ROOF VENT MODEL

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We can use the example (left) of a ‘big box’ (high volume warehouse) fire to explore the various fire dynamics we are likely to experience in such instances. Is roof venting the answer? How will it work? Is gas (smoke) cooling useful here? What conditions are we likely to experience and how will this impact on our firefighting deployments? There is one thing here that is certain and that is any fast-developing fire in a large volume warehouse or store, with a standard fire load, is going to be very difficult to deal with and could place firefighters at great risk. Taking a look at the following baseline example in a 600m2 (6458ft2) open-space warehouse using a simple zone computer model, provides us with some idea of the fire conditions we may encounter, the impact of roof ventilation and the heat exposure to firefighters we might expect, should we venture inside under various venting configurations.  

We will experience some two-way (Bi-Directional) exchange of air with smoke in the air flow path. Smoke will be leaving the upper part of the doorway with air flowing in below. The speed (velocity) in the smoke leaving the door gives us a guide as to how intense the fire inside is becoming.  

Either by firefighters, or by automatic ventilators, or by the fire itself. The size of the vent opening will determine the height of the bottom of the smoke layer from the floor. Without any venting at all the smoke is likely to be down to the floor. However, as the vent opening is created and enlarged, the air flow-path will gain momentum with more smoke and fire leaving the structure but with more air entering at floor level to increase the size of the fire.   

Providing there is an adequate fuel load, even with the 5m2 (54 ft2) entry door in the open position from the start, the fire will develop at a fast rate for the first seven minutes, at which point it will begin to slow as it becomes vent limited at around 7 Mega Watts (MW) in size with the smoke layer nearly down to the floor. Given adequate ventilation it will then continue on a growth curve and may soon outpace the suppressive capacity of a single hand-line (at around 20-30 MW).  

The smoke layer will be hot and dangerous (flammable but too rich in mixture to ignite in most parts) at high level but far cooler and less dense at ground level. As the fire progresses the bottom of a visible smoke layer may appear to ‘bounce’ up and down. This is a warning sign of impending ignitions in the smoke layer. The base of a very hot and well mixed smoke layer may also show some flaming.  

E- FIREFIGHTER LOCATIONS: When advancing inside, firefighters may be exposed to a warm kind of humidity and some heat flux from the smoke layer. Temperatures at roof level may be extreme but down at firefighter locations they will not appear as hot or dangerous. This may deceive firefighters who will be unaware of dangerous conditions existing and hidden by smoke above their head, due to ceiling height. In general, any heat flux in excess of 5 Kw/m2 is considered arduous and 10Kw/m2 as critical. 

F – HEIGHT OF THE BASE OF THE SMOKE LAYER: As firefighters advance in, the smoke layer will begin to descend at a rate of about 1.5 metre (5ft)/minute until it reaches the floor. At this point the fire has become ventilation limited and visibility may be severely restricted. To effectively raise the smoke layer away from the floor, very large (or several smaller) openings are required at high level. 

G – GAS (SMOKE) COOLING: It may be extremely beneficial to attempt to cool the smoke near the ceiling using brief bursts (3-5 seconds) and sweeps of a fog-pattern. This may help to cool and contract hot gases and raise the smoke layer by a metre (3ft) or so from the floor. However, moving the gases around extensively and entraining air in with fog patterns on constant flow should be avoided. However IMPORTANT – Don’t allow your efforts to cool the gases overcome the need to deal with the base fire!!! Straight stream fire attack may well be needed within the first few seconds. Make sure you have enough water with you (+650 L/min or +175 GPM) in the hose-line in such cases of large volume high fire loaded floor space. If there is high-rack storage near to the ceiling the dangers of becoming trapped by interior collapse of stock are great, so use the reach of a solid bore stream from a good distance and don’t advance between racks that can trap firefighters in a spider-like web of light steel as they fall. In this building, a well pumped hose-line stream should reach the base fire almost from the entry point, unless it is shielded by stock or partitioning. 

VENTILATION EFFECTS: If the roof was opened up by firefighters, the fire, or by automatic ventilators, the initial impacts in general will be to raise the smoke layer slightly away from the floor, reduce heat flux (exposure) from the overhead down on firefighter locations and increase visibility. Any positive effect of roof ventilation depends not only on the size of the opening created but also the size of the openings at floor level to enable adequate air exchange. In effect, the inlets should be 2-3 times the size of the roof openings to maximise good effect. However, at the same time smoke and fire gases are being released from the roof, the fire is being fed with much needed oxygen and ill intensify. Therefore, increasing ventilation increases fire size and more flow-rate is needed to suppress the fire and this may be resource dependent. 

1.5 m2 (4 x 4ft) roof opening (B) at 600 seconds will have little effect in raising the smoke layer (F) or reducing heat flux (E) and temperatures at the ceiling will surpass 500 C (932 F) at 1000 seconds. Temperatures at the lower layer will also be untenable for firefighters at 400 C (752 F) even with this roof opening and will force firefighters out from the warehouse. The under-ventilated conditions will create the potential for backdraft. Heat flux at (E) will be in excess of 20kW/m2. 

3.0 m2 (4 x 8ft) roof opening (B) at 600 seconds will raise the smoke layer to just under one metre (2.5ft) and although the upper layer temperature stays around 400 C (752 F) after 1000 seconds the lower layer temperature becomes tenable for firefighters at around 100 C (212 F). It is worth noting here that although roof vents will allow around 10-50% of additional air to flow into the high-level smoke layer, the oxygen level at the ceiling is less than 10% and insufficient to support combustion of the smoke layer. Heat flux at low level however is still high at 15 kW/m2 which will place firefighters in the ‘critical’ zone, even though temperatures at that level are tenable. 

6.0m2 (4 x 16ft) roof opening (B) at 600 seconds will bring the base of the smoke layer to around 1.8 metres (6ft) from the floor with an upper layer temperature of 360 C (680 F) after 1000 seconds with an upper layer oxygen content that is bordering on an ignitable mix at 15%. At this point, there may well be a danger of ignitions occurring within the gas layer, although they be unsustainable. This is when the base of the gas layer is seen to bounce up and down (warning sign to evacuate). Heat flux at firefighter locations will be down around 8 kW/m2 at this point (1000 seconds) but rising.  

10m2 (5 x 20ft) of roof opening (B) at 600 seconds will raise the base of the smoke layer just above head height but upper layer smoke temperatures at 325 C (617 F) could possibly support combustion as external ignition at the roof combines with an upper layer oxygen concentration of 16% to sustain some sort of ignition. Heat flux at firefighter locations is around 6-7 kW/m2 and stable but it is the uncertainty in the upper level gas mix that may be of concern. 

REMEMBER:  

• What appears ‘cool to warm’ at floor level may mean very hot at the ceiling. 
• The larger the roof vent, the greater the need for air to feed in at lower level and support effective ventilation. 
• The greater the air exchange becomes, the larger the fire may grow. · A point may be reached where the base fire is too large for the suppressive capacity available from one hose-line. 
• A point may come where the hot gases at the ceiling will ignite using air inflow from the roof vent made to support sustained combustion. 
• A repeated ‘rise and fall’ or ‘bouncing’ at the base of the smoke layer is a waring sign to evacuate. 
• The use of ‘drones’ with thermal image capability are extremely useful for this type of fire to establish where in the building the main fire is located. This information may assist in deploying from the nearest location to the fire and further assist when considering roof ventilation operations. EuroFirefighter.com