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Steam Turbine Engine

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Mechanical Features and Designs

A steam turbine engine, also known as a heat-force engine,[1] is a steam engine in modern form. The steam turbine differs from a steam engine in that highly pressurized steam or vapor is directed onto or towards vanes or a rotor to produce rotating motion, thereby transforming chemically stored energy into mechanical working energy.

Steam turbines were first used over 100 years ago to power plants, heat homes, propel ships, and run electricity.[2]

Modern steam turbines are used today to operate large power plants and still provide electricity. Coal, natural gas, nuclear reactors, wood, and municipal solid waste are the fuel sources used to generate the steam needed to run through a highly complicated system of multi-stage turbines that spin to an output shaft and drive a generator.[3] In the U.S., more than 88 percent of electricity is generated by the use of steam turbines.[4]

Contents

[edit] History

The principle of steam power was actually a concept first explored by Greeks in the first century A.D. Hero, a Greek engineer and mathematician, devised a crude steam engine or turbine type invention called an aeliopile that consisted of a steam-producing boiler attached to small hollow sphere with two tubes.[5]

When steam engines were invented and improved upon in the 1700s by Thomas Newcomen and James Watt, the invention of the steam turbine followed.

The development of a dynamo, an electrical generator, in 1830 by British physicist Michael Faraday spurred experimentation in devising a method by which a steam engine could create rotary motion in order to generate electricity, something that was quickly becoming a necessity. There were power limitations to steam engines in addition to being large and noisy. It was also discovered that there was a limit as to the number of revolutions a steam driven piston could actually achieve within a minute.[6]

By the mid-19th century, a number of engineers had proposed ways of directing steam from steam engines onto blades attached around the edges of a wheel. Early experimentation failed because the steel was not durable enough to withstand rapid rotation.[7]

[edit] Charles Parsons

In 1884, as a partner at Clarke, Chapman and Parsons, Charles Parsons finally met with some success when and fellow partners saw the possibility for a steam turbine engine to power electrical generators. As a result, he invented and patented a turbine fabricated out of compounded steel that turned a dynamo at 18,000 revolutions per minute. In 1890, his steam turbine and accompanying electrical generator were installed in the Fourth Banks power station. Steam turbine technology was being used across Europe.[8] Eventually, the technology was even used to power electric street lighting.[9]

Parsons also worked on developing steam turbines designed for marine propulsion. His first experiment, using a two-foot model boat towed by fishing line that was then scaled up to a full-sized vessel called Turbinia, was introduced in 1894 at Queen Victoria’s Diamond Jubilee.[10] With steam turbine propulsion, the Turbinia reached a top speed of 34 knots. This was a huge development considering the fastest destroyers only reached 27 knots.[11] Parsons' steam turbines were equally successful in the application of marine propulsion and he was later commissioned to fit a Royal Navy destroyer with a steam turbine.[12]

[edit] Carl Gustav Laval

It was a Carl Gustav Laval, a Swedish scientist and inventor, who spearheaded the development of high-speed turbines. In 1882, he built his first impulse steam turbine. Other advancements in the technology followed, including a reversible turbine for marine use that attained a speed of over 42,000 revolutions per minute.

Laval has also been attributed with inventing divergent nozzles used to deliver steam to turbine blades, a more flexible shaft that could prove to be dangerous at high speeds if it wobbled, and special helical gear.[13]

[edit] Charles Curtis

Charles Curtis, a graduate of Columbia University in civil engineering, worked as a patent lawyer before getting into the manufacturing of electric motors and fans. In 1899, he became the first American to design a gas turbine engine. This precipitated other substantial developments such as the Curtis Steam turbine in 1896. His steam turbine demanded only one tenth of the space and weighed one eighth as much as the machines it replaced. In 1901, he sold the rights of his turbine to General Electric.[14]

[edit] Features/How it Works

A steam turbine is an ideal prime mover for driving any type of machine that requires rotational mechanical power. It can deliver power at constant or variable speeds and is also capable of speed control. The use of steam turbines includes such drive applications as compressors, centrifugal pumps, ship propellers, and electric generators.[15]

A steam turbine works on the principle of thermodynamics known as the Rankine Cycle. The process involves the water being heated until it becomes a saturated liquid. From this stage it is compressed into a steam, which is then transferred to a turbine where its pressure is reduced by expansion over turbine blades. This process produces electricity while the low-pressure steam is condensed back to liquid form. Water, called return water, is mixed with new water or feedwater and pumped back into the boiler.[16]

The way a steam turbine works is according to its mechanical arrangement (single-casing, cross-compound, or tandem compound), steam flow directions (axial or radial), steam cycle (condensing, non-condensing, extraction, reheat, fossil fuel, and nuclear) and the number of exhaust flows.[17]

To maximize the efficiency of the turbine, steam must be expanded and generate work; this is accomplished in various stages. The extraction of the steam is used to define this set of stages and is either carried out using a reaction turbine or an impulse turbine. Most steam turbines today are a combination of both designs.

[edit] Impulse Turbine

The impulse steam turbine configuration involves steam at high pressure being used to drive an impulse turbine commercially known as a Pelton wheel. A number of buckets are fixed or welded to the rotor. High-pressure steam is exerted on the buckets through a divergent nozzle converting the steam into a high velocity jet.[18] The impulse spins the turbine and removes kinetic energy from the fluid flow. Just before reaching the turbine, the fluid’s pressure head is changed to velocity head by speeding up the fluid through the nozzle.[19]

[edit] Reaction Turbine

In a reaction turbine, the nozzles that emit the working fluid are attached directly to the rotor. The speed at which the fluid is discharged from the nozzles produces a reaction force on the pipes. This causes the rotor to move in the opposite direction of the fluid, creating an opposite reaction. The fluid pressure changes as it passes through the rotors blades. A pressure casement is needed to contain the fluid as it acts on the turbine.[20]

[edit] Types

There are different types of steam turbines.

[edit] Condensing Steam Turbines

These are steam turbines that condense steam below atmospheric pressure so that maximum energy output is gained from it.[21]

[edit] Non-condensing Steam Turbines

These turbines are also referred to as back pressure steam turbines. Steam is expanded over a turbine and the exhaust steam is used to meet a facility's steam needs. Simply put, steam is expanded to reach a pressure the facility can use.[22] It also means that steam leaves the turbine at above atmospheric pressure and is used for heating or other required processes before being returned to the boiler as water.[23]

[edit] Extraction Steam Turbines

These are steam turbines in which steam is extracted from the turbine at some intermediate pressure. The steam is used to meet a facility's needs and the remaining steam is expanded and further condensed.[24] These types of steam turbines are found at refineries, district heating units, and pulp and paper plants.

[edit] Reheat Steam Turbines

In this type of steam turbine, high-pressure, high-temperature steam is partially expanded through a turbine. Returning some steam to a steam generator and reheating it to its original temperature before feeding it back to the turbine can increase the efficiency of the steam. Reheat steam turbines are common in electrical power plants.[25]

[edit] References

  1. Steam Turbines. Thinkquest. 2008-09-09.
  2. Steam Turbines. NJCHP. 2008-09-09.
  3. Steam Turbines. NJCHP. 2008-09-09.
  4. How Steam Technology Works. Howstuffworks.com. 2008-09-09.
  5. Steam Engine History. 2008-09-09.
  6. How Steam Technology Works. Howstuffworks.com 2008-09-09.
  7. How Steam Technology Works. 2008-09-09.
  8. Sir Charles Agernon Parsons. University of Cambridge. 2008-09-09.
  9. Sir Charles Agernon Parsons. University of Cambridge. 2008-09-09.
  10. Sir Charles Agernon Parsons. University of Cambridge. 2008-09-09.
  11. Sir Charles Agernon Parsons. University of Cambridge. 2008-09-09.
  12. How Steam Technology Works. Howstuffworks.com. 2008-09-09.
  13. Carl Gustaf Patrik de Laval. Britannica.com. 2008-09-09.
  14. Curtis Bio. EdisonExploratorium.org. 2008-09-09.
  15. Steam Turbine. Answers.com. 2008-09-09.
  16. Steam Turbines NJCHP. 2008-09-09.
  17. Steam Turbine. Answers.com. 2008-09-09.
  18. [ImpuslseTurbine.com. 2008-09-09.
  19. [David Darling. Impulse Turbine. 2008-09-09.
  20. David Darling. Reaction Turbine. 2008-09-09.
  21. Turbine. Britannica. 2008-09-09.
  22. Steam Turbines. NJCHP. 2008-09-09.
  23. Steam Turbines. Britannica. 2008-09-09.
  24. Steam Turbines. NJCHP. 2008-09-09.
  25. Steam Turbines. Britannica. 2008-09-09.