Boiler Construction, Types and Classification
The two great families
Fire-tube boilers pass hot gases through tubes surrounded by water (e.g. Lancashire, locomotive, packaged shell boilers). They hold a large water volume, tolerate feedwater variations, but are limited in pressure and are dangerous on failure because of stored energy. Water-tube boilers pass water through tubes surrounded by hot gas; they suit high pressure and large capacity, respond quickly, but demand strict water treatment.
Shell and water-wall designs
A shell boiler is a pressure vessel with a cylindrical shell and flat or dished ends, with furnaces and tubes inside. Water-wall construction lines the furnace with tubes that absorb radiant heat, protecting refractory and increasing evaporation; it is typical of larger water-tube units.
Mountings and accessories - the examiner's favourite distinction
Mountings are fittings necessary for safety and attached directly to the boiler: safety valves, main stop valve, feed check valve, water-level gauges, pressure gauge, fusible plug, blowdown valve, manhole and inspection doors. Accessories improve economy or operation but are not essential to safety: economiser, air preheater, superheater, feed pump, injector, steam trap.
Traps
Questions swap mountings and accessories, or ask which fitting is not a mounting. Remember the fusible plug is a mounting and a last-resort protection, not a normal control. Also distinguish boiler proper from superheater and economiser surfaces.
Worked example
Classify an economiser: it recovers heat from flue gas to warm feedwater, improving efficiency, but the boiler can run safely without it - so it is an accessory. Classify the feed check valve: it prevents backflow and protects the boiler - a mounting.