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NATIONAL BUREAU OF STANDARDS REPORT

19^7

FIRE HAZARD TESTS OF SUBSTITUTE

DUCT MATERIALS

by

A- C. Hutton

and

D„ Gross

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Charles Sawyer, Secretary

NATIONAL BUREAU OF STANDARDS

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

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003“ 3b- 4721 September 25, 1952 19V7

FIRE HAZARD TESTS OF SUBSTITUTE

DUCT MATERIALS

by

A, Co Hutton

and

Do Gross

The publication, reprinting, or reproduction of this report in any form, either in whole or in part, is prohibited

unless permission is obtained in writing from the Office of the Director, National Bureau of Standards, Washington

25, D, C, Such permission is not needed, however, by a Government agency for which a report has been specifically prepared If that agency wishes to reproduce additional copies of that particular report for its own use,

FIRE HAZARD TESTS OF SUBSTITUTE

DUCT MATERIALS

ABSTRACT

The results of a number of tests performed for the

purpose of establishing criteria for evaluation of the

fire hazard of substitute materials for metal ducts are

presented. The procedures used to appraise the selfheating, ignition, flame spread and combustibility

characteristics are described. Included also is a description of a test on a large scale mock assembly of

a duct system demonstrating the flame spread hazard of

combustible ducts. Although the testing procedures are

by no means "standard", the use of the results obtained

is suggested as a basis for evaluation of the fire hazard

of substitute duct materials.
. - >

.V
- 2

1. INTRODUCTION

A request was made by the Housing and Home Finance Agency

that the Building Technology Division establish criteria for

evaluating substitute materials for metal ducts. The Fire Protection Section was asked to perform tests which would furnish

data which might be used as a basis for specification of criteria

for selection of such materials. Two materials were made

available for investigation with the suggestion that determinations be made of the temperatures at which self-heating and

ignition occur and of the flame-spread characteristics of

these materials.

2 . TEST MATERIALS

Material samples furnished for preliminary evaluation

consisted of:

"Sonoairduct Laminated Fiber Tubing " of 8- in. inside

diameter, 8 1/2- in. outside diameter, manufactured

by the Sonoco Products Companv, Hartsville, South

Carolina. It was supplied in M-ft and 18-ft lengths

and could be cut to any desired length with a hand

saw. To join straight sections, outside collars of

the same material and thickness and 12 in. long were

supplied by the manufacturer. The cellulose ply tubing

was analyzed as having approximately 50 percent

chemical wood and 50 percent ground wood and had a. paraffin-wax coating on both exposed surfaces, and

"Sal- Mo No .77 Asbestos Duct Board . " manufactured by

the Sail Mountain Company, Hamilton, Ohio. The board

comprised an admixture of asbestos and approximately

16.5 percent of combustible material including cotton.

Both surfaces were finished with a thin, black coating.

An attached label carried the following manufacturer's

specifications: "Size of sheet 33 in. x 48 in.;

Thickness 1/8 in. ; weight per sheet 7 lb; square feet

per sheet 11 sq ft; sheets per carton 16 sheets;

weight per carton 114 lb.

- 3 -

3 o TESTS

The primary purpose of the investigation was the performance

of tests from which criteria could be established for evaluation

of the fire hazard of substitute duct materials , The development

of standard test procedures and the determination of properties

of the particular materials submitted were considered secondary

,

Therefore, tests were designed to indicate the minimum safe requirements for duct materials in general., and were carried out

with the use of whichever of the two test materials proved more

suitable in form,

3.1 Self- Heating and Self-Ignition Characteristics

At the steady-state temperatures normally found in warm-air

ducts, it was not considered probable that self-ignition (ignition

in the absence of a directly applied high- temperature source) of

the duct, or of combustible materials in. contact with the duct,

would occur. Abnormal temperature conditions may prevail, however,

for circumstances where the bonnet protective limit control fails

or when Ignition of an accumulation of combustible trash in the

duct produces a localized heat source. Inasmuch as the performance

of a duct material may be influenced by the properties of combustible materials used as covering for, or in contact with the

duct, it seemed desirable to consider this influence on the

tendency of developing self-heating in the composite section.

With this in mind, a test was designed utilizing a short section

of a typical warm air heating duct assembly. This section was

judged to offer a degree of fire hazard which may be significant

in assemblies using substitute materials. For these tests, duct

materials of sheet form were used, although a similar arrangement

for round ducts could also have been used.

This testing arrangement (see Fig, 1) comprised a one-foot

length of a 3-in, by 12-in, duct section surrounded on the sides

by a single layer and on the top and bottom by a double layer of

1/2- in. insulating- fiber board sheet, a common insulating material 0 The ends were left open, A micro burner was mounted with its stem

projecting vertically 2 1/4 in, from the lower duct surface, A

thermocouple (A) placed at the center of the two upper insulating

boards enabled a continuous temperature recording to be made on

an. electronic potentiometer recorder. The temperature measured

between the two insulating boards for a given exposure resulted

from the composite effects of heat conduction through the duct

and backing and self-heating in the duct and backing. These heating

effects are influenced by the ability of the duct material both to

reflect radiant heat and to diffuse heat along the duct, as well as

by the tendency of the duct material to separate from the backing.

- b -

Two preliminary tests were run in which a pile of wood

chips and cotton lint was ignited and temperature curves were

obtained, These curves were then duplicated using a gas flame

which has the advantage of being reproducible . The "standard"

gas consumption history chosen is given in Fig, 1, The total

area of the stepped fuel-time chart was made equal to 0,400 ft^,

corresponding to a heat input of about bbO Btu for the natural

gas used. This is approximately 80 percent of the higher heating

value of the wood chip pile, the burning of which is known to be

not entirely complete.

Tests were run with Sal- Mo Asbestos Duct Board, asbestos

millboard (1/8 in, thick and containing less than 3 percent

combustibles), galvanized iron (U 0 S, standard gage No, 25),

black iron (U,S, standard gage No, 2b ) , aluminum (Brown and

Sharpe gage No, 2k), graphite coated aluminum and Sal- Mo

Asbestos Duct Board with an aluminum foil surface. The metal

ducts were stiffened by giving them a slight diagonal crimp

similar to fabrication practices in duct installations. These

ducts were fastened to the fiberboard backing at the center of

each edge to maintain the maximum contact area during heating.

Although this reduced somewhat the inward buckling tendency,

separation of the metal duct occurred in each test to varying

degrees, whereas very little separation was noticed for the

nonmetal ducts.

The results (Fig, 2) indicate a marked difference between

the characteristics of the metal ducts and the asbestos-base

ducts. Tests using the latter duct materials resulted in ignition

of the upper layers of fiberboard backing, whereas tests with

metal ducts resulted in peak temperatures no higher than about

212° F (100°C),

The effect of the emissivity of the duct surface was indicated in two of the tests. In one test, the emissivity of the

aluminum was increased by applying to it a coating of graphite

over a graphite plus varnish undercoat. When tested, a slightly

higher temperature at point A, Fig, 1, resulted than had been

obtained with the shiny aluminum duct. In a second test, Sal- Mo

Asbestos Duct Board was covered with a low emissivity surface by

cementing a sheet of aluminum foil to a duct which had previously

been given a coating of aluminum paint. This resulted in ignition

of the fiberboard, as with the plain Sal- Mo Asbestos Duct Board,

Thus the ability of the duct material to absorb radiant heat (as

measured by surface emissivity) appears to have only a slight

effect upon heat retention.

- 5 -

....

Xt aPPear s reasonable, therefore, to attribute the decided deferences between the local heat- transmission characteristics of metal and nonmetal ducts to the physical separation between he metal duct and its backing which results under fire exposure, thus, metal ducts covered with combustible materials present less lire hazard than similarly covered nonmetal ducts, including asbestos , when subjected to a fire such as might be caused by the ignition of an accumulation of wood chips, lint, etc,,

3.2 Flame-Spread Characteristics

, test procedures were used to investigate flame-suread cn racteristicss a flame-spread demonstration test, an ease-of~ ignition test, and a flame-spread comparison test.,

3 0 ~ (a ) Flame-Spread Demonstration Test of Sonoairduct

]ikPiv

h

nno

m

r terJ

al Wa

?

cJ??

sen since it was considered the more

/

demonstrating flame-spread hazard. The test was

characters ^ lsaal demonstration of the flame-spread

whir>h miffhi- • 1G damina ted tubing under a set of conditions whi.n might occur m warm-air heating applications.

4

sec iion of the tubing was embedded in dry sand en-

±-1/2 in

1

o/sandVth

1116 ],^ 11.-2

66 Fdgo 3)« There was approximately in. of sand

^ both above and below the duct and 2 1/2 in/

w

:

s

S

fittPb nf

Ch S1

?

e ’/ /

in ^ 90-degree galvanized-iron elbow

ovlr one end o? /p //te

A“o

Ubing C ° llar which fitted in turn

, ?ne

? f the duct. A 9-mo round to 9-in. by 12-in rec-

+ 0

n

f

Uh to a

r alumlnuTM transition piece completed the duct circuit

high

floor register with a 7 1/2-in. by 12- in. opening? 4n8-ft plasterboard panel placed vertically 6 in, from the register

represented

b

the £KdPla^rb0ard SbBet ' ^izontaUy

a coat o/a rSbb^-batfbll/pafnT

3 ^0 "1"617 ' The W&U WaS given

containld°a

h

2000

n

wa?f h/f

duot was enclosed in a chamber which

forced heated a?;

a

?

d through which an electric fan

permitted both thTF - he dact h

A removable plate-glass cover

velocit — ^

/readings?

HEreglster to obtal» outlet temperature and

- 6 -

When equilibrium conditions, 171° F (77° C) and 190 ft/min,

were established at the register, a handful of loose excelsior

was placed in the partly delaminated mouth of the duct at the

upstream side, The excelsior was ignited , As the flames progressed along the duct, the evolution of gas produced a mixture

toward the register end too rich for burning,, Increasing the

air velocity to approximately 400 ft/min accelerated the flame

spread through the duct and out of the register. Flames reached

a maximum height of about 7 ft up the wall, and then diminished

as the duct collapsed at its approximate mid-length, l6=mm color

motion pictures were taken of the demonstration.

Inasmuch as the laminated tubing was designed specifically

for embedment in concrete floor slabs, and since duct collapse

during the test was due to the sand (which was to have represented the concrete), a second test using concrete was performed.

Ordinary Portland- cement concrete (l;2sV) replaced the sand in

the wooden frame and, in addition, surrounded the galvanizediron elbow and transition piece up to the floor level to simulate poured concrete-slab construction. The concrete extended

the full length of the duct with the exception of 6 1/2 in, at

the duct mouth. The distance from the center of the register to

the duct mouth was 9 1/2 ft. Fig, 4- shows a view before test of

the floor and register, with asphalt-tile flooring partly laid,

A view of the concrete-embedded duct, heating chamber, fan, and

other details of the testing arrangement before test are shown

in Fig, 5° The test procedure was identical with that used in

the previous test.

Table 1 is a composite log combining observations taken

during the latter test with those read from l6~mm colored

motion- picture film taken of the first three minutes of the

test. The la.tter is available for projection upon request.

- 7

-

Table 1

Time

hr s mins sec

12 s 00 s 00

12 s 00 s 06

12 s 00 s 30

12 s 00 s 33

12 s 00 s 45

1.2 s OO eo 50

12 s 00 s 55

12 s 01s 00

12 s 01s 25

12s 01s 45

12 s 02 s 05

12 s 02 s 20

12 s 02 s 35

12 s 02 s 45

12 s 02 s 55

12 s 05 s 00

12 s 06 s 00

12 s 08 s 00

12 s 11s 00

12 s 20s 00

Test Log

Observation

Test begins with ignition of excelsior*

Equilibrium temperature 174°F (7 9°C)»

Air velocity 1.55 ft/min.

Smoke appears at register*

Smoke from burning excelsior stops*

Smoke from burning duct appears at register*

Smoke extends full width of panel (4 ft)

and reaches ceiling height (8 ft.)*

Fan speed increased to 470 ft/min* Flames

appear at register* Smoke decreasing*

Flames to 4- ft level, 2 ft wide.

Flames to 8- ft (ceiling) level, 3 ft wide.

Flames to 7- ft level, 2 ft wide*

Baseboard burning*

Flames to 6- ft level, 2 ft wide*

Flames to 4=- ft level, 2 ft wide.

Flames backing out of duct mouth cause

ignition of wooden plenum chamber. This

necessitates removing fan for a few seconds

to prevent damage to heating equipment*

This causes flames at register to cease.

Flames reappear to 3-ft level.

Flames to 4- ft level.

Flames to 1=2 ft level.

Flames to register level. Blistering and

burning of asphalt tile evident up to 5 in*

around a dull red grill.

Occasional licks of flame still visible

at transition piece; small pieces of duct,

still glowing, being blown through register.

Flames cease entirely with duct glowing.

Duct continues to glow. Water applied.

- 8 -

Fig .6 is a view of the concrete duct casing after the test,

showing the few charred pieces of an otherwise burned-out duct*

A front view of the floor and plasterboard wall, after the test

is shown in Fig* 7° Discoloration is evident to a height of 53

above the floor, and the baseboard shows charring which extends

6 in* to either side of the register* Blisters to 7 in* and

charring to b in. from the register appear on the asphalt tile

floor

3o2(b) EAse-of- Ignition Test

An ease-of-ignition test was performed to determine the

relative ease in obtaining a self-sustaining flame in the

laminated tubing, and to supplement the previous test described in 3.2(a). To indicate the relative ease of producing

a self-sustaining flame in the tubing, a test designed to

simulate the accidental dropping of a lighted match through a

wall or floor register on to an accumulation of oily lint, dust,

or other waste was devised*

The procedure consisted of placing a lighted match (wood

splint safety match) on oil- impregnated cotton batting at each

of two different duct locations - at a collared joint and on

the free duct surface (see points A and B, Figure 8). Varying

quantities of cotton impregnated with different quantities of

fuel oil were used to simulate accumulations representing

conditions which might be found in duct systems below furnace

level* A fresh portion of duct was used for each test, and

successive tests were designed to provide increasingly severe

igniting conditions. In this manner it was possible to determine

the conditions necessary to produce a self-sustaining flame

resulting in ultimate ignition of the duct* However, all of the

tests were conducted with an unheated duct and no air flow, a

situation less conducive to ignition than that of actual use.

Table 2 lists the tests performed, indicating the quantities

of No. 2 fuel oil and cotton batting used at each location.

j.

—

!*<«*£

- 9

-

Table 2 - Results of Ease of Ignition Test

POSIT ION A*

Test

Weight of

Cotton Batting

Volume of

No. 2 Fuel Oil Remarks*

gm cc

1A 0.0 0.0 No ignition

2A 0.0 5.0 do

3A 2.0 o o Ignition of

cotton only.

4-A 0.5 5o0 Ignition of

fuel - soaked

cotton only

5A © o

!

—

!

2o5 do

6A © 0

1

1 5.0 Self-sustaining

flame in duct

P OSITION B*

IB 0.0 0.0 No ignition

2B 0o0 5o0 do

3B 2.0 OoO Ignition of

cotton only

4B 2.0 2o5 Ignition of

fuel - soaked

cotton only

5B 1.0 5.0 do

6B 2.0 5.0 Self-sustaining

flame in duct

* See Fig . 8 ** The primary ignition source was the match which was only partly

consumed in all but those tests which developed a self-sustaining

flame .

- 10

It will be noted from the above table that flaming ignition

of the duct resulted when 5 cc of fuel oil added to 1 gram of

cotton batting placed at Position A (or when 5 cc of fuel oil

added to 2 grams of cotton batting placed at Position B) was

ignited by a matcho The tests have thus shown that ignition of

the laminated duct will result under conditions neither excessive

nor severe,

3.2(c) Flame-Spread Comparison Test

A flame-spread comparison test was made to determine the

flame- spread characteristics of the two products under comparable

conditions o Both materials were tested in the form of round ducts.

This was accomplished by molding a moistened sheet of Sal-Mo

Asbestos Duct Board around an 8- in. 0.D, metal pipe and and permitting it to dry completely to the desired circular shape. To

indicate the possible effects of the paraffin coating on the

Sonoairduct tubing, a duct of the same dimensions and comprising

3 layers of paraffin-free cardboard was likewise formed.

The test consisted of applying a continuous flame to the

inside surface of the duct, measuring the rapidity of flame

spread and observing the extent of surface involvement. A 3-ft

section of duct was inclined at a 20-degree angle with respect

to the horizontal. A Tirrell burner (3/8 in. tip), in which the

height of the flame with the burner vertical was adjusted to

5 in. with an inner cone 2 in. high, was mounted on a stand at

a 45-degree angle with respect to the horizontal. The stand was

placed so that the flame impinged at a 45~degree angle to the

side of the inside wall and at a point 4 in. from the duct mouth.

The point of application of the test flame, with the burner in

test position, was 3 in. from the tip of the burner stem,

measured along the axis of the burner stem. The flames followed

a helical path to the top of the duct.

Using a gas heating value of 1090 Btu/ft^ , the average heat

rate was 2905 Btu/hr . The gas flow to the burner was measured

with a wet gas meter. The results of the three tests, run in

the order shown, are given in Table 3, which includes calculated

average values of flame spread based upon the time necessary for

flames to extend the 36= in. length of duct.

- 11

Table 3

Test

Time __Flame

3- ft Flame Total Spread

Ma terial , Spread for Test1 Rate

» min; sec min; sec » in/min

Gas

Flow

Rate^

f t-Vmin

Heat

Rate

Btu/hr

j « a Sonoairduct , 5*10 ,

t » b Sal-Mo Duct , no spread , Board , ,

c Cardboard . 4; 00 ,

6 ;20 , 7 0*0452 2950

t 16; 05 no spread

y 0*0444 2910

5 = 55 9 0*0437 2860

The Sal-Mo Asbestos Duct Board exhibited no flaming tendency during the entire length of the test and only a 1 3/4- in.

by 3 1/2- in. white oval area surrounded by a blackened ring

(corresponding to the areas of flame impingement and dispersement) was affected* The white area resulted from the non-flaming

combustion of the black surface coating of the duct, whereas the

larger surrounding ring retained the coating which was further

blackened

o

The Sonoairduct section propagated flames at an average

rate of 7 in. /min, yielding flames at the upper end after

5 min 1.0 sec, while the cardboard duct spread flames at an

average rate of 9 ino/min, yielding flames at the upper end

after 4- min. The effect of the paraffin coating as indicated

by this test was to decrease the flame-spread rate, but only

slightly*

4* DISCUSSION AND CONCLUSIONS

The above tests give information on those qualities which

should be considered in developing criteria for self-heating,

ignition, flame spread and combustibility* Whereas the testing

procedures are by no means "standard , " a number of the results

obtained provide a basis for evaluation of substitute materials

for metal ducts*

12 -

The flame-spread tests illustrated the ease with which

a self-sustaining flame may be obtained, and the hazards

associated with the ignition of a combustible duct. The

conditions of test were not considered unlikely or impossible in general building construction, and the potential

hazards might suggest or allow even more severe testing

techniques. The tests gave convincing evidence that combustible ducts represent a distinct fire hazard.

The self-heating tests revealed that metals possess an

inherent superiority over .nonmetals with respect to the

localized heat transmitted through a duct with combustible

backing when the duct is subjected to a short-duration fire 0 The difference appeared to be due to metal buckling and

separation resulting from thermal expansion. There appeared

to be no difference between the Sal-Mo Asbestos Duct Board

containing 16.5 percent combustible and asbestos millboard

containing less than 3 percent combustible as evidenced by

these tests.

DUCT MATE-RIAL

A.B.C.D ARP

THERMOCOUPLE

LOCATIONS

12. IN.

(A.) CROSS -$E CTIOM OP 3*12 INCH DUCT

COVERED WITH WOOD PIBER80ARD

SHOWING COMBUSTIBLE TRASH PILE (SOLID)

AND GAS BURNER (DASHED LINES)

WHICH REPLACED IT.

TIME, MINUTE?

(B.) “STANDARD" GAS RATE HISTORY

(PIG I.) EXPERIMENTAL ARRANGEMENT EOR

SELP - WEATIN6 TEST
:
320

300

(PIG. 2. ) PPPPCT OP DUCT MATE- RIAL ON _ LOCALIZED MPAT TR AN9MITTE-D

TO PI8E-RBQARP BACKINGS

LEGEND: CURVE DUCT MATERIAL p-uel 280 1 . SAL-MO DUCT BOARD WOOD

Z GALVANIZED IRON n 3. SAL-MO DUCT BOARD GAS

A. ASBESTOS BOARD

Z60 5. ALUMINUM *

Q> GALVANIZED IRON M 7 aluminum witw black coat <3

8. BLACK IRON VI 240 a sal- mo plus Aluminum pqil M

IO 15" TIME- , MINUTE-S

GYPSUM

WALL

BOARD
.
Fig. 5 Concrete-embedded duct before test showing heating

chamber, fan and other details.

Fig.

6

Concrete

duct

casing

after

test.

A

few

charred

pieces

of

Fig.

7

Floor

and

plasterboard

wall

after

test,

duct

remain.

H

UJH205

CD

U.0y

10<r

OJ20h0

01

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<0I

THE NATIONAL BUREAU OF STANDARDS

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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

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