Method to evaluate UV dose of upper-room UVGI system using the concept of ventilation efficiency

Building and Environment 45 (2010) 1626–1631

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Method to evaluate UV dose of upper-room UVGI system using the concept of ventilation efficiency M. Sung a, *, S. Kato b a b

Department of Architecture, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 October 2009 Received in revised form 27 December 2009 Accepted 13 January 2010

Several countermeasures against the prevalence of infectious diseases have recently been issued, and one of them, the ultraviolet germicidal irradiation (UVGI) system, has been carefully considered for building environments especially. Besides experimental methods to evaluate the germicidal performance of upper air UVGI systems, this research introduces two numerical methods using the concept of ventilation performance, and illustrates the methods with a ward model. The first calculates the average residence time of air using the concept of local purging flow rate (L-PFR) which is multiplied by the average UV intensity of the upper area to obtain a UV dose. The other calculates the UV dose with the distributional UV intensity and deals with UV intensities as contaminant sources. The results of the illustrative cases with a ward model show that the method using the L-PFR concept could not clearly identify the difference in UV doses for each case with different exhaust opening setups, although the other could. The results from the method using the distributional UV intensity indicated the layout of ventilation openings and upper-room UVGI systems are important to optimize the germicidal performance. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Ultraviolet Disinfection Ward Purging flow rate CFD

1. Introduction The prevalence of influenza is consistently reported around the world by the World Health Organization (WHO), notwithstanding the improving hygienic environment [1]. Tuberculosis is also declared as a major threat of public infection even in advanced countries [2]. Furthermore, as the threat of accidents or bioterrorism involving infectious microbes in buildings has increased recently, ultraviolet germicidal irradiation (UVGI), germicidal technologies that use UV, have attracted attention – especially in Europe and North America. The ability of UV to disinfect microorganisms, especially bacteria and viruses, has been known about since about the beginning of the 19th century [3]. The use of UV for disinfection had been restricted primarily to industrial or laboratory applications at the early stage. Since the 1950s, however, research to apply UVGI systems to disinfection in hospitals had been conducted. Around the 1970s, Riley et al. performed several trial experiments to show the effective factors for the germicidal performance of UVGI systems using airborne microbes [4–6]. From the end of the 1990s, First and Miller also performed a series of experiments on the germicidal performance of upper-room UVGI system with a ward model

* Corresponding author. Tel.: þ81 3 5452 6431; fax: þ81 3 5452 6432. E-mail address: [email protected] (M. Sung). 0360-1323/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.01.011

chamber [7–9]. The results of their research showed that indoor environmental factors such as temperature, humidity, and air change rate would affect the germicidal efficiency of UVGI systems. These experimental approaches can illustrate the germicidal effect well in specific cases, but are hardly applicable when UVGI systems are considered for new projects and need to be optimized. Upper-room UVGI systems, one of the common applications of UVGI systems, are installed in the upper area of rooms and irradiate UV only to that area so as not to irradiate UV in the occupied zone. Therefore, airborne microbes need to be transported to the upper area to be disinfected by UV and airborne microbes could be delivered with airflow derived by forced and natural convection, such as via mechanical ventilation and thermal drafts respectively. The airflow can be estimated with sophisticated computational fluid dynamics (CFD) simulations. CFD simulation is capable of producing detailed information on the airflow and related data by modeling and simplified the airflow behavior including turbulence. Though there might be some uncertainties in the CFD simulation procedure, the CFD can be used to solve physical problems conceptually in various application methods. In this study, the methods used to calculate UV dose based on the concept of ventilation efficiency, which can be obtained by CFD simulation, were introduced to estimate the germicidal effect of an upper-room UVGI system in a ventilated room and a case study with a ward was illustrated.

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2. Methodology 2.1. Germicidal effect of UV From many previous experiments, disinfection efficiency is known to depend on the UV intensity, exposure time, and the type of microbes concerned, and several researchers have tried to model their relationship in terms of numerical equations [10]. The killing rate (KR) can be expressed by the empirical equation below.

KR ¼ 1  ekIt

(1) 2

2

where k is the UV rate constant [m /J], I is UV intensity [W/m ], and t is exposure time [s]. The UV intensity here is that of the UVC band ranging from 200 nm to 280 nm, which is the most effective wavelength for damaging DNA. The UV rate constants (k), which characterize how easily the microbe would be disinfected by UV, can be calculated using known UV intensity and exposure times from experiments. UV rate constants vary with the kind of microbes, but, in general, they are known to be higher for viruses and bacteria than fungi [11]. Once the UV rate constant is known, the killing rate can be estimated based on the UV dose, the multiplication of UV intensity and exposure time. Therefore, UV intensity and exposure time are the important points to evaluate the germicidal effect of UVGI systems, though there might be several environmental factors that also affect the germicidal effect such as temperature and humidity, etc [12]. 2.2. Distribution of UV intensity A method to measure the spatial UV fluence rate by actinometry is being developed and introduced in some research [13,14]. However, provided that reflectance is negligible, the value measured at a point facing the radiation source can approximate the UV intensity of that point. In this study, the UV distribution of an upperroom UVGI system (Air Shield, UK18) was measured using a UVC radiometer (DeltaOhm LP-471 UVC, effective from 200 to 280 nm) in a dark room with dimensions of 4  5  2.7 m3. The upper-room UVGI system was a wall-mounted type equipped with two compact low-pressure mercury lamps (Philips TUV PL-S, UVC output 2.3 W) emitting UVC rays mostly ranged about 254 nm as shown in Fig. 1. Several horizontal louvers were installed right in front of the UVC lamps to ensure horizontal UVC rays only radiate the upper area of the room. The UV intensities at about 300 points were measured facing the upper-air UVGI system at each distance, and horizontal and vertical angles. The air temperature of the dark room was from 24  C to 26  C, and the inside temperature of the upper-room UVGI system was around 40  C during the measurements.

Fig. 2. Diagram of the method to calculate UV dose using L-PFR.

time for air in a localized space is determined by air movements, which are driven by the momentum at the supply or exhaust openings and internal momentum such as natural convections, and these air movements can be estimated using CFD simulations. Purging flow rate (PFR) had been defined as the net flow rate at which a contaminant is driven towards the extract by Sandberg [15], and the concept of local purging flow rate (L-PFR) that is developed from PFR had been introduced at the same time as an index of ventilation efficiency in a local space. L-PFR can be calculated from the concentration simulation solving scalar transport equations using a method introduced by Davidson [16]. L-PFR [m3/s] could be calculated by Equation (2), where the uniform generation rate of a contaminant in a local space is qlocal [kg/s], and the average concentration of the contaminant in the space is Clocal [kg/m3].

L  PFR ¼

qlocal Clocal

(2)

L-PFR can easily be calculated using CFD simulation, but it is hard to determine experimentally, as the contaminant should be emitted uniformly in the local space. L-PFR can also be defined by the following equation from its definition.

2.3. Exposure time The time that the air containing microbial aerosols was exposed to the UV radiation could be estimated from the residence time – i.e. how long the air remained in the UV-irradiated space. Residence

Fig. 1. Inside of louvered upper-air UVGI system.

Fig. 3. Concept diagram of the method to calculate UV dose using the distributional UV intensity.

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Fig. 4. Model of four-patient ward.

L  PFR ¼

Vlocal Tlocal

(3)

where Vlocal is the volume of a local space [m3] and Tlocal is the average residence time [s] of the air in the local space. From Equations (2) and (3), Tlocal can be obtained as follows.

Tlocal ¼

Vlocal Clocal qlocal

(4)

In the case of a room where fresh air is supplied and exhausted, the average residence time of the air in a local space in the room from when supplied until exhausted can be calculated with this equation. Assuming the local space is the upper room area where UV is irradiated uniformly by an upper-room UVGI, the UV dose can be calculated with the average residence time of air using Equation (4) and average UV intensity in the upper room area. The UV dose in this case means the total UV dose of air irradiated from when supplied to the room until exhausted from the room as mentioned before.

DUV ¼ Tlocal Iave

(5) 2

where DUV is the total UV dose [J/m ] and Iave is the average UV intensity [W/m2] of the upper-room space. Fig. 2 shows a diagram of the method to calculate the UV dose using L-PFR and the average UV intensity.

upper area with a high UV intensity has a higher UV dose than one with low UV intensity even if the residence times of the two are the same. It can be inferred that the UV intensities vary with the distance and the direction from the UVGI system, which consequently forms a kind of distribution. In this method, UV intensity data was applied to the 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 the residence time of the air at that point in time as shown in Fig. 3. In this figure, the integrated value of the UVC intensity (I) multiplied by the residence time (t) at a spatial point eventually becomes the UV dose at the point, which expresses how much the air has been exposed to UVC rays until reaching that point. The concept of UV dose here is the same with the concentration of the contaminant that is the spatial flux of the contaminant multiplied by the residence time of the air. Where the kind of source is not defined and small enough to ignore gravity and inertia, 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 its averaged transport equation was solved as follows.

vD vUj D v nt vD ¼ þ vt vxj Sc vxj vxj

! þI

(6)

2.4. UV dose calculated with the distribution of UV intensity

where D is the UV dose [J/m2], I is the UV intensity [W/m2], Sc is the turbulence Schmidt number [-], and nt is the turbulence viscosity

Though the method using L-PFR is comparatively easy to apply with the average UV intensity of the upper room area, only the UV dose at the exhaust openings can be obtained using this method. Moreover, the UV dose might not be estimated exactly with the average UV intensity assumed, as a mass of air that resides in the

Table 2 Conditions of CFD simulation.

Table 1 Model cases. Case 1 2

A B A B

Conditions Mesh Turbulence model Boundary conditions

Exhaust opening

UVGI system

One on the ceiling near the door

On On On On

Two on the ceiling near the door and the window

the the the the

wall wall wall wall

near near near near

the the the the

door window door window

Thermal loads

About 100,000 cells Standard k-3 model kin ¼ 3/2  (Uin  0.05)2, 3 ¼ Cu  k3/2 in /lin Uin: inlet velocity, Cu ¼ 0.09, lin: length of opening/7 mass balanced no slip, standard wall function free slip Patients: 48 W (24 W/patient  2 patients) on the beds//Lighting: 329 W from the ceiling surface//Windows: 492 W from the window surface

6.0 5.0

Horizontal angle UVGI

4.0 Y

3.0

0° 20° 40° 60° Distance

2.0

Vertical angle

0 20 40 60

1.0

b

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

UVC intensity [W/m2]

a 2 UVC intensity [W/m ]

M. Sung, S. Kato / Building and Environment 45 (2010) 1626–1631

0.0 0

Vertical angle

Horizontal angle

0 UVGI

0° 10° 20°

Z

Distance

-90

1 2 3 4 5 Distance from UR-UVGI system [m]

1629

-60

-30 0 30 Vertical angle [ ]

20 40 60

60

90

By vertical angle

By distance

Fig. 5. UVC intensity distributions from measurements.

Fig. 6. Airflow distributions of four-patient ward model.

[m2/s]. The equation (6) was transformed from the standard scalar contaminant transfer equation. [17] 2.5. Case model A four-patient ward with dimensions 5.4  6.0  2.7 m was modeled for the CFD simulation (Fig. 4). As the ward had absolute symmetry, only half of it was modeled for the actual simulation. In accordance with the location of the exhaust openings and UVGI system, four cases were assumed as shown in Table 1. 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 a supply air velocity of 4.18 m/s in all cases. One exhaust opening was installed on the ceiling near the door in Case 1, but two exhaust openings were installed separately on the ceiling near the door and window in Case 2 to identify the effect of the position of the exhaust openings. Moreover, a UVGI system was installed on the upper wall near the door in Case A, and near the window in Case B, to identify the effect of the location of the UVGI system. 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 2 indicates the conditions of the CFD simulation and the thermal loads applied. 3. Results 3.1. Distribution of UV intensity The upper-air 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. Fig. 5a demonstrates that UV intensity fell drastically against the vertical angle, and in particular the UV intensities at 10 (vertical) decreased to just five percent of that at zero degrees. However, the differences in UV intensities at horizontal angles were not particularly significant compared to those at vertical angles (Fig. 5b). 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 2 m from the UR-UVGI system, which was about five times the width of the system. 3.2. Exposure time of air in upper area Fig. 6 shows the results of airflow calculations for the case model. Both cases show the air from the supply opening flows along the ceiling and is forced down at each corner near the patients. The air flows out through one exhaust opening in Case 1, but flows out through two exhaust openings in Case 2. The average exposure times of air calculated for each case using the concept of L-PFR are shown in Table 3. The exposure times (T) were almost the same notwithstanding the difference in exhaust openings. When the upper-room UVGI system was assumed to be mounted on the wall above the door at a height of 2.4 m and the

Table 3 Average UV doses using L-PFR method. Case

L-PFR [m3/sec]

T [s]

UV dose [J/m2]

1 2

0.155 0.149

65 67

4.26 4.45

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Fig. 7. UV dose distributions by the method using the distributional UV intensity.

UVGI zone was assumed to exceed a height of 2.1 m, the average UV intensity in the upper area of the case model was 0.066 W/m2 according to the results of previous UV intensity measurements for an upper air UVGI system. The UV doses for each case would be 4.26 J/m2 and 4.45 J/m2 respectively based on the average UV intensity. 3.3. UV dose calculated with the distribution of UV intensity UV distribution data was interpolated for application to CFD simulations. It was then applied to meshes for CFD calculations as scalar fluxes. Fig. 7 shows the spatial distribution of UV doses across sections (X ¼ 1.5 m and X ¼ 2.8 m) in each case. The values at each point indicate the average UV dose – i.e. how much the air has been exposed to UV until reaching the point from entering through the supply opening. All cases demonstrated high UV doses near to the UR-UVGI system, which meant that the high UV intensity provided an effective UV dose irrespective of the exposure time. Contrarily low UV doses were demonstrated near the supply openings, as fresh air not yet irradiated with UV is dominant in that area. Table 4 shows the average UV dose of each case. Comparatively low spatial average UV doses were indicated in Case 1-A, which is assumed to be the result of having one exhaust opening near the door. As the upper-room UVGI system was mounted above the door, the area near the door had a local area of high UVC intensity. In the case 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. Moreover, most of the air exposed to high UV was exhausted

through the exhaust opening without circulating. Contrarily, Case 1-B where the upper-room UVGI system is installed near the window shows the highest average UV dose because of the contrary reason to Case 1-A. Besides, the UV doses in Cases 2-A and 2-B, where there are two exhaust openings, were between those of Cases 1-A and 1-B. The air split between two exhaust openings seems to result in more circulation of highly exposed air near the upper-room UVGI system compared to Case 1-A. Notwithstanding these outstanding different results in each case, the average UV doses of the air at the exhaust openings were almost the same. The reason is assumed to be that the amounts of UV intensity in the ward and the air exchange rates are the same in all cases; so consequently, the average exhausted UV dose should be the same. This concept would also apply to the indoor contaminant emission problem. Based on these values, if the target microbe was Mycobacterium tuberculosis (k ¼ 0.4721 m2/J) which is known for its aerosol transmission, the overall killing rate in the ward would be 62–85% from Equation (1) as shown in Table 5.

Table 4 Average UV doses [J/m2] calculated using distributional UV intensity. Case

1-A 1-B 2-A 2-B

Room average

2.04 3.96 2.40 2.71

Over the bed

Exhaust

1

2

Average

1

2

2.16 3.82 2.53 2.75

1.88 4.30 2.20 2.83

3.87 3.81 3.86 3.82

– – 2.15 4.79

– – 5.58 2.86

M. Sung, S. Kato / Building and Environment 45 (2010) 1626–1631

Acknowledgements

Table 5 Killing rates [%] estimated with UV doses of Table 4. Case

1-A 1-B 2-A 2-B

1631

Room average

Over the bed

Exhaust

1

2

Average

1

2

62 85 68 72

64 84 70 73

59 87 65 74

84 83 84 84

– – 64 90

– – 93 74

4. Conclusions Wide variations were observed in the UV doses calculated using the distributional UV intensity field, though there was almost no variation in those using L-PFR. Moreover, the roomaverage UV doses calculated using L-PFR ranged between 4.26 and 4.45 J/m2 and were overestimated by about 8%–109% compared to those using the distributional UV intensity field. The reason is assumed to be that the method using L-PFR cannot account for the distribution of UV intensities, so the correlation between local UV intensities and residence times could not be considered. From the results of the UV doses calculated using the distributional UV intensity, it would be preferable to install an upper-room UVGI system far from the exhaust openings and divide the exhaust openings so as to increase the germicidal performance of upperroom UVGI systems. Noakes et al. have used CFD simulation to evaluate a UVGI system treating UV intensities as passive scalar sources [18]. In the calculation process of this study, UV intensities were also treated as scalar fluxes such as passive 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.

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