Abstract of a Paper by F. C. Gasche presented before the Steel Works Club of Joliet, 111.
In contradistinction to the reciprocating engine, the fundamental principle of the steam turbine lies in the fact that the reciprocating engine does work by reason of the static expansive force of the steam acting behind a piston, while in the case of the turbine the work is developed by the kinetic energy of the particles of steam, which are given a high velocity by the pressure to a lower. Steam turbines may be divided into two classes: Reaction turbines of which Hero's is an example; impact turbines, of which Branca's is an example; a combination of the two.
General principles applying to the design of steam turbines are the same as made use of in water turbines. The buckets and guides must have as little skin friction as possible and be so arranged that the steam may strike without sudden shock and have its direction of motion changed without sharp angular deflections.
One great difficulty that presents itself is the tremendous velocity of steam as compared with that of water under ordinary heads. In the reaction or Hero type turbine the peripheral velocity must be equal to that of the jet of steam to give us the ideal condition and in the impact turbine the ideal condition is when the peripheral velocity of the buckets is one-half that of the steam jet.
With high - pressuresteam discharging into a vacuum the velocities obtained are from 3000 to 5000 ft. a second. A turbine built on the lines just given would, therefore, have peripheral velocities far beyond the limits of strength of material. As an example, a 19-inch Hero engine would revolve at 75,000 revolutions a minute, and the ends of the arms carrying the steam nozzles would have a velocity of over 40 miles a minute. The great problem confronting the inventors of steam turbines has been to reduce this velocity and at the same time to utilize the maximum amount of the energy of the steam.
Several attempts have been made to improve the simple reaction turbine of Hero and produce one that will run at a slower speed. In 1862 Charles Monson patented a turbine having a succession of simple reaction wheels, each one in a separate chamber and arranged so that steam issuing from a wheel into its chamber passes through the hollow shaft into the next wheel and so on; Fig. 1 gives the idea of this invention. Steam enters the hollow arms in chamber A through the hollow shaft, passes out through the orifice at the end into the chamber A, whence it is conducted to the hollow arms in the chamber B through a second passage in the shaft, and so on to the last chamber.
In 1885 C. A. Parsons secured his first patent for a compound reaction turbine. In all his work the inventor, who was the first to place the steam turbine in large units on a commercial basis, has adhered to the reaction type and is responsible for the successful development of the compound reaction motor. At one time or another Parsons has patented most of the feasible arrangements for the compound turbine, until, at the presant time, the Parsons turbine manufactured by the Westinghouse Machine Co., is one of the most successful in the world. Fig. 2 shows the first Parsons turbine patented in 1885. Steam enters at the center A and passes to right and left between the series of guide vanes attached to the outer casing and the rotating blades attached to the inner drum which rotates on shaft BB. The steam escapes through the exhaust pipe E.
Fig. 1. Monson's Compound Reaction' Turbine.
In 1893 DeLaval secured his most important patent relating to the expanding nozzle to be used in combination with an impact type turbine wheel. By the use of this expanding- nozzle the energy of the steam under pressure was converted into velocity to impinge against a single row of moving vanes, a steam turbine was produced that for simplicity of construction has no equal.
In 1800 Mr. Curtis designed a steam turbine which was a combination of the DeLaval and the Parsons. A steam nozzle, which is a modified form of the DeLaval, is employed to expand the steam and give it a high velocity before it comes in contact with the blades, but a certain amount of expansion is left to take place in passing through the alternate rows of stationary and rotating vanes as in the Parsons.
Of the DeLaval turbine the unique features are the nozzle and the means by which the wheel is enabled to revolve upon its axis of gravity. DeLaval mounts his wheel nearer the center of a long light shaft capa-hle of being bent and returning to its original form. The shaft is mounted upon bearings of ordinary construction. This flexibility enables the forces set up by the revolving wheel to deflect so as to let the former revolve about its axis of gravity.
In the divergent nozzle the whole expansion of the steam is carried out. The steam at the mouth of the nozzle has the same pressure as the exhaust. In other-words the steam has its energy completely transformed into mass and velocity by the time it comes into contact with the buckets. This brings up another feature of this turbine, and that is that no parts with the exception of the nozzles, are subject to steam pressure.
With 150 pounds steam pressure and 26 in. vacuum the velocity of the steam leaving the nozzle is 3810 feet a second or 43 miles a minute. The nozzles are set at an angle of 20° to the plane of motion of the buckets so that the theoretical velocity of the buckets should be 47 per cent of the velocity of the steam, or 1790 feet a second or 20 miles a minute. It has been found difficult to produce a material for the wheels that with an ample margin of safety would withstand the strain produced by the centrifugal force at this high speed, hence it has been necessary to reduce the speed not to exceed 1350 feet a second.