A coaxial heat transfer tube (100), comprising a first inner heat transfer tube (110) having straight parts (111, 113, 115, 117) and curved parts (112, 114, 116), a first outer heat transfer tube (120) having straight parts (121, 123, 125, 127) and curved parts (122, 124, 126). The first outer heat transfer tube (120) radially surrounds the first inner heat transfer tube (110) at least within the first primary straight part (121) and the first primary curved part (122). Thermally insulating material (530) has been provided in between a curved part (122) of the first outer heat transfer tube (120) and a curved part (112) of the first inner heat transfer tube (110). Neither a straight part (121) nor a curved part (122) of the first outer heat transfer tube (120) comprises a longitudinal seam. A method for making a coaxial heat transfer tube (100).

A heat exchanger includes front and back walls forming a flue gas space such that a fluid flowing through a channel formed in the front and back walls can exchange heat with flue gas in the flue gas space, in use. The front wall includes a lower portion extending along the back wall, and an upper portion extending upwardly from an upper end of the lower portion and outwardly away from the back wall to form a combustion space of a flammable gas between the upper portion and the back wall. A length of the upper portion along a longitudinal direction thereof is longer than a length of the lower portion along a longitudinal direction thereof.

A tube and shell steam generator having an anti-clogging heat exchange tube bundle wherein the tube support plates within the tube bundle are designed with varying degrees of porosity thereby regulating local secondary side fluid conditions (velocity, quality, superheat, void fraction, etc.) in a manner to reduce the potential for clogging of the tube support plate lobes that are more prone to clogging.