[Illustration: FIG. 49.--PERIPHERAL VIEW OF THE BLADES OF A PARSONS TURBINE]
The Curtis turbine combines the De Laval and the Parsons principles. The steam enters through a series of nozzles which are of the expanding type, then it goes through a series of moving and stationary blades, as in the Parsons turbine, from which it enters another set of expanding nozzles and gains velocity and momentum before passing through the second series or stage of moving and fixed blades. (See Figure 50.)
Steam turbines are particularly adapted for use in electric power plants. The speed of rotation of the Parsons and Curtis types is much lower than that of the De Laval and hence the electric generators may be directly driven by them without the interposition of any gearing. They can be built of larger power than the reciprocating engines because they are so economical of space.
[Illustration: FIG. 50.--PERIPHERAL VIEW OF THE BLADES AND NOZZLES OF A CURTIS TURBINE]
A good comparison of turbine versus reciprocating engines is offered by the 74th Street power station of the Manhattan Elevated Railway, New York. This station, which was completed in 1901, was equipped with eight huge reciprocating engines, each developing 8,000 horsepower normally, and capable of delivering a maximum of 12,500 horsepower. The whole plant, therefore, had a maximum capacity of 100,000 horsepower. Gradually these units have been giving way to steam turbines of much higher power, and in 1919 there was installed one powerful turbine which alone was capable of developing as much power as the entire plant of 1901. This is a triple-compound turbine comprising one high-pressure turbine and a low-pressure turbine at each side. Steam enters the high-pressure turbine at 205 pounds pressure to the square inch and then exhausts into the low-pressure turbines, passing from them into condensers which operate under 29 inches vacuum. Each turbine drives a separate generator and the combined horsepower of the whole unit is about 100,000, while the floor space occupied is only 50 by 52 feet.
The economy of space and of fuel offered by the steam turbine is of great value in the power plants of ships, and this form of prime mover has been installed on modern high-speed passenger liners and also on high-speed war vessels. While in certain respects the turbine is ideal for such service, there are two handicaps which must be overcome. In the first place, the most efficient speed for the turbine is considerably higher than the efficient speed of the propeller and some means must be provided for stepping down the speed. In the second place, the turbine cannot be as economically controlled as a reciprocating engine and its direction of rotation cannot be reversed, so that difficulties are encountered in maneuvering the ship in harbors. It is no simple matter to gear down the high speed and enormous power of a turbine, However, an elaborate system of gearing has been provided for this purpose which has proved satisfactory even in powerful battle cruisers. The British battle cruisers with a power plant of 134,000 horsepower are driven by geared turbines. To reverse the propellers separate low-power turbines are used.