The tunnel ventilation system should ensure adequate air quality during normal operation and maintenance activities, as well as providing the desired smoke management in case of fire. Moreover, the escape routes should be kept free from smoke.
However, when selecting and operating a ventilation system, normal operation and smoke management cannot be considered independently.
During normal operation, it is required to keep the pollutant level inside the tunnel below defined threshold values for visibility due to particulates, or toxic gases in each section of the tunnel. Some tunnel ventilation systems can be primarily designed and operated in order to minimize the impact on the environment at the tunnel portals. Further information on design criteria for normal operation can be found in the specific section of the page on "Design and dimensioning".
Control for normal operation should be achieved at minimal operating costs. It is conventional to control the operation of the ventilation system through measurement of the relevant pollutants (including opacity and either or both CO and NOx), to optimise the use of the system. In some cases, tunnel operators need to use SCADA systems to check the proper functioning of automatic control systems. For further information see the page on "Control and Monitoring".
Other objectives are occasionally pursued e.g. to minimise the risk of condensation occurring on the windscreens of vehicles and preventing the entrainment of fog from outside the tunnel. For maintenance work in the tunnel, the tunnel ventilation has to ensure the air-quality criteria for longer exposure of workers.
In case of a fire in a tunnel, the ventilation system must be operated in order to establish and maintain appropriate conditions for self-evacuation and rescue operations.
During the self-evacuation phase (also called the self-rescue phase, during which tunnels users would, of their own volition, attempt to evacuate from the tunnel), the ventilation system aims to create and maintain a tenable environment for the evacuation of tunnel users. Specifically, this environment consists of acceptable visibility and air quality levels.
During the fire-fighting phase, the ventilation system operation has to be decided by the head of emergency operations, who should choose the best solution taking into account the possibilities of the ventilation system and the operational needs of the firemen.
Further information can be found in section 8.3 "Operation of smoke control systems" of the PIARC report 2007 05.16 "Systems and equipment for fire and smoke control".
In contrast with normal operation, where conditions in the tunnel change rather slowly, the conditions during a fire may change rapidly, leading to deteriorating conditions within the tunnel. The designer initially defines the objectives of the ventilation system, sizes the necessary equipment and proposes what actions the operator should take under various scenarios. The designer, therefore, needs to understand the behaviour of the smoke, what the tunnel user might do (human behaviour), and what the operator, and later emergency services, should do in response to an emergency.
Different ventilation strategies may be used in tunnels. The choice between them is generally guided by fire safety considerations; the use of the system in normal operation is adjusted to suit.
The PIARC report 1999 05.05 "Fire and Smoke Control in Road Tunnels" provides extensive details on smoke management and PIARC report 2011 R02 "Road tunnels: Operational strategies for emergency ventilation" discusses different ventilation methodologies and operational strategies..
For tunnel operators this is reflected in a series of choices that can be made about the configuration of existing ventilation equipment. Usually these choices are based upon pre-planned equipment usage configurations, hence the following questions are to be considered:
The objectives of the design should provide a useful means of characterizing the potential operational performance of the ventilation system, whereas the actual characteristics of the system define the achievable result from an operational perspective. In responding to a real incident this distinction may be critical – and the importance of the design-operator interface crucial.
In addition, it is essential to establish standard operating procedures for road tunnel fires. The development of an integrated Emergency Response Plan is an essential first step in planning the operational responses to tunnel emergencies. The plan should specify particular responses to various types of incidents, including the description of how the ventilation system should be used. For its preparation, a proper coordination and interaction between the tunnel designers (in some cases), the tunnel operators, and all outside agencies that might ultimately become active responders to an emergency incident within the tunnel, is required.
In the case of urban or complex underground systems, tunnel operators have to face additional challenges: during normal operation to properly control the fresh air flow inside the tunnel and make sure that all branches can received the required amount of air for pollutant dilution; and in case of incidents with fire, to minimise the harmful effects of smoke by isolating different tunnel sections to avoid smoke contaminating the whole tunnel network.
Further information on main issues related to ventilation of these tunnels can be found in chapter 6 "Ventilation" of the report 2016 R19 "Road Tunnels: Complex Underground Road Networks".