Cutoff (steam engine)

Steam expansion or steam expansion is called in thermodynamics, the adiabatic finite volume enlargement of a highly strained ( under pressure ) steam with a simultaneous reduction of pressure

This effect can for example be used to produce work in a steam turbine or a steam engine.

In the steam turbine process, the change in volume of the steam is converted into fluid energy which enables the turbine wheels of the unit in rotation, and thus produces mechanical work.

At a piston steam engine is moved through the steam expansion, a piston; The oscillating piston movement is generally converted by the crankshaft in a rotational movement, and thus also produces work. The higher the entrance of the vapor tension and the lower the final voltage at the outlet from the cylinder, is greater at a piston steam engine the theoretical work, can be made with a certain panel. Ie the ratio of inserted to the theoretically possible maximum amount of steam - - With a single-cylinder steam engine, the filling is generally only about 30%. By appropriate control of valves or valves being such that condensation in the cylinder, and surface damage is avoided and is reduced to the ambient pressure as possible to avoid losses of the steam pressure at the end of the working cycle.

With more powerful piston steam engines are compound machines with double or triple expansion, the latter consisting of high-, medium - and low-pressure part. So triple - expansion means in a steam engine that supplied highly strained working steam is gradually relaxed three times and thus better utilized. It may have higher vapor pressures - generally between 10 bar and 15 bar - and realized larger fillings and thus performance and engine efficiency is increased significantly.

In the multistage steam expansion is the amount of steam teilexpandierte, possibly via intermediate memory ( so-called pick-up or receiver ), is pushed in each case into the next cylinder. Around each cylinder to remove approximately the same power, the steam pressure and decreasing volume anwachsendem vapor a substantial enlargement of the cylinder diameter is accordingly required; the product of the piston surface and vapor pressure ( = force) is approximately the same in each cylinder. This results in space conditions arise so that the medium-pressure cylinder about 2.5 times, the low-pressure cylinder corresponds to about 6 times the capacity of the high pressure cylinder.

For reasons of space, especially on ships, so the number of cylinders is usually limited to a maximum of three. The remaining vapor pressure can then still be up 0.30 bar overpressure, which can be used for example in a downstream Abdampfturbine.

Since about 1880, the triple - expansion engine was the standard drive in the merchant shipping and they remained there until the 30s of the 20th century, after which it was replaced largely by internal combustion engines (diesel engines)

• Steam machine technology