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.".
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.
Air with low oxygen concentration is produced by hypoxic air generators, also known as air splitting units. There are three different types of hypoxic air generators: membrane-based, PSA-based and VSA-based ones. VSA-based hypoxic air generators have usually a lower energy consumption compared to PSA-based and membrane-based ones. Hypoxic air generators can be located inside or outside the protected rooms. Hypoxic air systems can be integrated with the building management system and can include systems to recover the heat generated by the hypoxic air generator that, otherwise, would be wasted.
Air with low oxygen concentration is transported to the protected volumes through dedicated pipes or, more simply, via an existing ventilation system. In the latter case, dedicated pipes or ducts are not required.
Hypoxic air fire prevention systems can also be used for purposes other than fire prevention, for example:
Combining fire prevention, indoor climate and reduction of artefacts/food degradation is a completely new approach for a fire safety system.
The benefits of preventing a fire instead of suppressing it makes hypoxic air especially suitable for applications where a fire would cause unacceptable damage and traditional fire suppression is unacceptable or unusable. Unlike traditional fire-suppression systems, dedicated pipes or nozzles are not required. In situations where the installation of a traditional firefighting system would pose severe problems, fire protection can be provided with hypoxic air.
Hypoxic air for fire prevention suits best for:
The reduction of artifact degradation and food deterioration is a plus for applications like food warehouses, storage and archives.
The inherent simplicity of hypoxic air systems facilitates integration of sustainable building design and fire protection engineering.
Fire-prevention systems which result in the oxygen content being less than 19.5% are not permitted for occupied spaces without providing employees supplemental respirators by federal regulation (OSHA) in the United States.
However, hypoxic air is considered by some to be safe to breathe for most people. Medical studies have been undertaken on this topic. Angerer and Novak's conclusion is that "working environments with low oxygen concentrations to a minimum of 13% and normal barometric pressure do not impose a health hazard, provided that precautions are observed, comprising medical examinations and limitation of exposure time." Küpper et al. say that oxygen concentration between 17.0-14.8% does not cause any risk for healthy people by hypoxia. It also does not cause risks for people with chronic diseases of moderate severity.
Pressurized aircraft cabins are typically maintained at 75 kPa, the pressure found at 2,500 m (8,200 ft) altitude, resulting in an oxygen partial pressure of about 16 kPa, which is the same as a 15% oxygen concentration in a hypoxic-air application at sea-level pressure. However, passengers are sedentary and crew members have immediate access to supplemental oxygen.
Hypoxic air is to be considered clean air and not contaminated air when assessing oxygen depletion hazards.
Information relating access to the protected areas i.e. oxygen-reduced atmosphere are illustrated:
Inspection body accreditation criteria are established according to ISO/IEC 17010 for third party verification of hypoxic air fire prevention system conformance to BSI PAS 95:2011 and VdS 3527en:2007