Figure 7. Air flowing through the doorway from one area to another.

The air flowing through the doorway from one zone to the other at a rate of 2 m/s generates a resistance of 6 Pa (Table 1). As we have a single doorway, the total resistance equals 6 Pa. We use mechanical exhaust ventilation. Therefore, no additional resistance is present in this case. The total resistance thus amounts to 6 Pa. We can state that all openings in the staircase require a resistance of at least 6 Pa so that the air passes through an open door in the direction of the exhaust grille of the smoke ventilation system.

The biggest opening in the staircase is an open emergency exit door outside the building. Let’s assume that its area is 2 m². In order to ensure a flow resistance of 6 Pa, the airflow rate must equal 2.0 m/s. It seems that — considering that the air should flow to the staircase at a rate of 2 m/s — we must supply 14,400 m³/h (we shall not include any other leaks in the staircase in this analysis). Therefore, the total amount of air required to meet the airflow rate criterion with such a mechanical exhaust is 14,400 m³/h + 14,400 m³/h = 28,800 m³/h. Now, we should consider a proper placement of air supply points in the staircase in order not to exceed door opening force at any storey, i.e. 100 N. This also applies to the door in the entrance hall to the zone engulfed in fire. Air is supplied to the entrance hall and staircase by two independent devices.

ATTENTION! In case of the pressure difference criterion leaks in the staircase may be significantly lower (Figure 1). The fan must therefore supply extremely different quantities of air depending on the number of open and closed doors. Smoke exhaust compensation and entrance hall air supply are carried out by a single device. Its output rate is thus always 14,400 m3/h, regardless of whether the door is open or closed. Only devices with valid certificates stating fan working ranges and system response times should be used. Additionally, the smoke ventilation system for the zone engulfed in fire shall be designed for the output rate of 14,400 m³/h.

e). Let’s assume that the entire air is being supplied to the entrance hall (Figure 8). The air flowing at a rate of 14,400 m³/h passes through the doorway between the entrance hall and the zone engulfed in fire. Subsequently, the air is being transported out of the building by the provided smoke ventilation system. Smoke exhaust compensation with the door closed is ensured by an independent device that supplies air to the zone engulfed in fire. Air supply systems (both for the entrance hall and the zone engulfed in fire) have the same output rate, but they alternate and are regulated by a pressure difference sensor. When the door is open, the device supplies air to the entrance hall, ensuring that the rate criterion of 2 m/s is met, while when the door is closed, the amount of air is drastically lowered in order to meet the pressure difference criterion (Figure 1). At the same time, the device that supplies air for the purposes of smoke exhaust compensation provides the absolute minimum amount of air (door open) in order to drastically increase it to 14,400 m³/h (door closed). Let’s analyse the resistance and system output generated with this airflow criterion.

e). Zakładamy, że nawiew całości powietrza następuje do przedsionka (Rysunek 8). Powietrze o wydatku 14 400 m³/h przemieszcza się przez otwór drzwiowy pomiędzy przedsionkiem i strefą objętą pożarem. W dalszej kolejności zostaje przetransportowane na zewnątrz budynku przez zaprojektowany system oddymiania. Do kompensacji oddymiania przy zamkniętych drzwiach przewiduje się niezależne urządzenie nawiewne do strefy objętej pożarem. Systemy nawiewne (zarówno do przedsionka, jak i do strefy objętej pożarem) mają ten sam wydatek, lecz pracują naprzemiennie, a ich regulacja odbywa się od czujnika różnicy ciśnień. Gdy drzwi są otwarte, urządzenie nawiewa powietrze do przedsionka, realizując kryterium przepływu 2 m/s, natomiast przy drzwiach zamkniętych ilość powietrza gwałtownie maleje, aby spełnić kryterium różnicy ciśnień (rysunek 1). W tym samym czasie urządzenie dostarczające powietrze do kompensacji oddymiania nawiewa minimalną ilość powietrza (otwarte drzwi), aby gwałtownie ją zwiększyć do 14 400 m³/h (zamknięte drzwi). Przeanalizujmy więc, jaki jest generowany opór i wydatek systemu przy tak realizowanym kryterium przepływu.

Figure 8. Air flowing through the doorway from one area to another

The air flowing through the doorway from one zone to the other at a rate of 2 m/s generates a resistance of 6 Pa (Table 1). As we have a single doorway, the total resistance equals 6 Pa. We use mechanical exhaust ventilation. Therefore, no additional resistance is present in this case. The total resistance thus amounts to 6 Pa. We can state that all openings in the staircase require a resistance of at least 6 Pa so that the air passes through an open door in the direction of the exhaust grille of the smoke ventilation system.