Steam generator

A process for operating a steam generator at loads from 5% to 100%, the steam generator comprising:

• a cylindrical vessel having an axis;
• an inlet for water, an outlet for a superheated steam, a fumes inlet and a fumes outlet:
• water/steam tubes passing through the cylindrical vessel from the inlet for water to the outlet for superheated steam;
• and
• headers for the superheated steam:
• wherein:
• the water/steam tubes are arranged in flat tube banks, and the flat tube banks form a sequence of flat tube banks that extend along a direction of the vessel axis, wherein the flat tube banks are crossed perpendicularly by a fumes flow that passes through the vessel from the fumes inlet to the fumes outlet;
• the water/steam tubes are arranged horizontally and contiguous within the flat tube banks and pairs of flat tube banks are connected to one another by obliquely extending tubes such that water/steam tubes in each pair of flat tube banks are exposed to fumes at different positions along the direction of the vessel axis;
• the water/steam tubes arranged in the flat tube banks are divided into two or more separate branches, wherein each branch is fed by a separate header;
• the headers for the superheated steam are grouped together in a bundle, and the bundle of headers is thermally insulated from a region external to the steam generator;
• and
• the steam generator is a once-through type steam generator with fumes flow in counter-current with respect to water flow;
• the process for operating the steam generator comprising steps of:
• feeding water into the water/steam tubes and fumes through the fumes inlet;
• flowing the fumes in counter-current with respect to water flowing inside the water/steam tubes thereby forming water/steam, the fumes and the water/steam exchanging heat through a heat exchange surface of the steam generator so that the fumes and the water/steam have respective temperature profiles along the vessel axis;
• maintaining, in the steam generator, the temperature profiles of the fumes and the water/steam in a common position along the vessel axis;
• and
• choking the heat exchange surface of the steam generator so that operation at loads lower than 30% takes place by excluding and then maintaining in a dry condition one or more branches of the water/steam tubes, up to having only one operating branch.