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Experimental Study of Electrostatic Capture Mechanisms in Commercial Electret Filters Francisco J. Romay a; Benjamin Y. H. Liu a; Soo-Jae Chae a a PARTICLE TECHNOLOGY LABORATORY, DEPARTMENT OF MECHANICAL ENGINEERING, UNIVERSITY OF MINNESOTA, MINNEAPOLIS, MN, USA First Published on: 01 January 1998

To cite this Article Romay, Francisco J., Liu, Benjamin Y. H. and Chae, Soo-Jae(1998)'Experimental Study of Electrostatic Capture

Mechanisms in Commercial Electret Filters',Aerosol Science and Technology,28:3,224 — 234 To link to this Article: DOI: 10.1080/02786829808965523 URL: http://dx.doi.org/10.1080/02786829808965523

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Experimental Study of Electrostatic Capture Mechanisms in Commercial Electret Filters Francisco J. Rornay*, Benjamzn Y H. Liu, and Soo-Jae Chae PARrICI E TECII\OI OGY 1 ABOKAIORY, D E P A R r M t Z r OF MLCllAhICAI LNGINf FRING, tJVlVERb1 I Y OF VINNESOTA, 1 11 LIiURC11 S I 5 E , M I N h t A P O I IS, MN, 55455. USA

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ABSTRACT. Electret filters are widely used in applications requiring high-filtration efficiency and low-pressure drop. These filters rely on electrostatic particle capture mechanisms in addition to the conventional mechanical capture mechanisms. This study reports experimental data collected on the performance of three types of commercially available fibrous electret filters: corona-charged fibrillated split-fiber media (type A), triboelectrically charged mixed-fiber media (type B) and corona-charged meltblown media (type C). The filtration efficiency of these filters was measured as a function of particle size (i.e., 0.05 to 0.5 p m ) and charge state (i.e., singly charged and neutral) for two face velocities. The same experiments also were performed on discharged electret filters to obtain information on the pure mechanical capture mechanisms. The magnitude of the effective surface electric field for each tested electret filter was estimated from a simplified Coulombic capture model, giving information on the ranking of the charge level for the tested electret filters. The single-fiber efficiencies for Coulombic and dielectrophoretic capture mechanisms were isolated by assuming negligible interaction among the different particle capture mechanisms. The single-fiber efficiencies were fitted by power law expressions to a combination of parameters suggested by dimensionless numbers derived from theory. The experimentally obtained power law exponents were in good agreement with those predicted by theory (Brown, 1981; Emi et al., 1987) using simplified charge configurations. No direct semi-empirical correlations in terms of dimensionless parameters could be obtained due to the lack of reliable information AND TECHon the charge density of the tested electret filters. AEROSOLSCIENCE NOLOGY 28:224-234 (1998) O 1998 American Association for Aerosol Research

INTRODUCTION Electret filters are fibrous filters made of dielectric materials that have a quasi-permanent electrical charge. These filters have a significant microscopic bipolar charge on the fibers and a very low net macroscopic charge. Electret filters are now commonly used in air cleaning applications requiring 'iCorresponding author.

high efficiency and low-pressure drop such as in disposable respirators, automotive cabin air filters, indoor air filters, and vacuum cleaner bags. " Electret filters arc manufactured by several processes including corona charging, triboelectric charging, and induction charging as reviewed by Brown (1993). The particle capture characteristics of these filters rely on a combination of conAero~olSc~cnccand Technology 28 224-274 (1998) 0 1998 Amerlcdn /l~--~atlonfor Acrocol Kcscdrch Pubhshcd by Else. c Cc i a c e Inc


Aerosol Science and Technology 2 8 3 March 1998

Experimental Study of Electrostatic Capture Mechanisms 225

ventional mechanical mechanisms (i.e., impaction, interception, and diffusion) and electrostatic mechanisms (i.e., Coulombic attraction and dielectrophoresis). During the early stages of filtration the electrostatic capture mechanisms are predominant even for uncharged particles due to the dielectrophoretic effect. Charged particles are captured by a combination of Coulombic attraction and dielectrophoretic forces. There arc several theoretical expressions available to describe the capture efficiency of electret filters due to Coulombic attraction and dielectrophoresis for different charge configurations (Pich, 1978; Brown, 1981; Pich et al., 1987; Lathrache and Fissan, 1987). All these theories assume a simple charge configuration on the fibers (i.e., uniformly charged fiber, line dipole charged fiber) and derive expressions for the singlefiber efficiency using analytical or numerical calculation schemes, in terms of dimensionless parameters. Experimental studies on the performance of electret filters have been conducted by a number of researchers (dc Haan ct al., 1956; Baumgartner and Loffler, 1986; Lathrache and Fissan, 1986; Emi et a]., 1987; Fjeld and Owens, 1988). More recent studies deal with the drop of capture efficiency when filtering different types of aerosols (Otani et al., 1993; Lehtimaki and Heinonen, 1994) and with the loading characteristics as a function of particle size (Walsh and Stenhouse, 1996). There are several problems that arise when comparing experimental results with the theoretical expressions. The main one is that the charge level and configuration for the real filters is not known with sufficient accuracy due to the lack of experimental techniques to measure the charge on the filters. Other difficulties include the fact that during testing mechanical and electrostatic mechanisms act simultaneously on the test particles, regardless of their charge state, and that Coulombic and dielectrophoretic effects also act simultaneously on charged particles. Therefore, it is difficult to make a direct comparison between the theoretical

predictions and the experimental observations. This study tries to resolve some of the difficulties mentioned above by testing the filters in their original charged state and after being artificially discharged. The tests were conducted with monodisperse aerosol particles of different charged states (i.e., neutral and singly charged). Therefore, aerosol methods are described to characterize commercial electret filters. Also, this study includes three types of commercial electret filter media as a way to compare their pcrl'ormance in light of the current theoretical understanding on electret filters. EXPERIMENTAL Electret Filter Media Corona-charged fibrillated split-fiber media, triboelectrically charged mixed-fibcr media, and corona-charged meltblown-fiber media were used for this study. The characteristics of these filters are given in Table 1. Types A and B are similar in basis weight, filter thickness, fiber dimensions, and packing density. Type C has a higher packing density and smaller fiber dimensions. Therefore, type C has a higher mechanical filtration efficiency than types A and B. Aerosol Generation System

A polydisperse sodium chloride aerosol was generated from a 0.1% wlv solution in DI watcr by a Collison atomizer. The dried and neutralized polydisperse aerosol then was fed to a differential mobility analyzer (Liu and Pui, 1974), to obtain a nearly monodisperse singly charged aerosol in the submicron range (i.e., 0.02 to 0.5 pm). The monodisperse aerosol was either left in the singly charged state or introduced to a bipolar charger/electrostatic condenser device to obtain neutral particles (i.e., zero charge). Filter Eficiency Measurement

The penetration of the test aerosol particles of known size and charge state through each



Aerosol Sciencc and Technology 28:3 March 1998


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228 F. J. Romay et al.

TABLE 2. Experimental Conditions Filter Media

Face Velocity (cm/s)

Electret Filter Charge State

10 and 50 10 and 50 5 3 and 25

Chdrgcd and Ncutral and D~scharged S~nglyChargcd Chdrgcd dnd Neutral and D~scharged S~nglyCharged Chdrged and Neutral and D~schargcd S~nglyCharged

-

A

B


C

Particle Charge State

---

where q is the single-fiber efficiency, d, is the mean fiber diameter, a is the filter packing fraction, L is the filter thickness, and P is the penetration. For an electret filter the overall singlefiber efficiency (i.e. q ) can be related to the mechanical and electrostatic singlefiber efficiencies by the following expression,

where q,, yo, and q, are the single-fiber efficiencies for mechanical, dielectrophoretic, and Coulombic capture mechanisms respectively. When testing a charged filter with neutral particles, the measured single-fiber efficiency is a combined mechanical and dielectrophoretic efficiency (i.e., q,, = q, + 7,). Likewise, when testing the same filter with singly charged particles the measured single-

In this equation, the uncertainty in the measured single-fiber efficiency increases with penetration for given constant uncertainties of all the other variables. The range of uncertainty for the estimated single-fiber efficiencies presented in this paper varies from 12% at low penetrations to 96% at a penetration of 0.1.

fiber efficiency is a combined mechanical, dielcctrophoretic, and Coulombic efficiency (i.e., qmoc= q, + qc,+ q,). Therefore, from these two measurements we can obtain the single-fiber efficiency for Coulombic capture as

By testing the discharged filter with neutral particles the single-fiber efficiency for pure mechanical capture mechanisms is obtained (is., q,). Therefore, the single-fiber efficiency for dielectrophoretic capture is given by

These expressions are valid if the interaction among all the particle capture mechanisms acting simultaneously is negligible. However, Emi et al. (1987) report that the Coulombic and dielectrophoretic capture mechanisms have a negative interaction when acting simultaneously. Experimental Uncertainty of Single-Fiber Eficiency Measurements

By applying uncertainty analysis to Eq. (1) the following expression is obtained for the uncertainty as a function of the relative errors of each of the variables involved,

Effective Electric Field at the Surface of the Fibers The capture of charged particles by Coulombic attraction is a function of the effective electric field at the surface of the fibers. This electric field is directly related to the charge level and configuration of the electret filter. A simplified way of estimating the magnitude of the surface effective field can be



derived by assuming charged particles being captured by the electret fiber by pure Coulombic forces as they approach the fiber at a migration velocity given by


where Z, is the particle electrical mobility and E, is the effectivc surface electric field. For a fibrous filter of cylindrical fibers the penetration of charged particles due to Coulombic capture then can be approximated by a simplified physical model in which

where P, is the penetration due to Coulombic capture, d, is the fiber diameter, 1, is the specific fiber length, and U is the filter face velocity. The specific fiber length is related to the filter basis weight by the following equation,

where W is the filter basis weight (kg/m2), and p, is the fiber density. Therefore, the effective surface electric field is given by

where P, is the penetration due to Coulombic capture given by

where P,,, is the total penetration of singly charged particles due to combined Coulombic, dielectrophoretic, and mechanical capture, and P,,, is the penetration of neutral particles due to combined dielectrophoretic and mechanical capture. Equation 10 is based on the validity of Eq. 2 for combining the single-fiber efficiencies due to different capture mechanisms. Dimensionless Parameters

For Coulombic capture by a line-multipole fiber the following dimensionless parameter

has been derived from the equations of motion of the particles (Brown, 1993),

where Z, is the particle electrical mobility, a is the fiber surface charge density, U is the face velocity, K, is the fiber dielectric constant, n is the number of elementary charges per particle, e is the elementary unit of charge, p is the gas viscosity, d, is the particle diameter, and E,, is the permittivity of free space. This dimensionless parameter is the ratio between the particle drift velocity due to Coulombic attraction evaluated at the fiber surface and the mean gas velocity. For dielectrophoresis capture by a linemultipole fiber the corresponding dimensionless parameter is (Brown, 1993),

2 ( K - 1) a2d$ Npo = 3 (K, + 2) ~,(1 + ~ ~ ) ~ d , (12) p ~ ' where K, is the particle dielectric constant, d, is the fiber diameter, and the other variables as defined before. This dimensionless parameter is the ratio between the particle drift velocity due to the induced polarization force evaluated at the fiber surface and the mean gas velocity. These dimensionless parameters have been used in several studies to obtain correlations of the single-fiber efficiency as a funetion of the Coulombic and dielectrophoretic parameters. However, to do this a charge level and configuration on the filter had to be assumed or fitted based on experimental measurements to calculate the magnitude of the dimensionless parameters.

RESULTS General Observations

Typical raw penetration measurements for one the filters evaluated are shown in Fig. 2. There are two curves for the charged electret filter with singly charged and neutral


Aerosol Science and Technology 2 8 3 March 1998

(1) Charged filter, Charged particles (2) Charged filter. Uncharged particles (3) Discharged filter. Charged particles (4) Discharged filter. Uncharged particles

FIGURE 2. Performance of Filter Media Type A (150 gim2) at U = 0.10 mls.

particles (curves 1 and 2), and two curves for the discharged filter with singly chargcd and neutral particles (curves 3 and 4). When comparing curves 1 and 4 it is evident that the effect of electrostatic capture mechanisms on the performance of electret filters is quite substantial. Curves 1 and 2 converge at the larger particle-size range because the Coulombic capture becomes negligible for large particles. Also, note how the dielectrophoretic effect becomes dominant for larger particles by comparing curves 2 and 4. The most penetrating particle size is also a function of the charge state of the particles, being in the typical 0.3 Fm range for singly chargcd particles and close to 0.1 Fm for neutral particles. This is due to the fact that the Coulombic effect increases with decreasing particle size and the dielectrophoretic effect increases with increasing particle size. These observations agree qualitatively with theoretical predictions of electrostatic capture mechanisms. Table 3 gives more specific information

on the pressure-drop characteristics and the maximum penetrations and their corresponding particle sizes for neutral and singly charged particles for all the filters tested.

Eflective Electric Field at the Surface of the Fibers

Table 4 shows the estimated mean surface electric fields calculated with Eqs. 9 and 10 for all the tested electret filters using the experimentally measured penetrations. The effective surface electric field of filters A, BI and C were estimated to be 9.9 C 5.9 X 10" Vim, 6.7 1 2.7 x lo5 V/m, and 2.5 f 0.9 X lo5 V/m respectively. The relatively large uncertainties are mainly due to the fact that Eq. 7 does not provide a sufficiently accurate physical model for the Coulombic capture mechanism as will be seen in the following sections. However, the estimated surface electric fields give a good indirect indication


Aerosol Scicncc and Technology 28:3 March 1998

TABLE 3. Pressure Drop and Maximum Penetration of Electret Filters Tested Filter Type

Specified Basis Weight (dmZ)

Pressure Drop AP (cm HZO) @ U (cmis)

Singly Charged Particles p,,,,, @ d, ( ~ m )

Neutral Particles P,,.,, @ dp


of the level of charge of the tested electret filters. Experimental Results on the Single-Fiber Eficiencies

Figure 3 shows the experimentally obtained single-fiber efficiency for pure Coulombic capture as a function of Z,/U for the threc types of media tested. Only the data from 0.05 to 0.3 pm were used since this mechanism becomes negligible for larger particles. From this figure it is clear that filters type A and B have very similar performance, while filters type C have a lower single-fiber efficiency for the same value of Z IU, indicating a lower charge density on t i c fibers. The experimental results can be fitted by a power law curve of the following form suggested by the Coulombic dimensionless parameter,

TABLE 4. Estimated Mean Surface Electric Field

Type

Manufacturer Specified Basis Weight (dmz)

Mean Effective Surface Electric Field (Vim)

Standard Deviation (Vim)

where a and b are obtained by least squares regression analysis. Table 5 gives the values of the coefficients a and b , with its 95% confidence intervals and the corresponding squared correlation coefficients R ~ . Brown's theory for Coulombic capture predicts a power law correlation with an exponent of 0.83, while thc work by Emi et al. (1987) predicts an exponent of 0.75. Therefore, the experimental measurements agree with the theories in the form of thc functional relationship between the single-fiber efficiency and the Coulombic dimensionless parameter. The coefficient b varies with the type of filter, particularly for filter type C. However, in all cases the coefficients are in agreement with theoretical predictions when considering the estimated confidcnce intervals. Figure 4 shows the single-fiber efficiency for dielectrophoresis capture as a function of Cd; for the threc typcs of media tested. Again, filter media types A and B behave similarly, while filter media C has lower single-fiber cfficiencies. Thc data can also be correlated by a power law curve of the following form suggested by the dimensionless dielcctrophoretic parameter,

where the resulting coefficients c and d are given in Table 6, with its 95% confidence



--+- Filter Media Type A --+-Filter Media m e B


+Filter Media Type C

r'

I

FIGURE 3. Experimental Single-Fiber Efficiency for Coulombic Capture as a Function of ZJU.

intervals and the corresponding squared correlation coefficients R'. The theory of Brown (1 981) as well as that derived by Emi et al. (1987) for dielectrophoresis capture predict a power law correlation with an exponent of 0.40. The filters tested show power law exponents that agree well with theoretical predictions when considering the estimated confidence intervals. In order to better compare the available theoretical expressions with our experimental data it is necessary to obtain quantitative information about the charge density and configuration in the tested filters.

TABLE 5. Power Law Regression Coeficients for Coulombic Capture Filter Type

Coefficient a (Vim)"

Coefficient b with 95% CI

RZ

A

exp (9.27 i 1.05) cxp (9.93 i 1.31) exp (11.87 i 1.60)

0.69 i 0.06 0.72 i 0.08 0.90 -+ 0.10

0.939 0.912 0.914

B C

CONCLUSIONS Electrostatic particle capture enhances conventional mechanical filtration mechanisms in the submicron-size range without increasing the pressure drop in electret filters. The charge state and the size of the particles strongly affect the penetration through electret filters. Singly charged particles were collected by simultaneous Coulombic attraction and dielectrophoresis in the 0.1 to 0.3 pm size range. For particles smaller than 0.1 pm the Coulombic effect was dominant. For particles larger than 0.3 pm the dielectrophoretic effect was dominant. Neutral particles were collected by combined mechanical and dielectrophoresis capture for particles larger than 0.1 pm. The use of a simplified model for Coulombic capture allowed to estimate the magnitude of the effective surface electric field for the tested electret filters (i.e., E, varies from 2.5 X lo5 to 1 X 10"Im). Based on the experimental results the charge level of the


1oO


I

-& Filter Media Type !A

:-+-

Rlter Media Type B

- ---6-- Filter Media Type C


L

FIGURE 4. Experimental Single-Fiber Efficiency for Dielectrophoretic Capture as a Function of CdgU.

tested electret filters from highest to lowest is for types A, B, and C respectively. Information on the single-fiber efficiency for Coulombic and dielectrophoresis capture was obtained by assuming additive effects of mechanical and electrostatic particle capture mechanisms. The scatter on the measured single-fiber efficiencies is probably due to the variable uncertainty of the efficiency measurements and to the statistical variation of the charge and media characteristics of the filter samples tested. The singlefiber efficiency for Coulombic capture is a power law function of N,, with an exponent that varied from 0.69 to 0.90 depending on the type of electret media tested (e.g., A, B or C). The single-fiber efficiency for dielecTABLE 6. Power Law Regression Coefficients for Dielectrophoresis Capture Filter Type

Coefficient c (llm-syl --

A B C

cxp (7.80 2 1.32) exp (10.00 i- 2.07) cxp (10.00 i- 1SZ)

-

Coefficient d with 95% CI

RZ

0.38 2 0.05 0.45 5 0.07 0.48 i- 0.05

0.870 0.808 0.886

trophoresis capture is a power law function of N,, with an exponent that varied from 0.38 to 0.48. These ranges agree well with theoretical predictions by Brown (1981) and by Emi et al. (1987) when considering the estimated confidence intervals and assuming idealized charge configurations. To obtain full semi-empirical correlations between single-fiber efficiency and dimensionless parameters for Coulombic and diclectrophoresis capture it is necessary to have reliable quantitative information on the charge level and configuration of the tested electret filters, a task that is under investigation. This research was sponsored 11y the Center for Fibration Research at the Univer.sily of Mimesola. .Memher:c.o f the Center include, 3M Company, AAF International, Donaldson Company Inc., Fleetpaxi Inc., W L. Gore & Associales Inc., Honeywell, and TSI hzc. The authors wish to thank the member con~punieso f the center for their support.

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electrostatic effects, Filtration and Separation 24:418-422. Lehtimaki, M., and Heinonen, K. (1994). Reliability of Elcctret Filters, Building and Environment 29353-355. Liu, B. Y. H., and Pui, D. Y. H. (1974). A Submicron Aerosol Standard and the Primary, Absolute Calibration of the Condensation Nuclci Counter, J. Colloid Intefuce Sci. 47: 155-17 1. Otani, Y., Emi, H., and Mori, J. (1993). Initial Collection Efficiency of Electret Filtcr and Its Durability for Solid and Liquid Particles, KONA Powder und Particle 11:207-214. Ouyang, M., Liu, B. Y. H., and Rubow, K. L. (1996). Performance Evaluation of Commercial Air-Cleaning and High-Efficiency Filter Mcdia, TAPH Nonwovens Conference and Technology Exhibition March 11-13, 1996, Charlotte, NC. Pich, J., Emi, H., and Kanaoka, C. (1987). Coulombic Deposition Mechanism in Elcctret Filters, J. Aerosol Sci. 18:29-35. Pich, J. (1978). Theory of Electrostatic Mcchanism of Aerosol Filtration, in F~mdamentalsoJ Aerosol Science (D. T . Shaw, cd.) Wiley Interscience, Ncw York, pp. 325-367. Walsh, D., and Stenhouse, I. (1996). Experimental Studies of Electrically Active Fibrous Filtcr Loading, Part. Part. Syst. Charuct. 13:47-53. Received 15 October 1996; accepted 17 October 1997.