PURPOSE: To obtain a system for producing water vapor with high pressure and temperature levels overcoming restrictions which exist in the current boiler designs by feeding a boiler with low quality fuel and/or fuel which is highly corrosive at high temperatures for a first phase of water heating and feeding conventional fuels to a superheating-reheating installation to raise the temperature of the water vapor for a second phase of heating.

CONSTITUTION: Natural gas, gas oil, fuel oil, coal and the like are fed to a heating unit 1 equipped with a series of burners 2 via a duct 3. The unit 1 includes a superheating unit 4, through which water vapor produced in a first phase of heating, that is, water vapor obtained by a boiler using problematic fuels, is circulated. Water vapor produced in the first stage of heating process enters an inlet 5 of the superheating unit, and water vapor heated to high- temperature level is obtained at the outlet 6 thereof. The unit 1 also includes a vapor-re-heating unit 7, and these units can be also provided with a second series of burners 9 and fed with fuels via the same supplying duct 3.