Buy CEN/TR SMOKE AND HEAT CONTROL SYSTEMS – PART 5 : GUIDELINES ON FUNCTIONAL RECOMMENDATIONS AND CALCULATION. exhaust ventilation systems (published as CR ). Part 6: Specification for pressure differential systems — Kits. Part 7: Smoke control. Design approaches for smoke control. in atrium buildings. G 0 Hansell*, BSc, PhD, CEng, MCIBSE, AlFireE H P Morgan, BSc, CPhys, MlnstP, AlFireE.
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This demarcation dimension was chosen arbitrarily and has no theoretical derivation.
The pressure difference across any small opening on to the route must be large enough to offset adverse pressures caused by wind, building stack effect and fire buoyancy. This flow is driven by the buoyancy of the smoke. As the layer gets deeper there is less height for the plume of smoke to rise before it reaches the smoke layer, hence less air is being entrained, wit’h the result that the temperature of the smoke layer increases with layer depth, even for a steady fire.
Nevertheless it remains generally true that this option is rarely found to be appropriate for most atrium buildings. Where sprinklers are present this will be clearly unrealistic and a value of kW for a 6 m perimeter fire may be more appropriate for designers wishing to adopt a fire-engineering approach to a design.
It is therefore important to identify the regime which applies. The primary purpose of the Report is to summarise in a readily usable form the design advice available from FRS at the time of its preparation.
The buildings adopting these changes often have included within their design large spaces or voids, often integrated with many of the storeys. This is explained in more detail in Chapter 5. The pressure drop criterion may be increased if the population of the building is adult and physically fit, to perhaps Pa 8 ms-I. There have been a number of purely qualitative papers, as well as papers on work using relatively simple models of smoke movement within atria see for example References Note that the effect of sprinkler cooling is to reduce the heat flux Q, without significantly changing the mass flux.
Should a different design fire be considered for whatever reason, the equations, figures, etc given here may no longer apply, and advice should be sought from experts. The surface of the plume in contact with the ambient atmosphere in the atrium will cause additional air to be entrainedI Figure 23 Throughflow ventilation of the atrium ’21 I into it Figure 24 a.
Thus the closer the screens may be installed to each other, the more the smoke base may be allowed to rise for the same heat flux. Recent research32into the ability of people to move through an exit against an opposing airflow has shown that movement is not impeded for airspeeds below 5 ms-‘, and is not seriously impeded below 10 ms-‘ although some discomfort was reported at these higher airspeeds.
This creates a large surface area for entrainment on both sides of the plume along its spill width Figure 24 bfor which reason they are also known as double-sided plumes.
Sprinkler systems vs smoke control: the EN approach – FMJ
The mass flow of gases entering this layer is equivalent to that flowing out through the exhaust system Figure Where Dd 2 1. Once the height of this layer base is chosen for a lowest-level fire, the height above the top of the opening or above the edge of the projecting canopy or balcony over the opening where relevant must be established where the fire is in an adjacent room. This Report is the culmination of a long-running collaborative project between the Fire Research Station of the Building Research Establishment and Colt International Limited on aspects of smoke movement and its control in atrium buildings.
The controls and wiring should of course be protected, to maintain the fn supply to the fans during a fire. T h e present Report is 12101- to serve the designers of smoke control systems for atrium buildings in the same way that the earlier Building Research Establishment Report, Design principles 11201-5 o r smoke ventilation in enclosed shopping centres, has 121001-5 designers of smoke ventilation systems in shopping malls.
112101-5 Experience of pressurisation designs suggests that the technique is well-suited to the protection of K I Heat radiation Figure 6 T h c onset of flashover 5 stairways used as escape routes in tall buildings, though it can also be useful in other circumstances. For a plain opening with no downstand obstruction Figure 14Dd can be considered as the rise of the plume beyond the balcony edge. This form of smoke control is that most readily understood by most as ‘smoke ventilation’ and is based upon a defined buoyant smoke layer being established at some point within the structure, with a ‘clean’ layer of air beneath.
In most practical compartments there is sufficient oxygen to support combustion in the first few minutes, and the fire growth and smoke production are controlled by the fuel, ie, fuel-bed control.
Throughflow ventilation Smoke ventilation throughflow ventilation is used when the fire is in ne same space as the people, contents or escape routes being protected, without it filling that space. Recent examples of this mode of fire-spread have been an office block in Siio Paulo7 and the Villiers building in London. Figures 15 and 16 give t h e mass flow values in graphical form for various opening heights and widths, using t h e above procedure. Sufficient air must enter the space below the layer to replace the gases being removed from the layer, otherwise the smoke ventilation system will not work.
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In practice the height of rise of the plume is usually chosen to permit safe evacuation, leaving only a dependency on the length of the line plume. The fire sizes on the previous page excluding a will be those used throughout. First though, it is worth reviewing the underlying principles of smoke ventilation and the general approach needed for successful design.
The Report does not exclude the options of using alternative methods where they are appropriate, or of using new techniques such as computational fluid dynamics as they are developed and validated.
The mass flow rate of smoke to be exhausted from the atrium roof will then be that calculated for the under-balcony condition A system of reasonably wide perhaps oneor two-metre slots surrounding a region of false ceiling could perhaps be used instead of screens below the false ceiling. However, the geometry of the opening on to the atrium has a crucial effect.
If the compartment is sprinklered and the water spray hits the glass, the localised heating of the glass by radiation from the fire and by the gas layer, combined with sudden cooling due to the water spray will increase the likelihood of the glass breaking.
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There have been several examples of this. A stage will be reached when the smoke temperature is insufficient to set off further sprinklers. Where sprinklers are installed and additional cooling of the smoke layer needs to be accounted for, the number of extraction points required will differ from those shown in Table 4.
In the present work the following, in terms of fire area and convective heat flux, are used to illustrate the calculation procedures adopted: Air mixes into the wn plume as it rises, giving a larger volume of smoky gases.