Design of fire test furnaces

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Design of fire test furnaces

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PRICE 25 CENTS DESIGN

OF

FIRE BY T. Z. HARMATHY REPRINTED FROM FIRE TECHNOLOGY V O L 5. NO. 2, MAY 1969 P. 140

-

150

RESEARCH PAPER NO. 411

O F T H E

DIVISION O F BUILDING RESEARCH

OTTAWA

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CALCUL DES F O U R S POUR

ESSAIS D'INCENDIE

SOMMAIRE

L1 auteur expose les donnees existantes sur lfapport de conlhustible ou la p u i s - s a n c e f o u r n i e aux f o u r s d ' e s s a i s o u ' a d'autres fours fonctionnant dans d e s con-

ditionsvariables. I1 montreque l a c o n -

s ommation df Cner gie 011 de conrbus tible d e s f o u r s d l e s s a i s depend fortement des

pr opri6 t 6 s thermiques d e 1' &chantillon essay&.

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FIRE TECHNOLOGY Vol. 5 No. 2

FT-

47

Design

of

Fire Test

Furnaces

T . 2. HARMATHY

Division of Building Research,

National Research Council (Canada)

Information concerning the power or fuel input into fire test fur-

naces or other furnaces operating under variable state conditions

has been developed. The power or fuel consumption of fire test furnaces depends considerably on the thermal properties of the test

specimen, as is indicated by data reported here.

I

NDUSTRTAL or laboratory furnaces and ovens are, as a rule, used for prolonged heating of materials a t some constant temperature level. As the length of time to bring the furnace or oven to hnperature is rarely critical, the design can be based on the calculated heat loss to the surroundings at maximum operating temperature under steady-state conditions.

Fire test furnaces, on the other 11and, by virtue of the temperature 'pro- gram they have to f o l l ~ w , ~ always operate under variable state conditions.

As vexy little information is available regarding the design of temperature-

programed furnaces, fire test furnacea are usually designed with a substantial margin of safety and consequently are expensive devices.

This paper discuwa in detail the power or fuel consumption of fire test

furnaces. Some formulas and conclusions may also prove useful to those concerned with the design of other temperature-programed devices, or with the analysis of the temperature history of building compartments

during actual fires. Those who are not familiar with fire tests and the op- eration of fire test furnaces may find it useful to consult comprehensive

publications concerning these ~ubjects.~. 2 '

I

H E A T B A L A N C E

The principle of conservation of energy is expressed (approximately) by equating the heat input into a system to the heat output. The system illustrated in Figure 1 (as the space surrounded by dashed line) is formed by the materials inside the chamber of a fire test furnace. For this system the heat balance is given by the following equation:

*

G F H P

+

G A H A

+

Q

=

G O H Q

+

Q C

+

h~

+

Q R

+

Q M

f

1)

*See tist of nomenclature on page 149.

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Furnaces 141

As the solid boundary surfaces of the system are formed partly by the

furnace itself (which, in turn, may also be built from several different

materials) and partly by the test specimen, the heat loss by conduction

can be divided into a number of terms:

fi

QC = Q c / +(lcs = qc/i + Q c ! s

- (2)

i l l

It is practical to regard the initial temperature of the system (in equi-

librium with the surroundings), T.,, as the datum temperature for the

sensible heat of all components of the system. TIlus

( H A ) -* = ( H B) = 0 (3)

Figure I . Heat balance for the furnace chamber of a fire h t furnace.

In addition to the sensible heat, the fuel also carries latent heat, i.e., the

heat of reaction. Consequently,

(HF)

= ( A H ) - d (4)

With Equations 3 and 4, Equation 1 becomes

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I

142 Fire TechnoIogy

where

H G

is referred to the T-, temperature level, and all terms represent

momentary values.

In the following discussions it will be assumed t h a t the entire system, including its boundary surfaces, is a t a uniform temperature that varies

according to the prescribed temperature program,

T

(t). In this way all

terms in Equation 5 are directly determined by, or calculable from, the

.temperature.program to be imposed on the furnace. It should be noted,

however, that this assumption may lead to significant errors when the

T (t) curve is very steep, the internal solid contents are of substantial

mass, and a significant portion of the solid boundary surfaces of the

furnace chamber is formed by a solid of high thermal conductivity.

Whenever such conditions prevail, the error should be estimated and taken

into account in the term q ~ .

I

C O N D U C T I V E H E A T L O S S

For an arbitrary T ( t ) curve, the conductive heat flow at some given

time, t, across the solid boundaries of a cylindrical furnace can be expressed

as follows:

where t =

AT

(7)

I

and T = AT (8)

Equation 6 is applicable as long as there is no noticeaMe temperature

rise at the outer surface of the materials surrounding the furnace chamber.

It has been shown4 that this condition is well satbfied as long as

This period is generally long enough to cover the most important part of the fire test.

For furnaces consisting mainly of plane surfaces, R--t m , Equation 6 reduces to where 1 =.

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- d j - z i - 0 7

and is uniquely determined by the temperature program, i.e., by the

T

(t)

function alone.

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d

Fire Tatf and is reproduced ?n Figure 2. The

-

T

(t) and +(t) func-

dt

tions corresponding t o this program are also shown in Figure 2.

Unless the furnace is definitely a cylindrical design, it is. permissible to

use the simpler expression given by Equation 10 and to take the d e c t of

corners, edges and curved surfaces into consideration by introducing a

correction

term,

t i c . Thus

Naturally, for surfaces whose center of curvature lies on the solid dde of

the surface,

E

is a negative number. As, in a ~ltrkt sense, is not constant

but (in absolute value) increases with time, its selected value must be

representative of the hime interval of interest.

Because of Equation 2, it is more accurate to introduce as many 5

factors as there are materials along the boundary surfaces of the furnace

chamber. Thus the total conductive heat flow can be expressed as follows:

-

i d 1

Since the specimen surface is, with rare exceptions, predominantly plane,

t, = 1 in most practical cases.

0 20 40 60 80 100 120

t min.

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144 Fire Technology T H E h r , q ~ , A N D q~ T E R M S

By assumption, the temperature of the furnace contents follows the temperature program imposed on the furnace. Their rate of heat absorption thus can be expressed as

and thus can be calculated from the temperature program (For fire tests, d

the

T

(t) function has been plotted in Figure 2.)

Heat l a s by radiation from the inside of the furnace chamber occum mainly through the obwrvation windows and through the flue opening; Although with the aid of the T ( t ) temperature program this heat loss can

be estimated, in practical problems it is generalIy permissible to combine

Q R with Q M and express them as a certain percentage of q c f .

The term q~ represents, primarily, the enthalpy loss from air currents entering the furnace through various holes (e.g., the holes of the electric heating element.3) and cracks and from an inadequate seal between the furnace and specimen. One may also use this

term

to correct for some errors introduced by the assumption of uniform temperature throughout the systern.

Accordingly

where q may be of considerable magnitude for smaller furnaces.

E L E C T R I C A L P O W E R R E Q U I R E M E N T

When the furnace is electrically heated, G p =

GA

= G O = 0. For this

case, with Equations 13, 14 and 15, Equation 5 becomes

In this equation the material properties should be selected to be representative of the temperature interval of interest.

To test the usefulness of Equation 16, a number 'of heat input studies

were performed in the Division of Building Research laboratory, using an electrical fire test flll"nace.3 The heating chamber of this furnace is formed

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the test specimen). The furnace is heated by twelve silicone carbide heat-

ing elements. There

is

a hollow Inconel plate inside the furnace chamber, and in one of the tests this was filled with crushed silicone carbide. The.

test schedule

b

given in Table 1. Information concerning the furnace and specimen geometry zind the properties of the rnateriak is listed in Table 2.

TABLE 1. Schedule of Fire Tests Performed in an Electric Furnace

Test No. Specimen Inconel plate

la brick wall, 6 in. thick

l b brick wall, 2% in. thick empty empty .

2 lightweight concrete wall, 6 in. thick empty

3a asbestos board, 36 in. thick empty

3b asbest- board, 44 in. thick a dwith

silicone carbide

Uaing this information and Equation 16, Q (t) curves have been calculated for the three different specimens. These curves are compared with

the experimental ones in Figures 3, 4, and 5. Although according to

Equation 9 the effect of the thickness of a 2x411. brick wall on its heat

absorption may begin to show up after 7 min, in practice this effect was

hardly noticeable up to the end of the test, i.e., up to 40 rnin.

As

the curves of Figure 5 indicate, the power input required to follow

the standard Fire Test temperature program is sensitive to the presence of objects

in

the furnace only during the first 10 min of fast m p e r a t u r e

rise.

ZOO

I I I I I I I I 1 I I I

-

6 In. wrll, oxperlmtntll

---

2 4 In. wall, rxperlmental

150

-

-a-• C ~ l c u l s t t d L t

-

= C 100

-

0 50 - - I I I I I 1 I I 1 I 1 I 0 20 40 60 80 I00 120 t mln.

i urn 3. Experimntal and c d c w heat input curves for an electric furnace with brick

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146 Fire Technology

T.48~13 2. Mi&uneous I n f o r d b n Concerning the Fwme and Test Specimens

Furace fire clay brick inaulatlmz firebrick -

Inconel - 37.0 - - -

-

0.114 -

silicone carbide heating bars - 17.5 - - - - 0.293 -

silicone carb~de fillina - 14.7 - - - _0.293 - Specimen brick lightweight concrete aabestoa board 200 I I I I I 1 I I 1 I 1 I

-

~ x p e r l m t n t r l

-.--

C a \ t u \ l l * d 150 - -

..

C

-

s C ,' 100

-

- .. u .r. -.---.-.-a- 50

-

I I I I I i 1 I I I I 1 0 20 40 60 80 100 120 t mln.

Figure 4. Experimental and colculaled heat input curves jor an electric ficrnace with a

6-in. lightweight concrete WQU specimen.

R A T E O F F U E L I N P U T

When the furnace is heated with fuel, Q = 0 and the rate of fuel input

can be expressed from Equation 5 after making substitutions from Equa- tions 13, 14, and 15 as follows:

There the numerator is. identical with Q for electrical heating

(see

Equa- tion 16). The G a / G p ratio is determined by the fuel and the exceas air

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I Furnaces 147 200 1 I I I 1 I 150

-

L E = . d

-

100

-

0

- Ewperlmentsl, plate t i l l e d with SIC

---

Experimental. plats wlthout S i c

50 -

.-.-.

Calculated, plate f i l l e d w i t h S i c

-

--

Calculated, plate without S i c

1

i i i 1 ₁ _I _t _I _{I .} 1 i 0 5 10 15 20 25 30 f t min. i

! Figure _%-in. 5. Experimental a d eahlctted heat input cwves for an eleckie furnace with a

asbestos board specimen.

I

As an example, the rate of fuel input into the previously examined furnace will be cdculabd. It is assumed the fuel is propane burned with

i

50 per cent excess air. The combustion reaction is as follows:

The heat of this reaction is at

T,,

68"

F

(AmG8 = 19,930 BfXZJb. By substituting the molecular w e i g h of the vaxiow compounds in this equation one finds that Ga/GF = 24.429 and the composition of the flue gas

by weight is 12.26 per cent carbon dioxide, 6.69 per cent water vapor, 7.43 per cent oxygen, and 73.62 per cent nitrogen.

To facilitate the calculation of the enthalpy of flue gases, the variation of the enthalpy of the most common combustion products with tempera-

ture (referred to 68'

F

datum temperature) has been plotted in Figure 6,

based on information presented by Perry.5 The calculated enthalpy of the flue gas of the example is also shown (by a dashed line) in Figure 6.

With the aid of this information, of the previously calculated

Q

(t)

curves (which represent the numerator of the expression in Equation 17,

and of the temperature program given in Figure 2, the

G F

(t) curves have been determined. They are plotted in Figure 7.

It may be of interest to observe that the fuel requirement depends con- sidhrably on the material of the test specimen, even though the s h c e of

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Fire

1000 10000

900

----

Flue gas of example 9000

800 6000 100 7000 . 600 6000 f! \ . a

-

& 500 5000 I 406 4000 300 3000 200 2000

This scale for Hz only

100 1000

0

0 500 1000 1500 2000

T O F .

Figure. 6. Enthulpy of various combustion pmducts.

t mln.

Figure 7. Fuel input curues for a hypothetical furnace: (a) brkk wall specimen; ( b ) cun-

crete block wall specimen; (c) asbestos board specimen, extm scl~one carbide in furnace:

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j

N O M E N C L A T U R E

c = specific heat a t constant pressure, Btu/lb F ;

G = mass flow rate, l b / h i

F = surface area, sq f t

it = rate of change of enthalpy of internal objects, Btu/hr

H

= specific enthalpy, Btu/lb

AH

= heat of reaction, Btu/lb

i

=0,1,2,3

,...

j = t/A7

k

= thermal conductivity, Btu/hr f t

"

F 1 = thickness of slab, ft

M

= number of kinds of internal objects

N

-

number of kinds of materiak forming the surface of the furnace

q = heat flow out of furnace, Btu/hr

ij = uncorrected heat flow, Btu/hr

Q

-

electric power, Btu/hr

R

= inner radius of cylindrical furnace, ft t = time, hr T = temperature, O

F

W = weight, lbs q = factor, dimemionless K = thermal diffusivity, sq ft/hr [ = factor, dirnensionlw p = density, Ib/cu ft

T = dummy time variable, hr

$ = function defined by Equation 11,

"

F/hrJ

S U B S C R I P T S

A = of air

c = by conduction through the boundary surface

1 = of or for the furnace, of or for a furnace material

= of the fuel = of the flue gas

; = of or for the i-th term or material

M = miscellaneous

-,

= fort <O = a t t = O = by radiation

,

= of or for the specimen or specimen material

T = a t t = ~

R E F E R E N C E S

"Shndard Methoda of Fire Tests of Building Construction and Materials,"

NFPA No. 251 (National Fire Protection Association, 1969).

2Shorter. G . W. and Harmath , T. Z., "Fire Research Furnaces at the National Research Council," Fue Study

d.

1, NRC 5732, 1960. Division of Building Re- search, National Research Council, Ottawa.

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150 Fire Technology a Blanchard, J. A. C. and Harmathy, T. Z., " S d - a c d e Fire Test Facilities of the National Research Council," Fire Study No. 14, NRC 8207, 1964, Division of Building

Research, National Research Council, Ottawa.

4 Harmathy T. Z. and Blanchard, J. A. C., "Transient Temperatures in Slabs Heated or ~ o o i e d on One Side," Canadian Journal of Chemical Engineering, Vol. 41, No. 128 (1963).

6 Perry, J. H., Chemical Engineers' Handbook, 4th ed., (McGraw-Hill, New York,

1963). Section 3, p. 116.

NOTE: The derivation of Equation 6 is available from the author.

ACKNOWLEDGEMENT: The experiments were conducted b Mr. E. 0. Porteous.

This paper is a contribution from the Division of Building &wearch, National Re-

search Council of Canada, and is published with the approval of the Director of the