What is a primary advantage of a once-through boiler?

A once-through boiler is designed to pass water through the heating surfaces only once, without the need for a steam drum or separation equipment. This design provides a key advantage: the ability to operate at high pressures. Since the water is continuously being heated and converted to steam without being recirculated, it can efficiently achieve higher operating pressures compared to traditional boilers that involve multiple drum systems.

Operating at any desired pressure is beneficial as it allows for flexibility in adjusting to different operational needs and in optimizing the performance of thermal systems. This capability is especially useful in applications requiring high-pressure steam for processes or power generation.

The other options highlight characteristics that are not advantages of once-through boilers, such as the need for drums and extensive blowdown operations, or slower startup times, which do not align with the efficient and streamlined operation of once-through systems.