Boiler Construction, Design, and Types
Where the water is decides everything
The most useful distinction on this exam is whether the water or the fire is inside the tubes. In a fire-tube boiler, hot gases pass through tubes surrounded by water; in a water-tube boiler, water passes through tubes surrounded by hot gases. Every consequence follows: fire-tube units hold a large water volume, steam slowly, and are pressure-limited (typically up to about 250 psig), while water-tube units are built for high pressure, fast response, and large capacity.
Know the family tree. Fire-tube: horizontal return tubular (HRT), Scotch marine, firebox, vertical tubular. Water-tube: D-type, A-type, O-type, and once-through designs. Plus cast-iron sectional boilers for low-pressure heating and waste-heat or electric boilers. Be able to name internals: shell, heads, tube sheets, furnace, through-stays, crown sheet, steam drum, mud drum, superheater, economizer, air preheater, and baffles.
Definitions and traps
Heating surface is the waterside area exposed to heat. One boiler horsepower equals 33,475 Btu/hr, or about 34.5 lb of steam per hour from and at 212°F. ASME Section I covers power and high-pressure boilers; Section IV covers heating boilers. Traps: confusing a superheater (raises steam temperature, no pressure gain) with an economizer (recovers flue-gas heat into feedwater); assuming bigger furnace volume raises pressure rather than improving combustion; forgetting that an HRT's crown sheet must stay covered - low water there is catastrophic, not merely inefficient.
Worked example
A 200 BHP boiler: 200 × 33,475 = 6,695,000 Btu/hr output, and about 200 × 34.5 = 6,900 lb steam/hr. At 80% efficiency, fuel input ≈ 6,695,000 ÷ 0.80 ≈ 8.37 MMBtu/hr.