United States
Environmental Protection
Agency
Environmental Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-045 Apr. 1984
&EPA Project Summary
Development and Evaluation of
Dilution Probes Used for
Sampling to Determine Source
Signatures
Joseph D. McCain and Ashley D. Williamson
The NEA, Inc., stack sampling system
was evaluated. It is designed for obtain-
ing emission source samples in two par-
ticle size ranges «2.5 ^m and 2.5-10 ^m
aerodynamic diameter) corresponding
to those obtained by dichotomous
atmospheric samplers. The stack gas
sample is drawn continuously through
a nozzle and probe and is diluted with
a measured flow of filtered air. A frac-
tion of the diluted sample is drawn
through collection filters in a dichoto-
mous sampler.
The equipment was laboratory tested
in a wind tunnel 10 cm in diameter.
These tests used monodisperse am-
monium fluorescein aerosol particles
(1.3-16 ^m in diameter) generated by a
spinning disk. The system was judged
to be reasonably well suited for defin-
ing emission source signatures. How-
ever, high losses of the larger particles
due to inertia! effects at the nozzle and
deposition of particles in the probe and
connecting Teflon hose resulted in
biased particle size distributions at the
filters. The losses could be partially com-
pensated by reducing the length of the
hose, using a right-angle precollector
with a larger cutoff diameter (12.5 urn),
and by controlled anisokinetic sampling
to over-sample the larger particles. The
loss in the hose of electrically-charged
particles was 4 times that of charge-
neutralized particles.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Atmospheric contaminants can often be
apportioned to specific sources with the aid
of elemental composition signatures for dif-
ferent source types. Such source signatures
preferably are obtained by sampling techni-
ques similar to those used for sampling the
atmosphere. NEA, Inc., has developed a
dilution source sampling apparatus (Figure
1) designed to obtain source samples in two
particle size ranges «2.5 f*m and 2.5-10 jj.m
aerodynamic diameter) corresponding to
those obtained in ambient sampling with
dichotomous sampler. In the NEA system,
a sample is drawn continuously from the
source (e.g., an electric power plant stack)
through a nozzle and probe and is diluted
with a measured flow of filtered ambient air.
A portion of the diluted stream is drawn in-
to a dichotomous sampler in which the gas
flow is divided and the particles are collected
on two filters designed for the desired parti-
cle size ranges.
For obtaining source signatures, this
equipment has two principal advantages
over conventional equipment: (1) The sam-
ple is obtained with the same filtration
technique for particle size fractionation that
is used in many ambient samplers, and (2)
the dilution process resembles the process
of cooling and dilution by entrainment of am-
bient air that occurs when stack gases are
discharged into the atmosphere, allowing
condensation of some chemical species that
are in the vapor phase in the stack.
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7. Nozzle
2. Probe
3 Hose
4. Diluter
5. Blower and Filter
6. Dichotomous Sampler Inlet
7. Dichotomous Sampler
0
Figure 1. NEA dilution sampling system
The question of most concern in the use
of this type of equipment is how nearly the
particle size distribution at the collection
filters resembles the original distribution at
the entrance of the sampling nozzle; that is,
how much of the particulate matter is lost
by deposition in system components ahead
of the filters. This question was addressed
in the present study through laboratory
testing.
Procedure
The tests were carried out in a wind tun-
nel 10 cm in diameter. A spinning disk
aerosol generator was used to produce
monodisperse ammonium fluorescein aero-
sol particles of 1.3 to 16 ^m in diameter. The
gas velocity in the tunnel could be varied
over the range of 5 to 20 m/s, which is
typical of the range encountered in stack
sampling.
The ammonium fluorescein particles are
dry and nonhygroscopic but readily soluble
in dilute ammonium hydroxide, producing a
solution in which the amount of the fluores-
cein can be measured by fluorimetry. Hence,
the amounts of particles deposited in each
component of the sampling system could be
determined by disassembling the equipment,
washing the internal surfaces of the com-
ponents with dilute ammonium hydroxide,
and fluorimetrically analyzing each wash.
Results
Tests with uncharged particles showed
low losses within the diluter itself (e.g., 0.7%
for 10-^m particles). However, losses in the
nozzle and in the probe and 3-m connecting
hose were unacceptably large at the high
sampling rate (76 L/min) initially used. The
loss in the hose alone amounted to 45% of
2-/jm particles and 99% of 15-pm particles.
At this sampling rate, the Reynolds number
of the flow in the hose and probe was
~8000, indicating a flow in the turbulent
regime, in which particle deposition rates
would be expected to be high. When the
sampling rate was reduced to 14 L/min, the
loss in the hose declined but was still
significant.
Further improvement was obtained by
replacing the standard gooseneck nozzle on
the NEA probe with a right-angle precollector
developed by Southern Research Institute.
This device was designed to have a
moderately sharp and predictable cut
diameter in the range of 12.5 pm at a sampl-
ing rate of 14 L/min. Using this device and
a 1.64-m hose gave losses nearer to accept-
able levels. For example, as shown in Figure
2, the fraction of 4-^m particles reaching the
sampler inlet increased from ~30% to
~75%.
A closer approximation to an unbiased size
distribution was attempted by controlled
anisokinetic sampling to oversample layer
particles. For 10-jum particles, a duct veloci-
ty of 15 m/s, a nozzle velocity of 4.65 m/s,
a nozzle diameter of 0.794 cm, and a sampl-
ing range of 14 L/min gave a measured
sampling loss that agreed within ± 10% with
the value of 28% calculated from theory
(Figure 2).
Paired tests using particles whose elec-
trostatic charges had and had not been
neutralized showed that charged particles
were subject to ~4 times more loss in the
nonconductive Teflon hose (see Figure 3).
Losses in the metal probe were not ap-
preciably sensitive to particle charge.
Conclusions and
Recommendations
This study showed that the NEA system
provides a reasonable method to collect
samples for defining source signatures.
However, the tests also showed substantial
losses of (1) large particles owing to inertial
effects and (2) particles of all sizes due to
electrostatic charges. These losses could in-
troduce serious bias in the particle size
distribution of a sample obtained with this
equipment. We found that the loss through
inertial effects could be reduced by sampl-
ing modifications and partially compensated
by controlled anisokinetic sampling (i.e. use
of an oversize nozzle to oversample larger
particles). There is no obvious way to solve
the charged particle problem through a
change in sampling techniques. The problem
might be avoided or reduced if the com-
ponents of the equipment were electrically
conductive.
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1 00
0,80
0.60
a
§ 0.40
0.20
A JO ft Hose, 76 Ipm isokinetic sampling
right angle precollector
O 5 ft Hose, 14 Ipm isokinetic sampling
_• Measured concentration fraction
using anisokinetic sampling: 5 ft hose
14 Ipm, 15.5 fps nozzle velocity, 50 fps
duct velocity
——Theoretical concentration fraction
for anisokinetic sampling conditions
0.5
1.0
10
20
30
Aerodynamic Particle Diameter, fim
Figure 2. Fraction of duct concentration, corrected for dilution, at dichotomous sampler inlet
vs. particle size.
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50
40
30
s
I
8'
o
20
70
Hose \ Without Charge Neutralizer
Probe ]
Hose
Probe
With Charge Neutralizer
8
o
a
_P i i i i i i i
1 2 3 4 5 6 7 8 9 10
Panicle Diameter, /urn
Figure 3. Probe and hose losses by particle size for charged and neutralized particles at a
sampling rate of 14 Ipm.
Joseph D. McCain and Ashley D. Williamson are with Southern Research
Institute, Birmingham, AL 35255.
Roy L. Bennett is the EPA Project Officer (see below).
The complete report, entitled "Development and Evaluation of Dilution Probes
Used for Sampling to Determine Source Signatures," (Order No. PB 84-164
284; Cost: $8.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, v'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
ps
°61"1
•A US. GOVERNMENT PRINTING OFFICE: 1984-759-102/928
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