The reheat concept is based on live steam run through a high-pressure turbine. Before entering the low-pressure turbine, the steam is returned to the steam generator to increase its temperature to the live steam parameters (pressure remains as is). Usually, the extraction pressure of the reheated steam is selected so that the steam can be reheated to the same temperature it had before entering the turbine.
For power output up to 60 MWe Siemens PG recommends the SST-PAC 400 reheat turbosets and for higher power outputs over 60 MWe SST-PAC 600 SST-PAC 800 reheat turbosets or the
SST-PAC 700 are the recommended solution.
Advantages of the reheat concept
- The heat is added to the steam cycle at a higher average temperature than to a non-reheat cycle. This improves the cycle efficiency and allows the steam turbine to provide a greater output for the same heat.
- There will be less steam going to the condenser and therefore lower cooling water losses and a smaller cooling system.
- The moisture content in the LP turbine will be reduced if the steam is reheated after it has expanded through the HP turbine. This will increase efficiency and also minimize erosion caused by water droplets.
- The reheat cycle allows higher inlet pressure and/or lower exhaust pressure while retaining the moisture content at low levels.
All these factors increase the steam cycle efficiency and reduce turbine aging.