Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Technical Note (National Research Council of Canada. Division of Building
Research), 1967
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC :
https://nrc-publications.canada.ca/eng/view/object/?id=0debed9c-3429-45a7-9dfd-7af665c9c180 https://publications-cnrc.canada.ca/fra/voir/objet/?id=0debed9c-3429-45a7-9dfd-7af665c9c180
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.4224/40001184
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Heat transfer measurements in furnaces and fires
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
e.
TEClHIN ][CAlL
NOTE
No.
508PREPARED BY T. T. Lie CHECKED BY G. W. S. APPROVED BY
PREPARED FOR
Novem.ber 1967
Record Purposes
SUBJECT HEAT TRANSFER MEASUREMENTS IN FURNACES AND FIRES
The temperature attained by a structure in a fire or furnace is determined by the heat transfer to the structure. In general, in a fire heat is received from luminous flames,
which have a high emissivity. When the thickness of the
flames is sufficient, heat radiation to the structure is considerable.
In gas-fired furnaces, the flames a:re often not so
luminous and the thickness of the flames is such that the heat transfer to the structure is influenced by the radiation from
the furnace walls, which are cooler than the gases. One may
therefore expect the heat transfer in these furnaces to be
generally lower than that from fire. It can be shown, however,
that above a certain limit, the coefficient of heat transfer is
not important. This limit depends on the propertie s of a
material and is, for materials used in building, about 15
Btu/ sq ft hr F. To estimate the heat transfer in the DBR
furnaces a number of measurements were made in these furnaces.
HEAT TRANSFER IN FURNACES
Heat transfer to an object from gases and furnace walls may be divided into heat transfer by convection and
heat transfer by radiation. The quantity of heat received
per unit area, unit time, and unit temperature difference between object and surroundings, depends on many factors.
2
-The most important are: temperature, composition, and
velocity of the gases, the thickness of the layer of gases between furnace walls and objects, the proportion between surface area of the object and inner surface of the furnace, and the emis sivity of the furnace walls and object.
The exchange of heat between the gases in a furnace, the furnace walls, and an object, may be de scribed as
follows.
The gases are continuOUSly transferring heat to walls and object, so that both attain a temperature dependent on the quantity of heat supplied to them.
The better the walls are insulated and the lower their thermal
heat capacity, the higher their temperature will be. Thus
through radiation more heat will be transferred from the
walls to the object. Heat transfer may also be increased
by enlarging the volume of the gases transferring heat to walls and object, because a thicker layer of gases gives more
radia-tion. A higher heat transfer may also be obtained by increasing
the emissivity of the gases.
Data concerning heat transfer in furnace s can be
found in References I to 3. セイゥョォウ and Mawhinney (Ref. 3)
describe a furnace with very well insulated walls, a 5-ft gas layer (emissivity 0.21) between specimen and walls,
and a gas temperature of 22000F, in which the heat transfer
to a specimen with a surface area one half the inner· surface area of the furnace was divided as follows:
h .
=
ca 4 Btu/ s q it hr F convectlon h=
ca 18 Btu/sq ft hr F radiation gas h = ca 24 Btu/ sq ft hr F radiation wallsh is the coefficient of heat transfer; hconvection changes with the velocity and temperature of the gases and the size
of the specim.en. The changes are relatively small and,
for the furnace under consideration, negligible with respect to hradiation'
3
-For an electrical furnace which contains heating elements in the walls, heat transfer is decided mainly by radiation from
the walls. Because a radiating wall may be considered equivalent
to a thick layer of luminous flame of the same temperature as, the wall, the heat transfer of the electrical furnace is expected to be comparable with that of a fire.
THE INFLUENCE OF THE HEAT TRANSFER ON THE TEMPERATURE IN A SLAB
When a slab is heated on one side the temperature
starts to rise at that surface. From the surface, part of
the heat is conducted to the inside of the slab, and part is
stored in the outer layers. The higher the heat flux at the
surface, the higher the surface temperature. Above a
certain level the temperature of the surface will usually
be almost equal to the furnace temperature. In this case
the coefficient of heat transfer hardly affects the wall
temperature. According to Reference (4) one may conclude
that this level occurs at a coefficient of heat transfer of
about 15 Btu/ sq ft hr F for concrete, and is lower for materials
with a lower conductivity. In Figure 1 the influence of the
coefficient of heat transfer on the surface セ・ュー・イ。エオイ・ is
shown for semi-infinite walls of asbe stos and concrete.
The graphs have been derived, by calculation, using data from Reference (5), for heat transfer coefficients of 10 and 60,
respectively. The latter is a value measured during an
experimental fire (6).
Although for asbestos the differences with the furnace temperature are small, whether the heat transfer coefficient is 10 or 60, for concrete the difference is considerable when
it is heated with a heat transfer coefficient of 10. By comparing
the area under the curve in the manner described in Reference (7) it can be estimated that the temperatures in an idealized wall (constant thermal pro-perties, no influence of moisture) are about 25 per cent lower than in a wall heated with a heat transfer coefficient of 60.
EXPERIMENT
The heat transfer to an object in a furnace depends on
the dimensions of the object and its properties. It is also
セ function of the temperature of the furnace and the object's
position in it. The heat transfer in the furnace will
there-fore vary with the test structure properties, position, and with time.
..
4
-For comparison, measurements were made in the DBR floor and wall furnaces and in the small-scale electrical furnace
using steel cylinders 2.5 in. in diameter and
1.
8 in. high. Anytemperature rise in the cylinders, which contained a thermocouple in the centre, was taken as the crIterion for heat transfer to the
cylinders. The cylinders' were centred in the furnace 5 in.
below the cover.
Temperatures were also measured by means of the thermocouple s belonging to the equipment of the furnace s,
and with bare thermocouples placed about 4 in. from the
cylin-der. Furnace temperature s were controlled according to the
temperature-time course given ·in ASTM Method E119, using
the readings from the furnace thermocouples. Heat transfer
coefficients were calculated using readings from the bare thermocouples near the cylinder.
In calculating the heat transfer to the cylinder the following assumptions were made:
a) that temperature is the same throughout the
cylinder;
b) that thermal propertie s of the cylinder are constant.
Equalization of the heat transferred to the cylinder with increase of the heat content of the cylinder give s
2 2 h (2TT r 1
+.
2 TT r ) 6e
6 t = p C TT r 1 セe.
1 or h =. p c r 1 6ei
2(r 1+
r 2) 6e
tlt whereh
=
the coefficient of heat transferr
=
the radius of the cylinder1
=
length of the cylinderp
=
density of the cylinder (500 1b/ cu it)c = specific heat of the cylinder (0. 12 Btu/lb F)
"
5
-セエ
=
'considered time period (;0 hour)セ Si = temperature rise of the cylinder in of
The values of h for the three furnaces are plotted in Figure 2. CONCLUSIONS
The results indicate that the heat transfer of the
electrical furnace is much higher than that of gas -fired wall .
and floor furnaces. After 20 min. the heat transfer coefficient
of the wall furnace reached 15 Btu/ sq ft hr F, the level above which the value of the heat transfer coefficient is no longer of
importance for the temperatures of the structure. It is
probable, therefore, that the heat transfer in the wall furnace is sufficient to give nearly the same results as for furnaces
with Cl: higher heat transfer coefficient.
The heat transfer coefficient of the floor furnace
remains below this level at about 10 Btu/ sq ft hr F. It
is expected that for light materials the results will be the same as those for a furnace with a higher heat transfer coefficient, but for concrete the temperatures of the structure may be lower by about 20 to 30 per cent.
It is possible that a higher heat transfer coefficient could be obtained by covering the walls of the furnace with a good insulating material.
6
-REFERENCES
1.
Gilchrist, J. D.King's College, Oxford, 1963.
Furnace s. Department of Metallurgy,
Newcastle upon Tyne. Pergamon Press,
2. Thring, M. W. The science of flame s and furnace s.
Univ. Sheffield, Chapman & Hall Ltd., London, 1962.
3. Trinks, W. and M. H. Mawhinney. Industrial furnaces.
Carnegie Inst. Technol., John Wiley & Sons, Inc.,
New York, 1961.
4. Harmathy, T. Z. and J. A. C. Blanchard. Transient
temperatures in slabs heated or cooled on one side. Can. J. Chern. Eng., Vol. 41, No.3, June 1963,
p. 128.
5. Carlslaw, H. S. and J. C. Jaeger. Conduction of heat
in solids. Oxford Univ. Press, 1959, p. 70.
6. Kawagoe, K. An experimental fire in a room with a
large opening. Bull., Fire Prevention Soc., Japan,
Vol. 8, No.2, 1959, p. 36.
7. Lie, T. T. Bekledingsmaterialen en Bouwconstructies
bij Brand. Heron, Tech. Univ. and Inst. T. N. O. for
Building Materials and Building Constructions. Delft,
3000 _--....__--...---
- - - , . - - - , r - - - . , . . - - - j2500
Asbestos
Concrete
J
Conductivity:
1
Diffusivity:
J
Conductivity:
1
Diffusivity:
0.07 Btu/ft hr F
O.Olsqft/hr
0.7 Btulft hr F
0.02 sq ft/h r
2000
70
60
50
Concrete (h
=10
Btu/sq ft hr
f)Furnace Temp •
30
40
TIME, MIN
20
Asbestos (h
=VVセo[[セセセ[[[セセセZ[
Btu/sq ft hr F):::
Asbestos
(h
=10
Btu/sq ft hr F)
10
500
1000
.
LLJe::::
:::> セ1500
e::::
LLJ 0-セ LLJ I -u. oFIGURE
1
INFLUENCE OF THE HEAT TRANSFER COEFFICIENT ON THE
SURFACE TEMPERATURE OF WALLS
,
.
-40 ,..._ _
- y - -_ _- - - , . ....---_ _- - y -_ _- - - . ..,..._ _..._ ...o
o
-•
-Wall Furnace
o
II
Electrical Furnace
I
/
I
/
/
0 /
/
/
/
/
/
1
0/
/
01
o
0/1
0 //
//
.
// .
. ; '-.
/.--
..
jJ
0 /. 0 _ - _ •...
--.
J' "/ 0 _ - - - - 0 ,.,- / セ - - - 0o
lセセセMMMMMMMMo
Floor Furnace
- - - 0 0 • 0...0o .",
o
0 J::16
8
28
36
12
セ 2032
u..24
...
-:::J...
CD4
4 812
16
TI ME, MIN 2024
28
FIGURE 2HEAT TRANSFER COEFFICIENT IN DBR FURNACES AS FUNCTION