Abstract: A modular system for heat exchange between fluids includes two end plates. At least one end plate is configured with inlets and outlets for fluids. The modular system includes a number of heat exchanger elements and a number of guiding elements. Each heat exchanger element includes a folded sheet material including a plurality of slits extending in a longitudinal direction of the folded sheet material, which longitudinal extending slits form the fluids flow paths. The folded sheet material is cast in one piece in an outer casing. A central opening of the outer casing covers an outer circumference of the folded sheet material, exposing a front side and a back side of the folded sheet material where two through holes, forming the inlets and outlets for each fluid, are provided on opposite sides of the central opening of the outer casing.