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informed him of his company's desire to build upon NIOSH's research with the design of its fi ltration and pressurization system with the goal of improving the air quality to miners working in its equipment. From this conversation, a coopera- tive working relationship was established with J.H. Fletcher Co. and this longstanding relationship continues to this day. Initially, shop testing was performed on a few different pieces of Fletcher's mining machinery at its headquarters and manufacturing facility in Huntington, W.Va. Next, engineers with Fletcher's metal/non-metal underground sector laid the groundwork for testing at an actual mine site to evaluate two new pieces of its equipment equipped with the newly designed fi ltration and pressurization system. This cooperating site was the Sidwell Mining Co. underground limestone mine, south of Zanesville, Ohio. NIOSH's point of contact was Ted DiNardo, the mine manager at this relatively new underground limestone mine. From November 2010, through July 2011, NIOSH performed research during 13 different visits to this mine. Unfortunately, at the end of December 2011, DiNardo informed the research- ers that the Sidwell underground mine was closing and all its underground assets, including the equipment, were being sold to the nearby Shelly Materials Co. (part of Oldcastle Materi- als/CRH). However, as part of the sales agreement, all of the original Sidwell underground employees, including DiNardo, were offered jobs with Shelly, which was opening a brand new mine only 4 miles away from the previous site. Once the new underground mine progressed to an extent where testing could be performed, NIOSH continued its research with the mine manager and underground mining crew. Fortunately, since the same two pieces of equipment tested at the previous mine — a single-boom face drill and roofbolter machine — were purchased, NIOSH was able to continue its research on the ex- act same equipment. From April 2013, through the completion of research in November 2014, NIOSH traveled to this under- ground limestone mine on 19 different occasions as a part of this cooperative research effort. Making research advances As a result of this cooperative research effort, advances were made in four areas. 1. The impact of a powered intake air unit versus a static design. It was determined that a powered intake air system was the optimal design over a static type system. On a powered unit, the intake air has its own fan, so the air is delivered at positive pressure through ductwork to the main HVAC unit. One benefi t to this is that a known quantity of intake air is always provided to the enclosed cab. Obviously, as the intake fi lter loads with dust, the intake air quantity decreases, but there is a known and reliable air quantity range from a clean to a fully loaded fi lter. The intake air is a critical component in the enclosed cab's positive pressurization level along with the cab's integrity (abil- ity to be reasonably sealed). In a static design, the actual intake air quantity is more variable based upon the loading rate of all the fi lters used in the system, and it is diffi cult to determine or control the intake-to-recirculation air ratio. Because of this, the actual amount of intake air delivered to the enclosed cab is variable and is dependent on numerous factors. Having a known quantity of intake air delivered to the enclosed cab is not only of great benefi t in creating positive cab pressurization, but is critical in regards to CO 2 levels and ensuring the equipment operator does not become asphyxiated while working in the enclosed volume. NIOSH researchers believe a minimum quantity of at least 25 cfm of intake/outside air per person should be maintained to dilute CO 2 quantities exhaled by each worker. Since almost all enclosed cabs in the mining industry are designed for a single operator, a recommended lower limit for pressurized intake air would be somewhere around the 40 cfm range in order to achieve a minimal cab pressurization, while also ensuring a level of safety in regards to the CO 2 issue. A good rule of thumb for an acceptable pressurized intake air range would be between 40 and 140 cfm. Another advantage with a powered intake air unit is it can J.H. Fletcher Co. face drill and roofbolter machine were evaluated at two different underground limestone mines over four years. AGGREGATES MANAGER / February 2017 31