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    The importance of Ventilation system

            Ventilation is required during the construction of any tunnel. This is true whether the tunnel is constructed by blasting, boring or placing prefabricated tubes in a trench. Temporary ventilation is necessary to provide a suitable, safe working environment for the construction workers. Since many flammable or airborne toxic gases, dust mist and fumes are released during the construction process, these contaminants can only be removed by ventilation. A dedicated, well tailored ventilation system is compulsory in the presence of natural gas in terms of flammability and asbestos in terms of toxicity. Ventilation systems for construction sites must be flexible in order to grow and move with the construction progress. They may therefore imply rather complex flow pattern including leakage flows, booster stations, filtering and cooling devices, conjunctions and disjunctions etc. Depending on the excavation length and method as well as the geothermal and ambient boundary conditions, cooling of the working environment may also be required. Considerable heat may be transferred to the tunnel air from the rock mass, from the boring machine, other machinery and vehicles as well as from cement setting. Especially heat transfer from the rock-mass is complex, because it is transient in time due to the gradual cool-down of the rock from the tunnel wall.