Abstract: A heat exchanger comprising a pressure vessel (1). A plurality of serpentines (8) convey a fluid to be heated through the pressure vessel (1) in one direction. A duct (9) surrounding the serpentines (8) conveys a second fluid in the opposite direction to give up its heat to the first fluid. The duct (9) is spaced from the pressure vessel (1) and is surrounded with thermal insulation (23). An opening in the duct (9) equalises the pressure between the inside and the outside of the duct (9) which is also supported against expansion caused by the pressure inside the duct (9) exceeding the pressure outside the duct (9).