Apparatus that provides and evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow

An apparatus that provides an evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow, the apparatus comprising:

• (A) a steam drum for feeding water to a downcomer pipe;
• (B) an evaporator in flow connection with the downcomer pipe, the evaporator comprising (i) a first evaporative section arranged at a first level and (ii) a second evaporative section, connected in parallel with the first evaporative section and arranged at a second level that is a higher level than the first level of the first evaporative section, wherein the first evaporative section and the second evaporative section are connected to a common inlet header, and wherein each of the first and second evaporative sections comprising multiple, in parallel connected, evaporation tubes for evaporation of the water to generate a mixture of steam and water;
• (C) a riser pipe in flow connection with the evaporator for conveying the mixture of steam and water to the steam drum;
• and
• (D) a vertically extending outlet collector for collecting the steam and water from the first and second evaporative sections and for feeding the collected steam and water to the riser pipe, the outlet collector comprising a lower portion and an upper portion above the lower portion,
• wherein (a) each evaporation tube of the first evaporative section makes a single, substantially horizontal pass across the vertical duct from the common inlet header to the lower portion of the outlet collector, (b) each evaporation tube of the second evaporative section makes three, in series connected, substantially horizontal passes across the vertical duct from the common inlet header to the upper portion of the outlet collector, such that at least one of the substantially horizontal passes across the vertical duct of each evaporation tube of the second evaporative section is arranged (i) downstream, in the gas flow, from a second of the substantially horizontal passes across the vertical duct of each evaporation tube of the second evaporative section, and (ii) at a higher level than the second of the substantially horizontal passes across the vertical duct of each evaporation tube of the second evaporative section, and (c) the evaporation cycle of the natural circulation steam generator provided by the apparatus is not associated with another external source of motive force, other than heat from the gas flow, to assist the evaporation of the water in the evaporator.