A further approach to the Rankine cycle can be made by using additional stages of reheat. A second stage of reheat involves similar additional cost and design considerations to those already described for single reheat. There are obviously greater difficulties in matching steam and metal temperatures during starting and load changes.
Figure 1.44 shows a simplified Mollier diagram for a single and double reheat supercritical cycle, and Fig 1.45 shows the T-S diagram. In practice a second stage of reheat is only normally justifiable with a supercritical cycle.
The critical pressure of steam
At 221.1 bar, water heated to 374°C turns directly into superheated steam without boiling in the accepted sense. The latent heat of vaporisation is zero and, since the densities of the water and steam are the same, there is no well defined water-to-steam transition interface.
A brief explanation from physical data source may be helpful. Water at high temperature consists of clusters of molecules held together by strong potential binding forces of short duration. These clusters permit angular or linear displacement which gives rise to the liquid characteristics.
Evaporation of a liquid into a gaseous space normally consists, at the molecular level, of a surface molecule in a liquid cluster acquiring sufficient thermal (and thus kinetic) energy to overcome the potential binding forces of its neighbouring molecules to break away clear of the cluster and leave the surface of the liquid to enter the gaseous space. The binding forces between the surface molecule and molecules well below the surface are not significant as potential forces drop off extremely rapidly with increasing distance. Liquids subjected to high pressures logically require higher thermal energy, and therefore higher temperature, for the surface molecule to break free of the molecular cluster. At supercritical pressures, the potential forces exerted between molecules in a molecular cluster are too large to be overcome by increases in thermal energy and hence no surface molecules escape from the molecular clusters. However, as the supercritical pressure fluid undergoes a temperature rise, the average size of its clusters diminishes. With further increases in fluid temperature, the molecular clusters are reduced to isolated molecules and all traces of any form of crystalline structure has disappeared. Thus the supercritical pressure fluid acquires its steam-like qualities not by evaporation of isolated surface molecules, but rather by the gradual diminution in the number of molecules contained within molecular clusters.
A double-reheat supercritical cycle can show approximately a 3.9% efficiency advantage over a single reheat subcritical cycle (242 bar/540°C/540°C compared with 166 bar/540°C/540°C).
For single-reheat, the reheat pressure is commonly about 25% of the initial pressure while for double-reheat the first reheat is usually at about 30% and the second 10% or less. There are no known proposals for triple-reheat, and it is most unlikely that the additional cost could be justified. There is also the difficulty of designing an LP turbine with the high inlet temperature required. This is already about 370°C for double-reheat machines.