STATE OF THE ART: 1971
INSTRUMENTATION FOR MEASUREMENT
OF PARTICULATE EMISSIONS
FROM COMBUSTION SOURCES
VOLUME III: PARTICLE SIZE
by
Gilmore J. Sem, John A. Borgos,
Kenneth T. Whitby and Benjamin Y.H. Liu
Thermo-Systems Inc.
2500 North Cleveland Avenue
St. Paul, Minnesota 55113
Contract No. CPA 70-23
Program Element No. 1AA010
EPA Project Officer: John O. Burckle
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, N. C. 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
July 1972
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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TABLE OF CONTENTS
VOLUME III
Page
FOREWORD 1
ABSTRACT AND CONCLUSION 2
A. INTRODUCTION 4
B. TECHNIQUES WITH SEPARABLE CLASSIFICATION AND SENSING ... .13
1. Particle Classification Techniques 13
2. Particle Sensing Techniques 44
3. Combinations of Classifiers and Sensors 44
C. TECHNIQUES WITH INSEPARABLE CLASSIFICATION AND SENSING . . .54
1. Optical Techniques 54
2. Impact and Momentum Sensors 61
3. Piezoelectric Single Particle Counter 62
D. LABORATORY POWDER SIZING TECHNIQUES 63
E. A DIFFERENT SIZING CONCEPT: PARAMETRIC MEASUREMENT 65
F. SUMMARY AND CONCLUSIONS 69
G. REFERENCES 72
111
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FOREWORD
The compilation of the information contained in this publication was per-
formed pursuant to Contract CPA 70-23 for the Environmental Protection Agency.
The work was sponsored by the National Environmental Research Center at Research
Triangle Park for the purpose of conception and evaluation of instrument systems
for particle size distribution applicable to the measurement of emissions from
stationary sources.
This report was prepared during the period from January to August, 1971, and
is the third of a four volume series. These volumes contain the following:
Volume I of this report is written for the engineer or planner who needs to know
a few basic facts about a particulate mass measurement technique and wishes to
minimize the time required to obtain this information. Volume I is intended for
use as a quick reference guide.
Volume II of this report is designed as a detailed in-depth report on operating
principles, techniques, historical data, and discussion of the more viable tech-
niques for particulate mass monitoring. Volume II is designed for the plant
engineer, abatement and control officials, and others who may not be familiar
with the detailed technology of these areas. Included are sections on power
plant emissions properties and extraction sampling probes.
Volume III of this report is a comprehensive survey of particle sizing techniques
which may be used by the plant engineer, abatement and control officials, and
others as a quick reference guide or as a source of more detailed information,
including references to original work.
Volume IV of this report describes an experimental evaluation of the beta radia-
tion attenuation technique for mass concentration measurements on a coal-fired
power generating plant. Problem areas requiring further developments are iden-
tified for personnel concerned with improving the techniques.
These reports have been issued as they were completed to make them available
to the public on a timely basis. Volume I and II were issued in September 1971.
Volume IV will be issued late this Fall.
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ABSTRACT AND CONCLUSION
Volume III (this volume) discusses candidate techniques for automatic
or semi-automatic measurement of particle size distribution in combustion source
effluents. Automatic or semi-automatic particle size measuring instruments do
not yet exist for this application. This report considers the application to
effluent streams of particle size measuring instruments used in other fields.
The discussions emphasize the particulate concentration parameter (mass, number,
surface area, etc.) which each technique senses as well as the method of classify-
ing particles into size ranges (aerodynamically, electrostatically, optically,
etc.) Included are descriptions of the basic operation of each technique,
discussions of limitations of each technique, suggestions of possible major
problems in applying each technique to effluent streams and an overall evaluation
of each technique relative to others.
The most promising approach for detailed size analysis on a routine basis
at this time is an aerodynamic particle size classifier combined with either
beta radiation attenuation, piezoelectric microbalance, or photometric concentration
sensors. The impactor classifies particles aerodynamically, the most useful method
for most air pollution applications. The beta and piezoelectric sensors (see
Volume I and II of this report for discussion of sensors) can detect the mass con-
centration, while a correctly-designed photometric sensor detects a parameter
(related to surface area) which could be used as an indicator of the effect of the
emissions on visibility. Although the more promising aerodynamic techniques can
classify particles in an approximate range from 0.2 to 30 microns, the particles
above and below these limits could be lumped into separate size categories, per-
mitting a gravimetric size measurement covering nearly the entire range of particles
found in effluent streams (from about 0.001 to above 100 microns).
Several techniques appear applicable to special sizing problems in effluent
streams. Cyclone classifiers can be used to separate the "respirable fraction"
of particles. Electrostatic and diffusion classifiers can possibly be used to
measure detailed size distributions in the range from 0.001 to 0.6 microns.
Modified optical particle counters may be useful for some research applications.
Holography offers the ability to photograph (in 3-dimension) the effluent par-
ticles in the stream without disturbing them in any way, but appears limited for
most sizing applications by its lower particle size resolution when operated across
stack distances, by its cost, and by its complexity.
Another concept looks promising for use as a continuous, routine monitoring
technique for effluent streams. The technique uses the size limitations of at
least three particle concentration monitors to measure particle concentration in
three or more size ranges. Three candidate concentration monitors measure: (1) the
mass concentration (sensitive primarily to 1 - 100 micron particles), (2) the opacity
of the effluent stream (sensitive primarily to 0.1 - 10 micron particles), and (3)
the number concentration (sensitive primarily to 0.001 - 1.0 micron particles).
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Respirable mass concentration is another promising candidate. Instruments which
measure most of these parameters have already been used in effluents so only a
moderate amount of hardware development appears needed. Simple analysis of such
measurements made simultaneously may provide sufficient particle size and con-
centration information for most routine monitoring applications.
Although not covered in this report, considerable research and development
must be done to develop sampling systems which can deliver truly representative
particle samples to tlie sizing instruments. No practical candidate for automated
sizing exists which has sufficient resolution to cover the most important effluent
size range and which does not require extraction of a sample from the effluent
stream.
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INSTRUMENTATION FOR
MEASUREMENT OF PARTICULATE EMISSIONS
FROM COMBUSTION SOURCES
VOLUME III: PARTICLE SIZE
A. INTRODUCTION
The concentration, size, and chemical composition of airborne particles
are the three most important properties defining the potential effects, harm-
ful or otherwise, of most particulate dispersions. Automatic measurement of
the concentration, specifically the mass concentration, of airborne particles
is the subject of Volumes I and II of this report. This volume discusses the
automatic measurement of size and size distribution of airborne effluent par-
ticles from large coal and oil combustion sources. Chemical composition and
its measurement is not a subject of these reports.
The size of an airborne effluent particle plays a very important part in
determining its future as an air pollutant. One of the primary purposes for
the measurement of particulate effluents is to monitor the potential harmful-
ness of the emissions so that the degree of control can be evaluated. A con-
tinuous measurement of particle concentration or emissions rate is usually not
a sufficient indication of the potential harmfulness of particulate emissions.
Measurement of the size of the particle greatly improves the estimate of the
potential harmfulness.
To comprehend the magnitude of the effect of particle size, consider one
microgram of particles that is made up of a single 100 micron (ym) diameter
particle with a density of about 2.0 grams per cubic centimeter. Nearly all
100 urn particles are collected by most control equipment on effluent sources.
The few 100 ym particles that are emitted to become air pollution settle to
the ground quickly, usually within a mile or two of the source. Their settling
velocity is about 50 centimeters per second. The settled particles result in
such harmful effects as dirty cars, houses, and streets, contamination of soil
resulting in harm to nearby farm crops, contamination of nearby lakes and rivers,
and the aesthetic degradation of the community. There is almost no direct health
hazard to humans from inhalation of such particles because very few reach the
person, very few of those that do reach him enter his respiratory system, and
nearly all which enter the respiratory system are collected by the entrance nasal
passage. The chemically-reactive surface area of 100 ym particles is relatively
small compared to the same mass of smaller particles.
If that same microgram of particles is made up of particles 1 ym in diameter,
there would be about 1,000,000 of them. Most particulate control equipment in use
today probably collects only about half of the 1 ym particles in an effluent stream.
Most cyclone collectors collect almost none of these particles while high efficiency
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electrostatic precipitators can be designed to collect most of them. The
settling rate of 1 ym particles is about 0.05 centimeters per second, almost
negligible in many cases. A 1 ym particle does not grow very rapidly by
agglomeration with other particles or by condensation of liquids on its surface.
Thus, these particles will travel long distances from the effluent source before
settling to the ground or depositing onto a surface. A large portion of such
particles emitted by effluent sources in industrial areas reach heavily-populated
centers where they scatter large amounts of light resulting in reduced visibility
and less sunlight reaching the ground, where they coat surfaces with grimy films,
and where people inhale them. One ym particles penetrate more deeply into the
respiratory system than nearly any other size. The chemically-reactive surface
area of 1 ym particles is about 10,000 times greater than an equal mass of 100 ym
particles. One ym particles, or slightly smaller, penetrate even the highest
efficiency fiber and membrane filters more easily than any other size. Thus, 1 ym
particles are a different type of air pollutant than 100 ym particles, and should
not be lumped with 100 ym particles for measurement purposes.
Now, if that microgram of particles is made up of 0.01 ym particles, there
would be 10-J-2 of them. Although no comprehensive study has been made, even the
highest efficiency electrostatic precipitator control equipment in use today
probably does not collect a significant fraction of these particles. It appears
that only high efficiency filters can collect them. Their gravitational settling
velocity is about 10"-' centimeters per second, completely insignificant when com-
pared with motions caused by forces such as wind, Brownian diffusion (random
molecular bombardment), temperature gradients, electrostatic fields and charges,
and Van der Waals molecular attractive forces. These particles can grow rapidly
by condensation of vapors, resulting in a higher liquid content of the particle.
They can also grow by agglomeration with other particles resulting in a lesser
number of larger particles. The size distribution of particles in this size range
often changes rapidly, making measurements difficult, and resulting in larger
particles (0.1 - 1.0 ym) after a period of time. In normal city atmospheres, this
growth process requires a few hours. Particles in the size range from 0.01 - 1 ym
make up most of the photochemical smog so prevalent in cities like Los Angeles.
Brownian diffusion causes most of the 0.01 ym particles to deposit in the upper and
middle portions of the respiratory system, causing them to penetrate less deeply
than 1 ym particles. These particles are highly reactive, having a surface area
about 100 million times greater than an equal mass of 100 ym particles. Particles
in the 0.01 - 1 ym range are often the nuclei for raindrops and ice crystal formation.
The effects of these small particles has little in common with the effects of 1 or
100 ym particles, and the concentration of such particles should be measured
separately.
Combustion effluent sources emit particles over an even broader size range:
from about 0.001 to over 100 ym. All portions of the size range are important in
defining the pollutant potential of particulate emissions. Prediction and con-
trol of photochemical smog formation requires information about the very small
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particles (from about 0.001 to 1.0 ym) . The study and prediction of mete-
orological effects such as modified raindrop production also need measurements
in this size range. Control of respirable particles requires measurement of
particles in the 0.1 - 10 ym range. The range from 0.1 - 10 ym affects visibility
most strongly. Control of dust fall near an effluent source requires measurement
of the particles larger than about 10 ym. Thus, the need .for measurements of
particle size over the entire range is established.
The size of a particle is usually defined in terms of a diameter or radius.
However, anyone looking at a sample of effluent particles with a microscope
realizes that most particles are not spherical and, hence, do not have a well
defined diameter. It is difficult, for example, to assign a single character-
istic dimension to a rod- or flake-like particle. Many conventions have been
used for classifying particles by size with a microscope, such as using one of
the following as the characteristic size: the longest dimension, the diameter
of a circle with cross-sectional area equal to the particle in question, or the
diameter of a sphere with volume equal to the particle in question.
However, the most useful method of classifying particles by size in most
air pollution systems is based on how rapidly the particle settles out of the
atmosphere. The relative harmfulness of a particle depends on several factors,
one of the most important being whether the particle remains airborne long
enough to reach high density human populations. Thus, the most reasonable size
classifying system places particles in categories depending on their relative
settling velocities. Particle size measured in this way is called the aero-
dynamic size, and is usually referenced to the settling velocity of a sphere
with a specific density of 1.0 gram per cubic centimeter. Thus, a particle
which settles at the same velocity as a 10 ym diameter spherical particle with
specific density of 1.0 gram per cubic centimeter is said to have an equivalent
aerodynamic size of 10 ym.
Particles with the same equivalent aerodynamic size, even though with
different specific densities and shapes, will have an equal chance of settling
to the ground within a given time period. For example, a 6.3 ym spherical
particle with specific density of 2.5 grams per cubic centimeter has an equivalent
aerodynamic size of 10 ym.
Equivalent aerodynamic size also characterizes the ability of a particle to
penetrate into the deepest portions of the human respiratory system after entering
the nose. Many of the dynamic properties of airborne particles, such as agglomeratior
collision, and reaction rates between particles, depend upon equivalent aerodynamic
size.
From the time the particle enters the atmosphere until It becomes firmly
attached to a surface, i.e., during its entire life as an airborne pollutant,
equivalent aerodynamic size is usually the most useful characteristic size. It
is our opinion that, although other methods of classification may be useful in
specific cases, for most pollution measurement applications, size classification by
aerodynamic methods is preferred.
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A look at the equations defining equivalent aerodynamic size will show
the important parameters which affect it. The gravitational settling velocity
v of a spherical particle in air is expressed in terms of particle and air
properties by the well-known Stokes equation (e.g., see Ref. 1333, p. 23):
v = T-m|- ' (A.I)
s 3irnD
P
where
m = mass of the particle
g = gravitational constant
n = viscosity of the air
D = diameter of the particle.
P
Equation (A.I) is valid only at atmospheric pressure and for particles having an
aerodynamic diameter larger than about 1 ym diameter. In the molecular slip flow
region below 1 ym, the Cunningham slip correction C to the Stokes equation applies.
The Cunningham slip correction takes account of the discontinuous nature of particle-
molecule collisions when the particle is comparable to the mean free path in size.
(See Fuchs^--" , p. 25, for detailed discussion.) Equation (A.I) then becomes:
(A.2)
With particle mass m expressed in terms of particle volume and density y >
Equation (A.2) becomes:
D^gC
(A.3)
To calculate the equivalent aerodynamic diameter of a particle which settles with
velocity vs, solve Equation (A.3) for D , which then becomes equivalent aerodynamic
diameter referenced to the particle density y Thus, Equation (A.3) defines
equivalent aerodynamic size.
We have now briefly discussed some of the various ways of defining particle size.
An equally important factor in expressing particle size distributions is how the
amount of particles in each size category is expressed. This is called the weighting
of the size distribution.
There are many ways to express the amount of particles in each size category.
One way often used in expressing atmospheric particle size distributions is the number
of particles per unit air volume within each size range. This is called the number
concentration size distribution. Other commonly used particle concentration
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8
weightings include particle mass, particle surface area, and particle volume.
Rather than expressing the actual particle concentration in numbers, milligrams,
square centimeters or cubic centimeters of particles per unit air volume, the
concentration weighting can be expressed as a percentage of the total which is
contained within each size category. For example, in the case of a particle mass
percentage weighting, the result would be a graph showing the percentage of the
mass of particles contained within each size category, or-more commonly, the per-
centage of the mass of particles contained by particles larger than (or smaller
than) a given size.
Figure 1 illustrates several of the more common ways of expressing a particle
size distribution in graph form. All nine graphs in Figure 1 represent the same
particle size distribution. Notice that each curve has a different shape or slope,
depending on which particulate parameter is represented and on how the investigator
chooses to represent it. There are many other ways to plot particle size distri-
bution, each resulting in a curve with a unique shape.
It is not always clear which way of expressing size distributions is most
advantageous in any given situation. Each has its limitations. A mass distri-
bution generally emphasizes large particles and deemphasizes or ignores small
particles which are important but do not contain significant mass compared to
the large particles. A number distribution, on the other hand, emphasizes small
particles which are present in large numbers. A number distribution ignores
the one or two large particles in a cloud of millions of small ones, even though
the one or two large particles may weigh more than the millions of small particles.
A concentration distribution shows the actual particle concentration level, an
important factor determining particle interaction rates and as essential factor
for pollution control monitoring. The percentage distribution, as well as several
other distributions, shows quickly, clearly, and without ambiguity, exactly which
size range contains the majority of the particles. Although one can usually con-
vert mathematically from one distribution to another, any measurement error becomes
magnified with each conversion. One cannot convert from a percentage distribution
to a concentration distribution without additionally knowing the total concentration
of the particle sample.
Thus, it is important to decide which method of expressing size distribution
is most useful for a given application before choosing the size measurement
technique. The investigator will then choose a size measuring technique which
yields the desired size distribution as directly as possible. Although a strong
case can be made for aerodynamic size as the most useful for most air pollution
measurements, the choice of the method of expressing particle concentration is not
as simple. One must be very careful not to ignore an important range of particle
sizes simply because of the choice of particle concentration weighting.
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c
o
cr
<
o_
SSVIfl V3dV
NOI1VU1N30NOO 310llHVd
Figure 1. Nine ways of presenting the same particle size distribution.
The distribution shown is not necessarily realistic, but is
made up entirely of particles in the 0.1 to 100 micron range.
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10
The discussion above is concerned primarily with complete size distri-
butions and methods of presenting them. There are ways of expressing important
features of a distribution as single numbers. A common method is to express an
average size (usually diameter) accompanied by a number representing the spread
in the distribution (usually standard deviation or geometric standard deviation).
The range of particle size, meaning the size of the largest and smallest particles
in the distribution, is another measure of the spread of a distribution. Other
"averages" include the mean, median, and mode of the distribution. The number
median and mass median diameters are commonly used. The mass median diameter is
the diameter which breaks the distribution into two size ranges, each containing
50% of the total mass.
This discussion has included only a brief summary of the most important aspects
of the definition of particle size and size distribution. Any reader contemplating
work in this area is strongly urged to refer to a text such as Herdan (Ref. 1357).
Many other aspects are covered in Herdan, including details on converting from one
distribution to another, how to choose the optimum number of size ranges to cover
a size distribution, other ways of presenting size distributions, hints on in-
terpreting a size distribution, etc.
At present, no instrument is available that can measure the complete range of
particle sizes in an effluent source. One reason is the inherent problem of the
choice of particle weighting discussed earlier. Thus, if the mass size distribution
is measured, small particles are ignored, etc. However, every instrument also has
practical limitations of size range. Few instruments can cover more than a range
of one-two decades in particle size. No instrument with potential for automation
can accurately cover more than 2 size decades. Since combustion effluents cover
at least 5 size decades (0.001 - 100 ym) , either several different instruments
must be combined to cover the complete size distribution, or one must choose the
size range of primary interest and find a suitable instrument for that range.
In the past, the range of sizes measured in combustion sources was dictated
by the available technique. Thus, almost all measurements were of mass concentration
in each of several size ranges from about 2 ym to about 100 ym. Particles below
2 ym could not be size classified by the technique and their mass could not be
detected in the presence of the larger particles. This has led to the conclusion
that there probably is no contribution to the mass emissions by particles below about
1 ym in combustion effluents; however, this has not been documented by direct measure-
ments. In the case of particles above 100 ym, most such particles settled out of
the sampling line before reaching the sizing instrument. Thus, most existing data
on particles above 100 ym also appears to be inaccurate. Most of the existing
particle size data from combustion sources is highly questionable for these reasons.
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11
Sampling statistics also often limit the validity of particle size data,
especially number count data. Much of the existing microscopic count size data
was obtained using a single microscopic magnification. With most atmospheric
and combustion-source aerosols, there are so many small particles that the likely-
hood of counting a large particle with this method is small, even if 1,000 - 10,000
particles are counted for a single size distribution. For example, there are usually
1,000 - 10,000 times as many 1.0 pm particles in atmospheric aerosols as there are
10 pin particles. Thus, for a reasonable statistical chance of obtaining an accurate
distribution covering only a single order of magnitude of size using a single
magnification requires the counting of about 100,000 particles, a formidable task
One method of avoiding this problem is to use several magnifications covering the
size range of interest. Each magnification should cover no more than a factor of
5 in particle size. Each magnification can then be chosen such that the smallest
size of interest is easily detectable and yet, the largest size of interest will be
counted enough times to result in a statistically valid distribution. This same
statistical limitation also applies to any instrument which counts the number of
particles in a distribution. The solution for such instruments is usually to count
enough particles to overcome the limitation.
Better sizing instruments must be made available so that measurements will
include the entire size range of interest. The increasingly recognized importance
of micron- and submicron-sized particles places special emphasis on the small particle
No single sizing technique can cover the entire size range. Thus, several techniques
will have to be used. The interpretation of the data obtained from several different
measuring techniques will be a problem in some cases. However, no method is foreseen
that will measure detailed distributions over the entire size range with a single
measuring technique.
The remainder of this report discusses specific particle size measuring
techniques in detail. The discussions are limited primarily to a basic
evaluation of each technique rather than a discussion of exact design details.
The reason for this is that most techniques are not developed for application
to measurements of combustion effluents, making a discussion of design details
rather speculative.
All particle sizing instruments must perform two distinct functions: (1)
classify the particles by size and (2) sense the amount of particles within each
size range. In some measurement techniques, the two functions can be considered
separately and developed hardware may consist of the two distinct components.
Section B discusses first classifiers, then sensors, and last, combinations of the
two components. It will be seen that most classifiers can be used with most sensors,
some combinations having better features than others. Section C discusses sizing
techniques which, while performing both functions, does not allow separation of
hardware for each function. Throughout Sections B and C, emphasis is placed on the
more feasible techniques.
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A number of particle sizing techniques exist for powders and liquid
slurries. Section D discusses possible application of these techniques to
combustion source aerosols. These techniques are deemphasized because of
the difficulty in relating the size of the slurry particles to the particles
in their airborne state.
Section E discusses a different concept of size monitoring for use in
combustion source effluents as well as elsewhere. This concept has strong,
practical appeal, but requires more study and a moderate amount of develop-
ment and testing.
Finally, Section F lists the most important conclusions and summarizes
this study.
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13
B. TECHNIQUES WITH SEPARABLE CLASSIFICATION AND SENSING
Many instruments for measuring the size distribution of an aerosol can be
broken into two independent components:
1. A classifier which separates the particles into several
size fractions, and
2. A sensor which measures the amount of particles within
each size range.
Conversely, practical sizing instruments can often be assembled by connecting
various classifiers upstream of various sensors. This discussion first con-
siders particle classification techniques, then considers particle sensors, and
finally considers the most reasonable combinations of the two.
1. Particle Classification Techniques
This chapter discusses all known particle classification techniques and
all known .forces which can act on airborne particles. The classification
techniques discussed below are:
a. Aerodynamic Classification
i. Gravitational Sedimentation
ii. Gravitational Elutriation
iii. Inertial Impaction
iv. Centrifugal Spectrometry
v. Cyclone Separation
vi. Centrifugal Elutriation
b. Electrostatic Classification
c. Sieve Classification
d. Filtration
e. Brownian Diffusion
f. Other Forces
a. Aerodynamic Classification
When a particle moves with respect to the surrounding air, the
force which resists that motion is called aerodynamic drag force.
For a spherical particle larger than 1.0 ym diameter moving with a
slow, constant velocity through air at atmospheric pressure, aero-
dynamic drag force F, can be expressed by the Stokes equation:
v (B.I)
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14
where:
n = viscosity of the gas,
D = particle diameter, and
P
v = particle velocity with respect to the gas.
The negative sign means that the force acts in a direction opposing
the motion of the particle with respect to the gas. For particles
between 0.1 and 1.0 pm (i.e., about the same size as the molecular
mean free path of standard air) and with all other conditions equal
to those for Equation (B.I), F, can be expressed by the Stokes -
Cunningham equation:
F, = - -^ CB.2)
where:
C = Cunningham slip-flow correction.
As D becomes large compared to the molecular mean free path, C->1.0
and PEquation (B.2) becomes Equation (B.I). Equation CB.2) is valid
with reasonable accuracy for particles with 0.1 ym
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15
Expressing m in terms of particle volume and density, Equation
(B.3) becomes:
P P
where:
18n
density of the particle.
(B.4)
Equation (B.4) shows that, for particles of constant density, the
settling velocity increases as the square of the particle size. Thus,
the length of time t^ it takes a particle to settle a given distance I
decreases with an increase in the particle size:
£_
V
(B.5)
In terms of hardware, a sedimentation size classifier is shown
schematically in Figure 2. The particles settle through the laminar
clean air streamlines and deposit onto the bottom of the chamber. The
sharpness of the size classification depends on the height of the
aerosol entrance duct with respect to the average settling distance, H,
and on the velocity profile of the aerosol and clean air flow.
Fine
Screeny
Aerosol
In
Trajectory of
non-deposited particle
Clean
Air
In
Trajectory of
deposited particle-
Filter
Figure 2. Schematic sedimentation size classifier with
horizontal air flow.
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16
If operated upstream of a concentration sensor, the sedimentation
size classifier would collect particles larger than a given size, and
particles smaller than that size would pass through to the sensor.
The particle size cutoff could be decreased by reducing the settling
height £ or by decreasing the horizontal flow velocity through the
classifier. The former results in a less sharp size cutoff and the
latter results in greater interference by convective currents. Thus,
any classifier of this type can be designed for only a narrow range
of adjustable particle size cutoffs, probably one order-of-magnitude
at most. The overall practical size range for sedimentation classifiei
is from about 1.0 |jm to about 50 pm. The lower size limit is set by
the practical limitations on the smallest values of £ and air flow rate
as well as by interference by convective currents. The upper size
limit is set by the difficulties in introducing large particles into
the entrance without losing them.
The sedimentation classifier dilutes the aerosol which passes
through by the ratio of clean air to entering aerosol. The upper con-
centration of entering aerosol is not a limiting factor for effluent
aerosols.
The Hexhlet dust sampler manufactured by
C. F. Cassella & Co., Ltd.
Regent House, Fitzroy Square
London Wl, England
uses a simplified sedimentation classifier which consists of parallel
plates through which the aerosol passes. There is no clean air sheath,
making the classification considerably less sharp, but making operation
easier. Other authors report other models of sedimentation classifiers
often called horizontal elutriators.274,834,1299,638,833,912
ii. Gravitational Elutriation
Elutriation is similar, in many respects, to sedimentation. The
principle of a gravitational elutriator is shown schematically in
Figure 3. Laminar air flows vertically within the elutriation tube.
Aerosol is introduced at the bottom of the tube. Large particles, i.e.
those with settling velocities (Equation B.4) greater than the vertical
air flow velocity, will be carried out with the exhaust. The particle
cutoff size is determined by the vertical air velocity in the tube.
The major factor causing degradation of the sharpness of the
particle size cutoff is the shape of the air-velocity profile within
the tube. Ideally, the air velocity profile would be flat at all
points. In practive, however, a boundary layer forms at the tube
wall, allowing small particles to settle to the bottom of the tube
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17
Air and
Fine Particles out
t t t
Air Distribution Screen
(large enough to pass
nearly all particles)
t
Large
Particles
out
Aerosol in
Figure 3. Principle of a gravitational elutriator.
through the boundary layer. Convective currents caused by thermal
gradients within the elutriator also modify the velocity profile.
The ability to overcome these two related problems determines the
ultimate sharpness of the particle size cutoff in practive. The
concentration levels within the elutriator must be low enough so that
large particles settling downward do not intercept appreciable numbers
of smaller particles. The applicable size range of elutriation is
about 1 to 100 ym. Other limitations are similar to those with the
sedimentation classifier.
-------
18
Several authors have considered gravitational elutriation in
more detail. 638, 833, 912,1069, 1333(p. 42,43).
A commercial gravitational elutriator for size classification
of powders is manufactured by:
Geoscience Instruments Corporation
435 East 3rd Street
Mount Vernon, New York 10553.
The advertised size range is 5 to 100 ym. This unit would not lend
itself to the sizing of stack effluent particles on a continuous
basis without considerable modification.
Gravitational elutriation size classification is a definite
candidate for application to effluent aerosols. Gravitational
elutriators operate over the approximate size range of 1 to 100 ym,
and may have the ability to classify even larger particles because
higher velocity in the entrance tube allows the larger particles
to enter the sedimentation classifier. Several elutriators designed
for different size cutoffs would probably be necessary to cover the
entire 1 to 100 ym range. A strong feature of this classifier is
that it results in an aerosol classified by aerodynamic equivalent
size.
iii. Inertial Impaction
A third method of using aerodynamic drag to classify aerosol
particles is inertial impaction. Figure 4-L340 shows an impactor
schematically. A flat plate is located at the exit of a nozzle,
perpendicular to the air jet. Air is sucked through the nozzle and
must turn a sharp corner to pass around the flat plate. Because of
their inertia, sufficiently large particles cannot turn the corner,
and therefore hit the plate. Smaller particles do not have enough
inertia to cross the air streamlines and are carried along with the
air stream.
As particles cross the air streamlines in an impactor, the force
which resists that motion is aerodynamic drag force expressed in Equatio
(B.2) with v being the component of the particle velocity perpendicular
to the streamlines.
When discussing impactors, it is convenient to define a parameter
known as Stokes number:
Y Cv D2
' (B-6)
where:
Stk = Stokes number,
v = average air jet velocity, and
W = jet width (rectangular jet) or diameter (round let).
-------
19
STREAMLINES
IMPACTION
PLATE
K
w
r
T
L
A
)
L-y
\
1
1
/ /
-r-
1
1
1
1
I
\\
A
\
\
\
\
K
JEf
JET EXIT
OF
IMPACTED PARTICLE
TRAJECTORY OF
PARTICLE TOO
SMALL TO
IMPACT
Figure 4. Schematic of an impactor showing particle trajectories.1340
A dimensionless particle size is defined as (Stk)1/2. Particles with
the same Stokes number have an equal chance of being collected by a
given impactor.
Ideally, an impactor should collect all particles larger than
a certain cutoff size and all smaller particles should escape as
shown by the dotted line in Figure 5 "AO In practice, however,
impactors have a characteristic "S" - shaped efficiency curve.
1.0
ui
O
u.
u.
ACTUAL
IDEAL
Figure 5. Typical ideal and experimental collection
efficiency curves of an impactor.13^0
-------
20
There are several reasons for this nonideal operation. These
reasons have been investigated in considerable detail in the
literature (see especially, References 1333 (pp. 151-159) and
1340), including:
a) the velocity profile at the jet exit is not flat
giving particles near the center of the jet greater
inertia than particles near the jet walls,
b) a boundary layer forms along the plate deflecting
particles which are near the edge of the jet more
than particles which are near the center line,
c) the high jet velocities needed to collect small
particles blow some of the larger particles off the
plates or cause them to bounce off,
d) some weakly-bound agglomerates of smaller particles
break up while passing through the high shear forces
within the jet or while striking the surface, and
e) with rectangular jets, the ends of the jet act like
a round jet: impactor while identical particles within
the central areas have significantly different Stokes
numbers.
Marple performed a careful theoretical and experi-
mental study of both round and rectangular jet impactors and
recommends the jet operating conditions shown in Table B.I as
the optimum for obtaining a sharp particle size cutoff. The
dimensions S, W, and T refer to Figure 4 and Re is the Reynolds
number based on the average jet velocity and the jet diameter
or width. Figure 6^-340 shows the impaction efficiency as a
function of (Stk)l/2 for each of the two optimum conditions
outlined in Table B.I. The rectangular impactor must be much
longer than it is wide. Re can vary from 500 to perhaps 100,000
without significantly changing the efficiency relationship. Values
of S/W greater than those shown in Table B.I (up to S/W = 5) affect
Table B.I Optimum jet configurations.
1340
'Jet
Round
Rectangular
S/W
1/2
1
T/W
1
1
Re
3000
3000
-------
21
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-------
22
the efficiency only slightly, but values less than those shown
cause efficiency to change quite drastically. In several reported
tests,13^0 varying the throat length from T/W = 1 to T/W = 10 did
not influence the efficiency curve significantly.
A number of other investigators have studied both round and
rectangular impactors. Tables B.2 and B.3-^ catalog the operating
conditions of each study.
Particles can be impacted into an air void ' rather than
onto a flat plate as described earlier. In this case, the impaction
"surface" consists of static air. The problem of particle blowoff is
eliminated by the use of a void. Large amounts of particles can be
impacted and collected for subsequent analysis. Particle sensors can
sample either the large-size fraction (from the void) or the small-
size fraction (non-impacted particles). Although this technique appears
to offer several significant advantages over flat-plate impaction, few
models have been built and little experimental data has been found.
Bird & Tole Ltd., Bledlow Ridge, High Wycombe, Bucks, England, manu-
factures a void impactor (called a cascade centripeter) based on the
design reported in Ref. 996.
Several impactors are often placed in series, each succeeding
stage having a smaller critical Stokes number and thus collecting
smaller particles than the preceding stage. The combination of these
impactors is called a cascade impactor.948,995,996,1002 Qne Q£ ^e
most commonly used methods of measuring the distribution of particle
mass as a function of aerodynamic particle size in the 0.2 - 30.0 ym
range is to classify and collect particles with a cascade impactor, and
then weigh the amount of material collected on each impaction plate.
This technique has been used for measuring the size distribution of
particles in effluent ducts. The primary features of this technique
are (1) aerodynamic size classification and (2) classification of the
particles inside the effluent duct at the conditions that exist in
the effluent stream. Equipment designed for stack sampling is
commercially available from:
2000 Inc. (modified Andersen impactor)
5899 South State Street
Salt Lake City, Utah 84107
Monsanto Company (modified Brink impactor)
800 N. Liridberg Boulevard
St. Louis, Missouri 63166
-------
23
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Cascade impactors can also classify particles within effluent
streams which have been pre-conditioned, e.g., by dilution and/or
cooling. Commercial models which might prove useful for conditioned
effluent streams are manufactured or marketed by:
Aerostatics Instrumentation & Research (modified Andersen impactor)
1081 East 2200 North
Logan, Utah 84321
Environmental Research Corporation (Lundgren impactor)
3725 North Dunlap Street
St. Paul, Minnesota 55112
Monsanto Company (Brink impactor)
800 N. Lindberg Boulevard
St. Louis, Missouri 63166
Scientific Advances, Inc. (Battelle impactor)
1400 Holly Avenue
Columbus, Ohio 43212
2000 Inc. (Andersen impactor)
5899 South State Street
Salt Lake City, Utah 84107
Union Industrial Equipment Corporation (Unico impactor)
150 Cove Street
Fall River, Massachusetts 02720
Willson Products Division, ESB Incorporated (Cassella impactor)
2nd & Washington Streets
Reading, Pennsylvania 19603
None of these commercial cascade impactors has an automated deposit
sensing or particle concentration sensing method. All are intended
to collect a classified sample for subsequent analysis.
The operable size range for cascade impactors in controlled
laboratory tests is generally, at best, from about 0.2 to 30 um
diameter, assuming particle density of 1.0 to 2.0 grams per cubic
centimeter and nearly spherical shape. The lower limit is set by
the difficulties imposed by:
a) very small nozzles which are difficult to drill and tend
to plug easily,
b) high jet velocities resulting in some particle blowoff,
-------
26
c) reentrainment of deposited particles limits the
amount of sample which can be collected, and
d) the small weight of small particles cannot be
weighed in the presense of the large tare weight
of the impaction plate.
The upper size limit can be larger than 30 ym depending on the type
of particle and the stickiness of the impaction surface. However,
such problems as particle deposition in the entrance regions of the
impactor, blowoff of previously-deposited particles, and bounce of
impacting particles, usually impose a practical upper size limit.
The lower size limit can be extended by low pressure impaction
for some applications. A major problem is the vaporization of
particulate materials by the low pressure. Further testing and
development is necessary to determine whether low pressure impaction
is practical for stack effluents.
A special case of low pressure impaction, often called an aerosol
beam, uses a sonic nozzle with a very low downstream pressure (<0.01
atmsophere). After particles pass through the sudden expansion region
just downstream of the nozzle, they are affected very much less by
fluid drag forces than in the atmospheric case. One study has looked
at the pattern of particulate deposit on a target placed as several
distances downstream of the nozzle.1273,1361 Another study looked at
stop distances of the particles in the aerosol beam.1362 Much work
remains to be done to understand aerosol beams. It is not clear
how the principle can be utilized for particle size classification.
One possible application may be to use the aerosol beam technique to
accelerate particles to a constant, known velocity and then use a
sensitive momentum transducer (e.g., see Ref. 252) to measure the mass
of individual particles. Much work remains to determine if this
technique, or any other aerosol beam technique, is practical for sizing
micron-sized particles.
Impactors, especially cascade void impactors, are definitely prime
candidates for size classifying particles in stack effluents over the
size range from about 0.2 to 30 um. Features include aerodynamic size
classification and the ability to classify particles at nearly any
environment condition including in-stack conditions. Cascade impactors
have been investigated extensively both theoretically and especially in
the laboratory. They have been used to classify stack effluent particles
with mixed results. Impactors appear to adapt easily to several particle
sensing techniques.
-------
27
iv. Centrifugal Spectrometry
A fourth method of using aerodynamic drag to classify aerosol
particles by size is centrifugal spectrometery.1333 (PP- 123-126),
123, 290, 354, 369, 546, 505, 649, 818, 925, 1119, 1120, 1341, 1342,
1 O / Q
This technique is equivalent to gravitational sedimentation
except that centrifugal acceleration speeds the process allowing its
application to small particles in the 0.03 - 3.6 ym equivalent aero-
dynamic diameter range.
649
Figure 7 shows the principle of operation of 2 centrifugal
spectrometer designs. In the first design, aerosol passes through
a spiral-shaped channel which rotates about its axis. Centrifugal
acceleration pushes particles radially toward the outer wall of the
channel while aerodynamic drag force (see Equation B.I) resists the
radial motion of the particles. The effective radius of the spiral
channel increases as the aerosol passes through the spectrometer.
Large, heavy particles deposit near the inlet where the centrifugal
acceleration is low, while small particles deposit near the exit
under the influence of high centrifugal acceleration. The spectro-
meter is normally precalibrated using aerosol particles of known
size and noting the position of their deposit on the outer wall of
the channel. The second design utilizes a sheath of clean air
between the aerosol stream and the deposit surface. The clean air
sheath improves the resolution of the spectrometer by forcing all
particles to travel nearly the same radial distance before being
deposited on the outer wall of the channel.
AEROSOL INLET
FLOW NOZZLE
(1) Spiral design
(2) Sheath air design
Figure 7. Two centrifugal spectrometer designs.649'1119
-------
28
This results in considerably sharper size classification than the
first design, but requires lower aerosol sampling rates.649,1119
The models which have been built used various types of driving
motors and bearing systems to allow high centrifugal speeds without
vibration. A commercial version of Stober design is marketed by:
Ivan Sorvall Inc.
(street address unknown)
Norwalk, Conn. 06856
It is difficult to imagine an automatic sensing technique for
use with centrifugal spectrometry. Additional problems for application
to stack effluents is the limited size range (essentially submicron)
and the small amount of material which can be collected. This technique
has seen considerable application in research studies of laboratory-
generated and atmospheric aerosols, and may be useful for some research
on stack effluents. However, its application to routine, automatic
stack effluent monitoring is doubtful.
v. Cyclone Separation
aerosol
A fifth method of using aerodynamic drag force to classify ae
particles by size is the cyclone separator.1333(pp.126-135), 1334,
1335,1216,675. . 0 , im , i -, * ^- c
Figures 8 and 9 ±J->-> show the principle of operation of
a cyclone. Aerosol enters the rectangular inlet tube A and passes into
the cylindrical part of the cyclone where it acquires a spiral motion.
Air spirals downward along the outer wall of the cylinder, throwing
large particles against the wall where they can slide to the bottom of
the cyclone. When the air reaches the bottom of the cyclone, it ascends
an inner spiral to exhaust through the upper axial tube C. Small
particles are carried out by the exhaust air stream. Large particles
can be collected for analysis from the bottom of the cyclone.
Figure 8. Cyclone.
1333
Figure 9. Motion of gas in a
-I OTQ
cyclone.
-------
29
Cyclones are used industrially as dust collectors on large
effluent streams. They are used in aerosol sampling as "respirable"
size range separators. In this application, a small cyclone separates
the large particles from the air stream, presumably simulating the
upper passages of the human respiratory system. The aerosol which gets
through the cyclone, containing only small particles, then simulates
the aerosol which penetrates deeply into the respiratory system. These
small particles are generally collected for analysis on a second stage
consisting of a high-efficiency filter. Figure 10 shows the "respirable
dust" curves which the cyclone attempts to simulate.1052 Qne Of the
curves was first developed at a conference at Los Alamos Scientific
Laboratory of the U.S. Atomic Energy Commissioners and was modified
and adopted as a standard by the American Conference of Governmental
Industrial Hygienists in 1968.1334 ^he other curve on Figure 10 was
defined earlier by the British Medical Research Council.1052 The
aerodynamic diameter in Figure 10 is based on a particle density of 1.0
gram per cubic centimeter. The vertical axis is percent of respirable
dust by weight of the particles.
I
LASL CONFERENCE CURVE
o ACGIH MODIFICATION
A MRC CURVE
345678
AERODYNAMIC DIAMETER (/im)
10
Figure 10. "Respirable dust" curves.
1052
-------
30
The relationship between the size of collected particles and
the dimensions and operating parameters of a cyclone can be approxi-
mated by:1333 (p. 129)
DP,min= 3 Ws-> ' (B'7)
v op
where:
D , = diameter of the smallest collected particle,
p,min r
n = viscosity of air,
R- = radius of the outer cyclinder (see Figure 8),
v = average inlet duct velocity,
Y = particle density, and
S = number of turns of the outer spiral in the
cylindrical part of the cyclone.
The difficulty in using Equation B.7 is in evaluating S. In most
practical cyclones, 1 < S < 3. Equation B.7 shows that for any
given cyclone, an increase in v results in the collection of
smaller particles. Practical cyclones can be designed such that
D is from about 0.5 to 20 urn.
P ,min
It is not easy to analyze cyclones mathematically. In fact,
most cyclones have been designed by use of experimental data.
Factors which influence the cyclone performance are the level of
turbulence in the cylinder, the relative size of the inlet and
exhaust (and thus the inner and outer spirals), redispersion of
particles which have struck the wall, the shape of the particles,
pulsations in air velocity, the dust concentration level, and the
roughness of the cylinder walls. The flow resistance through the
cyclone increases roughly as v . Although some cyclones have been
operated successfully in the inverted position, it is important
to define the orientation of the cylones. Cyclones, should always
be operated with the large particles moving downward unless its
operation in an alternative position is carefully investigated.
Reference 675 reports the results of an experimental evaluation
of cyclone operating conditions.
A set of six identical cylones, each operated at a different
flow rate, has been used recently to measure atmospheric particle 1354
size distribution in 6 size categories ranging from about 1-12 ym.
Walter C. McCrone Associates, Chicago, is currently investigating the
applicability of cascade cyclones to size distribution measurements in
stacks under contract to the Environmental Protection Agency.
-------
31
Cyclones are a definite candidate for effluent particle size
classification because of the operable size range, the ease of
continuous operation, and because the size classification is aero-
dynamic. The large particles can either be collected continuously
for analysis or can be eliminated continuously. As will be seen
later, cyclones adapt easily to the inlet of particle sensing instru-
ments, making automatic sensing of "respirable" range particles a
relatively simple task. A single cyclone has a limited operable size
range. Cyclones have already found application to separating the non-
respirable particle fraction from an air stream so that the respirable
fraction can be analyzed.
vi. Centrifugal Elutriation
Another particle size classifying method usind aerodynamic drag
force is centrifugal elutriation. This method, usually known as the
Bahco method, is often used to classify fly ash effluent particles by
size after they have been collected from the effluent stream. ^»^'
1332
Figure 11 shows the principle of operation of this method as
sold commercially for powder sizing. The entire assembly except compo-
nents 1-6 which make up the powder feeder assembly, rotates about a
vertical axis. Clean air enters past the throttle nut 13, past the
symmetrical disks 11, through the sifting chamber 10, radially toward
the axis and then outward through the fan vanes 8. Particles enter the
device through the feed nozzle 6 and enter the air stream through the
rotary duct opening 9. Large particles are carried outward by centri-
fugal force and are collected in the catch basin 12. Small particles are
carried with the air stream and emerge through the fan vanes 8 where
they impact on the inner wall of the rotor casting 7. Thus, the classi-
fication results in 2 size fractions, one with particles larger than the
cutoff size and one with smaller particles.
The size cutoff can be varied over a range of sizes by changing
the throttle nut 13 position by means of throttle spacer 14. This
changes the amount of air passing through the device and, in effect,
changes the aerodynamic drag force acting on the particles. To measure
the size distribution, the operator begins with the smallest size cutoff,
runs an analysis, weighs the large fraction which collects in the catch
basis 12, and then reruns the large size fraction with a somewhat larger
size cutoff setting. This is repeated as many times as necessary.
The ranges of sizes on the commercial unit is from about 50 ym down
to 1 or 2 ym. The lower limit is set by the impaction efficiency of
particles onto the rotor casting 8. Particles below a certain size will
pass out with the air stream and not be collected with the fine fraction.
With appropriate design, the fraction which is not collected on the rotor
casting 8 could be collected on a filter, resulting in a measurable
fraction of small particles. However, the difficulty in collecting small
particles is a serious limitation for particles below perhaps 5 ]jm or for
distributions which contain significant mass fractions below this size.
-------
32
Figure 11. Cross-sectional diagram - 1. hopper, 2. spring
plate, 3.brush, 4. orifice tube, 5. vibrator,
6. feed nozzle, 7. rotor casting, 8. fan wheel
vanes, 9. rotary duct opening, 10. sifting chamber,
11. symmetrical discs, 12. catch basin, 13. throttle
nut, 14. throttle spacer, 15. motor.1332
Another factor affecting the classification of any particles which
must be resuspended from a powder state is the agglomeration and/or
fractionation of particles. This is especially important for smaller
particles in the micron size range or smaller. Small particles often
adhere to larger particles and are classified with the bigger particles.
Thus, it may be desirable to investigate the operation of the centrifugal
elutriator on the suspended effluent particles as they emerge from the
stack. This may be possible with a redesigned system.
The quality of the classification also depends on the type of air
flow through the classifying regions and how the particles are introduced
into this region. The turbulence level, boundary layer formation, and
the geometry of the particle entrance region are all important. Complete
analysis of the existing units is beyond the scope of this report. Such
analysis could certainly result in improved performance,but considerable
performance testing would be required to evaluate the operation of any
such device.
This technique has been accepted by the ASME for measuring the size
of collected fly ash powder samples.239 j^g gahco apparatus was used to
obtain much of the size information on fly ash which is reported in the
literature, including much of the data reported in Volumes I and II of
this report.
-------
33
The commercial version of the centrifugal elutriator for
powders is manufactured by:
Harry W. Dietert Company
9330 Roselawn Avenue
Detroit, Michigan 48204
This company holds the American license from the Swedish Company:
AB Bahco.
The centrifugal elutriation technique is a candidate for effluent
particle classification. Although the technique is commonly used for
sizing fly ash particles, the present technique requires the collection
of a fly ash sample and redispersing the particles for the measurement.
This process is subject to large errors when submicron or micron-sized
particles are present in significant amounts. The lower size limit of
the present apparatus is in the 2 - 5 ym range. It would seem that
this technique could operate with the feeding of airborne particles
directly into the apparatus. Such a technique would probably require
. considerable redesign, but could result in a reasonable classifier for
the 1 - 60 Mm (approximate) range. This technique results in aerodynamic
equivalent diameter classification with its many advantages. This
technique appears to offer no significant advantage over cyclones and
impactors and is generally more expensive.
b. Electrostatic Classification
An aerosol particle which carries an electrical charge can be acted
upon by an electric field. The force acting on such a particle is expressed;
F = n e E (B.8)
e p
where:
F = force acting on the particle because of its
electrostatic charge,
n = number of elementary charges attached to the
particle,
-19
e = elementary unit of charge, 1.6 x 10 coul/elementary
charge, and
E = electric field intensity.
This force causes the particle to move through the gas in a direction
determined by the polarity of the field and the polarity of the charge
on the particle. The force resisting this motion is aerodynamic drag
force, expressed in Equation (B.I). Equating these two forces, one can
define the final speed attained by the particle:
n e E
' (B'9>
-------
34
Another term often used in electrostatic theory is the electrical
mobility of a particle Z defined:
n e
_P
V
p ~ E 3irnD C
(B.10)
Figure 12 shows the mobility of singly-charged particles in standard
air as a function of particle diameter D . The mobility of multiply-
charged particles is simply the value shBwn in Figure 12 multiplied by
the number of elementary charges n . The electrical mobility of a par-
ticle is the velocity of that particle when acted upon by an electric
field of unit intensity.
i-o
0-001
0-01 0-1
Particle diameter,
1-0
Figure 12. Electric mobility of singly-charged
particles at NTP.1211
-------
35
If aerosol particles can be electrostatically charged such
that n is a reproducible function of particle size D , then a
mobility classification is also a particle size classification.
Several mobility classifiers are shown in Figure 13.1211 xhe
mobility classifier in Figure 13.c is particularly suited to this
application. Unipolar-charged particles enter the-classifier as
an annular ring surrounding a core of clean air. A voltage with
polarity opposite the polarity of the particles is applied to the
central rod. The outer tube is electrically grounded. The charged
particles are attracted toward the central rod with a radial velocity
expressed in Equation (B.9). Particles with high mobility deposit
near the top of the rod while lower mobility particles travel further
down the rod before being deposited. If the geometry and electric field
intensity are chosen properly, particles with mobility less than a given
value (mobility cutoff) will not be deposited on the central rod, but
will pass out the bottom of the classifier where aerosol sensors can
detect the concentration. The mobility cutoff of a given geometrical
design can be varied by changing the electric field intensity or the
aerosol and clean air flow rates. Thus, in operation, one would measure
the. concentration at one mobility cutoff (corresponding to a pre-
determined particle size cutoff) and then at a different mobility cutoff
(corresponding to a different particle size cutoff). The concentration
of particles between the two sizes is the difference between the two con-
centration measurements.
Thus, an instrument based on this principle measures the number con-
centration within several size ranges. The size ranges are determined
by the relationship shown in Equation (B.9). The velocity v from Equation
(B.9) determines where the particle will land on the central rod.
Aerosol
(o)
Clean air
(b)
-Aerosol
Figure 13. Aerosol mobility classifiers.
1211
Two factors limit the particle size resolution of this type of
classifier. The first is the difficulty in placing an equal number of
charges on all particles of a given size. The second is the problem of
introducing particles at the top of the mobility analyzer in a very thin
annular ring so that all particles travel an equal radial distance through
identical flow conditions to reach the central rod.
-------
36
There are many ways to place an electrostatic charge on aerosol
particles. Most of these are reviewed in Reference 1211 and will not
be discussed here. The most successful for classifying 0.005 - 0.6 ym
particles is shown in Figure 14 and is called a sonic jet unipolar charger.
The details are discussed in Reference 68. The method of charging is
called diffusion charging and is discussed in Reference 1211 and 56. The
mobility of particles charged by this method allows good size classification
from 0.01 - 0.2 urn. The classification below 0.01 ym deteriorates because
only a very small fraction of such particles can be made to accept a single
charge. Above 0.2 ym, the mobility versus particle size relationship slowly
flattens until 2.0 ym particles have about the same mobility as 10.0 ym
particles. The electrical charge n placed on equally-sized particles is
quite narrow, resulting in relatively sharp size classification in the
0.01 - 0.2 ym range. For example, completely monodisperse particles
(geometric standard deviation of 1.0) have a measured geometric standard
deviation of 1.15 with an existing commercial instrument.1344
Figure 14. Sonic jet diffusion charger.
68
Another charging technique, field charging, could probably be used
to classify particles from 1.0 ym to 10 or 20 ym. However, this technique
has not been developed as thoroughly and other classifying techniques
appear more promising for the larger-sized particles.
-------
37
The second factor limiting particle size resolution is the introd-
uction of aerosol in as thin an annular ring as practically possible.
This assures that all charged particles of equal mobility travel radially
through identical conditions and their deposit is not spread out on the
collection surface. The need for a reasonable through-put requires an
annular ring of practical width. In practice, the ring width can be kept
down to about 10% of the radial distance which the particle must travel.
This technique has been used to measure particle size in a number of
studies of atmospheric and artifical aerosols.27,40,42,61,68,1276,1285,
1344,1390.
An aerosol charge-mobility classifier instrument is manufactured by:
Thermo-Systerns Inc.
2500 Cleveland Ave. N.
St. Paul, Minnesota 55113.
The instrument, the Whitby Aerosol Analyzer, can be used in several modes:
(a) as an automatic concentration versus size detector, (b) as a size
classifier, and (c) as a mobility classifier for an externally-charged
aerosol. The present commercial model is large and would not lend itself
readily to stack monitoring in its present form. The maximum aerosol temp-
erature must be kept below about 100 F on the commercial unit because of
contruction materials.
Electrostatic classification is a definite candidate for classifying
0.005 - 0.6 ym effluent particles. Recent improvements in the charger
design and the mobility operating conditions make the size classification
very good, especially in the 0.01 - 0.2 ym range.1344 with proper design,
the technique could probably be made to operate at either in-stack or out-
of-stack conditions. Although this technique does not lend itself to stack
applications in its present state of development and requires some develop-
ment for such application, it remains the most practical classifying tech-
nique in its size range. Although considerable development could probably
result in classification of particles above 1.0 ym, the availability of
other techniques makes such development unimportant.
c. Sieve Classification
One of the best known methods of classifying powder particles into
size fractions is sieving. This is usually done with a set of screens
mounted in matched holders and stacked such that particles encounter
progressively smaller openings as they fall through the screens. The
particles which remain on each screen are then weighed to obtain the
size distribution.
One of the greatest problems with sieves is getting all particles
which should pass a given sieve to actually fall through and keeping
those which should not from passing through. The method most often
used to make particles fall through is to agitate the particles
-------
38
violently enough to bounce each particle into many orientations
and to break up loose agglomerates. However, the agitation must
not be violent enough to break up the primary particles. These
techniques have extended the lower size limit of sieving techniques
down to about 10 ym diameter.
One such technique is acoustical agitation. An acoustical speaker,
mounted either below or above the stack of screens, causes the particles
to bounce violently. Such a unit is marketed commercially by:
Allen-Bradley Company
(street address unknown)
Milwaukee, Wisconsin
The lower size limit of the screens on this model is about 10 ym. The
largest screen size available has about 600 ym holes.
A second method of agitating the particles is to periodically blast
air upward through the screen. Such a unit is manufactured by:
Alpine American Corporation
3 Michigan Drive
Natick, Massachusetts 01762
The smallest advertised hole size of these screens is 15 ym. The largest
is greater than 100 ym.
Another method of agitating particles on the screens is to wash the
stack of screens in a liquid such as water during the sieving process.
This helps achieve better classification, but care must be exercised to
keep from losing or dissolving particles in the process.
One manufacturer of ordinary matched sieve sets is:
W. S. Tyler Company
(street address unknown)
Mentor, Ohio 44060
The holes in the sieves range from large sizes down to 44 ym.
An electromagnetic sieve shaker is manufactured by:
Geoscience Instruments Corporation
435 East 3rd Street
Mount Vernon, New York 10553.
-------
39
Although sieving could be adapted to airborne particles, little
application is expected for classifying stack effluents because it does
not operate on small enough particles and the technique does not lend
itself to automatic, continuous sensing of particle size. Some research
application may be found in the manual sizing of large particles collected
by effluent control devices. The agglomeration, deagglomeration, and
fractionation of powder particles is a major problem as is screen binding.
d. Filtration
The efficiency of a filter as a function of particle size generally
has a shape similar to Figure 15. If a particle distribution is located
primarily above or below the minimum point, this feature can be theoreti-
cally used to size classify the aerosol. Filtration theory has been
reviewed by many investigators, including References 1211 and 1333.
0.1
1.0
Particle Diameter, ym
10.0
100
Figure 15. Typical filter efficiency curve. The exact location
of the minimum shifts both horizontally and vertically
with changes in filter media, face velocity, filter
loading, and other factors.
If several filters are used, each with an efficiency curve falling
in a different size range, the filters can be placed in series similar
to a cascade impactor. The first filter will ideally collect everything
above a given cutoff size (50% point on the efficiency curve). Each
succeeding filter will collect a smaller size fraction.
An automated system may consist of cascaded filters with sensors
removing a small part of the material passing through each filter. The
sensor then measures the concentration of material below each filter's
cutoff size.
-------
40
This technique has several important problems. Filters become
plugged, changing the efficiency curve and thus changing the size
cutoff. The efficiency curve for most filters is not very steep,
making the size cutoff rather poor. The practical range of this
technique is from about 1.0 to 5 urn, a very narrow range. Size
classification in the range below 0.1 microns has not proven practical.
Since several other techniques appear to be considerably more promising
within the same size ranges, filtration classification will probably
not be used for stack effluents except for specific research applications.
e. Brownian Diffusion
For particles smaller than 1.0 pm, Brownian diffusion becomes an
important factor governing the motion of an individual particle. Brownian
motion of a particle is caused by the random bombardment of the particle
by gas molecules. A single molecular impact does not change the direction
and speed of an aerosol particle appreciably, but random collisions with
a large number of molecules causes motion such as that shown in Figure 16.
Fuchsl333 and Daviesl^H discuss Brownian diffusion in considerable detail
and much of the following discussion has been derived from them.
1333
Figure 16. Trajectories of gas molecules (a) and particles
undergoing Brownian motion (b).1333
The diffusion coefficient of a particle, a quantity characteriz-
ing the intensity of Brownian motion, can be expressed:
D = kTB
(B.ll)
-------
41
where:
D = diffusion coefficient,
k = Boltzmann's constant,
T = absolute temperature,
£
B = -T- = particle mobility,
P
C = Cunningham's correction factor,
n = viscosity of the gas, and
D = particle diameter.
P
1211
Table B.4 lists diffusion coefficients at normal atmospheric
conditions for particles from molecular size to 100 \im radius. Notice
that the diffusion coefficient is much larger for small particles than
for large ones.
As an aerosol flows through a tube, the Brownian motion of some
particles will cause them to cross fluid streamlines and hit the tube
wall. For particles in the size range where Brownian motion is important
(below 0.1 pro), they will adhere strongly to the tube and not be reentrained.
Thus, they are lost from the aerosol stream.
The rate of deposition of particles on the tube wall is a function of
the particle size; the concentration of small particles decreases more
rapidly than that of large particles. Considering only the deposition of
particles caused by Brownian motion, the concentration of aerosol leaving
a long, circular tube is :
~= 0.819exp(-14.63A)+0.0976 exp(-89.22A)+0.019 exp(-212A) (
0*
o
where:
C = average exit number concentration,
C = average inlet number concentration,
x = length of tube,
d = tube diameter, and
V = average gas velocity.
-------
42
Table B.4
The approximate molecular diffusion coefficients
of small particles in air at 760 mm and 20 C.1211
Particle radius
(ym)
-4
10 (hydrogen molecule)
5 x 10"4
io"3
5 x 10~3
io-2
2 x 10"2
5 x 10"2
io-1
2 x 10"1
5 x 10"1
1
2
5
10
20
50
100
Coefficient of diffusion, D
(cm /sec)
7 x 10"1
5-2 x 10"2
1-3 x 10~2
5-3 x 10~4
1-4 x 10"
3-6 x 10~
6-8 x 10"6
2-2 x 10~6
8-4 x 10"7
2-76 x 10"7
1-3 x 10~7
6-16 x 10"8
2-4 x 10~8
1-2 x 10"
5-9 x 10"9
2-4 x 10~9
1-2 x 10"9
1211
C/C is shown in Figure 17 as a function of A. Equation (B.12) assumed
(a)°Re <2000 (laminar flow), (b) the tube is long enough so that end effects
are negligible, (c) the velocity profile is parabolic, (d) the aerosol con-
centration is small enough so that particle interactions are negligible, and
(e) other effects such as electrostatic forces do not affect Brownian motion.
Equation (B.12) shows that
£-= F (A)
LI
o
where, using the relationship shown in Equation (B.ll):
(B.13)
A =
dv
(ir} Hh-
DP d2v
The first term in Equation (B.14) is a function of the gas, the second term
is a function of particle size, and the third term is related to the system
geometry and flow rate. Thus, if a monodisperse aerosol passes through
a given tube at known conditions, a measurement of C/C determines particle
size.
-------
43
0-5
0-001
0-005 0-01
0-05 0-1
0-5 I
Figure 17. Diffusion to the absorbing wall of a long tube through
which the flow is viscous and follows Poiseuille's
Law.1211
For a polydisperse aerosol, several tubes can be used, each with
different geometry and/or flow rajtes. Each tube has associated with it
a certain particle size of which C/C = 0.50. This particle size is
called the cutoff size with most particles larger than this size passing
through and most particles smaller being deposited. In this case, the
fraction of the size distribution located between two_given particle
sizes is simply the difference between the fractions C/C measured at
each condition.
The size range where this technique is useful is from 0.001 to 0.05 ym
diameter. Above 0.05 ym, Brownian motion becomes insignificant compared
to other motions. Below 0.00_1 ym is the molecular regime. The detector
normally used for measuring C/C is a nuclei counter (see Volume I of
this report). Since the size cutoff is not very sharp (see Figure 17),
size measurements made in this way are not very accurate. However, this
is the best known technique of size classifying 0.001 - 0.01 ym particles.
Husar suggests a somewhat different approach for using diffusion
for sizing particles. It also describes a very recent design of a sizing
instrument and measurements made by the instrument. Tables of design
criteria will prove useful for the design of other such instruments.
Several problems limit the use of diffusion techniques for classifying
effluent particles. Large particles above about 1.0 ym should be eliminated
before the smaller particles pass through the diffusion tubes. If the temp-
erature of the aerosol stream and the tubes are not in equilibrium, thermal
forces and condensation may confuse the deposition. Other deposition
mechanisms may also confuse the deposit, including gravity and even low levels
of turbulence.
-------
44
The diffusion technique may see application to stacks only for
classifying 0.001 - 0.01 ym particles. This will probably be only in
research application, although this technique is the only known size
classification method in the 0.001 - 0.005 pm range.
f. Other Forces
Many other forces can act on small particles. Some of these include
thermophoresis, diffusiophoresis, photophoresis (a form of radiometric
force), and magnetic forces (see, for example, Reference 1333 and 1211).
Although any or all of these forces may have an important effect on the
motion of an aerosol particle in a given system, no way is known to classify
particles by size using these phenomena in practical situations.
2. Particle Sensing Techniques
Volume I and II of this report discuss all known techniques for the sensing of
aerosol concentrations in practical situations. Although Volumes I and II are
oriented toward measurement of the mass of aerosol particles, all other comments are
appropriate for the present application. The reader is referred to Volume I for a
brief, but comprehensive, survey of all sensing techniques; and to Volume II for
detailed discussions of the viable techniques for effluent particle sensing.
3. Combinations of Classifiers and Sensors
Many factors must enter into the choice of a size classifier-concentration
sensor combination for any particle sizing application. A number of these factors
have been discussed in the introduction of this volume. After studying his appli-
cation carefully, the investigator must decide what particle size range is of
primary interest, what type of size classification he prefers, and which parameter
of the particles he wishes to sense. The remainder of this section presents those
classifier-sensor systems which appear most practical for automatic or semi-auto-
matic measurement of the concentration of particles within specified particle size
ranges in effluent streams.
Nearly any size classifier can be used with nearly any concentration sensor
to measure the size distribution of aerosols. It can be seen from the list of
particle size classifiers (at least 10 separate methods) and the longer list of
particle concentration sensors (at least 31 separate methods) that several hundred
possible combinations exist. Not all of these combinations are technically
feasible and most of them are not practical for effluent particle sizing. Table B.5
lists 27 combinations which appear to be technically feasible and which also appear
to be the most practical for application to effluents. Not all of the combinations
in Table B.5 are equally applicable, however, and most combinations have not yet
been developed. The remainder of this discussion will point out strengths and
weaknesses of the most promising of these combinations.
None of the combinations in Table B.5 covers the entire range of particle
sizes found in effluent ducts (from about 0.001 um up to at least 100 um diameter).
Thus, one must choose a smaller particle size range of primary interest, choose a
system which covers that range, and determine the limitations of the chosen system.
-------
45
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The only combination which can measure size distributions down to 0.001 ym
is the Brownian diffusion classifier and nuclei counter sensor as shown in Figure
18. Such a system has been used on artifical laboratory aerosols with some degree
of success in several research laboratories. The system consists of a set of
diffusion tubes of different lengths (called a diffusion battery) and a condensation
nuclei counter to sense the aerosol concentration entering and leaving each tube.
Each tube removes particles smaller than a given size, called the cutoff size. Each
tube has a different cutoff size. Thus, the concentration of particles within the
size range between two specific tube cutoff sizes is the difference between the two
exit nuclei counter concentrations. The sharpness of the size classification is
not very good, but this is the only potentially-automatic measurement method which
operates in this size range.
CONDENSATION
NUCLEI COUNTER
I
J CONDENSATION [__.
NUCLEI COUNTER Jlr
2 F
J CONDENSATION
-, NUCLEI COUNTER ^
i 3 r
J CONDENSATION |__
NUCLEI COUNTER ±2-
4 F
DIFFUSION TUBES
J CONDENSATION
"1 NUCLEI COUNTER
1 5
AEROSOL IN
Figure 18. Diffusion battery classifier with nuclei counter concentration
sensors. This technique could probably operate in the 0.001 -
0.01 pm size range. Condensation nuclei counter 1 measures
the concentration before aerosol passes through diffusion tubes
to the other condensation nuclei counters.
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47
The only alternative method (not automatic) is electron microscopy which
can be performed only on non-volatile solid particles (not liquid). Electron
microscopy in the size range below 0.01 ym is very difficult, if possible at all.
In using the diffusion battery, all particles greater than about 0.1 or 0.2 ym
should first be eliminated, e.g., by impaction. This reduces any interference
caused by the large particles. Although this technique has been used in laboratory
research studies, much development remains to make it useable for effluent particles.
At present, the optimum choice for classification in the size range between
diffusion batteries and aerodynamic methods is electrostatic classification. This
system consists of an electrostatic charger, an electrical mobility classifier,
and either a charge collector (e.g., a particle filter) or a condensation nuclei
counter to measure aerosol concentration downstream of the mobility classifier (see
Figure 19). As the mobility classifier is adjusted to collect particles with various
mobilities, the aerosol concentration is noted at each mobility. The concentration
within a given size range is equal to the difference in concentrations measured with
corresponding mobility cutoff settings. In the case of the charge collector sensor,
the current draining off the collected particles is measured by an electrometer. By
knowing how many charges have bled off each particle, one can calculate the aerosol
concentration which was responsible for the measured current flow. A commercial
system designed for operation in atmospheric aerosol has a high enough resolution in
the 0.01 - 0.3 ym range to measure a geometric standard deviation of about 1.15 when
sampling a completely monodisperse aerosol with a geometric standard deviation of 1.0.
The instrument has somewhat lower resolution over the complete 0.005 - 0.6 ym range.
The electrostatic sensor has proven superior to the condensation nuclei counter in
atmsopheric aerosols. Although considerable development and testing is necessary to
apply this technique to effluent streams, no basic problems are foreseen.
AEROSOL
t
IN
ELECTROSTATIC
DA DTI ^*l C
rAK riQ/Lc.
CHARGER
k.
ELECTRICAL
MOBILITY
CLASSIFIER
CONDENSATION
NUCLEI
COUNTER
AIR
OUT
ALTERNATE SENSORS
i
CURRENT
COLLECTOR
(PARTICLE FILTER)
AIR
OUT
ELECTROMETER
Figure 19. Electrostatic classifier with two alternate particle sensors:
(a) electrostatic sensor, and (b) condensation nuclei counter
sensor. This technique operates in the 0.005 - 0.6 ym size
range.
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48
There are several choices of classifier-sensor combinations for the 0.2 -
100 pm range. All of the aerodynamic classifier systems can be arranged with
the classifier upstream and in series with the sensor, as shown in Figure 20.
AEROSOL
IN '
AERODYNAMIC
CLASSIFIER
PARTICLE
CONCENTRATION
SENSOR
AIR
OUT
Figure 20. One possible arrangement of classifier and sensor for
all aerodynamic classification systems listed in
Table B.5.
In this case, the sensor detects everything passing through the classifier, i.e.,
the particles smaller than the designed cutoff. To measure several size fractions,
one must use several classifier operating conditions, or, more commonly, several
classifiers, each designed for a different size cutoff, with corresponding particle
sensors. In the case of the mass sensors, the concentration of particles passing
the last classifier stage can be lumped into one size fraction.
A look at several specific design concepts will make clear how other combina-
tions can be assembled. First, we will look at the impactor-sensor combination.
One system could be arranged as shown in Figure 21. In this design a continuous
flow of aerosol is drawn into the cascade impactor. Sensor #1 measures the total
concentration of the aerosol stream by sampling a small fraction of the impactor
flow. The aerosol entering the impactor passes through impactor Jet //I which
collects particles greater than size D . Sensor #2 measures the remaining con-
centration (particles smaller than D *j. The aerosol then passes through impactor
Jet #2 where particles larger than D _ (and
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49
IMPACTOR
STAGE
I
IMPACTOR
STAGE
2
IMPACTOR
STAGE
3
IMPACTOR
STAGE
4
AEROSOL,
IN
r
AIR
OUT
ft
SENSOR SENSOR
2
i! I
AIR OUT AIR OUT
SENSOR
3
4
AIROUT
SENSOR
4
4
AIR OUT
SENSOR
5
I
AIROUT
Figure 21. An impactor-sensor particle size measuring system using
aerosol concentration sensor behind every impaction stage.
Each sensor measures the concentration of particles smaller
than the preceding impactor cutoff size.
A second impactor design, using the impactor-piezoelectric sensor as an
example, is shown in Figure 22. In this design, all of the aerosol passes through
every impactor jet. The impaction deposition plates are piezoelectric quartz
crystals driven by external oscillators. In this case, each crystal senses the
size fraction larger than the size cutoff of its corresponding jet and smaller
than the size cutoff of the preceding impactor stage. A piezoelectric con-
centration sensor using electrostatic precipitation can sample the aerosol
passing the last impactor stage. The same arrangement could be used with beta
radiation attenuation and soiling potential. However, neither is as sensitive
as the piezoelectric technique.
A third practical impactor design uses the void-space impactor concept as
shown in Figure 23. As particles impact into the void (air) space, a sample
of the void air is drawn through a sensor. The sample flow of void air must be
much smaller than the air flow through the impactor jet. This technique avoids
the problem of particle blowoff which occurs as the impacted sample builds up on
conventional impaction plates. Nearly any sensor can be used with the void impactor.
A unique feature of this technique is the concentrating effect of the system. After
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50
IMPACTOR
STAGE
1
IMPACTOR
STAGE
2
IMPACTOR
STAGE
3
IMPACTOR
STAGE
4
AEROSOL
IN
1
SENSOR
ELECTRONICS
1
SENSOR
ELECTRONICS
2
SENSOR
ELECTRONICS
3
SENSOR
ELECTRONICS
4
CONCE
TRATK
SENSC
nr
AIR 0
Figure 22. Another impactor-sensor particle size measuring system using
an impaction plate which actively senses deposited particles.
This example shows piezoelectric crystal sensors. Beta
radiation attenuation or soiling potential could also be
applied with this design.
the system reaches steady-state operation, the aerosol in the void sample flow
becomes concentrated:
vs
o Q.
(B.15)
vs
where:
C = concentration of void sample flow,
C = initial concentration of the identical aerosol
size fraction,
Q. = flow rate through the impactor jet, and
J
Q = flow rate of the void sample to the sensor.
The concentrating effect can make some of the less sensitive sensors applicable.
The void-space impaction system responds more slowly to fluctuations in aerosol
concentration than the design shown in Figure 21 and 22 because of the damping
effect of the void space. Hardware of this type employing beta radiation
attenuation sensing is being fabricated by Environmental Research Corp., St. Paul,
under contract to Environmental Protection Agency/NERC.
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51
IMPACTOR IMPACTOR IMPACTOR IMPACTOR
STAGE STAGE STAGE STAGE
1234
AEROSOL
IN
J
SENSOR
2
T
SENSOR
3
T
SENSOR I AIR
OUT
SENSOR
4
AIR OUT AIR OUT AIR OUT AIR OUT
Figure 23. An impactor-sensor particle size measuring system
using an aerosol concentration sensor to measure the
.concentration of particles impacted into a void space.
The air flow rate passing out of each void space must
be very small compared with the air flow through each
impactor stage.
The cyclone classifier is particularly useful as a single-stage size
classifier in the 0.5 - 20 ym size range. It has been used in industrial hygiene
sampling applications as a simulator of particle retention in the upper respiratory
system in humans. Thus, anything passing through the appropriate cyclone (small
size fraction) is defined as respirable dust. The cyclone classifier with
appropriate particle concentration sensors will probably find a similar use in
stack effluent monitoring. The size classification with a- cyclone is not as
sharp as with an impactor, so detailed size analysis with several .size stages
would preferably be done with impaction. However, the cyclone has several
advantages for single-stage classification of effluent particles:
1. Can operate continuously for an indefinite time without any
deterioration in classification ability and without phenomena such
as particle bounce and particle blowoff which hinder continuous
operation of all impactors except void impactors,
2. Recognized as the standard simulator of the respiratory system for
industrial hygiene applications, and
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52
3. Simulates the operation of cyclone effluent control devices, making
cyclone classifiers especially useful in evaluating such control
devices.
A cyclone classifier with appropriate sensors is shown schematically in
Figure 24. The sensor which measures the concentration of small particles passing
through the cyclone can be nearly any sensor listed on Table B.5. Probably photo-
metry and piezoelectric quartz crystal sensors would be most useful because of
their high sensitivity. The elimination of the large particles would probably
eliminate any particle adhesion problems normally encountered with the piezo-
electric sensor. However, the sensor which measures the large particles coming
out from the cyclone must sense large, powder-like particles. The beta radiation
attenuation and soiling potential sensors, both with filter-tape particle
collectors, would probably be superior to other sensing techniques for this
measurement.
AIR OUT
I
SMALL PARTICLE
SENSOR
AEROSOL IN
LARGE PARTICLE
SENSOR
Figure 24. Single-stage cyclone classifier with two concentration
sensors: one for the small particle fraction and one
for the large particle fraction.
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53
Gravitational sedimentation has been used, particularly in England, to
simulate the deposition in the human respiratory system. This principle
could be used to separate large particles from an effluent stream as does
the cyclone. However, there does not seem to be a simple way to sense the
coarse particle fraction separately. One could sense the total concentration
and the fine fraction, and then subtract to obtain the coarse-particle fraction,
but this is not as accurate as sensing the coarse and fine fractions separately
as with the cyclone classifier.
Gravitational elutriation could be used in much the same way as cyclone
classification. Classification into two size fractions is relatively simple,
but the size cutoff is probably not sharp. Both the fine and coarse fractions
could be sensed directly as with the cyclone. Although this technique offers
promise as a single-stage classifier, it has not been developed for aerosols
such as stack effluents. Thus, development and testing is needed to more
fully evaluate its characteristics for this application.
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54
C. TECHNIQUES WITH INSEPARABLE CLASSIFICATION AND SENSING
On several aerosol size measuring techniques, the apparatus used to classify
the particles into various size ranges cannot be physically separated from the
apparatus used to sense the particle concentration within each size range. In
these cases the processes of classification and sensing occur simultaneously.
Most of these techniques involve the interaction of the particles with electro-
magnetic radiation and are discussed in considerable detail in Volume II of this
report. The discussions below summarize each technique and describe briefly how
each could be adapted to measurements of particle size in effluent streams. The
techniques include:
1. Optical Techniques
a. Optical Particle Counters
b. Angular Light Scattering
c. Multi-Wavelength Light Transmission
d. Light Scattering: Polarization Ratio Method
e. Holography
f. Automated Microscopic Method
2. Impact and Momentum Sensors
3. Piezoelectric Single Particle Counter
1211
1. Optical Techniques (See Hodkinson , pp. 316-317, Table III, on various ways
to use light scattering to measure particles)
a. Optical Particle Counters
The scattering of light by individual particles as they pass,
single-file, through a beam of light has been used extensively to
measure the size distribution of airborne particles. The principle
of operation can be seen in Figure 24. Aerosol is drawn through the
sampling tube by suction. As a particle passes through the sensing
volume, it scatters a pulse of light which is detected by the photo-
multiplier tube. The output of the photomultiplier tube is a series
of voltage pulses, one for each particle passing through the sensing
volume. The amplitude of the voltage pulse is proportional to the
size of the particle. By classifying the amplitudes of the voltage
pulses by means of a pulse-height analyzer, the size distribution of
the aerosol can be measured.
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55
PHOTOMULTIPLIER
CONDENSATION LENS
PROJECTION LENSi
AEROSOL
INTAKE
PUPIL LENS
SLIT
COLLECTION LENS
CHOPPER DISC
CHOPPER MOTOR
LIGHT PIPE
ENLARGED PETAIL
OF SENSITIVE
VOLUME
AREA
AEROSOL
EXHAUST
SUCTION (INLET) TUBE
."'^SENSITIVE
VOLUME
.98 CUBIC MM
ILLUMINATED
VOLUME
4 CUBIC MM
EXHAUST TUBE
Figure 25. Principle of operation of an optical particle counter
which senses light scattered at 90 from the incident
beam. (From a Royco Instruments, Inc. instruction
manual)
Many particle characteristics, in addition to size, affect the
amplitude of the scattered light, including the refractive index, the
shape, and the surface optical properties of the particle, the angle
between the incident light and the scattered light, the orientation
of a non-ideal particle in the light beam, and the wavelength character-
istics of the light source-photomultipler system. With any given instru-
ment design applied to a specific aerosol, most of these extraneous
effects can be minimized with appropriate calibration. An aerosol with
different optical properties requires a new calibration. If all of
these secondary effects are ignored, an optical counter measures the
number concentration within size ranges classified by particle surface
area.
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56
The design of the optics of an optical particle counter, including
the choice of scattering angle and the use of lasers instead of a multi-
wavelength light source, is quite controversial. Several commercially-
available designs detect forward-scattered light, i.e., light scattered
in the same direction as the light beam travels. This design usually
minimizes the effect of particle refractive index.143 with proper
design and calibration of a multi-wavelength 'system, the use of laser
light sources does not appear to offer significant advantages for most
applications.
The concentration of particles sensed by an optical particle counter
must be low enough so that the chance of more than one particle occupying
the sensing volume at one time (physical coincidence) is small. If more
than one particle is present in the sensing volume the scattered light
detected by the photomultiplier will be the sum of the light scattered
by both particles. Thus, the instrument will detect only one particle,
measuring it somewhat larger than either of the actual particles. For
application in effluent streams, dilution of the sample with clean air
is necessary to reduce coincidence errors to an acceptable level. Dilution
by 1,000 to 10,000 times would be required with present commercially-
available counters. If the size of the viewing volume can be significantly
reduced, less dilution will be required.
A problem analogous to physical coincidence is electronic coincidence.
Electronic coincidence occurs when the electrical signal from a second
particle arrives before the system has had sufficient time to process the
signal from the first particle. Possible solutions to this problem are
1) improvement in the speed of the electronics, 2) dilution, and 3)
reduction in the size of the viewing volume.
The theoretical lower limit of particle size which can be detected
is determined by the scattering of light by the gas in the sensing volume.
For practically-sized sensing volumes, this limit is approximately 0.1 -
0.2 ym diameter for carefully-controlled laboratory aerosols.1388 Most
commercially available optical counters operate down to 0.3 ym which is
nearly the optimum. If larger numbers of 0.1 ym particles are present in
the aerosol, the counter may count fluctuations in the concentration of
these particles as if they were particles in the range above 0.3 ym.""^
This could also limit the lower detectable size of an optical counter.
The upper size limit of the optical counter is determined by the
fluid mechanics problem of getting the airborne particles into the sensing
volume. Low flow rates and concentration are needed to reduce the size of
the viewing volume which in turn reduces coincidence losses and increases
the sensitivity of the sensor to small particles. With these low flow
rates, large particles (above 5 or 10 ym) tend to settle out of the
sampling tube before reaching the sensing volume. A carefully designed
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57
inlet flow system can greatly improve the chance of sensing large par-
ticles, but about 30 ym appears to be the practical limit with present
commercial instruments with carefully modified inlets. There is no
theoretical upper size limitation of the light scattering phenomena
itself.
Despite these important limitations, optical particle counters
will undoubtedly find use in research measurements of particle size
distributions in effluent streams. The optical counter will not cover
the entire size range of effluent particles, as is true of any sizing
technique, but it does cover an important size range. It appears that
the technique will not be used for routine monitoring of effluents be-
cause of the difficulties in bringing a representative sample to the
sensing volume. When used with other instruments so that a wide size
distribution is measured, the optical particle counter can be a powerful
tool for understanding the dynamics of any aerosol system.
The optical particle counter is covered in greater detail in Volumes
I and II of this report. A complete list of references is included in
Volumes I and II. Many good reviews of light scattering phenomena are
available, including those by Hodkinson,12H Hodkinson & Greenfield,1^3
Kerker,1215 and Van de Hulst.-^Ol Technical descriptions of commercially
available instruments are authored by Zinky,370 Qgle,580 Randall &
Keller,1210 Martens & Keller,756 Martens & Fuss,670 Martens,578 and
Sinclair.615 Other special purpose models are described by Thomas,
et al,120 Moroz, et al,U28 Whitfield and Mashburn,949 Neitzle,824
Nelson,276 Mumma,598 Gebhart, et al,1388 and Kiktenko, et al.1066
Most of these references discuss instruments designed for clean room
monitoring. Several discuss atmospheric air pollution measurements.
b. Angular Light Scattering
Angular light scattering refers to the scattering of light by an
aerosol particle at various angles with respect to the incident light
beam. To measure the angular distribution of scattered light, the
observer or light detector must measure the relative intensity of light
in each direction around a particle. The shape of the angular scattering
distribution changes as particle size varies. Thus, the shape of the
angular distribution of scattered light is a measure of particle size.
This technique is discussed in detail in Volumes I and II of this
report, by Kerker, et al,70 an(j by Kratohvil & Smart.842 Discussions
of the general principles of light scattering also cover the angular
distribution of scattered light.
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58
An instrument utilizing angular scattering for analysis of single
aerosol particles has recently been developed by:
Science Spectrum, Inc.
1216 State Street
P.O. Box 3003
Santa Barbara, California 93105
This technique has been used only in research laboratory appli-
cations . Considerably more theoretical and experimental work remains
to demonstrate the utility of this technique. The procedure is now
primarily manual with no simple method of automation available. The
data reduction technique is also manual. For these reasons, this
technique currently is probably unsuited for particle size distribution
measurements of effluent streams.
c. Multi-Wavelength Light Transmission
The extinction of a light beam as it passes through a cloud of
aerosol particles will change if the wavelength of the light beam
changes. The type and amount of change depends on the size of the
aerosol particles. This dependence, a rather complicated one, is
discussed in detail in Volume II of this report and by Kerkerl215 and
will not be repeated here. Measurement of the transmission of light at
several wavelengths through an aerosol cloud can result in an estimate
of the mean volume-surface particle diameter in the range of size from
about 0.2 - 2 ym diameter. The estimate is not very accurate unless
the size distribution is narrow. Effluent particle size distributions
are nearly always quite broad. However, the measurement is very simple
and inexpensive to make, requiring only a simple transmissometer with
variable wavelength. The wavelength variation can be performed by a
disk made up of several different filters. Several methods of data
reduction are described in Volume II.
Although this technique cannot measure the complete size distri-
bution, it can measure the volume-surface diameter with reasonable
accuracy in the 0.2 - 2 ym range. Its simplicity as applied to effluent
streams make further investigation desirable. If some way can be found
to automate the data reduction, probably by a computer technique, the
technique could result in useful particle size information at low cost
and complexity.
d. Light Scattering; Polarization Ratio Method
When unpolarized incident light is scattered by a particle, the
scattered light intensity can be described by two plane polarized
components: i, perpendicular, and i parallel to the plane of observa-
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59
tion. There is a relationship between the intensity of scattered
light and the wavelength of incident light, the particle size, shape,
size distribution, index of refraction, and observation angle. In
the polarization ratio method, the ratio i-,/io is a measure of the
particle size.
This technique, described in detail in Volume II, is practical
only for measuring size distributions with a standard deviation of
less than 0.3. Effluent particles have a much wider spread of particle
sizes, making this technique impractical.
e. Holography
Holography is a technique by which three-dimensional information
is recorded on a two-dimensional photograph. The technique consists
of photographing the interference'pattern that exists when a diffracted
or object field (Fresnel or Fraunhofer diffraction pattern of the object)
is allowed to interfer with a reference field or background wave. The
image can be later reconstructed and any part of the three-dimensional
reconstructed image can be focused or analyzed by simply placing the
focusing plane or analyzer in the reconstructed image. The photographed
object, e.g., a cloud of aerosol, is not disturbed in any way. The
hologram is a permanent, three-dimensional, photographic record of the
cloud.
Holograph is presently being used successfully by TRW Systems
Group under an EPA contract to study the spatial distribution of
clouds of particles within a coal-fired, steam-generating combustion
chamber.1257,1279 The limit of resolution of this system (a lensless
system) is about 25 pm and is determined by the photographic film
quality. The depth of field of the system is over 30 feet. A high-
quality telescope could increase the resolution to about 1 urn, but
the depth of field would then be only millimeters. For measuring the
size distribution of particles in effluent streams, a resolution of at
least 1 pm would be necessary. The lack of depth of field would be
severely restrictive.
The analysis of a reconstructed hologram of aerosol particles can
be automated by scanning the entire reconstructed image in three-dimensions,
The scanner could be a light-intensity sensor (photomultiplier) which
could have a data analysis system nearly identical to that used in optical
particle counters. The result would then be the number concentration of
particles in given size ranges. This size distribution could be measured
within various portions of the reconstructed image yielding the spatial
distribution of the particle size distribution. Although such a system
does not exist, it appears that nearly all components of such a system
have been developed for other uses. Although present holographic tech-
nology is limited for continuous monitoring applications by its resolution
and depth of field, such a system would be a very useful too] in many
research studies.
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60
Holography is a complex science in an infant state of development.
A chapter in Volume II discusses holography in more detail. Develop-
ments currently being made will undoubtedly make large improvements in
the applicability of holography to the measurement of individual effluent
particles.
f. Automated Microscopic Method
Microscopic methods of particle sizing have undergone considerable
automation in recent years. It is now possible to automatically perform
nearly all of the tedious counting and sizing process. One present
method requires the human operator to obtain a representative sample
which is compatible with the microscope. The operator places the sample
in the microscope and focuses on the desired portion of the sample. The
image is then relayed to a TV screen where a computerized electron beam
scans the image. The scanner detects the size of each object on the
screen by sensing dark and light spots. The computer then categorizes
the detected spots by size and prints out a particle size distribution.
The operator must choose other fields of view of the sample and other
magnifications. Electron microscope analysis can be performed in a
similar way.
It appears possible to automate the entire sizing system for a
given application. Nearly all parts of such a system have been developed
before for other applications. The prime obstacle appears to be the
rather extreme mechanical and electronic complexity of such a system.
The system would consist of an automated sampling system which would
obtain representative deposits of effluent particles on a suitable
surface and pass these sample deposits on to the microscopic system.
The automated microscopic system would then choose a suitable field of
view, focus on the field of view, perform its counting and sizing
procedure, and print out the results. The system would have to perform
analysis at several different magnifications to assure accurate statistical
analysis of all reasonable size ranges. The design of such a system would
appear to be a matter of connecting appropriate components and programming
a computer as a process controller and data analyzer. However, the cost
would be very high and the complexity would undoubtedly lead to poor
reliability.
The problem areas of such a system include the resolution limits
and automatic focusing complexities, the statistical limitations of
any microscopic study which requires examination of adequate numbers
of individual particles and several magnifications to cover the wide
particle size range of effluent particles, the complexity of the entire
system, and the prohibitive cost of the system. These restrictions
probably limit such a system to research programs and prohibit application
for routine continuous measurements.
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61
2. Impact and Momentum Sensors
When a moving particle strikes a surface, its momentum is transferred to
the surface. If the surface is suspended in such a way that it can vibrate,
the amplitude of vibration is proportional to the transferred momentum. This
principle has been used to detect the momentum of micrometeoroids striking
spacecraft.252 in most applications, a piezoelectric transducer is used to
convert the mechanical vibration into an electrical signal.
One of the most sensitive momentum transducers is shown in Figure 26.
When a moving particle strikes the target, the piezoeletric beams deflect
resulting in a damped oscillating electrical signal. The amplitude of the
oscillation is calibrated in terms of particle momentum. Thus, if the velocity
and density of the particle is known, the particle size can be calculated. The
piezoelectric beams used in this design^-1 are polycrystalline-modified lead
zirconate titanate ceramic. This material is described as being a nearly ideal
sensing element.252 fhe threshold sensitivity of the momentum transducer is
about 10 dyne-sec.
In an analytical evaluation of this instrument for use as a particle size
transducer for aerosol particles, it was found that particles below 30 microns
could not be accurately sized assuming the ultimate momentum resolution (10~^
dyne-sec.), reasonable particle velocities (10^ cm/sec.), and unit density
particles.2 It appears to be difficult to accelerate the particles to higher
velocities without introducing aerodynamic instability into the sensor. Perhaps
the aerosol beam technique-*^/3 >1361,1362 could be used for this purpose. However,
the inability of this sensor to detect particles below 30 microns makes it
relatively useless for effluent aerosols.
Target
Piezoelectric beams
Support -
Figure 26. Schematic of a particle momentum sensor which uses
piezoelectric beams as the transducer.252 Particles
strike the target, causing it to vibrate which, in
turn, causes an electrical signal in the piezoelectric
electronic circuit.
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62
3. Piezoelectric Single Particle Counter^
A piezoelectric microbalance can sense the addition of a single particle
to its surface.1223 Rather than measuring frequency shifts caused by a number
of particles added to the crystal surface as done the standard piezoelectric
microbalance (see Volume II of this report), the single particle counter
differentiates the frequency signal resulting in a pulse whenever a single par-
ticle becomes attached to the crystal surface. The magnitude of the pulse is
related to the mass of the particle. Thus, if particle, density is known, the
size can be calculated. A pulse-height analyzer can be used to classify the
pulses into particle size range.
The ultimate size resolution claimed for this technique is about one micron
179*}
for unit density spheres. ^ The particle must adhere solidly to the crystal
surface immediately upon contact, or the pulse magnitude will not be proportional
to particle mass. The concentration and sampling rate must be low enough to
prevent more than one particle from striking the crystal at one time. It appears
that no more than about 1()3 particles per second can be sampled. The crystal
sensor is somewhat sensitive to temperature, relative humidity, condensible vapors,
and any other foreign material which can adhere to the crystal surface.
One can also measure the mass concentration with the piezoelectric single
particle counter by monitoring the rate of change of frequency during the time
period of interest. Thus, it can also be used as a mass concentration monitor
as described in Volumes I and II of this report.
This technique has just recently been introduced to aerosol science and is
not yet very developed. Thus, it is not clear which or how many of the potential
problems will limit its use in effluent streams.
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D. LABORATORY POWDER SIZING TECHNIQUES
A number of semi-automatic techniques exist for the sizing of powder
particles in the laboratory. Most of these techniques require the dispersion
of the particles in a suitable liquid. The size analysis is then performed
on the liquid-suspended particles.
Several of these techniques could conceivably be automated for sizing par-
ticles in effluent streams. A representative sample would first have to be
collected from the effluent stream, dispersed accurately into the suitable liquid,
and then sized automatically by the sizing apparatus. At least two major problems
present themselves: (1) the mechanical complexity of a completely automated par-
ticle size monitoring system, and (2) the difficulty in defining the relationship
between the particles as they exist in the stack and as they exist in the dispersion
liquid. The first is primarily a mechanical design problem, but the second is a
more basic problem.
Several configurations are suggested here as possible solutions to the
problem of automating such a system. The system would consist of two major
components: (1) the particle size analyzer, and (2) the equipment needed to collect
a representative sample and bring it to the particle size analyzer suspended in
a suitable liquid.
Several semi-automatic particle size analyzers are commercially available for
sizing 0.5 - 100 \im particles suspended in a liquid. Two such instruments are
the Micromeritics particle size analyzer* and the Coulter counter**. These are
two of the most automated laboratory powder sizing instruments available.
The Micromeritics instrument classifies particles by size using the principle
of particle sedimentation through the liquid. The particle concentration is sensed
by an x-ray beam. A computerized readout system allows the adjustment of particle
density, liquid density, and viscosity. The result is plotted by an X-Y plotter
in terms of the "% by mass less than size" versus "equivalent particle diameter".
The particles must be introduced into the instrument in the form of a liquid
suspension.
The Coulter counter feeds the particles single-file through a small orifice
and measures a change in the electrical conductance across the orifice as each
particle passes through. The amplitudes of the resulting electrical pulses, one
for each particle, are proportional to the size (volume) of the particles. The
results, after appropriate classification of the electrical pulses, can be present-
ed in several forms, such as "% by number less than size" or "number counted per
size range" versus "equivalent particle diameter". As in the Micromeritics instru-
ment, the particles must be introduced into the Coulter counter in the form of a
liquid suspension. The Coulter counter liquid must be an electrolyte.
*Manufactured by: Micromeritics Instrument Corporation, 800 Goshen Springs Road,
Norcross, Ga. 30071.
**Manufactured by: Coulter Electronics Industrial Division, 590 West 20th St.,
Hialeah, Florida 33010.
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Each of the size analyzers has its own set of problems. These have been
quite well described in the literature and will not be discussed here. Some
of these problems may be limiting factors in the design of a fully-automated
system.
However, a major problem faced by all sizing systems of this type is the
design of the second component mentioned above: the equipment needed to collect
a representative sample of particles from the effluent stream and bring the sample
to the size analyzer suspended in a suitable liquid. No automated system of
this type is known and all known design concepts appear to lack the capability
of reliable, long-term, unattended operation. The two most feasible air-to-
liquid particle samplers are the LEAP and MSI samplers*. The LEAP sampler uses
electrostatic precipitation to collect particles from 300 to 15,000 liters per
minute of air onto a film of liquid. The liquid flows continuously over the
collection surface and then into a collection bottle. The MSI sampler is nearly
identical except that it uses impaction rather than electrostatic precipitation
and it operates at only one flow rate, 1000 liters per minute, as commercially
designed. Conceptually, one of these samplers could continuously collect particles
from the effluent stream and feed the resulting liquid suspension to the particle
size analyzer.
Even if the design problems could be satisfactorily solved, the problem of
relating the measured particle size in the liquid suspension to the particle size
in the effluent stream remains. Generally, one wants to measure the size of an
agglomerate just as it exists in the effluent stream. However, the agglomerate
may break up or grow in the liquid suspension. The control of the agglomeration
or deagglomeration process would be difficult. Also, any soluble particles would
be dissolved by the liquid, and liquid or partially-liquid particles would be
grossly changed in the liquid suspension. In instruments which use an electrolytic
suspension, the acidity of the stack gas could change the electrolytic properties.
Thus, the interpretation of data from -this type of sizing system would be difficult.
For these reasons, this technique of sizing effluent particles should be
approached with extreme caution.
*Manufactured by Environmental Research Corporation, 3725 North Dunlap Street,
St. Paul, Minnesota 55112.
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E. A DIFFERENT SIZING CONCEPT: PARAMETRIC MEASUREMENT
The measurement of a single particle concentration parameter has rather
severe limitations for characterizing an aerosol such as combustion effluent.
For example, the number concentration is usually dominated by the great numbers
of small particles below 0.1 ym diameter. The addition of enough 25 ym particles
to double the mass concentration will cause an undetectibly small change in the
number concentration. On the other hand, the mass concentration is usually
dominated by the great mass of large particles primarily above 1 ym diameter.
Doubling the number of 0.01 - 0.1 ym particles causes an undetectibly small
change in the mass concentration in such cases. Thus, mass concentration
measurements are sensitive to large particles while number concentration
measurements are sensitive to small particles.
The parametric particle size measurement method would use the inherent
size limitations of particle concentration sensing techniques to measure a
form of particle size distribution. Several concentration sensors would be
used, each sensing a different particulate parameter. For example, number,
surface, and mass concentration are three potential parameters. Number con-
centration would be highly sensitive to small particles, surface concentration
to medium-sized particles, and mass concentration to large particles. Although
this method would not offer enough detailed information for many scientific
research studies, it would appear to offer very useful measurements for source
monitoring applications.
Particle number concentration can be measured by a condensation nuclei
counter or by an electrostatic technique. Particle surface area, or a parameter
similar to surface area, is measured by well-designed optical transmissometers
and photometers. Particle mass is measured by beta radiation attenuation instru-
ments. All of these techniques are discussed in detail in Volumes I and II of
this report. Nearly all have been used in other applications, and adaptation of
at least one sensor in each group to effluent measurements appears easily possible.
Electrostatic sensors, optical transmissometers, and beta radiation attenuation
have all been used on large combustion effluent stacks in the past. Thus, the
design of equipment to make all three measurements simultaneously, on a continuous,
monitoring basis, does not appear to be an obstacle. Further development of
these techniques would not be prohibitively expensive and could probably result
in reliable instruments in about 2 years.
Even though this approach would result in the measurement of only three
particle concentration parameters, not in the measurement of size distributions,
much information about the particle size distribution would be available from
the three measurements. The number concentration measurement would be sensitive
to fluctuations in the concentration of small particles. The mass concentration
measurement would be sensitive to fluctuations in the concentration of large
particles. The surface area measurement would be sensitive to particles in the
middle size range. Thus, an increase in the emissions of large particles would
be detected by a corresponding increase in measured mass concentration and
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66
little change in the measured number concentration. The surface-area measure-
ment would resolve whether the particles were very large (perhaps 20 ym) or
close to 1 ym. An increase in the emissions of small particles would be detected
by a corresponding increase in the measured number concentration, but with little
change in measured mass concentration. Again, the surface area measurement would
resolve whether the additional particles were very small (approximately 0.01 ym)
or in the 0.1 ym range.
There are a number of other factors making this approach to particle size
monitoring of source effluents attractive. Both the large and the small particles
are important to air pollution. The particles above 10 ym settle onto nearby
surfaces, contaminating buildings, automobiles, streets, agricultural crops, trees,
rivers, lakes, etc., with possible damage due to corrosive effects. Particles
below 0.1 ym remain suspended for long periods of time, contributing to global
pollution, to rain nuclei, to condensation processes, to particulate-gaseous
chemical interaction including photochemical smog formation, etc. These small
particles grow, by condensation of vapors and by collision and agglomeration with
other particles, into the intermediate size range between 0.1 and 10 ym. Particles
of the intermediate size range penetrate most deeply into the human respiratory
system, penetrate filters more easily than other sizes, remain airborne for long
periods of time, are most visible, cause degradation of visibility, etc.
Thus, each size range is important for different reasons, and the measurement
of a single particle parameter does not characterize the entire range of particles
found in combustion sources sufficiently enough to relate source emissions to air
pollution effects.
Theoretically, if any form of the particle size distribution can be measured
with enough accuracy, any other form of the size distribution can be calculated
from it. However, the resolution limitations of practical instrumentation do not
allow accurate mathematical transformations to be made on aerosols such as
combustion effluents. For example, any measurement of particle mass concentration
within various size ranges will ignore the insignificant mass of the large numbers
of 0.01 - 0.1 ym particles. Any measurement of particle number concentration
within various size range will ignore the insignificant numbers of larger par-
ticles above 10 ym. Instrument accuracies to 8 or 10 significant digits of con-
centration and over 8 or 10 orders of magnitude of concentration would be necessary
to avoid this problem. Thus, measurement of the entire range with a single instru-
ment is not possible at this time and does not appear probable. Measurement of
a single parameter over the entire range with several instruments is also not
possible.
The mass concentration and the mass size distribution are nearly useless to
the meteorologist studying global pollution patterns and rain formation, to the
city air pollution official concerned with predicting the rate of photochemical
smog formation, and to the researcher studying the dynamics of pollution in the
submicron size range. These applications require a measurement which is sensitive
to the particles of primary interest, i.e., the submicron particles. The number
concentration and number size distribution are two such measurements.
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67
The number concentration and number size distribution are nearly as use-
less to the pollution control official concerned with dust fall on various
surfaces and the stack owner concerned with complying with pollution control
regulations written in terms of particulate mass. These persons require the
mass concentration and mass size distribution; parameters which are of primary
interest to them.
Likewise, the pollution control official concerned with reducing the visible
smoke from stacks and the visible haze over a city, and the people concerned with
reducing the harmful health effects of particulate pollution will be most con-
cerned with the particles in the middle size range: from 0.1 to 10 ym. Optical
measurements and particle surface area sensors are usually most sensitive to
particles in this size range.
This does not mean, however, that three separate and complete size distri-
butions would have to be measured to satisfy all applications. Rather, it appears
that sufficient data may be provided by three single concentration measurements
of these particle properties: (1) number concentration, (2) surface or cross-
sectional area concentration, and (3) volume or mass concentration. This would
satisfy most of the requirements for monitoring applications.
Measurements of other effluent particulate parameters may also prove useful.
For example, the mass of particles within the respirable range as defined by the
American Conference of Governmental Industrial Hygienists-^jS may be a useful
measure of the harmfulness of effluent particles from a health effects viewpoint.
The respirable mass concentration is essentially the portion of the total mass
concentration which remains airborne for long periods of time. The difference
between total mass concentration and respirable mass concentration is essentially
the fraction of particulate emissions which settles to the ground near the effluent
source. Thus, measurement of respirable mass concentration and total mass con-
centration provides another interesting combination for continuous monitoring
applications.
Some of the parameters which may be of interest for particle size monitoring
of source effluents are:
Number concentration
Surface concentration
Volume or mass concentration
Visible or visibility measurement
Respirable
Suspended
Settleable
Rain and snow nuclei
Potential photochemical smog aerosol formation.
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68
Theoretically, the result of this parametric approach appears to be
practical methods to measure not only the concentration of particulate
emissions in terms useful to a wide range of pollution control personnel, but
also much of the necessary particle size information. The necessary hardware
is either already developed or can be developed within about two years. The
equipment will not be prohibitively expensive, even for use as a continuous
monitor on every large combustion emissions source.
The authors recommend an intensive investigation into the potential use-
fulness of this approach. The response to each measurement in the system to
reasonable, expected effluent changes should first be studied. This would
isolate conceptual problems and define additional benefits of such measurements.
The conceptual evaluation should be followed or accompanied by development of
the appropriate concentration monitoring instruments capable of operating in a
total system. The development of concentration sensing instruments for this
application requires no development in addition to the concentration sensors
needed for the monitoring of single concentration parameters. Thus, particle
size information sufficient for most monitoring applications can be obtained
with little additional development of equipment.
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F. SUMMARY AND CONCLUSIONS
1. Particle size distribution is one of the three most important
parameters in defining the relative potential harmfulness of
particulate emissions. A single particle concentration measure-
ment cannot define the relative potential harmfulness of an effluent
stream without information about the relative concentration within
several size ranges. Particle size measurements are urgently needed
for research applications, for control equipment evaluation, and for
air pollution control monitoring.
2. Most size distribution measurements of combustion effluent particles
made in the past used available equipment which severely limited the
size range effectively sampled. The technique used for most such
measurements could not measure particles below 2 ym or over 100 ym
leading to the conclusion that nearly all effluent particles are
within the 2 - 100 ym size range.
3. Demonstrated size distribution measurements of combustion effluent
particles in the micron and submicron range are almost nonexistent.
This size range appears to be the most important for most air
pollution considerations.
4. There is no single preferred way of presenting particle size distri-
bution data which is useful for every application. The preferred
size measurement technique is usually the method which most directly
obtains the desired information in its final form. An important
consideration is whether particle number, surface area, volume, mass,
or some other parameter is needed for any given application.
5. No sizing instrument can classify particles over the entire particle
size range of interest (from 0.001 to over 100 ym). The alternative
is to combine several techniques to cover the entire range or to choose
the range of primary interest and find an appropriate technique for
that range. Both of these approaches require very careful interpretation
of the data.
6. Aerodynamic particle size is the most useful size parameter in most
applications.
7. Table B.5 lists the operable size ranges of various combinations of
size classifiers and concentration sensors. The recommendations for
particle size measurements of most combustion effluents for most air
pollution applications are outlined below:
a. An impaction size classifier (an aerodynamic classsifier) coupled
with a beta radiation attenuation, piezoelectric quartz crystal,
or photometric concentration sensor appears to be most applicable
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to detailed effluent stream particle size measurements at this
time. The impactor cannot classify particles less than about
0.2 or greater than about 30 ym. However, this appears to be
the range of primary interest for many, perhaps most, applications.
The particles below the last impactor cutoff can be lumped into
one size range and, with appropriate attention given to correct
design of the sampling system particles larger than the first
cutoff can be lumped into another size range. Thus, although all
size range cutoffs are between about 0.2 and 30 ym, the entire
range of size passing through the device is accounted for. At
the present, this approach appears to offer the most promose of
success as a single automated particle sizing tool in effluent
streams.
b. If size classification below 0.2 ym is necessary, electrostatic
techniques (0.005 - 0.6 ym) or Brownian diffusion techniques
(0.001 - 0.05 ym) must be used. Although not yet applied to
stack effluents, an electrostatic technique has demonstrated
high resolution in the 0.01 - 0.2 ym range on laboratory,
atmospheric, and small, flame-generated aerosols. The Brownian
diffusion technique, used only for specialized laboratory measure-
ments to date, is the only known method for sizing 0.001 - 0.005 ym
particles. Further research and development of both of these
techniques is necessary before automated instruments for stacks
could result. No other method of classifying particles by size
significantly below 0.2 ym is known.
c. The cyclone classifier can aerodynamically separate an effluent
aerosol sample into two size fractions. The size split must be
between about 0.5 ym and 20 ym and is not very sharp. The cyclone
classifier has been used extensively for separating the "respirable
fraction" from industrial hygiene aerosols and may prove useful for
similar application in effluent streams.
8. Although optical particle counters are severely limited for effluent
stream measurements with their present design, major modification may
significantly reduce the problems. The lower size limit of optical
particle counters is about 0.2 ym. The upper limit, determined by the
sampling system, is about 30 ym; and may be improved by development of
an instrument which can operate directly within the effluent stream.
Another problem at present is the high dilution with clean air required
to prevent coincidence.
9. Holography offers some promise for research sizing applications in
effluent streams. However, routine use appears limited by the cost
and complexity of the equipment. In effluent streams, its ability to
obtain a 3-dimensional record (photograph) of the effluent particles
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without disturbing the flow is highly attractive. However, present
holography is limited by the smallest particle which can be individually
resolved (a few microns) while still maintaining a practical depth-of-
field (perhaps greater than a centimeter) and distance-from-apparatus-
to-particle (greater than 3 meters for use in effluent streams).
Holography is developing rapidly and may prove more useful in the future
as the equipment cost and complexity decreases, and as resolution in-
creases .
10. Powder and slurry particle sizing techniques do not appear applicable
to effluent streams because of the difficulty in relating the particle
size measurement (made in a liquid) to the actual airborne particle.
11. A different technique, which appears to offer a practical method for
long-term, continuous monitoring in effluent streams uses the size
limitations of several different concentration sensors to effectively
measure particle concentration within several size ranges. The
technique would use (1) a beta radiation attenuation sensor to measure
the total particulate mass concentration (sensitive to D^, or large
particles in the 1 - 100 ym range), (2) a transmissometer to measure
opacity (roughly sensitive to D^, or particles primarily from 0.1 - 10 ym)
and (3) a condensation nuclei counter or electrostatic counter to measure
the particle number concentration (sensitive to the number of particles,
or to particles from 0.001 - 1.0 ym). Analysis of the 3 simultaneous
measurements would appear to offer sufficient particle size information
for most continuous air pollution monitoring applications. Measurement
of total mass concentration and respirable mass concentration offers
another interesting combination of particulate parameters. Nearly all
hardware needed for these measurements already exists. Further investi-
gation and testing of this approach is recommended.
12. Although this report addresses itself to the heart of particle sizing
apparatus (the size classifier and concentration sensor), the equally
important problem of particle sampling must also be considered. The
problem of delivery of truly representative samples of effluent to the
measuring instrument has not yet been solved. Although one can tolerate
some agglomeration of fragmentation of particles in a sampling system
for total concentration measurement, such changes in particle size cannot
be permitted in a particle sizing system. Questions related to the
conditioning of the effluent (dilution, heating, cooling, etc.) prior
to measurement by most sizing instruments must also be investigated
thoroughly. Emphasis must be placed on a thorough investigation
of the particle size changes which take place in effluent sampling
systems. This area merits at least as much attention as the particle
sizing instrument itself. The advantages offered by any potential
sizing instrument which does not require removal of a sample from the
stack must be emphasized. Unfortunately, all of the practical, available
techniques require sample extraction.
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957 Brink, J. A., Jr., "Cascade Impactor for Adiabatic Measurement",
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76
1066 Kiktenko, V. S. , Safronov, Y. P., Kuclryaut.sev, S. 1., Fedorov, B. F.,
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1119 Stober, W. , "Design and Performance cf a Si,'-.»- -Separating Aerosol
Centrifuge Facilitating Particle-Size Sp<">ct r^rr.otry in the Submicron
Range", Assessment Airborne RadioactIvitv, T'roc. Symp., Vienna,
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1120 Raabe, 0. G., "Calibration and Use of the ooetz Aerosol Spectrometer",
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1128 Moroz, W. J., Withstandley, V. D., and Anderson, G. W., "A Portable
Counter and Size Analyzer for Airborne Dust'!, \_eview of Scientific
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1201 Van de Hulst, H. C., Light Scattering by Small Particles, Wiley (1957).
1210 Randall, L. M., and Keller, J. D., "Electro-Optical Aerosol Counter
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1215 Kerker, M. , The Scattering of Ligi'T and Ctht-r F i t;c, I romagnetic Radiation,
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1216 Green, H. L., and Lane, W. R. , Particulate CJ.'Hids: Dusts, Smokes and
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Mass", Aerosol Science, V. 1, p. 111-114 0 ^Q) .
1257 Matthews, B. J., and Kemp, R. F., "Investigation of Scattered Light
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NAPCA Contract CPA 70-4 (Nov 3970).
1273 Israel, G. W. , "Investigations of Aerosol Beams", Staub-Reinhalt der
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77
1276 Whitby, K. T., and Liu, B.Y.H., "Atmospheric Particulate Data -
What Does It Tell Us About Air Pollution?", Paper Presented at llth
Conference Methods in Air Pollution and Industrial Hygiene Studies,
Berkeley, Calif., Mar. 30, 31, and Apr 1, 1970.
1279 Matthews, B. J., and Kemp, R. F., "Holographic Determination of
Injected Limestone Distribution in Unit 10 of the Shawnee Power
Plant", TRW Systems Group, Redondo Beach Calif., TRW Report No.
14103-6001-RO-OO under NAPCA Contract CPA 70-4 (Jun 1970).
1285 Paulus, H. J., and Peterson, C. M. , "Urban Aerosols: Count Size
Related to Meteorological Data", School of Public Health, University
of Minnesota, Minneapolis, Minnesota, Final Report (Nov 1969).
1299 Knight, G., "A Simple Method for Determining Size Distribution of
Airborne Dust by Its Settling Velocity", paper presented at the
American Industrial Hygiene Association Annual General Meeting,
Detroit, Mich., May 19, 1970.
1331 Todd, W. F., Hagan, J. E., and Spaite, P. W., "Test Dust Preparation
and Evaluation", Public Health Service, U. S. Department of Health,
Education,and Welfare, Cincinnati, Ohio.
1332 Graham, A. L., and Hanna, T. II., "The Micro-Particle Classifier",
Ceramic Age, (Sep 1962) .
1333 Fuchs, N. A., The Mechanics of Aerosols, Published by the MacMillian
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1334 Anon., "Threshold Limit Values of Airborne Contaminants for 1968",
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1335 Lippmann, M., and Harris, W. B., "Size-Selective Samplers for Estimat-
ing Respirable Dust Concentrations", Health Phys., 8:155-163 (1962).
1340 Marple, V. A., "A Fundamental Study of Inertial Impactors", Ph. D.,
Thesis, Dept. of Mechanical Engineering, University of Minnesota
(Sep 1970).
1341 Dyment, J., "Use of a Goetz Aerosol Spectrometer for Measuring the
Penetration of Aerosols Through Filters as a Function of Particle
Size", Aerosol Science, V. 1, p. 53-67 (1970).
1342 Khmelevtsov, S. S., "A Size-Separation Collector for Sampling Aerosols
From Curvilinear Flow", Dept. of the Army, Fort Detrick, Frederick,
Maryland, Clearinghouse No. AD 678 123 (Nov 1967).
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78
1343 Redkin, J. N., "Properties of Atmospheric Aerosol Measured with a
Centrifugal Spectrometer", Journal of Geophysical Research, V. 75,
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1344 Whitby, K. T., Husar, R., McFarland, A. R., and Tomaides, M.,
"Generation and Decay of Small Ions", University of Minnesota,
Minneapolis, Minnesota., Prepared for National Air Pollution Control
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1354 Lippmann, M., and Kydonieus, A., "A Multi-Stage Aerosol Sampler for
Extended Sampling Intervals", American Industrial Hygiene Association
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1357 Herdan, G., Small Particle Statistics, 2nd ed., Academic Press Inc.,
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paper presented at Second International Clean Air Congress, Washington,
D. C., Dec 6 - 11, 1970.
1362 Dahneke, B. E., and Friedlander, S. K. , "Velocity Characteristics of
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79
1370 Stern, S. C., Zeller, H. W., and Schekman, A. I., "Collection
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-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO. 2.
APTD-1524
4 TITLE AND SUBTITLE , , c ,-, , , i nTi
State of the Art: 1971
Instrumentation for Measurement of Particulate
Emxssions from combustion sources, Volume III:
Particle Size
,,UTHOR( Qilmore J. Sem; ; John A. Borgos;
Kenneth T. Whitby; & Benjamin Y.H. Liu
9. PERFORMING ORG-XNIZATION NAME AND ADDRESS
Thermo-Systems Inc.
| 2500 North Cleveland Avenue
St. Paul, Minnesota 55113
1.2. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
National Environmental Research Center
Research Triangle Park, North Carolina 27711
3. RECIPIENT'S ACCESSI ON- NO.
5. REPORT DATE
July 1972
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
1AA010
11. CONTRACT/GRANT NO.
CPA 70-23
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES .
Volume I was issued as APTD-0733
Volume II was issued as APTD-0734
Volume III (this volume) discusses candidate techniques for automatic
or semi-automatic measurement of particle size distribution in combustion source
effluents. Automatic or semi-automatic particle size measuring instruments do
not yet exist for this application. This report considers the application to
effluent streams of particle size measuring instruments used in other fields.
The discussion emphasize the particulate concentration parameter (mass, number,
surface area, etc.) which each technique senses as well as the method of classify
ing particles into size ranges (aerodynamically, electrostatically, optically,
'' s. Included are descriptions of the basic operation of each technique,
Discussions of limitations of each technique, suggestions of possible major
problems in applying each technique to effluent streams and an overall evaluation
of each technique relative to others.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
Particulate Emissions from Combustion Sour
Size Distribution Instruments
Mass Measurement Instruments
,
13 DISTRIBUTION STATEMENT
Release Unlimited
b. IDENTIFIERS/OPEN ENDED TERMS
ces
19. SECURITY CLASS (This Report/
Unclassified
20. SECURITY CLASS (This page)
Unclassified
c. COS AT I Field/Group
21. NO. OF PAGES
85
22. PRICE
EPA Form 2220-1 (9-73)
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