Once-through boilers

In this paper, the once-through boiler design for nuclear power plant, cogeneration, power plants and contained power plants for generation of coal is presented. The once-through boiler system is compared with other systems, namely the natural-circulation, controlled-circulation, once-through with superimposed circulation, and combined-circulation systems, under the aspects of the applicable pressure range, the influence of pressure and temperature on plant efficiency, and the suitability of the systems for fixed and sliding pressure operation. Reference is made to mass flux in the evaporator (cooling of the evaporator tubes), arrangement of the recirculation pump, recirculation and feed-water pump power, furnace wall design, main steam temperature pattern under pat-load operation and furnace slagging (feasible furnace outlet temperatures), required feed-water quality, dynamic behavior (start-up times, disturbances, load changes), preferred design practices, unit sizes, and advantages in specific applications. Special attention is focused on the water wall design including DNB, DO, thermal stresses and temperature differences between individual furnace tubes, steam preheating and water separation devices, start-up systems, and feed-water control. Finally three examples of large utility boilers and a special application for team soak or steam drive in oil fields are described