The heat exchanger is constructed with a single pressure vessel to contain the ducts for the hot gas flow and the heating surfaces for the secondary medium. The heat exchanger is provided with a central duct for the hot process gas and two parallel branch ducts through which sub-flows of the process gas pass. A throttle member is provided in at least one of the branch ducts in order to throttle the flow of hot gas therethrough. Additional hot gas can be bypassed from the duct section into the exhaust gas from the pressure vessel.