A water, or hydraulic, turbine is used to drive electric generators in hydroelectric power stations. The first such station was built in Wisconsin in 1882. In a hydraulic turbine falling water strikes a series of blades or buckets attached around a shaft, causing the shaft to rotate, this motion in turn being used to drive the rotor of an electric generator. The three most common types of hydraulic turbine are the Pelton wheel, the Francis turbine, and the Kaplan turbine. Toward the end of the 19th cent. two engineers, Sir Charles A. Parsons of Great Britain and Carl G. P. de Laval of Sweden, were pioneers in the building of steam turbines. Continual improvements of their basic machines have caused steam turbines to become the principal power sources used to drive most large electric generators and the propellers of most large ships.
A steam turbine typically consists of a roughly conical, steel shell enclosing a central shaft along which a series of bladed disks are spaced like washers. The blades are curved and extend radially outward from the rim of each disk. In some steam turbines the shaft is surrounded by a drum to which the rows of blades are attached. Between each pair of disks is a row of stationary vanes attached to the steel shell and extending radially inward. Each set of stationary vanes and the bladed disk immediately next to it constitutes a stage of the turbine; most steam turbines are multistage engines.
At the inlet end of the turbine high-pressure steam enters from a boiler and moves through the turbine parallel to the shaft, first striking a row of stationary vanes that directs the steam against the first bladed disk at an optimum speed and angle. The steam then passes through the remaining stages, forcing the disks and the shaft to rotate. At one end of the turbine the shaft sticks out and can be attached to machinery. A large steam turbine unit may actually be composed of several turbines that are all using the same shaft and steam. Such a unit might consist of a small, high-pressure turbine, connected to a larger, intermediate-pressure turbine, connected to a still larger, low-pressure turbine. After the steam leaves the turbine, it is sent to a condenser where it is converted back into water before being returned to the boiler.
Gas turbines are used mainly as aircraft engines. Some are used to drive electric generators, as in a gas turbine–electric locomotive, and high-speed tools. The term gas turbine is usually applied to a unit whose essential components are a compressor, a combustion chamber, and a turbine that resembles a steam turbine. The turbine drives the compressor, which feeds high-pressure air into the combustion chamber; there it is mixed with a fuel and burned, providing high-pressure gases to drive the turbine, the gases expanding until their pressure drops to atmospheric pressure. In a turboprop engine the turbine is used to turn a propeller as well as the compressor. In a turbojet engine only a small pressure drop is used to drive the turbine, the majority of the pressure drop occurring as the gases are expelled directly out of the engine. A variation of the turbojet is known as the turbofan engine.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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