Boiler Types, Construction and Design
Classify before you answer
Candidates should be able to classify boilers several ways at once: by firing (solid, liquid, gaseous), by construction (fire-tube, water-tube), by circulation (natural, forced, once-through), by pressure (low, medium, high), and by use (process, power, marine). Know the fire-tube family (horizontal return tubular, packaged Scotch, locomotive, vertical) and the water-tube family (straight-tube, bent-tube, D-type, Stirling). Understand why water-tube designs suit high pressure and rapid steam raising, while fire-tube designs store large quantities of water and respond slowly.
Construction details that get tested
Understand the pressure parts: shell, drum, tubes, tube plates, furnaces, combustion chambers, headers and staybolts. Know the purpose of stays (to support flat surfaces), the difference between direct and diagonal stays, and why tube expansion and welding are both used. Recognise the function of baffles, superheaters, economisers and air heaters, and where each sits in the gas path. Be ready to explain why a furnace is corrugated and how a Morison furnace differs from a plain one.
Traps
A frequent trap is confusing a component's location with its function. An economiser heats feedwater using flue gas; an air heater heats combustion air using flue gas; a superheater raises steam temperature. Another trap is mixing fire-tube and water-tube terminology: a fire-tube boiler has tubes through which hot gas passes, surrounded by water. Do not assume the highest-pressure design is always the most efficient.
Worked example
If asked which boiler suits rapid steam demand at high pressure, reason: rapid response needs small water content, and high pressure needs small-diameter pressure parts, so a water-tube design is indicated. Fire-tube boilers have large water content and slower response.
Rule of thumb
Match the design feature to the duty it serves, not to the boiler it happens to be fitted to.