How much water is typically evaporated in the radiant evaporator section of a once through boiler?

In a once-through boiler, the radiant evaporator section is designed to maximize heat transfer and efficiency by utilizing a significant portion of the heat produced to convert water into steam. Typically, this section of the boiler operates under conditions that allow for a large amount of evaporation to occur as water flows through it.

The correct choice reflects that around 85% of the water is generally evaporated in this section. This high percentage is a result of the boiler's design that ensures rapid and efficient steam generation, leveraging the intense heat in the radiant section. This design allows the once-through boiler to maintain high thermal efficiency and output, catering to systems that require quick steam generation.

In contrast, the other options represent lower percentages of evaporation that do not align with the operational characteristics of once-through boilers. These lower percentages would imply less efficient use of heat and might be indicative of different types of boilers or operational configurations that do not utilize the radiant section as effectively. Thus, the selected option accurately reflects the operational principle and efficiency expected in a once-through boiler system.