An exhaust steam turbine is a multi-stage, low-pressure steam turbine in which the exhaust steam from piston steam engines , which is still under slight overpressure, is expanded to the negative pressure in the condenser and thus energy is obtained from the pressure and heat gradient of the exhaust steam. Waste steam turbines were often used in ship propulsion in the first half of the 20th century. With the advent of diesel engines, however, the piston steam engines and with them the exhaust steam turbine came to an end.
Piston steam engines are multiple, but mostly triple expansion engines , consisting of high, medium and low pressure parts. 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. For reasons of space, especially on ships, the number of cylinders is usually limited to a maximum of three. The remaining steam pressure is then still between 0.10 and 0.40 bar overpressure.
In order to utilize the residual enthalpy still present in the exhaust steam after leaving the low-pressure cylinder, the idea arose at the beginning of the 20th century to use this in a steam turbine before the exhaust steam is fed to the condenser and liquefied there again. With these attached turbines, steam consumption and correspondingly fuel consumption can be greatly reduced with the same engine output, and thus the overall efficiency of such a system can be increased considerably.
The additional power gained by the steam turbine can, for example
- are used to drive auxiliary equipment (generator, compressor, fan, etc.),
- drive a separate propeller shaft. For this purpose, the steam turbines of several engines are combined via a gearbox or steam engines of several engines drive a single steam turbine. The latter was implemented on the Titanic (drive on the middle propeller shaft),
- also drive the main shaft, as described below for the Bauer-Wach steam turbine.
Bauer-Wach exhaust steam turbine
The so-called Bauer-Wach exhaust steam turbine, which was developed in 1926 by Gustav Bauer and Hans Wach at the Joh. C. Tecklenborg shipyard in Wesermünde (now a part of Bremerhaven), was widespread . After the takeover of Tecklenborg by the Bremen group Deutsche Schiff- und Maschinenbau AG (Deschimag) in 1926 and the closure two years later, the BW exhaust steam turbine including gearbox and coupling from the Deschimag shipyard AG Weser with over 900 units was mainly used successfully marketed worldwide in shipbuilding.
In the Bauer-Wach system, the steam flows from the low-pressure cylinder through a switchover flap into the exhaust steam turbine and drives it. A reduction gear and a fluid coupling (Föttinger system) transmit the turbine power via gear wheels directly to the propeller shaft. The engine output can thus, at best, be increased by up to about 10% or, with the same drive output, the fuel consumption can be reduced accordingly.
In reverse or maneuvering operation, the system is put out of operation or in idle operation by setting the switchover flap so that the exhaust steam reaches the condenser directly. The fluid coupling is emptied and the turbine is thus decoupled from the drive.
The reduction of the gear is designed in such a way that the turbine (4000-5000 / min) as well as the fluid coupling and propeller (100-200 / min) or the directly coupled steam engine work in the optimal speed range.
- H. Kedenburg: Steam turbines with condensation systems and Vulcan gears. Verlag Der Betriebsökonom, Verden / Aller 1953.
- Wilh. H. Eyermann: The steam turbine. A teaching and manual for designers and students. Oldenbourg, Munich 1906.