Efficiency of Multi-stage Turbomachines

In Chapter 7, we derived the equations for calculation of different losses that occur within a stage of a turbomachine. As shown, the sum of those losses determines the stage efficiency which shows the capability of energy conversion within the stage. The stage efficiency, however, is not fully identical with the efficiency of the entire turbomachine. In a multi-stage turbomachine, the expansion or compression process within individual stages causes an entropy production which is associated with a temperature increase. For an expansion process, this temperature increase leads to a heat recovery and for a compression process, it is associated with a reheat ash shown in Fig. 8.1. As a consequence, the turbine efficiency is higher and the compressor efficiency lower than the stage efficiency. The objective of this chapter is to describe this phenomenon by means of classical thermodynamic relations. The approach is adopted by many authors, among others, Traupel [1] and Vavra [2].