An evaporation cycle of a natural circulation steam generator. An evaporator is in flow connection with a downcomer pipe and includes a first evaporative section and a second evaporative section connected in parallel with the first evaporative section and arranged at a higher level than the first evaporative section. The evaporator cycle is not associated with another external source of motive force than heat from the gas flow to assist the flow of the water in the evaporator, and the evaporator has a vertically extending outlet collector for collecting the steam and water from the first and second evaporative sections to the riser pipe. The outlet collector includes a lower portion and an upper portion above the lower portion. The first evaporative section is in direct flow connection with the lower portion and the second evaporative section is in direct flow connection with the upper portion.

An evaporation cycle of a natural circulation steam generator. An evaporator is in flow connection with a downcomer pipe and includes a first evaporative section and a second evaporative section connected in parallel with the first evaporative section and arranged at a higher level than the first evaporative section. The evaporator cycle is not associated with another external source of motive force than heat from the gas flow to assist the flow of the water in the evaporator, and the evaporator has a vertically extending outlet collector for collecting the steam and water from the first and second evaporative sections to the riser pipe. The outlet collector includes a lower portion and an upper portion above the lower portion. The first evaporative section is in direct flow connection with the lower portion and the second evaporative section is in direct flow connection with the upper portion.

Disclosed herein is a once-through evaporator comprising an inlet manifold; one or more inlet headers in fluid communication with the inlet manifold; one or more tube stacks, where each tube stack comprises one or more inclined evaporator tubes; the one or more tube stacks being in fluid communication with the one or more inlet headers; where the inclined tubes are inclined at an angle of less than 90 degrees or greater than 90 degrees to a vertical; where each tube stack comprises a plurality of tubes arranged in a plurality of columns and a plurality of rows; where a plurality of tubes in a first column are offset from a plurality of tubes in a second column by a distance d2 and where a plurality of tubes in a first row are offset from a plurality of tubes in a second row by a distance d1; where d1 varies from 0.1d2 to 1000d2; one or more outlet headers in fluid communication with one or more tube stacks; and an outlet manifold in fluid communication with the one or more outlet headers.