EPA-600/2-76-118
April 1976
CASCADE IMPACTOR
CALIBRATION
GUIDELINES
by
Seymour Calvert, Charles Lake, and Richard Parker
A.P.T. , Inc.
4901 Morena Boulevard, Suite 402
San Diego, California 92117
Contract No. 68-02-1869
ROAPNo. 21ADJ-037
Program Element No. 1AB012
EPA Project Officer: Leslie E. Sparks
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
This report contains guidelines for routine calibration of
cascade impactors. The basic calibration technique discussed in the
report generating uniformly sized particles, testing individual stages,
determining particle number concentrations by light scattering, and
calculating efficiencies for given test parameters-. Each component
of the technique is discussed. The results of calibrations of three
cascade impactors and comparisons with published studies are presented.
11
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TABLE OF CONTENTS
Page
Introduction 1
2.0 Aerosol Generation 2
2.1 Calibration System 2
2.2 Particles 2
2.3 Particle Generator 4
3.0 Particle Concentration Measurement 8
3.1 Particle Counter 8
3.2 Counting Procedure 8
4.0 Calibration Procedure 11
4.1 General Considerations 11
4.2 Definitions 12
4.3 Particle Size Selection 13
4.4 Impactor Preparation 15
4.5 Measure Stage Pressure Drop 18
4.6 Particle Penetration 19
5.0 Results 22
5.1 Data Reduction 22
5.2 Typical Data 23
6.0 Discussion 29
6.1 Reproducibility 29
6.2 Accuracy 29
6.3 Stage Variations 30
6.4 Conclusions 30
7.0 References 34
8.0 Sample Calculations 35
111
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LIST OF FIGURES
No. Page
1 Calibration Apparatus 3
2 Collison Atomizer 5
3 Sampling System Arrangement 9
4 Aerodynamic cut diameter vs. impactor flow
for A.P.T. M-l cascade impactor 16
5 Aerodynamic diameter vs. diameter for
various densities 17
6 Efficiency vs. inertial impaction parameter
for A.P.T. M-l cascade impactor 24
7 Efficiency vs. inertial impaction parameter,
data from 4 separate U.W. Mark-Ill cascade
impactors (Stage 5) 25
8 Efficiency vs. inertial impaction parameter
for U.W. Mark-Ill cascade impactor,
Stages 4-7 26
9 Efficiency vs. inertial impaction parameter
for A.P.T. M-l cascade impactor, Stages 4-7 . 27
10 Efficiency vs. inertial impaction parameter
for Andersen non-viable cascade impactor,
Stages 4-7 28
11 Efficiency vs. inertial impaction parameter
for comparison 32
IV
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LIST OF TABLES
No. Page
1 Operating characteristics of a 3 Jet Collison
Atomizer 5
2 Available PSL particle diameters 14
3 Length/jet diameter (s/d ) for cascade
impactor 33
v
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1.0 INTRODUCTION
The following report has been prepared at the request
of the Particulate Technology Branch of the Environmental
Protection Agency (E.P.A.) Utilities and Industrial Power
Division. The contents are intended for use as a guideline
for experimental calibration of cascade impactors (C.I.)
and for Air Pollution Technology (A.P.T.) internal purposes.
The calibration process is based on approximately 4 years
of A.P.T. experience in connection with work for the E.P.A.
and other clients. In our experience, we have used University
of Washington Mark III, Andersen non-viable (not the in-stack
model), and A.P.T. M-l cascade impactors and have found that
calibration of these devices is necessary for accurate sizing
of particles in gas streams. Variation of hole diameter and
shape due to production machinery, corrosive chemical action,
and particle deposition can contribute to measurement inaccuracy
when using impactors. There are also discrepancies among the
published studies of inertial impaction devices. Many C.I.
manufacturers have not thoroughly calibrated their instru-
ments experimentally but use the experimental results of
Ranz and Wong (1952) even though Mercer and Stafford (1969) ,
and Stern, et al. (1962) report much different results. Con-
sequently, substantial uncertainties exist in the prediction
of C.I. performance from past studies and manufacturer's
specifications.
The basic calibration technique used by A.P.T. includes:
generating uniformly sized particles, testing individual stages,
determining particle number concentrations by light scat-
tering, and calculating efficiencies for given test
parameters.
The scope of this report covers the experimental
technique, results of calibrations of three C.I.s, and
comparisons with published studies.
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2.0 AEROSOL GENERATION
2.1 CALIBRATION SYSTEM
The calibration system (Figure 1) consists of an air-
liquid atomizer, aerosol drier, charge neutralizer, particle
counter, dilution air lines and appropriate metering and
flow equipment. Further details on the components are
included in succeeding sections.
2.2 PARTICLES
Monodisperse aerosols can be produced using suspensions
of polystyrene latex (PSL) spheres available from Dow
Chemical Corporation. Original suspensions as received are
concentrated, 10% by weight, and can be obtained with diam-
eters ranging from 0.087 to 2.0 microns (ym) with a standard
deviation of less than 0.01 ym, and a particle density of
1.05 g/cm3. Suspensions of PSL spheres larger than 2 ym
are available, however the standard deviations are much
larger and the suspensions are more polydisperse.
Particles of 0.5 ym to 2.0 ym diameter have provided a
sufficient size range for calibration of the lower 4 stages
of both U.W. Mark III and A.P.T. C.I.'s. This is also the
size range of most importance in fine particle control device
evaluation. Consequently, A.P.T. has been most concerned
with calibrations in this size range.
Useful suspensions of PSL can be made by diluting small
quantities of the original suspension with deionized water.
Any settling that occurs over a period of days can be
re-suspended by gentle agitation. The PSL is diluted to
a concentration sufficient to minimize the occurrence of
agglomerates. Dilutions of stock 10% solutions of PSL can
be estimated from a paper by Raabe (1969), however the
amount of dilution necessary actually depends on the
specific device used for spraying the hydrosol. Generally,
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To Counter
Filter
Pressure
Gauge
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Dilution
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Air
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Rotameter
Mixer Drying Tube
Atomizer
To Manometer and
Pressure Relief
Filter
Aerosol Dilution
Air Rotameter
Valve
> Figure 1. Calibration Apparatus
-------
the necessary dilution must be determined experimentally by
observing collected samples of dried aerosol with a micro-
scope. If doubles run more than 2 to 5%, the suspension
should be further diluted.
Concentrations of 0.004% to 0.03% (by weight) for
particles of 0.5 to 2.0 urn were found to be compatible
with the spray device in Figure 2.
2.3 PARTICLE GENERATOR
Drops containing PSL particles are produced from
suspensions with a Collison type atomizer as shown in
Figure 2. The operating range of this device is normally
170 to 350 kPa (10 to 50 psig) and operating character-
istics vary with each device. May (1973) has tabulated
some of the characteristics of a 3 jet model Collison
atomizer (Table 1).
Number concentrations and drop size distributions
produced by the atomizer are consistent for a given opera-
ting pressure, provided that the liquid level and concen-
tration are maintained. Evaporation losses will cause an
increase in the particle concentration and the number of
agglomerates. This has not been a problem for test periods
up to 3 hours in duration. For longer periods it would be
necessary to periodically sample and test for agglomerates,
and further dilute the PSL solution as necessary.
Drops leaving the atomizer are mixed with dry,
filtered air (approximately 45 &/min). This minimizes agglo-
meration of wet particles, dilutes the aerosol to a given
number concentration, and aids in drying out the wet particles
The aerosol is dried by passing it through a 1.2 m
(4 ft) section of a 3.8 cm (1.5 in) diameter glass tube
mounted horizontally with a layer of silica gel (~1.5 cm
deep) spread evenly along the bottom. Three "Staticmaster
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Compressed Air
Liquid
Drainag
0.159
cm ID
Liquid
Aerosol
Outlet
Baffle
Mason Jar
Figure 2. Collison Atomizer.
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2U500" 500 vie, Po210 alpha emitters (available from Nuclear
Products Company) are situated end to end at the "dry" end
of the glass tube to reduce the excess charge on particles
to the minimum level described by Boltzman's law. No
license is required to use these units.
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3.0 PARTICLE CONCENTRATION MEASUREMENT
3.1 PARTICLE COUNTER
Particle number concentrations are determined using a
Climet Cl 205 Particle Analyzer. Other commercially avail-
able instruments utilizing similar "electro-optical"
techniques are also satisfactory. The Climet device has
the capability of counting all particles with diameters
greater than a pre-set value (0.3, 0.5, 1, 3, 5, or 10 ym) .
Further discrimination can be achieved by using a potenti-
ometer to provide a continuous selection over the range
from 0.3 to 10 ym.
3.2 COUNTING PROCEDURE
The particle counter is used within a selected band of
particle diameters, centered about the known PSL diameter.
This reduces the effect of spurious counts resulting from
fine impurities and agglomerates. The particle count for
the larger diameter setting may be subtracted from that for the
smaller diameter setting to determine the number concen-
tration of particles within a desired size interval. It has
been our experience that spurious counts may still be a
problem within the size ranges available on the Cl 205.
Therefore, it is recommended that a potentiometer be used
to zero in as closely as possible to the actual PSL particle
size.
The maximum count allowed for the Cl 205 is 3.5 x 107 /m3
(106/ft3). The sample must be taken from a stream at or
very near ambient pressure. The C1 205 requires a flow
rate of 7 £/min.
The sampling inlet arrangement is illustrated in
Figure 3. A 4 mm OD tube is used for the inlet to the
particle counter. It is inserted a few millimeters into
the sampling tube (10 to 15 mm ID). Thus a sampling flow
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Sample
Counter
inlet tube
Excess
to vent
Particle
Counter
Figure 3. Sampling System Arrangement
-------
rate larger than 7 £/min is handled by letting the excess
flow exit through the annular space between the tubes.
This arrangement also ensures that the inlet flow to the
particle counter is at atmospheric pressure. Tygon tubing
and variable pinch clamps have been used satisfactorily
as throttles to control the sampling flow rate and the
flow rate through the impactor. They are shown as "A"
and "B" in Figure 1.
10
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4.0 CALIBRATION PROCEDURE
4.1 GENERAL CONSIDERATIONS
There are alternative approaches to calibration which
offer advantages and disadvantages in terms of the amount
of information gained versus the time, effort, convenience,
and simplicity. Before going into the details of the
calibration procedure, a few important principles will be
pointed out.
Determining the stage cut diameter is the primary
objective of the calibration. The computation of particle
size distribution can be based on stage cut diameters with
good accuracy, so long as particle bouncing on the upper
(larger cut diameter) stages is prevented. Therefore, the
calibration should concentrate on the particle size range
in the vicinity of the stage cut diameter. For all stages,
the inertial impaction parameter (defined in equation 1)
at the stage cut point has a value of approximately 0.2.
Therefore, it is suggested that the calibration of any
stage should cover an impaction parameter range of 0.1 to
0.3. These values are for round jet impactors.
The presentprocedure calibrates one impaction stage
at a time because it is simpler, in determining collection
efficiencies from inlet and outlet particle concentrations,
not to have to account for the contributions of two or
more stages in series. Interference from the upstream
plate, noted by Willeke and McFeters (1975), has been observed
mainly for large particles in rectangular jets. Very little
effect has been noted for small cut diameter stages with
round jets. In calibrating single stages, however, it is
necessary to ensure that the flow pattern is very nearly
the same as in actual operation. Therefore, the present
procedure requires an impingement plate to be placed
upstream of the jet plate being calibrated. This arrange-
ment is shown in Figure 1. Further experiments are
11
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presently under way to determine the influence of one or
more upstream impactor stages.
Some question has arisen as the expansion effects on
hole diameter under hot and cold flow conditions. The most
practical method for resolving this problem is to calibrate
the impactor at various temperatures.
4.2 DEFINITIONS
The inertial impaction parameter is defined as follows
d2 C1 p u d2
P P pa u lfr<,
K = = xiu (1)
P 9 yr d 9 yr d
u C u C
where K = inertial impaction parameter, dimensionless
C' = Cunningham slip correction factor =
1 + 4^ 1.257 + 0.40 exp (-1.10 d /2X)|
ap P J
X = mean free path of gas molecules, cm
d = aerodynamic particle diameter, ymA
Pa
p = particle density, g/cm
u = gas (particle) velocity through jet, cm/sec
Up = gas viscosity, poise, g/cm-sec
d = jet diameter, cm
c J '
ymA = ym (g/cm3)lz
Aerodynamic diameter is defined as:
dpa = dp (C'Pp) ^ 10"' ymA (2)
For the case where the stage is 50% efficient (i.e.
the cut point) , the following parameters are substituted
into equation (1).
K = inertial impaction cut parameter; K at 50%
PSO efficiency P
d = cut diameter or diameter (d ) at which stage
pc is 50% efficient p
d = aerodynamic cut diameter
pea
12
-------
Thus ,
dnr C' pu U dnra U
K = _££ E = J^a x 1Q a (-3-)
0.135 TT y d' N K
dc 9 ^G dc
4.3 PARTICLE SIZE SELECTION
.1 Obtain Theoretical Impactor Curves
Theoretical curves of cut diameter versus flow rate
through the impactor are usually provided by the manu-
facturers of commercially available cascade impactors.
These curves are generally based on some value of the inertial
impaction parameter at the cut-point (i.e. K in equation
(3) ). If these curves are not available, they may be
approximated using equation (3) written in the form
(4)
where Q = total flow rate,
N = number of jets or holes in an impaction stage
A good approximation may be obtained using K -0.2.
p 5 o
Curves of the aerodynamic cut diameter versus flow rate
for the seven-stage A.P.T. M-l cascade impactor are presented
in Figure 4 and are based on K =0.2.
Pso
.2 Determine PSL Particle Diameters and Flow Rates
for the Calibration
The PSL particle diameters available commercially are
listed in Table 2. The standard deviation is very large
for particles greater than about 2 ym. Therefore, 2 ym is
the largest size PSL particle suitable for use as a standard
aerosol for calibration.
13
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Table 2. AVAILABLE PSL PARTICLE DIAMETERS
(Source: Dow Chemical Company)
Avg. Diam.
In Microns
.087
.091
.109
.176
.234
.255
.312
.357
.364
.460
.481
.500
.527
.600
.721
.760
.794
.801
.804
.807
.822
1.011
1.099
1.101
2.020
5.7
15.8
One Std. Dev.
In Microns
.0046
.0058
.0027
.0023
.0026
.0022
.0022
.0056
.0024
.0048
.0018
.0027
.0125
.0030
.0057
.0046
.0044
.0035
.0048
.0056
.0043
.0054
.0059
.0055
.0135
1.5
5.8
Material
Styrene -Butadiene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Styrene -Butadiene
Polystyrene
Polystyrene
Styrene -Butadiene
Polystyrene
Polystyrene
Polystyrene
Styrene -Butadiene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polystyrene
Polyvinyl toluene
Styrene Divinylbenzene
Polystyrene
Density
g/ml
= 1.05
unless
other-
wise
noted
0.99
1.027
14
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The aerodynamic diameter corresponding to a 2.02 ym
diameter PSL particle can be calculated from equation (2)
and is equal to 2.13 ymA. From Figure 4, it can be seen
that for an aerodynamic cut diameter of 2.13 ymA, a mini-
mum flow rate of 20 &/min is required for Stage 4. There-
fore, 20 &/min is a convenient flow rate to use. From
Figure 4 it can be seen that a flow rate of 20 £/min
corresponds to aerodynamic cut diameters of about 1.1, 0.6,
and 0.4 ymA, for Stages 5, 6, and 7 respectively. The
most suitable PSL particle diameters for each stage can be
obtained from Table 2. Figure 5 is a convenient plot (from
Calvert et al. (1972) ) for the conversion between aerodynamic
and physical diameters.
For example, assume that 0.5 ym diameter PSL particles
are being used to calibrate Stage 7 of the A.P.T. M-l impactor,
From Figure 5, the corresponding aerodynamic diameter for a
particle density of 1.05 g/cm3 is about 0.58 ymA. The flow
rate required for a cut diameter of 0.58 ymA is obtained
from Figure 4 and equals about 8.5 £/min. This cut diameter
is for K = 0.2. Using equation (2) the flow rates
PSO
required for K =0.1 and K =0.3 can be calculated
Pso Pso
to be 4.3 £/min and 12.7 &/min respectively.
4.4 IMPACTOR PREPARATION
.1 Inspect and Clean Impaction Plates
Inspect the jet orifices with a microscope to ensure
that they are clean and round. If necessary, clear the
orifices with a wire. Clean the jet and impaction plates
in an ultrasonic bath with a detergent solution. Rinse
first with distilled water, then with acetone. Re-inspect
the jet orifices and repeat cleaning if necessary. Some
jets may be irregularly shaped because of poor manufacturing.
Such jets may be calibrated as they are, or returned to the
manufacturer.
15
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Convenient particle
diameter selection
for calibration
0.1
1.0
10
ACTUAL FLOW, i/min
50
Figure 4. Aerodynamic cut diameter vs. im-
pactor flow for A.P.T. M-l cascade
impactor.
16
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10.0
3.
r>
Pi
§
ii
Q
U
U
Ii
S
>H
Q
O
0.1
o-1 i.o 10.0
PARTICLE DIAMETER, d
Figure 5. Aerodynamic diameter vs. diameter for
various densities.
17
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.2 Grease Impaction Plates
Apply grease to the impaction plates or foil sub-
strates in the same manner as for normal laboratory or
field use of the cascade impactor. A.P.T. uses Dow Corning
high vacuum silicone grease or equivalent. If fibrous
substrates are being evaluated, they should not be greased.
Because the particulate concentration will be counted
upstream and downstream of the impaction plate, it is not
necessary to perform a gravimetric analysis of the sub-
strate and collected particulate. However, a gravimetric
analysis is suggested as a check on the particulate mass
balance of the system. If a mass balance check is being
conducted, it is necessary to record the flow rate and
time for the duration of the test. This will enable the
prediction of the total particulate mass entering and
leaving the system. The difference can then be compared
to the mass collected on the impaction plate.
.3 Assemble Impaction Stage
Place the impaction stage in the calibration tube as
shown in Figure 1. To provide the proper flow pattern,
place an impaction plate upstream and downstream of the
jet plate. The regular cascade impactor casing can be
used as the calibration tube if tubular spacers are made
to provide proper alignment and seals between the components.
Alternatively a special calibration tube could be constructed,
4.5 MEASURE STAGE PRESSURE DROP
.1 Measure Pressure Drop as a Function of Flow Rate
This provides data which enable the use of the stage
as an orifice flow meter during the calibration. It also
provides a means for checking the jet orifice size by
comparing the pressure drop flow rate data against data
18
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for a known jet plate. This is a useful check both in the
laboratory and in the field.
Measure the flow rate with a calibrated rotameter.
The rotameter should be calibrated against a wet test
meter every six months or whenever it is in disagreement
with secondary flow measurements (for example the pressure
drop measurements).
Measure the pressure drop across the impaction stage
with an open end manometer attached to the upstream side
of the impactor casing and downstream of throttle valve
"B" in Figure 1.
.2 Plot Pressure Drop Against Flow Rate
Pressure drop is conveniently plotted against flow
rate on log-log paper to give a straight line relationship.
A typical impaction plate will have a pressure drop directly
proportional to the square of the flow rate. For example,
Stage 5 of a U.W. M-III impactor has a flow resistance
which follows the relationship
AP (cm W.C.) = 0.2 (Q)2 (5)
4.6 PARTICLE PENETRATION
.1 Select Dilution Air Flow Rate
Select the dilution air flow rate necessary to dry the
aerosol and also to bring it to the desired particle con-
centration range for counting. The general flow rate range
is given in section 2.0 above.
.2 Start Dilution Flow to the Dryer
After checking the system to be sure that valves are
open and closed as required to allow the flow to pass
through the impactor stage, start the dilution air flow
to the aerosol dryer. Use the pressure regulator, throttle
19
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valves, and rotameter to adjust the flow rate.
.3 Load and Start Atomizer
Load the atomizer with about 500 ml of PSL suspension
diluted from roughly 0.5 ml of concentrated latex. Start
the atomizer, controlling the flow rate by adjusting the
pressure regulator and the pressure gage. The atomizer
flow resistance should be checked periodically by passing
air from a dry atomizer (no liquid present) through a flow
meter to detect any nozzle plugging.
.4 Adjust Flow Rate
Adjust the upstream sampling throttle "A" so that the
pressure drop across the impactor is proper for the desired
impactor flow rate, "Q". Keep throttle "B" open as much
as possible. Set the outlet dilution air flow rate to
provide more than 7 £/min total flow into the particle
counter.
When flow or concentration changes are made, approx-
imately five system volume change time intervals should
elapse before any data are taken to allow steady state
conditions to be reached. This can take several minutes
for some low flow rate impactors (e.g. Brink impactors).
.5 Measure Particle Concentration
Warm up the circuits of the particle counter for
several minutes as recommended by the manufacturer. Particle
counting should be done on the potentiometer setting as
close as possible to the PSL particle diameter. To check
the extent of agglomeration, it is helpful to use the next
highest channel also. In general, record counts in all
three channels, below, above, and at the PSL diameter.
The data recorded should include the following:
a. Stage identification
b. Particle identification parameters
c. Particle suspension (as used) specifications
20
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d. Barometric pressure
e. All rotameter readings
£. All pressure readings
g. Air temperature at impactor inlet
h. Particle counts on 2 or 3 channels for inlet and
outlet of impactor.
Make counts over the complete range of air flow rate
going both up and down. Plot the data obtained in the
simplest meaningful form so they can be checked for con-
sistency. Computation methods as discussed in Section 5.0
are used to obtain particle penetration. A plot of pene-
tration versus impactor pressure drop requires the least
computation and serves the purpose. Inspect the plot for
scatter of data and compare with the anticipated curve.
If it is unsatisfactory, make any worthwhile modifications
and repeat the run.
At this point it is advisable to inspect the impaction
plate by eye and with a microscope. A visual examination
can show whether the plate is overloaded. Microscopic
examination of light deposits enables the detection of
spurious particles.
Continue taking and plotting data until satisfied
that reproducible data have been obtained.
21
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5.0 RESULTS
5.1 DATA REDUCTION
.1 Compute inlet and outlet particle concentrations
by subtracting the concentration measured on the high
diameter channel from that measured on the lower channel.
For example, if counting 0.5 ym diameter particles, subtract
the concentration counted on the 1.0 ym channel from that
counted on the 0.3 ym channel (see sample calculation,
section 8.4). The net concentration should be equal to the
concentration measured at the potentiometer setting closest
to the PSL diameter. If this is not the case, spurious
counts may be a problem, and the concentration measured
closest to the PSL diameter should be used. If possible,
use the potentiometer to narrow the band around the PSL
diameter until no spurious counts are detected.
.2 Adjust outlet concentrations to the same (undiluted)
basis as the inlet concentration in cases where dilution
of the outlet sample has been necessary.
.3 Compute particle penetration, Pt, as the ratio of
outlet to inlet particle concentrations (undiluted basis).
Particle collection efficiency in fractional form is (1-Pt),
and in percentage it is 100 times the fractional efficiency.
.4 Obtain the impactor gas flow rate from the pre-
viously determined plot of pressure drop versus flow rate,
using the measured pressure drop for each penetration data
pair.
.5 Compute inertial impaction parameter from the
measured air flow rate and properties, particle properties,
jet hole size and number of holes by means of equation (1).
Jet velocity may be computed from the hole diameter, number
of holes, and air flow rate.
.6 Results can be plotted in several ways, depending
on their desired use. For checking the data quickly during
the calibration procedure, a plot of penetration versus
22
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impactor pressure drop is convenient. A plot of efficiency
versus impaction parameter is more useful for interpretative
purposes.
5.2 TYPICAL DATA
.1 Particle collection efficiency data points for
a single calibration run on Stage 7 from an A.P.T. M-l
impactor are plotted against impaction parameter in
Figure 6. The curve in Figure 6 represents the composite
of data points for several Stage 7 runs.
.2 Data for four separate U.W. M-III Stage 5 plates
are plotted in Figure 7.
.3 Figures 8 through 10 show the results of calibrations
of the A.P.T. M-l, U.W. M-III, and Andersen non-viable
cascade impactors in terms of collection efficiency versus
impaction parameter for each stage. The curves represent
the composites of several runs on different plates. Data
points are omitted for clarity.
23
-------
100
90
80
70
£40
w
PH
W
20
10
Composite for
several runs
0.10 0.20 0.30 0.40
INERTIAL IMPACTION PARAMETER, K
0.50
Figure 6. Efficiency vs. inertial impaction parameter
for A.P.T. M-l cascade impactor, stage 7
24
-------
0.1 0.2 0.3 0.4
INERTIAL IMPACTION PARAMETER, K
0.5
Figure 7 . Efficiency vs. inertial impaction parameter,
data from 4 separate U.W. Mark III cascade
impactors (stage 5).
25
-------
100
0.1 0.2 0.3 0.4
INERTIAL IMPACTION PARAMETER, K
0.5
Figure g. Efficiency vs. inertial impaction parameter
for U.W. Mark Ulcascade impactor, stages 4-7
26
-------
10
0.1 0.2 0.3 0.4
INERTIAL IMPACTION PARAMETER, K
f ~r\
0.5
Figure 9. Efficiency vs. inertial impaction parameter
for A.P.T. M-l cascade impactor, stages 4-7
27
-------
0.1 0.2 0.3 0.4
INITIAL IMPACTION PARAMETER, K
0.5
Figure 10. Efficiency vs. inertial impaction parameter
for Andersen non-viable cascade impactor,
stages 4-7.
28
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6.0 DISCUSSION
6.1 REPRODUCIBILITY
The data obtained with the procedure described above
have been quite reproducible, as illustrated by Figure 6.
Generally the data for a single stage fall within a band
width of inertial impactor parameter values ranging about
0.02 or 0.03. Thus, the scatter about a cut parameter is
about ħ8%. The corresponding scatter of cut diameter
values would be ħ4%. Similarly, a variation of ħ10% in
impaction parameter corresponds to a variation of about
ħ5% in particle cut diameter.
6.2 ACCURACY
One way to estimate the accuracy of the impactor
calibration is to compare the results with published
theory and experimental data. Figure 11 is an efficiency
plot which compares the averaged results for all four
stages of each impactor with a few published experimental
and theoretical results. It can be seen that the overall
average cut parameters are within a spread of about 0.03
(signifying about 8% spread of cut diameters) for every-
thing but the Ranz and Wong experimental data.
Curves "C" and "D" show the effect of jet length to
diameter ratio, s/d , ranging from 3 for "C" to 10 for "D",
from Mercer and Stafford's experimental data. The jet
length/diameter ratios for the impactor stages whose
calibrations are reported here are presented in Table 3.
It can be seen that the ratios range between 2.4 and 12.5,
with the A.P.T. M-l having the smallest variation.
Another factor whose effect is related to that of
(s/d ) is the shape of the orifice in the jet plate. Of
29
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the three impactors calibrated, only the A.P.T. M-l has
converging orifices in the jet plates. For the impactors
with cylindrical orifices the jet diameter and velocity
depend on jet plate thickness as well as (s/d ). While
detailed discussion of impactor design and theory is
beyond the scope of this report, it is important to note
that such factors do influence the performance of the
impactor and should be recognized when comparing
experimental and theoretical results.
6.3 STAGE VARIATIONS
Turning back to Figures 8, 9, and 10, it can be seen
that the cut characteristics are generally "sharp" (i.e.
efficiency rises steeply over a small impaction parameter
range). However, the fourth stages for both the A.P.T.
and the U.W. impactors show a pronounced decrease in curve
slope above the cut point. This suggests that particle
bounce may occur at higher velocities for these stages.
The variation of cut parameter values is least for
the A.P.T. stages and greatest for the Andersen. This is
believed to be due in part to the close control of (s/d )
v>
and the use of converging orifices in the A.P.T. M-l. It
was also noted that some of the jet holes in the Andersen
plates tested were not round, but roughly triangular.
Such non-uniformity of the jet holes could be responsible
for variations in cut parameter between impactor stages.
6.4 CONCLUSIONS
The procedure outlined in this report provides a simple
technique by which inertial impaction devices may be cali-
brated. Such factors as loading, particle bounce, wall
losses, electrostatic and condensation effects, which may
occur during source testing, are evaluated by this technique
30
-------
to some degree. The calibration method, therefore, pro-
vides a reliable, intrinsic efficiency and is applicable
to laboratory and field data obtained by careful C.I.
operation.
31
-------
100
o
o
>-
u
S
w
I-H
CJ
II
PL,
PL,
PU
*A. Ranz § Wong, theoretical,
s/d > 1.0
Wong, experimental,
3.0
* C. Mercer $ Stafford, s/d =3.0
Mercer g Stafford, s/d ^ 10.0
Stern, Zeller, Sheckman.s/d
A.P.T. M-l
U.W. Mark III
H. Andersen non-viable
0.1 0.2 0.3 0.4
INITIAL IMPACTION PARAMETER, K
0.5
0.6
Figure 11. Efficiency vs. inertial impaction parameter for
comparison.
32
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TABLE 3:
LENGTH/JET DIAMETER (s/d )
FOR CASCADE IMPACTORS
Impactor
A.P.T. M-l
U.W.Mark III
(New)
Andersen
(Non-Viable)
Stage - s/dc
4
3.1
4.0
4.7
5
2.4
6.2
7.3
6
3.5
9.2
9.8
7
4.6
12.5
9.8
33
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7.0 REFERENCES
May, K.R. The Collison Nebulizer: Description, Performance
and Application. Aerosol Science, Vol. 4. P. 235-243. 1973.
Calvert, S., Goldshmid, J., Leith, D., and Mehta, D. Scrub-
ber Handbook. Environmental Protection Agency, Contract No.
CPA-70-95. Vol. I. P. 4-148. 1972.
Ranz, W.E., Wong, J.B. Impaction of Dust and Smoke Particles
Industrial Engineering Chemistry. P.44, 1371. 1952.
Stern, S.C., Zeller, H.W., and Schekman, A.I. Collection
Efficiency of Jet Impactors at Reduced Pressures. Indus-
trial and Engineering Fundamentals. Vol. 1, No. 4. P. 273.
1962.
Mercer, T.T., Stafford, R.G. Impaction from Round Jets.
Ann. Occupational Hygiene. Vol. 12. P. 41-48. 1969.
Willeke, K. and McFeters, J.J., The Influence of Flow
Entry and Collecting Surface on the Impaction Efficiency
of Inertial Impactors, J. Colloid and Interface Sci.,
5_3, 121 (1975).
Raabe, O.G., Generation and Characterization of Aerosols,
from Inhalation Carcinogenesis, Proc. of the Biology
Division, Oak Ridge Nat. Laboratory Conf., Gatlinburg,
Tennessee, October 8-11, 1969. (reference from personal
communication from J. McCain, Southern Research Institute).
34
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8.0 SAMPLE CALCULATIONS
8.1 AERODYNAMIC DIAMETER:
PSL Diameter - 0.5 ym
Particle density, p = 1.05 g/cm3
Cunningham slip correction factor corresponding to a
diameter of 0.5 ym, is 1.33
d = d / C'p \ = 0.5 | (1.33)(1.05)1 = 0-59 ymA
P P \ P / \ /
(The use of Figure 4 yields a value of about 0.58 ymA)
8.2 GAS VELOCITY THROUGH JET:
Number of jets, N = 110
Diameter of jet, d = 0.0343 cm
Sample flow rate at conditions of operation,
Q = 20 £/min
4Q 4(20) (1000)
u = = = 3.28 x 103 cm/sec
Nirdc2 (110) (TT) (0.0343)2(60)
8.3 INERTIAL IMPACTION PARAMETER:
Gas velocity at conditions of jet, u = 3.28 x 103 cm/sec
Viscosity of Gas at conditions of jet, y~ = 1.8 x 10"4 poise
Jet diameter, d£ = 0.0343 cm
Aerodynamic diameter = 0.58 yiuA
35
-------
pa _8 (0.58) (3.28x10 ) (10 )
K = x 10 = =0.20
P 9Pd 9(1.8xlO~")(0.0343)
8.4 CONCENTRATION MEASUREMENT
Consider PSL particle diameter = 0.5 ym
Impactor Inlet
Concentration on 1.0 ym channel = 0.01 x 106 cm"3
Concentration on 0.3 ym channel = 1.01 x 106 cm"3
Net concentration measured = (1.01 - 0.01) x 106 cm"3
= 1.00 x 106 cm -3
Concentration at 0.5 ym potentiometer
setting = 1.00 x 106 cm"3
Sample flow rate before dilution =3.5 £/min
Sample flow rate after dilution = 7.0 £/min
Actual concentration entering impactor,
7 0 0 /mi n 6-3 63
n = '.'" *-/min £1.00 x 10 cm ) = 2.00 x 10 cm
3.5 £/min
Impactor Outlet
Concentration on 1.0 ym channel = 0.001 x 106 cm"3
Concentration on 0.3 ym channel = 0.50 x 106 cm"3
Net concentration measured = 0.50 x 106 cm"3
Concentration at 0.5 ym potentiometer
setting = 0.50 x 106 cm"3
Sample flow rate before dilution = 3.5 Jl/min
Sample flow rate after dilution = 7.0 &/min
Actual concentration exiting impactor,
n = 7-° (0.50 x 106 cm'3) = 1.00 x 105 cm'3
3.5 &/min
36
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8.5 EFFICIENCY and PENETRATION:
Inlet concentration, n^ = 2 x 106 particles/cm 3
Outlet concentration, n = 1 x 106 particles/cm 3
Pt - -
iTT ~
* 1Q6 n c r
x 106 = *5ğ fraction
n = 1 - Pt = 0. 5
Percent efficiency = 0.5 x 100 = 50!
8.6 CUT DIAMETER FOR A GIVEN STAGE:
Inertial impaction parameter at 50% efficiency, KT^I-ri= 0.20
3
Jet gas velocity, u = 3.28 x 10 cm/sec
Viscosity of gas at conditions of jet, yr = 1.8 x 10~ poise
Jet diameter, d = 0.0343 cm
\~-
pea
K
p50
ux
9yG
(10
dc\
-*)
"(0
^
.20)
(3.
(9) (1.8x10 ""
28xl03) (1Q-8
)(0
)
.034
3)
-,1/2
=0.58
37
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-118
3. RECIPIENT'S ACCESSIOr*NO.
4. TITLE AND SUBTITLE
Cascade Impactor Calibration Guidelines
5. REPORT DATE
April 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Seymour Calvert, Charles Lake, and Richard Parker
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
A.P.T. , Inc.
4901 Morena Boulevard, Suite 402
San Diego, California 92117
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADJ-037
11. CONTRACT/GRANT NO.
68-02-1869
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT.AND PEfllOD COVERED
Guidelines; 9/75-1/76
14. SPONSORING AGENCY CODE
EPA-ORD
is.SUPPLEMENTARY NOTESproject officer for this document is L.E. Sparks, Mail Drop 61,
Ext 2925.
16. ABSTRACT
The report contains guidelines for routine calibration of cascade impactors.
The basic calibration technique discussed in the report involves generating uniformly
sized particles, testing individual stages, determining particle number concentra-
tions by light scattering, and calculating efficiencies for given test parameters. Each
component of the technique is discussed. The results of calibrations of three cas-
cade impactors and comparisons with published studies are presented.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Air Pollution
Measurement
Calibrating
Impactors
Air Pollution Control
Stationary Sourc.es
Cascade Impactors
13B
14B
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
43
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
38
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