Basement Store Fire: Ventilation-Limited Burning and the Door Decision
At 02:15 a crew is called to a two-storey commercial building where fire is confined to a basement store. The store measures 6 m by 4 m with a 2.5 m ceiling and is reached through one 0.9 m wide timber door from a small internal lobby. That door has stayed closed since ignition. Smoke is pulsing out of a grille-covered vent of 0.8 m² at 2.0 m height on the outer wall: slow, dense, yellow-brown puffs, and between puffs the smoke appears to be drawn back in. The steel door handle is too hot to hold, paint is blistering and lifting at the door edges, and there is a low roaring sound with no flame visible from outside. A thermal imager shows 620 °C at the lobby ceiling and 180 °C at 1 m. A 500 mm by 300 mm plastic-louvred duct passes through the store wall at 2.2 m; the louvre has melted. One 45 mm hose line with a spray branch is charged at the lobby door. All occupants are reported out.
1. Estimate the maximum heat release rate the existing openings can support and decide whether the fire is fuel- or ventilation-controlled. State what that means for the next move.
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Step 1 - ventilation factor. Two openings now feed the store: the 0.8 m² vent at 2.0 m and the 0.15 m² melted duct (0.5 × 0.3) at 2.2 m.
A√H = (0.8 × √2.0) + (0.15 × √2.2) = 1.13 + 0.22 = 1.35 m^2.5
Step 2 - air mass flow (Kawagoe: ṁ ≈ 0.5 A√H):
ṁ = 0.5 × 1.35 = 0.68 kg/s of air
Step 3 - fuel the air can burn. Cellulosic/packaging fuel needs about 6 kg of air per kg of fuel, so the maximum burning rate is 0.68 ÷ 6 ≈ 0.11 kg/s. At roughly 16 MJ/kg that is about 1.8 MW.
Step 4 - judgement. The racking, packaging and plastics in a 60 m² store would release far more than 1.8 MW if freely burning, so the fire is ventilation-controlled. The observations fit: no external flame, dense yellow-brown smoke, a very hot ceiling layer with a low neutral plane, and pressure-driven pulsing.
Consequence. Unburnt pyrolysis gases and CO are accumulating. Opening the door roughly doubles the ventilation factor, so the fire will not simply 'stay the same size' - it will grow rapidly toward flashover or backdraught. The immediate priority is cooling the compartment gases and controlling the opening, not forcing entry.
2. List the backdraught indicators present and set out how the door should be opened, if it must be opened at all.
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Indicators present:
- No visible flame despite a roaring fire and 620 °C at the ceiling.
- Dense smoke pulsing from the vent and being drawn back in between pulses - the classic pressure/air-hunger sign.
- Heat at the door: handle too hot to hold, paint blistering and lifting.
- Restricted openings, high fuel load, fire burning in a sealed compartment for some time.
- Roaring or whistling through gaps, and smoke staining around the door edges.
Together these amount to a strong backdraught warning. The correct stance is that the door stays shut until the compartment is cooled and everyone is in place.
If entry becomes necessary:
- All crews in BA, charged spray branch at the door, branch operator behind the wall and to the side of the swing, never in front of the opening.
- Open the door from a protected position - door hook, line, or a controlled 45° crack - and hold it. Do not stand in the doorway.
- Watch the smoke behaviour through the gap; if smoke is being sucked in, close up, cool the gases with short spray pulses deflected off the ceiling, and repeat.
- Only enter when the smoke stops being drawn inward and the gas layer is knocking down.
- No positive-pressure fan at that door before entry.
3. The crew leader proposes starting the positive-pressure fan at the lobby door to clear smoke before entry. Decide and justify, and give an alternative way to cool the store.
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Decision: do not start PPV at the lobby door.
A fan at the only door would push air into a hot, fuel-rich, under-ventilated compartment and mix it with the accumulated pyrolysis gases. That is precisely the mechanism that produces a backdraught: air enters, the gas layer comes into the flammable range, and an ignition source anywhere in the compartment ignites it. The lobby the crew is standing in becomes the exit path for the fireball. PPV would also drive smoke through the lobby and up the internal stair, spreading the fire problem into the rest of the building.
Better sequence:
- Keep the basement door closed and keep everyone out of the doorway arc.
- Cool the compartment gases from outside through the existing vent using a spray from a protected position, angled up into the opening in short pulses, so the upper layer is cooled without adding much air.
- Protect the stair and upper floors: close doors, keep the stair clear, and consider positively pressurising the stair from the ground floor to keep the escape route tenable - this is a different objective from ventilating the fire compartment.
- Only after the fire is knocked down and the compartment is no longer fuel-rich should PPV be used, and then from a position where the exhaust path is controlled.