The Steam Turbine

The idea of the steam engine, which Charles Parsons patented in 1884, was not a new one. Hero of Alexandria had demonstrated a crude form of steam turbine around 130 BC. All steam engines, whether turbines or not, are designed to extract energy from high pressure steam and convert it into motion by allowing the steam to expand. All of the turbine designs prior to Parsons' invention were inefficient, and he saw that this was because the steam was expanded in a single step.

He realised that if steam was allowed to expand gradually through more than one set of blades then much higher efficiencies could be attained. His prototype had 15 stages of expansion. The steam expanded through successive rings of moving blades on a shaft and fixed blades in a casing, producing purely rotary movement. Charles Parsons also designed an electric generator to utilise the very high-speed of his turbine. The turbine generator was then established and became the world's most important means of producing bulk electric power.

Electricity generation, Dynamos and Steam turbines

Though the steam turbine was later put to other uses, most notably in marine propulsion, its first purpose was to generate useful electrical power. When Charles Parsons introduced his revolutionary steam turbine it was with an integrated high-speed dynamo. A dynamo is a machine that converts mechanical energy such as that produced by a steam or hydraulic turbine into electrical energy by means of electromagnetic induction. In this process a changing magnetic field induces an electric current in a closed conductive circuit. A changing magnetic field can be created either by varying the strength of the field or by moving the circuit or the field. In dynamos it is usually produced by moving the field relative to the circuit.

This modern turbine has three stages. The high-pressure section has small blades. They are small because the incoming steam has very high energy - about 2,500 pounds-per-square-inch at 1,000 degrees Fahrenheit. After the steam passes through the high pressure section, it is sent back to the boiler to be reheated to 1,000 degrees. The steam is then sent to the next section of the turbine, called the intermediate pressure section. The blades here are larger than those in the high-pressure section.

After passing through this section, the steam is sent to the low-pressure section of the turbine. Because much of the energy has already been removed from the steam, the blades here are the largest in the turbine. The steam exits the turbine through the bottom, where it is condensed back into water. From there it is sent back to the boiler to be made into steam again.

Electricity is created produced when a wire is moved through a magnetic field. It is this principle that is used to make electricity. In generators, the rotor is an electromagnet. The magnet is spun inside the stator which is composed of a large coil of copper wire. Each time one of the poles of the magnet "cuts across" a conductor, voltage is induced. As the pole of the magnet moves away from the conductor, the voltage decreases until it reaches zero. The North Pole of the magnet produces a positive charge on the conductor and the South Pole will produce a negative charge.

The picture above depicts a single phase generator. Most power plant generators have three sets of stator windings, and are called three-phase generators. One of the reasons three-phase generators are used is that no more than one of the three output voltages is at zero at any one time. Most generators in Ireland are used to produce 50 hertz (cycles per second) alternating current. A rotor magnet with two poles must make 50 complete revolutions per second and results in a turbine speed of 2,500 revolutions per minute.

The major by-product from producing electricity with a large generator is heat. Because high temperatures inside the generator will breakdown insulation inside the generator, this heat must be removed for continued safe and efficient generator operation. In order to cool the generator a system using hydrogen gas is employed. Hydrogen will remove more heat than an equal volume of air. Because hydrogen is so explosive, the concentration (purity) inside the generator is maintained above 87 percent. At those levels there is not enough oxygen for any explosion to occur.

In 1888, Charles Parsons installed his steam turbines at Forth Banks Station in Newcastle-on-Tyne. This was the earliest use of steam turbines in any public power station. The following year he set up his own manufacturing plant, C. Parsons & Co., Ltd, at Heaton Works, near Newcastle.

By the time of his death in 1931 Charles Parsons had the satisfaction of seeing his invention adopted for all major world power stations. The size of his generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. He knew that the total output from turbo-generators constructed by his firm and by their licensees, for land purposes alone, had exceeded thirty million horse-power.

Click here for the text of a lecture given by Sir Charles Parsons on the principles, development and applications of the steam turbine.

THE TURBINE (by Harriet Munroe, 1910)

Harriet Munroe was a well known literary editor in Chicago. Her poetry is not of the first rank, but the sight of the great Parsons turbine installed in 1912 to light the city inspired her to write a powerful tribute. In The Turbine , a technician describes the wonders of the installation, imagined as an imperious and temperamental female personality.

So there she sits,
Mounted on all the ages, at the peak
Of time. The first man dreamed of light, and dug
The sodden ignorance away, and cursed
The darkness; young primeval races dragged
Foundation stones, and piled into the void
Rage and desire; the Greek mounted and sang
Promethean songs and lit a signal fire;
The Roman bent his iron will to forge
Deep furnaces; slow epochs riveted
With hope the secret chambers: till at last
We, you and I, this living age of ours,
A new-winged Mercury, out of the skies
Filch the wild spirit of light, and chain him there
To do her will forever.

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