EP0708900A1

EP0708900A1 - Evaporation method and heating device therefor

Info

Publication number: EP0708900A1
Application number: EP95918659A
Authority: EP
Inventors: Jacques Pistien; Jean-Louis Giazzi; Robert Desage; Philippe Deblay
Original assignee: Superba SAS; Gaz de France SA; Cogia SA
Current assignee: Superba SAS; Cogia SA; Engie SA
Priority date: 1994-05-18
Filing date: 1995-05-18
Publication date: 1996-05-01
Anticipated expiration: 2015-05-18
Also published as: DE69506669T2; CA2167598A1; ES2128058T3; FR2720143A1; ATE174680T1; US5771845A; EP0708900B1; WO1995031674A1; DE69506669D1; JPH09500957A; FR2720143B1

Abstract

A device for evaporating a liquid, including at least one porous substrate (7a, 7b) exposed to a predetermined ambient pressure, a member (34) for supplying liquid to the substrate and loading it with same by circulating said liquid through the substrate from an upstream portion, and at least one energy source (19) for heating the substrate so that at least a part of the liquid is evaporated therefrom. Said member for supplying liquid to the substrate includes means (C1, C2) for pressurising the liquid to a pressure above ambient pressure to achieve a flow rate higher than the flow rate solely due to the capillary action and evaporation of the liquid, assuming said substrate is then in a horizontal position. The device is useful for making water evaporation apparatuses, particularly those with an electrical or gaseous energy source.

Description

"VAPORIZATION PROCESS AND ASSOCIATED HEATING DEVICE"

The present invention relates to a method and a device for vaporizing a liquid. It is known that, to vaporize a liquid, it is possible to use an electrical resistance immersed in a relatively large water depth. By this design, the heating time of the liquid necessary to vaporize it is relatively long and the vaporization yield is poor, especially in sequential regime. This is for example the case of a number of steam boilers using combustion gases.

By French patent FR 78 08 201, however, a steam generator is known which, although using an electrical resistance heating a porous body immersed at its base in a sheet of water, has the distinction of vaporizing relatively quickly the water contained in this porous body. Water renewal is ensured by the pumping capacity of the porous body. To obtain optimum performance, the height of the body of water is adjusted as a function of the density of heat flux transmitted to the body, this as a function of its pumping capacity. This process is applicable to all kinds of energies, such as for example combustion gases.

If the use of the pumping capacity of a porous body can therefore in certain cases improve the performance of steam boilers, limits exist, since the quantity of liquid contained in the body decreases with the pumping height. The first consequence of this phenomenon is to limit in practice to a few cm the height of a porous body and, as a second consequence, to maintain a small differential in liquid height variation, if one wishes to obtain an optimization of yield in depending on the density of the heating flow. DE-C-158 050 can in a way be considered to be an illustration of this type of steam boiler.

Thus, in this German patent, it is in particular known to provide the liquid vaporization device described:

- At least one porous substrate (that is to say with capillary property) exposed to a determined ambient pressure, - means for supplying liquid to the substrate so that it becomes charged with liquid, by circulation of said liquid in the substrate from an upstream portion,

at least one energy source for heating at least one vaporization zone of said substrate located downstream of the upstream portion, and the liquid which charges it, so that at least part of this liquid is vaporized there,

However, in this patent, several wicks are used as substrate supplied "by the effect of the suction force", that is to say by capillary pumping of said wicks.

Insofar as, as indicated above, the flow rate of a wick supplied by capillary pumping decreases with the height of the supply water body, FIG. 4 of this patent shows the advantage that there may be to use several feeding trays arranged at different elevation levels, the highest placing the substrate horizontally. Tests carried out with such a device however show that the yield remains poor, and that the quantity of liquid in the vaporization zone is quickly too low, that the time of arrival of the liquid in this vaporization zone is often too long. The device of DE-C-158 050 is also bulky, not conducive to industrial achievements Today (high efficiency, compactness, low manufacturing cost for large series, reliability over time ...).

The object of the invention is to provide a solution to many of the above-mentioned drawbacks and in particular proposes a process which can be industrially implemented under commercially attractive conditions, without exorbitant manufacturing and / or maintenance costs and also offers flexibility of use, performance and reliability adapted to current needs.

The solution of the invention consists in particular, for a given vaporization regime corresponding to determined energy supply conditions, in establishing in the substrate ("capillary") a liquid inlet flow rate greater than the inlet flow rate of liquid induced in the same substrate, in the position then assumed to be horizontal thereof, by the capillarity and vaporization of the liquid only, (the position of the first cited substrate not necessarily being horizontal).

To obtain this, the device of the invention provides that the means for supplying liquid to the substrate in question must include means for pressurizing the liquid in order to establish a pressure therein greater than ambient pressure.

To avoid any ambiguity, we have shown in FIG. 1 the manner in which we will advantageously proceed to measure the induced flow rate as indicated above, "in the same substrate, by capillarity and vaporization only (of course for the same conditions of supply of energy), in the position then assumed to be horizontal of the substrate ".

Thus, firstly place the substrate 7A considered horizontally with its environment so that In particular, the effect of gravity on capillary forces can be overlooked.

Then, we will bathe one end 7k ^ of this substrate in a container 5A containing a quantity Q- ^ of liquid to be vaporized (of course, it will be a "free" liquid: simply a sufficient quantity like liquid of which we filled a container).

The substrate will then be exposed to a given heating energy supplied by appropriate means 19A, "this heating energy":

- which should allow the vaporization (at least partially) of the liquid which has meanwhile "migrated" from the container 5A into the substrate, according to arrow 6,

- and must also be reproducible at 1 • identical on the device of 1 • invention including the heat exchange wall 21A, if one must be used to carry out the invention (if an electrical resistance is used, one must use, for comparison, the same resistance supplied with the same intensity; for gas, the same burner must be used and supplied under the same conditions).

Having placed the "dry" substrate 7A in the container, the knowledge, for a given time interval, of the quantity Q ₂ or of the weight of liquid which has entered the substrate (and which therefore has left the container) will make it possible to have the "induced flow" of liquid entering the substrate.

It will be noted that the "flow of liquid under pressure" specific to the invention now will be favorably obtained by the use of thin substrates (which may advantageously be of the order or less than 2 mm thick), which can only promote the evacuation of steam. This solution is also advantageous in terms of rapid vaporization of the liquid and more generally of yield. In the present invention, in order to obtain the desired flow of the liquid in the substrate, it is possible in particular to use the weight of a column of liquid, or alternatively to force the liquid in the substrate, for example by a pump.

Another solution can also consist:

- to incline the substrate relative to the horizontal,

- creating said column of liquid in the substrate, and

- create additional pressure in this column.

The additional pressure can in particular be created by a pump. According to yet another characteristic of the invention, it is advisable to produce the substrate of the present vaporization device with a thickness of between approximately 0.05 mm and 5 mm, and preferably less than 2 mm. In addition, advantageously, this substrate has a porosity of between approximately 5% and 90%, and a substrate will be used comprising empty volumes of liquid retention, so that this liquid can occupy between approximately 5% and 100% of said volumes empty. Thus, unlike very thick wicks specific to vaporization devices using (very essentially) capillary pumping, the use of a substrate which is almost like a thin porous film offers the advantage that the heat flux generated does not face it. , in or on the surface of the substrate, only a small thickness of liquid to be vaporized, with the consequence of a particularly rapid passage into the vapor phase, which can be a few seconds, with a yield which can be particularly high. In particular, by combining a substrate made as a thin film and a liquid flow meeting the aforementioned requirements of the invention, it has been found that for a constant heating flux density, there was variation in the vaporization yield at the same time as the flow rate of the liquid in the substrate was varied. Conversely, at a constant liquid flow rate, there was variation in this same vaporization yield at the same time as the density of the heat flux was varied. It was also possible to obtain a maximum vaporization yield for a state of equilibrium between the flow rate of vaporizable liquid entering the "film" constituting the porous substrate and the flow rate of the heating flow of this substrate, with then a vapor practically free humidity.

It also appeared that, to cover a wide range of heating flux densities, it was necessary to use different thicknesses of porous films constituting the substrate, while very advantageously respecting the characteristics mentioned above. For each of these thicknesses, and depending on the nature of the porous material, it has in particular been found that it was possible to obtain a range of use of the flow rate of liquid to be vaporized between a minimum content in transit and the maximum quantity of liquid saturating (or even supersaturing) the porous film. When one is not particularly looking for a very good vaporization yield, a characteristic of the invention also provides for increasing the flow rate range of liquid to be vaporized, by supersaturating the substrate in such a way that part of the liquid s 'flows on a free external surface thereof, being held against this substrate by interfacial tension.

A simple way to adjust the flow rate of liquid in the substrate is to tilt it relative to the horizontal. We can then create, as indicated above, a water column in the substrate area directly exposed to the heating flow and possibly above it, in order to take advantage of the effect of gravity. When the liquid flow rate is greater than the density of the thermal flow, the excess liquid flowing out of the lower part of the substrate can be recovered, to reinject it into its upper part.

When the effect of gravity is not directly used, another means of supplying liquid consists in injecting it under pressure into all or part of the passage section of the substrate. It is thus possible to arrange the substrate so that it bathes at two opposite ends (between which the liquid to vaporize flows along it), in a tube where said liquid will be brought by means of forced circulation. This could also be the case, for example, if the device was operated with substrates then arranged horizontally. On this subject, what follows first shows the influence of the pressure of a column of liquid on the flow of the latter in the porous substrate, then the influence of the inclination of the subtrate on the same flow of liquid , this in the case of a porous substrate consisting of a film for example of woven cotton 0.2 mm thick and 120 mm wide.

. Influence of the height of the porous film on the liquid flow rate:

Height of porous substrate per mm 30 60 90 130 180

Water flow rate in milliliters per minute 2.3 6.6 11 16 31

. Influence of the inclination of the porous film on the liquid flow rate:

Tilt angle relative to the vertical ... 0 ° 15 ° 65 ° 75 ° 90 ° Water flow in milliliter per minute 5.5 1.7 1.2 0.5 0.3

To supply the substrate of the device of the invention with the desired liquid flow rate, it is possible, in accordance with another characteristic of the invention, use drop-by-drop means, for example taking advantage of capillary pumping by immersing an additional porous body in a suitable reservoir. One can also consider using a spray nozzle which sends the liquid to be vaporized onto the substrate, or even immerse the porous substrate directly, at least part of which receives the flow of heating energy, in a sheet of water which can be vary the level. One can also consider using a pump to circulate the liquid under pressure in a locally bent pipe between two of the branches of which the porous substrate has been previously disposed, so that its ends are immersed in the liquid of the tube. It will also be noted that the vaporization of the liquid contained in the substrate can be obtained in particular by all or part of the following three heat transfer modes: radiation, conduction or convection originating both from combustion gases and from an electrical source, for example, this under operating conditions which can be both at lower pressure and at pressure greater than or equal to atmospheric pressure, the vaporization of a number of different liquids, such as water, alcohol, liquid petroleum or the like being able to be considered.

Concerning the porous substrate of the invention, it will also be noted that it could be produced from cotton fibers or threads, or even mineral fibers, such as for example glass or quartz fibers, or even metallic fibers, such as for example steel wires. One can also consider making the substrate from porous and permeable materials obtained by sintering metal powders.

In practice, two types of substrate may be preferred: a substrate formed like a canvas permeable flexible fibrous fabric, or a plate with a more rigid structure.

In the foregoing, reference has always been made to the use of a single substrate. However, the use of several substrates is completely planned. In particular, it will be possible in a certain number of cases to take advantage of replacing a single substrate with a surface determined by several substrates of smaller dimensions having as total surface that of said single substrate, each substrate of reduced dimensions being able in particular to be fed by its own liquid supply means, thus creating as many vaporization zones which can potentially be optimized individually. If several substrates are used, in certain applications, provision may in particular be made for placing these substrates in an inclined position with respect to the horizontal and for them to overlap in part, so that they are spaced apart from one another on a at least part of their surface, to leave between them a space favorable in particular to the circulation of steam.

Also, if at least two substrates are used, it may be advantageous for these substrates to extend on either side of the heating source by framing it.

Before describing the invention in more detail in relation to various accompanying figures, it will also be noted that in particular in the case where the energy source will comprise at least one gas burner, the device of the invention may advantageously comprise two hollow boxes delimiting between them a chimney in which the combustion products of the burner will then circulate, these boxes each containing at least one substrate. Now concerning the attached drawings, in addition to FIG. 1, by way of nonlimiting examples, these are organized as follows:

FIG. 2 shows a possible structure of the substrate in accordance with the invention, FIG. 3 is a perspective view of an assembly for measuring the flow rate in porous substrate, as a function of the height of its supply water body, FIG. 4 is a diagram showing the variation of the water flow rate of a porous substrate according to the invention as a function of the height of its water level, FIG. 5 is a side view (arrow V) of the generator shown in partial perspective, with cutaway, in Figure 6, Figure 7 is a diagram showing, for an installation of the type of that of Figures 5 and 6, the variation of the vaporization yield as a function of the heating power generated , this for several heights of water available and for two different thicknesses of porous substrates, FIG. 8 is a diagram showing, as a function of the power injected, the influence of the thickness of the substrate on the outlet temperature of the gases for a e installation still in accordance with that of FIGS. 5 and 6, FIG. 9 is a perspective view with cutaway of a steam boiler using an electrical resistance of cartridge type, FIGS. 10 and 11 are sectional views (respectively according to lines XI-XI and XX) of an electric vaporization device equipped with two porous substrates bathing locally in a pipe where circulates water to vaporize injected by means of a pump, FIG. 12 is a partial perspective view with cutaway of the drip feed of a device according to the invention, and FIG. 13 is also a partial perspective view with cutaway of another generator producing vapor by thermal radiation of a resistance.

In what follows, reference will only be made to the spraying of water although other liquids could be vaporized by the same devices.

FIG. 2 represents an example of design of a porous "film" 1 with capillary property, made of cotton, type "honeycomb" with square mesh 2 of approximately 30 to 50 mm. Like all substrates compatible with the invention, the latter therefore has a structure integrating empty volumes for retaining the liquid to be vaporized, these volumes being here constituted by the spaces between the wires of the mesh and by the structural empty volumes of the wires themselves same. The central part of the substrate 1 illustrated is a weaving of threads of different thicknesses according to the choice of the desired flow capacity. Peripherally, this substrate here consists of wicks three times thicker than those of the central part. Thus, a peripheral buffer for the reserve and diffusion of water is created towards the central parts of the mesh.

In the invention, the choice of the permeable substrate is important. In what follows, it will be seen that its thickness will always be between approximately 0.05 mm and 5 mm, with a porosity to the liquid to be vaporized between approximately 5 and 90%.

To better understand the modes of operation of the following exemplary embodiments, FIG. 3 is an example of an experimental device for feeding by pumping a fine porous substrate in the height of a body of water. This device makes it possible to adjust the flow rate of the liquid flowing through the effect of gravity right out of the feed tray. It consists of a balance 3, a container 5 for collecting the water flowing from the porous "film" 7 and a water tank 9 in which the upper part 7a of the porous substrate is immersed. To obtain a constant and free flow throughout the width of the passage section of the porous film, the film was cut at 11 at its bottom. The flow measurement consists in varying the height h of the water level of the tank 9. The table below indicates the characteristics of use of three thicknesses of porous substrates which may be square mesh of the type illustrated in FIG. 2.

Type of substrate thickness of storage capacity Density of the porous flow in cotton substrate of saturated water (g / cm2) for heating the substrate (W / cm2)

with small mesh 0.2 mm 0.104 from 1 to 2.5 with medium mesh 0.5 mm 0.142 from 2.5 to 4.5 with thick mesh 1 mm 0.196 from 4.5 to 10

FIG. 4 is a diagram which indicates the flow of water flowing in a vertical porous film with small meshes (that is to say of thickness <1 mm, for a unitary surface of mesh of the order of 0, 05 mm ² ) depending on the height of the water body. Curve (A) measures the flow of water which flows freely to the lower part of the substrate. Curve (B) measures the water flow when the same porous film is immersed at its bottom in 2 cm of water. Curve (C) measures the flow rate when the film is pressed against a metal wall without being immersed in its lower part.

Thus, depending on the conditions of use of the substrate, the flow rate of the liquid flowing therein can be varied in a ratio of 1 to 8 depending on the curve (A), in a ratio from 1 to 5, curve (B), and close to the latter, curve (C), when the porous film is pressed against an exchange wall.

In FIGS. 5 and 6, the steam boiler presented vaporizes water contained in porous films pressed against heat exchange walls 21. In this type of boiler, the heat transfer can take place, as well from a gas train type burner, such as 19, with atmospheric air supply, or with supply air, only from one or more radiant burners. In the first case, the heat transfer is mainly carried out by convection, while in the second it is mainly carried out by radiation.

Preferably, use will be made of several substrates 7a, 7b ... arranged in two separate chambers 23 each defined by two hollow metal boxes 29 of substantially parallelepipedal shape standing in two substantially parallel vertical planes, being spaced from one another so as to reserve between them an intermediate space 31 usable as a chimney for the evacuation of the fumes which can be produced by the burner, which will preferably be arranged in the lower part of the space 31, in a place where the space has a shape in the trunk of a pyramid, converging in the direction of the smoke evacuation. The chimney is closed laterally by walls (not shown).

In the present case, each partition 21 has been internally fitted with three porous films 7a, 7b, 7c extending in perspective over approximately half the height of the exchange wall, over 3/4 of the remaining height, and on the 1/4 of the highest part. On each porous film is applied a mesh 33 with large open meshes at a rate of 90% with a mesh surface of 4 cm ² to, on the one hand, ensure good thermal contact with the substrates and, on the other hand, leave a passage for the steam produced. Each enclosure 29 is also equipped with an upper tank 34 in which is immersed an "upstream" portion of the three porous films which are here of the same thickness. It will be noted that the porous film 7a, in order to reach the top part up to the tank 34, is kept apart from that marked 7b (space d). Thus, the entire column of water Cl stored on the thermally protected upper part of the film 7a will be used to feed under suitable pressure (higher than the ambient pressure prevailing in the enclosure considered) its lower part pressed against the partition 21, therefore fully active in terms of heat exchange and vaporization capacity. It is also the same for the film 7b, but with a column C2 of lesser height practically the entire column being here exposed to the heating flow. At the bottom of each of the enclosures, the water collected in an appropriate lower reservoir is shown, when the flow rate in the films is greater than that vaporizable by the heat flow. When this excess water reaches a predetermined level, it can be reinjected by a pump into the tanks 34.

FIG. 7 is a diagram which shows the influence of the number of substrates and the height of water on the vaporization yield as a function of the power injected, with either a single porous film of the aforementioned type "with small meshes" replacing the two substrates 7a, 7b, or these substrates themselves. For each case, the measure consists in varying the height of the water level of the tank 34, it being specified that the tank has in this case been placed approximately 4/5 of the height of the exchange walls.

On 1 • one of the curves, for a single porous body and with reference to a water height of H-9 mm relative to the maximum height H authorized up to the upper edge of the tank, there is an increase in yield of 0 , 30 g / h at 0.8 g / Wh, then a decrease in it up to 0.65 g / Wh, when the injected power is varied from 1.2 Kw to 2.4 Kw.

When the power is further increased, and the water height is also increased to H-4 mm, the same shape of curve is observed (marked by diamonds), with an efficiency increasing further up to 3 , 2 Kw, then a drop of it to 3.4 Kw. This drop in yield is even more significant if we place ourselves at H-2 mm, to reach the value of 0.9 g / Wh. By installing two porous bodies, reducing the water height to H-9 mm, the yield is further increased up to 1.10 g / Wh for a power of 4 Kw, to then maintain a similar yield of this power up to H-2 mm of water height. Thus, it is necessary to adjust the water height according to the power of the thermal source when one wants to obtain an optimum yield. On the other hand, beyond this optimum, the yield decreases when the power is increased, this due to a too low flow rate of liquid in the substrate. It can also be seen that the vaporization yield increases when there are two substrates for the same exchange surface.

We also note that, at a practically constant power of 2.4 Kw, the vaporization yield goes from 0.6 g / Wh to 0.8 g / Wh, ie a gain of 30% on the efficiency of the boiler. This gain is obtained when the water height is varied from H-2 mm to H-4 mm, then to H-9 mm.

For a steam boiler corresponding to that of FIGS. 5 and 6, the diagram of FIG. 8 shows the influence of the thickness of the substrate (s) on the temperature of the gases leaving the boiler, this depending on of the variation of its power. In this kind of heat transfer measurement, a porous film with "small meshes" gives a temperature difference of 120 ° C to 400 ° C while this difference is only from 300 ° C to 370 ° C for a porous mesh film thick. Figure 9 is a cutaway and perspective view of an alternative embodiment of a steam generator using an electrical resistance. It consists of a cartridge resistor 37 on the outside surface of which a fibrous substrate is applied and clamped, presenting itself as a flexible sleeve 39 sewn at 41 and 43 to form two half-surfaces 45a, 45b which extend towards the lower part of the enclosure 47 while being partially submerged in water in the upper part in an upper tank 49 the level of which could be varied (by a feed pump) and in the lower part in a lower collection tank 51. The enclosure 47 is also equipped at its upper part with an outlet for steam 53. With this type of electric resistance boiler, the same kinds of measurements were made as on the gas boiler in FIGS. 5 and 6. Each of the tables below shows, at constant water flow rate, the vaporization yield by varying the density of thermal flux for four thicknesses of porous film.

1) 0.2 mm thick porous film Flux density in W / cm2 2.50 3.30 5.00 6.10 6.90 8 Vaporization efficiency g / Wh 1.02 1.05 1.24 1.14

2) 1 mm thick porous film Flux density in W / cm2 2.50 3.30 5.00 6.10 6.90 8 Vaporization efficiency 0.89 1.04 1.16 1.20 1.24 1, 28

3) Film with parallel wicks, thickness 2 mm

Flux density in W / cm2 2.50 3.30 5.00 6.10 6.90 8 Vaporization efficiency 0.85 1.02 1.14 1.17 1.18 1.17

4) Film with parallel wicks, thickness 4 mm Flux density in W / cm2 2.50 3.30 5.00 6.10 6.90 8

Vaporization efficiency 0.85 1.02 1.10 1.11 1.10 1.08 Thus, for the same variations in heat flux density, there is a variation in the vaporization yield of 20% for a thickness of 0.2 mm, of 40% for a thickness of 1 mm, of 30% for a thickness of 2 mm and 25% for a thickness of 4 mm.

In the case of a porous substrate 1 mm thick, at the optimum yield of 1.28 W / cm ² , it is observed that there is practically no more liquid flow at the bottom of the substrate (assuming it is arranged vertically). There is then equivalence between the quantity of vaporizable liquid entering the porous film and the flow rate of the thermal source, for a flux density of 8 W / cm ² . In FIGS. 10 and 11, the porous film is locally immersed in the water to be vaporized which circulates in a closed circuit in a pipe. This kind of device can operate in different positions by the use of a pump and / or a regulating valve whose purpose is to ensure a pressurized supply of water to the substrate. The vaporization means comprise a rectangular resistor 59 with a power of 270 Watts. On the resistance is applied and tightened a fabric forming a woven film 61 sewn at 63 and 65 to form a sleeve extending downward, housed and integral with the interior of the lower part 67 of the pipe 69. This sleeve s 'also extends inside the upper part 71 of the same pipe 69. The resistance is housed in an enclosure 73 of vaporization. The vaporization enclosure comprises an outlet tube 75 for the vapor and a tube 77 for evacuating the excess water when the flow of circulating water is too high and a flange 79 integral with the resistance to be fixed in 81 at the enclosure. We have figured the mesh of a flexible canvas bathing in the upper and lower parts in the water through slots made in the tube 69 where the water comes in 83 to come out in 85 before being recycled. Connections 87 and 89 also allow the electrical resistance to be supplied.

In this device, the circulation of water is ensured by a pump 84 whose flow rate can be adjusted. The outlet 85 of the pipe is provided with a tap 86. This can ensure a slight overpressure in the pipe so that the liquid preferentially flows into the porous film. By adjusting the tap, the substrate can also be oversaturated with liquid, by creating a film of water maintained on the surface by the interfacial tension of the liquid on the faces of the porous film.

With the pump 84 and the tap 86, we will thus have means for putting the liquid at a pressure higher than the ambient pressure in the enclosure 73, this upstream of the zone where the substrate 61 is exposed to the heat of the resistance 59, thus making it possible to obtain the already stated conditions of flow in the substrate. The table below shows the results obtained with this installation at a constant flux density of 4.7 W / cm ² , when the supply water flow rate is varied and a substrate "at small meshes "of the type already presented.

Circulating water flow g / min 7 15 22 30 40 50 57

Vaporization efficiency g / Wh 1.31 1.32 1.30 1.26 1.16 1.12 1.10

Thus, the vaporization yield in the porous substrate is 20% higher when the incoming water flow is reduced from 57 g / min to 15 g / min. FIG. 12 is an example of a drip device for supplying a vaporization apparatus comparable to that of FIG. 8. By using a whole range of interchangeable porous bodies establishing constant liquid flow rates, it is then possible to obtain a good flow distribution over large widths of substrates and for very low flow rates. One can also easily adapt the liquid flow to the vaporization source, or even oversaturate the porous film. This device can also be used as trapping of salts contained in water, or even as an interchangeable liquid filter.

In this FIG. 12, a double woven substrate 111a, 111b surrounds, in pendant, a tubular electrical resistance 113, in the lower part of an evaporation enclosure 115. The upper part of the substrate is flared in "V" shape and rests on two supports. Its supply of liquid to be evaporated is therefore ensured by a drip, by means of two fine rectangular woven substrates 117, 119, hanging vertically and ending at their free lower end with fringes 120 favoring the drip and a good distribution of the liquid.

In the upper part, the substrates 117, 119 are immersed in a liquid supply tank 121, of variable liquid height, filled with a supply not shown. A chimney 123 allows the steam to escape.

FIG. 13 shows a device for vaporizing liquid using at least one 1 mm thick sintered stainless steel plate. In this embodiment, the liquid is vaporized by thermal radiation from an electrical resistance.

In this case, two plates 125, 127 in "S" of a sintered alloy of stainless steel with a porosity of 30% were used and arranged back to back to form an arch in "U" inverted around the resistance tubular 129. These rigid plates are joined at the top where they are held at 131 to be engaged in a sealed manner in a pipe 133 where the liquid 135 to be vaporized circulates. By this arrangement and by the fact of a slight pressure in the pipeline (thanks, for example, to a pump), the liquid will flow into the empty volumes enclosed by the two plates. In the center of the vault, and 10 mm from the two walls, the electrical resistance 129 has been located which extends over the entire length of the vault. In the lower part, the excess non-vaporized liquid is collected at 137 which can be reinjected at the inlet of the tube 133 to help feed the substrates.

If the porous walls 125, 127 and the reservoir tube 133 have been locally immersed, this vaporization element can also be supplied by the drip device of FIG. 12.

The process of the invention and its exemplary embodiments find their applications in particular in products from the craft sectors, the general public, DIY as well as in the processing and agro-food industries. Thus, using the combustion of natural gas or electrical energy, we can create steam generators ranging from a few kg of steam / hour to more than a ton / hour. These generators can be used, for example, in catering ovens, in bakeries, in consumer gas cookers, in the biscuit and pre-baking industry, in the textile industry for fiber processing, or even, for example, for steam pressing plants, or even in biology laboratories for sterilization. It is also possible, for example, to produce steam generators for individual irons or with a steam generator, or even for floor and wall cleaning devices.

Regarding the range of heating flux densities usable in the context of the invention, we note also that we can go from a few mW / cm ² to several tens of W / cm ² .

By the way, it should be clear that the device of the invention to operate both at atmospheric pressure and at overpressure or under vacuum, only the pressurization of the liquid having to be provided to ensure the flow conditions sought in the substrate.

Claims

CLAIMS 1.- Method for vaporizing a liquid, in which:

at least one porous substrate is supplied with said liquid to be vaporized, so that the substrate takes care of it,

- And exposing at least one area of said liquid-charged substrate to heating energy, to raise the temperature of at least part of said liquid to its vaporization temperature, characterized in that, for a given vaporization regime corresponding to determined energy supply conditions, a liquid inlet flow rate greater than the liquid input flow rate induced in the same substrate is established in the substrate, in a position then assumed to be horizontal thereof, by capillarity and vaporization of the liquid only.

2.- Method according to claim 1, characterized in that creates said liquid flow in the substrate through the weight of a column of said liquid.

3.- Method according to claim 1, characterized in that creates said liquid flow in the substrate by injecting said liquid under pressure.

4.- Method according to claim 1, characterized in that:

- the substrate is tilted relative to the horizontal,

a column of liquid is created in the substrate, and

- An increase in pressure is created in said column of liquid.

5.- Method according to any one of claims 1 or 2, characterized in that to establish said liquid inlet flow rate in the substrate, part of this substrate is immersed in a source of liquid disposed above said zone of the substrate exposed to the flow of heating energy, and the column of liquid is created in the substrate.

6.- Method according to any one of the preceding claims, characterized in that one adapts to the other the heat flow of heating and the flow of liquid supplying the substrate so that part of the amount of it liquid flows out of said substrate.

7.- A method according to any one of claims 1 to 6, characterized in that the substrate is supplied with liquid with a flow rate slightly greater than the flow capacity of the liquid inside the substrate, so that part of this liquid flows on a free surface of said substrate, being held against by interfacial tension.

8.- Method according to any one of the preceding claims, characterized in that at a second end opposite to a first end of the substrate where it is supplied with liquid so that this liquid flows therein while evaporating, bathe said substrate in a bath of the same liquid and the liquid contained in the bath is used to supply said first end of the substrate.

9.- Method according to any one of claims 1 to 5, characterized in that it is supplied with liquid at a first end so that practically all the liquid supplied to the substrate is evaporated before having reached a second end of said substrate.

10.- Device for vaporizing a liquid, in particular for implementing the method according to any one of claims 1 to 9, comprising:

- at least one porous substrate exposed to a determined ambient pressure,

means for supplying the substrate with liquid so that it becomes charged with liquid, by circulation of said liquid in the substrate from an upstream portion,

- at least one energy source for heating at least one vaporization zone of said substrate located downstream of the upstream portion and the liquid which charges it, so that at least part of this liquid is vaporized there, characterized in that that said means for supplying liquid to the substrate comprise means for pressurizing the liquid, to put this liquid at a pressure higher than ambient pressure.

11.- Device according to claim 10, characterized in that said pressurizing means comprise at least one liquid reservoir disposed above the heated area of the substrate.

12.- Device according to claim 10 or claim 11, characterized in that said pressurizing means comprise a column of liquid created in the substrate.

13.- Device according to any one of claims 10 to 12, characterized in that it comprises several substrates arranged in an inclined position relative to the horizontal at least for the most part of their heated area, said substrates overlapping partly by being spaced from each other on at least part of their surface to leave between them a space for the circulation of steam.

14.- Device according to any one of claims 10 to 13, characterized in that the (each) substrate is a permeable fabric of flexible fibrous fabric.

15.- Device according to any one of claims 10 to 13, characterized in that the (each) substrate is a plate with a rigid structure, such as metallic.

16.- Device according to any one of claims 10 to 14, characterized in that the (each) substrate includes fabric threads of different diameters.

17.- Device according to any one of claims 10 to 16, characterized in that the liquid supply means d (of each) substrate further comprises drip means.

18.- Device according to any one of claims 10 to 17, characterized in that it comprises at least two substrates having a width determined along at least most of which they are supplied with liquid, the two substrates extending on either side of the energy source (s), which extends over at least most of the width of the substrates.

19.- Device according to any one of claims 10 to 18, characterized:

- in that said energy source comprises at least one gas burner,

- And in that it further comprises two hollow boxes delimiting between them a chimney in which circulate the combustion products of said burner, said boxes each containing at least one substrate.

20.- Device according to any one of claims 10 to 19, characterized in that the (each) substrate is bathed at two opposite ends between which the liquid flows along it, in a tube where said liquid is brought by means of forced circulation.

21.- Device according to one of claims

10 to 20, characterized in that: - the substrate has a thickness of between approximately 0.05 mm and 5 mm, and preferably less than 2 mm.