Jet Fans in Tunnel Ventilation: How Thrust Is Measured and Specified

Jet Fans in Tunnel Ventilation: How Thrust Is Measured and Specified

In tunnel ventilation, jet fans do not move air through ductwork. Instead, they induce a longitudinal airflow by generating a concentrated jet of high-velocity air at one end of the tunnel. The critical performance parameter for this application is jet fan thrust — the axial force produced by the fan when accelerating air from inlet to outlet. Specifying a tunnel jet fan correctly requires understanding how this thrust is measured, how it is reported, and which standards govern the test procedure.

Thrust Measurement Principles

Thrust (often expressed in Newtons) results from the change in momentum of the air passing through the fan. Under free-field conditions — where the fan is not working against any ducted resistance — the net thrust is calculated from the mass flow rate and the difference between outlet and inlet velocities. In practice, thrust is measured by mounting the fan on a test rig equipped with load cells, or by calculating from velocity and pressure measurements taken upstream and downstream.

The recognised industrial procedure for this evaluation is ISO 13350, which specifies test methods for the performance of jet fans, including the determination of thrust, discharge velocity, and sound levels. This standard is essential because it defines the test environment (typically a free-air facility) and the correction factors needed to obtain repeatable, comparable figures. A tunnel jet fan manufacturer who quotes thrust values without referencing ISO 13350 is not giving a verifiable specification.

How Thrust Is Specified

For practical tunnel design, thrust is usually quoted at a defined operating point — often at maximum airflow under free inlet/outlet conditions. The specification must also state the fan diameter, rotational speed, motor input power, and whether the fan is reversible. In many tunnels, a reversible tunnel fan is required to switch airflow direction for smoke extraction or pressure relief. Reversibility inevitably changes the thrust characteristics, so the bidirectional thrust curve is a separate data set.

SINDFAN jet and tunnel fans are available in diameters from Ø500 mm to Ø1600 mm (50 Hz), covering models 50SJ to 125SJ. The thrust range spans 152 N to 1691 N. This range allows system designers to select a fan based on the longitudinal velocity required in the tunnel, the tunnel cross-section, and the pressure losses from traffic drag and gradients.

Key Mechanical Factors Affecting Thrust Durability

While initial thrust is a steady-state value, a tunnel fan must sustain that thrust under demanding conditions: high ambient temperatures, smoke exposure, and continuous vibration. The following construction details are directly relevant to maintaining thrust performance over the fan's service life.

• Impeller integrity: SINDFAN uses die-cast aluminium impellers, individually X-ray inspected to ASTM E155. This inspection verifies casting soundness, reducing the risk of blade failure that would destroy thrust balance.
• Motor thermal capability: Depending on the fire-fighting class, motors are rated for operation at 250°C for 1 hour, 300°C for 2 hours, or 400°C for 1 hour. A motor that fails at high temperature will immediately cut thrust during a critical ventilation event.
• Bearing durability: Bearings have a calculated L10 life of at least 20,000 hours, with a mean life of 100,000 hours per ISO 281. Premature bearing wear causes impeller sag and shaft deflection, degrading thrust alignment.
• Casing strength: The casing is formed from 3–6 mm medium-carbon steel, hot-dip galvanized. A rigid casing maintains the inlet bellmouth geometry and the axial clearance between impeller and diffuser, both of which affect thrust.
• Acoustic treatment: The standard construction includes a bellmouth inlet and either 1D or 2D silencers. Silencers do not alter theoretical thrust but ensure the downstream flow profile is uniform, which is important for accurate thrust measurement and for achieving the design thrust in the tunnel.

Test and Documentation Standards

Beyond thrust testing to ISO 13350, complete tunnel fan documentation includes aerodynamic performance to ISO 5801. Together, these standards provide a defensible basis for comparing bids from different manufacturers. When specifying a tunnel jet fan, always ask for a test report that shows thrust values, volumetric flow, measured input power, and the test facility conditions. The report should also indicate whether the figures apply to forward operation only or include the reverse direction for a reversible tunnel fan.

This level of documentation ensures that the fan will deliver the momentum flux needed to control smoke and maintain a tenable environment in a tunnel emergency.

Contact SINDFAN for Your Duty

If you are selecting a jet fan for a tunnel project and need to confirm thrust ratings, reversible operation, or test certifications, contact SINDFAN directly. Their engineers can match your tunnel duty — airflow, thrust, size, and fire-rating requirements — to a specific model in the 50SJ–125SJ range. Reach them on WhatsApp at +852 9524 8434 or email info@sind-tech.com with your duty specification.