Mechanical ventilation is required when a single tunnel face is driven more than 150 m. The duct installation must meet these rules:
1. For blow-in or exhaust systems, the intake or outlet should be outside the tunnel. The intake should be more than 20 m beyond the portal; the exhaust should be built like a chimney.
2. The distance from the duct to the working face depends on site conditions. For blow-in systems, the outlet-to-face distance is found by calculation.
3. With combined ventilation, when one fan moves forward the other fan's duct is extended. The overlap between the two ducts must be at least 20–30 m.
4. The duct should be laid straight with tight joints. Average leakage over 100 m must not exceed 2%. Bend radius must be at least three times the duct diameter.
When a building is sealed off from outside air, the short-term allowable concentration is 2–3%. Fresh air per person per hour: 30–40 m³ for normal use; 5–35 m³ for clean-mode wartime ventilation; 2–10 m³ for filtration-mode wartime ventilation.
Tunnel construction uses four ventilation methods:
1. Blow-in ventilation. The fan pushes fresh air through the duct straight to the face, displacing the smoke from blasted rock. The mix is then pushed out of the tunnel.
2. Exhaust ventilation. The fan intake sits near the face and pulls the smoke out of the tunnel directly. Fresh air flows in from the portal to replace it.
3. Combined ventilation. Blow-in and exhaust run together. It removes the dead zone at the face and pushes smoke out through the duct. It is the usual method for long tunnels.
4. Parallel heading ventilation. In long tunnels with a parallel heading, the heading and cross-cuts act as the air path. This lowers the fan pressure; local fans are added near the face.
Large tunnel axial fans are built for the supply and exhaust ventilation of very long tunnels through vertical or inclined shafts. They suit centralised supply, centralised exhaust and combined supply-exhaust layouts, and also semi-transverse, full-transverse and auxiliary heading ventilation. By changing the impeller diameter, or the blade angle at a fixed diameter, you meet the supply and exhaust airflow and pressure the design requires.