7.1.2 Cooling of IP rotors
The IP rotors of reheat turbines are subject to the same steam temperature at the inlet as HP rotors and, because of their larger diameters, are subject to larger centrifugal stresses. The bore of an IP rotor under the first few stages is therefore a critical region for creep.
On large machines operating with reheat steam at 565°C, forced cooling of the rotor by cooler steam is employed. This method is particularly effective in cooling the bore under the first wheel (Fig 1.109); with further increases in turbine size, additional cooling is needed to cool the bore of the second wheel. This can be effectively provided by introducing a second cooling steam flow between the first two stages, so that gland leakage steam is replaced by cooler steam.