Cornish boiler geometry: flue tube diameter relative to shell In a single-flue Cornish boiler, state the typical ratio of flue tube diameter to shell diameter. Choose the correct proportion.

Concept
A Cornish boiler has a single large furnace/flue. Typical practice makes the flue relatively large—about 0.6 of the shell diameter—to provide adequate grate and gas flow area while maintaining water volume for heat transfer.

Elimination
1/4 or 1/3 is more characteristic of multi-flue (e.g., each flue in a Lancashire boiler). 2/5 is small for the single-flue Cornish; 3/5 is the usual textbook value.

Final Answer
Three-fifths of shell diameter