Predicting the movement of smoke is a challenging task for the fire safety or building service engineer. Stewart Miles, a senior consultant at BRE’s Fire and Security Group, reviews the many different ways for controlling the hazards of smoke, and previews the latest development in this key area.
Smoke inhalation is the single biggest cause of loss of life from fire inside buildings. Furthermore, its presence can be a major hindrance to occupants attempting to escape or to fire fighters trying to reach a 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 source, 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 or tall buildings, and also in tunnels and underground facilities. Escape distances may be long, the occupant density high or there may be mobility issues (in hospitals, for example). Smoke management for fire-fighting and post-fire smoke clearance may also be required.
Buoyancy and entrainment forces generated by the fire will compete, in an often complex manner, with those due to external wind effects, stack effects, the air handling system, lift motion and so on. So predicting the movement of smoke is a challenging task for the fire safety or building services 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 control measures such as detectors and sprinklers, to the overall building design philosophy, and to people’s 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 below, and the design methodologies most widely used are described. Modern smoke management schemes have been developed since the 1940s, when procedures were adopted in North America to reduce the spread of smoke through air-handling systems. Later work, largely in North America and the UK, included the development of smoke dampers, pressurisation systems for stairwells and smoke exhaust venting schemes for atria and other large spaces. Research and development continues today, including some recent work at BRE. While smoke control designs are, in many instances, based on established guidance and regulatory information, physical or numerical modelling may sometimes be required.
Smoke Management Methods
There are various methods of containing and controlling the generation and movement of smoke(1):
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, e.g.(2) Controlling fire size, typically by means of sprinklers, may from part of the overall smoke management procedure. Here the suppression system would be expected to limit the heat release rate and control the spread of fire.
2. Compartmentation.
It is often overlooked that compartmentation, such as walls, fire doors and smoke screens, will in many cases form the first line of defence in controlling the movement of smoke. The provision for compartmentation will be closely linked to escape travel distances. Smoke leakage rates through compartment barriers may need to be included in the overall smoke management design.
3. Smoke exhaust ventilation.
Smoke management in large volume spaces such as atria and shopping malls is generally provided by a smoke exhaust ventilation system3,4. Hot smoke is collected at ceiling level and vented to the outside. A 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, for example, where some form 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.
4. Pressure differentials.
In contrast to smoke exhaust ventilation, pressurisation (or alternatively de-pressurisation) systems are designed to protect zones away from the fire source(5,6). The classic example is the protection of stairwells, where air is supplied to pressurise the stairwell and prevent smoke ingress. The design of pressure differential smoke control schemes is closely coupled to the provisions for compartmentation, and careful analysis of air leakage and relief paths is required for successful operation.
An alternative application of pressure differentials is provided by so-called zoned, or ‘sandwich’, smoke control systems for high-rise, generally commercial, buildings. Here the fire floor is de-pressurised relative to the adjacent floors, with aim of preventing smoke movement away from the fire floor. The building air-handling system is typically used, in fire mode, to provide the pressure differentials.
5. Dilution.
For a given exposure to smoke, it is possible to design a smoke dilution (or clearance) scheme, where fresh air is supplied and smoky air is 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.
6. 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 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, for example in escape corridors. Special attention is required in the context of buildings to allow for opening and closing doors, and to ensure that a fire is not unduly fed with additional oxygen.
7. Other methods.
There are a number of other ‘novel’ methods that have been employed such as 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 used 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.
1. Design guidance and calculations.
Various guidance publications and ‘hand calculations’ are available from organisations such as BRE(3), CIBSE(2) in the UK and ASHRAE(1) and NFPA(4,6) in North America. These provide valuable design assistance for smoke exhaust ventilation and pressurisation in particular.
2. Physical modelling.
A building may be geometrically complicated, or the proposed smoke management system deviates from ‘standard’ configurations, but it may still be necessary 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.
3. Computer modelling.
Computer models provide an alternative to physical modelling and three main types are available. For analysing smoke movement within a high rise building or tunnel network, 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, the use of a network air flow model is appropriate.
The next level of sophistication is provide by zone models. These were developed originally for room fires, where the geometry is divided into a small number of approximately homogeneous zones, for example, cool lower layer, hot upper layer and 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 desktop computers, they include approximations in respect of 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 and so on at each grid point are generated. However, they take longer to run compared to network and zone models.
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 Brussels Airport shopping mall and the European Parliament Building atrium. Figure 2 shows a hot smoke test in a multi-compartment building.
Current Research
Research into smoke management is an on-going activity, and a number of issues are being addressed currently in the UK and elsewhere, or have been identified as requiring further study. Examples include the effect of ‘plugholing’ where a ceiling level smoke exhaust system is rendered inefficient due the extraction of low level clean air through the smoke layer. Work into the interaction of smoke ventilation and the operation of sprinklers continues, and the appropriate use of ‘jet fans’ inside buildings (in particular car parks) are also the subject of continued analysis.
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 project7 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 approach studied was that of naturally ventilated smoke shafts serving the common access corridors. While this approach may protect the adjoining stair, the corridor itself is likely to become hazardous if exposed to smoke for an extended duration. This is illustrated in the CFD contour plot of temperatures shown in Figure 3.
Stewart Miles is a senior consultant at BRE’s Fire and Security group. For more information, contact Stewart Miles at [email protected].
References
1. Klote, J.H. and Milke, J.A., Principles of Smoke Management. ASHRAE, 2002.
2. The Chartered Institute of Building Services Engineers, Guide E – Fire Engineering, 2003.
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 Systems in Malls, Atria and Large Spaces, 2005 ed.
5. BSI/CEN, Smoke and heat control systems – Part 6: Specification for pressure differential systems, BS EN 12101-6, 2005.
6. National Fire Protection Association, NFPA 92A – Standard for Smoke-control Systems Utilizing Barriers and Pressure Differences, 2006 ed.
7. Miles, S., Clearing the decks. Fire Safety Engineering, October 2005, pp. 25-28.
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