A once-through evaporator for steam generation, comprising: a plurality of primary evaporator stages (110, 210, 310), each of the plurality of primary evaporator stages (110, 210, 310) including: one or more primary arrays (120, 210A, 210B, 210C) of heat transfer tubes, each of the primary arrays (120, 210A, 210B, 210C) of tubes having an inlet for receiving a fluid and arranged transverse to a flow of gas through the evaporator (100, 200, 300), an outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) coupled to the one or more primary arrays (120, 210A, 210B, 210C) of tubes; characterized in that each of the plurality of primary evaporator stages further includes a downcomer (137, 237, 337A, 337B, 337C, 337D) connecting said outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) to a secondary evaporator stage (150, 250, 320); and in that the secondary evaporator stage (150, 250, 320) includes one or more secondary arrays (160, 320A, 320B, 320C, 320D) of heat transfer tubes, each of the secondary arrays (160, 320A, 320B, 320C, 320D) of tubes coupled to an inlet (234, 334A, 334B, 334C, 334D) and arranged transverse to the flow of gas through the evaporator (100, 200, 300), the one or more secondary arrays (160, 320A, 320B, 320C, 320D, 320A to D) of tubes being arranged to receive a primary stage flow from the primary evaporator stages. a plurality of primary evaporator stages (110, 210, 310), each of the plurality of primary evaporator stages (110, 210, 310) including: one or more primary arrays (120, 210A, 210B, 210C) of heat transfer tubes, each of the primary arrays (120, 210A, 210B, 210C) of tubes having an inlet for receiving a fluid and arranged transverse to a flow of gas through the evaporator (100, 200, 300), an outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) coupled to the one or more primary arrays (120, 210A, 210B, 210C) of tubes; characterized in that each of the plurality of primary evaporator stages further includes a downcomer (137, 237, 337A, 337B, 337C, 337D) connecting said outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) to a secondary evaporator stage (150, 250, 320); and in that one or more primary arrays (120, 210A, 210B, 210C) of heat transfer tubes, each of the primary arrays (120, 210A, 210B, 210C) of tubes having an inlet for receiving a fluid and arranged transverse to a flow of gas through the evaporator (100, 200, 300), an outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) coupled to the one or more primary arrays (120, 210A, 210B, 210C) of tubes; characterized in that each of the plurality of primary evaporator stages further includes a downcomer (137, 237, 337A, 337B, 337C, 337D) connecting said outlet manifold (135, 235A, 235B, 235C, 335A, 335B, 335C) to a secondary evaporator stage (150, 250, 320); and in that the secondary evaporator stage (150, 250, 320) includes one or more secondary arrays (160, 320A, 320B, 320C, 320D) of heat transfer tubes, each of the secondary arrays (160, 320A, 320B, 320C, 320D) of tubes coupled to an inlet (234, 334A, 334B, 334C, 334D) and arranged transverse to the flow of gas through the evaporator (100, 200, 300), the one or more secondary arrays (160, 320A, 320B, 320C, 320D, 320A to D) of tubes being arranged to receive a primary stage flow from the primary evaporator stages.