Locomotive Boiler — Superheater Flue/Tube Size In a typical locomotive boiler, the approximate outside diameter of the large superheater flue (that houses the superheater elements) is closest to:

Introduction / Context:
Locomotive boilers contain two tube sizes: numerous small smoke tubes for convective heat transfer, and fewer large flues that contain superheater elements. Recognising typical dimensions helps visualise flow areas, gas velocities, and maintenance constraints.

Given Data / Assumptions:

• Conventional saturated-to-superheated locomotive boiler with Schmidt-type superheater elements.
• Metric approximations of traditional imperial sizes.
• Question refers to the large flue that carries superheater elements, not the small elements themselves.

Concept / Approach:
Small smoke tubes often have diameters around 40–50 mm, whereas superheater flues are much larger, on the order of about 125–140 mm to accommodate the multi-loop superheater elements and promote high-temperature gas flow. Therefore, an approximate size of 130 mm is the representative choice among the options provided.

Step-by-Step Solution:
Identify component: the “superheater tube” in many texts refers to the large flue carrying the elements.Recall typical sizes: superheater flues ~5 inches (≈127 mm).Pick the closest metric option: 130 mm.

Verification / Alternative check:
Historic locomotive data sheets list flue diameters near 5–5.5 inches, aligning with ~130–140 mm.

Why Other Options Are Wrong:
13 mm and 31 mm: too small; comparable to small tubing or the internal element legs, not the flue.230 mm: too large for typical locomotive boiler geometry.

Common Pitfalls:
Confusing superheater flue outside diameter with the superheater element tube diameter; the element tubes are smaller but reside inside the large flue.

Final Answer:
130 mm