4. The metro tunnel in context

4.1 Ventilation beyond smoke extraction

In the previous sections, the focus was on smoke extraction in the event of fire, and rightly so, as it generally determines the choice of ventilation modes, the number of plants and the electrical power to be installed. It is important, however, not to lose sight of the day-to-day, non-fire operation of the metro system. The subjects are thermal comfort for passengers waiting in stations, air quality, input data for calculating heating and possible air-conditioning of stations and trains, and, sometimes, the cooling requirements of trains' technical equipment.

For air quality, the strategy is dilution, i.e. bringing in fresh air, cleaner than the tunnel air, to reduce pollution levels (for the principle, see [2]). Fresh...