A boiler is a pressurized vessel in which heat generated through combustion is transferred to water to produce hot water or steam. The two traditional configurations for boilers in an industrial setting are known as water tube and fire tube. In both designs the boiler shell is a large cylinder, usually steel, lined with a suitable refractory material, with a series of tubes running through it. In the water tube design, hot combustion gases fill the space between the water filled tubes and the shell lining. In the fire tube design, the gases are in the tubes and the water in the surrounding space. Water tube boilers are more expensive to build but with greater space for the hot gases they can produce steam more quickly. Fire tube boilers hold more water and so take more energy to start producing steam but they are able to better tolerate fluctuations in demand. The requirements of the application determine which is the more appropriate design.
In 1922, an engineer named Mark Benson filed a patent for a supercritical steam generator. He proposed increasing the pressure and temperature of the water to the critical point at which it achieves a supercritical state. His design produces steam to drive a turbine without any bubbles forming, but it still became known as a Benson boiler.
This technology is now the basis for much electricity generation around the world. The process is continually being refined and improved to increase efficiency and reduce the emissions of harmful greenhouse gases in generators using fossil fuels. Following on from supercritical steam generation came ultra-supercritical steam generation where the temperatures reach 600C, pressure is 300bar and coal-fired plants achieve 46% efficiency. Advanced ultra-supercritical steam is being planned to reach temperatures of 700C and hit 50% efficiency in coal-fired plants. With even higher operating temperatures and pressures comes the need for new materials with which to build and insulate these generators.

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