VENTILATION.
In the economy of mining, particularly in coal where explosive gases are present, the subject of ventilation is no secondary consideration ; and the practical miner or engineer is not competent to the management of mines unless he is conversant with the scientific questions necessarily involved in the subject of the ventilation of deep and fiery mines.
It is not our purpose to discuss at present the science of ventilation ; this will be in volved in the application; but, in order to obtain a clear comprehension of the subject, and to present it practically, we must discuss its leading principles; and perhaps the best mode of doing so to the general reader is to make plain and every-day comparisons.
As the wind rushes in storms from the colder to the warmer districts by the increase and decrease in bulk and the ascent and descent of the condensed or rarefied portions in the strata of the air, so the air may be conveyed through any extent of building or subterranean passages. Rarefied air, being lighter than common air, ascends above the common strata, and this creates a commotion, and brings distant currents to replace the ascending column.
When common or natural ventilation is used, the ventilation of a mine is similar, in a limited sense, to that of a house. The air either passes in and through any passage open for its progress, or is drawn in a rapid current by the heat of a fire or a stove. That portion of the air which comes in contact with the fire is rarefied, and of course ascends rapidly through the chimney; and, as it escapes, fresh air takes its place; for "nature abhors a vacuum." In the same manner "furnace-ventilation" is conducted in deep mines.
But in our coal-mines the air is frequently carried through many miles of tortuous subterranean avenues, and too often through irregular channels, where the column is contracted to one-half or two-thirds of its bulk, and of course is to that extent retarded. The friction of air, though insensible when in slow motion, is very great when dragged mile after mile, through rough and jagged avenues, at the rate of 1000 feet per minute.
Through a straight, smooth passage it might be forced along with half the power re quired to propel it through the ordinary air-courses of the mine.
In most water-level mines, natural ventilation can be made available, entering the air at the lower level and returning it to the atmosphere at a higher point; but even in these, when the avenues are long and narrow, it is extremely difficult to keep up an even ventilation, owing to the constant variation of the temperature of the atmosphere, and the consequent changes in the currents and air-strata. Mechanical means are
frequently resorted to even in mines in the mountain-sides, where they are extensive.
The friction of air when carried violently through rough and intricate passages is greater than may readily be imagined. To carry 100,000 cubic feet of air per minute through a single avenue containing 10 square feet of area will require a far greater amount of power than to carry the same amount of air through ten avenues having one half the area, or 5 square feet to each; and this not only demonstrates the fact of fric tion, but points at the best mode of ventilating mines. Instead of carrying the air in one unbroken current through the entire mine, it is, therefore, best to split the air wherever convenient, and carry it by separate channels to the respective portions of the mine under operation. But there is another advantage to be gained by this process, in addition to the movement of the column of air. It is evident that all parts of the mine cannot be equally distant from the upcast and downcast shafts: consequently, it cannot be best to carry all the air through the most distant workings, in order to introduce it, with all its foulness, to the near workings. Nothing can be more evident than the economy and propriety of conveying a portion of the current to each part of the mine respectively, thereby not only decreasing the column of air, but giving to each set of workmen their share of pure air. This, however, cannot be done in our present system of mining; but it can be done, nevertheless, with great advantage to all interested, by dividing the mine into boundaries, and carrying a branch current to each, direct from the main column, by the most available way. In order to consider the different modes of ventilation used, we will discuss them under the heads of natural ventilation, furnace ventilation, steam-ventilation, and mechanical ventilation.