Tunnel ventilation system

The term « ventilation » combines several functions: pollution ventilation, smoke extraction, and sometimes ventilation for environmental protection purposes.

As explained on the page "Ventilation principles", in certain cases, ventilation in road tunnels can be achieved without the need of mechanical equipment (natural ventilation). However, in most of the tunnels with a length above some hundred meters, mechanical ventilation is unavoidable, which requires the design of a tunnel ventilation system (see the page on "Design and dimensioning").

The characteristics of the ventilation equipment to be installed strongly depend on the type of ventilation system for normal operation and fire scenarios.

Longitudinal ventilation systems utilize the tunnel tube as the “duct”. With the help of an air-jet placed in the tunnel air column, the flow resistance can be overcome by converting the jet momentum into static pressure.

The air jets can be placed at the tunnel entrance (Saccardo), blowing outside air into the tunnel or as ducted fans (usually called jetfans) along the tunnel, each one accelerating part of the tunnel air flow. Jetfans can work in both tunnel directions. (see section IV.2 “Longitudinal ventilation” of the PIARC report 1996 05.02 “Road Tunnels: Emissions, Environment, Ventilation”).

When a fan is installed into a tunnel a considerable decrease in the thrust occurs when the unit is close to the soffit or wall of the tunnel or in a niche. In those cases, it is recommended to use additional devices in order to maximize the installation factor. Examples are deflectors, inclined metal sheets, inclined silencers or nozzles. Section 4.4.4 "Ventilation" of the PIARC report 2017 R02 "Road Tunnel Operations: first steps towards a sustainable approach" provides some examples of efficiency improvement in longitudinal ventilation.

In some countries, air filtration systems have been put in place to mitigate the impact of vehicle emissions on ambient. Section 4.4.5 "Air cleaning" of the PIARC report 2017 R02 "Road Tunnel Operations: first steps towards a sustainable approach" provides some examples and approaches in different countries.

Jetfans operating in a tunnel can generate high noise levels, and can have adverse effects on speech transmission between people in the tunnel. This may become a safety issue when the noise level prevents the tunnel users from understanding what they are asked to do or when it makes it difficult for the firemen to communicate with each other. Therefore, some care must be taken in the assessment of sound emission by the jetfans.

Transverse ventilation uses ducts that run parallel to the tunnel. Two kinds of ducts are typically utilised:

• Fresh air ducts are used to inject fresh air into the tunnel in order to dilute the polluted gases produced by the vehicles;
• Extraction or exhaust ducts are used to remove vitiated air or smoke and hot gases produced by the fire from the tunnel volume. In some cases, the extraction capacity may be used in order to limit the longitudinal velocity in the tunnel under normal operation.

Extraction for smoke control is usually concentrated to a zone smaller than the length of the duct by the addition of motorized, remotely controlled dampers, also known as “point extraction”. The fans serving the ducts are often located in ventilation plants close to the tunnel portals or shafts; however, many variations can exist.

The extraction fans must be sized to ensure the required extraction airflow rates for all fire locations in the tunnel. In the past, extract ducts were typically connected to the tunnel with a number of small, evenly spaced open vents. This concept has since evolved by replacing the small, open vents with larger vents equipped with remotely controlled motorised dampers and spaced further apart. The use of thermal fuses and panels has been assessed and found to have adverse effects: the effectiveness of the smoke control system using such thermal devices was found to be compromised by the untimely opening of some dampers and/or the opening of dampers in non-optimal locations.

The thermal resistance of the fans must ensure that the extraction of the hot smoke is possible with any configuration. Cables, junction boxes and all other non-protected parts of the ventilation system should have the same fire resistance as fans. For more details on fire resistance of other equipment, see section VII.5 "Fire resistance of equipment" of the 1999 PIARC report 05.05.B "Fire and Smoke Control in Road Tunnels". Additional information on maximum temperatures measured during the Memorial Tunnel and the Zwenberg Tunnel fire tests can also be found in this PIARC report 1999 05.05.

To fulfil their function, dampers must be able to withstand normal tunnel environmental conditions and to operate under emergency conditions.

Ventilation equipment should meet a number of specifications, including resistance to fire and acoustic performance. Chapter 4 "Ventilation" of Report 2006 05.16 and its appendices 12.3 "Jet Fan calculation procedure", 12.4. "Smoke dampers" and 12.6. "Sound impact of jet-fans" provide additional information on ventilation equipment for longitudinal and transverse systems.

Life cycle aspects need also to be accounted for in the design and selection of tunnel ventilation equipment. Additional information can be found in the PIARC report 2012 R14 "Life Cycle aspects of electrical road tunnel equipment".

In some cases, as for complex or urban tunnels, specific considerations need to be considered in the equipment requirements, as described in chapter 7 "Other equipment & operating facilities" of the PIARC report 2016 R19 "Road Tunnels: complex underground road networks".   

Ventilation tests

Because the ventilation system plays a major role in tunnel safety, it is essential that it operates properly and effectively at all times. To achieve this goal, sets of tests have to be defined and adapted to specific tunnel specifications The primary objective of road tunnel ventilation system testing is twofold:

• to verify the functionality of all elements of the system, both at the time of ordering (factory and acceptance testing) and in-situ (functionality testing at specified intervals);
• to verify the in-situ performance of the system and its component parts by comparing it to the design specifications.

Three kinds of tests are usually performed in order to check the equipment and the safety objectives of the ventilation system:

• Reception (factory) tests: are aimed at checking that the equipment actual performance matches the specified requirements. The test guidelines generally point out the procedures for these test operations.
• On-site unit tests: are aimed at checking that equipment operation is in accordance with the project specifications.
• Integration tests: are aimed at checking that the safety objectives match, especially with regard to smoke control. The first set of integration tests may be performed without a fire in order to quantify the ventilation capacity, and a second set of tests may involve a calibrated fire in order to account for the buoyancy effects and to visualize the smoke development.

It is generally impossible to perform integration tests with fires as large as the design fires. Most commonly, the main purpose of these tests is to train tunnel operators and members of the fire brigade.

The list of tests and corresponding timetables must be adapted to each particular tunnel, and depends on the installed equipment, traffic volume and degree of utilisation of the installations, and cover several aspects, including:

• Visual Controls
• Electrical Measurements
• Airflow Measurements, Fan Pressure
• Airflow Measurements in Ventilation Exhausts
• Airflow Measurements at Smoke Dampers
• Noise Measurements: at air intakes and ventilation equipment outlets in order to determine the condition and aging of the silencers and the fans and in the machinery rooms to verify protection of workers.
• Shelters and ancillary rooms ventilation
• Fire Trials

Chapter 8 "Operational responsibilities for emergencies" of the PIARC report 2007 05.16 "Systems and equipment for fire and smoke control" describes in more detail the appropriate ventilation tests for ventilation systems.