WO2016083190A1

WO2016083190A1 - Steam generator

Info

Publication number: WO2016083190A1
Application number: PCT/EP2015/076774
Authority: WO
Inventors: André CHATROUX; Michel Planque
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2014-11-28
Filing date: 2015-11-17
Publication date: 2016-06-02
Also published as: FR3029270A1; EP3224542A1; EP3224542B1; DK3224542T3

Abstract

The invention relates to a steam generator (51) operating at constant pressure, which comprises a liquid intake (40) with a predefined flow rate comprised within a range, and a heating surface with a downwards slope allowing the flow of said amount of liquid and a temperature increase of said injected amount of liquid as it flows along the path until the complete evaporation of every mole of said amount of liquid. The heating surface is open so as to allow a direct discharge of the steam formed, and the linear thermal power applied by said heating surface is selected in accordance at least with the maximum length of the path and the thermal power necessary for evaporating all of the maximum flow rate of the range.

Description

STEAM GENERATOR

FIELD OF THE INVENTION The invention relates to the field of steam generators, and in particular to steam generators used in electrolysers of high temperature water vapor (EHVT). The invention relates more particularly to a device for converting a liquid into a vapor capable of providing a low steam flow rate, in particular a steam flow rate in particular between 10 g / h and 10 kg / h, and operating at constant pressure, in particular at atmospheric pressure or under a few tens of bar.

STATE OF THE ART

A high temperature steam electrolyser (EVHT), an acronym for High Temperature Steam Electrolysis (HTSE), is an electrochemical device for the production of high temperature water vapor. hydrogen from the water vapor by applying an electric current to a stack of electrolytic cells electrically connected in series and each consisting of two electrodes, namely a cathode and an anode, interposing a solid oxide electrolyte membrane. Globally, water vapor is introduced at the cathode of each cell supplied with electricity and an electrochemical reduction reaction of the water vapor leads to the formation of hydrogen on the cathode. In general, for a given operating point of the electrolyser, there is an electric current to be applied thereto and the flow rate of water vapor to be introduced into the electrolyser is calculated as a function of the intensity of the electric current. applied on the electrolyser. The current intensity can generally vary from 0 to 100% of the operating range of the electrolyser, it is therefore also necessary that the steam flow to be generated can also change linearly from 0 to 100% capacity, and be composed only steam. In addition, an electrolyser is a system that is very sensitive to inhomogeneities of current / gas flow, these homogeneities possibly causing premature aging of the electrolyser. For example, if the flow rate of steam varies around its setpoint, we can observe an instability of the operating point of the electrolyser which results in changes in the voltage of the cells, cause of premature aging. More serious, large variations in the steam flow result in pressure variations of a few tens or hundreds of mbar, which may be sufficient to damage the joints seal or crack the electrochemical cells themselves. It is therefore desired a flow of steam as homogeneous and regular as possible.

Regulated flow steam generators are commercially available and are suitable for generating high flows of water vapor, namely several kg / h. Such a steam generator is generally composed of a reserve of pressurized steam coupled to a steam flow control valve to generate the amount of water vapor required, and this steadily. There are also commercially available steam generators, suitable for lower flow rates, especially between 10 g / h to 10 kg / h. Since the steam flow control valves used for large flow rates are not adapted for this flow rate range, one solution is to regulate the flow of liquid water entering the generator so that the water vapor generated after evaporation corresponds to the desired amount of steam output.

According to one embodiment, these low-flow generators use a heating tube or a volume of which at least the bottom wall is heated, in which a controlled quantity of water is injected and heated until evaporation. However, it is observed that such generators cause "puffs" of steam and therefore overpressures. As the principle of the evaporation of liquid water is rather complex if the regularity of the output flow rate is taken into account, the difficulty in this solution is to control the heating of the water so as to avoid that a local boiling of the water pushes a part of the liquid in a zone too hot and is a source of boiling reaction of the boiling reaction. Since control of these phenomena is particularly difficult, in practice, low-flow evaporators are generally only constructed to generate a single dry vapor flow, the electric power and the evaporation surface are studied only for a single operating point. . According to another embodiment, other evaporators adapted for low flow rates use a carrier gas, for example nitrogen, which facilitates the spreading of the liquid and evacuate the steam produced. However, this solution does not make it possible to produce a dry vapor since it is impossible to separate the vapor from the carrier gas. SUMMARY OF THE INVENTION

The object of the present invention is therefore to propose a steam generator capable of producing, at constant pressure, a constant flow rate of vapor of a liquid, for example water, for the low flow rates of steam, and not requiring the use of carrier gas.

The subject of the present invention is thus a constant-pressure steam generator capable of providing a regulated steam flow rate, including for low steam flows, in particular between 10 g / h and 10 kg / h, and for a constant pressure of 0.degree. to several tens of relative bar.

According to the invention, this steam generator comprises:

a liquid flow regulator configured to generate a quantity of liquid, for example water, at a given instant following a constant flow of liquid within a predetermined range of flow rates, the maximum flow rate of this flow range is less than or equal to 10kg / h;

a device for converting the liquid into vapor operating at constant pressure and comprising:

o an enclosure provided with a liquid inlet connected to the liquid regulator and a steam outlet through which the steam escapes at a flow rate substantially identical to that of the liquid injected at the inlet, the enclosure having said constant pressure;

a downwardly inclining heating surface in the enclosure defining a flow path that allows gravity flow of the quantity of liquid along the path from the inlet, and a rise in temperature of the quantity of liquid to be measured its flow along the path until the total evaporation of each mole of this amount of liquid before the end of said course, this heating surface being open so as to allow direct evacuation of the steam formed; a source of energy capable of supplying a sufficient quantity of energy to the heating surface to apply to the liquid a linear thermal power along the path, this linear thermal power being selected as a function of at least the maximum length of the path and the thermal power required to evaporate all of a maximum quantity of injected liquid according to the maximum flow rate of the flow range. In other words, the invention is the combination of three parameters:

dimensions to ensure a flow of the liquid, in particular by gravity. It is understood for example that if the liquid is directed to a tank in which it is stored for evaporation by heating, it is difficult to precisely control the amount of steam produced. Through the flow, each volume of injected liquid receives "individually" and in a controlled manner a quantity of heat. The control of the quantity of steam produced as a function of the quantity of liquid injected is thus facilitated;

a heating of the liquid by an external source of energy which can be electric (heating resistance) or thermal (heat transfer fluid); and

- A path length adapted to obtain a complete evaporation of each mole of liquid injected before the end of the course.

Such a steam generator thus ensures control of the vaporization of each quantity of water injected. This vaporization is reflected in particular by a continuous evaporation of the liquid as it flows until its total evaporation before the end of the course, without formation of puff or liquid flow on a mattress (or bed) of steam inside the enclosure. Such a control of the evaporation process in particular avoids the presence of stagnant water in the chamber, a necessary condition for obtaining a regularity of the output steam flow, and makes it possible to directly supply a steady flow of vapor substantially identical to the flow of liquid. injected as input. In practice, the regulation of the flow of water (and therefore of steam) is controllable by a user by means of a command button. For example, the user can adjust the setpoint of the flow rate of liquid to be injected at the inlet to obtain a substantially equivalent flow rate of steam output.

Such a generator is thus capable of providing a constant dry vapor flow rate in the range 10 g / h to 10 kg / h, and this without using either carrier gas or electromechanical element, such as a steam flow valve for example. The manufacture of the generator and its operation are simplified.

For example, for a steam flow in the range lOg / h to 10kg / h, the maximum thermal power applied can be between 8 and 12 kW and the length of the path can be between 10 and 20m. In practice, for a flow rate below 300 g / h, the liquid is preferably injected dropwise. For example, a flow rate of the order of 10 g / h corresponds to approximately 1 drop of 36 μl of liquid every 13 seconds. According to one embodiment, the conversion device may further comprise a groove extending along the path and in which is housed the heating surface, the groove allowing a liquid flow by gravity and capillarity along the path.

In other words, the groove is sized to form a capillary channel, thus sucking the injected liquid. The gravity flow combined with the capillary flow allows in particular a uniform spreading of the liquid injected at the inlet in small flow, in particular drop by drop, over a large length of the heating surface throughout the course. It is thus formed a net of continuous water. The formation of drop train on the heating surface being avoided, the steam generation is therefore more regular and the output steam flow is constant even for a discontinuous water injection. This solution makes it possible in particular to produce a steady flow of steam from a non-linear injection of liquid, particularly drop by drop.

Of course, the liquid inlet may also be dimensioned to further allow a capillary injection of the liquid directly into the groove and prevent the formation of drops at the end of the liquid water injection tube. According to one embodiment, the end of a liquid water injection tube is in this case placed in contact with the groove.

In practice, the heating surface is disposed in an open groove on at least a lower portion of the path, preferably over the entire path, so as to allow the free escape of the steam generated.

In the embodiment corresponding to a flow by capillarity and gravity, the groove is provided in its bottom of the groove described above. In practice, this groove is open over its entire length towards the inside of the groove, and preferably has dimensions adapted for capillary flow. For example, for a heating surface of 3.5mm diameter and a flow range of 0 to 5 1 / h, the main groove has a height of 8.5mm for a depth of 17mm and the groove has a height and width 3.5mm allowing the matting of the heating surface. To direct the liquid towards the bottom of the groove, and thus concentrate the liquid on the heating surface at the bottom of the groove, the walls of the groove are preferably inclined, the inclination of each of the walls forming with the horizontal an angle inclination (a) greater than or equal to 30 ° C. In other words, the angle formed between the wall in contact with the circulating liquid, in the direction from the bottom of the groove to the opening of the groove, and the direction of gravity is non-zero and preferably lower or equal to 60 ° C. It ensures that the circulating liquid remains in contact with the heating surface which is preferably disposed closer to the bottom of the groove.

The groove is preferably made of a neutral material for the liquid, especially stainless steel particularly appreciated for its corrosion resistance.

In addition, this groove is advantageously helically shaped when the compactness is sought.

Alternatively, the heating surface may be located at the outer periphery of a cylinder.

The conversion device can thus be formed of a threaded part, the thread forming the groove. For example, the part may be a cylinder extending in the direction of gravity and the periphery of which is machined the helical groove. The helical groove preferably has a width, a depth, a length and a pitch suitable for the total evaporation of the injected liquid before the end of the path combined with the linear thermal power applied to the heating surface, as explained above. Furthermore, the opening of the groove is preferably dimensioned to ensure a direct evacuation of the vapor and limit an accumulation of steam above or in the groove which would be likely to disturb the heating or the gradual rise in temperature of the liquid . In particular, it is preferable that the steam does not push the liquid to an area too hot to avoid pressure puffs. For example, the vapor can evacuate substantially radially without disturbing the flow of water. Thus, the opening of the groove can be advantageously oriented in a direction different from that of gravity.

Advantageously, the enclosure may be formed of a thermally insulating outer envelope and a metal inner envelope maintained at a predetermined temperature, for example between 150 and 250 ° C for steam generation at atmospheric pressure, so as to avoid condensation on the inner walls of the enclosure. This internal envelope will be sized and qualified to operate up to the maximum pressure defined in the design of the steam generator. Preferably, this metal inner envelope is designed to operate at a pressure above atmospheric pressure or in the range of atmospheric pressure to a few tens of bar. According to a variant, in operation, the pressure in the chamber is that of the steam. According to an embodiment adapted in particular to a production rate of 0 to 5 kg / h at atmospheric pressure, the inner casing has a diameter of 220 mm for a height of 470 mm sheet 2 mm thick. The spiral with a linear length of 10 m is machined on a 200 mm diameter and 400 mm high tube, the distance between the opening of the groove and the inner wall of the enclosure being 8 mm. The volume in which the steam is present is about 6 liters.

According to another embodiment, the thermal power of the heating surface can be regulated via, for example, a temperature sensor capable of measuring the temperature at at least one point of the heating surface coupled to an energy regulator to be supplied to the surface. heating according to the measured temperature and a set temperature generally greater than or equal to 100 ° C.

In practice, a table giving the correspondence between the thermal power, the water flow and the temperature of the heating surface, is used for the regulation of the thermal power.

In another embodiment, it is possible to change the source of heat. For example, it is possible to use a coolant, for example an oil, which is circulated in a metal tube. This tube can be put in place of the electrical resistance, for example by the same matting technique. Knowing the heat capacity of the coolant, it is possible to calculate the nominal temperature ensuring that the maximum flow of liquid can be evaporated. Moreover, to ensure a regular flow of evaporation, it is advantageous to circulate the heat transfer fluid in the opposite direction to the flow of liquid water.

In addition, although the described device allows to produce dry steam, it is also possible to introduce, if necessary, one or more gases to achieve a mixture. In this case, a preheating of these gases is necessary before their introduction into the steam. This preheating can be obtained for example by a simple technique which consists of welding the spiral gas line on the generator enclosure.

The invention also relates to a device for converting liquid to constant pressure steam having all or part of the characteristics defined above. In particular, the device for converting liquid to steam is capable of providing a regulated flow rate of steam, in particular between 10 g / h and 10 kg / h, for a constant pressure of 0 to several tens of relative bar.

For example, this device for converting liquid to constant pressure steam comprises:

- a liquid inlet through which a quantity of liquid is injected at a given instant following a constant flow of liquid within a predetermined range of flow rates, the maximum flow rate of this flow range is less than or equal to 10 kg / h;

a downwardly sloping heating surface defining a flow path allowing the quantity of liquid to flow along the path by gravity from the inlet, and a rise in temperature of this quantity of liquid injected as it flows over the along the route until the total evaporation of each mole of this amount of liquid before the end of the course.

In addition, the heating surface is opened to allow direct evacuation of the formed vapor, and the linear thermal power applied by this heating surface to the liquid throughout the course is selected based on at least the maximum length of the course and the thermal power required to evaporate all of a maximum quantity of injected liquid according to the maximum flow rate of the flow range. In terms of process, the generation of steam or the conversion of liquid to vapor at atmospheric pressure or at a constant pressure with the generator or the device described above comprises in particular:

- The supply of energy to the heating surface described above so that the heating surface has a sufficient linear thermal power for the evaporation of a quantity of liquid to be injected at a predefined rate;

the injection of the quantity of liquid at a time t according to the predefined flow rate;

the flow, at least by gravity, of the quantity of liquid along the heating surface defining a flow path, and heating of the quantity of liquid as the liquid flows to its total evaporation before the end of the course. BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the appended drawings in which identical references designate identical elements and in which:

FIG. 1 is a schematic representation of a device for converting a liquid into a vapor according to one embodiment of the invention in which the groove is helical;

FIG. 2 is a partial diagrammatic representation in axial half-section of the helical groove of the steam generator of FIG. 1; and

Figure 3 is a schematic representation of the steam generator according to one embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The steam generator according to one embodiment of the invention is illustrated in FIG. 3 and notably comprises:

a liquid flow regulator 6;

a device for converting the liquid to vapor; and

a regulated energy source 7;

The liquid flow controller 6 is configured to provide the conversion device 10 with a quantity of liquid 50 at a given instant at a constant flow rate, for example between 10 g / h and 10 kg / h. In practice, the flow of liquid to be generated is set by the user by means of a set point 60 and corresponds substantially to the flow rate of steam to be produced by the conversion device 10. The liquid water flow regulator 6 may be a commercial regulator, for example a thermal mass flow controller or Coriolis. The device 10 for converting the liquid into vapor is configured to evaporate all of the injected liquid 50 and to generate steam 51 at a regular flow rate corresponding substantially to the injected liquid flow rate.

The structure of the liquid conversion device 50, especially water, steam 51 according to a particular embodiment illustrated in Figure 1 is described below. This conversion device consists in particular of an enclosure 4 provided with a vapor outlet 42 and a liquid inlet 40 intended to be coupled to the water flow regulator 6. The enclosure 4 is formed in particular of an outer casing 43 thermally insulating and an inner casing 44 which makes it possible to retain the vapor produced in the enclosure 4 and to channel this vapor on the vapor outlet 42 so that it can be used. This inner envelope 44 is in particular of metal material and is heated to be maintained at a sufficient temperature, for example 200 ° C, to prevent the appearance of condensate on the inner walls of the enclosure. This envelope 44 also serves to resist the maximum operating pressure of the steam generator. For example, for a production flow rate of 0 to 5 kg / h at atmospheric pressure, the inner casing has a diameter of 220 mm and a height of 470 mm made of 2 mm thick sheet metal.

According to a variant, the internal volume of the chamber 4 being in connection, via the vapor outlet 42, with the outside, the internal pressure of the chamber is consequently adjusted by the external pressure. For example, the output 42 is connected directly to the input of an electrolyser operating at atmospheric pressure, so that the internal pressure of the chamber 4 is equal to the atmospheric pressure, and therefore substantially constant. Of course, the external pressure, which sets the internal pressure of the chamber, may be different, especially higher. Similarly, a pressure regulator can be provided to directly regulate the internal pressure of the chamber to obtain a substantially constant pressure. According to another variant, in operation, the internal pressure of the chamber remains substantially identical to that of the pressure of the steam produced.

An open groove 2 of helical shape, for reasons of compactness, is disposed in this enclosure 4. This groove 2 is in particular machined at the outer periphery of a part 1, a stainless steel cylinder for example, which rests on a base 41 support bracket 4. This support base is in particular horizontal, that is to say substantially perpendicular to the direction G of gravity.

This groove according to this embodiment has a downward slope, for example between 1 and 4%, which allows a flow by gravity of the liquid water 50 from the inlet 40 of liquid. Moreover, a wired 3-wire electric heating element, for example of round section, serving as a heating surface, is inserted into the groove 2. This heating resistor 3 is positioned in the groove, and in particular near the bottom of the groove, so as to extend all along the throat to define a flow path of the liquid. This heating resistor 3 is powered by the source energy regulator 7 and is also intended to be brought into contact with the circulating liquid to bring it up to its evaporation temperature. In practice, to effectively transmit thermal energy to the liquid to be evaporated, the contact surface between the heating resistor and the liquid must be large. According to one embodiment comprising an electrical resistance of 10 m long and 3.5 mm in diameter, the contact surface is estimated at 550 cm ² .

More specifically, as illustrated in Figure 2, this groove 2 comprises in particular a groove bottom to the opposite of an opening and inclined walls. In particular, the opening is oriented so as to allow a substantially radial evacuation of the steam produced. The angle of inclination formed between the horizontal H and each of the walls 20 of the groove in the direction from the groove bottom to the opening is at least 30 °. This inclination makes it possible to direct the liquid towards the bottom of the groove and to ensure a permanent contact of the circulating liquid with the heating surface.

The bottom of the groove 2 also has a groove 21 in which is inserted the heating resistor. This groove 21 is open and extends in the extension of the groove. The groove 21 has dimensions, in terms of width and depth, smaller than those of the groove, and is especially sized to allow insertion by matting of the heating resistor and capillary flow of the liquid in the groove. For example, for a heating surface of 3.5 mm diameter and a flow range of 0 to 5 1 / h, the groove 2 has a height of 8.5 mm for a depth of 17 mm and the groove 21 has a height and a 3.5mm width for wired electrical resistance matting 3.

The capillary flow is particularly advantageous in the case of a low liquid flow, and in particular in the case where the liquid is injected drop by drop. The capillary flow makes it possible in fact a uniform spreading of the liquid to be evaporated on the heating surface, without formation of drop train, ensuring a regularity of the output steam flow. In addition, the water can also be injected by capillarity into the groove. For example, the liquid inlet may be a stainless steel tube positioned in contact with the groove and the heating resistor and dimensioned to allow capillary suction of the liquid in the groove, without the formation of droplets. Of course, depending on the type of liquid injection, drip or continuous, and the liquid flow rate to be introduced, it is possible to make the device without the groove. In this case, the heating resistor is disposed closer to the bottom of the groove and the liquid flows only by gravity. According to another variant, the wire heating resistor may be flat inserted for example in the bottom of the groove or in the groove, or a layer of a metal material covering one or more walls of the groove and / or the groove.

The heating resistor 3 is also coupled to the energy source 7, for example a regulated voltage source (not shown), to provide the energy necessary for the heating of the water during its descent along the throat to 'to its evaporation. The steam 51 produced is then discharged at the periphery via the opening of the groove without disturbing the flow to ensure regularity in the operation of the heater.

To ensure effective evaporation of all liquid introduced into the groove with or without boiling, it is necessary to heat the liquid homogeneously. In practice, the heating resistor is calibrated so as to ensure a rise, preferably regular, temperature of a given amount of water in the groove until it evaporates completely before the end of the course. The electric power to be supplied to the heating resistor can be optimized according to the quantity of water introduced, so as to distribute the best water over the entire length of the groove and to have a very stable steam production with a minimal energy expenditure. This optimization can be done by calculating the energy needed to heat up the water and then its vaporization, while taking into account heat losses.

In practice, it defines a maximum flow rate of liquid to be injected into the groove, for example 10 kg / h. This maximum flow rate defines a maximum quantity of liquid to be evaporated, here 10kg. The total thermal power required to vaporize this maximum quantity of liquid is calculated, taking into account, of course, the initial temperature of the liquid and the pressure in which the heating is carried out. For example, the total thermal power can be calculated to change liquid water from 20 ° C to a superheated vapor state at 150 ° C at atmospheric pressure. This calculation is for example carried out according to the relations:

P = m. This. (100 - 20) + m. L + m. Cp ₂ . (150 - 100) P = m. (C i (100 - 20) + L + Cp ₂ (150 - 100)) P = m. (4195. (100 - 20) + 2.258, 10 ⁶ + 2030. (150 - 100))

P = m. 2695100

P = 2.77. ÎO ^-3 . 2695100

P = 7486W with:

P: Minimum required thermal power [W];

m: mass flow rate of water [2.77.10 ^"3 kg / s];

- Cp ₁ : average specific heat of water between 20 ° C and 100 ° C [4195 J / (kg.K)]; - Cp ₂ : Average specific heat of water vapor between 100 ° C and 150 ° C [2030 J / (kg.K)];

L: Latent heat of vaporization of the water [2.258.10 ⁶ J / (kg.K)].

According to one embodiment, the power of the electrical resistance will preferably be chosen with a minimum of 30% additional heating capacity to have a better reactivity during changes in evaporation guidelines. In the presented case of a steam production rate of 10 kg / h, the recommended power for the heating resistor is (1, 3.P) = 9732 W, rounded to 10 kW.

The length of this resistance is calculated so as to limit the linear thermal power within the limit of the efficiency of linear heat transfer to liquid water, preferably in the range 0.5 to 1 kW / m. In the case of a steam production rate of 10 kg / h, or 10 kW of total power, the recommended length for this electrical resistance is therefore between 10 and 20 m.

In other words, the method for calculating the linear thermal power consists in particular of:

defining a maximum flow rate of the liquid to be injected, this maximum flow rate giving the maximum quantity of liquid to be evaporated;

calculate the total thermal power necessary to vaporize this maximum quantity of liquid, that is to say to raise the temperature of the quantity of liquid from an initial temperature to at least its evaporation temperature, to achieve the evaporation properly said and to overheat the steam produced; and

calculate the linear thermal power to be produced for a path length equal to the maximum length of the path.

For example, the minimum total heating power P needed to vaporize a quantity of liquid may in particular be obtained by the sum of the energy Pi necessary to heat this quantity of liquid to its boiling point, the energy P ₂ to achieve the actual vaporization and energy P ₃ to overheat the steam produced:

P = Pi + P ₂ + P ₃ In particular, the calculation of the power required to raise a given flow rate of liquid from an initial temperature to a final temperature, for example its evaporation temperature, is for example given by the relation:

P ₁ = m. Cp ^ AT ^ with:

Pi the power in Watt [W];

m the flow of liquid to be heated [kg / s];

Cpi the constant heat capacity at constant pressure of the liquid [J / (kg.K)];

- ΔΤι the temperature difference between the final temperature to be reached and the initial temperature [K].

The calculation of the power required to vaporize a given flow rate of liquid is for example given by the relation:

P ₂ = m. L with:

P ₂ the power in Watt [W];

m the flow of liquid to be heated [kg / s];

- the mass latent heat of vaporization at constant pressure of the liquid [J / kg].

The calculation of the power required to superheat a given flow rate of steam from an initial temperature to a final temperature, for example up to a temperature of 50 ° C higher than the evaporation temperature, is for example given by the relation:

P ₃ = rh. Cp ₃ . AT ₃ with:

P ₃ the power in Watt [W];

- the rate of steam to be heated [kg / s];

Cp ₃ the constant heat capacity at constant pressure of the steam [J / (kg.K)]; ΔΤ ₃ the temperature difference between the final temperature to be reached and the initial temperature [K]. Of course, this relationship can be weighted by taking into account possible heat losses, preferably by adding a minimum margin of 30% on the total power. Thus, the water to be evaporated is introduced into the groove via the liquid inlet and flows into the groove by gravity and / or capillarity. The water heats up during its descent until it evaporates. The steam is evacuated at the periphery of the groove without causing any disturbance to the flow, which guarantees the regularity of operation. According to one variant, it is possible to provide a regulation of the thermal power supplied by the heating resistor. In practice, the energy source 7 can be regulated according to one or more parameters measured or supplied during the operation of the generator. For example, it is possible to couple a temperature sensor to a voltage regulator. The temperature sensor, for example a thermocouple, for example measures the temperature 71 of the heating resistor at the end of the trip. The voltage to be supplied to the heating resistor 70 is then adjusted as a function of this measured temperature and a set temperature corresponding to the thermal power required to vaporize the water. In practice, there is a table or diagram giving the correspondence between the temperature, the water flow, and the electrical energy to be provided.

A heating element has been described as heating element. As a variant, the filamentary resistance is replaced by a tube traversed by a coolant, for example oil, the coolant being heated so as to provide a thermal power as described above.

Thus, the evaporator proposed in the present application is adapted to generate low steam flow, including flow rates of between 10 g / h and 10 kg / h, and does not require the use of carrier gas.

In particular, the proposed evaporator makes it possible to generate dry steam at a constant pressure, and the desired output steam flow rate is simply obtained by regulating the inlet liquid flow rate. In particular, it is possible to combine the liquid injection by drop and the generation of a steady flow of steam.

This is made possible thanks to:

a flow by gravity and / or capillarity which ensures a homogeneous distribution of the liquid on the heating surface; homogeneous heating of the liquid throughout the flow path which ensures a steady rise in temperature of the liquid and a regular evaporation of the liquid; and

optimal evacuation of the steam produced without disturbing the evaporation process.

Claims

Constant pressure steam generator comprising:

a liquid flow controller configured to generate a quantity of liquid at a given time at a constant flow rate of liquid between a predetermined range of flow rates, the maximum flow rate of said flow range being less than or equal to 10 kg / h;

a device for converting liquid (50) to vapor (51) operating at constant pressure comprising:

o an enclosure (4) provided with a liquid inlet (40) connected to the liquid regulator and a vapor outlet (42) through which the vapor (51) escapes at a flow rate substantially identical to that of the liquid (50) injected at the inlet (40), the enclosure (4) having said constant pressure;

a heating surface in the enclosure (4), with a downward slope defining a flow path allowing the gravity flow of said quantity of liquid along the path from said inlet (40), and a rise in temperature of said quantity of liquid as its flow along the path until the total evaporation of each mole of said amount of liquid before the end of said course, said heating surface is opened so as to allow direct evacuation of the steam formed;

a source of energy capable of supplying a sufficient amount of energy to the heating surface to apply to the liquid a linear thermal power along the path, said linear thermal power being selected as a function of at least the maximum length of the path and the thermal power required to evaporate all of a maximum quantity of injected liquid according to the maximum flow rate of the flow range.

Steam generator according to claim 1, characterized in that the conversion device further comprises a groove (21) extending along the path and in which is housed all or part of the heating surface, the free space contained in the groove (21) having dimensions allowing capillary flow of the liquid along the path.

Steam generator according to claim 2, characterized in that the liquid inlet is dimensioned to further allow a capillary injection of the liquid in said groove (21).

4. Steam generator according to claim 1 to 3, characterized in that the conversion device further comprises a groove (2) in which is disposed said heating surface, said groove (2) being open on at least a lower portion of said course, to allow the escape of steam.

5. Steam generator according to one of claims 2 to 4, characterized in that the groove (2) is provided in its bottom of said groove (21), said groove (21) being open over its entire length to the inside said throat.

6. Steam generator according to claim 4 or 5, characterized in that the groove has walls inclined so as to direct the liquid towards the bottom of the groove (2), said inclination of each of the walls forming with the horizontal a angle of inclination (a) greater than or equal to 30 ° C.

7. Steam generator according to one of claims 4 to 6, characterized in that the groove is made of a neutral material for the liquid, in particular stainless steel.

8. Steam generator according to one of claims 4 to 7, characterized in that said groove is helical.

9. Steam generator according to one of claims 1 to 8, characterized in that the heating surface is formed by a heating electric resistor (3) extending throughout the course.

10. Steam generator according to one of claims 1 to 8, characterized in that the heating surface is formed by a tube containing a heat transfer fluid extending throughout the course.

11. Steam generator according to one of claims 1 to 10, characterized in that the heating surface is located at the outer periphery of a cylinder.

12. Device for converting a liquid into a vapor according to one of claims 1 to 11, characterized in that the enclosure (4) is formed of an outer casing (43) thermally insulating and an inner casing ( 44) maintained at a predetermined temperature so as to avoid condensation on the inner walls of the enclosure (4).

13. Device for converting a liquid to vapor according to claim 12, characterized in that the inner casing (44) is designed to operate at a pressure above atmospheric pressure (4). 14. Device for converting a liquid into a vapor according to one of claims 1 to 13, characterized in that it further comprises:

a temperature sensor capable of measuring the temperature at at least one point of the heating surface; and

- A thermal power regulator to be applied to the heating surface according to said measured temperature and a set temperature greater than 100 ° C.

Device for converting a liquid (50) into vapor (51) operating at constant pressure, characterized

in that it comprises:

a liquid inlet (40) through which a quantity of liquid is injected at a given moment in a constant flow of liquid within a predetermined range of flow rates, the maximum flow rate of said flow range being less than or equal to 10 kg / h ;

a downwardly sloping heating surface defining a flow path allowing the flow of said quantity of liquid along the path by gravity from said inlet, and a rise in temperature of said quantity of liquid injected as it flows along the course until the total evaporation of each mole of said amount of liquid before the end of said course;

and in that

the heating surface is opened so as to allow direct evacuation of the steam formed; and

the linear thermal power applied by this heating surface to the liquid throughout the course is selected as a function of at least the maximum length of the path and the thermal power required to evaporate all of a maximum quantity of liquid injected at a maximum flow rate of the flow range. 16. Conversion device according to claim 15, characterized in that it conforms to the conversion device defined in one of claims 1 to 14.