Abstract: A heat-exchanger unit (13) for recovering heat from the flue gases from a cylinder steam boiler (10), said heat exchanger unit comprising a flue-gas tube (14) and a heat-exchanging tube (16) which is supported concentrically inside the flue-gas tube (14) by a bent inlet pipe (19) extending from a lower opening (20) in the peripheral wall of the flue-gas tube (14) to an inlet opening (21) in a lower outwardly bulging end wall (22) of the heat-exchanging tube (16), and an outlet pipe (23) extending from an outlet opening (24) in an upper part of the peripheral wall of the heat-exchanging tube (16) to an upper opening (25) in the peripheral wall of the flue-gas tube (14). The inlet pipe (19) has a first, horizontal, straight end portion (19A), a second, upwardly directed, straight end portion (19B) and also an intermediate, curved portion (19C). The bending angle (.alpha.) of the curved portion exceeds 90.degree. and the other end portion (19B) extends with an inclination determined by said bending angle (.

Abstract: A heat-exchanger tube (16) comprises a tubular body (17) and surface enlarging elements provided on said body (17) and consisting of a large number of pins (18) which are welded to the tube body (17) at the outer side thereof and which extend in outward directions from the tube body (17). The tube body (17) and the pins (18) are made of carbon steel. However, in order to reduce the risk of crack formations in the weld joints between the pins (18) and the tube body (17) or in portions of the pins (18) located adjacent to the tube body (17), the pins (18) are made of a material having a substantially lower carbon content than the material of which the tube body (17) is made.