Controlling Smoke
Smoke inhalation is the single biggest cause of loss of life in the event of fire inside buildings. Furthermore, its presence can be a major hindrance for occupants attempting to escape or for fire fighters trying to reach the fire. The effects of reduced visibility and toxic gases may be compounded by those due to thermal exposure, both from smoke gases in direct skin contact and from the effects of radiation from hot ceiling layers.
In large building complexes and underground structures smoke may be transported to locations far removed from the fire, for example through corridors and vertical shafts in high-rise buildings or across large undivided spaces in retail and warehouse premises. Controlling the production and transport of smoke is particularly important in large volume or tall buildings and also in tunnels and underground facilities. Escape distances may be long, the occupant density high or some occupants may be mobility impaired (e.g. hospitals). Smoke management for fire-fighting and post-fire smoke clearance may also be required.
Buoyancy and entrainment forces generated by the fire will compete with those due to external wind effects, stack effects, the air handling system, lift motion etc in an often complex manner, so that predicting the movement of smoke is a challenging task for the fire safety or building service engineer. A range of natural and mechanical measures will be available, and selecting an appropriate method requires a knowledge of fire science, fluid dynamics and building design. A smoke management system is likely to be coupled closely to other fire strategy measures such as detectors and sprinklers, to the overall building design philosophy and to people behaviour and their tenability. Issues surrounding commissioning and maintenance are also important.
The various methods
available for controlling the generation and transport of smoke are summarised
in this article, and the design methodologies most widely used are described. Modern
smoke management schemes have been developed since the 1940's when procedures
were adopted in
Smoke management methods
There are various methods of containing and controlling the generation and movement of smoke [1]:
Control of the fire source itself
An important consideration when designing a smoke management system is the appropriate choice of 'design fire'. The rate of growth and maximum size, generally defined in terms of heat release rate, is typically selected from recommended values in various design guides, such as CIBSE Guide E - Fire Engineering[2]. Controlling fire size, typically by means of sprinklers, may form part of the overall smoke management strategy, where the suppression system would be expected to limit the heat release rate and control the spread of fire.
Compartmentation
It is often overlooked that compartmentation, e.g. walls, fire doors, smoke screens, will in many cases form the first line of defence in controlling the movement of smoke. The provision of compartmentation will be closely linked to escape travel distances. Smoke leakage rates through compartment barriers, into for example escape stairs, may need to be included in the overall smoke management design.
Smoke exhaust ventilation
Smoke management in large volume spaces such as atria and shopping malls is generally provided by a smoke exhaust ventilation system [3,4]. Hot smoke is collected at ceiling level and vented to the outside. Supply of make-up air below the smoke layer is crucial, and must be included in the design along with the sizing of the smoke venting system. Natural or mechanical exhaust ventilators can be used, with the latter being required, for example, if external wind effects are likely to impede the passage of smoke. While smoke exhaust ventilation can be used in smaller compartments, it is generally reserved for large spaces, where some form of physical smoke containment may be required, e.g. smoke curtains, to create smoke reservoirs. Evacuation is generally assumed to take place within the fresh-air region below the smoke layer, and it is then important to calculate the clearance between head height and the smoke layer interface (as illustrated in Figure 1) as well as the smoke temperature and thermal radiation.,
Pressure differentials
In
contrast to smoke exhaust ventilation, pressurisation (or alternatively de-pressurisation)
syst
An
alternative application of pressure differentials is provided by so-called
zoned, or 'sandwich' smoke control syst
Dilution
For a given rate of smoke production (or amount of smoke), it is possible to design a smoke dilution (or clearance) scheme whereby fresh air is supplied and smoky air exhausted at a rate sufficient to reduce the smoke concentration to some specified level. The approach may be used, for example, to clear smoke that has infiltrated a protected space such as an escape corridor or refuge lobby. As an alternative to mechanical smoke dilution, schemes are sometimes designed using natural ventilation. However, the efficiency of the scheme is then highly dependant on atmospheric (wind) conditions.
Airflow
In tunnels, a common approach to smoke control is to use forced airflow to keep smoke to one side of the fire, allowing escape and/or fire-fighting access from the other. The critical design criterion is then whether ?back layering? is eliminated, i.e. whether there is a flow of smoke gases in the upstream direction. The method has been applied to a lesser extent in buildings, e.g. in escape corridors. Special attention is required in the context of buildings to allow for opening and closing doors etc, and to ensure that a fire is not unduly fed with additional oxygen.
Other methods
There are a number of other ?novel? methods that have been employed such as the use of air curtains, where a strong cross flow of air is designed to prevent the passage of smoke. Water curtains have been designed with a similar aim. Another approach that has been employed in metro tunnels, to protect parts of the network during maintenance, is the use of inflatable tunnel plugs that fill the cross section to prevent smoke passing.
Design methodologies
The engineer has a number of options available for evaluating the performance of smoke management schemes.
Design guidance and calculations
Various
guidance publications and 'hand calculations' are available from organisations
such as BRE[3], CIBSE[2] and BS PD 7974-2[8] in
the
Physical modelling
Where a building is geometrically complicated, or the proposed smoke management system deviates from ?standard? configurations, it may be necessary however to model the movement of smoke. One approach is provided by reduced-scale physical modelling, where the results can be extrapolated to full scale using Froude scaling.
Computer modelling
Computer models provide an alternative to physical modelling. Three main types of computer model are available. Where the smoke can be approximated as a cool gas and its movement is dictated primarily by the distribution of pressures associated with natural and mechanical forces, for example remote from the fire within a high rise building or tunnel network, the use of a network air flow model to analyse smoke movement is appropriate.
The next level of sophistication is provided by zone models. These were developed originally for room fires, where the geometry is divided into a small number of approximately homogenous zones, e.g. cool lower layer, hot upper layer, fire plume. Conservation equations are solved so that the pressure, temperature, gas concentration etc within each zone is predicted at discrete time intervals.
While network and zone models run very quickly on modern desk top computers, they include approximations in respect to the fluid dynamics and geometry. Computational Fluid Dynamics (CFD) provides a more rigorous, three-dimensional treatment where the underlying conservation equations are solved on a numerical grid containing typically tens or hundreds of thousands of points. A detailed solution 'picture' comprising temperatures, smoke concentrations etc at each grid point are generated. However, they take longer to run and require a higher level of user skill compared to network and zone models.
Commissioning and hot smoke tests
It
is sometimes necessary to test the completed smoke control system after
construction. This may include a 'hot smoke test' using hot, buoyant smoke, as
described in BR 368[3]. BRE has developed one such test, which burns
industrial methylated spirits cleanly in a steel tray to produce carbon
dioxide, water vapour and heat. The fire plume and transported 'smoke' is visualised
using oil-mist.It has been used to assess the smoke exhaust ventilation
system in the
Some current research at BRE
As part of a research programme in support of the development of Part B of the Building Regulations for England and Wales and its supporting guidance, BRE has recently completed a project[7] to examine the smoke ventilation of common access areas of flats and maisonettes and their relationship to the provision of Compartmentation and Means of Escape procedures. Smoke containment, provided by the provisions for compartmentation and smoke-rated fire doors, and coupled with limiting travel distances, plays a key role in the overall smoke management strategy for the common access areas. However, where smoke does enter these areas, natural and mechanical smoke management measures can offer varying levels of additional protection. A range of such measures were investigated using a combination of reduced-scale physical modelling and CFD calculations using BRE's JASMINE code.
One
example studied was that of naturally-ventilated smoke shafts serving the
common access corridors. The work showed that while this approach may protect
the adjoining stair, the corridor itself is likely to become hazardous if
exposed to smoke for an extended duration.
Research
into smoke management is an on-going activity. A number of issues are being
addressed currently in the
For more information, contact
References
1. Klote, J.H. and Milke, J.A., Principles of Smoke Management. ASHRAE, 2002.
2.
The Chartered
3. Morgan, H.P., et. al., Design methodologies for smoke and heat exhaust ventilation. BRE Report 368, 1999.
4. National Fire Protection Association, NFPA 92B - Standard for Smoke management System sin Malls, Atria and Large Spaces, 2005 ed.
5.
BSI/CEN,
Smoke
and heat control syst
6.
National Fire Protection Association,
NFPA 92A - Standard for Smoke-control Syst
7. Miles, S., Clearing the decks. Fire Safety Engineering, October 2005, pp. 25-28.
PD 7974 'Application of fire safety engineering principles to the design of buildings- Part 2 -Spread of smoke and toxic gases within and beyond the enclosure of origin (sub-system 2)', British Standards, 2002.
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