Steam superheater comprising: first and second compartment (301, 302), in which the first compartment is provided with a shell, a tube sheet, a rear end, tube bundle, baffle plates, and steam inlet adapted to shell, and in which the second compartment is provided with a shell, a tube sheet, a rear end, tube bundle, baffle plates, and steam outlet adapted to shell; a transition compartment that separates the first and second compartment and which is defined by a space in between tubesheets (303, 304); a channelling pipe passing through tubesheets (303, 304) and thereby through the transition chamber and which extends from the first compartment to the second compartment along the length axis of the steam superheater; a separation wall positioned between inlet chamber and outlet chamber; said transition compartment is provided with pprocess gas inlet which extends into inlet chamber of the transition compartment, the inlet chamber being confined between the wall of channelling pipe, the wall of tubesheet on the one side and into which tube bundle of the first compartment extends, and the wall of tubesheet on the opposite side and into which tube bundle of the second compartment extends; said transition compartment is provided with process gas outlet, which extends from outlet chamber of the transition compartment, the outlet chamber being confined between the wall of channelling pipe, the wall of tubesheet on the one side and into which tube bundle of the first compartment extends, and the wall of tubesheet on the opposite side into which tube bundle of the second compartment extends; and wherein the first and second compartments 301, 302 are connected in series with respect to the steam flow and in parallel with respect to the process gas flow. first and second compartment (301, 302), in which the first compartment is provided with a shell, a tube sheet, a rear end, tube bundle, baffle plates, and steam inlet adapted to shell, and in which the second compartment is provided with a shell, a tube sheet, a rear end, tube bundle, baffle plates, and steam outlet adapted to shell; a transition compartment that separates the first and second compartment and which is defined by a space in between tubesheets (303, 304); a channelling pipe passing through tubesheets (303, 304) and thereby through the transition chamber and which extends from the first compartment to the second compartment along the length axis of the steam superheater; a separation wall positioned between inlet chamber and outlet chamber; said transition compartment is provided with pprocess gas inlet which extends into inlet chamber of the transition compartment, the inlet chamber being confined between the wall of channelling pipe, the wall of tubesheet on the one side and into which tube bundle of the first compartment extends, and the wall of tubesheet on the opposite side and into which tube bundle of the second compartment extends; said transition compartment is provided with process gas outlet, which extends from outlet chamber of the transition compartment, the outlet chamber being confined between the wall of channelling pipe, the wall of tubesheet on the one side and into which tube bundle of the first compartment extends, and the wall of tubesheet on the opposite side into which tube bundle of the second compartment extends; and wherein the first and second compartments 301, 302 are connected in series with respect to the steam flow and in parallel with respect to the process gas flow.