Design Tunnel Ventilation

All road tunnels require ventilation to remove contaminants produced during normal engine operation. Normal ventilation can be achieved through natural means, traffic-induced piston effects, or mechanical equipment. The chosen method should be the most economical in terms of both construction and operating costs.

Additionally, ventilation must control smoke and heated gases from a fire in the tunnel. Effective smoke flow control is essential to ensure a safe environment for both evacuation and rescue operations. Emergency ventilation can be provided naturally by utilizing the buoyancy of smoke and hot gases, or mechanically.

Ventilation Modes: Various types of mechanical ventilation are typically used for road tunnels. These include normal, congested, emergency, and temporary modes, as discussed in the section on Tunnel Ventilation Concepts.

1.0 Natural Ventilation

Airflow in a naturally ventilated tunnel can occur in two primary ways: portal-to-portal (Figure 2A) or portal-to-shaft (Figure 2B). Portal-to-portal flow is most effective with unidirectional traffic, which generates a consistent, positive airflow. This results in relatively uniform airspeed within the roadway area and a gradual increase in contaminant concentration, peaking at the exit portal. Under adverse atmospheric conditions, airspeed may decrease and contaminant levels may rise, as depicted by the dashed line in Figure 2A.

Introducing bidirectional traffic into such a tunnel reduces longitudinal airflow and increases average contaminant concentration. In tunnels with bidirectional traffic, the highest contaminant levels may not necessarily be at the portal or the midpoint.

Portal-to-shaft ventilation (Figure 2B) is better suited for bidirectional traffic, though it too can be impacted by adverse atmospheric conditions. The effectiveness of the shaft depends on factors like air and rock temperatures, wind, and shaft height. Adding multiple shafts to a tunnel can be counterproductive, as contaminated air can become trapped between the shafts.

Naturally ventilated tunnels longer than 1000 feet require emergency ventilation to remove smoke and hot gases during a fire, as recommended by NFPA Standard 502. Tunnels between 800 and 1000 feet long should undergo engineering analyses to determine the need for emergency ventilation. These systems can also remove stagnant contaminants during adverse atmospheric conditions. Due to the uncertainties associated with natural ventilation, especially under adverse conditions, reliance on natural ventilation for tunnels over 800 feet long must be thoroughly evaluated. This is particularly important for tunnels with heavy or congested traffic. If natural ventilation is found to be inadequate, a mechanical ventilation system should be considered for normal operations.

In a naturally ventilated tunnel, smoke from a fire is primarily driven by the buoyant effects of hot gases, which tend to flow uphill. The steeper the grade, the faster the smoke moves, potentially restricting the ability of motorists trapped between the incident and a higher-elevation portal to evacuate safely. The Massachusetts Highway Department and Federal Highway Administration (MHD/FHWA) (1995) demonstrated how smoke moves in a naturally ventilated tunnel (see Table 2).

2.0 Longitudinal Ventilation

Longitudinal ventilation introduces or removes air from the tunnel at specific points, creating airflow along the roadway. This can be achieved through push-pull vent shafts, injection, jet fan operation, or a combination of injection and extraction at intermediate points in the tunnel. Injectors and jet fans are considered impulse systems because they impart momentum to the tunnel airflow, with the primary high-velocity jet diffusing out. At startup, this thrust accelerates the air in the tunnel until equilibrium is reached between this force and the opposing drag forces caused by viscous friction, pressure losses at the tunnel portals, traffic, wind, fire, and other factors.

Injection longitudinal ventilation, commonly used in rail tunnels, employs externally located fans to inject air into the tunnel through a high-velocity Saccardo nozzle (Figure 3A). This air injection, typically in the direction of traffic flow, induces additional longitudinal airflow. The Saccardo nozzle operates on the principle that a high-velocity air jet injected at a small angle to the tunnel axis can induce high-volume longitudinal airflow. The amount of induced flow depends on the nozzle area, discharge velocity, and angle, as well as downstream air resistances. This type of ventilation is most effective with unidirectional traffic flow.

With injection longitudinal ventilation, airspeed remains uniform throughout the tunnel, and the contaminant concentration increases from zero at the entrance to a maximum at the exit. Adverse atmospheric conditions can reduce system effectiveness, causing the contaminant level at the exit to increase as airflow decreases or tunnel length increases.

Injection longitudinal ventilation, with supply at limited tunnel locations, is economical as it requires fewer fans, places less operating burden on fans, and needs no distribution air ducts. However, as the tunnel length increases, disadvantages such as excessive air velocities in the roadway and smoke being drawn the entire length of the roadway during an emergency become apparent.

The main aerodynamic differences between jet fans and Saccardo injectors are that injectors impart thrust at one location in the tunnel, while jet fan systems distribute this thrust along the tunnel. Injectors use outdoor air as the primary flow, whereas jet fans pull primary airflow from within the tunnel. Saccardo injectors may operate in flow induction mode (low tunnel air resistance) or flow rejection mode (high tunnel air resistance); both modes are acceptable. Flow reversal at the nozzle position with flow exiting the near portal may occur, whereas jet fans always induce flow from one portal to the other. Flow under jet fans in a highly resistive tunnel may recirculate, but this is strictly a local feature.

Comparison of Longitudinal Impulse Ventilation Systems

1. Installation Costs: Jet fans have little to no civil engineering costs for installation but significant electrical cabling costs. Saccardo injectors require expensive civil engineering work to install the fans at the tunnel portal, with no cabling distribution costs.
2. Maintenance: Routine maintenance or emergency repairs on jet fans usually require disruption of normal tunnel service and availability; this is not the case for Saccardo injectors, which can be accessed externally.
3. Safety and Cost: Saccardo injectors eliminate electrical cabling in the tunnel, providing a clear safety and cost advantage over jet fans.
4. Space Requirements: Jet fans take up headroom in the tunnel ceiling, limiting the effective dynamic clearance envelope of the traffic, whereas Saccardo injectors are located outside the tunnel, making them ideal for tightly configured tunnels.
5. Performance Vulnerability: Saccardo injectors deliver thrust at a single point, making them vulnerable to local tunnel fixtures, such as traffic signs or lighting equipment near the outlet. Jet fans are less affected because their thrust is distributed.
6. Fire Safety: Jet fans are derated when operating at elevated temperatures during a fire (due to lower air density), whereas injectors are both safely outside the fire's reach and immune to thrust reduction because they use fresh air for intake. This makes Saccardo injectors ideal for emergency smoke clearance.

3.0 Longitudinal Ventilation Systems with Fans and Shafts

A longitudinal ventilation system with one fan shaft (Figure 3B) is similar to a naturally ventilated system with a shaft but provides a positive stack effect. Bidirectional traffic in such a tunnel causes peak contaminant concentration at the shaft. For unidirectional tunnels, contaminant levels become unbalanced.

Another form of longitudinal system has two shafts near the tunnel center: one for exhaust and one for supply (Figure 3C). In this arrangement, part of the air flowing in the roadway is replaced by the interaction at the shafts, reducing contaminant concentration in the second half of the tunnel. This concept is effective only for tunnels with unidirectional traffic flow. Adverse wind conditions can reduce tunnel airflow by short-circuiting the flow of air from the supply fan shaft/injection port to the exhaust fan/shaft, causing contaminant concentrations to increase in the second half of the tunnel.

Construction costs of two-shaft tunnels can be reduced if a single shaft with a dividing wall is constructed. However, this significantly increases the potential for short-circuited airflows from the supply shaft to the exhaust shaft; under these circumstances, the separation between exhaust and intake shafts should be maximized.

4.0 Jet Fan Longitudinal Ventilation

Jet fan longitudinal ventilation has been installed in many tunnels worldwide. In this scheme, specially designed axial fans (jet fans) are mounted at the tunnel ceiling (Figure 3D). This system eliminates the need for space to house ventilation fans in a separate structure or building but may require greater tunnel height or width to accommodate the jet fans outside of the tunnel's dynamic clearance envelope. However, as tunnel length increases, disadvantages such as excessive air speed in the roadway and smoke being drawn the entire length of the roadway during an emergency become apparent.

5.0 Smoke Control in Longitudinal Ventilation

Longitudinal ventilation is the most effective method of smoke control in a road tunnel with unidirectional traffic. The ventilation system must generate sufficient longitudinal air velocity to prevent backlayering of smoke (movement of smoke and hot gases against the ventilation airflow in the tunnel roadway). The air velocity necessary to prevent backlayering over stalled or blocked vehicles is the minimum velocity needed for smoke control in a longitudinal ventilation system, known as the critical velocity.

6.0 Semitransverse Ventilation

Semitransverse ventilation can be configured for either supply or exhaust. This type of ventilation uniformly distributes (supply) or collects (exhaust) air throughout the length of a road tunnel, making it suitable for tunnels up to about 7000 feet. Beyond this length, tunnel air velocity near the portals becomes excessive.

In tunnels with bidirectional traffic, supply semitransverse ventilation produces a uniform level of contaminants because air and vehicle exhaust gases enter the roadway area at the same uniform rate. With unidirectional traffic, additional airflow generated by vehicle movement reduces the contaminant level in the first half of the tunnel (Figure 4A). Since the airflow is fan-generated, this type of ventilation is not adversely affected by atmospheric conditions. Air flows the length of the tunnel in a duct with supply outlets spaced at predetermined distances. Fresh air is best introduced at vehicle exhaust pipe level to dilute exhaust gases immediately. The pressure differential between the duct and the roadway must be enough to counteract the effects of piston action and adverse atmospheric winds.

In the event of a fire, the supply air initially dilutes the smoke. Supply semitransverse ventilation should then be operated in reverse mode, allowing fresh air to enter through the portals and create a tenable environment for both emergency egress and firefighter ingress. Therefore, a supply semitransverse ventilation system should preferably have a ceiling supply and reversible fans to draw smoke up to the ceiling during a tunnel fire.

Exhaust semitransverse ventilation (Figure 4B) in a tunnel with unidirectional traffic flow produces a maximum contaminant concentration at the exit portal. In a tunnel with bidirectional traffic flow, the maximum concentration of contaminants is located near the center of the tunnel. A combination supply and exhaust semitransverse system (Figure 4C) should be applied only in unidirectional tunnels, where air entering with the traffic stream is exhausted in the first half of the tunnel, and air supplied in the second half is exhausted through the exit portal.

In a fire emergency, both exhaust semitransverse ventilation and (reversed) semitransverse supply create a longitudinal air velocity in the tunnel roadway, extracting smoke and hot gases at uniform intervals.

7.0 Full Transverse Ventilation

Full transverse ventilation is used in extremely long tunnels and tunnels with heavy traffic volume. It employs both a supply and exhaust duct system to uniformly distribute supply air and collect vitiated air throughout the tunnel length (Figure 5). Typically, such a tunnel is long and served by more than one mechanical ventilation system, configured into ventilation zones, each served by a dedicated set of supply and exhaust fans. Each zone can be operated independently, allowing the tunnel operator to change the direction of airflow by varying the level of operation of the supply and exhaust fans. This feature is crucial during fire emergencies.

In balanced operation, air pressure along the roadway is uniform, and there is no longitudinal airflow except that generated by the traffic piston effect, which tends to reduce contaminant levels. The pressure differential between the ducts and the roadway must ensure proper air distribution under all ventilation conditions.

During a fire, exhaust fans in the full transverse system should operate at the highest available capacity, while supply fans should operate at a somewhat lower capacity. This allows the stratified smoke layer at the tunnel ceiling to remain elevated and be extracted by the exhaust system without mixing. Fresh air entering through the portals creates a tenable environment for emergency egress and firefighter ingress.

In longer tunnels, individual ventilation zones should control smoke flow so that the zone with traffic trapped behind a fire is provided with maximum supply and no exhaust, while the zone on the other side of the fire (where unimpeded traffic continues) is provided with maximum exhaust and minimum or no supply.

Full-scale tests conducted by Fieldner et al. (1921) showed that supply air inlets should be at vehicle exhaust pipe level, and exhaust outlets should be in the tunnel ceiling for rapid dilution of exhaust gases under nonemergency operation. Depending on the number of traffic lanes and tunnel width, airflow can be concentrated on one side or divided over two sides.

8.0 Other Ventilation Systems

There are many variations and combinations of the road tunnel ventilation systems described. Most hybrid systems are configured to solve specific problems in the development and planning of a tunnel, such as excessive air contaminants exiting at the portals. Figure 6 shows a hybrid system developed for a tunnel with a near-zero level of acceptable contaminant discharge at one portal. This system is essentially a semitransverse supply system with a semitransverse exhaust system added in section 3 to minimize pollutant discharge at the exit portal, located near sensitive environmental receptors.

9.0 Ventilation System Enhancements

Single-point extraction is an enhancement to a transverse system, adding large openings to the extraction (or exhaust) duct. These openings include devices that can be operated during a fire emergency to extract a large volume of smoke as close to the fire source as possible. Tests proved this concept effective in reducing air temperature and smoke volume in the tunnel. The size of the duct openings tested ranged from 100 to 300 square feet (MHD/FHWA 1995).

Oversized exhaust ports are simply expanded exhaust ports installed in the exhaust duct of a transverse or semitransverse ventilation system. Two methods create this configuration: installing a damper with a fusible link or using a material that melts and opens the airway when heated to a specific temperature. Meltable materials showed only limited success in testing (MHD/FHWA 1995).

Conclusion

Effective tunnel ventilation is crucial for maintaining air quality and safety in road tunnels, as outlined in the ASHRAE standards. The choice of ventilation system—be it natural, mechanical, semitransverse, or full transverse—depends on factors such as tunnel length, traffic volume, and atmospheric conditions. Longitudinal ventilation, whether through injection or jet fans, provides economical solutions but may present challenges in smoke management during emergencies. According to ASHRAE guidelines, semitransverse and full transverse ventilation offer more control over air distribution and contaminant levels, particularly in longer tunnels or those with heavy traffic. Hybrid systems and enhancements like single-point extraction and oversized exhaust ports can address specific issues, such as minimizing pollutant discharge at portals. Ultimately, the appropriate ventilation system must balance cost, efficiency, and safety, ensuring that tunnels remain operational and safe for all users under both normal and emergency conditions.