Method of surface treatment for improved cooling coils

Abstract

A method of surface treatment of a cooling coil includes subjecting the cooling coil to one or more cycles of chemical etching to form a microstructured surface. Each of the one or more cycles of chemical etching includes a first etching step in which the cooling coil is immersed in an etchant for a first half-cycle reaction time, and a second etching step in which the cooling coil is immersed in the etchant for a second half-cycle reaction time. The method includes performing a chemical vapor deposition to coat the microstructured surface with a low-surface-energy material, in which the method further includes a step of rotating the cooling coil between the first etching step and the second etching step.

Description

METHOD OF SURFACE TREATMENT FOR IMPROVED COOLING COILSRELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore patent application no. 10202402886X filed on September 16, 2024, the contents of which are hereby incorporated by reference in their entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure relates to cooling coils and more particularly to surface treatment of cooling coils.BACKGROUND

[0003] Air dehumidification is one of the important processes in air conditioning systems. In the dehumidification process, air is first cooled to its dew point temperature through sensible heat removal. Further reduction in air temperature below the dew point temperature invokes latent heat transfer where moisture in the air is removed through a condensation process. The cooled and dehumidified air is then delivered to an indoor space. The realization of air sensible and latent heat removal is typically achieved by passing air through one or more plate fin-and-tube cooling coils where low-temperature water or a refrigerant is passed through the tube-side of the cooling coils.

[0004] Conventional cooling coils are typically made of intrinsically hydrophilic materials. The refrigeration system for supplying the tube-side fluid to the conventional cooling often operates with a relatively high energy consumption to supply the required tube-side fluid temperature (e.g., as low as 8 °C lower than the air dew point temperature). The fan used to generate the required airflow across the conventional cooling coil in a heat exchanger is also typically characterized by a relatively large power consumption as the air-side pressure drop across the cooling coil can be relatively high. Overall, the dehumidification efficiency of the conventional heat exchanger needs to be improved, especially considering the pressing global need to reduce overall energy consumption and carbon emissions.SUMMARY

[0005] In one aspect, the present application discloses a method of surface treatment of a cooling coil. The method includes subjecting the cooling coil to one or more cycles of chemical etching to form a microstructured surface. Each of the one or more cycles of chemical etching includes a first etching step in which the cooling coil is immersed in an etchant for a first half-cycle reaction time. Each of the one or more cycles of chemical etching includes a second etching step in which the cooling coil is immersed in the etchant for a second half-cycle reaction time. The method includes performing a chemical vapor deposition to coat the microstructured surface with a low-surface-energy material, in which the method further includes a step of rotating the cooling coil between the first etching step and the second etching step.

[0006] The first half-cycle reaction time and the second half-cycle reaction time of any one of cycles may be equal.

[0007] The step of rotating may include disposing the cooling coil in different orientations in the first etching step and in the second etching step.

[0008] The step of rotating may include rotating the cooling coil by 180 degrees about any axis on a horizontal plane.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] To aid understanding, various embodiments of the present disclosure will be described with reference to the following figures:

[0010] FIG. 1A is an image of a cooling coil after conventional etching.

[0011] FIG. 1B and FIG. 1C are images of different regions on a fin of the cooling coil of FIG. 1A.

[0012] FIG. 1 D is an image of another cooling coil after conventional etching.

[0013] with parts of the fins destroyed by etching.

[0014] FIG. 1 E is a schematic diagram of a cooling coil.

[0015] FIG. 2 is a schematic flow chart showing a method according to embodiments of the present disclosure.

[0016] FIG. 3 is a schematic illustration of a part of the method of FIG. 2.

[0017] FIG. 4 is a schematic illustration of another part of the method of FIG. 2.

[0018] FIG. 5 are images of etched surfaces after the chemical etching according to embodiments of the present disclosure.

[0019] FIG. 6 is a schematic diagram showing another aspect of the method, according to embodiments of the present disclosure.

[0020] FIG. 7 are scanning electron microscopy (SEM) images of the cooling coil at various stages of the present method.

[0021] FIG. 8 shows the SEM images of different regions of the cooling coil with micro / nanostructures successfully fabricated on the fin surface.

[0022] FIG. 9 shows images of uncoated fins and superhydrophobic fins of a cooling coil from one experiment.

[0023] FIG. 10 shows images of uncoated fins and superhydrophobic fins of a cooling coil from another experiment.

[0024] FIG. 11 shows images and results from a wind tunnel experiment.

[0025] FIG. 12 shows images from an experiment using conventional etching.

[0026] FIG. 13 shows SEM images and XPS results of cooling fins superhydrophobic-treated in accordance with a method of the present disclosure.

[0027] Fig. 14 shows images of cooling fins showing an evolution of the condensate droplets in an experiment.

[0028] FIG. 15 shows the maximum droplet diameter of condensate droplets on different types of cooling coil surfaces.

[0029] FIG. 16 shows different types of condensate droplet bouncing detachment possibilities.

[0030] FIG. 17 shows images of the cooling coils used in the further experiments.

[0031] FIG. 18 shows images of condensation on different fin surfaces.

[0032] FIG. 19 shows the bubble-like bridging edge movement observed.

[0033] FIG. 20 are images showing condensation on the conventional-0.1 mm and SHL-2Tier-0.06 mm surfaces.

[0034] FIG. 21 are images showing condensation on SHP-2Tier-0.06 mm surface with different air velocities.

[0035] FIG. 22 are images showing flying droplets on a SHP-2Tier-0.06 mm surface.

[0036] FIG. 23 is a chart showing the effect of water flow rate and different surface modification on heat transfer rate for the different cooling coils.

[0037] FIG. 24 is a chart showing the pressure drop for conventional and surface-modified samples as a function of air velocity under various coolant velocities.

[0038] FIG. 25 is a chart showing the heat transfer rate remeasured one day after the initial test and after seven days measured after the initial test.

[0039] FIG. 26 is a chart showing the pressure drop remeasured one day after the initial test and after seven days measured after the initial test.DETAILED DESCRIPTION

[0040] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0041] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise. In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0042] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0043] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0044] Terms such as “first”, “second”, "third", "fourth", etc., are used in the description and claims only for the sake of brevity and clarity, and do not necessarily imply a priority or order, unless required by the context.

[0045] Some methods may be described in terms of steps, stages, phases, or the like, merely to aid understanding and / or for convenient reference. The delineation between one step and another step may be merely for convenient reference in the present disclosure. It will be understood that in actual implementation there may not be a clear division or transition from one step to another subsequent step. There may be a certain amount of overlap among the steps and / or more than one step may occur or be performed concurrently in time, etc., unless the context dictates otherwise.

[0046] In the present disclosure, the terms “cooling coil”, “fin-and-tube cooling coil”, and “cooling coils” are used interchangeably to refer to a device with one or more tubes with a plurality of fins disposed about the one or more tubes. The plurality of fins may be disposed in parallel to one another along a length of the tubes. The plurality of fins may be disposed at regular intervals, e.g., with immediately adjacent fins being spaced apart by a relatively small spacing. The spacing between adjacent fins is also referred to herein as the fin pitch. The fin pitch is relatively small compared to the dimensions of the fins. For example, in a cooling coil, each of the plurality of fins may be about 250 mm (millimeters) x 200 mm, and the fin pitch may be about 3.0 mm. A cooling fin may be assembled with other components to form a heat exchanger.

[0047] Various methods have been explored to improve the performance of the cooling coil. There are several technical challenges to overcome, including the condensate droplet bounce hysteresis problem resulting from the localized andnanoscale Wenzel wetting state in micro-regions that may cause condensed water droplets to collect over time. It is believed that one reason for this flooding-like wetting phenomenon is that the surface is often uneven or non-uniform, with unetched defects caused by metal anisotropy. Condensation and the accumulation of condensed water on the fins obstructs the airflow passages, resulting in an increase in the cool coil's air-side pressure drop, which is a major contributor to fan power consumption. When operating in sub-zero temperature environments, the accumulated liquid condensate on the cooling coil resulted in pre-mature frosting and ice accretion, which further reduces the cooling coil efficiency.

[0048] According to embodiments of the present disclosure, a method of surface treatment of a cooling coil. The proposed method includes an improved and more uniform substrate for functionalization with superhydrophobic properties.

[0049] FIG. 1A shows an image of the cooling coil after the fins 220 were subjected to a conventional method of surface treatment. FIG. 1B and FIG. 1C are magnified images of different areas on the same fin in FIG. 1A. FIG. 1B shows an example of a region on a surface of cooling coil that was relatively uniformly etched, i.e., the microstructures in this region were observed to be relatively well-formed. FIG. 1C shows an example of another region on the surface of the cooling coil which was non-uniformly etched. The region in FIG. 1C shows regions where there are no microstructures and regions where there are microstructures. As mentioned, the non-uniformity may lead to uneven cooling and non-optimal performance of the cooling coil. FIG. 1D is an image showing another cooling coil 200 after a conventional chemical etching. Some parts of the fins 220 (e.g., upper left side of the image) appear intact. Some other parts of the fins 220' (circled) show signs of being over-etched. The fins have been completely etched away or destroyed by over-etching, leaving behind structures of smaller surface areas and irregular shapes. The over-etching exposes parts of the one or more tubes 210. Understandably, such a cooling coil would not be able to deliver at its designed optimal performance. As shown, the physical geometry of the cooling coil makes it difficult for chemical etching to be applied uniformly throughout both surfaces of each of the fins.

[0050] It was also found that when a sample was first immersed in an etchant, e.g. hydrochloric acid solution, random local regions on the surfaces of the cooling coil would begin to react with the etchant, whereas other regions may have no observable reaction with the etchant.

[0051] Additionally, when a part of a surface began to undergo etching, vapor bubbles were formed and adhered to the surface. The surface within the three- phase contact line of the vapor bubble (the interface region whereby the solid surface, acid solution, and vapor bubble) would be protected from the acid solution and would not react until the vapor bubble coalesces with another vapor bubble, and the vapor bubble was finally large enough to depart due to buoyancy forces. Overall, the surfaces experienced uneven etching (also referred to as a "high preferential etching phenomenon") with some regions being etched and some other regions not being etched.

[0052] FIG. 1 E is a schematic diagram of a cooling coil 200. The cooling coil 200 may include one or more tubes 210 and a plurality of fins 220 disposed thereon. The cooling coil 200 may be described as having two sides on either side of a cooling coil axis 202, the two sides being denoted as "A-side" and "B-side" solely for the sake of brevity and convenient reference.

[0053] FIG. 2 is a schematic flow chart showing embodiments of the proposed method 100. The method 100 includes forming a first plurality of first-tier microstructures. The method of surface treatment includes subjecting the cooling coil to a chemical etching that is configured to produce a substantially uniform surface or substrate for subsequent functionalization. The chemical etching may be a one-stage chemical etching or a two-stage chemical etching 300, 400. The method may further include further subjecting the etched cooling coil to a stage of forming boehmite. The surface of the cooling coil may be referred to as a microstructured surface after the etching and optionally the boehmitization 120 may be performed on the microstructured surface. The method may further include subjecting the cooling coil to a stage of chemical vapor deposition 130 in which a hydrophobic material is coated on the microstructured surface.

[0054] FIG. 3 illustrates a first etching stage 300 (also referred to as a "high concentration etching", "higher concentration etching", or "rapid etching") of the method which includes use of an etchant that may be generally considered to be a concentrated industrial acid, e.g., hydrochloric acid (HCI) of 2 mole (M) concentration. The cooling coil 200 may be disposed in a vessel so that the entire cooling coil may be fully immersed in the etchant.

[0055] In a first cycle 310, the first etching stage 300 includes a cycle in which, in a first cycle first step 311 , the cooling coil 200 is immersed in the etchant (e.g., acid solution) for a half-cycle reaction time (e.g., 1 minute but not limited to exactly such). The first cycle 310 includes a first cycle second step 312 of in which the cooling coil 200 is rotated (e.g., by a 180° rotation). The first cycle 310 includes a first cycle third step 313 in which the cooling coil 200 is immersed in the etchant for another half-cycle reaction time (e.g., 1 minute but not limited to exactly such). The two half-cycle reaction times of the same cycle are set to be of the same length of time or same reaction duration. The cooling coil 200 may be disposed with the cooling coil axis 202 disposed on a horizontal plane 101 , e.g., the cooling coil may be rotated by 180° about any axis on the horizontal plane. If the cooling coil 300 is A-side up (e.g., first side up) and B-side down (e.g., second side down) in the first cycle first step 311 , the cooling coil is B-side up (e.g., second side up) and A-side down (e.g., first side down) in the first cycle third step 313, and vice versa. The cooling coil may remain fully immersed throughout the step of rotating the cooling coil.

[0056] In a first intervening step 314 that is performed after the first cycle 310 and before a second cycle 320, the cooling coil 200 is rotated (e.g., by a 180° rotation).

[0057] The half-cycle reaction time of a successive cycle of the higher concentration etching is shorter than the half-cycle reaction time of a preceding cycle of the higher concentration etching. For example, in a second cycle 320, the first etching stage 300 includes a second cycle first step 321 in which the cooling coil 200 is immersed in the etchant for a half-cycle reaction time (e.g., 15 seconds) that is shorter than the half-cycle reaction time of the preceding cycle. The secondcycle 320 includes a second cycle second step 322 of in which the cooling coil 200 is rotated (e.g., by a 180° rotation). The second cycle 320 includes a second cycle third step 323 in which the cooling coil 200 is immersed in the etchant for the same half-cycle reaction time as the second cycle first step (e.g., 15 seconds). If the cooling coil 300 is A-side up and B-side down in the second cycle first step 321 , the cooling coil is B-side up and A-side down in the second cycle third step 323, and vice versa.

[0058] An intervening step of rotating the cooling coil between successive cycles is carried out. For example, in a second intervening step 324 that is performed after the second cycle 320 and before a third cycle 330, the cooling coil 200 is rotated (e.g., by a 180° rotation).

[0059] The half-cycle reaction time of a successive cycle of the higher concentration etching is shorter than the half-cycle reaction time of a preceding cycle of the higher concentration etching. For example, in a third cycle 330, the first etching stage 300 includes a third cycle first step 331 in which the cooling coil 200 is immersed in the etchant for a half-cycle reaction time (e.g., 10 seconds) that is shorter than the half-cycle reaction time of the preceding cycle. The third cycle 330 includes a third cycle second step 332 of in which the cooling coil 200 is rotated (e.g., by a 180° rotation). The third cycle 330 includes a third cycle third step 333 in which the cooling coil 200 is immersed in the etchant for the same half-cycle reaction time as the third cycle first step (e.g., 10 seconds). If the cooling coil 300 is A-side up and B-side down in the third cycle first step 331 , the cooling coil is B- side up and A-side down in the third cycle third step 333, and vice versa.

[0060] Another intervening step of rotating the cooling coil between successive cycles is carried out. For example, in a third intervening step 334 that is performed after the third cycle 330 and before a fourth cycle 340, the cooling coil 200 is rotated (e.g., by a 180° rotation).

[0061] The half-cycle reaction time of a successive cycle of the higher concentration etching is shorter than the half-cycle reaction time of a preceding cycle of the higher concentration etching. For example, in a fourth cycle 340, the first etching stage 300 includes a fourth cycle first step 341 in which the cooling coil200 is immersed in the etchant for a half-cycle reaction time (e.g., 5 seconds) that is shorter than the half-cycle reaction time of the preceding step. The fourth cycle 340 includes a fourth cycle second step 342 of in which the cooling coil 200 is rotated (e.g., by a 180° rotation). The fourth cycle 340 includes a fourth cycle third step 343 in which the cooling coil 200 is immersed in the etchant for the same halfcycle reaction time as the fourth cycle first step (e.g., 5 seconds). If the cooling coil 300 is A-side up and B-side down in the fourth cycle first step 341 , the cooling coil is B-side up and A-side down in the fourth cycle third step 343, and vice versa.

[0062] The cooling coil 200 was cleaned using deionized water to remove any hydrochloric acid remaining on the surfaces of the cooling coil, reaction products after etching, and / or other contaminants that may be attached to the surfaces.

[0063] FIG. 4 illustrates a second etching stage 400 of the method to be performed after the first etching stage 300. A less concentrated etchant is selected for use in the second etching stage 400. In some examples, the less concentrated etchant used was 0.6 M hydrochloric acid. The second etching stage 400 may also be referred to as a "low concentration etching", "lower concentration etching", or "slow etching" as it uses a less concentrated etchant compared to the first etching stage 300.

[0064] The second etching stage 400 may include only one cycle 410. That is, the method may include more than one cycle of a higher concentration etching and one cycle of a lower concentration etching.

[0065] The total reaction time of the lower concentration etching is longer than twice of the half-cycle reaction time of the longest half-cycle reaction time of the higher concentration etching. Any of the half-cycle reaction time of the lower concentration etching is longer than any of the half-cycle reaction time of the higher concentration etching.

[0066] For example, the second etching stage 400 (or the lower concentration etching) may include a second etching stage first step 411 in which the cooling coil 200 is immersed in the etchant for a half-cycle reaction time (e.g., 26 minutes). The second etching stage 400 may include a second etching stage second step 412 of in which the cooling coil 200 is rotated (e.g., by a 180° rotation). The second etchingstage 400 may include a second etching stage third step 413 in which the cooling coil 200 is immersed in the etchant for the same half-cycle reaction time (e.g., 26 minutes) as that of the second etching stage first step 411 .

[0067] The cooling coil 200 may be disposed with the cooling coil axis 202 disposed on a horizontal plane 101 , e.g., the cooling coil may be rotated by 180° about any axis on the horizontal plane. If the cooling coil 200 is A-side up and B- side down in the second etching stage first step 411 , the cooling coil is B-side up and A-side down in the second etching stage third step 413, and vice versa.

[0068] The total reaction time for the lower concentration etching or second etching stage 400 may be between 50 minutes and 55 minutes if the lower concentration etching is preceded by a higher concentration etching.

[0069] If the chemical etching was performed using only one concentration of an etchant, e.g., no higher concentration etching is performed, the chemical etching using only a lower concentration etchant (e.g., 0.6 M concentration hydrochloric acid) may be carried out for a total reaction time of 4 hours or about 4 hours.

[0070] The total reaction time of the lower concentration etching is longer than twice (or two times) of the first half-cycle reaction time of the higher concentration etching. For example, the total reaction time of the lower concentration etching may be longer than the total reaction time of the higher concentration etching. For example, any half-cycle reaction time of the lower concentration etching may be longer than any half-cycle reaction time of the higher concentration etching.

[0071] After the etching process, the cooling coil was cleaned with deionized water, e.g., to remove any hydrochloric acid remaining on the surfaces of the cooling coil, reaction products after etching, and / or other contaminants that may be attached to the surfaces.

[0072] In the first etching stage 300 including two or more cycles, the half-cycle reaction time of any cycle may be shorter than the half-cycle reaction time of an immediately preceding cycle. A total reaction time of the first etching stage 300 is preferably shorter than the total reaction time of the second etching stage 400, in which the etchant concentration in the first etching stage 300 is higher than the etchant concentration in the second etching stage 400.

[0073] FIG. 5 show samples of the fins from the upper, middle and lower sections of the cooling coil analyzed by scanning electron microscopy (SEM). The etched surfaces are visibly more uniformly treated than those after conventional etching (e.g., FIG. 1B and FIG. 1C). It can be seen in FIG. 5 that all regions of the surfaces were covered with microstructures. The images further verify that the two- stage etching results in a formation of a first plurality of first-tier microstructures.

[0074] Referring again to FIG. 2, after forming the first plurality of first-tier microstructures (e.g., after the chemical etching), the method 100 may further include forming a second plurality of second-tier nanostructures, e.g., via a boehmite forming process 120. The temperature of the boehmite forming process may be from 90°C to 95°C. The duration of the boehmite forming process may be from 50 minutes to 70 minutes. In some of the experiments conducted, the cooling coil was immersed in deionized water of 92°C for 1 hour to form boehmite.

[0075] After the first-tier microstructures and the second-tier nanostructures have been formed, the cooling coil was then subjected to chemical vapor deposition (CVD) treatment 130. The surfaces of the cooling coil had to be absolutely dry before the CVD treatment. Examples of low-surface-energy substances that may be employed in the CVD process include but are not limited to any one of the following: fluorides, silanes, and fatty acids. Examples of suitable fluorides include but are not limited to 1 H,1 H,2H,2H-perfluorodecyltrimethoxysilane and 1 H,1 H,2H,2H-Perfluorodecyltriethoxysilane, etc. Examples of suitable silanes include but are not limited to trimethylethoxysilane, methyltrimethoxysilane, hexadecyltrimethoxysilane, etc. Examples of suitable fatty acids include but are not limited to lauric acid and stearic acid, etc. Low-surface-energy substances used in the CVD process may be selected from materials that can be dissolved in volatile, non-polar organic solvents, such as toluene, tetrahydrofuran, etc. The mass percentage of low-surface-energy substances dissolved in organic solvents during CVD treatment may range from 3 wt% to 5 wt%. The CVD treatment of samples with low surface energy can take from 3 hours to 5 hours, and the temperature of the treatment can be from 90°C to 96°C. The CVD-treated cooling coil has surfaces which have been conferred superhydrophobic properties.

[0076] According to another embodiment of the present disclosure, the method 100 includes a lower concentration etching or a slow etching cycle using the etchant at a lower concentration, without involving the etchant of a higher concentration. It was found that the lower concentration etching provided sufficient time for the areas that may react slower (or experience reduced chemical reactions) to be etched so that the microstructures can be more uniformly generated on the entire surface. Additionally, it was found that the lower concentration etching, even if there were regions where etching is preferred, the fins would not be over-etched, i.e., there would not be a complete removal of the materials of the fins, and neither would the fins end up being too thin. A generally uniform surface microstructuring could be achieved and it was possible to facilitate or maximize Cassie-Baxter state droplets and promote jumping droplet condensation of condensates on the surfaces, even at the surfaces of the fins of the cooling coil.

[0077] FIG. 6 is a schematic diagram showing another aspect of the proposed method according to embodiments of the present disclosure. The method 100 may be described in terms of generating micro / nanostructures and functionalization of the micro / nanostructures formed.

[0078] The method 100 includes rinsing the cooling coil with deionized water, ethanol (e.g., available from Sigma-Aldrich, CAS no. 64-17-5) and isopropyl alcohol (e.g., available from Sigma-Aldrich, CAS no. 67-63-0), and then air dried.

[0079] Subsequently, the cooling coil is immersed in an etchant for a total reaction time of 4 hours. The cooling coil is similarly disposed in different orientations by rotating the cooling coil in the etchant. The cooling coil may be rotated once (e.g., one cycle) throughout the chemical etching or multiple times (e.g., more than one cycle). The half-cycle reaction time of the one or more cycles may be the same or equal within the same cycle.

[0080] The etchant may be any low concentrated industrial acid. Examples of industrial acids include but are not limited to hydrochloric acid, sulfuric acid, and nitric acid. In the experiments, a low concentration hydrochloric acid (Sigma-Aldrich, CAS no. 7647-01-0) of 0.6 M concentration was used. The total volume of acid per surface area of the fin may be determined based on a ratio of 384.9 ml / m2. A firstplurality of single-tier microstructures would be formed as a result of the single- stage etching process.

[0081] After the etching process, the cooling coil is thoroughly rinsed with deionized water, followed by ethanol, and finally by isopropyl alcohol, and then air dried.

[0082] The cooling coil may then be subject to a stage of forming boehmite, e.g., generating a second plurality of second-tier nanostructures (e.g., boehmite of about 300 nanometer dimensions) on the microstructured etched surface. For example, the cooling coil may be immersed in a deionized water bath of 95°C ± 3°C for 1 hour.

[0083] After the stage of forming boehmite (also known as a boehmitization process or an oxidation reaction), the cooling coil is cleaned. For example, the cooling coil may be cleaned by being rinsed with ethanol and isopropyl alcohol to remove any residual fluid and debris on the surfaces. The cleaned surfaces are then air dried. The cleaning also aids in the subsequent step of air drying the cooling coil.

[0084] After the cooling coil is completely dry, the cooling coil is subjected to a CVD treatment, e.g., functionalizing surfaces of the cooling coil by providing a low- surface energy nanocoating. In the course of the CVD treatment, the surfaces of the cooling coil may be coated with heptadecafluorodecyltrimethoxysilane (HTMS) (HTMS, Gelest, CAS #83048-65-1 ) and toluene solution (5% HTMS and 95% toluene) under atmospheric pressure, in which the air temperature is maintained at 90°C ± 3°C for 4 hours (e.g., by using an oven). A total HTMS solution volume of 307.9 pl_ / m2 was found to be sufficient to evenly coat the cooling coil.

[0085] For uniform coating of the HTMS vapor, a beaker containing the HTMS solution may be placed under the cooling coil with the cooling coil elevated above the base of the oven.

[0086] On completion of the CVD process, surfaces of the cooling coil would be conferred with superhydrophobic properties.

[0087] Images of row (a) of FIG. 7 show the SEM images of the microstructured surface post-etching. Images of row (b) of FIG. 7 show the SEM images of themicro / nanostructured surface after the boehmitization process. FIG. 8 shows the SEM images of different regions of the cooling coil with micro / nanostructures successfully fabricated on the fin surface.

[0088] In particular, it can be seen that the proposed method has successfully addressed the challenge of surface structuring process posed by over-etching and under-etching in different parts of the same cooling coil. Over-etching results in the destruction of the fins which are essential for condensation heat transfer. Underetching of fins would result in non-uniform surface microstructuring and negatively impact the condensation performance of the cooling coil. Additionally, the proposed method can be adapted for different applications which may have different design space and constraints, e.g., different sizes of cooling coils may be required for various applications. The method described herein can be applied to various shapes and sizes of cooling coils.

[0089] The proposed method takes into account the fin surface area of the cooling coil to determine the quantity of acid solution required. For example, the surface area may be calculated by obtaining the product of the fin length, fin width, and the number of fins, and multiplying the product by two to account for both sides of each fin (e.g., the A-side and the B-side) to obtain a total fin surface area. The volume of etchant may be obtained by applying a correction factor to account for the desired reaction coverage and efficiency. To illustrate, in some experiments, the total fin surface area was multiplied by a correction factor of 384.9 mL / m2to determine the volume of hydrochloric acid (as etchant) required, and water was added until the concentration (e.g., 0.6 M concentration) was obtained. In some examples, the chemical etching may have a total reaction time of 4 hours of immersing the cooling coil in a volume of hydrochloric acid of 0.6 M concentration, with the volume of the hydrochloric acid solution per surface area of the cooling coil being determined by a ratio (correction factor) of 384.9 mL / m2. Additionally, for the fabrication process to be more robust, the etching quality is controlled by the total volume of acid solution used. In other words, the total amount of acid is added simply based on the surface area of the cooling coil fins, while the concentration is kept low to minimize over-etching of regions of the fin surface. Thus, when most ofthe acid has reacted away in the solution, the resulting etched surface is uniformly structured for subsequent processes.

[0090] Experimental results

[0091] To evaluate condensate droplet repellence of the superhydrophobic cooling coil developed by the method according to the embodiments described herein, air dehumidification experiments were performed by running chilled water at 5°C through the tubes of the cooling coil and allowing the cooling coil surfaces to be exposed to ambient air of 28°C ± 1°C and relative humidity of 75% ± 5% for 6 hours. In addition, an uncoated commercial cooling coil was also tested for comparison.

[0092] FIG. 9 shows a comparison of the water vapor condensing on an uncoated conventional cooling coil (image (a) of FIG. 9). Image (b) of FIG. 9 shows a superhydrophobic cooling coil fabricated by the method of the first embodiment from 0 hour to 3 hours into the test. Image (c) of FIG. 9 shows a superhydrophobic cooling coil fabricated by the method of the first embodiment from 3 hour to 6 hours into the test.

[0093] It can be seen from image (a) of FIG. 9 that thick condensate film in the form of water bridges is formed on the inter-fin spaces of the uncoated conventional cooling coil. Such condensate formation is undesirable and leads to large thermal resistance that limits the dehumidification efficiency of the cooling coil.

[0094] In contrast, the superhydrophobic cooling coil shown in images (b) and (c) of FIG. 9 resulted in the formation of smaller and highly mobile condensation droplets, which potentially reduces the thermal resistance between the fin surfaces and surrounding air. The droplets were seen either rolling off the surfaces due to low surface adhesion or jumped off the surface by overcoming surface adhesion and gravitational forces when coalescing with neighboring droplets. In addition, the condensation phenomenon on the superhydrophobic cooling coil produced by the proposed method was found to be highly stable and was able to sustain 6 hours of continuous test duration.

[0095] Using similar experimental methodology, condensation on a superhydrophobic cooling coil developed by the method according to the second embodiment was conducted.

[0096] As shown in the image on the left in FIG. 10, the uncoated conventional cooling coil exhibited filmwise condensation which was unfavorable for cooling performance.

[0097] In contrast, as shown in the image on the right in FIG. 10, condensate droplets formed on the superhydrophobic cooling coil were highly mobile and exhibited exceptional droplet repellence by coalescence-induced droplet jumping. The surfaces of the cooling coil remain relatively dry free of condensate, indicating a significant reduction in fin-air thermal resistance. Furthermore, even after more than 5 hours of continuous tests, droplet jumping condensation mode was sustained, indicating good Cassie state stability.

[0098] To characterize the air-side pressure drop reduction during air dehumidification of the superhydrophobic cooling coils (cooling coils conferred with superhydrophobic properties in accordance with the first embodiment of the proposed method), experiments were performed using an open-loop wind tunnel, as shown in part (a) of FIG. 11.

[0099] Two cooling coils were connected in series, and three different cooling coil configurations were tested. They are (i) two uncoated conventional cooling coils connected in series, (ii) one uncoated conventional cooling coil and one superhydrophobic cooling coil connected in series, and (iii) two superhydrophobic cooling coils connected in series.

[0100] The experiments were conducted at ambient air temperature of 28°C ± 1 °C and relative humidity of 75% ± 5%. The chilled water inlet temperature supplied to the cooling coil was set at 10°C, and the flow rate was fixed 3 l / min. The pressure drop results are shown in part (b) of FIG. 11. It can be seen that the superhydrophobic cooling coils resulted in a significant reduction in air-side pressure drop for the tested air velocity range of 1 to 6 m / s, indicating the potential of the superhydrophobic cooling coil in reducing fan energy consumption.

[0101] Part (c) of FIG. 11 shows the percentage reduction of air-side pressure drop of two superhydrophobic cooling coils connected in series as compared to two uncoated conventional cooling coils over the range of air velocity tested. As the operating air velocity in commercial air handling units (AHU) typically ranges between 1.5 m / s and 2.5 m / s, it can be clearly seen from the chart that in this operating range, the superhydrophobic cooling coils demonstrated the highest pressure drop reduction of 15 % to 17%.

[0102] The experimental results show that the proposed method can promote condensate droplet formation and enhance condensate droplet repellence for improved performance of residential and commercial air-conditioning systems, cooling coils in automobile and electrified transportation systems, cooling coils in petrochemical and power generation plant systems, and various other engineering systems requiring air dehumidification and condensation processes.

[0103] More experimental results

[0104] FIG. 12 shows images from an experiment. A conventional cooling coil (e.g., as shown in part (b) of FIG. 12) from a conventional heat exchanger (e.g., as shown in part (a) of FIG. 12) was used in the experiments. The cooling fins were etched according to a conventional method in an attempt to make the cooling fins superhydrophobic and capable of water droplet repellency. Part (c) of FIG. 12 shows that the conventional etching process resulted in overetching. Part (d) of FIG. 12 shows that tiny water droplets could remain and grow on parts of the cooling fins owing to the presence of unetched areas. Parts (e1 ) to (e4) of FIG. 12 are SEM images taken from the fins, further showing the presence of unetched defects. Parts (c) to (e4) of FIG. 12 show that the conventional etching process produces many irregularities and defects. Although the fins may be superhydrophobic after the conventional etching process, the extent of the defects render the cooling fins unsuitable for use. In the present context, the term "unetched defects" refer to defects in the form of a non-uniformity, e.g., with parts of the cooling coil not etched and parts of the same cooling coil etched.

[0105] Condensation droplets were generated from gaseous droplets by nucleation growth. As a result, the size of the condensate droplets is typically verysmall. The unetched defects, or unetched areas on the non-uniformly etched cooling fins, were not able to repel the tiny water droplets well. As a result of the presence of unetched defects, the Wenzel wetting state was observed in the conventionally etched cooling fins, e.g., tiny water droplets remaining on the surface of the cooling fins and contributing to a greater risk of flooding.

[0106] Experiments involving a large-scale preparation of superhydrophobic cooling coils essentially free from unetched defects (using a method according to an embodiment of the present disclosure), enabling the cooling coil to achieve a maximum condensate droplet diameter before bouncing that is limited to 1 millimeter (mm). Cooling fins of dimensions in the scale of 25 cm * 24 cm x 9 cm size or larger with irregular shapes could be prepared according to embodiments of the present disclosure. That is, it was found that nanoscale Wenzel wetting state caused by unetched defects was eradicated and the probability of flooding was significantly reduced. The superhydrophobic cooling coils of the present disclosure could keep condensate droplets bouncing off the surfaces for 6 hours of continuous testing.

[0107] Parts (a) to (c) of FIG. 13 are SEM images of superhydrophobic-treated cooling coils with uniformly generated micro / nanostructures ("just-right superhydrophobic surface fully etched"). A relatively uniform growth of the nanoscale structures on the surface of the micrometer structures could be observed, and there are no unetched defects in these integrated micro- and nano- multilevel composite structures. The resulting superhydrophobic cooling coil surfaces had a water contact angle of 158°. From the XPS survey spectra shown in part (d) of FIG. 13, it can be seen that the chemical elements of the superhydrophobic cooling coils are mainly composed of C, O, F, Al and Cl. The high-resolution spectra of C 1s shows that the resulting superhydrophobic cooling coil surface was mostly composed of C-C / C-H bond at around 284.81 eV, C-0 bond at around 286.12 eV, C-CFx bond at around 288.82 eV, C-F2 bond at around 291.51 eV and C-F3 bond at around 294.11 eV, which were primarily derived from the superhydrophobic modification with HTMS.

[0108] To further demonstrate this point, the high-resolution spectra of F 1s were found mainly composed of C-Fx bond at around 688.66 eV and Al-F bond at around 685.01 eV. The detected Al-F bond indicated that discrete F atoms adsorbed on the surface of the superhydrophobic-modified aluminum coils during the hydrolysis of HTMS generated Al-F bonds under charge. As a result, superhydrophobic cooling coils were prepared by the coordinated action based on the micro- and nano- multilevel composite structures and the low surface energy of HTMS.

[0109] Fig. 14 shows images of cooling fins showing an evolution of the condensate droplets. The images were taken over a 6-hour period under the conditions of an inlet temperature of 5 °C, a room temperature of 28 °C, and a relative humidity of 80%.

[0110] Part (a) of FIG. 14 shows the superhydrophobic cooling coil with uniformly generated micro / nanostructures. After 6 hours of condensation testing, the superhydrophobic cooling coil with uniformly generated micro / nanostructures still maintained good droplet bouncing capability.

[0111] Part (b) of FIG. 14 shows the superhydrophobic cooling coil with unetched defects. After 2 hours of condensation testing, the superhydrophobic cooling coil with unetched defects begin to flood.

[0112] Part (c) of FIG. 14 shows a conventional cooling coil (bare aluminum cooling coil surface). In just 24 minutes, condensate droplets covered the bare aluminum cooling coil surface forming a water film (filmwise condensation). The condensate droplets began to exhibit a flooding behavior around 1 hour.

[0113] Referring to the experimental results shown in FIG. 15, in the cooling coil surface treated according to the proposed method, the maximum condensate droplet diameter could keep below 1 mm for a superhydrophobic cooling coil with uniformly generated micro / nanostructures (e.g., a fully etched superhydrophobic surface without unetched defects) that is not affected by droplet bouncing hysteresis caused by localized Wenzel phenomena at the nanoscale. The superhydrophobic cooling coils had a fin pitch of 1.3 mm and condensate droplets detached during growth due to coalescence-induced jumping. The maximum droplet diameter was lower than 1 mm before flooding occurs.

[0114] In contrast, in a conventional superhydrophobic cooling coil with unetched defects, condensate droplets accumulated at the unetched defects. The formation of nucleation sites sped up the growth of condensate droplets, leading to faster flooding.

[0115] FIG. 16 shows images and schematic diagrams of different types of condensate droplet bouncing detachment possibilities in the case of the superhydrophobic cooling fin without unetched defects, including self-jumping behavior, coalescence-induced jumping behavior, coalescence-induced rolling and then jumping. Part (a) of FIG. 16 shows images captured between 180 minutes and 181 minutes into the condensation test (after 3 hours of testing). It was observed that the mismatched condensate droplets could be rapidly detached by coalescence-induced jumping (parts (c) and (d) of FIG. 16). Part (e) of FIG. 16 shows that the large condensate droplets caused by coalescence were not able to remain on the superhydrophobic surface with uniformly generated micro / nanostructures. The condensate droplets continued to roll away from the superhydrophobic surface under gravity or under impact from other droplets, despite the fact that multiple mismatched droplets reach force equilibrium during the coalescent process and sometimes do not bounce in stages.

[0116] Further experimental results

[0117] FIG. 17 shows images of the cooling coils used in further experiments to study the impact of the proposed superhydrophobic surfaces on heat transfer performance in commercial fin-and-tube heat exchangers under dehumidification conditions. The cooling coils selected were of a wavy configuration with dimensions of 206 mm x 200 mm x 45 mm (L x H x W). The fin pitch was 1.3 mm, with two tube rows and eight tubes per row. The fins were made of aluminum, while the tubes were copper. The cooling coils were originally characterized by superhydrophilic surfaces. Superhydrophobic surfaces without unetched defects were formed from the originally superhydrophilic surfaces. The dehumidification performance of heat exchangers with such cooling coils were assessed. Five types of surfaces with varying surface wettability were assessed, including: SHP (superhydrophobic), SHL (superhydrophilic), UNC (uncoated), SLIPS (slippery liquid-infused porous surface),QLS (quasi-liquid surface). Their results showed that low-wettability surfaces (SHP and QLS) exhibited higher heat fluxes and collected more water compared to high- wettability surfaces (SHL and SLIPS).

[0118] The preparation of superhydrophilic (SHL) and superhydrophobic (SHP) surfaces involved the proposed multistep surface modification protocol described above. The cooling coils were first meticulously cleaned with water to remove surface contaminants. Subsequently, an oxidation-reduction treatment was used by immersing the surface in a 2 M hydrochloric acid solution for 3.5 minutes, followed by immersion in a 0.6 M solution for 60 minutes to establish microscale roughness structures. The microstructured samples were rinsed thoroughly with deionized (DI) water and subjected to boiled at 90 °C for 1 hour to induce in-situ growth of nanostructures on the microstructured surfaces. This hierarchical micro-nano fabrication process ultimately allowed the wavy fin surfaces with superhydrophilicity. To achieve superhydrophobicity, the SHL surfaces were further modified via chemical vapor deposition using HTMS at 90 °C for 3 hours. The sample surface must be thoroughly cleaned with DI water and dried before CVD treatment.

[0119] The modified heat exchangers were then installed in a wind tunnel using a bracket. Additionally, SHL and SHP heat exchangers without the etching procedure were also fabricated. The coating procedures and definitions of the heat exchangers are summarized in Table 1 below. Notably, the oxidation-reduction process reduced the fin thickness from 0.105 mm to 0.06 mm.Table 1

[0120] Experimental conditions of the wind tunnel for dehumidification performance testing are shown in Table 2 below.Table 2

[0121] In other words, the chemical etching described above was used to form superhydrophilic surfaces which were further coated via chemical vapor deposition to form superhydrophobic surfaces.

[0122] In the study, the influence of surface modification on dehumidification condensation flow patterns, heat transfer rate, and pressure drop in heat exchangers inside and outside a wind tunnel was examined. To investigate the dehumidification condensation flow pattern on the fin surface, photographs were first taken outside and inside the wind tunnel. Subsequently, the heat transfer rate and pressure drop were measured in the wind tunnel to evaluate the quantitative performance.

[0123] FIG. 18 shows images of condensation on different fin surfaces: (a) Conventional-0.1 mm under natural convection, (b) SHL-2Tier-0.06 mm under natural convection, (c) SHP-2Tier-0.06 mm under natural convection, (d) Conventional-0.1 mm under forced convection, (e) SHL-2Tier-0.06 mm under forced convection, (f) SHP-2Tier-0.06 mm under forced convection.

[0124] As shown in part (d) of FIG. 18, the droplet density on the conventional surface was much lower than that observed under natural convection. An interesting phenomenon — a bubble-like water bridge formation, movement, and disappearance (highlighted in the ellipse) — was observed between the fins. Based on video analysis, the bubble-like bridging appeared to form when condensation liquid drips from the heat exchanger tube due to gravity. With the aid of airflow, the bridge moved along the air stream, increasing in height as it absorbed condensed liquid from the fin surface. Once the bridge reached the condenser fin tip surface, its height continued to grow rapidly both upwards and downwards within the fin gap. Simultaneously, the absorbed condensed liquid flowed downward more easily with the assistance of the bubble-like liquid bridge. Eventually, the bridge thickness decreased, leading to rupture. The movement rate of the bridge’s edge could reach up to 0.2 m / s, as shown in FIG. 19.

[0125] FIG. 20 are images showing condensation on the conventional-0.1 mm and SHL-2Tier-0.06 mm surfaces. For the conventional surface, droplets covered the fin surface and could bridge the fin gaps. The droplet diameter was nearly equal to the fin pitch, and the droplets gradually flowed downward. The liquid-gas interface was clearly visible at the bottom of the heat exchanger.

[0126] For the front view of the SHL surface, the liquid film on the fin surface was not visible, except at the bottom. The bubble-like bridge could also be observed in the back view of both the conventional and SHL surfaces.

[0127] FIG. 21 are images showing condensation on SHP-2Tier-0.06 mm surface with different air velocities: (a) 1.2m / s, (b) 2.1 m / s, (c) 2.5m / s, (d) 3.1 m / s, (e) 4m / s, (f) 4.8m / s, (g) 6.2m / s, (h) 8.1 m / s. The droplet distribution on the SHP fin surface was similar to that observed under natural convection. As the airflow rate increases, the droplet diameter and distribution changed. When the air velocity was below 2 m / s, droplets covered the entire SHP surface. Most droplets had diameters smaller than the fin pitch, while larger droplets became trapped in certain areas between the fins. As the air velocity increased, the maximum droplet diameter gradually decreased, and droplets in the regions between the copper tubes beganto clear. However, even at an air velocity of 8.1 m / s, the droplets were not completely removed.

[0128] FIG. 22 are images showing flying droplets on a SHP-2Tier-0.06 mm surface. The detached droplets could travel with the airflow for nearly a meter from the sample, and the motion trajectories of some larger droplets can be seen in FIG. 22. When the air velocity exceeded 4 m / s, two distinct droplet distribution regions appeared: one with droplets and one without. As the air velocity increased, the droplet-free region between the tubes continued to expand, and the largest droplets rarely reached the fin pitch. This phenomenon may be attributed to two factors. First, the increased air velocity enhances droplet removal. Second, as heat transfer intensifies, the fin temperature in the regions between the tubes may no longer reach the dew point. Under these conditions, the fin surface may be divided into two zones: a mixed heat transfer region (latent and sensible heat transfer) and a sensible heat transfer region.

[0129] FIG. 23 is a chart showing the effect of water flow rate and different surface modification on heat transfer rate for the different cooling coils. The experiments were conducted at fixed water flow rates of 10.9 kg / min and 13.2 kg / min. At the lower water flow rate, the conventional sample exhibited the highest heat transfer rate compared to the SHL-2Tier-0.06mm and SHP-2Tier-0.06mm samples. Additionally, the performance of SHL-2Tier-0.06mm was slightly better than that of SHP-2Tier-0.06mm. The same trend was observed at the higher water flow rate, where at 13.2 kg / min, the conventional sample again outperformed the SHP-2Tier-0.06mm and SHL-2Tier-0.06mm samples, with SHL-2Tier-0.06mm performing slightly better than SHP-2Tier-0.06mm. This result contradicted the conclusions drawn under pure steam conditions, where the heat transfer rate of dropwise condensation was found to be several orders of magnitude higher than that of film condensation. It is evident that the heat transfer resistance of SHL was higher than that of SHP due to the additional film conduction resistance. The difference in heat transfer rate may be attributed to the accumulation of noncondensable gases on the SHP-2Tier-0.06mm fin surface, which inhibited the transfer of water molecules to the condensation nuclei. In contrast, the flowing liquidfilm covering the SHL-2Tier-0.06rrim surface not only increases the fin’s effective heat transfer area, improving fin efficiency, but also enhances air disturbance near the surface, preventing non-condensable gas accumulation. Additionally, the liquid film significantly increases the available condensation surface, especially when the coolant temperature is sufficiently low.

[0130] FIG. 24 is a chart showing the pressure drop for conventional and surface-modified samples as a function of air velocity under various coolant velocities. The chart shows that the pressure drop for the conventional samples was the largest among the four different samples. Additionally, the pressure drop for SHP was higher than that for SHL. This indicated that for heat exchanger samples used in dehumidification, when the fin thickness is the same, the surface modification method proposed herein enables the SHL surface to not only reduce the pressure drop but also enhance the heat transfer rate.

[0131] To evaluate the effect of time durability on surface modification performance, the SHL heat transfer rate and pressure drop were remeasured one day and seven days after the initial test. As shown in FIG. 25 and FIG. 26, the results indicate that time has minimal influence on both the heat transfer rate and pressure drop. This suggests that under laboratory conditions, the SHL surface modification remains effective over time, sustaining enhanced heat transfer and reduced pressure drop.

[0132] The foregoing shows that the method proposed herein could be used to produce either superhydrophilic cooling coils or superhydrophobic cooling coils. While the micro / nanostructuring processes to produce both superhydrophilic cooling coils or superhydrophobic cooling coils are the same, the superhydrophobic cooling coils are achieved with an additional chemical vapor deposition process. The superhydrophilic cooling coils showed good dehumidification performance with low air-side pressure drop under dehumidification conditions. The superhydrophobic cooling coils were shown to be capable of being flood-proof without having to sacrifice performance under dehumidification conditions.

[0133] The bubble-like liquid film movement observed in this study for both conventional and SHL wavy fin-and-tube heat exchangers. The velocity of the edgecan reach up to 0.2 meter per second, with the velocity of the bubble-like film interface on the SHL surface being higher than that on the SHP surface. Under the same conditions, the bubble-like liquid film density distribution on the SHL surface is lower compared to that of the conventional surface.

[0134] The heat transfer performance shows a significant influence of fin thickness and a slight influence of surface modification. The heat transfer performance improves as the fin thickness increases. Under the same conditions, the dehumidification heat transfer performance of the SHL surface is slightly better than that of the SHP and conventional surfaces. This difference became less noticeable as the coolant flow rate decreases.

[0135] The effect of surface modification on the heat exchanger surface could significantly influence the pressure drop in the heat exchanger. As the wettability of the surface increases, the pressure drop of the heat exchanger can be reduced appreciably, leading to better heat transfer performance. This is particularly evident for the wavy fin-and-tube heat exchanger with a pin pitch of 1 .3 mm.

[0136] A method of surface treatment of a cooling coil includes subjecting the cooling coil to one or more cycles of chemical etching to form a microstructured surface. Each of the one or more cycles of chemical etching includes a first etching step in which the cooling coil is immersed in an etchant for a first half-cycle reaction time. Each of the one or more cycles of chemical etching includes a second etching step in which the cooling coil is immersed in the etchant for a second half-cycle reaction time. The method includes performing a chemical vapor deposition to coat the microstructured surface with a low-surface-energy material, in which the method further includes a step of rotating the cooling coil between the first etching step and the second etching step.

[0137] The first half-cycle reaction time and the second half-cycle reaction time of any one of cycles may be equal.

[0138] The step of rotating may include disposing the cooling coil in different orientations in the first etching step and in the second etching step.

[0139] The step of rotating may include rotating the cooling coil by 180 degrees about any axis on a horizontal plane.

[0140] The cooling coil may be immersed in the etchant throughout the step of rotating.

[0141] The low-surface-energy material may be any one material selected from the group consisting of fluorides, silanes, and fatty acids dissolved in a solvent, in which the solvent may be selected from the group consisting of non-polar organic solvents.

[0142] The low-surface-energy material may be any one selected from the group consisting of: 1 H,1H,2H,2H-perfluorodecyltrimethoxysilane and 1 H,1 H,2H,2H- perfluorodecyltriethoxysilane, trimethylethoxysilane, methyltrimethoxysilane, hexadecyltrimethoxysilane, lauric acid, and stearic acid.

[0143] The solvent may be any one of toluene and tetrahydrofuran.

[0144] The method may further include a step of forming boehmite on the microstructured surface before performing the chemical vapor deposition.

[0145] Each of the first half-cycle reaction time and the second half-cycle reaction time may be 26 minutes, with the etchant being hydrochloric acid of 0.6 M concentration.

[0146] The chemical etching may have a total reaction time of 4 hours of immersing the cooling coil in a volume of hydrochloric acid of 0.6 M concentration. The volume of the etchant per surface area of the cooling coil may be characterized by a ratio (correction factor) of 384.9 milliliters per square meter.

[0147] The method may include two or more cycles of chemical etching, with an intervening step of rotating the cooling coil between successive ones of the two or more cycles of chemical etching.

[0148] The two or more cycles of chemical etching may include more than one cycle of a higher concentration etching. The two or more cycles of chemical etching may include one cycle of a lower concentration etching in which the etchant has a lower concentration than the etchant of the higher concentration etching.

[0149] The half-cycle reaction time of a successive cycle of the higher concentration etching may be shorter than the half-cycle reaction time of a preceding cycle of the higher concentration etching. The total reaction time of thelower concentration etching may be longer than twice of the half-cycle reaction time of a longest half-cycle reaction time of the higher concentration etching.

[0150] The first half-cycle reaction time and the second half-cycle reaction time of the one cycle of the lower concentration etching may be from 25 minutes to 27.5 minutes.

[0151] The method may be characterized by each of the first half-cycle reaction time and the second half-cycle reaction time of a first cycle of the higher concentration etching being 1 minute, with each of the first half-cycle reaction time and the second half-cycle reaction time of a second cycle of the higher concentration etching being 15 seconds, each of the first half-cycle reaction time and the second half-cycle reaction time of a third cycle of the higher concentration etching being 10 seconds, and each of the first half-cycle reaction time and the second half-cycle reaction time of a fourth cycle of the higher concentration etching being 5 seconds.

[0152] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the claimed invention.

Claims

CLAIMS1 . A method of surface treatment of a cooling coil, comprising: subjecting the cooling coil to one or more cycles of chemical etching to form a microstructured surface, each of the one or more cycles of chemical etching including: a first etching step in which the cooling coil is immersed in an etchant for a first half-cycle reaction time; and a second etching step in which the cooling coil is immersed in the etchant for a second half-cycle reaction time; and performing a chemical vapor deposition to coat the microstructured surface with a low-surface-energy material, wherein the method further includes a step of rotating the cooling coil between the first etching step and the second etching step.

2. The method as recited in claim 1 , wherein the first half-cycle reaction time and the second half-cycle reaction time of any one of cycles are equal.

3. The method as recited in claim 1 or claim 2, wherein the step of rotating comprises disposing the cooling coil in different orientations in the first etching step and in the second etching step.

4. The method as recited in any one of claims 1 to 3, wherein the step of rotating comprises rotating the cooling coil by 180 degrees about any axis on a horizontal plane.

5. The method as recited in any one of claims 1 to 4, wherein the cooling coil is immersed in the etchant throughout the step of rotating.

6. The method as recited in any one of claims 1 to 5, wherein the low-surface- energy material is any one material selected from the group consisting of fluorides,silanes, and fatty acids dissolved in a solvent, the solvent being selected from the group consisting of non-polar organic solvents.

7. The method as recited in claim 6, wherein the low-surface-energy material is any one selected from the group consisting of: 1 H,1 H,2H,2H- perfluorodecyltrimethoxysilane and 1 H,1H,2H,2H-perfluorodecyltriethoxysilane, trimethylethoxysilane, methyltrimethoxysilane, hexadecyltrimethoxysilane, lauric acid, and stearic acid.

8. The method as recited in claim 7, and wherein the solvent is any one of toluene and tetrahydrofuran.

9. The method as recited in any one of claims 1 to 8, further comprising a step of forming boehmite on the microstructured surface before performing the chemical vapor deposition.

10. The method as recited in any one of claims 1 to 9, wherein each of the first half-cycle reaction time and the second half-cycle reaction time is 26 minutes, and wherein the etchant is hydrochloric acid of 0.6 molar (M) concentration.

11. The method as recited in any one of claims 1 to 9, wherein the chemical etching comprises a total reaction time of 4 hours of immersing the cooling coil in a volume of hydrochloric acid of 0.6 molar (M) concentration, and wherein the volume of the etchant per surface area of the cooling coil is characterized by a ratio of 384.9 milliliters per square meter.

12. The method as recited in any one of claims 1 to 9, comprising two or more cycles of chemical etching, the method further comprising an intervening step of rotating the cooling coil between successive ones of the two or more cycles of chemical etching.

13. The method as recited in claim 12, wherein the two or more cycles of chemical etching comprises: more than one cycle of a higher concentration etching; and one cycle of a lower concentration etching in which the etchant has a lower concentration than the etchant of the higher concentration etching.

14. The method as recited in claim 13, wherein the half-cycle reaction time of a successive cycle of the higher concentration etching is shorter than the half-cycle reaction time of a preceding cycle of the higher concentration etching, and in which the total reaction time of the lower concentration etching is longer than twice of the half-cycle reaction time of a longest half-cycle reaction time of the higher concentration etching.

15. The method as recited in claim 13, wherein the first half-cycle reaction time and the second half-cycle reaction time of the one cycle of the lower concentration etching are from 25 minutes to 27.5 minutes.

16. The method as recited in claim 15, wherein each of the first half-cycle reaction time and the second half-cycle reaction time of a first cycle of the higher concentration etching is 1 minute, and wherein each of the first half-cycle reaction time and the second half-cycle reaction time of a second cycle of the higher concentration etching is 15 seconds, and wherein each of the first half-cycle reaction time and the second half-cycle reaction time of a third cycle of the higher concentration etching is 10 seconds, and wherein each of the first half-cycle reaction time and the second half-cycle reaction time of a fourth cycle of the higher concentration etching is 5 seconds.

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