Spring/Summer 2008 Journal of the International Society for Respiratory Protection, Vol. 25 53 A Computational Model of an Outward Leak from a Closed-Circuit Breathing Device Kathryn M. Butler Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8665 E-mail: kathryn.butler@nist.gov ABSTRACT losed-circuit breathing devices recycle exhaled air after scrubbing carbon dioxide and adding make- up oxygen from a tank of pure oxygen. This equipment provides up to four hours of use before oxygen cylinders and CO 2 -absorbent canisters need to be swapped out. Firefighting situations in which these devices would be useful include tunnels, mines, ships, high-rise buildings, and environments contaminated with biological or chemical toxins. Since firefighters may encounter environments containing open flame and high radiant heat, there is concern that a higher concentration of oxygen within the respirator may result in an enhanced possibility of fire ignition in the case of outward leakage around the facepiece. This study uses computational fluid dynamics (CFD) to investigate the flammability of the environment near a respirator leak from a closed-circuit self-contained breathing apparatus (CC-SCBA) during the breathing cycle. The physical boundary for the computational problem is defined by the combination of headform and respirator geometries obtained from 3D laser scanning. Velocity boundary conditions are defined along a narrow band representing a leak through the gap between respirator seal and head. Oxygen concentration fields and flow streamlines are presented for multiple combinations of fuel and air in the surrounding environment, for pure oxygen and air expelled from the leak, and for both normal and high stress breathing patterns. The flammability diagram for propane is used to estimate the flammable regions as a function of time. Keywords: closed-circuit SCBA, computational model, respirator leak, flammability, flammability diagram INTRODUCTION he open-circuit Self-Contained Breathing Apparatus (SCBA), which vents exhaled air to the atmosphere, is by far the most common respiratory protective device used in firefighting. The standard compressed air cylinders used in these devices contain a limited air supply with a duration of 30 min to 60 min when breathing rates are normal. Under heavy workloads, however, a compressed air cylinder may last as little as 10 min to 20 min. Although this may be adequate for fighting ordinary house fires, longer durations may be necessary in certain situations, including fires in tunnels, mines, ships, high-rise buildings, and environments contaminated with biological or chemical toxins. An alternative to the open-circuit SCBA is the closed-circuit SCBA (CC-SCBA), which recirculates exhaled air by absorbing carbon dioxide and adding fresh oxygen. Because oxygen needs are only around 5 % of air needs (oxygen consumption rate/ventilation rate), an oxygen cylinder of comparable weight to a compressed air cylinder can sustain the firefighter for a much longer period of time (Kyriazi, 1999a). Closed-circuit SCBAs, also known as rebreathers or four-hour sets, can be used for up to four C T