What Makes a Jet Fan Reversible

A reversible jet fan is an axial tunnel fan that can generate thrust in either direction along the tunnel axis. In normal service, jet fans mounted near the tunnel soffit add momentum to the air already inside the tunnel, maintaining longitudinal airflow without the large ductwork and plant rooms required by a central ventilation system. A bidirectional tunnel fan performs the same duty but can also be run in reverse, which is what makes it useful in a fire: the direction of thrust can be selected at the control panel rather than fixed by installation.

SINDFAN jet fans cover Ø500–1600 mm at 50 Hz across the 50SJ–125SJ range, with thrust from 152 N to 1691 N. The bidirectional reversible option is available across this range.

Symmetric Blade Design and Thrust Balance

Reversing an ordinary axial fan is not simply a matter of running the motor backwards. Most impellers are optimised for one flow direction, so reverse operation produces noticeably less thrust and a different efficiency point. A reversible jet fan uses a symmetric blade design: the blade section is essentially identical about its mid-chord plane, so the fan delivers near-equal thrust in both directions. This matters in tunnel fire ventilation, because the reverse direction is a design case in its own right. The smoke control scheme assumes the fan can deliver its rated thrust whichever way it is running.

For that reason, aerodynamic and acoustic data for a bidirectional fan should be read with forward and reverse values considered separately, and the thrust figure used for the smoke case should be the reverse-direction value where the fan will be reversed.

Fire Mode: Keeping Smoke Moving

Tunnel fire ventilation depends on fans that continue to operate in hot smoke rather than shutting down when they are needed most. On SINDFAN reversible jet fans in fire mode, the motors are rated up to 400°C for 1 hour, so the fan keeps driving smoke toward the extraction point throughout the critical evacuation period.

Direction is set by the smoke control strategy. Fans upstream of the fire may be reversed to push smoke away from an evacuation route, while fans downstream maintain longitudinal flow toward an extraction shaft or portal. Because thrust is near-equal in both directions, the same fan can serve either role, which simplifies spares and panel logic.

Construction Details That Support Bidirectional Duty

  • Impellers are die-cast aluminium and X-ray inspected to ASTM E155, so internal porosity that could affect balance or strength is detected before assembly.
  • Casings are 3–6 mm galvanized steel, giving corrosion resistance in damp tunnel atmospheres.
  • Silencers are available in 1D and 2D lengths. The 1D version is the shorter, lighter option; 2D gives greater attenuation where tunnel space and mounting capacity allow. Both add pressure loss, which should be accounted for when the fan thrust is selected.

Control and Reversal Sequence

Reversing a running fan requires the motor to decelerate, stop and then accelerate in the opposite direction; the drive and control logic must prevent any attempt to plug-reverse a fan still turning at speed. Panels for tunnel installations normally include this interlock, together with a defined ramp-down and ramp-up profile. Commissioning should confirm thrust in both directions at the actual installed position, since inlet and outlet clearances, mounting height and nearby obstructions affect installed performance.

Contact SINDFAN

If you are sizing a bidirectional tunnel fan for a road, rail or metro tunnel, send us the tunnel length and cross-section, the required longitudinal air velocity, the smoke control concept, ambient and fire temperatures, and the available mounting space. SINDFAN will match a 50SJ–125SJ model and confirm forward and reverse thrust. Contact us on WhatsApp +852 9524 8434 or email info@sind-tech.com with your duty.