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Boiler Circulation＆Water Treatment An adequate flow of water and water-steam mixture is necessary for steam generation and control of tube metal temperatures in all the circuits of a steam-generating unit. At supercritical pressures, this flow is produced mechanically by means of pumps. At subcritical pressures, circulation is produced either naturally by the action of the force of gravity, by pumps, or by a combination of the two. Fig-1 Simple natural-circulation circuit (diagrammatic) including primary steam separator in drum The force of gravity available to produce flow in natural circulation comes from the difference between the densities (1b/cuft) of the fluids in the downcomer (downflow) and riser (upflow) portions of the circuit (Fig-1). Maximum pumping effect occurs if the fluid in the downcomers is water at or slightly below saturation temperature and free of steam bubbles. Heat-absorbing rises at saturation temperature convey to the boiler drum a water-steam mixture of less density than that of the water in the downcomers. This difference in density establishes the force available for circulation. The flow in the various circuits of boiler units designed for forced circulation at subcritical pressures, is produced by mechanical pumps. There are two general types of forced-circulation systems, a “once-through ” system and a “recirculating” system. The “once-through” force-circulation type receives water from the feed supply, pumping it to the inlet of the heat-absorbing circuits. Fluid heating and steam generation take place along the length of the circuit until evaporation is complete. Further progress through the heated circuits results in superheating the vapor. Conventionally this type of force circulation requires no steam-and-water drum. A modification of the “once-through” type evaporates to partial dryness(90%quality) removeing the excess water in a separator. The “recirculation” forced-circulation-type unit has wate