Smoke control

Smoke control is the deliberate limitation and direction of smoke movement during a fire. It can use physical barriers, natural ventilation, mechanical extraction, pressurisation or a combination of these measures. The objective is to protect particular spaces or routes for a required period, rather than to remove every trace of smoke from a building.

Smoke control works with the processes that transport combustion products. Hot gases rise, ceiling jets spread sideways, and pressure differences drive flow through openings. Fans, vents and barriers can change those flows, but outdoor conditions, building leakage and other fire systems also influence the result.

The design must therefore state where smoke is intended to go, which spaces must remain usable and what conditions could defeat the system. Detection, make-up air, equipment operation and maintenance belong to the assessment. A fan or vent installed without those supporting functions does not establish smoke control.

Objectives and protected spaces

The 2005 International Fire Engineering Guidelines (IFEG) describe the general objective as removing heat and minimising smoke concentration in designated areas. This focuses attention on selected locations rather than an undefined whole-building outcome. The relevant locations might include escape routes, occupied zones or areas needed for firefighting.

Cooper identifies three objectives for designed smoke and heat venting. These are supporting escape, helping firefighters enter and see the fire, and reducing smoke and heat damage to property. A system may address more than one objective, but the corresponding conditions and duration must be made explicit.

A smoke-control zone provides a defined boundary for that objective. It can be separated from adjacent zones by partitions, floors, doors and dampers. NUREG-1805 notes that a smoke compartment is enclosed on all sides, including the top and bottom.

Protection of a boundary is not the same as maintaining clear air throughout the fire room. Pressurisation can restrict smoke entering a stair while leaving severe conditions in the fire compartment. The location of people and the intended place of safety determine whether that outcome is useful.

Extraction has a different spatial aim. It removes smoke from the hot layer to slow descent or reduce concentration in the protected volume. Its effectiveness depends on the incoming smoke flow and replacement air, so a rated fan capacity alone cannot specify the resulting layer height.

Smoke movement as the design basis

Smoke movement begins with the fire plume and its entrainment of surrounding air. The rising products carry smoke into an upper layer, after which ceiling flows and openings distribute it. The amount entering a smoke-control system therefore depends on fire size, plume geometry and air supply.

Pressure differences govern horizontal flow through openings. They can arise from hot gases, wind, building services or differences between indoor and outdoor temperatures. Smoke-control pressures act within that combined field rather than replacing all other influences.

The fire scenario must include the fuel and the geometry that shapes entrainment. IFEG notes that walls and corners restrict air entrainment into a plume. A model developed for an unrestricted fire may consequently misrepresent smoke production near such boundaries.

Layer behaviour provides another design condition. Smoke-layer height varies with time, fire size and exhaust rate. A calculated clear height belongs to those inputs and cannot be treated as a fixed property of a ventilation unit.

A mass balance explains why the smoke layer can continue growing while an exhaust fan runs. Over a short interval, the change in stored gas mass equals the mass entering minus the mass leaving. This form assumes that the stated flows remain constant over the interval.

$$\mathrm{\Delta }m=({\dot{m}}_{\mathrm{i}\mathrm{n}}-{\dot{m}}_{\mathrm{o}\mathrm{u}\mathrm{t}})\mathrm{\Delta }t$$

Symbol	Description	Unit
$\mathrm{\Delta }m$	Change in gas mass stored in the layer	kg
${\dot{m}}_{\mathrm{i}\mathrm{n}},{\dot{m}}_{\mathrm{o}\mathrm{u}\mathrm{t}}$	Mass flow entering and leaving	kg/s
$\mathrm{\Delta }t$	Time interval	s

Van Rossum illustrates this with 10 kg/s entering a smoke layer and 8 kg/s leaving. During 1 s, the layer gains 2 kg of gas. Extraction reduces accumulation in this example but does not balance the incoming plume.

$$\mathrm{\Delta }m=(10-8)\times 1=2\mathrm{k}\mathrm{g}$$

The calculation concerns the entire smoky gas mixture, including entrained air, rather than soot mass alone. Converting accumulated mass into layer depth also requires density and enclosure geometry. Temperature changes and heat losses can change that density, so equal volume flows do not necessarily represent equal mass flows.

A stationary layer requires a balance over the relevant interval. A growing fire changes the plume supply, and an opening or fan change can alter the removal rate. The simple balance is therefore an explanation of accumulation, not a complete transient smoke-exhaust design method.

Natural and mechanical ventilation

Smoke and heat exhaust ventilation (SHEV) can remove hot gases through openings near the ceiling or roof. Cooper distinguishes natural buoyancy-driven removal, fan-driven removal and combined arrangements. The system must also introduce relatively cool, uncontaminated air at a lower level.

Natural ventilation uses the buoyancy of the hot layer and the pressure conditions across vents. It does not require a fan to create every part of the flow, but it still depends on effective vent area, layer conditions and replacement air. Wind and external temperatures can alter those conditions.

Mechanical extraction uses fans to remove smoke from the upper part of a space. Australian Building Codes Board (ABCB) guidance describes extraction at or near the roof or ceiling, with make-up air at low level. That guidance concerns its national context and does not provide a universal design arrangement.

Make-up air replaces gases removed by the exhaust or vent. Insufficient replacement air can restrict the intended flow and change pressure conditions. It should be treated as part of the smoke-control system rather than as an unrelated opening.

Natural and mechanical venting both need a coherent airflow path. Based on Cooper (2002), p. 3-219 and Australian Building Codes Board (2020), p. 128.

Airflow paths should not short-circuit the intended smoke movement. Supply at low level and exhaust near the layer serve different functions in the cited venting approach. Their locations and operation must support a coherent flow from clean inlet to smoky outlet.

An exhaust inlet can also draw relatively clear air from below a thin smoke layer. This effect is called plugholing. The extracted mixture then contains less smoke than a calculation assuming pure layer extraction would suggest.

Van Rossum describes several ways of addressing this flow pattern. These include spreading extraction over more outlets, reducing flow per outlet, or allowing a deeper layer where the design permits it. Outlet orientation also matters, because the local suction flow determines whether air is drawn from beneath the layer.

Plugholing also changes the interpretation of an exhaust measurement. The total flow through a duct includes any clear air drawn into it. Only the part taken from the smoke layer contributes directly to removal in the layer mass balance.

Pressure differences and containment

Stair pressurisation supplies air to protect an escape stair from smoke infiltration. It creates a pressure difference between the stair and adjoining spaces. Open doors change the airflow and pressure, so the design must consider the relevant door conditions.

Zone pressurisation uses different pressures between a fire zone and adjacent zones. Supply to non-fire zones and extraction from the fire zone can both contribute. The intended direction is from protected spaces towards less protected spaces through leakage paths.

Pressurisation protects boundaries by controlling flow direction. Based on Evans et al. (2005), p. 70 and Australian Building Codes Board (2020), p. 127.

Excessive pressure is not a universal improvement. ABCB guidance explains that relief may be needed to avoid excessive door-opening forces or interference with automatic closing. The system must protect against smoke while preserving the operation of the escape route.

A Smoke barrier restricts smoke passage through a continuous construction or closure. NUREG-1805 notes that such a barrier may or may not also have fire resistance. Smoke containment and fire resistance are related building functions but are not interchangeable labels.

Dampers can complete a smoke boundary within ventilation ducts. Shutdown of air handling can limit recirculation and smoke transfer between compartments. A stopped fan alone may not seal an open duct, so the damper arrangement and its control still matter.

Reservoirs, boundaries and special volumes

A Smoke reservoir is a volume in which smoke can collect above the space being protected. Its boundaries help define where the layer is retained and from which region extraction occurs. Reservoir behaviour must be analysed together with incoming plume flow and replacement air.

Large open spaces differ from subdivided office floors. Cooper describes warehouse spaces that may be divided by ceiling screens and atrium-like spaces connected to adjacent rooms. These geometries affect smoke accumulation and the paths between the fire and occupied areas.

Atrium smoke control must account for the connected volume and the locations requiring protection. Smoke can collect above an atrium fire or arrive from an adjoining space. The relevant plume and layer model therefore depends on the actual fire location.

Tunnel ventilation is another application of smoke control. Van Rossum identifies tunnels as spaces in which fire-engineering analysis addresses conditions that differ from ordinary occupied buildings. Computational fluid dynamics, which calculates gas movement through a divided modelled space, can investigate the effectiveness of the proposed system.

System interaction and reliability

Smoke control often relies on detection to select the correct fire mode. Zone systems may need the alarm location as well as confirmation that a fire exists. An incorrect zone signal could produce pressure or airflow patterns that do not match the fire location.

Sprinkler operation can reduce heat and buoyancy while disturbing the smoke layer. The US National Institute of Standards and Technology (NIST) describes cooling and mixing in its discussion of building smoke management. This interaction can change the conditions assumed for smoke transport and heat-operated vents.

Cooper describes delayed activation of later sprinklers or vents when earlier sprays cool their elements. Such skipping makes the sequence less predictable. Interactions should be evaluated rather than assuming that each system reaches its design state independently.

Active systems can contain many components in series. ABCB gives illustrative reliability calculations using assigned component values, but these are simplified examples rather than universal system probabilities. Their purpose is to show how complexity and dependencies can reduce availability.

Normal building ventilation can either support or oppose the fire-mode arrangement. Recirculation can distribute smoke if the system does not change to its intended state. The design should document fan, damper and control positions for each relevant operating mode.

Commissioning and design limits

Commissioning should include an installation check, performance tests in all operating modes, repair of defects and retesting. ABCB presents this sequence because a completed installation is not necessarily a functioning system. Repeated testing may be needed to close identified defects.

Access for inspection and maintenance affects reliability over the system's life. Components that cannot be reached safely are harder to test and service. Documentation should identify required inspections and the mode in which each component is expected to operate.

Calculations carry uncertainty from the fire, geometry and flow paths. IFEG notes that smoke-spread calculations beyond the enclosure of origin are more difficult and uncertain than those within it. Results should not be given more precision than those inputs support.

Dilution alone should not be confused with every definition of smoke control. NUREG-1805's cited glossary definition distinguishes pressure- and airflow-based control from simple supply-and-extract dilution within an enclosure. Broader smoke-management classifications nevertheless include dilution as one technique.

Performance assessment

Performance should be assessed against the sequence of system states. Detection must select the intended mode, fans and dampers must respond, and airflow must then develop through the actual openings. Treating the final airflow as immediate would hide delays and incorrect intermediate states.

Smoke-control acceptance covers the complete operating sequence. Based on Australian Building Codes Board (2020), pp. 129–131.

Different door positions can create different smoke-control problems. A closed door may preserve a pressure boundary, while an open door requires airflow through its opening. Pressure relief and door operation therefore need to be tested for the scenarios used in design.

Mechanical equipment also has dependencies. A fan needs a suitable control signal and operating condition, while a damper must reach its intended position. Verification should follow those functions through to airflow at the protected boundary instead of stopping at an electrical signal.

Exhaust capacity and fire size are coupled design inputs. ABCB warns that smoke-control performance can be compromised if sprinklers fail to limit the fire as assumed. A larger smoke source may exceed the exhaust arrangement or damage exposed components.

Maintenance should preserve both airflow hardware and the physical building boundary. Later alterations to partitions, doors, ducts or ventilation can change the smoke-control zones. Those changes require comparison with the design basis even when the original fans remain in place.

A pressure measurement and a layer-height measurement answer different questions. The first helps establish flow direction across a boundary, while the second describes smoke accumulation in a volume. The acceptance record should use observations that correspond to the actual design objective rather than treating either as a universal indicator.

See also

• Smoke
• Protected escape route
• Fire safety strategy

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Australian Building Codes Board. (2020). Fire safety verification method data sheets (Version 1.1).
• Cooper, L. Y. (2002). Smoke and heat venting. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-219–3-242). National Fire Protection Association.
• Evans, D. D., Peacock, R. D., Kuligowski, E. D., Dols, W. S., & Grosshandler, W. L. (2005). Active fire protection systems (NIST NCSTAR 1-4). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire dynamics tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission fire protection inspection program (NUREG-1805). U.S. Nuclear Regulatory Commission.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.