Abstract: A heat exchanger including a plurality of flat sheets arranged in parallel and a plurality of profiled sheets, each of which including a number of straight segments and being arranged between two subsequent flat sheets and having a repeating profile. The profiled sheets and the flat sheets together create a plurality of parallel ducts arranged in layers. The parallel ducts are divided by the profiled sheets into ducts of a first type and ducts of a second type, the ducts of the second type neighboring the ducts of the first type. Each duct of the first and second type has a width w(d) which is a function of a distance d with d the distance from a first flat sheet.