Ventilating a tunnel means designing and operating a system that moves air through the tunnel in a controlled way so that exhaust gases, dust, heat, and smoke are kept within safe limits. The exact solution depends on tunnel length, traffic type, gradient, and regulatory requirements, but the basic principles are similar whether the tunnel is for roads, railways, metros, or mining.
The first step is to choose a ventilation concept. The most common types are longitudinal, transverse, and semi-transverse ventilation:
- Longitudinal ventilation moves air along the axis of the tunnel from one portal to the other. This is achieved using jet fans mounted in the tunnel crown and sometimes axial fans at portals or shafts. It is widely used in road tunnels and mining access tunnels because it is relatively simple and cost-effective.
- Transverse ventilation supplies and removes air through ducts running parallel to the tunnel. Fresh air is introduced through slots or vents along the tunnel, and contaminated air is extracted through separate slots connected to an exhaust duct. This provides very uniform air distribution but is more complex and expensive.
- Semi-transverse ventilation combines features of both methods, for example by supplying air uniformly while exhausting it mainly in the longitudinal direction using jet fans and shafts.
After selecting the concept, engineers calculate the required airflow and pressure to control pollutants and heat under worst-case traffic or train schedules, and to manage smoke in fire scenarios. These calculations determine the size and number of tunnel ventilation fans, which are usually axial jet fans inside the tunnel and larger axial or centrifugal fans at shafts and portals.
To ventilate a tunnel effectively, fans are combined with air shafts, ducts, and dampers. Shafts allow fresh air to be supplied or exhaust to be removed at intermediate points, which is essential in very long tunnels. Ducts may be used for supply or exhaust in transverse systems. Dampers and louvers control where air enters or exits the tunnel, and can be adjusted during normal operation and emergencies to direct airflow along planned routes.
Modern tunnel ventilation systems rely on monitoring and automation. Sensors measure carbon monoxide, nitrogen oxides, visibility, airflow, and sometimes particulates and temperature. A central control system uses this data to adjust fan speeds and operating modes. Under normal conditions, fans may run at low speed or only some fans may be operating. When traffic density or pollution increases, the system automatically increases airflow. In a fire, special smoke control modes are activated to direct smoke away from evacuation routes.
Noise and environmental impact must also be considered when ventilating a tunnel. Exhaust fans may discharge through stacks with silencers and, in some cases, filtration systems to meet noise and emission limits around the portals or shafts. Inside the tunnel, fan selection and placement aim to control noise levels for drivers and maintenance staff.
In summary, ventilating a tunnel involves selecting an appropriate ventilation concept, sizing and locating axial or centrifugal tunnel fans, providing shafts and ducts where needed, and using sensors and controls to manage airflow. A well-designed tunnel ventilation system protects users during daily operation and provides reliable smoke management during emergencies.
