Importance

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Abstract

An evaporator in a steam generator extracts heat from high temperature gases to convert heated water into saturated steam. The evaporator includes two sections--a once-through section and a circulation section, both of which include tubes located in the flow of hot gases. Heated water flows through the tubes of the once-through section at a rate sufficient to maintain the interiors of its tube fully wetted while enabling steam to develop in that water, with the steam having a quality of at least 20%. The circulation section includes a drum that is connected to the tubes of that section such that water from the drum circulates through the tubes and then back to the drum, with the circulation being such that the water in the tubes of the circulation section keeps the tubes fully wetted while steam develops in that water. The water from the tubes of the once-through section discharges into the drum as does the water circulating back from the tubes of the circulation section. Within the drum, the steam formerly entrained in the water escapes. Since the tubes of both sections remain fully wetted the heat transfer occurs most efficiently. The presence of the once-through section enables the circulation section to be reduced in size and this shortens start up. The drum confines most impurities to the liquid water that is circulated in the circulation section, and these impurities may be removed through a blowdown.

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First Claim