Water-tube boiler principle — flow paths of water and hot gases State whether the following is correct: “In water-tube boilers, water passes through the tubes while the surrounding space carries flames and hot gases.”

Introduction:
Boilers are broadly categorized into fire-tube and water-tube types. Understanding which medium (water or hot gases) flows inside the tubes is fundamental to identifying performance characteristics such as operating pressure, steam-raising rate, and response to load changes.

Given Data / Assumptions:

• Conventional water-tube designs (e.g., Babcock & Wilcox, Stirling, modern high-pressure units).
• Combustion occurs in a furnace volume external to the tubes.

Concept / Approach:
In water-tube boilers, the tubes carry water/steam mixture, and combustion gases flow around the outside of these tubes and headers. This configuration enables higher allowable pressures, faster steam generation, and better heat transfer control compared with fire-tube boilers, where hot gases pass inside the tubes and water surrounds them in the shell. Water-tube arrangements also facilitate superheaters, reheaters, and economisers in the gas path.

Step-by-Step Solution:

Verification / Alternative check:
Typical layouts show membrane walls and banks of generating tubes bathed in furnace radiation and convection gas passes, confirming the stated flow directions.

Why Other Options Are Wrong:

• No / fire-tube claim: reverses the roles and describes fire-tube boilers instead.
• Restrictions to vertical or economiser sections: inaccurate; the definition applies broadly across water-tube types and orientations.

Common Pitfalls:
Confusing “water-tube” with “water-wall” terminology; assuming small package fire-tube boilers are water-tube because water surrounds the tubes in the shell.

Final Answer: