In most modern road tunnels there is no giant duct blowing air from end to end; instead, rows of compact fans hang from the ceiling and quietly nudge the air along the bore. Each of those is a jet fan, and together they are how longitudinal ventilation works. A jet fan does not try to move all the air in the tunnel by itself - it throws a fast jet that drags the surrounding air along with it, so a modest fan moves a large volume of tunnel air. This guide explains how a jet fan induces longitudinal airflow, how the control system banks fans on to reach a target air velocity, and how reversal and critical-velocity logic use the same fans for smoke control in a fire.
Jet Fan Ventilation in one line: A jet fan is a ceiling-mounted, axial fan that generates thrust to induce longitudinal airflow along a tunnel rather than ducting air from outside. The high-speed jet it discharges entrains and drags the surrounding tunnel air, so a bank of jet fans can drive the whole air mass along the bore toward a portal or extraction point. Most tunnel jet fans are reversible so the same units can push airflow either way, which lets the control system stage them to a target air velocity in normal operation and reverse or ramp them for smoke control during a fire.
A jet fan is an axial fan, meaning air passes straight through it along the line of its shaft, mounted horizontally under the tunnel ceiling and aimed along the direction of the bore. What makes it effective is not the volume of air passing through the fan itself, which is small compared with the tunnel's cross-section, but the momentum of the fast jet it discharges. That jet shoots out at high velocity and, through friction and mixing, entrains the slower air around it and pulls it along. The net effect is that the fan imparts thrust to the entire column of air in the tunnel, gradually accelerating it in the chosen direction.
Because a single fan can only impart so much thrust, jet fans are installed in numbers, spaced out along the length of the tunnel and often paired side by side at each location. Spacing them out matters, because a fan needs a clear run of tunnel downstream to let its jet re-mix with the bulk air before the next fan adds its own thrust; placing them too close together wastes their effect. Distributing the thrust along the bore in this way lets the fans overcome the friction of the tunnel walls and the resistance of the traffic and steadily build the airflow the ventilation strategy calls for.
This induced-airflow approach is what defines longitudinal ventilation and sets a jet fan apart from the large supply or exhaust fans used in other schemes. There is no separate air-supply duct running the tunnel's length; the tunnel bore itself is the duct, and the jet fans are simply distributed thrust points within it. That makes the arrangement relatively simple and cost-effective to build, which is a large part of why longitudinal ventilation with jet fans is so common, and it is also why the fans have to be individually controllable so the control system can add exactly as much thrust as conditions require.
The control system does not run the jet fans flat out; it runs just enough of them to achieve the airflow the tunnel needs at that moment. That target is usually expressed as an air-velocity setpoint, a desired speed for the air moving along the bore, chosen so that pollution is swept out fast enough to keep gas and visibility readings within limits. Anemometers or airflow sensors in the tunnel measure the actual air velocity, and the control logic compares it with the setpoint to decide whether to add or shed fans.
Fans are brought on in banks rather than one continuous ramp, because most jet fans are on-off machines that run at fixed speed. To increase airflow the control system starts another fan or another pair; to decrease it, it stops one. This banked staging steps the total thrust up and down in increments, and the logic uses hysteresis and minimum run times so the fans are not switched on and off repeatedly around the setpoint. The result is a stepwise approach to the target velocity, with the system settling on the smallest number of running fans that keeps the air moving fast enough.
Staging in banks also spreads wear across the fan installation. Rather than always starting the same fans, the control strategy can rotate which units run first, so operating hours accumulate evenly and no single fan is overworked. Because the fans are distributed along the tunnel, the logic can also choose which sections to energise based on where pollution is worst, concentrating thrust where it does the most good. All of this is a normal-operation duty, keeping the measured air velocity near its setpoint as traffic and congestion push the ventilation demand up and down through the day.
The reason most tunnel jet fans are built to be reversible becomes clear in a fire. Reversibility means a fan can produce thrust in either direction, so the same units that push air one way in normal ventilation can push it the other way when the emergency strategy demands it. During a fire the ventilation goal changes completely: instead of diluting pollution everywhere, the system must control where smoke goes, and that often requires driving the airflow in a specific direction that may be opposite to the everyday flow.
The key concept in fire-mode jet-fan operation is critical velocity. Critical velocity is the minimum air speed along the tunnel needed to stop smoke from a fire from flowing backward against the airflow, a phenomenon called backlayering. If the air moves faster than the critical velocity in the chosen direction, the smoke is swept downstream and the tunnel upstream of the fire stays clear for people to escape and for responders to approach. The control system therefore ramps the jet fans, running many of them and reversing some as needed, to establish an airflow that meets or exceeds critical velocity in the direction that keeps the escape path clear.
This dual role is central to how jet fans fit into a tunnel's SCADA and field operations. The control system must know each fan's status and be able to command it individually and reliably, both to stage fans to an air-velocity setpoint day to day and to invoke the correct reversal pattern in an incident. Operators in a control room, backed by SCADA, monitor fan health, run periodic reversal tests to prove the fans will respond when needed, and after an event verify that every fan returned to normal service. Because the fire-mode duty demands full output and reversal on command, jet-fan reliability is maintained and monitored continuously, not just checked when pollution control happens to call for it.
A jet fan works by thrust, not by pumping the whole tunnel's air through itself. It discharges a high-speed jet that entrains and drags the slower surrounding air along with it, so a compact fan imparts momentum to the entire column of air in the bore. Installed in numbers and spaced along the tunnel, the fans together accelerate the whole air mass in the chosen direction.
Jet fans are made reversible so they can produce thrust in either direction along the tunnel. In normal ventilation this flexibility helps direct airflow toward the most convenient portal, but the main reason is fire safety: during a fire the system may need to drive smoke in a specific direction that is opposite to the everyday flow. Reversibility lets the same fans establish that directed, smoke-controlling airflow on demand.
Critical velocity is the minimum air speed along the tunnel needed to prevent smoke from a fire flowing backward against the airflow, a behaviour known as backlayering. If the air moves faster than the critical velocity in the chosen direction, smoke is carried downstream and the upstream side stays clear for escape and access. In a fire the control system ramps and reverses jet fans to reach or exceed critical velocity in the direction that protects the escape route.
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