Boiler Design, Construction, and Types
Core concepts
Boilers split into fire-tube (hot gases inside tubes surrounded by water - Scotch marine, firebox, HRT) and water-tube (water inside tubes, gases outside - D-type, A-type, O-type packaged units, once-through). Fire-tube units hold more water and respond slowly to load swings; water-tube units serve higher pressure and capacity and heat up faster.
Know the code boundaries: ASME Section I covers power boilers (steam above 15 psig, or water above 160 psig or 250 F); Section IV covers heating boilers at or below 15 psig steam and 160 psig / 250 F water. Recognizable construction details include the shell or drum, mud drum, waterwalls, superheater, economizer, air preheater, refractory and insulation, and steam drum internals (separators, scrubbers) that limit moisture carryover.
Ratings to master: boiler horsepower (1 hp = 34.5 lb steam/hr from and at 212 F = 33,475 Btu/hr), heating surface, MAWP, and design temperature.
Traps
Questions swap fire-tube and water-tube behavior, confuse the mud drum with the steam drum, or test which ASME section applies right at a boundary pressure. Read units carefully - psig versus psia changes the answer.
Worked example
A boiler generates 69,000 lb/hr. Boiler horsepower = 69,000 / 34.5 = 2,000 hp. At about 80% efficiency, fuel input is roughly 2,000 x 33,475 / 0.8 = 83.7 million Btu/hr.