Boiler construction: The Lancashire boiler (a horizontal, internally fired, fire-tube boiler) is characterized by multiple large internal flue tubes running through the shell. How many internal flue tubes does a Lancashire boiler have?

Introduction:
The Lancashire boiler is a classic shell boiler design used historically in small power and process plants. Recognizing its geometry is fundamental for distinguishing it from Cornish and other fire-tube boilers.

Given Data / Assumptions:

• Horizontal shell boiler with internal furnaces.
• Fire and flue gases pass through large-diameter tubes within a common water shell.
• Natural circulation around the tubes.

Concept / Approach:
The Cornish boiler has one large internal flue tube; the Lancashire boiler evolved the concept to two parallel furnace flues to increase heating surface and firing capacity while keeping manageable furnace dimensions. Both flues run from front to back and often split into smaller passes (Galloway tubes may be fitted as cross water-tubes for enhanced heat transfer).

Step-by-Step Solution:
1) Identify boiler type by furnace count.2) Cornish → one internal flue; Lancashire → two internal flues.3) Conclude: Lancashire boiler has two internal flue tubes.4) Extra features (Galloway tubes) do not change the count of main flues.

Verification / Alternative check:
Historical drawings and textbooks consistently depict two large parallel furnace flues in Lancashire boilers.

Why Other Options Are Wrong:
One/three/four: do not match the canonical Lancashire layout.

Two with a central superheater flue: superheaters are not part of the original configuration and do not constitute a third main furnace flue.

Common Pitfalls:
Confusing with Cornish boiler or miscounting cross water tubes (Galloway) as main flues.

Final Answer:
two