Function of Superheater : The steam produced in the boiler is not dry but contains some moisture. Such wet steam can damage/corrode and reduce the efficiency of the plant. Hence before supplying wet steam to turbine, it is passed through super-heater, where it is dried and superheated by the flue gases. Super-heater increases the overall efficiency of the plant.
Function of Economizer: It extracts the heat from flue gases and utilizes this heat to raise the temperature of feed water to the boiler, so that efficiency of overall system is increased.