STEAM TURBINE. The principles underlying the steam turbine are very simple, much more so, indeed, than those of the reciprocating engine. It is, therefore, not surprising that turbines of a single and crude type were in vented very early in history. The first steam turbine is believed to have been constructed by Hero of Alexandria, in the year 120 B. C. He boiled water in a cauldron, caused the steam to pass through pipes to two jets, fixed at op posite edges of a disc, the jets being turned at right angles to the plane of the disc and in opposite directions. The force of the escaping steam caused the disc to revolve. Nearly eighteen hun dred years later, in 1629 A. D. Branca turned a wheel by impinging a jet of steam on to paddles fixed to the circum ference. These two simple turbines il lustrate the two types now in use—the reaction turbine and the impulse tur bine, motion being produced in the for mer by the reaction of steam escaping from an orifice, and in the latter by the impact of particles of steam upon a mov able vane.
The next invention on these lines, af ter Branca's, was that of Wolfgang de Kempelen, who produced a reaction tur bine in the year 1784, James Watt con structing a similar machine almost simultaneously. It was not, however, un til the comparatively recent date of 1883 that the steam turbine received prac tical application. In that year De Laval designed a turbine which he used to turn an early model of the cream sep arator which is still associated with his name. In his machine, which is of the impulse type, a specially constructed nozzle causes a jet of steam to impinge on to buckets arranged on the rim of a revolving cylinder. The velocity of the steam is very high, as much as 2,500 feet per second, and the cylinder re volves at from 10,000 to 30,000 revolu tions per minute, the higher speed being used in machines of smaller size. Ow ing to this high velocity, this type of turbine is not used for marine engines, but finds a common application for driv ing dynamos.
C. A. Parsons, of England, built the first large turbine in 1884. This was ca pable of producing about 10 horse power and made use of both the impulse and the reaction principle. It consisted of a cylindrical case, containing a shaft or spindle, the diameter of this spindle being less than the internal diameter of the cylinder. An annular space was
thus left between the two. On the in side of the cylindrical case were nu merous rings of inwardly projecting blades, while, on the shaft, were mounted corresponding outwardly projecting blades. These two sets of blades occu pied the space between the cylinder and the spindle. When steam was admitted it met a ring of fixed blades on the wall of the cylinder. These blades deflected it in such a manner that it impinged upon the corresponding blades of the spindle, and imparted to them a rotary motion. The diameter of the cylinder increased in successive stages toward the exhaust. The blades in the first few rows are of copper, the others of a spe cial brass alloy.
The Curtis turbine is similar to the Parsons, but depends upon the impulse principle, steam being admitted through a series of nozzles. The steam is ex panded almost to exhaust pressure in the nozzles, so that there is no appre ciable difference of pressure between the front and back end. It follows that the end thrust, which has to be taken care of in the Parsons turbine, is almost en tirely avoided in this engine. Moreover, owing to the expansion of steam in the nozzles, the front blades are not sub jected to the action of superheated steam and so do not require to be con structed of special metal for resisting high temperatures. Regulation of speed is obtained by closing one or more noz zles.
Other types of turbine are the Riedler Stumpf, the Rateau, the Melins and Pfenniger, the Schulz, and the Zolly. While each one has its special features, they all follow the general principles outlined above. As an indication of the speed at which the shafts run in differ ent engines, the following figures may be taken as representative. The velo city of the tips of the rotating blades in the Parsons turbine is 100-150 feet per second at the front end, and at the back end, where the diameter of the shaft in creases, 300-350 feet per second. In the Rateau, the speed is 350-400 feet per second, and in the Curtis a little higher. On the other hand, the buckets in the Be Laval and the Riedler-Stumpf tur bines reach a velocity of 1,000 feet per second.