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Heat transfer measurements in furnaces and fires

(2)

DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

e.

TEClHIN ][CAlL

NOTE

No.

508

PREPARED 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.

(3)

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 walls

h 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'

(4)

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.

(5)

..

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. Any

temperature 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 ) 6

e

6 t = p C TT r 1 セ

e.

1 or h =. p c r 1 6

ei

2(r 1

+

r 2) 6

e

tlt where

h

=

the coefficient of heat transfer

r

=

the radius of the cylinder

1

=

length of the cylinder

p

=

density of the cylinder (500 1b/ cu it)

c = specific heat of the cylinder (0. 12 Btu/lb F)

(6)

"

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.

(7)

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,

(8)

3000 _--....__--...---

- - - , . - - - , r - - - . , . . - - - j

2500

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

.

LLJ

e::::

:::> セ

1500

e::::

LLJ 0-セ LLJ I -u. o

FIGURE

1

INFLUENCE OF THE HEAT TRANSFER COEFFICIENT ON THE

SURFACE TEMPERATURE OF WALLS

(9)

,

.

-40 ,..._ _

- y - -_ _- - - , . ....---_ _- - y -_ _- - - . ..,..._ _..._ ...

o

o

-•

-Wall Furnace

o

I

I

Electrical Furnace

I

/

I

/

/

0 /

/

/

/

/

/

1

0

/

/

0

1

o

0

/1

0 /

/

/

/

.

/

/ .

. ; '

-.

/

.--

..

jJ

0 /. 0 _ - _ •

...

--.

J' "/ 0 _ - - - - 0 ,.,- / セ - - - 0

o

lセセセMMMMMMMMo

Floor Furnace

- - - 0 0 • 0...0

o .",

o

0 J::

16

8

28

36

12

セ 20

32

u..

24

...

-:::J

...

CD

4

4 8

12

16

TI ME, MIN 20

24

28

FIGURE 2

HEAT TRANSFER COEFFICIENT IN DBR FURNACES AS FUNCTION

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