VENTILATION CONTROLLED FIRES

VENTILATION CONTROLLED FIRES

Under ventilation controlled conditions excess pyrolizate and flammable products of combustion present in smoke are a significant hazard to firefighters. Let’s go back to the fire triangle (see Figure 4) to examine the nature of this threat. While fuel, heat, and oxygen are present in proportion to support combustion where the fire is burning, the heat of the fire is pyrolyzing more fuel vapour than the fire can consume. In addition, incomplete combustion results in production of flammable gases such as carbon monoxide. The speed of fire development is limited by the availability of atmospheric oxygen provided by the current ventilations profile of the compartment or building.

When the fire is burning in a ventilation controlled state, any increase in the supply of oxygen to the fire will result in an increase in heat release rate. Increase in ventilation may result from firefighters making entry into the building (the access point is a ventilation opening), tactical ventilation (performed by firefighters), or unplanned ventilation (e.g., failure of window glazing due to elevated temperature).

It is essential to recognize when the fire is, or may be ventilations controlled and the influence of planned and unplanned changes in ventilation profile. Most compartment fires that progress into the growth stage are ventilation controlled when the fire department arrives. A bi-directional air track (smoke out the top and air in the bottom) is often a significant indicator of a ventilation controlled fire. As illustrated in Figure 5, as the neutral plane drops, smoke exiting through the opening reduces the size of the inlet opening, further restricting the air available for combustion and extent to which the fire is ventilations controlled.

The series of photos in Figure 5 also illustrates the influence of decreased oxygen supply on the colour and optical density of smoke. As combustion becomes more incomplete, smoke production increases, colour darkens, and optical density increases. When working inside a burning building, flames moving through the hot gas layer are also a strong indicator of a ventilation controlled fire (john moore, 2012).

VENTILATION INDUCED EXTREME FIRE BEHAVIOR

When the fire is ventilation controlled, increased air supply to the fire will result in increased heat release rate and depending on conditions may result in extreme fire behaviour such as flashover or backdraft. While similar, vent induced flashover and backdraft are different phenomena.

REFERENCE

P.J. Denno& W.D. Walton [Eds.], SFPE Handbook of Fire Protection Engineering, 3^rd Edition

Frantzich, Hakan, Rapport. (1997), “Fire Safety Risk Analysis of a Health Care Facility” Department of Fire Safety Engineering, Lund Institute of Technology, Lund University, Lund

Dodge, F. W., (1991),”Fundamentals of Fire Safety in High-Rise Buildings”, Skokie, IL : Concrete and Masonry Industry Fire Safety Committee.

Ayuba P, Olagunju, R.E. and Akande O.K (2012).Failure and Collapse of Buildings in      Nigeria: The Role of Professionals and other Participants in the Building Industry.

Interdisciplinary Journal of Contemporary Research in Business.Vol 4, No. 6.