Patent Description:
Aluminum offers a lighter, less expensive alternative to copper for the manufacture of heat exchangers. However, aluminum can be more susceptible to corrosion and fouling. For example, water cooled chillers can be exposed to a wide variety of water qualities that can cause corrosion and fouling of the water-bearing heat transfer tubes. Given the unique geometry, size, and weight of these tubes, it can be very difficult to efficiently and effectively apply a surface treatment to them. As manufacturers seek to utilize aluminum or other non-traditional metals (e.g. other than copper) for the manufacture of heat exchanger tubes, there remains a need in the art for new surface treatments and cost-effective methods of their application.

<CIT> discloses a method of, and apparatus for, anodizing an internal surface of an aluminum tube wherein the oxide layer produced has a uniform thickness along the length of the internal surface of the tube. <CIT> discloses a method of, and apparatus for, anodizing an internal surface of an aluminum or aluminum alloy hollow part. Again, the oxide layer produced on the internal surface of the part is of uniform thickness.

Disclosed is a method of anodizing the interior surface of a heat transfer tube, wherein the heat transfer tube comprises aluminum, the method comprising: placing a plurality of contact electrodes in electrical communication with, and along, an exterior surface of the heat transfer tube, inserting a counter electrode into an interior space of the heat transfer tube, providing an electrolytic solution to the interior space of the heat transfer tube, passing an electric current between the plurality of contact electrodes and the counter electrode through the electrolytic solution, forming an oxidation layer along the interior surface of the heat transfer tube, , stopping the passage of the electric current, removing the electrolytic solution, and applying a sealing solution to a surface of the oxidation layer to form a sealed oxidation layer along the interior surface of the heat transfer tube, characterized in that the oxidation layer has an oxidation layer thickness that decreases along a length of the heat transfer tube from one end to an opposite end.

The method may comprise configuring the counter electrode to have a decreasing electrical conductivity along its length, and wherein the decrease in the oxidation layer thickness along the length of the heat transfer tube corresponds to a decrease in electrical conductivity along the length of the counter electrode.

The configuring the counter electrode to have a decreasing electrical current flux along its length may comprise configuring the counter electrode to have a decreasing thickness of electrical shielding along at least a portion of its length.

The configuring the counter electrode to have a decreasing electrical conductivity along its length may comprise configuring the counter electrode to have one or more sections of electrical shielding disposed along its length, and wherein the one or more sections are arranged to having a decreasing electrical conductivity along the length of the counter electrode.

The inserting the counter electrode may comprise inserting the counter electrode to an insertion depth that extends partially into an interior space of the heat transfer tube, and wherein at least a portion of the decrease in the oxidation layer thickness along the length of the heat transfer tube corresponds to the insertion depth.

The forming the oxidation layer may comprises adjusting a flow of the electric current between the plurality of contact electrodes and the counter electrode to change the oxidation layer thickness along at least a portion of the length of the heat transfer tube.

The passing the electric current may comprise applying electrical energy to the contact electrodes and counter electrode thereby creating a voltage difference therebetween.

The method may comprise positioning the plurality of contact electrodes along the length of the heat transfer tube.

Further disclosed is a chiller comprising a plurality of heat exchangers, wherein at least one of the plurality of heat exchangers comprises a plurality of heat transfer tubes, wherein one or more of the plurality of heat transfer tubes comprises aluminum, wherein an oxidation layer formed on the interior surface of one or more tubes of the plurality of heat transfer tubes, and wherein the oxidation layer has an oxidation layer thickness that decreases along a length of the heat transfer tube from one end to an opposite end.

A significant challenge to deploying aluminum parts in HVAC systems can be the susceptibility of aluminum to corrosion and fouling. In order to reduce the rate of corrosion, a surface treatment 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 species). However, a challenge with the surface treatment of heat exchanger tubes can be the presence of surface features on the surface of the tubes. Surface features can include fins, spikes, or other protrusions recessing into or extending from the internal and/or external surface or the tube. These features can be configured to break up boundary layer flow and increase the local convective heat transfer coefficient. When coatings are applied after the formation of surface features the coatings can partially defeat the benefit of the surface feature by filling the recesses, and/or covering the protrusions of the feature thereby limiting its effectiveness.

In solving these problems, the applicants have developed the disclosed method and apparatus for anodizing the interior surface of a heat transfer tube. As shown in the attached figures, the disclosed method includes a first step <NUM> of placing a plurality of contact electrodes (30a, 30b, 30c) in electrical communication with, and along, an exterior surface of a heat transfer tube <NUM>. The contact electrodes (30a, 30b, 30c) can be wrapped around the exterior surface of the tube <NUM> and can be positioned with any desired spacing along the length of the tube <NUM>. For example, contact electrodes (30a, 30b, 30c) can be equally spaced along the axial length of the tube <NUM>, and can be wrapped substantially around the outside circumference of the tube <NUM>. Placing the plurality of contact electrodes (30a, 30b, 30c) can include any suitable method of engaging the contact electrodes to the outer surface of the tube <NUM>, such as sliding, wrapping, clipping, and/or sandwiching the counter electrodes over the tubes <NUM>, and the like. Fasteners, belts or straps and tensioners, or other mechanical securements can be used to attach and/or press the contact electrodes ((30a, 30b, 30c) against the outer surface of the tube <NUM> to enhance electrical communication between the electrodes and the tube <NUM>.

A second step <NUM> of the disclosed methods can include inserting a counter electrode <NUM> into an interior space <NUM> of the heat transfer tube <NUM>. The counter electrode <NUM> can be positioned along a centerline <NUM> of the tube <NUM>, or arranged about the centerline <NUM>, such that there is substantially equal distance between a surface of the counter electrode <NUM> and an interior surface of the tube <NUM> in all radial directions. One or more positioning guides <NUM> can be located within the tube interior space <NUM> to aid in positioning the counter electrode <NUM> on or about the centerline <NUM> of the tube <NUM>. Further, one or more centering holes <NUM> can be included in the positioning guide <NUM> to aid in positioning the counter electrode along or about the centerline <NUM>. The positioning guide <NUM> can include surrounding holes <NUM> which can allow fluid to flow through the tube <NUM> during the disclosed methods. The positioning guides <NUM> can be made of a dielectric material that is not electrically conductive, such that contact with the counter electrode <NUM> and the tube <NUM> will not short the electrolytic circuit created during anodizing.

The counter electrode <NUM> can include a single metal element or a plurality of metal elements (42a, 42b, 42c) arranged together to form the counter electrode <NUM>, e.g., as shown in <FIG>. The counter electrode <NUM> can include a metal that is more noble than aluminum, e.g., ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, copper, mercury, and rhenium. The one or more metal elements can include electrical shielding material (41a, 41b, 41c) disposed along at least a portion of its length. The electrical shielding material (41a, 41b, 41c) can include a dielectric material, e.g., thermoplastics such as polypropylene, polytetrafluoroethylene (PTFE), polyethylene (such as high density polyethylene, HDPE), and the like, configured to prevent the flow of electrical current through the counter electrode <NUM> along portions covered by the electrical shielding material <NUM>. Thermoplastics can be chosen based on their compatibility with the electrolyte solution <NUM> (e.g., chemicals that are inert or nonreactive when exposed to the electrolyte solution <NUM>, electrodes, components, and work pieces such as heat exchanger tubes), such as described by American Society for Testing and Materials (ASTM) D543-<NUM> in force at the time of filing the present application. An effective thickness of the electrical shielding material <NUM> can vary along the length of the counter electrode <NUM>. For example, the electrical shielding material <NUM> can be thickest at one end <NUM> of the counter electrode and can transition to a smaller thickness material, or to a bare, exposed, metal element at an opposite end <NUM>. The transition in thickness of the electrical shielding can be continuous or discontinuous, including a sloped transition, a stepped, and the like. For example, the counter electrode <NUM> can include a plurality of metal elements (42a, 42b, 42c) each having electrical shielding material (41a, 41b, 41c) covering one or more sections of the length of the metal elements (42a, 42b, 42c) to form a stepped transition in effective electrical shielding thickness. Thus, creating a stepped transition in radial current flow through an electrolytic solution <NUM> between the counter electrode <NUM> and the tube <NUM> which changes as a function of the tube length. In another example, e.g., as shown in <FIG>, the counter electrode <NUM> can include a single metal element <NUM> having an electrical shielding material <NUM> configured in a decreasing thickness along the length of the counter electrode <NUM>. Thus, creating a continuous transition in radial current flow through an electrolytic solution <NUM> between the counter electrode <NUM> and the tube <NUM> which changes as a function of the tube length.

Shielding current flow through the electrolytic solution can allow for anodizing to different depths along the interior surface of the heat transfer tube <NUM>. For example, allowing for changes in oxidation depth as a function of the length of the heat transfer tube <NUM>. Such methods can be used to provide additional protection to areas of the heat transfer tube <NUM> that are most susceptible to corrosion, such as at the hottest axial positions of the tube when used in a heat exchanger (e.g., sections of the tube that will be nearest to a hot inlet fluid stream, or hot side inlet manifold).

Variable coating thickness can be achieved through the use of a counter electrode <NUM> which extends partially into the heat transfer tube <NUM>, e.g., less than the full length of heat transfer tube <NUM>. Such methods can allow for localization and/or thickening of the surface treatment along portions of the interior surface of the heat transfer tube <NUM> where the counter electrode <NUM> is present (e.g., while little or no surface treatment will form along portions where the counter electrode is not present). For example, a counter electrode <NUM> can be inserted partially into a heat transfer tube <NUM> to form a surface treatment along the tube interior surface for a distance that corresponds to the depth of the insertion. In this way, the surface treatment can be thickest at one end of the heat transfer tube <NUM> and thinnest, or non-existent, at the opposite end.

Furthermore, electrical shielding material <NUM> can include one or more conductive sections and one or more partially non-conductive sections. The one or more conductive sections and one or more partially non-conductive sections can be arranged in any pattern along the length of the counter electrode <NUM>. The sections can include a dielectric material (e.g., thermoplastics such as polyvinyl chloride, polyethylene, and the like) the composition and/or thickness of which can be tailored to allow a desired current flux distribution (or current density distribution, e.g., distribution of current flow along the inner surface of the heat transfer tube <NUM>), or lack thereof, for each section. In these ways, the electrical conductivity profile along the length of the counter electrode <NUM> can be tailored to account for changes in corrosive and/or fouling conditions that may be present along the length of the heat transfer tube <NUM> when in operation.

A third step <NUM> of the disclosed methods can include providing an electrolytic solution <NUM> to the interior space <NUM> of the heat transfer tube <NUM>. An electrolytic solution <NUM> can include an acid (e.g., sulfuric acid, chromic acid, phosphoric acid, and the like) which can be provided to the interior space <NUM> of the heat transfer tube <NUM> using any suitable means. For example, as in <FIG> the electrolytic solution <NUM> can be pumped through the tube <NUM> in a flow process, or as in <FIG>, the tube <NUM> can be placed into a bath of electrolytic solution <NUM> in a batch process. The electrolytic solution can include an oxygen rich electrolyte. The electrolytic solution <NUM> can include dye, pigment, etching solution, or other chemicals which can be used to influence the physical characteristics of the oxide layer such as porosity, adherence to tube surface, and color.

Referring to <FIG>, a heat transfer tube anodizing apparatus <NUM> can include a pump <NUM> which can pump the electrolytic solution <NUM> from a source reservoir <NUM> through the heat transfer tube <NUM> to an accumulator <NUM>. The source reservoir <NUM> can include a heat exchanger <NUM> for heating or cooling the electrolytic solution <NUM> to a desired processing temperature. An inlet valve <NUM> and outlet valve <NUM> can be used to isolate the inlet flow line <NUM> and outlet flow line <NUM> from the heat transfer tube <NUM> while the tube is configured for processing. An inlet end cap <NUM> and an outlet end cap <NUM> can be used to fluidly connect the heat transfer tube <NUM> to the inlet flow line <NUM> and the outlet flow line <NUM> respectively. If the concentration of active species (e.g., sulfuric acid, chromic acid, phosphoric acid, and the like) in the electrolytic solution <NUM> at the accumulator <NUM> is sufficiently high then the solution can be optionally recycled back to the source reservoir <NUM> where it can be reused in the process.

Referring to <FIG>, a heat transfer tube anodizing apparatus <NUM> can be configured for the heat transfer tube <NUM>, provided with the counter electrode <NUM>, to be submerged into a tub <NUM> containing a volume of electrolytic solution <NUM> disposed therein. The interior surfaces of the tub <NUM> can be made of, or protectively coated with, a high dielectric, corrosion resistant material suitable for containing the electrolytic solution <NUM> such as plastic (e.g., polyethylene, polytetrafluoroethylene). If desired, a heat exchanger <NUM> can be used to heat or cool the electrolytic solution <NUM> within the tub <NUM> to a desired processing temperature.

A fourth step <NUM> of the disclosed methods can include passing an electric current between the plurality of contact electrodes (30a, 30b, 30c) and the counter electrode <NUM> through the electrolytic solution <NUM>. One or more power supplies (32a, 32b, 32c) can be configured in electrical communication with the one or more contact electrodes (32a, 32b, 32c) and one or more metal elements of the counter electrode <NUM>. The one or more power supplies (32a, 32b, 32c) can be used to create an electrical potential difference between the tube <NUM> and the counter electrode <NUM>. The one or more power supplies (32a, 32b, 32c) can be in control communication with a controller configured to adjust an electrical parameter of the one or more power supplies (32a, 32b, 32c) to maintain a desired output voltage of the power supply (e.g., electrical potential difference across the electrodes), a desired output current flow from the power supply through the electrolytic solution <NUM>, a desired power output from the power supply, or a combination including at least one of the foregoing. This electrical potential difference creates a driving force for current to flow from the counter electrode <NUM> through the electrolytic solution <NUM> and to the inner surface of the tube <NUM>. The inner surface of the tube <NUM> can act as the anode where oxygen gas is released and an aluminum oxide layer forms and grows while the counter electrode <NUM> can act as the cathode where hydrogen gas is liberated.

A fifth step <NUM> of the disclosed process can include forming an oxidation layer along the interior surface of the heat transfer tube <NUM>. The oxidation layer can form when the electrolytic solution <NUM> is present between the counter electrode <NUM> and the contact electrodes (32a, 32b, 32c) and a difference in electrical potential is created therebetween. The electrical potential difference, the concentration, the acidity, and the temperature of the electrolytic solution <NUM>, the current flow, or a combination including at least one of the foregoing can be controlled in order to provide the tube <NUM> with the desired oxidation layer. Further, the profile of the oxidation layer can be tuned to provide the desired corrosion resistance as a function of the length of the tube <NUM>, which can allow for optimizing the oxidation layer based on material properties such as heat transfer resistance effect (e.g., thermal conductivity) and corrosion resistance effect. For example, the oxidation layer can be thicker along a portion of the length of the heat transfer tube <NUM> that has an increased electrical potential applied thereto. The increased electrical potential applied to a section of the tube <NUM> can be the result of a higher electrical potential applied to the section or can be due to a reduction in the effective thickness of the electrical shielding material <NUM> layer(s) of the counter electrode <NUM> along that section, or a compositional change in the electrical shielding material <NUM> (e.g., resulting in lower shielding strength). The oxidation layer has an oxidation layer thickness that decreases along a length of the heat transfer tube <NUM>, having decreasing thickness from one end <NUM> to an opposite end <NUM>. The oxidation layer developed as described herein can have a maximum thickness at a point along the length of the heat transfer tube <NUM> of less than or equal to about <NUM> micometers (µm), or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>, or less than or equal to about <NUM>, or less than or equal to about <NUM>, or less than or equal to about <NUM>, or less than or equal to about <NUM>, or less than or equal to about <NUM>. In an example, an electrical potential difference of from about <NUM> volts direct current (VDC) to about <NUM> VDC can be applied between the contact electrodes (30a, 30b, 30c) in electrical communication with a heat transfer tube including substantially <NUM> series aluminum and the counter electrode <NUM> for a duration of from about <NUM> to about <NUM> minutes to form an oxidation layer having a maximum thickness of from about <NUM> to about <NUM> along the interior surface of the heat transfer tube <NUM>.

A sixth step <NUM> of the disclosed process includes stopping the passage of the electric current. Once the desired oxidation layer thickness is reached the applied electrical potential can be removed and current flow through the electrolyte solution <NUM> can be stopped.

A seventh step <NUM> of the disclosed process can include removing the electrolytic solution <NUM>. Removing can include separating the electrolytic solution <NUM> from the interior space <NUM> of the heat transfer tube <NUM> in any suitable way. For example, as in <FIG>, the flow electrolytic solution <NUM> can be stopped and a washing fluid (e.g., water), a sealing solution, or the like, can be used to flush the interior space <NUM> of the heat transfer tube <NUM>. In another example, as in <FIG>, the heat transfer tube <NUM> can be removed from the bath of the electrolytic solution <NUM> and placing in a separate washing tub containing a washing fluid (e.g., water).

An eighth step of the disclosed process can include applying a sealing solution (e.g., a corrosion resistive solution) to the oxidation layer that is formed on the interior surface of the heat transfer tube <NUM>. The sealing solution can help reduce the rate at which the oxidation layer is corroded thereby helping to improve durability. Examples of sealing solution can include, but is not limited to, an aqueous solutions of nickel acetate, potassium hexafluorozirconate and trivalent chromium sulfate (e.g., Trivalent Chrome Process (TCP)) and deionized water. Further, an aqueous nickel acetate sealing step can include exposing the interior surface of the heat transfer tube <NUM> to aqueous solution of from about <NUM> weight % (wt%) to about <NUM> wt% nickel acetate, at a temperature of from about <NUM> °F to about <NUM> °F, for a duration of from about <NUM> minutes to about <NUM> minutes. Further, a TCP sealing step can include exposing the interior surface of the heat transfer tube <NUM> to from about <NUM> wt% to about <NUM> wt% trivalent chromium sulfate, at about ambient temperature (e.g. <NUM> °F), for a duration of from about <NUM> minutes to about <NUM> minutes. Further, a deionized water sealing step can include exposing the interior surface of the heat transfer tube <NUM> to deionized water, at a temperature of boiling (e.g., <NUM> °F at <NUM> atmosphere of pressure), for a duration of from about <NUM> minutes to about <NUM> minutes.

The heat transfer tube <NUM> having an oxidation layer formed therein, as described herein, can be used in the manufacture of a heat exchanger. For example, the heat transfer tube <NUM> can be used in the manufacture of a shell and tube heat exchanger, finned tube heat exchanger, plate-fin tube heat exchange, and the like. The heat exchanger can be used in the construction of heating, air conditioning, and refrigeration equipment. For example, the heat transfer tube <NUM> can be used in the construction of a shell and tube heat exchanger that can be configured for use in a chiller of an air conditioning system. The oxidation layer formed as described herein can provide the heat transfer tube <NUM> with additional protection from corrosion and fouling over its lifetime of operation while minimizing impact of oxidation layer thickness on the thermal conductivity of the heat transfer tube <NUM>.

The numerical steps described herein are not intended to designate a corresponding temporal sequence or order of operations. Unless indicated otherwise, the steps can be performed in any order, separated into distinct temporal events, combined into a single temporal event, or can overlap temporally, without departing from the nature of, and still benefiting from, the present disclosure.

Claim 1:
A method of anodizing the interior surface of a heat transfer tube, wherein the heat transfer tube comprises aluminum, the method comprising:
placing a plurality of contact electrodes (30a, 30b, 30c) in electrical communication with, and along, an exterior surface of the heat transfer tube (<NUM>),
inserting a counter electrode (<NUM>) into an interior space of the heat transfer tube (<NUM>),
providing an electrolytic solution (<NUM>) to the interior space of the heat transfer tube (<NUM>),
passing an electric current between the plurality of contact electrodes (30a, 30b, 30c) and the counter electrode (<NUM>) through the electrolytic solution (<NUM>),
forming an oxidation layer along the interior surface of the heat transfer tube (<NUM>),
stopping the passage of the electric current,
removing the electrolytic solution (<NUM>), and
applying a sealing solution to a surface of the oxidation layer to form a sealed oxidation layer along the interior surface of the heat transfer tube (<NUM>),
characterized in that:
the formed oxidation layer has an oxidation layer thickness that decreases along a length of the heat transfer tube (<NUM>) from one end to an opposite end.