The best manner of expelling foul air from tunnels, according to the vacuum method, is by means of bell exhausters. This consists of two sets of bells connected by an oscillating beam and balancing each other. Each set consists of a movable bell, which covers and surrounds a fixed bell with a water joint. In the central part of the fixed bell there are valves which open upwards, and on the bottom of each movable bell there are valves which open from the outside. When one bell ascends, the valves at the bottom are closed, the air beneath is then rarefied, and a vacuum is produced; the valves in the central part of the fixed bell filled with water are opened, and there is an aspiratory action from the pipe leading to the headings, and the foul air is thus carried away. The apparatus makes about ten oscillations per minute, and the dimensions of the bells depend upon the quantity of air to be exhausted in a minute. In the St. Gothard tunnel, where these bell exhausters were used, they exhausted 16,500 cu. ft. of air per minute.
Plenum Method Contrivances.
—Fresh air may be driven into tunnels to dilute the carbonic acid by two different ways, viz., by water blast and by fans. Water when running at a great velocity produces a movement in the air which may be sometimes usefully and economically employed for ventilating tunnels. Water falling vertically is let run into a large horizontal zinc pipe having a funnel at the outer end; into this the air attracted by the velocity of the water is forced. By an opening at the bottom the water is afterward withdrawn from the pipe, and there remains only the air which is pushed forward by the air which is being continually sucked in by the velocity of the water.
The best and most common means of ventilation by the plenum method is by fans. There are numerous varieties of these fans in the market, but they all consist of a kind of fan wheel which by rapid revolution forces the fresh air into the pipe leading to the headings of the tunnel or to the working places. Instead of a large single fan, such as is used for mining purposes, it is better to have a number of small fans acting independently of each other, conveying the fresh air where it is needed through independent pipes.
Saccardo’s System.
—A new method of ventilating tunnels was devised by Mr. Saccardo for the ventilation of the Pracchia tunnel along the Bologna and Lucca Railway in Italy. At the highest end of the tunnel the mouth was contracted inward in a funnel shaped form so as to just admit a train. Immediately at this contraction, a lateral tunnel, 50 feet long, branched off from one side of the main tunnel. At the mouth of this lateral tunnel was installed a fan which forced air into the tunnel and with 70 revolutions per minute delivered 3.532 cu. ft. of air per second at a water pressure of 1 in. This air current was directed inward through a second contraction or funnel, parallel to the one at the entrance and 23 ft. beyond it. In operation the action of the artificial air current was to suck in a considerable volume of outside air, while the air pressure was sufficient to counterbalance the movement of air produced by a train moving at a velocity of 16.1 ft. per second. Mr. Saccardo’s method was employed in ventilating a tunnel on the Norfolk and Western Railway with satisfactory results.
Compressed Air.
—In the excavation of tunnels in hard rock a number of rock drilling machines are employed which are moved by compressed air at a pressure of not less than five atmospheres. At each stroke about 100 cu. ins. of compressed air are set free, and at an average of 10 strokes per minute there would be 5000 cu. ins. of air at five atmospheres or 25,000 cu. ins., or a little more than 175 cu. ft. of fresh air at normal pressure set free every minute by each of the machines employed. But the air exhausted from the drilling machine is foul.
Regarding ventilation by compressed air, Mr. Adolph Sutro, in a lecture delivered to the mining students of the University of California, said:
“I will note a curious fact which I have never seen explained, and which is worthy of close investigation by means of experiments. In the Sutro tunnel we found that the compressed air used for driving the machine drills, after having been compressed and expanded and discharged from the drills, was not wholesome to breathe, and the men and mules would all crowd around the end of the blower pipe to get fresh air. Whether the air in being compressed has parted with some of its oxygen or because vitiated from some other cause, I do not know, and I hope that this subject will at some future day be carefully examined into.”