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In-duct UV Systems Can Be Good for IAQ
October 23, 2014
In the absence of coil treatment, moisture build-up on the coil won't drain into the condensate pan but instead can blow off the coil and enter the airstream in aerosol form, particularly when thick biofilms cause a higher velocity airflow across the coils. If the HVAC system uses in-duct filters, one potential result of this situation is a soggy filter element that provides a potential home for microbial growth. A UV light source at the filter — an in-duct UVC system — is one way to further combat microbial growth.
However, there are factors to keep in mind to help the technology deliver the desired results. One of these factors is humidity levels. "With in-duct systems, the humidity levels can affect system performance," Stephen Martin, occupational and health safety engineer for the Center for Disease Control says. "When humidity levels are above 50 or 60 percent, the same amount of [ultraviolet] energy doesn't adequately kill microorganisms."
Another consideration is exposure time. In-duct systems have airflow that's usually measured in hundreds or thousands of cubic feet per minute, which shortens the exposure time of microbes to the UV light. But that's not to say that it is ineffective. Instead, manufacturers usually use more UV bulbs at a higher dosage (than what is typically used in a coil system application) to achieve the desired result.
Chapter 60 of the 2011 ASHRAE Handbook says, "In contrast to air disinfection irradiance levels, which may exceed 1000 µw/cm2, coil surface irradiance levels on the order of 1 µw/cm2 can be effective, although 50 to 100 µw/cm2 is more typical."
Several peer-reviewed studies during the last decade have indicated that the kill rate is above 98 percent for many — though not all — microbes. For those microbes, especially microorganisms in spore form, manufacturers often use a filter to capture them and expose them to UV light for a longer time.
Read the full article on UVC systems at www.facilitiesnet.com/14622BOM.