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:
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:
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.
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".