MXPA01006543A - Heat exchanger with a reduced tendency to produce deposits and method for producing same - Google Patents

Abstract

The invention relates to a method for producing a heat exchanger. Said method is characterised in that a metal polymer dispersion layer having a halogenated polymer is chemically deposited on a heat exchange surface in a currentless manner. The invention also relates to a method for producing a heat exchanger. Said method is characterised in that a metal phosphor layer having a thickness of 1 to 15&mgr;m is applied by currentless chemical deposition before the metal polymer dispersion layer is applied. The invention also relates to a heat exchanger which can be produced by an inventive method and to the utilisation of a coating which is produced by currentlessly, chemically depositing a metal polymer dispersion layer having a halogenated polymer in order to make the coated surfaces less likely to accumulate solid materials from fluids whereby depositsare formed.

Description

"THERMAL EXCHANGER WITH A REDUCED TENDENCY TO PRODUCE DEPOSITS AND METHODS TO PRODUCE IT" The present invention relates to a process for the production of thermal transfer devices comprising the anelectrolytic chemical deposition of a metal / polymer dispersion layer. The present invention also relates to thermal transfer devices according to the invention. The present invention also relates to the use of a metal / polymer dispersion layer, as a permanent scale inhibitor. In recent decades, all the ramifications of the industry have suffered from fouling in thermal transfer devices (Steinhagen et al. (1982), Problems and Costs due to Fouling of the Heat Exchanger in New Zealand Industries, Heat transfer Eng., 14 ( 1), pages 19-30). When designing heat exchangers, consideration must be given to increasing friction pressure loss and resistance to thermal transfer due to fouling. This results in oversizing of the thermal transfer devices by 10 percent to 200 percent.

The development of anti-fouling methods has therefore acquired considerable importance. Mechanical solutions have the disadvantage of being restricted to relatively large heat exchangers and also causing considerable increased costs. Chemical additives can result in unwanted contamination of the product and in some cases can contaminate the environment. Due to these reasons, ways to reduce the fouling tendency by modifying thermal transfer surfaces have been recently sought. Although surface coatings with organic polymers, such as polytetrafluoroethylene (PTFE), reduce the tendency to fouling, the known coatings themselves cause significant additional heat transfer resistance. At the same time, the reasons for durability mean that the thickness of the layer has a lower limit. Similar problems are also observed in methods involving the application of monolayer silane coatings to the surface to be protected (Polym, Mater.Sci.and Engineering, Proceedings of the ACS Division of Polymeric Materials Science and Engineering (1990), Volume 62, pages 259 to 263).

The problems associated with the use of polymer coatings do not occur in a process described in Patent Number WO 97/16692. In this process, the hydrophobicity of the surface is increased by ion implantation or by ion bombardment methods. Even though this results in a reduction in the fouling tendency, the use of this process, which always requires vacuum techniques, however, is very expensive. In addition, the processes described are not suitable for coating difficultly accessible or complex surfaces, or components with a uniform layer. The deposits whose formation must be prevented are inorganic salts, such as calcium sulfate, barium sulfate, calcium carbonate and magnesium carbonate, inorganic phosphates, silicas and silicates, corrosion products, particulate deposits, for example, sand ( river and sea water), and organic deposits, such as bacteria, algae, proteins, mussels and larvae of mussels, polymers, oils and resins and biomineralized compounds consisting of the aforementioned substances. An object of the present invention is to indicate a process for the production of a thermal transfer device that, on the one hand, reduces the tendency of the thermal transfer surface to accommodate solids deposits, causing fouling and, on the other hand, resulting in negligible thermal transmission resistance while having high stability (for example, for heating, corrosion and scour). At the same time, the surfaces treated by the process must have satisfactory durability. The process must also be economical for use on poorly accessible surfaces. We have found that this object is achieved by a process for the production of a thermal transfer device comprising the anelectrolytic chemical deposition of a metal / polymer dispersion layer, wherein the polymer is halogenated, on a thermal transfer surface. For the objects of the present invention, a thermal transfer device is a device having surfaces designed for thermal exchange (thermal transfer surfaces). Preference is given to thermal transfer devices that exchange heat with fluids, in particular with liquids. Heating elements and heat exchangers, in particular plate heat exchangers and spiral heat exchangers, are the preferred embodiments of thermal transfer devices. A halogenated polymer is a fluorinated or chlorinated polymer; preference is given to fluorinated polymers, in particular perfluorinated polymers. Examples of the perfluorinated polymers are the polytetrafluoroethylene (PTFE) polymers and the perfluoroalkoxy (PFA, in accordance with DIN 7728, Part 1, January 1988). This solution according to the invention is based on a process for the anelectrolytic chemical deposition of the metal / polymer dispersion phases known per se (Riedel: Funtionelle Vernickelung [Functional Nickel Plating], Eugen Leize publication agents, Saulgau, 1989, pages 231 to 236, ISBN 3-750480-044-x). A metal / polymer dispersion phase comprises a polymer, for the purposes of the present invention, a halogenated polymer, which is dispersed in a metal alloy. The metal alloy is preferably a metal / phosphorus alloy. The processes used so far to prevent the incrustation tendency resulted in surfaces that had greater roughness than electropolished steel (see Table 1). It has now been found that a coating that also reduces roughness carries out the same work. In addition, it has been found that the effect of the polymer component to reduce the fouling tendency is crucial, even though the polymer content in the dispersion layer is rather low, at 5 percent to 30 percent by volume. Furthermore, it has been found that the treated surfaces according to the invention facilitate good thermal transfer, even though the coatings may have a non-inconsiderable thickness of 1 to 100 microns. The treated surfaces according to the invention also have satisfactory durability, which also allows layer thicknesses of 1 to 100 micrometers which appear as appropriate; the thickness of the layer is preferably 3 to 20 micrometers, in particular, 5 to 16 micrometers. The polymer content of the dispersion coating is from 5 percent to 30 percent by volume, preferably from 15 percent to 25 percent by volume, especially from 19 to 21 percent by volume. In addition, the coatings used according to the invention are, as a result of the process, relatively inexpensive and can also be applied to surfaces that are difficult to access. These surfaces can be any of the thermal transfer surfaces, such as the internal surfaces of pipes, the surfaces of electrical heating elements and surfaces of the plate heat exchangers, etc., which are used to heat or cool fluids in industrial plants. , in private homes, in food processing plants or in the generation of energy or water treatment. "Thermal transmission" means the transfer of heat from the interior of the thermal transfer device to any coating present on the fluid side, the heat conduction within the coating layer, and the thermal transfer from the coating layer to the fluid (for example a salt solution). In a preferred embodiment of the process according to the invention, the metal / phosphorus alloy of the metal / polymer dispersion layer is copper / phosphorus or nickel / phosphorus, preferably nickel / phosphorus. In a further embodiment of the process according to the invention, the nickel / polymer dispersion layer is a nickel / phosphorus / polytetrafluoroethylene dispersion layer. However, fluorinated polymers such as perfluoroalkoxy polymers (PFA, tetrafluoroethylene copolymers and perfluoroalkoxy vinyl ethers, for example, perfluorovinyl propyl ether) are also suitable. If the heat transfer device is also to be operated at a relatively low temperature, the use of chlorinated polymers is also feasible. In contrast to electrodeposition, the electrons required for nickel / phosphorus chemical or autocatalytic deposition are not provided by an external energy source, but instead are generated by chemical reaction in the electrolyte itself (oxidation of a reducing agent). The coating is performed by immersing the workpiece in a solution of the metal electrolyte that has been previously mixed with a stabilized polymer dispersion. The immersion operation was preferably followed by conditioning at a temperature of 200 ° C to 400 ° C, in particular at a temperature of 315 ° C to 325 ° C. The duration of conditioning is generally from 5 minutes to 3 hours, preferably from 35 to 45 minutes. Examples of metal solutions that can be employed are commercially available nickel electrolyte solutions containing Ni11, hypophosphite, carboxylic acids and fluoride and, if desired, deposition moderators, such as Pb2 +. These solutions are sold, for example, by Riedel, Galvano-und Filtertechnik GmbH, Halle, Westphalia, and Atotech Deutschland GmbH, Berlin. The polymers that can be used are, for example, commercially available polytetrafluoroethylene dispersions (PTFE dispersions). Preference is given to PTFE dispersions having a solids content of 35 percent to 60 percent by weight and an average particle diameter of 0.1 to 1 micrometer, in particular 0.1 to 0.3 micrometer, where the particles have a spherical morphology, and containing a neutral detergent (eg polyglycols, alkylphenol ethoxylate or, if desired, mixtures of these substances, from 80 to 120 grams of neutral detergent per liter) and an ionic detergent (eg, alkyl- and halo- alkylsulfonates, alkylbenzenesulfonates, alkylphenol ether sulphates, tetraalkylammonium salts or, if desired, mixtures of these substances, from 15 to 60 grams of the ionic detergent per liter). Typical immersion baths have a pH of about 5 and contain approximately 27 grams per liter of NIOSO 4 x 6 H 2 O and approximately 21 grams per liter of NaH 2 P 2 x H 2 O, with a PTFE content of 1 to 25 grams per liter. The polymer content of the dispersion coating is affected mainly by the amount of dispersion of the polymer added and the selection of detergents. The present invention furthermore relates to a process for the production of the thermal transfer device having a particularly adherent, durable and heat resistant coating and therefore achieves the object according to the invention in a specific manner. This process is based on a process for the production of a thermal transfer device comprising an anelectrolytic chemical deposition of a metal / polymer dispersion coating, where the polymer is halogenated, towards a thermal transfer surface. This process further comprises applying a metal / phosphor layer with a thickness of 1 to 15 micrometers by anelectrolytic chemical deposition prior to the application of the metal / polymer dispersion layer. The anelectrolytic chemical deposition of a metal / phosphorus layer with a thickness of 1 to 15 micrometers to improve adhesion is carried out by means of the metal electrolyte baths described above, to which, in this case, none is added. stabilized polymer dispersion. The conditioning preferably is not carried out during this time, since this generally has a detrimental effect on the adhesion of the subsequent metal / polymer dispersion layer. After deposition of the metal / phosphor layer, the workpiece is introduced into the dip bath described above which, in addition to the metal electrolyte, also contains a stabilized polymer dispersion. The metal / polymer dispersion layer is formed during this operation. This is preferably followed by conditioning at a temperature of 200 ° C. at 400 ° C, in particular at a temperature of 315 ° C to 325 ° C. The duration of the conditioning is usually from 5 minutes to 3 hours, preferably from 35 to 45 minutes. In a further embodiment of the process according to the invention, the metal / phosphor layer has a thickness of 1 to 5 micrometers. In a further embodiment of the process according to the invention, the metal / phosphorus alloy in the metal / polymer dispersion layer of the metal / phosphorus layer is nickel / phosphorus or copper / phosphorus. In a further embodiment of the process according to the invention, the metal / polymer dispersion layer is a nickel / phosphorus / polytetrafluoroethylene dispersion layer. The invention also relates to a thermal transfer device that can be produced by a process according to the invention. The thermal transfer device according to the invention is preferably produced using a process according to the invention. In a further embodiment, the aforementioned thermal transfer device according to the invention is designed for the transfer of heat to fluids, in particular, to liquids. The appropriate heating elements here are all those that transfer heat to the fluids. In addition, heat exchangers, in particular plate heat exchangers and spiral heat exchangers, are preferred examples of these heat transfer devices. The invention further relates to the use of a coating produced by anelectrolyte chemical deposition of a metal / polymer dispersion layer wherein the polymer is halogenated in order to reduce the tendency of the coated surfaces to accumulate solids from the fluids, causing fouling. . The fluids are preferably liquids. The soiling whose formation is prevented according to the invention has already been described. Some advantages of the thermal transfer devices according to the invention or their coatings are indicated by the attached drawing, in which: Figure 1 shows the coefficient of thermal transfer through the boundary layer as a function of time, taking into account any coating layer present, in contact of several surfaces of the heat exchanger with a boiling salt solution, and Figure 2 shows the coefficient of heat transfer through the boundary layer as a function of time, taking into account any layer of present coating, in contact of several surfaces of the heat exchanger with a hot stream of the salt solution. Figure 1 shows the decrease in the heat transfer coefficient (a [/ m ^ K]) due to the CaS04 deposits as a function of time (t [min], abscissa) for several thermal transfer devices that differ in the nature of its surfaces. Reference number 1 refers to the measured values of the coating according to the invention of Example (* 7) The reference number 2 represents the measured values for an electropolished steel surface. The power per unit area is 200 kW / m2, the concentration of the CaS04 solution is 1.6 grams per liter and the temperature corresponds to the boiling temperature. Figure 2 shows the measured decrease in the heat transfer coefficient (a [W / m2K]) due to the CaS04 deposits as a function of time (t [min], abscissa) for several thermal transfer devices that differ in the nature of its surfaces. Reference number 1 refers to the coating according to the invention of Example (* 7). Reference number 3 represents an untreated steel surface. The power per unit area of the thermal transfer device is 100 kW / m2. A solution of CaS 4 having a concentration of 2.5 grams per liter flows beyond the transfer device at a rate of 80 centimeters per second and a temperature of 80 ° C.

Example The advantages of the coated coating surfaces according to the invention compared to the uncoated heating surfaces, the electropolished surfaces and the ion implanted or sputter surfaces were determined in the laboratory investigations. Table 1 contains a comparison of the measured values for surface roughness, surface energy and wetting angle of the heating surfaces investigated, and the relative decrease in the thermal transfer coefficients measured within the first 100 sheets of the experiment. It is evident that the thermal transfer devices according to the invention provide very low surface energy, a very large contact angle and a very good heat transfer compartment. Table 1: Energy angle Asperity, a ^ oo / cto Super- micrometer Contact tros [mJ / m2] * [°] * * i -k * k -k ~ k -k -k -k Not treated (steel! 84 65 0.14 0.4 Electropolished steel 86 62 0.08 0.65 Steel implanted with Si ion * 5 39 80 0.14 0.75 Steel implanted with F-ion * 5 37 82 0.14 0.9 Steel Ionically bombarded with DLC * 6 36 85 0.13 0.85 Steel implanted ionically with TiNF * 6 34 0.14 0.9 Steel / Ni-PTFE * 7 25 100 0.1 0.9 Table 2 shows the surface energy, contact angle and bacteria (Streptococcus thermophilus) deposited per unit area of the thermal transfer devices according to the invention compared to prior art thermal transfer devices. Table 2: Energy Angle Superfical cells of loglO / cm2 to * Not treated (steel) 84 65 5.7 Electropolished steel 86 62 5.5 Steel implanted with Si ion * 5 39 80 4.9 Steel implanted with ion F * 5 37 82 5.5 Steel bombarded ionically with DLC * 6 36 85 5.0 Steel bombarded ionically with CrC * 6 34 87 4.1 Steel / Ni-PTFE * 7 25 100 3.9 * Measurement by the method of A. J. Kinloch, Adhesion and Adhesives, Chapman & Hall, University Press, Cambridge, 1994 ** Measurement by the method of D. K. Owens, J. of Appl. Polym. Sci. 13 (1969) 1741-1747 *** Relative heat transfer coefficient after an operating time of 100 hours (by the method of Müller-Steinhagen et al., Heat Transfer Engineering 17 (1998), 46-63) * *** Surface roughness, Ra in accordance with DIN ISO 1302 * 5 Method as described by JW Mayer, "Ion Implantation in Semiconductors, Silicon and Germanium", Academic Press, 1970 (ISBN 75107563) * 6 Process for the application of DLC of diamond-like carbon in accordance with GB-A 9006073 * 7 First, a 5 micron anelectrolytic nickel layer containing 8 percent phosphorus was applied to improve adhesion by immersion in an electrolyte nickel electrolyte solution chemically. The Ni / phosphorus / PTFE dispersion is subsequently produced in an immersion bath consisting of a mixture of chemically anelectrolyte nickel electrolyte solution and detergent stabilized PTFE dispersion.

The nickel / phosphorus / polytetrafluoroethylene deposition was carried out at a temperature of 87 ° C to 89 ° C, that is to say at less than 90 ° C, and at a pH of the electrolyte solution of 4.6 to 5.0. The deposition rate was 10 micrometers per hour, and the thickness of the layer was 15 micrometers. The composition of the chemically anelectrolytic nickel / PTFE electrolyte solution is shown in Table 3.

Table 3: Concentration pH [gram / liter] NOSSO4X6H2O 27 4.8 aH2P02xH20 21 CH3CHOHCOOH 20 C2H5COOH 3 Na Citrate 5 NaF 1 PTFE (50%) 8 * 2-50 Chemically anelectrolytic nickel electrolyte solutions are commercially available (Riedel, Galvano- und Filtertechnik GmBH, Halle, Westphalia, and Atotech Deutschland GmbH, Berlin). After application of the nickel / phosphorus / PTFE layer, the work piece was conditioned at 300 ° C for 20 minutes. The polymer and phosphorus contents in the dispersion layer were 20 volume percent PTFE, which corresponds to 6 weight percent PTFE, and 7 percent phosphorus. PTFE dispersions are commercially available. The solids content and the average particle size were 50 weight percent and 0.2 micron respectively. The dispersion was stabilized by a neutral detergent (50 grams per liter) of Lutensol® alkylphenol ethoxylate, 50 grams per liter of Emulan® alkylphenol ethoxylate, manufacturer of both detergents is BASF AG, Ludwigshafen) and an ionic detergent (15 grams per liter of Lutensit® alkyl sulfonate, BASF AG, Ludwigshafen, 8 grams per liter of perfluoro-C3-C8-alkylsulfonate from Zonyl®, Dupont, Wilmington, USA). The concentration figures of 2-50 grams per liter is related to the amount of dispersion solution added. The measurement was carried out by the method of H. Müller-Steinhagen, Q. Zao and M. Reiß, "A novel surface of low fouling metal thermal transfer", 5th UK National Conference on Heat Transfer, London, 17th September 18, 1997. The culture of the cell is Streptococcus thermophilus.

Claims (7)

CLAIMS:

1. A process for the production of a thermal transfer device for heat exchange with fluids, wherein a) a metal / phosphor layer with a thickness of 1 to 5 micrometers is applied by chemical deposition anelectrolytic to a thermal transfer surface and b) a metal / polymer dispersion layer, wherein the polymer is halogenated, is subsequently applied by anelectrolyte chemical deposition to the metal / phosphor layer produced in step a), and the metal / polymer dispersion layer has a content of polymer from 5 percent to 30 percent by volume.

2. A process according to claim 1, wherein the metal / phosphorus alloy of the metal / polymer dispersion layer and the metal / phosphorus layer is nickel / phosphorus or copper / phosphorus, preferably nickel / phosphorus

3. A process according to claim 1 or 2, wherein the metal / polymer dispersion layer is a nickel / phosphorus / polytetrafluoroethylene dispersion layer.

4. A process according to any of claims 1 to 3, wherein the metal / polymer dispersion layer has a polymer content of 15 percent to 25 percent by volume, especially from 19 percent to 21 percent by volume.

5. A process according to any of claims 1 to 4, wherein the metal / polymer dispersion layer has spherical polymer particles having a median particle diameter of 0.1 to 0.3 micrometer.

6. A thermal transfer device produced by a process according to any of claims 1 to 5.

7. The use of a coating, produced by anelectrolyte chemical deposition of a metal / polymer dispersion layer, wherein the polymer is halogenated, to reduce the tendency of the coating surfaces to accumulate solids from the fluids, causing fouling.