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Efficient test method for evaluating gas removal performance of room air cleaners using FTIR measurement and CADR calculation
Building and Environment 47 (2012) 385e393
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Efficient test method for evaluating gas removal performance of room air cleaners using FTIR measurement and CADR calculation Hak-Joon Kim a, c, Bangwoo Han a, Yong-Jin Kim a, *, Young-Hun Yoon b, Tetsuji Oda c a
Environment and Energy Systems Research Division, Korea Institute of Machinery and Materials, 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Korea Environmental Industry & Technology Institute, 209 Jinheungno, Eunpyeong-gu, Seoul 613-2, Republic of Korea c Department of Electrical Engineering and Information Systems, School of Engineering, The University of Tokyo, 7-3-1, Hongo Bunkyo-ku, Tokyo 113-8656, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2011 Received in revised form 25 May 2011 Accepted 25 June 2011
We have developed a new test method for differentiating the gas-removal performance of indoor air cleaners by using Fourier-transfer infrared measurements and a clean-air-delivery-rate (CADR) calculation method in a closed test chamber (4 m3). Eighteen air cleaners were evaluated using both the new method and the current Korean and Japanese test methods, using ammonia, acetic acid, acetaldehyde, and toluene as test gases. The Association of Home Appliance Manufacturers’ statistical method for calculating regression line slopes of test chamber gas concentrations during air cleaner operation was used. The standard deviations of CADRs for ammonia, acetic acid, and toluene, gases that were easily removed by the air cleaners in the test chamber, were 3.2, 751.3, and 13.4 times higher, respectively, than the gas-removal efficiencies determined using the current arithmetic calculation method, which uses the ratio of concentrations after 0 and 30 min of air cleaner operation. The new test method clearly differentiated the gas-removal performances of various air cleaners, especially for gases that are quickly removed by indoor air cleaners. Also, the single-pass removal efficiency of the air cleaners was obtained with a simple calculation: CADR/flow rate/0.83. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Air cleaner Chamber Clean air delivery rate (CADR) Gas-removal efficiency VOC
1. Introduction Indoor air quality (IAQ) is a top environmental priority of environmental agencies such as the US Environmental Protection Agency (US EPA) and the European Environment Agency (EEA). IAQ is a global problem because people now spend much of their time indoors, and exposure to toxic gas pollutants such as NO2, CO, VOCs from fuel/tobacco combustion, construction and furnishing materials can be an issue in the workplace or home, or both [1,2]. Airborne indoor pollutants include particulates, allergens, and organic and inorganic gaseous pollutants [3]. In particular, some buildings, including new construction, contain such high concentrations of gas-phase pollutants that they are qualified as “sick” because exposure to the inside of buildings results in multiple sickness symptoms, such as headache, fatigue, skin and eye irritation, or respiratory illness, commonly described as “sick-building syndrome” (SBS) [4]. Solutions recommended by the US EPA to improve IAQ include combinations of actions such as removing pollutant sources such as dust and harmful gases, increasing ventilation rates and improving air distribution, and cleaning
* Corresponding author. Tel.: þ82 42 868 7475; fax: þ82 42 868 7284. E-mail address: [email protected] (Y.-J. Kim). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.06.024
indoor air [5,6]. The most efficient method of cleaning indoor air is the use of air cleaners, which remove airborne contaminants relatively quickly by several different processes of varying effectiveness, such as filtration, activated carbon, ionization, and photocatalytic oxidation [7e10]. US EPA explains that usually the best way to address this risk is to control or eliminate the sources of pollutants, and to ventilate a home with clean outdoor air. The ventilation method may, however, be limited by weather conditions or undesirable levels of contaminants contained in outdoor air. If these measures are insufficient, an air cleaning device may be useful [11]. In particular, to remove gas-phase pollutants, most air cleaners use an adsorption mechanism; activated carbon filters have been commonly used because they have a high adsorption capacity of gas-phase pollutants due to their highly developed porous structure and large specific surface area [12]. Many countries, including the US, Japan, and Korea, evaluate the performance of room air cleaners using their own standard test methods [13e15]. Brief descriptions of the methods commonly used in these countries to test air cleaner performance are listed in Table 1. For particle removal performance, all methods measure the clean air delivery rate (CADR; m3/min), which describes the equivalent volume of clean air provided to the space by an air cleaner. This is a universal and efficient metric for estimating the particle removal performance of air-cleaning devices in rooms of
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Table 1 Details of Korean, American, and Japanese test methods for evaluating particle and gas removal performance of household air cleaners. Test method
KACA
Experimental conditions Particle removal
Gas removal
Type of test particle
Initial particle concentration (#/m3)
KCl
108e1010 10
JEMA
Smoke Dust
7.07 10 3.54 10
AHAM
Cigarette Smoke Arizona Dust Pollen
2.4e3.5 1010 2.0e4.0 108 4.0e6.0 106
11
Size range (mm)
Measurement
Type of test gas
Size of test chamber (m3)
Measurement time (min)
Measurement
0.3
CADR
4
30
Removal efficiency
0.3
CADR
1
30
Removal efficiency
0.1e1.0 0.5e3.0 5e11
CADR
Ammonia, Acetic acid, Acetaldehyde (10e13 ppm) Ammonia, Acetic acid, Acetaldehyde from five tobaccos e
e
e
e
various sizes or in comparing air cleaning with ventilation as an IAQ control technique [16,17]. The only difference among the three performance tests listed above for particle removal is the experimental conditions of the test particles. EPA had explained that although AHAM uses the CADR concept to evaluate the performance of portable air cleaners in reducing particulate matter concentrations, the CADR can be applied equally to the removal of gases pollutants. The gas-removal efficiencies of 126 room air cleaners for ammonia, acetaldehydes, and acetic acid, evaluated using the Korean test standard, SPS-KACA-002-132, for 5 years, from 2005 to 2009, are shown in Fig. 1. The average removal efficiencies for ammonia, acetaldehyde, and acetic acid were 86.5%, 69.8%, and 98.1%, and the efficiencies for ammonia and acetaldehyde ranged from 20 to 100% with relative deviations of 17.8% and 23.8%, while those for acetic acid were mostly 100% with a deviation of only 3.5%. These results indicate that the test method for evaluating gasremoval performance has a technical limitation, namely an inability to differentiate among the time-dependent gas-removal performances of various air cleaners and test gases, especially gases that are easily removed by air cleaners, because removal performance is evaluated using only the gas concentrations at specific times (0 and 30 min) according to Korean and Japanese standards. To evaluate the time-dependent gas-removal performances of air-cleaning devices more efficiently, several research groups have developed test protocols that have not been standardized [12,18e23]. Chen et al. [19] developed a full-scale test method with a 54-m3 test chamber, gas chromatography/mass spectrometry
(GC/MS) measurements, and 17 volatile organic compounds (VOCs). They measured the CADRs of 15 air cleaners, but due to the scale of the test chamber and the measurement method, approximately 12 h were needed to obtain a single test result. HowardReed et al. [21,22] also developed a field test protocol with test houses of 85 and 340 m3 to compensate for the technical limitations of CADR evaluation in the field, given dynamic mass transport conditions such as weather-dependent humidity and temperature, using decane gas and GC/electron capture detector (ESD) measurement methods. These methods still required a test period of several hours because of the size of the test facility and the long measurement time of the analytic method. Using a relatively small test chamber (6.3 m3) and measurement time of approximately 30 to 90 minutes for a test, Niu et al. [18] tested 27 air cleaners with toluene gas; they proposed that this test method could quantify the initial cleaning capacity of an air cleaner for gaseous phase pollutants. To evaluate the time-dependent gas-removal performance of air cleaners for various gases more quickly (<30 min for a single test), we developed a novel method of measuring CADRs that uses the same statistical calculations as applied in ANSI/AHAM AC-1-2006, a relatively small closed chamber (4.0 m3), and a real time multigas measurement system with a Fourier-transfer infrared spectrometer (FTIR) that is used by the US EPA and the National Institute for Occupational Safety and Health (NOISH) as a standard analytic method for measuring organic and inorganic gas-phase gases [24e26]. In addition, we evaluated 18 air cleaners that are commercially available in Korea and Japan using the current Korean and Japanese test methods and compared the results to those of the new test method. 2. Experimental setup 2.1. Test gases and specimens
Fig. 1. Gas-removal efficiencies of 126 room air cleaners for ammonia, acetaldehyde, and acetic acid, determined using the Korean standard test method.
Table 2 lists the physical and chemical properties of the test gases used in this study. Four gases were selected for the gasremoval performance tests: ammonia (NH3), acetic acid (CH3COOH), acetaldehyde (CH3CHO), and toluene (C7H8); the first three are also used in the Korea Air Cleaning Association (KACA) and Japan Electrical Manufacturers’ Association (JEMA) standards, and toluene is a representative VOC. All four gases are sources of malodorous indoor irritants, and have been used in many previous studies [18,27e29]. Eighteen indoor air cleaners commercially available in Korea and Japan were tested in this study. Descriptions are provided in Table 3, where the cleaners are grouped on the basis of manufacturing company and type of filtration. All products were equipped with HEPA filters for particle removal and carbon filters
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Table 2 Chemical and physical characteristics of test gases used in this study. Gas
Phase
M.W. (g/mol)
Boiling Temp. ( C)
Melting Temp. ( C)
Density (g/cm3)
Solubility in water (g/100 ml)
Danger
Characteristics
Ammonia (NH3) Acetic acid (CH3COOH) Acetaldehyde (CH3CHO) Toluene (C7H8)
Gas Liquid Liquid Liquid
17.03 60.05 44.05 92.14
33.4 118.1 20.2 110.6
77.7 16.7 123.5 93.0
0.00068 1.049 0.788 0.8669
89.9 (0 C) Fully miscible Miscible 0.053 (20 C)
Toxic, corrosive Corrosive Toxic, carcinogenic Carcinogenic
Light colorless gas, irritating smell Colorless liquid, irritating smell Volatile colorless liquid, irritating smell Colorless liquid, unique smell
for gas removal, and the flow rates of the air cleaners ranged from 3.0 to 7.3 m3/min; the flow rates of the majority of household room air cleaners fall in this range.
2.2. Performance test and evaluation methods The experimental set-up for the measurement of CADR and gasremoval efficiency is shown in Fig. 2: a test specimen was placed in a closed chamber (4.0 m3), and an FTIR spectrometer (Model I4000, Midac, Costa Mesa, CA, USA) with a 20-m cell was used to measure the concentrations of ammonia, acetaldehyde, acetic acid, and toluene in the chamber. Wall surface adsorption of the test chamber was negligible during the test because when we checked the natural decay for an hour with known gas concentration of approximately 10 ppm of the test gases, there is no decrease in the initial concentration during the period. When the test chamber background level fell below the measurement limit of 0 ppm, a heated or bubbled gas was introduced to the chamber in clean compressed air at a rate of 2 L/min until the initial concentration of the test gas ranged between 8 and 13 ppm. This concentration was intentionally higher than actual field concentrations of the gas, so that the concentration decay curve could be accurately measured before it fell below the instrument detection limit [19]. When the required initial concentration was reached, the test specimen and the FTIR spectrometer were turned on, and gas concentration data were acquired at 20-s intervals for 30 min. The FTIR system had a 1-cm1 resolution with a 20-m gas cell, a He/Ne laser as a radiation source, a liquid-nitrogen-cooled mercury cadmium telluride detector, ZnSe optics, and a Ge/ZnSe beam splitter. The spectral wave frequency was in the range of 650e4500 cm1, with 32 scans, during which measurements were taken at intervals of 20 s per spectrum. The IR peaks that were used
to measure the concentrations of the gases were 917.45e940.38 cm1 for ammonia, 2629.78e2887.87 cm1 for acetaldehyde, 1146.49e1225.78 cm1 for acetic acid, and 722.06e735.63 cm1 for toluene, to avoid overlap with other peaks, such as those of water and carbon dioxide. The IR spectrums were analyzed using the commercial library databases supplied by Midac and the US EPA, and the FTIR system was purged with pure nitrogen gas before and after the measurements. Also, before the experiments for the gas removal performance tests, the FTIR was calibrated with the known concentrations of standard gases with approximately 10 ppm and 5 ppm for the all test gases which were supplied by the Korea Research Institute of Standards and Science. The concentrations of the calibration gases of ammonia, acetic acid, acetaldehyde and toluene were 11.6/6.3 ppm, 9.5/5.8 ppm, 10.8/ 6.1 ppm and 12.9/6.2 ppm with 5% uncertainty (k ¼ 2, 95% confidence interval), respectively, and the standard deviations of the concentration measured by the FTIR with the calibration gases were 0.05/0.03 ppm, 0.05/0.01 ppm, 0.14/0.09 ppm, 0.08/0.02 ppm for ammonia, acetic acid, acetaldehyde and toluene, respectively. The gas-removal performance of each room air cleaner was expressed as removal efficiency calculated using the arithmetic methods of KACA and JEMA [13,14]. Removal efficiency was calculated as follows:
hi ¼
Ci;0min 1 Ci;30min
! 100ð%Þ
(1)
where hi: removal efficiency of gas i Ci,0 min: initial concentration of gas i Ci,30 min: concentration of gas i after 30 min of air cleaner operation The Association of Home Appliance Manufacturers (AHAM) established a performance metric, a CADR, that is based on an air
Table 3 Descriptions of the air cleaners tested in this study. M
No.
Filter combination
Mass of activated carbons (g)
Flow rate (m3/min)
CADR
Removal efficiency
Single-pass efficiency Ammonia
Toluene
A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Porous activated carbon plate) HEPA filter þ Carbon filter (activated carbon sheet) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets)
180 280 320 180 180 280 280 320 320 300 300 320 320 235 300 21 180 130
3.5 5.4 6.4 3.2 3.6 4.3 5.7 5.4 6.5 5.8 5.8 7.2 7.3 3.8 5.0 3.0 6.4 3.3
O
O
X X X O O O O O O O O X X O X X O X
X X X O O O O O O X X X X X X X X X
B
C
*M : manufacturer, O : tested, X : not tested.
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Fig. 2. Experimental setup for the air cleaner performance tests under the new and current test methods.
cleaner’s reduction of particles in a closed chamber. In this study, gas-removal performance of a room air cleaner was also represented by the CADR calculated using the statistical method of the AHAM [15]. Theoretically, the regression of pollutant concentration follows a first-order decay model:
Ct ¼ C0 ekt
(2)
where Ct ¼ concentration at time t (ppm) C0 ¼ initial concentration at t ¼ 0 (ppm) k ¼ decay constant (t1) t ¼ time (min) The time-resolved decay constant k is calculated statistically using a linear regression of ln Cti and ti using the following formula:
k ¼
SXY SXX
(3)
where,
SXY ¼
n X
ti ln Cti
i¼1
¼
n X
ðti Þ2
i¼1
1 n
1 n n X
n X i¼1
! ti
!2
n X
! lnCti ; SXX
i¼1
ti
i¼1
Using Eqs. (2) and (3), the AHAM method for calculating CADR is
CADR ¼ Vðke kn Þ
(4)
where CADR ¼ clean air delivery rate (m3/min) V ¼ volume of test chamber (m3) ke ¼ total decay rate (min1) kn ¼ natural decay rate (min1) In this study, natural decay (kn) when the air cleaners were not operating was negligible, indicating that there was no leakage or diffusion loss of test gases in the test chamber during the 30-min test period. CADRs were calculated using the data from the gasremoval efficiency experiments shown in Fig. 2.
In order to more efficiently understand time dependent gas removal performances of the test air cleaners during the test which was performed with different initial concentrations of the various test gases, we computed the normalized concentrations by dividing the original data of the concentrations measured by the FTIR with the initial concentrations for each test gas just before the operation of the air cleaner. Also, we computed fitted lines of the first-order exponential equation with the normalized concentrations by using the commercial software, EXCEL 2007 version, to investigate the applicability of the CADR concept for the gas removal performance tests. In previous studies [18,19,29e32], if air flow rate and single-pass removal efficiency were known, CADR was obtained using the following formula:
CADR ¼ hs;i Q E
(5)
where hs,i: single-pass removal efficiency of gas i Q: air flow rate of an air cleaner E: short-circuit factor (E ¼ 1 under well-mixed conditions) By comparing the CADR measured experimentally in this study and the CADR calculated with Q and hs,i, we calculated E for this study. Single-pass efficiency (hs,i) was measured as shown in Fig. 3 in an open test duct (600 600 mm2) with 10 and 6 air cleaners for ammonia and toluene, respectively (Table 3). Single-pass efficiency was calculated using the fraction of the gas concentration removed from the air stream as it passed through the air cleaner The initial concentration was maintained at between 8 and 13 ppm, which was the range used in the closed chamber tests.
3. Results and discussion Fig. 4 shows normalized concentrations over time of the four studied gases for two air cleaners with different flow rates manufactured by the same company, B. After 30 min of air cleaner operation, the normalized concentrations of ammonia, acetic acid, and toluene had decreased to nearly 0% of their initial concentrations, while that of acetaldehyde was reduced to 40% to 50% of its
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Fig. 3. Experimental setup for the single-pass efficiency test.
Fig. 4. Gas concentration as a function of time for two air cleaners manufactured by the same company but with different flow rates: (a) 5 m3/min and (b) 7.3 m3/min.
initial concentration. Very volatile organic compounds, such as acetaldehyde, are known to be difficult to remove with sorptionbased materials due to low absorbability [19]. In particular, the air cleaner with a 7.3 m3/min flow rate removed all test gases more swiftly than another device, which had a 5.0 m3/min flow rate. Shown Table 3 and 4, the carbon filters manufactured by B company were carbon filters which were packed by cylindrical pellets of activated carbons, and the mass of activated carbons of the carbon filter for the air cleaner with the flow rate of 7.3 m3/min was larger than that for the air cleaner with 5.0 m3/min. The residence time of the air cleaner with 7.3 m3/min was also longer than that with 5.0 m3/min, while the circulation time which meant the time to treat whole air volume of the 4.0 m3 closed chamber, was shorter than that with 5 m3/min. Thus, the air cleaner with 7.3 m3/min removed the test gases more effectively than that with 5.0 m3/min since the gas removal performance of air cleaners is dependent on parameters related to the physical and operational parameters such as activated carbon mass (single-pass removal efficiency), residence time (single-pass removal efficiency and flow rate), and circulation time (flow rate) [18,19,29e31]. Fig. 5 shows normalized concentrations over time of the four studied gases for two air cleaners with different flow rates manufactured by different companies, A and C. Acetaldehyde concentration was reduced faster by the air cleaner with a flow rate of 3.2 m3/min than by that with a flow rate of 6.4 m3/min. Ammonia was approximately 90% removed by the 3.2 m3/min flow rate air cleaner, while acetic acid and toluene were 100% removed by both air cleaners after 30 min of operation. Shown Tables 3 and 4, the circulation time with the air cleaner with 3.2 m3/min was 2 times longer than that with 6.4 m3/min, while the mass of activated carbons was 1.1 times larger than that with 6.4 m3/min, and the residence time was 2.3 times longer than that with 6.4 m3/min. In this case, we expected that the gas removal performance of the air cleaners in the closed chamber was affected by the parameters related to single-pass removal efficiency such as the mass of activated carbons and residence time. The results indicate that the gas-removal performance of an indoor air cleaner depends on several parameters, such as the type of gas, flow rate, and type of filter. However, if evaluated only on the basis of gas-removal efficiency, calculated from the initial
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Table 4 Descriptions of physical and operational characteristics of the four air cleaners tested in this study.
B B A C
Carbon filter type
Width (m)
Length (m)
Depth (m)
Volume (m3)
Flow rate (m3/min)
Residence time (sec)
Circulation time (sec)
Pellets Porous activated carbon plate Pellets Pellets
0.38 0.27 0.24 0.26
0.36 0.27 0.37 0.32
0.0080 0.0090 0.0080 0.0094
0.00110 0.00068 0.00071 0.00079
7.3 5.0 3.2 6.4
0.00917 0.00811 0.01332 0.00738
33.33 48.00 75.00 37.50
concentration and the concentration after 30 min of air cleaner operation, the gas-removal performances of the four different air cleaners in Figs. 4 and 5 can be considered the same for acetic acid and toluene, which were swiftly removed by all the air cleaners, with somewhat different gas-removal performances for ammonia and acetaldehyde. Fig. 6 shows the log-normalized concentrations over time of the test gases for the same company air cleaners with different flow rates and their fitted equations. All lines fitted well using the firstorder model explained in Eq. (2) [12,18,19]. In particular, the decay constants of the regression lines for acetaldehyde, ammonia, toluene and acetic acid were 0.030, 0.128, 0.113 and 0.151 min1 for the air cleaner with a flow rate of 5.0 m3/min, while those for the air cleaner with a flow rate of 7.3 m3/min were 0.036, 0.169, 0.226 and 0.259 min1. All the decay
Fig. 5. Gas concentration as a function of time for two air cleaners manufactured by different companies and with different flow rates: (a) 3.2 m3/min and (b) 6.4 m3/min.
constants for each gas with a flow rate of 5.3 m3/min were much smaller than those of 7.3 m3/min, and the gas removal performance of each air cleaner for the four different gases was clearly differentiated with the difference of the decay constants. Fig. 7 shows the log-normalized concentrations over time of the test gases for the different company air cleaners with different flow rates and their fitted equations. the decay constants of the regression lines for acetaldehyde, ammonia, toluene and acetic acid were 0.064, 0.113, 0.182 and 0.299 min1 for the air cleaner with a flow rate of 3.2 m3/min, while those for the air cleaner with a flow rate of 6.3 m3/min were 0.027, 0.134, 0.241 and 0.240 min1. Differently from the comparison with the removal efficiency calculated by the current test method, explained in Fig. 5, all the decay constants for each gas were clearly different,
Fig. 6. Log-normalized gas concentration as a function of time for the same company air cleaners with different flow rates and the fitted equations: (a) 5 m3/min and (b) 7.3 m3/min.
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Fig. 8. Gas-removal efficiencies of 18 household air cleaners for the four study gases, obtained using the current test methods.
Fig. 7. Log-normalized gas concentration as a function of time for the different company air cleaners with different flow rates and the fitted equations: (a) 3.2 m3/min and (b) 6.4 m3/min.
even for toluene and acetic acid which were removed perfectly by the air cleaners within 30 min. Thus, these results indicate that it is possible to measure the time-dependent gas-removal performance, i.e., CADR, of various air cleaners for different test gases by applying the AHAM calculation method, and also to differentiate gas removal performance of various air cleaners for all the test gases by the CADR. Fig. 8 shows the gas-removal efficiencies of the 18 household air cleaners for the four study gases. Regardless of carbon filter type or flow rate, most of the efficiencies calculated using Eq. (1), which is used in the current Korean and Japanese tests, were near or in excess of 80%, except those for acetaldehyde. Efficiencies for ammonia, acetic acid, and toluene, which are easily removed by the air cleaners, even exceeded 90%. As shown in Figs. 4 and 5, the residual concentrations of ammonia, acetic acid, and toluene in the test chamber mostly decreased to the detection limit of the FTIR spectrometer after 30 min of air cleaner operation. Here, the current calculation method used in Korean and Japanese test standards clearly differentiated the removal performances of the air cleaners for acetaldehyde, but efficiencies for ammonia, acetic acid, and toluene are similar so that gas-removal performance of those three gases may be not differentiated by the current test method.
Fig. 9 shows the CADRs of the 18 models for the four test gases, indicating how quickly the air cleaners removed each gas from a closed chamber. The CADRs for all gases ranged widely, from 0 to 1.6 m3/min, and clearly differed according to gas type and air cleaner model. In particular, the CADRs for ammonia, acetic acid, and toluene, which were completely removed after 30 min by all air cleaners, varied from 0.2 to 0.8 m3/min, 0.5 to 1.6 m3/min, and 0.4 to 1.5 m3/min, respectively. Fig. 10 shows the relative standard deviations of the gas-removal efficiencies and the CADRs of the 18 air cleaners for the four gases. The relative standard deviations of the CADRs were 31.0%, 47.3%, 53.2%, and 32.2% for ammonia, acetic acid, acetaldehyde, and toluene, and those of the gas-removal efficiencies were 9.8%, 0.06%, 32.0%, and 2.4%, respectively. The standard deviations of the CADRs for ammonia, acetic acid, and toluene were 3.2, 751.3, and 13.4 times higher than those of the gas-removal efficiencies, respectively. These results indicate that the novel test method developed in this study is able to differentiate the gas-removal performance of various air cleaners better than the methods currently used in Korea and Japan, especially in the case of gases that are efficiently removed by air cleaners. The CADRs for particle and gas removal obtained from experiments and calculated using Eq. (5) are compared in Fig. 11. For the Eq. (5) calculations, single-pass removal efficiencies were measured
Fig. 9. CADRs of 18 household air cleaners for the four study gases, obtained using the new test method.
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H.-J. Kim et al. / Building and Environment 47 (2012) 385e393
method, which uses a 30.4-m3 test chamber, to be 0.86 (Fig. 11a). We also tested 10 models of air cleaners for ammonia and six models for toluene using a 4.0-m3 test chamber, and the shortcircuit factor for this new gas-removal performance test averaged 0.83 (Fig. 11b), 0.03 lower than that for the particle removal test. The short-circuiting, the re-entrainment of some of the cleaned air from the outlet of the air cleaner into the inlet of the air cleaner rather than mixing completely with air within the chamber, is related to the mixing characteristics in a closed space, and is difficult to single out [18,29]. Here, the difference may be related to the different sizes of test chamber, mixing fan and test agents such as particle and gas. 4. Conclusions
Fig. 10. Relative standard deviations of the gas-removal efficiencies and the CADRs of 18 air cleaners for four gases
using a test duct, as shown in Fig. 3. For 5 years, 2005e2009, we measured particle removal CADRs for the 126 air cleaners commercially available in Korea and Japan, and determined the short-circuit factor for the particulate removal test in the KACA test
The gas-removal performance of 18 room air cleaners that are commercially available in Korea and Japan was evaluated with four different gases, ammonia (NH3), acetic acid (CH3COOH), acetaldehyde (CH3CHO), and toluene (C7H8), which are irritating and malodorous in indoor environments, by using current standard test method in Korea and Japan, and a novel test method which measured the gas-removal performance of indoor air cleaners using the AHAM CADR calculation method and FTIR measurement in a closed chamber (4 m3). The evaluation test results indicate that the gas-removal performance of an indoor air cleaner depends on the type of gas, flow rate, and type of filter. However, if evaluated only on the basis of gas-removal efficiency of the current test method, calculated from the initial concentration and the concentration after 30 min of air cleaner operation, the gas-removal performances can be considered the same for ammonia, acetic acid and toluene near or in excess of 90%, with somewhat different gas-removal performances for ammonia and acetaldehyde. However, the CADRs for all gases with the novel test method ranged widely, from 0 to 1.6 m3/min, and clearly differed according gas type and air cleaner model. In particular, the relative standard deviations of CADRs for ammonia, acetic acid, and toluene, which were effectively removed by the air cleaners, were 3.2, 751.3, and 13.4 times higher than those of the gas-removal efficiencies, calculated using the current arithmetic calculation method. Moreover, with the CADRs obtained by the new test method, the single-pass removal efficiency of an air cleaner can be obtained with the simple calculation of CADR/flow rate/0.83. Therefore, it is concluded that our new test method can differentiate the gas-removal performances of various air cleaners, especially for gases such as ammonia, acetic acid, and toluene, which are removed quickly by indoor air cleaners. Acknowledgments This research was supported by a Basic Research Fund (NK163C) and the General Research Fund (SC0820) of the Korea Institute of Machinery and Materials. References
Fig. 11. Comparison of the CADRs of air cleaners for particle and gas removal obtained experimentally and through calculation: (a) CADRs for particle removal, (b) CADRs for gas removal.
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H.-J. Kim et al. / Building and Environment 47 (2012) 385e393 [6] US EPA. Indoor air quality e the inside story: a guide to indoor air quality. U.S. EPA/Office of Air and Radiation, Office of Radiation and Indoor Air: http:// www.epa.gov/iaq/pubs/insidest.html#Improve5, last updated on 10 October 2010. [7] Reisman RE, Mauriello PM, Davis GB, Georgitis JW, DeMasi JM. A doubleblind study of the effectiveness of a high-efficiency particulate air (HEPA) filter in the treatment of patients with perennial allergic rhinitis and asthma. Journal of Allergy and Clinical Immunology 1990;85(6): 1050e7. [8] Ao CH, Lee SC. Indoor air purification by photocatalyst TiO2 immobilized on an activated carbon filter installed in an air cleaner. Chemical Engineering Science 2005;60:103e9. [9] Alshawa A, Russell AR, Nizkorodov SA. Kinetic analysis of competition between aerosol particle removal and generation by ionization air purifiers. Environmental Science and Technology 2007;41:2498e504. [10] Grinshpun SA, Adhikari A, Honda T, Kim KY, Toivola M, Rao KSR, et al. Control of aerosol contaminants in indoor air: combining the particle concentration reduction with microbial inactivation. Environmental Science and Technology 2007;41:606e12. [11] US EPA. Guide to air cleaners in the home. U.S. EPA/Office of Air and Radiation, Office of Radiation and Indoor Air. http://www.epa.gov/iaq/pubs/airclean. html, last updated on 11 May 2011. [12] Bastani A, Lee CS, Haghighat F, Flaherty C, Lakdawala N. Assessing the performance of air cleaning devices e a full-scale test method. Building and Environment 2010;45:143e9. [13] JEMA. JEM 1467, air cleaners of household and similar use. Japanese Electrical Manufacturers’ Association (JEMA); 1995. [14] KACA. SPS-KACA002-132, room air cleaner standard. Korea Air Cleaning Association (KACA); 2006. [15] AHAM. ANSI/AHAM AC-1-2006, method for measuring performance of portable household electric room air cleaners. Association of Home Appliance Manufacturers (AHAM); 2006. [16] Shaughnessy RJ, Sextro RG. What is an effective portable air cleaning device? A review. Journal of Occupational and Environmental Hygiene 2006;3(4): 169e81. [17] Shaughnessy RJ, Levetin E, Blocker J, Sublette KL. Effectiveness of portable indoor air cleaners: sensory testing results. Indoor Air 1994;4:179e88. [18] Niu J, Tung TCW, Chui VWY. Using large environmental chamber technique for gaseous contaminant removal equipment test. ASHRAE Transactions 1998; 104(Part 2):1289e96.
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[19] Chen W, Zhang JS, Zhang ZZ. Performance of air cleaners for removing multiple volatile organic compounds in indoor air. ASHRAE Transactions 2005;111:1101e14. [20] Persily AK, Howard-Reed C, Watson S, Martys NS, Garboczi EJ, Davis H. Standards development for gas phase air cleaning equipment in buildings. NISTIR 2008;7525. [21] Howard-Reed C, Nabinger SJ, Emmerich SJ. Characterizing gaseous air cleaner performance in the field. Building and Environment 2008;43:368e77. [22] Howard-Reed C, Henzel V, Nabinger SJ, Persily AK. Development of a field testmethod to evaluate gaseous air cleaner performance in a multizone building. Journal of the Air and Waste Management Association 2008;58:919e27. [23] Nozaki A, Ichijo Y, Narita Y. A study on the deterioration of gaseous pollutant removal performance of room air cleaners. Proceedings of the International Symposium on Contamination Control, 2010, Tokyo, Japan. [24] EPA. Test Method 318, extractive FTIR method for the measurement of emissions from the mineral wool and wool fiberglass industries. US Environmental Protection Agency; 1999. [25] EPA. Test Method 320, measurement of vapor phase organic and inorganic emissions by extractive Fourier transform infrared (FTIR) spectroscopy. US Environmental Protection Agency; 1998. [26] NIOSH. Method 3800. Organic and inorganic gases by extractive FTIR spectrometry. 4th ed. NIOSH Manual of Analytical Methods; 2002. p. 1e47. [27] Shiratori S, Inami Y, Kikuchi M. Removal of toxic gas by hybrid chemical filter fabricated by the sequential adsorption of polymers. Thin Solid Films 2001; 393:243e8. [28] Hodgson AT, Destaillats H, Sullivan DP, Fisk WJ. Performance of ultraviolet photocatalytic oxidation for indoor air cleaning applications. Indoor Air 2007; 17:305e16. [29] Daisey JM, Hodgson AT. Initial efficiencies of air cleaners for the removal of nitrogen dioxide and volatile organic compounds. Atmospheric Environment 1989;23(9):1885e92. [30] Yu KP, Lee GWM, Huang WM, Wu CC, Lou CL. Effectiveness of photocatalytic filter for removing volatile organic compounds in the heating, ventilation, and air conditioning system. Journal of the Air and Waste Management Association 2006;56:666e74. [31] Waring MS, Siegel JA, Corsi RL. Ultrafine particle removal and generation by portable air cleaners. Atmospheric Environment 2008;42:5003e14. [32] Novoselac A, Siegel JA. Impact of placement of portable air cleaning devices in multizone residential environments. Building and Environment 2009;44: 2356e438.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Efficient test method for evaluating gas removal performance of room air cleaners using FTIR measurement and CADR calculation Hak-Joon Kim a, c, Bangwoo Han a, Yong-Jin Kim a, *, Young-Hun Yoon b, Tetsuji Oda c a
Environment and Energy Systems Research Division, Korea Institute of Machinery and Materials, 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Korea Environmental Industry & Technology Institute, 209 Jinheungno, Eunpyeong-gu, Seoul 613-2, Republic of Korea c Department of Electrical Engineering and Information Systems, School of Engineering, The University of Tokyo, 7-3-1, Hongo Bunkyo-ku, Tokyo 113-8656, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2011 Received in revised form 25 May 2011 Accepted 25 June 2011
We have developed a new test method for differentiating the gas-removal performance of indoor air cleaners by using Fourier-transfer infrared measurements and a clean-air-delivery-rate (CADR) calculation method in a closed test chamber (4 m3). Eighteen air cleaners were evaluated using both the new method and the current Korean and Japanese test methods, using ammonia, acetic acid, acetaldehyde, and toluene as test gases. The Association of Home Appliance Manufacturers’ statistical method for calculating regression line slopes of test chamber gas concentrations during air cleaner operation was used. The standard deviations of CADRs for ammonia, acetic acid, and toluene, gases that were easily removed by the air cleaners in the test chamber, were 3.2, 751.3, and 13.4 times higher, respectively, than the gas-removal efficiencies determined using the current arithmetic calculation method, which uses the ratio of concentrations after 0 and 30 min of air cleaner operation. The new test method clearly differentiated the gas-removal performances of various air cleaners, especially for gases that are quickly removed by indoor air cleaners. Also, the single-pass removal efficiency of the air cleaners was obtained with a simple calculation: CADR/flow rate/0.83. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Air cleaner Chamber Clean air delivery rate (CADR) Gas-removal efficiency VOC
1. Introduction Indoor air quality (IAQ) is a top environmental priority of environmental agencies such as the US Environmental Protection Agency (US EPA) and the European Environment Agency (EEA). IAQ is a global problem because people now spend much of their time indoors, and exposure to toxic gas pollutants such as NO2, CO, VOCs from fuel/tobacco combustion, construction and furnishing materials can be an issue in the workplace or home, or both [1,2]. Airborne indoor pollutants include particulates, allergens, and organic and inorganic gaseous pollutants [3]. In particular, some buildings, including new construction, contain such high concentrations of gas-phase pollutants that they are qualified as “sick” because exposure to the inside of buildings results in multiple sickness symptoms, such as headache, fatigue, skin and eye irritation, or respiratory illness, commonly described as “sick-building syndrome” (SBS) [4]. Solutions recommended by the US EPA to improve IAQ include combinations of actions such as removing pollutant sources such as dust and harmful gases, increasing ventilation rates and improving air distribution, and cleaning
* Corresponding author. Tel.: þ82 42 868 7475; fax: þ82 42 868 7284. E-mail address: [email protected] (Y.-J. Kim). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.06.024
indoor air [5,6]. The most efficient method of cleaning indoor air is the use of air cleaners, which remove airborne contaminants relatively quickly by several different processes of varying effectiveness, such as filtration, activated carbon, ionization, and photocatalytic oxidation [7e10]. US EPA explains that usually the best way to address this risk is to control or eliminate the sources of pollutants, and to ventilate a home with clean outdoor air. The ventilation method may, however, be limited by weather conditions or undesirable levels of contaminants contained in outdoor air. If these measures are insufficient, an air cleaning device may be useful [11]. In particular, to remove gas-phase pollutants, most air cleaners use an adsorption mechanism; activated carbon filters have been commonly used because they have a high adsorption capacity of gas-phase pollutants due to their highly developed porous structure and large specific surface area [12]. Many countries, including the US, Japan, and Korea, evaluate the performance of room air cleaners using their own standard test methods [13e15]. Brief descriptions of the methods commonly used in these countries to test air cleaner performance are listed in Table 1. For particle removal performance, all methods measure the clean air delivery rate (CADR; m3/min), which describes the equivalent volume of clean air provided to the space by an air cleaner. This is a universal and efficient metric for estimating the particle removal performance of air-cleaning devices in rooms of
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Table 1 Details of Korean, American, and Japanese test methods for evaluating particle and gas removal performance of household air cleaners. Test method
KACA
Experimental conditions Particle removal
Gas removal
Type of test particle
Initial particle concentration (#/m3)
KCl
108e1010 10
JEMA
Smoke Dust
7.07 10 3.54 10
AHAM
Cigarette Smoke Arizona Dust Pollen
2.4e3.5 1010 2.0e4.0 108 4.0e6.0 106
11
Size range (mm)
Measurement
Type of test gas
Size of test chamber (m3)
Measurement time (min)
Measurement
0.3
CADR
4
30
Removal efficiency
0.3
CADR
1
30
Removal efficiency
0.1e1.0 0.5e3.0 5e11
CADR
Ammonia, Acetic acid, Acetaldehyde (10e13 ppm) Ammonia, Acetic acid, Acetaldehyde from five tobaccos e
e
e
e
various sizes or in comparing air cleaning with ventilation as an IAQ control technique [16,17]. The only difference among the three performance tests listed above for particle removal is the experimental conditions of the test particles. EPA had explained that although AHAM uses the CADR concept to evaluate the performance of portable air cleaners in reducing particulate matter concentrations, the CADR can be applied equally to the removal of gases pollutants. The gas-removal efficiencies of 126 room air cleaners for ammonia, acetaldehydes, and acetic acid, evaluated using the Korean test standard, SPS-KACA-002-132, for 5 years, from 2005 to 2009, are shown in Fig. 1. The average removal efficiencies for ammonia, acetaldehyde, and acetic acid were 86.5%, 69.8%, and 98.1%, and the efficiencies for ammonia and acetaldehyde ranged from 20 to 100% with relative deviations of 17.8% and 23.8%, while those for acetic acid were mostly 100% with a deviation of only 3.5%. These results indicate that the test method for evaluating gasremoval performance has a technical limitation, namely an inability to differentiate among the time-dependent gas-removal performances of various air cleaners and test gases, especially gases that are easily removed by air cleaners, because removal performance is evaluated using only the gas concentrations at specific times (0 and 30 min) according to Korean and Japanese standards. To evaluate the time-dependent gas-removal performances of air-cleaning devices more efficiently, several research groups have developed test protocols that have not been standardized [12,18e23]. Chen et al. [19] developed a full-scale test method with a 54-m3 test chamber, gas chromatography/mass spectrometry
(GC/MS) measurements, and 17 volatile organic compounds (VOCs). They measured the CADRs of 15 air cleaners, but due to the scale of the test chamber and the measurement method, approximately 12 h were needed to obtain a single test result. HowardReed et al. [21,22] also developed a field test protocol with test houses of 85 and 340 m3 to compensate for the technical limitations of CADR evaluation in the field, given dynamic mass transport conditions such as weather-dependent humidity and temperature, using decane gas and GC/electron capture detector (ESD) measurement methods. These methods still required a test period of several hours because of the size of the test facility and the long measurement time of the analytic method. Using a relatively small test chamber (6.3 m3) and measurement time of approximately 30 to 90 minutes for a test, Niu et al. [18] tested 27 air cleaners with toluene gas; they proposed that this test method could quantify the initial cleaning capacity of an air cleaner for gaseous phase pollutants. To evaluate the time-dependent gas-removal performance of air cleaners for various gases more quickly (<30 min for a single test), we developed a novel method of measuring CADRs that uses the same statistical calculations as applied in ANSI/AHAM AC-1-2006, a relatively small closed chamber (4.0 m3), and a real time multigas measurement system with a Fourier-transfer infrared spectrometer (FTIR) that is used by the US EPA and the National Institute for Occupational Safety and Health (NOISH) as a standard analytic method for measuring organic and inorganic gas-phase gases [24e26]. In addition, we evaluated 18 air cleaners that are commercially available in Korea and Japan using the current Korean and Japanese test methods and compared the results to those of the new test method. 2. Experimental setup 2.1. Test gases and specimens
Fig. 1. Gas-removal efficiencies of 126 room air cleaners for ammonia, acetaldehyde, and acetic acid, determined using the Korean standard test method.
Table 2 lists the physical and chemical properties of the test gases used in this study. Four gases were selected for the gasremoval performance tests: ammonia (NH3), acetic acid (CH3COOH), acetaldehyde (CH3CHO), and toluene (C7H8); the first three are also used in the Korea Air Cleaning Association (KACA) and Japan Electrical Manufacturers’ Association (JEMA) standards, and toluene is a representative VOC. All four gases are sources of malodorous indoor irritants, and have been used in many previous studies [18,27e29]. Eighteen indoor air cleaners commercially available in Korea and Japan were tested in this study. Descriptions are provided in Table 3, where the cleaners are grouped on the basis of manufacturing company and type of filtration. All products were equipped with HEPA filters for particle removal and carbon filters
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Table 2 Chemical and physical characteristics of test gases used in this study. Gas
Phase
M.W. (g/mol)
Boiling Temp. ( C)
Melting Temp. ( C)
Density (g/cm3)
Solubility in water (g/100 ml)
Danger
Characteristics
Ammonia (NH3) Acetic acid (CH3COOH) Acetaldehyde (CH3CHO) Toluene (C7H8)
Gas Liquid Liquid Liquid
17.03 60.05 44.05 92.14
33.4 118.1 20.2 110.6
77.7 16.7 123.5 93.0
0.00068 1.049 0.788 0.8669
89.9 (0 C) Fully miscible Miscible 0.053 (20 C)
Toxic, corrosive Corrosive Toxic, carcinogenic Carcinogenic
Light colorless gas, irritating smell Colorless liquid, irritating smell Volatile colorless liquid, irritating smell Colorless liquid, unique smell
for gas removal, and the flow rates of the air cleaners ranged from 3.0 to 7.3 m3/min; the flow rates of the majority of household room air cleaners fall in this range.
2.2. Performance test and evaluation methods The experimental set-up for the measurement of CADR and gasremoval efficiency is shown in Fig. 2: a test specimen was placed in a closed chamber (4.0 m3), and an FTIR spectrometer (Model I4000, Midac, Costa Mesa, CA, USA) with a 20-m cell was used to measure the concentrations of ammonia, acetaldehyde, acetic acid, and toluene in the chamber. Wall surface adsorption of the test chamber was negligible during the test because when we checked the natural decay for an hour with known gas concentration of approximately 10 ppm of the test gases, there is no decrease in the initial concentration during the period. When the test chamber background level fell below the measurement limit of 0 ppm, a heated or bubbled gas was introduced to the chamber in clean compressed air at a rate of 2 L/min until the initial concentration of the test gas ranged between 8 and 13 ppm. This concentration was intentionally higher than actual field concentrations of the gas, so that the concentration decay curve could be accurately measured before it fell below the instrument detection limit [19]. When the required initial concentration was reached, the test specimen and the FTIR spectrometer were turned on, and gas concentration data were acquired at 20-s intervals for 30 min. The FTIR system had a 1-cm1 resolution with a 20-m gas cell, a He/Ne laser as a radiation source, a liquid-nitrogen-cooled mercury cadmium telluride detector, ZnSe optics, and a Ge/ZnSe beam splitter. The spectral wave frequency was in the range of 650e4500 cm1, with 32 scans, during which measurements were taken at intervals of 20 s per spectrum. The IR peaks that were used
to measure the concentrations of the gases were 917.45e940.38 cm1 for ammonia, 2629.78e2887.87 cm1 for acetaldehyde, 1146.49e1225.78 cm1 for acetic acid, and 722.06e735.63 cm1 for toluene, to avoid overlap with other peaks, such as those of water and carbon dioxide. The IR spectrums were analyzed using the commercial library databases supplied by Midac and the US EPA, and the FTIR system was purged with pure nitrogen gas before and after the measurements. Also, before the experiments for the gas removal performance tests, the FTIR was calibrated with the known concentrations of standard gases with approximately 10 ppm and 5 ppm for the all test gases which were supplied by the Korea Research Institute of Standards and Science. The concentrations of the calibration gases of ammonia, acetic acid, acetaldehyde and toluene were 11.6/6.3 ppm, 9.5/5.8 ppm, 10.8/ 6.1 ppm and 12.9/6.2 ppm with 5% uncertainty (k ¼ 2, 95% confidence interval), respectively, and the standard deviations of the concentration measured by the FTIR with the calibration gases were 0.05/0.03 ppm, 0.05/0.01 ppm, 0.14/0.09 ppm, 0.08/0.02 ppm for ammonia, acetic acid, acetaldehyde and toluene, respectively. The gas-removal performance of each room air cleaner was expressed as removal efficiency calculated using the arithmetic methods of KACA and JEMA [13,14]. Removal efficiency was calculated as follows:
hi ¼
Ci;0min 1 Ci;30min
! 100ð%Þ
(1)
where hi: removal efficiency of gas i Ci,0 min: initial concentration of gas i Ci,30 min: concentration of gas i after 30 min of air cleaner operation The Association of Home Appliance Manufacturers (AHAM) established a performance metric, a CADR, that is based on an air
Table 3 Descriptions of the air cleaners tested in this study. M
No.
Filter combination
Mass of activated carbons (g)
Flow rate (m3/min)
CADR
Removal efficiency
Single-pass efficiency Ammonia
Toluene
A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Porous activated carbon plate) HEPA filter þ Carbon filter (activated carbon sheet) HEPA filter þ Carbon filter (Pellets) HEPA filter þ Carbon filter (Pellets)
180 280 320 180 180 280 280 320 320 300 300 320 320 235 300 21 180 130
3.5 5.4 6.4 3.2 3.6 4.3 5.7 5.4 6.5 5.8 5.8 7.2 7.3 3.8 5.0 3.0 6.4 3.3
O
O
X X X O O O O O O O O X X O X X O X
X X X O O O O O O X X X X X X X X X
B
C
*M : manufacturer, O : tested, X : not tested.
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Fig. 2. Experimental setup for the air cleaner performance tests under the new and current test methods.
cleaner’s reduction of particles in a closed chamber. In this study, gas-removal performance of a room air cleaner was also represented by the CADR calculated using the statistical method of the AHAM [15]. Theoretically, the regression of pollutant concentration follows a first-order decay model:
Ct ¼ C0 ekt
(2)
where Ct ¼ concentration at time t (ppm) C0 ¼ initial concentration at t ¼ 0 (ppm) k ¼ decay constant (t1) t ¼ time (min) The time-resolved decay constant k is calculated statistically using a linear regression of ln Cti and ti using the following formula:
k ¼
SXY SXX
(3)
where,
SXY ¼
n X
ti ln Cti
i¼1
¼
n X
ðti Þ2
i¼1
1 n
1 n n X
n X i¼1
! ti
!2
n X
! lnCti ; SXX
i¼1
ti
i¼1
Using Eqs. (2) and (3), the AHAM method for calculating CADR is
CADR ¼ Vðke kn Þ
(4)
where CADR ¼ clean air delivery rate (m3/min) V ¼ volume of test chamber (m3) ke ¼ total decay rate (min1) kn ¼ natural decay rate (min1) In this study, natural decay (kn) when the air cleaners were not operating was negligible, indicating that there was no leakage or diffusion loss of test gases in the test chamber during the 30-min test period. CADRs were calculated using the data from the gasremoval efficiency experiments shown in Fig. 2.
In order to more efficiently understand time dependent gas removal performances of the test air cleaners during the test which was performed with different initial concentrations of the various test gases, we computed the normalized concentrations by dividing the original data of the concentrations measured by the FTIR with the initial concentrations for each test gas just before the operation of the air cleaner. Also, we computed fitted lines of the first-order exponential equation with the normalized concentrations by using the commercial software, EXCEL 2007 version, to investigate the applicability of the CADR concept for the gas removal performance tests. In previous studies [18,19,29e32], if air flow rate and single-pass removal efficiency were known, CADR was obtained using the following formula:
CADR ¼ hs;i Q E
(5)
where hs,i: single-pass removal efficiency of gas i Q: air flow rate of an air cleaner E: short-circuit factor (E ¼ 1 under well-mixed conditions) By comparing the CADR measured experimentally in this study and the CADR calculated with Q and hs,i, we calculated E for this study. Single-pass efficiency (hs,i) was measured as shown in Fig. 3 in an open test duct (600 600 mm2) with 10 and 6 air cleaners for ammonia and toluene, respectively (Table 3). Single-pass efficiency was calculated using the fraction of the gas concentration removed from the air stream as it passed through the air cleaner The initial concentration was maintained at between 8 and 13 ppm, which was the range used in the closed chamber tests.
3. Results and discussion Fig. 4 shows normalized concentrations over time of the four studied gases for two air cleaners with different flow rates manufactured by the same company, B. After 30 min of air cleaner operation, the normalized concentrations of ammonia, acetic acid, and toluene had decreased to nearly 0% of their initial concentrations, while that of acetaldehyde was reduced to 40% to 50% of its
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Fig. 3. Experimental setup for the single-pass efficiency test.
Fig. 4. Gas concentration as a function of time for two air cleaners manufactured by the same company but with different flow rates: (a) 5 m3/min and (b) 7.3 m3/min.
initial concentration. Very volatile organic compounds, such as acetaldehyde, are known to be difficult to remove with sorptionbased materials due to low absorbability [19]. In particular, the air cleaner with a 7.3 m3/min flow rate removed all test gases more swiftly than another device, which had a 5.0 m3/min flow rate. Shown Table 3 and 4, the carbon filters manufactured by B company were carbon filters which were packed by cylindrical pellets of activated carbons, and the mass of activated carbons of the carbon filter for the air cleaner with the flow rate of 7.3 m3/min was larger than that for the air cleaner with 5.0 m3/min. The residence time of the air cleaner with 7.3 m3/min was also longer than that with 5.0 m3/min, while the circulation time which meant the time to treat whole air volume of the 4.0 m3 closed chamber, was shorter than that with 5 m3/min. Thus, the air cleaner with 7.3 m3/min removed the test gases more effectively than that with 5.0 m3/min since the gas removal performance of air cleaners is dependent on parameters related to the physical and operational parameters such as activated carbon mass (single-pass removal efficiency), residence time (single-pass removal efficiency and flow rate), and circulation time (flow rate) [18,19,29e31]. Fig. 5 shows normalized concentrations over time of the four studied gases for two air cleaners with different flow rates manufactured by different companies, A and C. Acetaldehyde concentration was reduced faster by the air cleaner with a flow rate of 3.2 m3/min than by that with a flow rate of 6.4 m3/min. Ammonia was approximately 90% removed by the 3.2 m3/min flow rate air cleaner, while acetic acid and toluene were 100% removed by both air cleaners after 30 min of operation. Shown Tables 3 and 4, the circulation time with the air cleaner with 3.2 m3/min was 2 times longer than that with 6.4 m3/min, while the mass of activated carbons was 1.1 times larger than that with 6.4 m3/min, and the residence time was 2.3 times longer than that with 6.4 m3/min. In this case, we expected that the gas removal performance of the air cleaners in the closed chamber was affected by the parameters related to single-pass removal efficiency such as the mass of activated carbons and residence time. The results indicate that the gas-removal performance of an indoor air cleaner depends on several parameters, such as the type of gas, flow rate, and type of filter. However, if evaluated only on the basis of gas-removal efficiency, calculated from the initial
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Table 4 Descriptions of physical and operational characteristics of the four air cleaners tested in this study.
B B A C
Carbon filter type
Width (m)
Length (m)
Depth (m)
Volume (m3)
Flow rate (m3/min)
Residence time (sec)
Circulation time (sec)
Pellets Porous activated carbon plate Pellets Pellets
0.38 0.27 0.24 0.26
0.36 0.27 0.37 0.32
0.0080 0.0090 0.0080 0.0094
0.00110 0.00068 0.00071 0.00079
7.3 5.0 3.2 6.4
0.00917 0.00811 0.01332 0.00738
33.33 48.00 75.00 37.50
concentration and the concentration after 30 min of air cleaner operation, the gas-removal performances of the four different air cleaners in Figs. 4 and 5 can be considered the same for acetic acid and toluene, which were swiftly removed by all the air cleaners, with somewhat different gas-removal performances for ammonia and acetaldehyde. Fig. 6 shows the log-normalized concentrations over time of the test gases for the same company air cleaners with different flow rates and their fitted equations. All lines fitted well using the firstorder model explained in Eq. (2) [12,18,19]. In particular, the decay constants of the regression lines for acetaldehyde, ammonia, toluene and acetic acid were 0.030, 0.128, 0.113 and 0.151 min1 for the air cleaner with a flow rate of 5.0 m3/min, while those for the air cleaner with a flow rate of 7.3 m3/min were 0.036, 0.169, 0.226 and 0.259 min1. All the decay
Fig. 5. Gas concentration as a function of time for two air cleaners manufactured by different companies and with different flow rates: (a) 3.2 m3/min and (b) 6.4 m3/min.
constants for each gas with a flow rate of 5.3 m3/min were much smaller than those of 7.3 m3/min, and the gas removal performance of each air cleaner for the four different gases was clearly differentiated with the difference of the decay constants. Fig. 7 shows the log-normalized concentrations over time of the test gases for the different company air cleaners with different flow rates and their fitted equations. the decay constants of the regression lines for acetaldehyde, ammonia, toluene and acetic acid were 0.064, 0.113, 0.182 and 0.299 min1 for the air cleaner with a flow rate of 3.2 m3/min, while those for the air cleaner with a flow rate of 6.3 m3/min were 0.027, 0.134, 0.241 and 0.240 min1. Differently from the comparison with the removal efficiency calculated by the current test method, explained in Fig. 5, all the decay constants for each gas were clearly different,
Fig. 6. Log-normalized gas concentration as a function of time for the same company air cleaners with different flow rates and the fitted equations: (a) 5 m3/min and (b) 7.3 m3/min.
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Fig. 8. Gas-removal efficiencies of 18 household air cleaners for the four study gases, obtained using the current test methods.
Fig. 7. Log-normalized gas concentration as a function of time for the different company air cleaners with different flow rates and the fitted equations: (a) 3.2 m3/min and (b) 6.4 m3/min.
even for toluene and acetic acid which were removed perfectly by the air cleaners within 30 min. Thus, these results indicate that it is possible to measure the time-dependent gas-removal performance, i.e., CADR, of various air cleaners for different test gases by applying the AHAM calculation method, and also to differentiate gas removal performance of various air cleaners for all the test gases by the CADR. Fig. 8 shows the gas-removal efficiencies of the 18 household air cleaners for the four study gases. Regardless of carbon filter type or flow rate, most of the efficiencies calculated using Eq. (1), which is used in the current Korean and Japanese tests, were near or in excess of 80%, except those for acetaldehyde. Efficiencies for ammonia, acetic acid, and toluene, which are easily removed by the air cleaners, even exceeded 90%. As shown in Figs. 4 and 5, the residual concentrations of ammonia, acetic acid, and toluene in the test chamber mostly decreased to the detection limit of the FTIR spectrometer after 30 min of air cleaner operation. Here, the current calculation method used in Korean and Japanese test standards clearly differentiated the removal performances of the air cleaners for acetaldehyde, but efficiencies for ammonia, acetic acid, and toluene are similar so that gas-removal performance of those three gases may be not differentiated by the current test method.
Fig. 9 shows the CADRs of the 18 models for the four test gases, indicating how quickly the air cleaners removed each gas from a closed chamber. The CADRs for all gases ranged widely, from 0 to 1.6 m3/min, and clearly differed according to gas type and air cleaner model. In particular, the CADRs for ammonia, acetic acid, and toluene, which were completely removed after 30 min by all air cleaners, varied from 0.2 to 0.8 m3/min, 0.5 to 1.6 m3/min, and 0.4 to 1.5 m3/min, respectively. Fig. 10 shows the relative standard deviations of the gas-removal efficiencies and the CADRs of the 18 air cleaners for the four gases. The relative standard deviations of the CADRs were 31.0%, 47.3%, 53.2%, and 32.2% for ammonia, acetic acid, acetaldehyde, and toluene, and those of the gas-removal efficiencies were 9.8%, 0.06%, 32.0%, and 2.4%, respectively. The standard deviations of the CADRs for ammonia, acetic acid, and toluene were 3.2, 751.3, and 13.4 times higher than those of the gas-removal efficiencies, respectively. These results indicate that the novel test method developed in this study is able to differentiate the gas-removal performance of various air cleaners better than the methods currently used in Korea and Japan, especially in the case of gases that are efficiently removed by air cleaners. The CADRs for particle and gas removal obtained from experiments and calculated using Eq. (5) are compared in Fig. 11. For the Eq. (5) calculations, single-pass removal efficiencies were measured
Fig. 9. CADRs of 18 household air cleaners for the four study gases, obtained using the new test method.
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method, which uses a 30.4-m3 test chamber, to be 0.86 (Fig. 11a). We also tested 10 models of air cleaners for ammonia and six models for toluene using a 4.0-m3 test chamber, and the shortcircuit factor for this new gas-removal performance test averaged 0.83 (Fig. 11b), 0.03 lower than that for the particle removal test. The short-circuiting, the re-entrainment of some of the cleaned air from the outlet of the air cleaner into the inlet of the air cleaner rather than mixing completely with air within the chamber, is related to the mixing characteristics in a closed space, and is difficult to single out [18,29]. Here, the difference may be related to the different sizes of test chamber, mixing fan and test agents such as particle and gas. 4. Conclusions
Fig. 10. Relative standard deviations of the gas-removal efficiencies and the CADRs of 18 air cleaners for four gases
using a test duct, as shown in Fig. 3. For 5 years, 2005e2009, we measured particle removal CADRs for the 126 air cleaners commercially available in Korea and Japan, and determined the short-circuit factor for the particulate removal test in the KACA test
The gas-removal performance of 18 room air cleaners that are commercially available in Korea and Japan was evaluated with four different gases, ammonia (NH3), acetic acid (CH3COOH), acetaldehyde (CH3CHO), and toluene (C7H8), which are irritating and malodorous in indoor environments, by using current standard test method in Korea and Japan, and a novel test method which measured the gas-removal performance of indoor air cleaners using the AHAM CADR calculation method and FTIR measurement in a closed chamber (4 m3). The evaluation test results indicate that the gas-removal performance of an indoor air cleaner depends on the type of gas, flow rate, and type of filter. However, if evaluated only on the basis of gas-removal efficiency of the current test method, calculated from the initial concentration and the concentration after 30 min of air cleaner operation, the gas-removal performances can be considered the same for ammonia, acetic acid and toluene near or in excess of 90%, with somewhat different gas-removal performances for ammonia and acetaldehyde. However, the CADRs for all gases with the novel test method ranged widely, from 0 to 1.6 m3/min, and clearly differed according gas type and air cleaner model. In particular, the relative standard deviations of CADRs for ammonia, acetic acid, and toluene, which were effectively removed by the air cleaners, were 3.2, 751.3, and 13.4 times higher than those of the gas-removal efficiencies, calculated using the current arithmetic calculation method. Moreover, with the CADRs obtained by the new test method, the single-pass removal efficiency of an air cleaner can be obtained with the simple calculation of CADR/flow rate/0.83. Therefore, it is concluded that our new test method can differentiate the gas-removal performances of various air cleaners, especially for gases such as ammonia, acetic acid, and toluene, which are removed quickly by indoor air cleaners. Acknowledgments This research was supported by a Basic Research Fund (NK163C) and the General Research Fund (SC0820) of the Korea Institute of Machinery and Materials. References
Fig. 11. Comparison of the CADRs of air cleaners for particle and gas removal obtained experimentally and through calculation: (a) CADRs for particle removal, (b) CADRs for gas removal.
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