United States
Environmental Protection
Agency
Environmental Sciences Research",.
Laboratory '
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-086 Dec. 1982
Project Summary
Development of a Charged
Grid Sensor for Airborne
Carbon Fibers
S. Salmirs, J. Schrader, and A. Butterfield
This development project addresses
the sensing and measurement of
carbon fibers moving in ventilating
ducts or exhaust stacks of incinerators.
The sensor system employs a series of
five electrically charged grids with
different grid spacings for sensing
fibers, determining their lengths, and
presenting a count of the fibers
detected. The system senses carbon
fibers shorter than 0.1 mm moving at
velocities from 2 to 4 m/s and
measures fiber populations of up to
100 fibers/m3 in each of five lengths
ranging from 0.1 mm to 1.5 mm.
Areas requiring further development
are identified, as are design
requirements, alternative materials
and fabrication techniques for
producing a unit to be evaluated in a
field test. In a companion
development project, a mobile flow
test facility was produced for
evaluating and calibrating instrumen-
tation to measure fibrous aerosols.
This Project Summary was developed
by EPA's Environmental Sciences
Research 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
The selection of a measuring system
based on electrically charged grids
applied a well established technique to
the requirements of a continuous
monitor instrument for carbon fibers
emitted from stationary sources and
moving in ducts or stacks. Application of
the charged grids to an in-duct monitor
instrument required extending their
sensing capabilities to include fiber
lengths of 0.1 mm or less. The
developed system consists of five grids
in series, with each grid tailored to
sense fibers of a specific length range.
Figure 1 illustrates the system concept.
In operation, electrical discharges from
the grid wires eliminate fibers from the
gas stream; the combination of spacing
and applied voltage establishes a
threshold length for each grid so that all
fibers longer than the threshold length
are removed from the stream and
sensed by a specific grid. The last grid in
the system has a threshold length less
than 0.1 mm.
Prototype Instrument Charged
Grid System
The operating concept for the
measuring system is based on
experimental measurements of the
burnout of carbon fibers between
charged electrodes. For fibers contac-
ting the electrodes, burnout is assured
at field strengths of 2000 v/cm (2
kv/cm). A field strength of 10 kv/cm
precipitates an arc breakdown burnout
for fibers of lengths equal to half the
distance between the electrodes.
These results led to the concept of five
grids in series, with each grid sensing
and eliminating fibers of a specific
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Fan
(Optional)
Electric
Field
Grid Sensing
Minimum Length
1
2
3
4
5
0.1mm
0.2mm
O.4mm
0.8mm
1.6mm
Figure 1. Concept schematic diagram for the charged grid sensing and measuring system.
length range. NASA's experience with
fibers emitted from pool fires and with
measurement of fibers passing through
air filters in ventilating systems showed
an upper practical limit for fiber lengths
to be in the range of 3 to 4 mm. Data
from the processing of carbon fiber
composite scrap mixed in refuse-
derived fuel further confirmed an upper
practical limit of fiber length in the
range of 3 to 4 mm. All measurements
from fiber releases showed the majority
of the population to be less than 1 mm
long.
The discharge of a capacitor by
shorted grid elements provides a
triggering signal t:/a sensing circuit that
feeds a counter. The events at each grid
are counted separately; thus, the system
determines the length distribution as
part of the total count. A timer circuit
allows the count at each grid to be
related to a population of fibers moving
in the air stream. Thus, for any set of
flow conditions, the system determines
the presence of conductive fibers in the
stream together with their length distri-
bution and, over a period of time, deter-
mines an average volumetric population
of fibers moving in the stream.
Flow Test and Calibration
Facility
A mobile flow test facility was de-
signed, built, and made operational for
evaluating and calibrating the five-grid
sensor system. The facility employs a
recirculating low-speed wind tunnel to
simulate flow conditions representative
of industrial ventilating systems. The
fiber-dispensing equipment and
conductive fiber measurement
equipment developed for NASA was
installed as part of this system. The
principal features of- the facility are
shown in Figure 2.
Results of Testing and
Evaluation
Evaluation and calibration testing
established the system's operating
capability and identified areas requiring
further development. The testing
consisted of bench evaluations, which
did not produce quantitative data, and
operations in the flow simulation
facility, which generated quantitative
measurements.
Operational Evaluation
Testing
The operational evaluation testing
addressed the burnout of fibers at the
grids, the abilities of grids to respond to
fibers of the length ranges intended by
design, and the responses of the system
to a flow of fibers. These tests were
performed in a flow passage on a bench
top or within a small chamber. An exit
fan (see Figure 1) established the flow
velocities. The principal observations
and results from these evaluation tests
are included in the following paragraphs.
Burnout of Fibers. A grid discharge
that either shortened a fiber or cut it into
segments could introduce errors in
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Recirculating
Wind Tunnel
Laboratory
Area
Instrumentation
and Control
Fiber Chopper
and Dispenser
Office
Area
Figure 2. Mobile flow test facility.
counting. Such an effect would
necessitate statistical correction of the
counting results before measuring data
could be accepted. That effect did not
appear during chamber or fire-release
testing; nevertheless, operational
verification was considered necessary
for the five-grid sensor. For the
evaluation, the grids were operated
while small quantities of pre-cut fiber
were introduced into the fan-induced
flow. In this mode, cutting or partial
burning of fibers at the front grids (5 and
4 in Figure 1) would be followed by
counts at the downstream grids (2 and
1); any fibers less than 0.1 mm long
would be collected on a filter
downstream of grid 1 (and upstream of
the fan). Operation with fibers having
lengths greater than 0.8 mm (grid-4
spacing) did not produce any down-
stream counts (grids 2 and 1) or any
fiber residue on the filters, indicating
that any fiber that. precipitated a
discharge was consumed.
Fiber-Length Discrimination and
Sensing. This series of evaluations was
performed in a small chamber. Air-flow
through the instrument was induced by
the exit fan. Single fibers were lofted by
means of a pulsed air jet blowing
through a clump of fibers cut to lengths
of 3 mm (long), 1 mm (medium), and 0.5
mm (short). In qualitative testing, the
grids responded according to the length
distributions of the cut fibers.
Evaluation and inspection of the grids
indicated certain effects which compro-
mised the operation of the five-grid
sensor. Fibers adhered to the Teflon*
walls of the flow passages between the
grids; fibers contacted the grids before
the voltage had recovered from the
burnout; and there was evidence that, at
the higher velocities, fibers passed
through the field without making
contact or precipitating an arc. The
fallout of fibers by adherence to the
Teflon is a materials-related effect,
indicating that the surface selected as
the flow-passage liner requires
modification.
Operation in the Flow
Test Facility
Continuing operations in the flow
test facility evaluated length
distributions and sensitivity to
'Teflon is registered trademark of E I. DuPont de
Nemours and Co
population at flow velocities simulating
the conditions in ventilating ducts. For
this evaluation, a calibration rig was
constructed having the same flow areas
and flow resistances as the five-grid
sensor. The calibration unit was
designed to have the same flow
velocities at the same power settings as
the five-grid sensor, while capturing all
fibers on a filter element. The five-grid
sensor unit and the calibration unit
were installed in the test section so that
they were equidistant from the walls
and from each other. Velocity
measurements around the calibration
unit showed it passed 10% of the flow
associated with its frontal area;
therefore, the filter in the calibration
unit provided a means to measure the
population of fibers flowing in the
tunnel. The evaluation measurements
were performed at duct-stream
velocities of 1.52, 3.05, and 6.1 m/s,
using cut fiber lengths of 0.2, 1.0, and
2.0 mm. All grids operated and provided
data which show effects of fallout,
saturation, and velocity. Over a portion
of the operating regime tested,
extraneous effects did not appear
significant and, within that portion of
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the regime, the system accurately
measured the fiber populations.
Conclusions
The three principal objectives of the
development effort were met. A
prototype five-grid unit was designed,
built, and tested. The efforts to
configure a system required that
alternative manufacturing techniques
be evaluated. The system was tested
operationally in a flow facility that could
simulate the conditions of ventilating
ducts or stacks and inject a controlled
fiber population into the gas stream.
This sensor system is suitable for
continuous monitoring. The pertinent
supporting results may be summarized
as follows:
Prototype Demonstration of
Operation
A five-stage electrically charged grid
sensor capable of sensing airborne
fibers less than 0.1 mm long operates
effectively over a range of duct velocities
from 2 to 4 m/s. At lower velocities,
results are influenced by fallout; higher
velocities may not allow sufficient time
for a fiber to align with the field and
precipitate a discharge. Populations
from 10 to 100 fibers/m3 can be col-
lected within the length range sensed
by each grid. The present upper limit to
the population that can be counted is
determined by the sensing rate designed
into the electrical operation of the
system. At higher populations, the grids
can be saturated by multiple contacts,
allowing fibers to pass undetected.
Evaluation of Fabrication
Techniques
The prototype unit was fabricated from
materials compatible with the flow
environment at the exit of an incinerator
employing an active particulate control
system (controlled air-with-after-
burning incinerators which could have
exhaust temperatures exceeding 540°C
were not considered). Type 304
stainless steel was used for the grids.
Teflon for the grid supports and inter-
electrode insulator, and Sauereisen #8
cement for both the interelectrode
insulation and grid retention system.
Studies showed that either chromium-
nickel or chromium-nickel-iron alloys
were required for grid materials.
Several ceramics or cements showed
properties suitable for structure or
insulators. Conventional machining
techniques can be used in fabrications;
however, nonstandard techniques may
be required for joining electrical
connections.
Flow Simulation Facility
A recirculating wind tunnel wa
designed and built as a mobile sel
contained facility for evaluating an
calibrating fiber sensing instrumen'
under conditions simulating the flo'
velocities associated with ventilatin
ducts or exhaust stacks. The un
provides flow velocities covering th
range that would be encountered i
industrial operations. The test section i
the flow facility has dimensions con
parable to actual ducting and allov
operation of an instrument under sim
lated free-stream conditions. The sy
tern for introducing and maintaining
fiber population provides repeatab
fiber populations. As presently co
figured, the flow facility provides velo
ities ranging from 0.3 to 11 m/s. For
wide range of fibrous aerosols, fib
populations of more than 100 per cut
meter appear to be within the capabil
of the system.
S. Salmirs, J. Schrader, and A. Butterfield are with The Bionetics Corporation,
Hampton, VA 23666.
William D. Conner is the EPA Project Officer (see below).
The complete report, entitled "Development of a Charged Grid Sensor for Air-
borne Carbon Fibers," (Order No. PB 83-116 848; Cost: $10.00, subject to
change} will be available only from:
National Technical Information Service
5285 P6ff Royal Road
Springfield, VA 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
U S. GOVERNMENT PRINTING OFFICE: 1983 659-OI7/C
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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