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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False ceilings Where there is an unbroken false ceiling in the fire room or balcony it must be treated as the top of the smoke layer. Recent experience of fires in atrium buildings in the has shown the problem of flame travel internally through the atrium to be minor in comparison to that of hot and toxic gases accumulating and building down in the atrium – spreading throughout the building and affecting escape routes.
An easily understood way of achieving this is to ensure that the boundary between the room and the atrium is both imperforate and fire-resisting, and that the atrium base has only a very restricted use.
Hence the mass flow and heat flux within the smoky gases may be determined. For balcony reservoirs cool smoke can be expected to affect some nearby rooms under some circumstances, but would not significantly hinder safe escape.
Chapter eb Smoke ventilation within the atrium Smoke movement in the atrium When the smoke and heat cannot, for various reasons, be confined and removed from the room of origin or associated balcony space, the use of ‘throughflow’ or steady-state ventilation from the atrium itself is usually considered. I Figure 20 Under-balcony smoke reservoir venting into an atrium smoke reservoir Entrainment into smoke flows from compartments is being studied This results in large quantities of air being entrained and hence a very large mass flow rate of smoke entering the layer in the atrium roof.
The preferred 1101-5 of design fire would be a timedependent growing fire, to which the means of escape and evacuation time for the particular building occupancy could be related, allowing the increasing threat to occupants to be calculated as time progresses.
However, the geometry of the opening on to the atrium has a crucial effect. As the fire grows and declines, the mismatch in volume between the inlet air and the extracted fire-warmed air will also change. The 112101-5 of this present Report is to provide guidance only on design principles of smoke control and it is hoped 121011-5 support the code rather than to preempt it. For example, in buildings where the population is largely familiar with the escape routes; where the incoming air is entering the fire room directly, or where in the instance of the inlet air being supplied via the atrium the major escape routes are away from the atrium; then a less onerous parameter can be applied.
Unfortunately it is often very difficult, impracticable, or extremely expensive to fit a 11201-5 smoke extraction system to each and every room, however fn. These can be simply defined as follows: When designing smoke ventilation or depressurisation systems, the mass flow rate and heat flux developed in the room are major parameters in the calculation of the system requirements, changes in which can substantially affect all of the subsequent smoke flow conditions.
If this gas temperature or lower cannot be achieved then consideration should be given to: This suggests that inflow airspeeds should not usually exceed 5 ms-I. Where such structures exist and are an appreciable fraction of the overall layer depth, the depth below the obstacle 121001-5 be found e Table 3 b rather than 3 a. This is not usually a viable option where the opening between the room and the atrium is large for example, an open-fronted room or a room whose glazing has fallen away in whole or in large part.
Furthermore, when considering an unsprinklered office occupancy, there exists the potential for flashover to occur and for the entire floor to become involved in fire. Introduction This Report is intended to assist designers of smoke ventilation systems in atrium buildings. Values of Cd will vary for differing flow geometries. Other values may be appropriate for other circumstances. The minimum number of openings through the false ceiling required within a single subdivision can be found from Table 4.
Where sprinklers are installed and additional cooling of the smoke layer needs to be accounted for, the 1211-5 of extraction points required will differ from 1210-15 shown in Table 4. In the event of a fire, a fan of suitably larger capacity starts up and draws smoky gases into the chamber in a similar way. If the compartment is open to the atrium, 12011-5 the gases flow out immediately they reach the opening.
In practice, for a given mass flow rate and layer depth, Table 3 a3 b or Equation eh can be used to find the minimum value of accessible perimeter. Screens may be fixed or 121011-5 descend upon smoke detection. Therefore the role of a smoke control system is principally one of life safety. For any given size 112101-5 fire, an equilibrium can be reached where the quantity of gases being removed equals the quantity entering the layer in the fire plume – no significant mixing of air occurs upwards into the base of the buoyant smoke layer.
Effects of sprinkler systems in smoke reservoirs Offices, shops, assembly, industrial and storage or other non-residential purpose groups are now expected to have sprinklers if they have a floor more than 30 metres above ground level.
Air mixes into the fire plume as it rises, giving a larger volume of smoky gases. Once the layer temperature reaches approximately “C, then in most compartments the downward radiation from the gas layer is sufficient to cause auto-ignition of the remaining combustible materials in the compartment Figure 6. Sprinklers may also be required in other 121101-5 for insurance purposes.
Such air inflows through doors in public buildings could hinder Dg fm Figure 21 Local deepening at a transverse barrier As a consequence, with adequate validation, this type of modelling should have a wide applicability. 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 sn I into it Figure 24 a. In most practical compartments there is sufficient oxygen to support combustion in the first few minutes, and the fire growth and smoke production wn controlled by the fuel, ie, fuel-bed control.
It must also dn low enough to allow the escape doors to be opened with relative ease.
V VI “wind W WB X Y P A ADB e B 01 P PO Volumetric flow rate of gases m3s-1 Volumetric flow rate of gases from a reservoir m3s-I Design wind velocity ms-‘ Width of vertical opening m Width of balcony distance from vertical opening to front edge of balcony m Height from the base of the smoke layer to the neutral pressure plane m Height from the base of the fire to the smoke layer immediately above m Coefficient in critical exhaust rate equation kgrnp3 Empirical height of virtual source below a balcony edge m Additional smoke depth due to local deepening m Temperature rise above ambient of smoky gases “C Temperature rise above ambient of smoky gases under a balcony “C Temperature rise above ambient of smoky gases in a reservoir “C Temperature rise above ambient of smoky gases in a vertical opening “C Density of gases kgm-3 Density of ambient air kgmW3 vii.
Research in this area is highly desirable.
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It is suggested that either of the extreme values should be adopted in seeking a conservative design approach. It follows that to assist the fire services, the smoke control system should be capable of performing its design function for a period of time longer than that required ne the public to escape, allowing a speedier attack on the fire to be made after the arrival of the fire service.
Any screen fixed midway across a compartment opening will serve no purpose, since smoke will flow on both sides simultaneously.
The use of a computer is necessary since the technique involves the solution of tens of thousands of mathematical equations for every step forward the simulation makes.
In other words a smoke control system in the atrium is essential to make certain that escape is unhindered, by ensuring that any large quantities of thermally buoyant smoky gases can be kept separate sn people who may still be using escape routes, or awaiting their turn for evacuation.
This excessive entrainment can be reduced by restricting the sideways travel of the smoke under the balcony and hence reducing the length of the line plume.
BS en 12101-5
A potentially valuable bonus of such a system in a sprinklered building is that the sprinklers which are normally required in the space above the false ceiling will cool the smoky gases before they reach the fan.
The rate of burning, heat output and mass Qpw leaving the compartment are now strongly dependent upon the geometry of the opening, ie ventilation control Figure 7. Atria today are designed as undivided volumes within a structure, intending to create visually and spacially an ideal external environment -indoors6. Most small rooms will therefore take this value. 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.
Thus there appears to be a need for a properly designed smoke control system in atrium buildings.