Axial Fans for Tunnels, Metros and Car Parks: Ventilation and Smoke Control

Ventilation in tunnels, metro stations, and car parks does two very different jobs. Every day it keeps air clean by removing exhaust and heat. In a fire it must control smoke so people can escape and firefighters can work, and it must keep running at high temperature while doing so. The second job is governed by product certification, not just by airflow, and that shapes how these fans are specified. This page covers the daily and design duties, what smoke ratings mean, and where the boundary of our range sits.

Two Duties, Two Sets of Requirements

Duty	What it needs
Daily ventilation	Airflow to keep pollutants below limits, often controlled by gas sensors, and efficient part-load operation
Smoke control in a fire	Certified operation at a stated temperature for a stated time, with defined airflow and, in tunnels, often reversible flow

Many installations use dual-purpose fans that serve both. The daily duty determines energy use, and the emergency duty determines certification, motor arrangement, and control.

Car Park Ventilation

Enclosed car parks are ventilated to remove vehicle exhaust, especially carbon monoxide and nitrogen oxides, and to clear smoke in a fire. Required airflow comes from codes and local rules, usually as air changes per hour or per-space rates, so confirm the requirement for your project.

Illustration. A car park of 30,000 m³ ventilated at 6 air changes per hour needs 30,000 × 6 ÷ 3,600 = 50 m³/s, or 180,000 m³/h. The figure of six is for illustration only. A duty of this size is typically split across two fans of about 90,000 m³/h each, both within our range of 185,000 m³/h, so that ventilation continues if one is out of service.

Demand control. Cars are seldom present in numbers that need full ventilation. Carbon monoxide sensors can trigger ventilation only when needed, and speed control follows demand. Because power falls with the cube of speed, running at half airflow needs roughly one-eighth of the power in the ideal case, which makes demand control very effective. See adjustable pitch versus VFD control.

Tunnels and Longitudinal Ventilation

Road tunnels commonly use longitudinal ventilation, where fans push air along the tunnel in the direction of travel. Jet fans, a specialised type of axial fan mounted in the tunnel space, move air by momentum rather than through a duct. The thrust they produce depends on outlet velocity and area:

Thrust ≈ air density × outlet area × outlet velocity²

Example. A fan with a 1.0 m diameter and a 30 m/s outlet velocity, at 1.2 kg/m³, produces about 848 N of thrust before installation losses. Installed thrust is lower because of effects near walls, ceilings, and neighbouring fans, so tunnel designers apply installation factors.

Jet fans are tested to ISO 13350, and tunnel fans that must reverse flow in an emergency are a recognised category, since airflow direction may need to change to move smoke the right way. Axial fans can reverse by reversing rotation, at reduced performance, so the reverse duty must be specified and verified. See how to read an axial fan curve.

Metros and Station Ventilation

Metro systems combine tunnel ventilation fans, station extract fans, and plant room ventilation. Airflow is set by heat loads from trains and passengers, by smoke-control design, and by air quality requirements. Fans often run at low speed most of the time and at full duty only in an emergency, so efficient part-load control and reliable emergency starts both matter.

What Smoke Ratings Mean

Smoke-extraction fans are tested to EN 12101-3, which classifies them by temperature and duration. A class such as F300-120 means a fan has been shown to operate at 300 °C for 120 minutes, and F400-120 means 400 °C for 120 minutes. Suppliers publish these classes for axial fans and for centrifugal fans. Key points:

• The class is a tested and certified product rating, not a general claim about materials.
• The rating applies to the tested configuration, including motor and drive.
• Local fire regulations decide which class and duration a project needs.
• An emergency rating does not make a fan suitable for continuous high-temperature process duty.

Our guide to high-temperature axial fans sets out the distinction.

Motors, Cables, and Controls in Fire Conditions

A smoke fan is only as reliable as its power and control supply. Motors, cables, and control equipment that must operate in a fire need to be rated for it, and start-up should not depend on equipment that may fail in the same fire. Where the motor sits in the airstream, it is exposed to hot smoke, so the motor arrangement is part of the certified configuration. Variable-speed drives that do the daily duty may need a bypass for emergency full-speed operation.

Noise

Tunnel portals, car park exhausts, and station vents are near people, and noise limits often apply. Silencers add resistance that must be included in the system curve. See axial fan noise.

Where Our Range Fits

NextAir Systems manufactures axial fans with impeller diameters up to 1,600 mm, static pressure up to 2,400 Pa, and air volume up to 185,000 m³/h. That covers ducted and plant-room ventilation for car parks, station plant areas, and service tunnels. Smoke-rated certification to EN 12101-3, and jet-fan or reversible tunnel-fan designs, are confirmed case by case before you specify them, and we will say so early if a requirement is outside what we can supply.

What to Specify

1. Daily airflow requirement and how it was derived.
2. Emergency smoke duty: airflow, temperature class, and duration.
3. Whether flow must reverse, and the reverse performance required.
4. Control approach: sensor-based, speed control, or fixed.
5. Number of fans and redundancy.
6. Motor arrangement, power supply, and fire-rated cabling.
7. Noise limits and attenuation.

Tell Us the Duty and the Smoke Class

Send your daily airflow, smoke duty, and site details, and we’ll confirm what we can supply. Contact us for a free quote.

Frequently Asked Questions