A steam turbine is a machine that exploits the thermal energy of steam under pressure, converting it into useful mechanical energy through a thermodynamic transformation of expansion. Specifically, the steam turbine converts the internal energy of the vapor into kinetic energy of rotation.
The steam turbine, thanks to the highest thermal efficiency and the best power / weight ratio, has completely replaced the steam engine, which was an alternative engine invented by Thomas Newcomen and then significantly improved by James Watt.
From a thermodynamic point of view, the maximum efficiency occurs when steam expansion is an ideal process (reversible transformation) in which the vapor pressure decreases, becoming mechanical work, in an infinite number of stages. Watt's alternative machine was single-stage, and subsequent refinements were used in most of the two or three stages (double and triple expansion). In contrast, modern steam turbines achieve high thermal efficiency thanks to the presence of a greater number of stages in series.
Steam turbines are expensive and require advanced manufacturing processes and high quality materials. In addition, they have a high efficiency when operating at speeds of thousands of rpm, so if the load rotates at lower speeds, a gearbox is needed. However, if the installed power is high, the high investment costs are offset by the fact that the steam turbine consumes less fuel, requires less maintenance and is smaller compared to an alternative thermal engine of equal power.
Applications of steam turbines
Steam turbines can be used to produce electricity, coupled to generators, often without the need for gear boxes. In this case, they work in ideal regimes, since the generators have to turn at a constant speed (3000 rpm for 50 Hz networks and 3600 rpm for 60 Hz networks, in some cases, especially in nuclear plants, they use generators of 4 poles that rotate at half the speed). In addition, the steam turbine, being a rotating machine, is advantageous as the motor of an electric generator, since it does not require any mechanical member to transform the reciprocating movement into a rotating one.
Another typical field of application for steam turbines is in plants such as refineries, paper mills, desalination plants 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 the production of steam and that of electrical energy or mechanical work.
Steam turbines are also used as marine engines on ships, where limited dimensions are an advantage. Propeller steam turbine locomotives were also built, but their diffusion was very limited.
The power plants based on the production of high temperature steam (thermal power plants, nuclear power plants, geothermal power plants, some solar thermal plants, etc.) use steam turbines connected to electric generators to produce about 80% of the electricity of the planet.
The turbines for electrical production are usually directly coupled to their generator.
As the generators must rotate at synchronous speeds with respect to the frequency of the electrical system, the most common rotation speeds are 3,000 rpm for 50 Hz systems and 3,600 rpm for 60 Hz systems.
As nuclear reactors operate at lower temperatures and with less gas pressure than thermal power plants, they usually operate at half the speed of rotation, but with 4-pole generators.
On boats, steam turbine propulsion has multiple advantages over thermal explosion engines: smaller size and weight for the same power, less maintenance and less vibration. However, a steam turbine is only efficient at high rotation speeds (in the order of thousands of revolutions per minute), while most propellers are designed to operate at less than 100 rpm. This requires precise and complex transmissions (and at a high cost).
An alternative is the use of turbo-electric propulsion, in which the turbines generate electricity as in an energy plant and this is used to power electric motors that move the propellers. Although the manufacturing cost is higher, it is profitable, since the energy consumption and maintenance costs are lower than that of a combustion engine of equivalent power.
It must be said that diesel engines are capable of greater efficiency: steam turbines still do not reach an efficiency of 50%, while diesel cycle engines often exceed this level (especially in marine propulsion systems).