Ventilation is needed in tunnels because they are long, enclosed spaces where air cannot circulate freely. Whether the tunnel is used for road traffic, rail, metro, mining haulage, or service vehicles, engines and equipment inside the confined space emit exhaust gases, particles, and heat. Without a controlled ventilation system, these pollutants would accumulate to unsafe levels, making the tunnel hazardous or impossible to use.
In everyday operation, the primary goal of tunnel ventilation is to control exhaust gases and particulates. Vehicles emit carbon monoxide, nitrogen oxides, and fine particulates; trains and service vehicles may add diesel exhaust or brake dust. Ventilation fans—often jet fans mounted in the tunnel crown and larger axial fans at portals or shafts—create a longitudinal or transverse airflow that carries this contaminated air towards exhaust points. Fresh air then enters through entrances or dedicated supply shafts, keeping average concentrations below regulatory limits and maintaining visibility.
Ventilation is also needed to manage temperature and comfort. In long tunnels, heat from engines, electrical systems, lighting, and solar gain at portals can raise air temperature. Without ventilation, hot air would stagnate, affecting driver comfort, passenger safety, and the performance of sensitive electrical and signaling equipment. Forced airflow carries heat away, replaces hot air with cooler outside air, and reduces condensation on tunnel linings and equipment.
Perhaps the most critical reason for tunnel ventilation appears during a fire or emergency event. Smoke and hot gases from a vehicle fire can travel rapidly along the tunnel, severely limiting visibility and creating dangerous conditions for people trying to escape. Modern tunnel ventilation systems are therefore designed to perform a defined smoke control function: directing smoke in a predictable direction, protecting escape routes, and creating tenable conditions long enough for evacuation and for emergency services to intervene. This requirement strongly influences the sizing, temperature rating, and control strategy of tunnel fans.
Ventilation also contributes to long-term durability and maintainability. Continuous or intermittent airflow reduces moisture accumulation and helps control corrosion on reinforcement, cables, and support structures. Good air movement improves working conditions for maintenance crews and allows dust and fumes from maintenance activities to be diluted and removed more quickly.
In summary, ventilation is needed in tunnels to ensure acceptable air quality in normal operation, maintain reasonable temperatures, and provide reliable smoke control during emergencies. Tunnel ventilation fans, ducts, and dampers are therefore not optional extras; they are essential safety and performance components that allow modern tunnels to operate reliably and safely throughout their service life.
