Handbook for Methane Control in Mining - AMMSA

173The tunnel face is usually ventilated with much less air thanyou think. If 20,000 cfm goes down the shaft but only 2,000 cfmreaches the face, then as far as methane control is concernedthe tunnel is being ventilated with only 2,000 cfm.Figure 14–2.—Exhausting system of face ventilation. For clarity,the equipment is not shown.Figure 14–3.—Blowing system of face ventilation. If the blowingduct diameter is 2 ft, the maximum distance from the face would be20–30 ft.There are two categories of faceventilation: exhausting (Figure14–2) and blowing (Figure 14–3).The exhausting system is the lessefficient in clearing out gas fromthe face. For example, the faceventilation effectiveness 7 (FVE)of a 10,000-cfm, 24-in-diamexhaust duct located 10 ft from amine face is only about 0.10. Inother words, the concentration ofmethane measured near the faceis 10 times higher than theconcentration in the air passingthrough the duct [Wallhagen1977]. If the end of the exhaustduct is more than 10 ft from theface, the FVE is even less.Therefore, the end of an exhaustduct must always be 10 ft or lessfrom the tunnel face unless othermeans are used to ventilate theface, such as venturi air moverspowered by compressed air.Blowing face ventilation (Figure14–3) is better for clearing outgas than exhaust ventilationbecause the momentum of the airin a blowing jet carries it farther.However, blowing systems alsolose effectiveness as the face-toductdistance increases. The ductmust be kept as close to the faceas possible, with the end of the7 FVE is an indicator of the proportion of air reaching the last foot, i.e., a distance of 1 ft from the face.