Abstract: The present invention refers to a plate heat exchanger (1) for heat transfer between two fluids, comprising several permanently joined modules (2), each consisting of two outer heat transfer plates (3) and between these several principally rectangular inner heat transfer plates (4), which have inlet and outlet openings (12) for respective fluids. The outer heat transfer plates (3) show smaller inlet and outlet openings (12) than the inner heat transfer plates (4), so that the modules (2) can be welded with each other around the inlet and outlet openings (12) of the outer heat transfer plates (3).