In this respect it is remarkable that the deficiency is general. Boilers are almost always calculated too closely, and many efficient engines are crippled for want of a full supply of steam. Where the generation of steam is more an object than the first cost of machinery, as on ocean steamers, and where coal is costly, it is a matter of necessity or economy to provide engines calculated to do the most duty with the least amount of steam, as in the Cornish low-pressure engines, or in working steam expansively in high pressure engines; but in all other cases it is economy to provide an excess of steam boiler or heating surface. The object in such cases is to make a small amount of fire surface heat a great extent of boiler surface, or to expend the caloric in the most efficient manner.
But in colliery establishments, where coal is plentiful and cheap, the same objects are not directly sought, though it is equally important that a large boiler surface should be provided—not so much, however, with regard to the economy of fuel as to the efficiency of the power, though a surplus of boiler does not affect the quantity of coal used in an unfavorable manner; in fact, the result is quite the reverse. When there is a deficiency of boiler surface, steam is always low, and the engines cannot do their duty, while an excessive firing simply wastes the heat by passing it up the stack. Therefore, an extra boiler, with fires arranged for the most effective use of the flame and heat, and moderate firing, not only saves labor, coal, and material, but also renders the machinery fully effective.
Large single engines are not as useful in colliery establishments as smaller double connected engines where speed is required, as in nearly all our mining departments, except in pumping; and in this, large, heavy, and slow-moving engines are more useful than smaller and faster ones.
To present the matter clearly, we will state that a 40 horse-power engine, single, moving at 50 revolutions per minute, and consuming 16,000 cubic inches of steam (which is an excess) at 50 pounds pressure, will lift 6000 pounds 300 feet per minute.
But two 20 horse-power connected engines, running at 100 revolutions per minute, and consuming 32,000 cubic feet of steam, will lift double the weight, with more ease, 300 feet per minute. In the first instance the pinion or drum should be two feet in diameter, and in the second one foot diameter. But if the pinion remains the same the load is lifted at double the speed. To a certain extent this rule holds good notwithstanding the additional friction of the two cylinders, since the advantage gained by having one crank continually on the live centre more than compensates for the addi tional friction.
The heaviest loads to be lifted from our deep mines should not exceed 7 tons, or, if 10 tons, 3 tons may be counterbalanced by the descending cage and wagon: therefore 260 horse-power ought to be sufficient to do the work of hoisting from any single shaft or slope. But, as before observed, and on the principle advanced in the case of the 40
horse-power or the two 20 horse-powers, two 100 horse-power connected engines would have the same advantage over the 200 horse-power single engine that the two 20 horse-power engines had over the 40 horse-power engine. The load in both cases is, perhaps, more than should be attached. We calculate, however, the maximum duty of a single engine as the minimum duty of two connected engines of the same average power and supplied with a full amount of steam. A 40 horse-power engine will not lift more than 6000 pounds 300 feet high per minute, without great strain, running at 50 revolutions per minute. But two 20s, running at 100 revolutions per minute, will lift a greater load—say 8000 pounds—that distance with ease.
The weight to be lifted in a shaft 300 feet deep may be estimated thus:—coal, 3 tons; car, 1:1 tons; cage, I ton; rope, 1 ton,—since the pulleys should be some distance abuse the landing-point. This would give 51 tons as the load. But the cage and car would be counterbalanced, and would thus reduce the weight to be lifted by the engine to 33 tons at starting, 3 tons in the middle, and 21- at the top of the shaft; since in the middle the rope is balanced, and nothing but the coal is lifted, while at the top the rope counter balances an equal weight of coal. Therefore the maximum load is 8400 pounds, the average 6720 pounds, and the minimum 5040 pounds. This load can be lifted 500 feet per minute by two 20 horse-power connected engines, if geared to run at 100 revo lutions per minute.
In a shaft of 1000 feet depth, the load would be considerably increased with the same coal-car and cage, on account of the increased weight of the rope, and the tendency to render the counterbalance less effective by the increased depth. For instance, a balance with equal weights is poised on common levels, but if the weight on one side is lower than that on the other, the lowest side will always be the heaviest in effect, though of the same weight really : fifteen ounces may counterpoise a pound if the centre of gravity be not equal, and the same in relation to the counterbalance in shafts. The cars and the cages may be of equal weight, but, one side being 1000 feet lower than the other, they will not be equal in effective weight, even though the additional weight of the rope be added to the upper side. The difference may not be great ; still it adds to the load at starting, which is the most trying point to the engine.