Tunnel jet fans are selected by installed thrust, not by airflow. The total thrust from all running units must exceed the sum of wall friction, portal pressure loss, traffic piston effect and fire-driven buoyancy at the design critical velocity, which for most road tunnels falls between 2.5 m/s and 3.5 m/s. Individual road-tunnel jet fans typically deliver 400–1,600 N of thrust at 15–45 kW with nozzle velocities of 30–35 m/s, and are spaced 60–120 m apart — roughly 8–10 tunnel heights — so each jet has re-attached to the tunnel soffit before the next unit fires.
Step 1: Determine the critical velocity
Critical velocity is the longitudinal air speed that just prevents smoke backlayering against the airflow. It rises with fire heat release rate and tunnel slope, and falls with tunnel cross-sectional area. Typical design values are 2.5–3.0 m/s for a 20 MW car or van fire, 3.0–3.5 m/s for a 30 MW bus or truck fire, and 3.5–4.0 m/s for a 100 MW heavy goods vehicle fire in a large-section tunnel. Add roughly 0.1–0.2 m/s per percent of upward gradient, because buoyant smoke climbs the slope.
Multiply the critical velocity by the tunnel cross-section to get the volume flow the system must establish. A twin-lane tunnel with a 60 m² free section at 3.0 m/s requires 180 m³/s, which is 648,000 m³/h. No single fan does this; the flow is generated by the momentum of many jet fans acting on the whole air column.
Step 2: Build the pressure balance and convert it to thrust
The required total thrust is the sum of the resistances that oppose the design flow:
- Wall friction: proportional to tunnel length and to velocity squared; a 1,500 m tunnel at 3 m/s typically needs 250–450 Pa equivalent.
- Portal entry and exit losses: usually 0.5–1.0 velocity heads, roughly 3–10 Pa at 3 m/s.
- Traffic piston effect: helps in unidirectional tunnels but opposes flow in congested or bidirectional conditions; can swing ±100–300 Pa.
- Fire throttling and buoyancy: the hot gas layer adds resistance and, on an upgrade, a chimney effect of 50–200 Pa.
- Meteorological pressure difference between portals: commonly 20–80 Pa, occasionally 150 Pa in mountainous terrain.
Total required thrust in newtons equals the required pressure rise in pascals multiplied by the tunnel cross-sectional area in square metres. For 600 Pa across 60 m² that is 36,000 N. If each fan delivers 1,000 N of nominal thrust and the installation efficiency factor is 0.85, the effective thrust per unit is 850 N, so 43 units are needed — then add redundancy for the units inside the fire zone that must be assumed lost.
Jet fan classes and typical duties
| Duty class | Impeller diameter | Nominal thrust | Motor power | Typical application |
|---|---|---|---|---|
| Light, car park and short tunnel | 315–450 mm | 30–110 N | 0.37–1.5 kW | Car parks, underpasses under 200 m |
| Medium urban road tunnel | 630–800 mm | 350–700 N | 7.5–18.5 kW | Urban tunnels 300–1,000 m |
| Heavy road tunnel | 900–1,120 mm | 700–1,300 N | 18.5–37 kW | Motorway tunnels 1–5 km |
| Extra heavy / long tunnel | 1,250–1,400 mm | 1,300–2,200 N | 37–90 kW | Long alpine and subsea tunnels |
| Reversible heavy | 900–1,250 mm | 60–75 % of forward thrust in reverse | 18.5–55 kW | Bidirectional tunnels, twin-tube cross-passages |
Spacing and installation rules that determine real thrust
A jet fan only delivers catalogue thrust if the jet can attach to the tunnel soffit and entrain the surrounding air without hitting an obstruction. The installation efficiency factor — the fraction of nominal thrust actually transferred to the air column — is typically 0.80–0.92 and can fall below 0.70 with poor detailing.
- Longitudinal spacing of 8–10 tunnel heights, commonly 60–120 m, so the jet has decayed and mixed before the next unit.
- Soffit clearance of 0.10–0.15 times the fan diameter; too close and the jet stalls against the ceiling, too far and it drops into the traffic envelope.
- Silencers on both inlet and discharge, typically adding 200–600 mm per end, since bare jet fans produce 95–110 dB(A) at 1 m.
- Nozzle deflection of 4–8° downward where the ceiling profile is curved, to keep the jet attached without directing it at vehicles.
- Minimum 10 m clear of signage, gantries and lighting; a 1 m obstruction in the jet path can remove 15–25 % of the effective thrust.
- Grouped in pairs or triples so that losing one control circuit does not create an asymmetric thrust distribution.
Fire rating, motors and control
Jet fans in the smoke path must keep running while the fire develops. Certified classes to EN 12101-3 include F200 (200 °C/120 min), F300 (300 °C/60 min) and F400 (400 °C/120 min); road tunnel authorities commonly require 400 °C for 120 minutes, and some long tunnels specify 250 °C for 2 hours combined with fixed fire-fighting systems. Motors must be tested as an assembly with the impeller and casing, cabling must be fire-survival rated for the same duration, and starters should be soft-start or star-delta to limit inrush when 20 or more units start in sequence. Keruite manufactures tunnel jet fans at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every impeller is balanced to G2.5 and performance-tested before shipment.
When to choose which ventilation strategy
- Choose longitudinal jet fan ventilation for unidirectional tunnels up to roughly 3–5 km, where smoke can be pushed ahead of the fire and downstream traffic clears freely.
- Choose reversible jet fans for bidirectional tunnels and for tunnels where congestion can trap vehicles on both sides of a fire, and size the system on the lower reverse thrust.
- Choose transverse or semi-transverse ventilation with extract ducts when the tunnel exceeds about 5 km, when it is bidirectional with heavy traffic, or when portal emissions must be controlled.
- Choose point extraction with dampers plus jet fans for long urban tunnels where smoke must be removed locally rather than driven kilometres to a portal.
- Choose additional axial supply fans at a shaft when meteorological or piston-effect pressure differences exceed what jet fans can economically overcome.
FAQ
What is critical velocity in tunnel ventilation?
Critical velocity is the longitudinal air speed that just prevents smoke from backlayering against the airflow. Typical design values are 2.5 to 3.0 metres per second for a 20 MW fire and 3.0 to 3.5 for a 30 MW fire, increasing with upward tunnel gradient.
How is jet fan thrust calculated for a tunnel?
Total required thrust in newtons equals the required pressure rise in pascals multiplied by the tunnel cross-sectional area in square metres. Divide by the effective thrust per unit, which is nominal thrust times an installation efficiency factor of about 0.80 to 0.92, then add redundancy.
How far apart should tunnel jet fans be spaced?
Longitudinal spacing is normally 8 to 10 tunnel heights, which in practice means 60 to 120 metres. That distance lets each jet decay and mix with the main air column before the next unit adds momentum, preserving the assumed installation efficiency factor.
What fire rating do tunnel jet fans need?
Road tunnel authorities commonly require 400 degrees Celsius for 120 minutes, with 300 degrees for 60 minutes and 200 degrees for 120 minutes also recognised classes. The fan, motor and casing must be certified as a tested assembly, with fire-survival rated cabling for the same period.
Are reversible jet fans always necessary?
No. Unidirectional tunnels where downstream traffic clears freely can use single-direction fans. Reversible units are needed for bidirectional tunnels or where congestion may trap vehicles on both sides of a fire, and the system must then be sized on the lower reverse thrust.