EPA-650/2-74-014
October 1973
Environmental Protection Technology Series
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EPA-650/2-74-014
DEVELOPMENT OF A
LOW PRESSURE IMPACTOR
by
A. R. McFarland, H. S. Nye, and C. H. Erickson
Anderson 2000 Inc.
P. O. Box 20769
Atlanta, Georgia 30320
Contract No. 68-02-0563
Program Element No. 1A1010
EPA Project Officer: R. K. Stevens
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park , North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
October 1973
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EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
11
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ABSTRACT
A Low Pressure Impactor aerosol sampler was designed
fabricated and tested. The system injests a fixed aerosol flow rate
of 1 cfm at inlet conditions and causes the particulate matter to be
separated and collected on four atmospheric pressure and three
reduced pressure impaction stages and an after-filter. Cutpoint
sizes of the stages are 9. 7, 5.0, 2. 46, 1.21, 0.355, 0.141, and 0. 05
micrometers for spherical particles with a density of 2 gm/cm3.
Each of the impaction stages is fitted with a glass fiber media
collection substrate to facilitate gravimetric analysis of the collected
samples.
Experiments conducted with laboratory aerosols show the
system to have wall losses less than 6 percent when the mass median
diameter of the aerosol is 0.6 micrometers. For particles 6. 1 microns
in size, the wall losses on the upper stages are less than 11 percent.
Both particle rebound and re-entrainment from the collection surfaces
are shown to be negligible. Each low pressure stage can be loaded
with more than 10 mg of deposited aerosol without re-entrainment
occurring.
ill
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TABLE OF CONTENTS
Title Page Number
INTRODUCTION 1
INERTIAL IMPACTOR PERFORMANCE 9
LOW PRESSURE IMPACTOR DESIGN 16
EXPERIMENTAL PROGRAM 20
FIELD TESTS 37
SUMMARY AND CONCLUSIONS 43
REFERENCES 46
IV
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LIST OF ILLUSTRATIONS
Figure Title Page Number
1 Impactor Stage-Single Jet Type 3
2 Low Pressure Impactor Unit 6
3 Low Pressure Impactor System 7
4 Collection Characteristics of 10
Multi-Jet Inertial Impactor
for ReS 100
Effect of Jet Reynolds Number 15
upon KQ
Apparatus Employed in Aerosol 21
Tests
Size Distributions- Typical Test 23
Aerosols
Size Calibration of Low Pressure 25
Impactor
Particle Bounce Characteristics of 33
Stage LP-3 (0. 05/^m Cutpoint)
10 Loading Curve for Stage LP-3 36
(0. 05 j/m Cutpoint) of Low Pressure
Impactor
11 Size Distribution of Cigarette Smoke 38
12 Size Distribution of Atmospheric 40
Aerosol June 30-July 20, 1973 at
Urbana, Illinois
13 Size Distribution of Atmospheric 41
Aerosol July 21 -August 15, 1973
at" Urbana, Illinois
V
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LIST OF TABLES
Table Title Page Number
Low Pressure Impactor 19
Design and Operational
Parameters-1 cfm Inlet
Flow Rate
-24. 3 mm Hg Pressure
in Expansion Chamber
Wall Losses for Individual Z9
Impactor Components
Wall Loss Characteristics 31
of Original Stages "A" and
"B"
Vl
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LIST OF SYMBOLS
A = 1.23, a constant
C = Cunningham's Correction
D- = Jet Diameter
D = Particle Diameter
D = Particle Diameter for which Stage Efficiency is 50
percent
K = Inertial Parameter
K 5 = Value of Inertial Parameter for which Stage Efficiency
is 50 percent
KT = Value of Inortial Parameter Corresponding to Conditions
Employed in a Particular Test
L = Distance from Jet Exit Plane to Collection Plate
P = Air Pressure at Jet Exit Plane
Q - 0. 41, a constant
Re- - Jet Reynolds Number
T = Air Temperature at Jet Exit Plane
VQ = Air Velocity at Jet Exit Plane
b = 0.44, a constant
77 = Stage Efficiency
r/ - Value of Efficiency Obtained Experimentally
X = Mean Free Path of Air Molecules
M- = Dynamic Viscosity of Air
p = Density of Particulate Matter
P
vii
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INTRODUCTION
Background
The deleterious effects of atmospheric particulate matter are
numerous and varied. These include:
Reduction of solar radiation (which causes a decrease in
seasonal temperatures) Landberg1 reported that urban
areas receive up to 20% less insolation than rural areas.
Robinson2 and Holzworth3 demonstrated that there is a
relationship between visibility and the amount of particulate
matter in the atmosphere.
Corrosion of metals Hudson4 observed that industrial
locations with high concentrations of particles and oxides
of sulfur are more corrosive to steel and zinc than less
industrialized ares.
Interference with photosynthesis Particles settling on
vegetation interfere with light required for photosyn-
thesis thereby lowering starch production by the plant
(Czaja5 and Bohne6).
Human health hazard Atmospheric particles are suspect
as being a threat to human health' since these may be
intrinsically toxic, may carry an adsorbed toxic material
or may interfere with the clearance mechanisms in the
in the respiratory tract.
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In all of the above areas the knowledge of the size of the par-
ticle is vital to understanding its effects and determining the best
methods to control particulate emissions from man made sources.
In the case of examining the role of aerosols as public health
Q
hazards, the Task Group on Lung Dynamics noted the utility of a
sampler which can be used to determine the size of particles in aero-
dynamic terms. The degree of penetration and retention of particles
in the respiratory system is a function of aerodynamic size'.
Particles are, to a large extent selectively deposited by aerodynamic
size in the nasopharyngeal, tracheobronchial, and pulmonary areas
of the respiratory system. Final deposition site also depends on
variations in the respiratory air flowrate, and on physiological
considerations.
The use of aerodynamic sizing is of interest not only because
it allows simulation of the important size parameter in lung deposition
but also because it renders itself to the measurement of the aerosol
mass-size distribution. Although there are various types of apparatus
which could be used to acquire size distribution data based upon
aerodynamic size, the most widely used are cascade impactors.
With reference to Figure 1 the cascade impactor draws an aerosol
sample through a series of two or more stages made up of a jet or
orifice plate and a collection surface. As the air flows through the
jet it is accelerated to a specific velocity and directed towards the
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GAS
STREAMLINE
k\\\\\\\\\\\\\\\T\\\\\\\\\ >
COLLECTION
PLATE
-JET
.TRAJECTORY OF A
"HIGH INERTIA PARTICLE
TRAJECTORY OF A
LOW INERTIA PARTICLE
FIGURE I - IMPACTOR STAGE - SINGLE JET TYPE
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collection surface and through the following jet stage. Particles with
sufficient inertia cross the air flow stream lines and impact on the
collection surface. The smaller particles, which have insufficient
inertia, follow the stream lines into the next impactor stage. By
employing several stages in series it is possible to separate an
aerosol into size groupings. Samplers which characterize atmos-
pheric particles in this manner have been available since 1945 when
May10 developed a four stage cascade-type inertial impactor.
May's first impactor employed rectangular jets, however,
Ranz and Wong11 found better efficiency characteristics and sharper
cuts could be obtained with round jet impactors. In 1958, A. A. Ander-
sen12 developed a cascade inertial impactor for bacterial sampling
which employed multiple jets on each stage. Subsequently developed
versions of the apparatus have found application in atmospheric and
stack sampling.
Inertial impactors commonly used up to the present time
have had the capability of sizing particles to a lower limit of approxi-
mately 0.4 micrometers. This operational characteristic has
limited the study of the submicron components of aerosols such as
motor vehicle exhaust (which at cruising speeds consists of particles
of carbon, motor oil, aldehydes, ketones and lead) approximately
70 percent of which have an equivalent size for unit density particles
of less than 2 microns.13 Indeed, ninety percent of the lead by
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weight is associated with particles of sizes less than 0.5 micrometers.14
Other important submicron aerosols include, stationary source combus-
tion products, photochemical aerosols, oil mists and metallic fumes.
The limitation on particle size can be extended to much smaller
diameters if the impactor is operated at a reduced pressure. Basic
investigations by Stern and Zeller15 demonstrated the feasibility of
such an approach. Subsequently, McFarland and Zeller16 conducted
in-depth studies to determine the operational characteristics of a low
pressure impactor and, recently, Bucholz^ ' conducted tests with an
impactor operating at reduced pressure for separating particles of
0. 1 micrometers and smaller.
Purpose of Study
Although the feasibility of particle collection by low pressure
impaction has been demonstrated, the concept has not been extensively
used in air sampling. The reason is principally due to the lack of
specially designed apparatus.
The purpose of the present study was to design, construct and
test a prototype cascade impactor which can be used to determine the
mass-size distribution of atmospheric aerosols as small as 0. 05
micrometers for a density of 2 gm/crn3. The basic design of the
resulting device, which is shown schematically in Figure 2 and photo-
graphically in Figure 3, incorporates the configuration of a conventional
Andersen non-viable impactor* in four stages of a high pressure section
#Andersen-2000 Inc. Atlanta, Georgia
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INLET
INLET PORT
STAGE A
STAGE B-
STAGE C-
STAGE D
BASE
THROTTLIN
NOZZLES
EXPANSION
CHAMBER
u_
i
CE
UJ
DETAIL A
PRESSURE
TAP
— RUBBER GASKET
TEFLON GASKET
GLASS FIBER PAPER
TEFLON GASKET
JET PLATE
RUBBER GASKET
/EE DETAIL A
AFTER- FILTER
PRESSURE TAP
---SUPPORT BASE
OUTLET
FIGURE 2 -LOW PRESSURE IMPACTOR UNIT
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war PRESSURE
GAGE
LOW PRESSURE
iMPACTOR
COURSE
CONTROL
VALVE
FINE CONTROL
VALVE
FIGURE 3- LOW PRESSURE IMPACTOR SYSTEM
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8
(to separate particles larger than 1. 21 ^m) followed by three stages
in the low pressure section which effect separation of particles down
to 0.05 micrometers. The first four stages operate at atmospheric
pressure and the low pressure stages operate at approximately 1/30
atmosphere.
The unit has been laboratory tested to determine efficiency
characteristics and to evaluate the performance limiting character-
istics of (1) the loss of particles to internal surfaces other than the
collection plate (wall losses), (2) the particle rebound or bounce
characteristics for the stage -which has the highest air velocity,
and (3) the mass loading capability of the low pressure stage which
should be most susceptible to re-entrainment of a deposited sample.
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INERTIAL IMPACTOR PERFORMANCE
The size selection characteristics of an inertial impactor
stage are reflected by the efficiency with which the stage collects
variously sized particles. In turn, the efficiency, which is commonly
called the impaction efficiency, 77 , is a function of three dimension-
less parameters;
rj = f(K, L/Dj, RSj)
where;
K = Inertial Parameter
L/D. .= Aspect Ratio
Re- = Jet Reynolds Number
J
Of these dimensionless groups, K has the most significant influence
upon r\ - - the other two may be considered to be second order
variables .
R?nz and Wong11 carried out studies with impactors which
had single circular or slit jets on each stage and related the collections
efficiency to the inertial parameter. Later an experimental investi-
gation by McFarland and Zeller ! s showed the relationship between
K and 77 for a stage with multiple circular jets. A typical curve
relating these two variables is shown in Figure 4.
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LU
O
o:
UJ
00
80
O
z
UJ 60
O
LL_
U_
LU
40
10
a
o
o
0
O.I 0.2 0.3 0.4
INERTIAL PARAMETER , K , DIMENSIONLESS
FIGURE 4 - COLLECTION CHARACTERISTICS OF
MULTI-JET INERTIAL IMPACTOR
FOR Rej>IOO
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11
In this work, the inertial parameter is defined as:
Cp V D 2
K = Pop
•where;
C = Cunningham's slip correction factor
Q - particle density
V = jet exit velocity
D = particle diameter
IJL = air viscosity
D. = jet exit diameter
It is customary to attempt to represent the r\ vs- K curve by
a single value, namely that of the inertial parameter which corresponds
to 50 percent efficiency, K ,-• For a given stage operating with fixed
values of all variables involved in K other than particle size, the
7] vs. K curve can also be represented by the diameter of a spherical
particle which would be removed with 50 percent efficiency. This
parameter is called the stage cutpoint and is denoted by D (-•
In the design of an inertial impactor stage it would appeal-
possible to achieve small values of the stage cutpoint through either
increasing the jet velocity or reducing the jet size. However, this
approach does have its limitations. When velocities much greater
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12
than 3000-4000 cm/sec are used, small particles may tend to either
rebound or become re-entrained from the collection surface. In
addition, it is presently not practical to use jet diameters smaller
than approximately 0.01 inch. As a consequence, the smallest
cutpoint which can be reliably achieved through varying the jet
velocity and diameter is approximately 0. 5 micrometers.
Closer observation of the impaction parameter reveals that
if the value of Cunningham's correction, C, can be increased, it rnay
be possible to achieve a cutpoint diameter without resorting to high
jet velocities or extremely small jet diameters.
Cunningham's correction, C, takes into account the non-
continuum nature of gas flow about the particle. The factor C
increases directly with the increase in the mean free path length of
the molecules while increasing inversely with particle size.
l R
Milliken has shown C to accurately be represented by:
DP
A - mean free path of gas molecules
- 0.0685 micrometers at standard atmospheric
temperature and pressure
A - 1.23
Q = 0.41
.b = 0.44
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13
For small values of ^ , C ^ 1, where as for large values
DP
of —— , C ^ —- (A+Q). For a perfect gas, the mean free path X
P P
can be represented by:
\ = 1.70 x 10"5T/P
T = temperature, °K
P = pressure, mm Hg
It may be noted that X is inversely proportional to pressure.
Reference to the definition of the inertial parameter indicates that
a reduction in pressure will increase C thereby giving a larger
values for K and thus allowing for collection of smaller aerosols
with a given stage.
The effect of pressure upon C may be noted by the example
that the value is 1. 6 for a 0.3 micron particle at atmospheric pres-
sure whereas it is 23. 1 for the same particle at 1/30 atmosphere.
While the influence of the aspect ratio, L/D., and the jet
Reynolds numbers, Re., are of second order importance, they do
somewhat affect the collection efficiency of an impactor. The air
jet effluxing from the nozzle remains relatively intact, independent
of aspect ratio, as it approaches the collection plate. There is,
however, a small change in streamline curvatures as the aspect
ratio is varied and, as a consequence, a variation in collection
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14
efficiency. For values of L/D- between 0. 55 and 5, Marple shows
that the shift in K c is less than 3 percent.
Both the velocity profile at the jet exit plane and the boundary
layer on the collection plate are affected by the jet Reynolds number.
As Re- is decreased the value of K increases. McFarland and
Zeller, in their study with multiple-jet stages, made a linear
interpolation of experimental data to show this effect. A plot of
their results, adjusted to show K g = 0. 14 for large values of the
Reynolds number, is presented in Figure 5. In the design of an
inertial impactor, it is possible to take into account the shift in
K 5 caused by the Reynolds number effect, thus a well defined D
exists for a given impactor stage operated under fixed conditions.
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15
•Q o.te
cc
UJ CO
1- en
< 2
o: o
o_ en
z
2 UJ
_,„
O.fb
0.14
0.12
20 40 60 80 100
JET REYNOLDS NUMBER, Rejf DIMENSIONLESS
FIGURE 5 - EFFECT OF JET REYNOLDS NUMBER
UPON K05
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16
LOW PRESSURE IMP AC TOR DESIGN
The fundamental criteria employed in the development of the
Low Pressure Impactor were that the system should sample at a flow
rate of 1.0 cfm, that the outpoints of the first and last stages should
be approximately 10 and 0.05 micrometers, respectively, and that
the collected samples should be compatible with gravimetric analysis.
The approach selected was to use a system which is divided
into two sections-- a set of four impactor stages •which operates at
atmospheric pressure and a set of three impactor stages together
with an after-filter which operates at reduced pressure (See Figure 2)
The two sections are separated by a throttling plate which serves not
only to create a pressure drop but also limit the flow through the
system to an equivalent of 1 cfm at inlet conditions. To minimize
aerosol losses from jets effluxing from the throttling nozzles, an
expansion chamber with an axial length of over 1 ft. has been employed.
Basically, the upper four stages of the system are similar to
the Andersen non-viable sampler, however the unit has been modified
to the extent that the jets of the first two stages are few in number
(only 36 per stage) and have tapered inlets. Also, the collection
plates of these two stages are designed to permit air to flow both
collection
around the edges and through one-inch holes in the/plate centers.
Collection in the upper stages is effected upon 81 mm diameter
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17
substrates composed of glass fiber filter media.
The principal elements in low pressure stages of the unit are
three jet plates, a collection plate, an after-filter support and gaskets.
The stages are arranged such that air is passed through the jet plate
of the first stage and directed towards the second stage jet plate.
The holes in the two plates are offset to permit the second stage jet
plate to serve as the collection plate for the first stage. Particles
are deposited on a special glass fiber filter media collection substrate.
Proper values of the aspect ratio are obtained through the use of a
neoprene gasket to separate the jet plates. To preclude the glass
fiber media from adhering to either the gasket or the jet plate, thin
teflon gaskets are placed on either side of the media.
The air from the first stage impaction process is directed
through the jets of the second stage (which are situated under opening
in the first stage collection substrate) and the process is repeated.
For the third stage, impaction takes place upon a surface designed to
serve only the purpose of holding a collection substrate. After passing
the third stage, the air flows through a glass fiber filter and is dis-
charged from the system.
The system is setup such that the only variable that need be
controlled during sampling is the pressure level in the expansion
chamber. This value is measured with the aid of a percision Wallace
and Tiernangage and is to be maintained at 24. 3 mm of mercury.
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18
Stage outpoints for the system were calculated using the data
shown in Figures 4 and 5. The resulting design specifications for
the system are presented in Table 1. The four atmospheric pressure-
stages are denoted by A, B, C and D whereas the low pressure stages
are listed as L/P-1, LP-2 and LP-3. Selection of cutpoints was set to
provide a ratio of approximately two between successive stages in the
high pressure section and a ratio of approximately three between
successive low pressure stages.
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TABLE 1. Low Pressure Impactor Design and Operational
Parameters - 1 cfm Inlet Flow Rate
- 24.3 mm Hg Pressure in Expansion Chamber
Number of
Stage Jets
A (Modified) 36
B (Modified) 36
C 400
D 400
LP-1 600
LP-2 600
LP-3 1762
Aspect
Diameter Ratio
(inches) L/D.
J
0.
0.
0.
0.
0.
0.
0.
161 0.55
104 0.9
0295 3
0187 5
0547 2.2
0398 3.0
0208 2.9
Jet
Velocity
cm/sec
100
240
269
668
1606
2886
4158
Jet Reynolds
Number
Re.
254
393
127
200
45.9
61.4
40. 5
Stage Outpoints
(Dp>5,m)
9.7
5.0
2.46
1.21
0.355
0. 141
0.050
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20
EXPERIMENTAL PROGRAM
A set of laboratory tests was conducted with the prototype
Low Pressure Impactor to verify the design stage cutpoints and to
quantify the performance limiting factors of: losses to internal
surfaces of the impactor other than the collection surface (wall
losses), particle bounce and re-entrainment, and the mass loading
characteristics. Basically, these tests involved subjecting the
sampler to an aerosol which has known (or easily measurable)
properties and which is readily identifiable. The aerosols were
generated with two types of apparatus: a spinning disc atomizer
and a nebulizer, *" with the first device serving the purpose of form-
ing large (> 1 micrometer diameter) particles and the latter device
being used to generate the smaller aerosols. For both systems, the
aerosol generators formed a spray from a solution of 70 percent
uranine dye and 30 percent methylene blue dye dissolved on a solution
of 67 percent ethyl alcohol and 33 percent water. Evaporation of the
spray droplets produced the actual test aerosol. Particle size was
varied by changing the concentration of the dye solutions.
The basic layout of apparatus employed in the testing is shown
schematically in Figure 6. Aerosol from either the spinning disc or air
blast atomizer was passed through an 8-inch diameter duct. One
sample stream of the aerosol was drawn at a flow rate of 1 cfrn
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SPINNING DISC
AEROSOL
fAIR BLAST
ATOMIZER
1
W&T
PRESSURE
GAGE
A
VACUUM
PUMP
MEMBRANE FILTER HOLDER
LOW PRESSURE
1MPACTOR
MAIN CONTROL
VALVE
.VACUUM
PUMP
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
FIGURE 6 -APPARATUS EMPLOYED IN AEROSOL TESTS
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22
into the Low Pressure Impactor and a second sample stream, also at
a flow rate of 1 cfm, was drawn through a 47 mm glass fiber filter
mounted in a membrane filter holder. The purpose of the latter
sample was to monitor the constancy of aerosol output.
For each test performed with the spinning disc generator, a
sample of aerosol was collected on a membrane filter and a micro-
scopic size distribution was made. Figure 7 shows the results
obtained from such a determination. The characteristic aerosol
size was represented by a mass-average size which was obtained
by converting the microscopic data to a mass basis and calculating
the average value.
The nebulizer used to generate the submicron aerosols was
a Model 099 Dispos-A-Neb manufactured by Bio-Logics, Incorporated.
For determination of the particle sizes created with this device,
samples were collected on electron microscope grids using a
Thermosystems, Incorporated electrostatic sampler. These were
subsequently sized from photomicrographs taken with the aid of a
Hitachi HU-11 transmission electron microscope. Figure 7 shows
results obtained from sizing a typical submicron aerosol.
Stage Outpoint Sizes
To obtain the desired cutpoint of 0. 05 micrometers for the
final stage, LP-3, of the Low Pressure Impactor, it is necessary
that the pressure level at the jet discharge plane.be 22. 1 mm Hg.
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CL
Q
VI
CO
111
N
co
98
95-
9O
70
Q
UJ
5 50
o
CO
CO
CO 30
CO
>-
QQ
10
UJ
o
cc
UJ
CL 5
AIR BLAST ATOMIZER
AEROSOL GENERATOR
SPINNING DISC
AEROSOL GENERATOR
0.02 0.05 O.I 0.5 1.0
PARTICLE DIAMETER , Dp, MICROMETERS
FIGURE 7-SIZE DISTRIBUTIONS-TYPICAL TEST AEROSOLS
5.0
UJ
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24
Since the pressure level for the system is sensed at the expansion
chamber and there are pressure losses as the air flows through
each impaction stage, it was necessary to determine pressure
variations in the low pressure system. The results of such a check
show;
LOCATION PRESSURE
Jet Exit Plane of Stage LP-3 22. 1 mm Hg
Jet Exit Plane of Stage LP-2 23. 6
Jet Exit Plane of Stage LP-1 24. 2
Expansion Chamber 24. 3
Design cutpoints for stages LP-1, LP-2 and LP-3, which are shown
in Table 1, are based upon these pressure levels. The predicted
cutpoints are also shown graphically in Figure 8 wherein the cutpoints
are presented not only for a particle density of 2 gm/cm3 but also
for 1 and 4 gm/cm3. Operation of the impactor with the predicted
cutpoints is obtained when the expansion chamber pressxire is set
at 24. 3 mm Hg. Use of lower pressure levels will shift the cutpoints
of the low pressure stages to smaller values.
In order to verify the design cutpoint particle sizes of the
low pressure stages, detailed tests were conducted with stage LP-3.
Since this stage has the smallest jets, largest value of L/D., and
smallest value of Re. of the low pressure stages, it is anticipated
J
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0.01
ABC D LP-I LP-2
STAGE NUMBER
FIGURE 8-SIZE CALIBRATION OF LOW PRESSURE
LP-3
IMPACTOR
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26
that any deviations from design predictions would be most easily
observed by testing this stage.
The tests were conducted using the apparatus arrangement
shown in Figure 6 with the aerosol generated by the nebulizer. The
high pressure stages of the impactor were left in place during these
tests in order to strip the largest particles from the distribution.
The low pressure stages were re-arranged such that LP-3 was placed
above LP-1 and LP-2. During operation, aerosol was drawn through
the impactor system for a time sufficient to collect an easily measur-
able quantity of uranine dye. The amount of uranine collected by
stage LP-3 and the remaining components of the low pressure section
of the impactor was determined by washing the parts in distilled
deionized water to extract the uranine dye and subsequently analyzing
the wash water with a Turner Model 110 Fluorometer. From the.
resulting data, the test efficiency -n , of stage LP-3 was determined.
Next, a value of the inertial parameters, KT, which corresponds to
the test efficiency was taken from Figure 4 and the following equation
employed;
T
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27
This expression was solved for D ,- based upon knowledge
p. o
of the size and density of the test particles and upon an assumed
value of two for the density of the outpoint size particles.
This method was chosen because the cutpoint size can be
determined from a minimal number of tests. Reliance is placed
upon the impaction efficiency curve only to the extent that the slope
is utilized.
The results for triplicate tests with stage LP-3 are shown
in Figure 8 superimposed upon the curve •which gives the predicted
cutpoint size for the system. It should be noted the experimental
data verifies the calculated cutpoint of 0. 05 micrometers for the
last stage.
Similar tests were performed on stages C and D of the
high pressure section using particles produced by the spinning disc
aerosol generator. In this case the impaction stage to be tested
was placed first in the impactor and its efficiency determined.
Following the prodecure given above the values of D ,- were computed.
These results, which are also shown in Figure 8, support the predicted
cufcpoint sizes.
Wall Losses
Wall losses in cascade impactors can be attributed to the
following factor-s; high jet and other internal velocities, close
spacing of internal components and abrupt changes in air direction
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28
at locations other than collection surfaces. Use of an impactor with
any of these design deficiencies may result in a high ratio of internal
wall losses to sample collected. The relatively large geometric
scale of the low pressure impactor together with the multi-jet
principle renders it a device which has inherently low wall losses.
At the present time, the only method to reliably quantify wall
losses is through the use of controlled laboratory experiments. The
approach used to acquire these data for the low pressure impactor
involved the following procedure.
Prior to the onset of each test run, the entire impactor was
washed with a laboratory grade detergent and rinsed with distilled,
deionized water. The unit was then subjected to a heterogenous
uranine-methylene blue aerosol created by the nebulizer (geometric
standard deviation of approximately three). At the completion of
each test run, the individual collecting surfaces and the internal wall
surfaces were again washed with a measured volume of distilled,
deionized water to extract the dye. The wash water was then subjected
to fluoroscopic analysis to quantify the uranine mass which has been
deposited on the impactor surfaces.
The results obtained for aerosols with mean sizes of 0.3
and 0.6 micrometer are shown in Table 2. It may be noted that
totally the losses were less than 6 percent in each case.
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29
TABLE 2. Wall Losses for Individual
Impactor Components
Impactor Components
Inlet
Jet Plate A (Modified)
Jet Plate B (Modified)
Jet Plate C
Jet Plate D
Interface
Throttling Plate
Expansion Chamber
Jet Plate LPI-1
Jet Plate LPI-2
Jet Plate LPI-3
All other extraneous
surfaces
Total Wall Losses
Percent Wall Losses for given
Aerosol Size
0. 3 micrometers
0.
0.
0.
0.
0.
0.
0.
1.
0.
0.
0.
1.
5.
053
160
193
226
206
034
866
545
499
293
149
307
43
0. 6 micrometers
0.
0.
0.
0.
0.
0.
0.
1.
1.
0.
0.
5.
031
170
230
190
230
015
860
530
990
460
090
058
85
-------
30
Early in the experimental testing phase of the program it
was noted that stage A and B had inordinately high wall losses
(Table 3). To reduce this phenomenon a design change was under-
taken. For both stages A and B the number of jets was reduced
from 400 to 36 and, correspondingly the diameter of the jets was
increased (to 0. 161 inches for stage A and 0. 104 inches for stage
B). In addition the intake sides of jets were chamfered 60°. A one
inch diameter circle was cut in the center of the collection plates of
both stages A and B to reduce the volumetric flow rate (and hence
the velocity) of gas passing around the periphery of the collection
plate. These modifications provided a drastic reduction in wall
losses when tested with a monodisperse 6. 1 micrometers diameter
uranine aerosol (Table 4).
Wall loss data for the remaining upper stages, C and D,
were also acquired. Here it may be noted that the losses for each
stage are approximately 2 percent when the stage is tested with
particles of size similar to the stage cutpoint size.
Particle Bounce
Particle bounce can greatly reduce the efficiency of an
inertial impactor. If the collection surface is a smooth plate, as
the jet velocities are increased beyond 3200 cm/sec in the
lower stages of the impactor an increase in particle bounce and
re-entrainment can be expected. The problem may be partially
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31
TABLE 3. Wall Loss Characteristics of
Original Stages "A" and "B"
Jet
Stages
A and B
A and B
Sampler Flow
Rate (cfm)
1. 3
1. 0
Test
Size
6
6
Aerosol
(fim)
. 1
. 3
Wall Losses
Percent
32
40
. 4
. 0
TABLE 4. Wall Loss Characteristics
of Upper LPI Stages ("A" & "B" Modified)
-Flow Rate - 1 cfm
Jet
Stages
Test Aerosol
Size ( p. m)
Wall Losses
Pe rcent
A and B (Modified)
D
6. 10
3. 44
1. 02
10. 7
2. 2
2. 2
-------
32
controlled by employing thin viscous coatings on the collection
surfaces, since the coating will both create a condition of inelastic
impact and at the same time provide an adhesive force to retain the
particles on the collection surface. An alternate approach, which
serves to increase adhesion, is to employ glass fiber filters as the
collection substrates. This method offers a considerable advantage
over the use of viscous coatings in that less substrate preparation
is required and the substrates render themselves better to standard
analysis procedures.
To determine if the glass fiber filters used in the low pressure
section of the system were effectively preventing rebound of impacted
particles, a set of tests was conducted with stage L.P-3. This par-
ticular stage was chosen for detailed study because it has the highest
jet velocity, 4160 cm/sec. In conducting these tests, the stage was
operated in a manner in which the velocity could be varied yet the
predicted efficiency could be held constant.
If particle bounce were a problem, the expected result of
a plot of efficiency vs. velocity would show a decrease in efficiency
with increasing velocity as the particle bounce phenomenon comes
into play. The results of the tests, which are presented in Figure 9,
show the efficiency of the stage increases slightly with increasing
jet velocity (due to a jet Reynolds number effect not adequately
compensated for in the test conditions). The efficiency curve in the
-------
Ld
O
o:
UJ
CL
100
- 80
O
Z
UJ
h-
O
LU
_J
_J
O
O
60
40
20
o
3
LU
CD
2
,0
UJ i
Sli
2 UJ
CO CD
W<
UJ H-
Q CO
2000 4000 6000 8000
JET VELOCITY , Vj , cm/sec
FIGURE 9 - PARTICLE BOUNCE CHARACTERISTICS OF STAGE LP-3
(0.05 fjm CUTPOINT)
Co
UJ
-------
34
vicinity of the design velocity'did not drop which gives evidence that
particle bounce is not a significant problem with the low pressure
section of the impactor.
With respect to the high pressure stages, McGregor^ '
conducted bounce tests with a standard Andersen non-viable sampler
and found that a stage with a cutpoint of 1. 1 micrometers (for unity
density particles) could be operated at a velocity five times as large
as the velocity encountered in stage D of the present system with-
out a rebound phenomenon being noticeable. Based upon this result
it would be expected that the upper stages of the present design would
not be subject to a limitation caused by particle bounce.
Loading Characteristics
When substantial quantities of aerosol are deposited on the
collection plate of a given stage, it is possible that portions of the
collected material could be re-entrained by the air stream and be
subsequently re-deposited on lower stages. Under such circum-
stances, misleading size distribution data would be obtained.
The tendency of an impactor stage to be susceptible to
re-entrainment problems can be tested experimentally by subjecting
the stage to a known aerosol and studying the relationship between
the mass of material sampled and the collection efficiency. Should
re-entrainment occur, the efficiency would show an apparent drop
as the mass loading is increased. Tests of this type have been
-------
35
conducted with a standard Andersen non-viable sampler by
(20)
McGregor. His results, when applied to the upper stages
of the low pressure impactor system, indicate that the permissible
mass loadings of Stage A woxild be approximately 2 mg, that of
Stage C would be about 13 mg and that of Stage D should be approxi-
mately 8 mg. Loadings above these values do not show abrupt
re-entrainment effects, but rather a gradual decrease in efficiency
of the stage. In addition for the stages with smaller cutpoints, the
loadings limitation is more pronounced as the jet diameter is
decreased and the velocity increased. Since Stage LP-3 has both
the highest velocity and the smallest jet sizes of the low pressure
stages, it was selected for an investigation of the loading charac-
teristics. It was assumed that if the loading limitations of stage
LP-3 were acceptable, so would be those of stages LP-1 and LP-2.
The tests -were conducted by exposing stage LP-3 for varying
times to a dye aerosol generated by the nebulizer. After each run,
the efficiency of the stage as well as the total quantity of uranine
aerosol collected by the imp-actor was determined for each test.
The results, shown in Figure 10 demonstrate that the efficiency of
LP-3
stage/ is constant up to a sample load of over 10 mg, indicating
that overloading is not a problem.
-------
100
Ssok, « ^
or w O
UJ
a.
O 60
z
UJ
O
u_
U_
UJ 40
o
^ 20
O
o
| | _J I 1114 | | 1 | | 111^
0.2 0.5 1.0 2.0 5.0 IQ.O
MASS LOADING , mg
FIGURE 10 - LOADING CURVE FOR STAGE LP-3(0.05Mm OUTPOINT) OF LOW w
PRESSURE IMPACTOR ^
-------
37
FIELD TESTS
The Low Pressure Impaetor system was setup to simultan-
eously sample aerosol in parallel with a standard Andersen non-
viable unit. For the first experiments the two devices were exposed
to a well-mixed and diluted cigarette smoke. The mass collected
on the various stages of both impactors was determined through
measurement of the weight change of the glass fiber collection
substrates using a semi-micro analytical balance. All collection
substrates were conditioned for several hours to the laboratory
environment before the weight measurements were made. The
resulting data, which has been converted to cumulative distributions,
is shown in Figure 11. In this case the particle size parameter
represents that of eqxiivalent spheres of unit density. It may be
noted that there is good agreement between the data obtained from
use of the two devices for sizes larger than approximately 0.6
micrometers. Below this size the Low Pressure Impaetor tends
to show a greater relative abundance of small particles.
With respect to the mass of aerosol collected by each unit,
the sum of all differential •weights for the Low Pressure Impaetor
was 78.9 mg whereas that of the standard non-viable sampler was
79. 5 mg.
Both units were exposed to atmospheric aerosol for times
sufficient to collect several tens of milligrams in each unit. The
-------
o.
Q
VI
V)
LU
CO
X
Q
LU
I-
<
O
O
CO
CO
CO
CO
CD
LJ
O
(T
Ld
Q_
96
95
90
70
50
30
O STANDARD ANDERSEN NON-VIABLE SAMPLER
A LOW PRESSURE IMPACTOR (LPI)
PARTICLE DENSITY = I gm/cm3
10
0.05 O.I 0.5 1.0
PARTICLE DIAMETER, MICROMETERS ( jim)
FIGURE II- SIZE DISTRIBUTION OF CIGARETTE SMOKE
5.0
U)
00
-------
39
purpose of collecting these substantial quantities of mass was to
minimize any errors associated with the process of measuring
differential weights of the substrates. Results for two separate
atmospheric aerosol runs are represented in Figures 12 and 13.
In both runs the Low Pressure Impactor yielded data which indicates
greater percentages of particles of size less than 0. 6 micrometers.
Additionally, the data presented in Figure 12 shows that the Low
Pressure Impactor collected substantially greater fractions of the
very large particles. Nineteen percent of the aerosol mass was
associated with sizes larger than 16 micrometers. With reference
to Figure 13, it would appear that the Low Pressure Impactor
collected less material of large sizes than did the standard non-
viable unit. However, the cumulative distribution curve is misleading
for this test since it presents percentages rather than actual mass
values. The raw data showed nearly identical quantities of large
particles 'were collected by the two devices (for example the total
mass of all particles larger than 7 micrometers in size collected by
the Low Pressure Impactor was 3. 79 mg whereas that of the standard
non-viable unit was 4. 13 mg). But, the Low Pressure Impactor
System collected more total mass (41. 99 mg versus 31. 04 mg)
therefore the cumulative distribution of the Low Pressure Impactor
is shifted to the left.
-------
Q.
Q
VI
CO 90
UJ
N
CO
Q 70
LJ
h-
O
O
CO
CO
CO
CO
30
m
LU
DC 10
LU
O
A
STANDARD ANDERSEN NON -VIABLE SAMPLER
LOW PRESSURE IMPACTOR (LPI)
ASSUMED PARTICLE DENSITY
= |grn/cm5
O.i
0.5 1.0 5.0
PARTICLE DIAMETER, Dp, MICROMETERS
FIGURE 12 -SIZE DISTRIBUTION OF ATMOSPHERIC AEROSOL
JUNE 30 - JULY 20 , 1973 AT URBANA, ILLINOIS
10.0
20.0
-------
ex
Q
VI
-------
42
In addition to determinations of the aerosol size distribution,
comparative values of average aerosol concentrations were calcu-
lated. These results, presented in the following table, show that
Average Concentrations
Test Std. Non-Viable Unit Low Pressure Impactor
June 30 - July 20 39.0 ^g/m3 86.2 ^g/m3
July 21 - Aug. 15 30. 3 42. 9
the Low Pressure Impactor yields higher values of mass concen-
tration. This is due, at least in part, to the better collection charac-
teristics of the Low Pressure Impactor for large particles.
-------
43
SUMMARY AND CONCLUSIONS
Although the cascade impactor has been a useful tool in the
measurement of aerosol mass-size distribution, most systems in
current use are limited to a usable lower particle size limit of
approximately 0.4 micrometers. However, operation of specially
designed impactors at reduced pressures can extend this lower limit.
In the present study a Low pressure Impactor system has been de-
signed, fabricated and tested which has a particle cutpoint size for
the last stage of 0.05 micrometers for particles of density = 2 gm/cm3.
The impactor has four stages which operate at atmospheric
pressure and separate particles into fractions with size ranges of
>9.7, 5.0-9.7, 2. 46-5. 0 and 1. 21-2. 46 micrometers. These are
followed by three stages and an after-filter which separate the aersol
into size intervals of 0. 36-1. 21, 0.14-0.36, 0. 05-0. 36 and <.. 0. 05
micrometers.
Laboratory testing was performed to verify the predicted
cutpoints and to evaluate the performance-limiting characteristics
of a) wall losses b) particle rebound from collection surface and
c) collected deposit re-entrainment (mass loading limitation). The
results of these experiments confirmed the predicted cutpoints of
the stages which were tested. Data points for the tests are shown
in Figure 8 superimposed upon a curve which represents the calculated
cutpoint values.
-------
44
Wall losses for the entire system were shown to be less than
6 percent when tested with aerosols of 0. 3 and 0. 6 micrometers
median diameter which had geometric standard deviations of 3.
Initial tests with the upper stages indicated that substantial wall losses
resulted when the stages were used to sample large particles. A
re-design of the first two stages reduced these losses by 2/3; the total
loss for these stages, when tested with 6. 1 micrometer diameter
aerosol, is now 10. 7 percent.
Tests with the last stage of the low pressure section, a stage
which has a normal jet velocity of 4160 cm/sec, indicate that particle
rebound from the collection surface is not a significant problem. In
these tests, the jet velocity was increased to over 7000 cm/sec while
the other impaction parameters were adjusted in such a manner that
the predicted collection efficiency with the test aerosol would remain
nearly constant. Even at this high value of velocity, there was no
reduction in the efficiency of the stage which demonstrates that a
substantial fraction of the particulate matter did not bounce off of
the collection media during the impaction process.
The last impaction stage of the low pressure section has design
parameters (high velocity and small jet sizes) which make it the most
vulnerable of the low pressure stages to re-entrainment of collected
deposits. This-phenomenon is observed to occur in impactors when
substantial deposits are collected on a given stage. Portions of the
-------
45
deposits are subsequently eroded away during further sampling. Tests
with the last low pressure stage showed no tendency for re-enlrainment
for deposits as large as 10 mg.
The Low Pressure. Impactor system was operated in parallel
with a standard Andersen non-viable cascade impactor and used to
sample cigarette smoke and atmospheric aerosol. For cigarette
smoke, which lias tew large particles, the agreement between data
obtained from the two devices was excellent for particles larger than
0.6 micrometers. The Low Pressure Impactor showed a greater mass
fraction for the smaller sizes. Tests with atmospheric aerosol
showed the Low Pressx;re Impactor to yield larger fractions of small
( -•-. 0. 6 micrometer) particles and higher overall values of aerosol
concentration. In one case the Low Pressure Impactor collected a
substantially greater qxiantity of larger (>7 micrometers) particles.
-------
46
REFERENCES
1. Landsberg, H. , "Physical Climatology. " Znd edition, Gray,
DuBois, Pennsylvania, pp. 317-326(1958).
2. Robinson, E. , "Effects of Air Pollution on Visibility." Air
Pollution, Chapter 11, Vol. 1, 2nd edition, A, C. Stern (ed. ),
Academic Press, New York, 349-400 (1968).
3. Holzworth, G. C. , "Some Effects of Air Pollution on Visibility
in and near Cities. " Air Over Cities Symposium, U. S. Dept.
of Health, Education and Welfare, Robert A. Taft Sanitary
Engineering Center, Cincinnati, Ohio, Technical Report
A62-5, 69-88 (1961).
4. Hudson, J. D. , "Present Position of the Corrosion Committee's
Field Tests on Atmospheric Corrosion (Unpainted Specimens)."
J. Iron Steel Institute, Vol. 148, 161-215(1943).
5. Czaja, A. T. , "Uber das Problem der Zementstaubwirkungen
auf Pflanzen. " Staub, Vol 22, 228-232, (1962).
6. Bohne, H. , "Schadlichkeit von Staub aus Zimentwerken fur
Waldbestande. " Allgem. Forstz, Vol. 18, 107-111(1963).
7. "Air Quality Criteria For Particulate Matter." U.S. Depart-
ment of Health, Education and Welfare, Public Health Service,
Environmental Health Service, National Air Pollution Control
Administration Publication No. AP-49, 129-144(1969).
8. "Deposition and Retention Models for Internal Dosimetry of the
Human Respiratory Tract." Task Group on Lung Dynamics
Health Physics, Vol. 12, 173-207(1966).
9. Findeisen, W. , "Uber das Absetzen Kleiner in der Luft
suspendierten Leilchen in der menschlichen Lunge bei der
Atmung." Arc. Ges. Physiol. , Vol. 236, 367-379 (1935).
10. May, K. R. , "The Cascade Impactor; An Instrument for
Sampling Coarse Aerosols," Journal of Scienctific Instruments,
Vol. 22, 187-195 (1945).
11. Ranz, W. E. and Wong, J. B. , "Impaction of Dust and Smoke
Particles on Surface and Body Collectors, " Industrial and
Engineering Chemistry, Vol. 44, 1371-1381 (1952).
-------
47
12. Andersen, A. A. , "A Sampler for Respiratory Health Hazard
Assessment," American Industrial Hygiene Association
Journal, Vol. 27, 11. 160-165, 1966.
13. Mueller, P.K., Helwig, H. L. , Alcocer, A. E. , Gong, W. K. ,
and Jones, E. E. , "Concentration of Fine Particles and Lead
in Car Exhaust." American Society for Testing and Materials,
Special Technical Publication 352, pp. 60-73, 1964.
14. Lee, R. E. , Jr., Patterson, R. K. , Crider, W. L. , and
Wagman, J. , "Concentration and Particle Size Distribution
of Particulate Emissions in Auto Exhaust,", Atm. Env. 5,
225-237 (1971).
15. Stern, S.C., Zeller, H. W. , Shekman, A. I. , "Collection
Efficiency of Jet Impactors at Reduced Pressures, " Industrial
and Engineering Chemistry Fundamentals, Vol. 1, No. 4
pp. 273-277, 1962.
16. McFarland, A. R. , Zeller, H. W. , "Study of a large volume
impactor for high altitude aerosol collection. " Report of the
Division of Technical Information Extension of the USAEC,
TID-18624, 1963.
17. Buchholz, H. , "An Underpressure Impactor." Staub-Reinhalt,
Luft, Vol. 30, No. 4, 1970.
18. Millikan, R. A. , "The general law of fall of a small spherical
body through a gas and its bearing upon the nature of molecular
reflection from surfaces." Phys. Rev. 22, 1-23 (1923).
19. Marple, V. A. , "A Fundamental Study of Inertial Impactors."
Ph.D. Thesis, Univ. of Minn. (1970).
20. McGregor, F. R. , "Development of a Modified Andersen
Impactor, " M. S. Thesis, Univ. of Notre Dame, (1971).
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-650/2-74-014
3. RECIPIENT'S ACCESSION«NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
October 1973
Development of a Low Pressure Impactor
6. PERFORMING ORGANIZATION CODE
T. AUTHOR(S)
A. R. McFarland,
H. S. Nye and C. H. Erickson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
Anderson 2000 Inc.
P. 0. Box 20769
Atlanta, Ga. 30320
1A101C
11. CONTRACT/GRANT NO.
68-02-0563
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
Natioanl Environmental Research Center
Research Triangle Park, N. C.
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A Low Pressure Impactor aerosol sampler was designed fabricated and tested. The
system injects a fixed aerosol flow rate of 1 cfm at inlet conditions and causes the
particulate matter to be separated and collected on four atmospheric pressure and
three reduced pressure impaction stages and an after-filter. Outpoint sizes of the
stages are 9.7, 5.0, 2.46, 1.21, 0.355, 0.141, and 0.05 micrometers for spherical
particles with a density of 2 gm/cm . Each of the impaction stages is fitted with a
glass fiber media collection substrate to facilitate gravimetric analysis of the
collected samples.
Experiments conducted with laboratory aerosols show the system to have wall losses
less than 6 percent when the mass median diameter of the aerosol is 0.6 micrometers.
For particles 6.1 microns in size, the wall losses on the upper stages are less than
11 percent. Both particle rebound and re-entrainment from the collection surfaces
are shown to be negligible. Each low pressure stage can be loaded with more than
10 mg of deposited aerosol without re-entrainment occurring.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
Aerosol Sampler
Low Pressure Impactor
Re-Entrainment
Particle Sizing
Sub-Mi cron
c. COSATI Held/Group
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
54
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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