Introduction of the Steam Turbine.—The invention of the steam turbine has revolutionized marine engine practice, in re spect especially of the largest and fastest vessels. For the gener ation of electricity the turbine has a notable advantage in directly developing the high speed of rotation which a dynamo requires, and, when designed on a large scale, its efficiency is unrivalled by steam engines of other kinds. To Sir Charles Parsons we owe not only the main idea of the modern steam turbine, but also the in vention of many mechanical features and details essential to its practical success and general adaptation.
In the steam turbine, as we have seen, pressure, instead of being exerted on a piston, is employed in the first instance to set the fluid itself in motion. There is a conversion of pressure-energy into velocity-energy as a preliminary step towards obtaining the effective work of the machine. If this were done in a single step it would involve immensely high velocities in the steam jet and in the vanes on which the jet acts. Attempts to design a steam tur bine were made by numerous inventors, but fell short of practical success mainly because of the difficulty of arranging for high enough velocity in the working parts to utilize a reasonably large fraction of the kinetic energy of the steam. There was a further difficulty in getting the energy of the steam into a suitable kinetic form, namely, to get the stream of issuing particles to take a single direction, without undue dispersion, when steam was allowed to expand through an orifice from a chamber at high pressure into a space where the pressure was greatly less.
In 5889 Dr. Gustaf de Laval introduced a form of steam tur bine in which both of these difficulties were, to a considerable de gree, overcome, partly by the special form of the nozzle used to produce the steam jet and partly by features of design which al lowed an exceptionally high speed to be reached in the wheel car rying the vanes against which the steam impinged.
Parsons attacked the problem at an earlier date and in a dif ferent way, by his invention of the "compound" turbine. He di vided the whole expansion of the steam into a great number of successive and separate steps and thereby limited the velocity ac quired at each step to such an extent as to make it comparatively easy to extract the greater part of the kinetic energy as work done upon the moving blades, without making the velocity of these blades inconveniently high. Moreover, in Parsons's compound turbine the range of pressure through which the steam expands in each separate step is too small to cause any difficulty in the formation of the jets. The guide blades, which form the jets, are
distributed round the whole circumference of the revolving wheel, and all the revolving blades are consequently in action at once. The steam streams from end to end of the turbine through an annular space between a revolving drum and the casing which surrounds it. Parallel rings of fixed guide blades project inwards from the casing at suitable distances, and between these are rings of moving blades which project outwards from the drum and re volve with it. At each step in the expansion the steam streams through a ring of fixed guide blades, and the streams so formed impinge on the next ring of moving blades, and so on. The con struction, which is of great simplicity, is described, along with others, in the article TURBINE : STEAM. It lends itself well to the generation of power on a large scale, especially where a fairly high speed of rotation is wanted.
Parsons introduced his compound steam turbine in 1884. For some years it was made in small sizes only, and the steam was discharged to the atmosphere without condensation. So long, however, as this was done the steam turbine was sacrificing one of its most important advantages, namely, its exceptional capacity for utilizing the energy of low-pressure steam down to the lowest vacuum obtainable in a condenser. In 1891 it was first fitted with a condenser, and it then began to be used in electric supply sta tions. The first application to marine propulsion was in the "Tur binia," in 1897. The success of this little experimental vessel of 100 tons, which, with its horse-power of 2,100, made what was then a record in speed for any ship, was soon followed by the ap plication of the turbine to war-ships and other steamers. In mer chant vessels its use was at first limited to those of the highest speed, for the turbine shaft was directly coupled to the shaft of the screw propeller; but in 1910 Parsons introduced a mechanical reducing gear between the two, which allowed the turbine shaft to run much faster than the propeller shaft, to the great advantage in efficiency of both turbine and propeller. Later he followed this up by a "double-reduction" gearing which admitted of a still greater difference in speed of rotation between the propeller and the turbine. In most ships the single reduction system is suffi cient for the purpose : its introduction greatly extended the range within which the turbine could be advantageously substituted for the three-cylinder or four-cylinder compound engine of the piston type in ocean-going steamships.