Page:EB1911 - Volume 27.djvu/424

FIG. 12.-Boston Subway, third phase.

A i FIG. 13.-Boston Subway, longitudinal vertical section through shield. The first application of mechanical or fan ventilation to railway tunnels was made in the Lime Street tunnel of the London and North-Western railway at Liver ool, which has since been re laced by an open cutting. At a later diate fans were applied to the § evern and Mersey tunnels.

The principle ordinarily acted upon, where mechanical ventilation has been adopted, is to exhaust the vitiated air at a point midway between the portals of a tunnel, by means of a shaft with which is connected a Ventilating fan of suitable power and dimensions. In the case of the tunnel under the river Mersey (fig. I4) such a shaft could not be provided, owing to the river being overhead, but a ventilating heading was driven from the middle of the river (at which point entry into the tunnel was effected) to each shore, where a fan $0 ft. in diameter was placed. In this way the vitiated air is drawn rom the lowest point of the railway, while fresh air Hows in at the stations on each side to replenish the partial vacuum, as indicated by arrows in the accompanying longitudinal section of the tunnel. The principle was that fresh air should enter at each station and “ split " each way into the tunnel, and that thus the atmosphere on the station platforms should be maintained in a condition of urity.

p The fans in the Mersey tunnel are somewhat similar to the well known Guibal fans, with the exception of an important alteration in the shutter. With the Guibal shutter, the top of the opening

(Fmm n diagram in Proc. Insl. Ci1:.Eng.) FIG. 14.-Longitudinal Section of the Mersey Tunnel, showing Method of Ventilation.