This report has been prepared

for informin' and record

purposes and is not to be referei

in any publication.

NATIONAL BUREAU OF STANDARDS REPORT

^923

FIRE EXTINGUISHMENT BY MEANS OF DRY POWDER

by

C. S. McCamy

H. Shoub

T, G. Lee

ir

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Rureau of Standards at its headquarters in Washington,

D. C., and its major field laboratories in Boulder, Colorado, is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section carries out spe- cialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the

inside back cover of this report.

WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Tubes. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation.

Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology

and Lubrication. Engine Fuels.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment. AEC Radiation Instruments. Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metals Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Concreting

Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Analogue Systems. Application Engineering.

• Office of Basic Instrumentation ° Office of Weights and Measures

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services. Sun-Earth Relationships.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Radio

Systems Application Engineering.

Radio Standards. Radio Frequencies. Microwave Frequencies. High Frequency Electrical

Standards. Radio Broadcast Service. High Frequency Impedance Standards. Calibration

Center. Microwave Physics. Microwave Circuit Standards.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-12-1029 November 6, 1956 *+923

FIRE EXTINGUISHMENT BY MEANS OF DRY POWDER

by

C. S. McCamy

H. Shoub

T. G. Lee

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repri

unless permission Is ob

25, D. C. Such permii

cal ly prepared if that

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

part, is prohibited

tards, Washington

t has been specifirt for its own use,

FIRE EXTINGUISHMENT BY MEANS OF DRY POWDER1

ABSTRACT

The theory that dry chemical extinguishment

results from the release of carbon dioxide has

been disproved by successful use of inert

powders. Optical measurements of the effect

of dry chemical in shielding the fuel from the

radiant energy of the flame indicate that in full

scale fires, most of the energy can be blocked.

The minimum powder concentration required has

been measured. On a small fire this concentration

was of the order of 10"^ gm/cm3 0 Although the

effectiveness of dry chemical may be largely

attributable to the interruption of combustion

chain-reactions, these results indicate that

radiation shielding is a significant factor.

1. Introduction

A common type of commercial fire extinguisher is

charged with dry powder which is expelled by the discharge

of compressed gas. The powder, known in the trade as "dry

chemical", is finely-ground sodium bicarbonate with small

amounts of additives to improve its flow properties.

Numerous experiments have demonstrated the effectiveness

of such extinguishers in fighting flammable -liquid fires.

In a series of tests to determine the relative effectiveness

of various types of small portable devices, commercially

available four and five pound dry-chemical extinguishers

proved equal to 10 pound carbon dioxide extinguishers on

flammable liquid fires of ten different conformations. Tie

Factory Mutual Laboratories consider dry-chemical extinguishers

about twice as effective as carbon dioxide extinguishers

of equal weight capacity on the basis of the relative

areas of gasoline spill fires for which these devices are

considered adequate protection (1) .

"^This paper was originally presented at the Sixth

International Symposium on Combustion in August 1956.

2Numbers in parenthesis indicate references given at the

end of the paper.
-2 -

Although dry sodium bicarbonate has been used in one

manner or another since the beginning of this century, or

before, the prototype of our modern dry chemical did not

appear until 1928, when D. J. Block applied for a patent

on a dry chemical composed of sodium bicarbonate with enough

magnesium stearate added to keep the powder free flowing (2).

About 19^3 it was found that the effectiveness of the

extinguishers was somewhat improved by the use of more

finely powdered material. Fire-fighters generally believed

that the sodium bicarbonate decomposed when heated and

liberated enough carbon dioxide to extinguish the fire.

This belief seems to have been the basis upon which the

dry-powder method of extinguishment was developed.

As plausible as the carbon dioxide theory seemed, it

was not very satisfactory when considered quantitatively.

On this basis, how could five pounds of sodium bicarbonate

be as effective as 10 pounds of pure carbon dioxide? If

it were completely decomposed, the sodium bicarbonate would

liberate about 26 percent of its weight as carbon dioxide.

Samples of dry chemical were collected after an actual

extinguishment and were quantitatively analyzed. According

to this analysis less than 15 percent of the bicarbonate

had decomposed, therefore less than four percent by weight

of the powder was liberated as carbon dioxide. It appears

that the carbon dioxide theory which motivated the development of the dry chemical extinguisher does not account for

a large part of the extinguishing action.

The ultimate objective of the investigation of which

this work was a part was to determine the mechanism of

dry chemical extinguishment.

2. Mechanisms of Extinguishment

A brief consideration of the general nature of flammable

liquid fires suggests some possible mechanisms. Once the

vapors are ignited, the flame provides heat to vaporize

more fuel. The rising vapors are heated by the energy

liberated in the reaction zone until they are hot enough

to react and produce more heat. The reactions occurring in

hydrocarbon flames are not well known but spectral studies

and other experiments indicate that they are complex branched

chain type reactions involving short-lived free radicals

which carry the impetus of the reaction to succeeding

links in the chain (3). In a continuous cycle, the flame

heats the fuel and the heated fuel and oxygen feed the

flame. Any mechanism which interrupts this cycle extinguishes the flame.

There are a number of conceivable physical mechanisms

of extinguishment of flammable-liquid fires, including the

following:
1 . 2 »

-3~

Throttling e The influx of fuel is stopped.

Cooling o So much heat is removed that combustion

ceases o 3 , Blowing -out o The velocity of the gaseous fuel

or air exceeds the velocity of the combustion

wave and the combustion wave recedes, finally disappearing as a result of other mechanisms,

4-, Smothering , The influx of air or oxygen is stopped,

5® Diluting o Inert gas is mixed with the atmosphere,

6, Shielding o Material is interposed between the flame

and the fuel to prevent the transfer of energy by

radiation.

Chemical mechanisms of extinguishment include the

following s

1® Chain -breaking . Material is introduced into the

flame so that potentially reactive particles

encountering this material will not propagate the

reaction chain,

2, Preferential oxidation , A material with a strong

tendency to unite with oxygen is introduced into

the flame so that the available oxygen, combines

with the extinguishing agent rather than the fuel.

It is quite likely that this list of mechanisms is

incomplete, but it will serve as a basis for the present

discussion.

3* Preliminary Experiments With Various Powders

In view of the many conceivable mechanisms, it was

felt that the more likely ones should be determined by a

comparison of the effectiveness of a variety of powders in

extinguishing gasoline fires. Powders were selected on

the basis of their physical properties. The fineness ranged

from 85 percent through 200 mesh to over 85 percent through

4-00 mesh. Three tests were performed on each powder, under

actual fire-fighting conditions. The fuel, 5/8 gallon

of leaded gasoline, was placed on a slightly indented

4- x 4—ft concrete surface, ignited and allowed to burn

five seconds before the extinguisher was discharged into

the fire. An effort was made to keep the discharge operation uniform for various powders, the extinguisher being

aimed at the base of the flames and advanced slowly

while sweeping from side to side. The extinguisher was

a commercial ’’stored-pressure” device of 2,5 lb capacity,

pressurized with dry nitrogen at 3.50 psi gage. The powder

charge weighed 2 to 2,5 lbs, depending on the bulk density

of the powder. The average discharge rate ranged from

0,4- to 0,5 Ib/sec for the various powders.
Extinguishment efficiency was based on the average

time rate of extinguishment or extent to which flames were

suppressed, as evaluated by two observers,, The most

effective powders were rated +3, the least effective -3

and the others were rated between these values in algebraic

order . Positive numbers were used if the fire was

extinguished, negative numbers if note The most easily

dispersed powders were rated A, the least C, on the basis

of visual observation during use. Table 1 lists the

various powders, some of their properties, and their

extinguishment efficiencies . In the table, the powders

are listed in order of apparent effectiveness based on

efficiency and dispersibility®

To determine whether or not the discharge was merely

blowing the fire out, an extinguisher charged with dry

nitrogen at a pressure of 4-50 psi gage was tried* The

gas discharge fanned the flames and increased the intensity

of the fire* This result shows that the flame is not

merely blown out by the discharge of a dry chemical

extinguisher, and also indicates that the mechanism by

which such devices operate Is neither smothering nor

diluting, for these mechanisms should be provided as

well or even better by the inert gas*

It is noted that immediately after extinguishment

the fire could be reignited, indicating that the powder

was not forming a film on the gasoline and throttling

the fire.

The effectiveness of borax and silica proves that the

release of carbon dioxide is not the principal mechanism

of extinguishment by means of dry powder* The results

shown in table 1 seem to indicate that the combination of

fine particle size and good dispersibility is an important

requirement for extinguishment by means of dry powder*

The salts of the alkali metals were more effective than

other types of materials tested*

Among the materials tested, there was no observable

correlation between the effectiveness of powders and their

heat capacities* The amount of energy required to heat

one gram of each material from l8°C to 300°C was calculated

and is shown in table 1* Heat of reaction was included

where decomposition or dehydration would occur. With the

materials listed in order of effectiveness, there is no

apparent order among the heat capacities. It must be noted

that mechanisms involving the heating of particles are

not necessarily ruled out by this observation alone, since

heat transfer phenomena are not governed only by heat

capacity. Nonetheless, heat transfer theory shows that

heat transfer from gases to fine particles is very efficient

due to the high surface to mass ratio involved*
The results of these preliminary experiments indicated

that the mechanisms of throttling, cooling* blowing -out,

smothering, and diluting are of minor importance if

operative at alio Since the preferential oxidation theory

was inapplicable to the best dry powders tested, the

original list of mechanisms under consideration was

reduced to two principal mechanisms, shielding and chainbreaking e These mechanisms would depend upon the projected or other area of material in a cloud and therefore

would require fine particle size and good dispersibility,

in accord with the results of these experiments® There

is a considerable amount of corroborating evidence for

these conclusions, based upon combustion experiments not

simulating fire-fighting conditions (4-, 5? 6? 7, 8),

k 0 Optical Properties of Dry Chemical

In order to determine the possibilities of the

shielding hypothesis, measurements were made of some optical

properties of commercial dry chemical dispersed in the air.

Measurements were made to determine how much of the

incident radiant energy would be transmitted undeviated

through a cloud of powder and how much would be deflected

through various angles® It was felt that, with the aid of

the theory of scattering, the deflection measurements might

reveal information concerning the relative magnitudes of

the effective particle diameters and the wavelength of the

bulk of the radiant energy®

Since commercial dry-chemical extinguishers discharge

powder at a rate which is initially very high, and which

decreases very rapidly, they were not considered suitable

for use in controlled optical experiments® It was found

that commercial dry chemical would remain on a fixed 100

mesh sieve but would flow if the sieve were vibrated®

This fact made it possible to dispense a stream of powder

which could be irradiated and observed under controlled

conditions® The powder was placed in a light metal

cylinder 12 cm in height and 5 cm in diameter, open at

the top and bottom, resting on the center of a round 100

mesh sieve 20 cm in diameter® An electric vibrator was

used and the rate of powder discharge was controlled by

adjusting the voltage on the vibrator ® To avoid interference from particles floating around in the air, a

vacuum cleaner was used to remove the powder as it fell®

The vacuum, cleaner was equipped with a conical attachment

20 cm in diameter placed 20 cm below the dispensing sieve®

At this distance, the exhaust system had no apparent effect

on the free fall of the powder in the first 8 cm of fall.

The diameter of the falling powder stream contracted from

5-0 cm at the sieve to 4*0 cm at 2.5 cm below the sieve

and remained fairly tant for an additional 5 cm. On

the basis of previous estimates of the discharge rate
- 6 -

employed in fire-fighting, the powder was discharged so

that its flow rate was 0.012 gm/sec-cm2 in the region under

observation*

Measurements were made using radiant energy in the

visible region of the spectrum and in the infrared* For

the visible region, the optical system consisted of a

100 watt incandescent lamp, projection lenses, a photovoltaic cell, and a recording potentiometer. The photocell

was mounted so that it could view the powder stream from

various angles. The lamp and photocell combination gave

an effective spectral distribution similar to the relative

visibility curve for the normal human eye but shifted

about 0. 05 micron toward the long wavelengths/

Previous investigations had shown that the spectral

distribution of the energy emitted by hydrocarbon diffusion

flames is similar to that of a black body at l4-70°K,

having a maximum in the infrared at a wavelength of about

2 microns (9)® To approximate this distribution, an

electrically powered hot-wire source was operated at

1200° K and gave a spectral distribution with a maximum

around 2.4- microns. A radiation pyrometer was used as

a detector.

Measurements were made of the fraction of the incident

energy which was transmitted directly and the fraction

deflected through angles from 30° to 150° at 15° intervals.

The observed relative intensities of the deflected energy

are shown in figure 1. It was found that 85 percent of

the visible light and 8l percent of the infrared energy

were transmitted directly through the 4-.0 cm stream. There

was much more energy deflected at forward angles than at

backward angles and the observed relative intensity of

deflected energy (within the angular range covered) was

two to five times as great in the infrared as it was in

the visible. This would be expected if the particle

diameters were greater than the effective wavelength of

the infrared source. An independent analysis of the

particle diameter distribution with a Roller Analyzer

(an elutriation type separater) indicated three percent

by weight less than 10 microns, 14- percent from 10 to 20

microns, 38 percent from 20 to 4-0 microns, and 4-5 percent

above 4-0 microns.

In a gasoline spill-fire on an area four feet square,

flames often rise six to eight feet in the air. The powder

cloud used in extinguishing such fires would have a depth

of the order of 4-5 cm. If the concentration were the same

as that of the powder stream in these experiments and

Bouguer ' s exponential law of transmission may be assumed,
- 7

-

it appears that less than 10 percent of the energy radiated

in the direction of the gasoline would he transmitted

by the powder cloud* It would seem that this effect should

make a substantial contribution toward the extinguishment

of the fire*

If the shielding effect is a substantial part of the

mechanism, and the size distribution of particles in the

dispersion is that indicated above, one would predict on

the basis of optical principles that powders having a

larger part by weight in smaller particles would be more

effective if the small particles were discrete in the

dispersed state* The preliminary experiments seem to show

such a trend for the range of particle sizes used.

5 . Transmittance Measurements During Extinguishment

The uncertainties involved in interpreting laboratory

data in terms of fire-fighting conditions made it desirable

to simulate actual conditions. For this reason, a group

of measurements was made of the effect of dry chemical

on the radiant energy from a flame during actual extinguishment. The objective was to determine how much of the energy

radiated toward the fuel was transmitted by the dry

chemical cloud® The adverse conditions at the surface of

the fuel made it impractical to place a detector there,

so a burner was constructed with a window in the bottom.

The burner was a thin steel cylinder 10 cm in diameter and

6 cm deep, open at the top* The window in the bottom was

made of clear fused quartz*

The powder was placed in a cylinder 8 cm in diameter

and 4-5 cm in height with a compressed air inlet at the

bottom and a nozzle at the top (figure 2). The 1 x 10 mm

nozzle opening discharged a stream having a cross section

of greater width than height* To avoid the transient

flow conditions produced when the compressed air was

turned on or off, the extinguisher was allowed to discharge

continuously* Except when it was desired to apply powder

to the flame, the powder stream was caught by a funnel

connected to a vacuum dust collector. The funnel could be

pulled aside for a given time interval, by means of a

solenoid and an electronic timer* This arrangement provided

short bursts of powder with a nearly constant flow rate

during the discharge*

A lead sulfide infrared photoconductive cell was placed

below the window to detect the energy radiated toward the

fuel and another such ce*ll 'was placed above and to one

side of the burner to detect the energy radiated in that

direction. This arrangement is shown in figure 2. In
-8

-

order to limit the effect of radiation from other sources,

infrared filters were used to limit the spectral sensitivity of the detectors to the wavelength region from 0,7

to 2.6 microns

o

The fuel was a commercial mixture of heptanes with

a boiling point range of about 88°C to 100°Co This fuel

was chosen because it represents a major component of

gasoline, the most common hazardous flammable liquid,

but has a much more uniform burning rate in a spill-type

fire. The fuel was floated on a thin layer of water to

prevent overheating of the window and its mounting

gaskets. The thickness of the water layer was 1,1 cm

and the thickness of the fuel layer was 0,4- cm at the

start of the experiment. The metal rim of the container

extended about 3 cm above the fuel level. When the burner

had warmed up, the flame measured about 10 cm in diameter,

at its widest part, and about 40 cm in height.

In the part of the spectrum used, thin layers of

water and quartz transmit nearly 100 percent at all

wavelengths, A 4 mm layer of heptane transmits nearly

all energy in the wavelengths from the visible spectrum

to 1,6 microns with the exception of sharp absorption

bands at 1,25 and 1,45 microns, and about 77 percent from

1,6 to 2,3 microns, with the exception of a narrow absorption band at 1,7 microns. Thus the spectral sensitivity

of the lower detector was little different from that of

the upper one. There was practically no change in these

absorbing layers during the small fraction of a second

involved in making a measurement. Aside from these

considerations it was not necessary to take into account

the transmittance and reflectance of these various layers

because only relative values of the irradiance at the

receiver were required for transmittance measurements.

In the usual measurement of transmittance, a radiant

source of constant intensity is employed and a determination is mad© of the ratio of the relative amounts of

energy received with and without a sample Interposed

between the source and receiver. In the present case,

the source was a flame which decreased in intensity when the

powder cloud was injected. The upper detector measured

the decrease in intensity in the upward direction and it was

assumed that the intensity in the downward direction changed

in the same ratio. The transmittance, measured by the lower

detector, was then corrected for the decrease in flame

intensity.
-9-

Because the extinguishment occurred so quickly, it was

necessary to record the data with a high speed automatic

recorder* The recorder used was essentially a row of

2-inch cathode ray oscilloscopes that were photographed

simultaneously by a 35 mm camera in which the film was

exposed continuously as it moved at right angles to the

deflections of the spots on the tubes. The output of a

4-0 cps fixed frequency oscillator was applied to one channel

to provide a time scale.

The radiant intensity of the flames dropped very

rapidly after the appearance of the powder. The time

required to extinguish the flame averaged about 200 milliseconds as observed by means of the upper detector but

only about 170 milliseconds as observed by means of the

lower detector » During the time that the radiant intensity

was diminishing, slight fluctuations in the intensity

produced relative maxima and minima in the two radiometric

records* The measurements of transmittance were made at

the maxima because such corresponding points on the two

curves presumably represented events which occurred

s i mult aneou s ly

Thirty measurements of this Blind were made with the

powder concentration near the minimum value required for

extinguishment. The average transmittance was found to be

53 percent* This result seems to support the belief that

even in a small fire, the shielding effect of the powder

may contribute materially to the extinguishing process.

The cloud of powder used in these experiments had a depth

of about 6 cm, In larger fires, where thicker and more

concentrated clouds are used, a somewhat greater effect

would be likely®

6, Concentration Measurements During Extinguishment

Quantitative considerations of the mechanism of extinguishment hinge ultimately upon knowledge of the concentration of material required for extinguishment. The relative

effectiveness of different powders can be expressed quantitatively if the minimum, effective concentration of each is

known. The quantitative control of concentration during a series of experiments depends upon measurement of concentration, A satisfactory method of measurement should give the

concentration during extinguishment. Direct sampling

techniques were not employed because such methods disturb

the distribution of the powder and disrupt the normal

course of events during extinguishment. To avoid these

difficulties, optical methods were employed to measure

several related physical quantities from which the concentration was calculated®
-10 -

The average weight rate of discharge for a time period

of a few seconds could be obtained by direct weighing and

timing, but during a small fraction of a second the weight

change was too small to be differentiated from variations

in the recoil of the discharging cylinder. It was found

that the average rate of discharge was proportional to

the average optical density of the powder stream. (This

is a manifestation of Beer's Law, usually applied to the

relationship of optical density and concentration in

solutions.) This fact was utilized in calculating the

instantaneous rate of discharge from the optical density

of the powder stream measured by projecting a light beam

through the stream onto a photoelectric cell and recording

the response automatically.

Another such photoelectric system was placed a fixed

distance downstream and when a short burst of powder was

injected the elapsed time between the appearance of powder

at these two points was recorded. The burner was placed

below a point midway between the light beams as shown in

figure 2. The data obtained were used to calculate the

velocity of the stream at the burner. The average observed

velocity was ^3 cm/sec.

A photoelectric system such as those described was

used to scan the stream horizontally and vertically at the

burner position to determine the distribution of the

powder in the stream. The distribution determined in

this way appeared to be nearly uniform over an area approximating an ellipse and to drop rapidly to zero outside this

region. From the plotted density, an estimate was obtained

of the lengths of the axes of an ellipse which would

encompass all of the powder if it were uniformly distributed^ This ellipse had axes of 6 and 10 cm, giving an

area of b7 cm^.

The measured values of weight rate R, velocity v,

and cross sectional area A were used to calculate the

concentration of powder e by means of the equation

c = R/vAo All of these measurements, except the area

scanning were recorded simultaneously with the transmittance

measurements during extinguishment by the use of six

channels on the oscillograph. It was found that when the

concentration exceeded 2.8xl0“L(

' gm/cm3 the flame was

always extinguished but when the concentration fell

below 0.7x10“^ gm/cm3 the flame was not extinguished.

At concentrations between these values, both successes

and failures were recorded. These values may be applicable

to this flame only but the order of magnitude is established

for theoretical considerations. The technique described

here was the only practical method of concentration measure-
-11 -

ment found which met the conditions imposed by the nature

of the experiments 0

7o Conclusions

The theory of extinguishment involving the evolution

of carbon dioxide is not satisfactory because powders that

do not release carbon dioxide are effective agents. The

combination of fineness and dispersibility of powders

appears to have some correlation with their effectiveness.

Salts of alkali metals appear to be particularly effective.

The evidence indicates that the mechanism of dry-powder

extinguishment is not principally that of excluding

oxygen , diluting the atmosphere, cutting off the fuel

supply, or blowing put the, fire® A minimum concentration

of the order of 10“^ gm/cnP of commercial dry chemical

is required to extinguish small fires involving heptane

under the conditions of the experiments. In the light of

prevailing theories of combustion and flame propagation,

it appears that dry-powder extinguishment may be largely

attributable to the interruption of chain -re actions in

the combustion process. The rapidity of the action, the

apparent dependence of effectiveness on fineness, and

the small concentration required, as determined in these

experiments, seem to support this view® However, it

appears that a full account of the mechanism of dry-powder

extinguishment of flammable -liquid fires must also include

the factor of radiation shielding®
-11 -

8. References

(1) Approved Equipment for Industrial Fire Protection,

Factory Mutual Laboratories, 184 High Street, Boston

10, Massachusetts.

(2) Block, D. Jo : Fire Extinguishing Material

Patent 1,793 >4-20, filed September 22, 1928

, U. S.

•

(3) Lewis, B. , and Von Elbe, G.,: Combustion,

Explosions of Gases, Academic Press, Inc.,

(195D .

Flames and

New York

(4) Dufraisse, C., and German, M. : Compt. Rend

1221 (1938)o

20Z,

(5) Jorissen, W. P„ , et al: Rec. Trav. Chim.

2g, 403 (1933).

(Amsterdam)

(6) Dijksman, A. J. ; Chem. Weekblad, 3U 718 (1934).

(7) Jorissen, W. P., et al, Rec. Trav. Chim. (Amsterdam),

53, 957 (1940).

(8) Muraour, Henry, and Michel-Levy, Albert:

Rend., 218, 551 (194-4).

Compt.

(9) Roeser, W. F. , and McCamy, C. S., Principles of Fire

Detection in Aircraft Engine Spaces, WADC Technical

Report 54-307, Wright Air Development Center (1954).

TABLE 1

The Properties and Effectiveness of Various

No. Powder^ Special Characteristics

1 Potassium bicarbonate with 2%

zinc stearate

Decomposes on heating, fine

particles

2 Dry chemical (contains 95+%

sodium bicarbonate)

Decomposes on heating, free

flowing

3 Borax with 2% zinc stearate High heat capacity, loses water

on heating

4- Sodium carbonate^ High interior surface area

5 70% sodium carbonate

30% magnesium, carbonate

Better flow than magnesium

carbonate

6 50% sodium carbonate

50% Portland cement; dried at 120C

7 60% silicon dioxide

b0% sodium carbonate; heated

at 280C , 3 hr

Better flow than silicon

dioxide

8 30% silicon dioxide

70% sodium carbonate

Better flow than silicon

dioxide

9 Sodium carbonate (derived from

untreated sodium bicarbonate)

10

11

Sodium bicarbonate dried at 120C

Silica with 2% zinc stearate5

Poor dispersibility

Inert, hard & irregular surface

12 Borax with 2% zinc stearate Coarse particles

13 Gypsum plaster (CaSoi* 5 il^O) Loses water on heating

lb Portland cement dried at 120C Dark gray color

15 Talc Soft surface

16 Fire clay Light gray color, low heat

conductivity

17 Sodium bicarbonate, not dried Poor dispersibility

wexe cx i/uer btsuixu xxj cix ur c:nmrn R re. i h i gra.Cie.

^ASTM Standard Specification for Sieves for Testing Purposes, Ell-39*

3lncludes sensible heat and heat of reaction where applicable (cal/gm)

All sodium carbonate except a.s noted was derived from, commercial dry

^Improved performance was obtained in a later test when discharge
Powders for Fire Extinguishment

No * Sieve Mesh^

(passing 85%)

Estimated^

Heat Capacity

(18C-800C)

Dispersibility

Efficiency

1 270 est, 238 A 3+

2 325 259 A 3

3 270 est. 463 A 3

325 79 A 2

5 325 72 A 2

6 325 68 B 2

7 4-00 est. 67 B 2

8 325 74- B 2

9 170 est. 79 A 2

10 200 259 B 1

11 4-00 est. 60 B 1

12 170 est. 463 A -2

13 4-00 est. 61 B -2

14- 325 58 C -2

15 Impalpable 66 C -2

16 200 63 A -3

17 200 259 C -3

chemical, decomposed by heating,

pressure was increased about 25 percent.

>

c/>

tjj H2

UJ

>

H

<

..

J

tej oc

>»

boP

CD • P C’J H CD d

o

CM rd •H O a

n 0

!>» O ,d

-P r 1 o

•rH it 0 W £j i>j P on p

CD "d

-P 171 p P rH •H O 05

•r( •H

CD P O

> c3 P

•H 0

-P

cd .p H

1—1 tifl O

CD J3 O

p o

P ch

T3 Jm o

CD -P • > £ s fn T3 O

CD 0 -H W -P to rQ O fH O CD CD

i—I Pi

CD Ch W

X3 0-H

T3) TJ H

CDP

P

bO •H
ELECTRONIC ^

TIMER

Figure

2.

Equipment

used

to

measure

the

transmittance

and

concentration

of

powder

clouds.
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of the National Bureau of Standards ($1.25) and its Supplement '($0.75), available from the Superintendent of Documents, Government Printing Office. Washington 25, D. C.

Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25, D. C.
NBS