Appliance Design - May 2008 - (Page 38)

INDoor AIr QuAlIty Photocatalytic oxidation Another unique application of UV energy for improving indoor air quality, photocatalytic oxidation (PCO), has been under development for many years. It is not as widely used or commercially accepted as UVGI (Ultraviolet Germicidal Irradiation), but PCO use is growing. PCO has many uses and benefits, including the improvement of indoor air quality by removal of pollutants and odors from an air stream. In indoor air applications, PCO may be part of a standalone room air cleaner, or installed in larger ducted HVAC systems. PCO is a room-temperature, catalytic chemical reaction. In its most popular form, it uses short-wave UV light to supply energy to a catalyst covered substrate. The most common catalyst used is titanium dioxide, and the substrate may be a structure such as glass filter media, aluminum mesh, screen or honeycomb. The structure allows the air to pass over or through the substrate, and pollutants from the air are adsorbed onto the surface of the catalyst. The UV light, typically UV light between 200 and 380 nanometer, is applied to the surface. This creates electron-hole pairs and, with the moisture and oxygen in the air, generates free hydroxyl radicals that break down organic pollutants. Carried to completion, the reactions result in the conversion of the pollutants into carbon dioxide and water. Carbon is currently the accepted method of removing pollutants and odors. These are absorbed on the carbon and the carbon is changed when it reaches capacity. Unlike carbon, PCO is a catalyst so the pollutants, are converted rather than captured, so theoretically the catalyst can last indefinitely. Challenges facing the OEM employing PCO include: 4Presenting the catalyst on a substrate that allows sufficient surface area for contact with the air stream. 4Avoiding excessive pressure drop through or over the substrate. 4Applying the optimum level of UV energy to the catalyst while matching it with the quality of the catalyst substrate. Implemented properly, PCO can prove effective in air purifiers and HVAC systems. < uV system installed downstream of the air conditioning coil in a residential HVAC system (horizontal system). Residential UV systems are designed for simple, safe and convenient installation. 4The geometry of the system and placement of the lamps. 4The age of the lamps. UV output depreciates with age, depending upon the lamp. Many methods for designing a UV system are available. Lamp manufacturers provided some rough tables more than 50 years ago. Now, better application data is available, and UV energy requirements are better understood. Some manufacturers have formulas or proprietary modeling software to facilitate proper design of a system. Nonetheless, there are many different approaches by different manufacturers and standards have not yet been established. Currently, there is a great deal of work being performed on application and testing standards by organizations such as ASHRAE and IUVA. For moving-air-stream disinfection systems, the applications generally have many lamps mounted in the return air duct. The lamps should irradiate the air for a distance of at least 3 ft. to 4 ft. in order to provide a reasonable dosage. An added section of plenum may need to be added by the OEM. For irradiating surfaces, particularly airconditioning coils and drain pans, modest levels of UV will be adequate. Typically the lamps are installed downstream of the coils at a distance of 12 in. to 14 in. from the coil. Most large systems have this type of room available. The lamp fixture, or multiple fixtures, are installed in rows. The lamps should be installed end-to-end in order to cover the full width of the coil. Modular and track mounted fixtures can make this easier. Multiple rows may be required depending upon the height of the coils and the output of the UV lamps. The power of an individual lamp is not important. What matters more is the mounting of the lamps, the proper number of lamps, and the previously mentioned design factors, to achieve a good level and distribution of UV energy over the coils. Although standards have not been agreed upon and established to date, it appears that levels in the 50 microwatts to 150 microwatts of UVC per square centimeter are good for most applications. Installation hurdles When UV equipment is installed in HVAC/R systems, special considerations are necessary. To fully realize the benefits of the UV, certain precautions must be taken to 38 applianceDESIGN May 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - May 2008 Contents Editorial Shipments/Forecasts News Watch New and Notable Displays & Indicators Elastomers Quality & Standards Indoor Air Quality IHHS Highlights New Products Classifieds Design Marts Advertiser’s Index Association Report: PSMA

Appliance Design - May 2008

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