A boiler water cycle of a fluidized bed boiler, preferably, a supercritical once-through-unit (OTU) boiler, includes a drop leg, a number of horizontal inlet headers, substantially having a length of the length of a front wall of the furnace of the boiler, arranged below the furnace, and panels of water tubes of the front wall and a rear wall, extensions of the water tubes being directly connected to the inlet headers, wherein each inlet header is in flow communication with a drop leg by way of an inlet duct connected to the end of the inlet header. The inlet headers include a front wall chamber below the front wall of the furnace, a rear wall chamber arranged below the rear wall of the furnace, and at least one grid chamber below the center part of the furnace grid in a wind box of the fluidized bed boiler.
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