Ventilation in tunnels has two functions:
Historically, the first reason for installing ventilation systems in tunnels was the reduction of pollution levels. Although the emissions of pollutants by road vehicles have decreased dramatically over the last decades, this function is still important and must be paid close attention at the design stage. In some cases, natural ventilation due to the piston effect of moving vehicles may be sufficient to fulfil the air quality requirements in normal operation. The need for a mechanical ventilation system is assessed considering the length of the tunnel and the traffic type (bidirectional or unidirectional) and conditions (possibility of congestion). The technical report 2012R05EN "Road tunnels: vehicle emissions and air demand for ventilation" provides the data necessary for the design of the ventilation system for road tunnels in normal operation, defining the minimum quantity of fresh-air that is required to ensure adequate in-tunnel air quality and visibility thresholds. In addition, the report provides the values of admissible concentrations of toxic gases and particulate matter. The report takes into account the change in legislation which enforces more stringent emissions factors and also changes in vehicle technology, including emission factors specific to some countries which are presented together with the emission factors for the Euro standards.
Requirements for ventilation in emergency situations, especially fire are also related to the presence of other equipment or facilities, emergency exits for example, that should also be taken into account. Natural ventilation might be sufficient in some cases, but mechanical ventilation is often required for tunnels over a few hundred meters in length.
Different ventilation strategies may be used in tunnels. The choice between them is generally guided essentially by fire safety considerations; the use of the system in normal operation is adjusted to suit : see Chapter 5 "Ventilation for fire and smoke control" of report 05.05.B 1999
The longitudinal strategy consists in creating a longitudinal air flow in the tunnel, in order to push all the smoke produced by a burning vehicle on one side of the fire. If users are present on that side, they may be affected by the toxic gases and reduced visibility, so the use of this strategy in bidirectional and/or congested tunnels requires great caution. The minimum air velocity for successful smoke control depends on the design fire size and tunnel geometry (slope, cross-sectional area).
The transverse strategy takes advantage of the buoyancy of fire smoke: smoke tends to concentrate in the upper part of the tunnel space, from where it can be mechanically extracted. The system is designed so as to preserve a fresh air layer in the lower part of the cross-section (correct visibility, low toxicity) which allows self-evacuation. It is therefore important to keep the longitudinal air flow as low as possible in the fire region to avoid de-stratification and excessive longitudinal spread of smoke. This strategy is applicable to any tunnel, but the design, construction and operation of the system are more difficult and expensive.
The ventilation design process includes the computation of the minimum acceptable capacity of the system in terms of thrust and/or flow rates, the design of the ventilation network and the choice of appropriate ventilation equipment Chapter 4 of the Report 2006 05.16.B : Ventilation and its appendices 12.3 "Jet Fan calculation procedure", 12.4. "Smoke dampers" and 12.6. "Sound impact of jet-fans". Ventilation equipment should meet a number of specifications, including resistance to fire and acoustic performance.
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.
A prescriptive approach has generally been adopted with design fires. However, given the wide range of fire sizes experienced, it is evident that the selection of a design fire size for a particular tunnel is not straightforward. A consideration of several factors, such as the type of traffic allowed, the ventilation system, tunnel geometry and fire mitigation systems has to be taken into account, which suggests the possibility of applying methodologies based on a performance-based approach, as described in the PIARC report 2017R01EN "Design Fire Characteristics for Road Tunnels".
The optimisation of ventilation control for air quality considerations during normal operation is crucial to reduce energy consumption; it is an important issue since this consumption represents a significant part of the operational cost of a tunnel.
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 Report 2008 R07.
The environmental issues linked to ventilation, besides the energy consumption and the related carbon footprint, are linked to the localised, concentrated discharge of polluted air from the portals and stacks. Reducing their impact on the tunnel surroundings is part of good environmental design : see Section 4.3. "Tunnel air dispersion technique", Section 4.6. "Operational aspects" and Appendix D. "Overview of dispersion modeling in designing ventilation systems" of the Report 2008 R04.
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 report 05.16.B 2006.