Ventilation Facts

Ventilation is required in road tunnels to provide a safe, comfortable environment for motorists. To maintain a safe tunnel environment during normal operations, several factors must be considered:

Safe levels of vehicle-emitted pollutants such as carbon monoxide (CO) and oxides of nitrogen (NOx) must be maintained.
Visibility must be maintained for safe driving.
A tenable environment must be maintained for motorists escaping a fire emergency.

In tunnels, typically less than 240 m, the air pushed through the tunnel as a result of vehicular movement (piston-effect) is sufficient to maintain safe levels of pollutants. Fresh air is brought in the entrance portal and tunnel air is forced out the exit portal. For longer tunnels, up to 1.5 km and during congested traffic conditions, the piston-effect may not be sufficient. Therefore, mechanical ventilation using jet fans must then be employed to maintain a safe tunnel environment.

For this study, a jet fan ventilation system is proposed for the tunnels (greater than 240 m and less than 1.5 km). The number of jet fans has been determined assuming a 1000 mm inside diameter jet fan, and it assumes 3 lanes in each direction.

GreenLink Mechanical Ventilation Requirements

At this preliminary stage of concept development, the critical velocity is 3 m/s. The jet fan system must generate this velocity through the tunnel while overcoming airflow resistance due to stopped traffic, tunnel wall friction, resistive effects of a fire and an opposing wind.

The following table summarizes the mechanical ventilation requirements for the 6 proposed tunnels along the 6 km project alignment:

Tunnel Name	Length	Ventilation Type	Ventilation Requirements
Howard Avenue Community Tunnel	120	Natural	None
Mount Carmel / Villa Paradiso Community Tunnel	1,000	Longitudinal	42 jet fans, 21 jet fans in 7 sets of 3 equally spaced throughout tunnel in each direction
St. Clair College Community Tunnel	240	Natural	None
Oakwood Community Tunnel	1,220	Longitudinal	42 jet fans, 21 jet fans in 7 sets of 3 equally spaced throughout tunnel in each direction
Pulford Community Tunnel	230	Natural	None
Bellewood / Huron Estates Community Tunnel	1,030	Longitudinal	42 jet fans, 21 jet fans in 7 sets of 3 equally spaced throughout tunnel in each direction

The jet fan installations (3 in a set) can be evenly distributed over the length of the tunnel as long as they are not installed closer than 100 m to a portal.

In-tunnel Criteria

The following in-tunnel criteria was used for this study:

Carbon Monoxide: CO levels not to exceed 70 ppm. This is in accordance with the World Road Congress (PIARC) criteria.1
Nitrogen Oxides: NOx not to exceed 10 ppm. This is in accordance with the World Road Congress (PIARC) criteria.2
Visibility: Visibility shall be maintained so that a light beam passing through 100 m (328 ft) shall have greater than 50 percent of the emitting intensity (called Transmission). Visibility is also related to light extinction. A Transmission of 50 percent is the same as an Extinction Coefficient of .0070 m-1 (0.0021 ft-1). This is in accordance with the PIARC criteria.1
Fire Emergency: During a fire emergency, the direction of smoke movement should be controlled by preventing backlayering. The air velocity required to prevent backlayering is call the "critical velocity". The critical velocity equations are presented in NFPA 502, Annex C.3 The critical velocity varies as a function of tunnel cross-sectional area, height and grade, and fire heat release rate.
Air Velocity: The longitudinal air velocity in the tunnel must not exceed 11 m/s during emergency conditions (NFPA 502: A.7.17.3.3(6)).3 During normal operations, a maximum air velocity of 5 m/s was used. Excessive tunnel air velocities may produce extreme equipment power requirements for normal operations and should be avoided. The exception is if the movement of traffic generates higher airflows.

The number of jet fans was based on the ability of the ventilation system to generate "critical velocity" during a fire event. One of the main functions of a tunnel ventilation system is to provide a means for controlling smoke and heat movement during a fire emergency. In the case of a fire in a tunnel serving unidirectional traffic, it may be assumed for a limited access highway that the traffic ahead of the fire will proceed to the exit portal and the traffic behind the fire will come to a stop. Therefore, the ventilation system would be operated to force the smoke and hot gases in the direction of the empty tunnel. Thus, a clear and safe environment behind the fire is provided for evacuating people and for fire fighter access to the incident. The ventilation system accomplishes this objective by preventing the development of a smoke backlayer.

Note: Backlayering is the movement of smoke and heated gases back over the vehicles stopped behind the fire in the presence of a longitudinal airflow attempting to push the heat and smoke away from the stopped vehicles.

References

PIARC - World Road Association, PIARC Technical Committee on Road Tunnels Operation (C5), "Road Tunnels, Vehicle Emissions and Air Demand for Ventilation", France, 2004.
PIARC - World Road Association, "Road Tunnels", XXIst World Road Congress, Kuala Lumpur, 3-9 October 1999.
National Fire Protection Association, "NFPA 502 Standard for Road Tunnels, Bridges, and Other Limited Access Highways 2004 Edition" 2004, National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02169-7471.