Evaluation of UV dose of upper-room UVGI system in a ward using CFD simulation

Transcription

1 Indoor Air 2008, August 2008, Copenhagen, Denmark - Paper ID: 233 Evaluation of UV dose of upper-room UVGI system in a ward using CFD simulation M.K. Sung 1,*, S. Kato 1, T. Akutsu 2, H. Ida 2, M. Asai 2, R. Yanagihara 3 and U. Yanagi 4 1 University of Tokyo, Japan 2 Nihon Sekkei, Japan 3 Tokyo Electric Power Company, Japan 4 National Institute of Public Health, Japan * Corresponding mkii@iis.u-tokyo.ac.jp SUMMARY An ultraviolet germicidal irradiation (UVGI) system installed in the upper room (UR) area was considered for indoor air germicide. The germicidal efficiency of the system can be evaluated based on UV intensity, exposure time, and the types of microbes. In this study, the UV intensities of a UR-UVGI system were measured in a dark room, and the indoor airflow was calculated using a computational fluid dynamics (CFD) simulation. The UV dose can be defined by the residence time multiplied sum of the UV intensity to which the air containing microbes is exposed. UVC doses were issued using CFD calculations coupled with the distribution of UV intensities measured. The results for some ward cases demonstrated that the spatial distribution of UV intensities and the different airflow characteristics based on the composition of supply and exhaust openings had a considerable effect on the efficiency of the UR-UVGI system. KEYWORDS Upper room UVGI, UV dose, CFD, Ventilation efficiency INTRODUCTION As the threat of accidents and bio-terrorism involving infectious microbes in buildings has increased recently, germicidal technologies that use UV have attracted attention. Two types of application, the In-Duct UVGI (ID-UVGI) system and the Upper-Room UVGI (UR-UVGI) system, have mainly been considered, and are being applied to indoor air germicide (IUVA, 2005). The ID-UVGI system, which is installed in air conditioning units or ducts is primarily used to disinfect microbes that may have accumulated or grown on the surfaces of filters, coils, or the walls of such unit. But due to the limited exposure time, airborne microbes passing through the unit are not efficiently disinfected unless the UV intensity is high enough. On the other hand, the UR-UVGI system, which is installed in the upper room area, is more effective at disinfecting airborne microbes as a result of their comparatively long exposure, as the air flows slowly and diffusively, allowing sufficient time for such exposure. But as extended exposure to UV, especially the UVC band, is also harmful to occupants, the UR- UVGI system should be installed in the upper room area and limited to UV irradiation of the upper room area only not the area occupied by people. Safety guidelines on UV exposure for individuals subject to occupational exposure of UV rays were also introduced by the American Conference of Governmental Industrial Hygienists (ACGIH) and the International Radiation Protection Association (IRPA) (WHO 1994). Ultraviolet rays, especially UVC (in the range 100 nm ~ 280 nm) were demonstrated to have a germicidal effect more than a century ago (Kowalski 2001). In terms of the germicidal

2 Table 1. UV rate constants (k) for some representative microbes. Type Microbe UV rate constant (k) Reference Fungi Aspergillus niger Penicillium chrysogenum Luckiesh 1949 Luckiesh 1949 Bacteria Bacillus subtillis Mycobacterium tuberculosis Peccia 2001 Riley 1976 Virus Vaccinia virus Collier 1955 mechanism, it is known that UV photons absorbed by DNA cause functional cloning disorders. From numerous previous experiments, disinfection efficiency is known to depend on the UV intensity, exposure time, and the type of microbes concerned. The killing rate (KR) can be expressed by the empirical equation below. KR kit = 1 e (1) where k is the UV rate constant [m 2 /J], I is the UV intensity [W/m 2 ], and t is the exposure time [s]. Table 1 demonstrates the UV rate constants (k) of some representative microbes (with reference to IUVA, 2005), which characterize how easily the microbe would be disinfected by UV. The UV rate constants tend to be larger for viruses, followed by bacteria and fungi. As a high UV rate constant equates to a high potential for being killed by UV according to Equation (1), the potential for disinfection by UV has the same high order of probability. Sharp (1940) also conducted UV exposure experiments on airborne bacteria and found that aerial spores were more readily disinfected than those on an agar surface. METHODS Measuring UV distribution of UVGI system A method to measure the spatial UV fluence rate by actinometry was developed and introduced (Ronaldo et al. 2004). But, provided that the reflectance were negligible, the value measured at a point facing the radiation source could be appropriate as representative of the point. In this study, the UV distribution of a UR-UVGI system (Air Shield UK18) was measured using a UVC radiometer (LP-471 UVC) in a dark room with dimensions of m 3. The UR-UVGI system was a wall-mounted type equipped with two compact lowpressure mercury lamps emitting UVC rays mostly ranged in 254nm. The UV intensities at about 300 points were measured facing the UR-UVGI system at each distance, horizontal and vertical angle. Calculation of UV dose using CFD UVC intensities were applied to the Computational Fluid Dynamics (CFD) model as scalar fluxes like airborne contaminants of the same value. The air entering the room from supply openings flows around the UVGI and the non-uvgi zone before exiting through the exhaust openings. In this process, when air enters the UVGI zone, the accumulated total represents the degree of UV intensity multiplied by residence time of the air at that point in time. Where the kind of source is not defined and small enough to ignore gravity and acceleration, the UV intensity can also be assumed as a passive scalar matter transported along the airflow. In this study, the UV intensity was treated as a contaminant and the averaged transport equation of it as follow was solved.

3 C U jc + t x j = x j ν t C Sc x j + q (2) where C is the concentration of contaminant, ν t is the turbulence viscosity, Sc is the turbulence Schmidt number and q is the emission rate of contaminant. A model of a four-patient ward with dimensions [m] was created for the CFD simulation (Figure 2). As the ward had absolute symmetry, only half of it was modeled for the actual simulation. In accordance with the combination of supply and exhaust openings, four cases were assumed as shown in Table 2. An air supply opening was installed in the centre of the ceiling and adjusted to blow air towards each patient. The air supply flow rates were planned to provide a ventilation rate of 11 times per hour. This corresponds to the supply air velocity of 4.18 [m/s] in Cases 1 and 2. But in Cases 3 and 4, personal supply openings were also installed, the supply air velocity was 3.34 [m/s] and the air velocities for the personal supply openings were 1.38 [m/s]. The thermal loads from patients lying on the beds, lighting in the ceiling, and windows were considered, as it was assumed to be summer. The commercial CFD software, STAR-CD, was used for this study. Table 3 indicates the conditions of the CFD simulation and the thermal loads applied. Figure 2. Model of four-patient ward for CFD simulation. Table 2. Simulation cases. Case Supply Openings (initial temperature: 20 C) Exhaust Openings Supply from ceiling Supply from ceiling Supply from ceiling + personal supply Supply from ceiling + personal supply Exhaust 1 Exhausts Exhaust 1 Exhausts 1 + 2

4 Table 3. CFD simulation conditions. Mesh Turbulence model Boundary conditions Thermal loads Conditions About 100,000 cells Standard k-ε model 2 3 / 2 <Supply> k = 3 / 2 ( 0.05), in U in ε in = C u kin / lin U in : inlet velocity, C u =0.09, l in : length of opening/7 <Exhaust> U out : mass balanced Patients: 68 W (34 W/patient 2 patients) on the beds Lighting: 334 W from the ceiling surface Windows: 492 W from the window surface a) b) c) Figure 3. Measuring UVC intensity distribution of UR-UVGI system. a) Louvers in front of lamps, b) Visualized distribution, c) Measuring in a dark room. UVC intensity [μw/cm 2 ] Horizontal angle Vertical angle [ ] UVC intensity [μw/cm 2 ] Horizontal angle Distance from UR-UVGI system [m] a) b) Figure 4. UVC intensity distribution of UR-UVGI system. a) Based on vertical angle at a distance of 0.6 m, b) Based on distance. RESULTS UVC distribution The UR-UVGI system should irradiate a narrow strip to avoid irradiating the occupants below with UVC rays. Several black louvers in front of the lamps create a vertically narrow and horizontally wide beam (Figure 3). Figure 4a demonstrates that UVC intensity fell drastically

5 against the vertical angle, and in particular the UVC intensities at ten degrees (vertical) decreased to just five percent of that at zero degrees. However, the differences in UVC intensities at horizontal angles were not so significant (Figure 4b) compared to those at vertical angles. A vertically narrow beam can be assumed to experience negligible reflection from the ceiling surface. The UV intensity decreased with distance on the basis of the inverse square law beyond about two meters from the UR-UVGI system, which was about five times the width of the system. Position of UR-UVGI system mounted Over 25 [μw/cm 2 ] Figure 5. UVC intensities applied to CFD meshes (Clipped section below Z=2.4m). UVC dose UVC distribution data were interpolated for application to CFD simulations. They were then applied to meshes for CFD calculations as scalar fluxes (Figure 5). The UR-UVGI system was assumed to be mounted on the wall above the door at a height of 2.4 m. The UVGI zone was assumed to exceed a height of 2.1 m. Figure 6 shows the spatial distribution of UV doses across a section (X=1.5 m) in each case. All cases demonstrated high UV doses near to the UR-UVGI system, which meant the high UVC intensity provided an effective UV dose irrespective of the exposure time. Cases 3 and 4, which had two personal supply openings, demonstrated low UV dose areas near these openings. Cases 1 and 3 indicated comparatively low UV doses (Figure 6, 7), which is assumed to be the result of having one exhaust opening near the door. As the UR-UVGI system was mounted above the door, the area near the door had a local area of high UVC intensity. In cases where the exhaust opening was installed near the door, the air velocity was comparatively high in the area of high UVC intensity, and consequently the exposure time was shorter than otherwise. Cases 2 and 4 also featured an exhaust opening near the door, but at the same time had another opening on the opposite side. This caused the airflow to split into two, causing longer exposure near the UR-UVGI system than those of Cases 1 and 3. Wide variations were observed in UV doses calculated with the averaged UVC intensity and the distributional UVC intensity field (Figure 7). The average UVC intensity of the UVGI zone was [μw/cm 2 ]. The room average UV doses calculated with this value ranged between ~ [J/m 2 ] and were overestimated by about 51%~71%. The high level of UVC intensity clustered near the UVGI system and the low exposure time in that area can be assumed to cause such differences. Table 4 demonstrates that the UV doses of Bed 1 near the

6 UR-UVGI system were a little higher than those of Bed 2. But, it was inferred that personal supplies lessened such differences in Cases 3 and 4. Based on these values, if the target microbe were Mycobacterium tuberculosis (k=0.4721), the killing rate would be 67~73%. a) b) c) d) [μj/cm 2 ] Figure 6. UV dose distribution (section at X=1.5 m). a) Case 1, b) Case 2, c) Case 3, d) Case 4. UV Dose [J/ ] with distributional UVC intensity UV doses calculated with averaged UVC intensity Case 1 Case 2 Case 3 Case 4 Figure 7. Room average UV doses calculated with distributional and averaged UVC intensity. Table 4. Average room and local UV doses. Case Room average Bed 1 (over 20 cm) Bed 2 (over 20 cm)

7 DISCUSSION AND CONCLUSIONS In the calculation process, UV intensities were treated as scalar fluxes like airborne contaminants. As a result, UV doses near to the supply openings were low, where the air was contrarily fresh. In terms of ventilation efficiency, the age of the air needs to be short. But by contrast, it is desirable for the air to have a prolonged stay in the room, especially in the UVGI zone, to increase the UV dose. Since calculation of the UV doses starts after the air enters the room and the results are those in a steady state, application of this method might be considered to be limited to conditions where microbes enter the room through the supply opening. But if we don t know where the release points for such contaminants are, this method can be helpful in determining the optimal layout of supply and exhaust openings and the UR-UVGI system to increase the disinfection efficiency of UR-UVGI systems based on the same ventilation rate and other conditions. REFERENCES Farhad M. et al Analysis of Efficacy of UVGI Inactivation of Airborne Organisms Using Eulerian and Lagrangian Approaches. ASHRAE IAQ2004. IUVA IUVA Draft Guideline IUVA-G01A-2005, General Guideline for UVGI Air and Surface Disinfection Systems. International Ultraviolet Association. Kowalski W.J Design and Optimization of UVGI Air Disinfection Systems. Ph.D. Thesis, The Pennsylvania State University, 1-6. Noakes C.J. et al Use of CFD Modelling to Optimise the Design of Upper-room UVGI Disinfection Systems for Ventilated Rooms. Indoor and Built Environment, 15;4; Noakes C.J. et al Development of a numerical model to simulate the biological inactivation of airborne microorganisms in the presence of ultraviolet light. Journal of Aerosol Science, 35, Philip W.B. et al The Application of Ultraviolet Germicidal Irradiation to Control Transmission of Airborne Disease: Bioterrorism Countermeasure. Public Health Reports, Vol. 118, Ronald O.R Spatial Distribution of Upper-room Germicidal UV Radiation as Measured with Tubular Actinometry as Compared with Spherical Actinometry. Photochemistry and Photobiology, 80, Sharp G The Effects of Ultraviolet Light on Bacteria Suspended in Air. Journal of Bacteriology, 38, Shelly L.M. and Janet M.M Evaluation of a Methodology for Quantifying the Effect of Room Air Ultraviolet Germicidal Irradiation on Airborne Bacteria. Aerosol Science and Technology, 33, Sung M.K. et al A Study on the Efficacy of UVGI System Based on Indoor Air Flow Characteristics, Annual Convention of Architectural Institute of Japan 2007, WHO Environmental Health Criteria 160; Ultraviolet Radiation. International Programme on Chemical Safety.