A steam cooled chemical reactor (1) comprising a vertical vessel (2), a plate heat exchanger embedded in a catalytic bed, to cool the catalytic bed by evaporation of a cooling water flow, wherein a water inlet and a steam outlet are located underneath the heat exchanger, and the plates and related piping are arranged so that the path of the cooling flow comprises a first ascending path from bottom to top of the catalytic bed, and a second descending path from top to the bottom of catalytic bed, and wherein internal evaporation channels of the plates provide the second descending or the first ascending path, and water upcomers or respectively steam downcomers provide the other of said first and second path.