United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-86/027 Nov. 1986
c/EPA Project Summary
Field Evaluation of
RECEIVED Windscreens as a Fugitive
NOV241986 Dust Control Measure for
ENVIRONMENTAL PROTECTION AGENCY Material StO^gC PMCS
LIBRARY, REGION V
Robert A. Zimmer, Kenneth Axetell, Jr., and Thomas C. Ponder, Jr.
The Air and Energy Engineering Re-
search Laboratory (AEERL) has insti-
tuted a coordinated program for devel-
opment of a technology for the control
of fugitive particulate sources. The
AEERL has identified windscreens as a
promising control technique for one of
the major sources—storage piles. Be-
fore this technology can be effectively
applied, however, application criteria
need to be developed. These criteria are
screen porosity, screen width, screen
height, and the distance between the
screen and the pile.
AEERL and the Environmental Sci-
ences Research Laboratory (ESRL) have
completed an in-house study designed
to determine changes in windspeed
(not changes in emissions) due to wind-
screens. The ESRL wind tunnel was
used in experiments conducted to de-
termine the optimal windscreen poros-
ity, size, and location for control of fugi-
tive dust emissions from storage piles.
Before this information could be ap-
plied to the design of windscreens, rt
was necessary to conduct a field study
to validate the wind tunnel studies with
respect to windspeed changes, and to
determine the relationship between
changes in windspeed and changes in
fugitive dust emissions.
This field study was conducted dur-
ing the summer of 1985. Testing was
performed simultaneously on two cone
shaped piles of shredded topsoil 8 ft*
'Readers more familiar with metric units are asked
to use the conversion factors at the end of this
summary.
high with a base diameter of 25 ft. One
pile was exposed, and the other was
controlled with a 50 percent porosity
windscreen. Sensors were located at
the surface of each pile to determine
windspeed. Exposure profiling towers
with samplers at four heights were
used to determine total particulate
emissions. Windspeed data were col-
lected continuously by an onsite com-
puter. Profiler data were averaged for a
complete test, and selected fitters were
subjected to laser diffraction analysis to
obtain particle size data.
The windscreen parameters found to
be most effective in the field for reduc-
ing windspeed on the pile surface under
perpendicular winds were 1.25 pile
heights high, 1.5 pile heights wide, and
2.0 pile heights upwind. Both the wind
tunnel study and this study recorded
negative screen efficiencies in the lee of
the pile, but the field study showed this
result to a much greater extent.
Regression analyses of data for par-
ticulate emission reduction and other
variables showed a strong linear rela-
tionship between emission reduction
and windspeed for the largest particle
size ranges evaluated. These analyses
also showed that emission rates were
directly related to windspeed and in-
versely related to the moisture content
of the pile surface. These relationships
held regardless of the particle size frac-
tion considered.
Optimum windscreen design param-
eters recommended for permanent or
semi-permanent installations are
porosity = 50 percent, height = 1.0 H,
-------
width = 5.0 D, and distance = 2.0 H for
a pile of height H and diameter D.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing 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 Air and Energy Engineering Re-
search Laboratory (AEERL) has insti-
tuted a coordinated program for the de-
velopment of a technology for the
control of fugitive paniculate sources.
The AEERL has identified windscreens
as a promising control technique for
storage piles, a major source of these
emissions. Before this technology can
be effectively applied, however, appli-
cation criteria need to be developed.
Together, AEERL and the Environ-
mental Sciences Research Laboratory
(ESRL) have completed an in-house
study, designed to determine changes
in windspeed (not changes in emis-
sions) due to windscreens. The ESRL
wind tunnel was used in experiments
conducted to determine the optimal
windscreen porosity, size, and location
for control of fugitive dust emissions
from storage piles. Before this informa-
tion could be applied to the design of
windscreens, it was necessary to con-
duct a field study to validate the wind
tunnel studies with respect to wind-
speed changes and to determine the re-
lationship between changes in wind-
speed and changes in fugitive dust
emissions.
The three objectives of this study
were:
(1) To verify that the data collected in
the wind tunnel with respect to
changes in windspeed are accu-
rate under field conditions.
(2) To determine the relationship be-
tween changes in windspeed and
changes in paniculate emissions
by panicle size.
(3) To develop windscreen design
parameters.
Previous studies have yielded contra-
dictory results concerning the relation-
ship between particle uptake and wind-
speed. Similar contradictions were
found in the two studies performed to
investigate reductions in dust concen-
trations due to the use of windscreens.
The inhouse study and the one de-
scribed here are the first laboratory and
field studies that attempt to measure
windspeed or paniculate reductions at
or near a pile surface.
The inhouse study simulated, in a
wind tunnel, the effect of a windscreen
in reducing windspeed on the surface of
a storage pile. The scale-model storage
pile used was 4.3 in. high and was cov-
ered with gravel having diameters less
than 0.2 in. A variety of windscreen
parameters were evaluated during the
study, and isotachs of windspeed and
windspeed reduction were presented
for both unscreened and screened piles.
Field Sampling
The current study entailed a field ex-
ercise to evaluate the results of the in-
house study under actual field condi-
tions. The basic sampling protocol used
was to measure windspeed and panicu-
late concentrations on two identical
storage piles simultaneously. One pile
was controlled with a windscreen of 50
percent porosity, and one had no wind-
screen. A critical parameter in such a
test protocol is that the piles be identical
initially and throughout the test period
with respect to dust-emitting character-
istics. The piles were constructed out of
the same highly erodible material
(shredded topsoil) and were shaped
identically.
Field Site
The field site was located on a pri-
vately owned farm in the Wichita, KS,
area. This area had the desirable charac-
teristics of relatively high-speed winds
with a persistent direction. The 24-acre
field was in a rural area about 7 mi
northwest of the Wichita Mid-Continent
Airport. This level area was bounded by
a paved road on the western edge and
an unpaved road on the northern edge
(downwind).
The entire field was covered with
grass, which grew to a height of 2-6 in.
There were no continually active panic-
ulate sources in the upwind direction
(south) from the field, just additional
pastures and fields with mature crops.
Nor were there any tall windbreaks
within 0.5 mi to the south. Trees that
grew along the stream at the south end
of the field only extended 10-15 ft above
field level and were at least 500 ft from
the sampling area.
The identically constructed storage
piles consisted of dried, shredded top-
soil. The piles were conical, with a
height of 8 ft and a base diameter of
25ft and were located 150 ft apart. A
detailed test plot is shown in Figure 1.
The instrument trailer was 75 ft down-
wind of the piles. Screen widths up to
5 pile diameters were accommodated
with this layout.
Sampling Equipment and
Deployment
Windspeed was monitored with sev-
eral rotating-cup windspeed sensors
with pulsed output. The output was di-
rected through a windspeed translator
module that converted the signal to a
standardized analog voltage. This sig-
nal was translated to a digital signal,
through the use of an analog-to-digital
converter, which was then processed by
a personal computer.
Wind direction was monitored up-
wind with a wind direction sensor. The
signal was input to a translator module
and an analog-to-digital converter for
computer processing.
Ten windspeed sensors were used,
one of which was upwind at a height of
8 ft to correspond with the height of the
storage pile. Placement of the sensors
was guided by verification of wind tun-
nel testing: it was desirable to obtain
windspeed measurements at the same
locations as in the wind tunnel testing.
The wind tunnel testing, however, in-
volved 108 windspeed measurement lo-
cations, 9 sensors at a time. Pile rotation
to 12 positions yielded 108 measure-
ments. Such a protocol was impractical
for a field test because of the equipment
required and because wind direction ir
the field is not fixed as it is in a wine
tunnel.
Nine windspeed sensors were de
ployed downwind of the screen, five or
the uncontrolled pile and four on thf
controlled pile. The sensors were set a
a fixed position on the pile, about 6 in
above the surface of the pile and per
pendicular to the ground. The position!
were set relative to the prevailing wine
direction, and they remained fixed ove
the 8-week test.
So that identical instruments could bi
positioned at the same relative loca
tions on each pile, a true north point am
true south point were determined fc
the base of each pile. A string was the
run across the peak of the pile to cor
nect the two points. Vertical distance
were measured along the string, an
horizontal dimensions were measure
perpendicular to the string.
Particulate was measured with a tot
particulate exposure profiler head. Th
exposure profiler head consisted of
high-volume motor with a variable flo1
rate, a filter holder, and a circular intal
-------
Predominate
Wind
Direction
125ft
•50ft
0
8
*c>
1
Q
§
-------
windspeed data stratified by incoming
wind direction in 10° cohorts.
Composite u/ur values were calcu-
lated for the 10° wind direction cohorts,
as shown in Table 1. As the wind direc-
tion moves around the pile, the loca-
tions of the sensors are effectively
shifted to new positions. This same ap-
proach was used in the original inhouse
wind tunnel study. In the field study,
however, the wind direction was varied
rather than the pile orientation. Utiliza-
tion of the entire data base yields
55 data points. Because only 110° of the
compass is shown, a substantial portion
of the compass is left unresolved. Some
of the data points are plotted in Figure 2,
over the isotachs from the wind tunnel
study.
As shown in Figure 2, the data on the
front of the pile match reasonably well
for u/ur <0.8; however, the area where
the ratio is >1.0 appears to be larger
than that found in the wind tunnel. The
field data for the back side of the pile
yielded significantly higher u/ur ratios
than the wind tunnel study. In fact, the
highest ratios measured during the field
testing were on the back of the pile. The
testing suggests that the high wind-
speed flow lines not only extend around
to the back of the pile, but are reinforced
in some fashion.
The differences noted between this
study and the inhouse study can be at-
tributed to a number of factors:
(1) Ambient windspeeds and wind di-
rection measured in the field are
much more variable than those
observed in the wind tunnel and
resulted in higher turbulence.
(2) Actual pile configuration and
composition during the field test-
ing may not have been compara-
ble to the idealized scale model
pile used in the wind tunnel, pos-
sibly a significant factor.
(3) Experimental equipment used in
the two studies may not have
been comparable. The extent to
which the wind sensors corre-
spond to the laboratory thermis-
tors is unknown.
(4) Experimental errors between the
two studies may not have been of
comparable magnitude.
(5) The presence of reentrained dust
from the pile surface may have
some effect on the measure-
ments. For example, it may be
that the inherent kinetic energy of
the entrained particles from the
front of the pile can be transferred
Table 1. U/Ur Values for the Unscreened Pile 125° to 234° Wind Directions
Wind Direction, Position Position Position Position
10° Cohort 1234
125-134
135-144
145-154
155-164
165-174
175-184
185-194
195-204
205-214
215-224
225-234
0.82
0.68
0.65
0.56
0.46
0.53
0.57
0.60
0.67
0.82
0.88
7.08
7.08
7.73
7.07
7.09
1.09
1.11
1.08
1.11
1.16
1.25
0.72
0.71
0.83
0.81
0.82
0.84
0.88
0.86
0.88
1.02
1.07
1.36
1.34
1.35
1.21
1.07
0.94
0.74
0.45
0.33
0.20
0.28
Position
5
1.05
1.08
1.14
1.06
1.00
0.95
0.82
0.64
0.49
0.39
0.38
0.6
0.4
0.6
0.4
Figure 2. Composite u/u, values for an unscreened pile—field testing and wind tunnel data.
to the wind sensors behind the
pile and result in higher apparent
windspeeds.
Windspeed Control
Effectiveness for Windscreens
The wind tunnel windspeed data for
screened piles are presented as a series
of isotach lines. The isotach lines are
presented in the form of 1 - (u/u0),
where u and u0 are windspeeds with
and without a windbreak.
The field data were also manipulatec
into the 1 - (u/u0) format. Data were alsc
stratified by incoming wind directioi
and screen configuration. Data coult
not be presented as a series of isotacl
lines as in the wind tunnel report, be
cause only four data points on the pil'
were obtained (in contrast to 108 dat
points in the inhouse study).
A portion of the data were refoi
matted to indicate the maximum wine
screen wind reduction by incomin
windspeed and screen configuration.
-------
Several conclusions could be drawn
from the analysis. Windspeed reduction
was greatest for perpendicular screen
orientations. A 2.0-pile-height distance,
1.5-pile-diameter width, and a 1.25-pile-
height screen were found to be most
effective. For nonperpendicular winds,
on the other hand, the 3.0-pile-diameter
screen width was the most effective. In
the lee of the pile, negative control effi-
ciencies were recorded.
Findings of the inhouse study were
compared with those of the current
study. Both studies found the taller
windscreens to be most effective. The
inhouse study found a 3.0-pile-height
distance to be more effective than a 1.0-
pile-height distance. A 2.0-pile-height
distance was not evaluated. The current
study found a 2.0-pile-height distance to
be more effective than either the 1.0 or
a 3.0-pile-height distance. Both studies
found a 1.5-pile-diameter screen length
to be more effective than a 1.0-pile-
diameter screen length. Both studies
recorded negative screen efficiencies in
the lee of the pile, but the field study
showed this result to a much greater
extent. In general, the wind tunnel effi-
ciencies were higher than those mea-
sured in the field.
Objective 2—Comparison of
Windspeed Reductions and
Particulate Control Efficiencies
The second objective of the study was
to compare average windspeed reduc-
tions with paniculate emission reduc-
tions for entire tests as measured by the
exposure profilers. There were 42 valid
tests in the data set.
All applicable data for windspeed ver-
sus paniculate (TP) emission reductions
are shown in Table 2. A linear regres-
sion of windspeed (WS) versus total
paniculate (TP) reductions yielded a
correlation of 0.372, R2 of 0.138, signifi-
cance level of 0.015, slope of 0.841, and
y-intercept of -0.150. These results in-
dicated a significant relationship be-
tween the two variables that was nearly
1 to 1.
When the linear regression was
forced through zero (no windspeed re-
duction results in no TP reduction), the
correlation improved to 0.417, R2 was
0.174, the significance level was 0.005,
and the slope was 0.466. This relation-
ship was more significant than the one
above and indicated TP reductions
slightly less than half of the correspond-
ing windspeed reductions.
There were 15 negative TP reductions
Table 2. Comparison of Windspeed and Total Particulate Reductions
Windspeed, ft/min Total Particulate, lb/3.28 ft
Test
IB
2
3
4
5
6
7
9
10
11
12
13B
14
15A
15B
16
17
19A
19B
20
21A
21B
22
23A
23B
24
25A
25B
26
27
28B
29
30A
30B
31
32
33
34A
34B
35
36
105
Screened
1679
951
1039
791
969
545
598
473
350
312
257
198
477
661
641
493
386
256
350
343
402
494
512
562
230
426
714
680
902
577
424
375
419
433
313
305
174
214
198
310
256
408
Piles
Exposed
1755
1480
1653
1228
1354
1024
1026
1070
524
869
409
314
1018
1020
679
580
508
294
388
342
993
985
1010
1031
380
455
961
820
986
1009
721
609
680
620
545
356
289
275
272
410
380
389
Reduction
0.043
0.357
0.371
0.356
0.284
0.468
0.417
0.558
0.332
0.641
0.372
0.369
0.531
0.352
0.056
0.150
0.240
0.129
0.098
-0.003
0.595
0.498
0.493
0.455
0.395
0.064
0.257
0.171
0.085
0.428
0.412
0.384
0.384
0.302
0.426
0.143
0.398
0.222
0.272
0.244
0.326
-0.049
Screened
9.82
1.45
2.92
1.04
1.26
0.54
0.46
0.38
0.02
0.02
0.03
0.01
0.04
0.03
0.03
0.01
0.03
0.03
0.01
0.01
0.04
0.06
0.07
0.20
0.02
0.02
0.29
0.15
0.04
0.17
0.09
0.01
0.04
0.02
0.01
0.01
0.04
0.01
0.01
0.01
0
0
Piles
Exposed
8.36
1.34
2.34
0.98
2.52
0.66
0.49
0.47
0.04
0.04
0.03
0.01
0.06
0.02
0.03
0.01
0.08
0.02
0.01
0.01
0.04
0.34
0.91
2.15
0.04
0.03
0.19
0.10
0.03
0.12
0.13
0.02
0.03
0.03
0.01
0.01
0.03
0.01
0.01
0.01
0
0
Reduction
-0.175
-0.082
-0.250
-0.058
0.499
0.180
0.051
0.197
0.453
0.471
-0.061
0.371
0.394
-0.188
0.101
0.029
0.588
-0.596
-0.111
0.180
0.108
0.825
0.928
0.908
0.486
0.192
-0.509
-0.555
-0.431
-0.403
0.290
0.358
-0.599
0.301
0.357
0.063
-0.147
0.391
-0.071
0.218
0.000
0.000
in the 42 tests. A negative TP reduction
means that a higher emission rate was
measured on the screened pile than on
the exposed pile. Many of these tests in
which the screen appeared to increase
emissions could be the result of differ-
ences that were less than the measure-
ment error for exposure profiling, but
several of the differences in emission
rates were large enough to indicate the
probability that these measurements re-
flected actual occurrences of increased
emissions. This observation agreed
with the findings (discussed previously
herein and in the inhouse study) that the
windscreen actually produced in-
creased windspeeds on the lee side of
the pile.
If the negative TP reductions were as-
sumed to be due to measurement errors
and set at zero, the correlation was vir-
tually unchanged at 0.404, R2 was 0.163,
the significance level was 0.010, the
slope was 0.626, and the y-intercept was
0.175.
It was observed that many of the tests
were taken at windspeeds too low to
cause wind erosion, and (during other
tests) the moisture content of the pile
was so high that erosion would not
occur even with high windspeeds.
Twenty of these tests with negligible
emissions were eliminated to see
whether the windspeed/emission rate
relationship was stronger during tests
with wind erosion. The results of this
regression analysis were: R = 0.287,
R2 = 0.082, significance level = 0.184,
-------
slope = 0.859, and y-intercept
= -0.122. According to this analysis,
the tests with negligible emissions did
not appear to be distorting the first cal-
culated relationship between the two
variables.
The above regression analyses indi-
cate that a highly significant relation-
ship exists between windspeed and par-
ticulate emission reductions, and that
the relationship is approximately linear
with a slope less than 1 (1 percent re-
duction in windspeed results in less
than 1 percent reduction in paniculate
emissions). Also, in some instances
windspeed on the front face of the pile
appears to be reduced by the wind-
screen, but emissions from the pile ac-
tually increase as a result of higher
windspeeds on the back of the pile.
Particle Size Data
The emission rates of particles within
several size ranges were determined by
selecting a heavily loaded filter from
each profiling test and subjecting it to
laser diffraction analysis. The resulting
percentages of net sample weight by
particle size range were then multiplied
by the TP emission rate for the test to
obtain emission rate by particle size
range. The effectiveness of the wind-
screen in reducing emissions for each
size range was then calculated as 1 -
ERscl/ERexp for that range.
The particulate reductions by size
range were then compared with corre-
sponding windspeed reduction in the
same manner as with the TP reduction
data; i.e., by regression analysis.
None of the particle size ranges had
as significant a relationship with wind-
speed as TP did, partially because of the
smaller data sets available with the par-
ticle size data (19 tests instead of 42).
Particle size data could not be obtained
on many filters because of their light
mass loadings. The particle size emis-
sion reductions that had the highest cor-
relations with windspeed reduction
were 30 - 62 ^m and 62 - 176 \im. The
two small particle size ranges (<10 urn
and 10-30 pm) both had poor correla-
tions. Slopes of regression lines for the
two particle sizes that were reasonably
significant were higher than the slopes
for TP, which indicates an emission re-
duction (in those size ranges) almost
equal to windspeed reduction. No ex-
planation was apparent for differences
in variation with windspeed for the dif-
ferent particle sizes.
The same frequent negative emission
reductions were observed in all particle
size ranges, as was the case for TP. This
indicates that increased emissions were
actually occurring as a result of the
screen, rather than as the anomalous
result of sampling or laboratory analy-
sis errors.
Factors Other than Windscreen
Affecting Emission Rates
Multiple linear regression (MLR) anal-
ysis was used to identify external vari-
ables affecting emission rates. Only test
data from the unscreened pile were uti-
lized because these emission rates were
not altered by the presence of the wind-
screen.
The variables included in the analysis
were windspeed, moisture content, and
silt content of surface material. Particle
size was also a variable in that MLR
analyses were run with four different.
sets of emission rates (<10 n-m,
<30 fj,m, <62 (im, and TP) as dependent
variables to examine the effects of the
external variables on different size
ranges of particles.
These regression analyses showed a
strong linear relationship between TP
emission rate and windspeed and a
strong inverse relationship between TP
emission rate and moisture content of
the pile. Approximately the same rela-
tionship was observed between the two
variables and emission rates regardless
of the size fraction considered.
Objective 3—Development of
Windscreen Design Parameters
The final objective of the study was to
develop windscreen design parameters.
For the three windscreen design param-
eters that were varied by test, the field
test results should provide better infor-
mation than wind tunnel data for opti-
mizing design values. Conditions such
as wind direction variation, surface
moisture content, and crusting could be
incorporated in field testing, but not in
wind tunnel studies.
Stepwise MLR was the statistical test
employed. The three windscreen vari-
ables—height, length, and distance
from pile—were entered along with ex-
ogenous variables such as windspeed,
surface moisture content, and silt con-
tent. None of the exogenous variables
had such an overriding effect on relative
emission rates with and without the
screen that they obscured the impacts
of changes in screen parameters. By in-
cluding these variables in the MLR, the
relatively small effects of these vari-
ables were taken into account rather
than acting as interferences in the direct
comparison of test results.
Combining the results of the MLR
with graphical analysis led to several
design conclusions. Screen lengths of
5.0 D appear to be appropriate for per-
manent or semipermanent installations.
Given the wind direction variations that
occur in real situations, the 1.0 to 1.5 D
lengths tested in the wind tunnel are
probably too short. The 2.0 H screen-to-
pile distance was found to be optimum.
This distance yielded slightly greater
emission reductions than the 1.0 or
3.0 H distance. Both the wind tunnel
study and this study showed that the
0.5 H height windscreen was not as ef-
fective as screens of 1.0 H. Also, the
screen height of 1.0 H is nearly as effec-
tive as higher screens. In general, the
optimum design parameters appear tc
be: porosity = 50 percent; heighi
= 1.0 H; width = 5.0 D; and distance
= 2.0 H.
Recommendations
The field study has helped to identif
several important areas for further in
vestigation. Although the wind tunne
study and the field study are in genere
agreement for the front of the pile, th
results are contradictory in one signif
cant area. The field study showed the
large portions of the back of the pile ha
windspeeds higher than the referenc
windspeed. This observation was reir
forced by the particulate emission dat<
Negative emission reductions wer
noted for the screened pile in a larg
number of tests. This basic result is i
direct conflict with the bulk of the win
tunnel data. Although the inhouse stud
found some negative reductions, th
field study showed negative reductior
as large as 40 percent.
Some ongoing physical process c
processes must not have been adi
quately investigated. The results to dai
raise questions on the applicability i
windscreens for reducing emissior
from storage piles. Before windscreer
are recommended as a control me
sure, it is imperative that the observe
relationship between the use of win
screens and the emission rate be inve
tigated further.
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Metric Equivalents
Readers more familiar with metric
units may use the following to convert
to that system:
Nonmetric Times Yields metric
acre 4047 m2
ft 0.305 m
in. 2.54 cm
Ib 0.454 kg
mi 1.609 km
R. Zimmer, K. Axete/l, Jr., and T, Ponder are with PEI Associates, Inc.. Golden, CO
80401; Kansas City, MO 64113; and Arlington, TX 76012, respectively.
Dale L. Harmon is the EPA Project Officer (see below).
The complete report, entitled "Field Evaluation of Windscreens as a Fugitive Dust
Control Measure for Material Storage Piles," (Order No. PB 86-231 289/AS;
Cost: $16.95, 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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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United States
Environmental Protection
Agency
Official Business
Penalty for Private Use $300
EPA/600/S7-86/027
Center for Environmental Research
Information
Cincinnati OH 45268
0000329
PS
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