Locomotive boiler construction detail: Typical small-diameter fire tubes used in a locomotive boiler have approximately what outside diameter?

Introduction / Context:
Locomotive boilers are multi-tubular fire-tube boilers employing many small tubes to obtain large heating surface and rapid steam-raising capability. Knowing the typical order of tube diameters helps differentiate between fire-tube and water-tube proportions.

Given Data / Assumptions:

• Fire tubes carry hot gases through the boiler shell water space.
• Practical locomotive practice uses tube diameters of a few centimetres, not a few millimetres.
• Multiple parallel tubes are fitted to maximize heat transfer area.

Concept / Approach:
Fire-tube diameters are selected to balance gas-side friction losses, heat transfer, and soot deposition. Values on the order of tens of millimetres are common. Options of 4.75 mm, 5.47 mm, and 7.45 mm are unrealistically small for locomotive boilers; approximately 47.5 mm aligns with textbook and historic practice ranges (about 38–51 mm).

Step-by-Step Solution:
Reject sub-centimetre options as impractical for gas flow in locomotives.Compare with known range: around 40–50 mm typical.Select nearest listed value: 47.5 mm.Conclude that 47.5 mm is the correct scale for locomotive fire tubes.

Verification / Alternative check:
Historical locomotive boiler drawings and standard problems cite tube diameters roughly 40–50 mm, with large counts to achieve the required heating surface.

Why Other Options Are Wrong:
Very small diameters would create excessive gas-side pressure drop and maintenance issues; 75.0 mm is larger than typical locomotive fire-tube practice.

Common Pitfalls:
Confusing fire-tube diameters with superheater elements or economiser tubes; mixing outside and inside diameters.

Final Answer:
47.5 mm