PROBLEM TO BE SOLVED: To improve the absorption efficiency of heat by a method wherein the inside diameter of the furnace wall of a combustion gas passage part is reduced lower than that of the furnace wall of a pass smoke port part. ;SOLUTION: An annular upper header 1 is arranged at the upper part of a can body and an annular header 2 at the lower part thereof, and the two headers are coupled together through a number of water tubes 3. The periphery of a water tube train arranged in an annular shape is covered with a cylindrical furnace wall 4 to form a combustion gas passage 7 running between the water tubes 3 and a furnace wall 4. A part surrounded with the water tubes 3 is a combustion chamber 9 and a burner 10 is arranged thereabove. In the inside diameter of the furnace wall 4, that of an upper part wherein a pass smoke port 5 is arranged is higher than that of a lower part and a distance between the water tube 3 of a combustion gas passage 7 and a furnace wall 4 is decreased. Combustion gas by a burner 10 flows through a gap between the water tubes of the pass smoke port part in a direction crossing the water tube and flows down in the direction of a combustion gas passage 7. In this case, the water tubes high-efficiently heats the water tubes by turbulence generated due to an uneven surface formed on the surface of the water tube.;COPYRIGHT: (C)1997,JPO
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