A steam turbine is a machine that exploits the thermal energy of steam at high pressure. Thermal energy is converted into useful mechanical energy through an expansion thermodynamic transformation. The steam turbine transforms the internal energy of steam into rotational kinetic energy.
The steam turbine has completely prevailed over the steam engine thanks to the best thermal efficiency and the best power / weight ratio.
Maximum efficiency occurs when steam expansion is an ideal process. In this case the pressure of the steam decreases and becomes mechanical work, in an infinite number of stages.
One way to classify the different types of turbines is according to their degree of reaction:
Action turbines: The expansion is carried out on the fixed blades.
Reaction turbines: The expansion can be carried out both on the fixed blades and on the moving blades.
Steam turbines usually work in a closed circuit. The inlet of steam is regulated by the inlet valves. At the turbine outlet, there is usually a steam condenser.
What Are Steam Turbines For?
There are different uses of turbines depending on how you want to use the steam energy.
1. Thermal Power Plants and Electricity Generation
Steam turbines can be used to produce electricity, coupled to generators, often without the need for gearboxes. Some examples are nuclear power plants, coal-fired power plants, and some solar thermalpower plants.
Together, all thermal power plants generate about 80% of the electricity in the world.
In this case, they work in ideal regimes, since the generators have to rotate at a constant speed.
Furthermore, the steam turbine can also function as the motor of an electric generator. The main advantage is that it does not require any mechanical member that transforms the reciprocating movement into a rotary one.
Turbines for electricity production usually couple the turbine shaft to its generator.
As nuclear reactors operate at lower temperatures and with less gas pressure than thermal power plants, they usually operate at half the rotation speed.
2. Industrial Processes
Another field in which steam turbines are used is in plants such as refineries, paper mills and other plants where high levels of process steam are required.
The plant can be designed in such a way that it makes use of the steam turbine to obtain a synergy between:
That of electrical energy or mechanical work.
3. Steam Turbines for Transportation
Steam turbines are also used as marine engines on ships, where the limited dimensions are an advantage. Propulsion steam turbine locomotives were also built, but their diffusion was very limited.
In ships, steam turbine-based propulsion has multiple advantages over internal combustion engines:
Smaller size and weight for the same power
However, a steam turbine is only efficient at high rotational speeds. This requires precise and complex transmissions (and at a high cost).
An alternative is the use of turbo-electric propulsion. In this system, the turbines generate electrical energy as in a power plant. The energy obtained is used to power electric motors that drive the propellers.