Patent ID: 8677945

Claim:
A boiler, comprising: an upper header; a lower header; and a heat-transfer tube that connects the upper header with the lower header, the heat-transfer tube being provided with projections such that a heat-transfer area per unit length of the heat-transfer tube is smaller in an upstream region than that in a downstream region of combustion gas, wherein either or both installation pitches between adjacent upper and lower projections and projecting lengths of the projections from the heat-transfer tube are arranged such that the heat-transfer area per unit length of the heat-transfer tube is smaller in the upstream region than that in the downstream region of the combustion gas, and wherein the projections are fins provided on the heat-transfer tube at distances from one another in the vertical direction, the boiler further comprises a plurality of inner heat-transfer tubes which are arranged cylindrically between the upper header and the lower header, and which constitute an inner heat-transfer tube row, a plurality of outer heat-transfer tubes which are arranged cylindrically between the upper header and the lower header to surround the inner heat-transfer tube row, and which constitute an outer heat-transfer tube row, a plurality of inner closing portions provided to close a gap between adjacent inner heat-transfer tubes while leaving a lower end of the inner heat-transfer tube row as it is, a plurality of outer closing portions provided to close a gap between adjacent outer heat-transfer tubes while leaving an upper end of the outer heat-transfer tube row as it is, inner fins which extend outward from the inner heat-transfer tubes, respectively, on a surface constituting an outer peripheral surface of the inner heat-transfer tube row, and which are inclined upward in a circumferential direction of the inner heat-transfer tube row, and outer fins which extend outward from the outer heat-transfer tubes, respectively, on a surface constituting an inner peripheral surface of the outer heat-transfer tube row, and which are inclined upward in a circumferential direction of the outer heat-transfer tube row, and wherein projecting lengths of the inner fins and the outer fins from the heat-transfer tubes in a lower region of each of the heat-transfer tubes are set smaller than those in an upper region of each of the heat-transfer tubes, and in a case where the projecting lengths are the same, the installation pitches are larger in the lower region than those in the upper region of each of the heat-transfer tubes.