EMERGENCY EVACUATIONS
SAFE ESCAPE UNDER PRESSURE
A pressurisation system is intended to prevent smoke leaking through closed doors into stairs by injecting clean air into the stairwell. The intent is to facilitate pedestrian escape route and offer firefighting access, Mark Newcombe, Product Engineer, Axair Fans UK Limited explains.
WHAT IS EN 12101-6? The standard describes six differential pressure systems based on their function, ranging from facilitating a safe evacuation in the fire affected area only, through to keeping firefighting access smoke free while evacuation takes place.
SYSTEM CLASSES: Class A system: For means of escape: The design conditions are based on the assumption that the building will not be evacuated unless it is directly threatened by fire. The level of compartment means that it is normally safe for the occupants to remain inside the building.
Class B system: For means of escape and firefighting: This pressure system may be used to minimise the possibility of serious smoke contamination during evacuation and while fire fighters are extinguishing the fire.
Class C system: For means of escape via simultaneous evacuation: These systems are designed for all of the occupants of the building to be evacuated when the fire alarm is activated.
Class D system: For means of escape and risk to personnel that are sleeping: These systems are designed for buildings where the occupants may be sleeping.
Class E system: For means of escape via evacuation in phases: These systems are installed in buildings where evacuation is carried out in stages or phases.
Class F system: Fire protection system and escape means: These pressure systems are used to minimise the possibility of serious smoke contamination of staircases used by fire fighters at the same time that the building is being evacuated.
28 WHAT PERFORMANCE IS
REQUIRED? For all possible systems, at least two evacuation situations are considered that may occur:
a) With all the doors of the evacuation route closed: A differential pressure of 50Pa must be maintained between the protected route and the non- pressurised areas where the fire has broken out. In some cases there is an additional requirement where a differential pressure of 10Pa has to be maintained between the protected route and the non-pressurised areas when the exit door to the outside of the evacuation route is open. In the latter, the difference between class systems lies in the number of doors that are open inside the staircase whilst the exit door to the outside is also open.
b) When a door is opened to the fire zone: In this case a minimum air velocity through the open door must be maintained. This is 0.75m/s for systems designed for evacuating personnel and 2m/s for systems designed for assisting firefighters. The difference between the class systems is based on the air velocity through the open door to the fire affected area as well as the number of doors that are open inside the staircase.
Selecting the class of the pressurisation system will determine the design flow rate of the installation. It is essential to determine the correct size and system class in order to ensure correct operation of the fire protection pressurisation system.
PRESSURISATION SYSTEM
CONTROL A system must be installed that is capable of automatically and safely supplying the two air flows; excessive
air flow to the staircase must be prevented so that children or less able occupants can open the access door during an evacuation.
The most commonly used pressure system is the fan controlled by a frequency inverter, which takes signals from a sensor that measures the pressure difference between the evacuation route and the non- pressurised area. This controls the pressure during situations where the doors in the evacuation route are closed. In the case where a door is opened, the pressure in the staircase decreases and fan speed increases to regulate the pressure and supplies the maximum airflow when an evacuation or exit door is open.
www.axair-fans.co.uk www.tomorrowshs.com
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