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Aerosol Science and Technology
ISSN: 0278-6826 (Print) 1521-7388 (Online) Journal homepage: http://www.tandfonline.com/loi/uast20
(Absence of any) effect of the electric charging
state of particles below 10 nm on their
penetration through a metal grid
Manuel Alonso
To cite this article: Manuel Alonso (2017): (Absence of any) effect of the electric charging state of
particles below 10 nm on their penetration through a metal grid, Aerosol Science and Technology,
DOI: 10.1080/02786826.2017.1397267
To link to this article: http://dx.doi.org/10.1080/02786826.2017.1397267
Accepted author version posted online: 25
Oct 2017.
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Date: 26 October 2017, At: 04:02
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(ABSENCE OF ANY) EFFECT OF THE ELECTRIC CHARGING STATE OF PARTICLES
BELOW 10 nm ON THEIR PENETRATION THROUGH A METAL GRID
Manuel Alonso
National Center for Metallurgical Research (CENIM-CSIC), Avenida Gregorio del Amo, 8.
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28040 Madrid, Spain
Corresponding autor email: malonso@cenim.csic.es
ABSTRACT
The effect of image force on the penetration of nanometer particles through metal grids remains
a controversial issue. Experimental evidence of the existence and of the absence of such effect
have both been reported in the past. A careful experimental work to measure penetration of
particles in the mobility equivalent diameter range between 3.4 and 10 nm has been carried out.
The possible particle size change between the aerosol generator and the filter has been
considered, as well as the possible effect of particle number concentration on the filtration
efficiency. The geometric dimensions of the filter allowed attainment of the fully developed
parabolic flow velocity profile upstream the grid. Measurements were done at two values of the
fiber Reynolds number, 0.09 and 0.12, much smaller than 1, as demanded by the currently
accepted filtration theory. Penetration of charged particles, measured in three alternative ways,
has been compared with penetration of uncharged and neutral particles (the latter consisting of a
mixture of positive, negative and uncharged particles). Two main conclusions have been
reached: (1) the charging state of the particles does not affect their penetration through the metal
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grid; and (2) the experimentally measured penetrations are fairly well predicted by the fan filter
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model of Cheng and Yeh (1980).
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1. Introduction
Aerosol particle penetration through wire screens has received a relatively large attention in the
past, mainly because it constitutes an ideal prototype of a fibrous filter and, as such, has been
widely used to develop air filtration theories. Wire screens have also been used to measure
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aerosol particle size distribution (diffusion battery), although this specific application has lost its
former importance in favor of more precise and fast measurements provided by particle electric
mobility analyzers.
An important issue is the effect that the presence of electric charges on the particles may exert on
the filtration performance of the grids. Electrical effects can appear in two different ways. First,
if the aerosol concentration is high enough, mutual repulsion among particles charged with the
same polarity, especially in the case of particles with high mobility, can lead to a reduction of the
fraction of particles that penetrate the grid. Second, even in the case that the aerosol
concentration is low, a charged particle may induce a charge of opposite sign in the fiber, thereby
arising an attraction force (image force) between the particle and the fiber which thus also results
in a decrease in penetration.
Experimental studies dealing with the effect of image forces on aerosol penetration through a
metal grid in the particle size range below 10 nm are few in number. More important is that
contradictory results have been obtained and the topic remains controversial. Scheibel and
Porstendörfer (1984) found no difference in the penetration of singly charged and uncharged
particles in the particle size range between 3.5 and 100 nm. Otani et al. (1995) did experiments
with particles smaller than 10 nm, even with ions, and found no image force effect neither in
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wire screens nor in laminar flow tubes. The same findings (no image force effects) were reported
by Alonso et al. (1997) for particles below 7 nm and air ions. In these three studies, the
experimentally measured penetration agreed reasonably well with the fan filter model equation
of Cheng and Yeh (1980), which considers diffusion as the only deposition mechanism. Shin et
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al. (2008) did experiments with (globally) neutral particles with diameter between 3 and 20 nm
and found good agreement with the Cheng-Yeh model for temperatures up to 500 K; however,
no experiments were done with charged or uncharged particles and, therefore, from their results
it is not possible to withdraw any definite conclusion about the effect of image force on filtration.
Similar results were reported much earlier by Cheng et al. (1990) also working with neutral
aerosols between 4.6 and 20 nm. Note that in this particle size range, the fraction of uncharged
particles in a neutral aerosol is quite high, of the order of 90% or more and, therefore, it is
difficult to determine whether the presence of <10% charged particles has any effect on the
filtration of the globally neutral aerosol.
In contrast with the works cited above, there are a number of other experimental works in which
image force has been found to increase the filtration efficiency for nanometer-sized particles.
Heim et al. (2005) reported a slight increase in the filtration efficiency of charged particles
between 2.5 and 20 nm in comparison with that of (globally) neutral particles using nickel and
stainless steel meshes. Kim et al. (2006) measured penetration of charged, uncharged and neutral
particles in the size range 2--100 nm and found that the filtration efficiency for charged particles
were larger than for uncharged ones, and that the filtration efficiency for neutral particles lied in
between the other two, that is, filtration efficiency increased as the fraction of charged particles
present in the aerosol increased, a clear indication of the existence of image force effects. In this
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latter work, it was also observed that electrostatic attraction (image force) was affected by the
aerosol face velocity in such a manner that for high velocities particle penetration was not
affected by its charging state. Van Gulijk et al. (2009), working with neutral aerosols in the size
range 7--20 nm, suggested that the presence of electric charges increased the sticking probability
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of particles. Finally, Heim et al. (2010) performed penetration measurements for charged
particles and ions with mobility equivalent diameters between 1.2 and 8 nm; though they did not
do measurements for uncharged particles and, therefore, a comparison cannot be made, these
authors found that penetration was smaller than predicted by the Cheng-Yeh theory for very low
Peclet numbers and this was attributed to image force between the particles and the grid.
In view of these contradictory results, a careful experimental work has been carried out with the
intention to add new light on this controversial topic.
2. Current theory on particle filtration by simultaneous diffusion and image force
The effect of particle charge on filtration was studied experimentally by Lundgren and Whitby
(1965), and Yoshioka et al. (1968), using fibrous filters. They found that the single fiber
efficiency for the mechanism of image force was proportional to the square root of the image
force number, KIM , defined as
  1 
Cp2e2
KIM   f
,

  f  2  12 2 u 0d p d 2f


[1]
where  f is the dielectric constant of the fiber (taken as infinite for metal fibers),  0 the
dielectric constant of a vacuum, C the Cunningham slip correction factor, p the number of
charges on the particle, e the electron charge,  the gas viscosity, u the air flow velocity, d p the
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particle diameter, and d f the fiber diameter. KIM represents the ratio of the particle drift velocity
by image force to the fluid flow velocity. In the above cited works, experiments were done with
particles between 0.1 and 1 m in diameter and carrying a large number of charges, up to 320
elementary charges per particle. They proposed the following expression for the single fiber
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efficiency due to image force:
1/2
EIM  aKIM
.
[2]
The proportionality constant a was 1.5 (Lundgren and Whitby, 1965) and 2.3 (Yoshioka et al.,
1968).
More recently, Alonso et al. (2007) made experiments with smaller particles (diameter between
25 and 65 nm) carrying up to 3 elementary charges at most. They found the following expression
for the single fiber efficiency:
1/2
EIM  9.7KIM
.
[3]
Any of these two correlations, [2] and [3], can be added to the single fiber efficiency due to
diffusion, to calculate the total single fiber efficiency. According to the fan filter model of Cheng
and Yeh (1980), which has been successfully verified in many experimental works, the single
fiber efficiency for the mechanism of diffusion is given by
ED  2.7Pe2/3.
[4]
In the last expression, Pe is the Peclet number, Pe  ud f / D , where D is the particle diffusion
coefficient (calculated with the Stokes-Einstein equation), and u and d f have the same meaning
as in Eq. [1].
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If both deposition mechanisms, diffusion and image force, operate simultaneously the single
fiber efficiency can be determined by the approximate expression
E  ED  EIM ,
[5]
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because the cross term containing the product ED EIM can be safely neglected (Alonso et al.,
2007).
Penetration is finally given by
P  exp  nSE  ,
[6]
where n is the number of screens (one in the present work), and S is the so called screen
parameter (see Table 1 below for its definition).
3. Experimental method
Penetration through a metal grid was measured for three different electric charging states of the
aerosol: (1) positively charged particles; (2) uncharged particles; and (3) a globally neutral
aerosol containing positive, negative and uncharged particles, with a roughly zero net charge.
Figure 1 shows the general experimental setup employed for the measurement of penetration of
aerosols in the three charging states. An evaporation-condensation NaCl aerosol was charged in a
circular tube containing two thin foils of
241
Am, each with an activity of 0.9 Ci, and size-
classified with a differential mobility analyzer (TSI NanoDMA, length = 4.987 cm; electrodes
radii = 0.937 and 1.905 cm). The DMA was operated in open mode, i.e. no sheath recirculation,
at aerosol ( = sampling) flow rate of 2 l min1, and sheath ( = excess) flow rate of 20 l min1. The
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singly-charged monodisperse particles, with mobility-equivalent diameter selected between 3.4
and 10.0 nm, leaving the DMA were then passed through one of the two routes, A and B in the
drawing, in order to preserve or modify their charging state. Route A contains another
241
Am
neutralizer, with the same characteristics as the former, and an electrostatic precipitator (ESP).
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The ESP consisted of a circular grounded tube made of copper, 10 mm ID and 10 cm in length,
with a coaxial metal wire 2 mm in diameter to which a DC voltage, high enough to remove all
the charged particles, was supplied. When the ESP is turned on, the particles coming from route
A are all uncharged; when the ESP voltage is turned off, the aerosol sent to the filtration unit is
neutral, i.e. it contains uncharged particles and roughly the same amount of positive and negative
particles giving a zero net charge. Note that since the particles are very small (≤ 10 nm), the
fraction of uncharged particles in the neutral aerosol is rather high, above 90% for 10 nm
particles, and about 99% for the smallest particles tested. On its part, route B is a bypass route, a
conductive tube carrying directly the positively charged particles from the DMA to the filtration
unit.
The filter efficiency measuring system consisted of two geometrically identical holders made of
brass and electrically grounded, one containing one metal grid (‘filter’ in route C in Figure 1),
the other empty (‘dummy’ in route D). Each holder consisted of a central tube, 80 mm long and
36 mm ID, and two conical ends 40 mm in length and an inside diameter linearly changing
between 4 and 28 mm. The central tubes of the holders were equipped with a series of rings, each
5 mm long, 36 mm OD and 28 mm ID. The grid was placed near the holder outlet, between the
two last rings and in contact with them. The wire screen exposed to the aerosol flow was thus a
circle of 28 mm in diameter. The length of the straight central tube assured the attainment of a
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fully developed parabolic flow velocity profile upstream of the grid if the aerosol flow rate is
kept below 1.2 l min1. The test particles, either charged, uncharged or neutral, were alternately
passed through the filter and the dummy unit. Penetration through the grid was determined from
comparison of the particle concentrations measured at the outlet of the holders. In general,
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particle number concentrations were measured with a condensation particle counter (CNC, TSI
model 3786).
Particles were diverted to routes A or B, and C or D, by means of three way valves provided by a
local manufacturer and operated manually. Three metal T-junctions, shown as T-1, T-2 and T-3
in Figure 1, completed the general experimental setup. All the connecting tubes were made of
conductive plastic.
Since penetrations were measured by comparison of the number concentration of particles
coming from two alternative routes, C and D, one has to be sure that the two routes are perfectly
identical, so that the presence of valves, T’s, and tubes does not have any effect on the
measurements. The best way to assure the correctness of the measurements is to follow the
methodology proposed by Alonso and Borra (2016). This method consists in performing a
measurement with, say, the setup shown in Figure 1, yielding a penetration P1. A second
measurement is carried out after exchanging the filter and dummy units, so that the filter is now
placed in route D and the dummy unit in route C, keeping the rest of the setup unchanged. With
this second arrangement a penetration P2 is obtained. Penetration should finally be determine as
the geometric mean of P1 and P2. This method is admittedly tedious and time consuming, but it
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guaranties that the experimental measurements are not affected by the possible asymmetry of the
line accessories (valves, T’s, connections, etc.).
The metal grid, provided by Goodfellow, was made of stainless steel. Its characteristics are listed
in Table 1.
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Two evaporation-condensation polydisperse aerosols differing in their particle size distribution
were generated. One, obtained with the furnace operated at 560ºC, had a mean diameter of about
4.5 nm; from this aerosol, particles with diameters of 3.4, 4.1 and 5.1 nm were selected with the
nanoDMA. From the other polydisperse aerosol, generated at 600ºC and which peaked at 8.2 nm,
particles of diameters 5.8, 7.2, 8.8 and 10.0 nm were selected. In this manner the test
monodisperse aerosols used for penetration measurements were selected from near the peak of
the particle size distribution of the polydisperse aerosol in order to reduce the extent of sizing
errors (Alonso et al., 2014).
Experiments were carried out with two aerosol flow rates through the filter/dummy system, 0.8
and 1.0 l min1, giving aerosol face velocities of 2.17 and 2.71 cm/s, and fiber Reynolds numbers
of 0.09 and 0.12, respectively. To attain these flow rates, part of the incoming aerosol was
purged just before the second three way valve (see Figure 1).
For each aerosol flow rate, each particle diameter selected with the nanoDMA, and each electric
charging state of the aerosol, 10 penetration measurements were done by shifting consecutively
the three way valve 3WV-2 between routes C and D, and using the setup shown in Figure 1, i.e.
with the filter placed in route C and the dummy unit in route D. Subsequently, the filter and
dummy units were exchanged, as described above, and 10 additional penetration measurements
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were done. Finally, the geometric average of the two values thus obtained was calculated. These,
the geometric averages, are the penetration values reported in the sequel. It must be pointed out
that the values of the two penetrations measured, before and after filter/dummy exchange, were
very similar and, therefore the arithmetic mean practically coincides with the geometric mean
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(Alonso and Borra, 2016).
Additional experiments were also carried out to test possible sources of errors. First, the
possibility of a change in particle size as the aerosol travels through the charge conditioning
routes A and B was examined. For this, the setup shown in Figure 2 was used. The second DMA
was also a TSI nanoDMA operated under the same conditions as the first one.
Second, penetration of positively charged particles was measured in two additional ways, besides
the one performed with the setup of Figure 1. For these additional experiments, the three way
valve 3WV-1 and route A altogether were omitted (see Figure 3). Furthermore, since in this case
the aerosol only contains unipolarly charged particles, an aerosol electrometer (EM) can be used
instead of the CPC for concentration measurements.
4. Results and discussion
4.1. Particle size downstream the DMA
In the first place, the possible effect of the differences between routes A and B of the general
setup (Figure 1) on the particle size was assessed. The results of these experiments, carried out
with the setup shown in Figure 2, are presented in Table 2. The first column in the Table
represents the mobility-equivalent particle diameter inferred from the fixed voltage applied to the
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first DMA using the standard Knutson-Whitby equation. For a fixed voltage applied to DMA-1
the voltage in the second DMA was scanned and particle concentration measured with the CPC.
From the arithmetic mean voltage, almost coincident with the peak voltage, resulting from
DMA-2 measurements the corresponding mobility-equivalent particle diameter was calculated.
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The thus measured particle diameters of the aerosols coming from routes A and B are listed in
the second and third columns, respectively. It is first observed a slight increase of particle size
for both routes in comparison with the nominal particle diameter inferred from DMA-1.
However, this does not mean a real, ‘physical’ growth of the particles themselves by, say,
condensation or coagulation. Condensation can be neglected because the two air streams, the one
serving as sheath air and that use to generate the NaCl aerosol were previously dried by passing
them through silica gel beds. Likewise, coagulation can be ruled out because particle number
concentrations were below 5.104 cm3. Note also that for the two largest particles there is even a
slight decrease of the particle diameter. The observed mismatch between the DMA-1 fixed
voltage and the DMA-2 peak voltage is actually typical of tandem DMA experiments; these socalled voltage shifts have been observed quite frequently in the past. Nevertheless the differences
are very small and should not deserve any further consideration.
The important point is that the particle diameter of the test aerosols supplied to the filter/dummy
system is independent of whether the aerosol has passed through route A (neutralizer plus ESP)
or through route B (directly from DMA-1). Indeed, the second and third columns in Table 2 are
practically coincident.
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4.2. Penetration of positively charged particles measured in three different ways
Penetration measurements for positively charged particles were, in comparison with the other
two charging states, the most difficult of all. The concentration of the aerosols entering the
filter/dummy system depended on the polydisperse aerosol from where they were generated and
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also on the specific size of the particles under considerations. For the higher concentrated
aerosols, concentrations were reasonably constant during the first 5--7 minutes or so and
penetration measurements could be done with confidence; after that, concentrations underwent
large and erratic, non-periodic fluctuations with no definite tendency and measurements could
not be done. In these circumstances, the DMA voltage was set to zero, so that only clean air
passed through the experimental line. After half an hour or so, measurements could be done
again for another 5--7 minutes. For lower concentrated aerosols, stable concentrations lasted for
longer periods of time, about 10--15 minutes, after which clean air was passed through the
system to let the accumulated charges dissipate away. All the conductive parts of the
experimental line were grounded; the only dielectric parts where charges can accumulate are the
o-rings present in the three way valves to prevent leakage. In contrast, experiments with
uncharged and neutral particles did not give any trouble at all.
Table 3 shows that there are no significant differences in the penetrations of positive particles
measured in the three different manners explained in the Experimental Method and also in the
Table caption. The reported values of penetration are the geometric means of the two
penetrations measured before and after exchange of the filter and dummy units; in turn, each of
these two penetrations are the arithmetic averages of ten measurements. The standard deviations
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of the twenty measurements are also reported in Table 3. These values of standard deviation are
the largest obtained in this work: the corresponding standard deviations for neutral and
uncharged particles were smaller, possibly because concentrations were more stable than for
positive particles.
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4.3. Effect of concentration on penetration of positively charged particles
Penetration of positively charged particles through the metal grid was measured at two levels of
particle number concentration. In these experiments, a dilutor was inserted in the general setup
shown in Figure 1, between T-2 and 3WV-2. In the dilutor, a certain fraction of positive particles
coming from route B was diverted towards a bypass route in which a HEPA filter was placed.
The two streams, the one with particles and the clean one, merged in an additional T-junction
located just before the three way valve 3WV-2.
The results of these experiments are shown in Table 4. The number concentrations appearing in
the third and fifth columns are those measured for the route containing the filter, C in the first
series of 10 measurements, and D in the second series after exchanging of the filter and dummy
units. Penetrations in the fourth column correspond to the low concentration level (i.e. third
column), and penetrations in the last column are those obtained for the high concentration
aerosol (fifth column). It is seen that up to 3.104 cm3, particle concentration does not have any
effect on penetration. For the aerosol concentrations attained in this work (high level, no dilutor)
space charge effects can be safely neglected.
A more detail examination of the effect of concentration on the penetration of positively charged
particles is presented in Figure 4, which shows the raw data (particle number concentration at the
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outlet of the filter and of the dummy unit) obtained for 8.8 nm particles. The plotted data are the
averages of 20 measurements, 10 in a first series, and the remaining 10 after exchanging the filter
and the dummy unit. The data points lie in a straight line, the slope of which is the particle
penetration, and this means that the aerosol concentration has no effect on filtration, i.e. space-
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charge effects (mutual repulsion between the unipolarly charged particles) are completely
negligible, at least up to the highest concentration achieved in this work, about 5.104 cm3. For
the rest of the particle sizes tested, the concentrations were significantly lower than for 8.8 nm
and, as shown in Table 4, no concentration effect was observed either.
4.4. (Absence of any) effect of particle charging state on penetration
Figures 5 and 6 show the experimental results obtained for positively charged particles at two
different face velocities, along with the theoretical curves calculated with the equations described
above in section 2. The full line represents the prediction of the fan filter model, i.e. assuming
that diffusion is the only deposition mechanism operating in the filter. The dashed and dasheddotted lines were calculated with the full equation [5], i.e. considering particle deposition by
image force in addition to diffusion. The curve corresponding to the correlation of Lundgren and
Whitby (1965) is very close to that of Yoshioka et al. (1968) and has not been plotted to preserve
clarity. The curve D + IM including the correlation of Yoshioka et al. is also very close to the
curve for diffusion alone and, because of the experimental errors (given by the standard
deviations appearing in Tables 3 and 4), it is difficult to make a definite conclusion about the
presence or absence of image force effects in our experiments. In contrast, the curve D + IM
using the correlation of Alonso et al. (2007), which was obtained for particles much smaller than
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those used by Lundgren and Whitby and Yoshioka et al., is certainly far from the experimental
points and can be discarded.
From Figures 5 and 6 one is tempted to conclude that image force has no effect on the
penetration of particles under 10 nm through metal grids. This conclusion appears more strongly
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justified by Figure 7, in which the results obtained for positively charged particles (+) are
accompanied by those obtained for uncharged (0) and neutral (+0-) particles. Indeed, no
significant difference among the three charging states can be observed in this plot. Furthermore,
the data points are fairly well reproduced by the fan filter model, considering diffusion as the
only particle deposition mechanism, i.e. assuming EIM  0 in Eq. [5].
5. Conclusions
A careful experimental investigation has been carried out to examine the effect of the charging
state of particles with diameter below 10 nm on their penetration through a grounded metal grid.
From a source of positively, singly-charged particles withdrawn from a DMA, three aerosol
populations were generated, one consisting of 100% uncharged particles, a second one
containing a large fraction of uncharged particles and small amounts of positive and negative
particles (neutral aerosol), and a third population consisting entirely of positive, singly-charged
particles. First, experimental measurements have shown that the presence of a neutralizer and an
electrostatic precipitator does not modify the particle size, so that the three aerosols undergoing
filtration had essentially the same mean diameter. Second, it has also been shown that space
charge effects are negligible at the particle number concentrations attained in this work. And
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third, penetration of positive particles has been measured using three slightly different methods
which gave essentially the same results.
After these preliminary and essential checks, penetration experiments of charged, uncharged and
neutral particles were performed. The main results can be summarized thus: (1) the
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experimentally measured particle penetration through the metal grid agrees fairly well with the
fan filter model equation of Cheng and Yeh (1980) for diffusional deposition; and (2) image
force can be safely neglected for singly-charged particles having mobility equivalent diameter
smaller than 10 nm. These results are in agreement with those obtained in the past by some
groups, but are at odds with the results reported by other groups.
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References
Alonso, M., Kousaka, Y., Hashimoto, T., and Hashimoto, N. (1997). Penetration of NanometerSized Aerosol Particles Through Wire Screen and Laminar Flow Tube. Aerosol Sci. Technol.,
27: 471--480.
Downloaded by [Chalmers University of Technology] at 04:02 26 October 2017
Alonso, M., Alguacil, F.J., Santos, J.P., Jidenko, N., and Borra, J.P. (2007). Deposition of
Ultrafine Aerosol Particles on Wire Screens by Simultaneous Diffusion and Image Force. J.
Aerosol Sci., 38: 1230--1239.
Alonso, M., Gómez, V., and Borra, J.P. (2014). Determination of the Mean Mobility of Aerosol
Nanoparticles Classified by Differential Mobility Analyzers. Aerosol Sci. Technol., 48: 1217-1225.
Alonso, M., and Borra, J.P. (2016). A Method to Limit Uncertainties in Aerosol Properties
determined from Comparative Measurements. J. Aerosol Sci., 91: 15--21.
Cheng, Y.S., Yamada, Y., and Yeh, H.C. (1990). Diffusion Deposition on Model Fibrous Filters
with Intermediate Porosity. Aerosol Sci. Technol., 12: 286--299.
Cheng, Y.S., and Yeh, H.C. (1980). Theory of a Screen-Type Diffusion Battery. J. Aerosol Sci.,
11: 313--320.
Heim, M., Attoui, M., and kasper, G. (2010). The Efficiency of Diffusional Particle Collection
onto Wire Grids in the Mobility Equivalent Size Range of 1.2-8 nm. J. Aerosol Sci., 21: 207-222.
18
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Heim, M., Mullins, B.J., Wild, M., Meyer, J., and Kasper, G. (2005). Filtration Efficiency of
Aerosol particles Below 20 Nanometers. Aerosol Sci. Technol., 39: 782--789.
Kim, C.S., Bao, L., Okuyama, K., Shimada, M., and Niinuma, H. (2006). Filtration Efficiency of
a Fibrous Filter for Nanoparticles. J. Nanoparticle Res., 8: 215--221.
Downloaded by [Chalmers University of Technology] at 04:02 26 October 2017
Lundgren, D.A., and Whitby, K.T. (1965). Effect of Particle Electrostatic Charge on Filtration by
Fibrous Filters. Ind. Eng. Chem., 4: 345--349.
Otani, Y., Emi, H., Cho, S.J., and Namiki, N. (1995). Generation of Nanometer Size Particles
and their Removal from Air. Adv. Powder Technol., 4: 271--281.
Scheibel, H.G., and Porstendörfer, J. (1984). Penetration Measurements for Tube and ScreenType Diffusion Batteries in the Ultrafine Particle Size Range. J. Aerosol Sci., 6: 673--682.
Shin, W.G., Mulholland, G.W., Kim, S.C., and Pui, D.Y.H. (2008). Experimental Study of
Filtration Efficiency of Nanoparticles Below 20 nm at Elevated Temperatures. J. Aerosol Sci.,
39: 488--499.
Van Gulijk, C., Bal, E., and Schmidt-Ott, A. (2009). Experimental Evidence of Reduced Sticking
of Nanoparticles on a Metal Grid. J. Aerosol Sci., 40: 362--369.
Yoshioka, N., Emi, H., Hattorim M., and Tamori, I. (1968). Effect of Electrostatic Force in the
Filtration Efficiency of Aerosols. Kagaku Kogaku, 32: 815--820.
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Table 1. Characteristics of the stainless steel grid used in the experiments.
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1
66
Fiber diameter, df (m)
1202
Thickness, h (m)
1031
Opening (m)
Open area fraction (%)
371
2
Surface density, s (g/cm2) 0.031
1
Volume density, v (g/cm3) 7.96
3
Solid volume fraction,  (-) 0.324
Screen parameter, S (-)
1.1104
1
Datum provided by the manufacturer (Goodfellow);
2
measured;
3
calculated from   s / hv ;
4
calculated from S  4 h /  d f 1    .
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Table 2. Comparison between particle diameters of the aerosols coming from routes A and B
(setup of Figure 3).
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dp measured with DMA-1 (nm)
3.4
4.1
5.1
5.8
7.2
8.8
10.0
dp measured with DMA-2 (nm)
route A
route B
3.6
3.6
4.2
4.2
5.2
5.3
5.8
5.8
7.4
7.4
8.7
8.8
9.8
9.8
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Table 3. Penetration of positively charged particles measured in three different ways: (a) with
the setup shown in Figure 1, using the particles coming from route B; (b) with the setup shown in
Figure 3 using the condensation particle counter (CPC) to measure concentrations; and (c) with
the setup of Figure 3 using the aerosol electrometer (EM). Flow rate through the filter (or
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dummy) unit: 1.0 lpm.
dp (nm)
Pe (-)
(a)
P (-) (b)
(c)
3.4
4.1
5.1
5.8
7.2
8.8
10.0
3.8
5.6
8.5
11.2
16.9
25.3
32.8
0.28 ± 0.03
0.36 ± 0.04
0.50 ± 0.03
0.55 ± 0.03
0.64 ± 0.06
0.72 ± 0.04
0.76 ± 0.03
0.25 ± 0.04
0.37 ± 0.04
0.47 ± 0.03
0.55 ± 0.02
0.65 ± 0.03
0.74 ± 0.03
0.78 ± 0.03
0.26 ± 0.03
0.37 ± 0.04
0.51 ± 0.03
0.56 ± 0.03
0.67 ± 0.02
0.74 ± 0.02
0.77 ± 0.03
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Table 4. Effect of particle number concentration on penetration (positively charged particles).
dp (nm)
Pe (-)
N (cm3)
P (-)
N (cm3)
P (-)
3.4
4.1
5.1
5.8
7.2
8.8
10.0
3.8
5.6
8.5
11.2
16.9
25.3
32.8
280
675
1280
930
1690
3150
2110
0.26 ± 0.04
0.35 ± 0.04
0.50 ± 0.04
0.57 ± 0.03
0.62 ± 0.03
0.71 ± 0.04
0.75 ± 0.03
2520
5040
9530
9100
15300
32700
24100
0.28 ± 0.03
0.36 ± 0.04
0.50 ± 0.03
0.55 ± 0.03
0.64 ± 0.06
0.72 ± 0.04
0.76 ± 0.03
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Figure 1. Setup for the measurement of particle penetration through the metal grid contained
inside the filter. NaCl = polydisperse sodium chloride aerosol generated by evaporationcondensation; DMA = differential mobility analyzer; 3WV = three way valve;
241
Am = cylinder
containing two small circular foils of 241Am, each with an activity of 0.9 Ci; ESP = electrostatic
precipitator; T = metal T-junction; MF = mass flowmeter; dummy = cylinder of same
dimensions as the filter but with no metal grid inside; CPC = condensation particle counter.
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Figure 2. Setup for measuring the diameter of the particles at the end of the two alternative
routes A and B.
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Figure 3. Setup for two additional ways to measure penetration of charged particles. The two
ways differ in the apparatus used to measure particle concentrations, a condensation particle
counter (CPC) in one case, an aerosol electrometer (EM) in the other. In comparison with the
setup of Figure 1, note that one 3-way walve and one T-junction have been eliminated (3WV-1
and T-1, respectively).
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Figure 4. Effect of particle number concentration on the penetration of positively charged 8.8
nm particles at face velocity of 2.71 cm/s.
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Figure 5. Experimental penetrations obtained for face velocity of 2.17 cm/s, along with
theoretical curves. D = Cheng-Yeh fan filter model considering diffusion alone; D + IM =
diffusion plus image force using two different correlations, those of Alonso et al. (2007), and
Yoshioka et al. (1968).
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Figure 6. Experimental penetrations obtained for face velocity of 2.71 cm/s, along with
theoretical curves. D = Cheng-Yeh fan filter model considering diffusion alone; D + IM =
diffusion plus image force using two different correlations, those of Alonso et al. (2007), and
Yoshioka et al. (1968).
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Figure 7. Experimental penetrations for three particle charging states and two face velocities. +:
positively charged particles; +0-: globally neutral aerosol (i.e. containing positive, negative and
uncharged particles, and zero net charge); 0: uncharged particles. The curve represents the
prediction of the fan filter model, i.e. assuming that diffusion is the only deposition mechanism
operating.
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