Once-through steam generator

A once-through steam generator, comprising:

• a duct having an inlet end in communication with a source of a hot gas;
• a tube bundle installed in the duct and comprising multiple heat transfer tubes that each define a single path from a top end to a bottom end, the tube bundle being characterized as having an economizer section, an evaporator section, and a superheater section, with a feedwater stream being received at the top end in the economizer section and superheated steam being discharged at the bottom end from the superheater section;
• and
• a start-up module comprised of a set of heat transfer tubes positioned in the duct near the inlet end and in fluid communication with the feedwater delivery piping, wherein, as part of a dry start-up, a first stream of cold feedwater is delivered into the start-up module to initially generate superheated steam which is delivered to and enters into the economizer section as an inlet stream to begin a controlled cool-down, with the inlet stream transitioning from such superheated steam to hot water as a rate of the first stream of cold feedwater delivered to the start-up module is increased, with such hot water of the inlet stream then being mixed into a second stream of cold feedwater before it enters into the economizer section, thus continuing the controlled cool-down and minimizing thermal fatigue stresses near the top end of the once-through steam generator.