| 摘要 |
PROBLEM TO BE SOLVED: To make further improvement in the efficiency of a boiler, by effectively utilizing the entire heat transfer surface of a water tube that is provided with a circular fin. SOLUTION: A first annular water tube line 4 which comprises a plurality of water tubes 3 and is provided with a first opening 5, and a second annular water tube line 9 which comprises a plurality of water tubes 3 and is provided with a second opening part 10, are included. The second water tube line 9 is arranged outside the first water tube line 4, and a combustion chamber 7 is provided inside the first water tube line 4. A gas passage 12 extending from the first opening part 5 to the second opening 10 is formed between both of the water tube lines 4 and 9, and the upstream side of the second water tube line 9 has the structure of a water tube wall. The downstream side has a structure, in which a plurality of water tubes with circular fins C are spaced apart from one another at predetermined intervals, and a guide member 16 is provided outside each water tube with a circular fin C.
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