Jet Fan Ventilation: Powering Airflow in Tunnels and Underground Spaces

Modern tunnel and underground systems rely on jet fan – specialized axial fan – to circulate air and control smoke without extensive ductwork. Jet fan (also called impulse or induction fan) creates a high-velocity jet of air that entrains surrounding air and pushes pollutants toward exits. For example, images like the one below show ceiling-mounted jet fans in a roadway tunnel, illustrating how a focused air jet drives airflow through the tunnel. Unlike traditional ducted fans, a jet fan system uses the tunnel itself as the flow path, saving space and cost while moving a large volume of air.

Jet fans mounted on a tunnel ceiling. A jet fan ejects a high-speed air stream that entrains (draws along) surrounding air to create a longitudinal flow. Each fan produces thrust (measured in Newtons) by accelerating air; as that jet mixes downstream, its momentum carries more air and contaminants along the tunnel bore. By leveraging Bernoulli’s principle, a single jet fan can influence many times its own discharge volume. In effect, the jet induces a large-scale circulation without requiring large ducts – a key advantage in space-constrained tunnels and garages.

Jet fan is duty-specific device: it is installed in series along the ceiling of an enclosed space (like a road tunnel or parking structure) and controlled together. Sensors can trigger individual fans to dilute or extract polluted air, offering a “smart” ventilation strategy. Jet fan directs air to a common exhaust or exit point, meeting ventilation and smoke exhaust needs in a cost-effective, duct-free way.

Key Performance Parameters

Jet fan performance is characterized by several key parameters: thrust, airflow velocity, entrainment (induction) ratio, noise, and efficiency. Each parameter helps determine how well a jet fan will ventilate a given space.

• Thrust (Newton, N): Thrust is the force pushing the air. High-power tunnel jet fans can produce on the order of 1,000–3,000 N of thrust. For example, one manufacturer’s data shows a tunnel jet fan series with “Thrust Force Up to 3000 N”. Thrust determines how far and how fast the airflow can be driven along the tunnel. (Thrust is often measured per fan on a test bench according to standards like AMCA 250.)

• Air Velocity & Flow (m/s, m³/s): Jet fan expels air at high speed. Typical outlet air velocities range from 10 to 40 m/s in the jet core (often higher near the impeller). The volumetric flow rate depends on fan size and motor power. For example, a 1 m (1000 mm) jet fan might move 5–10 m³/s at full power (exact values vary by design). Higher velocity jets entrain more air.

• Entrainment (Induction) Ratio: Entrainment is the multiple by which ambient air is drawn into the flow. The induction ratio is defined as (total airflow delivered)/(fan’s own air discharge). Modern jet fans can achieve induction ratios of 10× to 30× or more, meaning each 1 m³/s from the fan nozzle can induce 10–30 m³/s total flow. (For example, one product claims a “28 Times Induction Ratio”.) A high induction ratio means a jet fan effectively ventilates a much larger volume than just its direct output.

• Noise (dB): Jet fan can be loud at high speed. Halving a jet fan’s speed typically cuts noise by about 15 dB. In practice, a full-power jet fan in a tunnel might generate on the order of 70–85 dB(A) at 1 meter (free-field), depending on size and design. To control noise, many tunnel fans have sound-attenuating inlet flanges or acoustic liners.

• Efficiency (N/kW): Thrust efficiency is a common metric (thrust per unit power). High-efficiency jet fans can reach on the order of 30–40 N/kW. For instance, a 1500 N jet fan drawing 50 kW has 30 N/kW. Better aerodynamic designs (tight blade profiles, optimized nozzles) maximize thrust for each kilowatt used.

These parameters must be balanced in selection. For example, increasing motor size raises thrust but also noise. Designers often use several smaller jet fans in series to achieve the required ventilation with acceptable noise and cost.

Jet Fan vs Other Ventilation Systems

Jet fans offer a longitudinal (ductless) ventilation solution, which differs fundamentally from traditional ducted systems and other fan types. Key contrasts include:

• Longitudinal (Jet) vs. Transverse (Ducted) Ventilation: In longitudinal systems, jet fans are mounted in the tunnel itself to push air along its length by impulse. By contrast, transverse ventilation uses large axial fans (often above-ground) to supply or extract air through ducts or shafts at intervals. In practice, a pure jet-fan system needs no lengthy ductwork – it simply uses the tunnel as its own “duct,” saving space and materials.

• Axial/Mixed Flow vs. Jet (Impulse) Fan: Jet fans themselves are typically axial-flow fans (with a short, tubular housing) that produce a narrow jet. Mixed flow fan (which blend axial and centrifugal flow) exists but is less common in long tunnels. Traditional large axial fans move huge air volumes in one place. Jet fans, conversely, are modular and distributed. In a parking garage, for example, classic designs once used multiple roof vents and grilles, but jet fans allow a duct-free solution.

• Roof/Industrial Fans vs. Tunnel Jet Fans: Roof fans and commercial/industrial fans serve buildings and factories, typically providing exhaust or general circulation. Jet fans are specialized for vehicular tunnels, underground garages, and similar spaces. The two systems are complementary.

In summary, jet fans are chosen over ducted systems when a linear, flexible ventilation is needed and space or cost precludes large ducts. They excel in long corridors under traffic (road, rail, parking), whereas conventional fans are used in different HVAC roles.

Applications of Jet Fans

Jet fans are versatile and find use in many underground and enclosed spaces:

• Road and Rail Tunnels: The most classic application is highway and railway tunnels. In single-bore or bi-directional tunnels, ceiling-mounted jet fans continuously sweep exhaust fumes out and fresh air in. In an emergency fire, these same fans create a longitudinal airflow to extract smoke. Jet fans are typically installed along tunnels to drive ventilation air via impulse.

• Parking Garages and Vehicular Access Ramps: Multi-story parking structures use jet fans to meet strict CO/NO₂ limits. Jet fan ventilates by distributing air with strategically placed units. Jet fan systems eliminate much of the complex ductwork required by older garage designs. Jet fan pushs air from fan to fan creating jet streams, which allow the elimination of ductwork in parking garages. This simplifies large parking ventilation tremendously.

• Tunnel Garage and Logistics Corridors: Any long subterranean passage can benefit. For example, utility tunnels, mining drift tunnels, underground cargo conveyor corridors, or military bunker passages use jet fans for normal ventilation and smoke control.

Ceiling-mounted jet fans in a tunnel. Here, three high-powered jet fans drive airflow along the tunnel. In a road or transit tunnel, a string of such fans is installed at intervals to maintain ventilation flow in the direction of traffic. These fans can reverse thrust if needed for bi-directional traffic or smoke control. Sensors and controls turn them on as required.

Overall, jet fans adapt to many underground environments – from city metro and highway tunnels to parking garages, access ramps, and long corridors in industrial plants.

Benefits of Jet Fan Systems

Jet fan ventilation offers several advantages that make it a popular choice:

Jet fan installation in progress. Jet fan units are typically ceiling-mounted with quick-attach brackets, so installation is relatively straightforward. Unlike ducted systems, a jet fan system requires no long runs of ductwork or large ventilation shafts. The low-profile design of jet fans “can result in significant savings in excavation and construction costs”, since ceiling height can be lower and less concrete work is needed. A modular array of jet fans (as shown above) can often be fitted into existing tunnels with minimal structural change.

• Lower Construction and Civil Costs: Without building long ducts or new ventilation boreholes, projects save time and money. Studies report “savings in excavation and construction costs” because jet fans mount in the existing ceiling. In parking garages, eliminating shafts and ductspace can also allow higher leasable floorspace.

• Elimination of Ductwork: As noted earlier, jet fans “push air from fan to fan creating jet streams,” meaning contaminants are carried along tunnel lines. This removes the need for complex supply/exhaust ducts. Fewer parts means simpler design and less leak risk.

• Dynamic Ventilation Control: Jet fan systems integrate with CO/NO₂ sensors and building controls. Only the fans needed are activated, lowering energy use. Fan networks can be dynamically regulated: “Sensors placed at optimum points… control which fans operate to dilute or extract contaminated air”. This smart operation improves efficiency and safety (since fresh air is pushed only where needed).

• Smoke Exhaust: In emergencies, jet fans can rapidly clear smoke. Effective ventilation is vital for firefighter access and evacuation, since harmful gases (CO, NO₂) and smoke can otherwise make tunnels deadly. Jet fan, by driving air toward exits, helps fulfill this requirement. As one industry source emphasizes, “an efficient ventilation system will help evacuation” in case of fire. By providing longitudinal airflow, jet fans support this smoke-control objective.

• Space Saving: Compared to large inlets or blade-type fans, jet fans occupy minimal space overhead. This compactness is especially important in retrofit or constrained sites. Project managers often cite the compact footprint of jet fans as enabling more flexible ceiling layouts.

• Maintenance Access: Individual jet fan can be serviced one at a time without affecting the whole system. Since each is accessible from the ceiling, routine maintenance is simpler than cleaning long ducts or large station fans.

Taken together, these benefits explain why jet fan systems are widely used despite often needing many individual units. The net result is a ventilation solution that is cost-effective, easy to install, and well-suited to smoke control.

SHUANGYANG FAN Solutions

SHUANGYANG FAN offers a full line of ventilation and smoke-control products to meet these needs. Their portfolio includes jet fans( images like the ones below) designed for robust tunnel ventilation and smoke extraction. Besides, for general ventilation and exhaust, they produce commercial fans, metro fans, industrial fans, and roof fans. For ultra-critical environments, their nuclear-grade fans are built to stringent safety standards. Complementing these, SHUANGYANG FAN supplies fire smoke dampers and control systems to integrate with any network.

Conclusion

Jet fan is the workhorse of underground ventilation, turning the tunnel itself into a ventilation duct through powerful, high-velocity air jets. By entraining ambient air, each jet fan moves far more air than it discharges, making it efficient for cleaning vehicle exhaust and controlling smoke. Key metrics like thrust (N), airflow velocity, and induction ratio determine a jet fan’s capability, while noise and efficiency influence design choices. Compared to ducted or axial systems, jet fan offers significant advantages in cost, space, and flexibility. It is widely used not just in metro/rail/road tunnels but in parking garages and industrial passages. The combination of ease of installation, minimal ductwork, and effective smoke extraction has led to its global adoption.

Note: The data is for reference only, please consult relevant companies for details. And the content of the article cannot avoid omissions and errors. Welcome to propose corrections.