Turbine

Turbines provide power for a variety of machines, including electric generators and water pumps. In fact, generators driven by turbines produce most of the electricity used to light homes and run factories. Turbines that power water pumps play an important role in irrigation projects throughout the world. Turbines are also used to turn the propellers of ships, and they are an essential part of jet-airplane engines.

The earliest known turbines date back to simple water wheels used by the ancient Greeks about 2,000 years ago. Today, turbines vary greatly in size and power, depending on their use. For example, a huge turbine that turns an electric generator can deliver nearly 750 million watts of power. But some turbines used to run shop machinery measure less than 1 inch (2.5 centimeters) in diameter and deliver under 750 watts.

As a fluid passes through a turbine, it hits or pushes against the blades or buckets and causes the wheel to turn. When the wheel rotates, the axle turns with it. The axle is connected directly or through a series of gears to an electric generator, air compressor, or other machine. Thus, the motion of the spinning rotor drives a machine.

The rotors of some turbines have only one wheel. But the rotors of others have as many as 50 or more. Multiple wheels increase the efficiency of the turbines, because each wheel extracts additional energy from the moving fluid. In a turbine with more than one wheel, the wheels are mounted on a common axle, one behind the other. A stationary ring of curved blades is attached to the inside of the casing in front of each wheel. These stationary blades direct the flow of fluid toward the wheels. A wheel and a set of stationary blades is called a stage. Multistage turbines have many stages.

Turbines are more commonly classified by the type of fluid that turns them. According to this method, there are four main kinds of turbines: (1) water turbines, (2) steam turbines, (3) gas turbines, and (4) wind turbines.

Water turbines are also called hydraulic turbines. Most water turbines are driven by water from waterfalls or by water that is stored behind dams. The turbines are used primarily to power electric generators at hydroelectric power plants. There are three main kinds of water turbines: (1) the Pelton wheel, (2) the Francis turbine, and (3) the Kaplan turbine. The type of water turbine used at a plant depends on the head available. A head is the distance the water falls before it strikes the turbine. Heads range from about 8 feet (2.4 meters) to more than 1,000 feet (300 meters).

The Pelton wheel is an impulse turbine. It is used with heads of more than 1,000 feet (300 meters). A Pelton's rotor consists of a single wheel mounted on a horizontal axle. The wheel has cup-shaped buckets around its perimeter. Water from a lake or reservoir drops toward the turbine through a long pipe called a penstock. One to six nozzles at the end of the penstock increase the water's velocity and aim the water toward the buckets. The force of these high-speed jets of water against the buckets turns the wheel.

The Francis turbine is used when the head is between about 100 feet (30 meters) and 1,000 feet (300 meters). A Francis turbine's rotor is enclosed in a casing. Its wheel has as many as 24 curved blades. Its axle is vertical. The wheel operates underwater. It is encircled by a ring of guide vanes, which can be opened or closed to control the amount of water flowing past the wheel. The spaces between the vanes act as nozzles to direct the water toward the wheel's center. The rotor is turned chiefly by the weight or pressure of the flowing water.

The Kaplan turbine is used for heads of less than 100 feet (30 meters). The rotor resembles a ship's propeller. It has from three to eight blades on a vertical axle. It works in a way similar to that of a Francis turbine. The Kaplan and Francis turbines are reaction turbines.

Steam turbines drive the electric generators in most U.S. power plants. They also power ocean liners and large machinery. Multistage steam turbines are among the world's most powerful engines. Some steam turbines produce nearly 750 million watts of power.

Steam turbines are run by steam. In most cases, the steam is produced by water heated in a boiler by burning such fuels as coal, oil, or natural gas. In nuclear power plants, however, heat produced by splitting atoms in a nuclear reactor changes water to steam.

Steam enters a turbine at temperatures as high as 1200 ºF. (649 ºC) and pressures as high as 3,500 pounds per square inch (250 kilograms per square centimeter). The high-pressure steam rushes through the turbine, causing the turbine wheels to spin rapidly. Steam turbines are designed to use the impulse principle, the reaction principle, or a combination of both.

Many modern steam turbines have 50 or more stages set on a horizontal axle. Each stage of the turbine consists of a wheel and a ring of stationary blades. The curved blades of both the wheels and the stationary rings are shaped so that the spaces between them act as nozzles. The nozzles aim the steam and increase its speed before it goes on to the next stage. The steam follows a zigzag path between the wheel blades of one stage and the stationary blades of the next.

As steam passes through a multistage turbine, it expands to as much as 1,000 times its original volume. Each successive stage of the turbine is therefore larger than the previous one in order to make efficient use of the expanding steam. This arrangement of larger and larger stages gives steam turbines their characteristic conical shape.

Steam turbines may be either condensing or noncondensing, depending on how the steam leaving the turbine is used. Steam from a condensing turbine goes directly into a condenser. Cold water circulating in pipes in the condenser cools the steam into water. A vacuum is thus created, because the volume of water is much less than that of steam. The vacuum helps force steam through the turbine. The water is pumped back to the boiler to be made into steam again. The exhaust steam from noncondensing turbines is not cooled into water. Instead, it is used to provide heat for buildings and for a variety of industrial processes.

Gas turbines burn such fuels as oil and natural gas. Instead of using the heat to produce steam, as in steam turbines, gas turbines use the hot gases directly. Gas turbines are used to power electric generators, ships, and high-speed cars. They are also an important part of the engines in jet aircraft.

Most gas turbine systems have three main parts: (1) an air compressor, (2) a combustion chamber, and (3) a turbine. The combination of the air compressor and combustion chamber is commonly called a gas generator. In most gas turbine systems, the air compressor and turbine are mounted at either end of a common horizontal axle, with the combustion chamber between them. Part of the turbine's power runs the air compressor.

The air compressor sucks in air and compresses it, thereby increasing its pressure. In the combustion chamber, the compressed air combines with fuel and the resulting mixture is burned. The greater the pressure of the air, the better the fuel-air mixture burns. The burning gases expand rapidly and rush into the turbine, where they cause the turbine wheels to rotate. Hot gases move through a multistage gas turbine in much the same way that steam moves through a steam turbine. Stationary blades aim the moving gas at the rotor blades and adjust its velocity.

Most gas turbine systems make use of the hot exhaust gases from the turbine. In some systems, some of the exhaust gases are circulated to a device called a regenerator. There, the gases are used to warm up the high-pressure air from the compressor before it enters the combustion chamber. Such preheating of the air reduces the amount of fuel needed for combustion. In jet engines, much of the gas stream is used to develop thrust.

Gas turbines run at even hotter temperatures than steam turbines. The hotter a gas turbine runs, the more efficiently it operates. The temperature in many gas turbines is 2000 ºF. (1093 ºC) or higher.

Wind turbines, which are commonly called windmills, are driven by the wind. They were developed about 1,300 years ago, and through the centuries, they have been used chiefly to grind grain and pump water. In the late 1800's, thousands of communities in the United States used windmills to draw water from the ground. During the 1970's, shortages of oil led to increased interest in wind turbines as a potential source of energy for generating electricity.

There are two basic types of wind turbines: (1) horizontal axis wind turbines (HAWT's) and (2) vertical axis wind turbines (VAWT's).

Horizontal axis wind turbines. Traditional HAWT's have rotors with multiple blades or sails. They include Dutch windmills and American windmills. Most modern HAWT's used to generate electricity have two propellerlike blades. The rotor of these HAWT's is mounted on a tower or mast that holds the blades high enough off the ground to catch the wind stream. In order for the turbine to operate efficiently, the blades need to face into the wind, and the axle must lie parallel to the wind stream. As the wind blows, the rotor turns due to the impact of the air on the specially shaped blades. HAWT's are designed to adjust to changes in the speed and direction of the wind. The angle of the blades can be changed to keep the turbine operating at a constant rate, no matter what the wind speed is. In addition, these turbines can be rotated around a vertical axis to keep the rotor blades facing into the wind.

Vertical axis wind turbines. The most efficient kind of vertical axis wind turbine was developed in the 1920's by a French inventor named Georges Darrieus. The Darrieus wind turbine looks like a giant eggbeater. It has two or three long curved blades attached at both ends to a vertical shaft. The Darrieus wind turbine can catch the flow of wind from any direction.

The first windmills were probably built in the A.D. 600's in Iran. These early windmills were used for grinding grain and irrigating crops. By the 1100's, they had spread to Europe. In the 1400's, people in the Netherlands began using windmills to drain marshes and lakes near the sea.

For many centuries, water wheels and windmills were the only useful turbines. The scientist Hero of Alexandria had built a small steam turbine about A.D. 60, but it had not been used to power anything. In 1629, Giovanni Branca, an Italian engineer, built an impulse steam turbine that was used in a stamping mill.

Early water wheels and windmills were less efficient than modern turbines, because much of the moving fluid escaped around the edges of the rotor blades. During the 1800's, engineers and inventors began developing more efficient, enclosed turbines. In 1824, Claude Burdin, a French engineer, introduced the word turbine in a scientific paper. It comes from the term turbo, the Latin word for a spinning object. Benoit Fourneyron, a French engineer, built the first successful enclosed water turbine in 1827. After Fourneyron's success, engineers soon overcame most of the problems that were involved in building efficient water turbines.

In 1849, an English-born inventor named James B. Francis built the first Francis turbine. The Pelton wheel, invented by an American mining engineer named Lester A. Pelton, began to be produced during the 1880's. Victor Kaplan, an Austrian inventor, developed the design for the Kaplan turbine during the early 1900's.

In 1884, an English inventor, Charles A. Parsons, developed the first reaction steam turbine. In 1897, he used steam turbines to power his vessel, the Turbinia. In 1895, Charles G. Curtis, an American inventor, patented a multistage steam turbine that worked by both reaction and impulse. This turbine started a revolution in power production, because it was extremely efficient for its size and weight. During the early 1900's, steam turbines replaced steam engines in electrical generating stations.

John Barber, an English inventor, was issued a patent on a simple gas turbine system in 1791. In 1930, an English engineer named Frank Whittle received the first patent on the application of a gas turbine to propel aircraft. The first airplane to be powered by a turbojet engine was built by the Heinkel Company of Germany in 1939.

Contributor: Marian Visich, Jr., Ph.D., Associate Dean of Engineering, State Univ. of New York, Stony Brook.

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Electric Power; Energy.

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