EP2912399A1

EP2912399A1 - Heat exchanger element with heat transfer surface coating

Info

Publication number: EP2912399A1
Application number: EP13785586.2A
Authority: EP
Inventors: Peter James Dobson; Mark Andrew EVANS; Alexander William Faris REIP
Original assignee: Oxford Nanosystems Ltd
Current assignee: Oxford Nanosystems Ltd
Priority date: 2012-10-25
Filing date: 2013-10-24
Publication date: 2015-09-02
Also published as: GB201219202D0; WO2014064450A1

Abstract

A method of improving the thermal transfer properties of a heat transfer surface (1) of a heat exchanger element by electroless deposition on the surface (1) of a coating (2) of a metal or a metal alloy, preferably of nickel or copper or their alloys, optionally with particulates of silver or chlorinated or fluorinated polymers. The electroless deposition is carried out for a shortened period of time, which creates a surface which is rough at the nano scale, including asperities of the order of hundreds of nanometres in size. The surface is also of low surface energy, highly hydrophobic and has a high surface area. The surface is a biomimetic hierarchical nanostructure.

Description

HEAT EXCHANGER ELEMENT WITH HEAT TRANSFER SURFACE COATING

The present invention relates to a method of treating a heat transfer surface of a heat exchange element to improve heat transfer across the surface, and to heat exchanger elements and products incorporating such heat exchanger elements such as boilers, heaters, refrigerators, air conditioners, heat sinks and the like.

The transfer of heat across surfaces is of importance in many products and systems. The mechanism of such heat transfer has been studied for many years and considerable work has been done in improving the heat transfer efficiency, for example by promoting or inhibiting particular modes of heat transfer. A typical example would involve the transfer of heat between a heat exchange element and a fluid in contact with the surface of that element. Heat exchange elements are frequently metallic to take advantage of metal's high thermal conductivity. Examples of such configurations abound: for example, heat exchangers (where there is a second fluid in contact with the reverse side of the heat exchanger element), boilers, radiators, refrigerators and so on.

Over the years a number of strategies and methods for improving heat flow across a surface have been developed. Generally an increase in surface area exposed to the fluid increases heat transfer and so changing the macroscopic configuration of the surface by adding fins or shaping it to form multiple channels is commonplace. Modifications at the microscopic scale (10^"6m) have also been explored. For example GB-A-2251363 discloses coating a heat transfer element with a sprayed mixture of metal and carbon particles of about 60 microns in size. This coating produces an increase in nucleation sites for boiling of the fluid in contact with the heat exchanger. JP-A-62-0000796 discloses a heat transfer tube coated with metal grains of 12-60 mesh (2.0 to 0.2 mm). Other microporous and microchannel heat exchangers have also been proposed for example using micro-sintered substances, porous aluminium layers and metal foams. More recently surface treatment at the nano- scale (10^"9m) has been explored and it has been shown that applying nano-particles or nano-tubes of metal oxide or carbon respectively can improve heat transfer. In the accompanying drawings Figure 1 illustrates how the heat transfer across a surface into a fluid varies with temperature. As illustrated there are several different modes of heat transfer. At lower temperatures the heat transfer from the hot (e.g. metal) surface to the fluid (e.g. water) works well through natural convection, especially if there is complete wetting of the surface (i.e. the surface is hydrophilic). As the temperature of the heat transfer surface increases the formation of bubble nuclei leads to heat transfer occurring by nucleate boiling. The temperature at which this starts is affected by surface roughness and the existence of regions that are strongly hydrophobic. The optimum surface has still to be properly defined, but currently it is thought that for good performance in this region the asperities on the surface should be less than 1 micron in dimension, the surface should be composed of a high thermal conductivity material such as a metal, and it helps if some of the micro-asperities of are a low surface energy material to give strongly hydrophobic regions. As temperature increases there is a transition to stable film boiling. In stable film boiling a layer of vapour exists next to the surface and heat transfer through this film is by conduction. In the transition from nucleate boiling to stable film boiling some areas of the surface display film boiling and some nucleate boiling. Because the thermal conductivity of the vapour is lower than the liquid, the heat flux across the surface tends to reduce in the transition boiling region before reaching a minimum at the onset of stable film boiling, and then increasing again with temperature difference.

Because of the number of complex processes which influence heat transfer and the relative lack of understanding of them, it is difficult to optimize heat transfer.

Consequently for different applications the various macro, micro and nano scale techniques mentioned above can be used individually or, of course, in combination.

The present inventors have found that a coating produced by carrying out electroless deposition for a limited period of time provides a surface having a hierarchical nanostructure, and that such coating has very good heat transfer properties.

Accordingly, application of such coating to the heat transfer surface of a heat exchanger element can improve the heat transfer properties of the element. The invention therefore provides a heat exchanger element having a heat transfer surface comprising a metal or metal alloy having a coating with a hierarchical nanostructure, the coating being on an exposed surface of the heat exchanger element. Typically, the coating is obtainable by electroless deposition onto the heat transfer surface, e.g. for a time sufficient to provide a hierarchical nanostructure.

The invention also provides a method of manufacturing a heat exchanger element according to the invention, comprising the step of electroless deposition on the heat transfer surface to produce a coating having a heirarchical nanostructure, the coating being exposed on the surface of the heat exchanger element. Typically, the electroless deposition is carried out for a time sufficient (i.e. sufficiently short) to provide a hierarchical nanostructure, i.e. deposition is stopped when a hierarchical nanostructure is present.

For example, the method may be a method of manufacturing a heat exchanger element having a heat transfer surface, comprising a step of treating the heat transfer surface to improve heat transfer across the surface, comprising the step of electroless deposition on the surface of a metal or metal alloy to produce a low surface energy, hydrophobic, high surface area coating.

The coating is a nano-rough coating. It is predominantly of low surface energy, is hydrophobic, and has a high surface area. The coating preferably has a high thermal conductivity, for example, by being metallic, and can be provided with anti-fouling or anti-lime scale properties. The surface wettability is preferably optimised for the heat transfer fluid which will be in contact with the surface.

Preferably the coating is applied by electroless deposition. Electroless deposition is typically used to provide surfaces which are uniform and smooth. However, by carrying out deposition for a limited period of time, a nano-rough surface can be provided. The present inventors have found that the electroless coating builds up by forming a hierarchical structure of asperities. As deposition continues for longer periods of time and the coating becomes thicker, the surface gradually becomes more uniform. However, if deposition is carried out for a shortened period of time, the resulting thin, nano-structured coating has a surface with excellent heat transfer properties.

Thus, the electroless deposition, when carried out for the desired length of time, produces a naturally uneven metallic surface with a multiplicity of asperities, e.g. reentrant cavities separated by protrusions on the nano-scale. The asperities (or cavities and protrusions) are typically of the order of 100-500 nm across, e.g. up to about 300nm across, more preferably 100-180 nm across, and up to 500nm, e.g. up to 300nm, more preferably up to 150nm, high/deep.

The uneven surface provides the improved heat transfer properties of the surface. To achieve this effect, the coating is exposed, i.e. it is an exposed surface on the heat transfer surface.

The average coating thickness is typically in the range of no more than 2μιη, preferably no more than 1 μιτι, e.g. no more than (e.g. less than) 500nm or no more than 300nm. Coating thicknesses which are too high generally indicate that the deposition has been carried out for a longer period in order to obtain the thicker layer. This longer deposition time leads to loss of the nanoscale structure as the coating gradually becomes more uniform over time. Thus, if the coating is too thick, it tends to have a less pronounced nanostructure. Average coating thicknesses can be determined either by mass-related techniques (e.g. by use of a quartz crystal microbalance). Alternatively the thickness at a predetermined number of sites in a sample coating can be determined by visualisation of the coating using SEM and calculating an average. For example, 10, e.g. 20 or 50 sites within a sample area, e.g. of 1mm² can be determined from an SEM image and an average calculated. The maximum coating thickness (the thickness at the highest point of a coating in a sample; Tma_X in Fig. 2) is typically less than Ι μιη, e.g. no more than (e.g. less than) 500nm. The maximum coating thickness can be determined using SEM techniques. The coating is preferably directly coated onto the heat transfer surface. In one aspect, a single layer coating is provided. The coating is in the form of a hierarchical nanostructure, i.e. structures formed one upon the other at successively smaller length scales. Such surfaces include those having a biomimetic morphology, i.e. flower-like or fruit-like, in that they have the same morphology repeated at smaller and smaller scales. For example they are formed by protrusions and cavities at successively smaller scales formed upon each other. Thus the surface coating may consist of a first structure (first level) of protrusions each of which are themselves coated with (second level) protrusions of ten or one hundred times smaller size. The surface may have a two-level hierarchy or multi-level hierarchy of structures. The hierarchical nanostructure is typically such that the first structure of protrusions, which is in direct contact with the heat transfer surface, is a nanoscale structure, i.e. the first level asperities (or cavities and protrusions) are typically of the order of 100-500 nm across, e.g. up to 300nm across, more preferably 100-180 nm across, and up to 500nm, e.g. up to 300nm high/deep, more preferably up to 150nm, high/deep. Second and, if present, further, structures have a smaller scale, such as asperities (or cavities and protrusions) which are typically of the order of 10-50 nm across, more preferably 10-20 nm across, and up to 50nm, more preferably up to 15nm, high/deep.

Preferably the electroless deposition is of nickel, copper or alloys thereof. Alternative materials for use in the electroless deposition include nickel/iron alloys and silver/nickel eutectoids, wherein the ratio of silvennickel is in the range 0: 100 to 100:0, for example from 10:90 to 90: 10, e.g. from 20:80 to 80:20. Silver and nickel do not form alloys. Co-deposition of silver and nickel will therefore tend to provide silver as the outer coating. To provide a more homogenous coating, the composition of the electroless bath can be varied during deposition, for example using a silver-rich bath and gradually increasing nickel content. Optionally particulates of silver (which imparts bactericidal properties) or nano- particulates of fluorinated or chlorinated polymers or polyethylene or polypropylene can be included to impart hydrophobicity. In one aspect, such fluorinated or chlorinated polymers or polyethylene or polypropylene are present in a maximum amount of 5% by weight, e.g. up to 3%, 1% or 0.5%. In another aspect, the coating is free of fluorinated polymers; in particular it is free of fluorinated or chlorinated polymers or polyethylene or polypropylene. The inclusion of polymer nanoparticles gives a very low surface energy hydrophobic surface. Preferably the sizes of the particulates included in the deposit are in the sub one hundreds of nanometre range.

The coating layer may optionally comprise cerium and/or lanthanum compounds, for example the oxides. These are, for example, incorporated in the form of

nanoparticulates dispersed within the coating. Where used, cerium or lanthanum compounds are typically present in an amount of up to 10% by weight, typically up to 5% by weight, for example up to 3%, 1% or 0.5% by weight.

In one embodiment of the invention, the coating is primarily metallic. For example, the coating may comprise at least 90% by weight metals, e.g. at least 95% by weight, 98% by weight or at least 99% or 99.5% by weight metals.

In one embodiment, the coating does not comprise silver, e.g. it does not comprise silver or gold, e.g. it does not comprise noble metals. Typically, the metals present in the coating are non-noble metals. The process for coating the substrate typically involves electroless deposition. This is typically carried out for a pre-determined period of time, and/or to lead to a predetermined coating thickness. Prior to deposition of the coating, it can be advantageous to activate the surface. Surface activation typically increases the number of nucleation sites. This can be beneficial since the coating will bind to a greater proportion of the surface area. However, the greater the number of nucleation sites, the more likely it is that uniformity will develop in the coating. Thus, if the surface is activated, the deposition time is typically reduced, and/or the coating thickness is reduced, to ensure that the desired nano-rough surface is achieved.

Activation can be carried out in any known manner, for example by immersion in a Pd containing solution such as PdCl₂. Alternative activating solutions (e.g. nickel acetate and sodium hypophosphate) are described below with respect to particular deposition examples, but these activation techniques can be used in connection with other types of deposition and are not limited to use with the particular deposition bath stated. Deposition of the coating is typically carried out at elevated temperature. For example, the temperature may be at least 40°C, e.g. at least 50 °C, at least 65 °C or at least 80 °C. The maximum temperature is 100 °C, typically 95 °C, since aqueous deposition baths are used. The most appropriate temperature may vary dependent on the coating being deposited, for example copper deposition is beneficially carried out at 46 °C, whilst nickel deposition is beneficially carried out at at least 75°, e.g. about 85°C.

The immersion time, or the length of time the surface is immersed in the electroless coating bath, is dependent on the thickness of coating required, and the nature of the coating and substrate material. Typically, coating thicknesses used herein are low to avoid uniformity generating in the coating structure. Thus, the immersion time is typically less than 1 hour, preferably up to 45 minutes or up to 30 minutes. To determine whether an selected immersion time provides the desired hierarchical nanostructure, SEM imaging can be carried out on the resulting samples to determine whether the surface contains first level asperities having second (and optionally further) level asperities formed thereon. Typically, a coating process which provides a coating of less than 500nm in thickness will have the required hierarchical nanostructure.

Typically, an immersion time of up to 1 hour, e.g. up to 45 minutes, e.g. up to 30 minutes is sufficient to provide a hierarchical nanostructure. The coating is suitable for any heat transfer element including those made of carbon steel, austenitic stainless steel, martensitic steels, aluminium and its alloys such as aluminium bronzes, aluminium silicon etc., copper and its alloys, titanium and zirconium. However electroless deposition is also suitable for coating non-metallic substrates such as carbon composites.

The coating provides an improved bubble nucleation property which enhances the formation of bubbles and gives a local convective mixing effect. Thus it particularly improves heat transfer in the convective, nucleate boiling and transition boiling regions. A key advantage of improving the thermal transfer properties is that a smaller heat exchanger can be used to transport a given amount of heat. This can save both space and weight.

The heat exchanger elements which can be coated in accordance with the invention include, for example, shell and tube heat exchangers, plate heat exchanger, plate and shell heat exchanger, adiabatic wheel heat exchanger, plate fin heat exchanger, pillow plate heat exchanger, fluid heat exchangers and dynamic scraped surface heat exchanger. The heat exchanger element may, for example, be designed for liquid to liquid heat exchange or gas to liquid heat exchange. In one aspect of the invention the heat exchanger element is a liquid to liquid heat exchanger element. The coatings may be used in heat exchanger elements incorporated in boilers, air conditioners,

refrigerators, radiators, heat sinks, solar collectors, and other types of thermal transfer component. Because the surface is hydrophobic it tends to reduce frosting in

refrigeration and air conditioning applications.

One or both of the opposing sides of a heat exchanger element may be coated in accordance with the invention. It is also possible to coat opposite sides of a heat transfer element with coatings of the same or different properties. Elements having different coatings on opposing sides are particularly useful where the heat transfer fluids on different sides of the heat transfer element are different.

In addition to improving the thermal properties of the heat transfer surface, the coating can provide corrosion and wear resistance thus enhancing the life of the components. The invention also provides the use of the electroless deposition coating described herein applied to a heat transfer surface to improve heat transfer across the surface.

The invention will be further described by way of example with reference to the accompanying drawings in which :-

Figure 1 illustrates the different modes of heat transfer from a surface to a fluid;

Figure 2 is a schematic representation in cross-section of the surface coating produced in accordance with the invention;

Figures 3 to 5 are micrographs showing the hierarchical nanostructures of the coatings produced in accordance with the invention; and

Figures 6 to 8 show results of tests comparing heat transfer efficiency into water for uncoated heat exchangers and heat exchangers coated in accordance with the invention.

Figures 9 and 10 depict images of the coatings of the invention obtained using FIB imaging.

In accordance with an embodiment of the invention a heat transfer element such as the surface 1 of a heat exchanger made from steel, aluminium or its alloys, copper or its alloys, titanium or zirconium, or a combination of these, is coated by the electroless deposition of layers of copper and/or nickel 2. The coating 2 has asperities formed by re-entrant cavities 4 and protrusions 5. As illustrated in Figure 2 the coating can be of the order of hundreds of nanometres thick and the asperities have a depth and width of the order of a few hundred nanometres. The protrusions themselves are not smooth but carry smaller scale asperities. Thus, a hierarchical nanostructure is present.

In more detail the substrate to be coated is cleaned and sensitised by and then immersed in an aqueous chemical solution containing the metal ions and any other phases of nanoparticles (for example silver, fluorinated or chlorinated polymers or polyethylene or polypropylene) in colloidal emulsion. Such nanoparticles are typically less than 0.2 microns in diameter. The solution also includes reducing agents, complexing agents and stabilisers, the choice of which depends on the details of the substrate and the materials to be deposited. Typical methods for specific coatings will now be explained. Thicknesses quoted in the Examples below are average thicknesses. These are calculated by determining the thickness at 10 random sites on a sample using SEM imaging and calculating the average.

Nickel Deposition Example A

Substrates which are composites of Silicon Carbide and Aluminium are first washed with acetone and then deionized water. These are then added to an activator solution made up of nickel acetate (60g/L), sodium hypophosphate (60g/L) and ethanol

(900mL/L) for 1 minute while sonicated to generate microturbulence at the surface. The substrate was then heated to 120°C to ensure the nucleating process of the nickel. The activated sample was then placed in an electroless nickel bath made up of NiS0₄.7H₂0 (25g/L), NaH₂P0₂.H₂0 (20g/L), C₃H₆0₃ (25ml/L), H₃B0₃ (20g/L), NaF (lg/L) and KI0₃ (0.003g/L). The substrates are left in the bath for a time which depends on the depth of coating required, typically 20 minutes for a Ι μπι depth.

Nickel Deposition Example B

Steel substrates were ultrasonically cleaned in acetone and then cathodically cleaned in 10% sodium hydroxide solution for 5 minutes. The substrates were then rinsed with deionized water and immersed in a sulphuric acid solution (50%) for 30 seconds. After deionized water rinse, the substrates were transferred immediately to a plating solution made up of NiS04.7H20 (21g/L), NaH₂P0₂.H₂0 (24g/L), C₃H₆0₃ (25ml/L),

CH₃CH₂COOH (3g/L) and Pb(CH₃COO)₂ (3ppm) for 1 hour to give a coating depth of 2μπι.

Copper Deposition Example A

The substrate was first washed with deionized water and then electrochemically degreased with H₂S0₄ for 30s. The substrate was then activated in a solution of PdCl₂ for a minute before being rinsed with distilled water and placed in the deposition bath made up of CuS04 0.024M, NiS0₄ 0.002M, H₃B0₃ 0.5M, NaH₂P0₂ 0.27M,

Na₃C₆H₅0₇ 0.052M and EDTA 0.026M at a pH of 9.2 for 30 mins for a Ι μιη coating. Copper Deposition Example B

To create a super hydrophobic coating a substrate of steel was first cleaned with deionized water and ethanol and then immersed in a solution of CuS0₄ (0.05M) at room temperature for 3 minutes. The substrate was then washed with deionized water and dried in a stream of N₂ before being immersed in a 1.0wt% ethanol solution of perfluorooctyltriethoxysilane for 1 hour at room temperature. Finally the substrate was heat treated at 140°C for 0.5 hours.

Copper and Nickel Deposition Example

The sample was first washed with deionized water and then electrochemically degreased with H₂S0₄ for 30s. It was then activated for 60 seconds with PdCl₂ and finally added to the deposition bath made up of CuS0₄-5H₂0 (0.03 mol/L),

NiS0₄-6H₂0 (0.0024 mol/L), NaH₂P0₂-H20 (0.24 mol/L), Na₃C₆H50₇-2H₂0 (0.05 mol/L) and H₃B0₃ (0.50 mol/L), dissolved in deionized water for 20min at a pH of between 7.5-9.5 to give a coating of between Ι μπι and 3μπι

In order to reduce surface energy and increase hydrophobicity then PEG 10000 (50ppm) can be added to the deposition baths mentioned above. Nickel and Silver Deposition Example

Aluminium alloy substrates were ultrasonically cleaned in acetone and then cleaned for 5 minutes in nitric acid and sodium hydroxide solution. The substrates were then rinsed with deionized water. After deionized water rinse, the substrates were activated with a PdCl₂ solution and then transferred immediately to a plating solution. The solution was made up of NiCl₂-6H₂0 (12g/L), HB0₃ (15 g/L), NH F (5 g/L), NaH₂P0₂.H₂0 (15 g/L). Ag nanoparticles (6g/L) were dispersed into the plating solution by sonication. The dispersed solution was heated to 85 degrees and the substrates then coated for 1 hour to give a 2μπι coating. PTFE-containing Deposition Example

Substrates were firstly cleaned with nitric acid and sodium hydroxide. The substrates were then activated with a Pd catalyst and finally placed for deposition into a bath made up of NiS0₄-7H₂0 (20g/L), NaH₂P0₂-H₂0 (25g/L), CH₃COONa/3H₂0 (30g/L), C₃H₆0₃ (20g/L) 60 vol% PTFE (5 g/L). The solution was first sonicated to fully disperse the PTFE particles and the substrate then added to bath for a 1 hour period to give a 2μηι coating. The coatings showed a PTFE wt% of 1.8 Wt %.

Ce oxide Containing Deposition Example

Aluminium alloy substrates were ultrasonically cleaned in acetone and then cleaned for 5 minutes in nitric acid and sodium hydroxide solution. The substrates were then rinsed with deionized water. After deionized water rinse, the substrates were activated with a PdC12 solution and then transferred immediately to a plating solution. The solution was made up of NiS0₄-iH₂0 (32g/L), Na₃C₆H₅0₇-aH₂0 (12 g/L), H₃P0₄ (5 mL/L),

NaH₂P0₂.H₂0 (12g/L) NaCi₂H₂₅S0₄ (0.1 g/L). Ce0₂ nanoparticles (20g/L) were dispersed into the plating solution by sonication. The dispersed solution was heated to 85 degrees and the substrates then coated for 1 hour to give a 2μπι coating.

Example 1: Imaging of Coatings

Aluminium alloy substrates were ultrasonically cleaned in acetone and then cleaned for 5 minutes in nitric acid and sodium hydroxide solution. The substrates were then rinsed with deionized water. After deionized water rinse, the substrates were activated with a PdCl₂ solution and then transferred immediately to a plating solution. The solution was made up of NiCl₂-6H₂0 (12g/L), HB0₃ (15 g/L), H₄F (5 g/L), NaH₂P0₂.H₂0 (15 g/L). The dispersed solution was heated to 85 degrees and the substrates then coated by immersion in the bath for 20 minutes. A JEOL 6480 LV SEM equipped with an Oxford Instruments X-MAX80 SD X-ray detector and INCA x-ray analysis system was used to image the samples and perform the analysis using EDX. The measurements are semi-quantitative. The SEM image is depicted in Figure 3, with a further magnified image shown in Figure 4. Repetition of the coating process described above but extending immersion time to 60 minutes led to a more uniform coating which is depicted in Figure 5. As is apparent from the Figures, as the deposition time is increased, the coating becomes more uniform and the hierarchical nanostructure is gradually lost.

Example 2: Comparison of Coated and Uncoated Heat Exchangers: Gas Boiler Testing The effect of the coatings described herein was studied by comparing the performance in a gas boiler of an uncoated heat exchanger element and a heat exchanger element coated in accordance with the invention.

The testing procedure was carried out in accordance with European Standard EN13203 (valid as of October 2013). An uncoated sample was put into a boiler and this was then connected to a test rig. The rig measured the temperature of the water going into the boiler, the temperature of the water leaving the primary heat exchanger as well as the flow rate of water and the energy used to heat it. The sample was run on the rig using a cycle that replicated 24 hours of light use. After such time the sample was replaced with a fully coated version and the test was rerun under the same conditions.

To obtain the coated heat exchanger elements, the inside surfaces of the heat exchanger element samples were firstly cleaned with sodium hydroxide, rinsed with water and then cleaned with nitric acid. The samples were then activated with a palladium solution before being heated in an oven to get it up to temperature for deposition to take place

(85°). The deposition solution was made up as for Example 1 above and then heated in a bath until 85 degrees C was reached. The solution was then pumped through the heat exchanger element using a diaphragm pump for 30 minutes. The sample was then washed with water and left to dry.

For coating of the outside of the samples, areas where the solution could leak were sealed with epoxy resin. The sample was then prepared and then heated in an oven as above. The solution was made up in the same way as described above and heated to deposition temperature of 85 °C. The sample was then added to the solution and left for 30 minutes for deposition to take place. The results for the standard (original) and coated heat exchangers are set out in Tables 2 and 3 below. The terminology is further explained in Table 1.

Table 1

t

/././.zso/ ozao/iad OSmO/HOZ OAV 2 continued

The results show that the test using a coated heat exchanger ended with warmer water (negative value of Tdom. - out improvement) demonstrating improved performance. Further, overall boiler efficiency improved by 0.3%, corresponding to a roughly 1% improvement in heat exchanger efficiency. Significantly, both waste energy and waste water (water which passes through the system before it gets to temperature) were reduced by over 60%.

Example 4: Comparison of Coated and Uncoated Heat Exchangers: Heat transfer to Water

Coating 1 samples were coated using the Nickel and Silver Deposition Example shown above. Four samples were obtained, referenced Coatings la to Id. A coating 2 sample was created using the Copper Deposition Example A shown above.

The samples were tested using a standing water test. To the coated side of the samples a glass tube was sealed to the coating using a silicone based sealant. The tube was then filled with 1 or 2mL water and placed on a hot plate. The hotplate was set to 100 degrees and the temperature of the water was recorded while the sample was heated.

Results are depicted in Figures 6 to 8 in which Figure 6 shows heat transfer of an uncoated aluminium silicon alloy heat exchanger fin, and improved heat transfer for the identical fin coated as described above. Figure 7 shows heat transfer for Coating 1 a to Coating 1 d in comparison with an uncoated heat exchanger. Figure 8 depicts an average of the four results for Coating 1 a to Coating 1 d. It can be seen from these results that the heat exchanger coated in accordance with the invention heats water significantly faster than an equivalent uncoated heat exchanger. For example, the coating of the invention can reduce time to heat a volume of lmL of water from 25 to 50°C to 3 minutes or less, typically to 2.5 minutes or less. Example 5: Focussed Ion Beam Imaging (FIB)

Further imaging was carried out on samples produced in accordance with Copper Deposition Example A above using focussed ion bean (FIB) imaging. The system used was a FEI FIB200 (FEI, Eindhoven, The Netherlands). The system is fitted with a high brightness Ga+ liquid metal ion source (LMIS) on a double lens column and an open electron multiplier detector (channeltron) is orientated at a fixed angle of 45 with respect to the sample normal. The detector aperture is 20 mm in diameter and the first grid electrode is located a fixed 15 mm distance from the gallium beam impact position. This detection system for imaging can select and detect either negatively charged particles (secondary electrons and secondary negative ions) or positively charged particles (secondary positive ions) emitted from the sample due to the impact of energetic gallium ions with the surface. The gallium ion beam current and scanning raster can be tailored to allow high sputtering rates at current densities of ~ 10 Acrn^"2.

In the sample preparation an ion beam current of ~lnA with an energy of 30 keV focused to a beam spot size of ~250nm was used for milling a crater on the copper- coating structure to produce a cross-section for imaging. A slice of 0.2-0.3 microns thickness was typically removed in a single milling and imaging cycle. Initially, an ion beam current of ~5-7nA with a beam spot size increased to about 600-700nm was used to remove a larger volume of material more quickly from the sample (one slice of 5-8 microns thickness was milled in this case). Secondary ion (SI) imaging and secondary electron (SE) imaging were carried out with an ion beam current usually in the range of ~50pA when the beam spot size is ~50nm.

Figure 9 shows the FIB image at 0° tilt, and Figure 10 shows a cross section image of the coating, taken by tilting the sample at 45° (relative to the milling direction).

Claims

1. A heat exchanger element having a heat transfer surface comprising a metal or metal alloy having a coating with a hierarchical nanostructure, the coating being on an exposed surface of the heat exchanger element.

2. A heat exchanger element according to claim 1, wherein the coating is obtainable by electroless deposition on the heat transfer surface.

3. A heat exchanger element according to claim 1 or claim 2, wherein the hierarchical nanostructure comprises first level nanoscale asperities, the first level nanoscale asperities having further asperities formed thereon.

4. A heat exchanger element according to claim 3 wherein the width and depth of the first level nanoscale asperities is less than 300 nanometres.

5. A heat exchanger element according to claim 3 or 4, wherein the width and depth of the further asperities is less than 50nm.

6. A heat exchanger element according to any one of the preceding claims, wherein the coating has an average thickness of less than 500nm.

7. A heat exchanger element according to any one of the preceding claims wherein the coating comprises nickel, copper, or alloys of nickel and/or copper.

8. A heat exchanger element according to any one of claims 1 to 6, wherein the coating comprises an alloy of iron and nickel or a silver/nickel eutectoid.

9. A heat exchanger element according to any one of the preceding claims wherein the coating comprises particulates of silver, fluorinated or chlorinated polymers, polyethylene or polypropylene.

10. A heat exchanger element according to any one of the preceding claims wherein the coating comprises particulates of cerium and/or lanthanum compounds.

11. A heat exchanger element according to Claim 9 or 10 wherein the particles forming the particulates are no larger than 100 nanometres.

12. A heat exchanger element according to any one of claims 1 to 8, wherein the coating is free of fluorinated polymers.

13. A heat exchanger element according to any one of the preceding claims, wherein the coating comprises at least 90% by weight metal(s).

14. A heat exchanger element according to any one of the preceding claims wherein the heat transfer surface is of steel, aluminium or aluminium alloys, aluminium silicon, copper and its alloys, titanium or zirconium.

15. A heat exchanger element according to any one of the preceding claims, wherein the heat exchanger element has two heat transfer surfaces and wherein a different coating is present on each of the heat transfer surfaces.

16. A boiler, air conditioner, refrigerator, radiator, heat sink or solar collector incorporating a heat exchanger element having a heat transfer surface in accordance with any one of the preceding claims.

17. A method of manufacturing a heat exchanger element according to any one of claims 1 to 15, comprising the step of electroless deposition on the heat transfer surface to produce a coating having a heirarchical nanostructure, the coating being exposed on the surface of the heat exchanger element.

18. A method according to claim 17, wherein the electroless deposition is carried out for a maximum time of 45 minutes.

19. A method according to claim 17 or 18, wherein the electroless deposition is carried out at a temperature of at least 40°C.

20. A method according to any one of claims 17 to 19, wherein the coating comprises nickel and the electroless deposition is carried out at a temperature of at least 75 °C and for a maximum time of 30 minutes.

21. A method according to any one of claims 17 to 20, which further comprises the step of incorporating the heat exchanger element into a boiler, air conditioner, refrigerator, radiator, heat sink or solar collector.