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Hydrophilic Zeolite Coatings for Improved Heat Transfer Ronnie A. Munoz, Derek Beving, and Yushan Yan* Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521
The evaporation time and wetting limit temperature for a water droplet on the surface of a bare, Zeolite-A-coated, or ZSM-5-coated stainless steel substrate have been experimentally studied. Three stainless steel substrates are prepared with different finishes. The surface finishing of a substrate is shown to cause observable changes in the wettability of a bare or ZSM-5-coated surface. Contact angle measurements reveal that Zeolite-A coatings are the most hydrophilic and that bare stainless steel is the least hydrophilic. The evaporation time and wetting limit temperature of a water droplet placed on the surface of the bare and coated substrates are examined as surface temperature increases. The zeolite coatings decrease the evaporation time and increase the wetting limit temperature on the bare stainless steel substrate, with Zeolite-A coatings offering the best improvement. For the bare substrate and the substrates coated with the same zeolite, as hydrophilicity increases, the evaporation time decreases and wetting limit temperature increases. Introduction Many industrial processes require the transfer of heat from a surface to water. In most cases an increase in the heat transfer rate across the solid-liquid boundary means higher system efficiency and lower energy consumption. Ultimately this improved heat transfer translates into an economic gain. In heat exchanger applications, the system efficiency improves as heat is moved more quickly from the surface to the contacting water. This rate of heat transfer can be increased by making the surface hydrophilic.1,2 Hydrophilicity increases the water droplet’s surface area at the solid-liquid interface, allowing for additional heat to be transferred to the same volume of water. In fin-tube heat exchanger applications, an increase in the surface hydrophilicity of the fins leads to a decreased pressure drop.3 In most fin-tube heat exchangers the distance between the fins is very narrow. As water condenses on the fins, the resulting droplets and bridging water begin to reduce the cross-sectional area for air flow. A hydrophilic surface causes the contacting water droplets to spread into a surface film. This film helps to reduce the pressure drop of the air forced through the system and decreases operation noise. The present paper not only reinforces the use of hydrophilic surfaces for improved heat transfer4 but demonstrates how zeolite coatings can be used in these applications. It is known that in many heat transfer processes corrosion is a problem.5,6 Galvanic coatings, and more recent plasma vapor deposition (PVD) coatings, are often used for corrosion protection.7 A major concern of galvanic coatings has been their negative environmental impact. PVD coatings have proved to be the most promising alternative for galvanic coatings. If a thick PVD coating is desired for corrosion protection, a thin undercoat often precedes the PVD to help alleviate the relatively high internal stress of the * To whom correspondence should be addressed. E-mail:
[email protected].
coating. This high stress can cause adhesion problems, and the additional process means that the environmental advantages are partially lost. Additional limitations are often imposed by the size and increased operational cost of the vacuum reactor required for PVD. In contrast, zeolite coatings are easily applied to complex geometries by a single-step, in-situ crystallization process that offers high adhesion and excellent corrosion resistance.8 Coatings that are both hydrophilic and corrosion resistant have found their way into automotive, refrigerator, air-conditioning, and other evaporator applications.4 A tradeoff often exists for these coatings: they have either good hydrophilicity or good corrosion resistance. Current improvement of hydrophilic and corrosion resistant coatings has focused on plasma or ionassisted reactions.1,9 Although much success has been made with these processes in rendering coatings hydrophilic, most of the techniques add an additional step to the fabrication process and are again limited by the size and increased operational cost of the vacuum reactor. By comparison, the zeolite coatings in this study have good hydrophilicity, as synthesized. Add to this the aforementioned benefits, and it is not difficult to see how this technology can be used for improvements that can translate into system and economic benefits. Experimental Section Application of Coatings to Substrates. The preparation of the substrate’s surface is a threefold process: finishing, cleaning, and coating. The surface of three large SS-304 sheets (3 in. × 6 in.) was finished with CAMI (Coated Abrasive Manufacture’s Institute) standardized silicon carbide sandpaper of 600, 1200, and 1500 grit. The SS-304 sheet finished with 1500 grit was prefinished with an initial 600 grit finish followed by a 1200 grit finish. The 1200 grit finish was prefinished with a 600 grit finish. After completion of the SS-304 finish, the sheets were cut into substrates measuring 2 × 3.5 × 0.061 cm. The individual substrates were then placed in 35 mL of 1 N HNO3 at 21 °C and cleaned with sonication for
10.1021/ie0489047 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/07/2005
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20 min. The substrates were rinsed under deionized water and dried with compressed air. The ZSM-5 synthesis solution contained a molar composition of 0.16 TPAOH/0.64 NaOH/1 TEOS/92 H2O/ 0.0018 Al (TPAOH is tetrapropylammonium hydroxide and TEOS is tetraethyl orthosilicate). A typical solution preparation began with the addition of 0.0126 g of aluminum powder (200 mesh, 99.95+%, Aldrich) to 100 g of double-deionized water; 6.33 g of sodium hydroxide (pellets, 97+%, Aldrich) was then added to the solution, which was stirred for 30 min. Then 297.03 g of double-deionized water and 20.12 g of TPAOH (40 wt %, Sachem) were added to the solution, and stirring was continued for 30 min; 51.5 g of TEOS (98 wt %, Aldrich) was added to the solution, and the solution was stirred for 4 h. Then two substrates, with 30 mL of ZSM-5 synthesis solution, were placed vertically into a sealed Teflon-lined autoclave (Parr Instrument Co.) and heated in a convection oven at 175 °C for 12 h. The coated substrates removed from the autoclaves were then rinsed under deionized water and dried with compressed air. The Zeolite-A synthesis solution contained a molar composition of 10 NaOH/0.2 Al2O3/1 SiO2/200 H2O. A typical solution preparation began with the addition of 1.182 g of aluminum powder (200 mesh, 99.95+%, Aldrich) into 364.4 g of double-deionized water; 87.5 g of sodium hydroxide (pellets, 97+%, Aldrich) was then added to the solution, which was stirred for 30 min. Next, 21.9 g of Ludox LS30 colloidal silica (30 wt %, silica, Aldrich) was added to the solution, and stirring was continued for 4 h. Then six substrates, with 150 mL of Zeolite-A solution, were placed into 250 mL polypropylene bottles (Nalgene) and heated in a convection oven at 65 °C for 12 h. An SS-304 substrate was floated vertically in the synthesis solution by inserting its corner into a slit made in a Teflon ball. The coated substrates removed from the bottles were then rinsed under deionized water and dried with compressed air. Characterization of Coated and Uncoated Substrates. The application of a Zeolite-A coating onto three differently finished substrate surfaces (600, 1200, and 1500 grit) was verified using X-ray diffraction (XRD, Siemens D-500 diffractometer using Cu KR radiation) and a scanning electron microscope (SEM, Philips XL30-FEG, operated at 20 kV). The same was done for the ZSM-5-coated substrates. The film thickness of all coated substrates was determined by exposing the profile of the coating using hydrofluoric acid. Film thicknesses were determined to be in the range of 34 µm for all substrates. The wettability of the coated and uncoated surfaces was determined by contact angle measurement using a VCA Optima XE. A 28 gauge blunt-tip needle was attached to a VCA Optima XE mechanically controlled micrometer for dispensing a 2 µL double-deionized water droplet onto the surface of a sample. Three contact angle measurements were taken (using the VCA Optima AutoFAST contact angle calculation software) on the center of the sample, which was then dried with compressed air so that the measurement could be repeated in the same location. Using right and left contact angles on the drop, 12 measurements were made for each sample and used to determine the average and standard deviation for the data. Contact angles were recorded for the three differently finished SS-304 substrates and the three differently finished ZSM-5-coated
substrates. Because the average contact angle of the three differently finished Zeolite-A-coated surfaces was approximately zero, a relative measurement of hydrophilicity was made using the terminal film size of a 5 µL double-deionized water droplet placed on the surface. Characterization of Improved Heat Transfer. A uniform heating surface was constructed for the heat transfer experiments. A 7.6 × 7.6 × 0.2 cm fiberglassreinforced silicon-rubber 90 W heating blanket was sandwiched between an 8.3 × 8.3 × 3.8 cm block of Aluminum-6061 and an 8.3 × 8.3 × 1.3 cm piece of silica board insulation. The heating surface was then placed on the VCA Optima XE stage and was used to control the surface temperature on the samples. The temperature at the aluminum block surface (upon which the samples were laid) was controlled by a PID temperature controller capable of maintaining the temperature of the samples to within (1 °C of the set point. After a sample had been placed between the heating surface and the VCA Optima mounted syringe or after the evaporation of a drop from the sample surface, the sample was given 5 min to reach its equilibrium temperature. This time was determined to be more than sufficient by real time infrared (Raytek MX4 with close focus and NIST calibration) measurements of the samples. The mechanically controlled syringe (same as above) was used to dispense a 3.75 µL double-deionized water droplet that dangled above the sample surface. The needle tip was mounted 1.5 mm above the sample surface, and the droplet was released from the needle tip by lightly tapping the VCA Optima with a finger. The volume of the drop was kept constant by using a motorized syringe system. The placement of the drop on the center of all substrates was ensured by aligning the substrate corners with four spots that had been marked onto the heating surface. The time the drop took to evaporate was recorded by a hand timer for periods of >3 s. For times of
