A heat exchanger comprising: a corrugated heat transfer sheet having a first surface and a second surface on opposite sides thereof, the corrugated heat transfer sheet comprising a plurality of ridges and grooves which alternate in a width direction, and have crests and troughs spaced apart in a depth direction, wherein the width direction, the depth direction and a longitudinal direction are mutually perpendicular directions; a first flow channel for a first fluid flow in a first flow direction parallel to the longitudinal direction; a second flow channel for a second fluid flow in a second flow direction parallel to the longitudinal direction, the second flow direction being opposite to the first flow direction, and a channel dividing system which provides the first flow channel and the second flow channel on opposite sides of the corrugated heat transfer sheet such that the first fluid flow is adapted to be in contact with the first surface, and the second fluid flow is adapted to be in contact with the second surface wherein the channel dividing system comprises a first channel dividing element and a second channel dividing element spaced apart in the longitudinal direction, the heat exchanger being characterized in that the first channel dividing element and the second channel dividing element having first blocking portions adapted to block top portions of the ridges, and second blocking portions adapted to block bottom portions of the grooves, wherein dimensions of the first blocking portions and second blocking portions in the longitudinal direction are less than or equal to 10% of a dimension of the corrugated heat transfer sheet in the longitudinal direction. a corrugated heat transfer sheet having a first surface and a second surface on opposite sides thereof, the corrugated heat transfer sheet comprising a plurality of ridges and grooves which alternate in a width direction, and have crests and troughs spaced apart in a depth direction, wherein the width direction, the depth direction and a longitudinal direction are mutually perpendicular directions; a first flow channel for a first fluid flow in a first flow direction parallel to the longitudinal direction; a second flow channel for a second fluid flow in a second flow direction parallel to the longitudinal direction, the second flow direction being opposite to the first flow direction, and a channel dividing system which provides the first flow channel and the second flow channel on opposite sides of the corrugated heat transfer sheet such that the first fluid flow is adapted to be in contact with the first surface, and the second fluid flow is adapted to be in contact with the second surface wherein the channel dividing system comprises a first channel dividing element and a second channel dividing element spaced apart in the longitudinal direction, the heat exchanger being characterized in that the first channel dividing element and the second channel dividing element having first blocking portions adapted to block top portions of the ridges, and second blocking portions adapted to block bottom portions of the grooves, wherein dimensions of the first blocking portions and second blocking portions in the longitudinal direction are less than or equal to 10% of a dimension of the corrugated heat transfer sheet in the longitudinal direction.