Appliance Design - July 2008 - (Page 20)

HEatING ElEmENtS coating is a thin-film coating that is applied at high temperatures and sprayed onto the glass surface during the float glass, or molten, stage of glass making. This is the most durable way to make a glass conductive and helps ensure even distribution of the metal oxide throughout the glass material. Artwohl says that the metal oxide coating can be custom formulated to get the optimal resistance and emissivity levels desired for the application. In one formulation, the glass can deliver just enough heat to prevent condensation. Other applications call for multiple layers of coatings that can break down the light and block the ultraviolet and infrared rays while allowing the visible light to pass through. This is important because it is the UV and IR light that can spoil food, and especially milk and other dairy products. In terms of tuning the glass to the appropriate temperature, Usinowicz says that different sheet resistances are used. In a freezer door, it is not uncommon to use 5 W to 8 W per sq. ft. to dissipate condensation. But, he says, in a food service application where a plate needs to be kept warm and the glass is in contact with the bottom of a plate, 20 W per sq. ft., might be required. For a towel warmer that reaches 150 to 170 DegF, 30 W per sq. ft. could be needed. Another method of using metal oxide is through application of the material as a film. One such company that does that is Saint Gobain Glass in Germany. The company’s Thermovit heatable laminated glass is a sandwich type construction. It comprises two or more sheets of glass interlaid with one or more films of polyvinyl butyral (PVB). It includes two 6 mm leaves of laminated safety glass, sandwiching a 1 mm layer of PBV in the middle. A conductive coating is applied to the inside surface of one of the glass panels. The laminated version is limited to 149 DegF, a non-laminated, single pane version could be allowed to reach 248 DegF. Whether coating or film, an electrical connection must be made to the conductive glass. EGP, Anthony International and other companies that use this technology place buss bars along two opposing edges. These serve to distribute the electrical current across the film and give consistent coverage through the glass. The distance between the buss bars, which is determined by the length of the glass, can create minco thermal-Clear heaters can be used in both the heads up display and the display in the portable electronic device. different amounts of available power requirements, says Usinowicz. For instance, if there are two pieces of glass, a 12 in. x 36 in. and a 12 in. x 24 in, and they both use the same coating formulation, the larger glass would require 3 amps, and deliver 142 W per sq. ft. and reach 192 DegF. The smaller pane with the closer buss bars requires 4.6 amps, can go to 338 W per square foot, and reach 305 DegF. So the smaller pane can generate more watts per square foot and reach higher temperatures. Of course, different metal oxide coatings can be selected, and the same coating wouldn’t necessarily be used on both pieces of glass, says Usinowicz. The goal is to use the least amount of power, in amps, to get the required power density, or watts per square foot. Some systems require transformers to step down the voltage. The transformers are individually wound and sized according to the specifications for glass temperature, size and other factors. For instance, the same transformer couldn’t be used on the 12 in. x 36 in. glass as compared to the 12 in. x 24 in. pane. Other units use line voltage as would be found in any U.S. home. EGP’s heated glass works with a GFCI outlet using 110 VAC or 240 VAC, says Fred Fowler, president of EGP. While heated glass is already established as a material for use in many applications, research is continuing. Companies that supply these materials are looking at different glass formulations, as well as ceramic materials. Other companies are developing glass doors with coatings that capture heat from light and do not require a heated system. The future, however, might be in the form of transparent, carbon nanotube sheets that are applied to glass. Researchers at the University of Texas – Dallas and at the Fraunhofer Technologies Institute in Stuttgart, Germany, are independently working on such technology. The researchers say that carbon nanotube sheets are more mechanically robust as compared to oxide films such as indium tin oxide and can offer substantially greater sheet resistance. Carbon nanotubes have been described as minute bits of string and trillions of the nanotube strings must be assembled to make a macroscopic product such as a sheet. Until recently, nanotube sheets were made in a week-long process akin to paper making, by filtering solutions of nanotubes and then peeling the nanotubes off the filter once dry. In Dallas, Prof. Ray Baughman is overseeing the work developing carbon nanotube sheets that speeds up production to about seven meters per minute. The researchers can now create forests of nanotubes, which are teased 20 applianceDESIGN July 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - July 2008

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