WO2008088217A2

WO2008088217A2 - Method for protecting a heat exchanger against corrosion, and heat exchanger and liquid composition for protecting a heat exchanger against corrosion

Info

Publication number: WO2008088217A2
Application number: PCT/NL2008/050031
Authority: WO
Inventors: Wouter Arthur Johan Scheffer; Joseph Adamzek
Original assignee: Triple E B.V.
Priority date: 2007-01-17
Filing date: 2008-01-17
Publication date: 2008-07-24
Also published as: WO2008088217A4; NL1033242C2; NL2001743C; NL2001743A1; WO2008088217A3

Abstract

A method for protecting a heat exchanger against corrosion, comprising a number of plate-shaped metal fins spaced apart with intermediate slits, which are in heat exchanging contact with a number of tubes, wherein the free external surface of fins and tubes is at least partly covered with a metal containing coating, wherein the coating is applied by flowing a metal containing coating agent as a liquid flow over the external surface.

Description

Title: Method for protecting a heat exchanger against corrosion, and heat exchanger and liquid composition for protecting a heat exchanger against corrosion.

The invention relates to a method for protecting a heat exchanger against corrosion, comprising a number of plate -shaped metal fins, spaced apart with intermediate slits, which are in heat exchanging contact with a number of metal tubes. Such a type of heat exchanger is generally known and is typically- used for cooling cooling medium, for instance water, flowing through a number of tubes, with the aid of a gas flow, for instance air, flowing through a number of slits, between a number of fins.

Frequently used applications are radiators, fin tubes and cooling batteries for air conditioning and power stations.

As a rule, with a fin tube, a single metal tube is concerned around which, on the outer casing, a single metal fin is wound while enclosing a single spiral shape. Here, the heat exchanging contact is effected by welding the fin by its edge onto the casing surface. As a rule, with a cooling battery or radiator, a packet of metal fins is concerned that is crossed by a multiple number of metal tubes. Usually, the heat exchanging contact between tubes and fins is realized in that the tubes are passed, with a force fit, through holes in the fins.

In many cases, the free outer surface of fins and tubes which is exposed to the cooling air flow remains uncovered. As a result, considerable corrosion can occur.

From the viewpoint of cost price, nowadays, the heat exchangers are often designed with aluminum fins and copper or aluminum tubes. As a result, the heat exchanger is highly sensitive to corrosion at the location of the transitional areas between fins and tubes. The fact is that when, at the location of the transitional area, there is a slit in which moisture is taken up, an electrolyte can be formed in the slit. In this manner, as a result of galvanic corrosion, the contact between fin and tube can be lost. As a result, the heat exchanging capacity of a cooling battery for an air conditioning unit which is exposed to sea air can, in some cases, decrease within one year by as much as 40%.

In order to protect the heat exchanger against corrosion, it has already been proposed to immerse it in its entirety in a bath in which a metal containing coating agent is present. Here, the object is to cover the free external surface of fins and tubes with the agent so that after the heat exchanger has been taken from the bath, the agent forms a thermally conductive coating. A known coating agent utilized in an immersion bath is Heresite/Bronzglow coating. A drawback of this method is that the coating agent is relatively viscous, so that parts of the external surface of the heat exchanger located further inwards are not covered, or only poorly so. The coating agent often stagnates between the fins of the heat exchanger and cannot go deeply enough into the heat exchanger.

An alternative method for protecting a heat exchanger against corrosion involves spraying the free external surface with a spray mist of metal containing coating agent. A known agent used thereto is Blygold or Alucoat, a coating agent based on polyure thane with aluminum particles. Although this method offers many advantages over leaving the external surface untreated, this method also has a number of drawbacks. Upon spraying, a considerable portion of the mist is lost in the atmosphere, which causes a substantial burden to the environment. This method is further insufficiently reproducible with respect to the distribution of the coating layer over the external surface. In particular at the location of the transitional areas of the heat exchanger between fins and tubes located further inwards, the coating layer has no or insufficient coverage.

As, in practice, the heat exchangers are often provided with the coating layer after being installed, these drawbacks are significant. The invention contemplates a method for protecting a heat exchanger against corrosion of the type mentioned in the preamble, wherein the drawbacks mentioned are obviated while maintaining the advantages mentioned. To that end, the invention provides a method for protecting a heat exchanger against corrosion, comprising a number of plate-shaped metal fins, spaced apart with intermediate slits, which fins are in heat exchanging contact with a number of tubes, while the free external surface of fins and tubes is at least partly covered with a metal containing coating, characterized in that the coating is applied by flowing a metal containing coating agent as a liquid flow over the external surface.

By flowing the metal containing coating agent as a liquid flow over the external surface, a good covering of the free surface can be obtained, also at the location of the parts of the heat exchanger located further inward. The coating agent flows along the individual fins and free parts of the tubes, whereupon it forms a coating, in a reproducible manner, for instance through evaporation of a solvent and/or curing of components.

Here, the coating agent can be applied onto the external surface as a liquid jet, for instance with the aid of a jet tube. By moving the jet along edges of the fins, the agent can be applied in an efficient manner. By following, upon application, the geometry of the heat exchanger along a path, it can be ensured that the entire free external surface is reached. The coating agent can flow, for instance under the influence of gravity, through the part of the heat exchanger located further inward.

By applying the coating agent abundantly, rapidly, a complete flowing-over of the surface can be achieved. Here, coating agent dripping from the fins can be recovered and, if desired, be recirculated.

It is preferred that the coating agent cures on the air for forming a coating.

With the aid of the method according to the invention, a coating with substantially constant thickness can be formed. The thickness of the coating agent is typically greater than the thickness that is obtained when the coating agent is sprayed on as a spray mist, and is typically smaller than the coating that is obtained with the aid of the coating agent of an immersion bath.

The invention also relates to a heat exchanger, as set forth in claims 16 - 22.

The invention further relates to a liquid composition for applying a coating on a material suitable for use in a method according to the invention.

The invention relates in particular to a liquid composition comprising: - 2.6 to 15% by weight, preferably 2.6 - 10% by weight of isocyanate, preferably an aromatic isocyanate prepolymer;

- metal particles, preferably in a concentration of at least 20 % by weight;

- at least 12.5 % by weight of aliphatic ester; and - at least 20% by weight of aromatic hydrocarbon.

A composition (applicable) according to the invention typically comprises a) at least one curable composition, b) particles for increasing the heat conductivity of the coating, and c) at least one solvent. Further components, such as, for instance, filler, may optionally be included in the composition.

In view of the ease of application of the composition (applicable) according to the invention, the din cup 4 viscosity as determined in ISO- standard 2431.1993 is preferably 40 sec or less, more preferably 35 sec or less, in particular 32 sec or less more particularly 28 sec or less. In view of the ease of application of a desirable layer thickness, the viscosity is preferably at least 20 sec, more preferably at least 22 sec, in particular at least 26 sec.

The curable compound typically comprises a prepolymer or monomer which polymerizes during curing. Herein, the term "prepolymer" is used for a polymerizable compound that is built up, at least conceptually, from at least two monomelic units.

The curable compound preferably comprises a dϋsocyanate monomer and/or an isocyanate prepolymer, in particular an aromatic diisocyanate monomer or a prepolymer of one or more aromatic diisocyanate units.

A composition according to the invention, comprising such a prepolymer and/or monomer is of advantage because it is curable through exposure to moisture (for instance from the air) without the presence of a polymerization aid, such as an initiator, being required. Further, a composition comprising a diisocyanate monomer and/or an isocyanate prepolymer is suitable for providing a heat exchanger with a coating which, if desired, is highly flexible and, therefore, has a high resistance against wear resulting from expansion and shrinkage of the underlying material. A surface layer based on an isocyanate also has a good resistance against UV-light.

Good results are achieved in particular with a prepolymer of diphenyl methane 4.4'diisocyanate. An example of another isocyanate is toluene diisocyanate. Diphenyl methane 4,4'diisocyanate or a prepolymer thereof is preferred from a viewpoint of safety.

The concentration of the curable compound can be selected within wide boundaries. For a low viscosity, the concentration is in general less than 15% by weight, based on the total weight of the composition, in particular at most 12% by weight or less, preferably 10% by weight or less, for instance 9% by weight or less.

The concentration of the curable compound is typically at least 2.6 % by weight based on the total weight of the composition, in particular at least 6% by weight, more particularly at least 8% by weight. The particles for increasing the heat conductivity are typically metal particles, in particular aluminum particles. Such particles are also of advantage for increasing the impermeability to water of the cured coating.

Preferably, a combination of sinking and non-sinking particles is used. Such particles are also known to the skilled person as "leaving particles" and "non-leaving particles".

The particles are typically microparticles, with an average diameter of, for instance, 20 μm or less, in particular 10-20 μm, for instance approximately 15 μm. The concentration of the particles can be selected within wide boundaries, depending on the desired effect. For an increased heat conductivity and/or impermeability of the surface layer, the concentration is preferably at least 20% by weight, based on the total weight. The concentration is typically at most, 30% by weight. The solvent can, in principle, be a known solvent in which the other constituent parts dissolve or can at least be dispersed. The solvents are typically selected from organic solvents and in particular from the group consisting of aliphatic hydrocarbons liquid at least at room temperature, aromatic hydrocarbons liquid at least room at temperature and esters liquid at least at room temperature.

Examples of aliphatic hydrocarbons are liquid alkanes, such as C5-C17 alkanes and, in particular, mixtures thereof such as liquid naphtha. Preferably, the aliphatic hydrocarbon content, in particular the alkane content is at most 10 % by weight based on the total weight. If present, it is preferably minimally 2.5 % by weight.

Preferably, an aromatic hydrocarbon is present, such as an aromatic hydrocarbon selected from the group of alkyl benzenes, in particular mono- di- and trialkylbenzenes. Here, the alkyl group can in particular be selected from the group of C1-C12, in particular from Cl to ClO alkylbenzenes. Examples of i v/1/ικi- z.υυσ * υ J w υ g i

7

suitable alkylbenzenes are xylene, 1,2,4-trimethylbenzene, C9 di and trialkyl benzenes and ClO di and trialkyl benzenes.

The aromatic hydrocarbon content is preferably at least 20% by weight based on the total weight. For a relatively low viscosity, it is, with particular preference at least 26% by weight. The aromatic hydrocarbon content is typically at most 60% by weight, based on the total weight, in particular at most 50% by weight.

Preferably, an aliphatic ester, an acetate ester, such as an ester selected from the group of n-butyl acetate, methoxypropyl acetate and methoxy methyl ethyl acetate is present. If present, the content is preferably at least 12.5% by weight, based on the total weight. For a composition with a relatively low viscosity, the concentration can be selected to be relatively high, such as at least 20% or at least 26%. The aliphatic ester content is typically at most 35% by weight. Highly suitable is a mixture of solvents. With it, the curing speed can be set as desired. Highly suitable is, for instance, a mixture of one or more aliphatic hydrocarbons (for instance derived from white spirit and/or naphta), one or more dialkylbenzenes and/or one or more trialkylbenzenes, and one or more aliphatic esters. It has been found that a composition containing such a mixture of solvents exhibits a favourable drying and/or curing behaviour, whereby drying- curing takes place in a phased manner, for instance whereby, within for instance one or several hours, more than 95%, for instance approximately 98%, of the eventual drying and/or curing takes place, and the remaining drying and/or curing takes place over a substantially longer period of time, such as at least a few days, for instance approximately a week. This is of advantage for obtaining a surface cover with a desired flexibility. Formulation example

n-Butylacetate 2.6 - 10.0%

Solvesso (™) 100 10.1 - 25.0% m-Xylene 10.1 - 25.0% White spirit 2.6 - 10.0%

Methoxy propyl acetate 10.1 - 25.0%

Diphenyl methane diisocyanate 2.6 — 10.0%

Aluminum particles 20 — 30%

(all percents by weight are based on total weight)

Further embodiments of the invention are represented in the subclaims. The invention will be further elucidated on the basis of a non- limitative exemplary embodiment that is represented in the Figures.

Fig. 1 represents in a schematic manner an embodiment of the method according to the invention;

Fig. 2 represents in a schematic manner an embodiment of a recirculation system for use in the method according to the invention;

Fig. 3 represents in a schematic manner an embodiment of the heat exchanger according to the invention.

It is noted that the Figures are only schematic representations of a preferred embodiment of the invention. In the Figures, identical or corresponding parts are indicated with the same reference numerals.

Fig. 1 represents in a schematic manner an embodiment of the method according to the invention. The method according to the invention is utilized for protecting a heat exchanger 10 against corrosion, comprising a packet 1 with a number Ia ... In of plate-shaped metal fins Ia ... In, spaced apart with intermediate slits 5, which are crossed by a number of tubes 3, thereby forming a heat exchanging contact. The fins are manufactured from, for instance, a metal comprising aluminum or aluminum alloy. The tubes are manufactured from, for instance, a metal comprising copper or an aluminum alloy. Such heat exchangers are produced, for instance, by pressing a system of mutually parallel fins Ia ... In on a system of tubes 3, whereby the fins are perforated thereby and the tubes reach through the fins. As a result, each fin comprises a number of transitional areas with tubes projecting from the plane of the fins.

For protecting the heat exchangers against corrosion, the free external surface of tubes and the fins Ia ... In is covered with a metal containing coating, wherein this coating is applied by flowing a metal containing coating agent as a liquid flow 7 over the external surface of the packet 1. Preferably, the coating agent is applied onto the external surface as a liquid jet 7. Within the context of this application, the wording "jet" is understood to mean at least a substantially continuous flow of liquid. For applying the coating agent in an efficient manner at least at the location of the transitional areas between the fins Ia ... In and the tubes, the coating agent is applied by moving the jet 7 along the edges of the fins. Preferably, this movement is carried out in a pattern 8a, which corresponds, in principle, to the configuration of the tubes 3 in the packet 1. The coating is formed in that the coating agent is cured in the air for forming a coating.

It is further possible that the method according to the invention is carried out in two steps, wherein during the first step the jet 7 is moved along one of the external surfaces of the packet (see Fig. 1, left hand side) and wherein during the second step, the jet 7 is moved along an opposite external surface of the packet (see Fig. 1, right hand side). In an advantageous manner, the heat exchanger is then disposed at an inclination.

It is possible to apply the coating agent in abundance onto the packet 1. In this case, the abundant amount of liquid flows downwards along the surfaces of the fins Ia ... Ib, and the superfluous liquid 4 drips from the packet. For collecting the superfluous amount of liquid, use can be made of a reservoir 6. The reservoir 6 preferably comprises means 2 for making the collected coating agent available for recirculation.

After the treatment of the heat exchanger with a liquid flow comprising coating agent, also, a follow-up treatment can take place. This follow-up treatment can comprise that the edges of the fins Ia ... In are finished by applying a second coating agent as a spray mist on the edges of the fins. This second coating agent is known per se and can, for instance, be the conventional Blygold or Alucoat. Fig. 2 represents in a schematic manner an embodiment of a recirculation system for use in the method according to the invention. The recirculation system 20 comprises a main reservoir 24 in which a liquid composition is included with a coating agent. The main reservoir 24 is in hydraulic communication with means 7a, 7b which are arranged for treating a heat exchanger with the liquid flow. The means 7a, 7b comprise, for instance, a jet tube. In order to apply the liquid flow in a controlled manner onto the heat exchanger to be treated, supply buttons 27a, 27b, respectively, and valves 29a, 29b, respectively, are provided between the means 7a and 7b and the main reservoir 24. For distributing the liquid flow from the main reservoir 24 to the two lines, the exit of the main reservoir is provided with a T-piece 28. For collecting the superfluous amount of the liquid, use is made of a reservoir 6. The reservoir 6, as is for instance outlined in Fig. 1, is in communication with a further reservoir 27, between which are included a filter 21, a quick coupling element 23 and a pump 25. By means of a recirculation system 20, a substantially closed circuit is provided for reuse of the liquid composition with the coating agent, which is cost-reducing and environmentally friendly.

Fig. 3 represents in a schematic manner an embodiment of the heat exchanger according to the invention. The heat exchanger 30 is formed by cooperation of a packet of fins 31 and a system of tubes 33. The fins of the packet 31 are placed with respective, intermediate slits, while the system of fins is crossed by the system of tubes 33. It is also possible that instead of the system of tubes 33, only one single tube is used. Also, if desired, one continuous fin can be used. Preferably, the fins are manufactured from aluminum or an aluminum alloy. Preferably, the tubes are manufactured from copper, aluminum of an aluminum alloy.

When the heat exchanger is treated according to the method of the invention as set forth hereinabove, a coating 35 is formed with substantially constant layer thickness. The layer thickness is in the range of approximately 20 to 40 micrometers, preferably approximately 25 to approximately 35 micrometers. By flowing the liquid with coating agent on the respective fins of the heat exchanger, typical breadths 35 are formed on the fins. Preferably, the coating is formed at least at the location of the transitional areas 37 between fins and the tubes. More preferably, the free external surface of the fins is completely covered with the coating. Within this context, "free external surface" is understood to mean the surface facing outwards of the cooperating fins and tubes.

The invention is not limited to the exemplary embodiment represented here. For instance, with this process, also heat exchangers of the "Fin tube" type can be protected against corrosion, or for instance the so-called "microchannel" heat exchangers, which have a structure that is more like a car radiator. Also, for instance, copper/copper heat exchangers can be used. Such variants will be clear to the skilled person, and are understood to fall within the scope of the invention as set forth in the following claims.

Claims

1. A method for protecting a heat exchanger against corrosion, comprising a number of plate-shaped metal fins, spaced apart with intermediate slits, which are in heat exchanging contact with a number of tubes, wherein the free external surface of fins and tubes is at least partly covered with a metal containing coating, characterized in that the coating is applied by flowing a metal containing coating agent as a liquid flow over the external surface.

2. A method according to claim 1, wherein the coating agent is applied to the external surface as a liquid jet.

3. A method according to claim 2, wherein the coating agent is applied by moving the jet along edges of the fins.

4. A method according to any one of the preceding claims, wherein the coating agent is applied in abundance.

5. A method according to claim 4, wherein coating agent dripping from the fins is collected.

6. A method according to claim 5, wherein collected coating agent is recirculated.

7. A method according to any one of the preceding claims, wherein at least the free external surface at the location of the transitional areas between fins and tubes is covered.

8. A method according to any one of the preceding claims, wherein the free external surface is completely covered.

9. A method according to any one of the preceding claims, wherein the coating agent is cured on the air for forming a coating.

10. A method according to any one of the preceding claims, wherein a coating with a substantially constant thickness is formed.

11. A method according to any one of the preceding claims, wherein, with the coating agent, a coating is formed with a thickness which is between approximately 40 and approximately 50 micrometers, in particular approximately 30 micrometers.

12. A method according to any one of the preceding claims, wherein the fins are substantially of aluminum or an aluminum alloy.

13. A method according to any one of the preceding claims, wherein the tubes are substantially of copper.

14. A method according to any one of the preceding claims, wherein the coating agent comprises an isocyanate prepolymer, metal particles, an aliphatic ester and an aromatic hydrocarbon.

15. A method according to any one of the preceding claims, wherein the edges of the fins are finished by spraying a second coating agent as a spray mist on the edges.

16. A heat exchanger with coating obtainable through the method according to any one of the preceding claims.

17. A heat exchanger, comprising a packet with a number of plate - shaped metal fins spaced apart with intermediate slits, which are in heat exchanging contact with a number of tubes, wherein the free external surface of fins and tubes is at least partly covered with a cured, metal containing coating, characterized in that the coating comprises metal particles and a cured polyisocyanate, wherein the weight ratio of polyisocyanate to metal particles is preferably in the range of 2 to 12, in particular 2 to 4.

18. A heat exchanger, comprising a number of plate-shaped metal fins spaced apart with intermediate slits, which are in heat exchanging contact with a number of tubes, wherein the free external surface of fins and tubes is at least partly covered with a cured, metal containing coating, characterized in that the coating has a thickness which is between approximately 20 and approximately 40 micrometers, in particular approximately 30 micrometers.

19. A heat exchanger according to claim 17 or 18, wherein the fins are substantially of aluminum.

20. A heat exchanger according to any one of claims 17 - 19, wherein the tubes are substantially of copper, aluminum or an aluminum alloy.

21. A heat exchanger according to any one of claims 17 - 20, wherein the coating covers at least the free external surface at the location of the transitional areas between fins and tubes.

22. A heat exchanger according to any one of claims 17 — 21, wherein the coating covers the free external surface completely.

23. A liquid composition for corrosion prevention of an external surface of a heat exchanger, in particular a heat exchanger provided with a number of plate-shaped metal fins spaced apart with intermediate slits, which are in heat exchanging contact with a number of tubes, comprising:

- 2.6 to 15% by weight, preferably 2.6 - 10 % by weight of isocyanate, preferably an aromatic isocyanate prepolymer;

- metal particles, preferably in a concentration of at least 20 % by weight;

- at least 12.5 % by weight of aliphatic ester,- and

- at least 20% by weight of aromatic hydrocarbon.

24. A composition according to claim 23, comprising

- 2.6 — 10 % by weight of isocyanate prepolymer;

- 20 — 30 % by weight of metal particles;

- 20 — 50% by weight of at least one solvent selected from the group of aromatic hydrocarbons; - 12.7 - 35% by weight of at least one solvent selected from the group of aliphatic esters, in particular acetate esters; and

- at most 10% by weight of at least one solvent selected from the group of aliphatic hydrocarbons, in particular alkanes.

25. A liquid composition according to claim 23 or 24, wherein the din cup 4 viscosity as determined by means of ISO standard 2431.1993 is 40 sec or less, preferably 35 sec or less, in particular 32 sec or less, more particularly 28 sec or less.