United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-125 Dec. 1981
Project Summary
Spray Charging and Trapping
Scrubber for Fugitive Particle
Emission Control
Shui-Chow Yung, Julie Curran, and Seymour Calvert
The control of fugitive particle
emissions (FPEs) with a Spray
Charging and Trapping (SCAT) scrub-
ber was evaluated both theoretically
and experimentally. The system uses
an air curtain and/or jets to contain,
convey, and divert the FPEs into a
charged-spray scrubber.
Experiments were performed on a
225 mVmin bench-scale spray
scrubber to verify the theory and
feasibility of collecting fugitive par-
ticles with charged water spray. The
effects of charge levels on drops and
particles, nozzle type, drop size, gas
velocity, and liquid/gas ratio on
collection efficiency were determined
experimentally. The results of the
experiments and the comparison
between theory and data are presented.
An air curtain was developed for
conveying the FPEs to the spray
scrubber. The design and air flow field
for the air curtain are presented.
A prototype SCAT scrubber was
built to study the effects of crosswind
and hot buoyant plume. Available data
revealed that the air curtain was
successful in deflecting crosswind up
to 15 mph and containing a hot
buoyant plume. Theories were devel-
oped for predicting the trajectories of
the air curtain jet stream and the hot
buoyant plume.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory. Research Tri-
angle 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
The Spray Charging and Trapping
(SCAT) scrubber system is a simple and
inexpensive way to control fugitive
particle emissions (FPEs). The SCAT
system uses air curtains or air jets to
contain, divert, and convey FPEs into a
charged spray scrubber located near the
source.
The SCAT system has two sections
arranged in a push-pull configuration
with the fugitive particle emission
source located between them (Figure 1).
The fugitive particles are contained by
air curtains and are pushed from the
source into the spray scrubber. The
scrubber has a low-pressure-drop
entrainment separator to remove the
spray drops.
Water from the entrainment separator
can be passed through a separation
process, such as a filter, to remove the
collected dust particles. The water may
then be recycled and the dust may be
disposed of to prevent its redispersion.
Alternatively, a blowdown stream of
dirty liquid may be directed to a disposal
system.
The major SCAT system feature,
suiting it to FPE control, is the use of air
curtains and/or air push jets. The use of
air curtains minimizes the requirement
for solid boundaries and enables access
to the source. Air curtains could also be
used to deflect the wind or to deflect a
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buoyant plume from a hot source. The
SCAT system is very compact and
portable.
Preliminary Experiments
The SCAT system has three basic
functions, to: (1) contain and convey the
fugitive emissions to the scrubber, (2)
remove the particles with charged
water sprays, and (3) collect the
particles and water drops. There is
insufficient published information on
the design and performance of an air
curtain and a charged-water spray
scrubber. To generate design data, the
air eurtain and the charged spray
scrubber were studied individually in
separate bench-scale experiments. A
prototype SCAT system was built and
used to study the effects of hot buoyant
plumes and crosswinds.
Air Curtain
An air curtain is a sheet of moving air
formed by round or rectangular jets. Air
curtains have been widely used in
industrial and commercial plants,
mainly to provide constant access or to
isolate a warm interior from the cold
outdoors or vice versa.
Most published information on air
curtain performance and design relates
to air conditioning and ventilation.
There is little published literature on
dust containment even though it has
been used for this purpose in industry.
The design of the SCAT system requires
information on the jet expansion angle,
air entrainment ratio, mixing of particles
in the curtain, and the effects of
crosswind and hot sources. Jet expan-
sion angle and particle mixing determine
the overall cross-sectional dimensions
of the spray scrubber. The air entrain-
ment ratio determines the volumetric
flow rate. Crosswinds and heat effects
dictate the nature and placement of air
curtains and sprays.
Ideally, the air curtain should have
small expansion angle, small air en-
trainment ratio, and a uniform velocity
distribution.
Experiment
The air jet nozzle of the air curtain
used in this study was a continuous slot
2.1 m (7 ft) long. The slot was formed by
two parallel plates which protruded
22.9 cm (9 in.) from one side of the duct.
The distance between the plates, which
is the slot width, could be adjusted. The
slot was divided by thin cross-plates at
5.1 cm (2 in.) apart, so that the air would
discharge perpendicularly to the longi-
tudinal axis of the duct.
The discharge distribution for this
manifold was uniform and the discharge
angle close to 90°.
The air curtain flow field was
measured for several slot widths and
slot exit velocities with the slot vertical.
Linear velocity was measured for three
vertical levels at several locations
downstream of the slot. The jet ex-
pansion angle and the air entrainment
ratio were calculated from the measured
velocity distribution. The results were
compared with the equations derived
from a two-dimensional jet exhausting
into still surroundings and jets with
two-sided expansion. Figures 2 and 3
show the measured centerline axial
velocity decay and entrainment ratio,
respectively. The measurements lie
between the predictions by Abramovich
and the present study and are equal t<
the average of the two predictions.
The measured jet expansion angk
was 20-28°, depending on air exi
velocity. The average of measured je
expansion angles agrees with tha
calculated for a pure momentum jet.
Charged-Spray Scrubber
For a spray system, collection b\
drops is the principal collection mecha
nism and the particle penetration for E
given size particle depends on the drop
diameter, the collection efficiency of a
single drop, and the ratio of liquid-to-
gas flow rates.
The instantaneous single-drop col-
lection efficiency has been determined
experimentally for uncharged drops
collecting uncharged particles. There is
no explicit expression for single-drop
efficiency when electrostatic, inertial.
Q=
Air Curtain
Hot Fugitive
Emission
Source
o
o
SCAT
Spray
Scrubber
0
SCA T system arrangment.
'-27 cm
A 2.54 cm
3.81 cm
300
500
2L-- Dimensionless
W
Figure 2. Measured and predicted centerline axial velocity decays.
TOOL
I
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and viscous forces are simultaneously
present. The equations of motion have
been solved numerically for the
collection of fine particles by a single
spherical collector with various combi-
nations of charge: the results can be
used for predicting particle penetration.
Experiment
Charged-spray scrubbers have been
studied experimentally by several
researchers. However, their data are
not suitable for verifying a mathematical
model because several important elec-
trical parameters were not defined.
To obtain design data under well
defined conditions, the charged-spray
scrubber system shown in Figure 4 was
built. The system consisted of a flow
straightening section, an inlet particle
sample section, a particle charging
section, a spray section, an entrainment
separator, and an outlet sampling
section.
The particle charger section consisted
of two rows of corona wires and ground
electrode tubes. The spray section
included two spray banks: the water
was charged by induction.
Experiments were performed to
determine the minimum water require-
•ments, to evaluate spray nozzles, to
study various drop charging methods, to
determine the effect of drop and/or
particle charging on particle collection
efficiency, and to verify published
theories. The particle collection effi-
ciency, of the charged-spray scrubber
was determined by injecting redispersed
dust to the blower inlet and by simul-
taneously measuring the particle size
distribution and mass concentration at
the inlet and outlet of the scrubber.
Particle charge was measured with a
Faraday cup which consisted of an
isolated, shielded, glass-fiber filter
connected to an electrometer. The filter
collected the particles and their charges
which were measured by leaking them
to the ground through the electrometer.
Charge/mass ratio was calculated from
the measured charge and particle mass
on the filter.
Drop charge was measured by placing
a drop collector in the scrubber. The
collector collected the drops and their
charges which were measured by
leaking the charge to the ground
through an electrometer. Thus, by
monitoring the current and sampling
time, and measuring the amount of
water collected, the charged level can
be calculated.
Figure 5 shows the measured charge
level on drops. Nozzles were hook-type
hollow-cone nozzles. Curve A is for a
water flow rate of 9.5 x 1CT4m3/s (0.3
gpm) per nozzle and a pressure of 450
kPa (50 psig). Curve B' is for a water flow
rate of 7.2 x lO^mVs (0.25 gpm) per
nozzle and a pressure of 380 kPa (40
psig). The drop diameter, measured and
sized photographically, was about 0.24
mm for both conditions.
Results
The scrubber was operated for four
conditions:
700
50
40
30
20
10
5
4
3
2
I I I I I
A 7.27 cm Slot Width
Q 3.8 cm Slot Width
5.1 cm Slot Width
i I I i
Derived from Prandtl
Eddy Viscosity Theory
-= 0.76 (—)05
McElroy
1
I
I i
I i t
70
20 30 40 50
100
x/w, m/m
200 300 400 500 1000
figure 3. Measured and predicted air entrainment ratio.
Power Supply
Flow
Straightening
Section
Inlet
Sampling
Section
Spray
IS action
Blower
Particle
Charging
Section
Outlet
Sampling
Section
I Vent
u
Sump
'Entrainment
Separator
Pump
Figure 4. Experimental apparatus for studying charged-spray section of SCAT
scrubber.
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(1) Uncharged particles/neutral drops
(UP/ND).
(2) Charged particles/neutral drops
(CP/ND).
(3) Uncharged particles/charged
drops (UP/CD).
(4) Charged particles/charged drops
(CP/CD).
Figures 6 and 7 show data for nozzles
A and B, respectively. The measured
mass median drop diameter, spray
angle, and discharge coefficient for
nozzle A (hooktype) were 0.24 mm,
100°, and 0.63, respectively. They were
0.35 mm, 87°, and 0.69, respectively,
for nozzle B (pigtail type). Only one spray
bank was used in all experiments and
nozzle pressure was maintained at 432
kPa (48 psig). One power supply was
used to charge both the particles and
the drops. The applied voltage was -10
kVDC. The measured charge/mass
ratio was -1.5 x 10~5 C/g for particles
with a mass median diameter of 3 /urn
aerodynamic diameter and a geometric
standard deviation of 2.5.
The collection efficiency of the spray
scrubber is improved by charging either
the water or the particles. Further
enhancement was measured when the
water and particles were oppositely
charged and it is greatest with sub-
micron particles. For particles with
diameters larger than 5 //m aerody-
namic diameter, charging the water
and/or particles has little effect on
efficiency.
The scrubber with nozzle A has better
collection efficiency at a lower liquid/
gas ratio than that with nozzle B. A
possible explanation Is that nozzle A
produced finer drops.
Comparison Between Theory
and Experimental Results
The drops from a hollow-cone nozzle
are localized at the edge of the spray
cone and do not travel parallel to the gas
stream, but at an angle which depends
on the spray orientation with the gas
stream. Therefore, in calculating the
single-drop collection efficiency, the
resultant relative velocity between the
gas and the drop must be used for
calculating the initial impaction param-
eter.
The spray nozzles in the spray
scrubber were equally spaced in the
duct so the drops travelled various
distances before striking the wall. To
simplify the calculation of particle
penetration, the average range of all
drops was used. Figure 8 shows the
predicted and the measured grade
penetration for the UP/NP condition. As
can be seen, the agreement is good. For
CP/ND and UP/CD conditions, the
theory predicted no improvement in
particle collection efficiency, which is
contrary to experimental findings.
When drops and particles are oppo-
sitely charged, the theory predicted an
increase in the collection efficiency.
Figure 9 shows the predicted scrubber
penetration along with that measured.
The agreement is good for particles
larger than 3 fjm aerodynamic diameter,
but not for those smaller than 3 /jm.
These discrepancies could be due to
the use of average drop range. Figure 10
5x70'6
~i—i 1—i 1 1 1—i 1—
-UG = 2.9 m/s
-1 Spray Bank
Nozzle/Grid Spacing —1.3 cm
Nozzle 'A'
B. QL= 7.2 x 70~4 m3/s
Pressure - 380 kPa-
: A
% 10'
I
5x10'
A. QL = 9.5 x 1CT* nf/s
Pressure = 450 kPa
024681012 1416 1820
Applied Voltage. -kV
Figure 5. Measured charge level
on drops.
7.0
0.5
c UP/ND '-
UG = 2.9 m/s
Ql/0G = 4 x 1
1 Spray Bank
Nozzle 'A'
Hydrated Lime Particles
0.05
0.01
0.7 0.5 7 5 70
Aerodynamic Particle Diameter, fjmA
Figure 6. Experimental spray scrub-
ber penetration.
CJ
CB
1.0
0.5
: 0.7
I
,0.05
i
0.07
UP/ CD
CP/ND'
CP/CD
,UP/ND
G — 2.9 m/s
QL/QG =7 x 10'* m3/mj
1 Spray Bank
Nozzle 'B-
Hydrated Lime Particles
0.1 0.5 1 5 10
Aerodynamic Particle Diameter, /jmA
Figure 7. Experimental spray scrub-
ber penetration.
1.0
05
-------
shows a plot of the predicted single-
drop collection efficiency for 1 //m
diameter particles vs drop range.
Electrostatic augmentation does not
have much effect until the drop has
been slowed down; i.e., drop range is
large. In performance predictions, drop
range was limited to 70 cm, which was
the average. In reality, drop range varied
between 9 and 97 cm, depending on the
location of the nozzle. Therefore, the
use of average drop range could cause
underestimation of electrostatic effects.
Other possible explanations are that
drops are not of uniform diameter and
charge level.
Prototype SCAT System
A two-section SCAT scrubber system
was designed and built for studying
crosswind deflection and hot source
control. One section housed the spray
scrubber; the other section had three air
curtains and one push jet. Both sections
were on casters so the distance be-
tween the air curtains and the spray
scrubber could be adjusted.
The spray scrubber had a cross
section of 2.44 x 1.83 m (8 x 6 ft). The
bottom 0.61 m (2 ft) was the scrubber
sump. Therefore, the active scrubber
cross section was 1.83 x 1.83 m (6 x 6
ft). There were 36 large pigtail nozzles
set concurrent with the gas flow at the
scrubber front surface. A zigzag baffle
entrainment separator was used to
remove the water drops.
The scrubber was designed for an air
velocity of 4.5 m/s (10 mph), at which
the pressure drop for the entrainment
separator is approximately 1.3 cm W.C.
(0.5 in. W.C.): an induced draft fan was
needed to overcome the entrainment
separator pressure drop. In some
applications (e.g., spray scrubber located
downwind from the FPE source), the fan
may be unnecessary.
In the other section, two air curtains
were vertical and about 1.83 m (6 ft)
apart; the third was horizontal and 2.7 m
(9 ft) above the ground. The air curtains
could be swivelled as needed to deflect
crosswinds and bouyant smoke plumes.
A propeller fan was at the center.
Even though the air curtain section
had three air curtains and one air-push
jet, they need not be operated simul-
taneously. Under calm conditions only
the air-push jet may be required to move
the dust into the spray scrubber. When
there is crosswind, one air curtain may
be enough to deflect the wind and to
convey the dust into the scrubber.
Crosswind Experiments
Under windy conditions, the SCAT
system spray scrubber can be put
downwind of the fugitive particle source
and the wind will carry the particles to
the spray scrubber. If the spray scrubber
cannot be put downwind, the wind can
be deflected from the FPE source with
wind screens or air curtains. Some-
times one air curtain can be used to both
deflect the wind and convey the particles.
Complete wind deflection, required to
maintain dust containment, occurs
0.4
i O-3
ti
I 0.2
.o
S 0.1
T
Drop Diameter = 250 fjrn
Particle Diameter — 1 fjmA
do = 1,800 cm/s
= 300 cm/s
Charged Particle/Charged Drop
Neutral Particle/Neutral Drop
10 20 30 40
Drop Range, cm
50
60
70
Figure 10. Single-drop collection efficiency as a function of drop range.
when the resultant air flow of crosswind
and SCAT air curtain jet flow bypasses
the SCAT scrubber. At this point the
blocking distance (range produced by
the SCAT air curtain) is just larger than
the distance between the air curtain and
the spray scrubber.
For wind deflection, the momentum
of the air curtain flow in the direction
opposing the wind must be equal to or
greater than the momentum of the
wind. Wind deflection depends on
several SCAT operating parameters: the
incident angle at which the air curtain
meets the wind is the most important.
Several formulas for correlating the
parameters mentioned above are avail-
able in the literature. Indoor experi-
ments were intended for identifying the
best correlation. Experiments were
performed by fixing the range, wind
speed, air curtain slot width, and slot
exit velocity, and varying the incident
angle for wind deflection. Actually, both
the wind speed and direction fluctuate.
For this reason, additional crosswind
experiments were done outdoors.
One outdoor experiment was for
determining the air curtain range. The
scrubber and air curtain were so
arranged that the wind direction was
perpendicular to the spray scrubber and
the air curtain jet discharged at 45°
against the wind. Smoke or tracer
particles were injected at various
locations for flow pattern observations.
The air curtain range was determined
visually as the distance from the air
curtain nozzle to the nearest location
where the tracer plume is disturbed by
the cross wind.
In a second experiment the jet stream
trajectories for various wind speeds,
incident angles, air curtain slot widths,
and exit velocities were determined.
If the air curtain is also to convey the
dust, then the spray scrubber needs to
be rotated to intercept the curved jet
stream. The third experiment, involved
measurement of the angle between the
spray scrubber frontal face and wind
direction for various crosswind and
wind/air curtain incident angles.
Measured air curtain range and
trajectory were compared with predic-
tions in Figures 11 and 12. The
agreement is good, so one can predict
the location of the air curtain and spray
scrubber relative to the crosswind and
fugitive emission source.
Hot-Source Experiments
Some metallurgical processes, such
as iron and steel manufacturing, emit
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750
fc/30
'-5 110
I
90
O
90 110 130 150
Measured, degree
Figure 11. Predicted and measured
incident angle for wind
deflection.
8
c
T3
0)
02468
Measured Air Curtain Range, m
Figure 12. Predicted and measured
air curtain range (out-
door data).
very hot plumes containing high con-
centrations of particles. The most
efficient and economical way to clean
these plumes is to capture them at the
source where the concentration is
highest and the gas volume smallest. In
many cases, practical reasons (e.g., the
presence of overhead cranes) make it
impossible to capture the plume at the
source or even vertically above the
source with fume hoods. In these
situations, an air curtain could be used
as the "ceiling" to contain the fumes
and dust and to horizontally displace the
plume into a receiving hood or scrubber.
Experiments were done to study the
feasibility of containing hot plumes with
air curtains. The hot source was
simulated with an open-top furnace
with an open-flame burner. The furnace
was at the center of the SCAT system
with 3.1 m (10 ft) between the air
curtain and the spray scrubber. Since
the operation of the burner was fixed,
the ceiling air curtain location was
adjustable so that the jet stream could
meet the hot plume at different tem-
peratures.
Most of the experiments were per-
formed with the ceiling air curtain exit
axis 61 cm (2 ft) above the top of the
furnace, where the peak plume rising
velocity and temperature were about
190cm/s(4.3 mph)and471°C(800°F),
respectively.
Experiments were done for three air-
curtain slot widths and four slot exit
velocities. Except for small slot width
(2.5 cm or less) coupled with low exit
velocity (20 m/s or less), the air curtain
contained the hot plume. Experimental
observation could be described by a
correlation for predicting the hot plume
trajectory which accounted for the
buoyancy and momentum of the plume.
Conclusions
FPEs can be controlled by using air
jets to contain and convey the emissions
into a nearby spray scrubber. The
collection efficiency of a spray scrubber
can be improved by charging the water
and/or the particles. Measured particle
penetration can be predicted for the un-
augmented scrubber but not very well
for the electrostatically augmented
scrubber.
The air curtain developed in this study
can achieve a smaller expansion angle
and a lower entrainment ratio than
those reported in the literature. Small
expansion angles and entrainment
ratios are beneficial to the control of
FPEs with the SCAT system.
A prototype SCAT system has been
built to study the effects of crosswind
and containment of hot buoyant plume.
Reasonable predictions of experimental
data on air curtain range and trajectory
in the presence of crosswind can be
made. The air curtain was successful in
containing a hot buoyant plume and the
trajectory of the plume can be predicted.
Recommendations
The theories and experimental data
presented in this research permit the
design of a SCAT system. However,
additional studies are required to
optimize the SCAT design. Future
research and development work is
needed in the following areas:
(1) The effect of obstacles on air-
curtain flow field. One of many
SCAT features which suit it for
fugitive emission control is un-
obstructiveness. Workmen and
equipment (e.g., cranes) can pass
freely and work on the source
during SCAT system operation.
The presence of workmen and
equipment may create turbulence
and change the air-curtain flow
field.
(2) The optimal design of the receiving
hood to intercept the air-curtain
jet stream.
A pilot study on an actual fugitive
emission source is recommended to
demonstrate the feasibility of using the
SCAT system for controlling the emis-
sions. Since electric arc furnaces, coke
ovens, copper converters, etc. are the
major fugitive emission sources and the
plumes from these sources are hot, an
ideal demonstration would be on one of
these.
Nomenclature
QG = volumetric gas flow rate, mVs
QG, = volumetric gas flow rate at
nozzle exit, mVs
QG« = average gas flow rate at "x"
meters downstream from
nozzle, mVs
QL = liquid volumetric flow rate,
mVs
UG - gas velocity, m/s
UGC - centerline gas velocity, m/s
UGJ = gas velocity at nozzle exit, m/s
UG» = average jet velocity at "x"
meters downstream from
nozzle, m/s
w = slot width, m
x = distance downstream from
slot, m
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5. Yung, J. Curran, and S. Calvert are with Air Pollution Technology, Inc., 4901
Morena Blvd., Suite 402, San Diego, CA 92117.
Dennis C. Drehmel is the EPA Project Officer (see below).
The complete report, entitled "Spray Charging and Trapping Scrubber for
Fugitive Particle Emission Control."(Order No. PB 82-115 304; Cost: $21.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•tt U.S. GOVERNMENT PRINTING OFFICE : 1 981 --559-092/3360
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
U S ENVIfc PROTECTION
REGION 5 UiaHA
,230 S DEARBORN
CH1CAGU II 60604
AGENCY
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