When as many as 5,000 cars, trucks, and buses may pass through a tunnel in an hour, it is essential to dilute exhaust gases, which include hazardous carbon monoxide, by means of mechanical ventilation.
Approximately 150 cubic feet per minute of fresh air is required in a two lane tunnel, and twice that amount is necessary in a four lane tunnel.
The ventilation system in New York’s Holland Tunnel, completed in 1927, set the pattern for many long vehicular tunnels in the United States and other countries. The 1.6 mile, twin tube tunnel runs under the Hudson River between Manhattan and New Jersey at a depth of 93 feet.
Four ventilation towers, two on each side of the river, contain enormous blowers that force fresh air through louvers in the sides of the buildings and into ducts running beneath each roadway. Air enters the ducts at a velocity of 60 miles per hour and is diverted into expansion chambers that run the length of the tunnel.
By an upward system, fresh air is emitted at curbside through a series of slots. The slot openings can be regulated from a central control point so that the amount of ventilation in the tunnel varies according to traffic conditions. Exhaust fumes and air escape through ducts in the ceiling of the tunnel, travel to chimneys in the buildings at either end, and empty into the atmosphere. In peak traffic, this efficient system changes all the air in the tunnel every 1.5 minutes.
Other methods of ventilating a tunnel include a longitudinal system, used in the Saint Cloud Tunnel in Paris: air is drawn in through ducts from the ends of the tunnel and issued out in the center with fans. By the upward transverse system, found in the Mersey Tunnel in Liverpool, England, the inlet is at curbside (as in the Holland Tunnel), but exhaust is channeled out through shafts at both ends.
The Velsen Tunnel in the Netherlands employs yet another method, a cross flow system, in which blowing ducts and exhaust ducts are situated on opposite sides of the tunnel to ensure proper ventilation.