Once-through steam generator with a vertical gas flue of essentially vertically disposed tubes

A once-through steam generator, comprising:

• a vertical gas flue having a given circumference in a horizontal section and being formed of adjacent tubes having tube centers and being gas-tightly welded to one another, and said gas flue being associated with fossil fuel burners,
• said given circumference being measured on lines connecting said tube centers of said adjacent tubes;
• said tubes of said gas flue being connected in parallel for a coolant flow, being essentially vertically disposed, having an internal tube diameter d, having an inner surface, and having fins on said inner surface forming a multiple thread;
• said internal tube diameter d being a function of a quotient K, wherein:
• said quotient K is defined as a summed mass through-put of all of said tubes at 100% steam output, divided by said given circumference;
• and
• when said quotient K, expressed in kg/s m, is from 3 to 7, said tube diameter d, expressed in mm, is from 1.975K+6.575 to 5.15K+6.82;
• when said quotient K is from 7 to 13, said tube diameter d is from 1.74K+8.50 to 4.15K+6.82;
• and
• when said quotient K is from 13 to 19, said tube diameter d is from 1.40K+12.4 to 4.15K+6.82.