EVAPORATION CYCLE OF A NATURAL CIRCULATION STEAM GENERATOR IN CONNECTION WITH A VERTICAL DUCT FOR UPWARD GAS FLOW

Abstract

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.

Claims

1-11. (canceled)

12. An evaporation cycle of a natural circulation steam generator in connection with a vertical duct for upward gas flow, the evaporation cycle comprising: a steam drum for feeding water to a downcomer pipe; an evaporator in flow connection with the downcomer pipe and comprising a first evaporative section and a second evaporative section connected in parallel with the first evaporative section and arranged at a higher level that the first evaporative section, each of the evaporative sections comprising multiple in parallel connected evaporation tubes for evaporating the water to a mixture of steam and water; a riser pipe in flow connection with the evaporator for conveying the mixture of steam and water to the drum; and 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 comprising a lower portion and an upper portion above the lower portion, wherein (a) each evaporation tube of the first evaporative section comprises a single substantially horizontal pass across the vertical duct from an inlet header to the lower portion of the outlet collector, (b) each evaporation tube of the second evaporative section comprises multiple in series connected substantially horizontal passes across the vertical duct from an inlet header to the upper portion of the outlet collector, and (c) 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.

13. An evaporator cycle according to claim 12, wherein the outlet collector is a vertically arranged elongated chamber.

14. An evaporator cycle according to claim 13, wherein the riser pipe is connected to a top portion of the outlet collector.

15. An evaporator cycle according to claim 12, wherein the second evaporative section comprises an inlet header arranged at a higher level than the lower portion of the outlet collector and at a lower level than the upper portion of the outlet collector.

16. An evaporator cycle according to claim 15, wherein the inlet header is a vertically arranged elongated chamber and the downcomer pipe is connected to a bottom portion of the inlet header.

17. An evaporator cycle according to claim 15, wherein each evaporation tube of the second evaporative section is arranged so that the overall water flow is parallel with the gas flow.

18. An evaporator cycle according to claim 17, wherein each evaporation tube of the second evaporative section comprises two passes across the vertical duct.

19. An evaporator cycle according to claim 12, wherein the first evaporative section comprises the inlet header arranged at the same height level as the lower portion of the outlet collector.

20. An evaporator cycle according to claim 19, wherein the inlet header is a vertically arranged elongated chamber and the downcomer pipe is connected to a bottom portion of the inlet header.

21. An evaporator cycle according to claim 12, wherein each evaporation tube of the second evaporative section comprises three passes across the vertical duct and the first evaporative section and the second evaporative section comprise a common inlet header.

22. An evaporator cycle according to claim 21, wherein the common inlet header is a vertically arranged elongated chamber and the downcomer pipe is connected to a bottom portion of the inlet header.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 shows a schematic diagram of an evaporation cycle in accordance with a first embodiment of the present invention.

[0029] FIG. 2 shows a schematic diagram of another evaporation cycle in accordance with a second embodiment of the present invention.

[0030] FIG. 3 shows a schematic diagram of an evaporation cycle in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] FIG. 1 schematically shows an evaporation cycle 10 of a natural circulation heat recovery steam generator (HRSG) according to a preferred embodiment of the present invention. The evaporation cycle 10 is arranged in connection with a vertical duct 12 for an upward flow of hot gas 14, such as exhaust gas from a gas turbine. The evaporation cycle 10 comprises a steam drum 16 for feeding water to a downcomer pipe 18, an evaporator 20 in flow connection with the downcomer pipe 18 for evaporating the water to a mixture of steam and water, and a riser pipe 22 in flow connection with the evaporator 20 for conveying the mixture of steam and water to the stream drum 16. Also, a feedwater line 24 for introducing fresh water to the steam drum 16 and a steam line 26 for discharging steam from the steam drum 16 are connected to the steam drum 16.

[0032] The evaporator 20 comprises a first evaporative section 28 and a second evaporative section 30 that are connected in parallel, i.e., the first and second evaporative sections 28 and 30 are in direct flow connection to the downcomer pipe 18. The first evaporative section 28 is arranged below the second evaporative section 30, which means that the first evaporative section 28 is arranged in the gas stream 14 upstream of the second evaporative section 30. The first, or lower, evaporative section 28 comprises multiple in parallel connected evaporation tubes 32, each of which makes a single horizontal pass 34 across the vertical duct 12 from an inlet header 36 to a lower portion 38 of an outlet collector 40. Correspondingly, multiple in parallel connected evaporation tubes 42 of the second, or upper, evaporative section 30 make two horizontal passes 44, 44 across the vertical duct 12 from an inlet header 46 to an upper portion 48 of the outlet collector 40. The second evaporative section 30 is connected parallel to the gas flow 14, i.e., the second horizontal pass 44 is arranged in the gas stream downstream of, or at a higher level than, the first horizontal pass 44. The evaporation tubes 32, 42 are usually finned tubes but, for the sake of simplicity, this is not shown in FIG. 1.

[0033] According to the present invention, the first mixture of steam and water collecting to the lower portion 38 of the outlet collector 40 flows through the lower evaporative section 28, due to the single pass configuration, with very low friction. Therefore, even a relatively low pressure difference between the fluid columns upstream and downstream of the first evaporative section, i.e., the difference of the hydrostatic pressure of water in the downcomer pipe 18 and that of the mixture of steam and water in the riser pipe 22 and in the outlet collector 40, brings about an intense flow of water through the lower evaporative section 28. The first mixture flows from the lower portion 38 of the outlet collector 40 upwards and combines with a second mixture of steam and water, formed in the second evaporative section 30, and acts as an internal ejector pump for the second mixture and assures a sufficient flow velocity of the combined mixture of steam and water. Based on the advantage described above, an evaporator cycle according to the present invention does not need any other source of motive force than heat from the stream of hot gas to assist the flow of the water in the evaporator.

[0034] The outlet collector 40 can be of different vertically extending shapes, comprising a lower portion and an upper potion, but advantageously, it is a vertically arranged elongated chamber. The riser 22 is connected to a top portion of the outlet collector 40, or, in practice, it can be a direct extension of the outlet collector.

[0035] The inlet headers 36, 46 of the first and second evaporative sections 28 and 30, which distribute water from the downcomer pipe 18 to the multiple in parallel connected evaporation tubes 32, 42, respectively, are advantageously vertically arranged elongated chambers. The downcomer pipe is preferably connected to a bottom portion of each of the inlet headers 36, 46, by pipe sections 50, 50 extending below the inlet headers, respectively.

[0036] Because the first evaporative section 28 comprises only a single substantially horizontal pass 34 across the vertical duct 12, the inlet header 36 of the first evaporative section is arranged substantially at the same height level as the lower portion 38 of the outlet collector 40. In practice, the substantially horizontal pass may be slightly slanted, typically, by at most two degrees, and connection tube sections between the evaporation tubes and the inlet headers and outlet collectors, respectively, may be slightly bent. Therefore, the bottom section 38 of the outlet collector 40 may be at a somewhat higher level than the inlet header 36.

[0037] Because the second evaporative section 30 is arranged parallel to the gas flow 14 and comprises two substantially horizontal passes 44, 44 across the vertical duct 12, the inlet header 46 of the second evaporative section is naturally at a lower level than the upper portion 48 of the outlet collector 40. Moreover, because the second evaporative section 30 is at a higher level than the first evaporative section 28, the inlet header 46 of the second evaporative section 30 is advantageously at a higher level than the lower portion 38 of the outlet collector 40 and at a lower level than the upper portion 48 of the outlet collector 40.

[0038] FIG. 2 schematically shows an evaporation cycle 10 of a natural circulation heat recovery steam generator (HRSG) according to another preferred embodiment of the present invention. The elements in FIG. 2 that are identical with the corresponding elements in FIG. 1 are denoted by the same reference numbers as those in FIG. 1.

[0039] The embodiment of FIG. 2 differs from that in FIG. 1 mainly in that the evaporation tubes 52 of the second evaporative section 54 make three substantially horizontal passes 56, 56, 56 across the vertical tube 12. Due to the three passes, more steam is produced in the second evaporative section of FIG. 2 than in that of the embodiment in FIG. 1. Because of the increased length and number of turns in the evaporation tubes, however, the friction of the fluid flow is increased, and there is an increased need for assisting the flow of the mixture of steam and water by the flow from the first evaporative section. Therefore, as in the embodiment shown in FIG. 1, the first and second evaporative sections 28, 54 have a common outlet collector 40, whereby the intense stream of steam and water from the first evaporative section 28 combines with the corresponding stream from the second evaporative section 54 and assures in all conditions a sufficient flow velocity of the combined mixture of steam and water.

[0040] The inlet ends of the evaporation tubes of the first and second evaporative sections 28, 54 are, in the embodiment of FIG. 2, on the same side of the vertical duct 12. Therefore, the first and second evaporative sections 28, 54 may have a common inlet header 58, advantageously, a vertically arranged elongated chamber.

[0041] FIG. 3 shows a third embodiment of the present invention that differs from that of FIG. 2 only in that, instead of a common inlet header, the first and second evaporative sections 28, 54 have separate inlet headers, 60, 62, respectively. The inlet headers 60, 62 are connected to the downcomer pipe 18 by pipe sections, 64, 64, respectively.

[0042] FIGS. 1 to 3 schematically show a vertical cross section of an evaporation cycle of a heat recovery steam generator (HRSG). In practice, a complete evaporation cycle extends through the depth of the vertical duct, and the upper and lower evaporative sections are multiplied correspondingly. Usually, about three or four upper and lower evaporative sections share common inlet headers and outlet collectors. Usually, there are also multiple inlet headers and outlet collectors, which may share common downcomer and riser pipes, or there may be multiple downcomer and riser pipes connected to single or multiple inlet headers and outlet collectors.

[0043] It should be understood that the invention is described by way of examples in connection with what are at present considered to be the preferred embodiments, but it is intended to cover various combinations of its features and other applications within the scope of the invention as defined in the appended claims. Described is an embodiment in which only one evaporative section is divided into upper and lower evaporative sections. It should be understood, however, that there can be plurality of evaporative sections that are divided into upper and lower evaporative sections. In addition, a plurality of steam drums can be considered.