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, among other factors, the length of the tunnel, traffic type (bidirectional or unidirectional) and conditions (possibility of congestion) and type of vehicles.
The same factors determine the requirements for ventilation in emergency situations, especially fire. The presence of other equipment or facilities (emergency exits for example), 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.
In addition to these major aspects, environmental issues linked to ventilation, issues relating to energy consumption and the related carbon footprint need to be considered. Those 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.
The following chapters describe key aspects to be taken into consideration for a proper design and operation of ventilation systems in road tunnels.
The page on Ventilation principles presents the strategic elements that must be taken into account before making a decision concerning the choice or the design of a tunnel ventilation system.
The page on Design and dimensioning describes the main criteria to be considered for the design and sizing of ventilation systems in road tunnels, covering both normal and emergency ventilation.
The page on Control and monitoring examines some specific aspects related both to the tunnel ventilation system and the control system and SCADA.
And last, but not least, additional considerations related to tunnel operation are given in the page on ventilation strategies . These are essential to ensure that all actions required are handled in a consistent and safe way, recognising that the level of safety provided for tunnel users is highly dependent upon the specific characteristics of the tunnel, but also depends strongly on operational procedures which need to be taken into account for each tunnel.