Handbook for Methane Control in Mining - AMMSA

175Figure 14–5.—TBM ventilation system with second duct exhaustingair from the face. Venturi air movers provide additional air movement.Auxiliary face ventilationsystems. A common way toventilate the tunnel face is touse an auxiliary ventilationsystem (Figure 14–6).Auxiliary face ventilationsystems ventilate the tunnelface with a fan and duct thatare separate from the mainventilation system. Auxiliarysystems are often calledscavenger fans.Figure 14–6.—Auxiliary system with adequate overlap (not to scale).Figure 14–7.—Auxiliary system with no overlap and poor methanedilution.A critical feature of auxiliarysystems is the requiredoverlap with the mainventilation duct, sinceauxiliary systems that donot overlap properly sufferhuge efficiency losses.Figure 14–6 shows a simpletwo-duct auxiliary systemthat is working properly.The main duct is on exhaust,with the fan on the surface;the scavenger, or auxiliaryfan, is blowing toward theface. Note that the inlet ofthe scavenger fan is in thefresh air stream of the mainventilation duct. Figure 14–7shows the same arrangement,but with no overlap. Theinlet of the scavenger fanpicks up contaminated airreturning from the facerather than fresh air,creating recirculation.Eddy currents betweenthe two inlets provide theonly air to the face, greatlyreducing the amount offresh air available todilute methane. To prevent this scenario, the two ducts must overlap by at least twice thetunnel diameter (Figure 14–6).