a substantially cylindrical jacket;

a plurality of heat transfer pipes extending longitudinally within the jacket, each of the heat transfer pipes having an inlet end and an outlet end;

an axially disposed bypass pipe having an inlet end and a cone-shaped outlet end;

a coolant inlet for introducing a coolant into the jacket;

an exhaust gas stream inlet for introducing a hot exhaust gas stream into the inlet ends of the heat transfer pipes and the bypass pipe, the exhaust gas stream passing through the heat transfer pipes in an indirect heat exchange with the coolant in the jacket, producing steam and cooling the exhaust gas stream;

a coolant outlet for removing the coolant and steam;

an exhaust gas stream outlet for removing the cooled exhaust gas stream, the cooled exhaust gas stream having an exhaust temperature; and

a control system controlling a gas passage velocity and quantity of the exhaust gas stream through the bypass pipe whereby the exhaust gas stream exhaust temperature is maintained within a predetermined temperature range, the control system including

a stopper disposed at the outlet end of the bypass pipe, the stopper having a head plate extending into the outlet end of the bypass pipe, the stopper being axially displaceable between closed and open positions, an outer surface of the stopper head plate contacting an inner surface of the bypass pipe outlet end when the stopper is in the closed position, the inner surface of the bypass pipe outlet end and the outer surface of the stopper head plate defining a gas passage having a gas passage cross-section that increases as the stopper is axially displaced from the closed position, the stopper being cooled by a cooling medium, and

a control device controlling the axial position of the stopper.