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for Air and Surface Disinfection Ultraviolet germicidal irradiation lamps can help clean coils and improve indoor air quality by W. J. KOWALSKI, PE and W. P. BAHNFLETH, PhD, PEDepartment of Architectural Engineering The Pennsylvania State University, University Park, PA 16802Lethal to microorganisms, ultraviolet radiation in the range 2250 - 3020 Angstroms is used in a variety of disinfection applications, a process referred to as Ultraviolet Germicidal Irradiation (UVGI). Since the first UVGI system was successfully implemented for disinfecting the municipal water system in Marseilles, France,1 in 1909 the disinfection of medical equipment disinfection with UVGI has been a common and reliable practice. But unlike water- and equipment-disinfection applications, however, the disinfection of air streams using UVGI has a history of varying success and unpredictable performance. The first laboratory studies on UVGI disinfection of air in the 1920's showed such promise that the elimination of airborne disease seemed possible. In 1936 Hart used UVGI to sterilize air in a surgical operating room2. In 1937 the first application of UVGI to a school ventilation system dramatically reduced the incidence of measles, with subsequent applications enjoying similar success3. Experiments by Riley and O'Grady4 resulted in the elimination of tuberculosis (TB) bacilli from hospital ward exhaust air. plethora of designs that were more imitative then engineered followed these early applications. The result was a mixture of successes and failures. This experience is reflected in various guidelines that decline to sanction the use of UVGI as a primary system. A 1954 study on the use of UVGI in showed a failure to reduce disease in London schools. Although limited data is available to determine the causes of earlier design failures, the apparent cloning of UVGI systems without regard to operating conditions probably doomed many installations from the start.A review of current industry design practices indicates that information on the design of UVGI systems lacks the detail necessary for engineers to ensure performance. This article addresses the factors that determine the design parameters of UVGI systems and discusses methods that can be used for sizing systems more effectively.
AIR-STREAM DISINFECTION APPLICATIONS The first step in the design of an airstream or surface disinfection system is to characterize the application. This includes describing the air stream, identifying the specific surface, and, sometimes, targeting specific microbes, such as TB. UVGI units are commonly located in an AHU downstream from the mixing box. Photo A shows a typical airstream disinfection system installed downstream from the filter bank and upstream from the cooling coils.Although UVGI systems can also be placed in the return-air duct to deal with recirculated, contagious pathogens, they are rarely placed in outside air supply duct. Spores, which hail from the outdoors, are more efficiently removed by filtration alone. An exception exists in cases such as AIDS clinics, where environmental bacteria from outdoors could threaten immunodeficient patients indoors. SURFACE-DISINFECTION APPLICATIONS UVGI for microbial-growth control has been undergoing much study recently and has enjoyed success in field applications5,6. Microbial growth may be comprised of fungi, bacteria, or even algae, but never viruses.
PHOTO C. (left) Microbial
growth on unirradiated filters. PHOTO D. (right) Microbe-free irradiated
filters. Photos courtesy of Airguard Industries, Louisville.
TYPES OF MICROORGANISMS The variety of microbes encountered by a given UVGI system is essentially unpredictable. It depends to some degree on the type of facility and geographic location. All viruses and almost all bacteria (excluding spores) are vulnerable to moderate levels of UVGI exposure. Because viruses are primarily contagious pathogens that come from human sources, they are found in occupied buildings. Bacteria can be contagious or opportunistic, with many found indoors; however, some are environmental. Certain facilities, such as agricultural buildings, may disseminate unique types of bacteria such as spore-forming actinomycetes. Spores, which are larger and more resistant to UVGI than most bacteria, can be controlled effectively through the use of high efficiency filters. The coupling of filters with UVGI is the recommended practice in all health care settings8 and for UVGI applications in general. MICROBIAL RESPONSE TO UVGI A basic review of the mathematics of UVGI disinfection will assist design engineers. The population S of a species exposed to any biocidal factor is described by the characteristic logarithmic decay equation:
where: k = standard decay-rate constant, cm2/microW-s I = Intensity of UVGI irradiation, microW/cm2 t = time of exposure, (sec) The standard decay rate constant, k, defines the sensitivity of a microorganism to UVGI, and is unique to each microbial species9. It can be thought of as the rate constant at an intensity of 1 microW/cm2, providing a basis for comparing pathogens. The rate constant for E. coli, commonly used for design purposes, is 0.000767 cm2/microW-s. Equation (1) omits two characteristics that may impact the disinfection process: the shoulder and the second stage. The shoulder represents the delay in response (or threshold dose) of a microorganism subject to UVGI exposure. If air velocity is too high and the dose is insufficient, the microbe may have a negligible response or even recover from the damage. Insufficient data exists to determine the shoulders, or threshold doses, of most airborne pathogens. Most microbial populations exhibit characteristic two-stage inactivation curves (Figure 3) in which each stage has a unique rate constant. The total survival curve is the sum of a fast-decay curve (the vulnerable majority) and a slow-decay curve (the resistant minority) as follows:  (2)
where :kf = rate constant for fast-decay population ks = rate constant for slow-decay population F = fraction of the total initial population subject to fast-decay response
The resistant fraction of most microbial populations is about 0.01 percent but some studies suggest it can be as high as 10 percent for certain species3. A distinction exists between the terms "disinfection" and "sterilization." Sterilization is defined as the complete destruction of all microbial species. Sterilization sometimes is considered to be 99.9999 percent eradication or a six log (base 10) reduction in microbial population. Disinfection, on the other hand, is merely the reduction of microbial population. Because airstreams are generally disinfected, not sterilized, this residual second stage usually can be ignored.
DESIGN PARAMETERS A number of parameters must be considered when considering UVGI products for HVAC designs. The most important factors are the airflow or HVAC equipment that will be disinfected, the lamp wattage and distance, and the ventilation system design itself. Air-stream characteristics The characteristics of an air-stream that can impact UVGI design are relative humidity (RH), temperature, and air velocity. Increased RH is commonly believed to decrease decay rates under ultraviolet (UV) exposure. However, studies on this matter are contradictory and incomplete at present. Fortunately, because most UVGI studies were conducted under normal indoor conditions, typical room and in-duct applications are not likely to differ greatly. Air temperature has a negligible impact on microbial susceptibility to UVGI10. However, it can impact the power output of UVGI lamps if it exceeds design values. Operating a UVGI system at air velocities above design will degrade the system's effectiveness because of the cooling effect of the air on the lamp surface, which, in turn, will cool the plasma inside of the lamp. UV output is a function of plasma temperature when power input is constant. Not all UVGI lamps have the same response to cooling effects. Some lamps have different plasma mixtures; overdriven power supplies that respond to plasma temperature; or UV-transparent, infrared-blocking shielding that limits cooling effects. Data from the manufacturer should be consulted to determine the cooling effects or the limiting design air velocities and temperatures within which the lamps can be efficiently operated.
Ventilation system design A number of ventilation system parameters can impact UVGI design. Air velocity and air mixing. Doses are determined by the time of exposure and UVGI intensity, both of which are dependent on the velocity profile and amount of air mixing in the airstream. The velocity profile inside the duct or chamber depends on local conditions and may be impossible to know in advance with any certainty. In any event, the design velocity of a typical UVGI unit is similar to that for filter banks -- about 400 fpm. Sufficient mixing will occur at these velocities to temper the effects of a non-uniform velocity profile.The amount of air mixing that occurs will affect system performance to a degree that depends on system configuration. This is illustrated in Figure 4, which compares the survival predictions for mixed- and unmixed-flow conditions in square ducts of increasing dimension. The error resulting from the assumption of complete mixing will decrease as system dimensions increase.In systems where the lamps do not span the entire duct width or length, the assumption of complete mixing also will result in larger differences, compared to unmixed flow. The important point is that system operation will lie somewhere between these two assumptions, which provide limits describing system efficiency.
FIGURE 6. Ray-tracing computer model of a cooling coil bank irradiated with a UVGI lamp. Rays are color-coded from blue to red in order of decreasing intensity. Image was generated using Photopia software from Lighting Technologies, Inc., Boulder, CO. Using reflectors. Reflectivity can be an economical way of intensifying the UVGI field in an enclosed duct or chamber. A surface with a reflectivity of 90 percent will reflect 9/10 of the light it receives. The results of a computer-generated analysis of reflectivity are shown in Figure 5. The components of reflectivity -- both direct and inter-reflected -- will clearly sum to greater than the initial direct intensity. This can occur whenever the surface is mostly enclosed and highly reflective. Such designs can considerably improve economics.
FIGURE 6b. Ray-tracing computer model of a cooling coil bank irradiated with a UVGI lamp. Twenty reflections are shown with 90 percent reflective surfaces. Image was generated using Photopia software from Lighting Technologies, Inc., Boulder, CO. Two types of reflective surfaces exist: specular and diffuse. Specular surfaces produce mirror-like reflections that are directionally dependent on the source, while diffuse surfaces produce non-directional reflections that spread equally in all directions. Non-glossy white paper is a good example of a diffuse surface. Most materials possess a combination of specular and diffuse properties and exhibit a degree of directional dependence. For UVGI design purposes the degree of directional dependence is usually not critical. Some materials reflect visible light, but not UV light. Polished aluminum is highly reflective to UV wavelengths, while copper, which reflects most visible light, is transparent in the UV range.
FIGURE 6c. Ray-tracing computer model of a cooling coil bank irradiated with a UVGI lamp. The staggered 5/4 coil tubes are 0.5 in. dia. with six fins per inch. Image was generated using Photopia software from Lighting Technologies, Inc., Boulder, CO. No simple method of calculating the three-dimensional UVGI-intensity field for specular reflectors exists. Ray-tracing routines using Monte Carlo techniques, are one approach, but the results do not easily lend themselves to analysis. However, they can be rather useful for examining complex geometries, such as when cooling coils are irradiated. Figure 6 shows ray-tracing diagrams of a UVGI lamp irradiating a bank of cooling coils from three perspectives. Note how few of the rays penetrate the coils, even after 20 reflections. Also note how the copper tubes absorb many of the rays -- although (pure) copper is transparent to UVGI the water inside is not. Combining with filtration. UVGI systems generally are used in combination with HEPA filters, a practice usually recommended for isolation-room applications. For other applications, however, HEPA filters do not offer a significant enough improvement in microbial-removal rates over high efficiency filters to warrant their exclusive use with UVGI. Recirculation systems. UVGI systems that recirculate room air or that are placed in a return-air duct or mixing-air plenum deliver multiple doses to airborne microorganisms. Although the effect is partially dependent on the air change rate, the result is an effective increase in removal rate in comparison wit a single-pass system. Calculations of removal rates for UVGI and filters in recirculation systems can be performed by evaluating the system minute-by-minute, including filtration rates, outside-air rates, and any microbial contaminants. Lamp considerations The hardest part of sizing a UVGI system is determining the lamp wattage for the stated disinfection goal. The intensity field caused by the lamp and the reflectors must be modeled and averaged before equation 1 is used to predict the disinfection rate.
Lamp-intensity field. An exact description of the lamp-intensity field is necessary to accurately determine the dose that is to be delivered to an airborne microorganism. Lamp ratings often are the sole parameter used for sizing a UVGI installation. Although this may be a conservative approach when distances to the lamp exceed 1 meter, oversizing and prohibitive economics can result. If complete mixing is assumed, then any intensity field can be described by the single value of average intensity. This requires computing the intensity at every point in a three-dimensional matrix defining the duct. We need to know the field caused by the lamp and, if necessary, the field caused by the reflections. Although the inverse-square law has been used for this purpose, it has proven to be inaccurate close to the lamp. An improved approach is to use the radiation view factor from a differential planar element to a cylinder as detailed in the sidebar Calculating the Intensity Field of a UVGI Lamp. Ignoring reflectivity, the average intensity field can be conservatively computed by applying Equation 3 to a three-dimensional matrix. There are view factors that can be used for computing the reflected intensity from flat parallel or perpendicular surfaces. Consult any thermal-radiation textbook for such view factors. First, use Equation 3 to determine the intensity at the flat surface. Then, use the appropriate view factor to determine the reflected intensity after multiplying by the reflectivity. Table 1 presents a comparison of UVGI systems that were sized using the view-factor method and may be used to approximate the performance of similar systems.
CONCLUSIONS Although simplistic, the methodology presented here is more accurate than any previously published method for sizing UVGI systems. The authors hope that these principles will lead to successful applications and avoidance of the design problems that have hampered the industry and perplexed engineers. Although the goal of eliminating airborne disease might remain unachievable, the information presented here may help lead the industry back to the path of continuous improvement. ACKNOWLEDGEMENTS The authors wish to thank the following for providing information and support for this article: Ultraviolet Devices, Inc., Lumalier, Inc., American Ultraviolet, Inc., Steril-Aire, Inc., Insect-O-Cutor, Inc., and Airguard Industries, Inc. REFERENCES Supplemental References (not printed in original article)
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