United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-97/006 April 1997
SEPA Project Summary
Development of Alternative, Non-
Halon Fire Protection System
Roger A. Patterson, Garth Gobeli, Robert E. Tapscott, and Philip J. DiNenno
With the phaseout of halon produc-
tion, two alternative technologies—wa-
ter misting and low-residue particu-
lates—have come to the forefront.
These technologies use water or dry
chemicals in reduced quantities to pro-
vide acceptable fire protection. A re-
view and an assessment of the state-
of-the-art for these technologies was
conducted. From this information, wa-
ter misting was recommended as the
most promising near-term technology.
An experimental program defined and
optimized the operating parameters for
a water mist system at laboratory-scale,
followed by room-scale testing. In the
laboratory, a water flux of 0.6 L/min-m2
effectively extinguished Class A and
Class B (heptane) fires. Below this wa-
ter flux level, the extinguishment times
varied significantly, while water fluxes
above this level did not increase extin-
guishment times in comparison to the
amount of water used. Room-scale ex-
periments demonstrated that scale-up
from the laboratory is straightforward
and can minimize the requirements for
room-scale tests. A cost comparison
of water mist systems with respect to
the equivalent halon system indicates
that water mist is competitive in many
applications. Water misting fire sup-
pression system design and costs are
estimated at $90 to $150/m3 across a
range of technologies. Low-pressure,
water-only mist systems, could be in-
stalled for below $30/m3. Halon sys-
tems now average $125/m3 in many ap-
plications.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search 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
In the late 1940s and during the 1950s,
low-residue particulate and water mist fire
suppression technologies were being de-
veloped as specialty applications substi-
tuting for dry chemical and water sprinkler
systems, respectively, in areas where
weight and materials compatibility prob-
lems were encountered. The introduction
of halons in the early 1960s caused these
systems to be set aside. With interna-
tional environmental agencies agreeing to
phase out halons, low-residue particulate
and water mist fire suppression systems
have re-emerged as possible alternatives
to halon fire suppression systems.
Dry chemical agents have been used
for many years to extinguish fires. Such
agents are at least as effective as halons
in suppressing fires and explosions in
many applications; however, they can
cause unacceptable levels of secondary
damage. Recent research indicates that
fine particulate aerosols, low-residue par-
ticulates, can effectively suppress fire while
eliminating some of the disadvantages
caused by larger dry chemical particles.
Studies of pyrotechnically generated aero-
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sols, that produce a fine particle by reac-
tion (combustion) between an oxidant and
a reductant, offer the ability to distribute a
particulate cloud uniformly throughout a
complex space and, if the particle size is
small enough, remain suspended in the
protected space for times on the order of
tens of minutes. These suspension times
could allow fine particles to act as "total-
flood agents," yielding significant advan-
tages over present dry chemical systems
and, potentially, some halon systems.
Alternatively, on a weight basis, water
is a more effective fire extinguishant than
halons if near-complete evaporation is
achieved. Water suppresses or extin-
guishes fires through three predominate
mechanisms: (1) heat extraction using
water's latent heat of vaporization and gas-
phase cooling; (2) oxygen displacement
by steam expansion; and (3) radiant heat
attenuation involving surface cooling by
surface wetting/evaporation and blocking
of radiant heat transfer. Water misting sys-
tems use fine water sprays to provide fire
protection, the mists are tentatively de-
fined as having droplets 200 micrometers
(|im) or less in size. Since small droplets
evaporate significantly faster than large
droplets, the small droplets produced with
water misting systems provide the above
capability while reducing water require-
ments and collateral damage.
Project requirements were threefold. (1)
Information on low-residue particulate and
water misting systems for fire protection
was collected, and the state-of-the-art for
low-residue particulate and water misting
systems with regard to fire protection was
assessed. Based on the evaluation, water
misting was selected as the most promis-
ing near-term technology. (2) An experi-
mental program to develop a water mist-
ing fire protection system was conducted.
The program had two phases, a labora-
tory-scale experimental study to determine
basic parameters needed for developing
the fire suppression system including se-
lecting and optimizing the components of
the water mist fire suppression system.
Followed by room-scale experiments us-
ing the optimized equipment and operat-
ing parameters to determine the overall
effectiveness of the fire suppression sys-
tem in actual use. The system's ability to
suppress fire, protect against reignition
and/or explosion, and prevent damage to
powered equipment, paper records, and
electronic data storage contained in the
room was assessed. (3) Finally for three
current halon applications and the equiva-
lent water mist fire suppression system,
an econometric analysis was conducted
to determine whether water misting sys-
tems could economically replace halon
systems.
Procedure
A literature review of low-residue par-
ticulate and water mist systems was con-
ducted. Although little information has been
reported in the open literature on low-
residue particulates, the review focused
on (1) a survey of existing compounds
and possible suppression mechanisms; (2)
quantifying the performance and qualities
of existing compounds; (3) measurement
of particulate size; (4) interaction of par-
ticulates with fire; and (5) damage to elec-
tronic equipment by the particulates. The
water misting review focused on (1) the
production of fine droplets; (2) measure-
ment of fine droplets; (3) types of nozzles
(e.g., dual-fluid, high-pressure); (4) inter-
actions of water droplets with fire (includ-
ing flame/plume penetration, evaporation,
and transportation phenomena); (5) dam-
age to equipment, particularly electronic
circuits, by water mists; and (6) systems
currently being investigated or tested in
the field. Following the technology review,
the state-of-the-art for low-residue particu-
late and water mist fire suppression sys-
tems was assessed, taking into consider-
ation the stage of development, engineer-
ing design requirements, operation, main-
tenance, overall performance potential, and
potential impact of these fire suppression
systems. It was proposed that water mist-
ing technology development be carried out
in subsequent tasks since it was the more
promising near-term technology.
In laboratory studies, the effects of drop-
let size, droplet size distribution, droplet
velocity, and obstacles in the path of the
spray were studied with respect to how
they affected the water flux needed to
extinguish incipient fires. The information
obtained was used to characterize and
optimize the operation of water mist spray
nozzles used in developing a water mist
fire suppression system. Heptane telltale
fires (50.8-cm diameter cups filled to within
2 mm of the top with water and 10 ml_ of
n-heptane) were chosen for this phase of
testing since they represented incipient
fires. Additionally, the literature and re-
searchers currently performing room-scale
fire extinguishment testing indicated that
these fires were the hardest to extinguish.
A range of single-fluid nozzles allowed
a wide spread in water mist characteris-
tics without the additional variables added
with dual-fluid systems. Selected nozzles
represented the range of products avail-
able — low pressure/high momentum
nozzles (2.7-mm orifice diameter), inter-
mediate pressure/momentum impingement
nozzles (1.0- and 1.4-mm orifice diam-
eter), and low-momentum humidification
nozzles (0.2- and 0.5-mm orifice diam-
eter). For each nozzle and test condition
the spray pattern and water flux in L/min-
m2 was determined. Based upon the dif-
ferent spreads (ranges) in water flux for
the nozzles, positions were chosen for the
placement of 50.8-cm telltales, which were
filled with water and 10 mL of heptane
and were ignited. After a 30-sec preburn,
the times required to extinguish telltales
with each system were recorded. A pri-
mary objective was to determine the criti-
cal concentration of water required to ex-
tinguish the fires. In this case, critical con-
centration was defined as the most effec-
tive use of water; i.e., a minimum regard-
ing the amount of water used and the
time required to extinguish the fire. Water
mist fire suppression system goals are to
minimize system requirements without add-
ing to collateral damage.
Following selection of a nozzle and de-
termination of the required water flux, ad-
ditional laboratory-scale water flux tests
were carried out to develop the optimum
nozzle spacing to provide a uniform water
flux across the entire protected space for
room-scale testing.
Room-scale experiments were proof-of-
concept and scale-up tests. Room-scale
testing of the selected and optimized wa-
ter mist fire suppression system was con-
ducted to determine the overall effective-
ness of the fire suppression system in
actual use. The ability of the system to
suppress and extinguish Class A (wood
and paper) and Class B (heptane) fires,
protect against explosion or reignition, and
limit the damage to powered equipment,
paper records, and electronic data stor-
age contained in the room was assessed.
Due to uncertainties in National Board
of Fire Underwriters requirements and
those of the forthcoming National Fire Pro-
tection Association Standard 750 on wa-
ter mist fire protection system installation,
operation, and testing procedures, the mar-
keter of the final nozzles tested was un-
willing to set system costs. To complete
the third phase of the project, three gen-
eralized water mist systems where enough
information was available to complete an
econometric analysis were used.
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The task evaluated three applications
of water mist fire suppression systems as
replacements for Halon 1301 total-flood
systems. Cost estimates were based on
the following assumptions: (1) where wa-
ter pumps are required, sufficient electri-
cal power is available; (2) all systems
assume Underwriters approval for all hard-
ware components; (3) a current Halon
1301 price of $50/kg; (4) cost estimates
based on approximate equipment list prices
and installation cost that are reasonable for
comparison purposes; (5) approximately
equivalent maintenance costs for both wa-
ter mist and Halon 1301 systems; and (6)
life cycle cost comparisons driven by the
probability of an accidental discharge, mini-
mal for water whereas Halon 1301 costs
are on the order of $10/m3 of protected
volume.
Results and Discussion
Low-residue particulate research is cen-
tered on development of (chemical) agent
formulations, determination of concentra-
tions required to extinguish Class A and
Class B fires, documentation or elimina-
tion of any potential acute inhalation toxic-
ity problems, and development of particle
generator systems.
Water mist fire suppression technology
is further along in its development, since it
has drawn upon the broad base of hard-
ware and theoretical knowledge developed
for controlling air pollution aerosols, in-
dustrial scrubbing, humidifying, air cool-
ing, dust suppression, foam control, moist-
ening, and water sprinkler fire suppres-
sion. At present, at least 17 water mist fire
suppression systems are available or are
being developed by different manufactur-
ers. Additionally, the potential suppliers of
nozzles and systems greatly exceed this
number if this area of application expands.
From laboratory-scale experiments, a
water flux of 0.6 L/min-m2 is the critical
concentration for extinguishing heptane
telltales (representing difficult to extinguish
incipient fires) with a water mist fire sup-
pression system. Water fluxes above 0.6
L/min-m2 did not significantly increase ex-
tinguishment times in comparison to total
water usage. While water flux levels be-
low this range were able to extinguish the
telltales, the extinguishment times became
longer and more erratic. Extinguishment
times for water fluxes between 0.025 and
0.60 L/min-m2 show standard deviations
on the order of their extinguishment times.
Water fluxes below 0.025 L/min-m2 were
not able to extinguish the fires. Crowding
the nozzles so as to increase the water
flux decreased their fire extinguishment
effectiveness, at least for heptane fires.
Room-scale testing for the water mist
system was proof-of-concept and involved
a center-fed ceiling system design with a
nozzle spacing of 40.6 + 5 cm; the array
was adjusted within these parameters for
a best fit to the room. For a nozzle spac-
ing designed to yield a uniform water flux,
the most efficient system for nozzles hav-
ing a small circular spray pattern was a
rhombohedral patterned array. Laboratory-
scale flow rate tests indicated a water flux
of 0.47 L/min-m2 at 3.45 MPa for the opti-
mized nozzle spacing. Increasing the op-
erating pressure to 6.90 MPa increased
the water flux to 0.76 L/min-m2 without
changing the droplet size distributions. The
initial water flux for the water mist fire
suppression system was below the critical
concentration of 0.60 L/min-m2, the
system's capacity allowed an increase in
water flux to levels beyond the critical
concentration. To allow a direct compari-
son to the laboratory-scale experiments,
the room-scale tests were conducted at
0.47 L/min-m2.
At a water flux level of 0.47 L/min-m2,
the water mist system was capable of
extinguishing all unobstructed, partially
obstructed, and fully obstructed Class A
(wood crib and paper) and Class B (32
and 292 kW pan) fires. Increasing the
water flux to 0.76 L/min-m2 showed that
water usage increased at a greater rate
(1.44 times) than did the decrease in ex-
tinguishment time (1.07 times). An operat-
ing personal computer, books, and news-
papers were exposed to unobstructed
wood crib fires during a room-scale extin-
guishment. Most of the damage to the
personal computer and paper related ma-
terials was caused by smoke, which was
easily cleaned off, and heat. The water
mist formed only a thin film on the com-
puter and papers, which evaporated quickly
after the water mist system was shut down.
Post-fire, long-term storage of the personal
computers show no adverse effects caused
by exposure to the water mist.
The third requirement of this project was
a direct system cost comparison of three
present halon applications and the equiva-
lent water mist fire suppression system.
Due to uncertainties in Underwriters re-
quirements and those of the forthcoming
National Fire Protection Association Stan-
dard 750 on water mist fire protection
system installation, operation, and testing
procedures, the marketer of the final
nozzles tested was unwilling to set sys-
tem costs. To complete the third phase of
the project, three generalized water mist
systems were considered, where enough
information was available to complete an
econometric analysis. The systems cho-
sen were (1) marine engine room and
machinery spaces (1500m3), where an in-
stalled water mist system would cost $120
to $147/m3 ($180,000 to $220,000) for
open and enclosed bilges, respectively,
whereas a Halon 1301 system would cost
$150/m3 ($225,000); (2) combustion tur-
bine enclosures (320 m3), where the order
of magnitude cost estimates would be
$150/m3 ($48,000) for the water mist sys-
tem compared to approximately $125/m3
($40,000) for Halon 1301; and (3) emer-
gency generator (320 m3), engine test cells,
and similar facilities costing $141 to $156/
m3 ( $45,000 to $50,000) for low and high
pressure water mist systems, respectively,
compared to $234/m3 ($75,000) for an
installed Halon 1301 system.
Conclusions
Low-residue particulate fire suppression
technology, particularly pyrotechnically
generated aerosols, is at a developmental
level. While the potential for low-residue
particulate fire suppression technology ex-
ists, its development is still in its infancy.
Meanwhile, water mist fire suppression
technology is further along in its develop-
ment since it has been able to draw upon
the broader base of hardware and theo-
retical knowledge developed for water
sprinkler fire systems and other applica-
tions. With this greater foundation to draw
upon, water mist fire suppression technol-
ogy was recommended as the most prom-
ising near-term technology.
The critical concentration for heptane
telltale fires was 0.6 L/min-m2 based upon
laboratory studies. The concentration was
initially proposed as the total mass of wa-
ter in droplets per unit volume (or area)
required to extinguish various classes of
fire. A better definition of critical concen-
tration would be the most effective fire
extinguishment concentration, represented
by the minimum concentration where ex-
tinguishment times versus water flux be-
come essentially constant. At this point,
fires are extinguished quickly, but with the
least amount of water and water related
collateral damage.
Water fluxes above 0.6 L/min-m2 did
not significantly increase extinguishment
times in comparison to total water usage.
For water flux levels below this range,
while able to extinguish the telltales, the
extinguishment times became longer and
more erratic. Extinguishment times for
water fluxes between 0.025 and 0.60 L/
min-m2 show standard deviations on the
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order of their extinguishment times. Water
fluxes below 0.025 L/min-m2 were not able
to extinguish the fires. Increasing water
flux rates for these nozzles had the oppo-
site effect on extinguishment times, indi-
cating interactions between droplets and
changes in droplet size and distribution
may be more important than the total
amount of water present in the protected
space.
Room-scale experiments demonstrated
that scale-up from the laboratory is straight-
forward. Significant findings from the room-
scale testing were (1) at a water flux of
0.47 L/min-m2, the water mist can neither
inert the space nor stop reignition of a
hydrocarbon pool fire; (2) upon reignition,
the water mist contained the fire during
repeated extinguishments; and (3) fires
can be extinguished without collateral dam-
age to books, papers, and energized elec-
trical (computer) systems.
The engineering design and cost of wa-
ter mist fire suppression systems indicate
a high-end cost estimate of $90 to $1507
m3 across a range of technologies. For a
low-pressure, water-only mist system, this
cost could be reduced to below $30/m3.
The cost of water mist systems should
decrease overtime as additional competi-
tors enter the market and R&D costs are
recovered. Given the high cost of avail-
able Halon 1301 (approximately $50/kg),
halon systems now average $125/m3.
Therefore, water mist fire suppression sys-
tems are cost competitive with Halon 1301
in many applications.
Recommendations
Potential future research involves study-
ing the extent of the interaction between
individual nozzles, the dependence upon
nozzle spacing, and the subsequent ef-
fect on drop size distribution on fire extin-
guishment and extinguishment time. Addi-
tionally, determining the droplet size range
that will allow significant amounts of water
mist to flow around obstacles in sufficient
concentration to extinguish fires will be of
great benefit. It was proposed that at higher
concentrations, the water mist coalesced
into larger drops, which then fall out of the
protected space. Additionally, these stud-
ies could be enhanced by determining the
size range of the drop that in addition to
falling into the fire, could be swept into the
side of the fire from a distance, and thereby
aid in extinguishing the fire by horizontal
flame penetration and cooling at the flame/
fuel interface.
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Roger A. Patterson, Garth Gobeli, and Robert E. Tapscottare with the University of
New Mexico, Albuquerque, NM 87131; and Philip J. DiNenno is with Hughes
Associates, Inc., Columbia, MD 21045.
Theodore G. Brna is the EPA Project Officer (see below).
The complete report, entitled "Development of Alternative, Non-Halon Fire Protec-
tion System," (Order No. PB97-147961; Cost: $41.00, subjectto change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management 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
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EPA
PERMIT NO. G-35
EPA/600/SR-97/006
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