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The recent introduction of the turbine may, however, effect a change in this class of machinery, as it is admirably adapted to high falls, and may be advantageously employed at a moderate cost. The great objection to its use in this form is the great velocity it attains on high falls, and the consequent reduction which would be requisite to work pumps at 10 to 12 strokes per minute, when the machine itself is moving at the rate of 400 to 500 revolutions per minute. This appears to be the only drawback, and it is not improbable that the simple cylinder here described may, under certain conditions, be best adapted to meet all the requirements of raising water from deep mines with the aid of convenient streams on high falls.
ON THE PROPERTIES OF STEAM.
Before considering the application of the steam engine as a prime-mover, it may be interesting to know something of the properties of steam by which it is moved, in regard to pressure, temperature, and density, as ascertained by various philosophers since the days of Newcomen and Watt. Of late years a great change has gradually taken place in the system of working the steam engine. At the time of the introduction of the doubleacting engine of Watt, the makers of engines never dreamed of employing steam at a greater pressure than 10 lbs. on the square inch, and up to 1840 that was the maximum pressure at which steam engines were worked, with the exception of a few constructed on Wolfs principle of double cylinders, where the steam is first admitted to the piston of the smaller cylinder at a pressure of 30 to 40 lbs. per square inch, and after having performed its office there, is allowed to expand into the second cylinder of three or four times greater capacity, and thus to unite its force with that of the small cylinder, as it moved from one extremity of the stroke to the other. To work this description of engine with high-pressure steam, it was necessary to proportion the strength of the parts of the engine as well as the boiler to a much greater extent of pressure than in the double-acting engine of Watt. Hence it was soon found that the waggon form for the latter, as employed by Watt, was not calculated to resist a pressure exceeding 10 or 12 lbs. per square inch without the introduction of numerous wrought-iron stays to retain it in form. To raise steam for the compound engine such a boiler was wholly inadequate, and a series of small boilers, with hemispherical ends, were introduced in its stead wherever steam of highpressure was required.
The single pumping engines of Watt, and the compound engines of Wolf, employed at the mines in Cornwall, gave, however, extraordinary results as regards the work accomplished for the quantity of coal consumed, which was less than half the 1 quantity used in the rotative engines employed in mills. It was also asserted that the double cylinder engine in use on the Continent (but chiefly made in this country) was performing a more satisfactory duty than could possibly be attained by the single cylinder low-pressure engine.
These assertions, often repeated, and the returns of Cornish engines, published from year to year, led to a close inquiry into the subject, first in my own works at Manchester, and subsequently before the British Association for the Advancement of Science, where the whole question was ably discussed, and ultimately led to a better system of working in factory engines, with a saving of one-half the fuel formerly consumed in effecting the same quantity of work. In these investigations it was found that the compound engine had no advantage over the single cylinder engine, as constructed by Watt, when worked at the same pressure of steam and the same rate of expansion; that is, a single cylinder engine, with properly constructed valves, having the power of cutting off the steam at any point of the stroke, is quite as effective, and more simple in construction, than the double cylinder engine. It is true, that at first the double cylinder engine had an advantage over the single cylinder engine in its greater uniformity of motion, but this is no longer the case, as an increase of the velocity of the piston from 240 to 320 and 360 feet per minute effectually remedies that evil, and increases the power of the engine in the ratio of the increase of speed.
Thus it will be seen that a great change has come over the system of employing steam; the pressure is quadrupled in factory engines, and more than doubled in marine engines. Every engine of recent construction is provided with boilers of great resisting powers, and on an average cuts off the steam in the cylinder at one-fourth, and at other times one-fifth or one-sixth of the stroke, the steam acting by expansion alone during the remaining three-fourths, four-fifths, or five-sixths, as the case may be. This system is found to be of great value, as the quantity of fuel consumed does about double the amount of work which could be got out of it on the low-pressure principle.
The important results already obtained by a judicious system of working steam expansively, has given a powerful stimulus to the extension of our commerce and manufactures, and the question naturally arises, whether or no we have attained the full benefit from the introduction of the methods of working now employed, or whether we may not reap a still greater advantage from progressing in the same direction and using steam of higher pressure, expanded to still greater lengths than has yet been attained in our present practice. This is a question which remains for solution, and it appears most desirable that we should ascertain by direct experiments to what extent of pressure and expansive action we fcmay safely venture with perfect security to the boilers and the working parts of the engines. Assuming for a moment that an increased pressure, accompanied by increased expansion, would in the same proportion increase the economy of working, we have then to consider the capabilities of our vessels for resisting those pressures. And lastly, the observation of the action of steam in expanding has led many to expect still further advantage from the use of superheated or gaseous steam. To make sure progress in either of the directions here indicated two things are necessary: we must cultivate a more intimate acquaintance with the resisting powers of materials, and the strength of vessels of different forms, before we can assure ourselves of success; and we must attain increased and increasing knowledge of the properties of the agent we employ under the various conditions of expansion and superheating. In regard to the first of these requisites a steady progress has been made, and experimental inquiries have been extensively carried on in regard to the resisting powers of vessels and the causes of their failure, and the difficulty of constructing boilers fxTresist^very high pressures has been greatly diminished. Our knowledge of steam has also rapidly increased, and many of the necessary questions relating to its properties have been for ever set at rest by the recent and classical labours of Eegnault, carried on at the instance and with the assistance of •the French Government. The questions of the density and law of expansion of steam, however, still require solution.