The increase in steam temperature has required the development of new materials, and the growing power output and main steam pressure has required improved mechanical design using existing materials. That is why efficiency was initially driven by a pressure increase up to supercritical conditions. With the development of high chromium steels, it was possible to raise steam temperatures up to 600°C and achieve ultra-supercritical parameters. For economic reasons, steam power plants in the 250 - 500 MW class have until recently remained primarily subcritical, with temperatures not exceeding 566°C.
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