A heat exchanger comprises a plurality of substantially similar plates 10, with a pair of ports. Each plate is turned 90 DEG in the same direction relative to the adjacent one so that the ports align to provide a pair of separate series flow paths which are interleaved between the plates. The angular alignment of the ports is controlled by the regular polygon shape of the plates. The plates are clamped together and sealed for example by furnace brazing. The ports are defined by oppositely extending stub tubes 54, 56. The plates preferably have dimples or other projections in opposite directions from both faces. Also disclosed is a method of producing substantially identical heat exchanger plates which have different patterns of projections (42, figure 1). The different patterns being obtained by locating or not locating projection forming punches in recesses in the dies used to form the plates. <IMAGE>
