FURNACE-VENTILATION.
In figure 147, a is the upcast slope, or shaft ; b, the main avenue, or gangway; c, the inlet air-course below the gangway; d, the return air-course above the gangway ; g, doors to keep the air in its proper course; and f, the furnace. The downcast slope is not lettered, but the arrows indicate the course of the air. The lower passage, c, is not indispensable, and may be omitted when the upper or return air-course is carefully preserved. By opening the door g in the gangway b, the air passes in through that avenue, dispensing with c altogether if the shutes are air-tight.
The furnace f may be placed to the left of the upcast a, perhaps with more pro priety than between the upcast and downcast, on account of the limited room around the bottom of the shaft, and the tendency of the furnace to weaken the pillars. But, wherever placed, the return air must enter the upcast shaft above the point reached by the flame of the furnace, or else, under certain circumstances, the gases brought out of the mine by the returning current of air might be fired. Even with the precaution of a "dummy drift," &c. from the furnace to the upcast, the gas is sometimes fired, by accident, or by the forcing back of the return gases, by the falling of the roof, or from other causes, on the flames of the furnace; and this is one of the fatal defects of the furnace as a ventilator.
A large area of grate-surface is necessary, and an immense amount of coal consumed daily, to ventilate a deep mine,—requiring say 100,000 cubic feet of air per day; and the attention demanded is perhaps greater than that where mechanical motors are used. In the furnace system the depth of the shaft is no impediment, so far as expe rienced, in effecting ventilation. The shaft, if dry, is heated, and retained in that con dition by the furnace and the hot air ascending from it; and the consequence is that the air continues in a rarefied state, and escapes upward rapidly, though the " drag" of the column may be considerable.
The following table, from the Transactions of the North of England Institute of Mining Engineers, will show the amount of coal and the estimated horse-power requisite to obtain a given amount of ventilation, as per experiments conducted at the Hetton, Elemore, and Eppleton collieries, in the Newcastle coal-field.
In the above table it will be noticed that a certain amount of steam is employed. Iletton has 4 furnaces, consuming 19 tons of coal in 24 hours, and 3 steam-boilers, pro ducing steam for jets, and consuming 16 tons of coal in the same length of time ; the whole equal to 109 horse-power, employed in mechanical means to produce a ventilation of 176,000 cubic feet per minute.
It will be found, on comparing this with the results of mechanical ventilation as pro duced by the suction-fan, that it requires double the power to produce the same venti lating column, under the same circumstances, with steam and furnace that it does with the fan. But in this case, though the ventilating power will compare with the best furnace-ventilation in England, the use of the steam-jet in connection with the furnace may rather add to the power required, in a greater proportion than the effect produced. It was decided, by a long series of experiments made in the Newcastle district in England, that furnace-ventilation was the most effective mode then in use ; and this decision was considered conclusive until 1863, when questions concerning fan-ventilation, as used in France and Belgium, were discussed among the mining engineers in that district, and the fan was generally conceded to be superior to the furnace so far as the proportion between power and result was concerned. But up to 1864 the fan was only occasionally used in the English mines ; and then, with but rare exceptions, the ponderous, costly, and imperfect French system was introduced.