Steam Driven Generators: Steam Turbines and Electric Generators
A steam turbine driven generator, sometimes known as “turbo generators”, can be best explained by understanding a steam turbine and a generator separately. A steam turbine is a steam driven driver. Water is heated at extremely high temperature to convert it into steam. Energy created by highly pressurized steam is converted to mechanical energy which rotates the blades in the steam turbine. And a generator is best described as a machine by which mechanical energy is transformed into electrical energy. The coiled wires used in a generator spin inside a magnetic field which causes electric current to flow through the wire.
When a steam turbine is connected to a generator, it produces electricity and is known as a steam turbine driven generator. The auxiliary systems built in them make them work safely and with greater efficiency.
Steam turbine driven generators are commonly used in solar thermal electric power plants, coal, geothermal, nuclear, waste incineration plants and natural gas power plants. They are also used extensively in cement, sugar, steel, paper, chemicals and other industries.
Steam turbine driven generators are generally high speed machines. Most of the electric power in the world is produced by steam turbine driven power plants. In United States alone around 85.0% of the electricity is produced using steam turbine generators.
Steam Turbine Driven Generators can range widely in size. They rarely exceed about 1,500 Megawatts (2 million horsepower) on the top end, and are used on a small scale as well, down to about 500 kw (670 horsepower) on the low end.
Working principle of a Steam Driven Generator
In a steam turbine driven generator, heat is generated from a source. There is a boiler which contains water and the heat is used to convert it into steam which is of high temperature and high pressure. Steam production depends on the flow rate, surface area of the heat transfer and the heat of combustion used. This steam from the boiler is pushed into the turbine through nozzles, which spins the blades mounted on a shaft. The steam turbine consists of a casing to which stationary blades are fixed inside and a rotor has moving blades on the periphery.
There are two basic types of steam turbine - impulse turbines and reaction turbines; whose blades are designed to control the speed, direction and pressure of the steam as it passes through the turbine. The generator is attached to the turbine and when the turbine blades rotate, it makes the generator work on the principle of magnetic induction and creates electricity. The method of retrieving the electrical energy depends on how it will be used. The steam is condensed usually into a condenser. Therefore, a boiler, steam turbine, generator and condenser form the main components in a steam powered generator plant. Steam Turbines can also be made to operate without a condenser as well, but provide a considerably lower power capacity for the same size of turbine.
- Impulse Turbines: Here, the rotor spins because of high force or the direct push of steam on the blades.
- Reaction Turbines: Here, the rotor spins from a reaction force rather than an impact or impulse force.
The reactive force comes from the change in steam pressure energy as the steam leaves the blades. This method generally operates at a higher efficiency than Impulse Turbines.
The working of a larger steam turbine may be complex and difficult to comprehend, since it uses a set of blades on rotor. Each set of blades is called a stage which works by either impulse or reaction. A mixture of impulse and reaction stage complicates its working since these set of blades are all mounted on the same rotor axle and all turning the generator at the same time.
Steam turbines regulate their speed with use of automated valves and a control governor, so they generate optimum power as needed at any particular time.
Turbines also vary in their cooling process of steam. Condensing turbines, usually used in large power plants to generate electricity, convert the steam to water using condensers which allows the steam to expand more and facilitates the turbine in extracting maximum energy from it. This makes the electricity generating process much more efficient. Non Condensing turbines don't have this feature and thus are rarely used except for small, auxiliary systems where only low power is needed.
In large steam turbines, in fossil-fuel power plants, the steam pressure can be as high as 20–30MPa (3000–4000 psi or about 200–270 times atmospheric pressure), but is generally operates at less than 1,000 psi. A typical power plant steam turbine rotates at 1800–3600 rpm
Efficiency of a steam driven generator
The efficiency of a steam driven generator depends on many factors such as the type of steam turbine, its size, the inlet steam pressure and temperature, the exhaust steam pressure and temperature, and the steam flow rate.
Steam turbines are suitable for large thermal power plants. They are made in a variety of sizes up to 1.5 GW (2,000,000 hp) turbines which are used to generate electricity. However, coal power plants and burning fossil fuels or nuclear power, used to generate electricity from a steam turbine generator has an adverse impact on the environment. They emit carbon dioxide and other pollutants into the air and water. They have slower startup than gas turbines.
Factors affecting a steam driven generator
Other than the basic safety procedures to be followed to operate a steam turbine powered generator, the following are key factors one must consider in the operation of the same to ensure optimum efficiency and long lasting reliability:
- Regular maintenance of system voltage and frequency
- Optimize operating pressure, temperature and speed limits of the plant components
- Ensure lubrication of components
- Following feeding process of fuel to the combustion chamber
- Maintaining condenser quality and Coolant in the generator,
- Type of generator used. Transformers and high voltage switching
- Warning for Overload protection, emergency shut down and load shedding
Steam driven generators working on constant load often lead to deposition from the steam on the fixed and moving blades. These deposits results in low efficiency and low output. Low efficiency of energy transfer due to this eventually restricts steam flow. This is a rare occurrence in generators where the load varies since there is blade washing effect.
Steam driven generators are commonly used in power generation, renewable energy, oil & gas and manufacturing industries and are manufactured by top brands such as Worthington, Hitachi, General Electric, Siemens, Elliot, and Westinghouse
There is a large market and huge demand for these kinds of generators since they are most widely used in producing electricity in the world. But one must consider other important factors such as price, quality, features, after sale services, replacement of parts etc. while buying a steam driven generator.
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