Fire ventilation

from cooking and bad/old air etc.) is prevented by the pressuredifference over the ventilation device (the hole in thewall). If the ventilation system stops working this pressuredifference will be lost, and fire gases can easily be spread viathe ventilation ducts between the different fire compartments/rooms.With functioning mechanical ventilationsystems there is little risk of spreading fire gases throughthe ventilation system. If the ventilation system stops workingthis can contribute to the spread of fire gases betweenfire compartments in buildings.A modern ventilation system can be an integrated partof the protection in the building, in which case the designand function of the system would then be based on a specificdimensioned fire pressure. By ensuring that the ventilationsystem works even during a fire, and as long as thisdimensioned fire pressure is not exceeded, the system canoften manage to prevent the spread of fire and fire gases forquite a long time. In such cases it can be important to makesure that the ventilation system maintains its function by,for example, the fire and rescue services putting in additionalmeasures to protect the system.Thermal expansionWhen there is a fire in a completely closed room there willbe a build up of pressure as a result of the heating and expansionof the air. For small or moderate differences in temperaturethis pressure will be small, i.e. when it is not burning,but in the case of fires where the temperature can reachmany hundred degrees this pressure difference can have asubstantial effect, especially if the fire develops quickly. Ifthe size of the fire remains constant the pressure will increa-Fires in rooms with only minor leakage openings(gaps) normally only produce pressures in themagnitude of several tens Pascal as a result ofinhibited thermal expansion.Air/smoke that is heatedup expands and takesup more space, whichcauses the pressure toincrease.AdiabaticWork or a process thattakes place adiabatically,takes place withoutany exchange of heatto the surroundings.39