Ventilation of Tunnels

In applying the Saccardo system, the tunnel was extended for 15 or 20 ft. by a structure either of timber or brickwork, the inside line of which represented the line of maximum obstruction of the tunnel, and this was allowed to project for about 3 ft. into the tunnel (fig. 1o). The space between this line and the exterior constituted the chamber into which air was blown by means of a fan. Considering the length of tunnel, it might at first be thought there would be some tendency for the air to return through the open mouth, but nothing of the kind happened. The whole of the air blown by the fan, 164,000 cu.ft. per minute, was augmented by the induced current yielding 46,000 cu.ft. per minute, making a total of 210,000 cu.ft.; and this volume was blown down the gradient against the ascending train so as to free the driver and men in charge of the train from the products of combustion at the earliest possible moment. Prior to the installation of this system the drivers and firemen had to be clothed in thick woollen garments, pulled on over their ordinary clothes, and wrapped round and round the neck and over the head.

The Saccardo system was installed in 1899 at the St. Gotthard tunnel with most beneficial results. The ventilating plant is situ ated at Goschenen at the north end of the tunnel and consists of two large fans operated by water-power. The quantity of air pressed into the narrow mouth of the tunnel is 413,00o cu.ft. per minute at a velocity of 686 ft., this velocity being much re duced as the full section of the tunnel is reached. After installa tion, a sample of the air taken from a carriage contained only 10.19 parts of carbonic acid gas per io,000 volumes.

In the Simplon tunnel (12.3 m. long), where electricity is the motive power, mechanical ventilation was installed at the time of construction at each end of the tunnel, both for construction pur poses and to serve as permanent equipment. It is based on a dif ferent principle than the Saccardo system. A steel sliding door is arranged at each entrance to be raised and lowered by electric power. After the entrance of a train the door is lowered and fresh air forced into the tunnel from the same end at considerable pres sure by fans. Since completion of the second tunnel following the World War an additional ventilation plant has been built at the north end of the tunnel.

The Moffat tunnel in Colorado (6.1 m. long) is ventilated by a mechanical plant based on the principle of forced or induced draft created at one end of the tunnel. A building was erected at the east portal which forms an extension of the tunnel and provides a fan chamber on each side. A vertical lift gate is provided to close

the portal. Two fans are provided, which differ for experimental reasons. Each has a diameter of 9 ft. and width of 6 feet. The motors are of 75o and Soo h.p. with capacities of 450,000 and 350,000 cu. ft. per minute, at velocities of 14 and 1 o m. per hour, respectively. Only one fan is operated, the other held in reserve. The ventilation is by forced draft for east bound trains and induced draft for west bound trains, which forces the smoke back along the train. The results are very satisfactory.

Other railway tunnels partially or completely ventilated by mechanical draft are the Giovi tunnel, in Italy; Hoosac tunnel, in Massachusetts ; East Mahanoy tunnel, in Pennsylvania ; Big Bend tunnel, in West Virginia ; Elkhorn tunnel, in West Virginia ; and the Gallitzen tunnel, in Pennsylvania.

Volume of Air Required for Ventilation of Railway Tunnels.—The consumption of coal by a locomotive during the passage through a tunnel having been ascertained, and 29 cu.ft. of poisonous gas being allowed for each pound of coal consumed, the volume of fresh air required to maintain the atmosphere of the tunnel at a standard of purity of 20 parts of carbon dioxide in Io,000 parts of air is ascertained as follows: The number of pounds of fuel consumed per mile, multiplied by 29, multiplied by Soo and divided by the interval in minutes between the trains, will give the volume of air in cubic feet which must be introduced into the tunnel per minute.

Ventilation of Tunnels for Electric Traction.

The intro duction of electric traction has simplified the problem of ventilat ing intra-urban railways laid in tunnels at a greater or less dis tance below the surface, since the absence of smoke and products of combustion from coal and coke renders necessary only such a quantity of fresh air as is required by the passengers and staff. For ventilation the shallow tunnels which form the under ground portions of the Metropolitan and District railways in Lon don, open staircases, blow-holes and sections of uncovered track are relied on. When the lines were worked by steam locomotives they afforded notorious examples of bad ventilation, the propor tion of carbonic acid gas amounting to from 15 or 20 to 6o, 70 and even more parts in 10,000. Since the adoption of electricity as the motive power the atmosphere of the tunnels has very much improved. Samples taken from the cars in 1905 after the adop tion of electricity gave as low as 11.27 parts in io,000.