 , with rounded edges, 3.5 cm wide and 1 mm thick. The gap between these strips is imposed by that between the stops constituted by the cusps of the rods. The rods  164   1,2,3  as well as the strips  168   1,2,3  and  170   1,2,3  are respectively engaged in the horizontal  150   1,2,3  and vertical sleeves  158   1,2,3  and  160   1,2,3 . The distance between these strips, which is held fixed by the U-shaped rods  164 ,  166 , determines the initial transversal pressure of the flexible plates  140   1,2,3 .  
         [0231]     Should rigid cellular panels be used in place of the flexible plates, thin sheets with a hydrophilic coating, identical to those used for the flexible plates, are previously glued then welded onto these panels, by weld lines similar to, but more widely spaced than, those producing the sets of ducts  142   1,2,3 , in order to ensure the reliability of the assembly thus constituted. This welding operation is more or less identical to that carried out in order to produce the flexible plates, which consists of pressing the elements to be welded, for a few seconds, between two thick metallic trays, provided with surfaces which are straightened then machined according to the weld lines to be produced, these trays being taken to an appropriate temperature, defined by the melting point of the polymer used. In the two cases, the edges of the sleeves of the rods and of the strips are welded whilst the edges of the cutouts at the ends of the horizontal sleeves and the exact positions of the connecting washers, presented hereafter, are marked to be put into place in a subsequent stage of the method of production of flexible or rigid plates. Each plate  140   1,2,3  comprises, in the diagonally opposite wide corners of its common top  148   1,2,3  and bottom channels  163   1,2,3 , washers  172   1,2,3  and  174   1,2,3  for feeding through these common channels. These washers and these common channels cooperate in order to ensure the distribution or the recovery of the heat-transfer fluid entering or exiting from these ducts. The dotted lines which link these washers in  FIG. 7 , represent the positioning of the odd- or even-numbered sets of feed devices (illustrated in  FIG. 8 ), which pass through the wide cutouts  152   1,2,3  of the sleeves  150   1,2,3 . Such rigid cellular plates possess plane walls.  
         [0232]     According to  FIG. 8 , the feed device of six even or odd hollow plates comprises a stack of six washers  172   1-6 , associated with a T-shaped coupling  180 , comprising a first tube  182 , coaxial with these washers, and a second at a right angle,  184 . This stack and this coupling are held in place by a tie rod  186 . Each of the washers  172   1-6  is a ring measuring, for example, approximately 17 mm in thickness and 4 cm internal diameter, in the case of hollow plates of one m 2  provided for a still with counter-current of water. Each ring is provided, in its central part, with a circular flange  188   1-6 , the side faces of which are welded to the internal faces of the walls  190   1-6  and  191   1-6  of a plate  140   1-6 ,  4  (see  FIG. 5 ) and the thickness of which is more or less equal to the internal thickness of these plates, i.e. approximately 2 to 3 mm. The downstream edge of the ring of each washer  172   1-6  comprises an external shoulder  171   1-6  and its upstream edge, an internal shoulder  173   1-6 . The circular flange  188   1-6  of each washer  172   1-6  has several horizontal holes  192 , 3.5 to 4 mm in diameter (8 holes, according to the drawing) which, on the one hand, open inside the washer and on the other, inside and in the lengthwise direction of the common trapezoid-shaped channel  148   1-6  (see  FIG. 5 ) which feeds the sets of ducts  142   1-6  of a plate  140   1-6 .  
         [0233]     The tie rod  186  comprises (1) a support base  194 , provided with an internal shoulder  195 , suitable for cooperating with the external shoulder  171 , of the downstream washer  172   1 , (2) a tapered rod  196 , the length of which is determined by the number of washers  172  to be stacked (a hundred, if appropriate) and (3) a threaded cylindrical end  198 . The tube  182  of the coupling  180  comprises, welded and/or glued at its two ends, supports respectively constituted by a cup  200 , with a hole through its centre and a ring  202 , provided with an external shoulder  203 , suitable for cooperating with the internal shoulder  173   6  of the upstream washer  172   6 . The support cup  200  is suitable for sliding on the end  198  of the tie rod  186 . This end  198  comprises a housing for an O-ring  204 . A nut  208 , engaged on the threaded end  198  of the tie rod  186 , makes it possible to keep the washers  172   1-6  clamped and transform their stack into a leak-free duct, for feeding the hollow plates  140   1-6 . Between the internal faces  191   1,3,5  and  190   2,4,6  of the walls of contiguous plates, which are welded to the circular flanges  188   1-6  of the washers  172   1-6  the high ends of the weld lines  144  (see  FIG. 7 ) of the intercalated odd plates appear as  193   1-65 , in  FIG. 8 .  
         [0234]      FIG. 9  represents, in cross-section, the top part of a set of nine flexible plates, comprising five odd-numbered cold plates  140   1,3.,5,7,9  and four even-numbered hot plates  140   2,4,6,8 , arranged alternating in a vapour-diffusion still, using the liquid to be distilled as heat-transfer fluid. These flexible plates are suspended from nine rods shaped as an inverted U  164   1-9 , engaged in trapezoid-shaped sleeves  150   1-9 , delimited by oblique weld lines  144   1-s9 . In this  FIG. 9 , the thin walls  210   1-9 , made of polymer (in particular of polypropylene) of the plates  140   1-9  as well as their hydrophilic coatings  212   1-9 , can be seen clearly. Between two contiguous plates, such as  140   1  and  140   2  or  140   8  and  140   9 , inserts  214   1-86 , preferably cellular, are arranged which go down to the top of the sets of ducts  142   1-9  (see  FIG. 7 ) of the plates  140   1-9 . The length of these inserts  214   1-8 9  equals the maximum width of the sleeves  150   1-9  of the suspension rods  164   1,9  of the plates  140   1,9 . The upper portion of each of the cold plates  140   3,5,7,9  as well as the two inserts, such as  214   2  and  214   3 , which border them, is covered with an impermeable cover, such as  216   3,5,7  which goes down to the lower edge of these inserts. This impermeable cover is produced by means of an impermeable sheet with a hydrophilic coating, identical to the material constituting the flexible plates, its hydrophilic coating  217   3,5,7  being in contact with that  212   2,4,6,8  of the even-numbered plates  140   2,4,6,.8 . The end inserts  214   1  and  214   8 , of a set of plates  140   1-9 , are separated from the plate  140   2  for the one and from the plate  140   8  for the other, by an impermeable sheet with a hydrophilic coating  218  and  220 . These sheets cooperate with two strips  222  and  224 , serving as support stops, in order to constitute the impermeable edges of a hydrophilic mat  226 , in contact with the upper portion of the hydrophilic coating of each of the hot plates  140   2,4,6,8  and of the hydrophilic coating of the protective covers  216   3,5,7  of the cold plates. This hydrophilic mat  226  is, for example, constituted by several layers of cotton fabric. Above this mat, spouts, such as  228 , are installed from place to place, suitable for pouring over it the hot liquid to be distilled. Between the slopes  162   1-9  which form the collecting liners of the liquids which flow from the external walls of the plates  140   1-9 , (see  FIG. 7 ) inserts (not shown) are arranged, identical to those  214   1-8  placed between the tops of these same plates. In order to constitute a still, the compact unit formed by the assembly of N hollow plates  140   1-N  is kept confined, by conventional clamping means, not shown, arranged around it.  
         [0235]     By means of the arrangements according to  FIGS. 7, 8  and  9  presented above, the hollow distillation plates of the vapour-diffusion stills according to  FIGS. 3 and 4  operate under the best conditions (the case of the hollow distillation plates of a still, according to  FIGS. 5 and 6 , will be dealt with in detail hereafter, in comments on  FIG. 13 ). Under the pressure of the heat-transfer liquid, the ducts of a flexible plate, as well as the common channels for distribution and recovery of this heat-transfer liquid in flexible or rigid plates, retain the correct thicknesses. Thanks to the lateral vertical strips, with fixed spacing, the ducts of the flexible plates can only have a limited internal thickness, of approximately 2 to 3 mm, in response to the pressure exerted by the heat-transfer liquid which circulates in them. Moreover, thanks to the inserts and clamping means referred to above, the top  148  and bottom  163  common channels are themselves preventing from collapsing under this same pressure. Under these conditions, the thickness of the free space between the plates  140  is kept at a correct value, namely approximately 5 mm.  
         [0236]     As regards the pitch of assembly of these plates  140 , it is equal to half the distance separating the internal and external shoulders of the coupling washers  172   1-69 , i.e. 8.5 mm. As regards these washers, It will be noted that under the action of the tie rod  186 , they are stacked tightly, making a leak-free line, of adjustable length. Moreover, the holes  192  make it possible, without appreciable loss of charge, to cause the heat-transfer fluid to enter into or exit from the top or bottom common channels of each hollow plate.  
         [0237]     Thanks to the arrangements according to  FIG. 9 , in a still with heat-transfer liquid, the coatings of the hot plates, assigned to the evaporation of the hot evaporation liquid to be distilled, are the only ones to be capable of being wet by this liquid. In fact, thanks to the impermeable covers  216  covering the tops of the cold plates as well as their two associated separation inserts  214 , the hot liquid to be distilled cannot reach them, whilst, under the action of the hydrophilic coating of these same covers, this hot liquid, which passes through the hydrophilic distribution mat  226  is brought, by gravity and capillarity, as far as the hydrophilic coatings of the set of hot plates.  
         [0238]      FIG. 10  represents in A and B, profile and top views of a compact, low-cost heat exchanger and in C and D, cross-sections of this exchanger and of the blank of its monobloc active element. According to  FIGS. 10A and 10C , the compact heat exchanger  250  comprises a casing  252  which completely surrounds an active exchange element  254 . This active element  254  is constituted by the stacking of a relatively high number (up to thirty, for example) of pairs of hollow plates  256   a - b , at the same time elongated, symmetrical and connecting. According to the section  10 C, the cross-section of the active element  254  is in the form of a fish&#39;s vertebral column, provided with hollow ribs  256   a - b , oblique and parallel to each other, which share a common central channel  258 . The internal thickness of these ribs  256 , and the distance separating them  260  and their common central channel  258  is small and more or less identical (2 mm, for example). The thickness of the walls of the active element  254  is thin (0.5 mm, for example).  
         [0239]     Each hollow plate  256   a - b  of the active element  254  comprises a rectilinear central part the length of which can vary from approximately 30 to 100 cm and the width from approximately 5 to 15 cm. A hollow plate  256   a  is connected to its symmetrical plate  256   b  by two couplings with hollow ends  262 - 264 , in the form of truncated semi-cones. The stacking axes of these truncated semi-cones coincide with the axes of the two lines which feed the different pairs of stacked hollow plates  256   a - b  and they end in the two coupling tubes  266 - 268  of the active element  254 .  
         [0240]     The casing  252  is represented transparent for the needs of the drawing of  FIG. 10A . It is formed of two half-shells  251 - 253 , with respectively convex and concave bases, assembled in tight fashion (welding, gluing or sealing device) by their assembly flanges  255   a - b  and  257   a - b.    
         [0241]     The gap between the casing  252  and the edges of the plates  256  of the active element  254  is small (1 mm, for example) but it is zero along the crest  270  of its convex wall and along the hollow  272  of its concave wall. The casing  252  possesses two coaxial coupling tubes  274 - 276  and two side openings through which the coupling tubes  266 - 268  of the active element  254  pass, the edges of these openings being welded, glued or assembled with a sealing device, at the root of these two tubes  266 - 268 .  
         [0242]      FIG. 10D  represents the cross-section of the blow-moulded blank  276 , from which the active heat exchange element  254  has been produced. This blank  276  comprises a stack of relatively long biconvex bellows  278 , provided with relatively short end couplings (see  FIG. 10A ) in the form of symmetrical truncated semi-cones. The stack of the bellows  278  is comparable to an accordion the bellows of which would have levelled-off crests  280  and narrow bases  282 , with sufficiently great depths of bellows before the large diameter of the end semi-cones, to allow the latter to constitute turned-over surfaces, involving a transition buckling when turned over. The transformation of the blank  276  into an active element  254  is carried out under the action of a controlled axial compression force. This force has the effect of causing each of the two symmetrical sides of each convex semi-bellow to pass from one stable state to another, becoming parallel to one of the two symmetrical sides of each concave semi-bellow which is associated with it. In the case of an active element made of glass, the transformation of the bellows from the blank into small parallel plates takes place at a particular temperature, giving the glass used an appropriate flexibility and elasticity. It will be noted that the flattening of the bellows of such blanks made of polymer or glass can be done without the tilting of one of the walls of the hollow end couplings and that an efficient monobloc heat exchanger is however produced, as taught in the PCT Application concerned, referred to above.  
         [0243]     The blank  276  makes it possible to produce a standard active heat exchange element. For a heat exchanger having to evaporate the liquid to be distilled, in accordance with said prior invention, the walls of a blank  276  made of polymer are provided with a thin hydrophilic coating  284 , preferably preformed, having a thickness of for example 0.1 mm. In this case, the blank  276  is, once again, produced by blow-moulding of a sleeve made of pasty polymer, flattened in shape, produced by an extruder, then introduced into a mould comprising multiple parallel grooves, previously provided with the coating  284 . In the case of an active element made of glass, the process for producing the blank is more or less identical to that used for the polymers. As regards the chemical treatment intended to give a dull finish to the internal and external faces of such an element made of glass, in order to make them wettable, this is carried out according to a technique which is perfectly familiar to glass makers. The coatings  284  (or the dull-finished faces) of the pairs of plates  254  of the active element  250 , are associated with a common layer of hydrophilic fabric  286 , which covers all the top end couplings  262  of this element (it is vertical in a still according to the invention). This common hydrophilic layer  286  is intended to uniformly distribute, over the coatings  284  of the plates  254 , the liquid to be distilled which is introduced into the casing  252 , through its top tube  274 .  
         [0244]      FIGS. 11 and 12  relate to a particular embodiment of a vapour-diffusion still module using a non-condensable gas saturated with vapour of the liquid to be distilled as heat-transfer fluid and compact distillation heat exchangers of the type described in FIGS.  10 A,B,C.  FIG. 11A  is an overall view of such a module.  
         [0245]      FIG. 11B  represents the details of this module and  FIG. 11C , a cross-section of one of the heat exchangers used. As regards FIGS.  12 A,B, they represent the details of the pipes and couplings of the different fluids which circulate in the still.  
         [0246]     According to FIGS.  11 A,B, the still  290 , presented as an example, is a module comprising firstly (1) eight compact distillation heat exchangers, vertically arranged,  292   1-8 , intended to ensure evaporation of the liquid to be distilled then condensation of its vapour, and (2) a simple compact heat exchanger  294 . According to  FIG. 12C , which is the section according to the plane C-C of  FIG. 11B , the active element  293   1-8  of each compact exchanger  272   1-8  comprises eight pairs of symmetrical, integral, thin, small hollow plates. According to  FIG. 11B , these pairs of plates are provided with a hydrophilic or wettable coating  284   1-8  and with a cover made of hydrophilic fabric  286   1-8 , ensuring a uniform distribution of the liquid to be distilled over all the coatings  284   1-8 .  
         [0247]     In this example of a still  290 , each plate of the eight symmetrical pairs of an active element  293   1-8  has a width of 10 cm, a length of 60 cm, an internal thickness of 2 mm, a wall thickness of 0.5 mm, a coating thickness of 0.1 mm and separation gaps of 2 mm. The surface area of each active element  2931 - 8  is approximately 1 m 2  and its total volume 2.5 dm 3 . The active volume V of a module of eight elements is 20 dm 3  and its total heat exchange surface area 8 m 2 .  
         [0248]     According to  FIG. 12C , the eight active elements  293   1-8  with vapour diffusion are combined in a single casing  296  but they could just as well be isolated or combined two by two or four by four in smaller casings. In any case, each active element  293   1-8  is associated with two coaxial inlet  298   1-8  and outlet  300   1-8  ports, provided in the part of the casing which surrounds it. According to FIGS.  11 B and  12 A,B, each active element  293   1-8  with vapour diffusion comprises, in its top part, a lateral inlet port  302   1-8  and, in its bottom part, a lateral outlet port  304   1-8 , diagonally opposite the preceding one. Similarly, the simple heat exchanger  294  comprises an active element  295 , provided with lateral inlet and outlet ports  305 ,  307  and a casing  308 , provided with two coaxial inlet and outlet ports  310 ,  312 .  
         [0249]     Above the still  290 , a seawater reservoir  314  is installed, linked by a tube  316   a - b  and a valve  317 , to a duct  318  which passes through a tube  320 , into which the eight outlet ports  304   1-8  of the active elements  293   1-8  with vapour diffusion and the outlet port  307  of the active element  295  of the simple heat exchanger  294  open. The duct  318  is linked to the inlet to the casing  322  of a counter-current heat exchanger  324  and the outlet from this casing is connected, by a tube  319 , to an antechamber  326 , preceding the inlet port  310  of the casing  308  of the simple compact exchanger  294 . This exchanger  324  is the subject, in  FIG. 11B , of a symbolic representation but, in  FIG. 12 , its representation conforms more closely to reality. This heat exchanger  324  is of the compact type and it comprises an active element  328 , the cross-section of which is represented in  FIG. 11C . The function of this element  328  is described in detail hereafter. The seawater which exits from the exchanger  324  passes through the heat exchanger  294  then exits from it, by its outlet port  312 , to enter a boiler  332 .  
         [0250]     According to  FIG. 12A , the boiler  332  comprises an inlet part  334 , extended by a heating tube  336 , itself passed through by a tubular radiator  338 . This radiator  338  possesses an inlet  340  and an outlet  342 , both external to the boiler  332 , and it is suitable for an appropriate heating fluid (hot gas or liquid from 105 to 120° C.) to pass through without damaging it. To this end, the radiator  338  can be made of a metal, suitable for resisting possible corrosion by the heating gas used, or of a polymer having good mechanical resistance to the temperature of the hot liquid. The heating tube  336  comprises at its downstream end (1) a dividing wall  344 , passed through by the tubular radiator  338 , (2) in the top part of this dividing wall  344 , one or more calibrated orifices  346 , suitable for engendering one or more vapour jets  347 , when the seawater arrives in this heating tube  336  and (3) in the bottom part of this same tube  336 , one or more holes associated with one or more short tubes  348 , with calibrated section, suitable for ensuring an appropriate withdrawal of this water.  
         [0251]     The boiler  332  is enclosed in an elongated cylindrical duct  350 , with a circular section, arranged horizontally, and the outlet ports  312  and  300   1-8  of the casings  308  and  296  of the heat exchangers  294  and  292   1-8  open into the bottom part of this duct. The inlet part  334  of this boiler occupies the upstream end of the duct  350  and it comprises, slightly after the outlet port  312  of the casing  308  of the exchanger  294 , a thick dividing wall  352 , with a truncated-cone-shaped opening  354  made in its centre, occupied by a closing device  356  with an identical profile, suitable for progressively closing this opening when it is pulled upwards. The closing device  356  is linked to a float  358  by two linking rods  359   a - b , between which the downstream end of a tubular radiator  338  passes. When the seawater reaches an appropriate level in the inlet part  334  and in the heating tube  336  of the boiler  332 , the float  358  brings the needle-valve closing device  356  to completely close the inlet opening  354  of the boiler, which thus operates at a constant level of seawater, situated above the tubular radiator  338 . In the bottom part of the duct  350 , under the heating tube  336  of the boiler  332 , a chamber  360  for superheating and supersaturation of the heat-transfer gas is installed, occupied by a narrow and slightly hollow tray, covered with several layers of hydrophilic fabric  361 . The seawater exiting from the heating tube  336  of the boiler  332  by the calibrated extraction tube  348 , flows over the downstream end of the tray and soaks the whole of the hydrophilic fabric  361 . In turn, this tray has eight calibrated holes, situated just above the eight outlet ports  300   1-8  of the casing  296  of the active elements  293   1-8  with vapour diffusion. A wick and/or a tube  362   1-8  engaged in each of the holes through the tray and in each of the outlet ports  300   1-8  of the casing  296 , establish a link between the hydrophilic coating  361  of the tray and the hydrophilic cover  286   1-8  of the end couplings  274  (see  FIG. 10A ) of the active elements  293   1-8 .  
         [0252]     The horizontal cylindrical duct  350 , surrounding the heating tube  336  of the boiler  332 , is linked by a bent tube  364  to another horizontal cylindrical duct  366 . The inlet ports  302   1-8  of the active elements with vapour diffusion  293   1-8  and the inlet port  305  of the active element  295  of the simple heat exchanger  294 , open into this duct  366 , whilst the outlet ports  304   1-8  and  307  of these same active elements open into the duct  320 . This duct  320  is linked by a bent tube  368  to another horizontal cylindrical duct  370 , into which the inlet ports  298   1-8  of the casing  296  of the active elements  293   1-8  open. The duct  370  comprises, at its downstream end, a dividing wall  371  which separates it from the antechamber  330  of the casing  308  of the simple heat exchanger  294 , the external wall of this antechamber extending that of the duct  370 .  
         [0253]     The distilled water which flows from the outlet ports  304   1-8  and  307  of the active elements  293   1-8  and  295  and that which has condensed on the external wall of the tube  318  passed through by the cold seawater accumulates at the base  372  of the horizontal duct  320 . The salt water which flows from the heat-transfer-gas inlet ports  298   1-8  of the casing  296  accumulates on the base  374  of the horizontal duct  370 . This base  372  is linked to the inlet to the active element  328  of the exchanger  324  (see  FIG. 12B ), by a tube  376 . The outlet from this active element  328  opens into a tube  378  and a drainage trough  379  of the distilled water, whilst the salt water which accumulates at the base  374  of the duct  370  is removed by a tube  380  and a trough  381 .  
         [0254]     Thanks to the embodiments given to the still according to the invention and to the boilers capable of feeding it, described in  FIGS. 11 and 12  commented on above, we have particularly useful distillation devices, with vapour diffusion and heat-transfer gas. The general functioning of the still described in  FIGS. 11-12 , is identical to that of the still, according to the third embodiment of the invention described in  FIG. 5 , which has been described in detail above. The N hollow distillation plates  54  are replaced by the eight compact exchangers with vapour diffusion  292  and the n auxiliary reheating plates, represented by the tube  66 , replaced by the simple heat exchanger  294 . The active element  84 , passed through by cold water to be distilled, of the heat exchanger  80 , replaced by the duct  318  similarly passed through, the casing  82  being replaced by the horizontal tube  320  and the exchanger  88  is replaced by the exchanger  324 . The trough  379  for removing the distilled water which has accumulated at the base  372  of the tube  320 , which has condensed in the exchangers  292   1-8  and  294  and in the tube  320 , and which exits from the active element  328 , replaces the trough  116  into which flows the distilled water collected at the outlet from the (N+n) plates  54 ,  63 - 66 , at the base of the casing  82  and at the outlet from the active element  114 . But the economic advantage of this second way of producing a still with vapour diffusion and heat-transfer gas, improved according to the present invention, is on the other hand much greater than that of the first, represented in  FIG. 5 . This superiority is due firstly to the form given to the heat exchangers used, and secondly to the means implemented for circulating the heat-transfer gas in these exchangers.  
         [0255]     It will be noted that the heat exchanger  80  or that constituted by the tube  318  and its casing  320  is an essential component of the still with vapour diffusion and heat-transfer gas, according to the present invention. Its function is to reduce the temperature of the heat-transfer gas exiting from the hollow plates by several tens of degrees, before causing it to enter the inter-plate spaces. This is in order to have, at the inlet to the inter-plate spaces, a local difference dH in enthalpy flows more or less equal to that engendered by the heat source between the outlet from these spaces and the inlets to the hollow plates, given the very large difference which exists between the apparent calorific capacities C p  of the saturated air at the temperatures concerned. On the other hand, it appears that the heat exchangers  88  and  324  have the objective of recovering the thermal energy from the distilled water to be removed, in order to optimize the C OP  of the still. In fact, the liquid to be distilled, entering the inter-plate spaces of the compact heat exchangers  294 , exits from one or more other heat exchangers  324  of the same type, arranged between the outlet or outlets from the heat exchangers  292 - 294  and the means  376  for collecting the distilled liquids which condense on the internal faces of the active elements of the heat exchangers  292  and  294  and on the walls of the duct  318  of the heat exchanger  318 - 320  or of its equivalents  250 . Doing away with these exchangers  88  and  324 , in the case of an inexpensive thermal energy, has scarcely any significance. The same practically applies to the n auxiliary hollow plates, represented by the tube  66  and its casing  63 , or by the exchanger  294 , producing an additional heat exchanger between the hot saturated heat-transfer gas and the liquid to be distilled, before the latter enters the heating chamber of the boiler  120  or  332 .  
         [0256]     To the advantages which can be attributed to the only compact exchangers used in the second still, will be added the use of one or more simple vapour jets for causing the heat-transfer gas to circulate. These vapour jets  347  are suitable for reheating to an optimum or simply efficient temperature T 1  and supersaturating the flow of heat-transfer air, which leaves downstream of the spongy mat  361  which is soaked with very hot water. Moreover these vapour jets provide this flow of air, by exchange of quantities of movement, with a sufficient pressure, to cause it to enter at the top and drop down inside the active elements  293  of the exchangers  292  and thus cause it to flow, opposite natural convection, in a looped circuit, through the hollow plates  256  of the active elements  293  and their inter-plate spaces  260 . By way of example, such a quantity of movement, capable of propelling a hot flow of air from the top of thin hollow plates downwards, then this same flow, cooled down, from the bottom of narrow inter-plate spaces upwards, overcoming the different losses of charge experienced during such a passage in a closed loop, can be obtained by taking to 102° C. the seawater in the heating tube  336 , which will engender one or more relatively powerful vapour jets, with 80 millibars of overpressure, ejected at 150 m/s. Such vapour jets make it possible to overcome natural convection and also to do away with the ventilator  92 , provided for this purpose in the still in  FIG. 5 . This has the consequence of also reducing the amount of investment to be carried out and appreciably simplifying the use of the equipment.  
         [0257]     The added advantages provided by the presence (1) of the float linked to the needle-valve closing device installed at the inlet to the heating tube of the boiler represented in  FIG. 12A  and (2) of the two groups of calibrated outlet holes, at the top and bottom respectively, made at the outlet from this heating tube, to allow the production of vapour jets and the extraction of the hot water, will be noted. Thanks to these components of the boiler, there is a heating tube with a constant level, pressure and flow. In fact, it is possible, by means of a flow of any sufficiently hot, gas or liquid, heating fluid, to take the seawater to be distilled, contained in this heating tube, to a temperature above its boiling temperature and thus to create, above the level of the water, a vapour under overpressure. The amplitude of this overpressure is determined by the heating power used. The heating tube  336  and the tubular radiator  338  constitute a heat exchanger for fluids confined in counter-current circulation. The characteristics of this exchanger (materials, diameters and lengths of the heating tube and of the tubular radiator), are determined as a function of the results to be obtained, given the respective characteristics (natures, flow rates, temperatures, calorific capacities) of the heating fluid available and of the liquid to be distilled. Such a production of vapour is obtained, for example, by means of a tubular radiator made of appropriate stainless steel, capable of resisting the different components of diesel-engine exhaust gas at 300° C. Should the heat-transfer fluid to be used be the cooling liquid (at approximately 110° C.) of a heat engine, the material used can be the same for both, (a polymer which is mechanically stable at these temperatures, for example). The same would apply if the heating liquid of the tubular radiator were thermal oil (of the type ESSO 500, for example) heated during the day by an appropriate solar boiler, equipped with a cylindrical parabolic reflector, and stored day and night at a high temperature (120 or 130° C., for example) and at atmospheric pressure, in a heat-insulated reservoir.  
         [0258]     When the boiler is stopped, the total pressure above the level of water in the heating tube is equal to the external pressure, and the flow of water through the extraction tubes is practically zero. When the boiler is in operation and an equilibrium temperature is reached (102° C., for example), the overpressure above the level of water is 80 millibar and the flow rates of water and of vapour are at their nominal values. The transition between these two states is very short since only the quantity of water present in the heating tube is to be heated. Any variation in the heating power leads to a variation in the temperature of the water and in the equilibrium pressure of the vapour in the heating tube. Consequently, any increase in the power of the heating power leads to a simultaneous increase in the flow of vapour and in the flow of water to be evaporated in the still, which can therefore comprise only a single control and therefore render the valve for adjusting the flow of salt water entering unnecessary.  
         [0259]     In  FIG. 5 , the embodiment of the boiler  120  has not been specified. In practice, it is possible to use one or other of the boilers described in  FIGS. 11 and 12 . It will be noted that the temperature of the water that it supplies is below its boiling temperature. In the absence of vapour under overpressure, the vapour jet  347 , used in  FIG. 12  to cause the heat-transfer gas to circulate, cannot therefore be created by the boiler  120 . Consequently, a mechanical means of propulsion, a ventilator  92 , must be used to cause this gas to circulate. The case of a boiler incapable of producing vapour under overpressure is, for example, that of a solar boiler without a reflector.  
         [0260]      FIG. 13  represents a perspective view of a still with vapour diffusion and heat-transfer gas circulating by natural convection, the distillation unit of which is a set of plane and flexible, thin hollow plates, of a model particularly well suited to this type of still. In fact, this  FIG. 13  describes in detail the production of a still according to  FIG. 6 , in which the solar boiler is replaced by a heating tube.  
         [0261]      FIG. 13  shows six thin hollow plates  400   1-6  which symbolically represent a distillation unit constituted by a large number of these same plates (several hundred or even several thousand, if appropriate) which can be installed on a framework (not shown) mounted in a heat-insulated treatment chamber  401 . This chamber  401 , like the chamber  48 ′ in  FIG. 6 , comprises three stages having approximately the same height: a lower stage for the bottom vent, a central stage for the distillation unit and an upper stage for the top vent. In  FIG. 13 , in order to facilitate the description and simplify the drawing, several walls of this chamber  401  are represented by their contours only.  
         [0262]     By way of non-limitative example, each hollow plate  400  measures 40 cm in width, 50 cm in height and 2 mm in internal thickness. Generally however, such plane, flexible and thin hollow plates, can have a maximum surface area per face of approximately 1 m 2 , a maximum width of approximately 80 cm and internal thickness of 5 mm at the most. Each plate  400  is formed from a fine fleece  402   1-6  made of polymer (in particular of polypropylene) having a good mechanical resistance to the maximum temperature (at the most 90° C.) of the heat-transfer gas. This fleece, identical to that used for producing the large plates  140  in  FIG. 7 , has a thickness of approximately 100 to 250 microns and it is provided with a hydrophilic or wettable coating of approximately 50 to 150 microns in thickness. In  FIG. 13 , each fleece  402   1-6  appears folded in two, carried by a suspension rod  404   1-6 , coating on the outside. The rods  4041 - 6  are made of polymer, with a rounded upper edge, and they are 2 mm in thickness, 4 cm in width and 50 cm in length. By means of one or more longitudinal weld lines, the top part of each fleece  402   1-6  is welded to its suspension rod  404   1-6 , and its bottom part similarly welded to a tension bar  406   1-6 . The rods  404   1-6  and the bars  406   1-6  are made of polymer identical to that of the fleece and they are all 2 mm in thickness and of 50 cm in length. The pressure bars  406  comprise at their ends supports  408   a - b  with co-planar upper edges. Between these two supports, through the tear  405  made in the front slope of the fleece  402   1 , can be seen the tension bar  406   1  which has an oblique lower edge  410 , connected slantwise to the end of this bar. At the bottom of the oblique edge  410 , there is provided in the thickness of each pressure bar  406 , a point  412  for extraction of the distilled water produced, constituted by a transversal cut, 1 mm deep and 3 mm wide, if appropriate, replaced or occupied by a flat wick. The upper edge of each pressure bar  406  is in the form of a very open V, jammed onto the cut  412 . The slopes of the fleeces  402   1-6  project over their tension bars  406   1-6 . These slopes are raised and folds formed slantwise, then compressed and held in place by any appropriate means, in particular by stitches. In this way, for each hollow plate  400 , two parallel and contiguous, inclined, flat ducts  414 , are constituted, making it possible to collect the salt water produced by each hollow plate of the still. Under the flat ducts  414  and perpendicular to the extraction points  412  of the assembly of hollow plates  400  thus formed, a single trough  416  is installed for collecting the hot distilled water produced.  
         [0263]     The salt water collected by the flat ducts  414  flows into a trough  418  provided with spouts  420   a - b , arranged above a heating tube  422 , covered with a thin hydrophilic mat  424 , with clear slopes. The length of this heating tube  422  corresponds to that of the assembly of thin hollow plates  400   1-6 , juxtaposed with inter-plate spaces  403   2-6  of the same thickness. The heating tube  422  is fed with heating fluid by a tube  423 , this fluid being capable of taking the temperature of the salt water which impregnates the mat  424 , to a maximum temperature of approximately 95° C. The tube  422  is installed in the bottom vent  426  of the still. This vent  426  is constituted between a thick panel of thermal insulation  428  which divides the lower stage of the treatment chamber  401  into two connecting parts. This panel  428  forms, with the similar panels, such as  430  (the only one shown), which constitute the insulation of the transversal walls of the lower stage of the treatment chamber  401 , on the one hand, the unoccupied part  432 , with a plane wall  433 , of this lower stage and, on the other hand, the bottom vent  426 , with a curved wall  427 . Perpendicular to the heating tube  422 , a reservoir  434  is arranged in which the salt water which flows hot from the mat  424  covering this heating tube  422  ends up. The hollow plates  400  are provided with a top vent  436 , constituted in the same manner as the bottom vent  426 . This top vent  436  opens into a passage  435 , formed between a block of thermal insulation  437  and the upper wall  439  of the treatment chamber  401 . In this passage  435 , one or more monobloc active heat exchange elements  438  are installed, passed through, with a counter-current of the air which circulates around, by the seawater to be distilled which enters the still by a tube  440  and exits from it by a tube  442 . By way of example, such a heat exchange assembly  438 , possesses an air/water exchange capacity of approximately 170 Watts/° C. and, for this purpose, it comprises thirty-four bellows 15 cm long and 5 cm wide, with internal thicknesses of hollow plates and inter-plate spaces of 2 mm. Beyond the space occupied by these elements  438 , can be seen the unoccupied part  443  of the upper stage of the treatment chamber  401 . Above the hollow plates  400 , an elongated device  444  (an open box as shown or a tube under slight pressure) for distribution of the tepid seawater introduced by the tube  442  is installed transversally. The base of the distributor  444  comprises two rows of holes, made at the pitch of assembly of the plates and passed through by spread wicks (not shown) fixed by a few clips, on top of the hydrophilic coating of these plates.  
         [0264]     The suspension rods  404   1-6  of the hollow plates  400   1-6  are fitted on two parallel horizontal girders, forming part of the framework installed in the heat-insulated treatment chamber  401 , and the pressure bars  406  of these plates under two horizontal pressure-control girders, similar and parallel to the preceding ones, connected to the framework by springs. The height of the hollow plates  400  determines the distance between these girders and the latter is fixed once for all. These girders, this framework and these springs are ordinary components which are not shown so as not to overburden the figure.  
         [0265]     Given the elasticity of each of the fleeces  402  and the rigidity of the springs integral with the two low girders, the individual force of tension of each fleece is approximately 200 to 400 grams, as a function of the wall thickness and of the height of the fleeces.  
         [0266]     At a distance of 5 cm from one of the ends of each of the rods  404 , a short spacer  448  is fixed squarely to it. This spacer  448 , which measures 22 cm in length, 2 cm in width and 2 mm in thickness, is free between the two slopes of the folded-up fleece  402 , its external edge coinciding with the external edges of these two slopes. Similarly, 5 cm from the opposite end of each of the pressure bars  406 , another short spacer  450  is also fixed squarely, under the same conditions, visible through the tear  451 , identical to  448 . Thus, at the bottom and at the top of the fleeces  402   1-6 , folded up over their suspension rods  404   1-6 , two diagonally opposite openings  452   1-6  and  454   1-6 , 20 cm high and 2 mm wide are provided, which constitute the inlets to and the outlets from the hollow plates  400   1-6 . These inlets and these outlets remain constantly well open and the internal thicknesses of these plates virtually constant, because of the tensions uniformly engendered in the clear slopes of the fleeces, by the springs integral with the girders resting on their pressure bars and because of additional gluing of the edges of the openings on the long spacers  456 , described hereafter.  
         [0267]     The hollow plates  400   1-6  are separated from each other or from the two assembly and maintenance panels referred to hereafter, by free spaces  403   1-7 , each of these spaces being bordered by a pair of long spacers, such as  456   2 , 2 mm thick and 2 cm wide, resting on the two girders of the framework. The inlets, such as  457   3 , to these inter-plate spaces  403  are visible in  FIG. 13  whilst their outlets are hidden. The assembly formed by the hollow plates  400   1-6  thus suspended and tightened, by the inter-plate spaces  403   2-6  and by the two free end spaces, bordered by long spacers, such as  456   2  et  456   7 , is assembled by two rigid panels (not shown) linked by means of clamping tie rods. In this way, the suspension rods  404 , the short spacers  448 - 450 , the long spacers  456  and the pressure bars  406  firmly clamp the fleeces  402  which constitute the hollow plates  400 , their inter-plate spaces  403  and the two free end spaces. Under these conditions, a distillation unit is constituted which has a completely sufficient lateral tightness, around the inlets  452  and outlets  454  of the hollow distillation plates  400  and, in the case of the inter-plate spaces  403 , on both sides of their inlets  457  and their outlets.  
         [0268]     The arrows  460 ,  462  and  464  represent the rising flow of air in the three stages of the treatment chamber  401 , namely in the bottom vent  426 , inside the hollow plates  400  and in the top vent  436 . The arrow  466  represents the flow of air along the walls of the monobloc active heat exchange element  438 , and the arrow  468  this flow in the collector space  443  of the upper stage of the treatment chamber. The arrow  470 , visible through the tear  472 , made in the rear slope of the fleece  402   1 , represents the downward flow of air in the inter-plate spaces  403 . As regards the arrows  474 , they represent the flows of air exiting from these inter-plate spaces  403  and entering the collector space  432  of the lower stage of the treatment chamber. The arrow  476  represents the flow of air which enters the bottom vent  426  of the chamber  401 . The arrows  478 ,  479 ,  480  represent the flow of seawater to be distilled which enters, passes through and exits from the active heat exchange element  438 .  
         [0269]     By means of these arrangements, this still according to  FIG. 13 , with vapour diffusion and non-condensable heat-transfer gas, circulating by natural convection, operates under exactly the same conditions as the still in  FIG. 6 . Moreover, with the new model of plane, thin and flexible, hollow plate used, all the functional advantages of the heat exchanger monodistillation unit, according to the present invention, referenced  250  in  FIG. 10A , are found improved. In fact, a set of hollow plates  400  possesses the same surface area of distillation heat exchange per unit volume, i.e. 400 m 2  per cubic metre, as a set of monobloc distillation exchangers, but in addition the thickness of the walls of these plates and of their hydrophilic coating is more than three times below that of these exchangers (0.15 instead of 0.50 mm). This considerably improves the Q/V ratio to be taken into account, in the calculation of the C IE  of the still, which then reaches the high value  297  indicated above. Moreover, if the production price of the main component of this new hollow-plates model  400 , (namely the fine fleece  402 , its suspension rod  404 , its pressure bar  406  and its spacers  450 ) is compared to that of a large flexible plate  140  in  FIG. 7  or even to the monobloc active element of a rigid distillation heat exchanger  250 , in relation to the same exchange surface area, it is noted that this price is remarkably low (less than 1 ε, for a plate of 50 dm 2 ) and several times lower than that of the two other models.  
         [0270]     Moreover, it will be noted that it is relatively easy to avoid any swelling of the plane and tightened walls of the flexible and thin, hollow plates,  400 , which can damage the efficiency of the still, by suitably choosing, on the one hand, the heights of the hollow plates and of the bottom and top vents of the still, and on the other hand, the thickness of these walls and their rigidity at the temperatures concerned as a function of the polymer used. This double choice has the objective of ensuring that the difference between the dynamic pressures of the rising flows of air in these hollow plates and downward flows in their inter-plate spaces, circulating in closed circuit, is practically negligible (of the order of 1 Pascal) before the mechanical pressure applied to the fleeces constituting the walls of these plates.  
         [0271]     In order to improve the overall C OP  of such a still, it is useful to add to the distillation unit, formed by the hollow distillation plates  400   1-6 , a unit for recovery of the heat of the hot distilled and concentrated liquids, produced by this still. This heat recovery unit comprises two groups of thin, hollow auxiliary plates, provided with hydrophilic coatings, installed vertically. The total surface area of the auxiliary plates of a heat recovery unit is approximately ten times less than that of the plates of the distillation unit with which it is associated. This ratio is an inverse function of the efficiency coefficient of the heat exchange carried out by these auxiliary plates. These auxiliary plates are rigid and suitable for supporting without deformation the hydrostatic pressures of the distilled and concentrated liquids which have to circulate in them. By way of example, these are rigid cellular panels of the type described above as a variant of the flexible panels  140   1-3  in  FIG. 7 , provided with coupling washers  172  and  174 . These washers form feed duct sections, assembled by tie rods such as that referenced  186  in  FIG. 8 . The end  184  of the bottom feed duct of each group of auxiliary plates constitutes the inlet to this group, connected to the suction tube of a siphon, and the end of its top duct, the outlet from this group connected to the drainage tube of this siphon. The heat recovery unit formed by these two groups of auxiliary plates and by the tubes of their siphons are not shown, so as not to overburden the drawing and because these tubes are ordinary components, added to original components, described in detail and also shown. The hollow plates of this heat recovery unit have the same lengths and widths as the hollow plates of the distillation unit, and they also possess inter-plate spaces with lateral edges, made tight by spacers. These two units are attached and their components are stretched and clamped by rigid end panels, connected together by assembly tie rods.  
         [0272]     Seawater, preferably at a temperature as low as possible (for example, cooled down by natural means or, by default, at T L1  rather than T L2 ), is poured over the coatings of the two groups of hollow auxiliary plates and part of the flow of air at temperature T 4  circulates from the top downwards along these coatings. The two siphon suction tubes dip in the trough  416  for collecting the distilled water and in the reservoir  434  for collecting the concentrated salt water respectively and they are connected to the inlets to the two groups of plates of the heat recovery unit. The two drainage tubes of these siphons are connected to the outlets from these hollow auxiliary plates and one of these drainage tubes opens a good distance below the level of the trough  416 , the other a good distance below the level of the reservoir  434 . The hot liquids which circulate in these hollow auxiliary plates from the bottom upwards cause the evaporation on the one hand of some of the seawater poured over their coatings. The flows of cooled air which circulate along these coatings from the top downwards carry along the vapour thus produced and, on this occasion, heat up again and become saturated. The two flows of saturated hot air, which exit from the inter-plate spaces of these two groups of hollow heat recovery plates, are added to those which exit from the inter-plate spaces of the hollow distillation plates. The mixture is then reheated and supersaturated and it reaches the temperature T 1 . Under these conditions, the temperatures of the distilled and concentrated liquids removed are relatively low, of the order of 40° C., i.e. 15° C. above the usual temperature T L1  of the liquid to be distilled. In the usual case where the quantities of distilled water and of salt water produced are equal, this has the result of causing the general C OP  of the still to increase to 20.  
         [0273]     As a result of all that has just been said, a domestic solar still with saturated hot air circulating by natural convection which comprises (1) a solar boiler having 1 m 2  of solar collection unit, which produces 7 kWh thermal per day, (boiler  120 ′ in  FIG. 6 ) installed in place of the heating tube  422  in  FIG. 13 , (2) a laminated distillation unit, formed from 100 flexible and plane, thin hollow plates (plates  400  with 20 dm 2  per face and a pitch of 4.5 mm) and (3) a heat recovery unit formed from some ten auxiliary hollow plates, can ensure a production of 200 litres of distilled water per day. With a small 35 kW gas burner, associated with one or more appropriate heating tubes  422 , installed between two symmetrical sets of distillation and heat recovery units, each unit comprising 500 hollow distillation plates and 50 recovery plates, identical or similar to the plates  400  in  FIG. 13  (each with 1 dm 3  of active volume), a still can be constructed for small institutions which will have (with a C OP  of 20) a production of distilled water of approximately 20 m 3  per day. An identical production of distilled water can be provided by a still provided, on the one hand, with a distillation unit of 2,000 tightened plane hollow plates, with 1 m 2  of surface area per face, a pitch of 4.5 mm and 10 m 3  of total active volume and, on the other hand, with a solar boiler equipped with a solar collection unit of 100 square metres, producing approximately 700 kWh per day. With this last distillation unit, it is possible to construct a still, associated with an average 350 kW boiler, which produces approximately 200 m 3 /day. Such a boiler can be the heat exchanger for cooling the diesel engine of a small generating station or of a ship. A fresh-water production of a few thousand m 3 /day is possible with a still with saturated hot air, circulating by natural convection, comprising a boiler of a few tens of MW, feeding in parallel the heating tubes of several distillation units, with the total active volume of a few hundred cubic metres, provided with so many heat recovery units having a few tens of cubic metres of active volume.  
         [0274]     The invention is not limited to the embodiments described.  
         [0275]     The efficiency of the stills according to the invention results from the maximum use of the heat which is supplied to them, which requires, in advance, an optimum insulation of their treatment chamber. In the case of solar stills, necessarily installed in the open, such insulation is generally produced on site, by means of a local construction (in cobwork, for example). In this case, the external wall of the still is a panel which is not very thick, delimiting the relatively tight enclosure of the still.  
         [0276]     In the case where the vapour-diffusion still with counter-current of water, according to  FIG. 3 , would not be able, for practical installation reasons, to operate by thermosiphon, a pump is used in order to ensure the circulation of the heat-transfer liquid.  
         [0277]     The heat exchanger  80  constituted by the coaxial ducts  318  and  320  in  FIG. 12B  can be replaced by a simple monobloc heat exchanger  250  or  438 .  
         [0278]     The thin and flexible, plane hollow plates  400 , with tightened walls, in  FIG. 13 , can clearly be used in order to constitute the distillation unit of a still according to  FIG. 5 .  
         [0279]     In the case of a still with natural convection and a solar boiler, according to  FIG. 6 , provided with a reservoir  63 ′ for collecting the hot salt water, ensuring an additional night-time operation, only the distilled water produced will be the subject of a heat recovery.  
         [0280]     In all the stills with heat-transfer gas circulating by natural convection, top and bottom vents of considerable height are necessary in order to engender this natural convection in a satisfactory manner and thus obtain an appropriate transit time t in the hollow distillation plates. Such heights can be inappropriate for a domestic still. But in this case, it is possible to correct this drawback by appreciably reducing these heights, whilst retaining the sought transit time t. This is done by installing, with a motor outside, the helix of a ventilator (identical to that  92  in  FIG. 5 ), upstream of the inlets to the inter-plate spaces, in the unoccupied top space  443  of the still in  FIG. 13 . The thrust exerted by this helix on the flow of cooled air, which has just passed through the hollow distillation plates  400  and the inter-plate spaces of the monobloc heat exchanger  438  with losses of charge, compensates for these losses and propels this flow with an appropriate speed and pressure in the inter-plate spaces and thus increases the flow rate of the flow of air which circulates in closed circuit. By adjusting once for all the speed of rotation of this helix, it is possible to control the dynamic pressure of this flow of air in the inter-plate spaces, in order to prevent any deformation of the walls of the hollow plates, which would be detrimental to the satisfactory circulation of this flow of air in closed circuit.  
         [0281]     Moreover, if it is desirable that such a domestic still could become a kitchen appliance, in the same way as a refrigerator, the heating tube  422  described in  FIG. 13  and its feed (which is a device generally absent from kitchens) are advantageously replaced by a particular heat source, easy to constitute in the kitchen of an apartment or on a pleasure boat. And this heat source, which will moreover have an additional function as a means of propulsion, is constituted by a heating tube, producing vapour jets, installed as the tube  422 . This tube will have a small internal diameter (2 cm, for example), it is closed at one end and provided with calibrated orifices, made at regular intervals (5 cm, for example) along a generating line. This tube is installed a good distance upstream of the inlets of the hollow plates, so that the vapour jets that it produces are, on the one hand, correctly directed and, on the other hand, capable of dispersing in the flow of gas before the latter enters the hollow plates. These vapour jets will, for example, have a temperature of 101° C. and a pressure just slightly higher (40 hPa) than atmospheric pressure. They are ejected at a speed of 110 m/s. And they will have a sufficient flow to be able to add 2 to 5° C., to temperature T 2  of the flow of air exiting from the inter-plate spaces, and thus saturate this flow of air while taking it to an optimum or simply efficient temperature T 1 , at the inlet to the hollow plates. Moreover, these vapour jets will produce an upward thrust, additional to that engendered by natural convection and, if appropriate, to the downward thrust produced by the helix of a ventilator. It will be noted that such a heating tube with vapour jets can (as an additional heat source, operating whenever necessary) be installed upstream of the inlets to the hollow plates, when the still comprises a solar boiler such as that referenced  120 ′ in  FIG. 6 .  
         [0282]     The vapour which will feed this heating tube with vapour jets is produced, completely safely, by a simple kettle connected to this tube by a heat-insulated tube. This kettle will contain distilled water and it is heated by any heating means available in the kitchen or, more generally, in the vicinity of the still. In the case where a production of distilled water is sought for a significant period (a few hours, for example), the kettle is a pot provided with a lid, suited to being fixed to it in tight manner. This lid will comprise a water intake and a vapour intake, intended to be connected by a tube to the free end of the tube with vapour jets. The water intake is extended by a duct, ending in a needle-valve closing device integral with a float (similar or equivalent to that  356 - 358  in  FIGS. 11 and 12 ), in order for this pot to be able to operate at a constant level. And the water intake of this kettle is fed by a tube open to the open air (similar to the tube  113 ′ in  FIG. 6 ), connected to the outlet trough from the still and provided with an overflow, opening above a reserve of distilled water. The quantity of distilled water thus consumed by the kettle, will reduce by one point the C OP  of the still. But this is scarcely significant, with a still according to the invention, such as that described in  FIG. 13 , which generally possesses a C OP  of at least 15. This solution can evidently also be used for stills for institutions, which are much more powerful, and this heating tube with vapour jets can then be used alone or associated with another heat source.  
         [0283]     Such a domestic still, provided with both relatively short top and bottom vents, of a tube with calibrated orifices producing vapour jets and, if appropriate, with a ventilator, constitutes a domestic device of modest size, producing distilled water under advantageous economic conditions. Such a device is particularly well suited for equipping pleasure boats and kitchens in blocks of flats in certain large modern coastal towns (such as Hong Kong or Singapore), where there is continually a certain shortage of fresh water and where, in order to meet this chronic insufficiency, seawater is also distributed for flushing toilets.  
         [0284]     When the temperature of the available water to be distilled is relatively high, above 35° C., for example, as is the case in certain deserts, the subsoil of which contain brackish water, it is necessary, in order for a vapour-diffusion still with counter-current of air to operate in optimum manner, to appreciably reduce this temperature before introducing it into the device. In order to do this, the large hollow rectangular plates  140 , with hydrophilic coating, described in  FIG. 7  will be used, converting it into a natural refrigerator. The liquid to be distilled will circulate by gravity inside these plates and, by gravity and capillarity, in their hydrophilic coating. By installing these plates in the shade, with a good distance between them, the dry air of the desert (or of any other arid region) will cause a continuous evaporation of a good part of the water which flows in the coating, which will have the effect of cooling down the water which circulates inside. The minimum temperature capable of being reached by such a natural refrigerator is the temperature of the dew point of the ambient air (i.e. below 10° C., for dry air).  
         [0285]     As has been said in the PCT Application relating to the prior invention, the non-condensable gas, used in a vapour-diffusion still, cannot be pure air but a mixture of air and of a gas capable of completely eliminating the infectious germs that could be contained in the water to be distilled entering a vapour-diffusion still according to the present invention. In fact, measurements carried out in an official laboratory have proved that a distillation, carried out by means of such a still, could transform the polluted water resulting from lagooning treatment of the used water of an average town into potable water.  
         [0286]     If the invention relates principally to methods and devices for producing fresh water from seawater, brackish water or polluted water, it is also of interest to the food and chemical industries, for producing concentrated liquids, such as syrups or brines. It is in fact particularly advantageous to recover the thermal energy from the hot effluents of the factories concerned, in order to economize on the considerable costs of evaporation of the different liquids to be concentrated.