The correct auxiliary fan location, as shown in Figure 11, is outby the last open crosscut where it can entrain and blow fresh air into the face area. The fan should also be positioned in the furthest upstream entry (in this case, the left) to create air movement that is traveling in the same general direction as the air being moved by the main mine fan. Being in the left-most entry, recirculation is reduced and the air sweeps the face. Using this configuration, virtually all of the air that was moved by the auxiliary fan was intake air, and 57 percent of the air at the last open cross-cut was pushed 400 feet to the face area. Figure 10 shows that the proper positioning of the auxiliary fan in the intake air significantly increases the fresh air quantity that moves to the face. Correct positioning of the fan yielded a mine ventilation efficiency of 45 percent when measured 400 feet inby the last open crosscut.
Both vane-axial and propeller fans should be positioned in such a way that they entrain, and then move, the maximum quantity of fresh intake air to the face. Generally, this is outby the last opening in the pillar. Both vane-axial fans and propeller fans should be positioned to entrain as much intake air as possible, keeping in mind that vane-axial fans entrain for up to 260 feet after the fan (Krog and Grau, 2006). The propeller fans should also be located in the intake air where entrainment takes place within 100 feet downstream from the fan. It should be noted that moving the fan further from the face, outby the last open crosscut in the long pillar, does not increase the airflow to the face, but increases the percentage of fresh air available at the last open cross-cut that is moved to the face.
Concerning the application of these fans, propeller fans work best in regional ventilation applications where they can move large slugs of air. Vane-axial fans work best in face and dead-end ventilation applications (due to better penetration and greater mobility) (Krog et al., 2006).
Unit ventilation systems
Figure 7 shows the transitioning of a split mining system to a unit mining system where the unit section is to the right of entries “D,” “E,” and “F.” In this scenario, the ventilation of the unit section was significantly affected by the positioning of two auxiliary fans. The fan situated at “A” is needed to push the air from the last open crosscut toward the face at “C” and to prevent short circuiting of airflow to the bottom of the figure. The second fan is situated near “B” between the last open crosscut and the “C” face. This fan, when directed to ventilate “C” face, allows only about 10 percent of the available air at “A” to reach the unit section through entries “D” and “E.” However, when the fan is positioned in the same location, but directed toward the unit section, the total ventilation air through “D” and “E” amounts to about 70 percent of the air coming through the last open crosscut near “A.”
Two important considerations are noteworthy when transitioning to unit ventilation. First, it is necessary that a mobile fan can be quickly adjusted, depending upon whether workers are in the uppermost “C” face or in the unit section. Portable diesel-powered fans are becoming popular in underground limestone mines and are best suited for such conditions. Second, as shown in Figure 7, a fan is necessary at “A” in all situations to ensure that fresh air passing through the last open crosscut is directed to the active faces.
These methods to improve ventilation airflows and ventilation efficiencies in large-opening mines include the use of the NIOSH Estimator to estimate the airflow required for proper DPM dilution, use of propeller fans where possible, and utilization of mine planning layouts that incorporate long stone pillars, stoppings, and auxiliary fans to direct airflow to active face areas. The use of long stone pillars is particularly effective in reducing leakage between intake airways and return airways, thus allowing the maximum amount of air produced by the main mine fan to reach the face area. Although various factors can impact face ventilation effectiveness, the best method to ventilate the face is to position an auxiliary fan outby the last open crosscut and in the furthest upstream entry, such that it blows through the intake entry. It is shown that auxiliary fans entrain and generate considerably more air quantity than the actual fan rating. Therefore, although actual conditions will dictate the proper position of the auxiliary fans, as a general rule, they should be located in the intake air, outby the last opening in the long pillar, entraining as much intake air as possible. This will lead to increased fresh air quantities moving toward the face. To reduce recirculation, the fan should also be located in the furthest upstream entry to create air movement that is traveling in the same general direction as the air being moved by the main mine fan.
The findings and conclusions in this report have not been formally disseminated by the National Institute for Occupational Safety and Health and should not be construed to represent any agency determination or policy.
Chekan G.J., Colinet J.F., Grau, III R.H.  Evaluating ventilating air movement in underground limestone mines by monitoring respirable dust generated from production shots. In: Ganguli R., Bandopadhyay S., eds. Mine ventilation: Proceedings of the 10th U.S./North American Mine Ventilation Symposium (Anchorage, Alaska, May 16-19, 2004). Leiden, Netherlands: Balkerma, pp. 349-355.