METHOD OF COATING HEAT TRANSFER COMPONENTS TO IMPART SUPERHYDROPHOBICITY
1. A method of coating heat transfer components to impart superhydrophobicity, the method comprising:
- conveying one or more heat transfer components to a cleaning station and cleaning the one or more heat transfer components with an organic solvent;
after the cleaning, conveying the one or more heat transfer components to a nanostructuring station and immersing the one or more heat transfer components in hot water for surface oxidation and roughening; and
after the immersion in hot water, conveying the one or more heat transfer components to a functionalization station and exposing the one or more heat transfer components to a heated precursor vapor comprising a hydrophobic species,wherein, during the exposure, the hydrophobic species is deposited on roughened surfaces of the one or more heat transfer components, thereby forming a superhydrophobic coating.
A method for coating heat transfer components to impart superhydrophobicity comprises conveying one or more heat transfer components to a cleaning station, where the one or more heat transfer components are cleaned with an organic solvent. After the cleaning, the one or more heat transfer components are conveyed to a nanostructuring station and immersed in hot water for surface oxidation and roughening. After the immersion in hot water, the one or more heat transfer components are conveyed to a functionalization station and exposed to a heated precursor vapor comprising a hydrophobic species. During the exposure, the hydrophobic species is deposited on roughened surfaces of the one or more heat transfer components, thereby forming a superhydrophobic coating. Prior to being conveyed to the cleaning station, the one or more heat transfer components may be attached to an automated conveyor system positioned to traverse the cleaning, nanostructuring, and functionalization stations.
- 1. A method of coating heat transfer components to impart superhydrophobicity, the method comprising:
conveying one or more heat transfer components to a cleaning station and cleaning the one or more heat transfer components with an organic solvent; after the cleaning, conveying the one or more heat transfer components to a nanostructuring station and immersing the one or more heat transfer components in hot water for surface oxidation and roughening; and after the immersion in hot water, conveying the one or more heat transfer components to a functionalization station and exposing the one or more heat transfer components to a heated precursor vapor comprising a hydrophobic species, wherein, during the exposure, the hydrophobic species is deposited on roughened surfaces of the one or more heat transfer components, thereby forming a superhydrophobic coating.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
The present patent document claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/662,354, which was filed on Apr. 25, 2018, and is hereby incorporated by reference in its entirety.
The present disclosure relates generally to surface coating technology more specifically to a method to impart superhydrophobicity to heat transfer components.
Air-coupled heat exchangers are widely used as evaporators in heat pumps and refrigeration systems. These components are susceptible to frost formation when they operate at low temperatures because natural water vapor in the air can condense and freeze or ablimate on the external surface. Frost formation on heat transfer components, such as air-source heat pump evaporators, can result in drastic efficiency penalties. The performance reduction is a result of the insulating nature of ice and the increased fan power required to pump air through the constricted channels between frosted fins. Furthermore, the need to defrost adds appreciable energy use to the system. Recently, researchers have made attempts to prepare superhydrophobic surfaces to inhibit frost formation. However, these efforts are plagued by hard-to-scale fabrication techniques, costly manufacturing methods and/or unrepresentative surface materials.
A semi-continuous method to coat large-size heat transfer components to impart superhydrophobicity and prevent frosting has been developed. The method comprises conveying one or more heat transfer components to a cleaning station, where the one or more heat transfer components are cleaned with an organic solvent. After the cleaning, the one or more heat transfer components are conveyed to a nanostructuring station and immersed in hot water for surface oxidation and roughening. After the immersion in hot water, the one or more heat transfer components are conveyed to a functionalization station and exposed to a heated precursor vapor comprising a hydrophobic species. During the exposure, the hydrophobic species is deposited on roughened surfaces of the one or more heat transfer components, thereby forming a superhydrophobic coating. Prior to being conveyed to the cleaning station, the one or more heat transfer components may be attached to an automated conveyor system positioned to traverse the cleaning, nanostructuring, and functionalization stations.
The method includes what may be described as cleaning, nanostructuring, and functionalization steps (explained below) that can be applied to a series of individual heat transfer components or to multiple batches of heat transfer components in a semi-continuous process. The method is described as semi-continuous since the heat transfer components may be conveyed without stopping or halted only briefly at treatment stations (e.g., cleaning, nanostructuring, and/or functionalization stations) while undergoing processing. The method may be carried out in a sealed or semi-sealed room, in ambient conditions. In other words, the method may be carried out in air at atmospheric pressure and room temperature (e.g., 18-25° C.), except where otherwise required during the process.
The “conveying” of the one or more heat transfer components referred to above and throughout this disclosure may be carried out manually or automatically. For example, the method may further comprise, prior to conveying the one or more heat transfer components to the cleaning station, attaching the one or more heat transfer components to an automated conveyor system configured to traverse the cleaning, nanostructuring, and functionalization stations. The automated conveyer system may be a conveyer belt or another automated transport system known in the art. The heat transfer components may be conveyed to each station individually or in batches. Given that the method is semi-continuous, the one or more heat transfer components may pass through a given station without stopping while undergoing processing or may be halted for processing at a given station, preferably for a short time duration. The conveyor system may be configured to lower or otherwise transport the heat transfer components into vats, drums and/or other chambers as needed during processing.
As illustrated in the schematic of
Preferably, the one or more heat transfer components are cleaned with more than one organic solvent. In this case, the cleaning station may include more than one vat and/or additional spray nozzle(s) in fluid communication with another organic solvent. The heat transfer component may be immersed in the two or more different organic solvents, preferably sequentially, to effect cleaning. For example, the heat transfer component(s) may be cleaned sequentially with two or more different organic solvents, as illustrated in
After cleaning with the organic solvent(s), the one or more heat transfer components may be rinsed thoroughly with water, more particularly with deionized (DI) water. The rinsing may comprise spraying with water (e.g., pressure washing), as illustrated for example in
Referring now to
The nanostructuring station may comprise a heated vat containing the hot water. Industrial boilers that are heated using gas or electricity may be suitable. In one example, the heated vat may take the form of a stainless steel drum wrapped with heating tape connected to a temperature controller. The vat may be a sufficient size (e.g., 55 gallon or about 208 liter capacity, or larger) to hold a batch of the heat transfer components (e.g., from 6-12 components). After the submersion in hot water, the heat transfer component(s) may undergo a drying process, either passively by ambient air or actively by exposure to a pressurized stream of nitrogen gas or air, as illustrated in
It may be beneficial to etch or activate surfaces of the heat transfer component(s) prior to the submersion in hot water. Thus, after cleaning and rinsing as described above, the one or more heat transfer components may be conveyed to a microstructuring station and exposed to an acid solution, as illustrated in
After cleaning, optional acid exposure, and nanostructuring (hot water treatment), the one or more heat transfer components may be conveyed to a functionalization station where a hydrophobic coating is applied to the roughened surfaces of the heat transfer component(s). Referring now to
The heated precursor vapor may be formed by heating a solution of toluene and a hydrophobic species such as a silane (e.g., heptadecafluoro-(tetrahydrodecyl)-trimethoxysilane (HTMS)) to a suitable temperature, such as 80° C. to 100° C. Typically, a volume ratio of the hydrophobic species to toluene in the solution is from 1:16 to 1:22. During the CVD process, the hydrophobic species is deposited on the nanostructured surface protrusions, and thus a superhydrophobic coating is formed on the one or more heat transfer components. Deposition of the hydrophobic species may take place over a period of typically two to four hours. The superhydrophobic coating, which is typically a silane coating, may be from a monolayer (>1 nm) to tens of nanometers (e.g., about 50 nm) in thickness. Typically, the thickness is from about 2 nm to about 10 nm, or from 2 nm to about 5 nm. The hydrophobic species may deposit (or build up) uniformly and conformally over the nanostructured surface protrusions, and may create a rough surface having a Cassie wetting state that allows water or other liquid droplets to coalesce and lift off the surface. Water droplets accumulate on the coated heat transfer component at reduced levels compared to an untreated component, thus preventing frost formation or significantly reducing frost build-up.
As indicated above, the method is applicable to metal-based components in general that may benefit from a superhydrophobic coating and to heat exchangers in particular. For example, the method may be used to coat fin and tube heat exchangers, shell and tube heat exchangers, double pipe heat exchangers, plate heat exchangers, and condensers, evaporators, and/or boilers. While the semi-continuous method is particularly beneficial for treating large-size, fully-manufactured and assembled heat exchangers, the process is also applicable to unassembled heat exchanger parts and other metal-based components, such as windmill blades, ducts, and refrigerator walls or other parts.
The simplicity of the coating process motivated the design and fabrication of equipment that could handle much larger samples. Initially, a design that allowed for coating smaller residential heat exchangers (e.g., up to about 36 cm×56 cm×15 cm) was investigated. Based on the success of the smaller design, equipment required to coat much larger heat exchangers (e.g., up to about 56 cm×81 cm×25 cm) was designed and built. Efforts were focused on designing a versatile system that would allow the nanostructuring and functionalization steps of the procedure to be performed at a low cost while maintaining tight control of temperature.
The prototype system can be used to coat up to six heat exchangers, depending on their size. However, the basic technology is scaleable to an industrial level, as described above. To summarize, the method can be performed on heat transfer components that are hung from a conveyor belt in a sealed/semi-sealed room. This way the chemical runoff can be collected and recycled. The evaporated acetone and ethanol can also be reclaimed through condensation. The heat transfer components may be submerged in hot water for a duration from five minutes to typically one hour during the hot water treatment process. The conveyer system that carries the heat exchangers can be easily lowered into vats of hot deionized water. The vats can be very simple industrial boilers that are fired using gas or electricity, preferably with temperature controllers. The heat exchangers can then be placed in an industrial furnace that has the ability to introduce the hydrophobic species during the CVD phase. A vent system can help capture any remaining gaseous chemicals before the next batch of heat exchangers are ready to be placed into the furnace.
In this example, large-size heat exchangers are cleaned manually. A spray bottle is used to spray acetone and ethanol on the heat exchanger. To ensure thorough cleaning, the heat exchanger was thoroughly sprayed in a preset pattern detailed below. This procedure was applied to heat exchangers of about 0.5 inch to about 1 inch in thickness. (1) Place the heat exchanger in an appropriately sized laboratory tray. The tray may be useful for collecting the cleaning agents for safe disposal. (2) To clean side 1 of the heat exchanger, hold the spray bottle 10-20 cm from the face of the heat exchanger and spray thoroughly so that all the fins are covered the cleaning agent. Hold the bottle at an angle to ensure that the spray hits the fin surfaces directly, and spray from top to bottom and left to right. (3) To clean side 2 of the heat exchanger, flip the heat exchanger and perform step 2 but from the opposite direction. (4) Repeat step 2 in the opposite direction. (5) Repeat step 3 in opposite direction. (6) Rinse the heat exchanger thoroughly with de-ionized water. (7) Dispose of the chemical the safely; it is noted that the acetone and ethanol used in this process can be reused up to 3 times if the heat exchanger is not very dirty.
Design of Boiler and Furnace
In order to make the design more economical, stainless steel drums were selected to serve as both the boiler (for the hot water treatment process) and the furnace (for the chemical vapor deposition process). Stainless steel drums are easy to procure and have high temperature and chemical tolerance. In one example, the system includes 55-gallon drums because the dimensions are sufficient to accommodate a variety of heat exchangers. In addition, heating elements for such drums are commercially available. Multiple (e.g., three in this example) heating tapes (e.g., 1440 Watt Briskheat) are wrapped around each drum. Each of the tapes has a temperature controller with a maximum temperature setting of 400° F. The heating tapes may cover the lower third or two-thirds of the drum surface area, as illustrated in
Hot Water Treatment—Nanostructuring
The boiler is filled with enough deionized water such that the heat exchangers are covered and that evaporation losses are accounted for. The temperature of the three heating tapes is initially set to 400° F. (approximately 204° C.). This setting is maintained until the temperature of the water reaches 90° C., or till boiling is observed, which typically takes three to six hours depending on volume. The clean heat exchangers are then placed into the hot/boiling water. The water can be allowed to flow into the heat exchangers. In case the internal components need to be protected from water, an option is to fill the heat exchanger with an inert liquid and seal it. The temperature of the controllers is then set to 250° F. (approximately 121° C.). The lid is placed on the drum. The heat exchangers are removed after an hour of treatment. They are dried and drained thoroughly, wrapped carefully in a plastic cover, and stored till they can be coated. The water is not reused; instead, it is allowed to cool and drained using a pump. The drum is then cleaned using a soap solution and a good amount of water. It is then ready for use as a furnace.
Hydrophobic Coating—Chemical Vapor Deposition (CVD)
The heaters are set to 400° F. (approximately 204° C.) and the drum is allowed to reach a temperature of 80° C. The heat exchangers can be kept in the drum during this heating process to evaporate any remaining water from the previous step. A solution of toluene and heptadecafluoro-(tetrahydrodecyl)-trimethoxy silane (HTMS) is measured out in a small beaker and placed in the drum with the heat exchangers once the internal temperature reaches 80° C. The lid is replaced and the sealed carefully using the quick release clamp. The heating tapes are set to 190° F. (approximately 88° C.). The heating tapes are turned off after three hours. In one example, a specific solvent/polymer (toluene/HTMS) volume ratio of 0.053 is used with a total volume of 40 ml (2 ml HTMS to 38 ml toluene) for the 55 gallon drum. This solution can be used to coat six heat exchangers (of 6 m2 area each) while maintaining a safety factor of 20. See Tables 1 and 2 below. Chemical vapor deposition has been found to work well from 80° C. to 100° C. The drum is allowed to cool overnight while it is sealed. This allows the remaining toluene and HTMS solution to condense. The heat transfer components at this point in the process may be superhydrophobic. They may be removed from the drum for use. The drum can be cleaned using toluene, acetone, and deionized water, in that order.
Multiple heat exchangers, ranging from automotive to residential evaporators and condensers from different commercial manufacturers have been coated using the inventive process. Some grades of aluminum may be resistant to the growth of the boehmite (which provides roughness for creating Cassie-Baxter droplet states) during the hot water treatment procedure. For those grades, an additional acid treatment, as described above (e.g., 2 M HCl for 15 minutes), has been shown to be beneficial.
Durability and Reliability
The coated heat exchangers have proved to be extremely durable. Heat exchangers coated one year ago have shown consistent results over multiple testing cycles. In addition, heat transfer experiments designed to induce condensation show a significant improvement in the average overall heat transfer coefficient for a superhydrophobic-coated heat exchanger compared to a heat exchanger with an unaltered surface, as can be seen from the data of
Frost Characteristics and Heat Transfer
In a further investigation, the inventive process was applied to a decimeter-scale, fully-assembled fin and tube heat exchanger, and the frosting characteristics and heat transfer performance were evaluated and compared with an unaltered heat exchanger and with a superhydrophilic heat exchanger. The heat exchangers were procured from Heatcraft Inc., USA. The tubes are made of copper and the fins are made of aluminum. The fins are brazed onto the 9.53 mm outer diameter copper tubes which make 12 passes through the heat exchanger in a staggered formation. The fin pitch is 2.85 mm and the thickness of each fin is ˜0.25 mm. The length, height, and depth of the heat exchanger are 25.5 cm, 15.2 cm and 2.5 cm, respectively.
In this investigation, two of the heat exchangers were sequentially submerged in acetone, ethanol and deionized water to clean the external surface. They were then exposed to a 2-molar hydrochloric acid solution to create microscale roughness features and placed in a 90° C. de-ionized water bath for one hour to generate a layer of aluminum oxy-hydroxide (boehmite). This process results in a surface with both micro- and nanoscale roughness features, which help promote hydrophilicity. One of the heat exchangers was set aside at this time to remain hydrophilic for characterization and testing. A conformal 2.5 nm thick layer of HTMS was deposited on the other heat exchanger surface using chemical vapor deposition at atmospheric pressure to produce a superhydrophobic surface. The conformal low surface energy coating in addition to the surface roughness promotes the formation of Cassie-Baxter type droplets during condensation. Scanning electron microscopy (SEM) images of the coated surface are shown in
Tests were carried out to evaluate frost accumulation and heat transfer. Frost formation on a heat exchanger ultimately reduces the heat transfer rate because of the increased thermal resistance across the ice layer. The total heat transfer rates of the unaltered, superhydrophilic and superhydrophobic heat exchangers when the coolant enters at −9.6° C.±0.1° C. and the air enters at 47.4° C.±2% are presented in
Results of an energy analysis for both the uncoated and superhydrophobic heat exchangers are shown in
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.
Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.