In complex infrastructures, the problem is rarely choosing a system in the abstract. The real issue is understanding how that system behaves inside the actual structure, with its geometry, its constraints, its aerodynamic losses and its operating conditions. Longitudinal ventilation in tunnels is a perfect example of this difference between nominal performance and actual performance.

Jet fans have been a widely used solution for road tunnel ventilation for decades. The reason is clear: they are versatile, relatively simple to install and compatible with longitudinal configurations that avoid more invasive plant works. But their widespread use should not make us forget one critical point: in a tunnel these systems do not operate under ideal conditions.

A significant part of the electrical power used is lost due to aerodynamic friction and interaction with the tunnel geometry. In other words, the useful thrust actually transmitted to the airflow is lower than what might be expected by looking only at nominal data.

The problem of real performance

The central point is that a fan cannot be assessed only on the basis of what it declares under test conditions. In a tunnel, other factors come into play: the shape of the section, installation position, jet development, proximity to the vault, friction losses and interference with the internal flow.

For this reason, when comparing different configurations, what really matters is not just bench thrust, but the ability to produce effective thrust inside the tunnel. It is precisely in this respect that shaped silencer solutions become interesting, as they are designed to divert the jet away from the tunnel vault and improve efficiency in operation.

Why geometry matters more than it seems

When two fans have substantially similar nominal data, one might think their behavior in a tunnel would be almost equivalent. In reality, this is not the case. If the jet is directed more favorably with respect to the real tunnel section, the distribution of the flow changes, and so does the thrust transferred to the air.

This means that efficiency does not depend only on the machine, but on the interaction between machine and infrastructure. And it is precisely here that an apparently secondary plant choice, such as the shape of the silencer, can produce concrete differences in performance.

A ventilation system, therefore, should not be read as the sum of isolated components, but as the result of a real configuration: fans, tunnel geometry, installation layout and operating conditions all work together and determine the final quality of the ventilation.

Measuring instead of assuming

The real leap in quality lies in measuring actual behavior rather than assuming it. When an on-site experimental campaign is carried out, with surveys distributed across the whole section, velocity measurements and differential pressure checks, a much more reliable reading of tunnel performance is obtained.

And this is precisely the point of greatest interest from an infrastructure perspective: real performance must guide both design choices and future decisions on upgrades, efficiency improvements and operating strategies.

More efficient ventilation does not simply mean better use of energy. It also means greater consistency between the installed system and the behavior expected under safety conditions. In a tunnel, this difference has a direct impact on reliability, sustainability and the infrastructure’s ability to react correctly under ordinary or critical conditions.

Conclusion

Tunnel ventilation cannot be evaluated only in terms of nominal data. It must be read as the overall performance of the installed system. Section geometry, machine position, flow development, friction, external influences and operating modes all contribute together to defining the real result.

When designing or upgrading ventilation systems, this difference between theoretical performance and in-tunnel performance is not a technical detail: it is a central issue of infrastructure safety.