FRACTIONAL AEROSOL FILTRATION EFFICIENCY OF
AIR CLEANERS
James T. Hanley1, Daryl D. Smith1, David S. Ensor1, and L. E. Sparks2
1 Research Triangle Institute, Research Triangle Park, NC 27707 USA
2 U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 USA
ABSTRACT
Under contract to the U. S. Environmental Protection Agency, the Research Triangle
Institute (RTI) is evaluating the fractional filtration efficiency of air cleaners. The test
duct accepts air cleaners up to 610 mm x 610 mm (24 x 24 inches) hi size and provides
test flow rates up to 470 L/s (1,000 cfm). Filtration efficiency is computed from
upstream and downstream aerosol concentration measurements performed with a
differential mobility analyzer, a laser aerosol spectrometer, and a white-light optical
particle counter. The measurements cover the particle diameter size range from 0.01 to
3 /im in 16 sizing channels. Air cleaners tested include furnace filters, pleated filters and
statically charged panel filters.
The efficiency of the air cleaners was often found to be highly dependent on particle size
and dust load. A minimum hi efficiency was frequently observed in the 0.1 to 0.5 /un
range. The presence of a dust load on the air cleaner frequently increased its efficiency.
However, for some air cleaners, little change or a decrease hi efficiency accompanied the
dust loading. The common furnace filter was seen to have a fractional efficiency of less
than 10% over much of the 0.01 to 1 /un size range.
INTRODUCTION
Air cleaner filtration efficiency ratings based on current standardized test methods do not
provide information sufficient to address indoor air quality concerns. In the United
States, for example, the filtration efficiency rating of in-duct ah- cleaners is most often
based on the ASHRAE (American Society of Heating, Refrigerating and Air-
Conditioning Engineers, Inc.) Standard 52-76 (1). This test provides an overall value of
filter performance for atmospheric aerosol and a coarser test dust. While such tests are
useful for relative ranking of filter performance, they do not quantify filtration efficiency
as a function of particle size.
The objective of this program has been to measure the fractional aerosol filtration
efficiency of in-duct air cleaners typically used hi residential and office ventilation
systems. The measurements have been made over the particle diameter range from 0.01
to 3 pm. Particles of this size are important because they are respirable, have relative
low settling velocities (thereby remaining airborne for long time periods), and this range
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AEERL-P-1043
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before complet
1. REPORT NO.
EPA/600/A-93/234
PB93-236636
4. TITLE AND SUBTITLE
Fractional Aerosol Filtration Efficiency of Air
Cleaners
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
J. T.Hanley, D. D. Smith, D. S.Ensor (RTI), and
L. E. Sparks (EPA/AEERL)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Research Triangle Institute
P. O. Box 12194
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
CR817083-01-0
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air and Energy Engineering Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; 1/91 - 12/92
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES .,_,_--... . ... . _ , . _ _ . -_. .- _ e A f\t f\ I
AEERL project officer is Leslie E. Sparks, Mail Drop 54, 9197
541-2458. Presented at IAQ '93, Helsinki, Finland, 7/4-8/93.
16. ABSTRACT
The paper gives results of an evaluation of the fractional filtration efficien-
cy of air cleaners. The test duct accepts air cleaners up to 610 x 610 mm in size and
provides test flow rates up to 470 L/s. Filtration efficiency is computed from up- •
stream and downstream aerosol concentration measurements performed with a dif-
ferential mobility analyzer, a laser aerosol spectrometer, and a white-light optical
particle counter. The measurements cover the particle diameter size range from
0.01 to 3 micrometer in 16 sizing channels. Air cleaners tested include furnace fil-
ters, pleated filters, and statically charged panel filters. The efficiency of the air
cleaners was often found to be highly dependent on particle size and dust load. A
minimum in efficiency was frequently observed in the 0.1 to 0. 5 micrometer range.
The presence of a dust load on the air cleaners frequently increased their efficiency.
However, for some air cleaners, little change or a decrease in efficiency accom-
panied the dust loading. The common furnace filter was seen to have a fractional
efficiency of less than 10% over much of the 0.01 to 1 micrometer size
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Air Cleaners
Filtration
Efficiency
Aerosols
Particles
Measurement
Pollution Control
Stationary Sources
Particulate
13 B
13 A, 131
07D
14G
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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includes the size of many indoor aerosol pollutants such as cigarette smoke, cooking
fumes, resuspended dusts, and radon progeny (2). Residential furnace filters, a
residential electrostatic precipitator (ESP), pleated paper-media filters, pocket filters,
panel electronic air cleaners, and statically charged panel filters have been tested.
METHODS
The air cleaners were tested in RTFs filter test facility (Figure 1). The duct
accommodates air cleaners with face dimensions up to 610 x 610 mm and operates at
flow rates up to 470 L/s (1,000 cfm). The system is equipped with a high efficiency
paniculate air (HEPA) filter bank on the blower inlet allowing the instrument-zeros and
background counts (e.g., particle shedding) to be quantified prior to each test. A second
HEPA bank is located on the exhaust to allow indoor discharge. The system operates at
positive pressure (i.e., the blower is upstream of the test filter) to minimize infiltration of
room aerosol. Because the downstream ducting runs back under the test section, the
challenge and penetrating aerosol test ports are located physically near each other,
thereby facilitating aerosol sampling. This duct configuration reduces overall particle
loss by using extra duct length (in which particle loss is low) to allow short sample line
lengths (in which particle losses can be high).
Three instruments were used to perform the fractional efficiency measurements: a TSI
Differential Mobility Particle Sizer (DMPS) for particle sizes from 0.01 to 0.09 jim, a
PMS Laser Aerosol Spectrometer (LAS-X) for sizes from 0.09 to 0.3 /xm, and a Climet
226/8040 Optical Particle Counter (OPC) for sizes from 0.3 to 3 /on. The DMPS
measures particle size based on the electrical mobility of the aerosol particles. The LAS-
X and OPC are wide-angle light scattering particle counters using a laser and white light
source, respectively.
The test aerosol used in the fractional efficiency tests was solid potassium chloride (KC1)
generated by drying a nebulized aqueous solution. KC1 was selected as the test aerosol
because of its relatively high water solubility, high deliquescence humidity, known cubic
shape, and low toxicity. Laskin and Collison nozzles were used to generate the
challenge aerosol. Upon generation, the aerosol was passed through a charge neutralizer
(TSI Model 3054) to neutralize any electrostatic charge on the aerosol (electrostatic
charging is an unavoidable consequence of most aerosol generation methods).
The loading dust was composed of 93.5% by weight of Standardized Air Cleaner Test
Dust Fine and 6.5% by weight #7 ground cotton linters. This dust is similar to the
loading dust specified in the ASHRAE 52-76 Standard except that the ASHRAE dust
also includes a carbon black component. We omitted the carbon black because its
presence makes the dust highly conductive and incompatible with the operation of
electronic air cleaners. While dusts having high conductivities may be encountered in
industrial applications, it appeared unlikely for the residential and office building
applications of interest to this study. The loading dust was disseminated by use of an
aspirator nozzle. Preweighed amounts of dust were fed until the desired pressure drop
across the air cleaner was achieved. Typically, the filters were loaded to levels of 125
and 250 Pa (0.5 and 1 in. water) pressure drop.
-------
Dry Filtered
Compressed
Air
>=fcv
Charge
Neutrallzer
Air Flow
Control
Valve
Exhaust Inlet
FIHer Finer
Bank Bank
Figure 1. Schematic diagram of fill's air cleaner test facility.
RESULTS
Figure 2 presents the fractional filtration curves for four clean (i.e., no dust load) filters
having ASHRAE Efficiencies ranging from 40 to 95%. The shape of the curves is a
consequence of filtration by diffusional collection being effective for particles smaller
than 0.1 fim diameter and interception and inertial collection being effective for particles
larger than 1 jun. In the region from 0.1 to 0.5 /xm these processes are less effective,
giving a minimum in filtration efficiency in this size range for most filters.
Figures 3, 4, and 5 show the clean and dust-loaded efficiency curves for a common
furnace panel filter, a pleated-paper filter (65% ASHRAE efficiency), and a charged-
fiber panel filter, respectively. The furnace filter had a clean efficiency of less than
approximately 10% for particle diameters between 0.02 and 1 fan. The efficiency
improves somewhat with dust loading, though it remained below 20 % over the 0.03 to
0.3 /*m range.
-------
0.01
0.1 1
Particle Diameter dim)
Figure 2. Fractional filtration efficiency of four pleated filters
having ASHRAE efficiencies ranging from 40 to 95%.
100
90
80
D 10 Pa: (clean)
A 125 Pa
O 250 Pa
0.1 1
Particle Diameter (um)
Figure 3. The fractional filtration efficiency of a furnace
filter for clean and dust-loaded conditions.
10
-------
2
'o
§
I
il
0.01
0.1 1
Particle Diameter (um)
10
Rqure 4. The fractional filtration efficiency of a pleated-paper filter
(65% ASHRAE efficiency) for clean and dust-loaded conditions.
-------
The efficiency of the pleated paper filter increased markedly with dust loading. The
charged-fiber filter showed a high initial efficiency but a marked decrease with dust
loading.
DISCUSSION
The test results illustrate the strong dependence of filtration efficiency on particle size
over the 0.01 to 3 /xm diameter size range. All of the filters exhibited a minimum in
filtration efficiency in the 0.1 to 0.5 /xm diameter size range. This is consistent with
filtration theory for particle collection by diffusion, interception, and inertial impaction
mechanisms.
The common residential furnace filter provided very little (averaging less than about
10%) collection of particles throughout the 0.01 to 3 |Am size range. Upon dust-loading,
the air cleaners showed a variety of responses with a decrease in efficiency for the
charged-fiber filter, a modest increase for the furnace filter, and a substantial increase for
the pleated media filter.
The strong dependence of filtration efficiency on particle size, combined with the
respirable and abundant nature of aerosol particles, requires that fractional filtration
efficiency tests be performed to fully assess the potential impact of an air cleaner on
indoor air quality.
RTI is currently completing construction of a new test rig to allow testing at particle
diameters up to 10 ^im and flow rates up to 1400 L/s (3,000 cfin). A wide range of air
cleaners are scheduled for testing including single- and two-stage ESPs, self-charging
(i.e. "passive") electrostatic panel filters, bag filters, charged-fiber filters, pleated-paper
filters, and spun-fiber furnace filters.
ACKNOWLEDGEMENTS
This work was funded by the U.S. Environmental Protection Agency, Air and Energy
Engineering Research Laboratory, Research Triangle Park, NC, under Cooperative
Agreement No. CR-817083-01-0.
REFERENCES
1. ASHRAE Standard 52-76. Method of Testing Air-Cleaning Devices Used in
General Ventilation for Removing Particulate Matter. The American Society of
Heating, Refrigerating, and Air-Conditioning Engineers, Inc., Atlanta, GA. 1976.
2. Owen, M.K., D.S. Ensor and L.E. Sparks. Airborne Particle Sizes and Sources
Found in Indoor Air. Atmospheric Environment, 26A(12):2149-2162. 1992.
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