Fire size and location in a tunnel are major factors in determining the smoke extraction system requirements to protect occupants downstream of a fire.
Introduction
Parsons Brinckerhoff (now part of WSP | Parsons Brinckerhoff) has been involved in the detailed design work on some recently completed major Australian road tunnels. A number of these unidirectional traffic tunnels, including the M7 Clem Jones Tunnel (Clem 7), Brisbane’s Airport Link Tunnel, and Legacy Way (under construction), employ a combined longitudinal and distributed smoke extraction ventilation (smoke duct) system for fire emergencies. This type of system can result in unique tunnel ventilation fan duty requirements. This article describes the challenges and analysis approaches to account for a wide range of parameters that can affect the fan requirements, including:
- fire location in the tunnel (distance to fans);
- fire heat release rate;
- thermal losses;
- tunnel section and grade at the fire site; and
- duct leakage.
Fan selection will be based on achieving multiple fan duties (airflow and pressure capacities), and analysis is required to ensure all parameters are accommodated.
Combined Ventilation Design
The longitudinal ventilation system provides air flow at or above the critical velocity upstream of the fire to prevent smoke from backlayering, and the smoke extraction system captures smoke downstream of the fire site. The longitudinal ventilation is primarily achieved by in-tunnel jet fans. The jet fans can operate with active controls (with a tunnel air velocity feedback system) to achieve a predetermined critical velocity. Excess longitudinal flow needs to be avoided to contain the smoke at the fire incident site (i.e., no overshoot).
The smoke extraction system is provided to protect occupants downstream of the fire during congested traffic conditions. This is achieved by opening sets of smoke dampers located immediately downstream of the fire. A specific mass of air flow has to be exhausted from the tunnel. This mass flow equates to the volume of air at ambient conditions required to prevent backlayering of smoke upstream of the fire and overshoot downstream of the fire. Values for this air volume flow are typically on the order of 3 metres (10 feet) per second and 1 metre (3.3 feet) per second respectively. This value is also known as the critical velocity, determined by CFD modelling and/ or empirical equations.
Figure 1 shows the typical configuration of this longitudinal ventilation and smoke extraction system. In order to minimise the smoke duct size, extraction is in the upstream and downstream direction in the duct. For example, Path 1 is the travel path of exhausted smoke and air mixture from the open dampers in the smoke duct near the fire location, to the exhaust fan upstream of the fire. This article focuses on fan selection using this type of configuration.
Fire size, location, and thermal effects
The fire size (heat release rate) and location of the fire in a tunnel are major factors in determining the required extraction, the smoke duct system pressure, and the fan duty. Dependent on project requirements, by increasing design fire size, a higher critical velocity of air needs to be supplied to prevent backlayering of smoke. Additionally, for fires located in larger cross sectional areas of the tunnel, or in caverns where there are traffic off/on ramps diverging/merging with the mainline tunnel, the critical velocity may still need to be maintained. Both of these factors will require a larger air flow volume and hence increased smoke duct system capacity. The fan selected must accommodate these multiple capacities.
Determining the fan capacity can be further complicated by the need to allow for varying air densities due to different fire sizes and heat losses. As the air is being heated at the fire site, the volume expands and density decreases. As the hot smoke travels along the smoke duct, heat transfer cools the smoke and the air density increases. Due to this, the fire size and location along the tunnel affects the pressure loss along the duct and the density of the air to be handled by the fans. For multiple extraction locations (i.e., different fire locations in the tunnel) this effect must be accounted for in the analysis to ensure the required extraction is achieved.
Smoke duct and damper leakage
A road tunnel smoke duct is not completely sealed. When the duct is in extraction mode it is at a lower pressure than the adjacent roadway, causing leakage of air between the tunnel roadway and the smoke duct. The air leakage occurs via cracks, construction tolerances, and gaps in closed dampers and duct slabs. Leakage is dependent on duct pressure and construction quality (i.e., there is no single leakage rate figure).
As an example, an increase of around 10 percent in air flow was estimated for a typical construction of “non-sealed” smoke duct for every 1000 metres (~3300 feet) of length for a particular configuration. The amount of damper leakage can be estimated via damper specifications and the expected local duct pressure. It needs to be noted that it is difficult to accurately estimate the impact of civil construction tolerances during the design stage of a project. Conservative estimates are often applied. The leakage effect both on flow rate and on duct pressure loss along the duct distance is shown diagrammatically in Figure 2.
Design and Analysis
The analysis to be undertaken on a smoke duct configuration has a number of elements. A numerical analysis can be used to account for incremental air leakage and air density changes along the duct, with simplified heat transfer models under steady state conditions. The results give a prediction of the required fan duties to achieve the air extraction mass flow rate at the fire site. This needs to be determined at multiple extraction locations.
As an example, Figure 3 shows a “flow split” between the two smoke extraction paths, Path 1 and Path 2, for a given fire size at two different fire locations (Case A and Case B, refer to Figure 1). Identical fan properties are used for both paths.
- Case A is for a fire located roughly at the aerodynamic midpoint between the extraction fans. By definition the airflow split is 50 percent between each end. In reality this may not occur exactly at the geometric tunnel mid-point, and depends on the upstream and downstream characteristics of the tunnel and the smoke duct. The important element is that the pressures are approximately equal on either side of the flow extraction point, regardless of fan sizes at each end of the path.
- Case B shows a fire located closer to the end of Path 2. In this example case, the pressures are approximately equal as the length of the airflow of Path 2 is decreased compared to air flow of Path 1.
Together, Case A and B show that the fan duty varies considerably with the fire location. Given that the required extraction will change with differing fire sizes and other locations, many fire locations will need to be assessed in order to determine the maximum flow and pressure the fan has to service.
The resulting fan duties for the two cases are shown parametrically in Figure 4 and show that the fan duty points vary considerably even when only two fire locations are considered. When assessing, many locations will need to be considered along with the varying fire size. The analysis is an iterative process. This will determine a range of fan duty limits. Practically, the final fan selection is based on the need for the fans to cover all possible fan duty points that may be encountered during operation. The fans ultimately selected will generate flows in excess of the required flows for many cases which will need to be considered and accounted for. Generally for smoke extraction duties, excess of design performance requirements is not an issue.
Conclusion
The process of sizing smoke ducts and determining the smoke extraction fan requirements is technically challenging, involving factors from many tunnel design disciplines, interactions of thermodynamics and fluid dynamics, and physical constraints. Fan requirements for the smoke extraction system are based on the possible fan duty points the system will encounter during operation.
Designs undertaken by WSP | Parsons Brinckerhoff have demonstrated the viability of smoke extraction systems for long road tunnels, by determining the necessary fan requirements and smoke duct requirements to achieve the required capacity. The design analysis and research the team has undertaken will benefit future smoke extraction system designs for long road tunnels, by ensuring that the installed systems can perform to their intended purposes and capacities. However, the actual performances of such systems are also dependent on the constructor’s final system design, the equipment suppliers, and the installer’s performance based on their project contractual obligations. The completed smoke extraction systems installed in the Clem7 and Airport Link tunnels are currently in service.
