It is, therefore, economical to use the expansive force of the steam; and to do this with the greatest effect a long cylinder or a series of cylinders is necessary. It is also necessary that the initial pressure be high, so as to take advantage of expansion to the fullest. In the compound system, therefore, an advantage is gained; and in the triple and quadruple systems a still further improvement arises from the use of steam at a higher pressure and expanded more fully, whereby less steam can be made to give a better mean result. A saving is thus effected, both in coal consumed and in boiler space. Pressures have now reached 160 lbs. and 170 lbs. per square inch, and will, no doubt, go higher still. The limit of economical working cannot, however, be far off, as with increased pressure difficulties will be met from the greater heat of the steam affecting the working parts. The whole question of economical working, in a word, depends upon the amount of effective work which can be obtained from the coal consumed. So that, not only is the engine subject to further improvement, but also the boiler and furnace. Combustion by forced draught, &c., are all elements in the question. From the combustion of 1 lb. of coal about 14,500 thermal units should be obtained. Roughly speaking, this energy may be stated to be expended as follows:-In the triple-expansion engines as now used at sea, we may take 11⁄2 lb. of coal consumed per hour as equivalent to one horse-power; but as one horse-power represents 33,000 foot-pounds of work done per minute, then the total work done by the combustion of the 1 lb. of coal in the hour is 33,000 × 60=1,980,000 foot-pounds. But it can be shown that from the combustion of one pound of coal 14,500 thermal units should be obtained, and consequently from 1 lb. 21,750 thermal units, which, multiplied by 772 (the mechanical equivalent of heat), gives 16,791,000 foot-pounds of work done in the hour; if, therefore, we compare the work actually done with the work which should be done if all the coal energy could be converted into useful work by the engine, we have this ratio: 1,980,000 or only a little over one-ninth of the energy 16,791,000' expended. The triple-expansion system of marine engines was first tried in the Propontis, built in 1874, by Messrs. J. Elder. The diameters of the cylinders were 23, 41, and 62 inches respectively; steam pressure 150 lbs. Mr. William Parker1 says: "The first engines made for sea-going purposes on the triple-expansion principle were those made in 1874, from the designs of Mr. A. C. Kirk, now of the firm of Messrs. R. Napier & Sons, for the S.S. Propontis. The next triple-expansion engines were those of the yacht Isa, of Newcastle-on-Tyne, in 1877, steam-power 120 lbs. In 1881 the Aberdeen was built and fitted with tripleexpansion engines by Messrs. R. Napier & Sons, the steam pressure being 125 lbs. In the Propontis and Aberdeen there were three cranks, in the Isa two cranks only, two of the cylinders being arranged tandem-wise. The diameters of the Aberdeen's cylinders are 30, 45, and 70 inches respectively, with 4 feet 6 inch stroke." Since then many sets of such engines have been made, the steam pressure rising to 160 and 170 lbs. A few quadruple-expansion engines have also been made. 1 See Trans. Inst. Naval Architects for 1887. The Industry in Bowling Harbour (the oldest steamer in the world). |