Method for coating the inner surfaces of a heat exchanger with a powdery solid catalyst

Abstract

The invention relates to a method for depositing a coating on inner surfaces forming cavities of a heat exchanger, characterized in that the coating comprises a liquid adhesive and a powder of a pulverulent solid intended to act as a catalyst for a physico-chemical reaction.

Description

PROCESS FOR COATING INTERNAL SURFACES OF AN EXCHANGER WITH A POWDERY SOLID Designation of the technical field concerned

[0001] The present invention relates to the production of a coating serving as a catalyst for a physicochemical reaction on internal metallic surfaces of a brazed plate and wave type heat exchanger, in particular a cryogenic exchanger. Technical problems addressed by the invention

[0002] A brazed plate and wave heat exchanger is typically a heat exchanger formed from a metal assembly of brazed plates and waves. This type of heat exchanger allows for very compact components offering a large exchange surface area and low pressure losses. This type of heat exchanger is formed from a set of parallel plates between which are inserted interlayer elements, in particular corrugated or wave structures. These corrugated or wave structures form cavities with internal metal surfaces. The stacked plates form between them a stack of flat passages for different fluids to be exchanged.

[0003] Thus, such a heat exchanger has internal metallic surfaces forming cavities through which at least one fluid circulates. The characteristic size of these cavities is of the order of a millimeter and has high aspect ratios (length / width ratio).

[0004] The plates and interlayers of this type of heat exchanger are typically made of metal.

[0005] There are many technologies for coating a metal substrate, but most of them become ineffective when it comes to coating an inner wall of a metal cavity of such a heat exchanger.

[0006] Chemical vapor deposition (CVD) uses a gaseous precursor of the coating to be produced. This precursor can be produced in the immediate vicinity of the surface to be coated (pack cementation) or transported via a gas to the surface to be coated (out of pack). When transported via a gas, the main limitations of this technology are the rapid depletion of the gas mixture in reactive species leading to heterogeneities in chemical composition and / or thickness, or even the absence of deposition for surfaces furthest from the carrier gas supply for large structures.

[0007] An improvement to this method consists of using a cement consisting of the powder of the metal to be deposited, an inert diluent and an additive, which may be a pickling flux, as described by EP2956566. The viscosity of the cement is adjusted so as to improve its flowability and allow the filling of the cavities to be coated. In another variant taught by EP3049545, the deposition is carried out by liquid means using an aqueous suspension comprising powder of the metal to be deposited and at least one additive which may be chosen from a binding agent, a dispersant, etc., the objective of which is to promote the wetting of the surface to be coated.

[0008] A thermally activated consolidation step is then necessary to extract the organic phase (debinding) and densify the coating. This operation is not always without effect on the mechanical properties of the structure to be coated, particularly if the treatment requires a high temperature.

[0009] The problem is all the more critical when the material constituting the coating to be deposited has a high reactivity with respect to the substrate, for example when the material of the coating to be deposited is based on iron oxide and the substrate is based on aluminum, this problem being linked to the combination of these two materials which is highly exothermic. In this example, maintaining an iron oxide deposit on the surface of the aluminum-based substrate during an assembly operation carried out at high temperature is therefore not easy, especially if the assembly is carried out by brazing.

[0010] This difficulty can be mitigated by implementing a diffusion barrier to isolate the coating from the substrate during consolidation. This approach significantly complicates the manufacturing process.

[0011] Another difficulty in implementing such coatings is the formation of a deposit whose growth is partly to the detriment of the substrate, particularly if the consolidation heat treatment leads to the formation of intermetallics between the substrate and the coating.

[0012] This constitutes one of the major drawbacks with regard to the regulation of pressure vessels, the mechanical strength of which is defined from a minimum thickness of the substrate which can take the form of a wall. This is all the more so if, by nature, the intermetallics formed between the substrate and the coating are of a fragile nature, such as those of the Fe-Al system.

[0013] While in the case of conventional equipment this can easily be avoided by adding an excess thickness at the wall level, this cannot be considered for a brazed plate and corrugated heat exchanger, whose performance is partly subject to the thickness of the walls constituting the plates and corrugated structures. Indeed, for this type of heat exchanger, the tolerated wall thinning is of the same order of magnitude or less than the wall thickness then consumed during the coating consolidation stage. Since the residual wall thickness cannot be controlled a posteriori, this requires perfect control of the conditions for depositing the precursor deposit.

[0014] Another disadvantage of these technologies is the formation of a completely smooth coating, free of roughness, allowing the reactive surface of the deposit to be increased.

[0015] To our knowledge, traditional coating technologies do not allow for an adherent deposit of iron oxide to be obtained on the surface of an aluminum cavity with a small cross-section and a high length to width ratio, greater than 1000.

[0016] Whether the deposition is carried out before assembly by PVD or CVD, or after assembly using a slip, the subsequent heating of the substrate, necessary for the consolidation of the coating or for assembly, leads to the formation of intermetallic compounds which are harmful to the mechanical integrity of the structure, especially since these intermetallics (Al-Fe) are known to be sensitive to hydrogen embrittlement.

[0017] The invention provides a new solution to these problems.

[0018] According to a first aspect of the invention, a method is proposed for producing a coating on internal metal surfaces forming cavities of a heat exchanger, the coating being intended to serve as a catalyst for a physicochemical reaction of ortho-para conversion of hydrogen, said method being characterized in that it comprises the following steps: a step of depositing a liquid adhesive on the internal metal surfaces, the liquid adhesive being aqueous, organic or inorganic based, and a step of depositing a powder of a pulverulent solid on the deposited liquid adhesive, the pulverulent solid being a mineral and the powder having a particle size of between 10 µm and 100 µm.

[0019] The heat exchanger can be a cryogenic exchanger.

[0020] The coating (liner) obtained on the internal metal surfaces of the exchanger according to the process therefore serves as a catalyst for a physicochemical reaction occurring in the exchanger during its operation. The powdered mineral solid is the catalyst. Thus, the exchanger combines the function of a conventional exchanger, with a transfer of calories between fluids, and the function of a physicochemical reactor.

[0021] The liquid adhesive allows powdered solids to adhere to the internal metal surfaces of the exchanger, i.e. to adhere a mineral phase to a metal surface.

[0022] According to an exemplary embodiment of the invention, the method comprises, after the step of depositing the powder, a subsequent step of maintaining the internal metal surfaces at room temperature during which the solvents of the liquid adhesive are, at least partially, eliminated by evaporation.

[0023] Depending on the nature of the adhesive, for example for a thermosetting glue, keeping it at room temperature may be sufficient for it to dry and acquire all its adhesive properties. The holding time is chosen to obtain the evaporation of all the solvents in the adhesive or the desired proportion of solvents. It depends in particular on the geometric characteristics of the exchanger and the nature of the adhesive. For example, it is from a few minutes to two hours at a temperature between room temperature and 100 °C.

[0024] According to another exemplary embodiment of the invention, the method according to the invention comprises, after the step of depositing the powder, a subsequent step of polymerizing the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal metal surfaces to interact with the powdered solid. Depending on the nature of the adhesive, for example for an organic binder, polymerization may be necessary for it to dry and acquire all of its adhesive properties. The adhesive is chosen such that the polymerization can be carried out in a temperature range sufficiently low so that the internal metal surfaces of the exchanger do not interact with the powdered solid.At higher temperatures, close to the melting point of the brazing material used in aluminum brazed plate and wave heat exchangers, the aluminum and brazing material become highly reactive with the oxide particles, or mixture of oxides, forming the powdery solid. This would result in the formation of intermetallic compounds between the substrate and the coating and a reduction in the thickness of the aluminum wall.

[0025] Thus, the liquid adhesive used to adhere the particles of the powdered solid to the internal metal surfaces of the exchanger is, depending on its nature, eliminated by evaporation or polymerized during a heat treatment.

[0026] According to an exemplary embodiment of the invention, the heat treatment is carried out at a temperature less than or equal to 200°C. By limiting the temperature at which the polymerization is carried out to 200°C, any risk of degradation of the mechanical characteristics of the aluminum is avoided and the legislation remains in compliance.

[0027] Advantageously, a gas flow is carried out in the cavities of the exchanger during the subsequent step of maintaining the internal metal surfaces at ambient temperature during which solvents of the liquid adhesive are, at least partially, eliminated by evaporation or during the subsequent step of polymerization of the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal metal surfaces to interact with the powdered solid.

[0028] For solvent evaporation at room temperature, the gas flow promotes solvent evaporation. The gas can be preheated to enhance evaporation. The gas is, for example, air or nitrogen.

[0029] Advantageously, with a powder of the pulverulent solid whose particle size is between 10 µm and 100 µm, the subsequent step of maintaining the internal metal surfaces at room temperature during which solvents of the liquid adhesive are, at least partially, eliminated by evaporation or polymerization of the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal metal surfaces to interact with the pulverulent solid, is carried out after a step of removing the powder from the pulverulent solid which does not adhere to the liquid adhesive deposited on internal metal surfaces of the exchanger.

[0030] After this removal, it is easier to proceed to the subsequent step of maintaining the internal metal surfaces at room temperature during which solvents of the liquid adhesive are, at least partially, eliminated by evaporation or of polymerization of the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal metal surfaces to interact with the powdered solid, which would otherwise be hindered by the presence of excess powder. Indeed, the presence of the latter would form a barrier to the evaporation of the solvents and would add material to be heated unnecessarily in the event of polymerization.

[0031] According to an exemplary embodiment according to the invention, the liquid adhesive may be polyvinyl alcohol and / or a polymer.

[0032] Advantageously, the liquid adhesive has a viscosity comparable to that of water, i.e. 1×10 −3Pa.s at 20°C. The liquid adhesive can thus flow inside the cavities of the exchanger and fill them and / or pass through them, covering all their internal metal surfaces.

[0033] In addition, the liquid adhesive has sufficient wettability on the internal metal surfaces to leave a layer of adhesive thereon after it has been brought into contact with the internal metal surfaces.

[0034] The wettability and viscosity of the liquid adhesive are such that after the liquid adhesive has flowed inside the cavities of the exchanger, adhesive remains on all the internal metal surfaces of the exchanger which have been in contact with the liquid adhesive and in the form of a layer of sufficient thickness.

[0035] According to the invention, the thickness of liquid adhesive remaining attached to the internal metal surfaces of the exchanger must be sufficient to correctly attach the powdery solid during the step of depositing the powder of the powdery solid on the deposited liquid adhesive. The thickness of the adhesive layer is for example between 5 and 50 µm.

[0036] According to the invention, the particle size of the powdered solid must not be too large so that the weight of the particles remains compatible with the adhesive strength of the liquid adhesive. Indeed, too large a particle size would lead to a powder that is too heavy and would not be properly held on the surface of the cavities of the exchanger. Thus, the maximum particle size of the powdered solid is chosen according to the density of the powdered solid, the adhesive strength of the adhesive and the thickness of the adhesive on the internal metal surfaces.

[0037] Furthermore, since the cross-section of the internal cavities is in the order of 1 mm² to 40 mm², the particle size of the powdered solid must remain limited so as not to excessively reduce the passage cross-section of the process fluid circulating in the cavities of the exchanger during operation of the exchanger.

[0038] In addition, a small particle size of the powdered solid is preferable because it allows the increase in the exchange surface of the coating resulting from the presence of the particles to be increased and therefore to develop the action of the powdered solid. The active surface of the powdered solid, taking into account the morphology of the particles, is thus much greater than that of the internal metallic surfaces of the exchanger that it covers.

[0039] The particle size of the powdered solid is first adjusted by grinding and filtration to obtain an appropriate particle size distribution. To obtain a powder with a small particle size, a high-energy mill or an attritor can be used.

[0040] According to an exemplary embodiment of the invention, the step of depositing the liquid adhesive on the internal metal surfaces is carried out by dipping the exchanger in a bath of liquid adhesive until the cavities are filled or by circulating liquid adhesive in said cavities.

[0041] Performing the operation by dipping is a simple way to coat the internal metal surfaces of the exchanger. The exchanger is placed in the bath in an orientation that allows the evacuation of air present in the cavities of the exchanger and the filling of these with the liquid adhesive. This prevents air from remaining trapped in the exchanger, which would lead to uncoated surfaces. The exchanger is kept in the bath for the time necessary for its filling, for example for about ten minutes.

[0042] Alternatively, the exchanger may be placed with its cavities arranged vertically, or inclined with a strong vertical component, and the liquid adhesive is poured into the exchanger from its upper part so that the liquid flows into the cavities of the exchanger by gravity flow. The lower part of the exchanger may be obstructed so as to effect a force-feeding of the exchanger by the liquid adhesive. The lower part of the exchanger may be free so that the excess quantity of liquid (which does not adhere to the internal surfaces of the exchanger) exits the exchanger from its lower end. The quantity of adhesive poured must be sufficient so that adhesive is deposited on all the internal surfaces of the exchanger on which a coating is desired.

[0043] According to the invention, once the internal metal surfaces are coated with adhesive, the powdery solid is deposited.

[0044] According to an exemplary embodiment of the invention, the step of depositing the powder of the powdery solid on the deposited adhesive is carried out by force-feeding or pouring. For example, this can be done by gravity pouring. As for the deposition of the adhesive, the exchanger is then placed with its cavities arranged vertically, or inclined with a strong vertical component, and powder of the powdery solid is poured into the exchanger from its upper part so that the powder flows into the cavities of the exchanger. It is also possible to proceed by force-feeding with powder, with the lower end of the exchanger obstructed. The quantity of powder poured must then be sufficient for powder to be deposited on all the internal metal surfaces of the exchanger on which a coating is desired.

[0045] According to another exemplary embodiment of the invention, the step of depositing the powder of the pulverulent solid on the deposited adhesive is carried out by placing the exchanger in a fluidization chamber in which the powder of the pulverulent solid has been previously suspended using a gas.

[0046] It would be very difficult, if not impossible, to immerse the exchanger in a tank containing only the powdered solid, as the mechanical resistance to the movement of the powder would be too great. By analogy, one can imagine the difficulty of immersing the exchanger in a sand tank.

[0047] By fluidizing the powdery solid, it behaves like a liquid. It then becomes possible to immerse the exchanger in the fluidized bed until it is completely covered. In addition, fluidization facilitates the flow of the powder into the internal cavities of the exchanger. The exchanger can be placed in the enclosure with its cavities arranged vertically, or inclined with a strong vertical component, to facilitate the flow of the powder into the exchanger.

[0048] Adjusting the descent speed of the exchanger in the fluidization chamber allows the cavities to be coated with a layer of particles of the powdered solid with a homogeneous distribution. The coverage rate of the internal metal surfaces by the powder of the powdered solid is thus very high, beyond the targeted minimum of 60%.

[0049] In the same way as the approach presented previously, the enclosure in which the exchanger is immersed can be a thermostatically controlled fluidization enclosure allowing the polymerization of the adhesive to be carried out.

[0050] Advantageously, the gas suspending the powder of the powdery solid comprises a reactive agent interacting with the powdery solid.

[0051] Advantageously, the reactive agent is intended to initiate or accelerate the polymerization of the adhesive. The reactive agent is, for example, hydrogenated nitrogen to reduce or control the degree of humidity of the powdered solid.

[0052] With the use of a very fluid liquid adhesive which perfectly covers all the internal metal surfaces of the exchanger and a powder of small particle size whose deposition method allows to cover all the previously deposited adhesive, the invention makes it possible to obtain a coherent coating of a powdery solid on all the internal metal surfaces of the exchanger. This provides a large surface area of ​​solid, i.e. catalyst, to promote the desired physicochemical reaction.

[0053] According to a second aspect of the invention, there is provided a cryogenic heat exchanger characterized in that it comprises internal metallic surfaces coated with a coating serving as a catalyst for a physicochemical reaction carried out according to the first aspect of the invention.

[0054] Advantageously, the internal metal surfaces of the exchanger form cavities whose length to width ratio is equal to or greater than 1000.

[0055] According to an exemplary embodiment of the invention, the exchanger is made of aluminum, or aluminum alloy, and the powdery solid is an oxide, a hydroxide, a mixture of oxides or hydroxides, or a mixture of oxides and hydroxides.

[0056] Advantageously, the oxide is Fe2O3 or the hydroxide is Fe(OH)3.

[0057] The invention is particularly advantageous for cryogenic exchangers of the brazed plate and wave type intended for the production of hydrogen, the coating of internal metallic surfaces obtained according to the invention of which acts as a catalyst for a physicochemical reaction of ortho-para conversion of hydrogen. This conversion is carried out while the hydrogen is in the liquid state and at a temperature of approximately -250°C. Brief description of the figures

[0058] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

[0059] is a schematic view of a coating method according to the second exemplary embodiment of the invention;

[0060] is a schematic and partial view of a cryogenic plate and wave exchanger according to an exemplary embodiment of the invention, after a step C of the coating process of the;

[0061] is a view similar to the, after a step D of the coating process of the;

[0062] is a view similar to the, after a step F of the coating process of the; and

[0063] is a schematic and partial view of an exchanger according to the invention to illustrate the ratio between the width and the length of its internal cavities.

[0064] As shown in, in step A, the powder of a pulverulent solid, i.e. the catalyst, to be deposited on internal metal surfaces of the heat exchanger 1 is first subject to an adjustment of its particle size by mechanical grinding followed by dry filtration to obtain and retain only particles whose particle size is suitable.

[0065] In step B, the powder obtained is placed in a fluidization chamber in which it forms a fluidized bed by the circulation of a gas.

[0066] In parallel, in step C, the preparation of the exchanger 1 is carried out, the internal surfaces of which form the substrate, or wall, to be coated. These surfaces can be stripped with a liquid acid solution for subsequent perfect adhesion of the adhesive and the powdered solid. Illustrates a partial sectional and close-up view of the exchanger 1 after this step C. Waves 2 are arranged between two separation plates 3. The waves 2 and the separation plates 3 form cavities 10 having metallic internal surfaces 11.

[0067] In step D, the adhesive is then coated on the surface of the cavities 10, by dipping in this embodiment. The exchanger 1 is thus immersed in a bath of liquid adhesive while being inclined so that the air present in the exchanger 1 can escape and be replaced by liquid adhesive. The temperature of the bath can be controlled to obtain a given viscosity of the adhesive, for example 1×10 −3 Pa.s at 20°C. The exchanger 1 is then removed from the adhesive bath and is held above it by arranging the exchanger 1 with its cavities oriented vertically. Thus, the excess adhesive flows by gravity out of the exchanger 1 and falls into the bath. The illustrates the partial sectional view of the after step D. Internal surfaces of the exchanger 1 are covered with adhesive 4.

[0068] In step E, the exchanger 1 is then immersed in the fluidized bed. Advantageously, the descent speeds of the exchanger 1 in the fluidized bed will be chosen to be between 0.5 mm / s and 1 m / s, and more preferably between 1 and 10 mm / s.

[0069] Exchanger 1 is kept in the fluidized bed for a few minutes before being extracted.

[0070] Advantageously, extraction speeds from the exchanger 1 of the fluidized bed will also be chosen between 0.5 mm / s and 1 m / s, and more preferably between 1 mm / s and 10 mm / s.

[0071] The excess powder is then removed in step F, which, in the example given, is done when the exchanger 1 leaves the fluidized bed. The exchanger 1 is arranged with its cavities 10 oriented vertically above the tank containing the powder. The particles which are not held by the adhesive thus flow by gravity out of the exchanger 1. To facilitate this flow, the exchanger 1 can be slightly shaken and / or a gas can be injected into the exchanger 1 for mechanical entrainment of the non-adherent particles. The shows the partial sectional view of the after step F. Particles 5 of the powdery solid are held on the internal surfaces of the exchanger 1 by the adhesive.

[0072] Note that the figures schematically illustrate an exemplary embodiment of the invention. The ratios between the dimensions of the elements shown are not necessarily representative of the actual ratios. Thus, the size of the particles 5 and the thickness of the adhesive 4 on the lane are not necessarily representative of their actual dimensions compared to those of the waves and separation plates shown and the cavities they form.

[0073] In step G, the exchanger 1 is then placed in a thermostatically controlled enclosure where it can be brought to and maintained for several tens of minutes at a temperature of 150°C in order to polymerize the adhesive.

[0074] In step H, exchanger 1 is then removed from the thermostatically controlled enclosure and brought back to room temperature.

[0075] The diagram shows a partial representation of an exchanger 1 according to the invention. The distance 6 illustrates the width of a cavity and the distance 7 its length. According to the invention, the cavities of the exchanger 1 coated with a catalyst have a length / width ratio equal to or greater than 1000. As shown in the enlargement 8b of the circular portion 8a of the exchanger 1, the wave 2 forms a cavity 10 with a separation plate 3, of a width 6 and a height 9. To assess the ratio between the width and the length of the cavity, the width of the cavity is considered to be the smallest dimension between its width and its height. In this example, the length / width ratio is thus equal to 7 / 6.

Claims

Method for producing a coating on internal metallic surfaces forming cavities of a heat exchanger, the coating being intended to serve as a catalyst for a physico-chemical ortho-para conversion reaction of hydrogen, said method being characterized in that it comprises the following successive steps: - a step of depositing a liquid adhesive on the internal metallic surfaces, the liquid adhesive being aqueous, organic or inorganic, and - a step of depositing a powder of a granular solid on the deposited liquid adhesive, the granular solid being a mineral and the powder having a particle size between 10 µm and 100 µm. A process according to claim 1, characterized in that it comprises, after the powder deposition step, a subsequent step of maintaining the internal surfaces at room temperature during which solvents from the liquid adhesive are, at least partially, removed by evaporation. A process according to claim 1, characterized in that it comprises, after the powder deposition step, a subsequent step of polymerizing the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal surfaces to interact with the powdered solid. A process according to claim 3, characterized in that the heat treatment is carried out at a temperature less than or equal to 200°C. A method according to any one of claims 2 or 3 characterized in that a gas flow is carried out in the cavities of the exchanger during the subsequent step of holding the internal surfaces at room temperature during which solvents of the liquid adhesive are, at least partially, removed by evaporation or during the subsequent step of polymerizing the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal surfaces to interact with the powdered solid. A method according to any one of claims 2 or 3, characterized in that the subsequent step of maintaining the internal surfaces at room temperature during which solvents of the liquid adhesive are, at least partially, removed by evaporation or of polymerization of the liquid adhesive by a heat treatment carried out in a temperature range which does not allow the internal surfaces to interact with the powdery solid, is carried out after a step of removing the powder from the powdery solid which does not adhere to the liquid adhesive deposited on internal surfaces of the exchanger. A method according to claim 1, characterized in that the liquid adhesive is polyvinyl alcohol and / or a polymer. A process according to claim 1 or 7, characterized in that the liquid adhesive has a viscosity comparable to that of water, i.e., 1×10 −3 Pa.s at 20°C. A method according to any one of claims 1 or 8, characterized in that the liquid adhesive has sufficient wettability on the internal surfaces to leave a layer of adhesive on them after it has been brought into contact with the internal surfaces. A method according to any one of the preceding claims, characterized in that the exchanger is made of aluminum, or aluminum alloy, and in that the powdered solid is an oxide or a hydroxide or a mixture of oxides or hydroxides, or a mixture of oxides and hydroxides. A process according to claim 10, characterized in that the oxide is Fe2O3 or in that the hydroxide is Fe(OH)3. A process according to any one of claims 10 or 11, characterized in that the powder of the pulverulent solid has a particle size between 50 nm and 1µm. A method according to any one of claims 1 to 9, characterized in that the step of depositing the liquid adhesive on the internal metal surfaces is carried out by immersing the exchanger in a bath of liquid adhesive until the cavities are filled or by circulating liquid adhesive in said cavities. A method according to claim 1, characterized in that the step of depositing the powder of the powdery solid onto the deposited adhesive is carried out by gavage or pouring. A process according to claim 1, characterized in that the step of depositing the powder of the pulverulent solid onto the deposited adhesive is carried out by placing the exchanger in a fluidization chamber in which the powder of the pulverulent solid has been previously suspended using a gas. A process according to claim 15, characterized in that the gas for suspending the powder of the pulverulent solid comprises a reactive agent interacting with the pulverulent solid. A process according to claim 16, characterized in that the reactive agent is intended to initiate or accelerate the polymerization of the adhesive. Cryogenic heat exchanger characterized in that it comprises internal metallic surfaces coated with a coating serving as a catalyst for a physico-chemical reaction carried out according to any one of claims 1 to 17. Heat exchanger according to claim 18, characterized in that the internal metallic surfaces form cavities whose length-to-width ratio is equal to or greater than 1000.

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