At what pressure are natural circulation boilers typically limited?

Natural circulation boilers are typically designed to operate under lower pressure limits than forced circulation boilers due to their reliance on the natural upward movement of heated water and steam. As water is heated and converted to steam, it becomes less dense and rises through the system, allowing for effective circulation without the need for mechanical pumps.

The correct answer indicates that natural circulation boilers are typically limited to below 21,000 kPa. This pressure threshold ensures that the natural convection process remains effective. Beyond this pressure, the density differences between the liquid and vapor phases become less significant, leading to a diminished natural circulation effect. Consequently, higher pressures may require forced circulation, where pumps are utilized to ensure adequate flow and heat transfer.

The other options reflect higher pressure limits, which are generally more applicable to forced circulation boilers designed for greater efficiency and capacity in high-pressure steam applications. Thus, these options would not accurately represent the standard operational limits for natural circulation boilers.