Operational ventilation

Interest of operational ventilation

In a fire, smoke represents five dangers (average mnemotechnics often used is word COMIX):

it is C haude;
it is O Passover;
it is M obile;
it contains fuels (gas of Pyrolyse, Suie), therefore I nflammable;
it is to' ic X' .

It can thus cause a fast Propagation of fire, and in particular of the thermal phenomena: Flashover generalized flash ( flashover ) and Explosion of smoke ( backdraft ).

It is thus necessary to seek to control the smoke, which is the real danger of the fires in closed medium. The first operation consists in creating a high opening, or “discharge system”, which will make it possible smoke to escape. Certain buildings are provided with a discharge system which opens with a simple type lever of “draw-release” located in bottom, or which opens in an automatic way when the alarm sets fire to starts. In absence of préinstallé discharge system, the firemen create one of them for example by breaking the roof, or by breaking the top of a window and by ensuring a passage to the window (even if it means to insert a door or a wall).

Once the discharge system created, natural pulling (draft) is often insufficient, and in all manners unforeseeable. One can however force it using a ventilator: it is about the ventilation to positive pressure (VPP) .

The space thus released of smoke is surer for the speakers as for the victims being saved: good visibility, lowers temperature, notable fall of the thermal accident risk.

The VPP is opposed to another technique, the “containment of the fire”, which consists in closing the doors so that smoke did not leave the room on fire.

Principles of operational ventilation

Two type of operational ventilation are used:

Ventilation with positive pressure

Ventilation with positive pressure consists in putting a large ventilator in front of an open door. The goal of the ventilator is not to create a draft, but to insert of the air in the part, which will create an overpressure. The Gradient of Pressure between the ventilated part and the discharge system will push smoke towards the discharge system.

For an optimal effectiveness, the ventilator must be in front of a door or a window, at a distance about equal to the diagonal of the opening, and tilted of 20 ° compared to the ground. The flow of air must represent several times volume to be broken down per hour.

One classifies the strategies of ventilation according to two axes: offensive, and horizontal/vertical defensive/.

The d'" terms are often employed; entrant" and of " sortant" when operational ventilation is used, indeed, leaving it is an opening carried out in the part on fire, unlike the discharge system which is always carried out partly high of a building.

Ventilation with depression

Ventilation with depression in general pre-is installed in the building. Vacuum cleaners with great flow are installed on the roof of the building. They are connected to the various corridors of the building by conduits builds out of matter nonflammable. In normal situation the conduits are closed by trap doors whose opening is ordered automatically by the system of detection of fire integrated into the building. The ordering of the vacuum cleaners on the roof are operated by remote control by an order specific and reserved to the firemen. All the electric cables (order and power) connecting the various bodies of this system are designed to resist fire an unquestionable time.

Defensive and offensive ventilation

Defensive ventilation consists in driving out smoke of the buildings not touched by fire. Thus, one can easily approach the part on fire (establishment and setting in water) and carry out the operations of rescue.

Offensive ventilation consists in driving out the smoke of the room on fire. One uses offensive ventilation to protect, by overpressure, a part which was not invaded yet by smoke, the combustion gases are expelled by the depression that created the ventilator.

Horizontal and vertical ventilation

Ventilation is known as horizontal when the discharge system is on the same level as the ventilator.

Ventilation is known as vertical when the discharge system is higher than the ventilator.

Risks of operational ventilation

The great risk of operational ventilation is to poke fire, to even generate a Embrasement generalized flash. For that, ventilation must be perfectly controlled, the perfectly known way of smoke.

It is particularly important to block the doors in open position: a door, while claquant, would modify the way of smoke and the fresh air, which could appear catastrophic.

History of operational ventilation

Of France, the first tests consisted in trying to aspire the air with ventilators to great flow (VGD), without much success. In 1977, colonel Legendre of the firemen of Paris and engineer Michel Lebey develop ventilation with positive pressure, but this one does not interest the staff.

The VPP is installation with the the United States, in Finland and at the the United Kingdom in the Années 1980 - 90. It is “redécouverte” by France in 1996.

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