Abstract
Inert gas agents have the potential to be widely used in fire suppression systems due to health and safety concerns associated with active chemicals. To suppress fire while minimizing hypoxic effects in an occupied area, the discharge quantity of inert gas agents should be carefully designed to dilute the oxygen concentration to a specific threshold level. In this study, the general expressions between oxygen concentration, the discharge rate of inert gas agents, and the ventilation rate of the air-agent mixture are derived first. Then, explicit formulas to calculate the discharge/ventilation rate and the required quantity of inert gas agents are given if the discharge rate and ventilation rate both are constants. To investigate the dilution and fire extinguishing efficiencies of inert gas agents, two scenarios with a discharge of inert gas agents into an enclosure are modeled using the Fire Dynamic Simulator (FDS). The simulation results show that the average oxygen mass fraction approximately reaches the design level at the end of the discharge period. Variation in oxygen concentration along the enclosure height is analyzed. For the scenario with a fire source, oxygen mass fraction decreases fast as oxygen is consumed by the combustion process. Thus, the fire is extinguished a little earlier than the end of the discharge period.
Highlights
The production of halons for fire protection was banned in most countries due to the stratospheric ozone layer depletion issue [1]
There is a potential for inert gas agents to be applied as a means of fire protection in some specific cases where health and safety related to toxicity is highly focused
Inert gas agents may be discharged into an enclosure to dilute the oxygen concentration to below the design level, which results in the combustion process self-terminating
Summary
The production of halons for fire protection was banned in most countries due to the stratospheric ozone layer depletion issue [1]. Research shows that lowering the oxygen concentration in air to below approximately 12% by volume will extinguish flaming fires [2,8]. Fire suppression systems are supposed to discharge a specific amount of inert gas agents into an enclosure to achieve the desired oxygen concentration within a specified time period. If the oxygen concentration is diluted much lower than the desired level, it is called over-discharge in the present study. Over-discharge of inert agent leads to hypoxia effects for occupants while under-discharge fails to dilute oxygen concentration to suppress the fire. The present paper first derives expressions to calculate oxygen concentration as a function of ventilation mass/volume rate, oxygen level, and discharge period. Explicit formulas are presented to calculate the discharge rate of inert gas agents to obtain a design oxygen level when the discharge rate and ventilation rate are constant. FDS has been widely used in numerical fire safety validations [21,22]
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