Turbine

I

Introduction

Turbine, rotary engine that converts the energy of a moving stream of water, steam, or gas into mechanical energy. The basic element in a turbine is a wheel or rotor with paddles, propellers, blades, or buckets arranged on its circumference in such a fashion that the moving fluid exerts a tangential force that turns the wheel and imparts energy to it. This mechanical energy is then transferred through a drive shaft to operate a machine, compressor, electric generator, or propeller. Turbines are classified as hydraulic, or water, turbines, steam turbines, or gas turbines. Today turbine-powered generators produce most of the world's electrical energy. Windmills that generate electricity are known as wind turbines.

II

Hydraulic Turbines

The oldest and simplest form of the hydraulic turbine was the waterwheel, first used in ancient Greece and subsequently adopted in most of ancient and medieval Europe for grinding grain. It consisted of a vertical shaft with a set of radial vanes or paddles positioned in a swiftly flowing stream or millrace. Its power output was about 0.5 horsepower (hp). The horizontal waterwheel (that is, a horizontal shaft connected to a vertical paddle wheel), first described by the Roman architect and engineer Marcus Vitruvius Pollio during the 1st century bc, had the lower segment of the paddle wheel inserted into the stream, thus acting as a so-called undershot waterwheel.

By about the 2nd century ad, the more efficient overshot wheel had come into use in hilly regions. Here the water was poured on the paddles from above, and additional energy was gained from the falling water. The maximum power of the waterwheel, which was constructed of wood, increased from about 3 hp to about 50 hp in the Middle Ages.

The transition from waterwheel to turbine is largely semantic. The first important attempt to formulate a theoretical basis for waterwheel design was made in the 18th century by the British civil engineer John Smeaton, who proved that the overshot wheel was more efficient. The French military engineer Jean Victor Poncelet, however, devised an undershot wheel, the curved blades of which raised efficiency to nearly 70 per cent; it quickly came into wide use. Another French military engineer, Claude Burdin, invented the term turbine, introduced as part of a theoretical discussion in which he stressed speed of rotation. Benoit Fourneyron, who studied under Burdin at the School of Mines at Saint-Étienne, designed and built wheels that achieved speeds of 60 or more rpm (revolutions per minute) and provided up to 50 hp for French ironworks. Ultimately Fourneyron built turbines that operated at 2,300 rpm, developing 60 hp at an efficiency of more than 80 per cent.

Despite its remarkable efficiency, the Fourneyron turbine had certain drawbacks as a result of the radial outward flow of water that passed through it. This created problems if water flow was reduced or load removed. The British-born American engineer James B. Francis designed a turbine in which the flow was inward, and the so-called reaction turbine, or Francis turbine became the most widely used hydraulic turbine for water pressures, or heads, equivalent to a column of water 10 to 100 m (33 to 330 ft). This type of turbine operates by expanding the pressure energy in the water during the flow through the blade passages, resulting in a net force, or reaction, which has a tangential component that turns the wheel.

For installations where water heads of about 90 to 900 m (300 to 3,000 ft) were available, the Pelton wheel, named after the American engineer Lester Allen Pelton, came into use during the second half of the 19th century. In this turbine, the water is piped from a high-level reservoir through a long duct, or penstock, to a nozzle where its energy is converted into the kinetic energy of a high-speed jet. This jet is then directed on to curved buckets, which turn the flow by nearly 180 degrees and extract the momentum. Because the action of the Pelton wheel depends on the impulse of the jet on the wheel, rather than on the reaction of the expanding water, this type of turbine is also known as an impulse turbine.

The increasing demand for hydroelectric power during the early 20th century led to the need for a turbine suitable for small water heads of 3 to 9 m (10 to 30 ft) that could be employed in many rivers where low dams could be built. In 1913 the Austrian engineer Viktor Kaplan first proposed his propeller turbine, which basically acts like a ship's propeller in reverse. Kaplan later improved his turbine by allowing the blades to swivel about their axis. These variable pitch propellers improved efficiency by optimally matching the blade angle to the head, or flow rate.

To maintain constant output voltage in a hydroelectric installation, the turbine speed must be kept constant regardless of variations in the water pressure acting on it. This requires extensive controls which, for both Francis and Kaplan turbines, act primarily to open or close the guide-vane passages to regulate the flow and, in the case of Kaplan turbines, vary the pitch of the propellers. In a Pelton wheel installation, the water flow is adjusted by opening or closing the supply nozzles. Here a temporary spill bypass nozzle has to be provided since rapid flow changes in long penstocks would induce pressure surges, called water hammers, which can be highly destructive. During adjustments, the total water flow through both the supply and spill nozzles must be kept nearly constant with the eventual closing of the bypass nozzle, which must be carried out very slowly to avoid water hammer.

III

Advances in Turbine Design

The trend in modern hydraulic turbine installations has been towards higher heads and larger units. Depending on the size of the unit, Kaplan turbines are now used with heads up to about 60 m (200 ft), and Francis turbines up to 610 m (2,000 ft). The world's highest head installation (about 1,770 m/5,800 ft), using a Pelton wheel, is at Reisseck, Austria, and the largest single units are installed in a plant at Itaipu, Brazil, where 18 Francis-type turbines sized at 700 megawatts (Mw) each have a total capacity of 12,600 Mw. The largest installations in North America are at La Grande on James Bay, in eastern Canada, where 22 units rated 333 Mw each have a total capacity of 7,326 Mw, and in the United States at the Grand Coulee Dam on the Columbia River, where the installation has a total capacity of about 6,500 Mw.

Many of the small dam hydroelectric systems built before 1930 were later abandoned because of high maintenance and labour costs. Increases in the cost of fossil fuels have focused renewed attention on these low head installations. With the development of standardized propeller turbines with nearly horizontal shafts, small installations have again become attractive.

Turbines can also be designed to run in reverse as pumps. This is done by inverting the electric generator to operate as a motor. Because electric power cannot be stored economically, the operation of the so-called pump-turbines with electricity generated from nuclear and fossil fuel power plants during off-peak hours enables additional water to be stored in a reservoir. It can then be reused to drive the turbine during peak periods. In recent years, pump-turbine technology has been developed to allow for heads up to about 600 m (2,000 ft) of water and for unit capacities of more than 400 Mw.

IV

Steam Turbines

The success of the water turbine inevitably led to consideration of the turbine principle for extracting power from steam. Where the Watt-type reciprocating steam engine utilized the pressure of steam, the turbine could achieve higher efficiency by utilizing the kinetic energy of steam flow. The turbine can be made smaller, lighter, and cheaper than a reciprocating steam engine of comparable power and can be made in far larger sizes than the conventional steam engine. Mechanically, it has the advantage of producing rotating motion directly without having to use a crankshaft or other means of transforming reciprocal to rotary motion. As a result, the steam turbine has supplanted the reciprocating engine as a prime mover in large electricity-generating plants and is also used as a means of jet propulsion.

Steam turbines are used in the generation of nuclear power and in nuclear ship propulsion, where they operate with fuel-fired boilers for power generation. In cogeneration applications requiring both process heat (heat used in an industrial process) and electricity, steam is raised at high pressure in the boiler and extracted from the turbine at the pressure and temperature required by the process. Steam turbines may be used in combined cycles with a steam generator which recovers heat that would otherwise be lost. Industrial units are used to drive machines, pumps, compressors, and electrical generators. Ratings range from a few horsepower to more than 1,300 Mw.

The steam turbine was not invented by any one individual but was the result of work by a number of inventors in the latter part of the 19th century. Notable contributors to the development of the turbine were the British inventor Charles Algernon Parsons and the Swedish inventor Carl Gustaf Patrik de Laval. Parsons was responsible for the so-called principle of staging, whereby steam was permitted to expand in a number of stages, performing useful work at each stage. De Laval was the first to design suitable jets and blades for the efficient use of the expanding steam.