3.2 Cycle efficiency and heat rate
The last section dealt with the theoretical aspects of steam cycles. An attempt is now made to investigate the differences between an ideal turbine expansion (one which is isentropic) and an actual turbine expansion, leading to expressions for turbine heat rate and efficiency.
The general cycle efficiency was illustrated with the use of the T-S diagram; for the turbine alone, efficiency is best discussed using the Mollier or enthalpy-entropy (H-S) diagram. A comparison of actual and isentropic turbine expansions is illustrated in Fig 1.31. The turbine expansion shown is that for an HP cylinder, i.e., wholly in the superheat region.