Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/91

Rh VENTILATION.] COAL 71 in a groove in a cast-iron plate, curved to the slope of the casing. By the use of the spiral guide casing and the FIG. 17. Guibal s Fan. chimney, the velocity of the effluent air is gradually reduced up to the point of final discharge into the atmosphere, whereby a greater useful effect is realized than is the case when the air streams freely from the circumference with a velocity equal to that of the rotating fan. The power is applied by steam acting directly on a crank at one end of the axle. In most of the newer examples, which are generally of large size, the power is divided, an engine being placed on each side. At Washington Colliery, Durham, a machine of 36 feet diameter, 12 feet breadth of face, and 13 feet diameter of intake passage, draws 120,000 cubic feet of air per minute, when making 38 revolutions Another at Usworth, 48 feet diameter and 12 feet breadth of face, driven by two high-pressure engines, with cylinders 3 feet in diameter and 3 feet stroke, equal to about 280 horse-power, exhausts 200,000 cubic feet per minute. The useful effect realized under the most favourable conditions is as much as 50 per cent, of that of the steam power employed. Waddle s fan, represented in fig. 18, is an example of FIG. 18. -Waddle s Fan. another class of centrifugal ventilator, in which a close cas ing is not used, the air exhausted being discharged from the circumference directly into the atmosphere. It con sists of a hollow sheet-iron drum formed by two conoidal tubes, united together by numerous guide blades, dividing it up into a series of rectangular tubes of diminishing sec tion, attached to a horizontal axle by cast-iron bosses and wrought-iron arms. The tubes at their smallest part are connected to a cast-iron ring, 10 feet in diameter, but at their outer circumference they are only 2 feet apart. The extreme diameter is 25 feet. A fan of these dimen sions atBrownhills in Staffordshire, in making 50 revolutions per minute, circulates 47,000 cubic feet of air through the workings. It has also been in use for some years in South Wales, and is found to work well; it is less expensive in first cost than Guibal s, although proportionally less economical from the smaller effect realized for the power expended. Another method of colliery ventilation is that by jets of steam blowing off at a high velocity into the upcast shaft, and producing a draught similar to that of the exhaust blast in the chimney of a locomotive. This plan found several advocates some years since, and was the subject of numerous comparative trials against the ventilating fur nace in the North of England, but the results were unfa vourable, the amount of air circulation produced being exceedingly small for the fuel expended. It seems probable^ however, that this want of success was in great part due to the defective character of the apparatus applied, and that, with properly-constructed aspirators and discharge passages, the steam jet may prove to be a very efficient means of ventilation. The comparative merits of furnace and machine ventila tion have long been discussed without any definite result. The former was at one time regarded in England as practi cally superior in every respect, but this opinion has been modified since the introduction of the improved forms of fans which have been worked to a considerable extent. In France and Belgium, on the contrary, machine ventilation has been more generally in favour. For a deep and ex tensive mine where the coal is not fiery, the furnace is undoubtedly the simplest and most efficacious method of producing a large circulation of air ; but for moderate depths, especially with fiery return air, a ventilating machine at the surface is in many cases to be preferred. There ia also an important advantage procured by the latter, namely, that of reserve power, so that a larger circulation may be obtained immediately in case of need, e.g., when the barometer falls suddenly, by merely increasing the speed of rotation, which cannot so readily be done with the furnace, which has a tendency to slacken at the time when the increased work is wanted. The quantity of air required for a large colliery depends Distribu- upon the number of men employed, as for actual respira- tion of air tion from 100 to 200 cubic feet per minute should be allowed. In fiery mines, however, a very much larger amount must be provided in order to dilute the gas to the point of safety. Even with the best arrangements a dan gerous increase in the amount of gas is not un frequent from the sudden release of stored up masses in the coal, which, overpowering the ventilation, produce magazines of explosive material ready for ignition when brought in con tact with the flame of a lamp or the blast of a shot. The management of such places, therefore, requires the most constant vigilance on the part of the workmen, especially in the examination of the working places that have been standing empty during the night, in which gas may have accumulated, to see that they are properly cleared before the new shift commences. The actual conveyance or coursing of the air from the intake to the working faces is effected by splitting or dividing the current at different points in its course, so as to carry it as directly as possible to the places where it is required. In laying out the mine, it is customary to drive the levels or roads in pairs, communication being made between them at intervals by cutting through the inter mediate pillar, the air then passes along one, and returns by the other. As the roads advance other pillars are driven through in the same manner, the passages first made being closed by stoppings of broken rock, or built up with brick and mortar walls, or both. When it is desired to preserve a way from one road or similar class of work ing to another, double doors placed at sufficient intervals apart to take in one or more trams between them whon closed are used, forming a kind of lock or sluice. These are made to shut air-tight against their frames, so as to