Abstract: The present invention is directed to a heat exchanger for reboiler or condenser service in a cryogenic distillation column (e.g. air separation) or other service in which small temperature differences between clean fluids are a primary objective. The exchanger comprises a stack of corrugated sheets having corrugations generally parallel to the diagonal between opposite corners of a sheet assembled with each sheet on an opposed diagonal to that of its neighbor, which provides for alternating condensing and boiling passages. These passages because of their configuration have increased primary heat transfer surfaces resulting in reduced temperature differences in the exchanger. Special edge configurations control boiling and condensing fluid flow patterns to maximize the heat transfer performance. Optionally, the heat exchanger can have an enhanced boiling surface applied to the boiling channel surface.