Types Of Turbines
While turbines can be classed as either impulse or reaction according to the way they function, there are four broad types of turbines categorized according to the fluid that supplies the driving force: steam, gas, water, or wind. Steam, water, and wind turbines are all used to generate electricity, and gas turbines are most often used by jet aircraft for propulsion. The steam turbine is mainly used by power plants that burn either fossil fuels or use nuclear energy to drive generators for consumer electricity. Steam turbines also power submarines and ships. The water or hydraulic turbine is used almost exclusively in hydroelectric plants to power an electric generator which then produces electric power for homes, offices, and factories. Wind turbines are the least common, but Scotland now uses the vertical machines called Darrieus turbines whose giant, bow-shaped blades look like huge egg beaters to generate electricity via the wind. The gas turbine is primarily used by jet aircraft.
Steam turbines transform the thermal energy stored in steam into mechanical work. The earliest steam turbine was also the earliest known steam engine. During the first century A.D., the Greek mathematician and engineer, Hero of Alexandria, built what was basically a novelty and produced no useful work, but was nonetheless the first steam turbine. It consisted of a small, hollow sphere with two nozzles or bent tubes sticking out of it. The sphere was attached to a boiler which produced steam. As the steam escaped from the sphere's hollow tubes, the sphere itself would rotate on its axis and continue to whirl. This was in principle a reaction steam turbine because the force of the escaping steam itself provided the thrust to make it spin. Steam was not considered in any type of turbine context again until the Italian Giovanni Branca published a work in 1629, in which he suggested the principle of the impulse steam turbine. In his book he details that it would be simple to convert the linear motion of a cylinder into the rotary motion needed for work by directing a jet of steam onto the vanes of a wheel, like water against a waterwheel. It is not known if he ever built such an engine.
Despite the advances made in understanding and managing steam that were gained in the eighteenth century, the steam turbine could not be built until the precision and strength of machining and materials had reached a certain level. In 1884, English engineer Charles Algernon Parsons (1854-1931) produced the first practical steam turbine engine. Although designed for the production of electric power, it was soon applied to marine propulsion and drove a ship named Turbinia in 1887. The spectacular speed and performance of this great ship opened a new era of steam propulsion at sea. Parsons overcame several major engineering difficulties involving stress, vibration, and balancing and truly deserves the title of father of the modern steam turbine. Besides their use at sea, steam turbines went on to generate an overwhelming proportion of the electricity used in the twentieth century. Today, the bulk of our electricity is generated by power stations using steam turbines. The steam is produced by the burning of fossil fuels (coal or gas) or by the use of nuclear energy. Most agree that steam turbines are still evolving and will play a considerable role in the generation of electrical power for some time to come.
Water or hydraulic turbines are identified with dams and the generation of hydroelectric power. When a turbine is operated by rapidly flowing or falling water, it is called an impulse turbine. The huge hydro-electric plant at Niagara Falls that was built at the end of the nineteenth century is this type of turbine. Water conditions usually determine what type of turbine is needed, and impulse water turbines require a constant flow of water to operate efficiently. Two aspects of this water flow is critical, its volume and its head. Water head is the distance water must fall before it strikes the turbine's wheel. With a sufficient volume and head like Niagara, the impulse turbine can have its wheel or rotor mounted on either a vertical or a horizontal shaft. The ends of the turbine's blades act like cup-shaped buckets, and as the water is directed at them at very high speeds by jets, the blades turn. As might be expected, most hydraulic turbines are of the reaction type since they are best suited to low-head situations. Here, the turbine is underwater and is turned by both the weight and speed of its flow. Its shaft is vertical and has either spirally curved blades or ones that resemble a ship's propeller. Unlike impulse turbines which achieve rotation by the acceleration of water from the supply nozzles, reaction turbines work because of the acceleration of water in the rotor or runner. Both then transform the energy from the rushing water into mechanical energy.
Wind turbines are the least common or significant of all turbine types, and many technical texts do not even mention them. Unlike waterwheels which directly led to the hydraulic turbine, the windmill has for the most part not evolved as a significant source of modern energy. As with the noted Darrieus turbines in Scotland however, wind turbines do exist and have proved useful in areas of high, continuous winds. Wind turbine clusters generate electricity in the Tehachapi Mountains near Barstow, California, as well as in certain areas of Hawaii and New Hampshire.
The best-known use for gas turbines is for jet engines. Gas turbines utilize hot gases as their names implies, and they are the newest type of turbine engine. Their gases are produced by the burning of some type of fuel, like kerosene. Air is then drawn into the front of the
turbine and passed through a compressor where the compressed air is mixed with fuel in a combustion chamber and is burned. This produces hot gases that expand and therefore rush through the turbine rotors, causing them to spin. This spinning can be used to power an electric generator or a pump, but in the case of a jet aircraft, the hot expanding gases are sent out at very high speed from the rear nozzle of the engine, producing thrust which then pushes the engine and the aircraft forward. Gas turbines attain temperatures higher than those of a steam turbine (the hotter a gas turbine is, the more efficiently it runs) and consequently cannot be built with ordinary metals.
Turbine engines are an example of an idea that could not be put into practice until technology had accomplished certain advances. Probably the most important technical advance was the widespread introduction of steel and its alloys that occurred during the second half of the nineteenth century. The popularity and use of certain types of turbine engines rises and falls as needs, priorities, and situations change. A good example is the use of steam turbines for ship propulsion. After dominating sea travel for many years, steam turbines declined after the 1973 oil embargo because the fuel to make steam became prohibitively expensive. Diesels moved in to take their place since they required less fuel. Diesels can only use liquid fuel however, and as oil becomes scarcer in the next century, steam turbines for ships may again be the choice, since they can be driven by coal-burning boilers.
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IEEE Guide for the Operation and Maintenance of Turbine Generators. Institute of Electrical and Electronics Engineers, 1990.
Meriam, J.L., and L.G. Kraige. Engineering Mechanics, Dynamics. 5th ed. New York: John Wiley & Sons, 2002.
Leonard C. Bruno