Steam expansion
In thermodynamics, steam expansion or steam expansion is the name given to the adiabatic, finite increase in volume of a high-tension (pressurized) steam with a simultaneous reduction in pressure.
This effect can be used, for example, to generate work in a steam turbine or a steam engine.
The maximum mechanical work performed is
In the steam turbine process, the change in volume of the steam is converted into flow energy, which sets the turbine impellers of the unit in rotation and thus generates mechanical work.
In a piston steam engine, a piston is moved by the steam expansion; the oscillating piston movement is generally converted into a rotary movement via the crankshaft and thus work is also generated. The higher the inlet tension of the steam and the lower the final tension at the outlet from the cylinder, the greater the theoretical work in a piston steam engine that can be performed with a certain filling. In a single-cylinder steam engine, the filling - i.e. the ratio of the inserted to the theoretically possible maximum amount of steam - is generally only about 30%. By means of suitable control of slides or valves, it is dimensioned in such a way that condensation in the cylinder and surface damage are avoided and, to avoid losses, the vapor pressure is reduced to the ambient pressure at the end of the work cycle as far as possible.
More powerful piston steam engines are compound machines with double or triple expansion, the latter consisting of high, medium and low pressure parts. In the case of a steam engine, triple expansion means that the high-tension working steam supplied is gradually expanded three times and thus better utilized. Higher steam pressures - generally between 10 bar and 15 bar - and larger fillings can be achieved, thus considerably increasing the performance and efficiency of the unit.
In the case of multi-stage steam expansion, the partially expanded amount of steam is pushed into the next cylinder, if necessary via an intermediate storage device (so-called sensor or receiver). In order to obtain approximately the same output from each cylinder, a considerable increase in the cylinder diameter is necessary in accordance with the falling steam pressure and increasing steam volume; the product of the piston area and the vapor pressure (= force) is roughly the same in each cylinder. This results in spatial relationships such that the medium-pressure cylinder corresponds to about 2.5 times the capacity of the high-pressure cylinder and the low-pressure cylinder about 6 times.
For reasons of space, especially on ships, the number of cylinders is usually limited to a maximum of three. The remaining steam pressure can then still be up to 0.30 bar overpressure, which can still be used, for example, in a downstream steam turbine .
The triple expansion engine had been the standard drive in merchant shipping since around 1880 and remained so until the 1930s, after which it was largely replaced by internal combustion engines (diesel engines)