Hypoxic air fire prevention system

Hypoxic air technology for fire prevention, also known as oxygen reduction system, is an active fire protection technique based on a permanent reduction of the oxygen concentration in the protected rooms. Unlike traditional fire suppression systems that usually extinguish fire after it is detected, hypoxic air is able to prevent fire.

In a volume protected by hypoxic air, a normobaric hypoxic atmosphere is continuously retained: hypoxic means that the partial pressure of the oxygen is lower than at the sea level, normobaric means that the barometric pressure is equal to the barometric pressure at the sea level. Usually 5%/10% of oxygen contained in the air is replaced by the same amount of nitrogen: as a consequence a hypoxic atmosphere containing around 15 Vol% of oxygen and 85 Vol% of nitrogen is created. In a normobaric hypoxic environment, common materials cannot ignite or burn. Thus, considering the fire triangle, a fire cannot occur because of the lack of sufficient oxygen. "However, at 15% oxygen level, risk for fire still exists, and the system cannot be seen as an alternative to extinguishing systems.".

The phenomenon of fire prevention at higher oxygen concentration than the oxygen concentration required for extinguishing an established fire has been observed and exploited for decades.

Air with a reduced oxygen content is injected to the protected volumes to lower the oxygen concentration until the desired oxygen concentration is reached. Then, because of air infiltration, the oxygen concentration inside the protected volumes rises: when it exceeds a certain threshold, low-oxygen air is again injected to the protected volumes until the desired oxygen concentration is reached. Oxygen sensors are installed in the protected volumes to monitor continuously the oxygen concentration.

The exact oxygen level to retain in the protected volumes is determined after a careful and accurate assessment of materials, configurations and hazards. Tables listing ignition-limiting oxygen thresholds for some materials are available in the fire safety literature. Alternatively the ignition-limiting threshold is determined by performing a proper ignition test described in BSI PAS 95:2011 - Hypoxic air fire prevention systems specification.

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