What drives natural circulation in a boiler?

• A. Electrical power of a circulating pump.
• B. Pressure of the atmosphere.
• C. Condensation of steam in the steam drum.
• D. Density differences in the boiler water.

Answer: (D)

Natural circulation is driven by buoyancy forces created by density differences in the boiler water. When water is heated in the risers, its density drops, making it lighter and causing it to rise toward the steam drum. Cooler, denser water flows down through the downcomers to replace it, forming a continuous loop. The resulting hydrostatic pressure differences along the loop push the water around without any external pump. Atmospheric pressure isn’t the driving factor, and while condensation in the steam drum affects heat transfer and water balance, it isn’t the mechanism that drives the circulation.