it is used in power generation plant
In a conventional boiler, water is heated in the furnace area and then rises up to the boiler drum where the steam separates and passes to the superheater stages. Any remaining water returns to the furnace tubes. It is the difference in the steam/water density that causes circulation. In a supercritical boiler (above 3,200 psi or 221 bar pressure) there is no difference in steam/water density so circulation is provided by pumps. The steam/water change occurs in the furnace tubes eliminating the need for a boiler drum. Advantages are higher efficiency and quicker start up times. Boiler drums are very thick and have to be heated and cooled relatively slowly to prevent stress damage. Forgot to mention that, in a supercritical boiler, the evaporation takes place in the tubes thus the water only flows through once - a once through boiler. In a drum type boiler, any water in the steam/water mix is recycled, thus passing through more than once.
