United States Industrial Environmental Research EPA-600/7-79-065
Environmental Protection Laboratory February 1979
Agency Research Triangle Park NC 27711
Proceedings: Advances
in Particle Sampling
and Measurement
(Asheville, NC, May 1978)
Interagency
Energy/Environment
R&D Program Report
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EPA-600/7-79-065
February 1979
Proceedings: Advances in Particle
Sampling and Measurement
(Asheville, NC, May 1978)
W.B. Smith, Compiler
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
Contract No. 68-02-2131
Task No. 21306
Program Element No. INE623
EPA Project Officer: D. Bruce Harris
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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PREFACE
The Symposium on Advances in Particle Sampling and Measure-
ment held in Asheville, NC, May 15-17, 1978, was sponsored by
the Process Measurement Branch, Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency. D. Bruce
Harris, EPA, served as symposium chairman. Session chairmen
were A.R. MacFarland, Texas ASM University; Dale Lundgren, Uni-
versity of Florida; W.B. Kuykendal, EPA; T.T. Mercer, University
of Rochester; and G.B. Nichols, Southern Research Institute.
A list of reports on the research supported by the Process
Measurements Branch can be obtained from Mrs. Judy Ford, MD-62,
Process Measurements Branch, Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Research
Triangle Park, NC 27711,
111
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ABSTRACT
The proceedings consist of 17 papers on improved instruments
and techniques for sampling and measurement of particulate emis-
sions and aerosols. Cascade impactors are described with low-
pressure stages that extend the useful range of these sampling
devices down to 0.02 ym? their use in measuring particle size
distribution of fly ash from coal-fired boilers and in testing
a smoke suppressant in a gas turbine is described. A new empiri-
cal equation simplifies calibration of cyclone collectors used
for sampling aerosols. Diffusion battery-nuclei counter com-
binations can be used for automatic monitoring of particle-size
distribution and number concentration of ambient aerosols. Trans-
missometers and instruments for measuring scattered light were
compared over a one-year period for continuous monitoring of
stack emissions at a secondary lead smelter. Instruments for
measuring electrical charge transfer and beta radiation attenua-
tion were less reliable in this use. A laser light back-scattering
instrument was compared in pulp and paper mills against a trans-
missometer in continuous monitoring of emissions from a wood-
waste fired power boiler, a kraft recovery furnace, and a lime
kiln. Other papers discussed a computer-based, data reduction
method for plotting curves of particle size distribution; tech-
niques and equipment for generating monodisperse aerosols and
their use in calibrating instruments; guidelines in selecting
laboratory methods for particle sizing; minimizing weight changes
in impactor collection substrates due to heat and sulfur oxides;
spectroscopic methods for determination of trace elements in
fly ash and their valence states; and sampling errors due to
aerodynamic effects. Applications of improved sampling and
measurement techniques to the evaluation of emission control
devices include a wet impingement technique for measuring par-
ticle sizes in wet scrubbers; the use of real-time aerosol in-
struments in studying the dynamic behavior of baghouses; and
the measurement of fractional collection efficiency of electro-
static precipitators for particles 0.01-10 ym in diameter, en-
abling the detection of transient fluctuations such as rapping
reentrainment of fly ash.
IV
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CONTENTS
Preface . iii
Abstract iv
List of Speakers and Chairmen vii
Paper 1. Inertia Effects in Sampling Aerosols
C.N. Davies and M. Subari 1
Paper 2. Cyclone Sampler Performance
Morton Lippmann and Tai L. Chan 30
Paper 3. Research on Dust Sampling and Measurement in our
Laboratory
Koichi linoya 52
Paper 4. Sizing Submicron Particles With a Cascade
Impactor
Michael J. Pilat. 74
Paper 5. Experience in Sampling Urban Aerosols With the
Sinclair Diffusion Battery and Nucleus Counter
Earl O. Knutson and David Sinclair 98
Paper 6. Selecting Laboratory Methods for Particle
Sizing
Ronald G. Draftz 121
Paper 7. Long Term Field Evaluation of Continuous
Particulate Monitors
A.W. Gnyp, S.J.W. Price, C.C. St. Pierre, and D.S. Smith. . . 122
Paper 8. An In-Stack Fine Particle Size Spectrometer:
A Discussion of its Design and Development
Robert G. Knollenberg 169
Paper 9. Optical Measurements of Particulate Size in
Stationary Source Emissions
A.L. Wertheimer, M.N. Trainer, and W.H. Hart 195
Paper 10. Studies on Relating Plume Appearance to
Emission Rate and Continuous Particulate Mass
Emission Monitoring
K.T. Hood and H.S. Oglesby. 211
v
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Paper 11. A Data Reduction System for Cascade Impactors
J.D. McCain, G. Clinard, L.G. Felix, and J. Johnson 228
Paper. 12. Aerosol Generation and Calibration of
Instruments
David Y.H. Pui and Benjamin Y.H. Liu . 260
Paper 13. Substrate Collectors for Impactors -
An Evaluation
D. Bruce Harris, G. Clinard, L.G. Felix, G. Lacey,
and J.D. McCain „ . 293
Paper 14. Particle Size Measurement for the Evaluation
of Wet Scrubbers
Seymour Calvert, Richard Chmielewski, and Shui-Chow Yung. „ . 301
Paper 15. Evaluation of Performance and Particle Size
Dependent Efficiency of Baghouses
D.S. Ensor, R.Go Hooper, G. Markowski, and R.C. Carr. .... 314
Paper 16. Evaluation of the Efficiency of Electrostatic
Precipitators
Wallace B. Smith, John P. Gooch, Joseph D. McCain,
and James E. McCormack. .......... 337
Paper 17. Some Studies of Chemical Species in Fly Ash
L.D. Hulett, J.F. Emery, J.M, Dale, A.J. Weinberger,
H.W. Dunn, C. Feldman, E. Ricci, and J.O. Thomson 355
Summary. Advances in Particle Sampling and Measurement
William Farthing 372
Metric Conversion Factors ............ 380
VI
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SPEAKERS AND CHAIRMEN
Mr. Richard Chmielewski
APT, Incorporated
4901 Morena Boulevard
Suite 402
San Diego, CA 92117
Phone: 714/272-0050
Dr. C.N. Davies
Department of Chemistry
University of Essex
Wivenhoe Park
Colchester C04 3SQ
ENGLAND
Phone: Colchester 44144
(STD Code 020 6)
Mr. R.G. Draftz
IIT Research Institute
10 West 35th Street
Chicago, IL 60616
Phone: 312/567-4291
Dr. David S. Ensor
Meteorology Research, Inc.
464 West Woodbury Road
Altadena, CA 91009
Phone: 213/791-1901
Dr. Alec Gnyp
Department of Chemical
Engineering
University of Windsor
Windsor, Ontario N9B3P4
CANADA
Phone: 519/253-4232
Mr. D. Bruce Harris
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Mail Stop MD-62
Research Triangle Park, NC
27711
Phone: 919/541-2557-8
Mr. Kenneth T. Hood
NCASI, Engineering Experiment
Station
Oregon State University
Corvallis, OR 97331
Phone: 503/754-2015
Dr. Lester D. Hulett
Oak Ridge National Lab
4500 North E-10
Oak Ridge, TN 37830
Phone: 615/483-0155
Dr. Koichi linoya
Department of Chemical
Engineering
Kyoto University
Kyoto
JAPAN
Dr. Robert G- Knollenberg
Particle Measurement Systems
1855 South 57th Court
Boulder, CO 80301
Phone: 303/443-7100
VII
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Dr. Earl Knutson
Health & Safety Lab
U.S. ERDA
376 Hudson Street
New York, NY 10014
Phone: 212/620-3655
Mr. W.B. Kuykendal
Environmental Protection Agency
Industrial Environmental
Research Laboratory
Mail Stop MD-62
Research Triangle Park, NC
27711
Phone: 919/541-2557
Dr. Morton Lippmann
Institute of Environmental
Medicine
New York University Medical
Center
New York, NY 10016
Phone: 212/679-3200
Dr. Dale Lundgren
Environmental Engineering
Department
University of Florida
410 Black Hall
Gainesville, FL 32611
Phone: 904/392-0846
Mr. Joseph D. McCain
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, AL 35205
Phones 205/323-6592,
extension 278
Dr. Andrew R. McFarland
Civil Engineering Department
Texas A&M University
College Station, TX 77843
Phones 713/845-2241
Dr. Thomas T. Mercer
University of Rochester
Rochester, NY 14642
Phone: 716/275-3821
Mr. Grady B. Nichols
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, AL 35205
Phone: 205/323-6592,
extension 361
Dr. Michael Pilat
Air Resources Engineering
University of Washington
Seattle, WA 98195
Phones 206/543-4789
Dr. David Pui
Mechanical Engineering
Department
University of Minnesota
Minneapolis, MN 55455
Phone: 612/323-2815
Dr. Wallace B. Smith
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, AL 35205
Phone: 205/323-6592,
extension 520
Dr. Allen Wertheimer
Leeds & Northrup Company
Dickerson Road
North Wales, PA 19454
Phones 215/643-2000,
extension 493
viii
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PAPER 1
INERTIA EFFECTS IN SAMPLING AEROSOLS
C.N. DAVIES
M. SUBARI
DEPARTMENT OF CHEMISTRY
UNIVERSITY OF ESSEX
ABSTRACT
A study is made of the aspiration coefficient of orifices
sampling aerosols, starting with a point sink and proceeding
to a tube of finite diameter. A critical analysis is made of
previous work and new experimental results are presented for
sampling at 90° yaw with a sharp-edged tube.
INERTIALESS PARTICLES
The trajectories of particles suspended in moving air may
differ from the streamlines of air flow either if their rate
of fall due to gravity is appreciable, compared with the air
velocity, or if the particles are sufficiently heavy to persist
in their original direction to some extent when the air flow
changes direction (Albrecht,1 Sell 2). Measurements of the
concentration of particles, by sucking a known volume of aerosol
and assessing the particles in it, are liable to error because
of these gravitational and inertial effects.
Consider, first, inertialess particles. These can be de-
fined as having a stop-distance, ds, at the local air velocity,
u, which is small compared with the distance, &, from any bound-
ing solid surface. That is
d = TU « l (1)
s
where T is the relaxation time of a particle and is equal to
the mass divided by the aerodynamic drag at unit velocity. Such
particles can still fall at an appreciable velocity, this being
v = rg. (2)
o
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Inertialess particle systems therefore have small TU and
large Jl, but the limiting size of the Stokes number of the sys-
tems,
Stu = TU/& « 1 (3)
depends on the value of Stu, being lower for high values of vs/uf
hence both Stu and vs/u need to be considered in deciding whether
a particle is inertialess and, of course, since u varies from
point to point in a flow field the particle may be inertialess
while it is in some parts of the field but not so in others.
The sampling of inertialess particles with an isolated small
tube yields accurate results (Davies3). When the airflow ap-
proximates to ideal flow (potential or irrotational) the par-
ticles themselves trace out a potential flow pattern, the concen-
tration of particles is the same at all points along a particle^
trajectory and trajectories do not intersect (Walton1*; Robinson ;
Levin6) .
There are two reasons why this does not mean that a sampling
orifice will always take a perfect sample of inertialess par-
ticles.
Firstly, the velocity of the particles entering the orifice
may not be the same as the air velocity, V, in the orifice.
Suppose that the normal to the plane of the sampling orifice
makes an angle \p with the vertical. Then the velocity with which
particles enter the orifice, resolved along the normal is
V + v cos ty (4)
s
Hence the concentration, c, calculated from the catch of par-
ticles (including those stuck on the internal surface of the
orifice) and the volume of air sampled, is related to the true
concentration, c0, by
cV = c0 (V + vg cos 4>) (5)
The efficiency of sampling or the aspiration coefficient of the
orifice is then, for inertialess particles,
A = c/c0 = (V + vo cos 40/V (6)
S
Hence when 41 = 0 and the orifice faces upwards, A = 1 + vs/V
and when \p = IT, with the orifice facing downwards, A = 1 - vs/V.
Only when the plane of the orifice is vertical and ty = ir/2 is
a true sample obtained.
The other factor causing error in the sampling of inertia-
less particles is the existence of dust shadows. These regions
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free from particles exist below and downwind of objects placed
in an aerosol. The effects are negligible in the case of a small
tube, but a bulky sampling head creates a shadow which sometimes
reduces the aspiration coefficient of its sampling orifice.
The writer has described the general effects (Davies7»8'9).
When the orifice in an isolated head faces upwards it always
has A > 1; at low rates of suction in calm air a dust shadow
exists below the head and particles settle upon the top, around
the orifice, with increasing aspiration rate the shadow region
climbs up over the top of the head so that particles settle only
upon a decreasing area around the orifice.
With an orifice in the under side of the head, in calm air,
no particles enter until a certain rate of aspiration is attained;
increasing above this value gradually raises the value of A until
it reaches unity at an infinite rate of suction. As A increases
there is no change in the deposition of particles on top of the
head. In a cross wind particles enter the head at a lower suc-
tion rate than is the case for calm air. The value of A remains
higher than the value for calm air, as suction is increased,
but the difference decreases until they become the same at a
fairly high rate of suction.
Dust shadows can reduce A when the orifice is not isolated,
even when its plane is vertical, if it is in the lee of an ob-
stacle; they may also affect the sampling of particles with
inertia (Davies ).
Since the concentration of inertialess particles is constant
along all trajectories, the values of A which are less than unity
result not from the aspiration of reduced concentrations but
because some of the particle trajectories which enter the orifice
originate from boundaries inside dust shadows and therefore have
zero concentration at all points along their lengths. This was
shown clearly by Walton1* who considered tubes of flow of inertia-
less particles, particularly in elutriators.
PARTICLES WITH INERTIA; POINT SINK
The trajectories of particles with inertia do not form a
system with a velocity potential; the concentration may vary
along a trajectory and trajectories may intersect (Voloshchuk
and Levin11). A true sample can still be obtained, when par-
ticles exhibit unlimited inertia, if their rate of fall under
gravity is negligible and if the air movement is solely due to
aspiration into a point sink. The concentration builds up as
the orifice is approached, due to the particles lagging behind
the air flow, and the analogue of a space charge around the ori-
fice is formed. Particles enter the orifice with a reduced ve-
locity which compensates for the excess concentration and a true
sample with A = 1 results (Davies 12).
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If the symmetry of this "space charge" is sufficiently
destroyed by wind, or by the settlement of particles under grav-
ity, the aspiration coefficient of the sampling orifice is re-
duced.
Levin6 has shown how particles which have very limited inertia
behave when approaching a point sink when they are sedimenting
under gravity and there is a wind.
In order to understand his paper it is best to start with
the equations for inertialess particles. His equations are
similar to those of Davies3 but are in polar coordinates. A
great simplification of the equations was achieved by using as
x-axis the direction of the vector sum of the wind velocity,
W, and the sedimentation velocity of the particles, vg. This
sum has magnitude
u0 = (W2 + v 2 + 2Wv cos <|>)^ (7)
S S
where $ is the angle between W and vs.
Since the particles are inertialess, the equations for the
trajectories are the same as those for the streamlines of flow.
The sink sucks with flow Q and creates radial velocities - Q/4Tir2
to which must be added the radial resolutes of u0. Hence the
equations for radial and tangential resolutes of a trajectory
are
u = - Q/4irr2 + u0 cos 6
; (8)
u = - u0 sin 9
By putting
H20 = Q/47TU0 (9)
these are rendered dimensionless,
ur/u0 = - A 02/r2 + cos 6
(10)
4 /ii . = — cinfl I
The solution of these equations is
(r2/2&02) sin2 6 + cos 9 = K (11)
where K is constant along a streamline or an inertialess trajec-
tory.
The formulation of these equations is shown in Figure 1.
Note the direction of the x-axis about which all trajectories
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1
Figure 1. The coordinate system. A sink at 0 draws a flow Q.
The curve with asymptote 2 at x/£0 = -°° is a critical
trajectory with stagnation point of both air flow and
particle motion at S.
x/C0
are symmetrical. The figure shows a meridian section and the
critical trajectory through the stagnation point, S, where
u =
r
8
= 0
(12)
It is necessary to locate the critical trajectory for the
calculation of the aspiration coefficient (Davies3). The equa-
tion of the critical trajectory is
or
/r2/2fce2J sin2 6 + cos 6 = 1
r/Jl0 = I/cos (e/2)
(13)
All particles with trajectories between the critical tra-
jectory and the x-axis enter the orifice; none of those outside
do so- The critical trajectory has the asymptote at x = -« (0
yoo/Ao = 2, (14)
hence, the rate of entry of particles into the sink is
TT (4fc2) c0u0 = Qc0 (15)
by equation (9), so that A = 1.
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The critical trajectory meets the x-axis at the stagnation
point, S, where 6=0, y=0, r=x and
x /ft0 = 1 (16)
S
When the particles have inertia it is necessary to consider
their acceleration in order to write down the dynamic equations:
m (dv /dt - VQ ae/dt) = 6TTan (-Q/4irr2 + Uo cos 9 - v )
re r
m (dv /dt + v dO/dt) = Siran (- u0 sin 6 + v_)
y r o
where vr and VQ are the radial and tangential resolutes of par-
ticle velocity. The equations are rendered dimensionless by
dividing through by Girari • ua and reducing t to dimensionless
t' and r to dimensionless r', so that
t = t1 JU/Uo and r = r1 fi,0
Since T = m/Girar), equations (17) in dimensionless form be-
come
'/dt1 - v.'-ae/at1) = -i/r|2 + cos e - v '
r 6 / r (19)
1 - v^-de/dt'J = - sin 0 + VQ '
where equation (9) shows that the dimensionless scale factor,
TU0/J,0 = TUO ^/4Tru0/Q = T V4TTu73/Q = k (20)
By integrating these equations the particle velocity (vr,
VQ) can be obtained at all points along a trajectory. Levin6
succeeded in obtaining solutions as series of rising powers of
k, for small values of k, by assuming that a particle stagnation
point existed at S, in the same position as the air velocity
stagnation point. At S, and nowhere else, both particles and
air have zero velocity. This is justifiable for small values
of k, since continuity must exist between air streamlines and
particle trajectories as k tends to zero. When k = 0 equation
(19) reduces to equation (10) .
His solutions are
V = cos 6 - 1/r" + k
.2 + , I2(l-r'2 cos 6)
r
3(1-3 cos2 6) , 20 cos 9 _ -, . , , & \ ,
r • t "*" ,6 ' J
4 sin 91 ,
T~l+ ' • •
r'6 )
v. = _ sin e - k SJJLJ. _ ka J6 sin 9 cos 6
9 .-1 3
(21)
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When k = 0 these equations reduce to equation (10) . They include
powers of 1/r ' = &0/r up to the sixth; since A0/r = 1 at the
stagnation point, the trajectories are increasingly accurate
for greater distances from the sink. This is important since
the calculation of the aspiration coefficient, A, depends on
the value of y at r = °°, 6 = IT (equations (14), (15)).
In order to obtain the trajectory equations it is necessary,
as in integrating equation (10) , to substitute in
dr/rde = v /VQ (22)
r W
from equation (21) and perform the integration. It is not clear
how Levin proceeded at this point but he obtained the asymptote
(r = oo) of the critical trajectory by successive approximation,
starting with the equation (13) for k = 0. This gave, in place
of (14), the formula
(yoo/S-°) = 4 - 3.2 k + 0.32 k2 (23)
Hence, since
then, using (9)
A = 1 - 0.8 k + 0.08 k2 (24)
By comparing this with the results of numerical integrations
of equations (19) , Levin6 concluded that the error in A, as given
by (24) was not more than 1% when k = 0.25 and not more than
2.5% when k = 0.5. It will be seen, later, that equation (24)
is not valid up to such large values of k as Levin claimed.
The writer (Davies12) made an analysis of the behavior of
particles with inertia approaching a point sink in otherwise
calm air. In the absence of appreciable rate of fall, series
solutions of the dynamic equations were obtained for large and
small inertia and connected by interpolation. This gave the
distribution of concentration in the spherical "space charge"
region near the orifice. Stepwise solutions, when the particles
were also falling under gravity, showed that some particles de-
scribed orbits about the sink. The calculations were made for
the condition
vs/V0 = 1 (25)
where VQ = 1/T (QT/4TT)1/3 (26)
was later defined as the dynamic sampling velocity (Davies,11*
1968) . This particular formulation arose because the dynamical
equations were rendered dimensionless by measuring lengths in
-------
units of the stop distance at velocity V , and times in units
of the particle relaxation time, t; hence
V0 = ds/T (27)
In order to compare these results with those of Levin it
is necessary to convert from Davies1 system to Levin's. This
is easily done. Since u = vs (W = 0), we have from (26), (29)
and (20)
Vs/VQ = TUJQT/47I)-1/3 = (TU/fi,)2/3 = k2/3 (28)
Hence the condition (25) also means that k = 1.
This of course is far outside the limit for Levin's results
and explains why his particles did not execute orbits around
the sink.
Davies found that the limiting trajectory failed to complete
an orbit while trajectories outside the limiting one did not
orbit at all. On this limiting trajectory the aspiration coef-
ficient was estimated to be 0.37. For k = 1, Levin's series
(24) gives A = 0.28; naturally, this is not the correct value,
since k is too large; in fact, for slightly larger values of
k the series (24) gives negative values of A.
The value A = 0.37 for vs/V0 = k = 1 would be correct if
all particles inside the critical trajectory entered the orifice,
even after orbiting. However, it is not yet clear whether this
is true. Davies found that a group of trajectories, inside the
critical trajectories, completed closed orbits and then escaped
from the sink. If this is correct, the value of A is reduced
from 0.37 to 0.23. However, ter Kuile13 believes that all tra-
jectories which execute orbits, these being inside the critical
trajectory, must enter the sink. He found that in order to plot
accurate trajectories, a large number of very short steps had
to be taken as the sink was approached and his computer program
was designed to do this. It is possible that the writer's escap-
ing particles only did so because too few steps were taken along
the trajectories and that they should really have been caught.
To settle the point, either more trajectories need to be computed
or recourse made to the theory of central orbits. Ter Kuile
and the writer agree, when calculated on the nearest non-orbiting
trajectory to the axis of symmetry through the sink, on the
assumption that all orbiting particles enter, that for vs/V0
= k = 1, then A - 0.37.
For heavy particles there is no stagnation point on the
x-axis, as was assumed by Levin for k < 0.5.
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It is important to notice that Levin's equation (24) is
derived by a theory in which the effect of the inertia of the
particles is dominated by their velocity of approach to the vi-
cinity of the point sink; this velocity is u0. In limiting the
result to low values of k, he has eliminated the influence upon
the value of A of the concentration "space charge" around the
sink and of particles with enough inertia to describe orbits
around the sink. Because of this limitation, it will be seen
that decreasing the rate of suction, Q, increases the value of
k, equation (20), so that the aspiration coefficient, A, de-
creases. Obviously this is because, when Q is small, more par-
ticles can shoot past the sink, due to their inertia upon the
wind and sedimentation velocities. This dependence of A on Q
was noted by Kaslow and Emrich,21 but its significance was not
realized and some confusion was created.
When inertia effects are larger, inside Levin's range, those
due to the aspiration velocity may preponderate and cause a re-
versal of the effect on A, so that increasing Q increases the
sampling error.
Confusion has also been caused by a failure to realize that
particles with unlimited inertia have A = 1 if the wind and sedi-
mentation velocities are both negligible. Examples of error
on this account are seen in the work of Kaslow and Emrich,21
Kim,22 Pickett and Sansone,17 Breslin and Stein,15 Agarwal,19
Gibson and Ogden,16 and, no doubt, others.
PARTICLES WITH INERTIA; FINITE ORIFICE
Levin's equation (24) applies to a point orifice. He thought
that it could be used for orifices of finite diameter, D, as
long as
D < 2A e. (29)
This means that the radius is less than the distance, OS, between
the orifice and the stagnation point (16).
A point sink has no orientation; on replacing it by a finite
orifice the direction of the normal to the plane of the orifice
introduces an additional variable. For simplicity, this will
be referred to as the direction of the sampling tube, ip, as shown
in Figure 2 relative to vs. An independent yaw angle, relative
to W, will be considered later.
Let W be the wind velocity, which is usually horizontal.
V is the mean air velocity in the sampling orifice and has the
direction of the sampling tube- Then
V = 4Q/7TD2 (30)
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Figure 2. The additional angle, \jj, which is necessary in proceeding
from a point sink to a sampling tube of diameter D. Note
that the angles >|/ and $ are not necessarily in the same plane.
The angle of yaw between V and W is not shown.
Stw = TW/D,
Two Stokes' numbers are involved, that on the wind velocity
(31)
and that on the sampling velocity
(32)
Stv = TV/D
The magnitude of the sum of the wind and sedimentation ve-
locity vectors is u0 (equation 7), as before, but, from (9) and
(30)
*„ = (D/4)
(33)
and, by (20), (31), (32), (33), and (7)
k = TU A = (4Tu /D) */u /V
V o / " o
^
oo
= 4Stw (W/V)^ {l + (vs/w)2 + 2 vg/W cos }3/*
(34)
10
-------
= 4Stv{(w/v)
2(w/v)« (vs/v)cos }3/"
(35)
Equation (35) is used when the wind velocity, W, is small.
Substitution of these values of k in equation (24) gives
A = 1 - 3.2 Stw f _"
W
-fr
COS
V
+ 1.28 S
3/2
(I)
2 cos
(36)
or
A = 1 - 3.2 St.
COS
(W
WV
+ 2
1.28
COS
3/2
(37)
According to Levin, the limits of validity of these equations,
which in fact are overestimated, are
within 1% for
within 2.5% for
k = 0.25,
k = 0,5,
A = 0.805
A = 0.62
(38)
(39)
It must not be forgotten that another angle, that between
the sampling tube and the vertical, ty, also has to be allowed
for. The values of A in equations (36) and (37) therefore need
to be multiplied by the factor of equation (6) (see Figure 2).
The final value becomes
A
(Vv)
COS
(40)
Note that the terms containing vs in equations (36) and
(37) express the loss of particles by the effect of sedimentation
in causing them to shoot past the orifice, whereas the vs in
(40) expresses the loss or gain due to sedimentation in the plane
of the orifice. These equations do not include losses due to
distortion of the symmetry of the concentration "space charge"
or to orbiting which come into operation at higher values of
k.
The value of A for a sampling tube of finite diameter depends
upon the yaw angle between V and w. This is not expressed by
equations (34) to (37) which hold only for very small values of
11
-------
k. When V and W have the same or opposite directions, yaw is
0° or 180° and the concentration "space charge" is symmetrical
about the sampling tube. Changing the yaw to 90° destroys this
symmetry and will be accompanied by a decrease in the value of
A. Some experimental results at 90° yaw are described below.
When the sampling tube is large there is a possibility of the
dust shadow reducing A at 180° yaw.
THE DAVIES SAMPLING CRITERION
The writer (Davies1 "*) proposed that accurate samples in
calm air would be obtained with an isolated small tube, the words
"small tube" meaning that the aspiration coefficient is inde-
pendent of the orientation of the tube, provided that
5(QT/4ir)1/3 £ D/2 < 0.2{Q/irgT)J5 (41)
The right hand term represents a maximum error of ±4% under
the worst conditions, with the tube facing up or down (i|j = 0
or IT, equation (5)), due to sedimentation.
The left hand term is derived from the excess concentration
in the "space charge" region being 1.6% at a distance of 5ds
from the center of the orifice (Davies12), where, by (26) and
(27)
= (QT/47T)
1/3
Considering the gravitational condition, use of equations
(2) and (30) shows that it makes
v /V < 0.04 (42)
s
The inertia condition reduces as follows, by (30) and (32) ,
5(D2VT/16)*/3 < D/2
or 5D/2 st1/3 < D/2 (43)
or stv < 0.016
When W = 0 and these values are inserted in equation (37)
we obtain
A = 0.9996
Hence, in absolutely calm air, the theoretical sampling error,
according to Levin, is only 0.04% which is obviously too strict
a criterion. However, the important effect of a low wind ve-
locity needs to be looked at. Davies1 * suggested that calm air
be defined as
W/V < 0.2 (44)
12
-------
Note that the limited size of orifice imposed by (29) re-
duces, by (33) to
u0/v)2 < 0.25 (45)
and that the value of u0/V defined by (42) and (44) for all values
of has
(u0/v)2 < 0.0576
so that Davies1 "small tube" is well inside Levin's maximum size,
(29). Putting W/V = 0.2 into equation (37) gives (W horizontal,
<|> = 90°)
A = 1 - 3.2 (0.016) (0.202 + 0.042)3/" + ...
= 0.995
showing an error of 0.5% due to a slight ambient wind. Great
care needs to be taken before relaxing the criterion too much
since the wind error increases rapidly. For example, if W/V
= 0.5,
A = 0.982
and the error has increased nearly four times for a two and half
times increase in wind speed.
If a 2.5% error is accepted as tolerable in "calm air",
that is for W/V = 0.2, then
0.975 =1-3.2 Stv (0.22 + 0.042)3/*
and
Sty = 0.085
Substituting this value in (41) changes the original Davies
criterion to
2.87 (QT/4TT)1/3 * D/2 < 0.2 (Q/irgr) * (46)
For a 4% inertial error at W/V = 0.2, Sty = 0.136. This
matches the sedimentation error and the criterion becomes
2.45 (QT/4TT)1/3 < D/2 < 0.2 (Q/Trgi) ^ (47)
In this case, the error when W = V is 41%. When W = 0.2V and
vs = 0.04V, with St = 0.136, the criterion (47) makes A = 0.95.
13
-------
The limit of W/V to which this can be extended while still
remaining within Levin's criterion (29) requires
W/V < 0.5
With this value, and the factor 2.45 of (47) instead of 5, cal-
culating as in (43) gives
(Sty/2)1/3 = 1/2.45 or Sty = 0.136
Hence, by (35)
k = 0.193
and by (24)
A = 0.85
which is within Levin's other limit of 1% error at k = 0.25,
(38). When the wind is not horizontal, the value of A is raised.
However, the experiments described below indicate that Levin's
estimate of 1% error at k = 0.25 is far too low when applied
to a finite tube diameter.
There are thus grounds for considering a change in the fac-
tor 5 of the inertial term of the Davies criterion to 2.45 so
that the new criterion is as in (47) , but lower values than this
will certainly involve larger errors. (Figure 3)
Several papers have been published in which the original
criterion is criticized as too severe. The experimental tech-
nique in some has been inadequate and in all the importance of
the reaction of the wind speed and the sedimentation speed of
the particles upon the inertia effect has been entirely over-
looked, although it was emphasized by the writer12 and is implicit
in Levin's equations (Figure 3).
EXPERIMENTAL RESULTS; CALM AIR
A very careful series of experimental measurements is due
to Breslin and Stein1b (1975). They measured aspiration coeffi-
cients with 398 < V < 5968 cm/sec and 0.04 < D < 0.4 cm, using
particles of coal dust with aerodynamic diameters between 2.9
and 41 ym. By means of a Coulter counter, the dust counts were
broken down into 14 size ranges so that the disadvantages of
employing an aerosol of wide size distribution were overcome.
Sampling was carried out with horizontal and vertical (facing
upwards) tubes; in most of the latter experiments the gravita-
tional error due to settlement into the orifice was small. Many
experiments resulted in low values of A, down to 0.4. The extra-
ordinary thing about these results is that Breslin and Stein
operated with an ambient wind velocity averaging 20 cm/sec near
14
-------
^
+
(Stv/2)
-1/3
Figure 3. A/[1 + (vs/V)] cos $ as a function of f(W2 + Vs^J/V2]1/2 and
(Sty/2)'1/3 according to equation (37) for horizontal wind. The
units on the axis of abscissae are (Stv/2)~1/3 since this is the value
of the constant multiplying the inertia term of the Davies sampling
criterion. The graphs enable the sampling error to be read off, for
various values of (W2 + Vg2)/]/2 and vs/V, according to the value
chosen for the constant, assuming that equation (37) is correct.
the sampling points without realizing that this contributed very
much to their values of A; in fact, they actually wrote that
the spread in their results "may indicate that the inertial
parameter alone is not completely adequate to characterize aero-
dynamic effects at sampling inlets".
Their experimental data are plotted on Figure 4, the aspira-
tion coefficient, as ordinate, against (Sty/2)"1/3 . This quan-
tity is chosen since it is equal to the numerical factor multiply-
ing the left hand term of the Davies criterion, namely 5 in the
original expression (41) and now, perhaps, reduced to 2.87 (46)
or 2.45 (47). How this comes about can easily be seen from (43).
In these experiments vs/V was negligibly small but W/V ranged
from about 0.05 to 0.004. By inserting these values in equa-
tions (37) the two curves, according to Levin, of A against
15
-------
(Stv/2)-1/3 have been plotted on Figure 4. Most of the experi-
mental points fall between these curves and it is evident that
the spread of the values of A as well as the low values are en-
tirely due to the 20 cm/sec ambient wind. If W = vs = 0 then
A = 1. In these experiments W » vs so that the reduction of
A below unity was due to W. Once again, it is the effect of
W and vs upon particles with appreciable inertia, which causes
a reduction in A. Particle inertia alone does not affect A for
isolated orifices, as long as the tube is reasonably sharp-edged
and particles deposited on the inner tube wall are included in
the sample. Note that Breslin and Stein use r0/ds, which is
the same as (Stv/2)-1/3, and is the radius of the sampling tube
divided by the stop distance of the particles from velocity V0,
equations (26) and (27) .
Some experiments have been performed by Gibson and Ogden;
they did not attempt to measure absolute concentrations but
i e
i.o
0.8
0.6
0.4
— 0.05 W/V
(Stv/2)
-1/3
Figure 4. The experimental points of Breslin and Stein. ^ The two curves
correspond to W/V = 0.004 and 0.05. They illustrate the importance
of the low ambient wind velocity of 20 cm/sec when the sampling
velocity was 6000 and 400 cm/sec, respectively; vs/V was very small.
16
-------
merely compared individual pairs of samples taken at differ-
ing values of Sty. Sampling was carried out with \p = = 0,
and W rarely exceeded 5 cm/sec. Particle aerodynamic diameters
were 9 to 39 pm. Unfortunately the experiments were incorrectly
planned, the authors falling into the old trap of failing to
recognize that the influences of particle inertia and of wind
plus sedimentation are complementary. In some pairs of samples
the inertia ratio was high and the wind plus sedimentation ratio
was low; in others, the reverse was true. In fact, without ex-
ception, the relative values of each ratio were such that the
defect of the aspiration coefficient, A, in one member of a pair,
due to one cause, was almost exactly balanced by the defect in
the other member of the pair, due to the other cause. Calcula-
tion of the ratio of the values of A for every pair by equations
(37) and (40) shows that it should be within 1% of unity in each
of the 26 pairs. The experimental ratios were between 0.51 and
1.15 and averaged 0,93, the differences from unity being due
to experimental error.
Pickett and Sansone17 carried out some experiments On
sampling coal dust, comparing the distributions of mass with
particle size in samples taken with nozzles 0.2 cm square and
1.3 cm diameter. They failed to detect any difference between
the mass distributions because most of the mass of particles
collected was in sizes too small for the sampling error of the
small orifice to be revealed; this was pointed out by Davies.18
Their criticism of the Davies criterion is invalidated by in-
adequate experimental technique and for the same reason no con-^
elusions can be drawn, as was done from the data of Breslin and
Stein,15 about the influence of the ambient wind velocity.
Agarwal19 also criticized the criterion but failed to realize
that the aspiration coefficient is a function of vs of specific
form, for horizontal wind, namely
A = f |stv|"(w2 + VS2)/V2~J3/*, vs/VJ (48)
by equations (37) and (6). Instead, he took the product
Stv - vs/V = Tvs/D
which is independent of V and is obviously incorrect.
EXPERIMENTAL RESULTS - SAMPLING IN A WIND AT 90° YAW ANGLE
Some experiments have been carried out in a small wind tun-
nel with the sampling tube at right angles to the wind direction;
in some of the experiments the wind was vertically downwards
so that $ = 0 and ^ = 90°. In others, the wind was horizontal,
making 4> = 90°, and the sampling tube sucked vertically upwards
so that v> = 180°.
17
-------
According to equations (6) and (37) , for values of A near
to unity, the aspiration coefficients can be calculated from
the experimental data as follows, for comparison with the actual
measured values of A.
When 3/2
/W2+v 2\
+1.28 Sty21 2 S I
(50)
Provided that both vs/W and vs/V are small, each case re-
duces to the same formula, namely
3/2 3
A = 1 - 3.2 Stv(|) + 1.25 Stv2(|) (51)
Levin's equation (24) relates to a point sink. The object
of the experiments was to find out whether its adaption to a
finite sampling tube in a given orientation, expressed by equa-
tions (36) and (37), was accurate.
Nearly uniform particles of di-2-ethylhexyl sebacate (den-
sity 0.92 g/cm3) were produced by a spinning disc generator and
had diameters between 15 and 30 ym. The wind speed, W, could
be varied from 74 to 306 cm/sec and the suction speed, V, from
188 to 2090 cm/sec. The sampling tubes had parallel bores with
thin walls tapering finely to a sharp edge at the orifice. Bel-
yaev and Levin20 have shown that the use of a thick-walled tube
reduces A when sampling with <|> = ip = 90° at zero yaw.
The experiments were difficult to perform accurately because
of fluctuations in the concentration of particles. vs/W never
exceeded 1.8% and vs/V 0.72%, hence equation (51) applies for
each of the orientations with an error which is barely percep-
tible in comparison with the scatter of the results. The best
way of displaying the data is to plot the aspiration coefficient,
A, as a function of Stv (W/V)3/2; according to equation (51),
A should be a unique function of this variable. The experimental
points are shown in this way on Figure 5.
Each sampling tube was attached to the second stage of a
standard cascade impactor. The particles deposited partly on
the inside of the sampling tube and partly on the cascade im-
pactor plate, the aspiration coefficient being calculated from
18
-------
0.8
0.6
0.4
0.2
.^^_ LEVIN'S THEORY, ADAPTED TO FINITE TUBE
DIAMETER, EQUATION (51)
O PATTENDEN AND WIFFEN23
DAVIES AND SUBARI, THIS PAPER
.O1
0.01
0.1
-W 1 3/2
Figure 5. Experimental results for A as a function of Stv(W/V)3/2, sampling
at 90° yaw angle with vs very small.
the sum of those in the tube plus those on the plate, divided
by the volume of air sampled. The particles inside the tube
were estimated either by weighing the tube before and after de-
position or by lining it with thin metal foil coated with a film
of magnesium oxide. The airborne concentration was measured
with a small isokinetic impactor, facing the wind, with a sharp-
edged slit orifice.
It is
to include
particles
particles
some solid
bounce off
lets of di
particles
incidence
essential, when measuring the concentration of aerosols,
particles deposited inside the sampling tube with
collected on the impactor plate or filter. Liquid
will usually adhere to the tube wall, on impact, but
particles, especially at high rates of suction may
In the experiments now described the liquid drop-
-2-ethylhexyl sebacate adhered to the metal tube, but
which struck the magnesium oxide layer at glancing
often rebounded and reached the sampling plate.
Ft is defined as the fraction of the total particles enter-
ing the sampling tube which were found adhering to the wall of
the tube. The experimental results are shown in Table 1. Ft
could be calculated by the empirical formula
Ft = 7 1 + erf
Stw (W/V)
0.2
0.15
(52)
19
-------
TABLE 1. FRACTION Ft OF PARTICLES ENTERING THE SAMPLING TUBE
WHICH DEPOSITS ON THE WALL OF THE TUBE
Method Stw
MgO 0.065
.065
.077
.077
.078
.078
.089
.089
.100
.100
.100
.100
.113
.160
.160
.160
.160
.178
.178
.178
.190
.190
.190
.190
.190
.204
.204
.220
.220
.220
w/v
0.093
.093
.110
.110
.400
.400
.400
.400
.150
.300
.400
.610
.400
.060
.120
.232
.470
.400
.400
.400
.070
.140
.270
.400
.530
.400
.400
.080
.170
.310
Ft
Calculated
0
0
0.001
.001
.009
.009
.023
.023
.013
.034
.048
.077
.094
.080
.155
.260
.404
.489
.489
.489
.202
.329
.472
.560
.622
.638
.638
.365
.532
.644
Experimental
0
0
0
0
0
0
0
0
0
0
0.10
.14
0
.05
.17
.18
.25
.29
.31
.27
.36
.31
.31
.58
.64
.53
.44
.43
.58
.61
(continued)
20
-------
TABLE 1 (continued)
Method St
MgO 0.240
.240
.256
.278
.278
.278
.290
.290
.346
.377
.377
.397
.397
.403
.462
.630
Weighing .630
.630
.630
.722
.722
.856
.856
.895
.895
W/V
0.351
.351
.400
.200
.400
.400
.475
.475
.400
.200
.200
.400
.400
.400
.400
.400
.200
.200
.400
.200
.400
.400
.400
.400
.400
Ff
Calculated
0.770
.770
.839
.798
.891
.891
.926
.926
.968
.955
.955
.987
.987
.989
.996
.999
.998
.998
.999
1.000
1.000
1.000
1.000
1.000
1.000
Experimental
0.75
.78
.81
.50
.54
.54
.92
.89
.85
.46
.57
1
1
.89
.87
1
1
1
1
1
1
1
1
1
1
and Table 1 shows that this agrees quite well with the experi-
mental measurements which cover the ranges 0.068 < St^ < 0.3
and 0.05 < W/V < 0.53. Equation (52) is graphed in Figure 6
and has been extrapolated beyond the experimental values to give
the curves up to W/V = 2. This was done, as an interim measure,
to give warning of losses that can occur upon the tube wall.
21
-------
0.8
0.6
0.4
0.2
T I \ I I I I I
W/V 2 1 0.5|0.2 0.
0.01
Figure 6. The fraction, Ft, of particles entering the sampling tube which
deposits upon the inside of the tube (experimental data in
Table 1).
The experimental data, for each value of A measured, are
given in Table 2 and the results are plotted in Figure 5, with
a separate point for each experiment. Also shown in Figure 5
are 14 points calculated from Table 2 of Pattenden and Wiffen's
paper23; each is the mean of about four measurements in a wind
tunnel at wind speeds between 2 and 6 m/sec, using solid
particles with aerodynamic diameters from 4.5 to 25 vim. The
sampling tube was vertical, in a horizontal wind, and it made
no difference whether the tube pointed upwards or downwards,
hence vs was negligible; the tube diameter was 1.25 cm, its end
was cut off square and the wall thickness was about 0.2 cm.
The mean inlet velocity was 70 cm/sec. Only their experiments
with a tube without a cover are discussed here. They found that
with a cylindrical rain cover over the tube there was a substan-
tial increase in the aspiration coefficient; this is an example
of May's stagnation point sampling, 2
-------
TABLE 2. EXPERIMENTAL RESULTS
Series
Ai
Ai
Ai
A!
Ax
A!
Ai
A!
A!
A!
A!
A2
A2
A2
A2
A2
A2
A2
A2
A2
A2
A2
A2
BI
BI
BI
BI
BI
BI
BI
BI
B3
B3
Orientation
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
stv
0.70
.70
.70
.70
.70
.70
.70
.70
.70
.70
.70
1.35
2.7
1.35
1.35
2.7
.36
.47
1.35
.35
.17
.26
.35
.17
.17
.17
.27
.27
.31
.31
.31
.89
1.02
W/V
0.110
.110
.093
.310
.232
.270
.150
.475
.475
.351
.351
.17
.08
.14
.14
.07
.53
.40
.12
.47
.61
.40
.30
.2
.2
.2
.2
.2
.2
.2
.2
.2
• 2
vs/V
0.001
.001
.001
.001
.001
.001
.001
.001
.001
.001
.001
.001
.000
.001
.001
.000
.002
.002
.001
.002
.003
.002
.001
.0018
.0018
.0018
.0018
.0018
.0018
.0018
.0018
.0034
.0034
A
0.498
.515
.425
.433
.378
.500
.580
.287
.556
.566
.504
.346
.365
.324
.390
.358
.638
.446
.343
.361
.493
.586
.570
.786
.609
.687
.516
.547
.448
.507
.408
.354
.364
/W+v\3/2
Stv\ V /
0.026
.026
.020
.121
.079
.099
.041
.230
.230
.146
.146
.095
.061
.071
.071
.050
.140
.120
.057
.113
.082
.066
.058
.015
.015
.015
.024
.024
.028
.028
.028
.082
.094
(continued)
23
-------
TABLE 2 (continued)
Series
Ci
C2
C3
c.
c,
C*
c*
c,,
B3
B3
B3
B3
B3
B3
B3
B3
B.,
B*
B.,
B*
G!
Ci
Ci
Ci
Ci
Ci
Ci
Ci
Ci
Ci
Ci
G!
C2
C2
C2
C2
Orientation
V
V
V
V
V
V
V
V
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
stv
0.086
.28
1.43
1.43
1.64
1.64
2.234
2.238
.89
.89
1.02
1.02
1.39
1.39
1.89
1.89
3.15
3.15
3.6
3.6
.086
.086
.086
.086
.086
.136
.136
.136
.136
.155
.155
.155
.195
.195
.22
.22
W/V
0.4
.4
.4
.4
.4
.4
.4
.4
.2
.2
.2
.2
.2
.2
.2
.2
.2
.2
.2
.2
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
vs/V
0.0034
.0068
.0021
.0021
.0021
.0021
.0021
.0021
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0036
.0036
.0036
.0036
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0034
.0023
.0023
.0023
.0023
A
0.602
.368
.135
.178
.139
.32
.154
.238
.342
.310
.337
.334
.364
.488
.371
.359
.259
.349
.326
.274
.747
.600
.690
.718
.744
.390
.600
.686
.686
.346
.450
.600
.509
.417
.403
.306
/W+v\'/'
stv (—)
0.022
.073
.365
.365
.418
.418
.571
.571
.082
.082
.094
.094
.128
.128
.173
.173
.289
.289
.289
.289
.022
.022
.022
.022
.022
.035
.035
.035
.035
.040
.040
.040
. 050
.050
.057
.057
(continued)
24
-------
TABLE 2 (continued)
Series
C3
C3
C3
C3
C3
C3
C3
C3
C3
cs
C6
C6
C6
C7
C7
C7
C7
C7
Orientation St
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
0.445
.445
.445
.51
.51
.69
.69
.28
-.28
.64
.865
.99
.99
1.01
1.16
1.58
1.58
1.81
W/V
0.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
.4
vs/V
0.0068
.0068
.0068
.0068
.0068
.0068
.0068
.0068
.0068
.0047
.0062
.0062
.0062
.0072
.0072
.0072
.0072
.0072
A
0.570
.512
.400
.475
.468
.581
.468
.520
.610
.645
.500
.331
.360
.151
.263
.183
.130
.160
/W+V\3/2
04. I = 1
btv\ v )
0.115
.115
.115
.132
.132
.180
.180
.073
.073
.165
.224
.256
.256
.262
.300
.409
.409
.469
Also shown in Figure 5 is the line of equation (51) as far
as A = 0.62, which is Levin's limit for 2.5% error and k = 0.5.
It is clear that this equation makes the value of A too large
compared with the experimental results at 90° yaw.
The results of Pattenden and Wiffen are in rough agreement
with Ours up to Stv(W/V)3'2 = 0.9. At this point their values
of A fall abruptly and maintain a low level, 0.05 to 0.08 for
0.13 St (W/V)3/2 < 3.
Since
Stv(W/V)3/2 = Stw(W/V)* (53)
the lower limit, 0.13, corresponds to
Stw = 0.13 (2.9)~35 = 0.08
and the higher to
Stw = 3 (8.6)~^ = 1.02
where their range of W/V was from 2.9 to 8.6.
25
-------
Looking at Figure 6, it is clear that all their measure-
ments which give the low values of A were made under conditions
such that a large proportion of the particles would be likely
to stick to the tube wall and fail to reach the filter. It is
possible that their employment of a thick-walled, blunt tube
had something to do with this.
Instead of A falling at an initial rate corresponding to
A = 1 - 3.2 Stv(W/V)3/2 + ... (51)
it seems that the equation should be more like
A = 1 - 10 Stv(W/V)3/2 + ... (54)
There are signs of minimum and maximum values of A at the ab-
scissae 0.8 and 0.16, respectively. The temporary reversal of
the trend of A, between these values, must be due to a change
in the mechanism of inertia effect, which was referred to above.
Bearing equation (53) in mind, the initial, Levin effect is due
to particles being whisked past the orifice and A is increased
if V is increased; however, at higher values of St the "space
charge" effect comes in and A increases as V is decreased, by
enough to give the small rise in A between the minimum and
maximum.
At still higher values of Stw and W/V the wind stream flow
tube has to expand considerably on entering the sampling tube
so that many particles shoot out of the flow tube, due to their
inertia, and A becomes small. This, actually, is a return to-
wards the point sink approximation. An increase in W lowers
the ratio of the cross section of the wind stream flow tube
to that of the orifice and A falls; an increase in V has
the opposite effect and A rises.
Of course, the tube diameter has to be constant during the
above changes, but this is looked after by the Stokes numbers.
CONCLUSIONS
Levin's equation for a point sink in a wind (24) can be
adapted for use with sampling tubes of finite diameter by in-
cluding as variables the angle between the tube axis and the
vertical (36, 37, 40) and the angle of yaw between the tube
axis and the wind direction.
His limits (38, 39) overestimate the accuracy of his equa-
tion, when adapted to finite tubes, because his theory takes
account neither of the ratio of the sedimentation velocity of
the particles to the mean sampling velocity, nor of the effect
of inertial build-up of concentration near the orifice, or "space
charge" which, in the presence of wind or sedimentation, may
26
-------
reduce A. Levin's assumption that a quadratic function of k
can represent A down to 0.62 (2.5% error) does not hold; a more
complicated expression, at least a cubic, is necessary in the
case of sampling at 90° yaw angle.
Quite low wind and sedimentation velocities have a substan-
tial effect in reducing A according to Levin's theory. That
they must be taken account of in assessing sampling error is
clear from the work of Breslin and Stein15 and from our own
experiments. Arguing from Levin's equation (24) adapted to a
finite orifice (36, 37), it is shown that the Davies sampling
criterion might be made slightly less restrictive for small tube
sampling with W/V < 0.2 by using, instead of the original form
(41),
2.45 (QT/4TT)1/3 < D/2 < 0.2 (Q/Trgi)^, (55)
which gives a maximum error of about 5% when both terms have
the most unfavorable values.
However, caution is necessary because the inertial term
is based on the adaptation (finite orifice) of Levin's equation,
which tends to
A = 1 - 3.2 f(Stv, W/V, v /V) ,
V o
as in equation (37), whereas the experiments suggest, at 90°
yaw, that the factor is 10 rather than 3.2. It is possible,
when Stv(W/V)3/2 is less than 0.01, that the curve.actually de-
creases its slope and approaches (A = 1, Stv(W/V)3/2 = 0) with
the factor 3.2, in which case the criterion (55) is correct.
Were the factor to be greater than 3.2 the original form of (55),
namely (41) might be better. Unfortunately, it is very difficult
to do experiments at small values of Stv and neither our own
nor those of Breslin and Stein15 are sufficiently accurate and
numerous to enable a decision to be taken. In place of the 2.45
in the criterion (55) they suggest 1.5 at which value their own
values of A range from 0.92 to 1.01 and an average error of 5%
might be anticipated. In any practical sampling system, though,
the wind is certainly not zero. The writer's view is that W/V
= 0.2 is a practical figure for a "calm air" situation and it
is on this that (55) was based.
Our work shows that when St^ exceeds 0.08 an increasing
deposition takes place on the inner surface of the sampling tube,
and with St^ > 0.3 almost all the sample will strike the tube
wall. This effect needs to be looked after in practical sampling.
It is possible that impact on the inside of the tube is
affected to some extent by turbulence in the wind stream. A
study is being made of the way in which the density of the deposit
is distributed in the tube. The Reynolds number of the wind
27
-------
tunnel, which was a smooth circular duct 10 cm in diameter, ranged
from 5000 to 20,000. The stop distance of the particles on the
wind velocity varied from 0.052 cm to 0.85 cm.
Clearly the particles with the larger values of stop-distance
would be little affected by turbulence and at low wind speeds
there would not have been strong turbulence. In the intermedi-
ate ranges of particle size and wind speed some enhancement of
deposition in the tube is possible but an appreciable effect
on the aspiration coefficient seems unlikely.
ACKNOWLEDGEMENTS
The apparatus was built by Sarah Weedon and M.J. Farncombe
with the aid of a grant from the National Coal Board. Mr. M.
Subari developed the apparatus and performed all the experiments.
He is grateful for financial support from the University of
Technology Malaysia, at Kuala Lumpur.
The apparatus is designed for experiments on the deposition
of particles between 2.5 and 15 ym aerodynamic diameter in the
human respiratory tract. Before this work could be started it
was necessary to study the entry of particles into a breathing
tube at 90° yaw angle to the moving stream of aerosol. Applica-
tions for a grant to support the inhalation experiments were
made to the Medical Research Council and to the Directorate-
General, Employment and Social Affairs, of the Commission of
the European Communities but were rejected, the latter on the
grounds that no funds were to be provided for basic research.
REFERENCES
1. Albrecht, F. Physik. Z. 32:48, 1931.
2. Sell, W. VDI-Forschungsh. 347: 1, 1931.
3. Davies, C.N. Trans. Inst. Chem. Eng. Suppl. 25:36, 1947.
4. Walton, W.H. Br. J. Appl. Phys. Suppl. No. 3, S29, 1954.
5. Robinson, A. Commun. Pure Appl. Math. 9:69, 1956.
6. Levin, L.M. Izv. Akad. Nauk SSSR Ser. Geofiz. 7:914, 1957.
7. Davies, C.N., and V. Peetz. Br. J. Appl. Phys. Suppl. No.
3, S17, 1954.
8. Davies, C.N. Br. J. Appl. Phys. 18:653, 1967.
9. Davies, C.N. Br. J. Appl. Phys. 18:1787, 1967.
10. Davies, C.N. Br. J. Appl. Phys. 11:535, 1960.
28
-------
11. Voloshchuk, V.M., and L.M. Levin. Izv. Akad. Nauk SSSR
Fiz. Atmos. Okeana 4{7):734, 1968.
12. Davies, C.N. Proc. R. Soc. London Ser. A 279:413, 1964.
13. ter Kuile, W.M. Rep. F. 1582. Inst. Milieuhygiene Gezond-
heidstechniek. 1G-TNO, Delft, 1977.
14. Davies, C.N. Br. J. Appl. Phys. (J. Phys. D), Ser. 2, 1:921,
1968.
15. Breslin, J.A., and R.L. Stein. J. Am. Ind. Hyg. Assoc.
36:576, 1975.
16. Gibson, H., and T.L. Ogden. J. Aerosol Sci. 8:361, 1977.
17. Pickett, W.E., and E.B. Sansone. J. Am. Ind. Hyg. Assoc.
34:421, 1973.
18. Davies, C.N. J. Am. Ind. Hyg. Assoc. 36(9):714, 1975.
19. Agarwal, J.K. Particle Technology Lab. Publication 265,
Univ. of Minnesota, 1975.
20. Belyaev, S.P., and L.M. Levin. J. Aerosol Sci. 3:127, 1972.
21. Kaslow, D.E., and R.J. Emrich. Lehigh Univ., Bethlehem,
PA. Dept. Physics Tech. Rep. 23, 1973; Tech. Rep. 25, 1974.
22. Kim, Y.W. Lehigh Univ. Dept. Physics Tech. Rep. 24, 1974.
23. Pattenden, N.J., and R.D. Wiffen. Atmos. Environ. 11:677,
1977.
24. May, K.R. Physical Aspects of Sampling. In: Airborne
Microbes, P.H. Gregory and J.L. Monteith, eds. Cambridge
Univ. Press, London, 1967. pp. 60-80.
29
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PAPER 2
CYCLONE SAMPLER PERFORMANCE
MORTON LIPPMANN
NEW YORK UNIVERSITY
INSTITUTE OF ENVIRONMENTAL MEDICINE
AND
TAI L. CHAN
GENERAL MOTORS RESEARCH LABORATORIES
ABSTRACT
Cyclones are finding increasing application in air sampling
both as pre-collectors in two-stage samplers and as stages in
multi-stage samplers for size-mass distribution determinations.
They are well suited for both applications because of their rela-
tively low cost, compact size, and independence of orientation
and loading conditions. Their applications have been limited
primarily because they have required extensive empirical calibra-
tions. A new predictive relation for cyclone collection effi-
ciency has been developed which permits extrapolation from a
limited experimental data base to a wide variety of operating
conditions. Constants for the equation are presented for eleven
small cyclones ranging in diameter from 10 to 152 mm.
INTRODUCTION
Cyclones are devices which remove particles from fluid streams
as the stream is caused to rotate within a cylindrical and/or
conical cross section. A conventional cyclone design is illus-
trated in Figure 1. Particles with sufficient momentum to cross
the flow streamlines and reach the outer wall are arrested, and
are either retained at the wall or migrate along the wall to
a collection zone at the bottom. In hydroclones, which are
cyclones for separating particles from aqueous suspensions, a
small percentage of the liquid effluent is withdrawn at the bottom
with the collected particles. The bulk of the liquid and the
undersized particles exit through the axial pipe at the top.
In air cleaning and air sampling cyclones, all of the air exits
at the top.
30
-------
Cyclones are widely used for removing particles of relatively
large size from fluids because of their relative simplicity and
relatively low costs in terms of construction, operation, and
energy consumption. They are frequently used as a first stage
or pre-collector upstream of a particle collector with a high
collection efficiency for the small particles not removed by
the cyclone.
Most of the literature on cyclones and their performance
relates to cyclones used in cleaning of fluid streams, and has
proven to be, unfortunately, of relatively little use in describ-
ing the performance of cyclones used in air sampling applica-
tions. The flow patterns and dust loading in the small cyclones
MAIN VORTEX
VORTEX CORE
Figure 1. Conventional double vortex flow pattern found in air
cleaning cyclones (from Hochstrasser24).
31
-------
used in sampling are generally quite different from those found
in air cleaning cyclones and hydroclones.
APPLICATIONS OF CYCLONES IN AIR SAMPLING
"Respirable" Dust Sampling
By far the greatest usage of small cyclones in aerosol sampl-
ing has been as the first stage of a two-stage sampler. In most
cases, they have been operated at flow rates which were believed
to permit cyclone penetrations equivalent to the penetration
of inhaled dust to the non-ciliated deep lung spaces in humans.
Such applications have been called "respirable" dust sampling,
and their rationale has previously been discussed.1'2'3
In "respirable" dust sampling, it is essential that the
cyclone cut-off characteristics be known and constant, and that
each cyclone be operated at a flow rate which produces the de-
sired cut-off. In the absence of a usable theory, it has been
necessary to design and calibrate air sampling cyclones empiri-
cally. Unfortunately, dependence on an empirical calibration
can lead to controversy and errors. The original calibration
of the Dorr-Oliver 10-mm nylon cyclone1* failed to include an
appropriate aerodynamic shape factor. With Kotrappa's5 shape
factor correction, the recommended sampling rate was reduced
to 1.8 Vmin, which is in good agreement with most of the more
recent empirical calibrations,6'7'8'9'10 which recommend either
1.7 or 1.8 Vmin. However, the Mine Safety and Health Adminis-
tration, which specifies acceptable samplers and their operating
characteristics in mine dust evaluations, still insists that
the 10-mm cyclone be operated at 2.0 &/min, on the basis of its
own calibration by Tomb and Raymond.11
There are conflicting calibration data for several of the
12.5 to 50-mm diameter stainless steel cyclones which have also
been used for size-selective sampling. Some of the differences
can be attributed to the same causes as for the 10-mm cyclone,
i.e., differences in calibration techniques, data interpretation,
and failure to use appropriate particle shape corrections.
Another factor is the dimensional variations between nominally
identical samplers which were documented by Lippmann and Chan.12
The steel cyclones are individually assembled, and the quality
control exercised by the manufacturers has not always been ad-
equate. The 10-mm nylon cyclones, which are injection molded,
have not been observed or reporter: to vary significantly from'
one to another.
The use of cyclones as the first stage in two-stage samplers
for "respirable" dust sampling received its first major impetus
in the atomic industry when, in January 1961, the U.S. Atomic
Energy Commission's Office of Health and Safety defined "respir-
able" dust as that portion of the inhaled dust which penetrates
32
-------
to the non-ciliated portions of the lung. Further, they defined
"respirable" dust specifically as follows:
Aerodynamic Diameter (ym) : >10. 5. 3.5 2.5 <2.0
% Respirable: 0 25 50 75 100
A variety of commercially available small cyclones were
found to have collection characteristics at appropriate flow
rates which were close to those of the AEC specification,* and
some of them were used extensively in sampling programs by AEC
contractors.
More widespread application followed the decision of the
American Conference of Governmental Industrial Hygienists (ACGIH)
in 1968 to issue a Notice of Intended Change in the threshold
limit value (TLV) for quartz, cristobalite, and tridymite (three
forms of crystalline free silica).13 The size-selection charac-
teristics for the first stage collector defined by ACGIH were
identical to those of AEC, except that they specified that at
< 2.0 ym, the % "respirable" would be 90% rather than 100%.
For all practical purposes, the two specifications are essentially
the same.
In 1969, the Federal Coal Mine Health and Safety Act speci-
fied a "respirable" dust limit for coal mine dust, and all com-
pliance sampling in the mines has been on "respirable" dust sam-
pling, with, most samples collected with a sampler including a
10-mm nylon cyclone.
Cyclone Samplers with Other Cut Sizes
The ambient atmospheric aerosol has two major mass modes.
The so-called "accumulation mode" contains some primary submicron
combustion-related particles, (from stationary and mobile sources),
and also the secondary particles which are formed in the atmos-
phere from gaseous precursors by gas phase chemical and photo-
chemical reactions. The mass median diameter of this mode is
generally between 0.3 and 0.5 ym, and little of the mass is in
particles smaller than 1.0 ym or larger than 2 to 3 ym. The
larger particles found in the ambient air, which form the so-
called coarse mode, are generally released by mechanical pro-
cesses, and differ substantially in chemical characteristics
and physical properties from the accumulation mode particles.
A variety of samplers designed to aerodynamically sort the
ambient particles into the two major mass modes have used cyclones
to collect the larger particles. There is no general agreement
on the appropriate cut size for this application, or whether
the cut-off characteristic should be sharp or broad. Some prefer
33
-------
a sharp cut at an aerodynamic diameter of 2, 2.5, or 3.5 ym.
Others prefer a 3.5 ym aerodynamic mass median with the ACGIH
respirable cut characteristic. Their rationale is that there
will be little practical difference in the size of the two mass
fractions for most atmospheric aerosols, and, therefore, little
justification for introducing a different sampler acceptance
standard.
A cyclone with a sharp cut-off characteristic and intended
for two-stage ambient air sampling has been described by John,
et al.1* The EPA has used some of the cyclone samplers developed
for "respirable" dust sampling for two-stage ambient air sam-
pling, primarily the 1/2-in. HASL cyclone described by Lippmann
and Harris.1*
Several instrument vendors market cyclone preseparators
for either source-test cascade impactors or high volume sam-
plers. While there are no data on their performance in the litera-
ture, calibration data on the Sierra Model 220CP cyclone have
been made available to us by Dr. John Olin of Sierra Instruments,
and they have been utilized in our analyses.
Virtual dichotomous samplers have been widely used in recent
years for two-stage sampling. The precise balance of the two
different parallel flow streams in such samplers makes them much
more expensive than cyclones. On the other hand, they put both
mass fractions on filters, and this is often advantageous in
terms of subsequent analyses of the large particle fractions.
Size-Mass Distribution Determinations
Size-mass distribution analyses of aerosol constituents
have been made using cascade impactor samplers for more than
thirty years, beginning with the original May impactor,15 and
the Laskin16 University of Rochester modification. They have
also been performed with aerosol centrifuges which deposit par-
ticles on a collection foil according to their aerodynamic size.
However, both types of sample collectors have significant limita-
tions when used for such applications. Solid particles tend
to bounce off or be re-entrained from impaction surfaces which
lack an adhesive layer, or whose adhesive becomes saturated.
Unfortunately, saturation usually is reached either before the
investigator is aware of it, or before the accumulated sample
is large enough for the subsequent analysis. Applications of
centrifuges are limited by their low sampling rates and large
physical size and cost.
The limitations of the other inertial classifiers for com-
positional analysis by particle-size fraction have led to the
development of samplers using cyclones in either parallel or
series arrays. The first development of a portable sampler using
a parallel array of cyclone-filter series samplers was described
34
-------
by Lippmann and Kydonieus.7 Each cyclone had a different cut
size and the overall size-mass distribution of the total aerosol,
or of any of its chemical constitutents, could be determined
from analyses of the collection on the filter following each
cyclone and a parallel filter sampler operated without a cyclone
precollector. The performance characteristics of an improved
version of this sampler were described by Blachman and Lippmann.9
Individual cyclones could be operated at flow rates of a, 1 to
22.5 fc/min, giving cut points ranging from ^ 6 ym to 0.25 ym.
A much larger version of this sampler for fixed-station ambient
air pollution sampling was described by Bernstein, et al.,17
having cut points at 3.5, 2.5, 1.5 and 0.5 ym. A parallel multi-
cyclone sampling train for stack sampling applications was de-
scribed by Chang,18 but sufficient calibration data were not
provided to characterize the cyclone's performance.
A series array of cyclones for in-stack sampling through
an eight-inch port was described by Rusanov19 in 1969. A more
compact series sampler which fit within a six-inch port and
sampled at 1 ft3/min was described by Smith, et al.20 in 1975.
It consisted of three cyclones and a back-up filter. The first
cyclone, with a reported cut point of 2.6 ym, was the T2A cyclone
used by Chang18 in his parallel array. The other two cyclones,
with nominal cut points of 1 ym and 0.5 ym, were designed and
constructed by Southern Research Institute (SRI) for this sampler.
A later development from SRI21 is a six-stage in-stack series
sampler which operates at 1 ft3/min and fits through a 10-cm
sampling port. The five cyclones were reported to have cut points
at aerodynamic diameters of 5.4, 2.1, 1.4, 0.65, and 0.32 ym.
The collection characteristics of the cyclones in this train
are notable in that their cut off characteristics are quite sharp,
as shown in Figure 2. For the SRI II cyclone, with a cut size
of 2.1 ym, the ratio of the particle sizes with 90% and 10% re-
moval, respectively, is 1.5. By comparison, for the Andersen
Stage 4 impactor, with a cut size of ^ 2.3 ym, the corresponding
ratio is also 1.5 (Figure 3).
Data processing to determine the particle size distribution
for these multi-stage samplers can be treated in a similar manner
as the cascade impactor.22 An alternate approach is the itera-
tion scheme that has been developed by Sinclair and Knutson based
on the approach of Twomey.23 It will be presented later in this
symposium.
AERODYNAMIC FLOW AND PARTICLE COLLECTION IN CYCLONES
Air Cleaning Cyclones
The conventional view of flow within a cyclone shown in
Figure 1 appears to be realistic for air cleaning cyclones.
There is a double vortex, with the air stream spiralling downward
near the outer wall and then upward in a tighter spiral and out
35
-------
o
LU
o
HI
Z
O
u
UJ
_l
o
o
100
80
60
40
20
CYCLONE I
CYCLONE II
CYCLONE III
CYCLONE IV
CYCLONE V
0
0.1
• I
1.0
10
PARTICLE DIAMETER,/zm
Figure 2. Collection efficiencies of Southern Research Institute cyclones
for a sampling rate of 28.3 S./min, a temperature of 25°C, and
a particle density of 1.0 g/crr>3 (from Smith and Wilson21).
through the axial exit tube. The particles may initially deposit
on the wall in a well-defined spiral pattern. With continued
operation, a dust layer will form and migrate along the wall
to a collection hopper in a still-air zone at the bottom of the
conical section. The concentration of particles in an air clean-
ing cyclone is usually quite high, which favors particle coagula-
tion while they are still airborne. Since collection probability
increases with increasing particle size, the collection effi-
ciency increases with increasing particle concentration.
Tangential inlet cyclones used in air cleaning are generally
at least six inches in diameter, have inlet velocities of 2500
ft/min or greater, and Reynolds numbers in the inlets of about
100,000 or more. Thus, the flow will be highly turbulent. Par-
ticles of about 5 ym and larger can be collected with relatively
low energy consumption. However, the energy input required to
capture smaller particles increases rapidly with decreasing size
and is rarely worthwhile economically, except when using plenum
housings containing large numbers of small diameter cyclones
operating in parallel. Particles as small as 2 ym can be col-
lected efficiently with cyclones of 2-inch diameter when operated
with a flow rate of about 900 A/min through each cyclone.
36
-------
UNCMTED STAM.CSS STEEl
PlATCS-SoM hflKIn
W.»SS FIDE* FILTER
1.0 2.0 3.0
AERODYNAMIC DIAMETER,
10.0
15.0
/am
Figure 3. Collection efficiencies for Andersen cascade impactor stages
at a sampling rate of 28.3 %/min {from Marple and Willeke22).
Air Sampling Cyclones
Inertial separation efficiency in cyclones increases with
decreasing body diameter because of the tighter turns of the
air path and the greater relative radial acceleration of the
airborne particles. As a result, the same collection efficiency
can be achieved with lower air velocities. With lower air veloci-
ties, the flow regime will change from highly turbulent to par-
tially turbulent. The flow is predominantly laminar in the small
cyclones used in personal "respirable" dust samplers. However,
when these same cyclones are used at higher sampling rates in
multi-stage particle size samplers, a flow transition occurs
and the flow patterns are affected by turbulence. The change
in the nature of the flow pattern within the cyclones with chang-
ing flow rate makes it difficult to develop a general description
or predictive relationship for cyclone performance, and makes
extrapolation from one operating condition to another somewhat
questionable.
Direct evidence for a change in airflow pattern with a change
in flow rate is provided by the abrupt changes in the pressure-
flow relations. Figure 4 shows pressure-flow data of Hochstrasser21*
for three different 19-mm diameter long-cone cyclones. As the
37
-------
I
CO
O>
k
a
n
3
2.5
2
•5 1.5
•2.
O
o
u
CO
C/9
O
cc
a.
O
cc,
a
HI
cc
I
uu
oc
a.
1.0
.9
.8
.7
.5
.4
.3
.25
.2
.15
.1
1 I I I
CYCLONE NO. 7
TF.CYCLONE
NO. 4 ~
TF
CYCLONE
NO. 1
STABLE FLOW
UNSTABLE FLOW
TF - LINE FOR TURBULENT
OUTLET PIPE FLOW
LF - LINE FOR LAMINAR
OUTLET PIPE FLOW
I
I
J I
I
I I I I
1 1.5 2 2.5 3 4 5 6 7 8 9 10
Q - FLOW THROUGH CYCLONE, m3/sec x 10'4
Figure 4. Pressure-flow relationships for three 19-mm diameter long-cone
cyclones (from Hochstrasser24),
flow increases, a point is reached where there is an abrupt re-
duction in pressure drop. With increasing flow thereafter, the
pressure drop increases again, but the slope of the curve is
lower. The same phenomenon was observed by Blachman and Lippmann
for the widely used 10-mm nylon cyclone of Dorr-Oliver.
Blachman and Lippmann9 presented indirect evidence for a
change in flow pattern within the cyclone in terms of an abrupt
change in the collection efficiency characteristic at a critical
38
-------
flow rate. Figure 5 shows that there is an abrupt change in
the collection characteristic between 5.0 and 5.8 jj/min. They
further reported that locations at which the particles were de-
posited within the cyclone change for the different flow regimes.
Since the iron oxide in their test aerosol was reddish in
color, it was possible to observe the pattern of particle collec-
tion along the walls of the white nylon cyclones. For flows
below 5 &/min, the collection was essentially all close to the
cyclone inlet, with no visible collection in the lower part of
the cyclone body. For higher flow rates and small particles,
the inlet deposit extended into a spiral along the tapered inner
wall. In addition, there was another separate "ring" deposit
in the lower part of the cyclone. At a higher sampling rate,
the location of the ring deposit was lower. Since the lower
portion of the cyclone is tapered, the particles forming the
ring were depositing in a region with a smaller diameter, and
this presents a complication in attempts to fit the data into
generalized models. The number of turns that particles traversed
before the cyclone's collection was completed was dependent on
the flow rate and varied from 1 at 0.9 Vmin to 5 at 9.2 H/min,
For higher flow rates, it was not possible to visualize the num-
bers of turns.
10
Q
<
E
«.
HI
N
1.0
o
UJ
0.1
SIERRA 220 CP
NH AEROTEC 3/4
AEROTEC 2
V
\-BK-152
-BK-76
UNI CO 240
10mm DORRCLONE
i i i i i 11 i t i i i i i 11
1.0
10
100
FLOW RATE, 8pm
1,000
10,000
Figure 5. Particle cut size, i.e., aerodynamic particle size for 50% cyclone
retention, as a function of flow rate through cyclone for eleven
small cyclones.
39
-------
Ring deposits in small cyclones have also been observed
by others21*'25 and have been associated with a uniform radial
motion of the airstream. Hochstrasser21* found ring deposits
only when there was a flow rate below the critical pressure drop
transition point. He then inserted a pressure probe into the
lower cone portions of his cyclones and demonstrated that there
was a stagnant air zone present when flow conditions produced
ring deposits, but not when conventional vortex flow existed.
The height of the stagnant zone appeared to be proportional to
the height of the ring deposit. Hochstrasser associates the
ring deposit mode of cyclone operation with a laminar flow con-
dition in the outlet pipe of the cyclone, as illustrated in Fig-
ure 6.
Since the flow pattern in the outlet pipe appears to criti-
cally affect the performance of the cyclone, Hochstrasser sug-
gests that restrictions in the outlet flow imposed by couplings
to the downstream filter or other second stage collector may
alter the removal characteristics of the cyclone itself, and
that lack of standardization of such couplings may account for
some of the reported discrepancies of small cyclone sampler cali-
bration and field performance.
Hochstrasser noted that the laminar outlet pipe flow con-
dition could only be produced in long-cone cyclones, i.e., having
a cone length of 5 cm in relation to his cyclone diameter of
1.9 cm. When the cone lengths were changed to 2.7 or 1.6 cm,
only the turbulent flow condition was observed.
John, et al.11* confirmed that abrupt flow pattern changes
cannot be produced in short-cone cyclones. Their cyclone has
a body diameter of 3.7 cm and a cone length of 4.6 cm. There
was a vortex in the outlet pipe under all test conditions.
John, et al. also studied the patterns of deposition and
retention of both liquid and solid particles within the cyclone.
They noted the following features:
1. An impaction spot on the wall of the cyclone body op-
posite the inlet, but slightly off a direct line.
2. A spiral trace of 1.25 to 1.5 turns.
3. A diffuse deposit on the cone walls increasing in den-
sity toward the bottom.
4. Large aggregates of particles in the collection cup
at the bottom.
5. A spiral trace in the outlet tube.
40
-------
LINE OF FLOW
LAMINAR OUTLET FLOW
ZONE OF STAGNATION
NEW THEORETICAL CYCLONE
LAMINAR FLOW PATTERN
PLANE OF TRANSITION
PARTICLE DEPOSITION
PATTERN
ZONE OF STAGNATION
PARTICLE DEPOSITION PATTERN
( LAMINAR FLOW CYCLONE )
Figure 6. Flow pattern and particle deposition in long-cone cyclones
operating in laminar outlet pipe flow condition (from Hochstrasser24).
41
-------
6. No measurable differences in the percentage of the aero-
sol penetrating to the back-up filter between liquid and solid
particles of the same aerodynamic size.
They also tested the cyclone for re-entrainment losses.
It was loaded with the equivalent of several days of expected
collection and then clean air was drawn through. The material
transferred from the cyclone walls to the outlet filter was about
1% per day or less.
The insensitivity to sample loading and the nature of the
sampled material gives the cyclone significant advantages over
the impactor for long-period sampling. Figure 3 shows that un-
coated impaction plates do not retain solid particles effi-
ciently, and that the use of filters as impaction surfaces may
result in even greater losses. Cyclones do not suffer from such
limitations and, as shown in Figure 2, the collection charac-
teristics can be as sharp as those of impactors.
In multi-stage samplers used for size-mass distribution
analyses, it is desirable to have the sharpest possible cut-off
characteristics for each stage. The work of Mercer26 and Marple
and Willeke22 has led to considerable refinement in the design
of jet impactors and characterization of their performance.
While comparable definitive work has yet to be performed on
cyclones, recent work, such as that illustrated in Figure 2,
has shown that it is possible to achieve relatively sharp cut-
off characteristics in properly designed cyclones. The sharpness
of the cut-off can be compared for cyclones of different sizes
by plotting cyclone retention as a function of a normalized
particle diameter (Dp-D50)/D50, where Dp is the aerodynamic
diameter of the test aerosol and D50 is the diameter at which
the cyclone retention is 50%. Figure 7 shows calibration data
for 5 Stairmand-type cyclones which have been developed and cali-
brated in recent years. The sharpest cuts are produced by the
SRI-V cyclone of Smith and Wilson.21
The particle collection characteristics of the conductive
airways of the respiratory tract are not sharp. Hence, it is
desirable that pre-collectors used for "respirable" dust sam-
pling should have similar collection characteristics. In this
sense, the 10-mm nylon cyclone, which is widely used for this
purpose, is less than ideal in that it has a relatively sharp
cut-off characteristic. For cyclones designed specifically for
"respirable" dust sampling, such as the 1/2-inch and 1-inch
diameter dual-inlet stainless steel cyclones developed by the
Health and Safety Laboratory of the U.S. Atomic Energy Commis-
sion, the potentially sharp performance characteristics of cy-
clones were deliberately degraded in order to produce samplers
with appropriately coarse cut-off characteristics. "*
42
-------
z
o
I-
1.0
0.8-
0.6
h-
LU
cc
111
O 0.4
0.2
I
-1.0
-0.5 0 0.5
NORMALIZED PARTICLE DIAMETER, Dp-D5o/D5o
1.0
Figure 7. Cyclone retention as a function of normalized particle diameter
for five small cyclones which have recently been calibrated.
Dp is the aerodynamic particle size of the test aerosol and
is the aerodynamic size for 50% cyclone retention.
The variation of experimentally determined particle cut
sizes with sampling rate is illustrated in Figure 5 for a variety
of sampling cyclones. For each cyclone, there are characteristic
relations between cut size and sampling rate- Most of the cali-
bration data are for aerodynamic particle sizes between 1 and
10 vim. Some of the cyclones, specifically the 10-mm Dorr-Oliver
and the SRI-V, have calibrations for cut sizes extending to 0.5
Mm and below. Furthermore, as shown in Figure 7, the SRI-V has
a relatively sharp cut-off characteristic for such small par-
ticles. Thus, it is particularly well suited for use as the
final inertial stage in a multi-stage size-mass sampling instru-
ment.
PREDICTIVE RELATIONS
Chan and Lippmann27 showed that previously proposed predic-
tive relations for cyclone efficiency (Rosin, et al.,ZB Lapple,29
Leith and Licht,31 and Beeckmans32) are inconsistent with experi-
mental observations in tests using low concentrations of mono-
disperse test aerosols. Using a hyperbolic tangent relation
43
-------
for cyclone performance first developed by Blachman and Lipp-
mann,9 they also described the empirical constants of this equa-
tion for four commercially available small cyclones which have
been used in air sampling and for several Stairmand-type cyclones,
It should be noted that the previous predictive theories were
developed from test data on, and have primarily been applied
to, cyclones used for air cleaning applications.
Hochstrasser21* and John, et al.11* also attempted to adapt
available predictive relations to their experimental data. Fig-
ure 8 shows the comparison of Hochstrasser's experimental data
with the predicted values for one of the cyclones he tested.
He concluded that for operational conditions where there was
turbulent flow in the outlet pipe, either the Earth equation
or the hyperbolic tangent formulation provided consistently good
predictions. On the other hand, for operational conditions where
the flow in the outlet pipe was laminar, only the Blachman and
Lippmann formulation was consistently reliable. Both the Leith
and Licht and the Lapple equations were unreliable for all cases.
John, et al.11* compared their data to predictions based
on the theoretical formulations of Lapple,2 Sproull,33 Leith
and Licht,31 and Barth.30 They found that none of them fit the
data, although the predicted slope of the efficiency curve from
the Barth relation was closer to that observed than were any
of the others. They did not attempt to fit their data to the
hyperbolic tangent relation of Blachman and Lippmann.9
The discrepancies between the conventional predictive theo-
ries and experimental performance could be due to any or all
of the following differences between the conditions within cy-
clones used in air cleaning as opposed to cyclones used in air
sampling: (1) in air cleaning, the concentrations are much higher
than the ambient aerosols; (2) in air sampling cyclones, the
particles usually remain on the wall where they were initially
deposited, while in air cleaning cyclones, deposited particles
are continually scoured from the initial deposition sites and
migrate to the collecting hopper; (3) the flow patterns in air
cleaning cyclones are always turbulent, while in the smaller
cyclones used in air sampling, the flows may be completely or
partially laminar.
The various cyclone theories are summarized in Table 1.
The complete expression for the hyperbolic tangent equation as
developed by Chan and Lippmann27 is:
n = 0.5 + 0.5 tanh
+ (A - 2B)—? + B _ A
KQn
44
-------
100
6?
*
O
UJ
U
UL
U.
UJ
O
111
O
O
UJ
s
U
U
30 —
20
10
LEITH & LIGHT
L APPLE
BARTH
LIPPMANN & BLACHMAN
DATA ( TURBULENT O.P. FLOW
DATA ( LAMINAR O.P. FLOW )
12345
dp (MMAD) - PARTICLE DIAMETER,,
Figure 8. Experimental cyclone collection data of Hochstrasser in comparison
with predictions based on various predictive relations (from
Hochstrasser24).
45
-------
The constants K and n are obtained from a plot of the ex-
perimentally determined particle cut sizes at several sampling
rates. The constants A and B are determined using a least-squares
fitting program for all of the pertinent experimental efficiency
data.
A summary of the four empirical constants and the ranges
of particle size and flow rate over which they apply is presented
in Table 2 for eleven small cyclones for which a sufficient cali-
bration data base exists.
SUMMARY
Cyclones have many advantages over other inertial-type par-
ticle separators for the in-situ classification of sampled aero-
sols. They are much smaller, more easily portable, and less
dependent on orientation than elutriators or centrifuges; they
are much less subject to errors by misclassification due to par-
ticle bounce or overloading of collection surfaces than conven-
tional jet impactors; and they are much simpler than virtual
dichotomous samplers. On the other hand, the performance char-
acteristics of properly designed and operated elutriators, cen-
trifuges, jet impactors, and dichotomous samplers are predictable
from first principles, while the prediction of cyclone perform-
ance is dependent on empirical calibration data. Therefore,
better predictive relations for cyclone collection efficiency
are needed in order to exploit the inherent advantages of cy-
clones for air sampling. A generalized predictive empirical
relationship for cyclone performance has been developed and
described. It has been applied to the available calibration
data for the eleven small cyclones for which reliable calibra-
tions exist, and the four characteristic constants have been
determined for each. The equation permits the calculation of
cyclone collection efficiency for any aerodynamic particle size
and flow rate within the range established by the original
calibration.
The cyclones evaluated vary considerably in their sharpness
of cut. Some have coarse cut-off characteristics which match
those of the ACGIH cut-off for "respirable" dust. Others have
much sharper cut-off curves, comparable to those of jet impactors,
and are, therefore, useful for multi-stage samplers for determin-
ing size-mass distributions.
46
-------
TABLE 1. SUMMARY OF CYCLONE COLLECTION THEORIES
Category
I. Conventional
II. Semi-empirical
III. Theoretical
Theory
Rosin (1932)
Lapple (1951)
Davies (1952)
Barth (1956)
Sproull (1970)
Leith and Licht (1972)
Beeckmans (1973)
Soo (1973)
Form of Equation
•>.. • ^°-5
T, - 1 - e£(D'Q)
n = 1 - (infinite series with
algebraic and exponential
terms)
Reference
28
29
34
30
33
31
32
35
IV. Empirical
Wagner and Murphy (1971)
Lippman and Chan (1974)
Chan and Lippmann (1977)
Beeckmans and Kim (1978)
D — b-n u
5 Q •— r*\£
n = FJTanh[f (D,D50)]}
n = F|Tanh[f(Q,D' '
n = F (St,Re)
36
12
27
37
-------
TABLE 2. EMPIRICAL CONSTANTS FOR CYCLONE EFFICIENCY CALCULATIONS
oo
Cyclone
Aerotec 2
UNICO 240
Aerotec 3/4
10-mm nylon
BK-152
BK-76
19-mm UC-1
AIHL
Sierra 220 CP
SRI IV
SRI V
D / ym, AED
2.5-4.0
1.0-5.0
1.0-5.0
1.8-7.0
1.0-1.8
0.1-1.0
2.0-5.0
1.0-3.0
2.0-5.0
2.0-7.0
1.0-10.0
0.5-3.0
0.3-2.0
Q, Jl/min
350-500
65-350
22-65
0.9-5.0
5.8-9.2
18.5-29.6
1150-2700
400-1100
6-25
8-27
7-85
7-28
7-28
K
468.01
123.68
214.17
6.17
16.10
178.52
4591.0
221.48
15.46
52.48
48.08
17.60
14.00
n
-0.80
-0.83
-1.29
-0.75
-1.25
-2.13
-0.98
-0.77
-0.636
-0.99
-0.86
-0.98
-1.11
A
2.02
1.76
2.04
3.07
1.19
0.74
0.914
0.507
2.927
2.906
2.163
1.580
4.928
B
-0.68
-0.82
-0.77
-0.93
-0.59
-0.07
-0.111
-0.026
-0.849
-0.547
-0.370
-0.202
3.347
Reference
12
4
37
24
14
—
21
-------
ACKNOWLEDGEMENT
This study was part of a center program supported by Grant
No. ES-00260 from the National Institute of Environmental Health
Sciences.
REFERENCES
1. AIHA Aerosol Technology Committee. Guide for Respirable
Mass Sampling. J. Am. Ind. Hyg. Assoc. 31:133-137, 1970.
2. Lippmann, M. "Respirable" Dust Sampling. J. Am. Ind. Hyg.
Assoc. 31:138-159, 1970.
3. Lippmann, M. Size-Selective Sampling for Inhalation Hazard
Evaluations. In: Fine Particles, B.Y.H. Liu, ed. Academic
Press, New York, 1976. pp. 287-310.
4. Lippmann, M., and W.B. Harris. Size-Selective Samplers
for Estimating "Respirable" Dust Concentrations. Health
Phys. 8:155-163, 1962.
5. Kotrappa, P. Revision of Lippmann-Harris Calibration for
Two-Stage Sampler Using Shape Factor. Health Phys. 20:350-
351, 1971.
6. Ettinger, H.J., J.E. Partridge, and G.W. Royer. Calibration
of Two-Stage Air Samplers. J. Am. Ind. Hyg. Assoc. 31:537-
545, 1970.
7. Lippmann, M., and A. Kydonieus. A Multi-Stage Aerosol Samp-
ler for Extended Sampling Intervals. J. Am. Ind. Hyg. Assoc
31:730-738, 1970.
8. Seltzer, D.F., W- Bernaski, and J.R. Lynch. Evaluation
of Size-Selective Presamplers. II. Efficiency of the 10-
mm Nylon Cyclone. J. Am. Ind. Hyg. Assoc. 32:441-446, 1971.
9. Blachman, M.W., and M. Lippmann. Performance Characteris-
tics of the Multi-Cyclone Aerosol Sampler. J. Am. Ind.
Hyg. Assoc. 35:311-316, 1974.
10. Caplan, K.J., L.J. Doemeny, and S.D. Sorenson. Performance
Characteristics of the 10-mm Cyclone Respirable Mass Sampler.
Part I. Monodisperse Studies. J. Am. Ind. Hyg. Assoc.
38:83-95, 1977.
11. Tomb, T.F., and L.D. Raymond. Evaluation of Collecting
Characteristic of Horizontal Elutriator and 10-mm Nylon
Cyclone Gravimetric Dust Samplers. Presented at 1969 Annual
Meeting, American Industrial Hygiene Association, Denver,
May, 1969.
49
-------
12. Lippmann, M., and T.L. Chan. Calibration of Dual-Inlet
Cyclones for "Respirable" Mass Sampling. J. Am. Ind. Hyg.
Assoc. 35:189-200, 1974.
13. Threshold Limits Committee. Threshold Limit Values of Air-
borne Contaminants for 1968. American Conference of Govern-
mental Industrial Hygienists, Cincinnati, 1968.
14. John, W., G.P. Reisehl, and J.J. Wesolowski. Final Report,
California Air Resources Board Contract No. A5-00487, Feb-
ruary 1978.
15. May, K.R. The Cascade Impactor, an Instrument for Sampling
Coarse Aerosols. J. Sci. Instrum. 22:187-195, 1945.
16. Laskin, S. The Modified Cascade Impactor. UR-129, Univer-
sity of Rochester Atomic Energy Project, Rochester, NY,
August, 1950.
17. Bernstein, D., M.T. Kleinman, T.J. Kneip, T.L. Chan, and
M. Lippmann. Development of a High-Volume Sampler for the
Determination of Particle Size Distribution in Ambient Air.
J. Air Pollut. Control Assoc. 26:1069-1072, 1976.
18. Chang, H.C. A Parallel Multicyclone Size-Selective Partic-
ulate Sampling Train. J. Am. Ind. Hyg. Assoc. 35:538-545,
1974.
19. Rusanov, A.A. Determination of the Basic Properties of
Dust and Gases. In: Ochistka Dymovykh Gasov v Promysh-
lennoy Energetike, A.A. Rusanov, I.I. Urbakh, and A.P.
Anastasiadi, eds. "Energiya". Moscow, 1969.
20. Smith, W.B., K.M. Gushing, G.E. Lacey, and J.D. McCain.
Particulate Sizing Techniques for Control Device Evalua-
tion. EPA-650/2-74-102a, U.S. Environmental Protection
Agency, Research Triangle Park, NC, 1975. 132 pp.
21. Smith, W.B., and R.R. Wilson. Development and Laboratory
Evaluation of a Five-Stage Cyclone System. EPA-600/7-78-
008, U.S. Environmental Protection Agency, Research Triangle
Park, NC, 1978. 66 pp.
22. Marple, V.A., and K. Willeke. Inertial Impactors: Theory,
Design and Use. In: Fine Particles, B.Y.H. Liu, ed. Academ-
ic Press, New York, 1976. pp. 411-446.
23. Twomey, S. Comparison of Constrained Linear Inversion and
an Iterative Nonlinear Algorithm Applied to the Indirect
Estimation of Particle Size Distributions. J. Coniput. Phvs
18:188-200, 1975.
50
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24. Hochstrasser, J.M. The Investigation and Development of
Cyclone Dust Collector Theories for Application to Miniature
Cyclone Presamplers. Ph.D. Thesis, Department of Environ-
mental Health, College of Medicine, Univ. of Cincinnati,
1976.
25. LOffler, F. The Calculation of Centrifugal Separators.
Staub Reinhalt. Luft (in English) 30:1-5, Dec. 1970.
26. Mercer, T.T. Aerosol Technology in Hazard Evaluation.
Academic Press, New York, 1973. pp. 297-304.
27. Chan, T.L., and M. Lippmann. Particle Collection Efficien-
cies of Air Sampling Cyclones: an Empirical Theory. Environ.
Sci. Technol. 11:377-382, 1977.
28. Rosin, P., E. Rammler, and W. Intelman. Principles and
Limits of Cyclone Dust Removal. Z. Ver. Deut. Ing. 76:433-
437, 1932.
29. Lapple, C.E. Dust and Mist Collection. In: Chemical En-
gineers' Handbook, J.K. Perry, ed. McGraw-Hill, New York,
1963.
30. Earth, W. Calculation and Design of Cyclones on the Basis
of Recent Tests. Brennst. Waerme Kraft 8:1-9, 1956.
31. Leith, D., and W. Licht. The Collection Efficiency of Cy-
clone Type Particle Collectors—A New Theoretical Approach.
AIChE Symp. Ser. No. 126, 68:196-206, 1972.
32. Beeckmans, J.M. A Two-Dimensional Turbulent Diffusion Model
of the Reverse Flow Cyclone. J. Aerosol Sci. 4:329-336,
1973.
33. Sproull, W.T. Air Pollution and Its Control. Exposition
Press. NY, 1970.
34. Davies, C.N. The Separation of Airborne Dust and Parti-
cles. Proc. Inst. Mech. Eng. London B (1):185-198, 1952.
35. Soo, S.L. Particle-Gas Surface Interactions in Collection
Devices. Int. J. Multiphase Flow 1:89-101, 1973.
36. Wagner, J., and R.S. Murphy. Miniature Liquid Cyclones—Effects
of Fluid Properties on Performance. Ind. Eng. Chem. Process
Des. Dev. 10:346-352, 1971.
37. Beeckmans, J.M., and C.J. Kim. Analysis of the Efficiency
of Reverse Flow Cyclones. Can. J. Chem. Eng., in press.
51
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PAPER 3
RESEARCH ON DUST SAMPLING AND MEASUREMENT
IN OUR LABORATORY
KOICHI IINOYA
DEPARTMENT OF CHEMICAL ENGINEERING
KYOTO UNIVERSITY
ABSTRACT
The presentation includes four topics of particle sampling
and measurement studied in our laboratory during the last few
years.
1. Anisokinetic sampling errors by round and slit probes
for wide ranges of Reynolds number and inertia parameter.
2. Sampling efficiency for particles by a ribbon which
has various inclination angles to the flow direction
for wide ranges of Reynolds number.
3. Fractional efficiency of impactor, taking into account
velocity distributions in the boundary layer at nozzle
outlet.
4. Effect of probe diameter on isokinetic sampling error
in experiments.
Details of the four research topics are given in the fol-
lowing sections.
ANISOKINETIC SAMPLING ERRORS OF PARTICLE CONCENTRATION BY ROUND
AND SLIT PROBES
In order to examine the errors in measuring particle concen-
tration due to anisokinetic sampling over a wide range of inertia
parameters, theoretical and experimental studies have been con-
ducted.1'2 In the experiments, a monodisperse methylene blue-
uranine aerosol generated by a spinning disc and lycopodium
52
-------
spores were sampled by a round and a slit type probe respectively
as shown in Figures 1 and 2. Figure 3 shows the details of these
two kinds of test probes. The experimental conditions are given
in Table 1.
1. SPINNING DISK AEROSOL GENERATOR
2. CORONA IONIZER
3. MIXING CHAMBER
4. BAUSCH AND LOMB PARTICLE COUNTER 8. SAMPLING PROBE
5. ORIFICE METER 9. VACUUM PUMP
6. FILTER 10. WET GAS METER
7. BLOWER 11. FILTER BOX
Figure 1. Schematic diagram of horizontal experimental apparatus for
methylene blue-uranine aerosol by spinning disc generator.
'. CONSTANT FEEDER
2. DISPERSER
3. CORONA IONIZER
4. SAMPLING PROBE
5. ORIFICE
6. WET GAS METER
7. VACUUM PUMP
8. BLOWER
9. SAMPLING FILTER BOX
II
Figure 2. Schematic diagram of vertical experimental apparatus for
lycopodium spores.
53
-------
-as—j«-25—JOB-
FILTER BOX
•—40
UI
Earth
BRASS PROBE
ACRYLIC PROBE
SLIT PROBE ROUND PROBE
Figure 3. Sampling probes and filter box.
TABLE 1. EXPERIMENTAL CONDITIONS
Particles
Lycopodium spores
Methylene blue-
uranine
Methylene blue-
uranine
Air Velocity,
u0 (m/sec)
Inertia Parameter,
P
Flow Reynolds
lumber
Geometric
Particle standard
size, deviation,
Dpso(Vm) ag
33 (24) 1.09
3,5 1.07
4.3 1.07
0.2 - 15.
0.05 - 3.
(1.1 - 81)
Particle
density,
Pp(9/cm3)
1.05
1.42
1.42
0
4
x 10 3
Concentration
in liquid
(wt %)
-
0.21
0.5
Theoretical results obtained under the assumption of poten-
tial flow are mostly in good agreement with experimental data
for both types of probe, as shown in Figures 4, 5 and 6. How-
ever, the anisokinetic sampling errors obtained from experiments
with the slit probe are smaller than the calculated values in
the large inertia region (lycopodium), as shown in Figure 7
In general, it is possible to use these figures to estimate*
54
-------
anisokinetic sampling errors. It is also recognized that the
concentration errors become smaller as the sampling velocity
(u) becomes extremely high. Therefore, the super high speed
sampling may be a practically simple method for particulate sam-
pling without the tedious procedure of isokinetic sampling.
o
I-
o
111
o
1
THEORETICAL
CURVE
METHYLENE BLUE
URANINE
WITH CORONA
IONIZER
ROUND PROBE
5m/s R=0.52cm P=0.05(-)3.47il
o u0 = 8m/s R=0.52cm P=0.12<-)4.28jU
A U(J = 6.5m/s R=0.42cm P=0.12(-)4.28M
A u0 = 15ms R=0.42cm P=0.29(-)4.28M
VELOCITY RATIO,— (-)
u
Figure 4. Anisokinetic sampling errors of round probe for smaller
particles (methylene blue-uranine particles).
o
O
IU
o
a P = 1.80
o p = 0.47
• P = 0.07
I Dp = 24 Mnr
LYCOPODIUM
SPORES
•WITH GRAVITY FORCE
USING SCHILLER-
NAUMANN EQUATION -
ROUND PROBE
VELOCITY RATIO,— (-)
Figure 5. Anisokinetic samp/ing errors of round probe for larger
particles (lycopodium spores).
55
-------
WITH CORONA IONIZER
P = 0.035
P = 0.18
P = 0.43
O U0 = 1 m/s Dp = 3.47 jum
METHYLENE BLUE - URANINE
— THEORETICAL CURVE
SLIT PROBE
VELOCITY RATIO,-^ (-)
Figure 6. Anisokinetic sampling errors of slit probe for smaller
particles (methylene blue-uranine particles).
2.5
O
cc
h-
z
O
u
1.5
0.5
o P = 6.22M
• P = 1.24(-}
n P = 0.25I-)
1 r
WITH CORONA IONIZER
LYCOPODIUM SPORES
THEORETICAL
SLIT PROBE Dp = 24 jim
0.5 1 1.5
VELOCITY RATIO,—(-)
2.5
Figure 7. Anisokinetic sampling errors of slit probe for larger
particles (lycopodium spores).
The amounts of particulate deposited in the probe and the
sampling tube are also examined, it is found that the deposit
shows a minimum value at Reynolds numbers of 2000 to 3000 for
56
-------
both probes, as shown in Figures'8 and 9, and that the amount
of deposit in an acrylic probe is much more than that in a brass
one, as shown in Figure 8.
Nomenclature
c : measured concentration
c0 : mainflow concentration
D : particle diameter
M : deposited mass in the probe and sampling tube
Mf : collected mass on the filter
P : inertia parameter = p D2u0/(18iaR) = 2¥
P P
R : probe radius
u : sampling velocity
u0 : main gas velocity
Re : Reynolds number in sampling probe
p : gas density
p : particle density
\i : gas viscosity
SAMPLING EFFICIENCY FOR PARTICULATES BY RIBBON
Average and local impaction efficiencies for a ribbon, at
various angles of inclination to the flow direction, have been
calculated for a wide range of particle Reynolds numbers from
the Stokes to Newton regions.3 The calculations were based on
a discontinuous potential flow model1*"10, which is in good agree-
ment with the observation of paraffin mist streamlines around
the ribbon as shown in Figure 10.
The experimental apparatus is shown in Figure 11. The cal-
culated sampling efficiencies agree well with the experimental
results of lycopodium spores, which have a settling velocity
of 1.76 cm/sec in air, as shown in Figures 12 and 13. The ef-
ficiency decreases as the particle Reynolds number increases
for the region beyond the Stokes one. The smaller the inclina-
tion angle of a ribbon, the higher the sampling efficiency.
This tendency is also observed beyond the Stokes region as shown
in Figure 14. Local impaction efficiencies are higher at the
57
-------
£•102
O
O
O.
UJ
Q
UJ
co
o
Cu
a.
50
30
.OLYCOPODIUM
SPORES
, METHYLENE BLUE - URANINE
(ACRYLIC PROBE)
10
5
QMETHYLENE BLUE
URANINE
\ (BRASS PROBE)
ROUND
PROBE
102 103 104
REYNOLDS NUMBER Re(-)
Figure 8. Relationship between deposition ratio and Reynolds
number in round probe.
+50 - O LYCOPODIUM
• METHYLENE BLUE
URANINE
(ACRYLIC PROBE)
103 1Q4
REYNOLDS NUMBER Re(-)
Figure 9. Relationship between deposition ratio and Reynolds
number in slit probe.
58
-------
Figure 10. Calculated streamlines superimposed on photograph
of wake.
AIR
4U
100
PARTICLES
I
-DISPERSER
AIR AND
PARTICLES/
RECTANGULAR,
DUCT
GRADUATION
OF DEGREE
M
VENTURI FLOWMETER
r^
in
m
GATE
VALVE
n
CYCLONE
BLOWER
GLASS WINDOW
WINDOW
Figure 11. Schematic diagram of experimental apparatus.
-------
UJ
u
u.
o
§
= 103
= 104
THIS WORK
(CALC.)
THIS WORK 30<4>< 160
I 0.2I-
0.5
12 5 10
INERTIAL SIZE PARAMETER
20
50
100
Figure 12. Relation between impact/on efficiency and inertia
parameter for a ribbon perpendicular to the flow.
INERTIAL SIZE PARAMETER
Figure 13. Relation between impaction efficiency and inertia
parameter for a ribbon inclined to the flow at = 10?.
60
-------
0.2 0.5 1 2 5
INERTIAL SIZE PARAMETER
(A)
= 0
0.5 1 2 5 10
INERTIAL SIZE PARAMETER
(B) $ = 103
20
50
0.5 1 2 5 10
INERTIAL SIZE PARAMETER
(C) * = 104
20
50
-)
Figure 14. Relations between impact/on efficiency and inertia parameter
for a ribbon inclined to the flow at 0 = 0 (Stokes flow),
103, and JO4.
61
-------
leading edge (x=l) than those at the trailing edge (x=-l) of
an inclined ribbon. The average sampling efficiencies are de-
termined along the transverse line at a distance of 20% of ribbon
width from the leading edge, as shown in Figure 15A. Therefore,
the microscopic observation of sampled particles should be con-
ducted along this line. Experimental local impaction efficien-
cies are generally in good agreement with the theoretical results
as shown in Figures 15 A and C. As the inclination angle becomes
smaller than 90 degrees, the local impaction efficiencies are
higher at the leading edge and lower at the trailing edge of
the ribbon than those of 90 degrees, and the efficiencies of
the inclined ribbons change little at the center line of ribbon,
compared with those of 90 degrees, as shown in Figures 15 A,
B, C.
Nomenclature
D : particle diameter
r : half width of ribbon
Re : particle Reynolds number for main gas velocity
P° (=Dppu0/u)
ufl : main air velocity
x : distance from the center line of ribbon
B : inclination angle of ribbon to flow direction
n : impaction (sampling) efficiency
n : local impaction efficiency
X
p : air density
p : particle density
p : air viscosity
$ : parameter which accounts for particle motion bevonrl
Stokes1 law = Re2 7(2^) *
po'
Y : inertia parameter for Stokes' law = D2p u /(36Mr)
FRACTIONAL EFFICIENCIES OF ROUND AND SLIT NOZZLE IMPACTORS
Separation mechanisms of an impactor have usually been stud'
by use of a potential flow model. However, there is some dis-
crepancy between such theoretical results and the experimental
data. In the present study, the theoretical impaction efficien
cies have been calculated, taking into account the velocity
62
-------
LEWIS ET AL. (CALC.)
THEORETICAL RESULTS
(THIS WORK)
• EXPERIMENTAL RESULTS
(THIS WORK, •v/2>JF= 0.9)
X AVERAGE EFFICIENCY
0 0.4 0.8 1
DISTANCE FROM CENTER
LINE OF RIBBON X(-)
(A)
= 90°
0 1
-J DISTANCE FROM CENTER LINE
OF RIBBON X(-)
(B) 0 = 10°
0 1
3 DISTANCE FROM CENTER LINE
OF RIBBON X(-)
(C) p = 30°
Figure 15. Local impaction efficiencies for a ribbon of various inclined
angles at $ = 0 (Stokes flow).
63
-------
boundary layer of incompressible gas at the nozzle outlets.
The theoretical results are in good agreement with the experi-
mental values for a round11'12 and a slit nozzle impactor, respec-
tively. Figure 16 shows the fractional efficiency of a slit
nozzle impactor as an example.
Effect of the clearance ratio (Rc), which is defined as
the ratio of the distance between nozzle and impaction plate
to the nozzle diameter, on the fractional efficiency has been
calculated by use of a potential flow model13 for a round nozzle
impactor, as shown in Figure 17.llf The calculated efficiencies
are in good agreement with experimental results for the ratios
of 2, 1, 0.5, 0.33 and 0.25 as shown in Figure 18 as an example.
Table 2 gives the approximate straight line equations of these
fractional curves.
Particle precipitating efficiencies on the inner wall of
a slit nozzle impactor have also been theoretically calculated
by use of a velocity distribution15 in the region where the
motion of a particle is mainly influenced by gravitational and
inertia forces. The nozzle and the calculated efficiencies are
shown in Figures 19 to 23.16 The experimental results obtained
by use of lycopodium spores and ragweed pollens are in good
agreement with the calculated precipitating efficiencies, as
shown in Figure 24.
TABLE 2. APPROXIMATE EQUATIONS OF FRACTIONAL EFFICIENCIES
FOR VARIOUS CLEARANCE RATIOS OF ROUND NOZZLE IMPACTOR
Approximate equations of fractional
Clearance ratio, R efficiency, nm
C J.
5 2.34/7 - 0.427
3 3.17/7 - 0.649
1-3 3.80/7 - 0.954 (Ranz)
1 4.74/7 - 1.05
0.5 5.63/7 - 1.26
64
-------
1
0.9
0.8
0.7
0.6
7 0.5
* 0.4
0.3
0.2
0.1
DC = 0.142cm
DC = 0.07 cm
POTENTIAL
FLOW
(DAVIES)
Rc>2
A YUU
Re = 0.5
• RANZ
Rc= 1-3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Figure 16. Relation between fractional efficiency and inertia parameter
for a slit nozzle impactor (S. Yuu).
Nomenclature
C : Cunningham slip correction factor
D : nozzle diameter or width at outlet
C
G : gravitational parameter or dimensionless settling
velocity = Cp Dpg/(18yu0)
L : distance from nozzle outlet to impaction plate
L : dimensionless nozzle length = fc/D
G
& : length of converging part of nozzle
R : clearance ratio = L/D
Re. : Reynolds number based on nozzle outlet
u0 : gas velocity at nozzle outlet
a : half-angle of converging nozzle
n : impaction (target or fractional) efficiency
65
-------
nwc : calculated precipitating efficiency on the inner nozzle
wall
¥ : inertia parameter = CD*p u0/(18UD )
c c C
EFFECT OF PROBE DIAMETER ON ISOKINETIC SAMPLING ERRORS
In order to examine the effect of probe diameter on iso-
kinetic sampling errors, experimental studies have been conducted
using methylene blue-uranine aerosol and lycopodium spores in
the small and large inertia regions respectively.17 It is found
that the standard deviation of concentration ratio, defined as
the concentration measured by a probe of any diameter divided
by that of a 1-cm diameter probe, becomes larger as the probe
diameter decreases. The following empirical equations are ob-
tained by use of the method of least squares, assuming that the
standard deviation is inversely proportional to the square root
of sampling amount W.
a = 9.19 x 10~3 W~°'5 d~°'31; for lycopodium (1)
a = 7.25 x 10"" W~°'5 d~°-28; for methylene blue-uranine (2)
Re = 0.50
I
Rc= 1.0
Re = 2.0
0.1 0.2 0.3 0.4 0.5
Figure 17. Relation between fractional efficiency and inertia parameter
for various clearance ratios of a round nozzle impactor.
66
-------
0.9
0.8
0.7
0.6
1 0.5
R-
0.4
0.3
0.2
0.1
17=5.63
0'
0.1 0.2 0.3 0.4 0.5
Figure 18. Relation between fractional efficiency and inertia parameter
for clearance ratio Rc = 0.5 of a round nozzle impactor.
67
-------
NOZZLE WALL
FLUID FLOW LINE
- CRITICAL TRAJECTORY
OF PARTICLE
Figure 19. Schematic diagram of two-dimensional nozzle.
A comparison of experimental data with the results calculated
by the above equations is shown in Figure 25.
It is also found in the small inertia region that the mean
values of concentration ratio measured by smaller diameter probes
differ with each other and significantly deviate from unity in
some cases, as shown in Figure 26.
c
GO
d
n
: mean measured concentration (g/m3)
: mean measured concentration for 1-cm reference probe
diameter (g/m3)
: inner probe diameter (cm)
: number of trial
68
-------
u0 : main flow velocity far upstream of probe (m/sec)
W : amount of sample (g)
a : nondimensional standard deviation of sample
a, : nondimensional standard deviation by reference probe
of 1 cm diameter
i
0.8
0.6
0.4
0.3
0.2
0.08
0.06
0.04
0.03
0.02
0.01
*
a.
L
0.36
0.2121(radianM2°
20
TAwcmaxj 0.8323
= 5
0.001 Q.003 0.006 0.01
G <-)
0.03 0.06 0.1
Figure 20. Calculated settling efficiencies for different Reynolds numbers,
taking account of air velocity distribution at nozzle entrance.
69
-------
0.06
0.04
0.03
o.o:
Re;
Re:
Rej = 50x
*
Oi
L
ffa/cmax
0.36
0.21 21 (radian)
12°
20
0.8823
I II II
0.001 0.003 0.006 0.01
G (-)
0.03 0.06 0.1
Figure 21. Calculated settling efficiencies for different Reynolds numbers
with uniform air velocity at nozzle entrance.
0.2
0.02
0.01
50
15
~f<0.36
Rej
a
L
^cmax
200
0.21 21 (radian)
120
20
0.8823
0.001 0.003 0.006 0.01
G <-)
0.03 0.06 0.1
Figure 22. Calculated settling efficiencies for various inertia parameters,
taking account of air velocity distribution at nozzle entrance.
70
-------
1
0.8
0.6
0.4
0.3
0.2
o 0.1
flo.08
0.06
0.04
0.03
0.02
Re;
*
a
200
0.36
0.21 21 (radian)
12o
L = 60
0.01
0.001 0.003 0.006 0.01
G (-)
0.03 0.06 0.1
Figure 23. Calculated settling efficiencies for various nozzle lengths,
taking account of air velocity distribution at nozzle entrance.
1
0.8
0.6
0.4
0.3
0.2
0.1
0.08
0.06
0.04
0.03
0.02
0.01
0.01
• RAGWEED
OLYCOPODIUM
0.003 0.006 0.1
0.03 0.06 1
Figure 24. Comparison of experimental settling efficiencies with
calculated ones.
71
-------
0
cc
Q 0.5
Z
w _
^ 0 0.1
£ P
DC <
X UJ
01 O
i ill'11! ' ' ' \ ' ' 'X
' OLYCOPODIUM ^A / -
SPORES /
%o° °^ AA
f & °° ~-
: ,0/e -
' / o AMETHYLENE
' / o° BLUE ' URANINE "
/ i , 1 i i i i 1 i i __i — 1 i i i i
0.05 0.1
0.5 1
STANDARD DEVIATION a (-)
CALCULATED BY EQS.(1) AND (2)
Figure 25. Comparison of experimental and calculated standard deviations
of concentration ratio.
j
T
>\
-------
REFERENCES
1. Yoshida, H., T. Osugi, H. Masuda, S. Yuu, and K. linoya.
Kagaku Kogaku Ronbunshu 2:336, 1976.
2. Yoshida, H., T. Osugi, H. Masuda, and K. linoya. Zairyo:
J. Soc. Material Sci. Japan. 25:667, 1976.
3. Ushiki, K., K. Kubo, and K. linoya. Kagaku Kogaku Ronbunshu
3:172, 1977.
4. Hess, J.L. J. Fluid Mech. 60:225, 1973.
5. Lewis, W., and R.J. Brun. NACA Tech. Note 3658, 1955.
6. Langmuir, I., and K.B. Blodgett. U.S. Army Air Force Tech.
Rep. No. 548, 1946.
7. Brun, L.D., and M. Vasseur. Rech. Aeronaut. 15:1, 1948.
8. Ranz, W.E., and J.B. Wong. Ind. Eng. Chem. 44:1371, 1952.
9. May, K.R., and R. Clifford. Ann. Occup. Hyg. 10:83, 1967.
10. Gregory, P.H., and Q.J. Stedman. Ann. Appl. Biol., 40:651,
1953.
11. linoya, K. In: Fine Particles, B.Y.Liu, ed. Academic Press,
New York, 1976. p 24.
12. Yuu, S., N. Miyake, and K. linoya. Kagaku Kogaku Ronbunshu
1:115, 1975.
13. Strand, T. J. Aircr. 4:466, 1967.
14. Yuu, S., and K. linoya. Kagaku Kogaku 34:427, 1970.
15. Rosenhead, H. Laminar Boundary Layers. Oxford Press, 1963.
p. 114.
16. Yuu, S., and K. linoya. Kagaku Kogaku 35:1251, 1971.
17. Yoshida, H., et al. J. Chem. Eng. Japan 11:48, 1978.
73
-------
PAPER 4
SIZING SUBMICRON PARTICLES WITH A CASCADE IMPACTOR
MICHAEL J. PILAT
DEPARTMENT OF CIVIL ENGINEERING
UNIVERSITY OF WASHINGTON
ABSTRACT
The UW Mark 3-4 Source Test Cascade Impactors and associated
sampling train have been designed, constructed, and used for
measuring the size distribution over the approximately 0.02 to
20 ym aerodynamic range. The UW Mark 4 Cascade Impactor utilizes
low absolute gas pressure which results in large Cunningham cor-
rection factors and accordingly enables sizing of particles of
0.02 to 0.2 ym diameter. The Mark 3-4 sampling train includes
the UW Mark 3 and Mark 4 Cascade Impactors followed by a 90 mm
diameter filter, a low pressure drop tabular coiled condenser,
a two-stage high vacuum pump, and an instrumental control box
for pressure and temperature gauges, heater controls, and valves.
The UW Mark 3-4C Cascade Impactors operate around 0.4 to 0.5
acfm whereas the UW Mark 3-4D model has a gas sampling flow rate
in the 1.6 to 1.8 acfm (stack conditions) range. Particle size
distributions measured at emission sources including the J.E.
Corette Power Plant (pulverized coal-fired boiler), UW Power
Plant (pulverized coal-fired boiler), and Kearny Power Plant
(Pratt and Whitney oil-fired gas turbine) are presented.
INTRODUCTION
Cascade impactors have been used since about 1968 to measure
the in-stack distribution (in the 0.2 to 20 ym diameter range)
emitted from stationary sources. Research at the University
of Washington has resulted in the development of the University
of Washington Source Test Cascade Impactor (models Mark 12
3, 4, 5, 10, and 20) which have been or are being patented by
Pilat1 and licensed out by the University of Washington (to Pol-
lution Control Systems Corporation of Renton, WA) for manufac-
ture and commercial sale.
74
-------
This paper reports on the Mark 3-4 Source Test Cascade Impac-
tor sampling train and its application for sizing particles
emitted from electric utility power plants. Pilat, Ensor, and
Bosch2 reported on the development and use of the Mark 1 and
2 UW Source Test Cascade Impactors. The Mark 1, 2, 3, and 5
models size particles in the general range of 0.2 to 20 urn aero-
dynamic diameter. The Mark 5 model has 11 stages followed by
a filter and is designed for use at high particle concentrations
such as exist at the inlet to control equipment. Pilat3 described
the earlier development work on the Mark 4 impactor. Pilat,
Fioretti, and Powell1* reported on the sizing of particles (in
the 0.02 to 20 ym diameter range) emitted from coal-fired power
boilers using the Mark 3-4 cascade impactor sampling train.
THEORY OF CASCADE IMPACTORS
Cascade impactors fractionate the particulate matter from
an emission source into size increments by inertial impaction
of the particles on a collection surface. This occurs at suc-
cessive stages within the impactor. The resulting index of
particle size is traditionally expressed by the particle size
collected with 50% collection efficiency for each stage, d •
The particle diameter has been related to the Stokes inertial
impaction parameter Y which is defined by Ranz and Wong5 as
c?dp vj
18u D.
(1)
where C is the Cunningham correction factor, p the particle den-
sity, dp the particle diameter, Vj the gas velocity in the jet,
u the gas viscosity and Dj the jet diameter. Solutions of the
equation of particle motion at various magnitudes of ¥ and experi-
mental studies have shown that the Stokes inertial impaction
parameter at 50% collection efficiency (Yso) for a particular
diameter (d50) ranges between 0.12 and 0.17 for circular jets.
These values were originally reported by Ranz and Wong5 and later
confirmed by McFarland and Zeller.6 Solving for the particle
diameter from equation (1) gives
fisi
t
.
Cp V.
P 3
h
(2)
Substituting an average value of 0.145 for Y50 provides an equa-
tion for d50,
75
-------
IB o
2.61p D.
(3)
Equation (3) provides an expression which relates the cascade
impactor stage d50 and the impactor parameters. These param-
eters can be appropriately altered to provide an even distri-
bution of dso's throughout the impactor stages. For sizing of
submicron particles, the Cunningham correction factor becomes
of particular significance due to the physical limitations in
further altering the other impactor parameters. The Cunningham
correction factor C is defined by an equation reported by Davies7
C = 1 +
2X
d
5 0
[l.
275 + 0.40 exp
(-1.
10
(4)
where X is the gas mean free path. Thus, it can be seen by exami-
nation of equations (3) and (4) that it is possible to select
the appropriate magnitudes of the impactor parameters necessary
to provide a stage dso as low as 0.02 Mm. Assuming a particle
density p of 1 g/cm3 and substituting into equation (3) provides
an equation for the aerodynamic cut diameter daso
h
da
5 0
2.61y D.
C V.
D J
(5)
UW MARK 3-4 CASCADE IMPACTOR INSTRUMENTATION
Sampling Train
The UW Mark 3-4 Cascade Impactor sampling train is shown
in Figures 1 and 2. Figure 1 illustrates the Mark 4D and 90
mm filter located inside the stack, whereas Figure 2 shows them
external of the stack. Heaters can be used to maintain the Mark
4D and filter holder at the desired temperatures. The Mark 4C
cascade impactor can also be used with the Mark 3-4D sampling
train. The UW Mark 3-4 Cascade Impactor sampling train includes
a UW Mark 3 impactor, a UW Mark 4 impactor, a filter holder,
a condenser, a leakless vacuum pump, a dry gas meter, and a'con-
trol box. The train can also be used with a BCURA cyclone up-
stream of the Mark 3 impactor. When physical constraints such
as too small a diameter of the sampling port exist, the sampling
train can be arranged with the Mark 4 located outside the stack
External heaters can be used to maintain the Mark 4 and filter
holder temperature above the dew point.
The control unit contains the instruments for measuring
the temperature and gas pressures, heater control, and pressure
tap valves. Pyrometers are used for the temperature readouts
measured by type J (iron-constantan) thermocouples. Thermocoupl
es
76
-------
LJCURA
CYCLONE
MARK 3
IMP ACTOR
90 mm
MARK 4 FILTER
IMPACTOR HOLDER
GAS FLOW
TO
CONTROL BOX
(Tt) THERMOCOUPLE
?y) PRESSURE TAP
STACK WALL
trr::
ooooooooo
CONTROL BOX
CONDENSER
DRY GAS
METER
VACUUM PUMP
Figure 1. U. of W. Mark 3-4D cascade impactor sampling train schematic.
-------
BCURA
CYCLONE
MARK 3
IMPACTOR
oo
3.
S
r
GAS FLOW
THERMOCOUPLE
PRESSURE TAP
STACK WALL TO
CONTROL BOX
STAGE
PRESSURE
TAPS
©T®T© MK 4 IMPACTOR
90 mm FILTER HOLDER
HEATING
INSULATION OR \ MANTLE
HEATING TAPE
CONTROL BOX
DRY GAS
METER
VACUUM PUMP
Figure 2. Mark 3-4D sampling strain with Mark 4D located outside the stack.
-------
are located for measurement of the gas temperature inside the
stack, at the Mark 4 inlet, Mark 4 outlet and filter outlet.
Gas pressures are measured at all Mark 4 jet stages, the filter
inlet, and the filter outlet. The gas velocity and static pres-
sures are indicated by Dwyer Magnahelic gauges. The external
heaters for the cascade impactors and/or filter holder are con-
trolled by rheostats housed in the control unit.
UW Mark 3 Cascade Impactor
The UW Mark 3 Cascade Impactor has 7 stages followed by
a filter holder (47 mm diameter filters). Figure 3 presents
a graph of the Mark stage da50's (calculated) as a function of
the gas volumetric gas sampling rate at stack conditions (as-
suming a gas pressure of 2S.92 in. mercury). The number of jets
per stage and the stage jet diameters are presented in Figure 3.
UW Mark 4C Cascade Impactor
The UW Mark 4C Cascade Impactor has jet stage and collection
plate dimensions similar to the Mark 3 model such that the cy-
lindrical casing diameter is about 2.9 in. (will fit through 4
in. diameter pipe sampling port). However, the Mark 4C has gas
pressure taps on the jet stages. Also the Mark 4C has a unique
arrangement of jet hole diameters and numbers per stage such
that the gas velocity does not reach Mach 1 (sonic velocity)
on any one stage until an upper limit gas sampling rate is reached,
This upper liir.it gas sampling rate is about 0.5 cfm for the Mark
4C. With this lower gas sampling rate, a 47 mm diameter filter
holder can be used.
UW Mark 4D Cascade Impactor
The Mark 4D Cascade Impactor model was designed in 1974
to have approximately 2 to 3 times the gas sampling flow rates
of the Mark 4C (or about 1 to 1.8 cfm at stack conditions).
The Mark 4D has a jet stage of about 3.55 in. diameter and a
cylindrical casing of 3.95 in. diameter and thus requires a mini-
mum 5 in. diameter pipe sampling port. Copper tubing of 1/8-
in. diameter is connected to the pressure taps on each jet stage.
This pressure tubing is connected to the pressure gauges in the
control box.
For collecting those particles passing the last stage of
the Mark 4D, a 90 mm filter holder is used. This 90 mm filter
holder was designed and constructed at the UW. The condenser
is connected to the sampling probe by a Teflon lined flex hose.
The condenser is to condense and collect water vapor and other
condensable vapors. The condenser is a 3/4-in. diameter stain-
less steel tube coil with a built-in water trap. It employs
dry ice as a coolant and is capable of handling a 5.0 scfm gas
79
-------
1.20
102
£
a.
6
in
Q
Ul
o
o
<
Q
O
cc
o
cc
10
10°
IO-
I i
FOR UPPER CURVE IN GROUP T = 500 °F
-FOR MIDDLE CURVE IN GROUP T = 300 °F
FOR LOWER CURVE IN GROUP T = 100 °F
I
10
rl
3 4567 8910°
FLOW RATE, Q, cfm
C/3
Ul 111
C3 "'
V) Z
1 1
3 12
4 90
5 110
6 110
CC
111
LLJ
— HI
Q I
0.7180
2 6 0.2280
0.0960
0.0310
0.0200
0.0135
7 90 0.0100
Figure 3. Calculated aerodynamic cut diameters for U. of W. Mark 3 stages.
flow rate. A two-stage, leakless vacuum pump is used with the
Mark 3-4.D sampling train. A Rockwell Model 250 dry gas meter
with temperature compensation to 70°F is used to measure the
volume of gas sampled and the gas sampling flow rate.
BCURA Cyclone
A BCURA (British Coal Utilization Research Association)
Cyclone is used to collect the larger sized particles which at
the higher particle concentrations can overload the inlet par-
ticle collection plates of the Mark 3 impactor. The BCURA Cyclone
is located at the inlet of the Mark 3 impactor, as illustrated
in Figure 1. The particulate sample is collected in the hopper.
80
-------
When physical constraints hinder the use of a 90° angle probe,
the BCURA Cyclone offers a convenient approach for enabling the
sampling nozzle to be faced upstream. The BCURA Cyclone (as
commercially available from Pollution Control Systems Corporation,
Renton, WA) can be used in sampling ports of 4 in. or larger
diameter (this commercially available model is slightly modified
from the British model).
The equation for calculating the aerodynamic cut diameter
da5o of the BCURA Cyclone has been reported by Hawksley, et al.,8
da
5 0
18 K a.p D
c i
P Q
(6)
where ai is the cross-sectional area of the cyclone inlet, D
the cyclone body diameter, y the gas viscosity, p the particle
density (assumed to be unity), Q the gas volumetric flow rate
through the impactor, and Kc a dimensionless cyclone constant.
Kc is defined by the equation
K = X/L (7)
\f
where L is the cyclone length and X is the "range" of the par-
ticles. The variable X is defined by the equation
X = u V/g (8)
where u is the Stokes velocity of the particle, V is the fluid
velocity and g the acceleration of gravity.
The aerodynamic cut diameter da5 0 for the BCURA Cyclone
as a function of the gas sampling flow rate at stack conditions
is presented in Figure 4.
UW MARK 3-4 CASCADE IMPACTOR SAMPLING PROCEDURE
Pre-Test Preparation
Prior to a particulate source test, certain information
about the emission source should be known. Since the aerodynamic
particle diameters (d50's) of the impactor stages are dependent
upon the gas flow rate through the impactor, approximate values
for the exhaust gas temperature and volumetric flow rate should
be known. From this information, a jet stage sequence can be
chosen to provide an even distribution of particles throughout
the impactors for the particle size range expected.
To reduce the tare weight for the particulate collection
plates, 0.002 in. thick stainless steel inserts are cut and formed
for the impactor plates. The inserts must be conditioned for
81
-------
10.0
10.0
ACFWI AT STACK CONDITIONS
Figure 4. BCURA cyclone aerodynamic cut diameter versus gas
sampling rate.
the particular emission source. They are first baked at a tem-
perature slightly higher than the expected exhaust gas tempera-
ture to alleviate the possibility of weight loss due to evapora-
tion of surface impurities. If the particulate matter to be
sized is of a non-adhesive nature (e.g., coal dust) the collec-
tion inserts can be greased with a thin layer of an adhesive
such as Dow Corning High Vacuum Grease prior to baking. This
minimizes bounce off and re-entrainment of particles within the
impactor. After the baking process the inserts and filters are
desiccated for at least 12 hours and weighed with an accuracy
of ± 0.01 mg. They are then stored in marked petri dishes and
are ready for shipment to the sampling site. Blank inserts are
prepared in order to identify weight changes caused by reasons
such as weighing balance out of calibration or grease volatiliza-
tion.
Certain consideration must also be given to the physical
restrictions of the sampling site. A probe length must be chosen
to be compatible with the particular stack or exhaust duct to
be tested. The size and location of the actual sampling plat-
form can also affect the lengths of reinforced flex hose required
to connect the components of the sampling train.
Prior to testing, it is important to thoroughly clean the
particulate sizing components of the train. An ultrasonic cleaner
is recommended for cleaning the impactor jet stages.
82
-------
Test Procedures
With the impactors cleaned and loaded, the sampling train
is assembled as described above. Temperature measurements are
performed and the gas humidity is calculated by the wet bulb-
dry bulb method. A type S pitot tube is used for velocity head
and stack static pressure measurements. A velocity profile
across the duct is selected for isokinetic sampling.
When the test parameters have been measured and calculated,
the probe is inserted inside the stack with the nozzle facing
downstream. Sufficient time (at least 15 minutes) is allowed
to pre-heat the impactors and filters to stack temperature.
The probe is then rotated so that the nozzle faces upstream and
the test is started.
After an isokinetic sampling rate has been established,
pressure measurements for the various stages of the Mark 4 impac-
tor are made and recorded. After all pressure readings have
been recorded, the pressure at the filter inlet is monitored
in order to detect an overloading of the filter. The tempera-
ture in the stack and at the impactor outlets is also monitored
and recorded on the data sheet.
Upon completion of the test the probe is removed from the
stack and allowed to cool. The impactors and filter holder are
disassembled and the collection inserts and filter are carefully
removed and stored in the appropriate petri dishes. The cyclone
catch is stored in a marked petri dish and the cyclone is washed
with acetone. The wash liquid is also stored in a sample bottle.
Post-Test Procedures
The collected samples (inserts and filters) are first baked at
250°F to drive off any moisture which might have been collected
in return shipment. The wash solutions are evaporated at slightly
elevated temperatures in cleaned and tared aluminum dishes.
They are then desiccated for at least 12 hours and weighed.
The particle weights are then recorded and used in the computer
program for data reduction.
The data reduction computer program RASS is used to cal-
culate size distributions, mass concentrations and other per-
tinent data to be used in the final analysis. It also generates
input information to be used in a particle collection efficiency
program which calculates the collection efficiency as a function
of particle size using data from simultaneous inlet and outlet
tests. Figure 5 presents a computer flow diagram for the data
reduction program which illustrates the calculation procedures.
83
-------
oo
No. of Holes
Per Jet Stage
Absolute Stage
Pressure
(in. Hg)
Hole Diameters
Per Stage
(in.)
40.
"jet
imp.
T;OKS
1 duct
Ma;r -= 2S.9
J
R = 8.31 x 10'
VISCOSITY
^ = 63 + 0.40T(°K)10-6
^impactor
A WEIGHTS FOR EACH COLLECTOR
PLATE PLUS FILTER
pp = 1.0 «P
= 0-145
(I M
P = 1 ATM
AIR MEAN FREE PATH
, _ M(82.057)T
0.499 pM
8RT
CUNNINGHAM i CORRECTION
V
jet
CUMULATIVE SIZE DISTRIBUTION BY WEIGHT
WT total on collection
plates with d50 d50 plate x % BY WEIGHT
(100) = LESS THAN
WT
total
d50 FOR PLATE X
^ \]\l j
SUBROUTINE:
LEAST SQUARES FIT FOR STRAIGHT LINE
THROUGH DATA (LOG NORMAL DISTRIBUTION),
95% CONFIDENCE LIMITS
ITERATIVE
LOOP
C = 1 +-F 1.257 + 0.40e
(-'•'•
1 meter
1 total
V
meter-
CONCENTRATION (GRAINS/SCF)
(15.43)
520° R
'meter
p
rmtr
% BY-WT-LESS-THAN
FOR 6 JET STAGES
MASS MEAN
DIAMETER
STANDARD
GEOMETRIC
DEVIATION
PLOT OF LN. PARTICLE DIA.
vs % BY-WT-LESS-THAN
(WITH 95% CONFIDENCE
LIMITS + STRAIGHT LINE FIT)
PRINTOUT
d50 FOR
EACH STAGE
CONCENTRATION
FOR TEST
Figure 5. Computer f/ow diagram for reduction of U. of I/I/. Mark
3 and Mark 4 cascade impactor test data.
-------
PARTICLE SIZE DISTRIBUTION RESULTS
J.E. Corette Power Plant
Boiler and Electrostatic Precipitator Description—
Unit No. 2 at the J.E. Corette Power Plant, Billings, MT,
burns pulverized coal in a Combustion Engineering boiler rated
at 163 MW electrical output. Sub-bituminous Grade C coal having
a gross heating value of 8,600 Btu/lb and a sulfur content of
0.7% is introduced in a tangential firing (swirling) pattern
from 16 burners located at four elevations in the boiler corners.
Combustion gases leaving the unit typically contain 80% of the
total ash load and pass through a regenerative air preheater
before entering a Research-Cottrell electrostatic precipitator.
The precipitator is designed for 600,000 acfm gas flow at 300°F
and 96% by weight particulate collection efficiency.
Sampling Site Description—
The inlet sampling site was located on the top of a short
horizontal section of duct connecting the air preheater and the
precipitator. The 4 in. diameter sampling port used was located
about 0.34 equivalent duct diameters downstream and less than
0.1 diameters upstream from major gas flow disturbances. The
arrangement of this port necessitated vertical insertion of the
sampling equipment.
Sampling Technique—
The Mark 3-4D Cascade Impactor sampling train was used for
the tests. Due to certain physical constraints at the sampling
ports, the Mark 4D impactor and the absolute filter were located
outside the stack as illustrated in Figure 2. Efforts were made
through thermostatically controlled heating tape and insulation
to maintain the Mark 4D and the filter at stack temperatures.
Due to the nature of the aerosol to be tested, the collec-
tion inserts were greased with Dow Corning High Vacuum Grease,
conditioned at 350°F and weighed. All other pre-test and testing
procedures as well as the post test analysis were performed in
the same manner as described above in this paper.
Five'tests (A through E, Tables 1 and 2) were conducted
at the site during September 29 and October 8, 1975. The first
four were performed at the outlet sampling port while the last
(Test E) was at the inlet to the precipitator. All of the tests
were conducted under normal operating conditions with the excep-
tion of tests C and D in which the soot blowers in the super-
heater section were curtailed.
85
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Test Results—
The distribution of particulate matter collected by the
various components of the Mark 3-4 sampling train is presented
in Table 1. The source test results are listed in Table 2.
The Pearson Correlation Coefficient, listed in Table 2, is a
measure of the degree of fit between the log normal approxima-
tion (a linear regression) and the actual particulate size dis-
tributions. The Pearson Correlation Coefficient is more pre-
cisely defined as the ratio of the covariance to the square root
TABLE 1. DISTRIBUTION OF PARTICULATE WEIGHT COLLECTED BY THE
UW MARK 3-4 IMPACTOR SAMPLING TRAIN FOR J.E. CORETTE TESTS
Particulate weight, g
Test
A
B
C
D
E
BCURA cyclone
hopper & wash Mark III Mark IV
0.0514
0.0332
0.1182
0.1850
0.9984
0.0559
0.0617
0.0888
0.0883
0.1808
0.0015
0.0032
0.0023
0.0045
0.0024
Condenser
Filter trap & wash
0.0006 0.2142
0.0007 0.0298
0.0007 0.0348
0.0018 0.0018
0.0029 0.0076
Total weight
excluding
condenser
0.1094
0.0978
0.2100
0.2796
1.1845
TABLE
2. SOURCE TEST RESULTS
FOR J.E. CORETTE TESTS
Test
A
B
C
D
E
Mass mean
diameter,
ym
4.9
3.5
10.4
35.7
210.6
Standard
geometric
deviation
5.17
5.32
5.10
12.97
12.73
Pearson
Correlation
Coefficient
0.985
0.974
0.967
0.953
0.910
Condensed
Solid particle particle
concentration, concentration,
gr/dscf gr/dscf
0.064
0.052
0.110
0.192
3.151
0.126
0.016
0.018
0.001
0.033
86
-------
of the product of squares of the variation in X and the variation
in Y. The actual particulate size distributions for each test
are illustrated in Figure 6.
Discussion—
The average size distribution for the four tests conducted
at the outlet of the precipitator (Tests A-D) shows that 13.4%
of the particle mass is less than 1.0 ym in diameter.
The actual size distribution for the inlet, Test E, shows
a marked skew to the right for particles greater than about 0.8
ym in diameter indicating a decreasing concentration of large
particles. The mass mean diameter for this test, calculated
from the log-normal approximation, is indicated as 210 ym. The
lack of additional actual data points for particles greater than
4.0 ym has skewed the log-normal approximation to the left and
given it a greater relative slope. Extrapolation of this
curve from the last actual data point of 7.5% of the cumulative
weight to the 50% points results in the unreasonably high mass
mean diameter. A further breakdown of the mass which was consoli-
dated in the last data point (approximately 90% of the total
mass) would eliminate the necessity of extrapolating the curve
to 50% of the cumulative weight and result in a more reasonable
mass mean diameter. For this particular test the BCURA cyclone
has a cut point diameter which was too small, thus collecting
and consolidating too much data into one point.
The vertical insertion of the sampling train into both the
inlet and outlet sampling ports meant that the UW Mark 3 and
4D impactors were oriented in an upside-down position. Disas-
sembly of the impactors after each test showed that some par-
ticles had fallen off the collection plates onto the preceding
jet stages. These jet stages were cleaned by washing with dis-
tilled water and the particle weights were determined (after
evaporation to dryness) and added to appropriate collection
stages. However, the extra lab work caused by the vertical
sampling probe alignment could be obviated by either a horizontal
or a right-side up positioning of the impactors.
UW Power Plant
Description of Source—
UW Power Plant Boiler No. 5 is a Riley Stoker Corporation
pulverized coal-fired boiler. It has a rated steam capacity
of 120,000 Ib/hr with outlet steam conditions of 400 psig and
600°F. The exhaust gases from Boiler No. 5 pass through a Re-
search-Cottrell electrostatic precipitator with an overall design
efficiency of 94% for particulate matter. The precipitator is
designed for a volumetric flow rate of 50,000 acfm at 400°F.
87
-------
10.0
II I
I I
1 I
E
6
in
D
OC
UJ
I-
ui
o
o
Q
O
cc
ui
<
at
u
cc
Q.
0.1
0.01
I
TEST A « OUTLET
TEST B A OUTLET
TEST C V OUTLET
TEST D O OUTLET
TEST E D INLET
0.01
1 10 50
% OF MASS LESS THAN STATED DIAMETER
90
Figure 6. Particle size distributions for tests A-E, J. E. Corette
Power Plant.
88
-------
The simultaneous samples were taken at the inlet and outlet
of the precipitator. The inlet sampling port is located at the
No. 5 Boiler exhaust duct prior to entrance into the precipitator.
The exhaust gases flow vertically upwards at the sampling point.
Sampling Technique—
The UW Mark 3-4 Cascade Impactor Sampling Train used was
identical to the one described above. The BCURA Cyclone, the
Mark 3 and Mark 4 Impactors and the 90 mm glass fiber filter
were inserted inside the duct. A Mark 4C was used at the pre-
cipitator inlet and a Mark 4D was used at the precipitator out-
let.
The pre-test preparation, the testing procedures and the
post-test analyses were conducted in the same manner as described
above. All pre and post test tasks were performed at the UW
Air Resources Lab since the UW power plant is directly adjacent
to the Civil Engineering Building. The two tests reported (A
and B) were simultaneous source tests performed at the inlet
and outlet of the electrostatic precipitator.
Test Results--
The size distribution and collection efficiency results
are summarized in Table 3. The overall collection efficiency,
including solid and condensible matter, was found to be 94.4%
and 90.3% for the two tests performed. The particle size dis-
tributions are presented in Figure 7.
Discussion--
These tests demonstrated that the Mark 3-4 impactor trains
were capable of simultaneous particle sizing (during the same
time period) at the inlet and outlet of particulate control de-
vices. Testing was performed at the inlet and outlet of the
control device at considerably different gas sampling rates pro-
viding particle collection efficiency data as a function of par-
ticle size over a 0.07 to 3.5 ym size range.
Kearny Generating Station
Test Site Information—
The primary objective of the source tests at the Public
Service Electric and Gas Company Generating Station, Kearny,
NJ, was to determine the effect of three different fuel additives
upon the particulate size distribution in the exhaust gas from
an FT4C-1LF Pratt and Whitney gas turbine engine. The three
different smoke suppressing fuel additives injected into the
turbine fuel supply are listed in Table 4. To avoid any con-
flict as to the industrial designation of the additives, they
89
-------
10.0
10 in
Q 1.0
111
<
Q
H
D
O
o
Z
Q
O
oc
TT
1 I
0.01
I I
0.01 0.1 10 30 60 80
% OF MASS LESS THAN STATED DIAMETER
Figure 7. Particle size distributions for U. of W. Power Plant.
90
-------
TABLE 3. SOURCE TEST RESULTS OF UW POWER BOILER TESTS
Particle mass concentration, gr/dscf Overall
Test
A-Inlet
A-Outlet
B-Inlet
B-Outlet
Mass mean
diameter ,
ym
25
12
23
34
Standard
geometric
deviation
12
12
9
21
Pearson
Correlation
Coefficient
0.92
0.91
0.91
0.90
Solids
0.708
0.033
0.784
0.050
particle
collection
Condensibles efficiency,
0.010
94.4
0.007
0.000
90.3
0.026
-------
TABLE 4. DESCRIPTION OF FUEL ADDITIVES
USED FOR KEARNEY TESTS
Additive
Test designation
A None
B I
C I
D II
Additive metal content,
% by weight
6
6
5
1
.75
.75
.34
.48
Fe
Fe
Mn
Ba
III 24.7 Mn
are defined as additives I, II and III. Additive I was an iron
based substance, additive II was an organic manganese-barium
compound and additive III was an organic manganese compound.
All additives were mixed with the fuel at a concentration of
approximately 50 ppm.
The FT4C-1LF Pratt and Whitney gas turbine which was tested
at the Kearney Power Plant is one of two engines included in
a TP4-2LF Twin Pac gas turbine installation. The two turbines
are located in opposite directions and simultaneously drive the
generator. The generator is also capable of operation at approxi-
mately half power with one turbine either windmilling or decoupled
from the generator. For all tests performed, turbine B was de-
coupled.
Test Schedule and Sampling Conditions—
The tests were performed January 21-27, 1976. The UW Mark
3-4 Cascade Impactor sampling train was arranged as shown in
Figure 2 with the BCURA Cyclone and Mark 3 Impactor located in-
side the stack and the Mark 4D Impactor and 90 mm diameter filter
holder outside the stack. The overall test preparation and pro-
cedures were the same as described above with certain considera-
tions given to the extreme conditions to be faced in this test
To alleviate any problem with weight changes in the stainless
steel collection inserts due to the extreme temperatures, pre-
test temperature experiments were performed. Type 304 stain-
less steel was found to exhibit a significant weight change due
to intergranular corrosion when baked at 950°F for a 2 hour
period. Type 316 ELC (extra low carbon) stainless steel showed
negligible weight change due to the elevated temperature and
was chosen for the collection insert material. All inserts
prebaked at 950°F for 2 hours, desiccated, and weighed, then
packed in marked petri dishes.
92
-------
Test Results—
The results from the five particulate source tests are pre-
sented in Table 5. The mass mean diameters are shown to vary
from 0.13 to 0.21 ym. Figure 8 illustrates the actual particle
size distribution for each test. The particles sized in all
five tests ranged from 0.03 ym to 3.7 ym in diameter. Excellent
correlation between the straight line approximation and the
actual size distribution is demonstrated by the Pearson Correla-
tion Coefficients listed in Table 5.
Discussion—
The source test results indicate that the turbine fuel ad-
ditives have a small effect upon the size distribution by mass
of the particulate emissions. The most apparent variation is
observed for the larger particles. The injection of additives,
particularly the organic manganese and the manganese-barium ad-
ditives, created a greater percentage of larger particles (greater
than 0.3 ym diameter). The additives had only a slight effect
upon mass mean diameter, the greatest change being for the two
manganese additives. The convergence of the size distributions
in the 0.1 and 0.2 ym diameter range indicates an insensitivity
for particles of this size to the additives. Reduction from
the industrial load schedule to the minimum load schedule for
the turbine, with the injection of iron based additive in both
cases, resulted in a very negligible change in the size distri-
bution.
Due to the particularly high exhaust gas temperatures (about
800°F), the Mark 4D Impactor was situated outside the stack.
The impactor and the exposed portion of the probe were maintained
at an elevated temperature by external heating. Despite these
efforts, a substantial amount of weight was obtained from the
probe washes. This weight is labeled as probe wash and not in-
cluded in the particulate size distribution since the exact
nature of this weight is not certain.
To alleviate such problems it is recommended that the con-
necting probe section between the Mark 3 and Mark 4 Impactors
be minimized and both impactors plus any connecting probes be
held at stack temperatures.
93
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1C.O
9 OUTLET
A OUTLET
A OUTLET
O OUTLET
B INLET
TESTA
TEST B
TEST C
TEST D
TEST E
10 15 20 30 40 50 60 70 80 85
% OF MASS LESS THAN STATED DIAMETER
Figure 8. Particle size distributions, tests A-E, Kearney
Generating Station.
94
-------
TABLE 5. SOURCE TEST RESULTS FOR KEARNY TESTS
Test
A
B
C
D
E
Turbine
load
Ind.
Ind.
Min.
Ind.
Ind.
Fuel
additive
None
I
I
II
III
Mass
mean
diameter ,
ym
0.14
0.13
0.13
0.17
0.21
Particle mass concentration, gr/dscf
Standard
geometric
deviation
3.5
4.5
4.7
7.1
4.6
Pearson
Correlation Solids
Coefficient
0.96
0.99
0.96
0.99
0.97
0.0033
0.0023
0.0027
0.0014
0.0014
Condensibles
0.0025
0.0162
0.0039
0.0043
0.0040
Probe wash
--
0.0027
0.0052
0.0010
0.0021
-------
CONCLUSIONS
An improved particulate sampling train was constructed to
size particles emitted from coal-fired power boilers in the range
of 0.02 to 20.0 pm in diameter. The new components were designed
and assembled to accompany the UW Mark 3 and Mark 4 Impactors.
It has been demonstrated that this equipment and technique is
capable of in-stack particulate sizing for the 0.02 to 20 ym
diameter range.
Two source test programs were performed on coal-fired power
boilers at the J.E. Corette Power Plant, Billings, MT, and at
the University of Washington Power Plant, Seattle, WA. A par-
ticulate source test was also performed on an oil fired turbine
at the Kearny Generating Station, Kearny, NJ. It has been shown
that the new sampling train is capable of sizing particles at
both the inlet and outlet of particulate control devices. It
was also demonstrated that two such sampling trains can be simul-
taneously operated at the inlet and outlet of a particle control
device on a coal fired power boiler during the same sampling
period to evaluate the collection efficiency as a function of
particle size.
ACKNOWLEDGEMENTS
This research was supported by the Electric Power Research
Institute, Palo Alto, CA. The assistance of G. Fioretti, G.
Raemhild, A. Thoreen, and E.B. Powell with the field testing
is appreciated.
REFERENCES
1.
2.
3.
4.
5.
Pilat, M.J. Source Test Cascade Impactor.
3,963,457, 1972.
U.S. Patent
Pilat, M.J., D.S. Ensor, and J.C. Bosch. Source Test Cascade
Impactor. Atmos. Environ. 4:671-679, 1970.
Pilat, M.J. Submicron Particle Sampling with a Cascade
Impactor. Paper No. 73-284, presented at APCA Annual Meetinq
Chicago, IL, 1973.
Pilat, M.J., G.M. Fioretti, and E.B. Powell. Sizing of
0.02 to 20 Micron Diameter Particles Emitted from Coal-Fired
Power Boiler with Cascade Impactors. Presented at APCA
Annual Meeting, Vancouver, B.C., 1975.
Ranz, W.E., and J.B. Wong. Jet Impactors for Determining
the Particle Size Distribution of Aerosols. AMA Arch
Hyg. Occup. Med. 5:464-477, 1952. '
96
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6. McFarland, A.R., and H.W- Zeller. Study of Large Volume
Impactor for High Altitude Aerosol Collection. USAEC TLD
18624, 1963.
7. Davies, C.N. Definitive Equations for the Fluid Resistance
of Spheres. Proc. Phys. Soc. London 57:259-270, 1945.
8. Hawksley, P.G.W., S. Badzioch, and J.H. Blackett. Measure-
ment of Solids in Flue Gases. British Coal Utilization
Research Association, Leatherhead, Surrey, 1961. 214 p.
97
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PAPER 5
EXPERIENCE IN SAMPLING URBAN AEROSOLS WITH THE SINCLAIR
DIFFUSION BATTERY AND NUCLEUS COUNTER
EARL 0. KNUTSON
DAVID SINCLAIR
ENVIRONMENTAL MEASUREMENTS LABORATORY
U.S. DEPARTMENT OF ENERGY
ABSTRACT
The Sinclair diffusion battery and nucleus counter have
been coupled to a data acquisition and control system for auto-
mated monitoring of ambient aerosol size distribution and number
concentration in New York City. Previous versions of this equip-
ment required recording of data manually or by strip chart.
The present version also incorporates a newly-designed rotary
sequencing valve and a nucleus counter with increased cooling
capacity and continuous alcohol feed. Unattended operation for
three days or more is possible.
Diffusion battery penetration data are obtained on computer-
compatible punched tape at 10, 20, 60 or 120-minute intervals.
Our standard practice is to resolve these data into an eight-
class particle size histogram spanning the diameter range 32
to 322 nm. Twomey's nonlinear iterative algorithm is used for
this calculation.
Approximately 1000 size spectra of the New York City urban
aerosol have been obtained with the present system in addition
to 233 spectra that were obtained with an earlier version. Typi-
cally, the geometric mean diameter and geometric standard devia-
tion are in the ranges 25 to 40 nm and 1.8 to 2.0 nm, respec-
tively. The spectra typically show a slight positive skew
relative to the log normal.
INTRODUCTION
This paper is a report on the status of the Sinclair dif-
fusion battery and nucleus counter and an account of our exper-
ience with this apparatus. Several events of significance have
98
-------
occurred since the last status report, given over two years ago:1
fourteen replicas of the diffusion battery were manufactured
by a commercial firm and distributed to subscribers worldwide;
the screen version of the diffusion battery is now commercially
available (Thermo-Systerns, Inc., St. Paul, MN); we have made
approximately 1000 measurements of urban aerosol size distribu-
tions and have made a number of changes in the apparatus based
on that experience.
The bulk of the information in the earlier report,1 includ-
ing calibration data, remains valid. The reader is referred
to that report and its references for descriptions of the basic
components of the system. Some additional information concern-
ing the system will appear in a forthcoming article on the New
York summer aerosol.2 The present paper will describe the modifi-
cations made to date, new peripheral equipment added for control
and data acquisition, data processing including the calculation
of size distributions from observed diffusion battery penetra-
tions, and the use of the system.
MODIFICATIONS TO PREVIOUSLY-DESCRIBED EQUIPMENT
Principal Dimensions of the Diffusion Battery
There now exist two manufacturing runs of the Sinclair dif-
fusion battery and these differ significantly in their principal
dimensions. The first sets of diffusion batteries were made
using diffusional separating elements supplied by the Brunswick
Corp. of Chicago. Brunswick used the term "Collimated Holes
Structures" to identify these elements. Each element consisted
of a circular disk, 4.44 cm in diameter, containing 14,500 holes
of 230 micrometers diameter. Eighteen disks of various thick-
nesses were used in the batteries.3
The second manufacturing run, which included the 14 bat-
teries distributed to subscribers, made use of diffusional ele-
ments manufactured by Nuclear Metals of Concord, MA. The ele-
ments were referred to as "Honeycomb Structures" by the manu-
facturer. These elements were disks, also 4.44 cm in diameter,
but they differed from the Brunswick version in the number and
size of holes in each disk. The Nuclear Metals disks contain
9,750 parallel holes of 380 micrometers diameter.
Table 1 shows the principal dimensions of the original and
recently manufactured diffusion batteries.
Sequencing Valve
In the previous version of the automated Sinclair diffusion
battery, air flow through the various stages of the battery was
controlled by a bank of 13 solenoid valves which were opened
in sequence by means of a cam and microswitch assembly. The
99
-------
TABLE 1. PRINCIPAL DIMENSIONS OF THE CHS AND
HS SINCLAIR DIFFUSION BATTERIES
Segment
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Tap*
No.
1
2
3
4
5
6
7
8
9
10
11
Segment Thickness (cm)
CHS**
0.343
0.310
0.640
1.029
1.313
1.880
2.57
2.61
1.228
2.56
2.55
2.60
2.58
2.56
2.58
2.56
2.62
2.57
HS
0.318
0.318
0.635
0.952
1.270
1.905
2.54
2.54
1.270
2.54
2.54
2.54
2.54
2.54
2.54
2.54
2.54
2.54
CHS: "Collimated Holes Structures" manufactured
by Brunswick Corp.
HS: "Honeycomb Structures" manufactured by
Nuclear Metals.
* Indicated tap follows the corresponding seg-
ment in the left column.
** The CHS version was used in all measurements
described in this paper.
valves were noisy, both acoustically and electrically. The elec-
trical noise was troublesome, since it caused spurious signals
in the digital circuitry used to record data. A second disad-
vantage of the arrangement was that it was not amenable to ex-
ternal control, such as timing signals for synchronization pur-
poses .
Based on our experience, specifications were drawn up for
a new sequencing valve. Particular attention was given to the
requirements of aerosol mechanics and to compatibility with
digital data-handling circuitry. The valve was designed and
constructed by the Instrumentation Division at EML.
100
-------
The new valve is shown in Figure 1. A configuration is
used in which 15 ports are arranged in a circular pattern. A
drive motor, which runs continuously, is coupled to the valve
rotor through a gear train and an electromechanical clutch.
Each actuation of the clutch causes the rotor to advance 1/15
of a revolution, stepping the valve to the next port.
To minimize aerosol losses, the valve was designed with
relatively large flow passages and a minimum number of changes
in flow direction. The smallest passage is the straight passage
through the rotor which has diameter of 0.64 cm. Downstream
of the rotor, the 15 ports are connected to a toroidal manifold,
with relatively large internal dimensions.
The time required to step the valve is one second so that
the interruption of flow is negligible. The clutch is activated
by momentarily (1/20 to 1/4 second) grounding a line from the
clutch solenoid, which is powered by an on-board 24 V DC power
supply. The solenoid current is only 220 mA and no interference
with digital data circuitry has been noted.
The fifteen ports include three spares, which are used to
obtain clean air readings (through an absolute filter) at the
beginning and end of each measurement cycle and to obtain a re-
dundant reading of the diffusion battery inlet concentration.
Table 2 shows the valve sequence used in the present system.
Port 0 is used as a "home" position, where the valve starts and
ends each measurement cycle.
Continuous Flow Nucleus Counter
As described elsewhere, "* the continuous flow nucleus counter
utilizes ethyl alcohol as the working fluid which condenses on
the aerosol particles. During the first extended outdoor test
of the nucleus counter, in August of 1976, we found that the
alcohol also served as a good desiccant. As long as the alcohol
is fresh, it dehumidifies the air sample stream. However, after
two to three days in humid weather, the alcohol becomes so diluted
with water that it is no longer an effective desiccant. Atmos-
pheric water vapor then enters the cold tube, where it forms
frost and blocks the light beam.
To counter the formation of frost, fresh alcohol is contin-
uously supplied to the alcohol tube of the nucleus counter.
The alcohol flows under gravity through a needle valve adjusted
to 150 cm3/day. The alcohol tube overflow is collected in another
receptacle, closed to the atmosphere.
A second difficulty encountered in August 1976, was that
of maintaining sufficiently low temperature in the cold tube
of the nucleus counter. We have found that it is necessary to
101
-------
Figure 1. Sequential muft/port samp/ing valve.
102
-------
TABLE 2. VALVE PORT ASSIGNMENTS
Valve Port Connection
0 Absolute Filter
1 Absolute Filter
2 Diffusion Battery inlet
3 Diffusion Battery inlet
4
5
6
7
8
9
10
11
12
13
14
Diffusion Battery
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
tap no.
1
2
3
4
5
6
7
8
9
10
11
maintain -15°C in the cold tube for proper nucleus counter opera-
tion. In the August sampling session, the temperature in the
equipment shed often reached 35°C and we had to supply additional
cooling by means of an air conditioner. Since then, a Model
2 nucleus counter has been built incorporating additional cooling
capacity. This version was found to be marginally adequate in
hot weather. To provide a reserve of cooling power, Model 3
(now in preparation) will incorporate two-stage thermoelectric
elements to increase the attainable temperature differential.
NEW PERIPHERAL EQUIPMENT
Timing and Control Circuit
Continuous cycling through the diffusion battery taps pro-
duce more data than is needed in most applications. Single
cycles initiated periodically give the preferred mode of opera-
tion. To facilitate correlating aerosol data from the diffu-
sion battery with other information, such as meteorological
variables, it is convenient to have these single cycles initiated
on the hour.
The Instrumentation Division at EML has devised a timing
and control circuit which provides the desired control signals
to the rotary valve. As indicated in the functional diagram,
Figure 2, all timing is based on counting pulses derived from
the 60 Hz power main. Counting commences from zero at the time
103
-------
115V
60 Hz
RUN/
STANDBY
SWITCH
TRANSFORMER
AND CLIPPING
CIRCUIT
SCALER CHAIN,
PULSE FORMING
3.1 SEC
1 PULSE/32 SEC
PULSE
COUNTING
AND
STEERING
TO MULTIPORT VALVE STEP
HOME DETECTOR
TO DATA RECORDER
TO EAA
PRESET
Figure 2. Functional diagram of timing and control circuit.
when the Run/Standby control is switched to Run. Cycles consist-
ing of fifteen 0.1-second pulses spaced 32 seconds apart are
started at one-hour intervals to step the multiport valve. A
provision is made to insure that the valve stops at home position
(port 0) after each cycle.
The same circuit also controls the data acquisition equip-
ment and an electrical aerosol analyzer (Thermo-Systems Model
3030). The latter is often run in conjunction with the diffusion
battery/nucleus counter for comparison.
The timing and control circuit has proved to be reliable
in two separate four-week periods of continuous operation. The
maximum observed error in cycle initiation time, using the tele-
phone company time as a standard, was 5 seconds. The important
point here is that the timing errors are not cumulative, since
the power company apparently adjusts its generators periodically
to keep the cumulative number of cycles on schedule.
In addition to the hourly schedule, the timing and control
circuit can be set to initiate cycles at 10, 20 and 120-minute
intervals.
Data Recording Apparatus
The Sinclair diffusion battery and nucleus counter generate
a large volume of data, even when operated on hourly cycles.
104
-------
Extended operation is feasible only in conjunction with apparatus
for reliable recording data in machine-readable form. We pres-
ently use a system assembled by the EML Instrumentation Division,
using some commercial components and some components which are
standard within our Laboratory. It is likely that this system
will change considerably in the near future to take advantage
of the rapid developments in commercial data handling electronics.
The main components of the present data recording apparatus
are two analog-to-digital conversion circuits, a time-of-year
clock, a teletype interface and a teletypewriter. The electronic
modules are housed in a standard full-width NIM bin, which also
houses the timing and control circuit.
The analog signal from the nucleus counter is converted
to digital form by means of a voltage controlled oscillator con-
nected to a sealer. A gating circuit keyed to the timing and
control circuit (described earlier) gates pulses from the oscil-
lator into the sealer during a five second period starting 17
seconds after each step of the rotary valve. The 17-second
period allows for the response time of the nucleus counter and
the five-second period provides signal averaging. The accumu-
lated count, which is directly proportional to the five-second
averaged analog voltage, is sent out to the teletypewriter on
the next step signal from the timing and control circuit. A
second analog channel is normally used to record data from the
electrical aerosol analyzer.
The data are recorded in the form of six-digit integers.
The teletype produces both a printed and a punched tape record
of the data. Due to a local convention in data handling, the
punched tape approach is the most direct method for us to enter
data into our main computer.
Teletype systems are widely used for data recording in our
Laboratory, mostly in temperature and humidity-controlled indoor
environments. We have logged about 2,500 hours of operation
in uncontrolled outdoor equipment sheds and we find that the
reliability of recording data varies with the weather. In cool
weather, the error rate is of the order of one mispunched bit
per day (1 bit in ^100,000). In hot weather, the error rate
may increase 1000-fold. We have found, however, that the punch-
ing errors are of such a nature that the actual numerical values
are seldom affected. By reprogramming our tape reading routine,
we find that even badly mispunched tapes can be read with good
reliability.
The main disadvantage of the teletype is that it is cumber-
some in field use. It is planned to replace the teletype in
the near future with a magnetic tape cassette system.
105
-------
DATA PROCESSING
A fully mechanized procedure has been developed for pro-
cessing the data from the Sinclair diffusion battery and nucleus
counter. The input is punched paper tape from the teletype and
the output is computer cards, one for each measurement cycle.
Each card is encoded with year-month-date-hour and eight values
of AN/A(loglOdp) from that hour's measurement. The cards are
suitable for filing and subsequent statistical studies. The
procedure also produces a printed summary, including a bar-graph
histogram, of each hour's results.
A flow chart of the computer program used for data processing
is shown in Figure 3. All elements of the program are straight-
forward, with the exception of the size distribution calculation.
For analyzing diffusion battery data, we sought an algorithm
which would be well suited for automatic calculations on large
volumes of data. Simplicity and speed of execution were impor-
tant. Other criteria were that the algorithm should be directly
applicable to field-grade data, without screening or smoothing,
and that it should have sensitivity and resolution sufficient
to bring out the main features of the size distribution.
Our criteria tended to rule out elaborate, mathematically
proper approaches such as that of Maigne5 and those based on
linear programming6 or nonlinear least squares.7 Four algorithms
were examined in detail and one of these, Twomey's nonlinear
iterative algorithm,8 was selected as the best compromise.
Twomey's Nonlinear Iterative Algorithm
The nonlinear iterative algorithm described by Twomey8 in
1975 was intended for the calculation of the aerosol size dis-
tribution from measurements of penetration through Nuclepore
filters. As described below, this algorithm adapted readily
to analysis of data from the Sinclair diffusion battery.
The diffusion battery taps 1 through 11 will be designated
by the index i. It is convenient to define two additional taps:
Tap 0, sampling the aerosol at the diffusion battery inlet and
tap 12, sampling the aerosol through an absolute filter. Thus,
the data from the diffusion battery consist of a set of 13 de-
creasing aerosol concentrations C|, i=0, 12. The last of these
is zero and is used primarily to check the zero reading of the
nucleus counter.
Determination of the aerosol size distribution involves
comparison of the 13 observed concentrations, C^, to the corres-
ponding trial concentrations, C*. The latter are given by the
integral expression: 1
106
-------
c
ENTER
CALL PENETR TO SET UP
HISTOGRAM SIZE CLASSES
AND COMPUTE THE PENE-
TRATION MATRIX
CALL OPTN TO ESTAB-
LISH COMMUNICATION
WITH THE TAPE
READER
THIS THE FIRST
LINE OF A SET
OF 15?
-------
c* = N r
1 o
;x)f (x)dx
(1)
where
N = the aerosol number concentration
f(x) = the (unknown) differential size distribution
x = the particle diameter
= the fractional penetration of aerosol particles
of diameter x through the diffusion battery seg-
ments preceding the i-th tap. Note that
<}>o (x) =1 and $! 2 (x) = 0.
The penetration functions i(x) are calculated as the product
of penetration expressions for each of the diffusion battery
segments preceding the i-th tap. For the latter, we use Thomas'
expression,9 which is simple and sufficiently accurate. Plots
of <|>j_(x) may be seen in reference 3 and, in an alternative form,
in Figure 4.
SINCLAIR DIFFUSION BATTERY AT 4.0 fipm
T
20 30 50 70 100
x, PARTICLE DIAMETER, nm
200 300
Figure 4. Penetration functions for the Sinclair diffusion battery.
108
-------
Although the algorithm can be applied to the observed dif
fusion concentrations directly, we find that convergence is
speeded by taking their first differences, D^, which are the
depositions in the corresponding diffusion battery segments:
Di = Ci-l ~ Ci' for i = 1' 12«
The corresponding trial differences are:
CO
D* = N f fl (x)f(x)dx, for i = 1, 12. (3)
i J
o
where
i = 1, 12.
The functions fi^(x), which represent the deposition in the dif-
fusion battery segments between taps i-1 and i, are shown in
Figure 5. The function ft12(x) represents deposition in the
absolute filter.
Twomey ' s algorithm requires preselection of a number of
discrete sizes to represent the continuous size distribution.
There are no fixed rules for selecting the number and spacing
of these sizes. The number and spacing should be such as to
reproduce the major features of the deposition curves, Figure
5, but using a larger number and smaller spacing is permissible.
The computation time, however, increases in proportion to the
number of discrete sizes selected.
We commonly use a set of eight discrete sizes, starting
with 42.2 nm and increasing to 237 in a geometric progression
with a common ratio of 10°*2S. Figure 5 suggests that it might
be possible to add one or two larger sizes, since ft12(x) is still
changing beyond 237 nm. However, this part of the curve is un-
certain due to the unknown effects of gravitational settling.
Furthermore, many aerosols have relatively few particles larger
than 250 nm diameter, which makes extension to larger sizes
unnecessary.
In terms of the discrete sizes, the trial depositions are:
n
D* =
. ..
13 3
109
-------
SINCLAIR DIFFUSION BATTERY AT 4.0 £pm
CURVES 4, 5, 6, 8, 9& 10
OMITTED FOR CLARITY
10 30
x, PARTICLE DIAMETER, nm
100
300
Figure 5. Deposition curves for the Sinclair diffusion battery.
where
x. = the j-th discrete size
N. = the particle concentration at the j-th discrete
-1 size
n
= the number of discrete sizes.
Twomey's algorithm starts with an initial guess for the
size distribution, then refines this guess by a series of small
changes to improve the fit to the observed depositions. For
the initial guess, we assume that each of the discrete sizes
has a concentration equal to 1/n times the observed concentration
at. tap 0; NJ = C0/n, j = l,n. The refined values, denoted by
N', are obtained by applying Twomey's nonlinear iteration
formula:
(5)
110
-------
There are no fixed rules concerning the sequence in which the
subscripts i and j in the formula are to be varied. We use the
following scheme:
1. The index i is varied in the sequence 1,2,3,...,12,1,2,3,...,
l-£fj-f£f-jf • • • •
2. At each step of the index i, Di is computed from equation
4 using the most recent values of N-;.
3. For each i, a complete cycle through the index j is per-
formed using equation 5 to obtain refined estimates of Nj.
This procedure is continued until there is satisfactory agreement
between D^ and D^.
The rationale of Twomey's algorithm may be seen as fallows.
Suppose, that for a particular value of i, the computed Dj^ is
smaller than Dj_. Equation (5), which is a sum of positive terms,
shows that the only way to close the gap is to increase at least
one of the Nj terms. The strategy used is to increase^all the
Nj terms, each in proportion to the impact it has on Di. Similar
reasoning applies if Di is larger than D^. By cycling through
the subscripts as previously indicated,Athe N- values are con-
tinually adjusted so as to bring each D- closer to the corres-
ponding D. .
Two further steps were taken in the computer program to
increase execution speed:
1. The factor ft^ in equation 5 was replaced by 0.8 ^ij/^^max'
where ^imax is the largest ftjj for the given value Of i.
This increases the size of the step made in each iteration.
2. No test is made for convergence. Experience has shown that
30 cycles through the double loop (i = 1,12; j = l,n) is
sufficient to achieve reasonable agreement between D* and
D.. Therefore, 30 cycles of iteration are used for each
dita set.
Computational Speed and Accuracy
The data processing just described is done in our Laboratory
using an IBM 360/30 computer and peripheral IBM equipment, such
as the paper tape reader, card punch and line printer. The
throughput is about four diffusion battery data sets per minute.
The calculation itself (the algorithm) requires eight seconds
per data set and the balance of the time is used in reading the
paper tape and the printout. This throughput is adequate for
oS? needs, but could probably be increased five or ten fold by
using faster equipment.
Ill
-------
All computations are done using single-precision IBM FORTRAN
(32 bit words). We think that the entire procedure could also
be done with a field microcomputer programmed in BASIC.
A convenient check on the internal consistency of the data
is provided by comparing the calculated concentrations, Cj, to
the observed concentrations, C^. With either field or labora-
tory data, we find that the root mean square relative difference
between these two is usually in the range 2 to 3%, which is be-
lieved to be comparable to the random perturbances in the meas-
ured concentrations. When larger relative differences occur,
they can often be traced to errors in data transmission or to
apparatus malfunctions.
The Twomey algorithm itself has been tested with simulated
data. It was found that simulated data from monodisperse aero-
sols calculates to a unimodal distribution with geometric stand-
ard deviation ^1.3. Simulated data from a mixture of two mono-
disperse aerosols calculates to a bimodal distribution provided
that the two sizes are separated by a factor of four or more.
This resolution cannot be improved by increasing the number of
size classes.
The algorithm also was tested with simulated data perturbed
by random errors. It was found that random relative errors of
2% had little effect, but that 5% errors produced obvious dis-
tortions in the computed size distribution. This shows that
data of good quality are required to obtain reliable size dis-
tributions.
A more comprehensive test of the algorithm was carried out
by analyzing real data obtained with near-monodisperse aerosols.
The body of data used in this test was obtained in a cooperative
project with personnel of the University of Minnesota Particle
Technology Laboratory, who generated the test aerosols using
their electrical aerosol generator. The two separate sessions
of this project are described in references 1 and 10.
Twenty-one of the sets of diffusion battery penetration
readings from the cooperative project were analyzed by means
of the Twomey algorithm. These data sets include several for
which the Pollak counter, General Electric counter, or an elec-
trical aerosol detector was used to measure diffusion battery
penetration.
The results of those tests for which the Sinclair nucleus
counter was used as the detector are summarized in Table 3.
It is seen that the geometric mean particle diameter extracted
by the algorithm agrees well with the size expected from the
electrical aerosol generator. The geometric standard deviations
are larger than expected, but this is a measure of the limited
112
-------
TABLE 3. RESULTS OF TWOMEY CALCULATION APPLIED TO DATA FROM
NEAR-MONODISPERSE Nad AEROSOLS
Test
No.
2
3A
14 + 15
17
19A
19B
Aerosol
Size From
BAG (nm)
42
42
42
75
24
24
Calculated
"Old" CFC Calibration
dg (nm)
36
47
49
80
23
24
°g
1.67
1.27
1.43
1.48
1.41
1.38
fit (%)
6.6
10.2
17.2
2.3
4.9
2.6
Results
"New" CFC Calibration
d (nm)
31
43
46
85
25
25
°g
1.49
1.11
1.27
1.35
1.18
1.20
fit (%)
12.2
10.3
5.0
3.6
18.3
8.0
CFC: Continuous flow condensation nucleus counter (Sinclair
nucleus counter)
EAG: Electrical aerosol generator
d : Geometric mean diameter
9
a : Geometric standard deviation
Fit: Root mean square relative difference between the observed
and trial values of aerosol concentration at the diffusion
battery taps.
resolution of the algorithm. The table also lists results ob-
tained using two calibration curves for the nucleus counter.
Although the curves were somewhat different, the calculated
particle size is nearly the same.
OVERALL SYSTEM: SERVICE AND MAINTENANCE
A block diagram of the complete aerosol measurement system
is shown in Figure 6, while a photograph showing the system in-
stalled in a roof-top air sampling station is shown in Figure 7.
"
-SBJ sDs-s- aus
113
-------
ROTARY
15-PORT VALVE
AEROSOL
IN
4 Cprn J U
SINCLAIR
DIFFUSION
BATTERY
4 gpmTO OTHER
INSTRUMENTS
60 £
BALLAST
TANK
SINCLAIR
NUCLEUS
COUNTER
STEP AND
HOME
SIGNALS
CONVERT
SIGNAL
TO PUMP
TIMING AND
CONTROL
CIRCUIT
ANALOG TO
DIGITAL
CONVERTER
Figure 6. Block diagram of aerosol measurement system.
tank is that some modification of the size distribution is in-
curred by losses and by coagulation. We have estimated mathe-
matically that losses by diffusion do not change the size dis-
tribution significantly, but coagulation (assuming a typical
urban aerosol concentration at the ballast tank inlet) causes
a 40% reduction in the concentration of 5 nm particles. For
particles larger than 10 nm, the loss (or gain) is insignificant.
Routine Service Schedule
When the system is in use for hourly sampling of the outdoor
aerosol, it is serviced once each weekday. On weekends, the
system operates unattended for 72 hours. Our recent experience
indicates that in cool or cold weather, the service interval
could be extended to one week.
The parameters which are checked during each visit are the
temperature in the nucleus counter cold tube and the alcohol
supply and flow rate. Other parameters, such as the nucleus
counter lamp intensity and the aerosol flow rate have been found
to be very stable. The cycle initiation time is also checked
each visit and has been found to be reliable. The punched tape
is removed twice a week to avoid handling awkwardly long tapes,
but this interval could be extended to one week if desired.
114
-------
Maintenance
A provisional maintenance schedule has been established
for the Sinclair diffusion battery and nucleus counter in which
the apparatus is disassembled and thoroughly inspected at ap-
proximate 1000 hour intervals of running time. Particular areas
of concern in the inspection are the diffusion battery segments,
the rotary valve seals and the lenses in the nucleus counter.
The segments should be inspected for discoloration or debris
on the upstream surfaces. (A slight accumulation of debris is
usually found on the first segment.) The segments should be
inspected for blocked holes by holding them up to a strong light.
If required, the diffusion battery segments are cleaned
by soaking in ethanol, acetone or a 20% solution of phosphoric
acid. The solvent treatment should not exceed one hour, since
the solvents may attack the aluminum and epoxy used to mount
the segments. The organic solvents are cleared from the segments
by drawing air through them.
The rate of fouling of the diffusion battery segments is
very low due to the low duty cycle used. In the normal one cycle
per hour schedule, there is flow through the diffusion battery
only 7.2 minutes per hour. The segments between taps 10 and
11, where the greatest mass deposition is expected, have a duty
cycle of 1 minute per hour. The nucleus counter is to a large
extent self-cleaning due to the continuous flow and condensa-
tion of alcohol in the cold tube.
Upon reassembly of the system, or after any move of the
system, it is advisable to check for leaks. One convenient check
is to place an absolute filter at the diffusion battery inlet,
then go through a normal measurement cycle. Leaks which con-
tribute 200 to 300 particles/cm3 are detectable in this way.
USE HISTORY AND FUTURE WORK
Three extended sampling sessions have been conducted to
date with the Sinclair diffusion battery and nucleus counter.
The first of these was in August 1976, on the roof of the dormi-
tory at New York University Medical Center using an earlier ver-
sion of the apparatus. Data transcription was done by hand and
233 hours of data were obtained. The results of this work, which
was part of a multi-laboratory study of the New York aerosol
under summer conditions, will be described in a forthcoming
article.2
The second and third sampling sessions were conducted on
the roof at EML during November 1977, and February 1978 The
full complement of equipment, as described in this report, was
used The two sessions resulted in 966 hours of data. At this
writing all these data have been reduced to cards and a few
statistical summaries have been done.
116
-------
Figure 7. Sine/air diffusion battery and nucleus counter mounted in
the rooftop sampling station. (The rotary multiport valve
is shown in the foreground and the nucleus counter is in
the left background. Note that the screen battery, rather
than the CHS or HS battery, is shown in this photograph).
115
-------
Figure 8 shows the grand average number-weighted aerosol
size distribution based on 400 measurements made on the roof
at EML during November and December 1977. The distribution is
seen to be unimodal, with the modal diameter at 30 nm. The curve
has a bell shape closely resembling that of a log-normal distri-
bution. The grand average number concentration was 26,900 cnT3.
Figure 9 shows the average diurnal pattern of concentration
for 42 nm particles. It is seen that the minimum and maximum
concentrations occur in the predawn hours and during the morning
rush hours, respectively. A second maximum occurred during the
afternoon rush hours. This latter maximum was not present in
the earlier data taken during the summer.2
Additional sampling sessions are planned and further appara-
tus modifications are contemplated to make the system more con-
venient to use. We anticipate that in the near future a second
system will become available so that sampling can be done simul-
taneously at two sites.
The most recent sampling session also indicated a need for
further development or calibration work with the nucleus counter.
This sampling was done during extremely cold weather. The con-
centrations measured with the nucleus counter were abnormally
low, possibly due to the reduced amount of alcohol vapor available
to condense on particles. We therefore plan to check the nucleus
counter calibration over a wide range of temperatures in a labora-
tory environmental chamber.
SUMMARY
The Sinclair diffusion battery and nucleus counter have
been improved and modified to make them suitable for extended
use in uncontrolled environments. A new sequencing valve, com-
patible with digital data recording circuitry and the require-
ments of aerosol mechanics, has been designed and constructed.
An accurate timing and control circuit and a data recording de-
vice have been added to make up a system which operates unat-
tended for three days or more.
Data processing techniques have been developed to conven-
iently handle data from the diffusion battery and nucleus coun-
ter The processing is largely automatic and results in a com-
puter card file of differential size distribution data.
Over 1000 hourly measurements of the urban aerosol in New
York City have been made with the system. Operation in winter
months has been found to be virtually trouble free. Operation
Tn hot, humid weather is more difficult, but it is anticipated
that the recent modification will prove to be adequate for sam-
pling planned for this summer.
117
-------
501
40 I
AVERAGE TOTAL CONCENTRATION
= 26,900/cm3
oo
CO
u
-ft
•o
5
vt
o
a.
•o
o
O)
_o
301
2Q\
10
AVERAGE SIZE DISTRIBUTION BASED
ON 400 HOURLY READINGS
10 30
PARTICLE DIAMETER, nm
100
300
Figure 8. Average number-weighted size distribution on EML roof,
November-December, 1977.
-------
601—
50
CO
3
o
40
30
20
10
AVERAGE DIURNAL PATTERN
FOR 42 nm PARTICLES, BASED
ON 17 DAYS OF DATA
Ol I I I I I I I i I I
I I I I I I I I
I I I I
MID
6 AM NOON 6 PM
LOCAL STANDARD TIME
MID
Figure 9. Average diurnal pattern for 42 nm particles on EML roof.
119
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ACKNOWLEDGEMENT
F. Guggenheim and M. Cassidy, both of EML's Instrumentation
Division, contributed the design and supervised the construction
of the multiport rotary valve and the timing and control circuit,
respectively.
REFERENCES
1. Sinclair, D., R.J. Countess, B.Y.H. Liu, and D-Y.H. Pui.
Automatic Analysis of Submicron Aerosols. In: Aerosol
Measurement. Proc. of Aerosol Measurement Workshop, March
24-26, 1976, to be published by University of Florida Press,
1978.
2. Knutson, E.O., D. Sinclair, and B. Leaderer. New York Sum-
mer Aerosol Study — Number Concentration and Size Distri-
bution. New York Academy of Sciences, to be published,
1978.
3. Sinclair, D. A Portable Diffusion Battery - Its Application
to Measuring Aerosol Size Characteristics. J. Am. Ind.
Hyg. Assoc. 33:729-735, 1972.
4. Sinclair, D., and G.S. Hoopes. A Continuous Flow Condensa-
tion Nucleus Counter. J. Aerosol Sci. 6:1-7, 1975.
5. Maigne, J.P., P.-Y. Turpin, G. Madelaine, and J. Bricard.
Nouvelle Methode de Determination de la Granulometrie d' un
Aerosol au Moyen d'une Battery de Diffusion. [New Method
for Determination of the Granulometry of an Aerosol by Means
of a Diffusion Battery.] J. Aerosol Sci. 5:339-355, 1974.
6. Cooper, D.W., and L.A. Spielman. Data Inversion Using Non-
linear Programming with Physical Constraints: Aerosol Size
Distribution Measurement by Impactors. Atmos. Environ.
10:723-729, 1976.
7. Mallove, E.F., and W.C. Hinds. Aerosol Measurement by Com-
bined Light Scattering and Centrifugation. J. Aerosol Sci.
7:409-423, 1976.
8. Twomey, S. Comparison of Constrained Linear Inversion and
an Iterative Nonlinear Algorithm Applied to the Indirect
Estimation of Particle Size Distributions. J. Comput Phys
18:188-200, 1975.
9. Thomas, J.W. Particle Loss in Sampling Conduits. in: IAEA
Symposium Proc. Vienna, 1967. pp. 701-712.
10. Sinclair, D., R.J. Countess, B.Y.H. Liu, and D.Y.H. Pui.
Experimental Verification of Diffusion Battery Theory.
J. Air Pollut. Control Assoc. 26:661-663, 1976.
120
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PAPER 6
SELECTING LABORATORY METHODS FOR PARTICLE SIZING
RONALD G. DRAFTZ
FINE PARTICLES RESEARCH LABORATORY
ILLINOIS INSTITUTE OF TECHNOLOGY RESEARCH INSTITUTE
ABSTRACT
In spite of continuing efforts to develop in-situ particle
sizing methods for process streams, there still remain many in-
stances when in-lab particle sizing is necessary or desirable.
The analyst has many methods available for particle sizing and
often faces the choice of which one is best.
This paper presented some general guidelines for selecting
a particle sizing method based on sample size, instrument size
range, analysis time, and measurement parameter and principle.
Suggestions were given for sample preparation and microscopical
methods for previewing samples to select the ideal sizing method,
121
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PAPER 7
LONG TERM FIELD,EVALUATION OF CONTINUOUS
PARTICULATE MONITORS
A. W. GNYP
S. J. W. PRICE
C. C. ST. PIERRE
D. S. SMITH
INDUSTRIAL RESEARCH INSTITUTE
UNIVERSITY OF WINDSOR
ABSTRACT
A critical field evaluation of five different particulate
monitors representing four distinctly different principles of
operation was carried out over a one year period at a secondary
lead smelter. Instrument performance was rated on the basis
of accuracy, reliability and maintenance as criterion variables.
The Lear Siegler RM41 and RAC transmissometer responses
were primarily dependent on particle size when aerodynamic diam-
eters exceeded 1 ym. Improvements in the long-term reliability
of the Environmental Systems Corporation PILLS V backscattering
device could make it potentially more useful than the best trans-
missometer because its calibration appears to be less sensitive
to changes in particle size, color and refractive index. The
Research Appliance Company transmissometer was adequate for moni-
toring emissions from the controlled reverberatory mode of opera-
tion but did not appear to be capable of detecting large par-
ticles whose diameters exceeded 25 pm during lead alloying kettle
production. Very limited potential for application to either
the reverberatory or lead alloying kettle modes of operation
was exhibited by the charge transfer IKOR 2710 particulate moni-
tor. Because of repeated mechanical failures in the sampling
module of the Research Appliance Company beta radiation attenuat-
ing device supplied for the study, it was impossible to criti-
cally evaluate its monitoring capabilities.
Reliable continuous monitoring is possible in stacks ex-
hausting gases that have passed through efficient control equip-
ment and contain particulate matter having constant physical
and chemical properties over long time periods. Continuous par-
ticulate monitors offer interesting possibilities for improving
122
-------
surveillance capabilities, validating manual compliance testing
and minimizing emissions from industrial sources on a 24-hour
real-time basis.
INTRODUCTION
Increasing awareness of the potential dangers arising from
the discharge of toxic materials into the environment from indus-
trial sources has prompted regulatory agencies to establish maxi-
mum emission levels. At the present time compliance is deter-
mined by periodic manual source testing programs that can provide
data that are valid, at best, only for the short-term testing
periods. Conventional manual stack sampling methods cannot pro-
vide records of any periodic excessive emissions resulting from
faulty operation of installed abatement equipment or process
upsets.
Assurance of long-term compliance with and enforcement of
regulations requires the availability of reliable continuous mon-
itoring capabilities. Although there are several promising par-
ticulate monitoring systems, their application has been restricted
by the lack of information on their accuracy, precision and long-
term reliability. In the absence of such data, it is not sur-
prising that industries have been reluctant to commit significant
capital outlays for the installation of large scale continuous
monitoring systems.
This study concentrated on the evaluation of the perfor-
mance characteristics of continuous particulate monitoring de-
vices because, with the exception of transmissometer systems,
there are no established performance specifications for any com-
mercially available monitors. The principal objectives of this
investigation were to assess the suitability, reliability, and
accuracy of continuous particulate monitors for stationary sources
on the basis of field experience at a secondary lead smelter.
Site
The secondary lead processing plant selected for this study
provided an opportunity for assessing the viability of using
continuous instrumentation for real-time monitoring of lead,
which had been designated as a hazardous pollutant by Environment
Canada. From a practical viewpoint, the stack was of suitable
size and construction to simultaneously accommodate five con-
tinuous monitors and three manual sampling trains. The two
distinctly different operating modes:
i. uncontrolled lead alloying kettle
ii. baghouse equipped reverberatory furnace
provided a wider range of emission characteristics than normally
obtainable from a time invariant process.
123
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Process Exhaust System
Reverberatory furnace gases, exhausted by means of an 18,000
scfm fan, were cleaned in a baghouse before entering the stack
through horizontal breeching at the 55-foot level as illustrated
in Figure 1. Gas temperatures to the bags were kept below 300°F
by means of ambient air drawn through the intakes in the ducting
from the reverberatory furnace. Cleaning of the baghouse was
initiated manually at 15-20 minute intervals. Mechanical shaking
of the bags in each of the three baghouse compartments for periods
of 25-30 seconds set up well-defined vibrations in the stack.
Pulses of particulate emissions were detected by all of the con-
tinuous monitors during bag cleaning.
Emissions from the lead alloying kettles by-passed the bag-
house. They were removed from the top of the kettles by fume
hoods located at the rear of each unit. These fume hoods were
connected to a common 30-inch, round, duct which led to a 24,000
scfm fan located at the base of the stack. During reverberatory
furnace processing both the baghouse and alloying kettle fans
were operational; otherwise the baghouse fan was not used.
The test plant had several kettle units. They were used
for remelting, alloying and refining purposes. Each consisted
essentially of a hemispherical cast steel bowl mounted over
burners fired with light industrial oil.
Instrument Selection
Instruments commercially available in Canada at the incep-
tion of this investigation were limited to systems employing
i. light attenuation
ii. light scattering
iii. charge transfer
iv. beta-radiation attenuation.
Instrument selection was dependent, partly, on the cooperation
of American and Canadian manufacturers and distributors. As
a result, the final complement consisted of the five monitors
listed in Table 1.
TESTING PROGRAM
Figure 2 illustrates the relative locations of the twelve
ports, installed at the enclosed sampling and exposed instrument
platforms, with respect to the breeching from the baghouse.
124
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STACK
xxxxx
SAMPLING LEVEL
INSTRUMENT LEVEL
DILUTION
AIR
EXHAUST FROM
REVERB FURNACE
OVERHEAD
FLUE
EXHAUST FROM
KETTLES
REVERBATORY
FURNACE
o
o
KETTLES
O
o
o
o
Figure 1. Plan and elevation of secondary lead smelter.
125
-------
EL 96' 0"
SAMPLING
PLATFORM
EL 75' 3"
EL 74' 9"
EL 74' 3"-
EL 73' 9'"
EL 70' 9"
/
//
EL 82' 4"
EL 81' 10"
-EL 81'4"
EL 80' 10"
EL 77' 3"
INSTRUMENT
PLATFORM
Figure 2. Elevations of ports and instrument and sampling levels.
126
-------
TABLE 1. CONTINUOUS PARTICULATE MONITORS SUBJECTED
TO FIELD EVALUATIONS
1 —
Principle of Instrument
operation model Manufacturer
Light Attenuation RM 41 No. 275 Lear Siegler Inc.
Environmental Technology
Div.
RAC No. 2700 Contraves Goertz Corp.
with
Research Appliance Co.
Light Scattering PILLS V Environmental Systems
Corp.
Charge Transfer 2710 IKOR Incorporated
Beta Radiation Automatic Stack Research Appliance Co.
Attenuation Monitor No. 2412
Sampling Trains
The basic components of the particulate matter sampling
trains, distributed by the Western Precipitation Division of
Joy Manufacturing and sold under the trade name "Emission Param-
eter Analyzer", are represented schematically in Figure 3. Prior
to use the S-type pitot tube, orifice rate meter and dry total
gas meter were calibrated in the University of Windsor labora-
tories according to EPS standard procedures.1
Aerodynamic particle size determinations were carried out
by means of an Andersen Stack Sampler in conjunction with a
second Joy sampling train. The standard sampling probe-pitot
tube arrangement was modified by extending the S-type pitot tube
in order that velocity determinations could be made in the vi-
cinity of the sampling nozzle. Figure 4 shows the arrangement
of the extended S-type pitot tube, Andersen impactor and sampling
nozzle.
In order to acquire samples for microscopic particle sizing
and to test the validity of single point monitoring and sample
acquisition a third sampling train was required. The arrange-
ment of components, illustrated in Figure 5, also facilitated
the acquisition of gas samples for on-site gas chromatographic
analysis.
Location of Ports
Because seven ports were required for the five particulate
monitors and four ports were needed for the manual sampling
127
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TEMPERATURE SENSORS
FILTER
HEATED AREA // HOLDER
THERMOMETER
PROBE
S - TYPE
PITOT TUBE
STACK
WALL
PITOT L
MANOMETER
THERMOMETERS
ORIFICE
BYPASS VALVE GAUGE
DRY
I TEST
V METER
AIR-TIGHT
PUMP
\ ___ /_^f^ _____ |
THERMOCOUPLE |~
l_
CHECK
VALVE
VACUUM
LINE
Figure 3. Paniculate matter sampling train.
128
-------
NOZZLE
S-TYPE
PITOT TUBE
ANDERSEN
CASCADE
IMPACTOR
THERMOCOUPLE
PROBE
SHEATH
Figure 4. Arrangement of extended S-type pitot tube with respect
to Andersen cascade impactor and sampling nozzle.
129
-------
MERCURY
BAROMETER
DRY GAS
METER
GAS SAMPLE
BULB
VACUUM PUMP
Figure 5. Components of membrane sampling train.
trains, it was not possible to locate all of them at one level
on the stack. Limitations imposed by scaffolding dimensions
and minimum requirements of the EPS source testing code1 dictated
that the manual sampling train ports be provided at the upper
level as shown in Figure 2. The arrangement of the continuous
monitoring instruments is shown in Figure 6. Under the test
conditions all monitors were at least four stack diameters down-
stream from the flow disturbance created by the baghouse breech-
ing. The manual sampling trains were 6 1/2 stack diameters from
the same flow disturbance and 3 1/2 diameters upstream from the
stack exit.
The placement of the instruments at the lower level was
designed to minimize mutual interference. The proximity to the
flow disturbance was not expected to handicap the sampling pro-
gram because submicron particulate matter would predominate
during the reverberatory furnace campaign. Introduction of
exhaust gases at the base of the stack during the lead alloying
kettle production mode provided more than 8 stack diameters of
separation between the sampling sites and the exit from the
induced draft fan.
Sampling Points
All particulate matter loading evaluations were determined
from samples collected at 12 points along each of the two per-
pendicular traverses as shown in Figure 7.
Aerodynamic particle size determinations with the Andersen
in-stack cascade sampler demand that isokinetic conditions be
maintained at the nozzle for representative sample collection.
130
-------
PILLS V
BAGHOUSE _
D
RAC
"RETRO-
MEMBRANE
TRAIN
NOT USED
BREECHING
LEAR SIEGLER
"RETRO"
STACK
DIAMETER = 45 FT
ELEVATIONS
O 81' 10"
A 82' 4"
O 8T 4"
D 80' 10"
ALL PORTS CAPPED
WHEN NOT IN USE
SAMPLING PLATFORM
N
INSTRUMENT PLATFORM
Figure 6. Arrangement of ports at the sampling and instrument levels.
-------
SAMPLING PORT
FOR MEMBRANE FILTER
SAMPLING
PORTS
TRAIN
PARTICULATE O
PARTICLE SIZE 9
MEMBRANE •
Figure 7. Location of sampling points.
In addition, a constant volumetric flow rate must be maintained
through the device because the impaction efficiency of each col-
lecting plate is a function of the gas flow rate. In order to
satisfy both conditions, it was necessary to locate sampling
points along both traverses where the stack conditions were such
that a constant flow rate provided, essentially, isokinetic
sampling conditions. Aerodynamic particle size sampling was
carried out at the points shown in Figure 7. The cumulative
method of sample collection was employed for both particulate
loading and particle size determinations.
Initially, the membrane sampling train was to be used to
collect particulate matter for microscopic particle sizing and
gas samples for chromatographic analyses. In view of its limited
traversing capabilities this train provided samples from a single
fixed point only. Because two of the continuous particulate
132
-------
monitors represented stack conditions in terms of measurements
at a single point, it was useful to check the validity of their
data with grain loading determinations derived from single point
sample acquisition with the membrane train. The sampling probe
on this train provided sample collection from the point £2 in
Figure 7. r ^
Sampling Time Per Point and Gas Sample Volume
Sampling times per point for particulate loading and particle
size determinations were designed to provide readily measurable
amounts of materials without overloading the filter or plates
of the Andersen impactor. Consequently, gas sample volumes cor-
responded to about 150 ft3 for particulate matter and approxi-
mately 125 ft for particle sizing tests. The membrane train
was kept at one point for the duration of any test.
CONTINUOUS PARTICULATE MONITOR DATA LOGGING
Although all the continuous particulate monitors were in-
stalled at the lower level, where they were exposed to adverse
effects of the environment, the remote control and recording
components had to be located in a protected area equivalent to
an industrial control room. The sampling level was modified
to control room specifications when it became apparent that
location of the remote control and recording devices in the field
office, at ground level, would not permit real time observation
of monitor performance in conjunction with the manual sampling
program. Before and after every test, each continuous particu-
late monitor system was zeroed and calibrated according to pro-
cedures outlined by the instrument manufacturers.
Optical Transmissometry
The electrical output from the optical head assembly of
the RAC transmissometer passed to the remote control panel which
provided:
i. direct readings for 0-100% opacity
ii. the operating mode selector switch (normal, span and
zero calibration modes)
iii electrical connections for both the data transmitting
cable and 115-volt single-phase power input.
The two-speed strip chart recorder provided with the remote con-
trol panel was not used because its scale was inadequate for
the scientific purposes of this study. Instead, the remote out-
put signal was fe^to a Hewlett-Packard, Model 7100B, strip chart
Recorder with a 17500 A multiple-span input module.
133
-------
In the Lear Siegler RM 41 monitor, the electrical signal
from the transducer unit was directed to the Optical Density/
Opacity Converter which provided, simultaneously, linear opacity
and optical density outputs. The convertor unit also provided
i. manual controls for remote zero and span calibration
checks
ii. a remote fault indicator for the transmissometer
iii. a shutter position indicator
iv. an over-range indicator for the selected measurement
range
v. a filter air-purge flow restriction indicator
vi. an automatic compensation for zero drift and exces-
sive compensation indicator
vii. an alarm level indicator and reset switches.
The four-inch strip chart recorder normally supplied with
the RM 41 system as an optional unit was replaced with a Hewlett-
Packard Model 7100B version, equipped with a 17500 A multiple-
span input module, for improved accuracy when integrating optical
density signals.
Backscattering
The ESC Particulate Mass Concentration Monitor was a com-
pletely self-contained unit with meter read-out and range selec-
tion located on the instrument panel at the monitoring level.
For remote data logging at the sampling level control room, the
output signal was fed to a Rikadenki B-161-361 Series multipen
recorder with a 50 ohm resistor connected across its input ter-
minals. The resulting 0-1 volt analog signal was integrated
with a Monitor Lab Model 8640 Mean-Value Integrator. A second
channel of the Rikadenki recorder produced a permanent record
of the output from this integrator. On the model supplied for
this investigation the manufacturer provided measurement ranges
of 0.05, 0.2, 1.0 and 5.0 gr/acf.
Charge Transfer
The continuous output signal from the IKOR 2710 sensor probe
was fed to a control unit consisting of
i. a power supply for the device
ii. a sensor output display
134
-------
iii. an integrating totalizer
iv. a test set function switch that is used for checking
sensor electronics.
The 0-10 volt output was fed to one of the Hewlett-Packard Model
7100B recorders located in the control room area. Five decades,
varying from 0.1 to 1,000, provided sensitivity selection for
mass flow sensing.
Beta Radiation Attenuation
The RAC Automatic Stack Monitor control console in associa-
tion with a Monroe, Model 1810, programmable printing calculator
regulated the entire operation of this system. Automatic gov-
erning of all system sequences, as well as individual component
functions, was performed by the master control module. Input
signals from the instrument subsystems were assimilated by the
control module and then transmitted to the mini-computer for
storage, calculation and printout.
The control console panel provided visual, digital readouts
of
i. gas sample volumes
ii. beta counts for filter taring and loading
iii. time intervals
Tape printouts of the above parameters and particulate matter
concentrations in appropriate units were obtained from the mini-
computer .
RESULTS
Thirty-two particulate matter concentration, particle sizing,
and membrane tests were carried out at the secondary lead smelter
during the nine month period from 4 June, 1976 to 7 March, 1977.
Twelve of these tests accounted for the reverberatory furnace
mode of operation. The remaining tests were conducted while
refining processes were in progress in the various lead alloying
kettles.
The simultaneous evaluation of the five continuous particu-
late monitors ended on 15 October, 1976 after the completion
of seventeen tests. A five month period of long-term reliability
evaluations terminated on 7 March, 1977.
135
-------
Particulate Matter Concentrations
Since particulate concentrations in the stack gases were
determined simultaneously with three different trains, it is
possible to compare single point (membrane) and limited travers-
ing (Andersen impactor) sample acquisition with the standard
EPS source testing procedures.
Figure 8 illustrates the close agreement between the data
obtained from the 24 point conventional grain loading tests and
values based on particle size determinations using 12 sampling
points. During some of the lead alloying kettle tests, a wide
range of particulate loadings in the stack gases was obtained
through the cooperation of the Smelter Supervisor, who provided
conditions simulating undesirable upset episodes that could occur
during normal processing.
With the exception of two tests during which obvious sam-
pling train failure occurred as a result of glass breakage, a
paired t-statistic test indicates a 92% level of confidence for
the particulate concentrations measured.
According to Figure 9, particulate concentrations determined
from the one point, membrane, sample acquisition technique are
approximately 35% higher than the mean values determined from
the 24 point and 12 point traversing tests. Deviations of this
magnitude indicate that single point sampling with membrane
filters can serve only as a semi-quantitative measure of con-
tinuous particulate monitor performance. It should be noted
that the mode of plant operation did not significantly bias the
particulate concentrations measured with the membrane filter.
Aerodynamic Particle Size Distribution Data
The experimental data were plotted on logarithmic-probability
(log-prob) paper as illustrated by Figure 10. Normal heating
of a molten charge during lead alloying kettle processing pro-
duced particulate emissions corresponding to the linear plot
exemplified by Test 24.
Disturbance of a kettle melt by the addition of sawdust
or sulfur was conducive to the generation of particles whose
cumulative log-probability size distribution plot showed two
linear segments comparable to the Test 13 representation in Fig-
ure 10. The break in the linearity seems to be indicative of
particulate production as a result of normal emissions from a
hot melt plus charring of granular additives dumped on to the
hot metal surface.
136
-------
20
Z .a
_i <5
11
GO fl
Z <
01 CC
£z
111 HI
0 O
Z Z
< O
O
16
i: 12
BEST LINEAR FIT
95% CONFIDENCE
CYCLONE FAILURE
8
12
16
20
PARTICULATE SAMPLING TRAIN
CONCENTRATION, arbitrary units
Figure 8. Comparison of standard and particle sizing train data.
137
-------
20
16
5 12
03
O
U
BEST LINEAR FIT
PERFECT AGREEMENT
95% CONFIDENCE INTERVAL H!
8
12
16
20
MEAN CONCENTRATION OF
SAMPLING TRAINS, arbitrary units
Figure 9. Comparison of paniculate concentration determinations from
single point membrane and two traversing techniques.
138
-------
E
a.
LU
<
Q
O
Q
O
cc
HI
50
40
30
20
10
8
1
0.8
0.6
0.4
0.2
1
—
—
—
—
—
_
•—
1
1 . I 1 1 1 , 1 , , , 1 , , 1,
RUN 13 - KETTLE / , |
WITH ARTIFICIALLY ' ' /
CREATED PEAK LOADS / / i
\ / i -
\ / / '
\ / °
V / RUN 24 - /
9 o KETTLE i
/ // '
/ ° / f
1 r i
/ /
? ' J\
1 D ^\
f ; y\
1 n f RUN 20 -
I / / REVERBERATORY
^\ i f
Y J( 4 FURNACE
/ ? /
1 / /
' /
/
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1
—
—
—
—
_
•—
|
0.1 125 10 30 50 70 90 99 99.9
PERCENT LESS THAN OR EQUAL TO STATED SIZE ON A MASS BASIS
Figure 10. Typical aerodynamic particle size distribution data.
139
-------
Test 20 illustrates the typical log-probability distribu-
tion of the particles collected during the reverberatory furnace
mode of operation. The two linear portions of the resultant
plot again suggest that the sample was a composite of emissions
from two different processes. It must be remembered that during
reverberatory furnace production the lead alloying kettles were
normally operational. As a result, reverberatory furnace test
samples really represent superimposition of baghouse emissions
on conventional kettle particulate matter. When the data from
Test 15 are excluded, on the basis of sample color and documenta-
tion of simultaneous lead-alloying kettle operation, from the
eleven other reverberatory tests, the arithmetic mean of the
aerodynamic particle diameters of the baghouse emissions is 0.43
ym with a standard deviation of 0.13 ym. The arithmetic mean
of the aerodynamic particle diameters for the twenty lead alloy-
ing kettle tests is 18.9 ym for the approximate range from 0.05
to 150 ym.
Particulate Matter Analysis
Identification of the major metallic components in the par-
ticulate emissions was required in order to determine whether
continuous monitor performance characteristics could be related
to particle compositions. A preliminary ESCA determination
showed that lead, tin and zinc were the three principal metals
in both the reverberatory furnace and lead alloying kettle stack
emissions. No other metals were present, consistently, in excess
of 0.5%.
The results of X-ray fluorescence analyses carried out in
the EPA laboratories at Research Triangle Park, North Carolina,
confirmed that lead, tin and zinc would be the predominant metal-
lic species in all the samples.
Due to the presence of tin oxides in the collected solid
materials, it was necessary to fuse the particulate matter before
atomic absorption analyses were carried out on all samples.
The data in Table 2 represent average values determined from
the combination of particulate matter recovered from the nozzle,
probe, cyclone, filter and impinger components of the sampling
train.
Continuous Particulate Monitor Performance
Figure 11 illustrates the Lear Siegler RM 41 instrument
response. It is impossible to correlate the results of the 32
tests by a single curve. However, three distinctly different
least square linear calibration plots can be recognized in terms
of the aerodynamic particle size of the emissions. The behavior
of the RAC instrument was essentially similar to the Lear Siegler
RM 41 as shown in Figure 12.
140
-------
TABLE 2. CHARACTERISTIC COLORS AND COMPOSITIONS
OF PARTICULATE SAMPLES
Test
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Color of particulate
on membranes
grey
blue
grey
grey
dark
grey
brown
black
brown
grey
black
brownish grey
white
grey
white
black
black
black
brownish
dirty
grey
membranes
black
brown
brown
whitish
grey
white
dark
ruined
grey
yellowish
yellowish
grey
grey
dark
brownish
black
black
silver grey
dark
grey
brown
grey
black
light grey
black
brown
Composition
by atomic
Pb
11
31
27
6.0
22
11
30
27
50
39
44
14
45
32
47
36
11
23
24
16
12
9.6
22
56
58
40
39
Sn
4
22
18
3.
21
6.
12
4.
5.
6.
5.
5.
7.
5.
15
14
3.
30
31
33
11
9.
20
8.
8.
3.
3.
of particulate matter
absorption analysis ,
weight %
5
7
3
8
9
6
1
0
9
6
4
6
7
0
6
Zn
0.
2.
2.
0.
*
•
•
12
1.
0.
*
3.
0.
.
•
2.
0 .
2.
2.
1.
2.
1.
2.
0.
•
•
•
33
0
6
23
39
95
95
2
28
56
3
75
70
75
5
30
1
1
9
4
5
2
11
11
13
14
Total
metal
16
55
47
10
44
18
42
43
57
46
50
22
53
38
63
52
15
56
57
51
25
20
43
65
66
43
43
141
(continued)
-------
TABLE 2 (continued)
Test
number
28
29
30
31
32
Color of particulate
brown light cream
membranes ruined
orange grey grey
black
black
Composition of particulate matter
by atomic absorption analysis,
weight %
Pb
28
16
19
33
34
Sn
2.9
2.4
4.2
4.6
19
Zn
0.24
.11
.21
.55
.62
Total
metal
31
18
23
38
53
JD
O
•
U)
O.
O
0.08 -
0.06 -
BEST LINEAR FIT
95% CONFIDENCE:?!
INTERVAL!
0.04 -
0.02 -
0 4 8 12 16 20
SAMPLING TRAIN CONCENTRATION, arbitrary units
Figure 11. Calibration data for Lear Siegler RM 41 transmissometer.
142
-------
CO
Q.
.O
o
73
CO
2
LU
Q
Q_
O
0.08 -
0.06 -
95% CONFIDENCE :i?i
INTERVAL^
0.04 -
0.02 -
04 8 12 16 20
SAMPLING TRAIN CONCENTRATION, arbitrary units
Figure 12. Calibration data for RAC transmissometer.
According to Figure 13, it would appear that there are two
distinctly different calibrations for the PILLS V continuous
particulate monitor. These lines correspond to periods following
initial installation and extensive field servicing by personnel
from ESC head office.
When the 31 test points during which the IKOR device was
operational are plotted in Figure 14, particle size considera-
tions once again provide a means of classifying instrument per-
formance according to the two linear plots.
It was extremely difficult to derive reliable calibration
curves for the RAC beta radiation attenuating device because
of repeated mechanical failures in the tape drive and clamping
systems. Figure 15 illustrates the instrument response during
the 13 tests when data could be collected. The two calibration
lines suggest that the performance of this continuous particulate
monitor was also dependent on the size of the particulate matter
being examined.
Continuous Monitor Reliability
Figure 16 and Table 3 summarize the results of the long-
term reliability evaluation of the five continuous particulate
monitors used in this study.
143
-------
12
c
3
GO
O
a.
GO
HI
cc
>
INITIAL INSTALLATION •
AFTER MAJOR SERVICING O
BEST LINEAR FIT
95% CONFIDENCE INTERVAL
I
6, 8 12 16
SAMPLING TRAIN CONCENTRATION, arbitrary units
20
Figure 13. Calibration data for ESC PILLS V monitor.
It is clearly evident that the Lear Siegler RM 41 transmissom-
eter operated throughout the entire testing period without any
major servicing. Only routine cleaning of filters and retro-
reflector and replacement of indicating bulbs were required.
The RAC transmissometer experienced a blower-motor failure
after about five weeks of operation. However, the capacity of
the retro-reflector blower provided sufficient purge air to permit
the instrument to function satisfactorily until its removal from
the stack.
The IKOR charge transfer device operated during the entire
testing program but required regular removal from the stack for
cleaning of the sensor. Although this was a relatively simple
procedure requiring minimal time, the conditioning of the sensor
following cleaning often took several hours.
Factory trained personnel were called to the site twice
during the testing period to make appropriate adjustments to
the ESC PILLS V backscattering monitor. Prior to Test 8, the
optical interrupters and a printed circuit board were replaced
in an effort to eliminate the excessive base line current signal
which made zeroing and spanning of the instrument impossible.
When the same problems reappeared after Test 14, the unit was
overhauled completely. The amount of servicing was consistent
with the field development of any new instrument. This was the
fourth PILLS V manufactured and the first subjected to rigorous
evaluation in an industrial stack.
144
-------
c
3
to
k.
4-»
15
O
a.
co
DC
O
POINT
AT 30
SUSPECTED ELECTRICAL SHORTS
BEST LINEAR FIT
95% CONFIDENCE INTERVAL ??
0
0246
SAMPLING TRAIN EMISSION RATE, arbitrary units
Figure 14. Calibration data for IKOR monitor.
145
-------
12
.5
'E
3
s 8
a.
O
LU
LU
CO
BEST LINEAR FIT
95% CONFIDENCE INTERVAL|H||
Dp<1
048
SAMPLING TRAIN CONCENTRATION, arbitrary units
Figure 15. Calibration data for RAC beta radiation monitor.
146
-------
TEST NUMBER
^
Mte-
I7v J. A zr
JZa \
TESTING
IKOR
RESPONSE
1 1
II
• 1
•f : :
I
LLS \l
ESPONSE
1 II [:|
TESTING
LEAR SIEGLER
TESTING
RAC TRANS
RESPONSE
TESTING
RAC B GAUGE
RESPONSE
1
1
1
TESTING PERIOD
OPERATIONAL
SERVICINGOR FAULT
(SEE TABLE 3)
NON OPERATIONAL
I
i
_L
_L
MAY
JUNE
JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER JANUARY FEBRUARY MARCH
1976 H 197? 1
Figure 16. Summary of servicing record and operational periods of
continuous monitors over a one year period.
-------
TABLE 3. SERVICING RECORD OF CONTINUOUS MONITORS
Instrument
Key
Servicing or fault
IKOR
PILLS V
Lear Siegler RM 41
Transmissometer
1. Installation complete.
2. Bullet sensor cleaned and turned
horizontally and exposed at sug-
gestion of Mr. A. Gruber.
3. Reading high and erratic, probe
cleaned.
4. Additional cleanings required. Mr.
A. Gruber and Mr. E. Allen visit
site.
5. Instrument functioning, however,
showed signs of severe dust build-
up.
6. Instrument removed from stack.
7. Installation complete.
8. Excessive base line current, manu-
facturer notified.
9. Impossible to zero or span, serviced
by Mr. B. Nuspliger.
10. Full scale deflection.
11. Returned to normal.
12. Optical assembly retracts and blower
fails.
13. Major servicing by Mr. Nuspliger and
Associate.
14. Optical assembly retracts, fail safe
switch placed in "on" position in
order to keep optics in stack.
15. Retracts in "auto" position, again
left in fail safe "on" mode.
16. Instrument removed from stack.
17. Installation complete.
18. Fault light, indicates dirty retro-
reflector cleaned by Mr. T. Novak.
19. Routine cleaning of filters and
automatic compensation check per-
formed by Mr. I. Kent.
20. Operate light burned out.
21. Light bulb replaced.
22. Instrument removed from stack.
(continued)
148
-------
TABLE 3 (continued)
Instrument
Key
Servicing or fault
RAC Transmissometer
RAC 3-Gauge
23. Installation complete.
24. Blower-motor failure,retro blower
sufficient to provide air to both
sides of transmissometer.
25. New blower arrived (not installed).
26. Instrument removed from stack.
27. Installation complete.
28. Tape drive control failure.
29. Tape drive fault causes "double
spotting".
30. Operational only when tape drive
advanced and controlled manually.
31. Low sample volumes.
32. Tape drive advancing continuously,
clamping device not sealing.
33. Major servicing by Mr. D. Wetick.
34. Tape drive malfunction. Clamping
device fails. Instrument not
operational.
35. Instrument removed from stack.
The RAC beta radiation attenuation monitor arrived after
the completion of Test 3 because of problems experienced by the
supplier during reconditioning after use at a potential site
in the U.S.A. Although it functioned for 13 tests, manual assis-
tance to the tape drive had to be provided for 6 of them. Ex-
tensive on-site servicing by a factory trained technician failed
to correct the major faults. Because of the repeated failures
in the Sampling Module the RAC 3-gauge was eliminated from the
program after Test 19.
Of the four remaining monitors the Lear Siegler RM 41 trans-
missometer, the ESC PILLS V and IKOR 2710 were retained for a
long-term reliability evaluation because they represented three
distinctly different classes of instrumentation. The RAC trans-
missometer was removed from the stack and further consideration
due to the blower-motor failure and satisfactory performance
of the alternate light attenuating instrument.
DISCUSSION OF RESULTS
The interpretation of continuous particulate monitor per-
formance depends on the reliability of the manual stack sampling
data used as a standard of comparison. This means that a thorough
appreciation of the errors and assumptions inherent in the de-
termination of particulate concentrations, mass emission rates,
149
-------
and aerodynamic and optical particle sizes is needed for a com-
prehensive analysis of instrument characteristics.
Random Error Analysis for Particulate Sampling Train
The acquisition of a truly representative sample from a
source stack involves a careful step-by-step process using a
sampling train consisting of several parts, each designed to
perform a specific task. Determinations of particulate concentra-
tions and mass emission rates require a series of calculations
employing various measurements which leads to a compounding of
errors. Systematic and random errors are compounded differently.
Systematic errors were minimized by:
i. calibrating the S-type pitot tube - sampling probe
assembly, integrating dry test meter, orifice rate
meter and stack temperature sensing element
ii. sampling isokinetically to within ± 10%
iii. following standard procedures outlined by the EPS
source testing code1
iv. assigning specific responsibilities to individuals
in the sampling team.
Random errors are inherent in any measurement. For any
stack sampling program they can be attributed to
i. source condition variation
ii. equipment and personnel variations in field procedures
iii. equipment and personnel variations in the laboratory
In the calculation of mass loading, there will be compounding
of 7 random errors involving
i. the variance in average stack gas velocity
ii. the variance in stack or duct cross-sectional area
iii. the variance in proportion by volume of water
iv. the variance in the average absolute stack gas tem-
perature
v. the variance in the absolute stack gas pressure
vi. the variance in particulate matter collected
vii. the variance in total gas sample.
150
-------
Table 4 summarizes the individual measurement variables
along with their standard deviations used to calculate the rela-
tive random errors in particulate concentrations and mass emis-
sion rates.
In this error analysis the measurement of average stack gas
velocity and total gas sample comprised approximately 85% of
the total random errors in the determination of the mass emission
rate except for tests involving particulate samples weighing
less than 50 mg. For low loading tests, the measurement of the
particulate catch was the principal source of random error.
Random error limits, corresponding to one standard deviation,
for the manual sampling program have been defined in Figures
11-15.
Particulate Matter Analysis
Results of ESCA, X-ray fluorescence and atomic absorption
analyses showed that lead, tin and zinc were the three most
prominent metals encountered in the stack emissions.
Although the ESCA procedures provided measures of composi-
tion at particle surfaces only, the results are in reasonable
agreement with the atomic absorption analyses.
It is difficult to comment on the X-ray fluorescence results
provided by the U.S. EPA laboratories because the techniques
employed in preparing and mounting membrane samples for analysis
were not described. When raw data in terms of pg/cm2 of filter
area were recalculated to percent of mass reported, the levels
of lead, tin and zinc were essentially comparable to those de-
termined by atomic absorption. However, the compositions ex-
pressed on a weight per cent basis by the U.S. EPA laboratories
are much lower than atomic absorption determinations. Because
of the extreme variations in the sample characteristics it is
suspected that matrix effects could be responsible for the dis-
crepancies in the X-ray fluorescence analyses.
The metal composition data in Table 2 represent the results
of replicate analyses of material recovered from the sampling
nozzle, probe, cyclone, filter and impinger components of the
sampling train. Each of these composite samples provided a measure
of the average composition of the stack emissions during the
period of any particulate matter concentration determination
in terms of the entire spectrum of particle sizes. Examination
of membrane samples by X-ray fluorescence neglected the signifi-
cant portion of particulate matter that had been collected in
the sampling nozzle ahead of the filter.
The recovery of material from dried beakers and filters
for the fusion step in the atomic absorption procedure represented
151
-------
TABLE 4. SUMMARY OF INDIVIDUAL MEASUREMENT VARIABLES AND
THEIR CORRESPONDING STANDARD DEVIATIONS
Error term
Individual measurements affecting error term
Symbol
Description
Standard deviation, a
NJ
Cross-sectional flow
area of duct, A
s
Mass of particulate
matter, M
Average stack
velocity, (Us)
based on 24 avg
local measurements
Stack gas moisture
content, B
wo
D.
W
M
Vi
Stack diameter
Gross weights of filter
and beakers including
errors resulting from
washing, drying, and
weighing
Tare weights of filters
and beakers
S-type pitot tube co-
efficient (by calibra-
tion)
Absolute stack gas
temperature
Stack gas velocity
head (inclined manometer)
Absolute stack gas
pressure
Molecular weight of
stack gas on wet basis
Final .volume of impinger
contents
Initial volume of im-
pinger contents
1/2 in.
4 mg
1/2 mg
0.005
5°R
0.01 in. H20
0.05 in. Hg
0.1 Ib /lb-mole
m
0.5 ml
0.5 ml
(continued)
-------
TABLE 4 (continued)
Ul
CO
Individual measurements affecting error term
Error
term
Average stack gas
temperature (T )
Absolute
pressure,
Total gas
(Vre£'
errors in
velocity,
stack gas
Ps
sample,
including
average stack
(U )
s avg
Symbol
Vm
P
m
Tm
Ts
Pbar
P
static
K0
DN
MH
Description
Dry gas metered volume
Absolute pressure at
the dry test meter
Absolute temperature at
dry gas meter
Local stack gas tem-
perature
Atmospheric pressure
at sampling site
Stack gas static pres-
sure
Calibration constant
for orifice
Nozzle diameter
Molecular weight of
Standard deviation, a
0
0
0
0
0 01 ft' f
mm 1
n
.02 ft3
.05 in. Hg
2°R
5°R
.05 in. Hg
.1 in. H20
lbm in. Hq
Ib-mole in. H2O,
0.001 in.
, lbm
1"
°Rj
m
stack gas on dry basis
Absolute pressure up-
stream of the orifice
Absolute temperature up-
stream of the orifice
Ib-mole
0.05 in. Hg
2°R
(continued)
-------
TABLE 4 (continued)
Error term
Individual measurements affecting error term
Symbol
Description
Standard deviation, a
(AH) setting
(1-B )
v wo'
Sampling time interval
Error associated with
setting the required
orifice pressure drop
Proportion by volume of
dry gas in stack gas
stream
0.5 sec
0.02 in. H2O
0.01
-------
80 to 92% of the particulate matter caught during any mass loading
test. Trial runs with laboratory formulations of lead, tin and
zinc compounds of compositions approximating those of collected
samples showed that better than 90% of each metal was accounted
for after the fusion process. On this basis it is believed that
the atomic absorption procedure provided the most reliable evalua-
tion of metal levels in stack emissions.
Particle Sizing Determinations
Because of the differences between theory and practice,
aerodynamic particle sizes must be used with caution. In prin-
ciple, cascade impactors provide discrete fractions as approxi-
mations to the true continuous particle size distributions that
are sampled. The uncertainty in particle size is compounded
by errors associated with wall losses, particle bounce and devia-
tions from theoretical calibration curves. Evaluations of the
severity of these errors are topics of current investigations.2
Particle bounce could have been significant during any test
because the impactors used for particle size determinations
employed no substrate material on the plates. It was decided
to work with bare metal stages to minimize any potential weighing
errors that could result from anomalous weight increases in glass
fiber substrates as a result of sulfate formation in the presence
of S02.3 The use of unlined impactor plates facilitated quantita-
tive recovery of collected particulate matter for future analytical
purposes.
According to McCain et al.,1* particle bounce can severely
distort size distributions of aerosols having large mass mean
diameters. As a result, the reported dimensions of distributions
with large mass median diameters could be smaller than the true
values. On the other hand, mass median diameters for small par-
ticle aerosols (whose mass median sizes are less than 10 ym) could
be systematically larger than the true values because of the
low resolution provided by cascade impactors.
Although it was recognized that sampling times as long as
4 hours could contribute to particle re-entrainment in the cascade
impactor, the particle sizing train was operated as long as the
particulate loading train
i. to provide a check on the mass concentrations which
served as the primary standard for instrument evalua-
tion
ii. to ensure a representative sample for the random
smelter process over the sampling period.
Because of particle bounce and re-entrainment considerations,
which cannot be readily evaluated, the aerodynamic diameters
155
-------
are subject to an uncertainty approximated as ± 25% of the re-
ported value. However, this accuracy is sufficient for the analysis
of continuous particulate monitor performance characteristics.
Aerodynamic particle sizes affect the performance of the
IKOR charge transfer and RAC g-attenuation devices because both
instruments depend on particle capture from a flowing gas stream.
The light attenuating Lear Siegler RM 41 and RAC transmissometers
and backscattering PILLS V monitor respond to the optically pro-
jected sizes of particles.
In principle, it is possible to convert aerodynamic parti-
cle sizes to optical dimensions using the relationship between
the true density of the particles under consideration and the
theoretical 1 g/cm3 standard. This conversion becomes virtually
impossible when dealing with particulate matter whose composition
could vary from test to test and during any specific test period.
The problem is further complicated by the fact that the geometric
standard deviation, aa, is required for computation of median
diameters on a number basis.5 Table 5 summarizes the lead, tin
and zinc compounds that could be found in varying proportions
in the stack emissions. The colors of the materials listed are
consistent with the characteristic colors of the membrane samples
reported in Table 2. Because of the additions of sulfur and
sawdust to the lead alloying kettles, elemental sulfur and carbon
also could have been present in substantial quantities in any
sample. Without a detailed evaluation of the density of any
test specimen it is not possible to derive reliable optical sizes
from the aerodynamic data.
Continuous Particulate Monitor Performance
For each instrument, calibration curves were expressed in
terms of the best linear fits wherever justified by the statisti-
cal "F" test. Error variances for each calibration curve were
evaluated and illustrated as 95% confidence intervals. Random
error limits for each manual particulate matter concentration
test have been indicated in Figures 11-15 in terms of one
standard deviation, a.
In Figures 11-15 particulate matter concentrations or
mass emission rates have been used as independent variables be-
cause they were the primary standards against which instrument
responses were compared.
Transmissometers
Figures 11 and 12 indicate that the Lear Siegler RM 41 and
RAC transmissometer responses are primarily dependent on particle
size when aerodynamic diameters exceed one micron. This behavior
is consistent with the theoretical predictions of Pilot and
Ensor5 and the experimental work of Uthe and Lapple.7
156
-------
TABLE 5. SUMMARY OF PHYSICAL PROPERTIES OF METALLIC COMPOUNDS
LIKELY TO BE EMITTED FROM A SECONDARY LEAD SMELTER6
Compound
Pb
PbCl2
PbCl2 .3PbS
Pb20
PbO
PbO
Pb30^
Pb203
Pb02
PbCl2.Pb(OH)2
PbCl2 -Pb(OH) 2
PbCl2. PbO. H20
PbCl2 -2PbO
PbCl2 -3PbO
Molecular
weight
207.21
278.12
995.95
430.42
223.21
223.21
685.63
462.42
239.2
519.35
519.35
519.35
724.54
947.75
Color
cubic silver
blue white
rhombic white
red
amorphous
black
tetragonal
yellow
rhombic
yellow
crystalline
red amorphous
powder
amorphous
yellow powder
tetragonal
brown
tetragonal
white
rhombic
monoclinic
prism colorless
to white
rhombic yellow
yellow
Refractive
index
2.199
2.260
2.665
2.535
2.51
2.61
2.71
2.3
2.04,2.15
2.15
2.146
2.24, 2.27, 2.31
Specific
gravity
11.3
5.85
8.342
9.53
8.0
9.1
7.21
6.24
6.05
7.08
(continued)
-------
TABLE 5 (continued)
m
00
Molecular
Compound weight
PbCl2. 7PbO 1840
PbS04 303
PbSCH. PbO 526
PbS208. 3H20 453
PbfHSOit) 2 . H20 419
PbS 239
Sn 113
SnCl2 139
SnCl2 • 2H20 225
SnCU 260
SnCli, • 3H20 314
SnO 134
Sno. xH20
Sn02 150
SnC-2 . xH2O
a stannic acid
.59
.28
.49
.39
.37
.28
.7
.61
.65
.53
.58
.70
.70
Refractive
Color index
yellow crystal
or powder
monoclinic 1.877
or rhombic white 1.882
monoclinic 1.93
white 1.99
2.02
deliquescent
crystalline
cubic black 3.912
metallic
a cubic gray
3 tetragonal
white metallic
Y rhombic white
rhombic white
white monoclinic
colorless liquid
monoclinic crystal
tetragonal (cubic)
black
white powder or
yellow brown crystal
tetragonal 1.997
white, brown 2.093
black
amorphous or
colorless precipitate
Specific
gravity
6.2
6.92
7.5
5.75
7.28
6.54
3.39
2.71
2.23
6.64
6.95
(continued)
-------
TABLE 5 (continued)
en
vo
Compound
Snoa • xH20
3 stannic acid
SnSC\
SnS
SnS2
Zn
ZnCl2
Zn(OH) 2
ZnO
ZnO
ZnOa + ZnO
ZnSOi,
ZnSO,, . 6H2O
ZnSCU • 7H20
Molecular
weight
214.77
150.77
182.83
65.38
136.29
99.40
81.38
81.38
178.76
161.44
269.54
287.56
Color
amorphous or
gelatinous white
yellowish white
crystalline powder
rhombic gray
black
hexagonal gold
yellow
hexagonal bluish
white metal
cubic white
deliquescent
rhombic
colorless
white or yellowish
amorphous powder
hexagonal
white
white yellow
powder
rhombic
colorless
monoclinic or
tetragonal
colorless
rhombic
colorless
efflorescent
Refractive
index
1.687
1.713
2.008
2.029
1.658
1.659
1.67
1.457
1.480
1.484
Specific
gravity
5.08
4.5
7.14
2.91
3.05
5.47
5.605
1.571
3.74
2.07
1.97
(continued)
-------
TABLE 5 (continued)
Compound
ZnS
ZnS
ZnS . H20
ZnS03 . 2H20
Molecular
weight
97.45
97.45
115.46
181.48
Color
hexagonal
colorless
cubic
colorless
yellowish
white powder
white crystalline
powder
Refractive
index
2.356
2.368
Specific
gravity
4.087
4.102
3.98
Ol
o
-------
In principle, the sensitivity of a transmissometer shows
a monotonic decrease with increasing particle size if particulate
matter having an absorptive index of refraction is involved.
However, a maximum in the response-particle size relationship
is to be expected according to the classical Mie theory for par-
ticles having only a real index of refraction. Figure 17 illus-
trates the effect of the absorptive refractive index on the
response-size relationship for a real refractive index of 1.5.
Because of these size and refractive index effects, transmissom-
eter s should not be capable of detecting very small, non-absorbing
particles, but would respond well to absorbing particles in the
same size range.
Most theoretical discussions involve monodispersed particu-
late systems which are seldom encountered in real life situations.
Figure 18 shows that as the polydispersivity of an aerosol in-
creases, as evidenced by increasing values of ag, the effect
of particle size on transmissometer response is diminished.
Because the stack emissions under investigation consisted
of the black and/or opaque whitish particulate matter and par-
ticle size distributions were of a highly polydispersed nature,
as indicated by the high slopes of the log-probability curves,
the effects of particle size on instrument performance should
have been minimized for particles smaller than about 1 urn and
larger than 25 ym optical diameter. Consequently, the typical
performance curves supplied by manufacturers showing the effect
of particle size on light attenuation are virtually useless for
the evaluation of the applicability of these instruments to any
specific source.
Both the Lear Siegler and RAC transmissometers satisfy the
U.S. EPA performance specifications.10 According to the operating
manual provided, the RAC instrument was designed with a smaller
angle of view. On the basis of collimation considerations the
RAC device should be better than the Lear Siegler RM 41 model.
However, a comparison of Figures 11 and 12 shows that the Lear
Siegler system exhibits higher sensitivities and provides cali-
bration curves with lower error limits. In actual fact, the
RAC transmissometer was incapable of detecting particles, during
this testing program, when the aerodynamic diameters exceeded
25 vim.
Environmental Systems Corporation PILLS V
The PILLS V monitor provided two calibration curves that
were relatively independent of particle size. The two data sets
were due, most likely, to a change in the instrument calibration
as a result of the replacement of the optics head and other ad-
justments made during the major overhauling of October 11, 1976.
161
-------
100
GEOMETRIC STANDARD DEVIATION = 2
0.1
0.01 0.1 1.0
GEOMETRIC MASS MEAN RADIUS,
10.0
Figure 17. Effect of absorptive refractive index on opacity-size
relationship.^
162
-------
ja
0)
D
O
O
GO
O
cc
HI
CO
z
O
o.
GO
LU
CC
Q
HI
O
Q
ill
CC
a.
35
30
25
20
15
10
05
CARBON M = 1.96- 0.66 i
X = 0.56 jum
0
0.05
I
0.2 0.5 1.0 2.0
VOLUME MEDIAN DIAMETER, j
5.0 10.0 20.0
Figure 18. Transmissometer response as a function of geometric
standard deviation and volume mean diameter.9
The calibration curve, corresponding to the data collected
after major servicing, exhibits a zero off-set and an increase
of approximately 2.3 in the "effective gain" or sensitivity.
Discussions with H.W. Schmitt, President of Environmental Systems
Corporation, confirmed that both of these effects could be at-
tributed to the replacement of the optics head and adjustments
made during the on-site reconditioning. The ESC personnel made
no attempts to quantify or adjust for the instrument changes
that could have resulted from their modifications.
Some of the scatter in the data collected after the major
servicing can be attributed to changes in both the real and ab-
sorptive components of refractive indices. Figure 19 illustrates
how a backscattering instrument would respond to particulate
matter of varying refractive index. Additional scatter in the
data can be attributed to mean diameters beyond the flat por-
tion of the PILLS V response curve and the general backscatter-
ing response curves as shown in Figure 19.
IKOR Particulate Monitor
The data in Figure 14 show that the IKOR monitor has limited
sensitivity to particles smaller than one micron in aerodynamic
163
-------
104E
« 103 -
= X = 0.904 /urn
" 102
V)
V)
10
EC
ID
0.
CO
o
Q.
CO
1.0
0.1
I—
8 = 167°
n\2 = 0
ag = 1.65
\
\
e = 167°
in-, = 1.96
- ag = 1.65
A = 0.904 jum
I
0.05 0.1
0.5 1.0
5.0 10.0 0.05 0.1
0.5 1.0
5.0 10.0
Figure 19. Response per unit mass vs. dj, mj = 1.33, 1.40, 1.50, 2.0;
rr>2 = 0, 0.1, 0.3, 1.96 for backscattering. 11
diameter. This is not surprising in terms of its design prin-
ciples. Charge transfer from particles to the sensor can occur
only if particulate matter impinges on the collecting head.
For the stack conditions encountered the impaction efficiencies
were generally so low that only that fraction of the particulate
matter with diameters exceeding 4 ym would make contact with
the sensor surface.
When it became apparent during the testing program that
particulate matter with diameters below one micron was being
missed by the IKOR sensor, the recommendation was made by A.
Gruber, Vice President, IKOR Inc., that the sensing element be
exposed and oriented with its axis perpendicular to the stack
gas stream. When this was done there did not appear to be any
significant effect on the instrument performance.
The wide scatter about the calibration curve for particles
whose diameters are larger than 5 ym can be due to variations
in
164
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i. electrical resistivity of the particulate matter
ii. stack gas humidity
iii. stack gas temperature
iv. particle precharge characteristics
v. probe surface conditions as a result of the accumula-
tion of particles or cleaning with abrasive cleansers,
as suggested by John.12
RAG Beta Radiation Monitor
The RAC beta radiation attenuating instrument supplied for
this program was a reconditioned unit which suffered from re-
peated mechanical failures in the tape drive, indexing and clamp-
ing components of the sampling module. Consequently, this moni-
tor functioned during only 13 of 22 possible testing periods.
According to design principles, a single calibration curve
should have been obtained. However, Figure 15 shows that this
instrument was more sensitive to smaller particles whose aero-
dynamic diameters were less than one micron. This behavior sug-
gests that the RAC 3-gauge was affected by particulate deposition
in the sampling nozzle and probe and the transport lines to the
filter tape. Such deposition was confirmed at the conclusion
of the testing program when substantial amounts of particulate
matter were recovered from the nozzle, probe and boundary layer
dilution screen.
Neither calibration curve passed through the origin. The
observed shift in these curves would result from
i. the previously discussed particulate deposition in
the lines
ii. in-leakage at the sampling head due to poor clamping
iii. filter tape losses as a result of de-fluffing under
vacuum during the sampling interval after taring
iv. filter tape losses as a result of abrasion during
tape transport between taring, sampling and final
counting.
CONCLUSIONS AND RECOMMENDATIONS
The performance characteristics of 5 continuous particulate
monitors were evaluated over a one-year period to assess their
165
-------
i. suitability
ii. reliability
iii. accuracy
with respect to their substitution for manual methods as a means
of ensuring long-term compliance with and enforcement of federal
regulations pertaining to emissions from secondary lead smelters
in the Canadian environment.
The Lear Siegler RM 41 transmissometer was more reliable
than the other four monitors. The experimental data indicate
that this device would be suitable for monitoring emissions from
controlled sources provided that the particulate matter had ap-
preciable absorptive indices of refraction. Its first order
dependence on particle sizes exceeding one micron aerodynamic
diameter makes it unsuitable for use where process variations
result in the generation of particulate matter of varying sizes,
colors and/or refractive indices. Transmissometer performance
could not be related to changes in the chemical composition of
the emissions.
The Environmental Systems Corporation PILLS V monitor per-
formance was handicapped by its poor reliability and high main-
tenance requirements. These deficiencies were consistent with
the difficulties experienced by prototype units undergoing long-
term field evaluations for the first time. Improvements made
to the optical interrupters, controlling the zero-span band,
and provision of an external standard for referencing the band
should eliminate many of the problems encountered during the
reliability determinations. If the long-term reliability were
improved, the PILLS V device could be potentially more useful
than the best transmissometer because its calibration appears
to be less sensitive to changes in particle size, color and re-
fractive index. Variations in the metal content of the particu-
late matter could not be directly correlated to the calibration
curve for this backscattering instrument.
Performance of the Research Appliance Company transmissom-
eter was adequate for monitoring emissions from the controlled
reverberatory mode of operation consistently producing particu-
late matter with median diameters approximating 0.5 ym. However
it does not appear to be capable of detecting large particles '
whose diameters exceed 25 pm.
The IKOR 2710 particulate monitor showed limited potential
for application to either the reverberatory or lead allovina
kettle processes. The particles passing through the baqhousP
were too small to impact effectively on the sensor On th t-h
hand, short, periodic bursts of high emissions durinq the nn
controlled refining operations necessitated frequent
of the sensing element. yuent
166
-------
Because of the repeated mechanical failures in the sampling
module of the Research Appliance Company beta radiation attenuating
device supplied for the study, it is difficult to comment criti-
cally on its possible application to measurement of emissions
from a secondary lead smelter.
Continuous particulate monitors have not been developed
to the stage where particle size, shape, composition and color,
which have pronounced effects on instrument performance, can
be accounted for in the calibration curves for the instruments.
Reliable continuous monitoring could be possible only if the
physical and chemical properties of the emissions remained suf-
ficiently constant over long time periods. These conditions
are approximated in stacks exhausting gases that have passed
through efficient control equipment.
REFERENCES
1. Standard Reference Methods for Source Testing: Measurement
of Emissions of Particulates from Stationary Sources. EPS
l-AP-74-1, Air Pollution Control Directorate, Environmental
Protection Service, Environment Canada, Ottawa, Ontario,
1974.
2. Harris, D.B. Procedures for Cascade Impactor Calibration
and Operation in Process Streams. EPA-600/2-77-004, U.S.
Environmental Protection Agency, Research Triangle Park,
NC, 1977.
i
3. Hemeon, W.C.L., and A.W. Black. Stack Dust Sampling: In-
Stack Filter or EPA Train. J. Air Pollut. Contr. Assoc.
22(7):516-518, 1972.
4. McCain, J.D., and J.E. McCormack. Non-Ideal Behavior in
Cascade Impactors. APCA paper 77-35.3, presented at 70th
Annual Meeting, Toronto, Ontario, June, 1977.
5. Pilat, M.J., and D.S. Ensor. Plume Opacity and Particulate
Mass Concentraion. Atmos. Environ. 4:163-173, 1970.
6. Handbook of Chemistry and Physics, R.C. Weast, Editor, 49th
Edition, The Chemical Rubber Co., Cleveland, OH, 1968-69.
7. Uthe, E.E., and C.E. Lapple. Study of Laser Backscatter
by Particulates in Stack Emissions. SRI 8730, Stanford
Research Institute, Menlo Park, CA, 1972. NTIS PB 212530.
8. Feldman, P.L., and D.W. Coy. Comparison of Computed and
Measured Opacities: Lignite Fired Boilers. APCA paper
73-168, presented at 66th Annual Meeting, Chicago, IL, June,
1973.
167
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9. Markowski, G.R., G.J. Woffinden, and D.S. Ensor. Optical
Method for Measuring the Mass Concentration of Particulate
Emissions. EPA-600/2-76-062, U.S. Environmental Protection
Agency, Research Triangle Park, NC, 1976.
10. Nader, J.S., F. Jaye, and W. Connor. Performance Specifica-
tions for Stationary-Source Monitoring Systems for Gases
and Visible Emissions. EPA-650/2-74-013, U.S. Environmental
Protection Agency, Research Triangle Park, NC, 1974.
11. Miller, A.C. Theoretical Modelling and Calibration for
a Particulate Mass Concentration Monitor. M.S. Thesis,
The University of Tennessee, Knoxville, TN, August, 1977.
12. John, W. Investigation of Particulate Matter Monitoring
Using Contact Electrification. EPA-650/2-75-043, U.S.
Environmental Protection Agency, Research Triangle Park,
NC, 1975.
168
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PAPER 8
AN IN-STACK FINE PARTICLE SIZE SPECTROMETER;
A DISCUSSION OF ITS DESIGN AND DEVELOPMENT
ROBERT G. KNOLLENBERG
PARTICLE MEASURING SYSTEMS, INC.
ABSTRACT
The development of an in-stack fine particle size measur-
ing device for the EPA has been underway at PMS for the past
several months. The instrument utilizes a laser fed optical
system with detection by near-forward light scattering. Sam-
ple volume is established through the use of a high resolution
imaging system using paired detectors viewing the sample plane
through a masked beam splitter. The instrument covers a 0.3
to 10.0 yrn size range with 60 channels resolution. Absolute
theoretical accuracy is ±20% of size for completely unknown
refractive index. The instrument is designed to operate con-
tinuously at in-stack temperatures up to 250°C at flow velocities
up to 30 m/sec. Flow velocities are determined from measured
particle transit times through the laser beam. Internal probe
components are cooled by a water jacket and external heat ex-
changer. The heat exchanger, probe pulse processing electronics,
data acquisition system and operating controls are housed in
a central electronics console. The instrument is being tested
on a local coal fired power plant. Its eventual use will be
directed at the characterization of particulate emissions of
stacks or other stationary sources and to qualitatively evaluate
the performance and collection efficiencies of particulate con-
trol devices now in operation.
INTRODUCTION
In September, 1977 Particle Measuring Systems, Inc. (here-
after PMS) was contracted by the Environmental Protection Agency
(hereafter EPA) to design, fabricate, and evaluate an in situ
Fine Particle Stack Spectrometer System (FPSSS). The FPSSS will
169
-------
be used to characterize the particulate emissions of stacks and
other stationary sources and to quantitatively evaluate the per-
formance and collection efficiencies of particulate control de-
vices now in use. The EPA design criteria that the FPSSS had
to satisfy were to provide in situ high resolution particle
sizing over a size range from 0.5 to 5.0 urn diameter with con-
centrations in this range above lOVcm3 and integrated partic-
ulate loading from 0.3 to 3.0 g/m3. The FPSSS is also required
to operate at temperatures from 20 to 250°C and at flow velocities
from 1 to 30 m/sec. An operational prototype instrument has
been designed and fabricated and initial performance testing
begun. This paper describes the current status of this develop-
mental effort.
The basic design approach followed in the FPSSS utilizes
existing technology. There are three major areas of prior work
which have provided the needed technology base upon which the
underpinnings of the FPSSS design could be established. These
are:
1) The PIONEER VENUS Large Probe Cloud Particle Size Spec-
trometer (LCPS),1 development work with joint involve-
ment of PMS and Ball Brothers Research Corporation
(BBRC).
2) The recent experience by PMS in stack measurements using
plumbed instruments.
3) The well developed PMS technique of utilizing high reso-
lution imaging systems with light scattering to perform
in situ single particle size measurements.
The current prototype FPSSS instrument would appear to meet
the original EPA design objectives. It has a near-forward light
scattering optical system with an expanded 60 channel 0.3 to
10.0 ym size range divided into four subranges of 15 size chan-
nels each. The instrument is capable of relatively accurate
measurements at concentrations up to 105/cm3 without regard to
refractive index. An optical velocimeter has also been designed
into the FPSSS. The instrument operates continuously at 250°C
temperatures utilizing a water-cooled head design and external
heat exchanger. Extensive theoretical modeling of thermal as
well as optical performance has been utilized in configuring
the FPSSS Probe head. Wind tunnel facilities at PMS have played
an extremely important role in measuring aerodynamic impacts
of the FPSSS sampling section. Calibration has included lab-
oratory and wind tunnel tests on particulates having known or
independently verifiable size distributions. Field tests on
an operating coal-fired power plant stack began in April, 1978
and some preliminary results are presented here.
170
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DESIGN APPROACH OF THE FINE PARTICLE STACK SPECTROMETER SYSTEM
(FPSSS)
A hot stack environment may appear to present possibly too
high concentrations for in situ single particle measuring devices;
however, measurements with a standard laboratory aerosol spectrom-
eter manufactured by PMS employing a ducted sample flow showed
this not to be the case. Today most sources must comply with
much more stringent emission controls. The standard compliance
device for coal-fired power plants is an opacity monitor which
measures total extinction. Permissible emissions generally com-
pute to extinction coefficients less than 3 x 10~Vcm. This
would correspond to a visibility of approximately 120 meters
and maximum concentrations around 5 x 103/cm3 for 1 ym radius
particles. Such concentrations are well within the range of
standard aerosol spectrometers offered by PMS. The real problem
is performing measurements in the hot environment. The stack
is often well above boiling temperature and if continuous mea-
surements are desired the instrument must be cooled to survive.
Our approach was to provide a reasonable thermal environment
for an existing PMS light scattering spectrometer electro-optical
system using an active cooling system.
The FPSSS design involves a laser fed high resolution imaging
optical system with particle detection and sizing by strong for-
ward light scattering (see optical system in Figure 1). The
imaging system provides accurate sample volume definition and
enables accurate particle transit time measurements to extract
flow velocity. The FPSSS sampling head design shown in Figure
2 reveals a water cooled insulated electro-optical system. The
probe head communicates to a standard PMS Data Acquisition System
(DAS) and thermal control system packaged in a control console
(see Figure 3). The FPSSS Probe has now been fabricated much
as shown in Figures 1 and 2; however, the subsystems within the
control console are still evolving and will eventually be re-
packaged.
The development of the FPSSS was not considered a high risk
task although it did require the application of diverse and de-
manding technology. The performance requirements are within
the range of standard PMS instruments (with the exception of
the high temperatures involved). Extensive use has been made
of prior success in developing technology to perform measurements
in even more severe environments. For instance, the Cloud Par-
ticle Size Spectrometer developed for NASA and the PIONEER VENUS
payload must withstand 450°C.1'2 This technology base has been
applied to design the FPSSS. An 80 node thermal model for the
probe head was developed by BBRC predicting cooling requirements,
thermal gradients and mechanically induced changes in optical
alignment. The design in Figures 1 and 2 primarily reflects
efforts to minimize alignment error predicted by the model.
The optical system design, scattering response, and thermal
171
-------
design will be detailed here. The actual hardware description
is deferred to the section in which the prototype is discussed,
below.
5mW He-Ne LASER
ADJUSTABLE BEAM
STEERING MIRRORS
DETECTOR
MODULE
BEAM
SPLITTER
SECONDARY
20- OBJECTIVE
7cm RADIUS
CONDENSING BEAM
FOLDING MIRROR
OBJECT PLANE
AR COATED
WINDOW
LASER BEAM DUMP
SPOT
Figure 1. Fine particle stack spectrometer probe optical system.
LASER MODEL 80-5T
STAINLESS STEEL BOOM (COHERENT RADIATION)
HIGH VOLTAGE REFERENCE DETECTOR \
LASER LEAD -^
OUTPUT ^
WATER LINE
SIGNAL _. 't_-
CABLES ^
INPUT
WATER
LINE
STAINLESS
STEEL INSULATION
RETAINER AND COVER
FIBERGLASS
INSULATION
HIGH REFLECTIVITY FLAT
MIRROR <2>
HEATER
PURGE AIR
PHOTO DETECTOR
ASSEMBLY
\
BEAM
SPLITTER
AIRFLOW
PURGE AIR
20x NIKON
MICROSCOPE
OBJECTIVE
/
HIGH REFLECTIVITY
MIRROR 7.5mm
RADIUS
LOW LOSS AR COATED
WINDOW
Figure 2. Fine particle stack spectrometer probe head.
FPSSS Optical System Design
The optical system became the primary design driver in the
FPSSS. Its alignment is the single most important optical param-
eter requiring control. The optical system shown in Figure 1
is essentially identical to that originally proposed to the EPA.
The output of a 5 mW high order multimode laser is directed to-*
wards a 7 cm radius condensing mirror by a pair of plane mirrors,
The condensing mirror focuses the laser beam down to approxi-
mately 125 ym diameter at the object plane of the light collect-
ing/ imaging system. The laser beam then expands to approxi-
mately 3 mm diameter before being absorbed by a dump spot on
172
-------
PROBE
ELECTRONICS
I SIGNAL _,
CABLE
TO FPSSS;
PROBE 'WATER'
LINES -~
MASTER
CONTROL
PANEL
THERMAL-CONTROLLER
RESERVOIR WATER PUMP
HEAT EXCHANGER
Figure 3. FPSSS data/control console.
the FPSSS window. Particles in the vicinity of the object plane
scatter energy through the window which is collected by the prime
objective (F/1.8, 50 mm f.l.) and relayed at 2:1 magnification
to the secondary objective which is a 20X Nikon microscope ob-
jective operated at 10X. The total magnification is thus 20X.
The final image is produced on a pair of exit faces of a beam
splitter. The reflected image face is masked with a 0.78 mm
wide vertical mask aligned in the direction of particle flow.
The second transmission prism face is unmasked. A pair of photo-
diodes and their associated preamps view the particle images
through each prism face.
The capabilities of this optical system are of a fundamental
nature in that they provide a means of defining a desired sample
volume effecting the in situ measurement capability.3'4'5 The
masked beam splitter derives two signals, which in conjunction_
with double pulse height analysis, provide a means of determining
if a particle's position is in the desired sample volume. The
relative size of the sample volume cross section with respect
173
-------
to the laser beam is depicted in Figure 4. The sample volume
includes only the region near the center of the laser beam.
The sample volume cross section is noticeably diamond shaped-
The center of this diamond cross section coincides with the ob-
ject plane of the collecting optics. The points of this diamond
cross section define the limiting depth-of-field of the FPSSS.
Both the width of the cross section and the depth-of-field vary
inversely with the magnification used in the collecting optics.
The diamond shaped sample cross section results from the
accept/reject criteria used in comparing signals at the masked
and unmasked detectors. The light transmitted on axis through
the beam splitter is the signal used to size the particles.
The light reflected at 90° forms its image on a circular aper-
ture with the central opaque vertical mask. Shown in Figure
5 are the size and positions of images formed by particles at
various positions in the illuminated volume. It is important
to understand that for small particles (essentially point ob-
jects) the image size is only a function of its axial displace-
ment from the object plane. The image size is linearly related
to the numerical aperture of the collecting optics and is given
approximately by:
Image size = N.A. x displacement from object plane-
It is apparent that only images that are near the object plane
and the center of the sample volume form images with light con-
centrated on the 'opaque mask as illustrated in Figure 5. Thus,
such images transmit little signal through the masked aperture.
Particles whose pulse amplitudes, as seen by the masked aperture
detector, are greater than those seen by the signal aperture
detector are rejected. This defines the diamond shaped sample
cross section and image relationships shown in Figure 6. We
use a gain ratio of masked aperture detector to signal aperture
detector of 4X for best noise immunity. The sample cross section
area is the product of the sample volume width and one-half the
depth-of-field.
One final aspect of the sample volume needs explanation.
As we have defined it the sample volume is only the region of
accepted measurements. The total view volume includes a much
larger volume where both accepted and rejected particles are
viewed. This view volume is as much as ten times larger than
the sample volume, and must be used for coincidence error esti-
mates. The ratio of the sample volume to view volume is a func-
tion of the inner numerical aperture of the collecting solid
angle (2°, N.A. = 0.03). This presents an obvious trade-off
between a design to collect very strong forward scattering and
one which maximizes measurements at the highest concentrations
174
-------
OBJECT PLANE
SAMPLE VOLUME
CROSS SECTION
1/E2 INTENSITY
(BEAM EDGE)
HIGH ORDER
MULTIMODE
INTENSITY
DISTRIBUTION
MAXIMUM VARIATION
IN INTENSITY OVER
SAMPLE VOLUME
Figure 4. Relative size and position of sample volume cross section.
Theoretical Optical Performance
The optical system discussed in the previous section was
obviously constrained by several factors. The FPSSS collecting
solid angle of 2 to 11° is smaller than that employed in standard
aerosol spectrometers offered by PMS (4 to 22°). This collecting
solid angle has primarily evolved from the alignment analysis
incorporated in the thermal model. The 2° inner angle can be
increased but the 11° outer angle cannot be increased without
moving the object plane closer to the probe window or increasing
the lens diameter. The latter choice may induce some flow errors,
Wind tunnel results of flow through the present sample volume
location insure sample flow velocities within 5% of free stream.
The option of increasing the inner angle remains and more will
be said in this regard in the section on future work; however,
all theoretical computational work thus far has used the 2 to
11° collecting solid angle.
The computed optical response curves along with some ex-
perimental data points are presented in Figure 7. The oscillat-
ing curves presenting the results of Mie scattering computations
for real indices should be viewed as worst case in that the ef-
fects of laser beam convergence and multimode characteristics
have not been included and all particles are nonabsorbing (any
175
-------
MASKED APERTURE
APERTURE AT
PRISM FACE
0.78 mm
- 2.50 mm
J APERTURE AT
PRISM FACE
IMAGE
PLANE
SIGNAL APERTURE
TO OBJECT PLANE
IMAGE PLANE
Particle Image at end of
Depth-Of-Field at center
of Sample Volume Width
Particle Image within
Depth-Of-Field and within
Sample Volume Width
Particle Image near
Object Plane and center
of Sample Volume Width
BEAM SPLITTER
Particle Image at end of
Depth-Of-Field but
outside Sample Volume
Width
Particle Image within
Depth-Of-Field but
outside Sample Volume
Width
Particle Image near
Object Plane but
outside Sample Volume
Width
Figure 5. Image sizes and positions on masked aperture detector.
OBJECT PLANE
MASKED APERTURE
SLIT WIDTH
T
LIMIT OF DEPTH-OF-FIELD
Figure 6. Image size as a function of sample volume position for the
case of 4X gain ratio of masked-to-signal aperture.
176
-------
BOUNDARY OF UNCERTAINTY
BEST FIT CALIBRATION CURVE
BOUNDARY OF UNCERTAINTY
0.5 ±0.12 urn
O LATEX PARTICLES
• GLASS BEADS
3.0
4.0
5.0
RADIUS,
Figure 7. Theoretical Mie scattering for 2-11 o real indices 1.4, 1.5,
1.6, and 1.7.
of these will reduce the amplitude of the oscillations). Thus,
the error bounds apply to all indices. Small amounts of absorp-
tion simply reduce the amplitude of the oscillations, as well
as the probable error, and few particles would be expected to
be completely nonabsorbing. The effect of absorption is illus-
trated in Figure 8 for a single real index of 1.4 and complex
index values of O.OOli, O.OOSi, and O.Oli.
The bounded results presented in Figure 7 represent sizing
tolerances for all particles, i.e., particles with higher and
lower indices or absorbing particles will still fit within the
envelope given. However, what we really desire is the average
error expected. Using the best fit calibration curve presented
this would be within 10% of actual size. The latex and glass
177
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particles, in fact, show errors to be considerably less than
that theoretically predicted. This is partially due to beam
convergence and laser multimode properties as well as the poly-
dispersion in the case of the glass beads. In general, the
results are better than expected considering the many compromises
involved in configuring such an instrument.
The choice of the collecting solid angle has been largely
dictated by other design constraints; however, it is nearly as
good a choice as any other set of angles. Strong forward scat-
tering is quite insensitive to particle shape because the
diffracted light contribution is itself a function only of cross
sectional area. It also provided the greatest .working distance
for viewing particles and thus enabled more nearly in situ mea-
surements.
Thermal Analysis
Single particle size spectrometers designed to operate at
high particle concentrations and at small size require an ex-
tremely small view volume. Ordinarily, the beam width must be
small if the view volume is to be small. A laser is required
to produce beam widths of 100 ym dimension if any useful energy
density is to be achieved. In the FPSSS this beam must stay
essentially fixed with respect to the imaging system if the beam-
splitter detector set is to view particles in the same region
of the laser beam cross section. Because of the high order multi-
mode beam intensity profile a movement of ±25% of the beam width
(25 )jm or 0.001 in.) is tolerable. The essential thermal problem
is thus to prevent a transverse beam movement of one mil over
a 250°C temperature range. The general problem of cooling the
laser electronics and optics to a tolerable temperature is trivial
by comparison. Looking at the optical system in Figure 1 and
the probe head in Figure 2 it can be seen that only two optical
components see the high temperature environment the 7.5 cm
radius mirror and the window. All other elements are within
the insulated water-cooled regions. The window has zero power
and thus cannot introduce alignment errors except by tilting,
which produces a negligible beam offset as long as it is per-
pendicular to the optical axis to within a couple of degrees.
The 7.5 cm mirror is thus the single element of concern.
The 80 node thermal model was used to predict the thermal
effects on alignment as well as cooling requirements. First
the optical effects of thermal-mechanical movement of the laser,
all mirrors and the windows were measured using an optical brass-
board with x, y, z translation stages. The thermal model then
provided the positional data for optical elements from which
changes in alignment were calculated.
178
-------
The most important results of the thermal analysis can be
summarized as follows:
a) Maximum cooling load of 765 watts is developed at 250°C
and 30 m/sec velocity with 20 feet of support boom (see
below).
b) The internal head can be maintained to within 3°C of
ambient outside air temperature (as seen by heat ex-
changer) at a 1 gpm coolant flow rate.
c) Internal head temperature gradients are less than 1°C.
d) The alignment is only sensitive to cross gradients be-
tween the two support arms holding the external 7.5
cm mirror mount.
The resulting FPSSS head design reflects these results in
subtle ways. The internal head body must be fabricated from
like material with high thermal conductivity and high thermal
mass aluminum was selected. The external mirror support
arms should have low thermal conductivity and coefficient of
expansion stainless steel was found adequate. Since these
support arms attach directly to the cooled aluminum inner head
body they must withstand the nearly full temperature gradient
from outside ambient to that inside the stack. It is always
desirable to maintain maximum gradients in the most stable ma-
terials, thermally.
DESCRIPTION OF THE PROTOTYPE FPSSS
At the time of this writing the FPSSS design is nearly com-
plete but only certain subsystems have been fabricated and tested,
The final FPSSS will contain a dedicated intelligent DAS instead
of the standard DAS shown in Figure 3, which will be capable
of real time computations of size and mass distributions, mass
loading, and extinction coefficient (or opacity). However, it
was found highly desirable to make preliminary in-stack measure-
ments to determine the scope of the DAS requirements, as well
as uncovering any probe problems early. For the preliminary
test work an existing Model CSAS-100 Classical Scattering Aerosol
Spectrometer electronics console satisfied the DAS requirements.
Furthermore, because of the great similarity in the FPSSS elec-
tronics to standard PMS instruments preliminary measurements
could be made with existing probe electronic subsystems as well.
Pulse processing electronics and pulse height analyzers were
fabricated and installed in the CSAS-100 electronics console.
Other fabricated subsystems include the FPSSS head, support boom,
thermal control system and port bearing assembly. These sub-
systems are shown in Figures 9 and 10.
179
-------
z
(3
CO
O
E
U i02
o) IU
co
LLJ
CC
101
10°
m = 1.4-0.0101-
m = 1.4-0.005i-
m = 1.4-0.001 r
f
f
1.0
2.0 3.0
RADIUS, jum
4.0
5.0
8. Theoretical Mie scattering for 2-1Jo absorbing particles with
1.4 real index.
FPSSS Head
The head of the prototype FPSSS illustrated in Figures 2
and 3 is shown in Figure 11 with the insulation retainer split.
The head is approximately 20 in. long, has a 2.25 in. x 3.5 in"
elliptical cross section and weighs seven pounds. The head body
is fed by two water lines which have circuitous paths runninq
the full length of the head body making several internal bends
180
-------
before running back to the rear and exiting. Thus, four water
lines are fed through the boom. Two high voltage laser leads,
a multiconductor signal cable, and an air line also feed through
the boom to the FPSSS head.
The optical system is identical to that described above.
The laser is mounted on "0" rings without adjustments. The only
adjustments are x-y adjustments on the first beam-folding mirror
and the 7.5 cm mirror. The adjustment on the 7.5 cm mirror was
not initially planned; however, it was found necessary when we
discovered that adjustment using the beam folding mirror had
near zero effect at the object plane by virtue of the object
plane being centered at the focus of the 7.5 cm mirror.
The head body was constructed in three sections to gain
access to the various optical element cells and for machineability,
"0" ring seals are used for the water paths and sealing is posi-
tive up through 100 psi. The photodetector assembly loads from
the rear end can be removed without splitting the head body.
Purge air was provided only for the 7.5 cm mirror initially.
This element has a small aperture and is easily purged. Con-
tamination on its surface is, however, more critical than on
the window. We eventually plan to purge the window, as well,
but did not initially in order to determine its potential ex-
posure and rate of contamination.
FPSSS Support Beam and Port Bearing
The FPSSS is designed to permit installation on a stack
port flange and penetrate into the stack to any desired length
in order to sample continuously across the stack. This has been
accomplished using a sectioned tubular support boom which can
be fed through a lateral bearing mounted to a stack port flange.
All water lines and electrical cables are continuous through
all boom sections. To add a section it is merely necessary to
screw it into the preceding one. The first boom section is nor-
mally left mounted to the probe head and all boom sections are
of 5 feet length. The boom sections are constructed of thin-
walled stainless steel tubing. Approximately 0.5 in. of fiber
glass insulation and an inner 1.125 in. I.D. PVC sleeve allow
sufficient room to slide the lines and cables when joining and
separating sections. Each boom section adds about 6 pounds of
suspended weight although the final section is slightly heavier
due to a larger wall thickness required for the increasing loads.
The port bearing design is of a clamshell nature and is
split to allow passage of the larger probe head. Once the head
is by the bearing it is closed forming a snug fit to the 2.5
in. O.D. stainless steel walls of the boom sections. The lateral
bearing has four sets of four concave stainless steel rollers.
181
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Thermal Control System
The thermal control system was rather simply constructed
using a small centrifugal pump, an automotive type heater radia-
tor, instrument blower and storage reservoir all packaged in
a single housing. The purge air pump is also currently housed
in the thermal control system. The water pump has a throughput
of 1 gpm with 30 feet of line and a corresponding pressure drop
of 7 psi. Quick disconnects are used on all lines.
FPSSS Probe Electronics
A block diagram of the FPSSS Probe electronics is shown
in Figure 12. The section enclosed in the box containing the
preamps and photodiodes is a separate module housed inside the
FPSSS Probe head. The amplifiers have a minimum bandwidth of
300 KHz. The second stage is a programmable amplifier used to
gain switch between size ranges. The two diodes with their
preamps provide the signals which make it capable of distinguish-
ing whether a particle is within the proper sample volume and
rejecting particles which are not. A reference detector con-
structed using a photodiode and an appropriate amplifier is also
inside the FPSSS Probe head. This reference detector provides
a reference signal proportional to the power output of the laser.
Changes in laser output are thus cancelled when the signal is
applied as the reference input of the pulse height analyzer.
Figure 9. Photograph showing the FPSSS head mounted to the section*!
boom and CSAS- WO electronics console.
182
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Figure 10. Photograph of FPSSS port bearing assembly and heat exchanger.
The remainder of the FPSSS electronics are contained in
the CSAS-100 Electronics Console. The two signal pulse streams
are first properly leveled with baseline restoration and DC off-
set circuitry. Dual peak readers then momentarily store the
pulse heights from both the signal and masked aperture detectors.
The peak readers are used to provide inputs to a comparator to
determine which of the two pulse heights from the preamps is
the greatest. This measurement determines when particles are
in the accepted sample volume.
A pulse height analyzer sizes the signal pulses of all par-
ticles passing through the beam. The pulse height analyzer has
a set of sixteen voltage comparators and latches. The reference
voltage to the comparators provided by the reference detector
is divided by resistive dividers. Because its reference voltage
is derived from the source of illumination, the entire system
has an effective automatic gain control (AGC).
The transmit time analyzer measures the half-width of delayed
pulses that are found to be in the sample volume. The transit
times are averaged over a great number of particles to provide
a true statistical velocity average (note that velocity is actu-
ally the reciprocal of transit time).
183
-------
Figure 11. Photograph of FPSSS partially disassembled revealing
construction insulation details.
between
The control
(accept/reject) logic provides the proper delay
an event and the strobe and reset pulses. This logic
also provides gating for valid and invalid strobes.
The pulse height analyzer is currently configured to provide
four size ranges of 15 channels each as follows:
Range 0 2.5
Range 1 1
Range 2 0.5
Range 3 0.4
10 Mm
5.5 pm
2.0 urn
0.85 ym
All size intervals are linear with maximum resolution in Range
3 being 0.03 ym. The ability to switch ranges has been found
to be particularly important in field measurements. Calibra-
tions with particles of known size have been performed previously
using latex spheres and glass beads with results shown in Fiqure
7.
RESULTS OF INITIAL IN-STACK TESTS
The first tests of the FPSSS were conducted at the Valmont
Power Plant owned and operated by the Public Service Company
of Colorado on April 24, 1978. This is a coal-fired power plant
184
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with general operation and control devices similar to that de-
picted in Figure 13. Measurements were performed at the 80 foot
level on the stack which afforded good access and at which a
Lear Siegler Model RM41 Opacity Monitor was also operating.
These initial tests were limited to an hour of measurement time
with the entire set-up, measurement, and pack-up completed in
less than two hours.
Measurements were taken at two foot intervals from 3 to
17 feet in-stack penetration. The stack had an I.D. of 21 feet
and we did not have sufficient boom to reach completely across.
However, the measurements revealed a symmetric profile of par-
ticle flux and the lack of measurement over the final four feet
was not regarded as significant.
The in-stack velocity profile revealed a greater amount
of turbulence than anticipated with gusts of 5 m/sec common near
the stack center. The mean flow varied from 5 to 12 m/sec with
the highest values at the center. No regions of reverse flow
could be identified.
The particle size distributions measured at 2 foot intervals
were surprisingly uniform as viewed on the CRT spectral display
of the CSAS-100 electronics console. In fact, once corrections
for variations in flow rate were made the distributions revealed
only minor spatial spectral differences which were of the same
order as temporal changes at a fixed location. For this reason
all measurements were integrated together to generate a single
average particle size distribution representative of the entire
stack (see Figure 14). The particle size distribution is seen
to be highest in number at the smallest size (0.4 urn) although
the slope is decreasing. The overall slope is quite steep.
In fact, it is steeper than observed ambient background in many
cases. A size distribution with such a steep slope is charac-
teristic of air passed through coarse filters.
The computed average distribution of extinction cross sec-
tion (2nd moment) as a function of particle size is given in
Figure 15. Thg extinction cross section was computed for a wave-
length of 5500A in an effort to make comparisons with the opacity
monitor operating with a white light source with its spectral
maximum at this wavelength. The extinction cross section is
seen to peak at sizes of 0.5 to 0.6 ym and has an integral value
of 3.0 x 10l*ym2/cm3. The integrated cross section over the 6
meter stack diameter provides a computed transmission of 0.83
or an opacity of 17%. The Lear Siegler RM41 had comparable opa-
city values of 14 to 16% during the period of measurement.
The close agreement between the Lear Siegler RM41 and the
FPSSS opacities must be regarded as fortuitous at the present
time. A certain amount of particle cross section also exists
below the 0.4 ym lower limit of sensitivity although probably
185
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MASKED
APERTURE
DETECTOR
SIGNAL
APERTURE
DETECTOR H>
BIAS
SAMPLE
VOLUME
VALIDATION
COMPARATOR
TRANSIT
TIME
ANALYZER
MEAN
VELOCITY
TO DAS
PULSE
HEIGHT
ANALYZER
ACCEPT
REJECT
LOGIC
Figure 12. FPSSS probe electronics block diagram.
less than 10% of the total. The exact spectral characteristics
of the Lear Siegler RM41 were not available and in this size
range extinction is a strong function of wavelength. Further-
more, an ideal transmissometer still gains contributions from
multiple scattering which were neglected in our treatment.
Finally, the soiling of the window resulted in a small but ob-
vious underestimate of particle size and opacity.
The average particle volume (mass) distribution (3rd moment)
is given in Figure 16. This is probably the most interesting
of all the spectral plots. It reveals several dominant mass
modes each being statistically valid. Perhaps most surprising
is the dominant mass mode at 0.65 ym. It is more striking when
presented with the full resolution of Range 3 as shown in Fig-
ure 17. Here there is little doubt that the FPSSS has captured
nearly all of the mass. The integrated mass loading is surprisingly
low being on the order of only 0.01 to 0.02 g/m3, depending on
the assumed density. The known undersizing due to the soiled
window should have only resulted in underestimates of mass less
than 30%.
We expected mass loadings nearer a tenth of a gram; however
the only way in which higher mass loading values would be com- '
patible with opacity values of less than 20% (regulatory com-
pliance value) would be with an increase in size and decrease
in concentration over that observed. A simple increase in size
of a factor of two provides an eight-fold mass increase but also
a four-fold increase in cross section and opacity Since the
186
-------
00
-O
MAIN STREAM
TURBINE
GENERATOR SWITCHYARD
COAL SUPPLY CRUSHER^
HOUSE
COLD REHEAT i
HOT REHEAT)Ife-r=i
FEED WATER PUMP
HEATERS
COOLING TOWER
PULVERIZERS
FORCED DRAFT
FAN
ASH DISPOSAL
REHEATER
Figure 13. Schematic drawing of a steam electric generating station.
-------
mass-to-cross section ratio is proportional to radius, or size,
it is necessary to increase size by a factor of ten and decrease
concentration by a factor of 100 to maintain the same opacity
but a factor of ten higher mass loading. Either of the above
is totally unreasonable from an instrumental standpoint. We
are thus left to conclude that this particular stack does indeed
have a low mass loading characterized by particles of about 1.3
ym diameter.
FUTURE WORK AND CONCLUSIONS
We are encouraged to believe that the FPSSS will be a satis-
factory instrument for making in situ size distribution measure-
ments in a hot stack environment. However, field testing has
only begun and final assessment must await much more exhaustive
work on other stacks and stationary sources where particulate
characteristics may be quite different. Results of measurements
ahead of the control devices in the Valmont stack are currently
being analyzed. Evaluation of control devices by performing
measurements up and down stream is a goal of the FPSSS develop-
ment. Suffice to say a great deal of work remains.
It is helpful when analyzing the performance of a new in-
strument to bear in mind the trade-offs made in its design.
With regard to the FPSSS two come to mind immediately. One has
resulted in the current size limit of 0.4 ym and the other may
prove to limit the maximum concentration to a lower level than
desirable. These two factors are strongly coupled.
The EPA initially asked for a size range of 0.5 to 5.0 ym.
We felt that a 0.3 ym sensitivity could be obtained. Our current
instrument has a 0.4 to 10 ym range. The difference between
0.3 and 0.4 ym is about a factor of five in signal. An expansion
of the collecting angles from 11 to 20° would pick up the dif-
ference. However, as mentioned in the previous section, this
would require moving the sample region closer to the prime objec-
tive since this objective's size cannot be increased by the rough
factor of two required and still allow the FPSSS head to fit
in standard 4 in. stack ports. Our wind tunnel results do not
preclude moving the sample region closer but it obviously will
not be as aerodynamically clean as it currently is. in view
of the highly turbulent conditions observed in the Valmont stack
clean aerodynamics may not be a profitable design feature and '
movement of the sample volume closer could indeed be a better
choice. A decision to move the sample region closer must also
await tests of a purge air fitting for the window which will
cause some reduction in available room.
The current instrument design can handle concentrations
up to 50,000/cm3 although coincidence losses must be corrected
for. An increase in magnification from 20 to 30X will nrovidA
measurements up to a maximum concentration of 105/cm3 ---
188
-------
105n
10*-
103-
n _.
I io2
10°
10'1
DIAMETER,
Figure 14. Average in-stack size distribution. The higher resolution data
for size range no. 3 has been summed into 0.1 IJJTJ size classes.
189
-------
105-
h
io3-
o
>
1Q2J
101
M ! n-h
3 4
DIAMETER, u,
Figure 15. Average in-stack extinction cross section distribution. The
higher resolution data for size range no. 3 has been summed
into 0.1 \im size classes. The mean cross section size is 0.79
190
-------
design goal. An increase in the inner collecting angle from
2 to 4° also doubles the maximum concentration where measure-
ments can be made. Halving the laser beam diameter has a four-
fold effect.
We previously mentioned that achieving a higher maximum
concentration limit and greater sensitivity were strongly coupled.
First, from glancing at Figure 14 it is obvious that increasing
the sensitivity automatically increases the number of particles
viewed and coincidence losses. Conversely, doubling the inner
collecting angle reduces the light collected. The overall sensi-
tivity is obviously reduced as well but there is a more subtle
effect. The 2° inner angle allows for very accurate measurements
out to 10 ym by collecting the diffracted light at small forward
scattering angles. A deterioration in sizing performance would
result if the inner angle were increased above 6° for 10 ym par-
ticles or above 10° for 5 ym particles. Of course, the latter
would require enlarging the outer collecting angle to achieve
adequate sensitivity. Halving the laser beam diameter aids sensi-
tivity as well as maximum concentration limits. However, here
one must be absolutely sure that alignment will be extremely
stable. To decrease the beam diameter a factor of two requires
either a condensing mirror of half the radius of the current
one or doubling the laser beam diameter input.
Probably the easiest and least controversial change would
be to increase the magnification to 30X. The only penalty paid
is in increasing the detector bandwidth (admitting more noise)
to see the relatively shorter pulses which would result. An
increase in bandwidth is required because the photodiode is sized
to view the full transit of a particle and any increase in magni-
fication will truncate the time a particle is viewed.
Work obviously needs to be done in all of the above areas
to determine optimum design; however, progress is complicated
by the impact of changes in one area of design or another. Con-
versely, other areas of continuing work should experience rapid
progress. The DAS design should be ready for fabrication in
July, 1978. From our field experience with actual stack measure-
ments valuable insight into desired packaging improvements af-
fording better portability and human engineering have been found.
For instance, we have found that five foot boom sections could
pose a problem when catwalks are narrow and guard rails inter-
fere. Four or even three foot boom sections may be preferable.
Anyone who has tried to make measurements around stacks knows
of the difficulties that can occur. In designing the FPSSS we
have tried to remain fully cognizant of the user's environment
and attempted to maximize .the usefulness of such an instrument.
The real utility of a device such as the FPSSS is not in extend-
ing its sensitivity to the smallest particles or its ability
to handle the highest concentrations, rather it is in providing
191
-------
105'
TOTAL PARTICLE VOLUME: 7.3 mm3/m3
10*-
fl
E
M
O
CO"
Il03-
LU
10H
10'
3 4
DIAMETER, |iim
Figure 16. Average in-stack volume (mass) distribution. The higher
resolution data for size range no. 3 has been summed into
0.1 pm size classes. The mean volume (mass) size is
1.31
192
-------
14,000^
12,000-J
10,000-j
SIZE RANGE NO. 3
0.03/Jm PER SIZE CLASS
8,000-
O
«r
O
6,000-1
4,000-1
2,000]
0.4 0.5
—r
0.6
0.7
DIAMETER,
0.8
0.9 1-0
Figure 17. Average in-stack volume (mass) distribution from 0.4
to 0.85 nm.
193
-------
measurements of sufficient spectral quality to gain insight into
processes which produce changes in particle size and number den-
sity while retaining a reasonable confidence in integrated proper-
ties such as mass loading and opacity.
One final remark is directed at the measurements reported
in the section on results, above. We reported that the velocity
variations were quite large. For the FPSSS this poses little
problem; however, it clearly must be problematic for direct sam-
plers using so-called isokinetic intakes. In our view it would
be impossible to follow the fluctuations observed with a pumping
system and achieve isokinetic sampling conditions. What effect
the stack gustiness has on such techniques is not currently known
but could possibly be explored by intercomparisons with the FPSSS.
ACKNOWLEDGEMENT
This work was funded by the Environmental Protection Agency
(EPA), IERL—Process Measurement Branch.
REFERENCES
1. Knollenberg, R.G., J. Hansen, B. Ragent, J. Martonchik,
and M. Tomasko. The Clouds of Venus. Space Sci. Rev.
20:329-354, 1977.
2. Colin, L. The Exploration of Venus. Space Sci. Rev.
20:249-258, 1977.
3. Knollenberg, R.G-, and R.E. Luehr. Open Cavity Laser "Active"
Scattering Particle Spectrometry from 0.05 to 5 Microns.
In: Fine Particles, B.Y.H. Liu, ed. Academic Press, New
York, 1976. pp. 669-696.
4. Knollenberg, R.G. Three New Instruments for Cloud Physics
Measurements: The 2D Spectrometer, The Forward Scattering
Spectrometer Probe, and The Active Scattering Aerosol Spec-
trometer. Presented at The Cloud Physics Conference July
26-August 6, 1976. Preprint available from the American
Meteorological Society.
5. Knollenberg, R.G. The Use of Low Power Lasers in Particle
Size Spectrometry. SPIE, Practical Applications of Low
Power Lasers, Vol. 92, 1976. pp 137-152.
194
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PAPER 9
OPTICAL MEASUREMENTS OF PARTICULATE SIZE
IN STATIONARY SOURCE EMISSIONS
A.L. WERTHEIMER
M.N. TRAINER
W.H. HART
LEEDS & NORTHRUP COMPANY
ABSTRACT
A new light scattering instrument is described for in-situ
measurements of particulates in the 0.1 to 10.0 ym diameter size
range. Two modes of scattering are used, each with two wave-
lengths of light, to generate five size fractions by volume from
a distribution of particulates. One mode measures polarized
light scattered in two orthogonal orientations at an angle of
90° to the optical probe beam. The second mode measures light
scattered in near forward angles (4 to 11°). Both modes allow
the extraction of size data when particles of different sizes
are present simultaneously in the sensing region. These prin-
ciples have been incorporated into a prototype portable stack
monitor, consisting of a 1.5 meter long, 9 cm diameter insertable
probe capable of withstanding temperatures up to 260°C. The
optical signals are carried through fiber optic cables contained
in the probe. An arc source and silicon photodetectors are
outside the stack at the end of the probe, while a digital micro-
processor analyzes the set of measurements and calculates the
size fractions. The microprocessor, an air purge system, the
lamp power supply, and a digital printer are housed separately
from the probe for ease of installation and service.
INTRODUCTION
Light scattering is ideally suited for measurements of par-
ticulates in real time situations. The characteristics of the
materials are not disturbed during the course of the measure-
ments, measured sizes can be correlated well with standard opti-
cal microscopy, and the technique is rapid and, in general, real
time. Frequently it is of interest to measure the mass or volume
195
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of particulates within a given size interval, and recent develop-
ments have produced low angle forward light scattering industrial
process instruments capable of real time measurements of particle
size and volume in either liquid or gases in the 2 to 200 ym
diameter range.1'2 These instruments are also unique for their
ability to simultaneously accommodate multiple sizes of particles
in the sensing region, so the loading level is considerably
higher than is normally possible with single particle counters.
One of the limitations in low angle forward scattering is
the difficulty in discriminating sizes much below the 1 ym level
with visible light. Prior to this work no comparable instrumenta-
tion was available to discriminate mass fractions in the 0.1
to 1 ym size range, the lower decade of the respirable region.
This paper discusses a new approach, based on scattering of light
at 90° to the direction of an optical probe beam for the genera-
tion of volumetric size fractions below the range normally covered
by forward light scattering. A combination of 90° and forward
light scattering has been incorporated to construct a prototype
stack monitor capable of measuring volume in five logarithmically
spaced size intervals in the range of 0.1 to 10 ym.
PRINCIPLES OF OPERATION
Light scattering is classically divided into regions of
analysis according to the relationship between size of the par-
ticle and the wavelength of light. For particles much larger
than the wavelength, scalar diffraction theory is adequate to
describe the interaction and define the angular distribution
of scattered light.3 Low angle forward light scattering is
commonly discussed using this analysis.
For particles much smaller than the wavelength, the angular
light distribution is independent of particle size. Equal amounts
of light are scattered into forward and backward lobes. This
is referred to as Rayleigh scattering,1* and is typical of scatter-
ing observed in the atmosphere or from suspensions of low and
intermediate molecular weight materials.
The most complicated analysis is required when the particle
is comparable in size to the wavelength. A full treatment for
all size ranges was first performed by Mie,5 and entire texts
have been written using this theoretical approach.6 The 90°
scattering approach described here is based on characteristics
of the transition between simple Rayleigh and rigorous Mie scat-
tering, and the analysis is included in the next section.
Principles of 90° Scattering
Mie theory will be used throughout this paper to describe
the angular distributions of flux for particles of different
sizes. Tables7 and computer programs8 are available to facili
tate rapid analysis using Mie theory, thus eliminating the need
196
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to decide if a simpler theoretical treatment would be sufficiently
accurate for the particular size and refractive index of interest.
Spherical shapes are usually assumed in light scattering theory
and have been used for ease of computation.
To illustrate some specific characteristics of light scat-
tering, Figures 1 and 2 were computed for spherical water drop-
lets of refractive index 1.33. A dimensionless size parameter,
a, is frequently used where
a = TTD/A, (1)
with X the wavelength of light. Figures 1 and 2 cover a range
of a = 0.5 to a = 40. Two orthogonal polarizations of incident
light are shown, referred to as ix and i2. The electric field
of ix is in a plane perpendicular to the plane containing the
probe beam and the observation point. The log of the flux per
particle is graphed, covering a hemisphere from 0° (light re-
turning directly toward the source) through 180° (forward scat-
tering direction).
For a = 0.5 and 1.0, Rayleigh scattering characteristics
dominate. Equal intensities are shown in both forward and re-
verse directions, and no light is scattered at 90° for the inci-
dent light designated as i2. As the parameter, a, increases,
the null at 90° disappears and the two curves tend to be more
alike. For larger values of a, the fine structure of the curves
changes slightly as the refractive index varies. However, one
of the more stable characteristics of the patterns is the null
at 90° for small values of "a" with i2, the light polarized so
that the electric field is in the plane defined by the direction
of propagation of the incident beam and the point of observation.
Finally, it should be mentioned that these curves describe
the light scattered from the individual particles. Suspensions
of N identical particles widely separated from each other produce
an angular pattern N times as intense as that of the single par-
ticle, if no significant shadowing or secondary scattering occurs.
This assumption is referred to as single scattering.
For small "a" values the differences between Figures 1 and
2 are greatest in the region of 90°. The relative differences
become smaller as the size parameter increases, where the dif-
ference in intensity can be calculated by subtracting the two
curves at 90°. At this point, it is appropriate to consider
the scattered flux per unit volume, since typical particle size
distribution curves frequently imply a vastly greater number
of small particles than large ones. When the flux difference
per particle, ii - i2/ is divided by the volume of the particle,
and plotted against the log of "a", a well-defined peak is ob-
served. Figure 3 shows such a curve, computed for spheres of
197
-------
4-p
60 90 120 150
ANGLE OF OBSERVATION, degrees
1.0
0.5
180
Figure 1. Log of scattered intensity per particle as a function of
observation angle, with forward direction at 180°. Two
orthogonal polarizations of incident light are shown, i\
in Figure 1, and /2 in Figure 2.
198
-------
4 -r
3 -•
30 60 90 120 150
ANGLE OF OBSERVATION, degrees
180,
Figure 2. Log of scattered intensity per particle as a function of
observation angle, with forward direction at 180°. Two
orthogonal polarizations of incident light are shown, i\
in Figure 1, and /2 in Figure 2.
199
-------
index 1.33. In order to simulate a realistic observation angle,
the flux difference per unit was computed over an angular collec-
tion range of 90° ± 10°.
The curve in Figure 3 has a peak at about a = 1.5, and the
width at half maximum covers a range from approximately a = 1
to 2. in order to identify a corresponding particle size, a
wavelength must be specified. For a helium-neon laser with
X = 0.6328 ym, the half width covers diameters from 0.2 to 0.4
ym, and the peak is located at 0.3 ym. Other wavelengths can
be used to measure volume components at other locations, as shown
in Table 1.
TABLE 1. VOLUME RESPONSE AS A FUNCTION OF
WAVELENGTH FOR 90° SCATTERING
Wavelength, i_im
0.45
0.6328
0.90
Particle diameter
at peak of response, ym
0.21
0.30
0.43
Diameter
range, ym
0.14 to 0.3
0.2 to 0.4
0.3 to 0.6
These numbers are computed for an index of 1.33. Other
indices produce slightly different curves, although the basic
characteristics are preserved. The curve for glass spheres,
for example (n = 1.55) has its peak at approximately the same
value (a = 1.5) and is quite similar in shape to the curve in
Figure 3 for the region around a = 1.5.
The approach, then, is quite simple. Scattered light in-
tensities are measured for two orthogonal planes of monochromatic
polarized light, at 90° to the probe beam, and the difference
is proportional to the volume of particles in the size range
where a = 1.5. This value of "a" translates to a size diameter
of approximately one half the wavelength of the probe beam, and
other size fractions can be defined by using other wavelengths.
For the probe described here wavelengths of 0.45 and 0.9 ym were
used to generate two submicron channels.
Principles of Forward Scattering
Forward scattering has been discussed in references 1-3,
so only a summary of the techniques and pertinent variations'
is included here. Low angle forward scattering is most readily
treated by using simple diffraction theory. Three discrete scat-
tered flux measurements are made at observation angles of approxi-
mately 4, 8, and 11 degrees in two wavelengths, 0.45 and 0.90
200
-------
ym. The signals are then algebraically combined to produce a
response per unit volume of material maximized for particles
at a particular size or size range and minimized or reduced to
zero for particles far from the size of interest. Since large
particle scattered flux is concentrated more toward the forward
angles, the largest particle response curves are dominated by
flux from the innermost zones. Smaller particles require higher
contributions from the outer zones. The nominal size range peaks
for the three forward scattering channels are listed in Table 2.
TABLE 2. FORWARD SCATTERING CHANNELS
Response peak
ym diameter, nominal
1.0
3.5
7.0
Wavelength,
0.90
0.45
0.90
urn
After the response shape was defined through scalar diffrac-
tion theory, which does not depend on the refractive index of
the material, the design was tested using rigorous (Mie) theory.
As expected, the two upper channels where the respective size
ranges are somewhat larger than the wavelength of the light
showed very little shape variation sensitivity to refractive
index of the material. The most sensitive peak was in the 1
ym range, with the diameter and wavelength nearly equal. Mie
calculations for all five response curves used in the instrument
are shown in Figure 4. These were calculated for an index of
1.5, typical of silica particulates, a common stack material.
The ripples in the forward scattering curves occur due to reso-
nances of light in the non-absorbing (n = 1.5 - O.Oi) material,
but will tend to be smoothed if small amounts of absorption are
present or if the light is not perfectly monochromatic.
EXPERIMENTAL VERIFICATION
The performance of the volumetric response functions in
the forward channels is based on knowledge and experience with
the Leeds & Northrup Microtrac™ Particle Size Analyzer. The
use of forward scattering, here, is merely an engineering exten-
sion of a previously tested system, so no fundamental testing
was required.
Major effort, however, was put into verification of the
90° response curves. Since the response function requires ac-
curate knowledge of signal differences, variations in beam in-
tensity had to be compensated. A ratio of ii/I0 ^nd ia/Io was
measured, where I0 was the strength of the probe beam. Then
the intensity difference was computed.
201
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a a
7Td\
X /
FigureS. Intensity difference per unit volume, (ii-i2)/a3, fora
scattering angle of 90°. The size parameter, a, is plotted
in equal log intervals.
202
-------
UJ
CO
Z
O
a.
CO
UJ
cc
ill
O
>
100% -
80% -
60% -
40% -
20%
0.10
10.0
PARTICLE DIAMETER,
Figure 4. Volumetric response curves, as a function of particle diameter,
for the five channels of the prototype. Curves 1 and 2 are obtained
with 90° scattering, and curves 3, 4, and 5 from forward scattering.
An experimental system was built, incorporating a lock-in
amplifier and an electronic ratio circuit, to accurately measure
90° scattering. This system recorded scattering curves for both
polarizations, ix and i2, concurrently. Hence, the response
curve for the function (i1 - i2) could be generated quite easily
and accurately.
The system, shown in Figure 5, consisted of a xenon arc,
grating monochromator, and two silicon detectors. The source
flux was chopped and a lock-in amplifier used to filter ambient
radiation noise from the relatively small 90° scattering signal.
The flux passing directly through the sample was focused onto
a reference detector. This signal was electronically filtered
to present a dc level to an analog divider, which ratioed the
scattered and reference signals to correct for variations in
source intensity. Therefore, the output of the divider was
directly proportional to ii or i2. While a rotating prism polari-
zer alternately selected i^ or i2, a synchronous motor scanned
the monochromator from 400 nm to 800 nm at a speed chosen so
that a pair of measurements, ^ and i2, were recorded once every
5 mm on a chart recorder. Variations in the size parameter,
a, were thus accomplished by changing wavelength rather than
size. Since the volume of particles remained the same, the flux
per unit volume was readily determined.
203
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OP AMP
ROTATING
G LAN-THOMPSON
POLARIZER
DETECTOR
XENON ARC'
SOURCE
Figure 5. Schematic drawing of laboratory system used to verify the
volumetric response functions for 90° scattering.
204
-------
This system was used to record data for 0.234 ym and 0.801
ym polystyrene spheres, dispersed in deionized water at 20 ppm.
The relative refractive index of polystyrene to water was 1.20.
These two particle sizes generated scattering data for "a" values
from 1.22 to 2.44 and 4.18 to 8.37, respectively. The angular
scattering field was 20° centered at 90° with respect to the
incident beam.
Further theoretical Mie scattering studies indicated that
particle absorption should reduce the secondary volume response
peaks significantly, but cause negligible changes in the primary
response peak. Experiments were then performed to confirm this
selective reduction of secondary response peaks. Data at 90°
scattering was recorded for dyed (absorbing) polystyrene spheres
of 0.234 ym and 0.801 ym diameter. This data is plotted along
with a theoretical curve for non-absorbing particles in Figure
6. The primary peak data (d = 0.234 ym) for absorbing and non-
absorbing particles are almost identical, but the secondary re-
sponse peaks (d = 0.801 ym) are greatly reduced for absorbing
particles. Notice the excellent agreement between experiment
and theory for non-absorbing particles. These experiments con-
firmed the theoretically generated response curves, and gave
us confidence to proceed with the design of the prototype in-
strument.
DESIGN OF THE PROTOTYPE
Real time measurements inside an industrial stack create
rather unique structural and environmental operation conditions,
with typical temperatures as high as 260°C, flow velocities up
to 50 feet per second, and atmospheres containing acid vapors.
Access to the measuring point is often limited to 10 cm diameter
ports, and sampling requirements make penetration well into the
stack a highly desirable goal. Anticipated loading levels are
in the range of 4 to 40 parts per billion by volume, assuming
a specific gravity of 2.5 for stack particulates. For represen-
tative sampling, a sensing zone containing a reasonably large
volume is required. The prototype design is based on these con-
straints, and Figure 7 shows a sketch of the instrument with
the associated electronics.
Optical Design
The interior stack temperatures rule out the use of in-situ
detectors, so a set of high temperature fiber optic cables is
utilized to sense the scattered light at five points (three for-
ward angles and two orthogonal positions at 90° to the probe
beam). These cables are installed inside a 1.5 meter long stain-
less steel tube, whose outside dimension is approximately 9 cm.
The active sensing region is towards the end of the probe and
is a slot 36 cm long by 3 cm wide. A lens located in the tube
tip collects scattered flux for the forward angles, while the
205
-------
O
CO
1.0 -
0.8
0.6
NON-ABSORBING (EXP)
ABSORBING (EXP)
THEORY FOR NON-
ABSORBING SPHERES
co 04
LU "••»
EC
0.2
123456
SIZE PARAMETER, a
Figure 6. Theoretical and experimental curves for 90° scattering showing
the flux difference, i\ - i2, per unit volume of material. Two
sizes of polystyrene spheres were used, 0.234 and 0.801 ju/n
in diameter, and the wavelength was varied to change the size
parameter, a.
10
TRANSCEIVER
PROBE
PRINTER
STACK GAS
CONTROL
CONSOLE
Figure 7. Prototype system in use in stack, showing control console
and printer with transceiver.
206
-------
two 90° fibers look directly through protective windows into
the sample region. Each fiber cable runs to its own detector,
located outside the stack at the end of the probe. A sixth de-
tector is used to monitor beam strength.
The optical probe beam is generated by a xenon arc source,
located in the transceiver end of the probe. Thin film inter-
ference filters with center wavelengths of 0.45 and 0.90 ym are
combined with linear polarizers in a filter wheel. The light
is then projected down the center of the tube in a nearly col-
limated beam of alternating blue or infrared polarized light,
with a beam diameter of approximately 1.25 cm at the end. To
avoid spurious signals the light is not reflected internally,
but exits the probe through a hole at the tip.
Electrical Design
A digital electronic approach has been taken, similar to
the single detector Microtrac system as described in Reference
2. For this system the six detectors generate electrical signals
in proportion to the light flux levels incident on them, and
after a pre-amplification stage, the electrical signals are fed
to an analog-to-digital converter. The microcomputer consists
of programmable read-only memories (PROM), random access memories
(RAM), and a central processing unit (CPU). Program instruc-
tions are inserted in the PROM's, and the CPU carries out the
instructions. Interim calculations and the final results are
stored in the RAM's until the point in the program where a dis-
play command is given to the output.
The cycle time is adjustable, but a full measurement cycle
can be completed in as little as four seconds. The digital
microprocessor handles twelve resultant input signals, five
angular scattering signals and one beam strength signal from
each of the two colors. The signals are first combined to pro-
duce the volumetric response curves shown in Figure 4, and then
a matrix decoupling routine is incorporated to remove most of
the effects of the response curve overlaps. A resulting nor-
malized five channel histogram of the volume distribution is
displayed on a digital printer. A mean size is also computed
from the histogram, as well as a number proportional to the total
volume of particulates.
Mechanical Design
All parts of the insertable section of the probe, except
for glass fibers and lens and window material, are fabricated
from stainless steel. The probe is mounted to the transceiver
with an insulating collar to minimize heat transfer.
207
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To insure cleanliness of the optical surfaces during measure-
ments, and to provide cooling of the enclosed arc lamp, a port-
able skid-mounted air blower is provided. Air is forced into
the probe portion through a set of four tubes running from the
transceiver end. These tubes terminate as specially designed
air curtains to flush across the faces of the two 90° windows
on the sides and the forward scattering lens system at the end
of the probe.
SUMMARY
This paper has described the principles and design of a
new light scattering instrument for real time measurements of
the size of source particulates. A photograph of the recently
completed prototype is included as Figure 8, and the operational
characteristics of the unit are defined in Table 3. The instru-
ment offers a method for in-stack analysis of particulate size
without dilution or extraction of material, and directly produces
a distribution of the particulates by volume fraction. This
device thus provides a unique and valuable measurement capability
for analysis and monitoring of particulates in source emissions.
Figure 8. Photograph of prototype system.
208
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TABLE 3. CHARACTERISTICS OF PROTOTYPE LIGHT
SCATTERING INSTRUMENT
Size range (particle diameter)
Size discrimination
Anticipated loading range
Operational range
Mode of operation
Power requirements
Operational temperature
Speed of measurement sequence
0.1 to 10.0 ym
Five volume fractions with
centers at 0.2, 0.4, 1.0, 3.5,
and 7.0 ym
0.01 to 0.1 grams of material/
meter3 or 4 to 40 parts/billion
by volume (with specific grav-
ity of 2.5)
4 to 400 parts/billion
Low angle forward scattering
and 90° polarization dependent
scattering
115 volts, 60 hertz, 45 amps
Probe portion in stack: up to
260°C (500°F)
Transceiver portion and other
electronics outside stack:
0° to 43°C (32° to 110°F)
Integration time selectable
from 4 to 256 seconds
ACKNOWLEDGEMENT
The development, design, and construction of the prototype
were funded by the U.S. Environmental Protection Agency, under
EPA contract 68-02-2447, "Instrumentation to Measure Particle
Size in Source Emissions."
REFERENCES
1. Wertheimer, A.L., and W.L. Wilcock. Appl. Optics 15:1616,
1976.
2. Wertheimer, A.L., et al. Effective Utilization of Optics
on Quality Assurance. SPIE Conference Proc. 129:49, 1977.
3. Born, M., and E. Wolf. Principles of Optics, Pergamon,
New York, 1959. Chapter 8.
209
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4. Kerker, M. The Scattering of Light and Other Electromag-
netic Radiation, Academic, New York, 1969. Chapter 1.
5. Mie, G. Ann. Phys. 25:377, 1908.
6. Van de Hulst, H.C. Light Scattering by Small Particles,
Wiley, New York, 1957.
7. National Bureau of Standards. Tables of Scattering Functions
for Spherical Particles, Applied Mathematics Series - 4,
U.S. Govt. Printing Office, Washington, 1948.
8. Dave, J.V. Subroutines for Computing the Parameters of
the Electromagnetic Radiation Scattered by a Sphere. Re-
port No. 320-3237, IBM Scientific Center, Palo Alto, CA,
1968.
210
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PAPER 10
STUDIES ON RELATING PLUME APPEARANCE TO EMISSION RATE
AND CONTINUOUS PARTICULATE MASS EMISSION MONITORING
K. T. HOOD
H. S. OGLESBY
NATIONAL COUNCIL FOR AIR AND STREAM IMPROVEMENT
INTRODUCTION
The National Council for Air and Stream Improvement has
participated in several studies incorporating extensive field
work and data analysis in investigations relating plume appear-
ance to continuous mass monitoring and emission rate. Emission
sources that have been studied with plume observing panels to
date include kraft recovery furnaces, power boilers, and a lime
kiln. The experience and results gained through evaluations
performed at these sources will be presented in the following
discussion.
Continuous particulate monitors have proven to be a valuable
tool in the development of relationships between plume appearance
and particulate concentration. The utility of these devices
has been greatly enhanced by the established linearity of their
response to particulate concentration determined through stack
sampling. A synopsis of the evaluation of a mass monitor relying
on the principle of laser light backscattering is offered as
a means of obtaining continuous particulate concentration data
in terms of grams per actual cubic meter.
THE RELATIONSHIP OF PARTICULATE CONCENTRATION AND OBSERVED PLUME
CHARACTERISTICS AT KRAFT RECOVERY FURNACES AND LIME KILNS
A study was organized by the National Council in the summer
and fall of 1974 to determine the relationship among continuously
measured light transmission properties of selected pulp and paper
process flue gases, the gravimetrically determined filterable
particulate concentration and the appearance of the plumes as
judged by certified observers. The entire results of this study
211
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can be found as NCASI Atmospheric Quality Technical Bulletin
No. 82. This relationship was derived to establish a scientific
basis for adoption of plume opacity regulations.
Previous studies reported on in NCASI Atmospheric Quality
Technical Bulletin No. 79 indicated the validity of using a
transmissometer as a continuous monitor of flue gas particulate
concentration at temperatures above the moisture dew point.
This conclusion was based on the instrument's linear optical
density response to particulate concentration. The transmissom-
eter enabled the subsequent association of particulate concentra-
tion to observer measured opacity determined through field test-
ing at two kraft recovery furnace sites in addition to the data
developed on a lime kiln source. The lime kiln and one of the
kraft recovery furnaces were sites located in the Northwest
United States. The recovery furnace at this site was a non-
direct contact evaporator system. The other kraft recovery
furnace was located at a Southern site incorporating a direct
contact evaporator.
The majority of the observing panel members taking part
in the study were technically oriented students complemented
by regulatory agency personnel with extensive plume reading ex-
perience. A transmissometer was used as an intermediate, and
as such opacity or optical density values were simultaneously
recorded corresponding to both the observer panel runs and the
particulate sampling tests.
The accuracy and reproducibility of observer read opacity
was characterized through data analysis that involved the cal-
culation of observations in accordance with EPA Method IX pro-
cedures. This entailed the arithmetic average of 25 readings
taken at 15-second intervals, which encompassed a six-minute
total time span. The result was a data base of approximately
1800 EPA Method IX observations from more than 300 observer panel
runs which formed the data base from which to work. The normal
size of the observing panel was thus an average of six indivi-
duals making and recording plume readings simultaneously.
An observer opacity lower detection limit in terms of par-
ticulate concentration was a parameter sought for the non-DCE
kraft recovery furnace Northwest site. The transmissometer was
used as a reference for the detection level depicted in Figure
1. The dotted lines in this figure encompass a range for the
transmissometer opacity response (corrected to a ten-foot path
length) where 25% to 75% of the observers reported "non-visible"
or zero opacity. Transformation of this range to the transmis-
someter calibration curve represented in Figure 2 provides a
comparison of the percent of observers reading non-visible at
the stack exit against the EPA Method V particulate concentra-
tion. The dotted lines in this figure indicate 25%, 50% and 75°
212
-------
of the observers reported non-visible opacity corresponding to par-
ticulate concentration levels of 0.041, 0.034, and 0.027 g/DSCM
(0.018, 0.015 and 0.012 gr/DSCF), respectively. The conclusion
obtained from this exercise was that below this range the ob-
servers did not see a plume, while above the concentration range
a plume was detected with a great degree of regularity. The
point defined as the observer detection limit corresponded to
that level where 50% of the observers reported zero opacity.
The particulate detection limit for the source studied was ap-
proximately 0.034 g/DSCM (0.015 gr/DSCF).
The relationship of plume appearance or opacity (based on
a ten-foot path length) to particulate concentration for the
Northwest non-DCE kraft recovery furnace site is presented in
Figure 3. The total observer member variation is represented
by the associated prediction intervals. Single observer opacity
is depicted here as a function of particulate concentration based
on data generated with clear or blue sky background conditions.
Use of the large field collected data bank which was analyzed
to derive this relationship indicated that single observer read
opacity could vary by ±13% opacity for the range of particulate
concentration studied at the 95% prediction interval. The equiva-
lent relationship of observer opacity to particulate concentra-
tion is presented in Figure 4 for the Southern site incorporating
dark mountain background viewing conditions. Total observer
variation was also found to be roughly ±13% opacity over the
measurement range of particulate concentration. The particulate
concentration that was indicated for the 20% observer opacity
level for both of the background conditions presented from the
two sites was approximately 0.080 g/DSCM (0.035 gr/DSCF).
Observer opacity depicted as a linear function of trans-
missometer opacity is shown in Figures 5 and 6 for the Northwest
and Southern sites, respectively. The variation in the observer
readings when compared to the instrument was ±10 to 15% opacity
for the measurement range studied. The second parameter denoted
as "B", or particulate concentration, on the X-axis was obtained
graphically from transmissometer calibration curves. Figures
3 and 4 presented previously incorporate this parameter to enable
their construction through a simple axis transformation.
The lime kiln opacity segment of the NCASI field study was
conducted in 1974 at a Northwest site. Two certified observers
were used in the nine test runs accomplished. During each test,
which ran approximately one hour, each observer recorded one
plume opacity reading per minute or 60 per test. Simultaneous
particulate sampling provided a comparison of mass concentration
to observed opacity. Two lime kilns each equipped with a venturi
scrubber operating in excess of 51 cm (20 in.) of water pressure
drop were routed to a common stack. The plume was read a few
213
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seconds after stack exit allowing the moisture to dissipate.
The plume diameter was judged to be about 6 to 8 stack diameters
at the point of observation. The stack was 1.8 m (4.5 ft) in
diameter.
100-,
75H
CO
a:
LU
tf>
CO
o
3?
25-
MEAN DATA POINTS
10
INSTRUMENT, % opacity
15
Figure 7. (NW) All data: relationship between observers reading
nonvisible and instrument opacity.
An exponential regression was assumed from theory to fit
the data with the resulting relationship displayed as Figure
7. The small sample size proved to be a limitation in the data
and chiefly responsible for the wide confidence interval noted.
Each observing run was characterized for plume type and back-
ground. The codes for the plume conditions experienced are in-
cluded in the figure for informative purposes.
The visibility threshold or the particulate level at which
50% of the observations were reported as non-visible was also
determined for the lime kiln source. The data indicated a value
of roughly 0.041 g/DSCM (0.018 gr/DSCF) for the detection limit
on this source. This was relatively close to the level found
for the kraft recovery furnace of 0.034 g/DSCM (0.015 gr/DSCF).
THE RELATIONSHIP OF PARTICULATE CONCENTRATION AND OBSERVED PLUME
CHARACTERISTICS AT COMBINATION WOOD AND FOSSIL FUEL-FIRED BOILERS
Another portion of NCASI studies concerning the relationship
of observed plume opacity and particulate concentration dealt
with combination wood and fossil fuel-fired boiler sources
214
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0.060-1
•6
VI
-a
^
o>
Z 0.045-
IT
I-
Z
O 0.030-
O
O
OBSERVATIONS NON-VISIBLE
EXTRAPOLATION
CONFIDENCE LIMITS 95%
0.01 5H
o
i-
cc
10
20
30
50-r
INSTRUMENT, % opacity
Figure 2. (NW): Detection range for observers.
OBSERVER PREDICTION
LIMITS: 95%
EXTRAPOLATED
0.01 0.02 0.03 0.04 0.05
PARTICULATE CONCENTRATION, gr/dscf
0.06
Figure 3. (NW) Clear data (Std): observer opacity vs. paniculate
concentration.
215
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50 T
40-
• 30
re
a
o
DC
LLJ
>
cc
LLJ
tn
CD
O
20--
10--
OBSERVER PREDICTION
LIMITS: 95%
0.02 0.04 0.06 0.08 0.10
PARTICULATE CONCENTRATION, gr/dscf
0.12
Figure 4. (S) Dark data (Std): observed opacity vs. particulate
concentration.
The study in its entirety can be found as NCASI Atmospheric
Quality Technical Bulletin No. 85. The two types of boilers
investigated in this project were a bark and coal combination
boiler located at a Middle Atlantic site and a bark and oil com-
bination boiler at a Southeastern United States site. The com-
parison of predicted single observer opacity and transmissometer
opacity for the Middle Atlantic location is shown in Figure 8
with the 80% and 95% confidence intervals. As noted, the ob-
servations were accomplished against a generally cloudy and hazy
background which contributed to the wide scatter in the data.
The values ascribed to the independent variable labeled "A" in
the figure are the particulate concentrations in grains per DSCF
obtained from the transmissometer's calibration curve. To obtain
units of grams per DSCM simply multiply by 2.29.
The relationship of predicted single observer opacity and
transmissometer opacity for the Southeastern site is represented
in Figure 9. The background described as primarily bright blue
sky was judged to result in less observer variation than the
cloudy and hazy background conditions experienced at the pre-
viously discussed site. Observer variability as denoted by the
95% confidence interval corresponded to approximately ±10% opacity
for the Southeastern site as compared to ±22% opacity at the
216
-------
50-r
40--
•o
a 30- •
o
cc
LLJ
CO
O
20- •
10- •
OBSERVER PREDICTION
LIMITS: 95%
10 20 30
INSTRUMENT, % opacity
40
0.0175 0.0304 0.0446 0.0615
PARTICULATE CONCENTRATION, gr/dscf
Figure 5. (NW) Clear data (Std) intermediate for Figure 3.
Middle Atlantic site over the measurement range studied. As
in the previous figure, the independent parameter denoted as
"A" in Figure 9 corresponds to particulate concentration in grains
per DSCF as obtained from the transmissometer calibration curve
for this source.
While the observer variability is relatively small as de-
picted in Figure 9 underlying the accuracy of the relationship,
the slope or change in observer readings was small for large
differences in transmissometer opacity response. The field data
collected suggested that observer reading only increased by 10%
opacity for a transmissometer increase of 45% opacity. The Middle
Atlantic site on the other hand reflected a 35% observer opacity
increase which corresponded to a 45% transmissometer increase.
Particle characteristics and plume background were judged to
be the chief contributors to such phenomena.
217
-------
50 T
40+
30
o
a
a
o
o
ai
> 20
LLJ
CO
m
O
PREDICTION LIMITS:
95%
80%
10 +
10 20 30
INSTRUMENT, % opacity
40
00175 0.0304 0.0446 0.0615
PARTICULATE CONCENTRATION, gr/dscf
Figure 6. (S) Dark data (Std) intermediate for Figure 4.
Fuel combination was one variable expressed by the investi-
gators as responsible for plume appearance changes causing ob-
lllver response variability. The magnitude of the fuel combina-
tion changes at the study sites was indicated by the ratio of
steam derived from the coal and bark which varied at the Middle
Atlantic site over a range encompassing 1.0 to 2.2 (coal/bark).
The ratio for steam derived from oil to that derived from bark
fuel was found to vary at the Southeastern site from 1.2 to 2.9
(oil/bark).
Particulate characteristic changes produced as a result
of fuel combination variability include particle density and
index of refraction. A range of densities between 2.0 and 4.0
q/cm3 were judged by the investigators as possible for coal-fired
power plants. In this context particle density, or maybe it
would be more accurate to say the physical size of the particle,
could have a pronounced effect on the opacity level while being
relatively independent of particulate concentration. The parti-
cle index of refraction is another parameter which affects plume
218
-------
opacity but is not necessarily a function of particulate concen-
tration. A small change in this value can lead to a substantial
shift in plume opacity.
The appearance or color of the plume is obviously caused
by the nature of the particulate 'matter in the plume. Particu-
lates emitted from an oil-fired boiler can be characterized into
three general types: black, white and colored. The black par-
ticles are judged to be chiefly carbonaceous material containing
traces of white and colored matter. The white portion of the
particulate is mainly in the form of irregular crystals or a
grouping of spherical particles containing primarily silicon.
The colored particles usually consist of a large fraction of
metallic elements and sulfur compounds in the form of oxides
and salts. The coal-fired boiler particulate emissions range
from milk white, yellow, gray, orange, and brown to black. Black
spheres found as particles consist of unburned hydrocarbons while
a colorless glassy sphere fraction was judged to have been burned
more efficiently. All of the particles contained a limited quan-
tity of metallic elements. A sand and char mixture was deter-
mined as the major constituent of particulate emitted from bark
boilers. While the sand is essentially ash which proceeds through
the system unchanged, the char is for the most part satisfactorily
described as unburned carbon whose quantity and nature is deter-
mined by boiler design and operation.
The background against which the various plumes were viewed
was an important factor in plume opacity judgements made by ob-
servers. The kraft recovery furnace plumes were reported by
the observers at a higher value when viewed against a dark moun-
tain or a blue sky background as compared to a cloudy sky. The
maximum contrast for the predominately "white" plume was the
controlling factor. The bark and coal combination boiler plume
was judged to yield the highest observer response when viewed
against a grey cloud background rather than dark mountains or
other conditions. The bark and oil combination boiler plume
was found to be most detectable with grey clouds or hazy sky
as background rather than white clouds or blue sky. Plume con-
trast was indicated as the major criterion to follow for maximiza-
tion of observer response resulting in readings comparable to
those measured by transmissometers.
FIELD PERFORMANCE OF A LASER LIGHT BACKSCATTERING SOURCE PAR-
TICULATE MASS MONITOR
A study was recently completed by the NCASI to evaluate
the applicability of the P-5A laser light backscattering mass
monitor to the measurement of particulate concentration prior
to and following control devices. Kraft recovery furnaces and
a wood-waste fired boiler were the sources used to determine
the monitor's performance by comparison to transmissometer opti-
cal density and EPA Method V sampling.
219
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•fi
•O
o
s>
o
o
<
oc
o
o
LLI
5
Z>
O
I-
cc
0.430 (% OPACITY)
CONG. = 0.0038e
OBSERVING CONDITIONS:
A- GOOD, BLUE
B -SLIGHT HAZE
C - INTERFERENCE
D - HAZE. LOOPING
/•• UPPER LIMIT OF 95%
CONFIDENCE INTERVAL
LOWER LIMIT OF 95%
CONFIDENCE INTERVAL
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0
OBSERVED % OPACITY
Figure 7. Paniculate concentration vs. observed opacity.
220
-------
The operating principles and theory of the laser monitor
manufactured by Environmental Systems Corporation can be found
in NCASI Atmospheric Quality Technical Bulletin No. 92 and in
several other publications. While beyond the scope of this paper,
Figure 10 is offered to illustrate the basic operation of the
instrument's optics head. Particles passing through the one-
ended beam generated by this optical complex causes light scatter
in all directions. As shown, the instrument has been designed
to allow light to be returned along small incident angles. The
light follows a path co-axial to the emitted beam to a light
collection lens. This lens serves to focus the light to a solid
state detector. Changes in the instrument response due to com-
ponent aging or temperature variation in the LED output power
are compensated for through a ratio of the signal detector re-
sponse to a reference signal. Interference from ambient light
sources is minimized through the use of a specific pulsed fre-
quency. Care must be taken, however, to shield the receptor
from artificial light (such as fluorescent) when adjustments
for zero and span are performed.
The first site on which the instrument was evaluated con-
sisted of emissions following a non-DCE kraft recovery furnace
source equipped with a high efficiency electrostatic precipi-
tator. A comparison at this site of the laser light monitor
to an LSI RM-4 transmissometer encompassed a 21-day time
interval during which 368 simultaneously hourly averages of mea-
surement response were obtained for each of the monitors. The
transmissometer was chosen for the evaluation based on the demon-
strated linear instrument response of optical density to gravi-
metrically determined particulate concentration found at this
site and in previous studies. Use of the transmissometer facili-
tated the compilation of a large data bank to enable the conduct
of more accurate and meaningful statistical analysis. The instru-
ment ranges chosen for the transmissometer correspond to a full-
scale value of 0.45 optical density while 0.1 g/ACM was selected
for the laser light monitor.
The relationship developed for the instrument comparison
is depicted in Figure 11 with the 95% confidence interval in-
dicated by the dashed lines. The regression equation corresponded
to:
PR AMC
= 0.00069 + 0.1861 (OPTICAL DENSITY)
The standard deviation calculated was 0.0018 g/ACM, which along
with a correlation coefficient of 0.88 projected a minimal
amount of scatter in the data over the range studied. This
range encompassed the lower half of the most sensitive setting
of the laser monitor with particulate concentration measurements
which corresponded to less than 0.229 g/DSCM (0.1 gr/DSCF) .
221
-------
50
40
I-
5
<
Q_
O
cc
UJ
>
cc
LLJ
CO
00
O
Q
LLJ
O
Q
LU
CC
a.
30-
20
10
0.011 0.022 0.034 0.047 0.060 0.074 0.089 0.105 0.122 0.141
PARTICULATE CONCENTRATION, gr/dscf
0 5 10 15 20 25 30 35 40 45
INSTRUMENT % OPACITY
Figure 8. Bark and coal combination boiler. Mainly cloudy
and hazy background.
The second portion of this study was carried out at a DCE
kraft recovery furnace operated under computer control. Evalua-
tion of the mass monitor was undertaken at two locations with
characteristic high mass concentrations, i.e., immediately pre-
ceding and following the direct contact evaporator. A laser
light monitor modified with a high temperature "ceramic" valve
assembly in the probe was employed for the testing accomplished
at the DCE entrance location to enable both gravimetric sampling
and short time interval RM-4 transmissometer comparisons to be
made. The laser light monitor was operated on the 2 g/ACM range
while the 0.9 optical density range was chosen for the trans-
missometer which utilized an actual four-inch measurement path
length which was mathematically adjusted to ten feet.
222
-------
50
> 40
Q.
O
cc
111
cc
111
Q
UJ
I-
o
Q
ai
cc
a.
30-
10-
0.
95% CONFIDENCE
LIMITS
20- - —
80% CONFIDENCE
LIMITS
(A) 0.029 0.039 0.049 0.059 0.071 0.083 0.096
(B) 0 5 10 15 20 25 30
(A) PARTICULATE CONCENTRATION, gr/dscf
(B) INSTRUMENT % OPACITY
0.109
35
0.124
40
0.140
45
Figure 9. Bark and oil comb/nation boiler. Mainly bright
blue sky background.
The calibration curve for the laser light monitor developed
through particulate sampling at the DCE entrance location is
depicted in Figure 12. This relationship corresponds to the
following regression equation:
Laser Monitor = 0.038 + 0.334 (Particulate Concentration)
with a standard deviation of 0.076 g/ACM and a correlation co-
efficient of 0.987.
The corresponding statistical analysis for the modified
transmissometer at this site was also calculated with a result
similar to that obtained with the mass monitor. A correlation
coefficient of 0.978 was found for this analysis. The recorder
223
-------
trace of the transmissometer visually possessed more noise than
that for the mass monitor, only a small portion of which might
actually be attributable to actual recovery furnace operation.
The mass monitor required essentially no maintenance over
a six-week study time interval. The transmissometer, on the
other hand, required daily attention with respect to lens clean-
ing and removal of particulate build-up in the slotted pipe used
to reduce the measurement path length of the instrument. The
information compiled at this site indicated the mass monitor
would operate satisfactorily in the range of approximately 3.75
g/ACM (5 gr/DSCF) found directly exiting the recovery furnace.
In this application the data generated could serve as an aid
to the operator in maintaining particulate emissions from the
furnace at a minimum.
A third phase of the mass monitor evaluation incorporated
a wood-waste fired spreader stoker power boiler site equipped
with two-stage multicyclones. The boiler was reported by the
mill to be rated at about 500,000 pounds of steam per hour.
A total of 142 hourly averages for the mass monitor and RM-4
transmissometer were compared with the resulting statistical
analysis represented as Figure 13. The relationship between
the two monitors was exceptional with a standard deviation of
0.002 g/ACM and a coefficient of variation of 0.986. The regres-
sion equation corresponded to:
GRAMS
ACM
= 0.005 + 0.154 (OPTICAL DENSITY)
LIGHT COLLECTION
LENS
BACKSCATTERED
BEAM
EMITTED
BEAM
SIGNAL
DETECTOR
Figure 10. P-5A optics diagram.
224
-------
0.04- •
u
^ 0.03- •
o
o
o
I 0.02
cc
LLJ
GO
0.01-•
UPPER & LOWER 95%
CONFIDENCE LIMITS
0.04 0.08 0.12 0.16 0.20
TRANSMISSOMETER OPTICAL DENSITY
(10 FOOT PATH LENGTH)
0.24
Figure 11. Non-DCE kraft recovery furnace relationship between
ESC laser light monitor and Lear Siegler transmissometer.
The equation for the statistical relationship determined for
the two monitors in this application was found to be surprisingly
close to that presented in Figure 11 for the non-DCE kraft re-
covery furnace equipped with an electrostatic precipitator. This
result addresses the similarity of the two monitors in their
response to particulate emissions with obviously different par-
ticle characteristics.
SUMMARY
The relationships developed between in-stack measured opacity
and simultaneous opacity measurements by trained observers have
been found to aid in the interpretation of regulations which
assume these opacity measurements to be equivalent. The relation-
ship between particulate concentration and opacity was defined
225
-------
/ / ,—» UPPER & LOWER 95%
CONFIDENCE LIMITS
0 1.0 2.0 3.0 4.0 5.0
PARTICULATE CONCENTRATION, 9/acm
Figure 12. West cyclone entry: ESC P-5A calibration curve.
over a practical range of particulate concentrations at the re-
covery furnaces studied. An additional study was completed to
determine the relationship of combination wood and fossil fuel-
fired boiler plume appearance, or opacity, and particulate con-
centration within a spectrum of existing and developing parti-
culate emission regulations. The findings for the fossil fuel-
fired boiler paralleled in many respects those made during in-
vestigations at the kraft recovery furnace. These include, among
others, a well-defined variability in observed plume character-
istics by trained observers viewing the same plume, and the need
to consider this variation if plume appearance is not to be the
controlling feature of comparison plume appearance mass emission
rate regulations. The findings differ in that the relationship
between plume appearance and particulate concentration is not
as precise for wood and fossil fuel combination boilers as for
kraft furnaces. Particle characteristics were discussed as
forming the probable cause of the lower precision encountered.
226
-------
An evaluation conducted on a laser light monitor designed
to measure particulate concentration as contrasted to opacity
monitors which measure the light transmission changes induced
by changes in flue gas particulate concentration displayed exemp-
lary results. When monitoring data were compared with simul-
taneously generated opacity monitoring records on controlled
sources from a kraft recovery furnace and a wood residue-fired
power boiler the laser monitor was judged to be equivalent in
quality of data and performance when used on sources with par-
ticulate concentrations of about 0.229 g/DSCM (0.1 gr/DSCF) and
less.
The laser light monitor was also found to perform satis-
factorily and unattended at particulate concentrations of about
3.75 g/ACM (5 gr/DSCF) found in the flue gas of a kraft recovery
furnace preceding an electrostatic precipitator. This appli-
cation was judged to be particularly attractive as a diagnostic
tool for operators in defining reasons for variability in par-
ticulate generation within recovery furnaces.
0.100
§ 0.075
cc
o
O 0.050
0.025
95% CONFIDENCE INTERVAL
0.15
0.30
0.45
, TRANSMISSOMETER OPTICAL DENSITY
(10 FOOT PATH LENGTH)
Figure 13. Relationship between a laser light monitor and
transmissometer for a power boiler.
227
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PAPER 11
A DATA REDUCTION SYSTEM FOR CASCADE IMPACTORS
J.D. MCCAIN
G. CLINARD
L.G. FELIX
J. JOHNSON
SOUTHERN RESEARCH INSTITUTE
ABSTRACT
A computer based data reduction system for cascade impactors
has been developed. The system utilizes impactor specific calibra-
tion information together with operating conditions and other
pertinent information such as stage weights and sampling duration
to determine particle size distributions in several forms for
individual runs. A spline technique is applied to fit a curve
to the cumulative size distribution obtained from each individual
impactor run. These fitted curves have forced continuity in
coordinates and slopes. Averages of size distributions for mul-
tiple runs are made using the fitted curves to provide inter-
polation values at a consistent set of particle diameters, irrespec-
tive of the diameters at which the data points fall in the original
individual run data sets. Statistical analyses are performed
to locate and remove outliers from the data being averaged, fol-
lowing which averages, variances, standard deviations, and con-
fidence intervals are calculated. The averages and statistical
information are available in tabular and graphical form in several
size distribution formats (cumulative mass loading, cumulative
percentage by mass, differential mass, differential number) .
The averaged data are stored in disk files for subsequent mani-
pulation. Additional programs permit data sets from control
device inlet and outlet measurements to be combined to determine
fractional collection efficiencies and confidence limits of the
calculated efficiencies. These results are available in graphical
form with a choice of log-probability or log-log presentations
and as tabular output. The program is set up to handle all com-
mercially available round jet cascade impactors, including common
228
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modifications, which are in current use in stack sampling. Other
round jet impactors can be easily substituted and slot type im-
pactors could be accommodated with slight program revision.
INTRODUCTION
Cascade impactors have gained wide acceptance as a prac-
tical means of making particle size distribution measurements.
These devices are regularly used in a wide variety of environ-
ments, ranging from ambient conditions to flue gas streams at
500°C (950°F). Specially fabricated impactors can be used for
more extreme conditions.
Because of their usefulness, the U.S. Environmental Protec-
tion Agency has funded research which has explored the theoretical
and practical aspects of impactor operation. As part of this
research, an effort has been made to design a comprehensive data
reduction system which will make full use of cascade impactor
measurements.
The cascade impactor data reduction system (CIDRS) described
here is designed to automatically reduce data taken with any
one of four commercially available round jet cascade compactors:
The Andersen Mark III Stack Sampler, the Brink Model BMS-11 (as
supplied and with extra stages), the University of Washington
Mark III Source Test Cascade Impactor, and the Meteorology Re-
search Incorporated Model 1502 Inertial Cascade Impactor. Pro-
vision is not made in this system for reducing data taken with
slotted jet impactors. With modification the computer programs
can accommodate any round jet impactor with an arbitrary number
of stages, and with more extensive revision, can be made to
handle data from slotted jet impactors.
The computer programs which comprise this data reduction
system are written in the FORTRAN IV language. The plotting
subroutines used were written specifically for the Digital Equip-
ment Corporation (DEC) PDP-15/76 computer, and these programs
are not compatible with other plotting systems. However, these
programs can be used as a guide when revision is made for use
with another operating system.
A broad outline of the program fundamentals is given here
with sufficient detail for anyone without a specialized knowledge
of computers to understand the methods and rationale of the pro-
gram. The program comprises two major blocks. The first block
treats data from individual impactor runs while the second treats
data from groups of runs, providing averages, statistical informa-
tion and fractional penetration (efficiency) results. The six
mainline programs which make up the data reduction system are
described in the overall program flow shown in Table 1.
229
-------
TABLE 1. PROGRAM FLOW
BLOCK 1. SINGLE RUN ANALYSIS
I. Impactor Program (MPPROG)
Takes testing conditions and stage weights to produce stage DSQ'S,
cumulative and cumulative % mass concentrations
-------
in several forms. The run analysis and output presentation are
accomplished by three main programs, MPPROG, SPLIN1, and GRAPH.
MPPROG and SPLINl perform analysis and manipulation while GRAPH
is totally devoted to various forms of graphical presentation
of the calculated distributions. The routines used in GRAPH
are specifically for use on a PDP-15/76 computer and are not
compatible with most other computers without modification. How-
ever, the general structure of GRAPH should serve as a useful
base for programming to achieve similar graphical output from
other computing systems.
MPPROG
In MPPROG, sampling hardware information, sampling condi-
tions and particulate catch information are used to determine
the effective cut sizes of the various impactor stages and the
concentrations of particles caught on these stages. The output
is organized into several tabular forms and stored on a disk
file for latter use.
Input Data to MPPROG—
Because individual impactors, even of the same type, do
not necessarily have precisely the same operational character-
istics, the program calculates stage cut diameters on an impactor
specific basis. Hardware data are stored within the program
which include, for each impactor to be used, the number of stages,
the number of jets per stage, the jet diameters, the stage cali-
bration constants, and flow-pressure drop relations for each
stage.
Run specific input data to MPPROG are listed in Table 2.
The maximum particle diameter must be measured by microscopic
examination of the particles collected on the first stage. Gas
analysis must be made at the same time the impactor is run.
Output from MPPROG--
Both input information pertaining to individual impactor
runs and calculated results are listed on the line printer.
A sample of output from MPPROG generated to the line printer
is shown in Table 3.
Input information for each run is given in the first five
lines of the printout. {Impactor pressure drop in inches of
mercury is calculated within the program; gas composition is
input as dry fractions and output as wet gas composition by per-
cent.) The mass collected on each stage in milligrams is listed
in the tenth line of the table.
231
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TABLE 2. INPUT DATA TO MPPROG
1. Impactor identification (required to call up hardware
information)
2. Gas composition (C02, CO, N2, 02, H20)
3. Impactor flow rate (acfm at stack conditions)
4. Stack pressure
5. Stack temperature
6. Gas temperature within impactor
7. Duration of sampling
8. True density of particles
9. Maximum particle diameter present in sample
10. Masses of catches by stage
The mass loading is calculated from the total mass of par-
ticles collected by the impactor and the total gas volume sam-
pled, and it is listed in four different units after the heading
CALC. MASS LOADING. The units are defined as:
GR/ACF - grains per actual cubic foot of gas at stack condi-
tions of temperature, pressure, and water content.
GR/DSCF - grains per dry standard cubic foot of gas at engineer-
ing standard conditions of gas. Engineering dry stan-
dard conditions in the English system are defined as
0% water content, 70°F, and 29.92 inches of mercury.
MG/ACM - milligrams per actual cubic meter of gas at stack condi-
tions of temperature, pressure, and water content.
MG/DNCM - milligrams per dry normal cubic meter of gas at engineer-
ing normal conditions of the gas. Engineering dry
normal conditions in the metric system are defined
as 0% water content, 21°C and 760 mm of mercury (torr)
232
-------
TABLE 3. SAMPLE CALCULATIONS
M
U)
CO
HYPOTHETICAL ANDFRSF.N
IMPACTOR F.UOW»ATE • O.soo ACFM
IMPACTOW PRtSSUPE nRUf « 0.3 TN, oF HG
ASSUMED PARTICLE DENSITY B 1.35 GH/CU.CM,
GAS COMPOSITION (PERCtNT) C02 s
CALC. MASS LOADING a 8.07UE-03 GH/ACF
IMPACTOP. STAGE
STAGE INDEX NI.JMRFP
050 (MICROMETERS) 10
[ MASS (MILLIGRAMS) o
IMPACTOR TMPERATUHE « aoo.o F • 2oa,a c SAMPLING DURATION • 20,00 CIN
STACK TEMPEHATURE
STACK PRESSURE •
1 ,9U CO »
l.OTaeE-02
31 32
I 2
.72 9,93 6.
,72 O.ao 0.
> ano.o F «
26,5" I.N, OF
0,00
GR/DNCF
S3 3a
3 a
35 a, is
53 0,09
20tt,a C
HG MAX. PARTICLE DIAMETER « 100,0 MICROMETERS
N2 « 76,53 02 • 20, S3 H20 • 1,00
l,BU70Etai MO/ACM 3,3718E + 01 M"0/ONCM
0
LL
1-
D
a.
85 36 37 88 FILTER
5 6 T 8 9
2.21 1.2B 0,67 0,33
0,3a 1,Oa 6,02E'04
1.21E-03 1.10E-OJ
LU
H
D
U
LL
U.
5
GEO, MEAN DIA, (MICROMETERS)
(NO,
J,27t+01 !,03EtOl 7.9«EtOn
7.93E+01 1.7flEt01
5,03F*07
5,15E.»00 3,OaEfOO 1.68E + 00
3,2aE+00 8.96E+00 3.95E+01
3.35E*07 a,52E+08 1.18E+10
9.SOE.OI 4.75E.01 2.36E-01
2,94f.tOl 8,56f>01 B,«7EtOO
5.I8EMO 1,13E*10
-------
Below the run condition data summary, the information per-
tinent to each stage is summarized in columnar form in order
of decreasing particle size from left to right. Thus, SI is
the first stage, S8 is the last stage, and FILTER is the back-
up filter. If a precollector cyclone was used, a column labeled
CYC would appear to the left of the SI column and information
relevant to the cyclone would be listed in this column. Beneath
each impactor stage number is listed the corresponding stage
index number, which also serves as identification for the stage.
Directly beneath these listings are the effective stage
cut diameters. The effective stage cut diameter is assumed to
be equal to the particle diameter for which the stage collection
efficiency is 50%. This diameter, D50, is calculated from an
equation of the form
so
(1)
where
D
5 0
= effective cut size,
k = stage calibration constant,
5
y = gas viscosity,
d = jet diameter,
p = particle density,
C = Cunningham slip correction factor, and
v = jet velocity.
Because the particle diameter, D50, enters the equation
for C, the solution of Equation 1 is obtained by an iterative
process. If the particle density, pp, is set equal to the true
density of the particles, as is the case in the sample output
in Table 3, the resulting diameter calculated from Equation 1
is the Stokes diameter, DS- If Pp is set equal to 1.0 the result-
ing diameter is the aerodynamic diameter DA as defined by the
Task Group on Lung Dynamics.1 If both pp and C are set equal
to 1.0, the resulting diameter is the aerodynamic impaction diam-
eter, DAI, as defined by Mercer.2 Unless otherwise specified,
MPPROG will automatically provide parallel output in terms of'
DS and DA- Parallel results in terms of DS and DAI or in terms
of DA and DAI are available if called for.
The stage weights, in units of milligrams as input, are
likewise listed for the respective stages on the line labeled
MASS. The mass loadings from each stage follow and are labeled
MG/DNCM/STAGE (milligrams per dry normal cubic meter per stage)
234
-------
The percentage of the total mass sampled contained in par-
ticles with diameters smaller than a particular D50 is called
the CUMULATIVE PERCENT OF MASS SMALLER THAN D50. It is the cumu-
lative percentage of total mass accumulated in the stage j.
The cumulative mass loading of particles smaller in diameter
than the corresponding D5? is listed in four different units:
milligrams per actual cubic meter, milligrams per dry normal
cubic meter, grains per actual cubic foot, and grains per dry
normal cubic foot. Note that these are the same units used for
calculating the total mass loading. They represent both metric
and English units and both stack conditions and engineering dry
standard conditions of temperature, pressure, and water content.
The geometric mean diameter for the particles collected
on each stage is then listed in micrometers. The geometric mean
diameter of a given stage may be expressed as the square root
of the product of the D50 of the given stage and the D50 of the
previous stage. In calculating the geometric mean diameter of
the first stage (or cyclone if applicable), the maximum particle
diameter is used instead of the "D50 of the previous stage."
In calculating the geometric mean diameter of the filter, one-
half the D50 of the last stage (stage eight, here) is used in-
stead of the "D5U of the given stage." (There is no D5a for the
back-up filter since all remaining particles are captured by
this filter .)
Finally, an approximate differential particle-size distribu-
tion is listed as DM/DLOGD, in milligrams per dry normal cubic
meter, and as number concentration, DN/DLOGD, in number of par-
ticles per dry normal cubic meter.
Differential size distributions may be derived two ways:
1. Finite difference methods may be used based on the DSQ'S
(abscissa) and the particulate masses on each stage
(ordinate). This technique was used to generate the
differential size distribution data in Table 3.
2. Curves may be fitted to the cumulative mass distribution
from which the differential curves (slope) for each
test can be calculated. This method is preferred and
is described in the following paragraphs.
SPLIN1
In many, if not most, sampling programs, a number of impactor
runs will be made. Frequently, these runs will be made using
several impactors, having different performance characteristics.
The latter may be true even if the same type of impactor is used
throughout a sampling program. This behavior results both from
manufacturing variations which cause calibration differences
235
-------
and run-to-run variations in sampling rates, which cause shifts
in the DSQ'S. Averaging results from such testing to obtain
a representative composite size distribution requires that the
distributions be broken down into like size intervals for all
the runs to be averaged. The same requirement for like size
intervals also holds for using inlet and outlet data from control
device sampling programs to obtain fractional efficiencies.
This requires curves to be fit to the data for each run to permit
interpolation to obtain values at common diameters for all runs
to be compared or averaged.
Before making the final selection of the spline technique
for fitting curves to the size distribution data, consideration
was given to a number of alternate fitting methods, and several
of them were tried. It was concluded that any attempt to fit
a predetermined functional form (e.g., log-normal) to the data
was generally not proper. Because the slope of the cumulative
distribution curve, the differential distribution, is the re-
quired quantity for calculating fractional efficiencies, con-
sideration was also given to fitting curves to the AM/AlogD ap-
proximations of the true differential distribution, which are
estimated directly from the stage loadings and D50's. However,
the magnitudes of the steps in D30 are large enough in most im-
pactors as to frequently make AM/AlogD a poor approximation to
dM/dlogD. Moreover, the boundary conditions are more difficult
to handle in fitting curves to AM/AlogD than in fitting to the
cumulative distributions. It was ultimately concluded, after
many trial fitting methods were tested, that the best use was
made of the data if the fitting was done to the cumulative dis-
tribution curve by means of a spline method.
SPLIN1 operates by fitting a curve which is continuous in
X and Y and the first derivative of Y with respect to X to the
cumulative mass concentration size distribution data. The result-
ing fitted curve is similar to that which one would draw through
the data points using a "French curve" or mechanical spline.
This fitted curve invokes no a priori assumptions as to the shape
of the distribution (e.g., power law or log-normal). The manner
in which the spline fits are made is described below.
Initial attempts at using the spline technique on the set
of points defining the cumulative distribution curve obtained
directly from the DSO'S were not satisfactory. The difficulty
occurred as a result of the inability of the method to generate
sufficiently rapid changes in curvature when the curve to be
generated was defined by a small number of points. A satisfactory
fit could be obtained by adding a set of interpolated points
between the original data points of the measured cumulative curve
These points are generated by means of a series of parabolas
through consecutive sets of three adjacent data points of the
original cumulative curve defined by the impactor stage data
236
-------
The fitting is done using log (concentration) and log (particle
diameter) as variables and begins with the segment containing
the smallest D50 in the data set.
The sequence of operations by which the interpolated points
are generated is shown in Figure 1. A series of parabolas are
fit through consecutive sets of 3 data points beginning at the
smallest D50 as shown in Figures la and Ib. In this description,
three interpolation points between each pair of D50's will be
assumed. However, the program will accommodate up to 5 interpola-
tion points. The use of more points will improve the accuracy
of the fitting, but will require more storage capacity. The
interpolation points are located along the parabolas, between
the lower pair of the three Dso points used to generate the pa-
rabola. The interpolated points are spaced evenly in log diam-
eter between the pair of original points. A similar process
is used to generate interpolated points between consecutive pairs
of Dso's up to the segment which terminates at the D50 of the
first collection stage as illustrated in Figures Ic to le. A
slightly different procedure which will be described later, is
used for segments which include the first collection stage D5o-
Since the fitting is for a cumulative curve, negative slopes
are not allowed. Therefore, a check is made for negative first
derivatives of the interpolation parabola at the bounds of each
segment within which the interpolated points are to be generated.
If a negative derivative is found in any segment other than the
first (the segment including the smallest D50) a straight line
interpolation between the segment bounds is used rather than
parabolic interpolation. If a negative first derivative is found
in the first segment to be fitted, a fictitious point is generated
and used to form a parabola which has no negative derivatives
in this segment. This fictitious point has the same concentra-
tion value as that of the first point on the cumulative curve
and has a diameter defined by
_ _ (Dso of last stage)2
fictitious (D50 of next to last stage)
The interpolated values for the segment between the last two
D50's on the cumulative curve are then generated from the pa-
rabola which passes through this fictitious point, and the points
for the last two stages on the cumulative distribution curve.
In the region about the first stage Dso, three sets of inter-
polated points are generated. The first are generated by parabolic
interpolation using a parabola through DMAX, D50 (stage 1) and
D50 (stage 2) as was done in the case of the previous segments.
However, in addition to these, two more points are generated
along the parabola above the first stage D50- These additional
points are spaced evenly in log (diameter) at the same intervals
in log (diameter) as the interpolated points between D50 (stage
237
-------
z
Q
CO
1
LLJ
>
<
_l
D
u
D50(6) DgQlS) D50{4) D50(3) D50(2) D50(1) DMAX
PARTICLE DIAMETER
Figure 1a. Cumulative size distribution from raw impactor data.
z
Q
- FIRST INTERPOLATION
- PARABOLA
o
a/
INTERPOLATED POINTS
' I ' | ' ' ',1 II r_l 1 I I | I I II . . '..in
D50<6) D50(5) D50(4) D50(3) D50(2) D50(1) DMAX
PARTICLE DIAMETER
Figure Ib. Start of development of interpolated points between first and last Den.
238
-------
z
O
LU
>
D
5
o
SECOND
INTERPOLATION
PARABOLA
INTERPOLATED POINTS
4-
I I i i
i i i i 111
i i i 111
H-1-
D50<6> D50<5) D50<4> D50<3) D50<2) ^o'1' DMAX
PARTICLE DIAMETER
Figure 1c. Continued generation of interpolated points
o
D50<5> D50<4> D50<3> D50<2' D50<1>
PARTICLE DIAMETER
Figure 1d. Continued generation of interpolated points
239
DMAX
-------
5
1
LU
>
<
_l
D
O
INTERPOLATED POINTS ON
FINAL PARABOLA
FINAL INTERPOLATION
PARABOLA
oo'
_LI 1
' , ' I ' ' '."I ,_! |_-L_L
D50<6> D50<5> D50<4> D50<3> D50<2> D50<1>
PARTICLE DIAMETER
Figure 1e. Generation of interpolated points on parabola
which includes DM AX.
j 1 I I )IM
DMAX
5
<
o
CO
GO
D
5
o
SLOPE = 0
HYPERBOLA AND
HYPERBOLIC
INTERPOLATION POINTS
BETWEEN
D50 (1) and DMAX
I » i i
I ' ' | I'll
-L_L_LJ
DMAX
D50(5) D50(4) D50(3) D50(2) D50(1)
PARTICLE DIAMETER
Figure If. Generation of interpolated points on hyperbola through
D5Q(1) and DMAX
240
-------
1) and D50 (stage 2) as shown in Figure le. These points are
used in generating the final curve fit up to the point on the
cumulative distribution curve defined by the first stage D50.
The third set of points is illustrated in Figure If.
Note that the cumulative mass distribution used in the illus-
trations of Figure 1 is one in which a large step in concentration
occurs between D50 (stage 1) and DMAX. This is typical of a
cumulative curve for a bimodal distribution in which one mode
has a median diameter substantially greater than first stage
D50. The interpolation parabola through DMAX, D50 (stage 1 and
D50 (stage 2) does not properly represent the shape of the true
distribution curve in this region. In particular, the true curve
must have zero slope at DMAX. It was empirically determined
that a hyperbolic interpolation equation fit in terms of linear
concentration and linear diameter between DMAX and D50 (stage
1) with the hyperbola asymptotic to the total loading at infinite
particle size resulted in acceptable results for the final spline
fits. Therefore, a seven point hyperbolic interpolation is used
in addition to the previously described parabolic interpolation
over this segment of the curve. This hyperbolic interpolation
is illustrated in Figure If. The use of the two sets of inter-
polated points in the final interval will be discussed later.
Generation of the Final Spline Fit—
The original data points, defined by the Dso's, together
with the interpolated points just generated, form a set of points
along a continuous curve (if one disregards the two sets of
points in the final segment) which has no negative slopes. How-
ever, the derivative of the curve in most cases will not be con-
tinuous at the D5o points. The spline fit to be described is
a smoothing technique which generates a series of parabolic seg-
ments that approximates a continuous curve through the complete
set of points defining the cumulative distribution. The segments
to be generated now will pass near or through those points and
will have forced continuity in both coordinates and first deriva-
tives. The technique is applied first to cover the interval
between the first and last stage D50's and then a second time
to cover the interval between the first stage D50 and DMAX.
From this point on in the discussion, no distinction is made
between the original points defined by the D50's and the inter-
polated values located between them.
The spline fit is generated by joining successive parabolas
at locations determined by the x (or log diameter) coordinates
of the points which now represent the cumulative distribution
curve (original points at the D50's plus the interpolated points).
These parabolas have continuity in slope forced by the fitting
procedure and are generated in such a fashion as to pass near
or through the points on the cumulative distribution curve.
241
-------
The procedure is illustrated in Figure 2. The spline fit
is begun at the lowest point, 0, on the distribution curve (at
the D50 of the last stage). The parabola used to generate the
interpolated points between the last two stages is assumed to
be the fitted curve up to the first interpolated point. (Point
1 in Figure 2a.) This parabola, a, is followed until the x-co-
ordinate at point 1 is reached. At the point A, located on this
parabola by the X-coordinate of point 1, a new parabola is fitted
as shown in Figure 2b. This parabola, b, is forced to pass
through point A with the same slope at A as the'parabola used
to define point A, and is forced to pass through the third point
above point 1 in the set of points defining the cumulative curve,
i.e., point 4. The parabola, b, is followed to the point defined
by the X-coordinate of point 2, thus locating a point B. At
B a new parabola is fit with forced slope continuity with b
passing through the third point ahead of point 2, i.e., point
5, as shown in Figure 2c. From C this process is repeated using
point C and 6 to generate a new parabola, d, and termination point
D, e, and E, etc., until a termination point at the D50 of the
first collection stage is reached. The last three points ob-
tained by parabolic interpolation are used in generating the
spline fit parabolas up to the first collection stage D50. The
coefficients of the fitting spline fit parabolas for the segments
a, b, c, d, . . etc., are saved for future use. These now rep-
resent the smoothed curve and will be used henceforth to define
the cumulative curve for that run.
The final spline fit starts by picking up at the point on
the fitting parabola which terminated at the D50 of the first
stage. The same procedure as before is followed, except that
the third point ahead determined by the hyperbolic interpolation
is now used for fitting, and the fitting parabolas are followed
to X-coordinates defined by the hyperbolic interpolation points.
The curve generated in this second zone of the spline fit [i.e.,
between D50 (stage 1) and DMAX) is an extrapolation which has
been found to be reasonably good to diameters equal to about
2 to 3 times the first stage D50. By using the second, third
(as illustrated), fourth, etc., point ahead in generating the
final parabola segments one can influence the amount of smoothing
provided by the program.
The cumulative concentration and slope of the cumulative
curve, dm/dlogD, can be calculated for any arbitrary particle
size by locating the fitting coefficients for the spline segment
containing that size. The boundary locations of each of the
parabolic segments, 0, A, B, C, . ., and the fitting coefficients
for each segment are stored in a disk file for subsequent use
by other programs (e.g., GRAPH, STATIS, etc.).
242
-------
z
Q
u
6
O
5
O
I I
I l I I I
D5o D50
(LAST STAGE) XA = X-\ (LAST-1 STAGE)
PARTICLE DIAMETER
Figure 2a. Start of the curve fitting procedure. Cumulative mass loadings
derived from stage catches are represented by solid circles.
Interpolated values are shown with open circles.
243
-------
o
2
Q
<
O
V)
V)
h-
<
3
5
I ll I I
J I
J 1 JIM
D50
(LAST STAGE)
U50
(LAST-1 STAGE)
PARTICLE DIAMETER
Figure 2b. Second step in the curve fitting procedure. Cumulative mass
loadings derived from stage catches are represented by solid
circles. Interpolated values are shown with open circles
244
-------
o
6
o
z
5
<
O
in
V)
D
5
D
O
t I I I I I I
D50
(LAST STAGE)
xb xc D50
(LAST-1 STAGE)
PARTICLE DIAMETER
Figure 2c. Third step in the curve fitting procedure. Cumulative mass
loadings derived from stage catches are represented by solid
circles. Interpolated values are shown with open circles.
245
-------
Problems Resulting from Extremely Close Stage Cut Diameters (D5o's)—
When two stages are used on an impactor which differ only
slightly in Ds0, the second of the two will collect too much
material because of the finite slope of real impactor stage col-
lection characteristics. The simplest example of this effect
would be obtained if two identical stages are used sequentially.
If that were the case, in an ideal impactor the second stage
should collect no material; however, because of the finite slope
of the real stage collection efficiency curve, it will collect
some particles. This would lead to the formation of a step in-
crease (infinite slope) in the cumulative concentration curve.
The severity of the effect is reduced as the spacing between
the D50's increases but can be sufficiently severe to cause
significant errors in the size distribution curves if it is not
properly accounted for. Calibrations indicate that the effec-
tive cut diameters, or D50's, at the first two stages of several
impactors suffer from this problem. The program MPPROG, because
of this, ignores the presence of the second stage of Andersen,
MRI, and University of Washington impactors in generating the
cumulative mass concentration curve from which the fitted curves
will be made by SPLINl. This procedure effectively nullifies
the problem. However, if calibrations of future versions of
these impactors do not show the small spacing in D 5 0, MPPROG
should be modified appropriately so as not to lose good informa-
tion when the curve fits are made.
GRAPH
Program GRAPH is dedicated entirely to presenting data from
single impactor runs. The output forms available on call are
cumulative mass loading versus Dso, AM/AlogD versus geometric
mean diameter, and AN/AlogD versus geometric mean diameter as
calculated in MPPROG. The latter are available on Stokes
aerodynamic and aerodynamic impaction diameter bases. As an
option, up to ten runs can be superimposed on a single plot.
Plots and tabular output of the fitted curves from SPLINl are
also available. The fitted curves from SPLINl are plotted super-
imposed on the data points from MPPROG, but only as single run
plots. The plots are all made on log-log grids.
The tabular output includes cumulative percent mass loading
less than particle diameter generated from the SPLINl fitted
curves, dM/dlogD versus particle diameter, and dN/dlogD versus
particle diameter generated by differentiation of the SPLINl
fitted curves.
ANALYSIS OF GROUPED DATA
STATIS
STATIS is a program for combining data from multiple impactor
runs under a common condition. The program tests data from £
series of runs (specified by the user) tor outliers, flags and
246
-------
removes outliers from the set, and then provides output in the
form of averaged size distributions with confidence intervals
as desired in both tabular and graphical form. The program is
set up to provide 50% confidence intervals; nowever, changes
can be made for the calculation of other confidence intervals
as desired (e.g., 90% or 95%).
The input data to STATIS are the fitted polynomial segments
generated from MPPROG by SPLINl which now define the cumulative
mass loadings for each run. The particle diameter basis for
averaging (i.e., aerodynamic, aerodynamic impaction, Stokes)
is user specified on control cards used to execute STATIS.
The fitting equations from SPLINl are differentiated at
preselected particle diameters to obtain the quantity (dM/dlogD.)
where i refers to particle diameter and j refers to the sequenc^ ^
number of a particular run in the set to be averaged. The values,
at each particle diameter, D^, are subjected to an outlier analy-
sis based on the deviations of the values of dM/dlogD for indivi-
dual runs from the mean for all runs.
The outlier test used is that for the "Upper 5% Significance
Level". A curve fitted to the tabular list of critical values
for excluding an outlier is used to generate the table. The
test is based on the quantity
- X
being greater than a critical value, c , which is
S n'
a function of the number of points in the data set.
X. = individual value
X = mean of all values
S = standard deviation of the data set.
The application of this test requires that there be three
or more runs in the sequence to be averaged. This outlier test
is repeated after discarding any outliers already identified,
provided there are at least three runs remaining in the set of
retained points.
After discarding outliers, a final average, standard devia-
tion, and confidence interval are calculated for each (dM/dlogDi)
These values are output on the line printer and are plotted on
call by the user.
Cumulative size distributions on a mass basis or percentage
basis are derived from the averaged dM/dlogD values by integra-
tion of these values. The choice of integrating the dM/dlogD
curve rather than direct computation of the cumulative averages
from the individual cumulative distributions was based on the
247
-------
fact that an error in a single stage weight is propagated forward
throughout the cumulative curve for all stages subsequent to
the one on which the error occurred. This would cause substantial
quantities of good data from other stages to be discarded by
the outlier analysis. Integration of the averaged differential
distribution, on the other hand, allows the data from the re-
maining, error free, stages to have their proper influence on
the averaged cumulative distributions. These cumulative dis-
tributions are again output in tabular form and, on call, in
graphical form.
The cumulative distributions can be obtained either includ-
ing or excluding particles smaller than 0.25 ym in diameter.
The option of excluding the particles smaller than 0.25 ym is
made available because of the fact that in a significant percent-
age of sampling situations, impactor back up filter catches can
be dominated by oversize particles because of bounce and/or re-
entrainment. This results in a filter weight gain which can
be many times higher than the weight of the fine particles, which,
ideally, should be the only material present. In those cases,
omission of the material which is nominally smaller than 0.25
um from the cumulative distributions will make the result a much
better representation of the true size distribution. This, of
course, is true only when the D50 of the last impactor stage
is about 0.25 to 0.5 ym as is usually the case with the commer-
cially available impactors.
Standard deviations and confidence limits for the cumulative
distributions are calculated from the approximation that the
variance (and square of a confidence interval) for a sum, A +
B, is given by the sums of the variances (and squares of the
confidence intervals) for A and B separately, i.e.,
Variance „ = variance, + variance^
A ~t~ 15 f\ o
and (confidence interval)^ + Q = (confidence interval)'
A
+ (confidence interval)2
B
The averaged differential size distributions generated by
STATIS are stored in a disk file for use by the programs PENTRA
or PENLOG in calculating control device fractional efficiency
curves.
Tabular and graphical output from STATIS includes cumulative
mass loading versus diameter, cumulative percentage on a mass
base versus diameter, dM/dlogD versus diameter, and dN/dlogD
versus diameter. The graphical presentations are made on log-
log grids with the exception of the cumulative percentage plot
which is made on a log-probability grid. All output forms, graphi-
cal and tabular, include confidence limits. The choice of diam-
eter definition used is left to the user. An index of runs which
248
-------
were rejected through the outlier analysis before averaging is
also printed. Rejection at any one particle size does not result
in the run being excluded at all particle sizes.
Programs PENTRA/PENLOG
These two programs are virtually identical and provide
tabular and graphical output of control device penetration and/or
efficiency versus particle size for a preselected series of par-
ticle sizes from about 0.25 to 20 ym. The only difference between
the two programs is in the form of the graphical output. In
the case of PENTRA, the fractional efficiency curves are pre-
sented on a log-probability grid while in PENLOG they are presented
on a log-log grid.
The calculations are made from averaged sets of inlet and
outlet data developed by STATIS. The user identifies the pair
of averaged data sets from which the efficiency is to be cal-
culated together with the diameter basis required (i.e., Stokes,
aerodynamic, aerodynamic impaction). The program retrieves the
appropriate averaged data sets and calculates the fractional
efficiency as
efficiency. (%) = 1
(dm/dlogD.)
i'outlet
(dm/DlogDi)inlet
x 100
where i refers to the i particle diameter in the preselected
diameter sequence. Simultaneously, if both the inlet and out-
let data sets included two or more runs, confidence limits are
calculated based on a method described by Y. Beers.1* The con-
fidence level associated with the limits generated by the program
as provided are 50% levels; however, other levels can be generated
by simply changing values of three constants used to generate
the appropriate t-table.
Test Cases and Exemplary Results
Tests of the final fitting process were made by generating
fictitious impactor runs having known size distributions. These
runs were generated by calculating the D50's associated with
several sets of sampling conditions. The stage weights required
to produce exact unimodal and bimodal log normal distributions
were then generated for these sets of particle diameters. The
program was exercised on these artificially constructed runs
and results obtained from the fitting procedure were then com-
pared to the original distributions. Figures 3 and 4 show ex-
amples of two such tests in the form of differential size distri-
butions. Figure 3 illustrates the input distribution and re-
covered distribution for an aerosol having a mass median diameter
of 4 ym and geometric standard deviation of 3.0. Recovered dis-
tributions from the spline fit and approximation results, AM/AlogD,
249
-------
10"
t
103
u
•G
•&
Q
O
o
Q
102
« UIMIMODAL LOG NORMAL DISTRIBUTION
D DM/DLOGD BASED ON CURVE FITTING
4 AM/ALOGD BASED ON STAGE WEIGHTS
MMD = 4.0 jum; a = 3.0
101
10-
10°
102
PARTICLE DIAMETER, ,um
Figure 3. Approximate differential size distribution based on stage weights
and same distribution based on spline fitting are compared to a true
unimodal log normal distribution.
250
-------
u
^
I
cf
O
Q
10°
10
1-1
10-'
10
1-1
u
"I
Q"
O
Q
10° 101
PARTICLE DIAMETER, jum
102
10
r3
-•- BIMODAL LOG NORMAL DISTRIBUTION
a dM/dlogD BASED ON CURVE FITTING
A AM/AlogD BASED ON STAGE WEIGHTS
MMD'S = 2.0 urn AND 10.0 Mm; a = 1.5
Figure 4. Approximate differential size distribution based on stage
weights and same distribution based on spline fitting are
compared to a true bimodal log normal distribution.
251
-------
taken directly from the stage weights both show excellent agree-
ment with the input distribution. The fitted results for diam-
eters larger than 7 ym represent an extrapolation to sizes larger
than the first stage D50. Figure 4 illustrates a similar test
for a bimodal distribution having equal amplitude modes, mass
median diameters of 2 ym and 10 urn, and geometric standard devia-
tions of 1.5. Again, beyond about 7 ym, the fitted points represent
extrapolations. Note that the AM/AlogD approximations derived
directly from the stage weights lie very close to the input curve
in regions where the slopes are not large but fall significantly
above the true curve in regions of high slope. Errors expressed
as percentage deviations from true values are shown for two cases
each for unimodal HMD = 4, ag = 2, and bimodal MMD = 2, ag = 1.5,
and MMD = 10, ag = 1.5 distributions in Figures 5 and 6. Note
that the results from the fitted curves generally fall within
±10% or better of the true values in the size interval covered
by the impactor stage D50's and are for the most part within
±50% of the true values in the extrapolation region above the
first stage D50. Much larger errors occur with the AM/AlogD
approximations to the differential distributions obtained di-
rectly from the stage weights. The errors shown in Figures 5
and 6 result only from the fitting procedure and do not include
any effects from non-ideal behavior in the impactors. Errors
arising from the latter can be much greater, as described by
McCain and McCormack.5
Examples of some of the graphical output formats available
from the program are shown in Figures 7 through 10. Figure 7
illustrates a single run cumulative mass distribution with the
original data points and fitted curve from SPLINl. Figure 8
shows the differential distribution obtained for the run shown
in Figure 7. Figure 9 illustrates a cumulative mass distribution
on a percentage basis with confidence limits obtained from the
average of several runs similar to that shown in Figures 7 and
8. Figure 10 illustrates a control device penetration curve
with confidence limits obtained from sets at averaged inlet and
outlet runs.
SUMMARY AND CONCLUSIONS
CIDRS represents a powerful, versatile tool for reducing
and managing data obtained with cascade impactors. It provides
the capability for single and multiple run data analysis with
varying degrees of smoothing of single run data available and
averaging and statistical analysis of multiple runs. Results
from the program are not biased by forced fits to arbitrary
distribution forms. Finally,.the program makes possible a very
significant time saving in handling and processing field data
obtained with cascade impactors.
252
-------
30
20
10
ro
Ul
Co
o
O
cc
cc
-10
-20
-30
— dM/dlogD BASED ON CURVE FITTING TO AN ANDERSEN RUN
A AM/AlogD BASED ON STAGE WEIGHTS OF SAME ANDERSEN RUN
... dM/dlogD BASED ON CURVE FITTING TO A BRINK RUN
D AM/AlogD BASED ON STAGE WEIGHTS OF SAME BRINK RUN
MMD = 4.0 pm; a = 3.0
0.1
1.0
10
PARTICLE DIAMETER, j
Figure 5. Percent error of approximate differential size distributions
based on stage weights and the same distributions based on
spline fitting from a uni modal log normal distribution.
100
-------
Ln
70
60
50
40
30
20
10
0
-10
o
g -20
DC
HI
-30
o
o
-40
-50
-60
-70
0.1
dM/dlogD BASED ON CURVE FITTING TO AN ANDEBfcEN RUN
A AM/AlogD BASED ON STAGE WEIGHTS OF SAME ANDERSEN RUN
... dM/dlogD BASED ON CURVE FITTING TO A BRINK RUN
D AM/AlogD BASED ON STAGE WEIGHTS OF SAME BRINK RUN
MMD'S = 2.0 urn AND 10.0 jum; a = 1.5
I
I
1.0
10
10
PARTICLE DIAMETER, //m
Figure 6. Percent error of approximate differential size distributions
based on stage weights and the same distributions based on
spline fitting from a bimodal log normal distribution.
-------
ICDLO-y i-13-76 1544 POTS 4,5.6
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s
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M
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I I I MM| 1—I I I HIM
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PARTICLE DIA^€:TER (MICROMETERS)
Figure 7. Single run cumulative mass distribution with original
data points based on stage weights and fitted curve
from SPLIN1.
255
-------
101D-37 1-19-76 1544 PORTS
W = e.4D M/CC
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:
-
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1 1 1 1 lllH 1 1 1 1 Mill 1 1 » I I "il
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PARTICLE DIAN€!TER (MIO30METER5)
Figure 8. Differential size distribution obtained for the run in
Figure 7 based on stage weights and based on curve fitting.
256
-------
OOS WOW 1IE5T FOR M0SBU
99.39
3 " ^>3 "3T
OwJ • 3 i
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994
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PARTICLE DIAMETER (MICRDNCTER5)
Figure 9. Cumulative mass distribution on a percentage basis with
confidence limits obtained from the average of several
runs similar to that shown in Figures 7 and 8.
257
-------
PE^ETRATIa^l-EFFICIE^CY
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F/5»tv/'e /O. X\ control device penetration curve with confidence limits
obtained from sets of averaged inlet and outlet runs.
258
-------
ACK NOWLEDG EMEN TS
This work was conducted under U.S. Environmental Protection
Agency contracts 68-02-0273 and 68-02-2131. The continued sup-
port and assistance of Dr. D.B. Harris, the EPA project officer
on these contracts is gratefully acknowledged. Additional ap-
preciation is also expressed to Mr. B. Gaston and Ms. A. Henry
for their assistance with the programs; to Mr. K. Gushing whose
earlier program was used as a basis for program MPPROG; and Dr.
W.B. Smith for his assistance and guidance during the program
development.
REFERENCES
1. Morrow, P.E. Deposition and Retention Models for Internal
Dosimetry of the Human Respiratory Tract. Health Phys.
12:173-208, 1966.
2. Mercer, T.T., M.I. Tillery, and H.Y. Chow. Operating Charac-
teristics of Some Compressed Air Nebulizers. J. Amer. Ind.
Hyg. Assoc. 29 :66-78-, 1968 .
3. Quality Assurance Handbook for Air Pollution Measurement
Systems, Vol. 1. Principles. EPA-600/9-76-005, U.S. Environ-
mental Protection Agency, Research Triangle Park, NC, 1976.
4. Beers, Y. Introduction to the Theory of Error. 2nd Edition,
Addison-Wesley, Reading, MA, 1957.
5. McCain, J.D., and J.E. McCormack. Non-ideal Behavior in
Cascade Impactors. Paper 77-35.3, 70th Annual Meeting,
APCA, Toronto, 1977.
259
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PAPER 12
AEROSOL GENERATION AND CALIBRATION OF INSTRUMENTS
DAVID Y.H. PUI
BENJAMIN Y.H. LIU
MECHANICAL ENGINEERING DEPARTMENT
UNIVERSITY OF MINNESOTA
ABSTRACT
Standardization and calibration of aerosol measuring in-
struments generally require the use of monodisperse aerosols.
In this paper, a review is first given of various aerosol gener-
ators that can be used for instrument standardization and cali-
bration. The basic principles of several generators are described
which can be used for producing monodisperse aerosols of primary
standard quality and polydisperse aerosols of high concentration
stability. Particular attention is devoted to the aerosol genera-
tors developed at the Particle Technology Laboratory, University
of Minnesota; these include the vibrating orifice monodisperse
aerosol generator, the mobility classifier monodisperse aerosol
generator, the ultrafine condensation monodisperse aerosol genera-
tor, and the constant output atomizer aerosol generator. With
the use of these generators, monodisperse and polydisperse aerosols
can be generated from 0.002 ym to 50 ym in diameter at various
concentration levels up to 106 particles/cm3 in certain size
ranges. Further, the particle size and concentration from the
monodisperse aerosol generators are known to a high degree of
accuracy (2% for size and 5% for concentration). The accuracy
has been verified by using independent microscopic and other
particle sizing and counting techniques.
In addition, the paper reviews various aerosol measuring
and sampling devices that have been calibrated by means of the
monodisperse aerosol standards. These include the diffusion
battery, the condensation nuclei counter, the electrical aerosol
analyzer, the optical particle counter and the inertial impac-
tor. The calibration procedures, precautions, and results are
briefly outlined. In addition, comparative studies for some
of the calibrated instruments are described. It is shown that
good agreement can be obtained when the instruments are properly
calibrated and standardized.
260
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INTRODUCTION
Recent advances in experimental aerosol science and instru-
ment development have made possible the rapid, in situ measure-
ment of particle size distribution. Instruments are now avail-
able for size determination of aerosols from 0.002 to over 50
ym in diameter. The successful application of these instruments
to experimental aerosol studies requires that the instrument
be properly standardized and calibrated. A well-calibrated in-
strument is essential for the correct interpretation of experi-
mental data, and for obtaining the highest possible accuracy
from a given device.
Standardization and calibration of aerosol measuring instru-
ments can best be accomplished by the use of monodisperse aero-
sols. The size, and sometimes the concentration, of the cali-
brating aerosol must also be known to a sufficiently high degree
of accuracy. The quality of the calibration is often dependent
upon the quality of the calibrating aerosol used and the accuracy
to which the particle size and concentration are known. Figure
1 shows the size range of some of the commonly used aerosol size
distribution measuring instruments, and the size range and other
pertinent characteristics of the monodisperse aerosol generation
techniques that can be used for instrument calibration.
AEROSOL GENERATION
Monodisperse Aerosol Generation Techniques Available Before 1970
Prior to 1970, the most commonly used techniques for generat-
ing monodisperse aerosols for instrument calibration and experi-
mental aerosol studies were the condensation aerosol generator
of La Her and Sinclair1 and the several variations of this gen-
erator as reported by Rapaport and Weinstock,2 Liu et al.,3 and
others;1* 6 and the spinning disc aerosol generator as originally
reported by Walton and Prewett7 and May,8 and subsequently by
others.9"13 These aerosol generators are capable of generating
aerosols of a moderate degree of monodispersity (og - 1.10).
However, both the size and the concentration of the aerosol must
be determined by an independent technique, such as electron or
optical microscopy. Therefore, the accuracy of the aerosol par-
ticle size and concentration is limited by the accuracy of the
size and concentration measuring techniques used, which is often
not very high due to the inherent limitation of the technique
itself, the tedious nature of the process involved, or the re-
quirement of high operator skill.
A relatively simple technique of generating a monodisperse
aerosol of a known size is that based on the use of monodisperse
suspensions of polystyrene and other latex spheres available
from Dow Diagnostics (P.O. Box 68511, Indianapolis, IN 46268).
These spheres are prepared by an emulsion polymerization process
261
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PARTICLE DIAMETER, ;um
0.001
0.01
0.1
10
100
AEROSOL
GENERATION
GENERATION
METHOD
CONDENSATION-
COAGULATION
ELECTROSTATIC
CLASSIFICATION
VIBRATING
ORIFICE
to
ag ~1.4
Sizing Accuracy 10%
Concentration Accuracy 10%
Maximum Obtainable
Concentration, particles/cm^ 10°
104
-1.05
2%
5%
106
105
,1.03
1%
5%
103
AEROSOL
INSTRUMENT
CONTINUOUS
MEASURING
DEVICE
CONDENSATION NUCLEI COUNTER
AND DIFFUSION BATTERY
OPTICAL PARTICLE COUNTER
ELECTRICAL AEROSOL ANALYZER
SAMPLING
DEVICE
INERTIAL IMPACTOR
Figure 1. Size range of aerosol generation, measuring, and sampling
devices.
-------
and are very uniform in size, with ag generally less than 1.06.
Both the size and the ag of the particles are determined by the
manufacturer by electron microscopy. Although these particles
are widely used in several applications, for example, in cali-
brating optical particle counters, there are several problems
associated with the use of these particles which are worth point-
ing out.
One problem results from the fact that the latex particles
in liquid suspension must be aerosolized, which is usually done
by means of an atomizer. During the atomization process, a large
number of empty water droplets are produced, along with those
containing the latex spheres. When these empty droplets are
evaporated, residue particles are formed from the dissolved im-
purity in the liquid. The residue particles are usually orders
of magnitude higher in concentration than the primary latex
spheres (see Whitby and Liullf; Langer and Lieberman15) . Thus,
the use of the latex aerosol for instrument calibration is limited
to those calibrations for which the presence of the residue par-
ticles is unimportant. For example, the latex aerosol can be
used for calibrating optical particle counters, which generally
do not respond to the residue particles. However, it cannot
be used to calibrate the electrical aerosol analyzer or the
condensation nuclei counter, both of which are sensitive to the
small residue particles present.
For a detailed discussion of the various aerosol generating
techniques including those described above, the reader is referred
to the review papers by Fuchs and Sutugin16 and Raabe.17
Modern Techniques for Generating Monodisperse Aerosols--
Generation of Monodisperse Aerosol Standards
A new generation of monodisperse aerosol generators has
been developed during the pa'st few years, primarily as a result
of research at the Particle Technology Laboratory, University
of Minnesota. The aim of this research was to develop generators
for producing monodisperse aerosols of a known size and/or con-
centration, which can be used as standards of calibration for
aerosol instruments and for general aerosol research. As a re-
sult of this research, solid and liquid aerosols of extremely
narrow size distributions can now be generated. Further, the
size and concentration of the monodisperse aerosol generated
can be calculated to a high degree of accuracy from the basic
generator operating conditions. The monodisperse aerosols gen-
erated by these devices can be referred to as "aerosol standards".
The particle size range and other pertinent characteristics of
these aerosol generators have been summarized in Figure 1, men-
tioned earlier.
263
-------
Vibrating Orifice Monodisperse Aerosol Generator —
For generating monodisperse aerosols in the size range from
0.5 ym to over 50 ym, a vibrating orifice monodisperse aerosol
generator has been developed by Berglund and Liu. The generator
is based on the instability and uniform break-up of a liquid
jet under periodic mechanical disturbance. The generator, shown
schematically in Figure 2, is composed of a droplet generation
and dispersion system and an aerosol dilution, neutralization,
and transport system.
To generate an aerosol with this system, the material to
be aerosolized is first dissolved in a suitable solvent, e.g.,
NaCl in water, or DOP or other oil in alcohol. The solution
is then supplied to the droplet generator by a syringe pump.
In the droplet generator, the liquid flows through a small (5,
10 or 20 ym diameter) orifice, which is vibrated by a piezoelec-
tric ceramic. The liquid jet is broken up by this vibration
into uniform droplets which are then quickly dispersed by a
turbulent air jet. The dispersed droplets are then mixed with
a much larger volume of filtered dry air to evaporate the solvent
from the solution droplets. The monodisperse aerosol formed
is then passed through a vertical drying column containing a
radioactive 85j(r source for neutralizing the particle electro-
static charge. The monodisperse aerosol is then ready for use.
The diameter, Dp, of the aerosol produced by the vibrat-
ing orifice generator can be calculated from the equation
Dp = (6QjiCvAf) (1)
where Q^ is the liquid solution flow rate through the orifice,
Cv is the volumetric concentration of the non-volatile solute
in the solvent, and f is the' vibrating frequency. The quantities
Q^ , Cv, and f can all be measured accurately. The overall un-
certainty of the calculated particle size is about 1%. Further,
since the rate of droplet production is equal to the vibrating
frequency, the theoretical particle concentration, Nfc^ , is given
by
Nth = f/Qa (2)
where f is the vibrating frequency and Qa is the total air flow
in the drying column. The actual concentration is slightly less
than the theoretical concentration because of particle loss in
the system. When the losses are properly determined, the genera-
tor can be used as a secondary concentration standard with un-
certainty of about 5%. The operating conditions of the generator
are summarized in Table 1.
264
-------
DROPLET GENERATOR DETAIL
DISPERSED
DISPERSION DROPLETS OR|RCE
ORIFICE ^
-------
TABLE 1. CHARACTERISTICS OF THE VIBRATING ORIFICE MONODISPERSE AEROSOL GENERATOR
(MODEL 3050 BERGLUND-LIU MONODISPERSE AEROSOL GENERATOR,
TSI, INC., P.O. BOX 3394, ST. PAUL, MN 55165)
Diameter of liquid Nominal frequency, Droplet diameter, Particle diameter, Nominal concentration,
orifice, urn kHz yma range, ymb particles/cm3c
5 450 15 0.6 - 15 273
^ 10 225 25 1.0 - 25 137
CTl
^ 20 60 40 1.8 - 40 36
a
Continuously adjustable over an approximate 25% range by varying the frequency.
b
Obtainable by the solvent evaporation technique.
Theoretical concentration based on the nominal aerosol output of 100 liters per minute.
-------
according to electrical mobility. The generator, shown schemati-
cally in Figure 3, consists of an atomizer, a diffusion dryer,
a 85Kr "neutralizer", a differential mobility analyzer, and an
electrometer current sensor.
To generate a monodisperse aerosol with this system, a poly-
disperse aerosol is first produced by spraying a liquid solution
with the atomizer and drying the particles with the diffusion
dryer. The aerosol is then passed through the 85Kr "neutralizer"
to obtain a Boltzmann equilibrium charge on the aerosol particles,
This aerosol is then classified electrostatically by the dif-
ferential mobility analyzer.
2 2
The differential mobility analyzer, described in detail
by Liu and Pui20 and by Knutson and Whitby,23 is in the form
of two concentric metal cylinders. Aerosol and clean air are
introduced into the apparatus near the top and flow down the
annular gap space between the cylinders in the form of laminar
streams. A dc voltage is applied to the inner cylinder—the
collector rod—and the outer cylinder is grounded. The applied
dc voltage causes particles of the opposite polarity to be at-
tracted to the collector rod, and those that have the correct
mobility, and hence the particle size, will be attracted to the
ROTAMETER
COMPRESSED AIR
DRYER
ABSOLUTE
FILTER
H.V. POWER
SUPPLY
D-
LINEAR
FLOWMETER
SILICA
GEL
ROTAMETER |
DILUTION
DRY AIR
DIFFERENTIAL
MOBILITY ANALYZER
EXCESS
AIR
r<*3—B—'
MONODISPERSE
AEROSOL
|
-------
vicinity of the slit where they are swept out by the small air
stream flowing through the slit. The mean electrical mobility
of the particles extracted by the device, Zp, can be calculated
from the operating conditions and the dimensions of the analyzer
by the equation
q + -r
^/~i y
•'c
V L
where qc is the flow rate of clean sheath air in the mobility
analyzer, qa is the flow rate of the polydisperse aerosol enter-
ing the analyzer, qs is the flow rate of the monodisperse aerosol
leaving the analyzer, r^ and r2 are the inner and outer radii
of the annular gap space in the analyzer, L is the length of
the collector rod between the aerosol entrance and the exit slit,
and V is the applied voltage on the collector rod. Knowing the
aerosol mobility from the above equation, the particle diameter,
Dp, can then be calculated as follows:
DP ' T-TZ;
where e is the elementary unit of charge, C is the slip correc-
tion for the particle, and y is the gas viscosity. An accuracy
of 2% in the calculated particle diameter can be achieved upon
carefully measuring the flow rates, the voltage, and the dimen-
sions of the mobility analyzer.
In addition to providing a standard for particle size, the
mobility classifier also provides a standard for particle con-
centration. This is based on the fact that the particles leaving
the analyzer are predominantly singly charged. By sampling these
particles into a Faraday cup and measuring the corresponding
current flow, I, the particle concentration, N, can be calculated
as follows:
N = (I / e qe) K (5)
where qe is the aerosol flow rate into the Faraday cup (the elec-
trometer current sensor shown in Figure 3) and K is a factor
near unity to correct for the presence of multiply charged par-
ticles in the output aerosol stream. Using a high sensitivity
electrometer (Gary 401 vibrating reed electrometer, Varian In-
strument Division, 611 Hansen Way, Palo Alto, CA 94303), currents
as small as 10~~16 A can be measured. Using the standard aerosol
flow rate of qe = 22 £pm, particle concentrations as low as 2
particles/cm3 can be determined. The accuracy in the measured
particle concentration, after making a small correction for the
interference effect of multiply charged particles (see the last
column of Table 2 for the value of the correction factor, K) ,
is about 5%. Other characteristics of the aerosol generator'
are summarized in Table 2.
268
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TABLE 2. CHARACTERISTICS OF THE MOBILITY CLASSIFIER GENERATOR FOR
PRODUCING SUBMICRON AEROSOL STANDARDS
(MODEL 3071 ELECTROSTATIC CLASSIFIER,
TSI, INC., P.O. BOX 3394, ST. PAUL, MN 55165)
Output
Diameter ,
ym
0.013
0.018
0.024
0.032
0.042
0.056
0.075
0.10
0.13
0.18
0.24
0.32
0.42
aerosol
Nominal
maximum
concentration,
cm~3
5,000
10,000
50,000
150,000
300,000
600,000
600,000
80,000
80,000
100,000
200,000
150,000
40,000
Input
Material
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
OOP
OOP
OOP
OOP
OOP
OOP
aerosol
Solution
concentration,
% by wt
0.005
0.01
0.05
0.1
0.5
1.0
1.0
0.01
0.04
0.1
0.6
0.8
1.0
Operating conditions
mobility analyzer
Plow
rate,
qc £pmb
20
20
20
20
20
20
20
20
20
7
7
7
7
Collector Rod
Q
voltage , V
U of M
65
123
215
373
624
1064
1807
2994
466C
2750
4243
6381
9174
TSI
71
134
234
406
680
1159
1968
3260
5075
2995
4620
6949
9990
of
K
factord
1.0
0.99
0.98
0.96
0.93
0.90
0.86
The input aerosol to the generator is obtained by atomizing an NaCl solution at
the indicated concentration with a Collison atomizer operating at 241 kPa, or
in the case of the DOP aerosol, obtained by the use of the vaporization-condensa-
tion technique described by Liu and Lee.21* ag - 2 for the NaCl aerosol and
Cg - 1.2 for the DOP aerosol.
qc is the clean sheath air flow in the mobility analyzer. Other flows are:
qa = qs - 0.1 qc, where qa = flow rate of the polydisperse aerosol at input and
qs = flow rate of the output monodisperse aerosol.
The collector rod voltage given is based on the following dimensions:
U of M prototype: r: = 0.950 cm, r2 = 1.907 cm, L = 45.52 cm
TSI Model 3071: r1 = 0.937 cm, r2 = 1.958 cm, L = 44.44 cm
and assuming T = 21°C and P = 1 atmosphere.
K is a correction factor to account for the effect of doubly charged particles in
the calibration of condensation nuclei counters. K is the ratio of the actual
particle concentration and the nominal concentration calculated by assuming all
particles are singly charged.
269
-------
In order to obtain a monodisparse aerosol with the above
stated accuracies in particle size and concentration, several
conditions must be met:
(1) The number median diameter of the polydisperse aerosol
at the classifier input should be kept equal to or
smaller than the desired diameter of the output mono-
disperse aerosol.
(2) The 85Kr "neutralizer" should have sufficient radio-
activity and neutralizing volume to insure that the
input aerosol is at Boltzmann charge equilibrium.
(3) For generating particles with diameter larger than
0.08 ym, the a of the input aerosol should not exceed
1.4. g
(4) The flow must be laminar in the mobility analyzer,
and the flow ratio, (q +qj/q^f should be kept small.
a. S G
(5) Conducting tubing should be used to transport charged
particles in the monodisperse output aerosol stream
and, in addition, the charged particles must be neu-
tralized before being introduced into the test instru-
ments.
Conditions (1), (2), and (3) are necessary in order to re-
duce to a minimum the concentration of the multiply charged par-
ticles of a larger particle size that have the same electrical
mobility as the primary singly charged particles.
Condition (1) can be met by using the proper solution concen-
tration in the atomizer for generating the input polydisperse
aerosols. The solution concentrations to be used with the Col-
lison atomizer to obtain the indicated number median diameters
are shown in Table 2. In no case should a more concentrated
solution be used, since this would lead to a higher proportion
of multiply charged particles in the output aerosol.
In the case of condition (2), the criteria described by
Liu and Pui22 can be followed to design the charge neutralizer
to obtain Boltzmann equilibrium on the input polydisperse aerosol.
In the case of condition (3), moderately monodisperse aero-
sols, such as those produced by the condensation aerosol gen-
erator can be used.21*
Condition (4) is necessary in order to reduce the range
of mobility, and consequently the range of particle diameters
passing through the classifier. To maintain laminar flow in
the analyzer, the critical Reynolds number should not be exceeded
and the flow rates qa and qc must have the proper ratio.
270
-------
Condition (5) is necessary in order to avoid particle pre-
cipitation due to charge build-up on insulating tubing. Pre-
ferably, metal tubing should be used to transport the charged
particles. If plastic tubing is to be used, the tubing should
be rinsed with a detergent or glycerine solution in order to
form a conductive layer on the inside surface.
Generation of Ultrafine Monodisperse Aerosols—
An aerosol generator is currently under development at the
Particle Technology Laboratory for producing monodisperse aerosol
below 0.01 ym in particle diameter. '26 The generation method
is based on the rapid condensation of NaCl vapor in a cold air
stream and the controlled coagulation of the resulting ultrafine
aerosol. The aerosol generator, shown schematically in Figure
4, consists of two major components: the droplet generation-
vaporization section and the aerosol condensation-coagulation
section.
To generate an aerosol with this system, an NaCl solution
of known concentration is atomized with the liquid atomizer to
form a polydisperse spray. The spray is then passed through
the diffusion dryer to evaporate the water from the droplets.
The aerosol then enters the evaporating tube in which the NaCl
particles are vaporized. The vapor is then injected into the
mixing nozzle, where it encounters a cold, primary dilution air
supplied to the nozzle from a filtered, dry compressed air source.
The rapid mixing of the vapor and the primary dilution air causes
a fine NaCl aerosol to be formed. This aerosol is then allowed
to coagulate and grow under controlled conditions. It is sub-
sequently mixed with a large volume of secondary dilution air
to "freeze" the particle size distribution and to transport the
aerosol to the testing instruments. Particles as small as 0.003 vim
have been generated with this system.
Aerosol Generators of High Stability
Several aerosol generators of high stability have been de-
veloped for use in a variety of aerosol studies. These include
the constant output atomizer21* for filtration, electrostatic
precipitation and other laboratory aerosol studies, the sulfuric
acid aerosol generator27 for studying the health effect of sul-
furic acid aerosol particles, and the constant-feed fluidized-
bed dust generator28 for generating coal dust and other solid
particles for use in instrument calibration and other studies.
Only the constant output atomizer described by Liu and Lee21t
will be further considered below.
The constant output atomizer is shown schematically in
Figure 5. The generator makes use of a syringe pump to provide
a constant flow of liquid to a pneumatic atomizer to achieve
a stable aerosol output. In the conventional atomizer, the
271
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DIFFUSION!;
DRIER
COLLISOIM
ATOMIZER
LINEAR
FLOWMETER
0-3 £pm 0-1 £pm TUBE
FURNACE
MIXING
'•7753 CHAMBER
Kr-85
NEUTRALIZER ^ ^ABSOLUTE
VALVETEXCESS FILTER
AIR
DILUTION
SYSTEM
°| ROTAMETER
0-30 gpm
COMPRESSED
AIR
ABSOLUTE
FILTER DRIER I FILTER
PRESSURE
REGULATOR
Figure 4. Schematic diagram of system for generating ultrafine
monodisperse aerosols.
HEATING TAPE (192W AT 115V)
ASBESTOS TAPE INSULATION
PYREX GLASS TUBE (19 mm I.D.)
ABSOLUTE
FILTER
DRY
COMPRESSED
AIR PRESSURE
REGULATOR
•j- ML i t
—CHS
SCREEN (40 MESH)
115V
VARIABLE TRANSFORMER
EXCESS AEROSOL
VALVE
MIXING NOZZLE
SYRINGE PUMP
EXCESS
LIQUID
RESERVOIR
ROTAMETER
0-100 Cpm
VALVE
DILUTION AIR
KRYPTON-85
CHARGE
NEUTRALIZER
AEROSOL OUT
Figure 5. Schematic diagram of the constant output atomizer and
condensation aerosol generator.
212
-------
solution being atomized is recirculated and, due to solvent
evaporation, the liquid in the atomizer reservoir becomes more
concentrated with time, causing the output particle size to
increase. In the present device, the liquid is not recirculated.
The output particle size consequently remains very stable. The
device can also be used to generate monodisperse aerosol of high
stability. With the addition of a heater and condenser, mono-
disperse DOP (di-octyl phthalate) aerosols in the 0.03 to 1.3
pro diameter range can be generated.
Particle Size Accuracy of Monodisperse Aerosol Generators
Among the several aerosol generation techniques discussed
above, three are capable of producing particles with a high
degree of accuracy in the generated particle size. The mono-
disperse aerosbls from the vibrating orifice generator, the
mobility classifier generator, and latex suspensions all have
a stated accuracy in particle size of better than 2%. Therefore,
it would be of interest to see whether such accuracy in size
can be realized in actual practice.
Two types of comparisons have been made in our Laboratory.
In one comparison, monodisperse aerosols of DOP were generated
by the vibrating orifice generator and their size determined
by the differential mobility analyzer using an optical particle
counter as the particle detector. The size of the particles
was also calculated from the generator operating conditions using
equations (1) and (4). This comparison will show whether the
vibrating orifice generator and the differential mobility analyzer
were operating with the stated accuracy.
In a second comparison, monodisperse aerosols of polystyrene
latex (PSL) were generated by aerosolizing the monodisperse latex
suspensions using the usual spray-drying technique (see Whitby
and Liu14). These latex suspensions were sized by their manu-
facturer (Dow Diagnostics, P.O. Box 68511, Indianapolis, IN
46268) by electron microscopy. By measuring the size of the
latex aerosols with the differential mobility analyzer, and com-
paring it with the size supplied by the manufacturer, one can
obtain some ideas as to the reliability of the particle size data
supplied by the manufacturer.
The results of the comparisons are shown in Table 3. We
found good agreement between the vibrating orifice and the mo-
bility classification methods. The maximum difference between
the calculated particle diameters based on these two methods
was 1.9%. This difference is well within the combined uncertain-
ties of these two independent methods.
A larger discrepancy was found between the sizes of the
polystyrene latex particles supplied by Dow Diagnostics and that
determined by the differential mobility analyzer, the maximum
273
-------
NJ
TABLE 3. COMPARISON OF THE PARTICLE DIAMETER MEASURED BY THE DIFFERENTIAL MOBILITY
ANALYZER WITH DIAMETERS GIVEN BY THE MANUFACTURER (PSL PARTICLES) AND THOSE
GIVEN BY THE VIBRATING ORIFICE MONODISPERSE AEROSOL GENERATOR
Particle diameter, ym
Aerosol
DOP
DOP
DOP
PLS (LS-1010-E)
PSL(LS-061-A)
PSL(LS-1020-E)
PSL(LS-1029-E)
PSL(LS-1117-B)
PSL(LS-1166-B)
Generation method
Vibrating orifice
Vibrating orifice
Vibrating orifice
Spray-drying
Spray-drying
Spray-drying
Spray-drying
Spray-drying
Spray-drying
Given
by Dow Co.
—
—
—
0.357
0.365
0.481
0.500
0.79
1.10
Calculated
(vibrating orifice)
0.890
1.094
1.701
—
—
—
—
—
—
Measured
(differential mobility)
0.888
1.115
1.695
0.338
0.345
0.517
0.528
0.820
1.182
% Difference
+0.27
-1.9
+0.35
+5.6
+5.8
-6.9
-5.3
-3.7
-6.9
-------
difference being 6.9%. A large part of the discrepancy can
probably be attributed to the error in the particle size data
of the latex suspensions supplied by the manufacturer. Several
investigators have found similar discrepancies in the particle
sizes supplied by Dow and the sizes determined by their indepen-
dent techniques. Table 4 summarizes these findings. It is in-
teresting to note that most techniques give particle sizes which
deviate from the sizes given by Dow Chemical Co. in the same
direction (either larger or smaller) as that given by the mobility
classification method. Further, the average of the sizes given
by these investigators agrees with the size determined by the
mobility classification method to about 2%.
Some of the frequently mentioned causes for the possible
sizing errors by electron microscopy are:
(1) Due to the use of emulsifier in the latex suspension,
a thin layer of the emulsifier is usually present on
the surface of the particles. The thickness of the
emulsifier shell would vary with the amount of dilution
used in preparing the suspension.
(2) The particles, upon continuous electron bombardment
in the microscope, have been found to change in size-
Nevertheless, a few percent error in size determination
for the PSL particles is usually not too serious for most appli-
cations. Consequently, the nominal particle size supplied by
the manufacturer can be used in these cases. When more accurate
particle size data are needed, the actual particle size should
be determined by an appropriate technique capable of giving the
required accuracy.
CALIBRATION OF AEROSOL MEASURING AND SAMPLING INSTRUMENTS
A variety of aerosol measuring and sampling devices have
been calibrated by means of the monodisperse aerosol standards
described above. These include the condensation nuclei counter,
the diffusion battery, the electrical aerosol analyzer, the opti-
cal particle counter, and the inertial impactor. The calibra-
tion studies have been reviewed in several papers contained in
the book, Fine Particles: Aerosol Generation, Measurement, Sam-1
pling, and Analysis, edited by Liu.36 Therefore, only a few
of these calibration studies will be reviewed here to illustrate
the application of the monodisperse aerosol standards and the
precautions in the use of the various aerosol generators for
instrument calibration and evaluation studies.
Condensation Nuclei Counter
Several commercial condensation nuclei counters have been
calibrated by means of the mobility classifier monodisperse aero-
sol generator and the calibration system shown in Figure 3.
275
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TABLE 4. DIAMETERS OF PSL PARTICLES GIVEN BY THE MANUFACTURER (DOW DIAGNOSTICS)
AND THOSE MEASURED BY DIFFERENT INVESTIGATORS
PSL I.D.
LS1010-E
LS061-A
LS1020-E
LS1029-E
LS1117-B
LS1166-B
Dow
Co.
0.357
0.365
0.481
0.500
0.79
1.10
Mobility
classification
0.338
0.345
0.517
0.528
0.820
1.18
Average
of values
on right
0.334
0.340
- - -
0.516
_ _ _
- - -
Particle diameter, ym
Electron microscopy Light scattering Centrifuge Optical array
(1) (2) (3) (4) (5) (6) (7)
0.33 0.318 0.354
0.336 0.342 0.339 0.343
0.488
0.50 0.499 0.55
Investigators:
(1) Heard et al.29
(3) Davidson and Haller30
(5) Dezelic and Kratohvil31
(2) Porstenderfer and Heyder
(4) La Mer and Plesner3"*
(6) Stbber and Flachsbart35
33
(7) Davidson and Collins
32
-------
In this system, the particle concentration can be varied continu-
ously by varying the ratio of aerosol and clean air flows. A
critical orifice in a vacuum line is used to draw a portion of
the flow (3 fcpm) through the electrometer. The remaining aerosol
flow (20 Apm) is then available for delivery to the nuclei coun-
ter being calibrated.
Due to the pulsating inlet flow common for most nuclei
counters, care must be taken to insure that the flow rate of
the calibrating aerosol is larger than the peak suction flow
in the counter. Further, in order to minimize precipitation
of particles in the sampling line and within the instrument due
to the charge on the particles, the test aerosol should be neutral-
ized before being introduced into the counter. The particle
loss inside the neutralizer should be evaluated and taken into
account in the concentration calculation.
Figure 6 shows the calibration results for an Environment/
One condensation nuclei counter.20 The response of the counter
is seen to be linear at concentration levels only up to 130,000
particles/cm3, above which the response becomes non-linear.
Further, the indicated concentration of the counter is lower
than the true concentration by a factor ranging from 2.5 in the
lower linear range to 5.3 at the upper concentration limit,
600,000 particles/cm3, used in the calibration.
Since all commercial nuclei counters have been referenced
by their manufacturer to the photoelectric counter of Pollak,37
the above result suggests that the Pollak counter calibration
may be in error. However, in a subsequent study,38 a Pollak
counter was evaluated with the mobility classifier aerosol gen-
erator. Figure 7 shows the results of the calibration study.
The particle concentration determined by the Pollak counter using
the original Pollak calibration is seen to be in good agreement
with that determined by the mobility classification method, the
maximum discrepancy being 17% at the upper concentration limit
of 250,000 particles/cm3 used in the study. The source of error
in the Environment/One counter calibration, therefore, must be
attributed to the calibration error made at the factory.
The response of the condensation nuclei counters has also
been evaluated as a function of particle size. The result for
/the case of the General Electric counter is shown in Figure 8.25
The counting efficiency (ratio of indicated concentration to
input aerosol concentration) of the GE counter is found to drop
rapidly below 0.02 um, reaching a value of 50% at a particle
diameter of 90 A.
Similar studies on the counting efficiency of the GE and
Environment/One counters by Cooper and Langer39 have resulted
in a similar finding. The study by Bricard et al.*0 on the con-
tinuous flow counter using alcohol as the working fluid also
277
-------
CMC NO. 1 (ENVIRONMENT/ONE, SERIAL NO. 149)
0
50 100 150 200 250 300 350 400 450 500 550
ACTUAL NUCLEI CONCENTRATION, 103 (PARTICLES/cm3)
600
Figure 6. Response of the Environment/One, Rich WO condensation
nuclei counter to monodisperse aerosols produced by the
mobility classifier aerosol generator.
gives similar results. Recently, Porstendorfer and Soderholm
have also found the size dependence of the Environment/One count-
er using a photographic counting-diffusion battery technique. **l
The cause of this non-ideal behavior has not yet been identified
at the present time, but it seems that this non-ideal behavior
is an inherent characteristic of most of the automatic condensa-
tion nuclei counters.
Diffusion Battery
The mobility classifier aerosol generator has also been
used for calibrating the diffusion batteries described by Sin-
clair et al."2'1*3 In the first calibration study, the diffusion'
battery comprised of the "collimated holes structures" was used.
These are metal discs with uniform size holes. Since the number,
the diameter, and the length of these uniform tubular holes can
be measured accurately with a microscope, the penetration of
a monodisperse aerosol of a known particle size through the bat-
tery can be predicted from the diffusion theory. In the cali-
bration study, monodisperse aerosols of various diameters were
generated by the mobility classifier generator, and the particle
penetration through the battery was measured with a condensation
278
-------
co
UJ
o 100
t
a.
Z
o
a.
10
z
UJ
O
O
o
8
oc
UJ
0.1
I I I I I I I
I I
I I I I
-o
O 0.05 urn NaCI
• 0.088 jum NaCI
A 0.032 jum NaCI
A 0.025 /L/m NICHROME
O 0.032 Aim NICHROME
• 0.11 ,um NaCI
O 0.15 jum NaCI
IDEAL
RELATIONSHIP
ACTUAL
CALIBRATION
Np = 0.971xN£
0.972
I I I I I 111
0.1 1 10 100
AEROSOL CONCENTRATION(ELECTRICAL), NE, 103 PARTICLES/cm3
Figure 7. Calibration of the Pollak counter with the mobility
classifier aerosol generator.
279
-------
1.0
0.9
0.8
z
2 0.7
cc 0.6
Z
0.5
0.4
0.3
0.2
0.1
0.0
GECNC
CAT 112L428G1
NO 3264181
O SINGLY CHARGED PARTICLES
A NEUTRALIZED PARTICLES
0.00 0.01 0.02 0.03 0.04 0.05 0.06
PARTICLE DIAMETER, jum
0.07
0.08
0.09
Figure 8. Counting efficiency of the General Electric condensation
nuclei counter as a function of particle size.
nuclei counter. Figure 9 compares the particle penetration
determined experimentally with that predicted from theory. The
agreement is seen to be very good in the tested size range from
0.024 ym to 0.110 urn. The good agreement constitutes a direct
verification of the theory of diffusion battery.
Electrical Aerosol Analyzer
The electrical aerosol analyzer is an aerosol size distribu-
tion measuring device operating on the principle of unipolar
diffusion charging and mobility analysis. •* ** The analyzer operates
by first placing a unipolar charge on the aerosol being measured,
and measuring the resulting mobility distribution of the charged
particles by means of a mobility analyzer. Although the charge
acquired by an aerosol particle under a specific set of charging
conditions can be calculated theoretically using the available
charging theories,^5 the uncertainties in the theoretical results,
particularly that caused by the particle loss in the instrument,
require that the instrument be calibrated. In the calibration
study reported by Liu and Pui,41* the mobility classifier gen-
erator was used to generate monodisperse aerosols of a known
size and concentration for the study.
Some sample results from the calibration are shown in Fig-
ure 10. The figure shows the empirical voltage-current for the
electrical aerosol analyzer upon exposure to monodisperse aerosols
280
-------
BATTERY NUMBER
9 10
O
UJ
Q.
Aim DIAMETER
Aim DIAMETER
Aim DIAMETER
0375 Aim DIAMETER
Aim DIAMETER
2345
EQUIVALENT LENGTH, km
Figure 9. Comparison of theoretical (solid lines) and experimental
penetrations through the Sinclair diffusion battery using
monodisperse aerosols from the mobility classifier generator.
281
-------
D
§
X
LL
O
1-
z
111
DC
OC
U
OC
UJ
O
OC
U
SMOG I* Dp = 0.0105 urn
Dp = 0.0125
0.018
0.024
0.032
0.042
0.056
0.075
70V
1000
10000
100000
COLLECTOR ROD VOLTAGE, V
Figure 10. Response of electrical aerosol analyzer to monodisperse
aerosols.
of various sizes. These curves can be used to construct a cali-
bration matrix which can be used in a data reduction algorithm1*6
for the inversion of instrument data to obtain the original size
distribution.
Optical Particle Counter
Several commercial optical particle counters have been eval-
uated by means of the latex aerosols and aerosols produced by
the vibrating orifice generator.1*7 Since the response of the
optical counter is a function of the size, the refractive index,
and the shape of the measured particle, a variety of test aero-
sols is needed in order to determine the instrument response
empirically. The ability of the vibrating orifice generator
to generate aerosol from a variety of materials has made this
an ideal generator for studying the response of optical counters.
Materials with a variety of refractive indices, such as the
Cargille index-of-refraction liquids, methylene blue, sodium
chloride and India ink, may be dissolved or diluted in a suitable
solvent and used in the generator to produce aerosols with the
desired optical properties.
282
-------
Some typical results for the Royco 220 particle counter
are shown in Figure 11. The response of the counter is shown
as a function of particle size and particle refractive index.
Such information can be used for determining the effect of the
refractive index on the measurement accuracy of the instrument.
Calibrations of the optical particle counters using irregular
particles have also been performed. In one study, coal dust
particles obtained by aerosolizing a finely ground coal powder
with the fluidized bed were used. 8 The polydisperse coal aero-
sol generated by the fluidized bed aerosol generator was passed
through a differential mobility analyzer to extract a monodis-
perse fraction, and the resulting monodisperse aerosol was then
applied to the optical counter for calibration. In another
study, inertial impactors of various cut sizes were placed at
the inlet of the optical counter and the response of the optical
counter was determined, from which an "aerodynamic" calibration
could be obtained.1*9
Inertial Impactor
The inertial impactor has found widespread use in aerosol
studies because of its simplicity and well defined operational
characteristics. Various designs of impactors are available
commercially for different applications.50'51 An extensive theo-
retical analysis of the inertial impactor has been performed
by Marple52 and Marple and Liu.53'5% Although the impaction
efficiency can now be accurately predicted by theory, experi-
mental evaluation still must be made in order to characterize
the real impactor behavior, including problems due to particle
reentrainment and bounce.
Many impactors have been calibrated by means of the vibrat-
ing orifice monodisperse aerosol generator. Two different ap-
proaches can be used in the calibration. In one approach, an
optical counter is used as a particle detector. By counting
the particles upstream and downstream of the impactor, the col-
lection efficiency can be obtained. In another approach,the
particles are tagged with a fluorescent tracer, and the particles
deposited on various stages of the impactor, including the after
filter, are measured fluorometrically, from which the impaction
characteristics are determined. This latter technique is especi-
ally useful for evaluating the multistage impactors, since the
wall losses can also be evaluated.
Figure 12 shows the calibration results for a single-stage
impactor and a two-stage impactor consisting of two identical
impaction nozzles.1*9 The experimental results are seen to agree
well with the theoretical prediction. The monodisperse aerosols
from the vibrating orifice generator have also been used to eval-
uate the Lundgren impactor, the Andersen impactor, and the Sierra
impactor55"57 as well as the virtual impactor.51 Using the latex
283
-------
10
o
QC
W
Z
D
O
O
.1
_ j I 11II I I I I
REFRACTIVE INDEX, m
O 1.6
• 1.6 (PSD
A 1.4
6 ~
EXPERIMENTAL
m = 1.49
I I ml
ROYCO PC 220
I I 1 I I Mil
1 10
PARTICLE DIAMETER, jum
Figure 11. Experimental calibration curve for the Royco 220 optical
particle counter.
aerosols, Rao and Whitby have also studied the particle bounce
phenomena inside impactors.5tf>59
Comparative Studies Between Aerosol Measuring Instruments
In the preceding sections, we have discussed the calibration
of various aerosol measuring instruments with monodisperse aero-
sol standards. If the instruments are properly calibrated and
the data properly analyzed, different instruments should give
the same measurement results when they are used on the same
aerosol. Such comparative studies involving the simultaneous
aerosol measurement of the same aerosol by different measuring
instruments are the subjects of this section.
The electrical aerosol analyzer (EAA) is capable of measur-
ing particle size distribution in the 0.005 to 1.0 urn diameter
284
-------
HI
u
2
HI
u
LL
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
—I 1 1 T
S/W = 1.4
Re = 850
T/W = 7
THEORETICAL
A - SINGLE
B - DOUBLE
UNIFORM PARTICLE
DISTRIBUTION
ACROSS ENTRANCE
C- DOUBLE
PARTICLES IN CENTRAL!
80% OF FLOW
AT ENTRANCE
EXPERIMENTAL
O SjumVOMAG ORIFICE
D 10 pm VOMAG ORIFICE
I
I
I
I I
0.1
0.2
0.3 0.4
0.5
0.6
0.7
0.4 0.5 0.6 0.7
Figure 12. Comparison of theoretical and experimental efficiency
curves for two impactors.
range. The measured distribution can be integrated to yield
the number and volume concentrations. Therefore, the EAA may
be compared with a condensation nuclei counter (CNC) and a gravi-
metric analysis technique for number and volume concentration
determinations. Such comparisons have been made, the detail
of which will be described in a forthcoming paper by Mulholland
et al.60 Some preliminary results of this study are described
below.
For number concentration determinations, polydisperse aero-
sols of NaCl,(NH^)2SO^, DOP, and sucrose were generated by the
constant output atomizer and supplied to two EAA's and a GE CNC.
In Figure 13, the total number concentrations measured by.the
EAA's are compared with that measured by the CNC. It is seen
that good agreement is obtained between the two different measur-
ing techniques. It should be mentioned that the CNC had been
calibrated several months prior to the experiment, and that the
285
-------
IDEAL RELATIONSHIP
13 10 30
AEROSOL CONCENTRATION (CNC), NC, 103 PARTICLES/cm3
Figure 13. Comparison of BAA and CNC for total number concentration
measurements obtained in the first experimental series.
100
286
-------
EAA concentrations were calculated by the simplified data reduc-
tion procedure of Liu and Pui"*1* using the instrument sensitivi-
ties for monodisperse aerosols.
In the second series of experiments, the CNC was recali-
brated, and a simplex minimization program of Liu and Kapadia61
was used for reducing the EAA data. The results of the compar-
ison are shown in Figure 14. Very good agreement is obtained,
the maximum discrepancy being about 10%.
CO
I
111
m
o
o
Z
O
Z
o
100
50
111
o
o
u
_l
o
CO
o
cc
10
I I I I I
DATA REDUCTION TECHNIQUE
O POLYDISPERSE SENSITIVITY (LIU & KAPADIA, 1977)
• MONODISPERSE,SENSITIVITY (LIU & PUI, 1975)
IDEAL RELATIONSHIP
5 10 50
AEROSOL CONCENTRATION (EAA), NE, 103 PARTICLES/cm3
100
Figure 14. Comparison of EAA and CNC for total number concentration
measurements obtained in the second experimental series.
287
-------
SUMMARY AND CONCLUSIONS
In this paper, we have reviewed the recent developments in
monodisperse aerosol generation. Some of the techniques developed,
such as the vibrating orifice generator and the mobility classi-
fier generator, are capable of generating monodisperse aerosols
of an accurately known particle size and concentration. These
aerosols can be referred to as "aerosol standards" since they
can be used as standards of measurement or calibration in the
field of aerosol science. The application of these monodisperse
aerosol standards to instrument calibration and evaluation has
also been briefly described and some sample results presented.
It is shown that with proper calibration of the instruments used,
aerosol measurement can be made with good accuracy as in many
other fields of science and technology.
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292
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PAPER 13
SUBSTRATE COLLECTORS FOR CASCADE IMPACTORS
AN EVALUATION
D. BRUCE HARRIS
PROCESS MEASUREMENTS BRANCH
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY-RTP
U.S. ENVIRONMENTAL PROTECTION AGENCY
AND
G. CLINARD, L.G. FELIX
G. LACEY, J.D. MCCAIN
SOUTHERN RESEARCH INSTITUTE
The use of cascade impactors to obtain particle size in-
formation on process streams has increased dramatically in the
past few years. The Process Measurements Branch (PMB) of EPA's In-
dustrial Environmental Research Laboratory-RTP has been involved
in investigating their performance and application for almost
a decade. Early in this effort we recognized the inherent po-
tential for error caused by the small sample weights on the heavy
metal collection stages being used in commercially available
impactors, and proposed and implemented the use of light weight
substrates. Many problems were met and resolved during the evo-
lutionary development of the methodologies of using cascade
impactors.
Substrates have generally been made of filter media (most
often fiber glass) and thin metal foils. An early study of sub-
strate usage was included in a test of a then-available commercial
impactor at a coal-fired power plant in 1973 and has been re-
ported in detail in EPA report "Field Measurements of Particle
Size Distribution with Inertial Sizing Devices," EPA-650/2-73-
035 (NTIS No. PB 226292). The advantages of substrates were
demonstrated and the method became the one recommended for IERL-
RTP use.
Weight loss problems were reported by some users of fiber
glass substrates. A detailed study of this problem was made
by Southern Research Institute under contract to EPA's PMB.
It found that most of the trouble was due to careless handling
and suggested procedures to be used to limit the errors to a
reasonable level. The detailed report is available: "Andersen
Filter Substrate Weight Loss," EPA-650/2-75-022 (NTIS No. PB
240720).
293
-------
Many investigators opted not to use filter media: instead,
thin metal foils, usually aluminum or stainless steel, were used
to solve the weight loss problem previously noted with fiber
glass. Many found that these solid metal surfaces had a problem
which had been observed with the original collection surfaces
supplied by the manufacturer — particles tended to bounce off
the collection surface. An obvious solution was to increase
the stickiness of the surface, usually by coating it with grease.
This appeared to solve much of the problem; however, a grease
that worked at one source might not work at another test.
In the meantime, people were reporting anomalous weight
gains with the fiber glass substrates, including variations with
temperature (Figure 1) and between various fiber glass filter
types (Figure 2). As the depth of this problem became evident,
EPA's PMB again asked SoRI to investigate the whole problem of
substrate use in cascade impactors. A detailed laboratory study
was conducted, utilizing many fiber glass filter media and metal
foil greases.
- COAL FIRED POWER BOILERS
0 PORTLAND CEMENT KILN
100
200 250
GAS TEMPERATURE. °C
300
Figure 1. Anomalous mass increases of Andersen glass fiber impact/on
substrates at different flue gas temperatures.
294
-------
1.5
1.0
Ol
o
V)
V)
0.5
100
• GELMAN TYPE A (OLD)
O GELMAN SPECTRO GRADE TYPE A
£ MSA 1106 BH
0 REEVE ANGEL 900 AF
CEMENT
PLANT
50 PPM
SO2
COAL FIRED POWER BOILER X
550 PPM SO2 X ^
X
X
X
X
200 300
TEMPERATURE, °C
400
Figure 2. Anomalous mass gains of various 47 mm diameter glass fiber filters
at different temperatures (60 minute samples at flow rates of 0.25 acfm).
295
-------
Many investigators had reported extraneous mass gains on
ambient filter media due to S02/S03 conversion on the filter
to sulfates. The laboratory screening tests were run to under-
stand what occurs during this mass gain and to select a material
sufficiently inert for use as instack impactor substrates. A
suitable material would be one which has stable low mass charac-
teristics and is mechanically strong enough to resist cutting,
tearing, and loss of material. Two laboratory test methods were
used: one employed a flow of gaseous S02 through the filter
media (Figure 3); the other involved soaking the material in
hot sulfuric acid solution. The results of the gas flow studies
are summarized in Table 1, which includes two materials which
had been soaked in sulfuric acid. One fiber glass material
(Reeves Angel 934AH) stands out for use as substrate media.
Since manufacturers sometimes change processes without changing
the media name, the substrates should be checked when received
for conformity with previous batches. A pH stability test, for
long term levels around 7, can be a good indicator.
Control of particle bounce and deposit scouring from metal
foil substrates has been demonstrated for a number of grease
coatings. The principal difficulty noted has been the deleterious
effect of elevated temperature on their physical and chemical
characteristics. Changes in viscosity sufficient to remove the
grease from the collection surface and chemical breakdown to
hard brittle masses have been noted.
A laboratory screening test was used, consisting of heating
each grease in an oven for several hours, followed by examination
of the material for changes in consistency, color, and mass.
Of the many materials tested, Dow Corning silicone and Apiezon
greases were the only ones to exhibit sufficient stability to
warrant field testing.
The field test was conducted at a coal-fired power plant.
Each impactor was loaded with the greased stages and the normal
backup filter. A Gelman 47 mm prefilter was attached to the
inlet to ensure that only gas (no particulate) would flow through
the impactor, which was horizontal in the stack.
The two Dow Corning greases (Molycote III and Silicone High
Vacuum) lost measurable weight from the stages and added weight
to the backup filter. It was decided that these did not have
the stability needed for instack use. The Apiezon tests are
summarized in Table 2. The only grease not to lose weight was
Apiezon H, which actually gained. Close examination of the
stages revealed a fine even layer of dust, probably deposited
during loading and unloading of the unit. Jet patterns were
barely visible on most stages and a uniform coating of grease
remained. No evidence of overflow was noted. An extended test
296
-------
w
to
vo
-J
SO2
AIR
PREHEAT
COILS
1
SAMPLE CONDITIONING
CHAMBER
OVEN
HUMIDIFIER
1
AIR-SO2 EXHAUST
HEATER TAPE
WATER HEATER
AIR FLOW DIRECTION
SO2 FLOW DIRECTION
AIR-SO2 MIXTURE
FLOW DIRECTION
Figure 3. Diagram of experimental set-up for filter substrate
conditioning experiment.
-------
TABLE L. MASS GAINS OF 47 mm GLASS FIBER FILTER SUBSTRATE MATERIALS
FROM LABORATORY CONDITIONING3
b
Material
Reeve Angel
934AH
Gelman AE
Gelman Spectro
Glass
Whatman
GF/A
Reeve Angel
934AH
Gelman AE
Gelman Spectro
Glass
Whatman GF/A
Reeve Angel
934AH
(Acid Washed)
Gelman AE
(Acid Washed)
Conditioning Mass Before Mass After
Batch Number of 47 mm Time Conditioning Conditioning
Number Filters Conditioned (hours) (grams) (cjrams)
3307 20
8204 20
8192- 20
20232
3563 20
3307 20
8204 20
8192- 20
20232
3563 20
4292 20
8206 20
26 2.1888 2.1881
26 2.6644 2.8735
26 2.6717 2.8160
26 1.7695 1.8349
26 2.2149 2.2166
26 2.6266 2.8375
26 2.6522 2.8051
26 1.7361 1.8272
18 2.0968 2.0975
18 2.6939 2.7699
Mass Gain
per Filter Mass Gain
(mg) (percent)
-0.04 -0.03
10.46 7.84
7.22 5.40
3.27 3.70
0.09 0.08
10.55 8.03
7.65 5.77
4.56 3.64
0.04 0.03
3.80 2.82
a. 1% S02, 3-5 ppm SO3, saturated H20, 220°C (428°F).
b. Filter materials are listed in order of conditioning in a stainless steel Alundum thimble holder.
-------
TABLE 2. SUMMARY OP FLUE GAS TEST RESULTS. THE FLUE GAS TEMPERATURE AVERAGED
ABOUT 149°C (300"F) FOR ALL TESTS, IMPACTOR FLOW RATE WAS 0.7 acfm
V£>
Substrate
Set
1
(Apiezon L)
Grease on
Substrate,
mg
Stage
0
1
2
3
4
5
6
Back-up
Blank
25.
13.
38.
11.
39.
12.
05
48
33
93
36
06
D
P
D
P
D
P
-
Mass Change
mg
+0.23
-4.96*
-7.29*
-27.98*
-0.64
-24.188
-0.35
b
+0.15
*
2
(Apiezon L)
Grease on
Substrate
mg
Blank
19.8
54.1
73.0
5.3
61.4
2.9
o.r
35
7
37
9
23
3
.23 D
.51 P
.24 0
.14 P
.23 D
.69 P
'
4a^s Change
ag
+0,
-3.
+0.
-28
+0.
-13
+0.
+0.
13
25a
33
.54*
15
.OS3
34
b
17
*
3
(Apiezon M)
Grease on
Substrate,
Mass Change
Jig nig
BUnk
9.2
4.4
71.3
1.6
44.6
3.9
£
0.2
21.
3.
19.
4.
26.
7.
69 D
19 t
03 D
36 P
09 D
10 P
__
i-O.
-0.
+0.
-7.
+0.
-10
-0.
+4.
08
02°
04
14°
10
,11°
05
a
40
4
_
0.1
1.3
37.5
2.3
38.8
0.7
f
3.1
4
(Apiezon T)
Grease on
Substrate, t
B:
23.
1,
3,
24,
24,
3,
tog
Lanfc
.63 D
.89 P
.25 P
.87 D
.78 D
.69 P
"
lass Change
mg
+0.
-1.
-0.
-0.
-0.
-1.
+0.
+5.
04
10°
25
21
61
07C
09
A
13
»
5
(Apiezon H)e
Grease on
Substrate
mg
Blank
4.6
13.2
6.5
2.5
4.3
2.4
f
3.6
41
4
3
28
30
4
.55 D
.18 P
.31 P
.04 D
.13 D
.87 P
-
, Macs
Change
mg %
+0.
+1.
+0.
K>.
+0.
+0.
-0.
+4.
01
56
82
80
44
53
05
•,
36
6
(Apiezon N)
Grease on
Substrate,
mg
Blank
3,
19
21,
1
1.
1,
3,
.8
.6
.0
.6
.8
.0
-
.0
19.19
4.41
19.00
3.37
18.79
4.14
"•
D
P
D
P
D
P
Mass
mg
+0
-1.
+0.
-1.
-0.
-1.
-0.
+4.
.03
52°
05
75
06
73°
10
74
Chan
1
.
8.
1.
9.
1,
9.
2.
3.
a. Greaae lost to overflow, not recovered.
b. Beeve-Angel 934AB Filter Material, preconditioned.
c. Some grease lost to overflow, most recovered and deposited back on foil.
d. Gelman AE Filter Material, preconditioned. Weight gains may be partially due to flue gas reactions.
e. Substrates contaminated with ambient dust, but probably not enough to alter results significantly.
t. Percent mass change in filter calculated using final filter weight.
Note: D means that the grease-toluene mixture was dropped on with a medicine dropper
(20 to 25 drops).
P means that the grease-toluene mixture was painted on with a small camel's hair brush
(3 to 4 times).
-------
program of Apiezon H was run at a coal-fired power plant: re-
sults are given in Table 3. As a result of these tests, Apiezon
H grease was approved for use in IERL-RTP programs for tempera-
tures up to 177°C (350°F).
The complete results of this study can be found in EPA
report "Inertial Cascade Impactor Substrate Media for Flue Gas
Sampling," EPA-600/7-77-060 (NTIS No. PB 276583).
This paper has briefly reviewed the development of the use
of collection substrates in cascade impactors. We will continue
to monitor operational procedures for impactors and recommend
techniques to satisfactorily use them; such as "Procedures for
Cascade Impactor Calibration and Operation in Process Streams,"
EPA-600/2-77-004 (NTIS No. PB 263623), currently being used by
IERL-RTP personnel and contractors.
TABLE 3. APIEZON H BLANK MASS GAINS FOR UNIVERSITY OF
WASHINGTON MARK III SOURCE TEST CASCADE IMPACTORS3
Stage
0
1
2
3
4
5
6
Filterb
Run 1
5/17/76
0.22
0.06
0.04
-0.03
0.02
-0.06
-0.14
-0.32
Run 2
5/17/76
0.08
0.09
0.01
-0.11
-0.08
-0.13
-0.12
-0.18
Run 3
5/19/76
0.16
-0.18
0.04
0.09
0.09
0.05
0.13
0.02
Run 4
5/20/76
0.55
0.11
0.10
-0.13
-0.05
-0.03
-0.05
0.08
Run 5
5/21/76
0.65
0.85
0.64
0.64
0.47
0.31
0.16
0.34
X
0.33
0.19
0.17
0.09
0.09
0.03
-0.004
-0.01
a
0.25
0.39
0.27
0.32
0.22
0.17
0.14
0.25
a. Coal-fired power boiler source, cold side precipitator
operating at 107°C (225°F), 30 minute runtime. All masses
in milligrams.
b. Reeve Angel 934AH 47 mm disc.
300
-------
PAPER 14
PARTICLE SIZE MEASUREMENT FOR THE
EVALUATION OF WET SCRUBBERS
SEYMOUR CALVERT
RICHARD CHMIELEWSKI
SHUI-CHOW YUNG
AIR POLLUTION TECHNOLOGY, INC.
INTRODUCTION
Under the sponsorship of the EPA, A.P.T. has performed per-
formance tests on several industrial scrubber systems to obtain
data on the fine particle collection efficiency of air pollution
control scrubbers. The scrubber performances in different situa-
tions are reported in terms of grade penetration curves and the
A.P.T. cut/power relationship.1 This information was used to
evaluate the scrubbers and to verify, improve and develop design
equations for the scrubber systems.
The measurement of the size of small wet particles presents
unique problems which were not entirely appreciated until recent
studies were specifically conducted towards these measurements.
This paper presents our experience with this problem including
possible errors due to evaporation of water from the particulate
as well as a single stage impactor design which has proved satis-
factory,
PARTICLE SIZE DISTRIBUTION MEASUREMENTS
The most important experimental measurements in the perfor-
mance tests are those regarding particle size and concentration
in the inlet and outlet of the scrubber. For accurate determina-
tion of particle size distribution, the measuring devices should
be able to measure the particles as they exist in the scrubber.
The devices should cause neither formation of particles nor break-
up of aggregates. Cascade impactors are the most commonly used
particle classifier in scrubber performance tests.
In a cascade impactor, particles are classified by inertial
impaction according to their mass. The larger ones are collected
301
-------
on the plate opposite the first stage and smallest on the plate
opposite the last stage. Cascade impactors are found to be use-
ful for measuring dry particle size distribution and concentra-
tion for particles with diameters from a few tenths to several
microns.
In the outlet of a scrubber the particles are usually wet.
Even though cascade impactors have been used for particle frac-
tionation at the scrubber outlet, there is doubt whether the
measured size distribution is representative of the wet particle
size distribution leaving the scrubber. This doubt surfaced
when we tried to measure the "grown" particle size distributions
in flux force/condensation scrubbers with cascade impactors.
Flux force/condensation (F/C) scrubbers utilize diffusio-
phoresis, thermophoresis and condensation of water vapor to im-
prove particle collection. A.P.T. has operated an F/C demonstra-
tion plant as well as several pilot plants.2 5 Particle growth
due to condensation of water vapor was demonstrated by the im-
proved performance of the scrubbers as well as by observation
of condensation clouds.
Wet Particles
Measurement of the grown particle size distribution with
multistage cascade impactors indicated some growth; however,
the measured size distributions were not as large as predicted.
The particles act as condensation nuclei and are predicted to
grow to a size which is dependent upon the amount of water con-
densed and the number of particles present. In typical applica-
tions the problem becomes one of measuring wet particles with
an aerodynamic diameter on the order of 1 ymA.
It was suspected that evaporation of water from particles
occurred inside the cascade impactor due to the pressure reduc-
tion from stage to stage. To study the evaporation from par-
ticles within the cascade impactor we used the experimental ap-
paratus shown in Figure 1. A 5% by weight sodium chloride solu-
tion was atomized using a Collison atomizer. The gas was mixed
with dilution air which passed through a conditioner which either
humidified or dried and heated the dilution air. The aerosol
was then passed through an impactor to obtain size distribution
data. A specific ion probe for Cl was used to determine the
mass of salt collected in these experiments.
The first part of the experiment involved drying the aerosol
generated by the atomizer by mixing it with dehumidified and
heated dilution air. The size distribution obtained was that
of the dry residual salt particles. Observation of the substrates
indicated that no moisture was present for these runs. Figure
2 shows the size distribution obtained for the dry particles.
302
-------
DILUTION AIR
CONDITIONER
ATOMIZER
PRESSURE *7
REGULATOR A
I
COMPRESSED
AIR
Figure 1. Sketch of experimental equipment.
303
-------
8.0
6.0
4.0
d
•o
of 2.0
HI
Q
UJ
g 1.0
OC
a 0.8
0.6
0.4
5% SALT SOLUTION
dpg = 1.7
O
0.2
10
20
40
WT%
-------
The size distribution of the parent wet particles can be
obtained from the size distribution of the residual salt par-
ticles since the initial salt concentration is known. The initial
drop diameter is related to the salt particle diameter by the
following equation:
dd = dp
where: d, = wet particle physical diameter, pm
d = salt particle physical diameter, ym
p = salt density, 2.16 g/cm3
c = salt concentration in solution, weight fraction
This equation is good for diluted salt solutions where the
density of the solution is close to that of pure water. The
predicted wet particle size distribution is shown in Figure 3.
Note that the aerodynamic diameter is shown and that conversion
to physical diameter is required for application of equation
(1). Also shown on this figure are lines which represent size
distribution which would exist if a given fraction of the water
were evaporated from each of the particles. The upper curve
shows the parent wet particle distribution while the lowest curve
represents the measured dry size distribution.
Wet Size Measurement
The second part of the experiment involved measurement of
the wet drop size distribution. For this case the dilution air
was first saturated in a fritted disk bubbler so that particle
drying would not occur. The results of several sampling runs
with the University of Washington cascade impactor are shown
in Figure 4. As can be seen, the wet size distribution, instead
of being close to the predicted parent wet particle size distri-
bution, is close to the size distribution of dried salt residues.
This indicated that evaporation from particles occurred. Since
the air was saturated with water vapor, evaporation from par-
ticles could not occur before the cascade impactor. Thus, it
could be deduced that evaporation occurted within the cascade
impactor.
To verify that evaporation did not occur before the cascade
impactor, a second series of experiments were conducted using
a series of glass impactors which allowed observation of the
jet and impaction plate. Figure 5 shows the experimental setup.
Each glass impactor consisted of one jet. By properly selecting
the jet diameter, the pressure drop across each stage could be
kept at a minimum.
305
-------
8.0 -
•RY SALT - 108% EVAPORATION
0.4 -
5 10 20
40 60
WT%
-------
10
5.0
=*• 1.0
a
•o
0.5
0.1
II Mini 1—I I I I i
/o /
*&*
/ ~x
PREDICTED Q /
PARENT / '
DROPS
, /
/
?'
/
/ DRY SALT
- I*/
GLASS IMPACTOR DATA
/ A
D
> O
U. of W. IMPACTOR DATA
I 1
I.M.I I , I . I | I I
2 5 10 20 40 60 80 90
WT% UNDERSIZE, %
Figure 4. Measured wet size distribution, U. of W. impactor
and series glass impactor.
307
-------
HEATER
ROTAMETER
FILTER
PRESSURE
REGULATOR
P T
P T
— >
^^
A
F
IL1
_L_
TO
VACUUM PUMP
PER
STAGE 1 STAGE 2 STAGE 3
COMPRESSED
AIR
ATOMIZER
Figure 5. Experimental setup for glass impactors.
A solution of 5% by weight salt was atomized. The aerosol
was dried by mixing it with heated dilution air. The dry salt
size distribution confirmed the previously obtained dry size
distribution from the University of Washington cascade impactor.
Several runs were then run without dilution air. The results
are shown in Figure 6 along with the predicted wet particle size
distribution. As can be seen, there is a reasonably good fit
between the glass impactor data and the predicted wet particle
size for diameters below 3 ymA. There is slight deviation for
drops larger than 3 ymA in diameter. The parent drop size dis-
tribution was calculated from the dried salt distribution measured
with the University of Washington cascade impactor.
The wet size distribution measured with the University of
Washington impactor differs from that of the glass impactors
for particles smaller than 3 ymA in diameter. The agreement
between the two for larger particle sizes is fairly good. Ap-
parently, significant evaporation from small particles occurred
within the University of Washington impactor. The results shown
in Figure 4 show that particles of 1 ymA in diameter have their
diameters reduced to 0.6-0.7 ymA by evaporation.
The reason for the evaporation from the particles is probably
due to the pressure reduction in the cascade impactor. The pres-
sure drop across the glass impactor was relatively low. There-
fore, little or no evaporation from particles occurs and the
measured wet particle size distribution is close to prediction.
308
-------
8.0
6.0
4.0
O RUN 7/2L
A RUN 7/3L
D RUN 7/4L
O RUN 7/5L
• RUN 7/8L - DRY
WET
2.0
PREDICTED PARENTS
E
3.
1.0
0.8
0.6
0.4
OAd
/
.;
/ 00
SALT PARTICLES
AVERAGE Ap cm w.c.
WET DRY
AP2 = 1.8 1.0
7.1 67
0.2
I
II I
5 10 20 40 60 80 90 95
WT%
-------
Pressure Drop
Another observation made in the lab and field experiments
is that the pressure drop experienced in a cascade impactor is
generally larger for wet particles than for dry particles for
comparable loading and sampling rates. This is usually attributed
to a higher pressure drop across the final filter if water is
present. Careful measurement of stage to stage pressure drop
in the glass impactor shows a higher pressure drop for wet par-
ticles than for dry particles as shown in Figure 6.
The results of these experiments show that the size distri-
bution of small wet particles (>3 ymA) obtained with a cascade
impactor can be in error due to evaporation of water from the
particles as they experience successive pressure reductions pass-
ing through the impactor.
Single Stage Impactor
A single stage impactor has been used in order to get data
on the size of the grown particles in an F/C system. The objec-
tive of the single stage device was to minimize the pressure
reduction and the evaporation time. The initial sizing system
was composed of a Greenburg-Smith impinger with a nozzle size
and sample flow rate chosen to provide a cut point in the area
of interest. The impinger was followed by a total filter so
that a single cut point was obtained.
In one application the predicted grown particle size dis-
tribution indicated that a minimum particle diameter on the order
of 0.5 ymA should occur with a given amount of condensation.
Figure 7 shows the data obtained with the single stage impactor
as well as the initial particle size distribution and predicted
grown size distribution. These data demonstrate that small wet
particles could be measured if enough care is taken and the prob-
lems are recognized.
Entrainment Measurement
A related problem in determining the performance of wet
scrubbers is measurement of entrainment. Scrubbing involves
the energetic interaction of the gas stream and the scrubbing
liquid. Entrained drops containing suspended or dissolved solids
can cause excessive particulate emissions. The contribution
which liquid entrainment makes to particulate emission can vary
widely from system to system. There is no clear cut boundary
separating "wet" particulate from entrainment; entrainment will
generally be larger in size ranging from about 10 um to several
millimeters in diameter. The advantages and disadvantages of
three common methods for measuring entrainment are briefly dis-
cussed below.
310
-------
2.0
1.0
0.8
0.6
0.4
0.2
1 I
I I I I I T
INITIAL SIZE
I I I I
10 20 30
WT%
-------
The second method is to use a cascade impactor. This can
produce correct results for drops larger than a few microns.
However, many of the commercially available impactors have cut
points which are below the range of interest for entrainment
measurement, i.e., for drops with diameters larger than 20 urn.
The third method for entrainment measurement is a hot-wire
anemometer. An electrically heated wire is placed in the gas
stream. Drops hitting the wire will attach to the wire, evapo-
rate, and cool the wire. The amount of cooling is dependent
on the drop size and is detected by monitoring the resistance
of the wire. The hot-wire anemometer can yield information on
drop size distribution as well as drop concentration. However,
we found the hot wire to be very fragile. It could not detect
large drops accurately and there were calibration problems.
CONCLUSIONS
The cascade impactor is useful for measuring dry particle
size distribution and concentration. It has a useful range of
diameters extending from a few tenths of a micron to several
microns. Due to evaporation of liquid from particles it is not
generally suitable for measuring small wet particles.
A single stage impactor can provide information on the size
of wet particles more accurately than a cascade impactor.
Liquid entrainment drop size distribution and concentration
can be measured with hot wire anemometer, impactor and chemical
stain methods. None of these techniques gives completely satis-
factory results. Better techniques are needed.
ACKNOWLEDGEMENT
The work upon which this paper is based was performed pur-
suant to Contracts 68-02-2124 and 68-02-2191 with the Environmen-
tal Protection Agency.
REFERENCES
1. Calvert, S., et al. A.P.T. Field Evaluation of Fine Par-
ticle Scrubbers. Presented at the Second Fine Particle
Scrubber Symposium, May 2-3, 1977, New Orleans.
2. Calvert, S., et al. Feasibility of Flux Force/Condensation
Scrubbing for Fine Particulate Collection. EPA 650/2-73-
036, U.S. Environmental Protection Agency, Research Trianale
Park, NC, 1973. NTIS PB 277307. ^<"cn iriangie
312
-------
3. Calvert, S., et al. Study of Flux Force/Condensation Scrub-
bing of Fine Particles. EPA 600/2-75-018, U.S. Environ-
mental Protection Agency, Research Triangle Park, NC, 1975.
NTIS PB 249297.
4. Calvert, S., et al. Improved Design Method for Flux Force/
Condensation Scrubbing. Presented at the Second Fine Par-
ticle Scrubber Symposium, May 2-3, 1977, New Orleans.
5. Chmielewski, R., et al. Flux Force/Condensation Scrubbing.
Presented at the Engineering Foundation Conference, November
1977, Monterey, CA.
6. Chilton, H. Elimination of Carryover from Packed Towers
with Special Reference to Natural Draught Water Cooling
Towers. Tr. Inst. Chem. Eng. 30:235, 1952.
313
-------
PAPER 15
EVALUATION OF PERFORMANCE AND PARTICLE SIZE
DEPENDENT EFFICIENCY OF BAGHOUSES
D.S.ENSOR
R.G. HOOPER
G. MARKOWSKI
METEOROLOGY RESEARCH, INC.
AND
R.C. CARR
ELECTRIC POWER RESEARCH INSTITUTE
ABSTRACT.
Baghouses have several operational characteristics which
should be considered during field testing. These factors are
high collection efficiency, variation in pressure drop and outlet
concentration due to bag cleaning, integrity of the bags, and
the operation of the emission source. Real time aerosol instru-
mentation at both inlet and outlet, gas monitors and pressure
transducers, all logged on a common time base, are used to track
the dynamic behavior of both baghouse and source. Any effect
of mismatch in sampling times between inlet and outlet manual
sampling trains can be evaluated with the real time data. A
light scattering detector installed at the outlet is also useful
in detecting operational problems, such as bag leaks and valve
malfunctions. Experience has shown that a systems approach,
including both source and baghouse, is required for a meaningful
evaluation.
INTRODUCTION
Background
Baghouses are capable of unusually efficient collection
of particulate matter. Compared to other types of gas cleaning
314
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equipment, baghouses are relatively independent of the composi-
tion and size distribution of the material collected. As emis-
sion particulate mass limitations become more stringent and
"clear stack" opacities are required, baghouses will have greater
application.
In the past there has been little incentive to study the
details of fabric filtration because of the more than adequate
collection efficiency. However, with the potentially large scale
application to electric power utilities, the basic data needed
to optimize baghouses on full scale units must be obtained in
an effective manner.
Objectives of Paper
The objectives of this paper are to describe test techniques
and approaches used to measure the detailed performance of full
scale baghouses. The major emphasis is on measuring particle
size dependent penetration or collection efficiency. The actual
test methods for particle sizing may vary depending on site
specific problems. The details of organization and management
of a large test group using state-of-the-art methods in a remote
location will not be discussed.
The techniques described in this paper have evolved in pro-
jects performed by Meteorology Research, Inc. for the Electric
Power Research Institute. The objectives of these studies have
been the complete evaluation of specific gas cleaning equipment.
A systems approach utilizing continuous monitoring instrumenta-
tion during the field testing and a parallel engineering study
of costs and maintenance problems has resulted in an integrated
analysis of the technology for specific installations.
TEST STRATEGY
Description of Baghouse
Baghouses have unique aspects when compared to other gas
cleaning equipment. A cut away drawing of a baghouse is shown
in Figure 1. The dirty air enters through a duct at the hopper
level. The gas is filtered through the bags and the cleaned
gas exists through ducts at the top of the baghouse. The bags
are attached to the cell plate. Baghouses are usually arranged
in individual compartments. Each compartment has valves allowing
isolation for cleaning with a reverse flow of air or mechanical
shaking of the bags. The compartments may be cleaned on a timed
cycle or when a limiting pressure drop is reached. Bags are
typically 20 to 35 feet long and 12 inches in diameter. A wide
variety of fabrics may be used for bag material. Fiberglass
fabric coated with graphite or Teflon to reduce wear is the usual
choice for utility boiler bags.
315
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Other designs of baghouses include filtration of the gas
on the outside of the bags or the use of fabric panels instead
of bags. However, baghouses have in common the removal of par-
ticulate matter by filtration of the gas through a fibrous ma-
terial and provision for cleaning the filtering material when
the pressure drop becomes excessive.
DAMPER SHUT
CLEANING
BLOWER
DAMPER OPEN
SHAKING
MECHANISM
$# DUSTY
wf.7 INLET
• TO EXHAUST FAN
DAMPER OPEN
Figure 1. Typical baghouse.
316
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Test Considerations
There are several aspects of baghouse behavior which should
be considered during a field test of a baghouse. Failure to
consider the following items during the development of a test
plan may yield misleading data.
Inspection—The integrity of the bags and cell plate is
monitored for interpretation of the test results. For example,
leaks could mask effects of bag cleaning and gas flow in a full
scale unit. Samples of new and used bags should be obtained
for laboratory testing.
High efficiency—The very efficient collection capabilities
of fabric filtration place a heavy demand on the ability to mea-
sure gas streams with a 10,000 fold difference in particulate
concentration. The outlet concentration of a baghouse can be
as low as 0.0001 grains/ft2. Thus, long sampling times with
high flow rate test equipment and very sensitive instrumentation
are required. Also special test and data reduction schemes are
required to compute particle size dependent penetration under
these conditions.
Batch process—Baghouses effectively behave similar to a
"batch process" because of the need to clean the collected par-
ticulate matter from the fabric when the pressure drop becomes
excessive. The outlet emissions will vary as compartments are
removed from service and cleaned. During the test program, the
cleaning cycle must be verified.
Time constants—The response of a baghouse is not instanta-
neous and in some cases several days could be required to obtain
steady operation. The slowest changing parameters are those
dependent on the ash layer in the bags. New bags require several
weeks of conditioning to reach a stabilized collection efficiency
and pressure drop. Process changes will introduce a time lag
before steady conditions in pressure drop and collection effi-
ciency are reached.
Effects of Fabric Particulate Matter Loading on Baghouse
Performance
The basic filtration parameters are the pressure drop and
collection efficiency as a function of particle size. Both of
these are dependent on the properties of the collected ash cake
and the supporting fabric. The pressure drop mechanisms are
best understood and will be used here for illustration of the
effects of ash loading on the filter.
The pressure drop through a fabric filter is usually de-
scribed by the following equation
317
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AP = S Q/A
where AP is the pressure drop through the supporting fabric and
ash layer, S is the drag, Q is the volumetric flow rate, and
A is the filter area. The parameter Q/A is often called the
air-to-cloth ratio, or face velocity. The drag is usually re-
lated to the fabric areal loading w by a number of equations,
including the linear form suggested by Robinson et al.,l
8 = 5,-, + K2w
£j
where SE is the effective filter drag and K2 is the specific
resistance coefficient for the dust. A nonlinear model has also
been proposed by Davis et al.2 for the cake formation zone with
the form
S = a w
where a and b are regression coefficients.
A drag versus loading curve is shown in Figure 2. An explana-
tion of the linear-nonlinear behavior has been advanced by Dennis
et al.3 from laboratory studies. The drag behaves in two stages
depending on the history of the filtration cycle. Nonlinear
drag and the cake formation occur during the stage where the
particles deposit within the fabric. When the fabric is saturated
and the cake forms on the surface, then a linear drag is observed.
The nonlinear section is not truly a "cake repair" behavior as
described in the earlier literature.1 Dennis et al.lf have de-
veloped a pressure drop model using the laboratory observation
that ash tends to break off in sheets or slabs during cleaning.
Therefore the filter cake is not homogeneous and has areas with
behavior in both Zones A and B.
There are a considerable number of expressions for the
specific resistance, K2. One is the Kozeny-Carman equation
K2 = (25/6) y (1 - e) S 2/£ne3 (1)
c f
where y is gas viscosity, e is the filter cake porosity, Sp is
the surface-to-volume ratio and &„ is the particle density.
K2 could be expected to be a sensitive function of particle size.
However, the determination of K2 from experimental data indicated
a different dependence on particle diameter than predicted above
and a dependence on face velocity.5
At the present time, the areal dust concentration cannot
be directly measured in the field without shutting off the bag-
house and removing a bag. An indirect method to estimate the
bag loading is to estimate the loading from the inlet concentra-
tion and cleaning frequency.
318
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Measurable Parameters
The measurable parameters are listed in Table 1. In the
typical full scale field test as many of these parameters are
measured as possible.
The fabric measurements which are performed in the laboratory
are difficult to relate to the operation of the full scale bag-
house.
TABLE 1. MEASURABLE PARAMETERS
Baghouse:
Pressure drop
Cleaning cycles
Gas flow rate
Temperatures
Fabric:
Permeability
Weight
Weave
Material
Plow Stream Properties, IN/OUT
Particulate concentration
Particulate size distribution
Opacity
Gas composition
Particulate composition
319
-------
c
'E
+j
H-
C3
CO
C5
<
cc
Q
O
E
m
CAKE
FORMATION
HOMOGENOUS CAKE DEPOSITION
TERMINAL
DRAG, ST
AS
AW
DUST RESISTIVITY K2 = AS/AW
-EFFECTIVE DRAG, SE
-RESIDUAL DRAG, SR
RESIDUAL FABRIC LOADING, WR
AREAL FABRIC LOADING, W, Ib/ft2
Figure 2. Fabric filter resistance curve.
MEASUREMENTS
Pressure Drop
Pressure drop is one of the fundamental variables describing
the operation of the baghouse. During the fabric cleaning cycles,
the pressure drop can be quite variable as shown in Figure 3.
The pressure drop across individual modules during the operat-
ing/cleaning cycle is useful to trouble shoot and interpret
operations. Two items may complicate pressure drop data:
. The overall pressure drop data reported for full scale
baghouses is the flange to flange value. The overall pres-
sure drop includes the pressure drop through the ducts and
valves in addition to the pressure drop through the fabric.
Although the flange to flange pressure drop is significant
in the design of the process, it cannot be related to lab-
oratory studies without assumptions or independent measure-
ments of the pressure drop through the baghouse due to the
components other than the bags.
• Often only the peak pressure drop for the baghouse is easily
obtainable. Although this pressure drop is important for
the fan designer, it is not descriptive of the pressure
£°?K K°U! filtering bags and depends on the design
of the baghouse. ^
out
be specifed
be specified.
data tO be usefu1' the gas volume through
it ^ Ope'atin9 cvde during measurement must
Most modern baghouses are well instrumented for
320
-------
o
o
12
11
10
9
10
10
10
10
10
10
12
12
12
12
12
12
14
14
14
14
14
14
16
16
16
16
16
16
10'
18
20
18
20
18
20
18
20
18
20
18
20
PRESSURE DROP ACROSS BAGHOUSE, in. W.G.
Figure 3. Typical pressure drop trace.
321
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pressure drop measurement and additional transducers can often
be added if needed.
Mass Concentration and Gas Velocity
Flue gas velocity and mass concentration are typically mea-
sured using EPA Methods 2 and 5, respectively.
Particles Larger than 0.5 um
Cascade impactors have been used for source testing to
characterize particles larger than 0.5 \an for a number of years.
An example of early work was reported by Pilat, Ensor and Bosch.6
The impactor designed at MRI to facilitate sampling of par-
ticulate matter in stacks is shown in Figure 4. The design is
a simple annular arrangement of jet and collectors reported by
Cohen and Montan.7 The simple jet-collection plate geometry
has been thoroughly studied by many investigators (for example/
Marple and Liu8). The use of a collection disc allows flexi-
bility in choice of substrates. For example, either a greased
surface or a filter mat can be used to enhance particle collec-
tion. "0" rings under compression seal the jet plates and body
sections.
NOZZLE
FIRST STAGE
JET PLATE
COLLECTION DISC
"O" RING
FILTER
Figure 4. Cascade impactor.
322
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The performance of a cascade impactor is strongly dependent
on the surface on which the particles are collected. The two
most popular substrates have been glass fiber filter mats and
greased surfaces. A greased surface is preferable because glass
fiber filter mats sometimes react with stack gases (Smith et
al.9) Also, Rao reported that filter mats have poor collection
efficiency. 1 °
A large number of materials have been screened as coatings
to enhance particle collection. Apiezon-L high-vacuum grease,
a high-molecular weight hydrocarbon, had the best properties
of the materials tested in terms of stability and particle ad-
hesion and it has been used in tests less than 300°F.
The discs are then weighed on a Cahn 4100 analytical balance
to 0.01 mg. Typical weights of the discs are approximately 630
mg. Weighing by substitution is used to improve the accuracy
of the weighing. The discs are stored in labeled 15 x 60 mm
petri dishes.
Quality control for the impactor tests include blank runs
where filtered stack gas is sampled through the impactor to detect
chemical reactions with the collection surface. In addition,
field controls are used to verify the precision of the weighing.
The weighing precision of the discs is consistently within
0.01 mg. The weight changes of the substrates when subjected
to filtered stack gas are typically less than 0.05 mg for flue
gas from the combustion of low sulfur coal.
The wide mismatch in inlet and outlet concentrations (a
factor of typically 10•*) precludes simultaneous impactor testing.
The approach used is a series of inlet tests a few minutes each
within the time period, several hours, of two outlet tests.
Blank tests at the inlet and outlet are also conducted.
The data is analyzed with a procedure reported by Markowski
and Ensor.11 The cumulative size distributions are fitted with
a numerical interpolation technique. Differential size distri-
bution curves are used to compute penetration. Error bounds
on both differential size distribution and penetration are cal-
culated.
Submicron Particle Sampling
Electrical Aerosol Size Analyzer—
The Model 3131 Electrical Size Analyzer (EASA) manufactured
by Thermo-Systems, Inc., St. Paul, Minnesota, is the only com-
mercially available instrument capable of measuring aerosol par-
323
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ticle size distribution in the 0.01 - 1.0 ym diameter range.
The EASA consists of two inter-connected modules: a flow module
and a control module. It was commercially introduced in 1973
and is a greatly improved version of the Model 3000 which was
commercially introduced in 1967. The EASA is described in detail
by Liu, Whitby, and Pui12 and by Liu and Pui.
The EASA, shown schematically in Figure 5, consists of a
charger, an electrical mobility analyzer which removes particles
smaller than a selectable size, and an electrometer which measures
the electrostatic charge carried by the particles which pass
through the mobility analyzer.
Aerosol is drawn into the EASA at a flow rate of 50 Jlpm,
of which 4 Jlpm is measured by the instrument, 1 Jlpm is filtered
for use as charger sheath air, and 45 Jlpm is filtered for the
mobility analyzer core air. The diffusion charger exposes the
aerosol to positive ions, which charge the particles as a func-
tion of particle size. The charger sheath air prevents particles
from entering the ion source chamber where they can acquire ex-
cessive charges. The 5 Jlpm of charged aerosol (4 Jlpm aerosol
+ 1 Jlpm charger sheath air) enters the mobility analyzer through
an annular opening near the outer wall of the mobility analyzer,
separating the charged aerosol stream from the negatively charged
central rod.
The particles are attracted toward the central rod, the
smaller ones with high mobility depositing near the entrance.
Particles smaller than a certain size are collected, while larger
particles pass through. The cutoff size depends on the voltage
on the central rod. A high efficiency filter collects the larger
particles and the associated current is measured by an electrom-
eter.
By cycling the central rod voltage through a number of pro-
grammed steps, each of which represents a factory precalibrated
particle size, one obtains a series of electrometer current
measurements. The higher current values occur with low central
rod voltages and small particle cutoff sizes. The difference
of electrometer current between two consecutive central rod volt-
ages is a measure of the particle concentration within the cor-
responding particle size range.
For field operation, the operator manually or automatically
steps the voltage of the mobility analyzer collector rod through
eleven internally programmed steps and records eleven correspond-
ing electrometer current values, a procedure which requires about
two minutes. Liu and Pui describe the calibration of the in-
strument, including aerosol sensitivity as a function of size
and the collector voltage-particle size relationship.13
324
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CONTROL MODULE
ANALYZER OUTPUT SI8NAL-
DATA READ COMMAND - -
CYCLE START COMMAND -
CYCLE RESET COMMAND -
AEROSOL FLOWMETEH READOUT
CHARTER CURRENT READOUT
CHARGER VOLTAGE READOUT
AUTOMATIC HIGH VOLTAGE CONTROL AND READOUT
ELECTROMETER (ANALYZER CURRENT) READOUT
..---- -TOTAL FLOWMETER READOUT
U>
FORCES ££ BARTICLE
f,»CLtCTROST»TIC
ftf'ACROOYNAMIC DRAG
>> EXTERNAL ,
• DATA ;
•^ACQUISITION'
-i SYSTEM •
TO VACUUM PUMP
Figure 5. Flow schematic and electronic block diagram of
the electrical aerosol size analyzer.
-------
Extractive Sampling—
MRI uses dilution systems to precondition the stack aerosol
to allow measurements with the EASA. Inlet aerosols to a bag-
house may require dilutions from 10 to 5000:1. A coal-fired
utility boiler emission usually requires from 100 to 200:1.
In a typical system the aerosol is sampled through an impactor
precutter through a heated line into a three-stage diluter.
Each diluter consists of a venturi meter for the aerosol, an
orifice meter for the dilution air and an orifice plate dilution
chamber. An in-stack diluter is used at the outlet of a control
device where dilutions of up to 20:1 may be required. The emis-
sions from a baghouse may be so dilute that only a diffusional
dryer may be used to remove the water from the emissions to pre-
vent condensation in the particle measuring instrument.
Opacity
Plant Process Visiometer—
The real time measurement of the inlet and outlet opacity
is very useful to monitor the operation of the baghouse. The
outlet opacity is a sensitive indication of problems such as
broken or leaking bags or valve leaks. Also the outlet opacity
is important from a regulations standpoint.
It has been our experience that the emissions from a properly
operating baghouse are so dilute that the opacity may be a few
tenths of a percent, which is below the detectability of a stack
transmissometer. A Plant Processing Visiometer, a hardened
integrating nephelometer, is used instead.
A diagram of the instrument is shown in Figure 6. The
aerosol is removed from the stack with a stainless steel probe
and transported into the measurement chamber. The aerosol par-
ticles in the chamber are illuminated by a flash lamp with an
opal glass filter. The scattered light is detected by a photo-
multiplier tube at approximately right angles to the flash lamp.
The optics have been designed so that the output of the photo-
multiplier tube is proportional to the extinction coefficient
due to scattered light. The instrument is a physical analog
of the following equation:
ir
bscat = 2 * f 3(9) sin e de
326
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SAMPLE FLOW
LIGHT TRAP
FLASH
LAMP
DIFFUSER
1L— - ELECTRONICS
PHOTO MULTIPLIER
ASPIRATOR
Figure 6. Diagram of the plant process visiometer (PPV).
327
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where
h = the scattering coefficient due to scattered light
scat
g(6) = volume scattering function
9 = scattering angle
If there is no light absorption, the scattering coefficient is
identical to the extinction coefficient. The extinction coef-
ficient is related to plume opacity with Beer's Law.
Opacity (percent) = Fl - exp (-bextL)J 10°
where
b = extinction coefficient, m"1
ext
L = stack diameter, m
The instrument is spanned with an internal calibrator con-
sisting of an opal glass lens of known scattering coefficient.
The lens is mechanically placed in the view of the detector for
calibration and retracted into a sealed chamber between cali-
brations. The PPV is described in detail by Ensor et al.1"1
Two aspects of the PPV performance are shown in Figure 7.
The ability to adjust the gain over a wide range permits the
use of two units: one to characterize the incoming ash and one
to monitor the outlet of the baghouse. The outlet data indicates
the sensitivity possible with the unit. The Freon 12 calibra-
tion point corresponds to a scattering coefficient of 3.6 x 10~"m~1
(8 mile visibility, 0.036 percent opacity on a 1-meter diameter
stack).
Gas Sampling
Continuous gas analyzers are preferred to manual sampling
for monitoring utility boilers because of the ability to detect
upset conditions. The flue gas must be conditioned before intro-
duction to the instruments. The gas is withdrawn from the stack
with stainless steel probes with in-stack filters. The condensate
is removed in a chilled dropout jar. Polypropylene tubing is
used to transport the gas to the analyzers. Multiple probes
at different locations in the duct are used to obtain a repre-
sentative sample of stack gas and to sample for stratification.
On site span and zero gas are used to calibrate the analyzers.
Typically, 02, S02, NO and CO are measured. Orsat samples are
used to check the 02, C02 and CO determinations.
328
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OUTLET OF BAGHOUSE
100
uj
UJ
O
o
UJ
o
CO
6.7< ID'3)
4.9(10-3)
3.1(10-3)
1.26(10-3)
0
UJ
o
\L
u.
UJ
O
O
(3
E
Ul
u
CO
1.44(10^)
0.96I10'1)
0.48(1(r1)
0 M
INLET OF BAGHOUSE
100
1600 1500
1400 1300 1200
LOCAL TIME, hours
1100 1000 900
Figure 7. Example of PPV data taken at Nucla baghouse.
329
-------
The Goksoyr/Ross (G/R) controlled condensation method is
used to measure SOa/HaSO^. The stack gas is pulled through a
glass probe maintained at 450°F. The particulate matter is re-
moved from the gas by filtration through a quartz filter main-
tained above 450°F. At this temperature the S03 is all in vapor
phase and only particulate matter is removed. A quartz filter
is used to reduce conversion of S02 to S03. The H2SCK is then
removed in a condenser maintained at 140°F. The condensate is
titrated with NaOH to determine the acid concentration. The
method is accurate to 16 percent.15
Data Integration
The numerous and diverse measurements performed on a bag-
house pose some problems in assembling the information. Our
approach to reduce difficulties in data analysis is to data log
all continuous output on a magnetic tape data logger. The single
time base allows correlation of the various parameters. A sche-
matic of such an arrangement is shown in Figure 8. In this
scheme a central data logger is used with backup strip charts.
This arrangement allows a field test control room where the test
parameters can be monitored from one location.
EXAMPLES OF DATA OBTAINED
Process Charts
Process charts of the main parameters plotted on a common
time basis are useful to determine the effects of process changes
or upsets on the baghouse- An example is shown in Figure 9.
Efficiency as a Function of Operating Parameters
Ideally the test should include some variation of air-to-
cloth ratio and the cleaning cycle to develop the limits of
operating parameters. An example of variation of emission with
air-to-cloth ratio is shown in Figure 10. This information is
useful for engineering design.
Penetration
The cascade impactor and EASA data are used to compute the
particle size dependent penetration through the baghouse. The
data reduction technique reported by Markowski and Ensorl* is
used for the cascade impactors. The EASA data is reduced by
computing an inlet and outlet size distribution and dividing
the outlet by the inlet distribution. An example of penetration
data obtained at a baghouse is shown in Figure 11.
330
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u>
S TYPE PITOT
PROBE WITH PRESSURE
TRANSDUCER
I
GAS
ANAJ.YZERS
AND PANELS
I TRAILER
TEMPERATURE
GAS LINES
FROM SAMPLE
LOCATION BEFORE
AIR HEATER
Figure 8. Integrated baghouse sampling system.
-------
GENERATOR
OUTPUT
MEGAWATTS
11.0 11.0 11.0 11.0 10.7
10.6
10.8 11.1 10.8
i i i
SUBMICRON OUTLET
PARTICLE i I
TESTING
CASCADE IMPACTOR
INLET PORT NO.
CONCENTRATION
(103 mg/m3)
4 6
4.7 14.7
INLET INLET OUTLET INLET
1 4
1.6 2.1
OUTLET
CONCENTRATION
M ill Uh -
4.0
3.0
s^
2.0 ^-
U.
r™
u
1.0 *
0.8
0.6
Q.
a
O
X
O
O
CO
cc
5
500
400
300
200
100
CO CONCENTRATION
(INLET)
1200 1300 1400 1500 1600 1700 1800 1900 2000 2100
LOCAL TIME, hrs
Figure 9. Process variation during November 12, 1975 (Ensor et al. 1
332
-------
V)
cc
X
Q
UJ
cc
111
UJ
Q
UJ
V)
O
00
X
O
O
cc
X
O
Ul
a.
SYMBOL
LOAD MW
CLEANING
FREQUENCY
PRESSURE
DROP
BETWEEN
CLEANING
IN. W.G.
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
O 6 NONE 3.0
D 11 HOURLY 3-4.5
A 12 HOURLY-CONTINUOUS 3-4.5
-
-
-
i
™
-
-
-
T
T i
? I
J_ J-
DESIGN SIX COMPARTMENTS
-
-
-
-
-
-
k
-
-
-
-
-
-
-
7,"
99.86
99.87
99.88
99.89
99.90
99.91
99,92
99.93
99.94
99.95
99.96
99.97
99.98
99.99
O
UJ
LL
LL.
Ul
0 0.5 1.0 1.5 2.0 2.5 3.0
AIR-TO-CLOTH RATIO BETWEEN COMPARTMENT CLEANING, acfm/ft2
Figure 10. Penetration as a function of air-to-cloth ratio with one
standard deviation limits (Ensor et al. 16),
333
-------
10
I-2,-
UJ
CO
D
O
I
CQ
O
O
CC
I
CC
111
10-4
10'5
SULFURIC ACID
NUCLEI
hHI
OVERALL
PENETRATION = 2.75 x 10'4
EFFICIENCY = 99.973%
NOVEMBER 14, 1975
6 MW LOAD
NO CLEANING CYCLES
PRESSURE DROP 3.0 IN. W.G.
• ELECTRICAL AEROSOL SIZE
ANALYZER
D CASCADE IMP ACTOR
ONE STANDARD DEVIATION LIMITS
PARTICLE DENSITY = 2.0 g/cm3
99.0
99.9
O
ID
o
99.99
99.999
0.01
0.10 1.00
PARTICLE DIAMETER,
10.0
Figure 11. Fractional penetration through baghouse (Ensor et a/.16).
334
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SUMMARY
The field measurements are complicated by the variability
of the outlet particulate concentrations, the dilute emissions,
the time required to obtain stable conditions, and the conditions
or the bags. The testing of baghouses is facilitated with the
use of continuous instruments for both particulate matter and
trace gases, and data logging.
REFERENCES
1. Robinson, J.W., R.E. Harrington, and P.W. Spaite. Atmos.
Environ. 1(11):499-508, 1967.
2. Davis, W.T., D.E. Noll, and P.J. LaRosa. A Predictive Per-
formance Model for Fabric Filtration Based on Pilot Plant
Studies. In: Proceedings, User and Fabric Filtration
Equipment II, Specialty Conference of the Air Pollution
Control Association, Niagara Falls, 1975.
3. Dennis, R., R.W. Cass, and R.R. Hall. Observed Dust Dis-
lodgment from Woven Bags and its Measured and Predicted
Effect on Filter Performance. Paper No. 77-32.3, presented
at the 70th Annual Meeting of the Air Pollution Control
Association, Toronto, Canada, June, 1977.
4. Dennis, R., R.W. Cass, D-W. Cooper, R.R. Hall, V. Hampl,
H.A. Klenn, J.E. Langley and R.W. Stern. Filtration Model
for Coal Flyash with Glass Fabrics. EPA-600/7-77-084, U.S.
Environmental Protection Agency, Research Triangle Park,
NC, 1977.
5. Cooper, D.W., and V. Hampl. Fabric Filtration Performance
Model. In: Proceedings, Conference on Particulate Collec-
tion Problems in Converting to Low-Sulfur Coals. G.B. Nichols,
ed. EPA-600/7-76-016, U.S. Environmental Protection Agency,
Research Triangle Park, NC, 1976. pp. 149-185.
6. Pilat, M.J., D.S. Ensor, and J.C. Bosch. Source Test Cascade
Impactor. Atmos. Environ. 4:671-679, 1970.
7. Cohen, J.J., and D.M. Montan. Theoretical Considerations,
Design, and Evaluation of a Cascade Impactor. J. Am. Ind.
Hyg. Assoc. 28:95-104, 1967.
8. Marple, V.A., and B.Y.H. Liu. Characteristics of Laminar
Jet Impactors. Environ. Sci. Technol. 8:648-654, 1974.
9. Smith, W.B., K.M. Gushing, and G.E. Lacey. Andersen Filter
Substrate Weight Loss. EPA-650/2-75-022, U.S. Environmental
Protection Agency, Research Triangle Park, NC, 1975.
335
-------
10. Rao, A.K. Sampling and Analysis of Atmospheric Aerosols.
Particle Technology Laboratory, Mechanical Technology De-
partment, University of Minnesota, Publication 269, 1975.
11. Markowski, G.R., and D.S. Ensor. A Procedure for Computing
Particle Size Dependent Efficiency for Control Devices from
Cascade Impactor Data. Presented at the 70th Annual Meeting
of the Air Pollution Control Association, Toronto, Canada,
June, 1977.
12. Liu, B.Y. H. , K.T. Whitby, and D.Y.H. Pui. A Portable Elec-
trical Analyzer for Size Distribution Measurement of Sub-
micron Aerosols. J. Air Pollut. Contr. Assoc. 24:1067,
1967.
13. Liu, B.Y.H., and D.Y.H. Pui. On the Performance of the
Electrical Aerosol Analyzer. J. Aerosol Sci. 6:249, 1975.
14. Ensor, D.S., L.D. Bevan, and G- Markowski. Application
of Nephelometry to the Monitoring of Air Pollution Sources.
Paper No. 74-110, presented at the 67th Annual Meeting of
the Air Pollution Control Association, Denver, June, 1974.
15. Maddalone, R.F., and N. Garner. Process Measurement Pro-
cedures-Sulfuric Acid Emissions. TRW Report 28055-6004-
RU-00 to Environmental Protection Agency, Contract No.
68-02-2165, February 1977. 36 pp.
16. Ensor, D.S., R. Hooper, and R.W. Scheck. Determination
of the Fractional Efficiency Opacity Characteristics, and
Engineering and Economic Aspects of a Fabric Filter Operating
on a Utility Boiler. MRI Final Report No. FR-1411 to Elec-
tric Power Research Institute, Contract RP534-1, 1976.
336
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PAPER 16
EVALUATION OF THE EFFICIENCY OF ELECTROSTATIC PRECIPITATORS
WALLACE B. SMITH
JOHN P. GOOCH
JOSEPH D. McCAIN
JAMES E. McCORMACK
SOUTHERN RESEARCH INSTITUTE
INTRODUCTION AND SUMMARY
A system is described that can be used to measure the
particle-size distribution in process streams at control device
inlets and outlets to evaluate their collection efficiency.
The fractional efficiency is determined from 0.01 to 10 ym and
real-time systems are used to discriminate transient events and
nonideal behavior.
Typical results are shown illustrating particle size distri-
butions and collection efficiency with and without rapping.
It was found in a series of tests that rapping emissions may
constitute up to 80 percent of the total emissions; the higher
emissions are observed at hot precipitators.
EQUIPMENT AND PROCEDURES
Figure 1 is a schematic illustrating the operating principle
of electrostatic precipitators. Ions, created near the high
voltage corona electrode, attach themselves to the aerosol par-
ticles which in turn are collected on the grounded plates by
electrostatic forces as they pass nearby. Ideally, the collec-
tion efficiency of a precipitator can be made arbitrarily high
by increasing the residence time of the aerosol (the precipitator
dimensions).
As a layer of collected particles accumulates on the grounded
plate, the plates are periodically "rapped" to dislodge the layer,
which falls into the hoppers below. Sometimes a fraction of
337
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HIGH-VOLTAGE
SUPPLY
PLATE
RAPPING
SYSTEM
DUST
LADEN
AIR
GROUNDED
COLLECTION
ELECTRODES
CLEANED
AIR
CORONA WIRES
DUST COLLECTION
HOPPERS
Figure 1. Schematic drawing of the electrostatic precipitation process.
the previously collected dust is reentrained into the aerosol
stream to be recollected or to escape into the atmosphere. The
rapping frequency and power are adjusted to minimize such losses;
but, rapping reentrainment is an important factor in limiting
precipitator performance.
A second, nonideal factor that influences precipitator per-
formance is "sneakage" or passing of a part of the aerosol stream
through areas where the particles are not charged, such as through
the hoppers, or the electrode suspension areas. Sneakage is
minimized through the precipitator design.
338
-------
Figure 2 shows curves illustrating the nature of precipi-
tator fractional efficiency; i.e., efficiency vs. particle diam-
eter. Because of the relative electrical mobility of particles
of different sizes, the collection efficiency is highest for
very fine and very large particles with a minimum value for par-
ticles of approximately 0.5 pm diameter. The curves also show
how sneakage and rapping reentrainment degrade the performance
of the precipitator. Sneakage allows particles of all sizes
to penetrate the precipitator, while rapping puffs contain pri-
marily large agglomerates of particles that were collected.
Thus, rapping reentrainment should not be considered penetration
of the primary aerosol, but rather the creation of a different
aerosol. For proper evaluation of a precipitator performance,
it is desirable to isolate and identify the emissions due to
normal penetration, sneakage, and reentrainment. Other, more
subtle nonideal effects exist but are generally less important.
Figure 3 shows the main objectives of precipitator evalua-
tion tests and the requirements to achieve these objectives.
For even the most rudimentary diagnostic tests it is necessary
to acquire knowledge of the gas velocity distribution, the main
gas constituents, and characteristics of the dust such as ap-
proximate size distribution, concentration, and electric resis-
tivity. Also, the electric operating parameters must be noted
and an assessment of the mechanical condition made. Tests de-
signed to acquire data for research generally require more tests
and better accuracy and resolution.
100
>- 90
UJ
O
u.
u] 80
O
o
111
O
CJ
70
60
• IDEAL PERFORMANCE
EFFECT OF SNEAKAGE
EFFECT OF REENTRAINMENT
0.01
0.1 1.0
PARTICLE DIAMETER, jum
10
Figure 2. Typical precipitator performance curve showing negative
effects of sneakage and reentrainment.
339
-------
Assure Compliance Obtain Design
with EPA Optimize Performance Data for
Objective of Tests Regulation of Control Device Control Device
Tests Required
Mass Concentration
Opacity
Gas Composition
Gas Temperature
Gas Volume
Pressure
Velocity Distribution
Particle Size Distribution
Control Device Data
Operating
Technical Considerations
(Decisions/Problems)
Adequate Space,
Laboratory Space
Number of Tests Required
Isokinetic Sampling
Condensible Vapors/
Volatile Particles
Mass Concentration/
Sampling Time
Traverse Strategy
Aerosol Gas Velocity
Process/Emission
Variations
Select Particle Sizing
Method;
Select Mass Train Type
Select Gas Analysis Methods
Real-Time Monitors Needed
Filter Mass Stability
Sample Preservation ..
•
v
3
x
c
o
c
c
•0
P
v
x
r
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for Modeling Process and
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— in -
1 0
1 0
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v
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D
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Key:
- Outlet
- Inlet
- Required
- Decision based on specific site or test objectives
- Must be considered
vs. Particle Diameter
Figure 3. Objectives of precipitator performance evaluations and tests
required for completion.
340
-------
In Table 1, the participate sampling methods used by South-
ern Research for precipitator analysis are listed. Cascade im-
pactors are used more than any other type of sampling device
for measuring particle-size distributions. Modified Brink im-
pactors are used when particle concentration is high (inlet)
and Andersen or University of Washington impactors are used when
the concentration is lower (outlets). Greased foils are pre-
ferred as collection substrates, but for temperatures above 300°F,
glass fiber filter material is used.
If larger samples are required for analysis, cyclones can
be used in a series arrangement to obtain sized particulate frac-
tions in quantities of several tens of milligrams to several
grams. The EPA/Southern Research cyclone system has five small
cyclones with D50's of 0.3, 0.6, 1.2, 2.5, and 5.6 ym.
Cascade impactors and cyclones yield samples that are ac-
cumulated over periods of time ranging from several minutes to
several hours; thus, the size distributions measured with these
devices are time averaged. Optical particle counters may be
used to yield real time or instantaneous data on particle concen-
tration and size. Disadvantages of these counters are: 1) They
are designed for laboratory use, and the samples must be diluted
and cooled. 2) They do not yield aerodynamic particle size;
consequently, some knowledge (or assumptions) of the optical
properties of the dust must be known to calculate the aerodynamic
sizes from optical data. Alternately, the optical particle
counters may be calibrated for the particles of interest to yield
aerodynamic sizes. Marple1 has designed special impactors for
calibration of optical particle counters, while McCain2 has used
sedimentation to perform calibrations. We have used the Climet
207 and Royco 225 optical particle counters in our field test
programs.
Figure 4 shows schematically an optical particle sizing
system for large particles that is used to count particles from
1-30 ym in five size fractions. In the diluter, the particles
pass from the inlet jet, through a turbulent zone, toward the
outlet jet. Small particles are deflected more than the larger
particles; thus a larger fraction of the large particles is
counted. It is necessary to dilute the small particles to a
number that is within the range suitable for the counter, but
the large particles require little or no dilution. Figure 5
shows a special plugin unit designed by Southern Research for
use with optical particle counters. It is a five channel pulse
height analyzer. The data are seen as analog readings on the
front of the plugin, and also analog recorder outputs are pro-
vided for each channel. Linear or logarithmic responses are
chosen by means of toggle switches on the front of the device.
The unit will plug directly into the Royco 225 optical particle
counter, or can be used in a special chassis with any optical
counter with which it is compatible.
341
-------
TABLE 1. MAIN COMPONENTS OF THE SOUTHERN RESEARCH PARTICLE
SIZING SYSTEM
Method
Size range (diameter, urn)
Remarks
Impactors
Cyclones
Optical particle
a
counter
Electrical mobility
analyzer3
Diffusion batteries
and condensation
nuclei counters9
0.5 - 10
0.3 - 6
0.3 - 1.5
1-30
0.01 - 0.5
0.01 - 0.2
Time-averaged
sample
Time-averaged
larger samples
Real-time data,
not aerodynamic
sizing
Time-averaged
sample
Time-averaged
size distri-
bution; some
time resolution
possible
Sample extraction and dilution required.
For the ultra-fine particle size range (0.01-0.5 pm) elec-
trical aerosol analyzers (TSI Model 3030) or diffusion batteries
with condensation nuclei counters are used. Again, these are
laboratory instruments and sample dilution and cooling is re-
quired. The diffusion batteries are the parallel plate type,
and are of our own design. We have used General Electric, Gard-
ner, and Environment One condensation nuclei counters.
A more detailed description of the procedures used to de-
velop and implement precipitator evaluation programs has been
given elsewhere.3
QUALITY ASSURANCE AND CONTROL
Three important problems related to quality control of par-
ticulate sampling data have been addressed as part of our sam-
pling program. Harris et al. have discussed the first problem,
the selection of suitable impactor substrates, in an earlier
paper of this symposium. A second problem has been the formation
of a sulfuric acid fume in our sample extraction and dilution
system. The formation of the fume is detected by a very non-
linear increase in submicron particles as the dilution factor
is reduced. This phenomenon is observed at sites when the S03
concentration is high and when low dilution factors are used.
342
-------
SAMPLE
FLOWRATE
MANOMETER
s s
/ <
;<
<
ff
~/
•«*
**
•»
»
I
\
GAS FLOW
\
PROCESS EXHA
PRORF i IMC
HEATER
DILUTION AIR
MANOMETER
AND ORIFICE
0.02 urn
FILTER
BLEED
VALVE
?=&=
CONDENSER FLOW
POT '
PUMP
BLEED
VALVE
VERTICAL
ELECTRICAL
LEADS, ETC.
LARGE PARTICLE
COUNTER
MAIN FRAME
Figure 4. The large particle sampling system.
343
-------
Figure 5. A special pulse height analyzer plugin developed by
Southern Research Institute.
Figure 6 shows our sample extraction-dilution system with dif-
fusion absorbers for condensible gases. The absorbers consist
of stainless steel cylinders, with a smaller, screen cylinder
inside. The void between the cylinders is filled with granular,
activated charcoal and the aerosol sample passes through the
inner cylinder. This technique has been very successful at
eliminating the formation of fumes at sources when the SO 3 con-
centrations are quite high.
A third potential problem is the distortion of particle
size distributions by electric charges that exist on the par-
ticles, particularly at the outlet of precipitators. An exten-
sive study of this problem has been made and the results will
be published later. Typical results, however, are shown below.
344
-------
U)
ft.
en
^
CYCLOI
PUMP
ATED PROBE \
UJ
X
/
PROCESS EXHAUST LINE
CHARGE NEUTRALIZER
CYCLONE
ORIFICE WITH BALL AND SOCKET
JOINTS FOR QUICK RELEASE
DUMP
TIME
AVERAGING
CHAMBER
BLEED DILUTION DEVICE
CHARGE NEUTRALIZER
SIZING
H INSTRUMENT
SOX ABSORBERS (OPTIONAL)
HEATED INSULATED BOX
RECIRCULATED CLEAN. DRY, DILUTION AIR
FILTER BLEED NO. 2
COOLING COIL
PRESSURE
BALANCING
LINE
DRYER
BLEED NO. 1
MANOMETER
Figure 6. The sample extraction-dilution system.
-------
Figure 7 shows one of the experimental setups used for the
charged particle experiments. Aerosols of 1.3, 0.69, and 1.0
urn mass median diameter were generated in concentrations that
were comparable to those found in process streams by nebuliza-
tion. The particles were passed through an electric corona
charging device, and into the test impactors (Andersen impactors
with glass fiber substrates). Polonium-210 devices were used
to discharge the particles in one aerosol stream and low voltage
ion precipitators were used to remove ions. The cascade impactors
were used as Faraday cages and the charge accumulated was mea-
sured by electrometers. Care was taken to insure that the aerosol
was properly split between the impactors and that losses in the
sampling lines were minimal. In all of the experiments the
results were extremely reproducible, indicating a high degree
of precision.
Typical results from the charged particle experiments are
shown in Figures 8 and 9. In Figure 8 a 1.01 pirn mmd aerosol
was charged to approximately the value that would be acquired
in a conventional electrostatic precipitator. The level of
charge relative to a precipitator is indicated in the figure.
Very little difference was found between the charged and un-
charged particle distributions.
A highly charged aerosol was sampled and the results plotted
in Figure 9. In this experiment, a much larger, and significant,
difference was found in the charged and uncharged aerosols.
Experiments were performed with different impactors, different
substrates, grounded and ungrounded impactors, monodisperse
particles, and different charge levels. Analysis of the results
is complete and a report is now being written (EPA Contract No.
68-02-2131).
The results of the charged particle experiments indicate
that for moderate levels of charge, and where using glass fiber
substrates, the interferences due to electrostatic charges are
small. When sampling highly charged aerosols, however, the size
distribution can be distorted significantly. Unfortunately no
suitable charge neutralizing device is available for in-stack
use.
TYPICAL RESULTS OF PRECIPITATOR EVALUATIONS
Figures 10 through 13 show results from field test measure-
ments illustrating the type of data that is obtained using the
systems described and their application to precipitator evalua-
tions and source characterization.
346
-------
DISPERSION
CHAMBER
DILUTION ROTAMETER
VALVE DRYER
COMPRESSED AIR LINE
ATOMIZER
ABSOLUTE
VALVE F|LTER
ATOMIZER ROTAMETER
REGULATOR
*— MIXING-DRYING COLUMN
ELECTROMETERS
Lj
CASQADE IMPACTORS
METERING ORIFICES
DIFFUSIONAL DRYERS
ELECTROSTATIC SHIELD
PO210 CHAMBERS
ION PRECIPITATORS
INSULATED
FROM GROUND
HIGH VOLTAGE
POWER SUPPLY
MERCURY MANOMETERS
WATER MANOMETERS
Figure 7. Experimental setup for the charged particle investigations.
347
-------
1BIS7U U>*0 KlflHUZBDIWCnK HUH. OWBB6
M = U5»ff EJOIttl»lH5IHIH .SHOW
10°
I
CHARGED
NEUTRALIZED
H—I I 1 IIH| 1—I I I MM| 1 1 MIN
1CT1 10°
PARTICLE DIAMETER (MICROMETERS)
Figure 8. Particle size distribution for moderate charging conditions
for a mass median diameter of 1.02 pm.
348
-------
TEISlS-lfl UHW KU1HU2ED UnciS
M = L£HKC GOJIEW9SLSIWM JSUQDG
101::
iff-
10'
»*».
s V
^
!f
•li
**
J*
r
[*
*
L
i
CHARGED
NEUTRALIZED
I I I Mill 1—I I I III
10°
101
H—I I I Mill
PAF?TICLE DIAMETER (MICROMETEF5S)
Figure 9. Particle size distribution for high charging conditions
for a mass median diameter of 1.01 /im.
349
-------
CO
ui
o
1,000,000
m
|
o
<
Q
O
CO
750,000
500,000
250,000 —
A.
1
O DIFFUSIONAL DATA PARTICLE DIA. 0.11 p
A OPTICAL DATA PARTICLE DIA. 0.35-0.6 urn
10
20
TIME, minutes
30
Figure 10. Particle concentration versus time in the effluent
from an asphalt batching process.
20,000
15,000
o
10,000 p
o.
O
5,000
-------
17 um AND LARGER PARTICLES
I I I I
SPARKING
OR
SLOUGHING
SPARKING
SLOUGHING
U)
Ul
7.8- 17 um PARTICLES
kj
'JUul il 1 11 1 Li 1 1 i lllJil
I
J J.I i l | i 1 II IkJl Lull 1 1 1 | I^IV HI 1,
I 1
j oJlL-dlLL
i^l l^ i li Ju iJ,U i , L H kl It 1...
1 1 L
3.8 - 7.6 um PARTICLES
28001
1.8- 3.6 Jim PARTICLES
Jw~*A*~K-**«--^»*-*^^ ^^^J^jui^^^j^^
0.67 - 1.t um PARTICLES
Figure 11. Chart recordings from the large particle sampling system.
-------
1011
1010
number'/DN m3
%
Q
O
0
•J
^
" 108
107
1C
I .
4 • ' I • T ' I • '
"i ~JL ,
~ 1 ^ 5
i f " :
'
« £
— o
§3
5 <
S UJ
O • NO RAP
& A RAP
— 5-
, 1 , . 1 . 1
-2 10'1 10° 10
GEOMETRIC MEAN DIAMETER,
12. The size distribution of the effluent from a precipitator
installed on a pulverized-coal fed boiler. Measurements
were taken with and without rapping.
352
-------
2
CL
10
io"H
-2.
±90.0
-•99.0
u
_
b
A
f
-•99.9
I1
OPEN SYMBOLS - NO RAP
CLOSED SYMBOLS - RAP
&AULTRAFINE
O9IMPACTOR
10
,-2
i i i IIIHI i i MIHI|—i i i mn| 99.99
10
-i
10P
101
PARTICLE DIAMETER (MICROMETERS)
Figure 13. Fractional efficiency of an electrostatic precipitator
showing the negative effects of rapping reentrainment.
353
-------
Figure 10 shows data obtained with the sample extraction-
dilution system and the optical and diffusional sizing systems
in the effluent stream from an asphalt batching process. The
need for size resolution in real time is clearly illustrated
as the two size fractions fluctuate in concentration, sometimes
in phase and sometimes out of phase.
Figure 11 shows particle size and concentration recordings
from the large particle sampling system. Emissions are shown
in real time for 5 discrete size ranges. From these data, it
was possible to identify and analyze the contribution of the
rapping puffs to the total emissions.
In Figure 12, the size distribution measured at the outlet
of a precipitator installed on the effluent stream from a coal-
fired boiler is shown. Measurements were made with cascade im-
pactors and an electrical aerosol analyzer, with and without
rapping. Rapping emissions can be seen to constitute a large
part of the total emissions in the size range from 1 to 10 ym
diameter.
Data of the type illustrated in Figure 12 are taken at pre-
cipitator inlets and outlets to determine the collection effi-
ciency versus particle diameter (fractional efficiency). The
collection efficiency of one precipitator is shown in Figure
13. The classical dip in efficiency near 0.5 ym is clearly
shown. Also, the increase in penetration from rapping reentrain-
ment is seen to be quite large in the 1 to 10 ym size range.
Particle sizing measurements are a valuable tool in control
device evaluations, and yield necessary data for diagnoses of
poor performance, model development, and for design purposes.
Efforts are continuing to refine the techniques in order to im-
prove accuracy and to reduce their complexity and cost.
REFERENCES
1. Marple, V.A. The Aerodynamic Size Calibration of Optical
Particle Counters By Inertial Impactors. Part. Tech. Lab.
Publication 306, University of Minnesota, 1976.
2. McCain, J.D., K.M. Gushing, and W.B. Smith. Methods for
Determining Particulate Mass and Size Properties: Labora-
tory and Field Measurements. J. Air Pollut. Contr. Assoc.
24(12):1172-1176, 1974.
3. Smith, W.B., K.M. Gushing, and J.D. McCain. Procedures
Manual for Electrostatic Precipitator Evaluation. EPA-600/7-
77-059, U.S. Environmental Protection Agency, Research
Triangle Park, NC, 1977.
354
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PAPER 17
SOME STUDIES OF CHEMICAL SPECIES IN FLY ASH
L.D. HULETT
J.F. EMERY
J.M. DALE
A.J. WEINBERGER
H.W. DUNN
C. FELDMAN
E. RICCI
ANALYTICAL CHEMISTRY DIVISION
OAK RIDGE NATIONAL LABORATORY
AND
J.0. THOMSON
THE UNIVERSITY OF TENNESSEE
ABSTRACT
Fly ash specimens from coal fired steam plant precipitators
and sluice ponds have been studied. In precipitation ashes from
two different plants the distributions of trace elements in the
matrix materials that make up the ashes have been found to be
the same. Alkali, alkaline earths, and rare earths are more
concentrated in the mullite phase; transition metals such as
V, Mn, Cr, Co, and Mo are more concentrated in the magnetic
phase; carbon and sulfur are enriched in a cinder-type phase.
Ash samples extracted from sluice ponds had brown iron-rich
phases that were highly concentrated in As and Mo and enriched
in other elements. The brown phase was not found in the precipi-
tator ash, so we speculate that a transformation of the ash has
occurred in the pond. Some initial results of a glow discharge
spectroscopy technique for arsenic speciation are reported.
Mossbauer spectroscopy has been used to determine iron chemical
states in both the precipitator ash and the pond ash. Engineer-
ing studies of a fly ash treatment process that would generate
iron compounds to scavenge and fix trace elements in sluice ponds
are recommended.
355
-------
INTRODUCTION
Because of increased dependence on coal as a source of
energy the disposal of ash from its burning presents an increas-
ingly difficult problem. The amount of fly ash generated in
the United States is estimated to be in excess of 4 x 10 tonnes
per year. One can visualize a quantity such as this as a pile
one meter high, covering an area approximately 4% km square.
Many schemes for the disposal of this material are under study.
A primary concern is whether or not trace elements such as arsenic,
lead, and many others will be leached from the ash and contami-
nate the environment. The detection of the trace elements, both
in the ash and its effluent, and the determination of their con-
centrations is rather easily done, but the determining of their
chemical species (valence, compound form) is not worked out at
all. A knowledge of the chemical species of elements in fly
ash is considered very important in order to predict toxicities
and the manner in which they will react with the environment.
For example, it is believed that arsenic in the +3 state is much
more toxic than that in the +5 form. The determination of va-
lences and compound forms of trace elements in fly ash is the
subject of this study. The results and conclusions that we will
present should be considered rather tentative. Our study is
less than one year old and our ash samples have been taken from
only two TVA steam plants, Bull Run and Kingston, in the immediate
vicinity of Oak Ridge National Laboratory.
The chemist has made some rather impressive accomplishments
in molecular spectroscopies capable of determining chemical
species: infrared, ultraviolet, x-ray photoelectron spectroscopy
(XPS), x-ray diffraction, and many others. All of these are
available to our efforts, but in very few cases are sensitivities
sufficiently high to be able to determine chemical states directly
in fly ash. XPS, for example, is capable of determining valence
states, but it is a surface method, and surface concentrations
must be in excess of 0.1 monolayer for most elements to be de-
tected. Most of the trace elements in fly ash are more dilute
than this. There is no "magic spectroscopy" to which we can
expose fly ash and determine chemical states without disturbing
the specimen. This leads to the necessity of performing chemical
and physical separations on fly ash, which immediately opens
up the criticism that the system is being disturbed and probably
altered. We argue that with present analytical capabilities
there is no alternative to this.
Fly ash is a complicated mixture composed mostly of alumi-
num, silicon, iron, titanium, alkali and alkaline earths, and
oxygen. These elements are present in the form of both amorphous
glasses and crystallized minerals. The authors suspect that
when the chemical species problem of trace elements is eventually
solved it will be found that the "chemical state" of a trace
element will be defined by the matrix in which it is dissolved.
We do not think that it is likely, for example, that lead will
occur in an isolated pure form of PbO or Pb02 or that Cu will
356
-------
be found as CuSO^. It is for these reasons that we are direct-
ing a lot of our efforts toward studying matrix types. We are
attempting to determine if certain trace elements are preferen-
tially associated with certain matrices.
EXPERIMENTAL PROCEDURES
Ash specimens from two TVA steam plants, Bull Run and Kings-
ton, located near Oak Ridge National Laboratory, have been col-
lected for study. Both of these plants use the tangential firing
method which requires that the coal be pulverized to a fine
particle size. The pulverizing step effects the removal of a
large percentage of the sulfur-containing pyrites. The flue
gas from the furnaces is passed through cyclone and electrostatic
precipitators. The ash that is produced is sluiced to ponds.
Specimens from the cyclones and precipitators of both of these
plants have been collected. No attempt to collect the submicron
material that escapes from the stack has been made at this point.
Some specimens from the uppermost strata of the Bull Run plant
sluice pond have also been collected.
Mossbauer spectroscopy measurements were made by means of
a specially designed spectrometer located at the University of
Tennessee Physics Department. Another specially adapted analytical
instrument is the glow discharge spectrometer developed by Feld-
man.1 This method involves the excitation of the optical spectrum
of arsine (AsH3) under very low background conditions. Arsenic
in solutions as low as 0.1 ppb can be detected. By adjusting
the pH of the solution being analyzed, one can cause arsine to
be evolved from arsenic in the As"1"3 state only or from both the
As+3 and As+5 forms. At pH less than 1.5, both the +3 and +5
states are reduced to arsine. At pH above 5, only the As+3 form
is reduced. We thus have a method of differentiation between
As"1"3 and As"1"5.
We have adapted a special oxalate extraction method for
removing iron, and those elements associated with it, from the
work of Williams, et al.2 It appears that this will allow us
a method of speciating arsenic on fly ash surfaces. Arsenic
is displaced from the fly ash surfaces into solutions by the
oxalate extraction. The solutions are then analyzed by glow
discharge spectroscopy. Oxalate does not appear to interfere
with the method, but one must use deaerated solutions to prevent
oxidation of the arsenic.
Most of the quantitative elemental analyses performed on
the specimens of this study were done by neutron activation
analyses, but .x-ray fluorescence analysis was also used for many
qualitative and semiquantitative measurements. Carbon and sulfur
analyses were done by means of the Leco analyzer.
EXPERIMENTAL RESULTS
When we examine the Bull Run and Kingston cyclone and pre-
cipitator ashes by the optical microscope and test their magnetic
357
-------
properties we are able to distinguish at least three types of
matrices. Their gray colors are due to white transparent and
translucent particles interspersed with black magnetic and non-
magnetic particles. We have separated some of the black particles
from the white ones by individual "particle picking" and by
magnetic extraction.
X-ray diffraction patterns of the white phase indicated
that quite a bit of material had mineralized into a crystal struc-
ture of the mullite type. Mullite is a ceramic having a highly
variable composition (3 Al203-2 Si02-2 Al20?'Si02). Its dif-
fraction pattern closely resembles sillimanite: Al203*SiO2.
Mullite exists in the glass as well as the crystalline form.
In our specimens most of the white matrix material is probably
glass, but the crystalline mullite is quite prominent. Through-
out this paper we will refer to both the glass and crystalline
forms of the white matrix material as the "mullite" phase. X-
ray diffraction patterns of the non-magnetic black particles
also indicate a mullite structure, but in addition, there is
an intense, broad-lined background due to elemental carbon in
a very finely divided state. Diffraction patterns of the mag-
netic phase show it to have a well-known spinel structure which
is usually assigned to Fe30,,.
Mossbauer spectra were taken of the white matrix and mag-
netic material. For the white material the spectra suggest that
iron in both the Fe+z and Fe*3 forms is present; two sets of
broadened quadrupole-split doublets were found, one narrow doublet
characteristic of Fe+3 and one with larger splitting similar
to what is found for Fe+2, but with a smaller isomer shift than
usual. The peaks corresponding to the magnetic phase were ana-
lyzed in terms of two six-line hyperfine splittings with fields
characteristic of magnetite, as would be expected for the spinel
structure of this phase. There is a slight departure, however,
of our spectra from that of high purity magnetite. Figure 1
is a Mossbauer spectrum of some as-received fly ash, showing
peaks of both the magnetic and non-magnetic phases. Those peaks
corresponding to the magnetic phase are labeled FejO^ and those
of the non-magnetic are labeled Fe+2 and Fe+3.
Table 1 shows that trace elements in the alkali, alkaline
earths, and rare earths are higher in concentration in the mul-
lite phase than in the magnetite phase. These data are averages-
of three measurements on different size fractions from a Kingston
plant sample. Table 2 shows an identical trend for these ele-
ments for a Bull Run fly ash sample. The level of concentrations
for the trace elements in the Bull Run sample is lower than for
Kingston. The elements V, Mn, Cr, Co, Mo and U appear to be
enriched in the magnetic phase, but only by a factor of 2. Table
3 shows the composition of the black cinder phase for both the
Kingston and Bull Run plants. This phase is unique in its en-
hanced concentrations of both carbon and sulfur.
358
-------
o
1
V)
oc
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 110.00 120.00
CHANNELS
Figure 1. Mossbauer spectrum of Allen Steam Plant fly ash.
359
-------
TABLE 1. DISTRIBUTION OF ELEMENTS IN MULLITE AND MAGNETIC PHASES
OF KINGSTON STEAM PLANT FLY ASH, CONCENTRATIONS IN PPM
Element
Al
C
K
Mg
Ti
Ca
Na
S
Ba
Sr
Ce
La
Nd
Sc
Th
Sm
Dy
Cs
Hf
Eu
Lu
Element
Fe
V
Mn
Cr
Preferential Distribution in Mullite
Mullite Matrix
153,000
50,000
24,800
13,300
9,240
9,300
2,073
1,900
1,236
1,050
173
100
89
36
26
18
14
10
7
3
3
Preferential Distribution in Magnetic
Mullite Matrix
3,200
295
179
148
Matrix
Magnetic Matrix
64,000
4,800
4,200
6,000
3,586
4,600
355
1,500
425
443
68
33
0
17
12
10
8
4
3
1
2
Matrix
Magnetic Matrix
430,000
445
473
439
(continued)
360
-------
TABLE 1 (continued)
Preferential
Element
Co
MO
U
Distribution in Magnetic Matrix
Mullite Matrix
45
0
15
(continued)
Magnetic Matrix
115
23
22
Random Distribution
Element
Rb
Ga
As
Se
W
Ta
Mullite Matrix
196
67
214
18
7
2
Magnetic Matrix
175
62
283
17
0
2
361
-------
TABLE 2. DISTRIBUTION OF ELEMENTS IN MULLITE AND MAGNETIC
PHASES OF BULL RUN STEAM PLANT FLY ASH,
CONCENTRATIONS IN PPM
Element
Al
K
Mg
Ti
Ca
Na
Ba
Sr
Ce
La
Nd
Sc
Th
Sm
Dy
Cs
Hf
Eu
Lu
Element
Fe
V
Mn
Cr
Preferential Distribution in Mullite
Mullite Matrix
155,000
20,100
11,800
9,475
2,500
1,225
515
360
150
84
32
30
24
15
10
10
6
3
2
Preferential Distribution in Magnetic
Mullite Matrix
17,600
206
82
170
Matrix
Magnetic Matrix
62,700
5,100
5,300
3,300
1,900
370
260
142
63
33
0
18
11
8
6
2
2
2
1
Matrix
Magnetic Matrix
465,000
304
270
260
(continued)
362
-------
TABLE 2 (continued)
Preferential
Element
Co
Mo
U
Distribution in Magnetic Matrix
Mullite Matrix
28
0
9
(continued)
Magnetic Matrix
230
100
13
Random Distribution
Element
Rb
Ga
As
Se
W
Ta
Mullite Matrix
160
27
25
16
3
2
Magnetic Matrix
160
31
47
15
7
2
363
-------
TABLE 3. CONCENTRATION OF ELEMENTS IN CINDER MATRIX OF KINGSTON
AND BULL RUN STEAM PLANTS, CONCENTRATIONS IN PPM
Element
C
Al
K
S
Mg
Ti
Ca
Na
Ba
Sr
Ce
La
Nd
Sc
Th
Sm
Dy
Cs
Hf
Eu
Lu
Fe
V
Mn
Cr
Co
Mo
U
Rb
Kingston Plant
450,000
78,*00
7,833
3,000
5,800
4,466
7,3f'Q
753
426
433
33
52
41
15
17
10
7
3
4
2
1
9,223
92
48
1£5
11
->
6
iS>
Bull Run Plant
241,200
60,200
5,550
4,150
3,880
3,510
1,050
378
155
250
65
35
0
13
10
7
5
4
3
2
1
6,850
90
24
180
13
7
4
75
(continued)
364
-------
TABLE 3 (continued)
Element
Ga
As
Se
W
Ta
Kingston Plant
10
64
12
3
3
Bull Run Plant
6
20
35
2
1
Samples were collected from the Bull Run sluice pond by
careful core drilling of the surface so as not to disturb the
stratification. A specimen six centimeters deep was extracted
and divided into four strata. The uppermost stratum was tan
in color. The tan color was less for the second stratum. The
lower two strata were slate gray. The data in Table 4 show quite
clearly why the color changed from tan to gray; the iron concen-
tration decreased with depth of the strata. Note that the alumi-
num concentration, and some other elements common to mullite,
increased slightly with depth. Since these data were taken by
neutron activation analysis, we do not have a direct measure
of silicon content. There is a remarkable regularity in the
manner in which certain trace elements change in concentration
as a function of stratum depth. As, Se, Br, Mo, Sb, Ba, Ga,
W, and Mn decreased uniformly with depth, while Sc, Cs, La, and
Nd increased slightly.
Oxalate extractions were done on the stratified samples
of Table 4. Most of the iron was removed, leaving the residues
qray in color. Essentially all of the arsenic and molybdenum
were removed from the specimens by the iron extraction Process.
Figure 2 shows x-ray fluorescence spectra of the oxalate extract
and the residue of the uppermost stratum. The arsenic and molyb-
denum peaks of the residue are not measurable, but in the dried
oxalate extract they are quite prominent.
w.
"
tofor BuS Run sample, because of the higher property of
magnetic phase.
365
-------
TABLE 4. DISTRIBUTION OF ELEMENTS IN STRATIFIED SLUICE POND
ASH. BULL RUN PLANT, CONCENTRATIONS IN PPM
Element
Al
K
Mg
Ti
Ca
Na
Ba
Sr
Ce
La
Nd
Sc
Th
Sm
Dy
Cs
Hf
Eu
LU
Fe
V
Mn
Cr
Co
Mo
U
Rb
Ga
As
Se
W
Ta
Top Stratum
128fOOO
17,000
14,800
5,900
5,700
1,600
780
400
120
69
87
25
19
12
9
9
4
3
4
88,000
277
248
135
53
129
15
134
80
395
33
11
1
2nd Stratum
137,000
22,000
16,300
6,400
5,000
1.800
700
540
130
76
84
27
20
13
9
8
4
3
4
64,500
269
240
133
53
59
14
200
70
267
33
9
1
3rd Stratum
147,000
23,000
17,200
7,100
5,400
1,750
680
530
135
72
102
30
21
13
9
10
5
3
5
46,000
269
223
125
47
15
12
71
64
125
18
7
2
Bottom Stratum
157,000
23,000
17,500
7,500
3,500
1,300
350
217
140
74
123
32
23
13
9
10
5
3
5
17,500
248
104
125
44
12
10
220
46
30
11
8
2
366
-------
FLY ASH
AFTER
LEACHING
OXALATE
LEACH
Figure 2. Oxa/ate extraction studies of fly ash. Lower spectrum: x-ray
fluorescence spectrum of dried oxalate extract. Upper spectrum:
x-ray fluorescence spectrum of fly ash residue after extraction.
o
e/s
CA
Fe304
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 110.00 120.00
CHANNELS
Figure 3. Mossbaucr spectrum of Allen Steam Plant fly ash after
oxalate extraction.
367
-------
Glow discharge spectroscopy measurements of oxalate extracts
from Bull Run ash before and after it contacted the sluice pond
have been made. The ratio of As+3/As+5 for the precipitator
ash, before pond contact, was 0.020. After pond contact (upper
stratum, Table 4) the ratio was 0.030.
Another sluice pond specimen, very rich in the brown iron-
containing component, was acquired. Using sedimentation tech-
niques, the finely divided brown component was concentrated into
a fraction relatively free of the coarser, gray material. X-
ray fluorescence showed again that this iron-rich component had
high concentrations of As and Mo. This component was examined
by Mossbauer spectroscopy at room temperature and liquid nitrogen
temperature. No appreciable change in the spectrum at the lower
temperature was observed, indicating that the material was prob-
ably not a simple hydrated ferric oxide. Hydrated ferric oxide
becomes magnetic at liquid nitrogen temperatures and exhibits
multiplet splitting in its Mossbauer spectrum.
We suspect that the iron-rich component in the sluice pond
is produced by a transformation of the fly ash, possibly the
oxidation of Fe30,,. We have separated some ash into a magnetic
and a non-magnetic component and subjected both to aeration in
a water extract of fly ash in the laboratory. The pH of the
water extract is 4-5. After about five weeks of aeration, the
solution exposed to Fe30i, is acquiring a slight tan color. The
solution exposed to the non-magnetic material is still colorless.
SUMMARY AND CONCLUSIONS
Our thesis that we stated in the introduction of this paper,
that "chemical species" of trace elements in fly ash might be
determined by the matrices that contain them, is given a great
deal of support by our data. There does appear to be a preference
for the mullite matrix over Fe304 for the trace elements Ba,
Sr, Ce, La, Nd, Sc, Th, Sm, Dy, Cs, Hf, Eu, and Lu. It is true
that the magnetic phase also has appreciable concentrations of
these elements, but is is not completely free of mullite, and
we think that this might actually be the carrier of the trace
elements instead of the Fe30,, matrix. Table 1 shows that there
is quite a bit of aluminum in the magnetic fraction which we
assume is associated with mullite. The data in Table 2 are from.
a totally different steam plant, but they are remarably similar
to those of Table 1. This suggests that there may be a common
pattern in the way trace elements speciate themselves in fly
ash matrices. Perhaps solubilities play a role. The above trace
elements are members of the alkali, alkaline earth, and rare
earth families. Since these are soluble in glasses it is not
surprising that they concentrate in the mullite matrix.
Since the mullite phase is by far the largest component
of fly ash, it is obviously very important to determine more
368
-------
specifics about how trace elements are contained in it. Linton
Evans, Williams, and Natusch3 have shown that many of them occur
in the outer surface layers of the particles. This should be
explored further. Also, studies should be done on how trace
elements are distributed between the amorphous and crystalline
portions of mullite. These are now in progress in our labora-
tory.
Transition elements such as V, Cr, Mn, Co, Ni, Cu, and Mo
are known to be soluble in iron oxides and many are capable of
forming solid solutions in F^O,,. The mixed spinel, NiFe20^,
for example, is also magnetic and has an x-ray diffraction pattern
very similar to FejO,,. This led us to speculate that some of the
transition elements may seek the magnetite matrix in preference
to the mullite. The data for V, Cr, Mn, Co, and Mo support this
(our analyses for Cu and Ni are not complete) , but the enrich-
ments are only by a factor of 2. It appears that the transition
elements do favor the magnetite matrix, but they are not exclu-
sively associated with it. The fact that the Mossbauer spectrum
of the magnetic phase departs slightly from that of pure Fe304
suggests that solid solutions may have occurred.
The cinder phase that we isolated was very rich in carbon
and sulfur. Perhaps the sulfur was adsorbed on the surfaces
of the carbon; our x-ray diffraction patterns showed that it
was finely divided. In past work, we have shown that sulfur
is associated with fly ash surfaces.1* We could find no clear
cut evidence of other trace elements being preferentially concen-
trated in the cinder phase. At the Asheville meeting, where
this paper was presented orally, we stated that the cinder phase
appeared to be rich in Se and Br; but since that time, we have
examined other cinder specimens and found Se and Br to be much
lower.
Fly ash researchers may have a lucky break with regard to
arsenic speciation. Previous work by ourselves1* and others show
that arsenic is primarily associated with fly ash surfaces.
Treatment of fly ash with oxalate appears to elute a large por-
tion of the arsenic. Feldman's glow discharge spectroscopy
method is capable of distinguishing As+3 and As . Another
ORNL group is also using the glow discharge spectroscopy method
to speciate arsenic in runoff waters from ash disposal sites.
The ratio of As+3/As+s was found to be slightly greater for ash
from precipitators than for ponded ash, but this does not appear
significant to us. Since the ratio in both cases is rather small,
we tentatively conclude that most of the arsenic is oxidized
to +5 before it contacts the pond.
We think that in sluice ponds fly ash can undergo trans-
formations such that a new iron-rich phase is generated. Arsenic
and molybdenum can be translocated to this phase. The color
of the Bull Run pond ash samples was considerably different from
369
-------
that of the precipitator specimens. The upper strata of the pond
samples were deep tan because of high iron concentrations, and
there was a definite correlation of the concentrations of As,
Mo, and other trace elements with iron concentrations. The fact
that As and Mo ate removed simultaneously with iron, when it
is extracted with oxalate, suggests strongly that they are located
on the surface of the iron-rich phase. There is a correlation
of other elements, Se, Br, Sb, Ba, Ga, W, and Mn with iron con-
centration in the stratified pond samples, but we have not yet
shown a definite association with iron, as for As and Mo. Theis
has also reported correlations of certain trace elements with
both iron and manganese.5
More work should be done to determine the exact nature of
the iron-rich material formed in sluice ponds. Our Mossbauer
data show that is is not "hydrated iron oxides." Also, engineer-
ing studies should bo considered for the processing of fly ash
to promote the formation of iron compounds that are efficient
in fixing trace elements in sluice ponds and disposal sites.
Rather than allow the magnetite and other oxidizable iron com-
pounds to lie "useless in the pond, perhaps it would be practical
to transform them into trace element scavengers.
ACKNOWLEDGEMENT
The authors would like to express their sincere apprecia-
tion to the Electric Power Research Institute, Palo Alto, Cali-
fornia, for the funding of this work. We are especially indebted
to Dr. Ralph Perhac of that Institute for directing our attention
to the importance of fly ash chemical speciation. Professor
Roger Minear of the University of Tennessee Chemistry Department
has also contributed to our overview of the fly ash problem.
Dr. Perhac and Dr. Minear have worked together to facilitate
contacts between many individual researchers.
REFERENCES
1. Feldman, C. Annual Progress Reports, 1975, 76, 77, Analyt-
ical Chemistry Division, Oak Ridge National Laboratory,
ORNL 5100, ORNL 5244, ORNL 5360. Improvements in the Arsine
Accumulation-Helium Glow Detector Procedure for Determining
Traces of Arsenic, submitted to Analytical Chemistry.
2. Williams, J.D.H., J.K. £yersr S.S. Shukla, R.F. Harris,
and D.E. Armstrong. Environ. Sci. Technol. 5(11) :113-
1120, 1971.
3. Linton, R.W., Peter Williams, C.A. Evans, and D.F.S. Natusch.
Determination of the Surface Predominance of Toxic Elements
in Airborne Particles by Ion Microprobe Mass Spectrometry
and Auger Electron Spectroscopy. Anal. Chem. 49:1514, 1977.
370
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4. Hulett, L.D., T.A. Carlson, B.R. Fish, J.L. Durham. Studies
of Sulfur Compounds Adsorbed on Smoke Particles and Other
Solids by Photoelectron Spectroscopy. In: Determination
of Air Quality, G. Mamantov and W.D. Shults, eds. Plenum
Press, New York, 1972; L.D. Hulett, H.W. Dunn, J.M. Dale,
J.F. Emery, W.S. Lyon, P.S. Murty. The Characterization
of Solid Specimens from Environmental Pollution Studies
Using Electron, X-ray and Nuclear Physics Methods. Mea-
surement Detection and Control of Environmental Pollutants,
International Atomic Energy Agency Conference (IAEA SM-206/1)
Vienna, Austria, 1976.
5. Theis, Thomas L., and J.L. Wirth. Sorptive Behavior of
Trace Metals on Fly Ash in Aqueous Systems. Environ. Sci.
Technol. 11(12):1096-1100, 1977.
371
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SUMMARY
ADVANCES IN PARTICLE SAMPLING AND MEASUREMENT
WILLIAM FARTHING
SOUTHERN RESEARCH INSTITUTE
The technology for particulate sampling in process streams
has been expanded and improved over the last several years.
New instruments have been developed, traditional devices and
methods have been refined, and standard procedures have been
established. The Process Measurement Branch of the Environmental
Protection Agency's Industrial Environmental Research Laboratory
sponsored a symposium in Asheville, NC, on recent developments
and related research.
The symposium began with papers concerning inertial effects
of sampling and sizing by inertia. C.N. Davies reviewed theo-
retical and experimental work concerning the aspiration coef-
ficient of sampling tubes and presented some new findings. His
primary emphasis was on the inability of the existing theory
to account for all of the effects of a finite wind velocity on
the aspiration of particles with finite inertia by a sampling
tube with finite diameter. An experimental test of one expres-
sion, a quadratic equation of the Stokes number, was presented
for tubes sucking at right angles to the wind. The comparison
showed that the level of aspiration predicted by the quadratic
was too high and there was evidence that at least a cubic equa-
tion is necessary. A potentially useful criterion was also given
for accurate sampling at low wind speeds relative to the sampling
velocity.
In a paper presented by Koichi linoya, the observed and
calculated effects of anisokinetic sampling were presented for
round and slit probes. It was suggested that the results pre-
sented could be used to estimate sampling errors. The concen-
tration errors were smaller at extremely high sampling velocities,
so super high speed sampling could be a simple method without
the tedium of isokinetic sampling. The amount of particles de-
posited in the probe and sampling tube was found to be minimized
when the Reynolds number was between 2000 and 3000. In a study
372
-------
of the effect of probe diameter on sampling errors with isokine-
tic sampling, it was found that the reproducibility, measured
in terms of the standard deviation of the deduced concentration
from many tries, of sampling experiments decreases as the probe
diameter decreases. In those experiments the average deduced
concentration increased as the probe diameter decreased.
Other work described by Dr. linoya concerned particle impac-
tion efficiencies on ribbons suspended at different orientations
across a pipe and impactors with different clearance ratios,
defined as the ratio of the nozzle to plate distance over the
nozzle diameter. In the ribbon study, calculations based upon
a discontinuous potential flow model agreed well with observed
efficiencies over a wide range of particle Reynolds numbers
extending from the Stokes to Newton regions. (These velocities
are somewhat higher than employed in typical impactors for sam-
pling process streams.) The purpose of the work concerning
impactors was to theoretically account for the effects of the
velocity boundary layer at the nozzle outlet of impactors using
a potential flow model. Good agreement with measured efficiencies
was claimed at ratios of 2, 1, 0.5, 0.33, and 0.25.
The performance of cyclone collectors and samplers was de-
scribed by Morton Lippmann. Traditionally used for control de-
vices, cyclones are now being routinely used for sampling of
"respirable" aerosols for assessment of health effects. More
recently, cyclones have been introduced for in-situ classifi-
cation of aerosol size distribution. Cyclones have been designed
with sharp collection efficiency versus size curves comparable
to impactors down to particle diameters of 0.5 ym while offering
several advantages such as no deterioration in performance with
large samples and much simpler operation than with dichotomous
samplers. The major problem is the lack of an accurate theory
to be used for design purposes. One important point of this
paper was the evidence that caution must be exercised in coupling
cyclones in series because it may affect flow and thus the be-
havior. An empirical relation for efficiency was presented in
this paper with four constants. The constants have been evaluated
for eleven cyclones for which reliable calibrations exist cover-
ing a range of particle sizes from 0.3 to 10 ym and gas flow
rates from 0.1 to 2700 Jl/min.
Recently developed systems for particle sizing by impactors
were described by Michael Pilat. The University of Washington
Mark 4 Cascade Impactor utilizes low absolute gas pressure re-
sulting in large Cunningham correction factors permitting in-
ertial sizing of particles from 0.02 to 0.2 ym in diameter.
In conjunction with the University of Washington Mark 3 Impactor,
two systems, one with 0.5 acfm and another with 1.8 acfm flow
rate, have been assembled and tested which size particles from
0 02 to 20 urn. A new impactor for high concentrations, using
many stages (28) to reduce overloading, has also been constructed
for sizing particles from 0.05 to 30 ym.
373
-------
Recent work with the Sinclair diffusion battery was described
by Earl Knutson. It has been coupled to a data processing sys-
tem which enabled unattended runs of several days with hourly
sampling. The penetration of particles through the battery was
measured with a new continuous flow condensation nuclei counter.
Experience has revealed several improvements in operating proce-
dures. An iterative mathematical technique was studied and
chosen to calculate size distributions from the data obtained.
Nearly 1000 size spectra of New York City urban aerosol have
been analyzed showing typical geometric mean diameters from 0.025
to 0.050 ym with geometric standard deviations from 1.8 to 2.0.
Although in-situ sizing of particles is available, many
instances still arise when laboratory sizing is desirable. Ronald
Draftz described the instruments available for such work. Such
measurements can be very sophisticated and can include shape
measurements in short times. The electronic image analyzers
are extending the practicality of the many useful measurements
possible with a light microscope. The problem of particle ag-
glomeration remains unsolved.
A recent field evaluation (at a Canadian lead smelter) of
five real time particulate monitors, all commercially available,
was described by A.W. Gnyp. Two transmissometers, a mass monitor
based on the backscattering of light, a charge transfer monitor,
and a beta radiation attenuation device were rated according
to accuracy, reliability, and maintenance over a one year period.
Accuracy was based upon the proportionality of the instrument
signal to aerosol mass concentration determined as a reference
by a manual sampling train. The sensitivity of the monitor out-
put to particle size distribution and composition was evaluated
using impactors and laboratory analysis of samples as references.
The overall conclusion was that reliable continuous mass monitor-
ing is now possible in stacks if the aerosol physical and chemi-
cal properties are approximately constant, as found in process
streams that have passed through efficient control equipment.
A real time in-situ particle sizing probe utilizing small
angle light scattering is being developed under the sponsorship
of the Process Measurement Branch of USEPA/IERL. The field
prototype of the instrument exhibited at the symposium is now
being tested. Robert Knollenberg described the operation prin-
ciple and some preliminary results from a coal-fired power plant.
The instrument covers a 0.3 to 10.0 ym size range with 60 chan-
nels resolution. The major uncertainty in sizing spherical par-
ticles with the instrument, performed by relating size to flux
scattered at small forward angles by single particles, is the
particle refractive index giving at most an error of ±20% and
normally within ±10% of actual size. The concentration range
for accurate measurements limited by coincidence counting in
the present model is 5 x 10"/cm3. Normally, the main effect
374
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of higher concentrations is to decrease the effective size ranqe
An optical velocimeter is also designed into the instrument.
The present design permits temperatures up to 250°C and veloc-
ities up to 30 m/sec. The results of an initial in-stack test
at a coal-fired power plant with an electrostatic precipitator
and a scrubber were reasonable. Calculated opacity from the
measured particle size distribution was about 15% while measured
opacity was 17%. The calculated mass loading was 0.01 to 0.02
g/m with a volume average diameter of about 1.3 Mm. It appears
that the instrument was capable of resolving several size modes
in this test.
Another in-situ portable light scattering instrument being
developed under EPA sponsorship to determine size distribution
was described by A.L. Wertheimer. This device measures flux
scattered from many particles simultaneously at three small
angles relative to the forward direction, 4°, 8°, and 11°, and at
a range of large angles of 80°-100°. Each measurement is per-
formed at two wavelengths, 0.45 and 0.9 urn, and the large angle
scattering is measured at two orthogonal polarizations. The
instrument relates the small angle signals dominated by Fraun-
hofer diffraction to the volume of particles in three size ranges,
centered at 1.0, 3.5, and 7.0 ym. For the lower end of the size
distribution, the difference in the two 90° signals at two
orthogonal polarizations obtained with the 0.9 urn (0.45 pm) wave-
length is related to the volume of particles in a size range
centered about 0.4 ym (0.2 ym) . The size range, mass loading,
and temperature range are 0.1 to 10 ym, 4 to 400 ppb by volume,
and 0° to 260°C. The prototype had just been delivered to EPA
to be tested in a wind tunnel facility.
Kenneth T. Hood reported the results of studies performed
by the National Council for Air and Stream Improvement concerning
the relationships of plume opacity, in-stack opacity, and contin-
uous mass monitors to mass emission rate as defined by filter
samples. A previous study had indicated the validity of an in-
stack transmissometer to monitor particulate concentrations in
flue gas, and calibration curves had been obtained for each site
to provide mass concentrations from transmissometer opacities.
In these studies observer opacity was frequently related to par-
ticulate concentration through use of simultaneous transmissom-
eter measurements and their calibration curves. The opacity
studies involved five different sites: a lime kiln, two kraft
recovery furnaces (one with a direct contact evaporator, and
one without) , and two wood and fossil fuel-fired boilers (one
with coal and another with oil) . The observer opacity under
good contrast conditions (dark or blue sky background) was found
to vary between ±10 and ±15% opacity from the average. The rela-
tionships of plume opacity to in-stack opacity were generally
found to be linear; however, the slopes were not one as might
be expected and varied from site to site, and the curves did
not intercept the origin. At the non-direct contact evaporator
system, 75%, 50%, and 25% of the technically trained observers
375
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read a non-visible plume at instrument opacity levels of 6%,
8%, and 10%, respectively. In tests of the mass monitor based
upon backscattering of light, it was compared to transmissometer
behavior at two kraft recovery furnaces and a wood-waste fired
spreader stoker boiler. The mass monitor was found to be equi-
valent to the transmissometer in quality of data and perform-
ance except that the former required less attention at high load-
ings (3.76 g/acm) at which the latter required frequent cleaning.
After three sessions on instruments for sampling and measur-
ing aerosols, the topics of the meeting turned toward procedures
for using the data and quality control. A computer based data
reduction system for cascade impactors, developed under EPA
sponsorship, was described by Joseph D- McCain. The system of
programs, becoming available for general use, utilizes calibra-
tion information for each impactor with operating conditions,
time, and stage weights to determine particle size distributions
in any one of several forms for individual runs or averages there-
of. The first program in the series utilizes a spline technique,
described as an "automated French curve", to fit a curve to the
cumulative size distribution. If differential mass or differen-
tial number size distributions are desired, these are obtained
from the cumulative curves which are forced to be continuous
in coordinates and slopes. Statistical analyses, including the
location of outliers, are performed on the data. Additional
programs permit data sets from inlet and outlet measurements
of control devices to be combined to determine fractional ef-
ficiencies. The program in its present form handles all com-
mercially available round jet cascade impactors, including common
modifications, which are in current use in stack sampling. Other
round jet and slot type impactors could be accommodated with
slight program revision.
In the area of instrument calibration, D.Y.H. Pui described
various aerosol generators now available. Three instruments
provide monodisperse aerosols for primary standards over a broad
range of sizes: the vibrating orifice aerosol generator for
diameters of 0.5 - 50 ym, the mobility classifier monodisperse
aerosol generator for diameters of 0.01 - 0.5 ym, and the ultra-
fine condensation monodisperse aerosol generator for smaller
sizes. The constant output atomizer aerosol generator provides
polydisperse aerosols of high concentration and stability. In
addition, some calibration measurements of various aerosol measur-
ing and sampling devices were reviewed, and comparative studies
showed good agreement between different types of instruments.
Quality control work by EPA was presented by D. Bruce Harris
and Gene Tatsch. Particle bounce or blow off is perhaps the
major problem associated with particle sizing with cascade im-
pactors. The use of substrates on collection plates such as
grease or glass fiber filter material has been employed to reduce
bounce. Tests were performed to ascertain which of the available
substrates are most suitable for flue gas sampling. Of nineteen
376
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greases tested, only Apiezon H grease was found to perform satis-
factorily at temperatures above 149°C (300°F). At higher tem-
peratures glass fiber material appeared to be more stable. How-
ever, large mass changes, induced by reactions related to SO*
concentrations, were observed with glass fiber and to a lesser
extent with grease. After various laboratory and field tests
a treatment procedure for glass fiber was identified which signi-
ficantly reduces mass gains. This treatment involves washing
the substrates in I^SO,, followed by rinsing, drying, baking
and in-situ conditioning before use in impactors. Reeve Angel
934AH glass fiber filter material performed best among those
tested. It was pointed out, however, that filter materials
change from batch to batch. The details of the results of this
study are available in a recent report.
In another area of quality control, Mr. Tatsch described
problems which have arisen because different procedures are pres-
ently utilized to reduce and present data, specifically that
from impactors. Methods are being sought to alleviate such
problems. It is hoped that the computer programs described in
the paper by Mr. McCain will be a practical means for uniform
data reduction and presentation with the required versatility.
The last session of the symposium contained three papers
concerning the procedures by which the available technology is
utilized to evaluate particulate control devices: scrubbers,
baghouses, and electrostatic precipitators. The first paper,
by Richard Chmielewski, was concerned with identification and
solutions of problems which arise in sizing wet particles. It
was suspected that because of the pressure reduction from stage
to stage, evaporation of water from particles occurred inside
cascade impactors. This was indicated by disagreement between
measured size distributions and those predicted from scrubber
theory. Laboratory experiments with a University of Washington
cascade impactor and a series of glass impactors verified this
suspicion. The jet diameter of the glass impactors could be
adjusted to minimize pressure drop. It was also found that pres-
sure drop across a stage is higher for wet particles than dry
ones. Based on the laboratory results a single stage impactor
usable in the field with a variable cut point was constructed
using a Greenburg-Smith impinger to minimize pressure reduction
and evaporation time. In one application the measurements agreed
with the prediction that the condensed particle size distribution
should have a minimum aerodynamic diameter on the order of 0.5
urn. Also discussed were problems which arise in measurements
of'entrainment where particles above 10 urn are involved. None
of the attempted techniques hot wire anemometer, cascade im-
pactor, and chemical stain methods—give completely satisfactory
results.
377
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A presentation of the measurements necessary for evaluation
of fabric filter control devices by D.S. Ensor stressed the im-
portance of a systems approach and real time monitors for a
meaningful evaluation. The overall mass loading is normally
very low and cleaning processes cause large fluctuations in
loading at some units one compartment per minute receives a
burst of air in the reverse direction. This is accompanied by
large changes in the velocity distribution of the stream. Other
aspects causing variations are bag integrity, bag conditioning
time, and valve malfunctions. The problems of matching data
sets obtained at different locations with different instruments
are reduced with a mobile control room containing an automatic
data logging system with a common time base. Sizing instruments
utilized in evaluations are diffusion batteries, optical particle
counters, electrical mobility analyzers, and cascade impactors.
In addition, gas monitors, pressure transducers, and a light
scattering nephelometer are used to track the dynamic behavior
of both the baghouse and the source.
The presentation concerning electrostatic precipitators
by Wallace B. Smith also stressed the importance of monitoring
transient events. Rapping reentrainment of short duration has
been found in many installations to be responsible for most of
the total emissions. If, for diagnostic purposes, collection
efficiency due to electrostatic behavior is sought, the measure-
ment program must differentiate between the different types of
emissions: simple penetration, rapping reentrainment, sneakage
around the collection fields, and boil up from the hoppers.
A measurement system was described which determines collection
efficiency for particles from 0.01 to 10 ym diameter. It in-
cludes cascade impactors, mass trains, electrical mobility ana-
lyzers, diffusion batteries, and optical particle counters.
A specially designed extraction-dilution apparatus is used to
condition the aerosol for compatibility with the latter three
instruments. Laboratory experiments were performed to determine
the effects of particle charge upon impactor behavior. The
experiments showed that, at high charge levels, size distribu-
tions determined with impactors are significantly skewed toward
larger sizes. However, at charge levels produced in electro-
static precipitators on process streams (where fields of 4 -
8 x 103 V/cm are employed), the effect of particle charge was
not significant.
378
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Research into the chemical state of fly ash was the last
topic on the program. L.D. Hulett reported some results from
a program to develop methods for trace element analyses in fly
ash and in waters with which it has been in contact. The samples
studied came from a precipitator hopper and a sluice pond dis-
posal site at a coal-fired power plant. The analytical procedures
discussed were x-ray fluorescence, scanning electron microscopy,
optical microscopy, x-ray diffraction, x-ray photoelectron spec-
troscopy, Mossbauer spectroscopy, flow discharge spectroscopy,
and chemical separation. None of these techniques can discrim-
inate chemical states of trace elements from fly ash matrix,
so the indirect approach of oxalate leaching was utilized. This
treatment separated iron in the +2 and +3 valence states accom-
panied by nearly all of the arsenic and molybdenum present espe-
cially after exposure to the pond. It appears that these were
associated with the iron on the fly ash surfaces. Most of the
lead was found in a phase, termed "mullite", consisting mainly
of ferroaluminosilicate glass. Selenium was concentrated in
a black cinder phase consisting of a combination of carbon and
"mullite". A phase, termed "magnetite", was identified as rela-
tively free of trace elements. A method was developed using
glow discharge spectroscopy to determine the ratio As+3/As+s
in water exposed to fly ash. Other trace elements observed on
the surfaces were zinc, copper, selenium, and bromine. A search
for the valence states of other elements will be continued.
379
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METRIC CONVERSION FACTORS
To convert from;
Ib
gr/ft3
ft3/min (cfm)
lbs/in.2
°F
ft2/1000 cfm
in. WG
gallons
ft
in.
tons
in.3
ft3
gal/min
ft2
in.2
gal/1000 ft3
grams
ft/min
ounces
oz/yd2
grains
gr/ft2
Ib force
lb/ft2
in. H20/ft/min
Btu
To;
kg ,
g/m3
m /sec
kg/m2
°C
m2/(m3/sec)
mm Hg
liters
m
m
kgs
cm
m3
I/sec
m2
cm2
1/m3
grains
cm/sec
grams
g/m2
grams
g/m2
dynes
g/cm2
cm H20/cm/sec
calories
Multiply by;
0.454
2.29
0.000472
703.
(°F-32) x 5/9
0.197
1.868
3.785
0.3048
0.0254
908.
16.39
0.028
0.0631
0.0929
6.452
0.135
15.43
0.508
28.34
33.89
0.0647
0.698
44.44 x 105
0.488
5.00
252
380
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TECHNICAL REPORT DATA
(flease read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-79-065
3. RECIPIENT'S ACCESSION- NO.
4. T,TLE AND SUBTITLE proceedings: Advances in Particle
Sampling and Measurement (Asheville, NC, May 1978)
5. REPORT DATE
February 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
W.B. Smith, Compiler
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
INE623
11. CONTRACT/GRANT NO.
68-02-2131, Task 21306
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 PERIOD COVERED
Proceedings; 1/77 - 9/78
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES IERL-RTP project officer is D. Bruce Harris, MD-62, 919/541-
2557.
16. ABSTRACT rj,ne proceedings consist of 17 papers on improved instruments and tech-
niques for sampling and measuring particulate emissions and aerosols; e. g. , cas-
cade impactors, cyclone collectors, and diffusion-battery/nuclei-counter combina-
tions. Transmissometers and instruments for measuring scattered light were used
for continuous monitoring of emissions from a secondary lead smelter and from
three pulp and paper mill sources (a wood-waste-fired power boiler, a kraft reco-
very furnace, and a lime kiln). Also discussed were: computer-based data reduction
for plotting curves of particle size distribution; equipment for generating aerosols
and its use in calibrating instruments; analysis for trace elements in fly ash; and
sources of sampling errors. Applications of improved techniques for evaluating
emission control devices included: a wet impingement technique for measuring par-
ticle sizes in wet scrubbers; use of real-time instruments in studying the dynamic
behavior of baghouses; and measurement of fractional collection efficiency and
detection of fly ash entrapment in electrostatic precipitators.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
cos AT I Field/Group
Pollution Diffusion
Dust Transmissometers
Aerosols Light Scattering
Sampling Lead
Measuring Instruments Smelters
Impactors Paper Industry
Cyclone Separate
ia. "DISTRIBUTION STATEMENT
Unlimited
Pollution Control
Stationary Sources
Particulate
Cascade Impactors
Nuclei Counters
Lime Kilns
Baghouses
19. SECURITY CLASS (ThisReport)
Unclassified
13B
11G
07D
14B
131
07A
20N,20F
07B
11F
11L
21. NO. OF PAGES
390
20 SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
381
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