Abstract: A heat exchanger includes a plurality of sets of fluid channels, each fluid channel including first and second end portions and an intermediate portion between the first and second end portions. The first end portions in a plane perpendicular to a direction of fluid flow in the channels have respective end perimeters which are in a first configuration, where adjacent end portions of different sets of fluid channels have a total first shared heat transfer length, this being a summation of lengths of mutually opposed perimeters of the so adjacent end portions of the different sets. The intermediate portions of the channels in a plane transverse to the direction of fluid flow have respective intermediate channel perimeters, the intermediate portions having a second configuration with a total second shared heat transfer length being a summation of lengths of mutually opposed channel perimeters of the adjacent channels of the different sets.