Steam and steam turbines
Steam and steam turbines play a crucial role in the generation of electricity by converting thermal energy into mechanical energy. The process begins with the creation of steam from various energy sources such as fossil fuels or nuclear energy, which is then channeled into a steam turbine. Inside the turbine, high-pressure steam flows through nozzles and passes over rotating blades, converting its thermal energy into mechanical energy as the blades turn.
Steam turbines are designed in stages, allowing for efficient energy conversion by utilizing a process called pressure compounding. This involves a series of nozzles and blades that maximize the energy extracted from steam, minimizing thermal energy loss. The amount of power generated depends on the steam flow and the pressure levels at both the inlet and outlet of the turbine. Connecting the turbine to a condenser can significantly enhance its efficiency by creating a vacuum, allowing the turbine to operate at lower outlet pressures.
In power generation, steam turbines are typically linked to electric generators, with nuclear power plants capable of producing substantial amounts of electricity, sometimes exceeding 1,300 megawatts. The design and construction of steam turbines include essential components such as bearings that require lubrication and seals to ensure efficient operation. Overall, steam turbines are integral to modern energy systems, providing a reliable means of electricity generation.
Steam and steam turbines
A steam boiler converts the chemical energy in fuel into the thermal energy of steam. A steam turbine converts this thermal energy into the mechanical energy of a rotating shaft. This shaft can drive an electric generator or other device.
Background
Fossil fuels such as oil and coal contain chemical energy. Uranium contains nuclear energy. Either of these forms of energy can be converted into thermal energy (heat), and this thermal energy can be used to make steam in a boiler. A steam turbine can be used to convert the thermal energy of steam into the mechanical energy of a rotating shaft. When the turbine shaft is used to drive an electric generator, electricity is produced. Although electric generators can be driven by diesel engines, gas turbines, and other devices, most electricity is generated using steam turbines.
Principles of Turbine Operation
High-pressure, high-temperature steam enters a steam turbine through a throttle valve. Inside the turbine the steam flows through a series of nozzles and rotating blades. As it flows through a nozzle, the pressure and temperature of the steam decrease, and its speed increases. The fast-moving steam is directed against rotating blades, which work something like the blades on a pinwheel. The steam is deflected as it passes over the rotating blades, and in response the steam pushes against the blades and makes them rotate. As the steam flows over the rotating blades its speed decreases.
Large turbines are composed of many stages. Each stage has a ring of nozzles followed by a ring of rotating blades. The slow-moving steam leaving the rotating blades of one stage enters the nozzles of the next stage, where it speeds up again. This arrangement is called pressure compounding. The energy of the steam is converted to mechanical work in small steps. Less of the steam’s thermal energy is wasted or lost if it is converted in small steps.
The amount of power produced by a turbine depends on the amount of steam flowing through it and on the inlet and outlet steam pressures. Steam flow is constantly regulated by the throttle valve, but the steam pressures are fixed by the design of the system. Inlet steam pressure is determined by the operating pressure of the boiler that supplies it. Outlet pressure is determined by where the steam goes when it leaves the turbine. If the steam simply escapes into the atmosphere, the outlet pressure is atmosphere. If the outlet steam pressure is made lower than atmospheric pressure, the turbine produces more power. This is accomplished by having the steam leaving the turbine flow into a condenser.
Cooling water passing through tubes inside the condenser removes heat from the steam flowing around the tubes and causes it to condense and become liquid water. Since water occupies a much smaller volume as a liquid than as steam, condensing creates a vacuum. When a turbine is connected to a condenser, the outlet steam pressure can be far below one atmosphere.
Details of Turbine Construction
Inside the steel turbine casing, stationary partitions called diaphragms separate one turbine stage from the next. Each diaphragm has a hole at the center for the rotor shaft to pass through. Nozzle passages are cut through the diaphragms near their outer rims, and the steam is forced to pass through these nozzles to get to the next stage.
The rotor of a turbine is made up of solid steel disks that are firmly attached to a shaft. Rotating blades are mounted around the rims of the disks. Where the shaft extends from the casing at each end, it is supported by journal bearings and a thrust bearing. The journal bearings are stationary hollow cylinders of soft metal that support the weight of the rotor. A thrust bearing consists of a small disk on the shaft of the turbine that is trapped between two stationary disks supported by the casing. If the rotor tries to move forward or back along its own axis, the rotating disk presses against one of the stationary disks. Thrust and journal bearings must be lubricated by a constant flow of oil that forms a thin film between the rotating and stationary parts of the bearing and prevents them from making direct contact. Without this film of oil, the bearing would wear out in a few seconds.
A seal must be provided where the shaft of the turbine passes through the casing. At one end of the casing the steam pressure inside is high. Outside the casing the air pressure is only one atmosphere. If there were no seal, steam would rush out through the space between the casing and the shaft. At the other end of the turbine, the pressure inside may be below atmospheric. Here air would rush in if there were no seal around the shaft.
Electric Power Generation
Most electric power is produced by steam turbines driving electric generators. This is true whether the source of the steam is a nuclear reactor or a boiler burning fossil fuel. The turbines in power stations are extremely large. In nuclear plants the turbines may produce as much as 1,300 megawatts of power. Power stations are often located near rivers so that water from the river can be used as cooling water in the condensers that receive steam from the large turbines.
Bibliography
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