Firing arrangement in water-tube boilers: Water-tube boilers (with water/steam inside tubes and hot gases outside) are generally classified as which type with respect to firing location?

Introduction / Context:
Boiler classifications include not only the path of water and gas but also the location of the furnace relative to the pressure parts. This affects maintenance, access, and allowable firing rates.

Given Data / Assumptions:

• Water-tube: water/steam inside tubes; hot gases flow outside over the tube banks.
• Furnace chamber is typically refractory-lined and separate from the pressure shell.
• Examples: Babcock & Wilcox, Stirling, modern utility boilers.

Concept / Approach:
Water-tube boilers place the furnace outside the main pressure shell, within a separate, water-cooled or refractory-lined chamber. This is termed externally fired. In contrast, many fire-tube boilers are internally fired because the furnace is inside the shell (e.g., Cornish, Lancashire, Scotch marine).

Step-by-Step Solution:
Identify furnace position: outside the pressure shell for water-tube boilers.Apply terminology: this is “externally fired.”Eliminate other options: internally fired describes many shell fire-tube units; “both” is not standard for classic water-tube designs.Choose the correct classification.

Verification / Alternative check:
Plant layouts show large membrane-wall furnaces external to drums/tubes pressure boundary, confirming external firing.

Why Other Options Are Wrong:
Internally fired applies to shell boilers; “both/none/top-fired only” do not reflect standard classification practice.

Common Pitfalls:
Confusing gas path classification (fire-tube vs water-tube) with firing location terminology.

Final Answer:
externally fired