ASHRAE Journal - August 2008 - (Page 29)

eliminate viable TB bacilli from the exhaust air of a hospital ward.6 This famous work, along with more recent studies documenting the effectiveness of UVGI7 have contributed to the renewed enthusiasm regarding UVGI applications that we see today. Government agencies and the HVAC community are increasingly aware of the beneﬁts of UVGI applications. The U.S. General Services Administration requires that UVC be included in cooling coil air-handling units for all new facilities and alteration projects to maintain coil cleanliness and improve air quality.8 The Centers for Disease Control and Prevention supports the use of UVGI as an adjunct to mechanical ventilation and ﬁltration to prevent and control the spread of tuberculosis.9 Similarly, the Federal Emergency Management Agency10 and Environmental Protection Agency (EPA)11 note that UVGI technologies can be used to provide protection against bioterrorism. UV Dose and the Microbial Response eralizing the use of Equation 2 for heterogeneous microbial populations complicated. Even accurately determining S for one speciﬁc microorganism can be difﬁcult, as the reported kvalues for the same species sometimes differ signiﬁcantly. For example, published k-values for Mycobacterium tuberculosis irradiated in air range from 0.077309 cm2/μJ down to 0.002132 cm2/μJ.6,12 Variation of measured k-values may relate to differences in conditions under which UV irradiance was conducted (air, water, surface), methods used to measure the irradiance level, and errors relating to culture-based measurements of microbial survival. Research to obtain reliable k-values for UV system design is ongoing. In the meantime, systems are usually designed conservatively using an average or worst-case value, depending on the disinfection goals. UVGI Design Guidelines UVGI effectiveness depends primarily on the UV ﬂuence or dose (DUV, μJ/cm2) delivered to the microorganisms: DUV It (1) where I is the average ﬂuence rate or irradiance in μW/cm2, and t is the exposure time in seconds (Note: 1 W = 1 J/s). Although Equation 1 seems quite simple, its application can be complex, for example, when calculating the dose received by a particle following a tortuous path through a device in which the ﬂuence rate varies spatially. The dose is interpreted as that occurring on a single pass through the device. Although the effects of repeated exposure of microorganisms entrained in recirculated air may be cumulative, this effect has not been quantiﬁed and it is conservative to neglect it. The survival fraction (S) of a microbial population exposed to UVGI is an exponential function of dose: S e k DUV (2) where k is a species-dependent deactivation rate constant (cm2/μJ). The resulting single pass inactivation rate ( ) is the complement of S: UVGI system design from the early 1900s until recent times was more art than science, as there were limited design criteria to follow. During the last half-century, the scientiﬁc community has gained a better understanding of how UVGI inactivates microorganisms. UVC lamp technology improved dramatically over this same period. Unfortunately, UVGI system design has not advanced at the same rate. Some of the ﬁrst guidelines for UVGI air-disinfection system designs were published in the 1940s. Additional guidelines published by General Electric (1950) and Philips (1985) are still used by many system designers today.13,14 More recently, Kowalski and others have made meaningful advances in the analysis and modeling of UVGI systems that have improved guidance for system design.15 However, no consensus guidelines yet exist that comprehensively address all aspects of UVGI system design required to ensure good performance. UVGI system design today relies on performance data from lamp manufacturers, the experience of system designers, and the recommendations of UVGI equipment manufacturers. Most equipment manufacturers have detailed methods for estimating the UV dose delivered, which may include using tabulated data charts, mathematical modeling, and complex formulas. Like most HVAC components, UVGI systems are typically oversized About the Authors Stephen B. Martin Jr. is an engineer for the U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Division of Respiratory Disease Studies in Morgantown, W. Va. and is an architectural engineering graduate student at the Pennsylvania State University, Indoor Environment Center, Department of Architectural Engineering in University Park, Pa. Chuck Dunn is president of Lumalier Corporation in Memphis, Tenn. James D. Freihaut is an associate professor of architectural engineering, William P Bahnﬂeth is a professor of . architectural engineering and director of the Indoor Environment Center, and Josephine Lau and Ana Nedeljkovic-Davidovic are architectural engineering graduate students at the Pennsylvania State University, University Park, Pa. 1S (3) and is a commonly used indicator of overall UVGI effectiveness, representing the percentage of the microbial population inactivated after one pass through the irradiance ﬁeld(s). Measured k-values for many species of bacteria and fungi have been published in scientiﬁc literature. As shown in Figure 1, bacteria are generally more susceptible to UVGI than fungi, but this is not always the case. Reported k-values for different species of bacteria and fungi vary over orders of magnitude. Consequently, choosing which k-value to use is often difﬁcult and confusing. The variation in reported k-values makes genAugust 2008 ASHRAE Journal 29

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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