It is key because of the very large spatial requirements it imposes on underground spaces, typically at station ends or intermediate shafts and which, consequently, is generally associated with large capital and operating costs.
It is also of vital importance because tunnel ventilation systems normally have a life safety function. Attaining balance between the key functions of the system to minimise risk whilst maximising value is the primary objective in the process of defining the tunnel ventilation system for an underground metro or railway.
Typically, tunnel ventilation systems are specified to deal with large amounts of smoke and to help control passenger comfort. There could be secondary considerations, too, such as the removal of fumes or dust during maintenance works. In the case of dealing with large amounts of smoke, the sizing of tunnel ventilation is normally driven by how the fire and life safety strategy defines the size and nature of the fire. There are other important considerations that affect smoke control such as the vertical rail alignment and location of stations.
To help achieve passenger comfort functionality, the sizing of the tunnel ventilation system is driven by aspects such as passenger comfort expectations in a particular country; the types of heat sources; and sinks that exist in the underground; whether platform screen doors are present, and so on. Typical tunnel heat sources that impact on tunnel ventilation equipment requirements could be train characteristics such as the provision of air-conditioning, or timetable patterns which, for example, can result in heat generated by large amounts of braking and accelerating.
Aurecon uses state of the art modelling techniques to help determine the most effective tunnel ventilation systems. Depending on the nature of the project, several different types of tunnel ventilation modelling may be conducted.
Complex tunnels such as metro tunnels and stations are modelled using the US Department of Transportation’s Subway Environment Simulation (SES) tunnel airflow network program. We have our own version of SES, modified to give it the ability to model features such as Saccardo nozzles and platform screen doors and with in-house post-processing tools to make the output easier to visualise.
Saccardo nozzles are, in effect, large steel or concrete ducts installed at one or both ends of platform tunnels, which are sometimes necessary, to deal with the characteristics of a particular rail alignment. For example, using modelling techniques, one could understand how they could be used to force smoke downhill along a steep alignment or whether they are necessary, at all, in the first instance.
Another benefit of using modelling could be to understand the aerodynamic interaction between the underground stations and the tunnels, as a result of installing full height platform screen doors. Another tool that can be used to help understand this interaction from a fire-and-life safety perspective is Computational Fluid Dynamics (CFD). CFD can aid in the assessment of specific problems associated with underground environments by providing a three-dimensional point of view of how smoke, heat and visibility affect a platform tunnel. This is useful, for example, to help determine the spatial requirements of over-track exhaust systems, to achieve the required evacuation criteria.
At Aurecon, we also have extensive expertise in the practical aspects of tunnel ventilation design such as defining structural fatigue loads from passing rail traffic and conducting full-scale site tests, to prove the performance of systems to stakeholders.