Publisher’s version / Version de l'éditeur:
Technical Note (National Research Council of Canada. Division of Building Research), 1973-04-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
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 [email protected].
Questions? Contact the NRC Publications Archive team at
[email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
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/10.4224/20353508
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Positive Pressure Fire Resistance Furnace
McGuire, J. H.
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.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=46b8824e-93cb-4138-b099-a40a275b3129 https://publications-cnrc.canada.ca/fra/voir/objet/?id=46b8824e-93cb-4138-b099-a40a275b3129
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'f
E
C
1HI N II CAlL
NOTE
'I
No.
TN570
PREPARED BY J. H. McGuire BY A.G. W.
DATE April 1973
PREPARED FOR Limited Distribution
SUBJECT POSITIVE PRESSURE FIRE RESISTANCE FURNACE
Positive pressure differentials in the range 0.01 in.to 0.1 in. W. G. are not unlikely to appear across partitions during the course of a fire in either buildings or ships. Such pressure differentials can significantly reduce fire resistance times, hence, it would not be unreasonable to require a positive pressure differential across a partition during a fire te st. Such a requirement has recently been
specified for marine applications by the Marine Regulations Branch of the Department of Transport1.
Unfortunately, the existing full-scale fire resistance furnaces at NRCL do not currently permit testing to the DOT specification. This note is concerned partly with modifications that would result in compliance, but primarily its concern is with the design of a furnace intended to operate over a range of pressure differentials. In the first instance, interest is confined to the testing of walls, bulkheads and doors.
MODIFICATION OF EXISTING WALL FURNACE
The existing wall furnace uses burners that consume propane and aspirate their air supply. Since the stoichiometric propane -air mixture is 1:25, it is not surprising that the burners will not operate against any appreciable pressure differential. The effective height of
.;;
-..., '..ｾ . ,
｜［ＮｾＧ ｾＮ ｾＧｩ ＾ｾｾ ｾＬ［Ｇｾ［
ｾ ... ｾＮＬＭ ＮＮＺＮＬｾ ,-' ｾ ｾ ｾＮ［ＢＮＮ Y':.:./I' tt ...,.•' J ... ｟Ｎｾ ...
-2-the furnace is 10 ft; shortly after being ignited -2-therefore, a total stack effect of about O. 1 in. W. G. can be expected. Only at the very top of the furnace can a slightly positive pressure be pennitted, the neutral pressure plane then being near to this level and a negative pressure differential of about 0.1 in. W.G. prevailing at the lowest level.
Two obvious means of establishing an over-all positive pressure differential are the construction of enclosure s around the unexposed face of the specimen or, alternatively, around the burner air intake region. The first approach, involving the establishment of any desired negative pressure differential within the enclosure, is undesirable primarily because it no longer leaves the unexposed wall surface to cool to a free atmosphere. Near to the failure tem-perature, the unexposed surface of the specimen may be nearly 250°F higher than ambient, and the air flow needed to dissipate the associated heat flux, even if an air temperature rise of 20°F were accepted,
would be over l, 000 efm.
The alternative approach of constructing and pressurising an enclosure around the burners involves modulating the air flow to this region as the burner requirement varies, so that an appropriate pressure differential is maintained across the burners. This action is essential to permit the burners to function satisfactorily but it gives a variable pressure differential across the test specimen. To maintain this latter constant would either involve modulation of the flue opening or the (modulated) injection of further air into the furnace proper. If this supply were, in turn, taken from the pressurized burner enclosure, it would simplify modulation requirements. The flue restriction implicit in this approach would also make the
pro-vision of blow-out panels highly desirable.
BASIC DESIGN OF A POSITIVE PRESSURE FURNACE
If a furnace is required to operate under an over-all positive pressure, the first design feature to be incorporated is the arrangement of the flue outlet at the bottom rathe r than at the top of the furnace. This action alone, provided high-level leakage does not develop, will achieve a positive differential at all levels. Leakage from the furnace, or more probably from the specimen, is likely to develop, however, and some means of maintaining a positive differential under these
circumstances is desirable. A suitable approach would be to continuously inject air and appropriately modulate a flue damper.
With a view to minimizing modulation, it would seem advisable to maintain a fixed air supply to the burners and not modulate it as fuel
would minimize flue damper modulation requirements.
Maintaining a fixed air supply to the burners would affect the choice of furnace temperature control technique. A popular approach, that of continuously regulating the total fuel supply, becomes
impractical, as a satisfactory gas/air mixture can only be maintained over a small range of gas supply.
Fortunately, another factor associated with constancy of air supply makes more practical the simple expedient of control by complete shutdown of the fuel to a proportion of the burners. To reduce the effective heat input to the furnace from 100 per cent design value to 0 per cent, it is no longer necessary to reduce the fuel supply to zero. With constant air supply, flue losses will now always constitute 40 per cent of the maximum design heat input instead of being reduced virtually to zero as fuel input is reduced to zero. It thus only becomes necessary to reduce the fuel supply to about 40 per cent of its maximum value. This can be conveniently achieved by shutting down the fuel supply to 60 per cent of the burners without disturbing the uniformity of temperature within the furnace. The number of burners to be shut down to effect satisfactory control can, in fact, be reduced by reducing gas pressure within the narrow limits prevailing.
Basically, therefore, the furnace should consist of an appropriate fire chamber with a dampered flue outlet at the lowest level, a constant air supply, and control by complete shutdown of fuel to selected burners.
DETAILED DESIGN
(a) Heat Input Requirement
The test specimen, to be mounted vertically in the first
instance, should measure 8 ft in height by 6 ft 3 in.
in
width. Assuming the specimen to have as high a (Kpc) product as is generally encountered, i. e., that of common brick, heat flow analysis 2 suggests that 10 minutesafter the start of a test a heat input of about 700,000 Btu/hr would be necessary to cover loss to the specimen and walls. Using natural gas and a 30 per cent excess of air, a further 500,000 Btu/hr flue loss would
pre-vail, giving a total requirement of 1.2 million Btu/hr. Comparison with an existing gas-fired small-scale furnace 3 suggests that a heat input of 1.4 million Btu/hr would be desirable.
Considering extremes, the testing of a water-filled steel
bulkhead would call for an input of the order of 4 million Btu/hr. Such a heat input was found to require a higher gas flow than was available, and further consideration to provide this facility was dropped. It was decided to provide a burner complex with a normal total operating
•
-4-output of about 1.5 million Btu/hr, but with a potential capability (with increased air and gas supply) of 2 million Btu/hr.
(b) Choice of Burner
For a previous furnace operating at positive pressure3 , a satisfactory burner was developed by merely terminating a I-in. gas pipe with a fire clay block 7 in. long conically shaped with diameters of 1 1/4 in. and 2 1/2 in. at each end respectively. The
I-in. pipe constituted the air supply and a few inches prior to the cone a gas pipe was teed into it. The burner consu:m.ed about 1 cfrn of natural gas, thus having an output of about 60,000 Btu/hr, the heating value of natural gas being about 1,000 Btu/ cu ft. To meet the heat input requirement specified in the previous paragraph for the projected furnace, more than thirty burners would be required this would seem to be an unnecessary extravagance.
The next available block size accepts a 1 1/2-in. gas pipe (1 1/2 in. internal diameter) and can be expected to offer over twice the heat output of the previous blocks. A pattern of 15 burners in
five rows of three would seem appropriate and would also be convenient from the ignition point of view, being satisfied with two V8 internal
combustion engine ignition systems. Each burner could provide an output of about 140, 000 Btu/hr giving a total potential of a little over 2 million Btu/hr with a total consu:m.ption of 35 efm of natural gas.
(c) Gas Supply
The gas supply to building M59 is 2 in. ID and the associated velocity head at a delivery of 35 efm should be about 0.1 in. W.G.
It is said that, with straight pipe, one velocity head is lost every 55 diameters4, suggesting that there will be a I-in. W.G. drop every 90 ft of straight pipe. The gas supply pressure being 10 in. W .G., it is to be hoped that the supply rate will just be sufficient despite
the fact that the gas meter is only rated for a delivery of 15 efm. Unless it is possible to install a pressure regulator near to the furnace, however, substantial pressure variation is to be expected as burners are shut
down to control furnace tempe rature. (d) Pipe Sizes
Working to a velocity head of 0.1 in. W.G., corresponding to a velocity of 27 ft/ sec appears to be convenient for the gas flows, and as already stated, a 2 -in. gas pipe would provide the total re-quirement of 35 efm natural gas under these circu:m.stances. Applying this criterion to each individual burner would give a pipe requirement of 0.5 in. (ID).
For the air supply, a flow velocity of 100 ft/ sec, glvmg a velocity head of 2.4 in. W. G., is feasible, but about half of this value m.ight be m.ore appropriate. Neither velocity head is
sufficient for air injection into the gas lines where the two system.s com.bine near to each burner.
Allowing a provision of 100 per cent excess air instead of the approxim.ate 30 per cent that would constitute a bare m.inim.um., the total air requirem.ent could be as high as 700 dm. total and about 45 dm. per burner. The 100 ft/ sec criterion would give pipe sizes of 4.5 in. and 1.2 in. respectively. Such choice would require a high pressure blower. Choice of 6-in. and 1. 5-in. pipes would lower the velocity head to 0.8 in. W.G.
(e) Flue Size
The design velocity head of the flue gases should not exceed 0.1 in. W.G. Their absolute tem.perature m.ight be four tim.es am.bient, or about 900 °C. Taking the effective density as roughly a quarter that of air at am.bient tem.perature, therefore, a design velocity of 40 ft/ sec is arrived at.
Flow rate will be approxim.ately 700 scfm. or 2,800 dm. at four tim.es the tem.perature; on a design velocity of 40 ft/ sec, a circular flue with nearly a l5-in. diam.eter is required or, alter-natively, a square one of about l3-in. side.
(f) Safety Feature s
The principal hazard to guard against is the ignition of a quantity of unburned gas in the furnace cham.ber. It is not suggested that this be elim.inated by sophisticated flam.e detection device s
interlocked with the gas valves. An adequate level of safety can be provided by m.ore sim.ple but less certain m.eans of reducing the likelihood of an incident, backed by the provision of blow-out
panels to elim.inate the development of a dangerous explosion. Such a com.bination, in fact, constitutes m.ore satisfactory over-all
prote ction.
The likelihood of attaining a flam.m.able m.ixture throughout the furnace cham.ber can be m.inim.ized by the following two m.easures. Firstly, power to the gas solenoids should only be available following the switching on of the air supply blower, and should also be controlled by an air-flow switch incorporated in the air line. Such a switch,
designed in the fire research section and consisting of a flow-sensing vane and a m.ercury switch, has functioned satisfactorily in the positive pressure furnace previously constructed and referred to earlier.
•
-6-is flowing. Autom.obile ignition system.s can be conveniently and inexpensively adapted for this purpose.
The subject of explosion relief is discussed in the NFPA Handbook5 and by various other authorities, including Rasbash6. The NFPA recommends that there be a 1 sq ft vent for every 30 cu ft of space for sm.all, solidly constructed enclosures. The Rasbash recom.m.endations depend on various variables, including acceptable pressure rise, and shape of enclosure.
To com.pletely elim.inate flam.e im.pingem.ent on a specim.en, it m.ight be de sirable to have the fire com.partrnent 3 ft deep,
giving a volume of 150 cu ft. The NFPA criterion would then call for 5 sq ft of vent. The Rasbash criterion, taking an overpressure of 5 psi as acceptable, would call for 2 sq ft.
A convenient arrangem.ent would be to have three one-sq ft panels in the base of the furnace, one being in the flue outlet, and a fourth at the top, at a location that m.ight be used as a flue outlet should negative pressure operation be desired. The lower panels should be designed to give way first. A convenient construction would be asbestos paper supporting asbestos fibre insulation.
COST
No attem.pt has yet been m.ade to estim.ate, accurately, the cost of the projected furnace. It can be 'said, however, that the following will constitute the m.ost expensive com.ponents:
15 burner blocks ($62.50 each) $ 950.00
10 gas solenoids ($50.00 each) 500.00
2 1 1/4-in. gas solenoids ($150.00 each) 300.00
2 large gas valves 300.00
15 sm.all gas valves 50.00
I 700 cfm. blower, 6 ozs. 350.00
Bricks
CONCLUSION
50.00 $2,750.00
Subsequent to the initial preparation of this note, the request for the furnace has been withdrawn. The note is printed however, for record purposes, as it indicates the simplicity of the design of a gas fired furnace capable of operating at positive pressure.
In the event of a requirement for a furnace operating over a whole range of both positive and negative pressures it is worth mentioning that this can be readily achieved by directing the low level flue outlet into a chimney. To reduce the chimney height needed to achieve appreciable negative pressures it would be desirable to select a lower velocity head (flue velocity) than that
suggested in this note. When intending to operate at a positive pressure it would also be convenient to bottom vent the chimney heavily so as to largely eliminate its influence.
REFERENCES
(1) Fire Testing Procedures for Materials for Use on Canadian Passenger Vessels. Marine Regulations Branch, Department of Transport, April 1968.
(2) Harmathy, T. Z. Design of Fire Test Furnaces. Fire Technology, Vol. 5, No.2, NRCC 10965, May 1969. pp. 140-150.
(3) McGuire, J .H. and P. Huot. Fire Tests Concerning the Penetration of Walls by Horizontal Plastic DWV Pipes. TN 557, DBR/NRC, January 1971.
(4) Perry, J. H. Chern. Eng. Handbook 4th ed. 1963. Tables 5-9, pp. 5-20.
(5) NFPA "Fire Protection Handbook", 13th edition 1969. pp. 17 -61-(6) Rasbash, D. J. "The Relief of Gas Vapor Explosions in Dome stic
Structures." The Structural Engineer. Vol. 47, No. 10, October 1969. pp. 404-8.
