Method of anodizing the interior surface of a heat pipe

Abstract

The present invention is titled "Method for Anodizing the Inner Surface of a Heat Transfer Tube". It discloses a method for anodizing the inner surface of a heat transfer tube, comprising: placing a plurality of contact electrodes in electrical communication with and along the outer surface of the heat transfer tube; inserting a counter electrode into the internal space of the heat transfer tube; providing an electrolyte solution to the internal space of the heat transfer tube; allowing current to flow between the plurality of contact electrodes and the counter electrode through the electrolyte solution; forming an oxide layer along the inner surface of the heat transfer tube, wherein the oxide layer has an oxide layer thickness decreasing along the length of the heat transfer tube; stopping the flow of current; removing the electrolyte solution; and applying a sealing solution to the surface of the oxide layer to form a sealed oxide layer along the inner surface of the heat transfer tube.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 706594, filed on August 27, 2020, the contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] Exemplary embodiments relate to the field of anodizing aluminum parts. More specifically, this disclosure relates to anodized aluminum heat exchanger parts and methods for manufacturing such parts. Background Technology

[0004] Aluminum offers a lighter and cheaper alternative to copper for manufacturing heat exchangers. However, aluminum may be more susceptible to corrosion and fouling. For example, water-cooled coolers may be exposed to a wide variety of water qualities, which can lead to corrosion and fouling of the water-containing heat transfer tubes. Given the unique geometry, size, and weight of these tubes, it can be very difficult to apply surface treatments to them efficiently and effectively. As manufacturers attempt to use aluminum or other non-traditional metals (e.g., besides copper) to manufacture heat exchanger tubes, there remains a need in the art for new, cost-effective methods for surface treatments and their applications. Summary of the Invention

[0005] A method for anodizing the inner surface of a heat transfer tube is disclosed, comprising: placing a plurality of contact electrodes in electrical communication with and along the outer surface of the heat transfer tube; inserting a counter electrode into the internal space of the heat transfer tube; providing an electrolyte solution to the internal space of the heat transfer tube; allowing current between the plurality of contact electrodes and the counter electrode to pass through the electrolyte solution; forming an oxide layer along the inner surface of the heat transfer tube, wherein the oxide layer has an oxide layer thickness decreasing along the length of the heat transfer tube; stopping the passage of the current; removing the electrolyte solution; and applying a sealing solution to the surface of the oxide layer to form a sealed oxide layer along the inner surface of the heat transfer tube.

[0006] In addition to one or more of the aspects disclosed above, or as an alternative, the reverse electrode is configured to have a decreasing conductivity along its length, wherein the decrease in the oxide layer thickness along the length of the heat transfer tube corresponds to a decrease in conductivity along the length of the reverse electrode.

[0007] In addition to one or more of the aspects disclosed above, or as an alternative, configuring the counter electrode to have a current flux that decreases along its length also includes configuring the counter electrode to have an electrical shielding thickness that decreases along at least a portion of its length.

[0008] In addition to one or more of the aspects disclosed above, or as an alternative, configuring the counter electrode to have a conductivity that decreases along its length further includes configuring the counter electrode to have one or more portions of an electrical shield disposed along its length, and wherein the one or more portions are arranged to have a conductivity that decreases along the length of the counter electrode.

[0009] In addition to one or more of the aspects disclosed above, or as an alternative, the insertion of the counter electrode further includes inserting the counter electrode to an insertion depth that partially extends into the interior space of the heat transfer tube, and wherein at least a portion of the decrease in oxide layer thickness along the length of the heat transfer tube corresponds to the insertion depth.

[0010] In addition to one or more of the aspects disclosed above, or as an alternative, forming the oxide layer also includes adjusting the flow of current between the plurality of contact electrodes and the counter electrode to change the thickness of the oxide layer along at least a portion of the length of the heat transfer tube.

[0011] In addition to one or more of the aspects disclosed above, or as an alternative, the passage of the current also includes applying electrical energy to the contact electrode and the counter electrode to generate a voltage difference between them.

[0012] In addition to one or more of the aspects disclosed above, or as an alternative, the plurality of contact electrodes are positioned along the length of the heat transfer tube.

[0013] A cooler is also disclosed, comprising a plurality of heat exchangers, wherein at least one of the plurality of heat exchangers includes a plurality of heat transfer tubes, wherein an oxide layer is formed on the inner surface of one or more of the plurality of heat transfer tubes, and wherein the oxide layer has an oxide layer thickness that decreases along the length of the heat transfer tube.

[0014] In addition to one or more of the aspects disclosed above, or as an alternative, the heat transfer tube is substantially composed of aluminum.

[0015] A heat transfer tube anodizing device is also disclosed, comprising: a plurality of contact electrodes configured to be electrically connected to and positioned along the outer surface of the heat transfer tube; a counter electrode including an electrical shield with a thickness decreasing along at least a portion of its length; a power supply including a positive terminal and a negative terminal, wherein the plurality of contact electrodes are configured to be electrically connected to the positive terminal and the counter electrode is configured to be electrically connected to the negative terminal; and a controller configured to adjust electrical parameters of the power supply, wherein the electrical parameters include output power, and output voltage, output current, or a combination of at least one of the foregoing.

[0016] In addition to one or more of the aspects disclosed above, or as an alternative, the counter electrode comprises a metal wire having an electrically shielded portion extending along its length.

[0017] In addition to one or more of the aspects disclosed above, or as an alternative, the counter electrode comprises a plurality of metal wires having an electrical shield extending along at least a portion of its length.

[0018] In addition to one or more of the aspects disclosed above, or as an alternative, the electrical shielding includes a plurality of electrical shielding portions disposed along the length of the counter electrode, and wherein at least two portions have different conductivity values.

[0019] In addition to one or more of the aspects disclosed above, or as an alternative, the plurality of electrically shielded portions are arranged to have a conductivity value that decreases along the length of the counter electrode. Attached Figure Description

[0020] The following description should not be considered as limiting in any way. Referring to the accompanying drawings, the same elements are labeled with the same numerals:

[0021] Figure 1 It is a diagram illustrating the disclosed method steps.

[0022] Figure 2 This is a schematic diagram of a heat transfer tube and a counter electrode with multiple metal elements.

[0023] Figure 3 This is a schematic diagram of a heat transfer tube and a counter electrode with a shielding material of reduced thickness.

[0024] Figure 4 This is a schematic diagram of a heat transfer tube in a heat transfer tube anodizing device with a flow process.

[0025] Figure 5This is a schematic diagram of a heat transfer tube in a heat transfer tube anodizing device with a batch process. Detailed Implementation

[0026] This document presents a detailed description of one or more embodiments of the disclosed apparatus and methods by way of example and not limitation, with reference to the accompanying drawings.

[0027] A significant challenge in deploying aluminum components in HVAC systems may be aluminum's susceptibility to corrosion and scaling. To reduce corrosion rates, surface treatments can be applied to protect the base aluminum or aluminum alloy material from corrosive interactions (e.g., with water and / or impurities therein, such as chlorine, fluorine, and other dissociated ionic substances). However, a challenge in surface treatment of heat exchanger tubes may be the presence of surface features on the tube's surface. Surface features may include fins, spikes, or other protrusions recessed into or extending from the inner and / or outer surfaces of the tube. These features can be configured to disrupt boundary laminar flow and increase local convective heat transfer coefficients. When a coating is applied after the surface features have been formed, the coating can partially negate the benefits of the surface features by filling in the recesses and / or covering the protrusions, thus limiting their effectiveness.

[0028] To address these issues, the applicant has developed a disclosed method and apparatus for anodizing the inner surface of a heat transfer tube. As shown in the accompanying drawings, the disclosed method includes a first step 100 of placing a plurality of contact electrodes (30a, 30b, 30c) in electrical communication with and along the outer surface of the heat transfer tube 10. The contact electrodes (30a, 30b, 30c) may be wound around the outer surface of the tube 10 and may be positioned along the length of the tube 10 using any desired spacing. For example, the contact electrodes (30a, 30b, 30c) may be equally spaced along the axial length of the tube 10 and may be wound substantially around the outer side of the tube 10. Placing the plurality of contact electrodes (30a, 30b, 30c) may include any suitable method of engaging the contact electrodes to the outer surface of the tube 10, such as sliding, winding, clamping, and / or holding the counter electrode onto the tube 10, etc. Fasteners, belts or strips and tensioners or other mechanical fasteners may be used to attach and / or press the contact electrodes (30a, 30b, 30c) onto the outer surface of tube 10 to enhance electrical communication between the electrodes and tube 10.

[0029] The second step 120 of the disclosed method may include inserting a counter electrode 40 into the internal space 12 of the heat transfer tube 10. The counter electrode 40 may be positioned along or around the centerline 8 of the tube 10 such that the distance between the surface of the counter electrode 40 and the inner surface of the tube 10 is substantially equal in all radial directions. One or more positioning guides 59 may be located within the internal space 12 of the tube to aid in positioning the counter electrode 40 on or around the centerline 8 of the tube 10. Furthermore, one or more centering holes 71 may be included in the positioning guides 59 to aid in positioning the counter electrode along or around the centerline 8. The positioning guides 59 may include a surrounding aperture 72 that allows fluid flow through the tube 10 during the disclosed method. The positioning guides 59 may be made of a non-conductive dielectric material such that contact with the counter electrode 40 and the tube 10 will not short-circuit the electrolytic circuitry generated during anodizing.

[0030] The counter electrode 40 may include, for example, Figure 2The diagram shows multiple metal elements (42a, 42b, 42c) arranged together to form a counter electrode 40, or a single metal element. The counter electrode 40 may include metals more expensive than aluminum, such as ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, copper, mercury, and rhenium. One or more metal elements may include an electrical shielding material (41a, 41b, 41c) disposed along at least a portion of their length. The electrical shielding material (41a, 41b, 41c) may include a dielectric material, such as a thermoplastic like polypropylene, polytetrafluoroethylene (PTFE), polyethylene (such as high-density polyethylene HDPE), etc., configured to prevent current from flowing along the portion covered by the electrical shielding material 41 through the counter electrode 42. The thermoplastic can be selected based on its compatibility with the electrolyte solution 61 (e.g., whether it is an inert or non-reactive chemical substance when exposed to the electrolyte solution 61, electrodes, components, and workpieces such as heat exchanger tubes), such as that described in ASTM D543-20, which is valid at the time of filing of this application. The effective thickness of the electrical shielding material 41 can vary along the length of the counter electrode 40. For example, the electrical shielding material 41 may be thickest at one end 45 of the counter electrode and transition to a thinner material or to exposed, bare metal elements at the opposite end 48. The transition in thickness of the electrical shield can be continuous or discontinuous, including sloping transitions, stepped transitions, and so on. For example, the counter electrode 40 may include multiple metal elements (42a, 42b, 42c), each metal element having an electrically shielding material (41a, 41b, 41c) covering one or more portions of the length of the metal element (42a, 42b, 42c) to form a stepped transition in the effective electrically shielding thickness. Thus, a stepped transition is generated in the radial current through the electrolyte solution 61 between the counter electrode 40 and the tube 10, the stepped transition varying according to the tube length. In another example, for example... Figure 3 As shown, the counter electrode 40 may include a single metal element 42 having an electrically shielding material 41 configured to decrease in thickness along the length of the counter electrode 40. Therefore, a continuous transition is generated in the radial current through the electrolyte solution 61 between the counter electrode 40 and the tube 10, the transition varying according to the tube length.

[0031] The shielding current of the electrolytic solution can be used to account for anodizing at different depths along the inner surface of the heat transfer tube 10. For example, the oxidation depth can be taken into account as varying with the length of the heat transfer tube 10. This method can be used, for example, to provide additional protection for the most corrosion-prone areas of the heat transfer tube 10 at the hottest axial location of the tube (e.g., the portion of the tube closest to the hot inlet fluid flow or the hot-side inlet manifold) when used in a heat exchanger.

[0032] Variable coating thickness can be achieved by using a counter electrode 40 (e.g., less than the full length of the heat transfer tube 10) that extends partially into the heat transfer tube 10. This method allows for the localization and / or thickening of the surface treatment along the portion of the inner surface of the heat transfer tube 10 where the counter electrode 40 is located (e.g., while little or no surface treatment is formed along the portion where the counter electrode is not located). For example, the counter electrode 40 can be partially inserted into the heat transfer tube 10 to form a surface treatment along the inner surface of the tube up to a distance corresponding to the insertion depth. Thus, the surface treatment can be thickest at one end of the heat transfer tube 10 and thinnest or absent at the opposite end.

[0033] Furthermore, the electrical shielding material 41 may include one or more conductive portions and one or more partially non-conductive portions. These conductive and partially non-conductive portions may be arranged in any pattern along the length of the counter electrode 40. The portions may include dielectric materials (e.g., thermoplastics such as polyvinyl chloride, polyethylene, etc.), the composition of which and / or thickness may be tailored to allow for a desired current flux distribution (or current density distribution, e.g., current distribution along the inner surface of the heat transfer tube 10) or the absence of such a current flux distribution for each portion. In these ways, the conductivity distribution along the length of the counter electrode 40 may be tailored to account for variations in corrosion and / or fouling conditions that may exist along the length of the heat transfer tube 10 during operation.

[0034] The third step 140 of the disclosed method may include providing an electrolyte solution 61 to the internal space 12 of the heat transfer tube 10. The electrolyte solution 61 may include an acid (e.g., sulfuric acid, chromic acid, phosphoric acid, etc.), which may be provided to the internal space 12 of the heat transfer tube 10 in any suitable manner. For example, as... Figure 4 As shown, electrolyte 61 can be pumped through pipe 10 during flow, or as... Figure 5 As shown, tube 10 can be placed in a bath of electrolyte 61 during a batching process. The electrolyte solution may include an oxygen-rich electrolyte. Electrolyte 61 may include dyes, pigments, etching solutions, or other chemicals that can be used to influence the physical properties of the oxide layer, such as porosity, adhesion to the tube surface, and color.

[0035] refer to Figure 4The heat transfer tube anodizing apparatus 300 may include a pump 52 that pumps an electrolyte solution 61 from a source reservoir 50 through the heat transfer tubes 10 to a collector 60. The source reservoir 50 may include a heat exchanger 51 for heating or cooling the electrolyte solution 61 to a desired processing temperature. An inlet valve 54 and an outlet valve 57 may be used to isolate the inlet flow line 53 and the outlet flow line 58 from the heat transfer tubes 10 when the tubes are configured for processing. An inlet end cap 55 and an outlet end cap 56 may be used to fluidly connect the heat transfer tubes 10 to the inlet flow line 53 and the outlet flow line 58, respectively. If the concentration of an active substance (e.g., sulfuric acid, chromic acid, phosphoric acid, etc.) in the electrolyte solution 61 at the collector 60 is sufficiently high, the solution may optionally be recycled back to the source reservoir 50, where it can be reused in the process.

[0036] refer to Figure 5 The heat transfer tube anodizing apparatus 300 can be configured to immerse a heat transfer tube 10, which is provided with a counter electrode 40, in a tank 70 containing a volume of electrolyte solution 61 disposed therein. The inner surface of the tank 70 may be made of or protectively coated with a high-dielectric, corrosion-resistant material suitable for containing the electrolyte solution 61, such as plastics (e.g., polyethylene, polytetrafluoroethylene). If desired, a heat exchanger 51 can be used to heat or cool the electrolyte solution 61 within the tank 70 to a desired processing temperature.

[0037] The fourth step 160 of the disclosed method may include passing an electric current through an electrolyte solution 61 between a plurality of contact electrodes (30a, 30b, 30c) and a counter electrode 40. One or more power sources (32a, 32b, 32c) may be configured to be electrically connected to one or more metallic elements of the contact electrodes (32a, 32b, 32c) and the counter electrode 40. The one or more power sources (32a, 32b, 32c) may be used to generate a potential difference between the tube 10 and the counter electrode 40. The one or more power sources (32a, 32b, 32c) may be controlled in communication with a controller configured to adjust the electrical parameters of the one or more power sources (32a, 32b, 32c) to maintain a desired output voltage of the power source (e.g., a potential difference across the electrodes), a desired output current from the power source through the electrolyte solution 61, a desired power output from the power source, or a combination including at least one of the foregoing. This potential difference generates a driving force for current to flow from the counter electrode 40 through the electrolyte solution 61 and to the inner surface of the tube 10. The inner surface of tube 10 can act as an anode, where oxygen is released and an aluminum oxide layer is formed and grown, while the counter electrode 40 can act as a cathode, where hydrogen is released.

[0038] The fifth step 180 of the disclosed process may include forming an oxide layer along the inner surface of the heat transfer tube 10. The oxide layer can be formed when the electrolyte solution 61 is present between the counter electrode 40 and the contact electrodes (32a, 32b, 32c) and a potential difference is generated between them. The potential difference, concentration, acidity, temperature, current, or combinations thereof of the electrolyte solution 61 can be controlled to provide a desired oxide layer to the tube 10. Furthermore, the profile of the oxide layer can be adjusted to provide a desired corrosion resistance that varies with the length of the tube 10, taking into account material properties such as thermal resistance effects (e.g., thermal conductivity) and corrosion resistance effects to optimize the oxide layer. For example, the oxide layer may thicken along a portion of the length of the heat transfer tube 10, which has an increased potential applied thereto. The increased potential applied to a portion of tube 10 may be a result of a higher potential applied to that portion, or it may be due to a decrease in the effective thickness of the electrical shielding material 41 layer(s) of the counter electrode 40 along that portion, or a change in the composition of the electrical shielding material 41 (e.g., resulting in lower shielding strength). The oxide layer may have an oxide layer thickness that decreases along the length of the heat transfer tube 10, for example, having a decreasing thickness from one end 45 to the opposite end 48. The oxide layer formed as described herein may have a maximum thickness at a point along the length of the heat transfer tube 10 of the following: less than or equal to about 10 micrometers (μm), or from about 1 μm to about 8 μm, or from about 1 μm to about 7 μm, or from about 1 μm to about 6 μm, or from about 2 μm to about 8 μm, or from about 2 μm to about 7 μm, or from about 2 μm to about 6 μm, or from about 3 μm to about 8 μm, or from about 3 μm to about 7 μm, or from about 3 μm to about 6 μm, or less than or equal to about 5 μm, or less than or equal to about 4 μm, or less than or equal to about 3 μm, or less than or equal to about 2 μm, or less than or equal to about 1 μm. In one example, a potential difference of from about 12 volts DC voltage (VDC) to about 18 VDC for a duration of from about 15 to about 30 minutes can be applied between the contact electrodes (30a, 30b, 30c) and the counter electrode 40, which are electrically connected to a heat transfer tube substantially comprising 6000 series aluminum, to form an oxide layer having a maximum thickness of from about 3 μm to about 6 μm along the inner surface of the heat transfer tube 10.

[0039] The sixth step 200 of the disclosed process includes stopping the flow of current. Once the desired oxide layer thickness is reached, the applied potential can be removed, and the current flowing through the electrolyte solution 61 can be stopped.

[0040] The seventh step 220 of the disclosed process may include removing the electrolyte solution 61. Removal may include separating the electrolyte solution 61 from the internal space 12 of the heat transfer tube 10 in any suitable manner. For example, as... Figure 4As shown, the flow of the electrolyte solution 61 can be stopped, and cleaning fluids (e.g., water), sealing solutions, etc., can be used to flush the internal space 12 of the heat transfer tube 10. In another example, as... Figure 5 As shown, the heat transfer tube 10 can be removed from the bath of the electrolyte solution 61 and placed in a separate cleaning tank containing a cleaning fluid (e.g., water).

[0041] The eighth step of the disclosed process may include applying a sealing solution (e.g., a corrosion-resistant solution) to the oxide layer formed on the inner surface of the heat transfer tube 10. The sealing solution may help reduce the rate at which the oxide layer is corroded, thereby contributing to improved durability. Examples of sealing solutions may include, but are not limited to, aqueous solutions of nickel acetate, potassium hexafluorozirconate, and trivalent chromium sulfate (e.g., the trivalent chromium process (TCP)) and deionized water. Furthermore, the aqueous nickel acetate sealing step may include exposing the inner surface of the heat transfer tube 10 to an aqueous solution of nickel acetate ranging from about 0.5 wt% to about 3 wt% for a duration ranging from about 15 minutes to about 30 minutes at a temperature ranging from about 190℉ to about 210℉. Additionally, the TCP sealing step may include exposing the inner surface of the heat transfer tube 10 to trivalent chromium sulfate ranging from about 10 wt% to about 30 wt% for a duration ranging from about 5 minutes to about 15 minutes at about ambient temperature (e.g., 72℉). In addition, the deionized water sealing step may include exposing the inner surface of the heat transfer tube 10 to deionized water at a boiling temperature (e.g., 212℉ at 1 atmosphere) for a duration of about 30 minutes to about 45 minutes.

[0042] As described herein, the heat transfer tube 10, in which an oxide layer is formed, can be used to manufacture heat exchangers. For example, the heat transfer tube 10 can be used to manufacture shell and tube heat exchangers, finned tube heat exchangers, plate-finned tube heat exchangers, and so on. This heat exchanger can be used in the construction of heating, air conditioning, and refrigeration equipment. For example, the heat transfer tube 10 can be used in the construction of shell and tube heat exchangers, which can be configured for use as coolers in air conditioning systems. The oxide layer formed as described herein provides the heat transfer tube 10 with additional protection against corrosion and fouling during its operational life, while minimizing the impact of the oxide layer thickness on the thermal conductivity of the heat transfer tube 10.

[0043] The digital steps described herein are not intended to specify a corresponding temporal order or sequence of operations. Unless otherwise indicated, the steps may be performed in any order, separated into different time events, combined into a single time event, or may be performed overlappingly in time, without departing from the nature of this disclosure and while still benefiting from it.

[0044] The term “about” is intended to include the degree of error associated with measurements based on a specific quantity of equipment available at the time of filing this application.

[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the terms “comprising” and / or “including” as used in this specification designate the presence of stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, and / or groups thereof.

[0046] Although this disclosure has been described with reference to one or more exemplary embodiments, those skilled in the art will understand that various changes can be made and equivalents can replace its elements without departing from the scope of this disclosure. Furthermore, many modifications can be made to adapt particular situations or materials to the teachings of this disclosure without departing from the essential scope of this disclosure. Therefore, it is intended that this disclosure is not limited to the specific embodiments disclosed as the best mode contemplated for carrying out this disclosure, but that this disclosure will include all embodiments falling within the scope of the claims.

Claims

1. A method for anodizing the inner surface of a heat transfer tube, comprising: Multiple contact electrodes are positioned to be electrically connected to the outer surface of the heat transfer tube and extend along the outer surface of the heat transfer tube. Insert the counter electrode into the internal space of the heat transfer tube. An electrolytic solution is provided to the internal space of the heat transfer tube. The current between the plurality of contact electrodes and the counter electrode is passed through the electrolytic solution. An oxide layer is formed along the inner surface of the heat transfer tube, wherein the oxide layer has an oxide layer thickness that decreases along the length of the heat transfer tube. Stop the passage of the current. Remove the electrolyte solution, and A sealing solution is applied to the surface of the oxide layer to form a sealed oxide layer along the inner surface of the heat transfer tube.

2. The method of claim 1, further comprising configuring the counter electrode to have a conductivity that decreases along its length, and wherein, The decrease in the oxide layer thickness along the length of the heat transfer tube corresponds to a decrease in conductivity along the length of the counter electrode.

3. The method according to claim 2, wherein, Configuring the counter electrode to have a decreasing current flux along its length also includes configuring the counter electrode to have an electrical shielding thickness that decreases along at least a portion of its length.

4. The method according to claim 2, wherein, Configuring the counter electrode to have a conductivity that decreases along its length also includes configuring the counter electrode to have one or more portions of an electrical shield disposed along its length, wherein the one or more portions are arranged to have a conductivity that decreases along the length of the counter electrode.

5. The method according to claim 1, wherein, Inserting the counter electrode further includes inserting the counter electrode to an insertion depth that partially extends into the interior space of the heat transfer tube, and wherein at least a portion of the decrease in oxide layer thickness along the length of the heat transfer tube corresponds to the insertion depth.

6. The method according to any one of claims 1 to 5, wherein, Forming the oxide layer also includes adjusting the flow of current between the plurality of contact electrodes and the counter electrode to change the thickness of the oxide layer along at least a portion of the length of the heat transfer tube.

7. The method of claim 1, wherein, Passing the current also includes applying electrical energy to the contact electrode and the counter electrode, thereby creating a voltage difference between them.

8. The method of claim 1, further comprising positioning the plurality of contact electrodes along the length of the heat transfer tube.

9. A chiller comprising a plurality of heat exchangers, wherein, At least one of the plurality of heat exchangers includes a plurality of heat transfer tubes, wherein an oxide layer is formed on the inner surface of one or more of the plurality of heat transfer tubes, and wherein the oxide layer has an oxide layer thickness that decreases along the length of the heat transfer tube.

10. The cooler of claim 9, wherein, The heat transfer tubes are essentially made of aluminum.

11. A heat transfer tube anodizing device, comprising: Multiple contact electrodes are configured to be in electrical communication with the outer surface of the heat transfer tube and are positioned along the outer surface of the heat transfer tube. A counter electrode, the counter electrode comprising an electrical shield with a thickness decreasing along at least a portion of its length, A power supply, comprising a positive terminal and a negative terminal, wherein the plurality of contact electrodes are configured to be electrically connected to the positive terminal, and the reverse electrode is configured to be electrically connected to the negative terminal, and A controller configured to adjust electrical parameters of the power supply, wherein the electrical parameters include output power, output voltage, output current, or a combination of at least one of the foregoing.

12. The anodizing apparatus according to claim 11, wherein, The counter electrode comprises a metal wire having an electrical shield extending along at least a portion of its length.

13. The anodization apparatus of any one of claims 11-12, wherein, The counter electrode comprises a plurality of metal wires having an electrical shield extending along at least a portion of its length.

14. The anodization apparatus of any one of claim 13, wherein, The electrical shielding includes a plurality of electrical shielding portions disposed along the length of the counter electrode, wherein at least two portions have different conductivity values.

15. The anodization apparatus of claim 14, wherein, The plurality of electrically shielded portions are arranged to have a conductivity value that decreases along the length of the counter electrode.

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