Furnace tube arrangement for steam generator

A steam generator comprising a combustion chamber having a polygonal cross-section defined by a plurality of connected combustion chamber walls at least one of which has a furnace tube arrangement comprising:

• a plurality of furnace tubes adapted for passage of an evaporatable flow medium disposed generally vertically to form a generally planar structure;
• at least one burner throat let into the planar structure at at least two vertically spaced levels, each burner throat defined by a throat wall into and around which tubes in the vicinity of the burner throat pass in order to leave the burner throat open;
• wherein the burner throats at the respective levels are so disposed that a vertical mid-line of a throat at a first level is laterally offset from a vertical mid line of a corresponding throat at a second level,
• wherein each burner throat is configured such that some tubes pass into and around a perimeter wall of the throat in a portion of the throat essentially fully exposed to thermal radiation in the throat in use, and some tubes pass into and around a perimeter wall of the throat in a portion of the throat subject to reduced exposure to thermal radiation in the throat in use;
• such as to define three groups of furnace tubes respectively:
• a) tubes disposed entirely along the planar wall;
• b) tubes that pass out of the plane of the structure and into and around the throat wall in the exposed portion;
• c) tubes that pass out of the plane of the structure and into and around the throat wall in the shielded portion;
• wherein the furnace tubes are so arranged as between a first burner throat at a first level and a corresponding second burner throat at a second level that at least some of the furnace tubes disposed such as to constitute tubes in group b) at said first level are disposed such as to constitute tubes not in group b) at said second level.