Pittsburgh, PA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2014-117, (RI 9695), 2014 Mar; :1-63
Background: One of the initial and persistent concerns over the use of mobile refuge alternatives (RA) is the temperature rise inside the RA from the metabolic heat of the occupants and the heat released by the CO2 scrubbing system. Moreover, the humidity within the RA will increase as the occupants lose water through respiration and perspiration. The National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and Health Research (OMSHR) in its 2007 report to Congress on refuge alternatives recommended that refuge alternatives be designed and deployed to ensure that a temperature-humidity metric known as apparent temperature not exceed 95 degrees F [NIOSH 2007]. Subsequently the Mine Safety and Health Administration (MSHA) adopted this recommendation. Notwithstanding the above process, a standard method to determine compliance with this metric does not exist. The heat transfer process in an RA, including the contributions of the human occupants, is highly complex, and is not easily defined analytically or experimentally. Initially, regulatory agencies accepted the certification of registered professional engineers that the manufactured RAs met the apparent temperature requirement. In 2007, NIOSH tested four mobile RAs in its Lake Lynn Experimental Mine, and found that two failed to meet the apparent temperature criterion by a wide margin. These tests used artificial heat and humidity sources to simulate the heat and humidity loading of human occupants. Another approach to testing the apparent temperature criterion would be to place an RA into mine ambient conditions, fill the RA to its rated occupancy with human subjects, and record the interior ambient temperature and relative humidity over the 96-hour period mandated in 30 CFR4 7.506 for breathable air sustainability. In consideration of this approach, a team of experts including physicians, biomedical researchers, and engineers endeavored to develop and obtain approval of a human subjects protocol for such testing. Ultimately it was determined that the experiment would place the subjects at an unacceptably high risk, and was not approvable by a Human Subjects Institutional Review Board (IRB). Thus, it became necessary to develop experimental and analytical methods to determine if refuge alternatives, as built and deployed, meet the apparent temperature requirement. NIOSH initiated research in 2008 with the goal of developing a technical foundation for such analytical and experimental procedures. Based on a significant amount of preliminary work at the Lake Lynn Experimental Mine, at NIOSH's Pittsburgh Research Laboratory, and at manufacturer facilities, the project was focused to address four research questions. The first and likely most significant question is: Does the mine behave as an infinite heat sink? The engineering assumption that a mine does behave as an infinite heat sink was applied in the calculations originally used to certify mobile RAs, and is being applied in the design of tests to the present. If a mine can be assumed to behave as an infinite heat sink, then the temperature rise within an RA would be significantly less for a given configuration than if the mine does not behave as an infinite heat sink. The second question is: Does the facility in which the test is conducted impact the resulting temperature rise? The manufacturers of RAs conduct tests to demonstrate that their RAs meet the 30 CFR 7.504 apparent temperature requirement, but they do so under varying conditions. The ability of an RA to dissipate heat could be different in a large open room (i.e., a high bay) as compared to a confined space, and accordingly the temperature rise predicted would be different. The third question is: Will the moisture generated by the occupants reduce the air temperature within the RA? It has been suggested that condensation on the interior surfaces of the RA could significantly increase the heat loss, which would in turn reduce the internal air temperature. The fourth question is: Could occupancy derating values be used for RAs that are rated and approved for use at one mine ambient temperature, but are deployed in a mine with a higher ambient temperature? Experimental and analytical studies, described in this report, were designed to answer these questions. Each of the studies contributed incrementally to the overall understanding of the problem, and the knowledge gained in one step was applied in the next to further the understanding of temperature rise in RAs. A recently completed "capstone" study provided data to answer the research questions and to validate the numerical model developed in the project. The in-mine component of this capstone study was conducted in an underground coal mine using a 10-person tent-type RA in a test area that was isolated from the mine ventilation system using brattice cloth and plastic sheeting to prevent airflow through the test area. The RA, the mine air, and the mine strata were instrumented to measure temperatures and other relevant parameters. The heat input from human occupants was simulated with specially designed containers that mimicked the heat and humidity loading equivalent to a 165-pound male. Two additional heat sources were placed in the RA to account for heat that would be generated by the CO2 scrubbers. To examine the effect of including moisture generation on the RA interior environment, the in-mine tests were conducted both dry (without moisture generation) and wet (with moisture generation). Lastly, an additional experiment was performed with the RA located in a large high bay to determine if the measured RA internal temperature rise was affected by the test facility. All tests were conducted for 96 hours. Summary of Findings: The mine strata temperatures were observed to increase throughout the 96-hour in-mine tests. The strata temperatures near the surface of the roof, rib, and floor increased more than the temperatures deeper into the strata, and, as depth into the strata increased, the strata temperature and its rate of change decreased. The strata temperature beneath the RA was observed to increase to a depth of 48 in (121.9 cm) into the mine floor. These findings demonstrate that the mine cannot be assumed to behave as an infinite heat sink, and provide a definitive answer to the first research question. This 96-hour test was repeated with the RA placed inside of a large high bay. The internal air temperature rise for the dry tests conducted in the high bay was compared to the internal air temperature rise for the dry tests conducted in the underground mine. The air temperature rise in the RA for the tests in the high bay was 21.0 degrees F, whereas for the in-mine tests, the rise was 25.2 degrees F. These findings demonstrate that the test location can have a significant impact on the results. In this case, the test conducted in the high bay underestimated the temperature rise by approximately 20%. This aligns well with the finding that the mine strata will not behave as an infinite heat sink - if it did, the in-mine results would have closely matched those from the high bay. The answer to the second research question is that the test location will affect the observed temperature rise. Significant condensation and pooling occurred within the RA during the in-mine wet test. Water puddled at every low spot on the tent floor and a layer of water that was roughly one-half-inch deep covered the bottom of the metal section of the RA. For the in-mine wet test, the temperature rise for the air inside the RA was 22.4 degrees F, which represents a 2.8 degrees F, or an 11% decrease compared to the in-mine dry test. The mine air temperature increased more during the wet test than during the dry test, whereas the temperature increase in the mine strata was less. These results indicate that condensation within the RA could indeed reduce the air temperature, but the results are confounded by the way in which the moisture from the "simulated miners" was injected into the RA. On subsequent examination, it was observed that the location of the injection tubes may have "short circuited" the heat transfer path from the interior of the RA to the RA roof to the mine air. The close proximity of the tubes to the roof of the RA may have resulted in the warm moisture condensing directly on the roof of the RA. Thus, it is impossible from the findings of this study to provide a definitive answer for the third research question. Notwithstanding, the indication is that the moisture contributed by the miners to the RA environment should be accounted for experimentally or analytically because it may have a small limiting effect on the temperature rise within the RA. A thermal simulation model, developed in this project, was evaluated using the actual parameters of the RA and the in-mine conditions under which it was tested. The model correctly predicted the observed temperature rise to within 1 degree F (0.6 degrees C). The validation of this model increases the confidence that it can be used to study temperature rise characteristics as well as to evaluate and certify RAs, as long as appropriate steps are taken to benchmark the model. This model was also used to develop derating tables for the RA used in this study. The answer to the fourth research question is, given a properly validated model that has been benchmarked for baseline conditions, tables can be developed to define the reduced occupancy to ensure that the apparent temperature criterion is not exceeded when the RA is placed in mines with ambient temperatures that are higher than the ambient temperature in which the RA was originally tested. Summary of Discussion and Recommendations: The four research questions posed at the beginning of the study were addressed, and this new knowledge and understanding can be used to improve the procedures used to determine if an RA meets the apparent temperature criterion specified in 30 CFR 7.504. RA apparent temperature determinations should be based on a standardized and published experimental method and supplemented by the use of validated and benchmarked numerical models. The experimental and analytical methods should not employ an assumption that mines will behave as an infinite heat sink. Moreover, the original engineering calculations that assumed this characteristic will underestimate the temperature rise in the RA. Furthermore, experimental tests must be conducted in a setting that will approximate the heat transfer characteristics found in a mine. A large and open room with a high ceiling will tend to behave as an infinite heat sink, and any tests conducted in such a high bay will significantly underestimate the air temperature rise in the RA. In addition, tests conducted in a test facility that attempts to mimic an infinite heat sink by using an HVAC system to maintain the air or the interior walls of the test facility at a constant temperature will also underestimate the air temperature rise within an RA. The apparent temperature in the tent-type RA tested in this study will exceed the statutory limit at a mine ambient temperature of 60 degrees F (15.6 degrees C), and consequently, the number of occupants would have to be reduced. It has been widely assumed that derating due to ambient conditions is of concern only for "hot" mines. The finding in this study indicates that occupancy derating could become necessary at temperatures lower than previously considered. It should be emphasized that this finding strictly applies only to the tested RA. However, based on first principles, similar results would be expected for RAs with comparable volumes and surface areas per miner. The exact ambient temperature at which an RA will exceed the 95 degrees F apparent temperature limit will depend on the manufactured characteristics of the RA and characteristics of the mine strata, and therefore these would need to be determined experimentally, analytically, or both. The experimental methods used in the capstone study establish a foundation for a standardized test method. The thermal simulation model is a powerful tool to predict temperature rise, and its use, in conjunction with the standardized test method, is recommended. This will allow a limited number of experimental tests to be leveraged analytically so that a wide range of RAs and operating conditions can be evaluated. Occupancy derating tables could be developed and used to account for the use of mobile RAs in varying mine ambient temperatures.