Tunnel Ventilation Systems: Longitudinal, Semi-transversal and Transversal
Ventilation is one of the key elements in the design and operation of a road tunnel.
During normal operation, its purpose is to maintain suitable air quality by controlling pollutants generated by traffic, including carbon monoxide, nitrogen monoxide, exhaust gases and particulate matter.
In an emergency, however, ventilation takes on an even more critical role: controlling smoke and supporting safe evacuation in the event of a fire. For this reason, tunnel ventilation cannot be approached with a single standard solution. Tunnel length and geometry, traffic conditions and evacuation strategy all influence the choice and design of the ventilation concept.
Among the main configurations used in road tunnels, three approaches can be identified: longitudinal, semi-transversal and transversal ventilation. But how do they differ?
Longitudinal ventilation
In this configuration, reversible Jet Fans are installed along the tunnel vault. Rather than using dedicated ventilation ducts, the tunnel itself becomes the path through which air is moved.
The fans generate airflow along the longitudinal axis of the tunnel, making this configuration comparatively straightforward from an installation point of view.
What happens in the event of a fire?
During an emergency, the role of the Jet Fans changes. The reversible fans are used to drive smoke towards the tunnel portal closest to the fire, while maintaining suitable conditions for evacuation.
One of the key parameters in this scenario is the critical velocity: the airflow velocity required to control smoke propagation.
Finding the correct balance is essential. If the velocity is too low, backlayering may occur, causing smoke to propagate against the intended airflow direction. If the velocity is too high, excessive turbulence can increase the mixing of smoke and fresh air. Critical velocity therefore needs to be determined according to the specific characteristics of the tunnel and the expected fire scenario.
Semi-transversal ventilation
Unlike longitudinal ventilation, this solution introduces a dedicated duct located above the tunnel.
Openings positioned along the tunnel, typically at intervals of approximately 50–100 metres, connect the duct with the traffic area. Each opening is equipped with an ON/OFF motorized damper.
Depending on the design, the system can be used to supply fresh air or extract polluted air, with a dedicated fan serving the ventilation duct. This configuration makes it possible to manage airflow at specific points along the tunnel rather than relying exclusively on longitudinal air movement.
What happens in the event of a fire?
In an emergency, the system can concentrate smoke extraction around the location of the fire. Only the dampers close to the fire remain open, while the others are closed. Extraction is therefore concentrated over a section of approximately 200–250 metres.
The result is a more localized approach to smoke management, focusing extraction where it is most needed.
Transversal ventilation
The main difference is the presence of two separate ventilation ducts.
An upper duct extracts polluted air from the tunnel, while a second duct located at road level supplies fresh air. As in semi-transversal systems, openings are distributed along the tunnel approximately every 50–100 metres and equipped with motorized ON/OFF dampers.
Two dedicated fans serve the supply and extraction ducts respectively. This arrangement allows fresh-air supply and exhaust-air extraction to be managed independently along the tunnel.
What happens in the event of a fire?
In the event of a fire, the two ventilation ducts perform different but complementary functions. In the upper extraction duct, the dampers are closed except for those located close to the fire. This concentrates smoke extraction over a section of approximately 200–250 metres around the affected area.
At road level, the fresh-air supply follows the opposite strategy: the lower dampers located close to the fire are closed, while the remaining dampers continue to supply fresh air along the tunnel.
This allows smoke extraction to be concentrated where it is needed while maintaining fresh-air supply away from the fire area.
Engineering ventilation around the tunnel
Tunnel ventilation is not simply a matter of selecting a fan.
It requires understanding how air and smoke will behave inside a specific infrastructure, under both normal and emergency conditions, and then defining the most appropriate combination of fans, ducts, dampers and control strategies.
At CBI Group, our experience in tunnel ventilation includes CBJET reversible Jet Fans for longitudinal airflow management and AXL axial fans for ducted ventilation systems.
From longitudinal systems to more complex semi-transversal and transversal configurations, the objective remains the same: providing ventilation equipment engineered around the specific requirements of each tunnel.
Because in tunnel ventilation, the right solution always starts with understanding the infrastructure.