Smoke exhaust systems provide a means of increasing occupant evacuation time from a space affected by fire and smoke.
Active smoke exhaust and pressurisation systems are complex, expensive and can be challenging to commission. Therefore, it is recommended that smoke control systems should only be considered where alternative passive measures cannot provide sufficient additional occupant escape time or where mandated by regulation.
Smoke control systems will require specific building architectural design to accommodate the various functions and will need to be maintained and tested for the entire life of the system.
Given the recent focus on fire safety in buildings, it’s a good time to demystify some of the assumptions made regarding the adoption of smoke control measures within buildings and enclosed spaces.
Smoke control has been a life safety feature adopted in some types of buildings for over 50 years. Over time, there have been regulatory changes made to specific technical and performance parameters, but the basic concept of the smoke control system remains the same.
The purpose of smoke control is to reduce the build-up of smoke within an enclosed space in order to provide the occupants of the space sufficient time to evacuate to a safer area of the building or to exit the building. Smoke control systems are also designed to reduce the rate of growth of the ‘hot smoke’ layer in a fire affected zone such that it is maintained at above a typical person’s height for a sufficient period to allow occupants to escape the hazardous area.
A smoke control system is not designed to extinguish or control a fire or to protect the building contents from smoke damage. It may be used by firefighters to provide better conditions to fight a fire and to help clear residual smoke from an area following a fire event.
Application of smoke control
Smoke control systems are normally required for buildings more than 25 metres in effective height in accordance with the National Construction Code (NCC). There are other situations where smoke control systems are mandated, including some public buildings, large isolated buildings (large floor areas) and buildings with atria spanning more than two storeys. In addition, smoke control systems may be included where a fire engineered risk based design solution is utilised in lieu of a deemed to comply design.
Buildings over 25 metres also normally include stairwell pressurisation systems. Whilst not a smoke venting provision, stair pressurisation systems provide many of the same benefits of a smoke control system as they actively pressurise emergency escape stairs to limit prevent the ingress of smoke during evacuation.
Smoke control system design requirements are covered in Australian Standard AS/NZS 1668.1. This standard includes information on construction of elements, fire protection of openings, various different modes of equipment action in fire mode, pressurisation and air velocity requirements to ensure the effectiveness of the smoke control system.
Types of smoke control
The key types of smoke control systems are:
- Shutdown System: This is where the air handling plant is shut down upon a fire alarm in order to prevent smoke from being rapidly spread from the fire source through the occupied space via the air conditioning system fans. This would normally be found in buildings less than 25 metres tall.
- Zone pressurisation system: This setup utilises pressure differentials between fire zones to limit the migration of smoke from a fire affected zone to a non-fire affected zone. The pressure differentials are normally achieved by running smoke exhaust fans in the fire affected zone and having natural air makeup, or fan assisted air makeup on the non-fire affected floors. The pressure differential driving air away from non-fire affected floors to limit smoke spread and therefore facilitate a staged evacuation of the floors. This system is normally found in multi-storey office buildings.
- Hot layer smoke control system: This setup utilises a means of smoke containment within an overhead smoke reservoir to ensure that the air space below the smoke reservoir remains non-toxic and allow sufficient time for occupants to escape the fire affected area. Extraction fans are utilised to continuously draw smoke from the smoke layer whilst makeup air is introduced to the occupied zone to maintain tenable conditions for evacuation. These would normally be found in buildings with atria.
- Air purge system: This set up is more commonly used for spaces where zone pressurisation may be difficult to achieve due to large openings between fire zones or high leakage in the building. The system introduces outside air to the fire zone whilst simultaneously extracting smoke quickly. This system relies on air movement to direct smoke away from escape paths and non-fire affected adjacent zones. These systems would normally be found in buildings with large horizontal openings such as airports or hospitals.
The physics behind smoke control
Effective smoke control relies on the properties of hot air. Air which is significantly hotter than the surrounding air will become buoyant and rise above its surroundings. This is the principle used to generate uplift in hot air balloons.
In the case of a fire, the smoke and hot air emitted by the fire will rise to the highest spaces of the room and will not readily mix with other air until it cools significantly. A smoke control system utilises the behaviour of buoyant hot air to try and maintain the hot smoke layer above the breathing zone of building occupants so they have sufficient time to escape the fire affected space.
The majority of deaths in a fire occur due to breathing of toxic smoke, and not due to direct effect of the fire. Therefore, it is crucial to prevent toxic smoke from quickly polluting an occupied space.
A smoke control system will promote the hot layering effect of a fire through one or more of the following means:
- Shutting down the air conditioning to prevent the air diffusers mixing room air with the smoke, thereby dispersing smoke to lower levels in the room
- Continuously extracting smoke from the hot smoke layer, thereby reducing the speed at which the hot layer builds up in a room
- Introducing fresh outside air to the space to flush smoke from the occupied zone and replacing extracted air from the smoke exhaust fans.
Smoke reservoirs can be used to help capture and contain the hot smoke layer and prevent its spread through the highest level of a space. Zone pressurisation or zone airflow control can direct smoke away from areas not in the fire zone, thereby assisting in containing the spread of smoke through a building.
Smoke zones are created by utilising smoke impermeable construction and these can be used to limit the size of a zone which may be affected by smoke during a fire event.
The key elements of a smoke control system include:
Smoke sealing: Smoke seals are used on doors and to seal around any penetration from one smoke zone to another smoke zone.
Smoke reservoirs: Smoke reservoirs can be thought of as upside down reservoirs which are used to contain and limit the spread of the hot smoke layer.
Smoke dampers: These special dampers include smoke seals and close during a fire alarm to prevent smoke from migrating from one smoke zone to another smoke zone via the building ductwork.
Pressurised lobbies and stairs: Pressurisation of exit paths is used to prevent ingress of smoke to these zones.
High temperature fan construction: Smoke control fans need to be constructed to enable handling of smoke at high temperatures. These fans have special motors and are made of metal to provide extended operation during a fire event. The fan may ultimately fail, but by then the occupants should have been safely evacuated form the building.
Fire rated power supply: In order to ensure that the smoke control fans and controls operate during a fire event, the electrical and control components must be rated such that a fire does not make them inoperable.
Essential power supply: The NCC requires that the power supply for smoke control and pressurisation systems be supplied from the essential side of the main switchboard. This segregates the power supply for essential life safety equipment from normal air conditioning and ventilation systems. In some cases, the essential power supply may also be generator backed to ensure supply upon loss of mains power.
Fire brigade control: Smoke control systems are activated by signals from the building’s Fire Indicator Panel (FIP). Where there are active fans utilised for smoke control, the FIP also needs to provide a means of manual override of the smoke control fans operation, by the fire brigade. This is called a Fire Fans Control Panel (FFCP) and is most commonly utilised to either assist with fire fighting efforts (by manually controlling zone air flow and pressure) or to clear smoke following a fire event.
BMS system interaction: AS1668.1 recommends that all fire mode control functions be directly activated by the FIP input signals and not rely on the signals from an automatic control system such as a BMS system. Where a sensor or control component must be wired through an electronic controller, then it must be wired such that loss of the control signal will result in fail safe mode of operation (ie. does not prevent the smoke control from operating.
Fire engineering solutions
Fire engineering alternative solutions are often utilised to specify the smoke control system application, configuration and design smoke quantities, in order to achieve performance based building egress and evacuation requirements. Fire engineering solutions will often deal with complexities around atria, large horizontal spaces, and evacuation models and safe zones.
The fire engineering scope will model a specified fire event scenario, with the aim of achieving or bettering fire evacuation times for a space.
Testing and Commissioning
It is essential that smoke control systems are designed by an experienced mechanical engineer and fully commissioned and proven by a qualified mechanical services contractor. This includes full functional testing to activate a fire alarm for a particular fire/smoke zone and then witness sequence of actions for the smoke control system.
Ideally, this should be witnessed by the installation contractor, system designer, main contractor, and building manager. Anomalies witnessed during the test should be recorded and follow up testing should be provided until the system has been demonstrated as functioning correctly.
Ongoing maintenance and testing
Smoke control systems are classified as an essential services measure and therefore need to be maintained in accordance with the Building Act.
Building owners must ensure that an essential safety measure is maintained so that it operates satisfactorily. Building owners must prepare an annual essential safety measures report on the buildings essential safety measures.
The owner must also keep records of maintenance checks, safety measures and repair work so a municipal building surveyor or chief officer of the fire brigade can inspect them. These documents must be made available to the municipal building surveyor or the chief officer on request after 24 hours’ notice has been given. Annual essential safety measure reports and records of maintenance checks, safety measure and repair work must be made available for inspection on request after 24 hours’ notice has been given.