VENTILATION.
In long tunnels, especially when excavated in hard rock, proper ventilation is of great importance, because the air cannot be easily renewed, and the amount of oxygen consumed by miners horses and lamps during construction is very large. The gases produced by blasting also tend to fill the head of excavation with foul air. Pure atmospheric air contains about 21% of oxygen and only 0.04% of carbonic acid; when the latter gas reaches 0.1% the fact is indicated by the bad odor; at 0.3% the air is considered foul, and when it reaches 0.5% it is dangerous. It is generally admitted that the standard of purity of the air is when it contains 0.08% of carbonic acid.
A large quantity of carbonic acid in the air is easily detected by observing the lamps, which then give out a dim red light and smoke perceptibly; the workmen also suffer from headache and pains in the eyes, and breathe with difficulty. Naturally, miners cannot easily work in foul air and, therefore, make very slow progress. It is, therefore, to the interest of the engineer to afford good ventilation, not only because of his duty to care for the safety and health of his men, but also for reasons of economy, so that the men may work with the greatest possible ease, thus assuring the rapid progress of the work.
It would be impossible to change completely the atmosphere inside a tunnel, as the gases developed from blasting will penetrate into all the cavities and gather there, but the fresh air carried inside by ventilation has a very small percentage of carbonic acid, mixes with that which contains a greater quantity, and dilutes it until the air reaches the standard of purity. We have not here considered the gases developed from the decomposition of carboniferous and sulphuric rocks, which may be met with in some tunnels, and which render ventilation still more necessary. Tunnels may be ventilated either by natural or artificial means.
Natural Ventilation.
—It is well known that if two rooms of different temperatures are put in communication with each other, e.g., by opening a door, a draft from the colder room will enter the other from the bottom, and a similar draft at the top, but with a contrary direction, will carry the hot air into the colder room, thus producing perfect ventilation, until the two rooms have the same temperature. Now, during the construction of tunnels the temperature inside may be considered as constant, or independent of the outside atmospheric variations; hence during summer and winter, there will always be a draft affording ventilation, owing to the difference of temperature inside and outside the tunnel. In winter time the cold air outside will enter at the bottom of the entrances and headings, or along the sides of the shafts, and the hot air will pass out near the top of the headings or entrances or the center of the shafts; in summer the air currents will take the contrary direction.
Natural ventilation in tunnels is improved when the excavation of the heading reaches a shaft, because the interior air can then communicate with the exterior at two points, at different levels. In such cases a force equal to the difference in weight between a column of air in the shaft and a similar one of different density at the entrance of the tunnel, will act upon the mass of air in the tunnel and keep it in movement, thus producing ventilation. Consequently, during winter, when the outside air has greater weight than that inside, the air will come in by the headings and go out by the shaft, and in the summer it will enter at the shaft and pass out at the entrance. Sometimes to afford better ventilation shafts 8 or 12 in. in diameter are sunk exclusively for the purpose of changing the air. When the inside temperature is equal to that outside, as often happens during the spring and autumn, there are no drafts, and consequently the air in the excavation is not renewed and becomes foul; then fires are lighted under the shaft and a draft is artificially produced. The hot air going out through the shaft, as through a chimney, allows the fresh air to come in as in ordinary ventilation.
When the head of the excavation is very far from the entrances, or when the mountain is too high to allow excavation by shafts, it is quite impossible to secure good natural ventilation, especially during the spring and autumn months, and the engineer has to resort to some artificial means by which to supply fresh air to the workmen.
Artificial Ventilation.
—Artificial ventilation in tunnels may be obtained in two different ways, known as the vacuum and plenum methods. Their characteristic difference consists in this, that in the vacuum method the air is drawn from the inside and the vacuum thus produced causes the fresh air from the outside to rush in, while the plenum method consists in forcing in the fresh air which dilutes the carbonic air produced inside the tunnel by workingmen and explosives. In the vacuum method the pressure of the atmosphere inside the tunnel is always less than the pressure outside, while in the plenum method the pressure within is always greater than that outside. Ventilation is the result of this difference of pressure, as the tendency of the air toward equilibrium produces continuous drafts. Both these methods have their advantages and disadvantages; but in the presence of hard rock, when explosives are continually required, the vacuum method is considered the best, because the gases attracted to the exhaust pipes are expelled without passing through the whole length of the tunnel, thus avoiding the trouble that a draft of foul air will give to the workmen who are within the tunnel. In both these methods it is necessary to separate the fresh air from the foul one; and this is done by means of pipes which will exhaust and expel the foul air in the vacuum method, or force to the front a current of fresh air when the plenum method is used. Artificial ventilation may also be obtained by compressed air which is set free after it has driven the machines, especially in tunnels excavated through rock, when rock drilling machines moved by compressed air are employed.
Vacuum Method Contrivances.
—The most common of the vacuum appliances consists in the simple arrangement of a pipe leading from the head of the tunnel out through the fire of a furnace. The air in the pipe is rarefied by the heat of the furnace and then set free from the other end of the pipe, thus creating a partial vacuum in the pipe, into which the foul air of the head rushes, the fresh air from the entrance taking its place, and thus ventilating the tunnel. A similar arrangement may be used with shafts, and the foul air may be driven out by a furnace which is placed either at the top or bottom of the shaft. Such furnaces act the same as those commonly used for heating purposes in the houses, with this difference, that, instead of fresh air being forced in, foul air is expelled. Another simple arrangement for producing a vacuum is by means of a steam jet which is thrown into the pipe, and which helps the expulsion of the air by heating it, thus producing a different density which originates a draft besides that mechanically originated by the force of the steam jet, which tends to carry out the foul air of the pipes.
Foul air may also be expelled by means of exhaust fans which are connected with pipes near the entrance of the tunnel. The fan consists of a box containing a kind of a paddle wheel turned by steam or water power and arranged so as to revolve at a high speed. The air inside the pipe is forced out by blades attached to the wheel, and thus the foul air of the front is driven away and fresh air from the entrance rushes in to take its place, and perfect ventilation is obtained.
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.”
In the December, 1901, number of “Compressed Air,” a magazine especially devoted to the useful application of compressed air, is read:
Compressed air wasted from power drills is so contaminated with oil from the cylinders that it cannot be taken into consideration as ventilation. It is as important to displace it with pure air as it is to drive out or draw off other vitiated air. The ventilation should be an independent supply provided by fan or blower, delivering by pipe at the point where miners are working.
Quantity of Air.
—The quantity of air to be introduced into tunnels must be in proportion to the oxygen consumed by the men, the animals, and the explosions. It is allowed that the quantity of air required for breathing purpose and explosions is as follows:
| 1 | workman with lamp | needs | 240 | cu. yds. | of fresh | air | in 24 hours. |
| 1 | horse | „ | 850 | „ | „ | „ | „ |
| 1 | lb. gunpowder | 100 | „ | „ | „ | ||
| 1 | lb. dynamite | 150 | „ | „ | „ |
In a long tunnel excavated through hard rock the number of workmen all together may be assumed at 400 at each end, and each workman is supposed to be furnished with a lamp. No less than ten horses are employed, and the average quantity of dynamite consumed is 600 lbs. per day. From the data given the consumption of air by workmen and lamps would be: 240 × 400 = 96,000 cu. yds.; the consumption of air by horses would be 850 × 10 = 8500 cu. yds.; the consumption of air by dynamite would be 150 × 600 = 90,000 cu. yds.; making a total consumption of air per day of 194,500 cu. yds., or about 8000 cu. yds. per hour.
To obtain good ventilation, then, it will be necessary to furnish every hour a quantity of fresh air amounting to not less than 8000 cu. yds. Since, however, a large quantity of pure air is expelled with the foul air, it is necessary greatly to increase this quantity.
It may be observed, in closing, that the water having its particles divided, as in a fog or mist, rapidly precipitates the gases produced by explosions. Now, when hydraulic machines are used, there is a hollow ball pierced by holes that are almost imperceptible, from which the compressed water spreads in very subtile particles, and this causes the fall of the gases from explosions. Such a method of precipitating gases is very good, but does not have the advantage of supplying new oxygen to replace that consumed by the men, animals, lamps, and explosions; besides, it has the defect of increasing the quantity of water to be removed. In tunnels the pipes used either for conveying the fresh air or for carrying away the foul air, are of iron, having a diameter of about 8 in.; they are fixed along the side walls about 3 ft. above the inverted arch.