Boiler Classification by Circulation Mechanism When circulation of water within a boiler occurs due to convection currents naturally set up by heating (without the use of a pump), what is the boiler called?

Introduction / Context:
Boilers are often classified by how water and steam circulate through their heating surfaces. Circulation strongly affects heat transfer, tube sizing, allowable heat flux, and the ability to operate at higher pressures. Understanding the difference between natural and forced circulation is foundational for boiler selection and design.

Given Data / Assumptions:

• No circulation pump is installed.
• Heating establishes density gradients between hot risers (steam–water mixture) and cooler downcomers.
• Geometry provides separate paths for upward and downward flow.

Concept / Approach:
Natural circulation relies on buoyancy. As water in riser tubes is heated, it forms a steam–water mixture with lower density than the relatively cooler water in downcomers. The resulting hydrostatic head difference creates a loop that circulates fluid without mechanical assistance. This mechanism is adequate for many subcritical water-tube and fire-tube boilers at modest pressures and heat fluxes.

Step-by-Step Solution:
Identify driving force: density difference between hot mixture (riser) and cooler liquid (downcomer).Recognize flow path: risers convey mixture upward to the steam drum; downcomers return denser water downward.Name the class: such units are called natural circulation boilers.Contrast: if a pump forces water through evaporator tubes and separators, it is a forced circulation boiler (e.g., La-Mont, Benson).

Verification / Alternative check:
Drum-type subcritical boilers (e.g., Babcock & Wilcox) exemplify natural circulation, while once-through designs use forced circulation or operate at supercritical pressures.

Why Other Options Are Wrong:

• Internally/externally fired: describe furnace location, not circulation method.
• Forced circulation: requires a pump; contradicts the premise of convection-driven flow.

Common Pitfalls:
Assuming natural circulation suffices at all pressures. At very high pressures, density difference diminishes, necessitating forced circulation or once-through designs.

Final Answer:
natural circulation boiler