EP1690058B1 - Heat exchanger - Google Patents

Abstract

The invention relates to a heat exchanger, particularly an evaporator for air conditioners in motor vehicles, having a number of heat transfer surfaces made of metal, in particular, aluminum or aluminum compounds. The aim of the invention is to provide this heat exchanger with a surface coating that is improved compared to the prior art. To this end, a number of layers are applied to its heat transfer surfaces, and nanoparticles are used for the coating.

Description

The invention relates to a heat exchanger with surface-treated heat transfer surfaces. Furthermore, it relates to a process for the surface treatment of heat exchangers.

In order to meet the requirements of the industry for components, such as those of the automotive industry on heat exchangers or transmitters, a treatment of the material surfaces is often unavoidable. With a surface treatment, the specific components are to be given specific properties that protect them in particular from environmental influences in favor of improved performance and extended life. In particular, the specific area of application and structural conditions must be taken into account.

Heat exchangers, in particular evaporators, which are used in air conditioning systems - especially in motor vehicles - usually consist of several successive lined and fluid-tight interconnected discs or tubes, between which tightly packed corrugated fins are arranged. On the one hand, these allow optimum heat transfer between the refrigerant flowing through the panes or pipes and the air flowing through the corrugated grid network, but on the other hand are predestined for the precipitation of condensate and dust or dirt. This moist, dirty heat transfer surface provides an ideal breeding ground for microorganisms whose settlement can result in undesirable odors. In addition, damage caused by corrosion is favored by the wet soiling in particular.

In order to avoid the accumulation of water and dirt on a surface, the surface of an object is usually rendered hydrophobic. Due to the fact that spherical droplets of water form on the surface as a result of the hydrophobic design, these pearls are basically dirt and water-repellent. With a hydrophobically equipped surface of the heat exchanger described, however, the water droplets can not bead off because of the very tightly packed corrugated rib structure. Instead, they hang between the adjacent, narrow ribs and gills. Thus, the desired self-cleaning effect is just prevented by the hydrophobic design. This also usually leads to a decrease in the overall performance of the heat exchanger.

In order to solve this problem while maintaining the design of the heat exchanger, a hydrophilic finish of the heat transfer surfaces is desired.

The hydrophilicity of a substance is characterized inter alia by its polarity, a low interfacial tension with respect to water and a good wettability with water, which results from the fact that the adhesion forces acting between the molecules of the same substance at an interface are large compared to the cohesive forces, the between the Molecules of the same substance act. If a surface is readily wettable, a drop of liquid on it forms a contact angle which is less than 90 °, ie the liquid can spread more or less on the surface. A hydrophilic finish of a surface thus leads to the formation of a thin, closed liquid film. The closed liquid film allows the dust and dirt particles to flow away, thus reducing the permanent accumulation of dust and dirt. In addition, since the corrugated fin surface dries faster due to the comparatively thin film formation of water, the settling of microorganisms on the heat exchanger surface is also reduced.

For example, in the CN 13242732 discloses an aluminum heat exchanger provided with a layer containing inter alia nanoparticles based on macromolecular surfactants and crosslinkable, unsaturated monomers and having anti-corrosive and hydrophilic properties.

Furthermore, from the EP 1 154 042 A1 a heat exchanger is known in which the heat exchanger surface is provided after an acid cleaning with a chromium or zirconium-containing conversion layer and a hydrophilic polymer-based layer containing silicate particles with a diameter between 5 and 1000 nm.

From the EP 0 128 514 A2 For example, a method is known for treating heat exchange surfaces provided with a corrosion protective coating. For the application of a firmly adhering hydrophilic coating, the corrosion-protective coating is aftertreated with a dispersion, the dispersion containing alumina fine particles with a particle size of 1 to 100 nanometers.

By this type of coating compromises are usually necessary, so that, for example, an optimal corrosion resistance and a simultaneously durable hydrophilic surface for self-cleaning can not be achieved in the same quality.

The invention is therefore based on the object to provide a heat exchanger of the above type, the heat transfer surfaces of aluminum or aluminum compounds, are provided with a surface coating, which is improved over the prior art. Furthermore, a particularly suitable method for such a surface coating of said heat exchanger should be specified.

With respect to the heat exchanger, the object is achieved according to the invention by applying a plurality of layers to its heat transfer surfaces, with nanoparticles being used for the coating.

The invention is based on the consideration that the design goals that are equally pursued in favor of a long service life and improved performance for the heat exchanger can not be achieved by a single layer or at least not satisfactorily achieved. This is especially true for actually divergent interpretation goals, namely z. B. on the one hand for optimized corrosion protection and on the other hand for a hydrophilic surface finish. In principle, it is precisely a hydrophilic or water-attracting and therefore moist surface that favors the damage or destruction of materials by chemical or electrochemical reactions. In order to avoid corrosion, it is therefore fundamentally desirable to prevent a contact of material and water with a hydrophobic finish. While for effective self-cleaning of the heat transfer surfaces, as described above, a hydrophilic finish of a surface is desired to promote the formation of a thin, closed liquid film which allows the dust and dirt particles to flow away.

In order to meet several, often even contrary, design goals, a multi-layer coating is therefore provided, each layer being upgraded for its own specific property. For a layer can namely, defects in the layer expose the metal, so that this location of the metal, especially in the case of a hydrophilic layer, ie a liquid-attracting layer, offers a suitable surface for corrosion damage. For multiple layers, the likelihood of defects in the layers lying directly above each other and exposing the metal is less. This has a correspondingly positive effect on a reduction of corrosion damage.

In the use of materials for the layers, tailor-made structures for the desired functions of the coating systems, such as, for example, the adhesion forces acting between the molecules of different substances play an important role. For the formation of functional coatings, the dimensions or orders of magnitude of individual components and mixtures are largely responsible. Particularly small particles, especially those with a size of a few millionths of a millimeter, are called nanoparticles. The smallest nanoparticles are clusters of a few hundred molecules and are subject to the laws of quantum mechanics, while for the larger of their kind the rules of traditional solid state physics apply. Nanoparticles have a much lower number of construction errors compared to larger particles of the same chemical composition. Due to their geometric and material-specific characteristics, they therefore offer a particularly large and versatile range of effects. For this reason, nanoparticles are used for coating.

Nanoparticles can be produced, for example, by plasma processes, laser ablation, gas-phase synthesis, sol-gel processes, spark erosion or crystallization and others.

Nanoscale particles are characterized by a particularly high surface-to-volume ratio. Because the adhesive force and the binding of the particles increases with increasing surface, layers produced therewith are generally particularly scratch and abrasion resistant. As a result, the thus-equipped surface provides no attack surface for damage to the protective coating, whereby, for example, corrosion damage can be minimized. Substantially selected nanoscale additives also improve corrosion protection. Due to their hydrophilicity and the comparatively large surface area, these particles are hygroscopic. Thus, their surface is moist and provides a thin film of liquid, which allows both a flow of dust and dirt particles as well as the rapid drying of the thin liquid film reduces the colonization of microorganisms. Each layer of the heat exchanger therefore contains materially different nanoparticles.

In order to ensure improved performance and a longer service life of the heat exchanger, at least one layer has anticorrosive properties and at least one further layer has hydrophilic and thus self-cleaning properties.

In a particularly advantageous embodiment, for corrosion protection reasons, it is preferable for a corrosion-inhibiting layer, and advantageously a hydrophilic layer, to be arranged thereon first. In order for a particularly effective self-cleaning effect to be achieved, the hydrophilic layer preferably forms the cover layer of the multiple coating. Advantageously, the layer having hydrophilic properties has a wetting contact angle with water of less than or equal to 60 °, preferably less than or equal to 40 °. The wetting contact angle is determined by the so-called sessile drop method, which is an optical Contact angle measurement for determining the wetting behavior of solids represents.

In a particularly advantageous embodiment of the heat transfer surfaces chromium-free, non-toxic additives are used for surface coating expediently. For this purpose, the nanoparticles are preferably made of organic and / or inorganic compounds of aluminum, silicon, boron and / or transition metals, preferably IV. And V. Subgroup of the Periodic Table, and / or cerium dissolved and / or dispersed in inorganic and / or organic solvents and / or dispersed Form used for coating.

For use of the heat exchanger in air conditioning systems, especially in motor vehicles, a particularly thin coating is expediently provided for efficiency reasons, which leads to no significant increase in volume and weight. Therefore, each layer thickness is advantageously less than 1.5 μm or equal to 1.5 μm, preferably less than 1 μm or equal to 1 μm, and the total layer thickness is less than 5 μm or equal to 5 μm.

With regard to the process for the surface treatment of heat exchangers, the stated object is achieved by applying a plurality of layers to a number of heat transfer surfaces of aluminum or aluminum compounds, wherein nanoparticles are used for the coating.

In this case, advantageously nanoparticles of organic and / or inorganic compounds of aluminum, silicon, boron and / or transition metals, preferably the IV. And V. subgroup of the periodic table, and / or cerium in inorganic and / or organic solvents dissolved and / or dispersed form used for coating.

The layers are advantageously applied by dipping, flooding or spraying, the individual layers, in particular for a particularly rapid layer structure, being applied directly one after the other, in a so-called wet-on-wet technique, with a single drying.

In an alternative embodiment of the method, the individual layers are preferably applied in separate treatment steps with respective intermediate drying.

The advantages achieved by the invention are in particular that a heat exchanger is provided by a multiple coating of heat transfer surfaces, which are used for coating nanoparticles, which ensures various, sometimes divergent requirements. The selected use of nanoscale particles of different materials achieves the desired functionality of the heat transfer surfaces. In this way of surface coating, for example, the corrosion protection or hardness and scratch resistance can be improved, and self-cleaning and antimicrobial surfaces can be produced. For both an improved corrosion protection and at the same time an improved self-cleaning effect by hydrophilization of the heat transfer surfaces, at least one corrosion-resistant layer and at least one further, in particular disposed thereon, hydrophilic layer is provided. As a result of the aforementioned improved properties increased use and / or performance of the heat exchanger is achieved.

As an exemplary embodiment, a heat exchanger, in particular an evaporator for motor vehicle air conditioning systems, is provided with a double coating of its heat transfer surfaces made of aluminum substrate. The nanoparticles for the respective layer are produced by a sol-gel process.

Of course, depending on the desired profile of requirements, a multiple coating can also be applied to the heat transfer surfaces, and of course the nanoparticles that differ materially for each layer can also be processed by processes other than the sol-gel process, such as the plasma process, laser ablation, gas phase synthesis, spark erosion or the crystallization u. a., can be produced.

In the exemplary embodiment, the application of a first corrosion-resistant and non-hydrophilic layer or the appropriately designed base layer by immersion treatment in an organically modified inorganic sol-gel layer with water-based solvent. By subsequent drying at a temperature in the range 100-150 ° C for 10 minutes, it is cured. The generated layer thickness is less than 1 micron. As a second layer or the cover layer, a further organically modified inorganic sol-gel layer with water-based solvent is applied by immersion treatment. It differs in chemical composition from the underlying layer. The second layer or topcoat is again cured at 100-150 ° C for 10 minutes. Its surface has a hydrophilic character and has a wetting contact angle with water of less than 40 °. This hydrophilicity is also resistant to permanent effects of condensation, so that the contact angle even after a condensate water load of over 1000 hours after the condensate constant climatic test is still below 40 ° according to DIN 50017-KK. The Gesamtschichtdikke of the layer structure of base and cover layer is a maximum of 2 microns.

Thus, the first layer or the base layer ensures optimum corrosion protection, and the generation of the functional hydrophilic cover layer improves the water drainage on the heat transfer surface. This favors the drainage of dust and dirt from the surface, and by the relatively thin film formation of water faster drying of the surface is ensured. These self-cleaning and rapid drying properties minimize the growth of microorganisms. All these factors improve the performance and / or performance of heat exchangers with such coated heat transfer surfaces.

Claims (8)

1. As heat exchanger, in particular an evaporator for air conditioning systems in motor vehicles, comprising a number of heat transfer surfaces made of aluminium or aluminium compounds onto which a plurality of layers is applied, wherein nanoparticles are used for coaling, wherein at least once layer has anticorrosive properties and at least one further layer, which is preferably disposed thereupon, has hydrophilic properties, characterized in that each layer contains nanoparticles which differ with respect to material.
2. The heat exchanger according to claim 1, in which the layer having hydrophilic properties has a wet contract angle of less than or equal to 60°, preferably less than or equal to 40°.
3. The heat exchanger according to one of the claims 1 to 2, in which the nanoparticles comprising organic and/or anorganic compounds of aluminium, silicon, boron and/or transition mentals, preferably of the IV and V subgroup of the periodic table, and/or cerium in anorganic and/or organic solvents in dissolved and/or dispersed form are used for coating.
4. The heat exchanger according to one of the claims 1 to 3, in which the thickness of each layer is less than 1.5 µm or equal to 1.5 µm, preferably less than 1 µm or equal to 1 µm, and wherein the total layer thickness is less than 5 µm or equal to 5 µm.
5. A method for the surface treatment of heat exchangers, in particular according to one of the claims 1 to 4, in which a plurality of layers is applied onto a number of heat transfer surfaces made of aluminium or aluminium compounds, wherein nanoparticles are used for coating, characterized in that each layer contains nanoparticles which differ with respect to material.
6. The method according to claim 5, in which the nanoparticles comprising organic and/or anorganic compounds of aluminium, silicon, boron and/or transition metals, preferably of the IV and V subgroup of the periodic table, and/or cerium in anorganic and/or organic solvents in dissolved and/or dispersed form are used for coating.
7. The method according to claim 5 or 6, in which the layers are applied by way of immersion, flooding or spraying, wherein the individual layers are applied directly one after the other without intermediate drying.
8. The method according to claim 5 or 6, in which the layers are applied by way of immersion, flooding or spraying, wherein the individual layers are applied in separate handling steps with intermediate drying.