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Comment PDF Heat Transfer

Understanding Boiler Circulation

By V. Ganapathy Boiler consultant |

Boilers generate steam using different methods to circulate the steam-water mixture through the evaporator tubes. These methods include natural circulation (Figures 1, 2a–2c), forced circulation (Figure 3a), and a once-through design. Figures 3b, 4 and 5 show examples of a waste-heat water tube and fire-tube boilers with external downcomers and risers. Most boiler systems have evaporator tubes heated by hot fluegases produced either from the combustion of fuels (such as oil, gas or solid fuels,) or by kilns, furnaces, gas turbines or catalytic crackers, or by other hot gas sources (such as waste-heat boilers). Boiling occurs in the evaporator tubes, and generates wet steam. FIGURE 1. Boilers may operate using a variety of methods to circulate the steam-water mixture throughout the evaporator tubes. A typical natural-circulation system is shown here (CR = circulation ratio)   The density difference between the colder water in the downcomers and the hotter steam-water mixture in the evaporator tubes ensures the circulation of the steam-water mixture back into the drum. External downcomers, as shown in Figure 2c, are unheated, while in package boilers…
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