Cost-Effective Ventilation System for a Light Rail Transit Project

CFD analysis was used to determine emergency ventilation requirements for 2 new LRT stations and connecting tunnels in the event of a train fire.

An underground light rail transit system project in the U.S. includes two new stations and connecting tunnels of 3.3 miles in length (5.28 kilometers) that require emergency ventilation to provide a tenable environment along the egress path in the event of a train fire. 

The cut-and-cover station is a center island platform that is 395 feet (120 meters) long. Each train consists of four cars. The platform level is served by two sets of escalators, one at each quarter point of the platform, that lead up to the mezzanine level. From the mezzanine level, two stair/escalator combinations lead up to two separate entrances at the street level. In addition, enclosed emergency exit stairways located at both ends of the platform lead to exits at grade level (see Figure 1). Due to the similar design of both stations, only one station ventilation analysis is presented here. 

Figure 1 – Section View of Station

Original Ventilation Concept 

Figure 2 shows the airflow schematic for the original ventilation concept with two ventilation systems: 

1. Station ventilation system - The intent of the station ventilation system design is to exhaust smoke and hot gases from a fire on a train stopped at the station. The smoke would rise up into the atrium and the station ventilation system would extract the smoke near the top of the atrium via the station ventilation dampers. The station ventilation system includes four bi-directional fans each delivering 100,000 cfm (50 m³/s). 
2. Tunnel ventilation system - The intent of the tunnel ventilation system design is to exhaust smoke and hot gases from a fire on a train stopped in the tunnel between stations or between the station and portal. The tunnel ventilation dampers are located near the ends of the station platforms to extract the smoke before it enters the station public area. The tunnel ventilation system includes four uni-directional fans each delivering 250,000 cfm (125 m³/s). 

Figure 2 – Airflow Schematic for Original Ventilation Concept

The original station design consists of ventilation fan plants located directly over the trainways at the ends of the station platform. There is a large atrium in the middle of the station where station ventilation dampers connect the station ventilation fans to the atrium area via dampers in the fan room level walls. At the ends of the station, there are tunnel ventilation dampers located in the ceiling of the trainway that connect the trainway region to the tunnel ventilation fans. 

Modified Ventilation Concept with Station Ventilation Fans Eliminated 

A computational fluid dynamics (CFD) analysis was performed to determine if the four station ventilation fans can be eliminated by re-configuring the tunnel ventilation fans and associated ducts and plenums so that the tunnel ventilation fans can exhaust smoke and hot gases from a tunnel fire or from a station fire (Figure 3). The design requirements precluded the need to design for simultaneous station and tunnel fires. 

Figure 3 – Airflow Schematic for Modified Ventilation Concept

The CFD analysis used to model the station, fire, and ventilation system was a software package FDS¹. The ventilation criteria is to maintain a tenable path of egress from the incident train to a point of safety for at least six minutes, which is the maximum time it should take passengers to evacuate from the platform to a point of safety². The design fire for this station is a 13.2 MW fire that follows a medium growth rate fire curve and reaches peak fire heat release rate at 17.7 minutes. The average soot yield of the fire is 0.1245 kg_soot/kg_{fuel burnt}. The fire properties used are representative of the light rail vehicles. The ventilation system is served by four fans, two fans at each end of the station, each fan delivering 250,000 cfm (125 m³/s). The fans are activated two minutes after the fire starts, and reach full operational capacity after 180 seconds. 

Figure 4 shows the CFD results at the platform level, 8.2 feet (2.5 meters) above the platform for a fire ignited on a train stopped in the station. The simulation results for the original ventilation concept and the modified ventilation concept are shown in comparison. Only the contours of visibility are shown, as that is the controlling criterion. Un-shaded regions are within the visibility criteria, while regions that are shaded violate the criteria. 

Figure 4 – Comparison of Visibility at Platform Level

The result of the CFD analysis shows that the modified ventilation concept performs just as well as the original ventilation concept for the critical first six minutes during passenger evacuation. It outperforms the original ventilation concept when the fire has reached its maximum fire heat release rate. This is due to the increased ventilation capacity of the tunnel ventilation fans over the station ventilation fans. 

Conclusion 

The CFD results were presented to the ‘authority having jurisdictions’ (AHJ). The AHJ approved the modified ventilation concept with the elimination of station ventilation fans. The modified ventilation design saved approximately U.S. $6 million in mechanical and electrical costs, in addition to lowered maintenance costs due to less equipment. Additional savings were realized by the elimination of fan room space and ventilation shafts. 

The CFD analysis also provided insight into key station design elements that impact the effectiveness of the ventilation system. For this type of station, the large atrium functions as a smoke reservoir and locating the smoke extraction dampers near the top of the atrium is effective in removing smoke from the station during the evacuation period. In addition, locating the tunnel ventilation dampers at the ends of the station is effective in preventing the spread of smoke from a tunnel fire to the station.

¹Fire Dynamics Simulator (FDS) Version 5.5.3, 2010, (CFD Software), Building and Research Laboratory, National Institute of Standards and Technology (NIST), 100 Bureau Drive, Stop 8600, Gaithersburg, MD 20899-8600, USA.

²NFPA 130, "Standard for Fixed Guideway Transit and Passenger Rail Systems", 2010 Edition, published by the National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02269-9101, USA, August 2009.