VENTILATION OF TUNNELS Ventilation of Steam Railway Tunnels.—The simplest method for ventilating a railway tunnel is to have numerous wide openings to daylight at frequent intervals. If these are the full width of the tunnel, at least 20 ft. in length, and not farther apart than about Soo ft., a tunnel can sometimes be naturally and ade quately ventilated. Such arrangements are, however, frequently impracticable, especially in long and deep tunnels, and then re course must be had to mechanical means. Not only long tunnels, but often those relatively short, require artificial ventilation when on a steep gradient, as the smoke and gases have a tendency to travel up the grade with the locomotive, which is then working at full capacity. Natural ventilation depends for its action upon the difference of temperature within the tunnel or ventilation shaft, and the outer air. In winter the draft is upward and in summer often downward. In the spring and autumn there are often periods when there is little difference in temperature and, in consequence, little circulation of air. Most steam railway tun nels are poorly ventilated, except where the difference in elevation of the ends or the prevailing winds, creates a natural draft through them. The need for ventilation is far greater with steam locomo tion than with electric motors.
The nrst application of mechanical or fan ventilation to railway tunnels was made in the Lime street tunnel of the London and North Western railway, at Liverpool, which has since been re placed by an open cutting. At a later date fans were applied to the Severn and Mersey railway tunnels.
Where possible, 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 Mersey river such kind of 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 4o ft. in diameter was placed. In this way the vitiated air is drawn from the lower point of the railway, while fresh air flows in at the stations on each side to replenish the partial vacuum.
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 platforms should be kept pure. In the Mersey tunnel there are five fans; two are 4o ft. in diameter by 12 ft. wide and two 3o ft. in diameter by io ft. wide, one of each size
being erected at Liverpool and at Birkenhead respectively. In addition there is a high speed fan, 16 ft. in diameter, in Liver pool, which throws 300,00o cu.ft. per minute. The ventilation of this tunnel was satisfactory up to a train schedule of 300 trains per day, or one each way every five minutes. As the traffic increased the air shaft became coated with soot several inches thick and arrangements were made to instal electric power.
The central point of the Severn tunnel lies toward the Monmouthshire bank of the river and ventilation is effected from that point by means of one fan placed on the surface at Sudbrooke Monmouth, at the top of a shaft which is connected with a hori zontal heading leading to the centre of the tunnel. This fan, which is 4o ft. in diameter by 12 ft. in width, removes from the tunnel some 400,000 cu.ft. per minute, and draws in an equivalent volume of fresh air from the two ends.
About 1896 an excellent system was introduced by Signor Sac cardo, an Italian engineer, which to a great extent minimized the difficulty of ventilating long tunnels under mountain-ranges where shafts are not available. This system, which is not applicable to tunnels in which underground stations exist, is based upon the principle of the ejector, and is illustrated in fig. io, which repre sents its application to the single-line tunnel through the Apen nines at Pracchia. This tunnel is one of 52 single line tunnels, with a gradient of 1 in 40, on the main line between Florence and Bologna, built by Thomas Brassey. There was a great deal of traffic which had to be worked by heavy locomotives. Before the installation of a ventilating system, under any condition of wind, the state of this tunnel, about 9,00o ft. in length, was bad; but when the wind was blowing in at the lower end at the same time that a heavy goods or passenger train was ascending the gradient the condition of affairs became very much worse. The engines, working with the regulators full open, often emitted large quanti ties of both smoke and steam, which travelled concurrently with the train. The goods trains had two engines, one in front and the other at the rear, and when, from the humidity in the tunnel, due to the steam, the wheels slipped and possibly the train stopped, the state of the air was indescribable. A heavy train with two engines, conveying a royal party and their suite, arrived on one occasion at the upper exit of the tunnel with both enginemen and both firemen insensible ; when a heavy passenger train came to a stop in the tunnel, all the occupants were seriously affected.