January 21, 1916. _________________________________________________________________________________________________________________________________ THE COLLIERY GUARDIAN. 131 NOTES ON MINE VENTILATION.* By John Shanks. In Western Canada the principles of ventilation, which should be considered when opening up a mine, have been frequently ignored, with results not contri- butory to efficiency or economy; a consideration of the subject at the present, time is moreover believed to be opportune, in view of recent mine legislation in Alberta bearing on it. The Alberta Mines Act of 1913, amended by Order in Council on November 4, 1914, enacts (General Regula- tion 16) that in every mine, unless it is naturally wet throughout, the cars shall be so constructed and main- tained as to prevent as far as practicable dangerous coal dust escaping through the sides, ends, or floors of the cars, but any cars which were in use in any mine at the time of the passing of these regulations may, not- withstanding that they are so constructed, be continued to be in that mine until January 1, 1920. Systematic steps shall be taken to prevent as far as practicable explosions of coal dust occurring or being carried throughout the mine. General Regulation 9 states that after July 1, 1915, there shall, in every mine in which a mechanical con- trivance for ventilation is used, be provided and main- tained in a condition to be put into immediate opera- tion, adequate means for reversing the air current. General Regulation 16 aims at preventing as .far as practicable the accumulation of dangerous coal dust. It is not clear whether the word “ dangerous ” refers to very fine or impalpable dust; possibly the Government advisers are not yet converted to the idea that all coal dusts are dangerous under certain circumstances. The author understands that the Government officials intend having the coal dusts from the different mines in the province tested, with a view to their classification. If this classification is intended to make a discrimination between what is dangerous and what non-dangerous coal dusts, the task is an extremely difficult one: and Rule 16 can only become operative after the results shall have been published. Since the results have been published of the laboratory experiments of Dr. Bedson in 1910-11, Fig. 1. Fig. 2. miles from Mine Mouth. SPLIT No. 4 200 TONS 25.000 CU. FT SPLIT No 2 200 TONS 26 000 CU. FT SPLIT No. 5 200 TONS 2S.000 CU FT SPLIT No. 1 200 TONS 25.000 CU FT. AIR ■> i::l Fig. 3. and of the experimental gallery tests at Altofts and Eskmeals, in England, and elsewhere, mining men now have very little doubt concerning the explosibility of coal dust. Whatever difference of opinion there may be, it is generally conceded that the finer the coal dust, the more readily it gives off its volatile gases; and the lower the temperature at which it gives off these gases, the more dangerous the dust. The following table gives the results of laboratory experiments with different dusts of equal fineness. The experiments were carried out some four years ago by Prof. Bedson, f Armstrong College, Newcastle-upon- Tyne, England. The coal dust charges were projected, by a blast of compressed air, over a glowing platinum wire. The impulse communicated by the explosion was indicated by the extent to which a pendulum arrange- ment was deflected. nrown minous Lignite. coaL splint. Fineness ...............(meshes) 2^0 ... 200 ... 200 Weight ................(milligr.) 500 ... 500 ... 500 Amperes required to the platinum wire ......................... 14 ... 14_ ... 14 Scale deflection............ (cm.) 60 ... 45'7 ..: 41’5 Impulse ........................ 100 ... 78’6 ... 71’0 Owing to the large tonnage of lignite coal mined in Western Canada, particular note should be taken of the impulse due to brown and lignite coal dust. Coal dust on roadways is only dangerous because of the possibility of its being forced into suspension in the mine atmosphere by strong currents of air, caused by a local gas explosion; or due. to a misjudged charge in a short hole; or to a fall of roof over a large area. Authorities have computed that 101b. of coal ground to impalpable dust, represents an absorbing surface of something between 10,000 and 60,000 sq. yds., and 3 oz. of coal dust thoroughly disseminated in 1 cu. yd. of air form an explosive mixture. Judging from these figures one can see how difficult it is to overcome the coal dust danger. The main sources of the coal dust found on roadways are :—(1) Dust carried into the mines from the screen- ing plant, and the tops of loaded mine cars in transit by the ventilating current; (2) leakage of dust from loaded * Bulletin of the Canadian Mining Institute, f Trans. Inst. M.E., vol. 39, part 5. cars during transit, a portion.of which falls on the rails and gets ground into impalpable dust. Making cars tight on the sides and bottom, while coal is allowed to be piled above the top of the cars, is only a prevention in a small degree. The coal above the top of a mine car is swept, as a rule, by the main intake current of air. This current carries away, in suspen- sion, large quantities of fine coal dust; hence the stronger the current, and the finer the coal, the greater will be the quantity of coal carried oft in such a manner. As already stated, it is only when in a state of suspension that coal dust is dangerous; accumulations are only dangerous because of the possibility of their being forced into suspension. The author believes that more stress should be placed by engineers and mine managers to prevent, as far as possible, an unnecessary high velocity of the air current in the haulage roads. This can only be accomplished by increasing, the area of haulage roads that are used as main intakes; or by maintaining a separate main intake, and ventilating the main haulage road as a split, carrying only sufficient air to supply the men and horses working on it. The size of main intake and return airways requires careful consideration from the point of economy, apart from the question of safety which accompanies low velocity air currents. Many properties have been hampered in their development by starting with unduly small airways. At the present time new properties arc being opened, having airways far too small for the economical passage of the air necessary for future requirements. Of course, there is an economical limit to large airways,..which is reached in thin seams. The following calculations will help to emphasise the writer’s points. Suppose there are in a mine in which a 5 ft. seam is worked, two airways, X and Y. X has been driven 10 ft. wide in the 5 ft. seam, and has not been brushed. Y has been driven 12 ft. 6 in. wide in the coal, and 3 ft. has been brushed out of the roof to make a height of 8 ft. The pressure, H, required to pass the same quantity of air in X and Y, other things being ! * O equal, varies as & where 0 represents perimeter of airways, and A area. Let IT, 0, and A represent their respective value in airway X; and H, O, and A similar values in airway Y; then, since H varies as AL we J A3’ have :— O A, 3 _ 30jx 1003 _ 6.85 ______ Ht A3 ax3 CL A 3 41x5b2 ° The pressure required for the small airway will there- fore be 5-85, or nearly six times as large as that required to pass the same quantity in the large airway, where the roof has been taken down. Small airways mean : (a) a larger fan engine ; (b) greater coal consumption; (c) dangerous velocities; and (d) high ventilating pressure and greater leakages. It is therefore apparent that the question of required size of an airway is important, and should not be arrived at without consideration of all future requirements. Rule 9 of the General Regulations makes it compul- sory to provide means whereby the ventilating current can be reversed. The reversal of the air current will be of more use in the case of underground fires in shafts, main airways, underground stables, etc., than in the event of an explosion, where the state of affairs under- ground is unascertained. It is a debatable question, whether, after an explosion, the fan should be re-started or not. Some authorities contend that it should be started only after preliminary exploration had been made by explorers wearing oxygen apparatus, who would ascertain conditions. To reverse the air current imme- diately after an explosion, without any preliminary examination of the workings, cannot be justified for a moment. The intake airway is usually the road to the surface; therefore, the workmen naturally proceed to reach this path, where, if the air current is following its usual course, they naturally expect to meet breathable air. For the past few years ventilating machines in Western Canada and United States have usually been so arranged that the air current can be reversed. This arrangement was originally made, not so much with a view to safety, as to economy. Because of the low temperatures prevailing in this country in winter time, great difficulty is experienced in operating wet shafts and haulage roads, owing to the rapid accumulation of ice. The heated air of the mine is frequently made to return along the haulage roads in order to keep the tem- perature above freezing point. To accomplish this it was found necessary to instal a reversible fan. British engineers have adopted the plan of reversing the air current by means of drift and door arrangements. (See figs. 1 and 2.) The fan remains an exhaust fan (as is the usual practice there) at all times. There is some thing to be said in favour of the exhaust fan, as, against the force fan, especially in gassy mines having a large goaf area. With a force fan, the gases in the goaf area are always under a state of compression, due to the positive water gauge of the fan. If anything happens to the fan oilengine to produce a sudden stop- page, the sudden release of pressure causes an expansion of the gases in the goaf, and they are likely to spread into the main arteries or airways of the mine, and cause serious trouble. The greater the goaf area, and the greater the positive ventilating pressure, the, greater is the danger. Some authorities hold that the goaf area of a mine is about one-sixth of the space originally occupied by the coal. If this be so, then when a venti- lating machine breaks down in a mine working a 6 ft. seam, which has a goaf area of one square mile, and bears an average positive ventilating pressure on the goaf of 3 in. w.g., i.e., 15’6 lb. per sq. ft., 27,878,400 cu. ft., of goaf gases expand inversely with the change in the absolute pressure. The calculation is as follows :— Lb. per sq.ft. Absolute pressure fan running 14’7 x 144 + 15’6_____ 2,132’4 Absolute pressure fan standing 11’7 x 114 ........ 2,116’8 27.878,400 * 2,132'4 io non om i £ £ ------o --------= -8,683,85c cu. ft. = new vol. of goaf gases 2,116’8 This causes a difference in volume of 205,453 cu. ft., which can only find space by moving into the airways. This quantity of gas would charge, to its most explosive point, an airway 100 sq. ft. in area, and approximately four miles long. Another danger arises from the general use of a forcing fan. Suppose a fire takes place in an airway, and, in order to keep the smoke and fumes from passing on to the workmen at the faces, it is found necessary to quickly reverse the air current, the total change in pres- sure will be approximately double the initial w.g. The change will be, say, from +4 to —4. This would cause a considerable expansion of the goaf gas, which in all probability would pass into the main airway, and in this way would be carried in a cloud over the fire on its way to the fan. In selecting a fan for a mine some engineers prefer to instal a unit near the shaft or mine openings, capable of meeting all future requirements. In doing so, the equivalent orifice A of the mine at its maximum resist- ance has to be assumed; then a fan selected, whose orifice of passage O suits the proved principle that, for high efficiencies, O should approximate to 3 A. Such a fan involves a large initial capital expenditure; and, in the early stages of development, when the equivalent orifice of the mine is large, the efficiency of both fan and engine must be low. Other engineers prefer to start with small units work- ing near their capacity, and therefore full efficiency. The cost of foundation, etc., for motor-driven fan units having capacities up to 75,000 cu. ft. of air on low water gauges is not great; at least, not such as to make their installa- tion and replacement by larger units unprofitable, pro- vided a future use can be found for them when discarded in the first instance. The fans at the mines in charge of the author are Jeffrey high-speed fans, and are guaran- teed to give 150,000 cu. ft. of free air against a pressure of 3 in. w.g. They are belt-driven by separate 75 horse-power induction motors, capable of giving the fans a capacity of 100,000 cu. ft. of free air against a 2Jin.' w.g. This would represent the fan and motor as giving a combined efficiency of about 55 per cent. The air passing in the mines is regulated down to 59,280 cu. ft.; and the fan works against a pressure of 2 in. water gauge. The meters show that the motor takes 85 amperes at an average voltage of 435. This represents 85 horse-power going into the motor for 18’68 horse- power in the ventilating current : a very low efficiency caused by using a motor and fan too large for present requirements. These fans were installed in the spring of 1914. They are built so that they can be used as either forcing or exhausting machines. This coal field • has a long outcrop line, which rises in some parts to a considerable height above the mine mouths. It was originally intended to take advantage of openings in the . high ground, and use the fans forcing in the winter months, and exhausting in the summer months. By doing so, advantage would have been derived from the assistance of the natural motive column during most of the year. The author found it impossible to use the fans forcing during the winter months, since the fan drift is also the water level. This drift would have choked with ice in a very short time. The idea of reversing the air current for purposes of economy is worth considering in large coal fields in the mountains, where the outcrop is frequently reached at a considerable altitude, and where the total ventilating pressure is low. Of course, if the total ventilating pres- sure is high, the percentage of w.g. due to motive column will be low. In a coal field situated as above, where the equivalent orifices of the mine splits are large, it appears to the author that economical use can be made of small fan units, placed on the outcrop line. Some such arrangement as is suggested in fig. 3 seems practicable. The location of the outcrop fans would greatly depend on the configuration of the country, and the amount of overburden or glacial drift to be driven through. A fan of moderate capacity would have ,to be placed at the mine mouth, capable of ventilating the main levels and a few splits. Suppose the plan represents the workings of a 7 ft. . seam. Assume : Length of the main intake = 8,000 ft.; area of the main intake (12 ft. X 7 ft.) = 84 sq. ft.: 100,000 cu. ft. per minute passes in main airways and