Fresh Air Flow Required to Maintain Safe Carbon Dioxide Levels and Provide a Breathable Air Environment in a Refuge Alternative

Federal mining regulations in the United States mandate that underground coal mines install refuge alternatives (RA) for miners to seek refuge after an inescapable disaster. RAs are required to isolate and protect occupants from hazardous conditions and to provide a life-sustaining, breathable air environment for a minimum of 96 hours. According to federal RA regulations, an RA’s oxygen levels (%O2) must be maintained between 18.5%–23% with carbon dioxide levels (%CO2) less than 1%. Once an RA is occupied, due to human breathing, the %O2 can decrease, and %CO2 levels can increase quickly. One method of providing an RA with a breathable air environment is to use a borehole air supply (BAS) to provide fresh air from the surface, purge existing harmful gases, and prevent harmful gas build-up. RA regulations require air supplies to provide air at 12.5 cubic feet per minute (cfm) per person. To investigate the minimum fresh air flow (FAF) rate needed to maintain interior %O2 and %CO2 within the mandated ranges, researchers conducted testing in a modified shipping container that represented the volume of an RA. During these tests, propane (C3H8) combustion and additional CO2 supplied from cylinders were used to match human O2 consumption and CO2 generation. The FAF rate supplied to the shipping container was varied to determine the minimum FAF rate required for the %CO2 inside the shipping container to stabilize below 1%. The test results showed that the minimum FAF rate was between 1.76–2.12 cfm per person. Therefore, the mandated per-person FAF rate provides a 6x–7x safety factor. Test results also showed that the %O2 range requirement was satisfied for the entire range of tested FAF rates from 1.76–12.5 cfm per person. In this paper, researchers from the National Institute for Occupational Safety and Health (NIOSH) provide a repeatable test method that can be used to evaluate the FAF rate versus interior gas concentrations (%CO2 and %O2) for various occupancy levels to ensure a breathable air environment within a refuge alternative. This paper also discusses federal RA regulations and previous NIOSH research. Additionally, this paper provides an experimental concept and set-up description, including the C3H8 combustion and supplemental CO2 delivery with gas flow rates used to simulate human breathing, data collection sensors, laboratory modifications, and safety measures. Lastly, the paper discusses test results, including the amount of time taken to reach hazardous interior %CO2 and %O2, as well as %O2 and %CO2 resulting from several FAF rates that have been used to validate a predictive model. This test method could be adopted to evaluate breathable air environments in refuge alternatives and confined enclosures in various industries.