Formation of Smoke Layer in Industrial BuildingPrinciples governing Smoke Layer FormationOn the outbreak of fire, there is a marked difference in the principle of formation and spread over of smoke layers in an open large industrial building when contrasted with that of a closed compact space in a residential house. The closed space usually found in a residential house suffocates the fire and reduces its fast growth. Whereas in an open industrial building due to the abundant availability of oxygen fire rages and grows rapidly generating a huge plume of smoke which initially shoots up to the roof and then starts logging all over the space available.
During this period if sufficient extraction mechanism is not provided to ooze out the smoke from the confines of the industrial building then the extent of damage it could cause to the occupants of the building can be colossal. The characteristics of smoke is such that it can spread with a velocity of up to 5m per second whereas an occupant can walk only 1-2 m per second and can manage to run at 7.5 m per second.
So if proper exit access routes are not available for the occupants to make their way in the event of a raging fire the ensuing smoke layer, containing toxic substances and asphyxiates, could disorientate them in seconds and even kill them in minutes by engulfing them all over. Typically studies have shown that in a situation where there is no ventilation even a space spanning a volume of 10, 000 cubic meter can be smoke logged within minutes. A smoke layer study made within an area of 13, 000 cubic meters resulted in smoke logging of the whole area in less than two minutes.
The three figures in the following page shows three successive stages depicting how even large buildings get smoke logged within few minutes of a raging fire. 1. Smoke rising to roof. 2. Smoke travels laterally. 3. Smoke-logged in minutes. Controlling Smoke LayerSmoke can be controlled by limiting the spreading of smoke throughout the industrial building and by providing means of extracting heat and smoke. This requires that there be openings or fans at higher levels in the building to extract and exhaust the smoke out of the building.
It also requires the existence of effective barriers to prevent the smoke from spreading through the building. Further well positioned inlet ventilators are required to supply replacement air in place of the extracted smoke layer. Care should be taken that all such inlet ventilation takes place below the smoke layer failing which it could mix with the smoke layer causing it to cool down and getting deepened inside the building itself instead of getting extracted or exhausted.
Ideally the smoke layer should be designed not to be less than one-tenth of the height between the floor and the ceiling. Also if the smoke layer is at a very low level then replacement air will be sucked into the middle of the ventilator rather than the smoke thereby reducing its efficiency. The containment of smoke is essential for effectiveness of smoke ventilation systems. If the lateral flow of smoke is not restricted then the ventilators might be rendered less effective in driving out the hot smoke. This may cool the smoke and deepen their levels towards the ground which would stifle escape roots and endanger life.
To prevent this smoke reservoirs have to be created by the use of smoke curtains resulting in the containment of smoke. The smoke reservoirs help in increasing the time available for occupants to flee the building since they reduce the travel distances. They are a cost-effective mechanism for dividing the building to zones. It also aids the emergency services by containing and channeling the smoke into predetermined areas. Smoke reservoirs limit the travel and cooling of smoke thereby preventing their descent downwards which could lead to obscuring of vision.
The three figures below show how smoke curtains help contain the smoke layer.