Flame deflectors

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Flame deflectors

F U M E DEFLECTORS

by T. 2. Harmathy

Conventionally, the f i r e protection of buildings has been directed against hypothetical f i r e s that spread by either of the following two

mechanisms: (i) conduction of heat through the boundaries of the space

on f i r e , followed by the ignition of the combustibles in the neighbouring space, ( i i ) collapse or partial failure af a boundary element and subsequent d i r e c t penetration of flames into an adjoining s p a c e . Consequently, to

check the spread of f i r e , the boundary elements of a l l constituent s p a c e s have been r e q u i r e d to exhibit specified ' I f i r e resistances", that is, proven

abilitiee to r e s i s t heat conduction and structural d a m a g e f o r specified

periods.

It has been r e c o g n i z e d f o r some time that the spread of f i r e is, in f a c t , p r i m a r i l y a convective-radiant p r o c e s s . The f l a m e s a r e driven

by p r e s s u r e differences f r o m one s p a c e to another e i t h e r horizontally through gaps around d o o r s , through left-open d ~ o r s and other openings, or v e r t i c a l l y t h r o u g h ducts, shafts, openings in ceilings, and by flames

issuing f r o m windows and p r o p a g a t i n g to the next f l o o r above.

In this paper the last mentioned mechanism of f i r e spread w i l l be examined briefly and a simple solution to overcome the danger of expansion

of fire by this mechanism w i l l be offered. Spread of Eire Along the Facade of Buildings

It w a s believed e a r l i e r that a horizontal projection of at least 2 f t w i d e , b u i l t above a window, could provide s u f f i c i e n t protection against the vertical spread of fire f r o m window to window. A few t e s t s performed in ~ r i t a i n ( l ) indicated, h o w e v e r , t h a t a 2-ft projection w a s inadequate. After passing such a narrow projection, the f l a m e s c u r l back to the face of the

building and may c a u s e the breakage of windows and ignition of cutnbus tibles on the s t o r e y above.

S o m e f u r t h e r e x p e r i m e n t s conducted in ~ u s t r a l i a ( ~ ) clearly showed

that, to provide some p r o t e c t i o n , the projection must be at least 3 f t wide, p r e f e r a b l y 4 ft or m o r e , i f the f i r e is expected to be v e r y s e v e r e . Such wide projections not only keep the flames away from the face of the building, but a l s o cut do-*rn the r a d i a t i o n hazard to an acceptable level.

Obviously, continuous balconies and open corridors a r e very

useful d e v i c e s in protecting buildings against m a s s i v e fires. Unfortunately, o p e n c o r r i d o r s are rarely used nowadays. Balconies are still popular in apartment buildings, but they are g e n e r a l l y built in smaller units, d e s i g n e d

t o s e r v e one [or two, at the most) apartment only. The dislike f o r continuoas balconies is understandable. They not only c u t down the natural daylight

=aching the interior, but also substantially increase the c o s t of the building, and often produce aesthetically undesirable effects.

Even though the isolated balconies are very beneficial f r o m the point of view of facilitating the r e s c u e of the occupants, their role in protecting the building against the vertical spread of the flames is obviously limited. In the following a simple device, referred Lo as "flame deflector1', w i l l be described which, if mounted above a l l windows of the building (except those of

the

continuous balconies, but at subs tantially lower costs and without the draw- backs mentioned.

Flame Deflectors

The technical details of a possible version* of the device are illustrated in Figure 1. Figure la shows the device from the front, b

f r o m the direction of the wall along which it is mounted, c is a section to

show some essential details, and d illustrates the deflector when activated

by

of

of

of

by

T-e

angles

along

of

be placed in the space formed

by

The panel assembly c a n turn around the hinge formed

by

6 ,

stud,

position

9 ;

placed under the nut. W i t h the removal

of

a

the

but,

0 The _version shown 'in the figare is probably one of the s isnplea t. The. designer may want to modify this ~ o h t i o a ta satisfy certain specific requirements.

For

of

**

some protection against the spread of flames, was based on studies on simple shelf-like projections. I t is possible

that

above the window, the effective width of the projection could be some- what reduced.

w i l l retain the released panel in a horizontal position. One end of these

cables is fastened to angle 3, the other to eyes 13, held in the wall.

A s flames issue from the w i n d o w below, nut 11 melts away and the panel becomes "activated", i. e . with the aid o f spring 11, it i s f o r c e d

t o a s s u m e a horizontal position, as s h o w n in F i g u r e

la.

exceeds 14005F and since the panel is cooled from the top by air currents, the a v e r a g e temperature of t h e sheet would not normally r i s e higher than

800 to 9 0 0 "I?. Although at these temperatures the strength of steel is still considerable, large temperature variations w i l l undoubtedly result in substantial warping. The angle 3 , which is not in contact with the p l a t e , is e x p e c t e d to remain w e l l below the 500 "F l e v e l and this can p r e v e n t the w a r p e d panel f r o m collapsing.

The u s e of an a s b e s t o s or plaster board is essential only if the f i r e i s expected to be ventilation controlled, in other w o r d s , long and hot. The purpose of using these boards is to cut down thermal radiation

from the panel assembly to t h e window above.

The types of f l a m e deflectors shown in F i g u r e 1 are placed 2 to

6 in. f r o m the w a l l s u r f a c e . They can be installed without too much

difficulty en als eady existing buildings.

T h e panels would usually be surfaced with baked-on enamel. The

designer would select a colour that matches the o t h e r s u r f a c e s of the

b u i l d i n g , or could u s e contrasting colours and use the p a n e l s as decorative elements.

Naturally, the p r o b l e m of corrosion must be taken into account i n selecting the materials o r f i n i s h e s for all components of the deflector.

In the case of newly designed buildings the designer may w a n t , in

o r d e r to produce some a e s t h e t i c effect, to mount the panels flush with the w a l l s u r f a c e . Obviously, as far as the d e f l e c t o r s a r e concerned, t h e r e are no practical difficulties in doing this, but the work involved in providing the required w a l l surface would result in higher c o s t s .

F i g u r e 2 shows an o f f i c e building equipped with flame deflectors that f o r m d e c o r a t i v e girdles between the floors. T w o of the deflectors h a v e been activated due to a fire on the third floor,

The total cost of equipping a building w i t h such simple flame d e f l e c t o r s is estimated to amount t o 1/2 to 1 per cent of the total

b u i l d i n g cost. Part of this extra c o s t could be recovered by saving on the quality of wall surfacing under the panels, and, possibly, by relaxing

M a r e Complex Deflector Designs

The f l a m e deflector d i s c u s s e d so f a r w a s d e v i s e d for activation by the f i r e i t s e l f . It may s e e m desirable under c e r t a i n c i r c u m s t a n c e s to

install a simple electric d e v i c e that could c a u s e the d e f l e c t o r to t u r n down

even before reached by the f l a m e s . Thus the deflector c o d d be activated by a signal r e c e i v e d f r o m a f i r e d e t e c t o r in the r o o m , or manually from conveniently located emergency s w i t c h boxes ( a s w e l l as by the f i r e ) .

Facilities for manual activation would s e e m especially desirable if the flame deflectors are designed to fulfil s o m e additional functions. It has b e e n s u g g e s t e d , f o r example, that s u c h activated deflectors could provide t e m p o r a r y bridges between isolated balconies a n d thus could

g r e a t l y i n c r e a s e the chances for s u r v i v a l of people trapped in the balconies of a burning section of the building,

N a t u r a l l y , the p r o v i s i o n of e l e c t r i c a l ( o r mechanical] override would

i n c r e a s e the c o s t of the flame deflector system. The c o s t s would further g r o w i f t h e deflectors a r e designed strong enough t o suppost people. Y e t ,

the extra advantages these more complex deflector systems offer may justify their u s e u n d e r certain circumstances.

R E F E R E N C E S

(1)

H.

b a s e d on an unpublished report, dated July 1960.

( 2 ) Horizontal Projections in the Prevention of S p r e a d of Fire f r o m Storey to Storey. Commonwealth Experimental Building Station,