Lancashire boiler — proportion of internal flue (furnace) tube diameter to shell diameter The internal furnace (flue) tubes in a Lancashire boiler have a diameter approximately equal to what fraction of the shell diameter?

Introduction / Context:
Geometric proportions in classic boilers are standardized to balance heating surface, gas velocity, and water space. The diameter of the twin furnace tubes in a Lancashire shell is a well-known rule-of-thumb ratio relative to the shell diameter.

Given Data / Assumptions:

• Two large internal furnace tubes carry hot gases through the water-filled shell.
• Adequate water space and circulation are required around and above the flues.

Concept / Approach:
Empirical design practice places each furnace tube diameter at roughly one-third of the shell diameter. This provides sufficient gas cross-section without starving surrounding water space or weakening the shell. It also leaves room for stays and bridges between the flues and shell crown.

Step-by-Step Solution:

Verification / Alternative check:
Design sketches and textbook examples consistently adopt the one-third rule for Lancashire furnaces, sometimes adjusted slightly to suit draught and capacity.

Why Other Options Are Wrong:

• One-fourth: flues become too small, raising gas velocity and pressure drop excessively.
• Two-fifth or one-half: flues consume excessive shell space, reducing water volume and strength.
• Three-fourths: physically infeasible within a two-flue shell.

Common Pitfalls:
Forgetting that the ratio is per flue, not combined; ignoring the need for adequate water space above the furnaces to prevent overheating.

Final Answer: