Vapor generating technique

A method of operating a vapor generating system, at substantially constant vapor pressure over a load range, including a once-through vapor generator in which heating fluid is directed through the tubes at substantially constant flow rate, and a moisture separator external and separate from the vapor generator which comprises:

• passing, in the upper portion of the load range of the system, a vaporizable fluid in one pass through the vapor generator in indirect heat exchange relation with a heating fluid to convert the vaporizable fluid into a wet vapor, and passing the wet vapor to the moisture separator to separate the moisture from the vapor;
• passing, in the lower portion of the load range, the vaporizable fluid in one pass through the vapor generator in indirect heat exchange relation with a heating fluid to convert the vaporizable fluid into a superheated vapor, passing the superheated vapor from the vapor generator to the moisture separator, providing vaporizable liquid injection into the superheated vapor between the vapor generator and the moisture separator, and separating the moisture from the wet vapor in the moisture separator.