BR112020001450A2 - heat exchange element, method for transferring heat to or from a fluid, and process for producing a heat exchange element - Google Patents

Abstract

The invention provides a heat exchange element comprising a substrate and a coating, wherein the coating is present on at least part of a flow path defined by the heat exchange element. The coating comprises a metal and has a structure comprising dowels having a length of up to 100 µm; the average length of the dowels varies across the entire coating. The invention also provides a method of transferring heat to or from a fluid which comprises providing the fluid to a flow path of the heat exchange element of the invention. The invention further provides a process for producing a heat exchange element of the invention, wherein the process comprises providing an autocatalytic deposition solution to a surface of a substrate. The invention further provides a flow process for producing a heat exchange element and a heat exchange element obtained or obtainable by that process.

Description

1/75 HEAT EXCHANGE ELEMENT, METHOD TO TRANSFER HEAT TO OR FROM A FLUID, AND, PROCESS FOR

PRODUCING A HEAT EXCHANGE ELEMENT FIELD OF THE INVENTION

[001] The present invention provides a heat exchange element that facilitates efficient heat transfer. The invention also provides a method of transferring heat to or from a fluid using a heat exchange element of the invention. Suitable processes for producing heat exchange elements, including the heat exchange element of the invention, are also provided, in particular an autocatalytic flow deposition process. The invention also provides a heat exchange element obtained or obtainable by the autocatalytic flow deposition process.

BACKGROUND OF THE INVENTION

[002] The transfer of heat across surfaces is of importance in many products and systems. Examples of such systems include cooling systems (such as air conditioning systems or cooling systems) and heating systems such as boilers. Other examples of such systems include heat recovery systems. A typical configuration of a heat exchange apparatus in such a system involves the transfer of heat between a heat exchange element and a fluid in contact with the surface of that element. A wide variety of sources can be used to provide heat to the heat exchange element. Examples of such configurations include, for example, heat exchangers (where the heat source of the heat exchanger is a second fluid in contact with the reverse side of the heat exchanger element), boilers, radiators, refrigerators and so on.

[003] It is therefore desirable to provide a heat exchange element that has very good heat transfer properties. It is particularly desirable to provide a heat exchange element that can transfer

2/75 efficiently heat to a fluid such as a liquid in contact with said element. However, the processes involved in transferring heat across a surface, and particularly from a solid surface to a liquid, are complex and misunderstood. Therefore, it is not a simple matter to produce a heat exchange element that has good heat transfer properties or to optimize existing surfaces to improve its heat transfer properties.

[004] Previous efforts have been made in this field. The approach taken typically involved maximizing the surface area of an object intended for use in heat transfer.

[005] Some previous authors tried to control the heat transfer capacity of an object by providing its surface with a specific humectability. In one example, WO 2011/149494 describes a heat exchange surface having a pre-selected contact angle with a particular liquid. The surface is produced by providing hydrophilic nanostructures on a substrate. The surface nanostructure is formed by depositing oxide-based nanomaterials on a substrate, and the nanostructures have an average square root roughness or height of 200 to 600 nm on average. The surface thus produced is said to be useful in free-boiling experiments.

[006] Other previous authors tried to control the heat transfer capacity of a surface by providing precisely designed structures on it. The designed structures are usually made of silicon. An example is found in “Surface structure enhanced microchannel flow boiling”, Zhu et al., Journal of Heat Transfer, Vol. 138, pp 091501-1 to 091501-13. A microchannel having a set of silicon micropylons is provided, and is said to promote heat transfer in a flow boiling regime.

[007] The present inventors previously provided a

3/75 nano-rough surface having a hierarchical nanostructure for use in heat transfer, described in WO2014 / 064450. It was found that, by performing autocatalytic deposition for a limited period of time, a coating having a hierarchical nanostructure could be produced on a substrate. Typically, the hierarchical nanostructure comprised a first-level structure coated with a second level of structure ten or a hundred times smaller in size. Typically, the first level structures were up to 500 nm high and the second level structure comprised elements up to 50 nm high. These surfaces were shown to effect heat transfer in flow boiling experiments.

[008] It is an object of the invention to provide a heat exchange element having heat transfer properties comparable with or better than the heat exchange elements discussed above, and which is suitable for heat transfer to a wide range of fluids . Improved heat exchange properties allow a heat exchange element of the invention to cool a heat source faster and via a smaller element, saving space and weight.

[009] In addition to providing a heat exchange element with good heat transfer properties, it is an object of the invention to provide a process for producing a heat exchange element. Many known methods of producing heat exchange elements are laborious and expensive. These known methods are commonly methods for increasing the surface area of an object intended for use in heat transfer.

[0010] An earlier example of a method of producing a heat transfer surface is described in “Surface structure enhanced microchannel flow boiling”, Zhu et al., Journal of Heat Transfer, Vol. 138, pp 091501-1 to 091501- 13. In that method, the design of silicon structures on a surface was used to increase the surface area of a

4/75 object. Methods used to design silicon structures include ion etching a silicon substrate and bonding a silicon wafer to a silicon surface.

[0011] The present inventors have previously described the use of autocatalytic deposition to create a coating on a heat exchange element (WO2014 / 064450). In this case, the autocatalytic deposition process created a nano-rough surface and the method involved placing a substrate in an autocatalytic deposition solution bath. However, the autocatalytic deposition of a metal in a bathing process suffered from a bubble adhesion problem. The autocatalytic deposition of a metal on a substrate usually produces bubbles of hydrogen gas on the surface of the substrate. It was found that, during the bathing process, hydrogen bubbles produced during autocatalytic deposition adhered to the substrate surface and caused the coating to form around the bubbles. This had two particular adverse effects. First, the rough structure of the coating formed by the autocatalytic deposition process was disrupted by the presence of bubbles, causing gap in the coating and / or portions of the coating not having the desired rough structure. Second, the coating formed by autocatalytic deposition was formed on the bubbles, leading to portions of the coating not in contact with the substrate. These non-adherent portions of the coating have been found to be fragile and often peel off the substrate over time, for example when using the coated substrate in exchange for heat. This was undesirable as it reduced the effectiveness of the coating's heat exchange. In addition, the exposed portions of the substrate caused by the hydrogen bubbles were subject to corrosion during use of the coated object as a heat exchange element.

[0012] Autocatalytic deposition methods remain desirable to produce heat exchange coatings as they can produce rough structures at low temperature, which reduces the cost of

5/75 process. Autocatalytic deposition is also desirable as it can be used to provide a coating containing metal, and thus have good heat exchange properties, which is desirable in a coating for a heat exchange element. In addition, autocatalytic deposition processes require less material than hot dip galvanizing processes (which are also referred to as galvanic deposition processes) that are carried out in a liquid metal bath.

[0013] It is an objective of the present invention to provide a process that can be carried out inexpensively and quickly, ideally at a low temperature to minimize energy costs. It is also desirable to provide a process that can be carried out on an existing heat exchanger in situ, so that the process can be advantageously used to retrofit the heat exchange element of the invention on an existing heat exchanger. Furthermore, it is an object of the invention to provide a process for providing a suitable coating for a heat exchange element (i.e., a rough coating comprising a metal) to a substrate, said process having the advantages of an autocatalytic deposition process, but avoiding the difficulties mentioned above.

SUMMARY OF THE INVENTION

[0014] The inventors have found that a heat exchange element having a coating comprising sharp pins in the micrometer size range, in which the length of the pins varies over a surface of the heat exchange element, has particularly high heat transfer properties advantageous. The invention therefore provides a heat exchange element comprising a substrate and a coating, in which the heat exchange element defines a flow path for fluid flow and in which at least part of the flow path is coated with the coating, wherein: the coating comprises a metal;

6/75 the coating comprises a plurality of dowels up to 100 μm in length; the coating comprises a first region at one end of the flow path where the average length of the pin is S1 and a second region on the flow path where the average length of the pin is S2; and S1 is greater than S2.

[0015] The heat exchange element of the invention is particularly well suited to promote efficient heat transfer between the pinned surface and a fluid. Accordingly, the invention provides a method of transferring heat to or from a fluid which comprises passing the fluid along a flow path of a heat exchange element.

[0016] A coating as comprised in the heat exchange element of the invention can be conveniently formed by autocatalytic deposition. The invention, therefore, further provides a process for producing a heat exchange element of the invention wherein the process comprises providing an autocatalytic deposition solution on a substrate surface.

[0017] The inventors have further discovered, surprisingly, that draining an autocatalytic deposition solution comprising a metal ion on a substrate quickly removes hydrogen bubbles from the surface and thus reduces the problems of fragility and / or peeling of the deposited coating that are associated with bathing processes. In addition, the flow of autocatalytic deposition solution still unexpectedly provides a rough surface suitable for promoting heat transfer from the surface. This finding is unexpected because previously it was thought that the flow process would lead to an irregular coating structure, which would not have such

7/75 good heat exchange properties. Furthermore, it is found that the process of deposition in autocatalytic flow is capable of producing a heat exchange element having regions of different pin length according to a modality of the invention's heat exchange element. Particularly advantageously, this autocatalytic flow deposition process can be used to retrofit an autocatalytically deposited coating in an existing heat exchanger in situ and without disassembly. The deposition solution can be provided only to those parts of a heat exchanger that require coating, thus minimizing material waste.

[0018] The invention, therefore, provides a process for producing a heat exchange element comprising a substrate and a coating, wherein: the coating comprises a metal; and the process comprises draining an autocatalytic deposition solution on a substrate surface.

[0019] This autocatalytic flow deposition process produces an object having a rough coating, the coating comprising a metal. The coating comprising a metal is a good heat conductor and has a large surface area, which increases the contact between the heat exchange element and the surroundings, promoting heat transfer between the element and the surroundings. Therefore, the heat exchange element produced by this process is suitable for use in exchange for heat.

[0020] The invention also provides a heat exchange element obtained or obtainable by this process in an autocatalytic deposition flow.

BRIEF DESCRIPTION OF THE FIGURES

[0021] Figure 1 illustrates the different modes of heat transfer from a surface to a fluid.

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[0022] Figure 2 illustrates the coating diagrammatically on the heat exchange element of the invention. Figure 2a shows model dowels in different orientations. Figure 2b shows a coating (1) on a substrate (2). Figure 2c shows an arrangement of groups on a surface and the pore between them. Figure 2d shows a coating (1) on a substrate (2), the coating having a gradual pin length across the surface of the substrate. Figure 2e illustrates diagrammatically a heat exchange element comprising a coating (1) on a substrate (2), the coating formed on a substrate by the autocatalytic flow process of the invention. Figure 2f shows a cross section of a heat exchange element (5) comprising a coated flow channel and an uncoated flow channel.

[0023] Figure 3 contains SEM images of coatings according to the invention. Figure 3 (a) shows a coating comprising pegs 1 to 3 μm in length; Figure 3 (b) shows a coating comprising pegs 4 to 5 μm in length; and Figure 3 (c) shows a liner comprising pegs 8 to 10 μm in length.

[0024] Figure 4 is a SEM image of a coating applied to the interior of a heat exchanger, comprising pins that are approximately 3 μm in length. This coating was produced by the autocatalytic flow deposition process of the invention.

[0025] Figure 5 contains SEM images of coatings according to the invention in which the coatings comprise groups. Figure 5 (a) shows a coating comprising pegs approximately 7 μm in length arranged in groups. Figure 5 (b) also shows a coating of pegs arranged in groups, the lowest resolution.

[0026] Figure 6 is a SEM image of a coating according to the invention comprising dowels approximately 7 μm in length applied to a wire mesh of 75 μm in diameter.

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[0027] Figures 7 and 8 show the heat flow in kW m-2 from a surface for an organic refrigerant as a function of wall overheating (ΔΤc) for several different surfaces. In Figure 7, one surface is a polished surface and the other is a coated surface according to the autocatalytic flow deposition process defined here.

[0028] Figure 9 shows the heat transfer coefficient in W m-2 K-1 for a coated surface according to the invention (by the autocatalytic flow deposition process), and an uncoated surface, in a variety of flow rates of soda.

[0029] Figure 10 shows the approximate height of the pin (upper dashed line) and the base radius of the pin (lower full line) obtained by an autocatalytic flow deposition process according to the invention over time.

[0030] Figure 11 shows a test equipment used to compare the heat exchange performance of an evaporator (heat exchanger) with that of an uncoated evaporator. The coated evaporator was coated according to the autocatalytic flow deposition process of the invention to provide a heat exchange element according to the invention.

[0031] Figure 12 shows the heat exchange coefficients as a function of the heat transfer rate of the coated and uncoated evaporators tested on the equipment shown in Figure 11.

DETAILED DESCRIPTION OF THE INVENTION Heat transfer through a surface

[0032] In the attached drawings, Figure 1 (top image) illustrates how the transfer of heat through a surface to a fluid (in this case, a liquid) varies with temperature. The Figure illustrates the several different heat transfer modes available. At lower temperatures, heat transfer from a hot surface (for example, metal)

10/75 for a fluid (eg water) works well through natural convection, especially if there is complete wetting of the surface by the fluid. As the temperature of the heat transfer surface increases, the formation of bubble cores leads to the heat transfer occurring by nucleated boiling. The temperature at which the beginning of such nucleated boiling occurs is affected by the surface roughness and the existence of regions that are strongly hydrophobic. As the temperature increases further, there is a transition to boiling in a stable film. In boiling in a stable film a vapor layer exists close to the surface and it is thought that the heat transfer through this film occurs by conduction. In the transition from nucleated boiling to stable film boiling, some areas of the surface have film boiling and some, nucleated boiling. As the thermal conductivity of the vapor is lower than that of the liquid, the heat flow through the surface tends to reduce in the boiling transition region before reaching a minimum at the beginning of the boiling in a stable film and then increases again with the temperature. The bottom image in Figure 1 also illustrates how the heat flow from the surface to the fluid (qw) varies with the surface overheating (ΔΤc, which is the difference in temperature between the surface temperature and the fluid temperature).

[0033] Without wishing to be bound by theory, it is speculated that the advantages of the heat exchange element of the invention may be attributable to various aspects of the structure of the coating.

[0034] It is believed that in the nucleated boiling regime, an important factor contributing to the heat transfer efficiency is the sharpening of the bolts in the coating. It is speculated that the sharper the dowels, the more efficiently the processes required for nucleated boiling heat transfer occur, including: - formation of a bubble at one end of the dowel;

11/75 - transfer of the bubble down to the side of the pin in a cavity or pore; - bubble growth in a cavity or pore by adding steam; and - detachment of the bubble and rewetting of the surface.

[0035] The pegs on the surface of the heat exchange element of the invention are sharpened, and thus promote the efficient performance of the steps above and thus heat transfer. In addition, the pin concentrates the heat flow at the tip of the pin, promoting the above processes.

[0036] Other preferred characteristics of the coating also contribute to improvements in heat transfer, in particular in the nucleated boiling regime. These include the size and shape of the pegs, the density of the pegs, the pore or cavity sizes on the surface and the density of pores / cavities (pores or cavities are spaces between pegs; these are discussed in more detail below). It is believed that the surface has an advantageous balance between the presence of enough pins to create bubbles and the presence of enough pores / cavities to store and grow bubbles.

[0037] It is believed that bubble nucleation occurs at or near the tips of pins and so that the high density of pins in the coating advantageously provides a large number of nucleation sites, enabling the efficient creation of bubbles. During bubble growth, heat is transferred to the bubble by boiling liquid to produce the gas in the bubble. Bubble growth is advantageous for heat transfer and is promoted by the existence of bubble growth sites such as pores and cavities.

[0038] The coating of the invention has a pore / cavity density large enough to promote bubble growth, but not so large that the number of bubble nucleation sites is

12/75 compromised. It is also believed that the size of the pores / cavities in the coating of the invention is appropriate to promote efficient bubble growth. The surface is also easily rewetable to detach bubbles from the surface easily. It is believed that controlling the size of the dowels, and the size of the pores or cavities between the dowels, is therefore advantageous in promoting heat transfer through a boiling regime.

[0039] The heat exchange element of the invention provides variation in the length of the pins in different regions over a flow path defined by said element. A flow path is a route along which a fluid can flow over a surface of the heat exchange element. In one region, at one end of the flow path, the average pin length is longer than in another region at a different point on the flow path. Different regions are believed to be suitable for different types of heat transfer. The first region, having a longer average length of the pin and, therefore, deeper cavities / pores, is better suited to promote nucleated boiling of a fluid passing along the flow path of the heat exchange element. The second region is less suitable for promoting nucleated boiling and more suitable for promoting film boiling of a fluid passing along the flow path of the heat exchange element. Thus, the heat exchange element of the invention advantageously provides regions suitable for at least two types of heat transfer regime.

[0040] It is further speculated that the arrangement of these regions along the flow path of the heat exchange element of the invention helps to promote heat transfer during boiling in flow. The region of longer dowels is beneficially arranged at one end of the flow path, where a fluid to be cooled can begin its flow along the flow path. Nucleated boiling can be started in this region. Measure

13/75 As the fluid drains along the flow path, it finds the second region of shorter dowels which can assist in establishing an efficient film boiling regime along the flow path. Heat exchange element

[0041] It should be noted here that reference to the "heat exchange element of the invention" indicates a heat exchange element as defined in claim 1. Reference to a heat exchange element indicates a heat exchange element that can be formed according to the autocatalytic deposition process of the invention. In preferred embodiments, the heat exchange element comprises all the features of claim 1 and is a heat exchange element of the invention.

[0042] "Heat exchange element" means a solid object suitable for transferring heat from itself to its surroundings. The surroundings can be, for example, a solid or fluid adjacent to (and usually in contact with) the heat exchange element. The heat exchange element is capable of absorbing heat from a source. The heat source can be, for example, a solid or fluid adjacent to (and usually in contact with) the heat exchange element. Thus, the heat exchange element of the invention is a solid object capable of transferring heat from a source, by itself, to its surroundings. In particular, the heat exchange element of the invention is suitable for transferring heat to a fluid, particularly to a liquid because a liquid is capable of boiling.

[0043] The heat exchange element of the invention defines a flow path for fluid flow. A heat exchange element that is not a heat exchange element of the invention can also define a flow path for fluid. The flow path for fluid flow, also referred to as a flow path, is a route along which a fluid can flow over a surface of the heat exchange element. Thus, the flow path comprises at least part of an exposed surface of the exchange element

14/75 heat. An exposed surface of the heat exchange element is one that can come into direct contact with the surroundings. The flow path must be exposed so that fluid can come in contact with it. For example, where the heat exchange element is in the form of a plate, the flow path can be any route over a surface of the plate. In some embodiments, the flow path can pass through the heat exchange element. For example, the heat exchange element may comprise one or more channels (including an open channel or a closed channel, i.e., a tube) to allow fluid to flow through the element; in this case, the flow path can include at least part of the one or more channels through the heat exchange element.

[0044] The flow path is typically all or part of the transfer area of the heat exchange element. "Transfer area" means the area of the heat exchange element that can contact a fluid to which heat must be transferred (such a fluid is referred to as a working fluid or a refrigerant). In some embodiments, the flow path is the entire surface area of the heat exchange element that can contact a fluid to which heat must be transferred (a refrigerant). In other embodiments, the flow path may comprise only part of the surface area of the heat exchange element that can contact a fluid to which heat must be transferred. Where the heat exchange element comprises a channel (or flow channel) to charge the fluid to which heat is to be transferred, in some embodiments the flow path comprises part of the surface of the flow channel. In other embodiments, the flow path comprises the entire surface of the flow channel.

[0045] Where the heat exchange element comprises a flow channel, one end of the flow path may be located at one end of the flow channel. For example, one end of the flow path may be located at an entrance to the flow channel.

15/75 Alternatively or additionally, one end of the flow path may be located within a flow channel (not at its end), for example, away from an entrance to the flow channel. Typically, one end of the flow path is in a position on the heat exchanger that is first contacted by a fluid to which heat must be transferred. Typically, one end of the flow path is located where the fluid to which heat must be transferred first comes into contact with the coating described here.

[0046] A cross section of a heat exchange element (5) is illustrated in Figure 2f. The heat exchange element (5) has a first flow channel and a second flow channel, each having an inlet (3). The first flow channel comprises a coating (1) while the substrate (2) is not coated on the second flow channel. The first flow channel defines a flow path (4) through it.

[0047] The flow path is usually continuous, meaning that it is a path along which fluid can flow while in continuous contact with the heat exchange element. The flow path may comprise more than one surface of the heat exchange element, for example, an inner surface and an outer surface.

[0048] It should be noted that a flow path does not necessarily constitute a path along which fluid is directed to flow. On the contrary, the flow path constitutes at least part of an exposed surface of the heat exchange element with which fluid can contact and thus on which fluid can flow.

[0049] The heat exchange element (for example, the heat exchange element of the invention) can be incorporated into a product. In one embodiment of the invention, the heat exchange element is incorporated into a heat exchanger. In another embodiment of the invention, the heat exchange element is incorporated into an air conditioning unit,

16/75 refrigerator, heat recovery system, radiator, heat sink, solar collector, boiler, or heat exchanger such as a mini heat exchanger or microchannel heat exchanger. Coating

[0050] The heat exchange element comprises a coating that promotes heat transfer from the heat exchange element. The heat exchange element is therefore able to transfer heat to its surroundings efficiently via the coating. Thus, typically the coating of the heat exchange element is present on a heat transfer surface of the heat exchange element. A heat transfer surface is a surface of the heat exchange element suitable for transferring heat to the surroundings. A heat transfer surface can contact the surroundings directly or indirectly; for example, a heat transfer surface may have one or more layers on top of it that separate it from direct contact with its surroundings. Typically, the coating is present directly on the heat transfer surface.

[0051] An exposed surface of the heat exchange element is a surface that is exposed in the vicinity. Typically, at least a part of the coating is an exposed surface. However, in some embodiments, another layer is present on the coating in such a way that the other layer (s) forms (s) the exposed surface.

[0052] In the heat exchange element of the invention, at least part of the flow path is coated with the coating. In some embodiments, the entire flow path is coated with the coating. This modality can be preferred because it allows the heat exchange element to maximize the heat transfer to a fluid in contact with, for example, flowing along the flow path.

[0053] The coating can also be present anywhere

17/75 on the heat exchanger (this is different, along the flow path). Advantageously, the coating can be present only on those parts of the heat exchange element that are to transfer heat to a fluid. This minimizes material waste.

[0054] The coating can be a continuous coating, in which the entire portion or all coated portions of the substrate are covered with the coating. Alternatively, the coating may be discontinuous, such that there are gaps in the coating on the coated portion of the substrate where the coating is not present on the substrate. Preferably, the coating is a continuous coating, as shown in Figure 2b.

[0055] In the heat exchange element of the invention, the coating comprises dowels. However, not all coating material is necessarily arranged in pegs; the coating may also comprise material distributed on the substrate surface. Thus, a continuous coating does not require pegs to be placed side by side without gaps.

[0056] In the heat exchange element of the invention, the coating comprises a plurality of dowels. "Peg" means a structure having a thicker portion at the end of the structure that tapers to a thinner portion at the other end of the structure. A pin can therefore be described as a structure having a pointed top or as a tapered structure. The thickest part of the peg (base) is typically at the end of the peg closest to the substrate and the thinnest part of the structure (tip) is typically at the end of the peg most distant from the substrate.

[0057] The base of the pin is defined as the smallest cross section of the pin that intersects the end of the shortest side of the pin. For example, when a peg extends at right angles to a surface

18/75 flat, all sides of the pin (that is, distances from one end of the pin to the other) will be the same length and the tip of the pin is located along a line forming a right angle with the base. The base of the pin is, therefore, the cross section of the pin in its plane of contact with the substrate. This is illustrated in the image on the left of Figure 2a. However, when a pin extends 45 ° from a flat surface, its tip may not be located on the base as a whole. In that case, the length of the peg sides as measured from the substrate to the tip of the peg will vary depending on where they are measured. The base of the pin is, therefore, positioned on the base of the shortest side and inclined at 45 ° with the base (image on the right of Figure 2a).

[0058] The pin is usually approximately cone shaped. That is, the pin can be approached to a cone having a circular base and a point located along an axis extending at right angles to the base. The approximately circular base is taken to be the smallest circle that contains the real base.

[0059] • The length of a pin is taken as the distance from the center of the circular base to the tip of the pin in this approximate cone shape.

[0060] • The radius of the base of a peg is the radius of the approximate circular base.

[0061] • The cone angle is the angle that the sides of the cone make with its central axis, measured at the end of the pin.

[0062] The coating comprises a plurality of dowels up to 100 μm in length. In general, the coating comprises dowels having a length of at least 1 μm. The coating also generally comprises dowels having a length of up to 50 μm. Thus, the coating typically comprises a plurality of dowels having a length of at least 1 μm and no more than 50 μm.

19/75 Preferably, the pegs are 1 to 15 μm long, for example 2 to 10 μm, for example 3, 4, 5, 6 or 7 μm.

[0063] The angle of the pin cone in the plurality of pins is typically small. The cone angle is generally less than 40 °. In some embodiments, the cone angle is 2 ° to 30 °, for example, approximately 5 ° or approximately 10 ° or approximately 20 °. In the context of cone angle, an approximate value can vary by ± 5 °, for example ± 2 °.

[0064] The angle of the pin cone remains within the ranges specified above regardless of the pin length. Consequently, the peg base radius of the pegs in the plurality of pegs increases with the length of the peg. Typically, the base radius of the pin is less than 5 μm. For example, the base radius of the pin can be 0.05 μm 3 μm, preferably 0.1 μm to 2 μm or 0.2 μm to 1 μm. In some embodiments, the length of the pin is 1 to 15 μm and the base radius of the pin is 0.2 to 3 μm. In some embodiments, the length of the pin is 1 to 10 μm and the base radius of the pin is 0.1 to 2 μm. In some embodiments, the length of the pin is 2 to 10 μm and the radius of the pin is 0.2 μm to 1 μm.

[0065] The length of the pin, the cone angle and the base radius of the pin can all be calculated by taking a SEM image of the coating and fitting approximate cones in the pin or pins seen in this image. The fitting can be done with the naked eye or by computer modeling.

[0066] The dowels are generally arranged close to each other. The density of dowels in the coating (ie the number of dowels per unit area) will usually vary with the base radius of the dowel, as a smaller base radius will allow dowels to be more closely compacted. 5 or more dowels per

20/75 100 μm², for example, at least 10 pegs per 100 μm², or at least 20 pegs per 100 μm². The base radius of the dowels also imposes an approximate upper limit on the density of dowels; however, where the dowels are arranged in groups (see below) the dowel density can be increased. Thus, in general the coating comprises no more than 500 dowels per 100 μm², for example, no more than 200 dowels per 100 μm². Preferably, the coating comprises from 5 to 500, for example, from 5 to 200, dowels per 100 μm².

[0067] The coating comprises a first region in which the average pin length is S1 and a second region in which the average pin length is S2. "Average pin length" means "average pin length". The average pin length can be calculated by establishing the length of each pin in a region and averaging from it. Most conveniently, average pin length can be calculated based on a representative sample of pins in a region. S1 and S2 can take values 100 μm. S1 and S2 are generally at least 1 μm. Also S1 and S2 are generally 50 μm or less. Thus, typically S1 and S2 are at least 1 μm and no more than 50 μm. Preferably, S1 and S2 are from 1 to 15 μm, for example, 2 to 12 or 2 to 10 μm.

[0068] "Region" means an area of the coating on the surface of the substrate. The region is typically an area of at least 20 μm², for example an area of at least 50 μm².

[0069] The average length of the pin in the first region (S1) is greater than the average length of the pin in the second region (S2). Typically, S2 is 95% of S1 or less. In some embodiments, S2 is 90% S1 or less, for example, 80% S1 or less. Usually, S2 is at least 10% S1, for example at least 40% S1. For example, S2 can be 10% S1 to 95% S1 or 50% to 90% S1.

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[0070] S1 can be up to 100 μm. Generally, S1 is 1 to 50 μm. In a preferred embodiment, S1 is 1 to 20 μm, for example, 2 to 15 μm. S2 can be up to 100 μm. Generally, S2 is 0.1 to 50 μm. In a preferred embodiment, S2 is 0.2 to 10 μm, for example, 0.5 to 10 μm. In one embodiment, S1 is 1 to 20 μm and S2 is 0.2 to 12 μm, for example, S1 is 2 to 12 μm and S2 is 0.5 to 10 μm. In some embodiments, the difference between S1 and S2 is 0.1 μm or more, for example, 0.5 μm or more or 1 μm or more. For example, the difference between S1 and S2 can be 0.1 to 5 μm.

[0071] The first region is located at one end of a flow path defined by the heat exchange element. For example, where the flow path comprises a flow channel (for example, a tube) through a heat exchanger, the first region can be located at the end of that flow channel. However, in some embodiments the flow path may not start precisely at the end of such a flow channel and thus the first region may be located in some way within the flow channel. The second region is located anywhere on the flow path to the first region. For example, the second region can be located at the center of a flow channel.

[0072] The first and second regions can be isolated from each other. For example, in a heat exchange element of the invention the first and second regions can be located on different plates or fins, or on different flow channels. That is, in some embodiments of the invention, the first and second regions may exist in portions disconnected from the coating. In other embodiments of the invention, the first and second regions can be located on the same coating portion.

[0073] In a particular embodiment, the heat exchange element of the invention comprises a coating in which the coating comprises a first region at one end of the flow path in which the

22/75 mean pin length is S1 and the mean pin length decreases over at least part of the flow path, starting at the first region. In this mode, the second region can be any region (different from the first) along said part of the flow path. In a preferred aspect of this embodiment, the average pin length is graduated along at least part of the flow path in such a way that the largest average pin length occurs at the end of the flow path and the average pin length decreases over along the flow path away from that end. This aspect is illustrated in Figure 2d. A coating according to this aspect of the invention can conveniently be obtained by the method of the invention. For example, the time for a substrate to be exposed to an autocatalytic deposition solution can be varied along the flow path by immersing the substrate very slowly in the solution. Alternatively, an autocatalytic deposition solution having a concentration gradient that varies along the flow path can be provided to the substrate.

[0074] Thus, in one embodiment, the average length of the pin is graduated along all or part of the flow path. “Graduated” means that the average pin length shows a variation in a constant direction, that is, a gradual or incremental variation instead of in stages. For example, the average pin length measured at a series of neighboring positions along the flow path can be successively longer at each position. In one aspect of this embodiment, the average length of the pin increases from one end of the flow path to the other. Where a flow path coincides with a flow channel or part of a flow channel, the average pin length can increase from one end of the flow channel to the other.

[0075] The coating of the heat exchange element of the invention may comprise a third region in which the average length of the

23/75 pin is S3. The third region can in one example be located on the flow path defined by the heat exchange element. S3 can be up to 100 μm. Generally, S3 is 1 to 50 μm. Preferably, S3 is from 1 to 20 μm, for example, from 1 to 15 μm, for example, 2 to 12 or 2 to 10 μm.

[0076] The average pin length in the third region (S3) is typically of an order of magnitude similar to the first region (S1) and greater than the average pin length in the second region (S2). Typically, S2 is 95% of S3 or less. In some embodiments, S2 is 90% of S3 or less, for example, 80%) of S3 or less. Usually, S2 is at least 10% S3, for example at least 40% S3. For example, S2 can be 10% S3 to 95% S3 or 50% to 90% S3. S3 is typically 95 to 105%, for example 99 to 101%, of S1.

[0077] In one embodiment, the third region is located at one end of the flow path. In this embodiment, the coating comprises a first region at one end of the flow path in which the average length of the pin is S1, a second region over the flow path in which the average length of the pin is S2, and a third region in a another end of the flow path where the average pin length is S1, where S1 is greater than S2. S1 and S2 can be as defined above. In this modality, the average length of the pin shows a gradual decrease and a gradual increase along the flow path.

[0078] The plurality of pins comprised in the coating have sharp points. As explained above, the sharpening of the pins promotes nucleated boiling. Thus, the pins are usually thin at the tip. Generally, a thickness at the tip of the dowels is 100 nm or less. This means that the maximum pin diameter, which occurs at the end of the pin (and the tip of the approximate cone) is 100 nm or less. For example, the thickness at the tip of the pins can be from 0.1 to 100 nm. Preferably

24/75 thickness at the tip of the dowels is 60 nm or less. For example, the thickness at the tip of the pins can be from 1 to 50 nm.

[0079] In some embodiments of the invention, the dowels are arranged in groups. The invention, therefore, provides a heat exchange element comprising a substrate and a coating as described herein, wherein the coating comprises a plurality of pins arranged in one or more groups. Each group comprises two or more pegs. Particularly good heat transfer efficiencies have been observed using heat exchange elements comprising groups.

[0080] The number of pins above, two in a group, is not particularly limited. Preferably, a group comprises five or more pegs. Typically, a group comprises between 5 and 500 dowels.

[0081] A group is typically a flower-like peg arrangement. A group comprises two or more dowels protruding from a node. A node is a volume of coating material from which the pins in that group protrude outward. The node may be approximately spherical or hemispherical in shape. The pins protrude from the node in an approximately radial manner (that is, approximately in directions along the radii of a spherical or hemispheric node). However, significant deviations in radial orientation are possible and so each peg may not be perfectly located along the radius of the sphere or hemisphere. A group and a node are illustrated in Figure 2c *.

[0082] The node can usually have a diameter (corresponding to a diameter of a sphere or hemisphere approaching the node) of up to 50 μm. Generally, the node diameter is 0.05 to 50 μm, for example, 0.1 to 20 μm, or 0.5 to 10 μm. Where the node is very small, it can be more conveniently thought of as a point from which the pins protrude.

[0083] A group can be described as having a height and a

25/75 diameter. Height is the longest distance perpendicular to the substrate. The diameter is the diameter of the smallest circle in the plane of the substrate that encloses the group when viewed from perpendicularly above the group. The height and diameter of a group can be determined by taking a SEM image of the coating comprising the group and fitting the height and diameter into it, either with the naked eye or via computer modeling.

[0084] The diameter of a group is generally less than 200 μm. The diameter of a group is typically 1 to 200 μm. Preferably, the diameter of a group is 2 to 100 μm, more preferably 5 to 50 μm or 10 to 50 μm, for example, 10 to 40 μm.

[0085] The height of a group is generally less than 200 μm. The height of a group is typically 0.5 to 150 μm. Preferably, the height of a group is from 1 to 100 μm, more preferably from 2 to 50 μm, for example, from 5 to 30 μm.

[0086] The density of groups (ie the number of groups per unit area) will vary with the diameter of the group. Where the coating comprises groups, the density of groups is generally up to 100 groups per 100 μm². Preferably, the density of groups is 0.5 to 50 groups per 100 μm², for example, from 1 to 25 groups per 100 μm².

[0087] Where the coating comprises groups, the first region may comprise a greater density of groups than the second region. Similarly, where the coating comprises groups, the average diameter of the groups in the first region may be greater than the average diameter of groups in the second region. Average diameter in this context means diameter on average. The average diameter of the group can be calculated by establishing the diameter in a region and averaging from it. Most conveniently, average group diameter can be calculated based on a representative sample of sample groups in a region.

[0088] The presence of dowels and / or groups in a coating gives

26/75 origin of cavities and pores. “Poro” means a space between adjacent groups; “Cavity” means a space between adjacent pegs. Cavities are therefore typically smaller than pores. The shape of a pore is not particularly limited. A pore can be described as having a depth, a width and a length. The pore depth is the maximum distance (in a direction perpendicular to the substrate) from the portion of the pore closest to the substrate to the maximum height of a neighboring group. The pore length is the largest straight linear extension of the pore in the plane of the substrate surface. The pore width is the largest straight linear extension of the pore perpendicular to its length in the plane of the pore. As with the other characteristics of the surface, these parameters can be determined by taking a SEM image of the coating and fitting the parameters, either with the naked eye or via computer modeling.

[0089] Generally, the depth of a pore is approximately equivalent to the height of the adjacent groups. Thus, the depth of a pore is generally less than 200 μm. The pore depth is typically 0.5 to 150 μm. Preferably, the depth of a pore is from 1 to 100 μm, more preferably from 2 to 50 μm, for example, from 5 to 30 μm.

[0090] Generally, the pore length is less than 500 μm. The pore length is typically 2 to 250 μm. Preferably the length of a pore is 10 to 100 μm, for example, 20 to 85 μm.

[0091] Generally, the pore width is less than 100 μm. The pore width is typically 0.5 to 50 μm. Preferably the pore width is 1 to 25 μm, for example, 4 to 20 μm.

[0092] It is speculated that different types of coatings (for example, varying in pin length or the presence or absence of groups) may be suitable for the most efficient heat transfer for different fluids. The density of dowels and cavities / pores, and their

27/75 sizes, determine the bubble formation density and the places where these bubbles move and grow. This affects the heat transfer properties. The presence of groups provides pores, and thus heat exchange elements having groups are best suited for transferring heat to fluids to which heat can be efficiently transferred by the formulation and growth of larger bubbles. Thus the coating on the heat exchange element of the invention can be varied to provide heat transfer optimized for a range of different fluids having different heat transfer characteristics. Fluids can include various species such as organic soft drinks, water, liquid N2 or CO2 and so on. For example, organic refrigerants can include hydrofluoroolefins (HFOs), hydrofluorocarbons (HFCs), fluorocarbons (FCs) and hydrocarbons. Another example fluid is ammonia.

[0093] The thickness of the coating on the heat exchange element is not particularly limited. The coating thickness can be defined as the longest perpendicular distance from the substrate to the edge of the coating material. Typically the coating thickness is at least 1 μm, for example, at least 2 μm. The coating thickness is generally 200 μm or less. Preferably, the thickness of the coating is 1 μm to 100 μm, for example, 2 to 50 μm. In one embodiment, the invention provides a heat exchange element where the thickness of the coating is 10 μm or more. In another embodiment, the invention provides a heat exchange element in which the thickness of the coating is 2 to 50 μm.

[0094] The exact weight of the coating per unit area of the substrate will depend on the structure of the coating and the materials therein. Generally, the weight of the coating per unit area of substrate is at least 10 g m-2. Generally, the weight of the coating per unit area of substrate is no more than 900 g m-2. Usually, the weight of the coating

28/75 per unit area of substrate is 20 to 500 g m-2, preferably 30 to 400 g m-2.

[0095] The coating comprises one or more metals. The coating generally comprises one or more transition metals. Preferably, the coating comprises one or more of vanadium, chromium, manganese, cobalt, nickel and copper. In a preferred embodiment, the coating comprises copper, nickel or a copper and nickel alloy. In a particularly preferred embodiment of the invention, the coating comprises copper. In another particularly preferred embodiment, the coating comprises an alloy of copper and nickel.

[0096] The coating usually has a high metal content, that is, it is primarily metallic. Usually, the coating contains at least 50% metal by weight of the coating. In a preferred embodiment, the coating contains at least 70% metal by weight of the coating. In one embodiment of the invention, the coating comprises 80% metal by weight of the coating. In a particularly preferred embodiment, the coating has a very high metal content, for example, at least 90% or at least 99% metal by weight of the coating.

[0097] The coating of the heat exchange element of the invention having a structure and properties as described above can conveniently be obtained by autocatalytic deposition. Thus, in an embodiment of the invention, the coating can be obtained by autocatalytic deposition. In one aspect of this embodiment, the coating is obtained by autocatalytic deposition. For example, the coating is typically obtained or can be obtained by an autocatalytic flow deposition process as defined here.

[0098] In one embodiment of the invention, the coating comprises one or more, for example, one or two, surface layers on the coating. “Surface layer” means a layer of material on

29/75 the surface of the coating. A surface layer is therefore a layer of material on the side of the coating opposite to the side of the coating that is in contact with the substrate.

[0099] The material of a surface layer is not particularly limited. For example, a surface layer can comprise one or more metals or one or more polymers. A surface layer can comprise one or more hydrophobic materials and / or one or more hydrophilic materials to adjust the wetting of the heat exchange element. A surface layer can comprise one or more protective materials to protect the coating from wear and tear or the influence of acres coolants such as ammonia.

[00100] Preferably a surface layer comprises one or more transition metals, particularly nickel or titanium. In a preferred embodiment, a surface layer consists of nickel, titanium, or an alloy comprising nickel and / or titanium. Preferably, a surface layer is a nickel layer. Typically the surface layer comprising a transition metal is on an exposed surface of the heat exchange element, that is, it comes in direct contact with the fluid flowing along the flow path.

[00101] Typically, a single surface layer is present, preferably a single layer comprising a transition metal as discussed above.

[00102] The surface layer (s) is / are thin layer (s) in order to preserve the advantageous structure of the coating. Generally the total thickness of any surface layer present is 500 nm or less. For example, the total thickness of the surface layer (s) present can be from 1 to 250 nm, for example, from 10 to 200 nm. Substrate

[00103] The substrate is a solid object. The substrate usually takes the

30/75 in the form of a typical heat exchange element or part thereof or a heat exchanger or part thereof which is then coated according to the invention.

[00104] Examples of suitable substrates include: shell and tube heat exchangers, plate heat exchangers, brazed plate heat exchangers, packaged heat exchangers, plate and shell heat exchangers, adiabatic wheel heat exchangers, plate fin and plate heat exchangers, plate and pad heat exchangers, fluid heat exchangers, scraped dynamic surface heat exchangers, mini heat exchangers and microchannel heat exchangers. Other examples of suitable substrates include parts of heat exchangers such as a fin, plate, coil or tube that is part of a heat exchanger. Still other examples of suitable substrates include heat exchangers or parts of heat exchangers suitable for incorporation into a boiler, air conditioner, refrigerator, radiator, heat sink, solar collector or other type of heat transfer component.

[00105] The substrate is preferably a heat conductor. The substrate can therefore comprise metal. In one embodiment, the substrate is a metal object including a metal or metal alloy. For example, the substrate can be an object such as a heat exchange element made from one or more of carbon steel, austenitic stainless steel, martensitic steel, aluminum and its alloys such as aluminum bronzes, silicon aluminum etc., copper and its alloys, titanium and zirconium. Preferably the substrate is an object comprising stainless steel or titanium, or the substrate consists of stainless steel or titanium. These metals are preferred as they resist corrosion.

[00106] The substrate may be non-metallic, and comprise a semiconductor such as silicon, or gallium nitride. It can, for example, comprise a carbon composite that has a high conductivity

31/75 thermal. In one embodiment, the substrate can be made of a carbon composite.

[00107] The substrate may comprise one or more outer layers. In the heat exchange element (for example, the heat exchange element of the invention), where an outer layer is present, all or part of an outer layer of the substrate is located between the substrate body and the coating. In one embodiment, a single outer layer is present that is in contact with the substrate body and the coating. In other modalities, two or more outer layers are present.

[00108] Typically if an outer layer is present, a single outer layer is present. More preferably, there is no outer layer in such a way that the substrate is in direct contact with the coating.

[00109] An outer layer, when used, typically comprises one or more metals or metal alloys. For example, an outer layer can be a metallic layer. An outer layer is usable in improving the corrosion resistance layer of the substrate, particularly where the outer layer comprises titanium, nickel or stainless steel. An outer layer can also improve the formation of the coating during the manufacture of the heat exchange element of the invention, and can improve the adhesion of the coating to the substrate on the heat exchange element.

[00110] The heat exchange element is suitable for transferring heat from its surface to a fluid in contact with its surface. As discussed above, the coating structure promotes efficient heat transfer by nucleated boiling and / or film boiling of a liquid, and thus the heat exchange element of the invention is particularly suitable for transferring heat to a liquid. Often, therefore, the substrate is an object suitable for or adapted to transfer heat to a liquid. In some embodiments, the substrate is an element of exchange of

32/75 heat, heat exchanger or part of a heat exchanger that is designed for transferring heat to a liquid. In one embodiment, the substrate is an appropriate heat exchanger for transferring heat to a liquid.

[00111] In a particular embodiment of the invention, the heat exchange element may be suitable for fluid heat transfer fluid, for example heat transfer from gas to liquid or heat transfer from liquid to liquid. Thus, the substrate can be a heat exchanger or part of a heat exchanger designed for fluid to heat transfer fluid, for example heat transfer from gas to liquid or heat transfer from liquid to liquid.

[00112] A heat exchange element suitable for transferring heat to a fluid such as a liquid has an appropriate surface or surfaces for contacting a fluid. A fluid to which heat is transferred by a heat exchange element can be referred to as a "working fluid" or "refrigerant". Typically, but not essentially, a heat exchange element suitable for transferring heat to a fluid such as a liquid comprises one or more flow channels suitable for charging a fluid to which heat is to be transferred. Typically, but not essentially, therefore, the substrate comprises one or more flow channels suitable for loading a fluid such as a liquid to which heat must be transferred.

[00113] A fluid from which heat is transferred to a heat exchange element can be referred to as a "heat transfer fluid" or "heating fluid". A heat exchange element that is suitable for receiving heat from a fluid such as a liquid typically comprises one or more flow channels suitable for transferring heat from a fluid to the heat exchange element. Thus, typically, but not essentially, the substrate comprises one or more flow channels suitable for carrying a liquid from which heat can be transferred

33/75 to the substrate.

[00114] In one aspect of the invention, the heat exchange element (for example, the heat exchange element of the invention) is a fluid-to-fluid heat exchanger, preferably a fluid-to-liquid heat exchanger such as a gas to liquid or liquid to liquid heat exchanger, or part of it. Typically, therefore, the substrate comprises one or more flow channels suitable for loading a fluid (e.g., a liquid) from which heat can be transferred to the substrate and one or more flow channels suitable for loading a fluid (preferably a liquid) to which heat can be transferred from the substrate.

[00115] "Flow channel" means a channel along which fluid can pass through the substrate. A flow channel comprises one or more openings through which fluid can enter and / or leave the flow channel. Such an opening can be referred to as an entrance. In most configurations, a flow channel is closed on all sides along its length (i.e., a tube) and comprises openings at one or more ends. However, as the converse will recognize, other configurations of a flow channel are possible.

[00116] In an embodiment of the heat exchange element, the flow path comprises a flow channel and the coating is present on at least a part of the surface of said flow channel. Typically the coating substantially covers the surface of the flow channel. The flow channel surface means the internal surface of the flow channel. The inner surface of a flow channel will come in contact with fluid flowing through the flow channel. The flow path may lie entirely on the flow channel. That is, the flow channel can extend beyond the flow path. Alternatively, the flow path can extend to or even beyond one or more openings (entrances) to the flow channel.

34/75

[00117] Since the coating is particularly useful for promoting heat transfer to a refrigerant (for example, a liquid), the coating is advantageously present on a surface or surfaces (such as the surface of a flow channel) that a refrigerant will contact when the element is in use. In order to avoid wasting material, the coating can be present only on the surface or surfaces of the heat exchange element that can contact a refrigerant.

[00118] The heat exchange element may comprise one or more flow channels with no coating present on its surface. Such uncoated flow channels can be useful for carrying a heat transfer fluid when the heat exchange element is in use. Such flow channels for heat transfer fluid are called "first flow channels" here. Typically, the heat exchange element is arranged in such a way that the heat transfer fluid can transfer heat to the refrigerant via the heat exchange element.

[00119] Where the heat exchange element of the invention comprises a flow channel having coating present on the surface of the flow channel, the first region (having an average length of pin S1 therein) and the second region (having a length medium of pin S2 therein) can both be located within the flow channel. Alternatively, one region may be located in the flow channel and the other may not be. In one embodiment, the first region is located at or near an entrance to the flow channel and the second region is located at a greater distance from the entrance than the first region. For example, where the first and second regions are within a flow channel, the liner may comprise more long dowels at or near the inlet and shorter dowels spaced along the flow channel, for example, the dowel length may gradually decrease from at or near the entrance to a remote point along the channel. An exchange element

35/75 heat according to this embodiment can be conveniently produced by the process of the invention.

[00120] In a preferred embodiment of the invention, the heat exchange element is suitable for transferring heat to a refrigerant, preferably to a liquid refrigerant. When the heat exchange element is in use, it can comprise one or more refrigerants; for example, a refrigerant may be present in one or more flow channels in the heat exchange element. Thus, in one embodiment, the invention provides a heat exchange element that contains a refrigerant. That is, the invention provides a working heat exchange element in which the working heat exchange element comprises a heat exchange element of the invention and a refrigerant. In another aspect of this embodiment, the heat exchange element also comprises a heat transfer fluid (for example in one or more flow channels of the heat exchange element).

[00121] The refrigerant is a fluid, preferably a liquid, suitable for receiving heat. A wide variety of liquids are suitable including CO2, nitrogen, ammonia, water, aqueous solutions, organic liquids including halogenated alkanes such as CFCs, and sulfur-based refrigerants. The heat transfer fluid is a fluid, usually a liquid capable of providing heat to the heat exchange element. A wide variety of liquids are suitable including water and organic liquids such as oils. Heat transfer efficiency of the heat exchange element of the invention

[00122] The heat exchange element of the invention is capable of highly efficient heat transfer. The heat transfer efficiency of the heat exchange element of the invention is similar to or better than the heat transfer efficiency of a comparable substrate having a

36/75 sintered surface. Advantageously, however, much less metal is needed to create the coated heat exchange element of the present invention than is needed to prepare a comparable substrate having a sintered surface.

[00123] The heat exchange element of the invention facilitates heat transfer more efficiently than a comparable substrate having a polished surface. In one embodiment, the heat exchange element of the invention has a heat transfer coefficient at least 20% higher than a comparable substrate having a polished surface, for example at least 30% higher or 50% higher than a comparable substrate having a polished surface. The heat transfer coefficients are typically calculated for the same system (that is, having the same heat source and the same refrigerant and the same temperature or temperatures) in order to make the comparison. The heat transfer coefficients for the purpose of this comparison are typically calculated at a boiling flow regime and at a heat flow of less than 200 kW m-2, for example, at 100 kW m-2. The polished surface is a polished surface with 1200 grade sandpaper. The polished surface typically has an average roughness of 0.04 μm or less, measured on a Taylor Hobson surface profiler (Taylor Surf Series 2, using Taylor Hobson ultra software).

[00124] In one embodiment, the heat exchange element of the invention has a heat transfer of 7000 W m-2 K-1 or more at a heat flow of approximately 80 kW m-2.

[00125] In one embodiment, the heat exchange element of the invention has a superheat that is 50%) or less than that presented by a comparable substrate having a polished surface, under the same test conditions. Preferably, the heat exchange element of the invention has an overheat that is 30% or less than that presented by a comparable substrate having a polished surface, under the

37/75 same test conditions. Typical test conditions include a free boiling experimental regime, and a heat flow of up to 500 kW m-2, for example, 20 kW m-2. Overheating is the difference (in Kelvin) between the surface temperature from which heat is being transferred to a fluid and the temperature of the fluid.

[00126] In one embodiment, at a heat flow of up to 500 kW m-2 the heat exchange element of the invention has a superheat of 10 K or less. In one aspect of this embodiment, at a heat flow of up to 200 kW m-2 the heat exchange element of the invention has an overheating of less than 10 K. Typical test conditions include a free-boiling experimental regime. Heat transfer method

[00127] The heat exchange element of the invention is capable of facilitating efficient heat transfer to its surroundings. In particular, the heat exchange element is capable of facilitating efficient heat transfer to a fluid in contact with the element and particularly to a fluid in contact with the coating of the element. The heat exchange element defines a flow path along which fluid can contact the element and which is coated with the coating, along all or part of its length. Thus, the invention provides a method of transferring heat to or from a fluid which comprises providing the fluid to a flow path of a heat exchange element of the invention.

[00128] As discussed above, the coating is adapted to promote heat transfer facilitating the boiling of a liquid (in a nucleated and film boiling regime). Consequently, in preferred embodiments of the method of the invention, the method comprises transferring heat to a liquid. In these embodiments, the method comprises providing a liquid to a flow path of a heat exchange element of the invention. In one embodiment, the method is a method of transferring heat to

38/75 from a solid to a liquid. In another preferred embodiment, the method is a method of transferring heat from a fluid to a liquid, for example, from a gas to a liquid or from a liquid to a liquid. In a particular embodiment, the method is a method of transferring heat from a liquid to a liquid.

[00129] In some embodiments, the heat transfer method comprises passing a fluid (preferably a liquid) along a flow path of a heat exchange element of the invention.

[00130] Generally, the method comprises transferring heat to a refrigerant, the method comprising passing a refrigerant along the flow path of a heat exchange element of the invention. For example, the method may comprise passing a refrigerant through a first flow channel (which defines the flow path) of the heat exchange element of the invention. In some embodiments, the method comprises transferring heat from a heat transfer fluid by passing a heat transfer fluid over a second flow channel of the heat exchange element of the invention. In a preferred embodiment, the method is a method of transferring heat from a heat transfer fluid to a refrigerant, the method comprising passing said heat transfer fluid through a second flow channel of the heat exchange element and passing said refrigerant through a first flow channel of the heat exchange element. Flow channels are typically arranged to conveniently enable heat transfer from the heat transfer fluid via the heat exchange element to the refrigerant.

[00131] The temperature at which the method of the invention is carried out is typically less than 500 ° C. The temperature at which the method of the invention is carried out depends on the refrigerant (if used). Typically, where a refrigerant is used, the method is performed at a temperature within 20 ° C of the refrigerant's boiling temperature, for example, 0 to 10 ° C

39/75 above the boiling temperature of the refrigerant.

[00132] Also provided by the invention is the use of a heat exchange element according to the invention as a heat exchanger. In one embodiment, the invention provides for the use of a heat exchange element in a heat exchange method as described herein. Process for producing heat exchange element

[00133] The coating of the heat exchange element of the invention can conveniently be created by autocatalytic deposition. Thus, the invention provides a process for producing a heat exchange element according to the invention, the process comprising providing an autocatalytic deposition solution to a surface of a substrate. In one aspect, the autocatalytic deposition process is a bath process. In another aspect, the autocatalytic deposition process is a flow process. These specific aspects will be described in more detail in later sections; the following comments regarding autocatalytic deposition processes apply equally to bath processes and flow processes. The heat exchange element of the invention and the substrate are as defined here.

[00134] The autocatalytic flow deposition process of the invention, however, can provide heat exchange elements that are not heat exchange elements of the invention. That is, the autocatalytic flow deposition process can provide coatings that differ from that of the invention's heat exchange element. The invention, therefore, provides an autocatalytic flow deposition process to produce a heat exchange element, and heat exchange elements obtained or obtainable by that process. In a preferred aspect, the autocatalytic deposition process of the invention is a process for producing a heat exchange element of the invention. The heat exchange element and the substrate are as defined here.

[00135] Autocatalytic deposition involves the reduction of metal ions in solution to produce metal atoms deposited on the surface of the

40/75 substrate to form a coating comprising metal, as described above. The autocatalytic deposition process is a non-electrolytic process. The autocatalytic deposition process does not require molten metal, unlike hot dip galvanizing processes.

[00136] Advantageously, autocatalytic deposition can be carried out at low temperatures. Typically, the autocatalytic deposition process is carried out at a temperature of 20 ° C to 120 ° C. In one embodiment, the autocatalytic deposition process is carried out at room temperature.

[00137] Preferably, the autocatalytic deposition process is carried out at a temperature of 100 ° C or less, for example 20 ° C to 100 ° C or 50 ° C to 100 ° C, for example, approximately 60 ° C, 70 ° C or 80 ° C. It should be noted that performing autocatalytic deposition within this temperature range means that the autocatalytic deposition solution is maintained within the above-mentioned temperature range, typically 20 ° C to 120 ° C, during autocatalytic deposition. Previous and subsequent method steps can be performed at temperatures within or outside the range mentioned above.

[00138] The structure of the coating produced by the autocatalytic deposition process is influenced by the conditions of autocatalytic deposition on the substrate surface. In particular, varying the deposition time, the concentration of ions in the autocatalytic deposition solution and the temperature will influence the structure of the coating formed by the autocatalytic deposition process.

[00139] The autocatalytic deposition solution comprises one or more metal ions. The metal ion (s) is / are deposited on the surface of the substrate to form a coating comprising metal during the autocatalytic deposition process. The metal ion (s) is / are usually selected from one or more of the vanadium, chromium, manganese, cobalt, nickel or copper ions. Preferably, deposition

41/75 autocatalytic comprises copper and / or nickel ions. Particularly preferably, the autocatalytic deposition solution comprises copper ions. For example, the autocatalytic deposition solution can comprise one or more of Cu2 +, Cu + and Ni2 +.

[00140] The autocatalytic deposition solution typically comprises a reducing agent. The choice of reducing agent will depend on the one or more metal ions in the solution and the nature of the substrate. Suitable reducing agents include, for example, one or more of iodate; oxyphosphorous ions such as phosphate, phosphite and hypophosphite ions; or borate.

[00141] The autocatalytic deposition solution is typically an aqueous solution. However, the autocatalytic deposition solution may comprise solvents other than water such as alcohols or ethers.

[00142] Usually, the primary solvent in the autocatalytic deposition solution is water. The autocatalytic deposition solution can also comprise one or more complexing agents and / or one or more stabilizers and / or one or more modifiers, the choice of which depends on the substrate and the materials to be deposited.

[00143] The structure of the coating formed by the autocatalytic deposition process is influenced by the concentration of metal and reducing ions on the substrate surface. A higher concentration of metal ions tends to increase the amount of material deposited by autocatalytic deposition. ("A higher concentration of metal ions" means a higher concentration of metal ions that are deposited on the surface during the autocatalytic deposition process). A higher concentration of metal ions also tends to increase the thickness of the coating formed. A higher concentration of metal ions also favors the formation of larger surface characteristics. For example, a higher concentration favors the formation of longer dowels and / or a higher density of dowels on the surface. Thus, the formation of a

The coating according to the invention (comprising a first region in which the average length of the pin is greater than that in a second region) can be obtained by providing variation in concentration of the autocatalytic deposition solution on the substrate surface. Thus, in some embodiments, the invention provides an autocatalytic deposition process which comprises: providing an autocatalytic deposition solution having a C1 concentration at a first substrate region; providing an autocatalytic deposition solution having a C2 concentration to a second region of the substrate; where C1 is greater than C2.

[00144] C1 and C2 are the concentrations of metal ions that can be deposited on the substrate in the solution or autocatalytic deposition solutions.

[00145] The structure of the coating formed by the autocatalytic deposition process is also influenced by the time by which autocatalytic deposition is allowed to occur. Generally, the longer the time allowed for autocatalytic deposition, the thicker the coating will be. Similarly, a longer deposition time favors the formation of larger surface characteristics. For example, a longer deposition time favors the formation of longer dowels and / or a higher density of dowels on the surface. Usually, the process involves providing the autocatalytic deposition solution with a substrate surface for at least 15 minutes. Also, usually, the process comprises providing the autocatalytic deposition solution to a surface of a substrate for a period of 12 hours or less. In one embodiment, the process of the invention comprises providing the autocatalytic deposition solution to a surface of a substrate for a period of 0.5 hour to 10 hours. For example, the period can be from 1 hour to 5 hours.

43/75 Of course, the autocatalytic deposition process can coat a heat exchange element in an advantageously short time.

[00146] In the context of an autocatalytic flow deposition process, reference to "providing autocatalytic deposition to a surface" should be taken to mean "draining the autocatalytic deposition solution onto a surface". That is, "to provide" should be taken to mean "to drain" or "to drain on". For example, where the process is a flow process, the process usually comprises draining the autocatalytic deposition solution on a substrate surface for at least 15 minutes. Also usually, the process comprises draining the autocatalytic deposition solution on a substrate surface for a period of 12 hours or less. In one embodiment, the process of the invention comprises draining the autocatalytic deposition solution on a substrate surface for a period of 0.5 hour to 10 hours. For example, the period can be from 1 hour to 5 hours.

[00147] In some embodiments, the autocatalytic deposition solution is not reconstituted during the autocatalytic deposition process. If the autocatalytic deposition solution is not reconstituted, the autocatalytic deposition solution will become depleted over time during the process of the invention. “Exhausted” means that the concentration of metal ions and reducers in the solution has dropped below its initial value (the value at the beginning of the process). The autocatalytic deposition solution can be said to be depleted of 5% since the concentration in the solution of metal ions to be deposited has dropped from at least 5%, to 95% or less of its original value. That is, if the autocatalytic deposition solution is suitable for depositing copper ions, the solution is said to be depleted of at least 5% since the concentration of copper ions in solution has dropped to 95% or less of its original value.

[00148] It should be noted that localized variations in concentration

44/75 can occur. Depletion is therefore considered in terms of the depletion of the autocatalytic deposition solution taken as a whole. For example, multiple samples of the autocatalytic deposition solution can be taken to indicate depletion in the solution taken as a whole. For example, in a bathing process, multiple samples can be taken from the bath while the autocatalytic deposition solution is stirred. In another example, where the process comprises draining autocatalytic deposition solution from a reservoir on the surface and back to the reservoir, one or more samples can be taken from the reservoir. Similarly, one or more samples can be taken from the solution flowing from the reservoir to the substrate. Alternatively or additionally, one or more samples can be taken from the solution by flowing from the substrate back to the reservoir.

[00149] Since the autocatalytic deposition solution is considerably depleted (for example 50% or more), the ion concentration in solution can be low and thus the deposition process will become undesirably slow. Consequently, the autocatalytic deposition process of the invention is usually continued until the autocatalytic deposition solution is exhausted by 50% or more.

[00150] In one embodiment, the process for producing a heat exchange element comprises providing an autocatalytic deposition solution to a substrate surface for a time T, where T is the time taken for the autocatalytic deposition solution to become exhausted. from 5 to 50%. That is, until the metal ion concentration to be deposited has dropped to 50% to 95% of its original value. Preferably, T is the time taken for the autocatalytic deposition solution to become exhausted by 10 to 40%, or no more than 30%. For example, the process for producing a heat exchange element (for example, a heat exchange element of the invention) may comprise draining an autocatalytic deposition solution over a

45/75 substrate surface for a time T.

[00151] The extent of depletion can be determined, for example, by noting the variation in time in the ion concentration or conductivity of the autocatalytic deposition solution. Thus, in one embodiment, the process involves measuring the ion concentration of the solution. In another mode, the process involves measuring the conductivity of the solution. "Measuring" may comprise monitoring the variation at a particular value over the course of the autocatalytic deposition process.

[00152] The converse would recognize that a wide variety of methods are appropriate to measure the ion concentration of the autocatalytic deposition solution, and thus determine the extent of depletion. These methods can be performed on one or more samples taken from the autocatalytic deposition solution. Alternatively they can be carried out online, for example, inside a bath or reservoir of autocatalytic deposition solution which is used for autocatalytic deposition. Suitable methods include optical methods such as colorimetry. Colorimetric methods are particularly well suited for Cu2 + ion solutions that are highly colored. Other methods include anodic extraction, voltammetry, ion chromatography and ion emission spectroscopy (eg, inductively coupled plasma optical emission spectroscopy, ICP-OES). So in some embodiments, the process involves measuring the ion concentration in the autocatalytic deposition solution, for example by any of the methods above. In the context of this measurement, “ion concentration” includes the concentration of metal ions that are deposited on the substrate during autocatalytic deposition.

[00153] Where the process comprises flowing autocatalytic deposition solution onto a substrate, ICP-OES can be used to analyze the autocatalytic deposition solution flowing over the substrate both before and after it contacts the substrate. This can reveal the concentration of

46/75 maximum and minimum ion to which the substrate is exposed. The ion concentration of the solution after it has contacted the substrate can be used to determine whether the solution flowing from the substrate back to a reservoir needs to be dosed with additional ions to adjust the concentration. In the flow process of the invention, the ion concentration in the reservoir can be measured periodically to determine whether the reservoir itself needs to be dosed with additional ions to adjust its concentration. Similarly, the ion concentration of a bath determined during a bathing process can be used to determine whether dosage is necessary to increase the ion concentration.

[00154] The desired pin length in the coating of the heat exchange element of the invention can be obtained by adjusting the initial ion composition of the autocatalytic deposition solution, and / or the time period for the deposition process to be allowed.

[00155] The amount of time for autocatalytic deposition to be allowed to occur affects not only the pin length in the coating formed by the process, but also the incidence of groups of pins in the coating. The longer the time for autocatalytic deposition to be allowed to occur, the greater the likelihood that groups of pegs will form. Thus, the process for producing a heat exchange element can be adjusted to form groups by increasing the amount of time for autocatalytic deposition to be allowed to occur.

[00156] The exact amount of time required to form groups will vary with the substrate, the composition of the autocatalytic deposition solution and the temperature.

[00157] The process can also be adapted to promote the formation of groups by repeating the process more than once. A process for producing a heat exchange element of the invention having a coating comprising groups can therefore comprise:

47/75 (i) carrying out the process of autocatalytic deposition of the invention as described herein; (ii) activating the coating surface of the heat exchange element thus produced, for example by immersing the heat exchange element in a PdCl2 solution to form an activated heat exchange element; and (iii) repeating the autocatalytic deposition process of the invention. Bathing process

[00158] In one aspect, the process of the invention is a bathing process (to produce a heat exchange element of the invention). In a bathing process, the autocatalytic deposition solution is provided on the surface of a substrate by placing the substrate in an autocatalytic deposition solution bath. In some embodiments, the autocatalytic deposition solution is agitated during the autocatalytic deposition process. Stirring the autocatalytic deposition solution reduces variation in the composition of the autocatalytic deposition solution (such as variation in local metal ion concentration) across the entire bath.

[00159] In some embodiments, the entire surface or all surfaces of the substrate are coated during the bathing process. In other embodiments, part of the substrate surface or surfaces are protected before the substrate is placed in the bath in such a way that the protected parts are not coated. In still other embodiments, part of the surface or surfaces of the substrate are not able to adhere to the coating and thus are not coated during immersion in the bath.

[00160] In some embodiments, multiple heat exchange elements according to the invention can be produced simultaneously by placing two or more substrates in a bath.

[00161] In one aspect, the bathing process can be used to provide a coating having a first region where the length of the

48/75 pin is S1 and a second region in which the pin length is S2 adjusting the time span of the autocatalytic deposition process. For example, the substrate can be slowly immersed in the bath in such a way that the part or parts of the substrate surface that enter the bath first experience a longer exposure to the autocatalytic deposition solution than part or parts of the substrate surface that enter in the bath subsequently. The coated part or parts of the heat exchange element so produced that spent the longest time exposed to the autocatalytic deposition solution will typically have the longest average pin length. If the substrate is immersed in the bath at a constant low speed, the coating produced on the substrate may show a slight variation in the average pin length. In another example, a protective coating can be removed from a portion of the substrate partially through the autocatalytic deposition process, leaving that portion of the substrate to be coated during the remaining immersion time.

[00162] In another aspect, the bathing process can be used to provide a coating having a first region in which the pin length is S1 and a second region in which the length of the pin is S2 allowing the concentration of the solution of autocatalytic deposition varies across the surface. Where a substrate comprises a channel into which the autocatalytic deposition solution can flow, solution will usually flow into said channel when the substrate is immersed in the solution bath. As the solution flows into the channel, autocatalytic deposition occurs and the concentration of metal ions to be deposited in the solution flowing through the channel is decreased. Fresh solution into the channel from the bath over time, but it always has the highest concentration of metal ions to be deposited (roughly corresponding to the highest metal ion concentration in the surrounding bath) at the entrance to the flow channel. So a gradient of

49/75 concentration is formed, a higher concentration of metal ions to be deposited being found at the entrance or entrances to the flow channel and a lower concentration of metal ions to be deposited occurring more in the flow channel. This effect is particularly pronounced where the flow channel is long and / or narrow. The longest dowels are formed where the concentration of metal ions to be deposited is the highest; shorter dowels are formed anywhere. Flow process

[00163] In one aspect, the process for producing a heat exchange element comprises draining an autocatalytic deposition solution on a substrate surface. In some embodiments, this autocatalytic flow deposition process produces a heat exchange element according to the invention.

[00164] Flowing an autocatalytic deposition solution on a substrate surface comprises providing a flow of autocatalytic deposition solution moving on a substrate surface at a non-zero flow rate. The solution flow is understood to be “on” a substrate surface if it is in contact with said substrate surface. The flow can be predicted on one or more surfaces of the substrate, for example external and / or internal surfaces of the substrate, an internal surface being, for example, the surface of a channel (for example, a tube) passing through the substrate. The solution flow can be provided over the entire substrate or over only part of the substrate.

[00165] In an autocatalytic deposition process that occurs in a static environment such as a bath, there is usually no direction resulting from the flow of autocatalytic deposition solution over any point on the substrate surface. The solution is typically agitated to provide solution movement within the bath, but that movement typically has no resulting direction in the course of the autocatalytic deposition process

50/75 static (eg, autocatalytic deposition bath process). In a static environment, the flow direction of the autocatalytic deposition solution over any point on the substrate surface can vary frequently and randomly during the course of the autocatalytic deposition process.

[00166] The autocatalytic deposition flow process provides a direction resulting from the flow of autocatalytic deposition solution over any point on the substrate surface subjected to the flow. The flow direction of the autocatalytic deposition solution over any point on the substrate surface subjected to the flow typically does not vary during the deposition process. The flow direction is usually constant during deposition in an autocatalytic flow. However, the predicted flow direction for the substrate surface can be deliberately varied during the course of the flow process.

[00167] As the expert will recognize, the precise details of the process used to provide a flow of autocatalytic deposition solution on a substrate (for example the apparatus) may vary. In a particularly simple implementation, the process may comprise pouring the autocatalytic deposition solution from a container onto a substrate surface. More usually, the process will comprise generating flow of autocatalytic deposition solution using a flow generator (for example, a pump), and providing flow of autocatalytic deposition solution to a substrate surface via a conduit.

[00168] In the flow process of the invention, the autocatalytic deposition solution flows over the surface of a substrate. The path taken by the solution flow over the substrate surface is referred to as "the solution flow path". In some embodiments, the coating is formed over the entire solution flow path. In other modalities, such as those in which part of the substrate on the solution flow path is

51/75 masked to prevent autocatalytic deposition, the coating is formed over part of the solution flow path. Preferably, the coating is formed over the entire solution flow path.

[00169] The flow of autocatalytic deposition solution on a substrate surface creates a concentration gradient on said substrate surface. The autocatalytic deposition solution provided on the surface contains metal ions to be deposited. The concentration of such metal ions in the autocatalytic deposition solution before it is provided on the surface of the substrate can be referred to as C1. Once the autocatalytic deposition solution contacts the substrate (that is, once the solution contacts a part of the substrate surface that is likely to be coated by autocatalytic deposition) autocatalytic deposition will occur. This causes metal ions to come out of the solution and reduces the concentration of metal ions to a lower value than C1. So as the autocatalytic deposition solution drains over the substrate, it is depleted. A concentration gradient forms. The concentration gradient is located along the solution flow path. The concentration of metal ions in the solution decreases as the distance that the solution has moved over the substrate surface increases.

[00170] As discussed above, the concentration of metal ions tends to affect the thickness of the coating and the size of surface characteristics formed by the process of the invention. Thus, the process of the invention can provide a coating that varies in thickness, and / or that shows characteristics of variable surface size along the path taken is by the flow of solution over the substrate surface. A characteristic surface is a structure in the coating such as a pin. In one embodiment, the coating thickness decreases along the solution flow path. The coating may be thicker at or near one end of the solution flow path, usually the end at which

52/75 the autocatalytic deposition solution first contacts the surface of the substrate during the process of the invention. In another embodiment, the size of the surface characteristics decreases along the solution flow path. The surface characteristics can be greatest at or near one end of the solution flow path, usually the end where the autocatalytic deposition solution first contacts the surface of the substrate during the process of the invention.

[00171] The formation of longer dowels is favored by a higher concentration of metal ions in the autocatalytic deposition solution. Thus, the autocatalytic flow deposition process can produce a coated heat exchange element, the coating comprising a first region in which the average pin length is S1 and a second region in which the average pin length is S2. The autocatalytic deposition flows along a route through the surface or surfaces of the substrate, which is the flow path of the autocatalytic deposition solution. Typically, the first region is at or near the end of that flow path, and the second region is also over the flow path.

[00172] In some embodiments, the process comprises draining an autocatalytic deposition solution on a substrate surface along a single flow path in a single direction. In other embodiments, the flow process comprises a step of varying the flow direction. The flow process can additionally or alternatively comprise a step of varying the flow path of the autocatalytic deposition solution on the substrate surface or surfaces.

[00173] Where the process comprises varying (for example, inverting) the flow direction or the flow path of the autocatalytic deposition solution, the process comprises changing the direction of the concentration gradient on the substrate surface. Thus, the position or region on the surface of the

53/75 substrate on which the thickest coating or the greatest characteristics (for example, the longest dowels) form will move to the new position, corresponding to the new position of the substrate surface that is in contact with the solution autocatalytic deposition having a concentration gradient, C1. This can cause the creation of another region, a third region, having an average pin length different from or the same as that in the first or second regions.

[00174] For example, where the flow of autocatalytic deposition solution is provided along a flow channel, the flow channel may comprise a thicker coating and / or greater characteristics at or near the end of the flow channel where the autocatalytic deposition solution entered the channel, and a thinner coating and / or smaller features further along the channel. If the process comprises draining an autocatalytic deposition solution at one end of a flow channel of a substrate (the inlet) and out of another end of the flow channel (the outlet), the thickest coating / greatest features will be located at or near the entrance and the thinnest coating / smallest features will be located at or near the exit. However, if the flow direction is reversed during the autocatalytic deposition process, the thinnest coating / the smallest features will be located at or near the middle of the channel and the thickest coating / the largest features will be located at or near each end of the channel.

[00175] For example, where the flow of autocatalytic deposition solution is provided along a flow channel, the first region may be located at or near one end of the flow channel and the second region may be located further along the flow channel. Thus, the flow channel may comprise longer dowels at or near the end of the flow channel where the autocatalytic deposition solution entered the channel,

54/75 and shorter dowels further along the channel. If the process comprises draining an autocatalytic deposition solution at one end of a substrate flow channel (the inlet) and out of another end of the flow channel (the outlet), the longest dowels will be located at the or near the entrance and the shortest pegs will be located at or near the exit.

[00176] However, if the flow direction is reversed during the autocatalytic deposition process, the shortest pegs will be located at or near the middle of the channel and the longest pegs will be located at or near each end of the channel.

[00177] The thickness of the coating and / or the size of the characteristic, for example, the length of the pin, produced in the process of autocatalytic deposition can be adjusted by controlling the flow. The faster the flow, the thinner the coating and / or the less the surface characteristics produced. For example, the faster the flow, the smaller the dowels produced. Additionally, increasing the flow reduces the size of the concentration gradient and thus reduces the variation in coating thickness and / or size of the characteristic (for example, pin length) along the flow path of the autocatalytic deposition solution.

[00178] In some embodiments, the process of the invention comprises continuously providing fresh solution for the substrate. In other embodiments, the autocatalytic deposition solution is recycled. In such an embodiment, the process comprises: draining the autocatalytic deposition solution from a reservoir of autocatalytic deposition solution on the substrate surface; and returning the autocatalytic deposition solution to said reservoir.

[00179] "Reservoir" means a volume of deposition solution

55/75 autocatalytic. Typically the composition of the autocatalytic deposition solution in the reservoir is not adjusted by an external source. For example, the reservoir is typically not filled from an external source of autocatalytic deposition solution (during the autocatalytic deposition process).

[00180] Generally, the autocatalytic deposition solution is provided on the surface of the substrate by pumping.

[00181] During autocatalytic deposition, the process generally comprises draining autocatalytic deposition solution on the substrate surface at a flow rate of at least 10 mL / min. Typically, the process comprises flowing the autocatalytic deposition solution onto the substrate surface at a flow rate of at least 50 ml / min or at least 100 ml / min, preferably at least 1 L / min. The aforementioned flows are generally maintained for at least one minute, for example for at least ten minutes, for example, for at least 30 minutes or 1 hour.

[00182] The process may involve varying the flow rate of the autocatalytic deposition solution. In one embodiment, the process comprises: draining an autocatalytic deposition solution on a substrate surface at a first flow rate F1; and draining an autocatalytic deposition solution on said substrate surface at a second flow rate F2.

[00183] F1 and F2 are typically different. F1 and F2 are typically at least 10 ml / min. For example, F1 and F2 can be at least 50 ml / min or 100 ml / min. In one mode, F2 is greater than F1. For example, F2 can be twice as much as F1 for example, F2 can be 2 to 50 times greater than F1. In another mode, F1 is greater than F2. For example, F1 can double F2. for example, F1 can be 2 to 50 times greater than F2. This last modality can be useful to ensure that the substrate is quickly covered with an autocatalytic deposition solution, before the flow rate is adjusted to a

56/75 appropriate deposition flow.

[00184] In some embodiments, F2 is large enough to reduce the adhesion of hydrogen bubbles from the substrate during autocatalytic deposition. F2 can be big enough to force hydrogen bubbles away from the substrate surface during autocatalytic deposition. In some embodiments, both F1 and F2 are large enough to reduce the adherence of hydrogen bubbles from the substrate during autocatalytic deposition, and / or to force hydrogen bubbles away from the substrate surface during autocatalytic deposition. In some embodiments, F1 and / or F2 are large enough to force autocatalytic deposition solution into a flow channel of the substrate.

[00185] In one aspect, the process comprises: pumping an autocatalytic deposition solution from a reservoir over a substrate surface at a first flow rate F1 and returning the autocatalytic deposition solution to the reservoir; and pumping an autocatalytic deposition solution from a reservoir on said substrate surface at a second flow rate F2 and returning the autocatalytic deposition solution to the reservoir.

[00186] In some cases, the process comprises draining an autocatalytic deposition solution on a substrate surface at a first flow rate F1 for a period of initiation before varying the flow rate to F2. For example, the solution can be provided at an F1 flow rate until all substrate surfaces that are to be coated are covered with the solution.

[00187] In some embodiments, the process comprises pumping the autocatalytic deposition solution to make the autocatalytic deposition solution flow over a substrate surface. For example, the process may comprise pumping solution from a reservoir to create a flow of autocatalytic deposition solution on a surface of the

57/75 substrate. Alternatively or additionally, the process may comprise pumping autocatalytic deposition solution out of the substrate, for example out of one or more flow channels in the substrate. A suitable pump is any type of device suitable for creating a flow of a fluid, particularly a liquid.

[00188] In some embodiments, the substrate comprises one or more flow channels, and the process comprises draining an autocatalytic deposition solution through said one or more flow channels. That is, the solution flow path comprises one or more of said flow channels. In some embodiments, the substrate comprises a flow channel, and the process comprises flowing an autocatalytic deposition solution through said flow channel.

[00189] Flow channel means a path through which fluid can pass through the substrate. In these embodiments, the process comprises contacting the autocatalytic deposition solution with the surface of said flow channel, usually the internal surface of said flow channel.

[00190] In one aspect of this embodiment, the process may involve coating one or more flow channels that are suitable for and / or intended to charge refrigerant. In a preferred aspect of this embodiment, the process comprises flowing an autocatalytic deposition liquid only onto a surface or surfaces of the substrate which are to contact a refrigerant. For example, the substrate may be suitable for use as a fluid-fluid heat exchanger having a region for charging the refrigerant fluid (a fluid to which heat is transferred from the heat exchange element) and a region for the cooling fluid. heat transfer (a fluid that transfers heat to the heat exchange element) and the process comprises flowing autocatalytic deposition solution only onto one or more surfaces that are suitable for and intended to contact a refrigerant (i.e. the refrigerant fluid). This modality is advantageous because it ensures that no

58/75 autocatalytic deposition solution is wasted on coating surfaces not intended for heat transfer.

[00191] The autocatalytic flow deposition process is advantageous because it allows the autocatalytic flow deposition solution to be controlled in such a way that only the substrate surfaces that are intended to be coated can come into contact with the solution. This reduces waste of the solution. The solution flow can be controlled, for example, by connecting a solution reservoir to the inlet or inlets of these flow channels that are intended to be lined. Other flow channels can be blocked. Advantages of the autocatalytic flow deposition process (flow process)

[00192] As explained above, the flow process of the invention reduces the effect of hydrogen bubbles on the coating of a substrate by autocatalytic deposition. The flow process therefore provides a heat exchange element that is robust, durable and resistant to corrosion.

[00193] The flow process has several other advantages. For example, the flow process can advantageously be used to coat substrates having small depressions (for example, small holes or channels in them) that may not be properly coated by an autocatalytic deposition bath process. Where a substrate having small depressions is placed in a liquid bath, air pockets can become trapped in these depressions, preventing the autocatalytic deposition solution from contacting the substrate hidden under the trapped air. The flow process of the invention is generally carried out at a flow high enough to force these air pockets away from the surface of the substrate and thus to ensure that all parts of the substrate that are exposed to the autocatalytic deposition solution are coated. The flow process of the invention is therefore suitable for coating, for example,

59/75 substrates comprising very narrow channels, particularly substrates suitable for use in cooling electronics.

[00194] Another advantage of the deposition process in autocatalytic flow is that it reduces waste of deposition solution. In the flow process of the invention, the flow path of the autocatalytic deposition solution on the substrate surface can be controlled. Consequently, autocatalytic deposition solution can be provided only for those parts of the surface that are to be coated in the process of the invention. In contrast, in a bathing process, the entire substrate is usually immersed in the autocatalytic deposition solution and any parts of the substrate surface that are not intended to be coated are masked by a protective coating. This can result in autocatalytic deposition occurring on the protective coating, wasting material.

[00195] Another advantage of the flow process of the invention is that several process parameters can be controlled in order to adjust the obtained surface structure. For example, the flow rate of the autocatalytic deposition solution on the surface, the temperature at which the process is carried out, the composition of the autocatalytic deposition solution and so on can all be changed to adjust the structure of the coating produced by the process. Particularly advantageous heat exchange structures are obtained where the autocatalytic deposition solution comprises copper ions, due to the high conductivity of the resulting copper coating.

[00196] The process can create a wide range of coatings suitable for heat transfer under various conditions. For example, it can be used to produce a heat exchange element suitable for transferring heat to a fluid, preferably to a liquid. In one embodiment, the process is to produce a heat exchange element with a coating suitable for use in an evaporative heat exchanger (a

60/75 heat exchanger that cools a fluid by heating another fluid to the evaporation point). In another embodiment, the process is to produce a heat exchange element with a suitable coating to transfer heat by boiling a liquid.

[00197] A particular advantage of the flow process of the invention is that it can be used to retrofit a coating autocatalytically deposited in an existing heat exchanger in situ and without disassembly. Aspects of the autocatalytic flow deposition process

[00198] The following specific aspects of the autocatalytic flow deposition process are provided.

[00199] 1. A process for producing a heat exchange element comprising a substrate and a coating, wherein: the coating comprises a metal; and the process comprises draining an autocatalytic deposition solution on a substrate surface.

[00200] 2. A process according to aspect 1 in which the process is carried out at a temperature of 20 ° C to 120 ° C.

[00201] 3. A process according to aspect 1 or aspect 2 in which the autocatalytic deposition solution is an aqueous solution.

[00202] 4. A process according to any preceding aspect in which the autocatalytic deposition solution comprises copper and / or nickel ions.

[00203] 5. A process according to any preceding aspect in which the process comprises: draining the autocatalytic deposition solution from a reservoir of autocatalytic deposition solution on the substrate surface; and return the autocatalytic deposition solution to said reservoir.

61/75

[00204] 6. A process according to any preceding aspect in which the process comprises draining the autocatalytic deposition solution on the substrate surface for a period of 0.5 hour to 10 hours.

[00205] 7. A process according to aspect 5 or aspect 6 in which the process comprises draining the autocatalytic deposition solution over the substrate surface for a time T, where T is the time taken for the autocatalytic deposition solution if become exhausted from 5 to 50%.

[00206] 8. A process according to any preceding aspect in which the process comprises monitoring the ion concentration in the autocatalytic deposition solution.

[00207] 9. A process according to any preceding aspect in which the process comprises: draining an autocatalytic deposition solution on a substrate surface at a first flow rate F1 and draining an autocatalytic deposition solution on said substrate surface at a second flow flow F2.

[00208] 10. A process according to aspect 9 in which F2 is greater than F1.

[00209] 11. A process according to any preceding aspect in which the process comprises pumping the autocatalytic deposition solution to make the autocatalytic deposition solution flow over a substrate surface.

[00210] 12. A process according to any preceding aspect in which the substrate comprises a flow channel, and the process comprises flowing an autocatalytic deposition solution through said flow channel.

[00211] 13. A process according to any preceding aspect wherein the process comprises: (i) providing an acid to a substrate surface, and / or (ii) activating a substrate surface,

62/75 in which steps (i) and / or (ii) are carried out before flowing an autocatalytic deposition solution on the substrate surface according to any preceding aspect.

[00212] 14. A process according to any preceding aspect wherein the process comprises applying a surface layer, preferably a surface layer comprising nickel, to a heat exchange element produced according to any one of aspects 1 to 13 .

[00213] 15. A heat exchange element that can be obtained or obtained by a process according to any preceding aspect. Additional process steps

[00214] Usually, before the autocatalytic deposition, the surface of the substrate is prepared by exposure to acid and an activation solution. Thus, in one embodiment, the process for producing a heat exchange element comprises: (i) providing an acid to a surface of the substrate; and / or (ii) activate a substrate surface, in which steps (i) and / or (ii) are performed before providing an autocatalytic deposition solution on the substrate surface.

[00215] The function of the acid is typically to clean and optionally also attack the surface. Suitable acids include sulfuric acid, hydrochloric acid or nitric acid. Where the substrate is a steel substrate, the acid used is preferably sulfuric acid. Where the substrate is a copper substrate, the acid used is preferably hydrochloric acid. The acid used is typically a strong acid, for example 20% acid or above. Generally, the acid exposure step is carried out at room temperature or above, for example from 20 ° C to 120 ° C, usually from 50 ° C to 100 ° C. Usually the substrate is exposed to acid for one minute or more, preferably from 1 minute to 1 hour. The substrate is usually rinsed

63/75 with water after exposure to acid.

[00216] The step of activating the surface may involve providing a solution containing metal on the surface, for example an aqueous solution comprising metal ions. An exemplary activation solution is PdCl2 solution. Activation is generally carried out at a temperature between 0 ° C and 100 ° C, usually at room temperature. Typically the substrate is rinsed after the activation step and before autocatalytic deposition.

[00217] Other steps that can be performed before the autocatalytic deposition process include, for example, applying a protective mask to a part of the substrate surface to prevent application of coating to that part of the substrate.

[00218] In some embodiments, the process for producing a heat exchange element of the invention comprises one or more steps carried out after the autocatalytic deposition process. In one embodiment, the process comprises applying a surface layer, preferably a surface layer comprising a metal, for example, nickel or tin (preferably nickel), to a heat exchange element produced by a method as described herein. Experimental protocol

[00219] Exemplary methods of applying a coating to a substrate to produce a heat exchange element of the invention are described below.

1. Bathing process

[00220] i. Any part of the substrate that is not intended to be coated is protected, for example by applying break lacquer to it. This step may be unnecessary if it is intended to apply coating to all parts of the substrate that are going to be placed in the bath.

[00221] ii. The substrate is preheated to a temperature of 80 ° C

64/75 by submerging the substrate in an 80 ° C water bath.

[00222] iii. The substrate is then transferred to a 20% sulfuric acid bath at 80 ° C and left for 15 minutes. The substrate is then rinsed in deionized water.

[00223] iv. The substrate is placed in a PdCl2 solution (1 g L-1) for 2 minutes. The substrate is then rinsed in deionized water.

[00224] v. The substrate is then reheated to 80 ° C as in (ii).

[00225] vi. The substrate is then placed in a nanoFLUX autocatalytic deposition solution bath at 75 ° C for two hours. The nanoFLUX solution comprises 0.01M to 0.1M CuSO4, 0.001M to 0.01M NiSO4, 0.1M to 0.5M NaH2PO2, 0.001M to 0.1M Na3C6H5O7, 0.1 lM to 1M HBO3, Janus Green 0- 700 ppm, PVP 0-200 ppm, CTAB 0-300 ppm, SBS 0-500 ppm and 0 to 200 ppm PEG. The solution in the bath is stirred continuously during this time. The bath contains at least 10 liters of solution per square meter of substrate surface to be coated.

[00226] vii. The coated substrate is removed from the bath and rinsed in deionized water.

[00227] viii. The coated substrate is dried in an oven.

[00228] The above experimental protocol can be modified to produce a coating comprising groups by adjusting step (vi) in such a way that:

[00229] vi. The substrate is then placed in a nanoFLUX autocatalytic deposition solution bath at 75 ° C for four hours. The solution in the bath is stirred continuously during this time. The bath contains at least 50 liters of solution per square meter of substrate surface to be coated.

2. Flow process

[00230] This flow process protocol is for coating one square meter of a substrate surface.

65/75

[00231] i. Any part of the substrate that is not intended to be coated is protected, for example, by applying break lacquer to it. This step may be unnecessary if the intended flow path of the autocatalytic deposition solution contacts only the part or parts of the substrate that are intended to be coated.

[00232] ii. The substrate is preheated to a temperature of 80 ° C by submerging the substrate in an 80 ° C water bath.

[00233] iii. The substrate is then transferred to a 20% sulfuric acid bath at 80 ° C and left for 15 minutes. The substrate is then rinsed in deionized water.

[00234] iv. The substrate is placed in a PdCl2 solution (1 g L-1) for 2 minutes. The substrate is then rinsed in deionized water.

[00235] v. The substrate is then reheated to 80 ° C as in (ii).

[00236] vi. At least 10 liters of nanoFLUX autocatalytic deposition solution as defined above are heated to 75 C in a reservoir. The autocatalytic deposition solution is continuously pumped from the reservoir over a substrate surface and returned to the reservoir for a period of two hours.

[00237] vii. The coated substrate is rinsed in deionized water.

[00238] viii. The substrate coated substrate is dried in an oven.

[00239] The above experimental protocol can be modified to produce a coating comprising groups adjusting step (vi) in such a way that:

[00240] vi. At least 50 liters of nanoFLUX autocatalytic deposition solution are heated to 75 ° C in a reservoir. The autocatalytic deposition solution is continuously pumped from the reservoir on a substrate surface and returned to the reservoir for a period of four hours.

3. Flowing process to coat a flow channel of a

66/75 heat exchanger

[00241] This flow process protocol is for coating a flow channel of a heat exchanger. The heat exchanger comprises a first flow channel having two ends (which is intended to be coated) and a second flow channel having two ends (which is for carrying a heat transfer fluid). In this protocol, the flow channel to be coated has a surface area of one square meter.

[00242] i. The second flow channel of the substrate (ie, the flow channel that must not be coated) is connected at each end to a water source that is maintained at 80 ° C. Water is pumped through the second flow channel continuously.

[00243] ii. The first flow channel of the substrate (ie, the flow channel that must be coated) is connected at each end to a 20% sulfuric acid bath reservoir at 80 ° C. Sulfuric acid from the reservoir is pumped through the first flow channel for 15 minutes.

[00244] iii. The pumping of water and acid is stopped.

[00245] iv. All acid is drained out of the first flow channel.

[00246] v. The first flow channel is then connected to a source of deionized water. Deionized water is pumped through the channel for 5 minutes or until the water that leaves the first flow channel runs clear. All water is then drained out of the first flow channel.

[00247] vi. The first flow channel is connected at both ends to a source of PdCl2 solution (1 g L-1) at room temperature. PdCl2 solution is pumped in the first flow channel and left for 2 minutes. Every PdCl2 solution is then drained from the first flow channel.

[00248] vii. The substrate is then rinsed in deionized water as in step (v).

[00249] viii. Pumping water at 80 ° C through the second channel

67/75 flow is restarted.

[00250] ix. The first flow channel is connected at both ends to a reservoir containing at least 10 liters of nanoFLUX autocatalytic deposition solution (as defined above) maintained at 75 ° C. The autocatalytic deposition solution is continuously pumped from the reservoir into the flow channel slowly, such that the first flow channel is filled with the autocatalytic deposition solution after a period of five minutes or more (for example 0, 1 L min-1 for a heat exchanger having a volume of 1 L).

[00251] x. The pumping rate of the autocatalytic deposition solution is increased by a factor of ten (eg to 1.0 L min-1). This pumping rate is maintained for two hours.

[00252] xi. The pumping of autocatalytic deposition solution is stopped and any autocatalytic deposition solution is drained out of the first flow channel.

[00253] xii. The coated substrate is rinsed in deionized water as in step (v).

[00254] xiii. The coated substrate is dried in an oven.

[00255] The above experimental protocol can be modified to produce a coating comprising groups in the first flow channel: adjusting step (ix) in such a way that the first flow channel is connected at both ends to a reservoir containing at least 50 liters of nanoFLUX autocatalytic deposition solution maintained at 75 ° C; and adjusting step (x) in such a way that the highest pumping rate is maintained for a period of four hours.

[00256] The converse will recognize that the specific characteristics of the above processes, such as temperature and pumping rate, can be varied.

68/75 Examples

1. Preparation of sample coatings.

[00257] Small copper specimens were coated according to the method of the invention using the bath process protocol (protocol 1), above. The coating times were varied to obtain different pin lengths. At least 100 ml of nanoFLUX solution was used in each case. The resulting heat exchange elements according to the invention were images using scanning electron microscopy (SEM). The results are shown in Figure 3 (common protocol) and Figure 5 (protocol modified to produce groups).

[00258] Figure 3 (a) shows a coating comprising pins 1 to 3 μm in length, obtained by subjecting the copper specimens to a bath of nanoFLUX solution for up to 1 hour. Figure 3 (b) shows a coating comprising pins 4 to 5 μm in length, obtained by subjecting the copper specimen to a bath of nanoFLUX solution for approximately 2 hours. Figure 3 (c) shows a coating comprising pegs 8 to 10 μm in length, subjecting the copper specimen to a bath of nanoFLUX solution for up to 4 hours.

[00259] Figure 5 (a) shows a coating comprising pins of approximately 7 μm in length arranged in groups. This coating was obtained by subjecting the copper specimen to a bath of nanoFLUX solution for approximately 3 hours. This was reactivated and then coated for another 3 hours. Figure 5 (b) also shows a coating of pegs arranged in groups.

[00260] A steel wire mesh specimen comprising wires 75 μm in diameter was also coated according to the method of the invention using the bath process protocol (protocol 1), above. The wire mesh was subjected to the nanoFLUX solution bath for

69/75 approximately 3 hours. The product was converted to an image using SEM and the results are shown in Figure 6.

2. Preparation of a heat exchanger.

[00261] A brazed plate heat exchanger made of 316 stainless steel with a copper braze, having a channel for a heat transfer fluid and a channel for a refrigerant, was coated according to the method of the invention using the process flow to coat a channel of a heat exchanger (protocol 3 above). This produced a heat exchanger having a coating comprising pegs approximately 3 μm in length along the inside of the channel for a refrigerant. A SEM image of the interior of this channel is shown in Figure 4.

[00262] This example shows that the process of the invention can be used to retrofit a heat exchange element of the invention into an existing heat exchanger. The process can be used to coat all or part of an existing heat exchanger to prepare a product according to the invention.

3. Heat transfer efficiency of the free-boiling heat exchange element.

[00263] A heat exchange element was produced according to the process of the invention, by coating a copper specimen in accordance with protocol 2, above, except that in step I saw the specimen was placed in a bath of nanoFLUX solution at 75 ° C for up to 4 hours. The heat exchange element was similar to that shown in Figure 3 (c) (comprising pegs 8 to 10 μm in length). It was tested for its ability to transfer heat to an organic refrigerant in a free-boiling experiment. The free-boiling experiment is described in “Compound effect of EHD and surface roughness in pool boiling and CHF with R-123”, Ahmad et al., Applied Thermal Engineering, vol. 31, pp.

70/75 1994-2003, 2011, and in “Pool boiling on Modified Surfaces Using R-123”, Ahmad et al., Heat Transfer Engineering, Vol 35, Issue 16-17, 2014. The results are shown in Figures 7 and 8.

[00264] Figure 7 shows the heat flow in kW m-2 from a surface for an organic refrigerant as a function of wall overheating (ΔΤc). Results are given for a coated copper heat exchange element as shown in Figure 3 (c) and for a polished oxygen-free copper surface. The results for the heat exchange element according to the invention are shown in blue and appear on the abruptly inclined line to the left of the graph. The results for the polished surface are shown in black and the most flattened line appears along the bottom of the graph. Figure 7 shows that the element of the invention can obtain a high heat flow from the refrigerant element while maintaining a low superheat. Even when a high flow of heat is provided to the surface, the element transfers heat to the refrigerant so efficiently that its temperature does not rise much above that above the refrigerant. The surface worked so well that the heating units providing heat to the surface could not keep up with the rapid loss of heat from the surface. In contrast, the polished surface transfers heat to the refrigerant slowly. The heat flow to the surface heats the polished surface to a high temperature above that of the refrigerant as heat dissipates from the surface and to the refrigerant slowly.

[00265] Figure 8 shows the heat flow in kW m-2 from a surface for an organic refrigerant as a function of wall overheating (ΔΤc) for several different surfaces: (i) a polished surface (the black line with the most flat slope). (ii) a surface coated with a copper coating such as

71/75 such as that described in WO2014 / 064450, comprising copper surface structures of the order of 500 nm in height (green line, almost flat to a ΔΤc of approximately 11 ° C and then rising abruptly). (iii) a heat exchange element according to the invention comprising a copper substrate coated with a coating comprising copper pegs 1 μm long (red line, almost flat to an ΔΤc of approximately 8 ° C and then rising abruptly). (iv) a heat exchange element according to the invention comprising a copper substrate coated with a coating comprising copper plugs 10 μm long (blue line, rising abruptly from ΔΤc of approximately 2 ° C).

[00266] The heat exchange elements of the invention produced a very high heat flow, to the point where the test equipment failed. Furthermore, the heat exchange elements of the invention (particularly the one having 10 μm long pins) maintained a very low superheat even at a high heat flow, illustrating the excellent efficiency of heat transfer from the surface to the refrigerant.

4. Heat transfer efficiency of the boiling heat exchange element in flow.

[00267] A heat exchange element according to the invention comprising a thin metal tube as a substrate was prepared by the autocatalytic flow deposition process of the invention. A coating was provided on the inner surface of the tube just like the coating shown in Figure 4. An organic refrigerant was drained through the tube as it was heated. The flow rate of the organic refrigerant was adjusted to be 200 kg m-2 s-1, 300 kg m-2 s-1, 400 kg m-2 s-1 and 500 kg m-2 s-1 at a time. The heat transfer coefficient at each flow was measured The heat transfer coefficient of an uncoated tube at each flow

72/75 was also measured. The experimental protocol for measuring the heat transfer coefficient is described in “Flow Boiling Heat Transfer In A Vertical Smalk-Diameter Tube: Effect Of Different Fluids And Surface Characteristics”, Al-Gaheeshi et al., Conference: Proceedings of the 4th International Forum on Heat Transfer, IFHT2016, November 2-4, 2016, in Sendai, Japan. The heat transfer coefficient in W m-2 K-1 is a measure of a measurement of heat transfer efficiency. A high heat transfer coefficient shows that the heat exchange element transfers heat more efficiently.

[00268] The results are shown in Figure 9. As can be seen in the Figure, the coated tube had a higher heat transfer coefficient than the uncoated tube at each flow. The coating to produce a heat exchange element according to the invention, therefore, improved the heat through the surface.

5. Variation in deposition time.

[00269] The autocatalytic deposition process of the invention can be adjusted to vary the size of the deposited structures. Figure 10 shows the approximate height of the pin (upper dashed line) and radius of the pin at the base (lower full line) obtained by varying the time for a substrate to be subjected to the autocatalytic deposition solution, for example, to a flow of deposition solution. autocatalytic. The length of the pin increases with time, as does the radius of the base.

[00270] The pins are approximately conical in shape. The length and radius above are obtained by approaching the pin of a cone whose axis passes through the base of the cone at right angles. The base of the cone is the circle in the plane at the base of the shortest side of the cone. Thus, the length of the pin is the approximate axis length of the cone from the base to the tip, and the radius of the base is the radius of the circular base in this approach.

[00271] It was noticed while carrying out these experiments that the

73/75 angle of the cone (ie the angle that the side of the cone makes with the base) is not affected by the time frame for autocatalytic deposition to be carried out. Thus, the process of the invention can produce long, sharp pegs; the dowels do not become less sharp as they become longer during the process of the invention.

6. Comparison of coated versus uncoated heat exchanger heat transfer efficiency

[00272] A coated evaporator (ie, a heat exchanger) was prepared according to Protocol 3 above. As explained in that protocol, the flow channel from the evaporator to receive refrigerant was coated according to the process of the invention. However, the other flow channel of the evaporator to receive water was not coated.

[00273] An uncoated evaporator was also obtained for comparative purposes. This uncoated evaporator was identical in structure to the other evaporator, but no flow channel of the uncoated evaporator was coated according to the invention.

[00274] The coated and uncoated evaporators were incorporated into test equipment each time. The test equipment is shown in Figure 11.

[00275] The system used R245fa as the working refrigerant. The evaporator's ability to transfer heat from water to the refrigerant was then assessed. Coolant and water were circulated through the evaporators according to the normal use of a heat exchanger. During the experimental tests carried out in this regard, the rate of heat transfer from water to the refrigerant was assessed. Multiple experiments were carried out to test the coating of the coated evaporator under different flow rates. The experiments were repeated using the uncoated evaporator, but using the same flow parameters to enable a comparison.

74/75

[00276] Based on the experiments performed, the heat transfer coefficient (AU) was calculated. This is the proportionality constant between the heat flow through a surface and the temperature differential that existed across the surface. Thus, where the surface has a high heat transfer coefficient, it can efficiently transfer heat even where the temperature difference across the two sides of the surface is small. Surfaces with lower heat transfer coefficients require a greater difference in temperature across the surface (ie, a greater driving force) before the surface will allow an appreciable flow of heat. Methods of calculating the heat transfer coefficient are given by Fernando et al. in “Propane heat pump with low refrigerant charge: design and laboratory tests”, International Journal of Refrigeration, 27 (1), pp.761-773, 2004, and by Dutto et al. in “Performance of brazed plate heat exchanger set in heat pump” Proceedings of the 18th International Congress of Refrigeration, new challenges in refrigeration “, Montreal, Quebec, Canada, vol. 3 (10-17 August 1991).

[00277] The heat transfer coefficient as a function of temperature for the two evaporators is shown in Figure 12. The refrigerant flow through the evaporators in both cases was 0.0121 kg / s.

[00278] The UA of the uncoated evaporator is shown by the square markers, and is in the region of 200 W / K. However, the coated evaporator, shown by circular markers, obtains heat transfer coefficients in the region of 300 W / K. For example, comparing the calculated heat transfer coefficient to a heat transfer rate on the water side of the 2.25 kW evaporators shows that the UA of the coated evaporator is 51.74% higher than that of the non-evaporator coated. This is a noticeable improvement.

[00279] Furthermore, this was achieved with very little pressure drop on the refrigerant-coated side of the evaporator during

75/75 operation.

The pressure drop is the difference in flow pressure along the flow path (for example, between the heat exchanger entry point and the exit point). The pressure drop is caused by turbulence in the fluid flow along the flow path.

A high pressure drop means the pump pumping fluid (eg refrigerant) through the heat exchanger needs to work harder to force the fluid through it.

It is therefore advantageous that a high pressure drop is avoided.

Claims (37)

1. Heat exchange element, characterized by the fact that it comprises a substrate and a coating, in which the heat exchange element defines a flow path for fluid flow and in which at least part of the flow path is coated with the coating, wherein: the coating comprises a metal; the coating comprises a plurality of dowels having a length of up to 100 μm; the coating comprises a first region at one end of the flow path where the average length of the pin is S1 and a second region on the flow path where the average length of the pin is S2; and S1 is greater than S2. Heat exchange element according to claim 1, characterized in that the pins have a length of at least 1 μm and no more than 50 μm. Heat exchange element according to claim 1 or 2, characterized by the fact that S2 is 50% to 90% of S1. 4. Heat exchange element according to any one of the preceding claims, characterized by the fact that S1 is from 2 μm to 10 μm. Heat exchange element according to any one of the preceding claims, characterized in that the pins have a tip thickness of 100 nm or less. Heat exchange element according to any one of the preceding claims, characterized in that said plurality of dowels is arranged in one or more groups, each group comprising two or more dowels. Heat exchange element according to claim 6, characterized by the fact that the diameter of each group is 10 to 50 μm. 8. Heat exchange element according to any of the preceding claims, characterized in that the thickness of the coating is 10 μm or more. Heat exchange element according to any one of the preceding claims, characterized in that the coating comprises copper. Heat exchange element according to any one of the preceding claims, characterized in that the coating comprises 80% metal by weight of the coating. Heat exchange element according to any one of the preceding claims, characterized in that the coating can be obtained by autocatalytic deposition. 12. Heat exchange element according to any one of the preceding claims, characterized in that the average length of the pin is graduated along all or part of the flow path. Heat exchange element according to any one of the preceding claims, characterized in that the coating comprises a surface layer on the coating. 14. Heat exchange element according to any one of the preceding claims, characterized by the fact that the substrate is a metal object. Heat exchange element according to any one of the preceding claims, characterized in that the substrate is an appropriate heat exchanger for transferring heat to a liquid. Heat exchange element according to any one of the preceding claims, characterized in that the flow path comprises a flow channel and in which the coating is present on at least a part of the surface of said flow channel. 17. Heat exchange element according to claim 16, characterized by the fact that the first region is located at or near an entrance to said flow channel and in which the second region is located at a greater distance from the entrance than the first region. 18. Heat exchange element according to any of the preceding claims, characterized in that the heat exchange element contains a refrigerant. 19. Method for transferring heat to or from a fluid, characterized in that it comprises providing the fluid to a flow path of a heat exchange element as defined in any one of claims 1 to 18. 20. Process for producing a heat exchange element as defined in any of claims 1 to 18, the process characterized by the fact that it comprises providing an autocatalytic deposition solution to a surface of a substrate. 21. Process according to claim 20, the process characterized by the fact that it is a bathing process. 22. Process according to claim 20, the process characterized by the fact that it comprises draining an autocatalytic deposition solution on a substrate surface. 23. Process for producing a heat exchange element comprising a substrate and a coating, characterized by the fact that: the coating comprises a metal; and draining an autocatalytic deposition solution on a substrate surface. 24. Process according to claim 23, characterized in that the heat exchange element is as defined in any one of claims 1 to 18. 25. Process according to any of claims 22 to 24, the process characterized by the fact that it comprises: draining the autocatalytic deposition solution from a reservoir of autocatalytic deposition solution on the substrate surface; and returning the autocatalytic deposition solution to said reservoir. 26. Process according to any one of claims 22 to 25, the process characterized by the fact that it comprises: draining an autocatalytic deposition solution on a substrate surface at a first flow rate F1; and draining an autocatalytic deposition solution on said substrate surface in a second flow F2. 27. Process according to claim 26, characterized by the fact that F2 is greater than F1. 28. Process according to any one of claims 22 to 27, the process characterized by the fact that it comprises pumping the autocatalytic deposition solution to make the autocatalytic deposition solution flow over a substrate surface. 29. Process according to any one of claims 22 to 28, characterized in that the substrate comprises a flow channel, and the process comprises flowing an autocatalytic deposition solution through said flow channel. Process according to any one of claims 20 to 29, the process characterized by the fact that it comprises: (i) providing an acid to a substrate surface; and / or (ii) activating a substrate surface, in which steps (i) and / or (ii) are carried out before providing an autocatalytic deposition solution on the substrate surface. 31. Process according to any of the claims 20 to 30 characterized by the fact that the autocatalytic deposition solution is an aqueous solution. 32. Process according to any of claims 20 to 31, characterized in that the autocatalytic deposition comprises copper and / or nickel ions. 33. Process according to any one of claims 20 to 32, the process characterized by the fact that it is carried out at a temperature of 20 ° C to 100 ° C. 34. Process according to any one of claims 20 to 33, the process characterized by the fact that it comprises providing the autocatalytic deposition solution to a surface of a substrate for a period of 0.5 hour to 10 hours. 35. Process according to any one of claims 20 to 34, the process characterized by the fact that it comprises providing the autocatalytic deposition solution to a surface of the substrate for a time T, where T is the time taken for the deposition solution autocatalytic become depleted by 5 to 50%. 36. Process according to any one of claims 20 to 35, the process characterized in that it comprises applying a surface layer, preferably a surface layer comprising nickel, to a heat exchange element. 37. Heat exchange element, characterized by the fact that it is obtained or can be obtained by a process as defined in any one of claims 23 to 36.