Transcript
Proceedings: Indoor Air 2005
DEVELOPMENT OF AIR PURIFICATION SYSTEM IN AIR-CONDITIONED SPACES
CHS Kwan1*, DWT Chan1 and LKC Law1 1 Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,
ABSTRACT Indoor air purification becomes a popular topic since the outbreak of Severe Acute Respiratory Syndromes (SARS) in 2003. Bacteria and viral contamination in indoor air has become a serious concern in healthcare facilities. In addition, there was a market boom in air purification products during the outbreak. This paper reviews major technologies available for bacteria removal, including ultraviolet (UV) light, photo-catalytic oxidation, HEPA filters; and describes the testing results of an air purification system in a University clinic. The system was installed with UV lamps and titanium dioxide honeycomb section for photo-catalytic oxidation. Total bacteria count was measured to test the effectiveness of the system. Without the use of purifier, the bacteria count could reach 500-600 cfu/m3 when there are 18 persons or more in the clinic waiting area. The air purifier can reduce the bacteria count to 400-450 cfu/m3 under similar occupancy condition. In addition, two bacteria decay profiles were measured after clinic service hour. The one with the use of air purifier could achieve a decay rate of 4.72 hr-1, comparing with the natural decay rate of 0.95 hr-1. INDEX TERMS Air purification, Ultraviolet Light, Titanium dioxide, Photo-catalytic Oxidation (PCO), Bacteria Count, Clinic INTRODUCTION Indoor air pollution is an environmental risk to human health. More attention is attracted on typical contaminants including particulates, volatile organic compounds (VOCs) and radon. The local Guidance Notes of Indoor Air Quality and the latest Indoor Air Quality Certification Scheme [1] outline the concentration requirement of those pollutants. Although indoor bacteria count is a parameter in the above scheme, there is comparatively less concern on the microbial loading in built environment in the past. In fact, airborne transmission is an important mode of pathogen among sporadic cases of nosocomial infection [2]. The outbreak of Severe Acute Respiratory Syndrome (SARS) gave the alarm signal to the world that the power of bacteria and virus has strengthened in this era. The disease was quickly spread through the droplet nuclei from SARS infectious patient. Over 8000 people were infected and it claimed over 900 lives worldwide. The infected cases in Hong Kong are about 20% of the total cases in the world [3]. The incident attracted the concern on the ventilation system performance in healthcare facilities. Responding to the disease outbreak, the fresh air system in buildings would be operated at its maximum fresh air rate, attempting to enhance the dilution effect. However, this practice was not an “active” approach, without any interaction with the pathogen itself, but passively increase the “chance of removal” of pathogens away from the occupied space. This approach allows quick implantation and does not require system alteration, but incurring a significant increase in energy consumption, and the operation cost. Air purification could be an alternative. DEVELOPMENT OF AIR PURIFICATION IN UNIVERSTIY CLINIC The air purification unit was installed in the clinic of the University. Major components include a titanium dioxide filter section; and 4 separate UV-C tubes installed vertically above the titanium dioxide section. The components inside the purifier are shown in figure 1. After entering the louver at the bottom of the purifier, a typical coarse filter with approximate efficiency of 25-30% is provided to prevent dust deposition on UV lamps and the titanium dioxide surfaces. The second section is a titanium dioxide coated on an aluminum honeycomb, illuminated with 7 x 8W UV-A lamps. The third section is installed with 4 x 12W UV lamps vertically. The final section is a variable speed fan. The unit was installed in the waiting area of University clinic.
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Corresponding author email:
[email protected] 3430
Proceedings: Indoor Air 2005 The area of the space is 84m2, and the ceiling height is 2.2m. There is a vertical air duct connecting the body of air purifier together with air distribution system above the ceiling. Based on velocity measurement at that duct section, the air purifier was found to deliver 160 Litre per second of air to the indoor space at 30 Hz operating fan speed. The air purification system is ducted to the return air plenum of the 2 fan coil units and distributed to the indoor space (figure 1).
Figure 1. Air purification unit RESEARCH METHODS The aim of the study is to evaluate the effectiveness of the air purifier on bacteria removal. Since the clinic was under normal operation during the time of study, from the health and safety point of view, it is not feasible to inject a specific type of bacteria as a testing source. The human source and the bacteria exists in the environment were used as an indicator. Cross-section analysis approach was adopted at the first stage. Total bacteria count was measured repeated at different occupancy condition. The fan coil units were kept at medium fan speed. The measurement started before the installation of air purifier, to allow an understanding the number of bacteria count in the clinic. The measurement was repeated, when the air purifier was in operation. By comparing two sets of data, any improvement due to the use of air purifier could be identified. Bacteria samples were obtained on agar plate inserted in Burkard Air Sampler, with an airflow rate of 20.0 Litre per minutes and a sampling duration for 9 minutes. The clinic area was divided into 2 zones as indicated in figure 1. The sampler was placed on the tables inside clinic, with a height of 30 cm above ground. The sampling points were located near and under the supply louver of the fan coil units (as shown in figure 1 above) at the time of measurement. Two samplers were used such that the bacteria count on both zones could be monitored at the same time. The patients waiting for physician consultation were sitting around the tables. The number of occupants in each zone was separately recorded. The agar samples collected were incubated for 48 hours under a controlled temperature of 35 °C. The number of colonies formed on the surface of agar was counted afterwards. RESULTS & DISCUSSION The number of patients was separately counted in each location. For instance at 10:00 a.m. there were 20 patients in the waiting area, 12 of them in zone A and 8 of them in zone B. In addition, one clinic clerk and one nursing staff were normally staying at the waiting area and serving the patients. One major finding is, when there were more than 10 persons in a particular zone, the bacteria count in the waiting area would exceed 500 cfu/m3, or the “good” 3431
Proceedings: Indoor Air 2005 class requirement on total bacteria in the Indoor Air Quality Certification Scheme and the Guidance Notes for Indoor Air Quality [1] issued by the Hong Kong Environmental Protection Department. In occasions the total bacteria count exceeded the 500 cfu/m3 good class limit even if there was less occupants. The difference in the quantity of bacteria emission from person to person may explain. On the other hand, after the installation of air purifier, the bacteria count could be maintained at a level below 500 cfu/m3. The air purifier was switched ON at 30 minutes before the first measurement, and operated in the whole day. On the evidence of the findings below, larger reduction of total bacteria count can be achieved in zone A. The bacteria sampling location at zone A was adjacent to the supply air diffuser and closer to the suction air grille of the purifier; while zone B was further away to the air purifier. Figure 2 (zone A) and 3 (zone B) illustrate the daily bacteria count profile, and compare the difference in total bacteria count if the purifier was in use. Total Bacteria C ount at Location A Air P urifier ON Air P urifier OFF E P D Good C lass
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600 Bacteria C ount (cfu/m3)
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Figure 2-3. Daily profile of total bacteria count at zone A and B respectively The curves (without using purifier) show that the bacteria inside clinic would not accumulate with time, instead the influence due to change in occupancy are more significant. Since the occupancy profile for the two days was not identical, the bacteria count at the same hour could be even higher when the purifier was in use. In particular, at 3:00 p.m., location A, there were only 4 patients when there was no purifier, but 13 patients when the purifier was in use. Such difference in occupancy in the two cases, and also the difference in quantity of bacteria emission from different persons, may not facilitate a clear identification on the air purifier performance. The relationship between bacteria count and the number of occupants was illustrated in figure 4 (zone A) and 5 (zone B). The purifier can successfully lowering the total bacteria count to a level below 500 cfu/m3 at both zones, even if there was more than 10 persons inside a particular zone. Total Bacteria Count at Location B
Total B acteria C ou nt at Location A Air Purifier ON
Air Purifier ON y = 25.968x + 252.75
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y = 20.472x + 333.84 2
R = 0.4118 500 400 y = 14.717x + 225.27 300
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Figure 4-5. Bacteria count in clinic vs. no. of patients at zone A and B respectively The first stage of our experiment shows that the number of occupants is an influencing factor to the bacteria concentration in clinic. However, the occupancy profile was not in our control during clinic service hours. In the second stage the night-time bacteria decay profile was observed. Since no patient or clinic staff would be staying in the clinic after clinic service hour, the influence due to change in occupancy could be eliminated in the second test. The measurement was performed in two days, in the first day the purifier was not used and the second day the 3432
Proceedings: Indoor Air 2005 purifier was operated at 30 Hz (160 L/s). The fresh air fan setting was not changed during the whole study. For the bacteria source, stage 1 results show that, without the use of purifier, the indoor bacteria count exceeds 500 cfu/m3(in zone A) at 5:00 p.m., which exceeds the EPD “Good Class” requirement [1]. Since a considerable amount of bacteria was accumulated from day-time clinic operation, and to satisfy the health and safety regulations in the University clinic, it was decided not to inject a specific type of bacteria for the test. The air purifier was switched OFF before the measurement in both days for bacteria source accumulation. Bacteria samples were sampled at a single point, which was near the suction louver of the purifier. Figure 6 shows the bacteria decay profile.
Night-time Bacterial Decay Bacteria Count (cfu/m3 )
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Figure 6. Night time bacteria decay profile respectively The bacteria removal rate can be calculated by using 1st order decay model: C(t)=C(0)e-kt Where
C(t) = C(0) = t = k =
(1) 3
concentration of bacteria at time t, cfu/m concentration of bacteria at t=0, cfu/m3 time spent in UV zone, hr-1 bacteria removal rate, hr-1 (decay constant).
The decay rates for the two trials are summarized in table 1. The use of air purifier facilitates a faster decay on bacteria. Table 1. Decay Rate and Indoor Conditions in Clinic (after service hours) Day1 Day2 Air Purifier OFF ON Temperature 21.2 oC – 22.1 oC 20.5 oC - 21.9 oC Relative Humidity 57.1 % - 72.1 % 60.3 % - 74.5 % Initial Bacteria Count 533 cfu/m3 672 cfu/m3 3 Bacteria Count after 1 hour 206 cfu/m 6 cfu/m3 -1 Bacteria Decay Rate 0.95 hr 4.72 hr-1 CONLUSIONS The combination of titanium dioxide filter and UV lamp installation in our air purification system could enhance the decay of indoor bacteria count. The improvement is more significant under high occupancy condition. The results also indicate that bacteria concentration varies with the number of occupants. Field study method was established to illustrate the effectiveness of the air purifier installation. However, there is a lack of design information from the supplier of titanium dioxide filter, such that the results obtained from this study could not be universally applied to all titanium dioxide filter products. The effectiveness cannot be predicted at the design stage until field measurement can be performed after installation. To facilitate air purification system design and further research study, it is necessary to establish testing protocol for the effectiveness of air purification material under a common platform. ACKNOWLEDGEMENTS The study was supported by a Dean’s Reserve Fund of the Faculty of Construction and Land Use of the Hong Kong Polytechnic University.
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Proceedings: Indoor Air 2005 REFERENCES Beggs CB. 2002. A Quantitative Method for Evaluating the Germicidal Effect of Upper Room UV Fields. Journal of Aerosol Science, vol.33, pp. 1681-1699 Guidance Notes for the Management of Indoor Air Quality in Offices and Public Places 2003. The Government of Hong Kong SAR, Indoor Air Quality Management Group. SARS Expert Committee 2003. SARS in Hong Kong from Experience to Action. Hong Kong SAR Government.
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