Session: 23-08 Last Stage Blades and General Design Aspects of Steam Turbines

Paper Number: 79398

79398 - History of Steam Expansion Machines Seen as Polytropic Efficiency Achievements 

Among the heat to power cycles the historically first successful technologies used cycles with steam expansion machines in its core. In the first phase as of the eighteenth century, reciprocating steam engines dominated. As of 1890 steam turbines were developed, which replaced the reciprocating steam engines until around 1930 with the exception of steam locomotives and some other niche applications. Up to now in steam cycle technology mostly the isentropic efficiency definition for expansion is used. Such numbers depend both on the dissipation and on the pressure ratio.

In history the overall pressure ratio of steam cycles has developed from low single digit values to over 3000. Thus it is attractive to consider the achievements of this technology especially for larger pressure ratio with an efficiency definition, which is completely independent of the pressure ratio. This definition exists in the form of the so-called polytropic efficiency. This is the ratio of the energy converted to mechanical work divided with the totally available energy of the steam pressure drop in question (called Exergy). One minus the polytropic efficiency is the ratio of the dissipated energy divided with the totally available energy of the steam pressure drop in question. The latter could also be called the dissipation rate of the steam expansion process. A constant polytropic efficiency expresses that this dissipation rate is assumed constant during the expansion interval.

A constant dissipation rate is a nearby assumption. Engineers love it because in both steam engines and in early turbomachine design often scaled repeat designs were used. And in modern 3D designed blading the dissipation rate is kept close to a limit (in the order of 6 to 10%) given by the used technology. This has fostered the application of a polytropic efficiency in gas turbine technology because of the nearly ideal gas behavior of the flue gases. In other words: A polytropic efficiency of a turbomachinery component indicates the part of converted energy without any exergy loss while the isentropic efficiency hides an exergy loss, which needs an extra consideration.

The aim of this paper is to disclose the historic development of the polytropic efficiencies from the early steam engines up to modern steam turbines based on historic and recent data publications of steam engines and turbines. With current methods the polytropic efficiency can easily be determined for steam expansion also across the wetness limit with the only assumption of equilibrium within the wet steam. These methods should become more visible.

This paper shows the polytropic efficiencies of selected historic steam expansion machines up to the current level in typical modern steam turbine plants. As an outlook also the thermodynamic potential of a notional Brayton cycle with steam as the flue gas in a single expansion path is drafted and its challenges are mentioned. With the future use of hydrogen for green energy storage such cycles may become an option.

Presenting Author: Hans E. Wettstein HEW Consulting

Presenting Author Biography: Hans holds a PhD. from ETHZ. He was working 1977-2011 on Gas Turbine development in various leading positions in Baden (BBC, ABB and ALSTOM). Inventor for 38 Gas Turbine related patents. 2006 until 2014: Gas turbine lectures at ETHZ. Since 2011 self-employed for Gas Turbine and Energy related Consulting. Company Name HEW. Since 2011 sixteen peer-reviewed turbine technology related publications.

Authors:

Hans E. Wettstein HEW Consulting