In the last article, we discussed the B class pressure differential system and the airflow rate criterion for the air supplied to a zone engulfed in fire, which is 2 m/s in case of the gravitational exhaust ventilation. Now, we will focus on a solution based on the mechanical exhaust ventilation, the purpose of which is extracting hot smoke from a zone engulfed in fire.

The PN-EN 12101-6 standard defines six classes of pressure differential systems: A, B, C, D, E and F. A designer working with an expert should establish which class to use in a given building. Depending on the choice, certain system design criteria introduced by the standard must be met. In total, we can distinguish 5 versions of the system based on the mechanical exhaust ventilation. Today, we will discuss three of them while the subsequent two will be described in the next article.

Mechanical Exhaust Ventilation

With this solution we need to seek the lowest possible underpressure in the zone engulfed in fire when the door to the entrance hall is closed, because the underpressure value directly affects the door opening force, which cannot exceed 100 N.

a) Let’s assume that the entire air is supplied to the staircase (Figure 4). The air flowing at a rate of 14,400 m³/h passes through the doorway between the staircase and entrance hall, and then 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. An automatic opening window of 2 m² effective area is planned for smoke exhaust compensation when the door is closed. Let’s analyse the resistance and system output generated with this airflow criterion.

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, so 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 entrance hall and 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. 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.

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, so 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 criterion with such a mechanical exhaust ventilation is: 14,400 m³/h + 21,600 m³/h = 36,000 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. Since the smoke exhaust compensation is carried out by the air supplied to the entrance hall, in case of the pressure difference criterion (Figure 1) the output rate is 21,600 m³/h. The air supply device supplies the constant amount of air at all times — when the door is both closed and open. 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 21,600 m³/h.