What a Longitudinal Ventilation System Does

In a longitudinal ventilation system, air is pushed along the tunnel axis rather than distributed through ducts. Jet fans, mounted at or near the tunnel crown, discharge a high-velocity jet that entrains the surrounding air and drives the whole cross-section in one direction. The system must overcome two main resistances: aerodynamic drag from vehicles moving through the bore (aided or opposed by the piston effect) and the natural stack effect created by temperature difference and portal elevation. Because longitudinal systems use the tunnel bore itself as the air path, they need no ductwork and no additional cross-sectional area for ventilation, which is why they are common in road and rail tunnels, metro running tunnels and enclosed car parks.

Thrust and Spacing in Tunnel Ventilation Design

Total installed thrust is the starting point, not fan quantity. The design calculation begins with the required air velocity for the design case (normal traffic, congested traffic, or fire), converts it into total pressure loss, and then divides by the net thrust each fan delivers inside the tunnel. Net thrust differs from the nominal value because the jet must be corrected for the incoming airflow velocity and for the number of fans installed.

Spacing follows from thrust: fans are arranged in groups, and each group is positioned so that the jet from the upstream group still has enough velocity to entrain air when it reaches the next group. Groups placed too far apart allow the jet to decay to ambient velocity and lose induction effect; groups too close together waste thrust that could be distributed elsewhere in the bore.

In fire design, the target velocity is usually set by the critical velocity needed to prevent smoke back-layering upstream of the fire. That value often drives the total thrust more than the normal-traffic case, so the fan bank is normally sized on the fire scenario and then checked against congested traffic.

Jet Fan Placement Principles

  • Mount the fan so the discharge jet clears the traffic envelope and stays parallel to the tunnel axis. Misalignment reduces usable thrust.
  • Keep the inlet clear of the tunnel wall and of the previous fan's jet. Recirculation at the inlet lowers both thrust and efficiency.
  • Space fans in the same group laterally so jets do not merge prematurely and so the entrained flow covers the full cross-section.
  • Place groups where the local cross-section is regular; abrupt changes in shape disturb the jet.
  • For fire cases, confirm that fan groups upstream of the fire can be stopped or reversed without pushing smoke into the evacuation route.

Fans are typically mounted in pairs at the crown or on side brackets, with the number of fans per group limited by tunnel width and by maintenance access.

Reversible Operation and Fire Management

Reversible jet fans let the operator set the airflow direction to match the fire scenario, pushing smoke away from evacuating occupants or holding it in place while they exit. Reversibility must be designed as a duty, not as an occasional feature: blade angles and motor sizing must deliver the required thrust in both directions, and the control logic must be able to switch the whole bank in a defined sequence.

Fan Construction and Testing

SINDFAN jet fans are available from Ø500 mm to Ø1600 mm with thrust from 152 N to 1691 N. Motors are rated for 250°C/1 h, 300°C/2 h and 400°C/1 h to suit different fire-resistance requirements. Impellers are die-cast aluminium and X-ray inspected to ASTM E155; casings are 3–6 mm steel, hot-dip galvanized. 1D and 2D silencer options are available where portal noise limits apply. Fans are tested to ISO 5801 and ISO 13350. Applications include road and rail tunnels, car parks and metro systems.

Contact SINDFAN

Send your tunnel length, cross-section, design air velocity, fire case and required thrust to SINDFAN on WhatsApp +852 9524 8434 or email info@sind-tech.com, and we will match fan size, thrust and placement to your duty.