Forced Circulation for Supercritical or Subcritical Boilers
The main objective of a good natural circulation design is to have wet steam at the outlet of all the tubes making up the water walls of the combustion chamber. Provided the dry out and critical heat flux are not reached the boiling inside the tube occurs in a nucleate regime, where the heat transfer coefficient is in the range of 1010kW/m2•C. Owing to such high internal heat transfer coefficients, the temperature of the tube walls remains very close to the saturation temperature of the water. This makes the task of material selection and circulation design easy. Another advantage of this natural circulation is that circulation rate increases either when pressure decreases or when heat flux increases. The former enables inherent flow stabilization during load rejection as long as the level of water is correctly controlled in the drum. The latter allows natural uniformity of flows in adjacent tubes receiving different heat fluxes.
KeywordsHeat Flux Critical Heat Flux Natural Circulation Supercritical Pressure Water Wall
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