Tunnel ventilation systems

Among the means used to fight fires in road tunnel, smoke control systems are important economic and strategic considerations. The main purposes of smoke control systems are to:

• Keep people as long as possible in a smoke-free part of the traffic room. This means that either the smoke stratification must be kept intact, leaving more or less clear and breathable air underneath the smoke layer (this is applicable to bidirectional or congested unidirectional tunnels) or/and to completely push the smoke to one side of the fire (this should preferably be applied to non-congested unidirectional tunnels where there are normally no people downstream of the fire).
• Allow people, in any cases, to reach a safe place in a reasonably short time and covering a reasonably short distance (see page Self-Rescue). Therefore facilities such as emergency exits or fireproof shelters should be provided whenever necessary. 
• Keep smoke out of the air in non-incident structures (escape routes, twin traffic tube, etc.)
• Produce good conditions for fire-fighting.

A longitudinal ventilation system keeps the area upstream of the fire smoke-free, which means that, in theory, there is no need for escape routes. However, emergency exits may be required to account for the unexpected, such as the fire developing to a size that the ventilation system can no longer handle, or an explosion occurring.

Smoke extraction in transverse or semi-transverse ventilation systems are based upon the following three principles:

• Achieve a concentrated smoke extraction at the highest possible efficiency by mean of extraction at ceiling level,
• Maintain the natural stratification of smoke and preserve largely smoke-free conditions at road level,
• Confine smoke, in longer tunnels, to a section of the tunnel near to the fire, by applying higher pressures in the zones situated to each side of the fire.

Smoke removal systems of this type will usually have a smoke extract duct, with openings or dampers for the capture of smoke, connected to extract fans. Additional information on the ventilation equipment and their specifications can be found in page Ventilation.

See pages Ventilation principles and Design and dimensioning for further information on the smoke control principles and design criteria.

The design of appropriate ventilation control scenarios for each possible fire situation is a very important part of the process: see Technical Report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation". These scenarios can be simple, especially when the longitudinal strategy is applied, or involve a large number of measurement and ventilation devices in complex, transverse-ventilated tunnels (page Control and Monitoring provides additional information on this topic). 

The interactions of the ventilation system design with other elements of a tunnel are numerous and diverse. In the case of transverse ventilation, for example, the required flow rates may impact the excavated section, with a potentially important impact on the construction cost. Ventilation also accounts for a large part of a tunnel's power supply requirements. It interacts closely with other safety equipment such as fire detection and fire fighting systems : see Chapter 5 "Fixed fire fighting systems in the context of tunnel safety systems" of the PIARC Report 2008 R07.

Finally, other parts of a tunnel than the main traffic space may require ventilation, most notably the emergency exits : see Section 5.3. "Escape route design" of PIARC report 2007 05.16 "Systems and equipment for fire and smoke control".