Mine plan layouts should be developed with considerations given to future ventilation needs as the mine expands in size. Three methods to ventilate a large, underground limestone mine have been documented and tested by NIOSH: perimeter, split, and unit ventilation (Grau et al., 2002). These methods are designed to create stable and measurable airflows at the working faces. Older large-opening mines that have limited preplanned ventilation systems often attempt to ventilate using the perimeter ventilation system. This system is designed to keep the active faces continuously supplied with intake air, which is accomplished by separating the active mining areas from the rest of the operation using air walls of stoppings or rectangular pillars, as shown in Figure 5. The disadvantage of this system is that all sections are on one ventilation circuit, creating possible air quality issues with multiple faces. Since the mining front continually expands, a second air wall is developed at least four entries beyond and parallel to the first air wall. In an older developed mine that is trying for the first time to establish a ventilation system, the first air wall needs to be developed by erecting fabric stoppings. However, the second air wall could be developed by minimizing crosscut development through the use of long stone pillars. As mining progresses beyond the second air wall and a third air wall is developed, crosscuts in the second air wall can be fully developed and the stone recovered. Expanding the perimeter in this way allows for better ventilation to the faces. Split mine ventilation is designed to split the mine into two parcels, intake and return, separated by an air wall (Figures 1 and 6). Face ventilation with this system is similar to the perimeter ventilation method, in that air is coursed by air walls. However, this system can be used during mine start-up and allows the ventilation air to be divided into multiple splits, if desired. Also shown in Figure 6 is a projected truck route situated in return air.

Unit mining is generally used in combination with other ventilation plans such as split ventilation systems (Krog et al, 2004). The unit ventilation method is a series of “units” or “sections” which make up the active mining areas, as shown in Figure 7. The units described in this method are pre-planned mining blocks of several pillars that contain the working faces and that are surrounded on four sides by long air walls incorporating stone stoppings. The air walls have only a few openings or check curtains, which allow for ventilation control and haulage. One advantage of this method is that it allows for these units to be at least partially removed from the main mine ventilation circuit when mining is completed.

Using auxiliary fans

Auxiliary propeller fans are becoming more popular in underground limestone mines. NIOSH studies have found that auxiliary, free-standing vane-axial and propeller fans have different airflow patterns which affect the positioning of fans for effective face ventilation (Krog et al., 2006). The study found that, due to momentum transfer, both types of fans entrain and move considerably more air than the rated capacity of the fan, as shown in Figure 8. The 8-foot-diameter propeller fan was rated at 115,000 cfm and was powered by a 30-horsepower motor. The vane-axial fan was rated at 22,000 cfm, it was equipped with a 23-inch diameter discharge reducer, and was powered by a 25-horsepower motor. The propeller fan generated airflow of 536,000 cfm. However, this was a slower-moving air mass that interacted differently with the surrounding air as compared to the airflow developed by the vane-axial fan. The air exited the propeller fan outlet at 2,500 feet/minute and expanded rapidly to cover the entire cross-section of the drift. The velocity profile was much closer to uniform (i.e., being more evenly distributed across the drift) than was observed with the vane-axial fan. The vane-axial fan had an exit velocity of 7,600 feet/minute and entrained air as far as 260 feet downstream from the fan, whereas the propeller fan entrained air only for about 100 feet. Both fans showed similar reductions in airflow quantity with distance.

These test results verified those by Dunn et al. (1983), who found that a free-standing vane-axial fan entrained nine to 15 times the rated capacity of the fan. Due to these different entrainment levels, propeller and vane-axial fans have different placement criteria when used as auxiliary fans. Kissell (2007) reported several studies showing that ventilation efficiencies in dead-end entries were improved when free-standing vane-axial fans were equipped with a reducing nozzle and were tilted slightly towards the roof.

Placing auxiliary fans

Although a long pillar air wall can assist in delivering large air quantities to the last open crosscut, the correct placement of auxiliary fans plays a vital role in moving the air from the last open crosscut to the face. To better understand this concept, NIOSH performed a series of in-mine tests to determine the impact that auxiliary fan positioning has on the percentage of intake air at the last opening of the long pillar that is delivered to the face (Grau and Krog, 2008).

Figure 9 shows an auxiliary fan improperly positioned because it is inby the last pillar opening. A fan at this location provides only marginal improvement in ventilation. In this scenario, 74 percent of the air produced by the main mine fan reached the last open crosscut, while only 5 percent of the air was measured 400 feet from the last open crosscut. A small amount of face ventilation is achieved because the air mass moving down the intake entries carries momentum which pushes it a short distance into the face area. Also, a small amount of ventilation arises from the motion of both the loader and trucks at the face. However, even with the fan and the equipment movement, the face ventilation in this scenario is minimal.

The fan being positioned inby the main ventilation air stream increases recirculation while providing minimal fresh intake air to the face. Furthermore, the position of the fan in the middle entry tends to promote excessive recirculation as the air reverses both in entry “A” and is non-directional in the entry at “B,” with the recirculated air acting as a substitute for fresh air. It should be noted that not all recirculation is detrimental, with previous studies showing that recirculation is a problem only when it replaces the quantity of fresh air moving to the face (Kissell and Bielicki, 1975).

Figure 10 shows the ventilation efficiencies measured along the stone pillar air wall to the last open crosscut and to the face where the highest efficiency drop occurs. The total efficiency is highly dependent upon the placement of an auxiliary fan or fans near the last open crosscut. An auxiliary fan improperly positioned inby the last open crosscut provided a ventilation efficiency of 5 percent measured at a location 3,000 feet from the intake portal or 400 feet inby the last open crosscut.