Article found at: http://www.firetactics.com/AIR_TRACK_MANAGEMENT.ht
|Air Track Management at Fires|
One of the most critical objectives at any structure or compartment fire is incident stabilization. The very basic principles of 3D Firefighting concepts are rooted in the overriding objectives of ‘Air Track Management’.The ‘air-track’ in a building fire, scientifically referred to as a ‘gravity current’, denotes the pathway or route taken as air enters the structure or compartment, heading towards the fire, and leaves the compartment via the same route back, or by alternative openings that occur, pre-exist or are created by firefighters during tactical venting actions.Paul Grimwood writes ………
In 1992 (Fog Attack) I provided first hand testimony, from archaic dusty texts I had located in the British Museum, of 19th century fire chiefs who had led London’s firefighters to understand the importance of maintaining control of the ‘air track’ in a structure fire.
‘The men of the fire brigade were taught to prevent, as much as possible, the access of air to the burning materials. What the open What the open door of the ash-pit is to the furnace of a steam boiler the open street door is to the house on fire. In both cases the door gives vital air to the flames’. Chief James Braidwood (1866)
‘It is not necessary that every fireman should be profoundly versed in the study of the atmosphere known as pneumatics; but as he has to constantly deal with such substances (air) it is absolutely indispensable that he should thoroughly understand certain principles by which he is able to control their use’. Chief Sir Eyre Massey Shaw (1876)
Another wrote a few years later ‘An American fire chief told me that he rubbed in the principle of ventilating by making his recruits extinguish a fire in a ‘drill’ building with all ventilation shut-off; they had a grueling time of it. Then he gave them a similar fire with the building vented. They never forgot the lesson. Ventilating must be done at the right time; air must not be encouraged to flow into a building until lines of hose are laid out and sufficient water is available’. Chief Aylmer Firebrace CBE
Management of the ‘air-track’ in a structure fire was clearly an important issue for these early London Fire Brigade Chief Officers.
A firefighting tactic and commonly used method by experienced firefighters, termed anti-ventilation, is to control this air track by isolating the flow of air feeding the fire. This tactical approach is a starting point from which firefighting operations should begin. For example, before we create openings we should locate any pre-existing air-track and assess where and if the fire is being fuelled by additional air. Would the restriction of this air-flow assist our tactical objectives?
How many times have firefighters entered a burning structure, ahead of the primary placement of a hose-line, to attempt the rescue of trapped occupants on upper floors, and walked right past the door to the fire room … leaving it open? A simple action of closing the door and isolating the fire from the escape route may well have saved many lives.
In many other instances there may have been a delay in getting water onto the fire. During this period it may be absolutely critical that we manage the air-track by closing doors and preventing the creation of vent openings, in an effort to stabilize the fire environment and restrict fire spread, at least until we are able to get sufficient flow-rate into position to deal with the fire.
Entering a fire-involved compartment or structure should incorporate the following sequence –
As an example of ‘air track’ management I wrote extensively in 1999 (Flashover & Nozzle Techniques) and again in 2005 (3D Firefighting) about how firefighters can take effective control of the air track in several ways ….
“As we crawled into the room, the fire’s roar was somewhat disconcerting. The thick smoke from the fire’s plume was banking down setting an ‘interface’ at about 4 feet from floor level and the heat radiating downwards from the ceiling could clearly be felt through the substantial layers of our protective clothing. I looked directly above our position, into the darkness of the smoke, and noted some yellow tongues of flame rolling the ceiling, detaching themselves from the main body of fire that blazed in the furthest corner of the compartment. We had advanced about 4 feet into the room as I reached for the nozzle of the high-pressure booster-line and discharged the briefest ‘pulsation’ of water-fog into the overhead.
There was no drop-back in terms of water particles and the series of ‘popping’ sounds suggested that the fog was ‘doing its thing’ in the super-heated gas layers. The tongues of flame dispersed for a few brief seconds before resuming their eerie ‘snake-like’ dance towards the open access point (doorway) situated behind us. “Hold the water” shouted Miguel over the SCBA comm’s radio. As we inched further into the room I realized then that I was placing my deepest trust in the man.
The smoke continued to bank down around us and I watched in awe as several ‘balloon-like pockets of fire gases ignited, each for a brief second, in front of my eyes about three feet from the floor. I could sense the moment of compartmental flashover’ was fast approaching and I instinctively reached for the nozzle again. “WAIT”, shouted Miguel – he laughed as he reached back and kicked the access door almost shut. I felt extremely vulnerable but then, as if turned off by a tap, the fire suddenly lost its ‘roar’ and the rolling flames in the plume above dispersed completely. Everything went dark as the fire ‘crackled’ and the smoke banked right down to the floor. There was an eerie silence within this blinding experience that seemed all too familiar to the ‘firefighter’ in me. Miguel took the nozzle out of my hands and discharged several brief ‘pulsations’ of water-fog, on a wide setting, into the upper portions of the room.
Again, there was no ‘drop-back’ and you could almost sense the minute particles of water suspending themselves within the super-heated flammable gas layers. The steam ‘over-pressure’ and humidity was negligible and any air movement went unnoticed. More importantly, the thermal radiation from above had lessened considerably reducing the likelihood of a flashover. Then I heard Miguel’s voice over the fire-ground comm’s calling for an exterior tactical venting action and almost instantly the smoke layer began to rise as firefighters in the street vented the window serving the room. The fire in the corner of the room became visibly active again as it increased in intensity, however this time the tongues of flame in the ceiling layer were heading towards the open window and away from our position.
Jose Miguel Basset was the Chief Fire Officer of the Valencia (County) Fire Brigade in Spain. He was a practical man who had learned much about fire and its behavior under various conditions. He had ‘played’ with fire over a number of years, experimenting alongside his trusty team of firefighters, pushing ventilation parameters to their limits in an attempt at gauging their effect on fire growth. Within the fiery depths of this acquired structure training situation He had demonstrated quite clearly here how firefighters may utilize tactical venting actions to attack a fire’s progress and that simply by closing the access door or opening a window at its highest level you can avert or delay a backdraft or flashover situation. He also showed how firefighters can reduce thermal radiation from above by reversing the direction of a fire’s plume away from the access point, as described.”
Air Track Velocity & Fire Gas Accumulation
Where an air-track is allowed to develop, the analogy of a running track can be used to describe various effects. If a running track were to exist inside a bedroom it would be difficult to get up any speed as there is insufficient space to run across the room. However, a long hallway or corridor might provide enough space to gain quite a speed if running. Similarly, the velocity of an air track might be greater in a ‘point to point’ pathway that exists between an inlet doorway and an outlet window as opposed to a room window that provides both air inlet and smoke outlet. The role that ‘momentum’ and ‘pathways’ play in the physical process of gravity current and air-tracks was discussed extensively HERE in 1999 by the author.
In Flashover & Nozzle Techniques (1999 – Grimwood) the author described computer modeling by fire investigator David Birk that demonstrated the varying effects that different access door openings have on fire growth & development in a room fire. With the ventilation controlled fire initially restricted to a burning chair he reports time to flashover as being greatly affected by such openings as follows –
Where ‘point to point’ openings exist, or are created, in a fire-involved structure, the velocity or momentum of the air-track will increase. Where a forced draft (wind) is involved the velocity of the air-track may increase further still. What this means is that where the first opening is created in a structure, a second opening may increase the air-track’s velocity and this may lead to rapid fire development, or some sort of fire phenomena. In effect the second opening is often the catalyst, although a single opening is sometimes all that is needed. If we vent a window with an open entry point behind us then we may set the air-track in motion. If we close the door behind us, the single opening of a window may not be enough to increase velocity in the air-track and air-inflow is more controlled.
However, where fires are isolated in this way, by closing entry (or other) doors, there are distinct advantages and disadvantages:
Advantages of Fire Isolation –
Disadvantages of Fire Isolation –
So there is a conflict here as by restricting air-flow into the compartment/structure, we may assert greater control over the fire’s development but cause CO levels to rise and Oxygen levels to drop, as well as increasing heat levels as thermal balance is destroyed and causing smoke to drop down and hit the floor. This is a call that only the nozzle or search team can make – with an officer sited at the entry doorway, feeding hose in and filling the door control assignment.
ATM FIREFIGHTING TECHNIQUES