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Commensurability problems in fire endurance testing
NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH
COMMENSURABILITY PROBLEMS IN F I R E ENDURANCE TESTING T. Z
.
H a r m a t h y F i r e Study No. 31 of the Division of Building R e s e a r c h Ottawa November 1973COMMENSURABILITY PROBLEMS IN FIRE ENDURANCE TESTING
by
T. Z. Harmathy
ABSTRACT
A simple method i s described by which c h a r a c t e r i s t i c s of the performance of f i r e t e s t furnaces can be determined m o r e con- veniently and accurately than with methods so f a r employed. The commensurability of the r e s u l t s of f i r e t e s t s obtained by furnaces of different design i s discussed and a possible solution t o putting the f i r e t e s t procedure on a m o r e r e a l i s t i c basis i s described.
COMMENSURABILITY PROBLEMS IN FIRE ENDURANCE TESTING
T. Z. H a r m a t h y
In r e c e n t y e a r s t h e r e h a s been considerable i n t e r e s t i n the heat t r a n s f e r c h a r a c t e r i s t i c s of f i r e t e s t f u r n a c e s . In E u r o p e t h i s i n t e r e s t h a s sprung f r o m the problem of c o m m e n s u r a b i l i t y of f i r e t e s t r e s u l t s obtained by v a r i o u s testing l a b o r a t o r i e s . In North A m e r i c a the need t o analyse the t h e r m a l p e r f o r m a n c e of t e s t f u r n a c e s h a s a r i s e n i n connection with suggestions concerning
t h e r e v i s i o n of s o m e e s s e n t i a l f e a t u r e s of the s t a n d a r d f i r e endurance t e s t method, ASTM
E 1 19.
P r e v i o u s Work
Two unpublished r e p o r t s w e r e r e c e n t l y d i s c u s s e d a t a
m e e t i n g of C o m m i s s i o n W. 14 of CIB in Stockholm, Sweden, (1 5-1 9 M a y 1972) with r e s p e c t t o the evaluation of heat t r a n s f e r in f i r e t e s t f u r n a c e s . Although the applied e x p e r i m e n t a l techniques w e r e different, the method of evaluation w a s based i n both r e p o r t s on the s a m e philo-
sophy: the heat flux into the boundary e l e m e n t s of the f u r n a c e was s e l e c t e d t o quantify the p e r f o r m a n c e of the furnace.
T h e r e a r e considerable difficulties i n performing heat f l u x m e a s u r e m e n t s . Any heat flux m e t e r , the t e m p e r a t u r e and m a t e r i a l
c h a r a c t e r i s t i c s of which do not m a t c h c l o s e l y those of the surrounding r e g i o n s , will a t t r a c t heat fluxes d i f f e r e n t f r o m those passing through the surroundings, and t h u s the m e a s u r e d heat flux values become meaningle s s
.
The technique of determining the heat flux by m e a s u r i n g sub - s u r f a c e t e m p e r a t u r e s i n the t e s t specimen and f u r n a c e w a l l s a l s o p r e s e n t s s e v e r a l problems. T h e s e a r e a s s o c i a t e d with the variability of the t h e r m a l p r o p e r t i e s of specimen and furnace m a t e r i a l s and with the f a c t t h a t the p r o c e s s of heat conduction i s a m a r k e d l y non-steady-state p r o c e s s f o r the e n t i r e duration of the f i r e
t e s t . E v e n if the heat fluxes could be evaluated f a i r l y a c c u r a t e l y f r o m s u c h m e a s u r e m e n t s , the values would r e f l e c t conditions a t
only a few i s o l a t e d r e g i o n s along the boundary s u r f a c e s . It i s well known, however, t h a t the heat flux is subject t o substantial l o c a l variation, e s p e c i a l l y during e a r l i e r s t a g e s of the f u r n a c e operation when the convective contribution t o heat t r a n s f e r is s t i l l significant.
The p r e s e n t p a p e r will d e s c r i b e a simple method by which the o v e r - a l l p e r f o r m a n c e of f i r e t e s t f u r n a c e s can be d e t e r m i n e d m o r e a c c u r a t e l y than by m e t h o d s used i n e a r l i e r studies. A s
introduction, i t s e e m s d e s i r a b l e t o m a k e a few r e m a r k s concerning the p r o b l e m of commensurability of f i r e t e s t r e s u l t s , the underlying i s s u e in a l l studies of f u r n a c e p e r f o r m a n c e .
P r o b l e m of C o m m e n s u r a b i l i t v of R e s u l t s
In g e n e r a l , one c a n divide the v a r i a b l e s t h a t d e t e r m i n e the
p e r f o r m a n c e of f i r e t e s t f u r n a c e s into t h r e e groups: heating c h a r a c t e r - i s t i c s , p r o g r a m c h a r a c t e r i s t i c s , and f u r n a c e c h a m b e r c h a r a c t e r i s t i c s .
The heating c h a r a c t e r i s t i c s a r e r e l a t e d t o such a s p e c t s of f u r n a c e operation a s f u e l used, type of b u r n e r and r a t i o of a i r to fuel input. T h e s e a r e the f a c t o r s that p r i m a r i l y d e t e r m i n e the luminosity of the f l a m e and the d e g r e e of turbulence i n the f u r n a c e , in other w o r d s , the c h a r a c t e r - i s t i c l e v e l of radiant and convective heat t r a n s f e r f o r a given f u r n a c e design.
The p r o g r a m c h a r a c t e r i s t i c s include p a r a m e t e r s that d e - s c r i b e the shape of the t e m p e r a t u r e v e r s u s t i m e o r heat input v e r s u s t i m e curves. Finally, a l l variables concerning the geo- m e t r y of the furnace chamber (the furnace p r o p e r and the t e s t
specimen) and the p r o p e r t i e s of a l l m a t e r i a l s forming the bound- a r i e s of the chamber a r e included i n the group of furnace chamber c h a r a c t e r i s t i c s .
In a standard ASTM E l 1 9 t e s t the average r a t e of heat penetration into various sections of the furnace chamber depends on s o m a n y v a r i a b l e s that it i s a l m o s t impossible to find a
sufficiently a c c u r a t e mathematical e x p r e s s i o n f o r it. The following e x p r e s s i o n m a y prove useful, however, a s a b a s i s f o r discussing the problem, at l e a s t in a qualitative sense. 1, 2 , 3
where
,
C
= f (heating c h a r a c t e r i s t i c s , t i m e ) ( 3 )5
=
f ( p r o g r a m c h a r a c t e r i s t i c s ) (4) As long a s the standard t e m p e r a t u r e -time curve i s followed,5
i s a s s u m e d t o be the s a m e f o r a l l furnaces.At e a r l y stages of the f i r e t e s t q and q a r e determined C f C s
p r i m a r i l y by the value of
5 .
As soon, however, a s radiation becomes1. S e e Norr~cnclature f o r symbols.
2. Equations ( 1 ) and ( 2 ) a r e implichd I>y Equation (10) and liofercncc. (1). 3 . In Equation ( 2 ) c s m e a n s the apparent specific heat of the specimen.
If endothermic reaction develops in the surface l a y e r of the s p e c i - m e n , the apparent specific heat m a y be very high. With exothermic
the predominant heat t r a n s f e r m e c h a n i s m and the o v e r -all heat
2 4
t r a n s f e r coefficient e x c e e d s about 1 5 ~ t u / f t h r O F ( 2 ) the r a t e
of h e a t penetration b e c o m e s m a i n l y dependent on the p r o p e r t y
g r o u p s , kpc, f o r the f u r n a c e and specimen. Consequently, f o r f i r e t e s t s of r e l a t i v e l y s h o r t duration, s a y l e s s than 1 -1/2 h r , the
t e s t r e s u l t s a r e v e r y sensitive t o the heating c h a r a c t e r i s t i c s , 5 e s p e c i a l l y f o r s p e c i m e n s with high values of the product kpc.
Satisfactory commensurability of f i r e t e s t r e s u l t s is not achievable without adequate standardization of f i r e t e s t facilities. The s h o r t e r the f i r e endurance of a specimen, the p o o r e r i s the commensurability.
Heat Balance f o r F i r e T e s t F u r n a c e s
Before introducing the recommended method of m e a s u r i n g the t h e r m a l p e r f o r m a n c e of f i r e t e s t f u r n a c e s , the h e a t input -output
r e l a t i o n s f o r a f u r n a c e c h a m b e r ( m o r e exactly, the space surrounded by dashed line i n F i g u r e 1) will be briefly examined. A s h a s been d i s c u s s e d i n Reference ( I ) , the heat balance can be w r i t t e n in approxi- m a t e l y the following f o r m :
w h e r e q r e p r e s e n t s radiant h e a t l o s s e s (mainly through the observation
R
windows) and q r e p r e s e n t s m i s c e l l a n e o u s l o s s e s a s s o c i a t e d with the M
p r e s e n c e of holes and c r a c k s and with inadequate s e a l between the f u r n a c e and the specimen.
4. F o r f u r n a c e s heated by liquid fuel t h i s condition i s n o r m a l l y achieved within the f i r s t 20 m i n of f u r n a c e operation owing t o the high liminosity
of the f l a m e s . In g a s - f i r e d f u r n a c e s the heat t r a n s f e r coefficient r i s e s m o r e slowly, the r a t e of r i s e depending t o a l a r g e extent on the f u r n a c e
design, e s p e c i a l l y the location of the b u r n e r s .
5. F o r f u r t h e r d i s c u s s i o n s of the possible effects of v a r i o u s f u r n a c e c h a r a c t e r i s t i c s on the r e s u l t s of f i r e t e s t s s e e R e f e r e n c e s ( 3 ) and ( 4 ) .
ln f u r t h e r discussions i t will be assumed that appropriate
steps will be taken t o eliminate radiant and
miscellaneous^'
10s se sp r i o r t o the furnace performance test. It will a l s o be assumed that no e l e c t r i c heating will be applied. Consequently, the t e r m s
Q,
q, and q can be left out of Equation (5).M
It i s practical t o r e g a r d the initial temperature of the system T (in equilibrium with the surroundings) a s the datum temperature
0
f o r the sensible heat of a l l components of the system. Thus
In addition t o sensible heat, the fuel c a r r i e s chemical energy, i.e. the net heat of combustion reaction. Consequently,
Equation (5) can be r e -written m o r e simply: 6
where
GG
=
GF
+
G A
(9)
The total heat flow into the boundary surfaces of the furnace can thus be expressed as:
This i s the basic equation that will be used in the method now described f o r determining the performance of f i r e t e s t furnaces.
6 . In a l l following discussions it i s tacitly assumed that the furnace chamber was c o r r e c t l y designed, in other words that i t s s i z e s a r e sufficiently large t o ensure complete combustion of the fuel within the furnace chamber. If this condition i s not fulfilled, some por- tion of the chemical energy of the fuel will a l s o appear among the output t e r m s .
Experimental and Calculation Procedure
To determine q by Equation (10) one m u s t know the values of
C
(AH)o, GF, GA and H The evaluation procedure should normally
G'
s t a r t with an examination of the composition of the fuel and the com- bustion p r o c e s s and with the estimation of the net heat of combustion. The experimental techniques and calculation methods to be followed have been described in many engineering handbooks (for example, in Reference (5)) and a r e , no doubt, well known to chemical
engineers. F o r simplicity, propane has been selected for the illustration of the calculations involved. The combustion reaction f o r propane i s a s follows:
The heat of combustion (at room t e m p e r a t u r e ) can be calculated with the aid of tabulated values of the heats of formation f o r the compounds involved in the reaction.
F o r propane combustion these a r e a s follows (5): f o r C H
The heat of combustion i s now obtained a s the difference between the heats of formation of products and reactants:
(AH)o
=
[ 4 x 104,036+
3(1-
m ) x 169,294+
3m x 47,549-
44,676]/44.09=
19,944-
8248m where 44.09 is the molecular weight of propane.m can be evaluated f r o m the composition of the combustion products; g a s chromatography is a convenient method. It i s believed that samples collected f r o m the flue every 5 m i n can give satisfactory insight into the p r o c e s s of combustion.
The expression for m is:
Y c 0
The measurement of G the flow r a t e of fuel, does not present
F 8
any practical problems. F o r gaseous fuels the u s e of a m a s s flow m e t e r i s recommended, s e v e r a l types of which a r e now available f r o m
commercial sources.
If premix burners a r e used, the flow r a t e of a i r can also be measured on the input side. If, on the other hand, the burners a r e injector type, information on G will not be available directly. It
A
can be evaluated f a i r l y accurately, however, f r o m the composition of the combustion products. F o r propane heating, for example, the evaluation procedure is a s follows:
According to Equation ( l l ) , the combustion of one mole of p r o - pane produce s
N
=
25.8+
23.8n+
1.5m (14) m o l e s of combustion products. Equation (1 1) a l s o shows that themoles of the three oxides, H 0, CO 2 and C 0, always add up t o 2
seven, r e g a r d l e s s of the values of n and m. The total number of m o l e s of products formed by the combustion of one mole of propane can thus be calculated from the r e s u l t s of the flue g a s analysis as:
7
By combining Equations (1 3), (14) and (1 5), the excess a i r factor can be expressed a s
and the flow r a t e of a i r a s
where 32 and 28 a r e the molecular weights of 0 and N respectively. 2 2 a
It i s possible to calculate the flow r a t e of a i r using this procedure at any d e s i r e d accuracy(by m e a n s of the reaction scheme, Equation ( 1 1) ) since the accuracy of the combusion g a s analysis can easily be
checked and corrected, if necessary.
An additional check of the calculated a i r flow values can be secured by measuring the velocity of the combustion products in the flue opening (with the aid of an Inconel Pitot-tube o r commercially available flow probes) and the temperature of the g a s e s at the s a m e place, TG# using thermocouples sufficiently shaded t o minimize
radiant heat exchange with the surroundings. Making u s e , again, of the information concerning the composition of the combustion products, G can be evaluated.
The l a s t piece of information t o be obtained f o r solving
Equation (10) i s the value of H the enthalpy of the combustion
GI
products a t the flue entrance. T h i s c a n be calculated by utilizing the known value of the g a s t e m p e r a t u r e (T ) a t the flue e n t r a n c e
G
and by using the information given in F i g u r e 2, together with t h a t
7
concerning the composition of g a s e s . F i g u r e 2 indicates, however,
t h a t the enthalpy of the combustion g a s e s i s expected t o depend v e r y little on the actual composition.
The information developed f r o m t h e s e investigations r e
-
l a t e s t o the r a t e of t o t a l heat absorption,
qc'
by the f u r n a c echamber. Of p r i m a r y i n t e r e s t t o testing l a b o r a t o r i e s i s the r a t e of h e a t penetration into the t e s t specimen. T h i s c a n be d e r i v e d by combining Equations ( I ) , (2), and (10):
-
- s v s ' S S
qcs A k p c
+
~ , / k - - p T
q cd f f f S s s s
Again, i t is obvious that Equation (1 8) i s only of approximate validity.
Use of Information Developed
The information s o f a r developed can be utilized i n a number of different ways, the m o s t important probably being t o c o m p a r e the "fire severity" c h a r a c t e r i s t i c s of various t e s t f u r n a c e s . Such a c o m - p a r i s o n could be m a d e i f e a c h f i r e t e s t l a b o r a t o r y w e r e t o a g r e e t o conduct a t e s t on a conveniently selected t e s t specimen, f o r example a n 8-in. thick s l a b of clay b r i c k , reinforced if n e c e s s a r y . By plotting the values of q /A obtained by the v a r i o u s l a b o r a t o r i e s against t i m e ,
C s s
the obvious w e a k n e s s e s of s o m e individual t e s t f u r n a c e s could be detected.
7. Before using F i g u r e 2, the concentration of the v a r i o u s components of the combustion g a s e s should be e x p r e s s e d in weight fraction.
If i t i s deemed n e c e s s a r y t o e x p r e s s the r a t e of heat flow into the t e s t specimen in t e r m s of heat t r a n s f e r coefficient, i t can be done by determining the surface t e m p e r a t u r e on the exposed
-
side of the specimen, T
,
preferably f r o m m e a s u r e m e n t s performed sa t s e v e r a l points on the surface. Figure 3 i l l u s t r a t e s a possible way 8
of fastening the thermocouples. Using the "average furnace temperature1' Tf, m e a s u r e d a s defined by ASTM E119, a s the nominal temperature of the heat sources in the furnace chamber, the heat t r a n s f e r coefficient may be obtained by the familiar e x p r e s -
The wisdom of characterizing f i r e t e s t furnaces in t e r m s of heat t r a n s f e r coefficients rather than heat flux and, even f u r t h e r , of trying t o divide the heat t r a n s f e r coefficient into radiant and
convective contributions i s , however, v e r y much in doubt. Accurate information on the average surface temperature of the specimen i s very difficult to procure. There a r e usually substantial local variations in the temperature of the gaseous mixture in the furnace chamber, so that selecting the "average furnace temperature" a s the reference temperature f o r the mixture i s not accurate. The
8. Using radiation pyrometry f o r determining the surface temperature of the specimen m a y prove r a t h e r deceptive. The e r r o r s associated with the unknown o r variable value of the surface emissivity, with
reflected radiation originating from the r e f r a c t o r y surfaces, with direct radiation f r o m the combustion products (even i f they appear transparent t o the eye), and with a number of other factors may be quite substantial.
mechanism of heat t r a n s f e r i s extremely complex and separating the heat t r a n s f e r coefficient into radiant and convective contributions m u s t always be somewhat a r b i t r a r y . 9
It should be emphasized that the magnitude of the coefficient of heat t r a n s f e r t o the specimen i s not a permanent c h a r a c t e r i s t i c of a f i r e t e s t furnace. As implied by Equations ( I ) , ( 2 ) , (18) and (19), i t m a y depend substantially on the individual specimen tested (in addition t o the program characteristics).
New Concepts i n F i r e Testing
The information developed on compartment f i r e s over the past two decades i n Japan, the United States, Britain, Sweden and Canada clearly indicates that the f i r e load concept [described, f o r example, in Ref.
( 6 )
1
on which various f i r e protection m e a s u r e s have been based over the past forty y e a r s i s an oversimplified one.Although the newer, m o r e r e a l i s t i c concepts will affect the future of f i r e protection mainly on a regulatory level, they will no doubt have some influence on the philosophy of f i r e testing a s well. The need t o revise this philosophy has clearly been admitted by the appointment of a new t a s k group of Committee E5 (ASTM) with the broad and open assignment of studying alternatives t o the temperature-time curve of the f i r e t e s t standard, E l 19.
Although the r e s u l t s of hundreds of compartment burning t e s t s have proved beyond doubt that the present temperature-time curve of
9.
In general, the radiant t r a n s f e r mechanism i s predominant after a few minutes' operation of the furnace. It has been found ( 2 ) that in computer simulations of f i r e t e s t s it i s possible t o regard the standard f i r e exposure a s equivalent t o radiant heat t r a n s m i s s i o n t o the speci- m e n f r o m a black body whose temperature follows the prescribed temperature v e r s u s time curve of ASTM E l 19. F o r poorly designed f u r n a c e s this assumption m a y be g r o s s l y inaccurate.E l 1 9 does not represent realistic conditions, it i s not possible to adjust the testing procedure to simulate actual f i r e s m o r e r e a l i s t i - cally unless the basic concept of standardization, namely the require-
ment of commensurability of t e s t results, is completely waived. The
best the appointed task group can do i s find an acceptable compromise between the aspects of accurate simulation of building f i r e s and the principle of commensurability of t e s t results.
It i s known that during the period when compartment f i r e s a r e fully developed, the r a t e of burning, o r r a t e of heat input, i s m o r e o r l e s s constant. Consequently, a f i r e t e s t procedure based on constant rate heat input into the t e s t furnace would represent a m o r e accurate simulation of compartment f i r e s than the present procedure, which i s based on a prescribed temperature program. The main problem i s t o decide what t h i s constant r a t e heat input should be. F r o m previous discussions it i s already c l e a r that i t
may be quite different for furnaces of greatly different characteristics. A m o r e o r l e s s equitable solution might be found i f , following
I t
a somewhat modified version of Odeents suggestion (7), a l l testing laboratories could agree t o conduct a few calibration t e s t s on a selected specimen (possibly, again, an 8-in. brick slab with some reinforcement). One of the m o r e research-oriented laboratories could lay down the groundwork by determining the constant heat input requirement (for i t s particular furnace) that would yield
approximately the same f i r e endurance f o r the selected specimen a s would the standard temperature -time curve. l o This laboratory would also advise the other laboratories regarding the fuel input
10. It would seem practical t o select the new t e s t conditions in
such a way that for "average" types of building elements the same r e s u l t s would be obtained a s for present conditions.
required to produce approximately the same r e s u l t s in their respective furnaces. Some of the laboratories might have t o conduct two o r
possibly three t e s t s until the fuel input requirement could be accurately determined. It i s suggested that the constant fuel input s o determined (together with other t e s t conditions such a s setting of burners, a i r line valves, dampers) should thereafter by employed in a l l other f i r e tests.
The obvious advantage of this t e s t procedure would be to put constructions employing lining m a t e r i a l s that have good insulat- ing qualities o r absorb heat in decomposition reactions into a m o r e advantageous position, while penalizing those lined with m a t e r i a l s that undergo exothermic reactions. Thus t e s t r e s u l t s would reflect m o r e closely the m e r i t s and d e m e r i t s of various constructions in
actual building fires. The question would still remain, however, whether the r e s u l t s obtained by various laboratories on various
constructions would be commensurable. Unfortunately, the answer i s not very encouraging.
By applying Equation (18) to the rate of heat absorption in both the calibration t e s t and another c a r r i e d out by identical rate of fuel input, the following approximate equation i s obtained:
/ ks s s ~ c
+
A s / ks s ~s cThis equation indicates that f o r a given specimen (given k
,
p c , c ) thes S
facility a s the A A ratio and the thermal properties of the furnace
B
flining. kt, pf, cf. At the usual range of these variables, however, the differences
in
g c s resulting from use of different furnaces a r e not expected to amount to more than f 10 per cent. This finding
clearly underline s the need for a higher degree of standardization of the operating and design characteristics of fire t e s t facilities before
any
meaningful progress can be made in the standardization of test procedure.Nomenclature
A surface area, ft 2
c apparent specific heat, Btu/lb " F G m a s s flow rate, lb/hr
h heat transfer coefficient, Btu/hr ft2 " F H enthalpy, ~ t u / l b
AH net heat of combustion reaction, ~ t u / l b k thermal conductivity, Btu/hr ft
"
F
m factor characterizing the completeness of combustion, dimensionless
exce s s a i r factor, dimensionless
number of moles of combustion products produced by the combustion of one mole of fuel, moles/mole
heat flow, Btu/hr electric power, Btu/hr temperature, " F
surface temperature, " F
mole fraction o r volume fraction, dimensionless factor, defined by Equation (3), dimensionless
: 1/2 factor, defined by Equation (4), " ~ / h r
density, lb/ft 3
Subscripts A of a i r
C by conduction through the boundary surface f of o r for the furnace, of furnace material
F of the fuel
G of the gaseous combustion products
M miscellaneous
Subscripts (conttd)
R by radiation
s of o r f o r the specimen, of specimen m a t e r i a l Super s c r i p t
*
i n the t e s t p e r f o r m e d on the calibration specimen R e f e r e n c e s1. Harmathy, T. 2. Design of F i r e T e s t F u r n a c e s , F i r e Technology, 5, NO. 2, 1969, p. 140-150,
-
2. Harmathy, T. Z. T h e r m a l P e r f o r m a n c e of Concrete M a s o n r y Walls i n F i r e . A m e r . Soc. Test. Mats. NO. 464, 1970, pa 209.
3. Seigel, L.G. Effect of F u r n a c e Design on F i r e Endurance T e s t Results. Arner. Soc. Test. Mats., No.
464,
1970, p. 57.4. Lie, T.T. F i r e and Buildings. Appl. Sci. Publ., England, 1972, p. 42.
5. P e r r y , J.H. (Ed). Chemical Engineers1 Handbook. Third ed., McGraw Hill, New York, 1959, p. 236.
6 .
Robertson, A. F. and G r o s s , D. F i r e Load, F i r e Severity,and F i r e Endurance. A m e r . Soc. Test. M a t s . No. 464, 1970, p. 3.
1 I
7. Odeen, K. Theoretical Study of F i r e C h a r a c t e r i s t i c s i n Enclosed Spaces. Royal Institute of Technology, Division of Building Construction, Stockholm, Sweden, 19 63.
F i g u r e
I
H e a t b a l a n c e f o r t h e f u r n a c e c h a m b e r o f a f i r e
t e s t f u r n a c e
F i g u r e 2
E n t h a l p y o f v a r i o u s c o m b u s t i o n p r o d u c t s
