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Hazard

Uncontrolled ventilation

Hazard Knowledge

The rate of development of any fire is directly linked to the supply of oxygen available to it. Establishing control over ventilation should form a key part of the overall incident plan.

Incident commanders should be aware that any increase in the supply of oxygen to a fire will accelerate the development of the fire. Experience has shown that where ventilation is not properly controlled or coordinated, firefighter safety has been compromised and serious consequences have followed.

Ventilation is one factor amongst the many tactical considerations that the incident commander will need to consider and implement as part of their overall incident plan. When planned and performed correctly ventilation can contribute to, and assist in, saving lives, improving firefighting conditions to support firefighter safety and reducing damage to property.

The incident commander must balance the benefits of controlled and coordinated tactical ventilation, in line with their service policy and training, with the hazards associated with accelerated fire growth and the introduction of oxygen into under-ventilated fire compartments.

Various natural or mechanical phenomena are associated with ventilation as well as being inherently linked to fire development, which can have an impact on any planned ventilation strategy. It is important that firefighters and incident commanders have an awareness of these phenomena and the potential impact when developing an overall ventilation and firefighting strategy, including:

  • Wind-driven fires
  • Coandă effect 
  • Piston effect
  • Trench effect 
  • Stack effect 
  • Dust explosions

Wind-driven fires

The term 'wind-driven fire' has no formal definition under ISO or in UK fire and rescue service manuals. It is, however, becoming the standard generic term for fires that may also be referred to as force draught, wind-assisted, force vented or blowtorched.

A wind-driven fire may be described as one where external wind (or ventilation-forced) pressure causes strong air movements, affecting the severity of fire spread.

Fires can be affected by wind pressure and high-velocity air movements. The impact can be experienced in open fires or wildfires, while in buildings the greatest impact is usually experienced with fires in high-rise structures. Where windows have failed through exposure to heat, allowing external wind to affect the speed and direction of fire development, firefighters located in the flow path between the air inlet and air outlet are potentially in great danger, as temperature layering balances out across all levels, floor to ceiling.

Coandă effect

The 'Coandă effect' is described as the tendency of a stream of fluid or gas to stay attached to a nearby surface rather than follow a straight line in its original direction. In firefighting terms, this is the tendency of a fast-moving stream of air to deflect to nearby surfaces. The airstream's static pressure tends to decrease, which causes a pressure difference between the wall and areas far from the wall. This bends the stream towards the surface and tends to keep it attached to that surface.

The Coandă effect will influence hot gases escaping from compartments involved in fire. The effects of convection, fire compartment pressurisation and the wind will cause smoke and hot gases to be expelled from an external opening and usually move vertically. In some instances, the Coandă effect also influences downward fire spread.

The Coandă effect will encourage the venting products of combustion to be drawn back towards the face of the building, which will generate fire spread to other compartments or areas of the structure.

While this effect is commonly considered to occur at high-rise incidents, the same effect is often responsible for the spread of fire from ground floor compartments to upper levels when uncontrolled ventilation occurs.

 

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Coanda effect

Source: Building Research Establishment

 

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Post-fire damage illustrating the result of the Coandă effect

Source: Building Research Establishment

Piston effect

The 'piston effect' is a phenomenon that creates a potentially large movement of air in a shaft or tunnel when an object moves in the enclosure. The effect is more pronounced when the object's sides are close to the enclosure walls and if the object moves at speed.

For example, a train, when moving in an unrestricted location, displaces air around it except in the direction of the ground. If the same train enters a tunnel, the displaced air is confined by the tunnel walls. An area of higher pressure is created in front of the train as well as around the sides. Behind the train, an area of lower pressure is created, which is filled by the pressurised air escaping from around the sides of the train and equalised by the flow of air from all sides of the area of low pressure.

As the train exits a tunnel, into an underground station for example, the pressure wave, or movement of air felt by passengers standing on the platform, is the pressure front created by the moving train. This effect is similar to the operation of a mechanical piston in an engine or pump, hence the term 'piston effect'.

The same effect and impacts can be created by the movement of a lift in a lift shaft. The piston effect can influence the movement of air not only close to the lift shaft but also in the wider area of the building or structure. These air movements will affect the ventilation flow paths throughout the structure and can induce undesirable air movement in relation to wind-driven fires, blowtorch effect, flashover, backdraught or fire gas ignitions.

Incident commanders and fire crews should be aware of, and manage, these flow paths to minimise the hazards that may be experienced during a fire where there is the potential for sudden and rapid fire growth.

Trench effect

The 'trench effect' is a phenomenon that can produce a developing fire plume that accelerates up an inclined surface. It is influenced by two separate physical effects, the Coandă effect and flashover.

The trench effect can occur when a fire develops on or close to an inclined surface (approximately 25°). The flames deflect towards the surface (Coandă effect) and heat the combustible materials further up the incline. These materials will begin to be heated, leading to pyrolysis and subsequent ignition. Rapid fire development continues towards the top of the inclined slope until the fuel is depleted.

The trench effect can be exacerbated by flow paths in buildings and structures as well as by prevailing climatic conditions. The piston effect can also intensify the trench effect.

 

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Diagram of the trench effect

Source: Building Research Establishment

Stack effect

The 'stack effect' is the movement of air into and out of buildings, structures and chimneys and is driven by buoyancy. Buoyancy occurs because of a contrast between external and internal air density caused by temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and height of the structure, the greater the buoyancy force (stack effect).

Buildings are invariably constructed with provision for natural ventilation. Generally, air in the building is warmer than the external air temperature. This warmer air rises up through the building and exits through open windows, ventilation openings and through other forms of leakage. The rising warm air creates an area of lower pressure in the lower section of the building, allowing cooler external air to be drawn in through open doors, windows or other ventilation openings.

 

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Diagram of the stack effect

Source: Building Research Establishment