the water enters the boiler through a section in the convection pass called the economizer. from the economizer it passes to the steam drum and from there it goes through downcomers to inlet headers at the bottom of the water walls. from these headers the water rises through the water walls of the furnace where some of it is turned into steam and the mixture of water and steam then re-enters the steam drum. this process may be driven purely by natural circulation (because the water is the downcomers is denser than the water/steam mixture in the water walls) or assisted by pumps. in the steam drum, the water is returned to the downcomers and the steam is passed through a series of steam separators and dryers that remove water droplets from the steam. the dry steam then flows into the superheater coils.