Boilers and Boiler Systems
What this domain tests
Boilers and Boiler Systems covers the construction, classification, and operating principles of fire-tube and water-tube boilers, the steam-water circuit, drum internals, superheaters, economisers, air heaters, and the safety accessories that protect the pressure envelope. Questions mix descriptive recall with operating-scenario reasoning.
Core rules to have at your fingertips
- Classification: fire-tube boilers (e.g. Lancashire, Scotch marine) pass hot gas through tubes surrounded by water - limited to lower pressures (typically below about 2 500 kPa). Water-tube boilers (e.g. Stirling, Babcock & Wilcox) pass water through tubes surrounded by hot gas - suitable for high pressure and large capacity.
- Circulation: natural circulation relies on the density difference between the water/steam mixture in the risers and the cooler water in the downcomers. Forced circulation uses a pump.
- Drum internals: steam drum contains baffles, cyclone separators, and scrubbers to reduce carryover; the water level must be maintained within the visible gauge glass range.
- Superheater: raises steam temperature above saturation; attemperators (spray or surface) control final steam temperature.
- Economiser: recovers heat from flue gas to preheat feedwater - reduces fuel consumption.
- Air heater: recovers heat from flue gas to preheat combustion air - improves combustion and efficiency.
- Safety accessories: safety valves (set at or below design pressure), low-water fuel cutoff, high-pressure limit, and gauge glass are mandatory.
Worked example
A water-tube boiler operates at 4 000 kPa with a steam drum at 250 °C saturation. Feedwater enters at 105 °C, passes the economiser and leaves at 180 °C. Flue gas enters the economiser at 340 °C and leaves at 200 °C.
Economiser heat recovered per kg feedwater = 4.187 × (180 − 105) = 314 kJ/kg. If the boiler evaporates 8 kg/s, heat recovered = 8 × 314 = 2 512 kW. This heat is effectively recovered from flue gas, so fuel input is reduced by 2 512 kW / LHV of fuel - a direct efficiency gain.
Traps that cost marks
- Confusing fire-tube and water-tube limitations - fire-tube boilers are not high-pressure units.
- Assuming economiser water can boil; it must remain below saturation to avoid steaming in the economiser.
- Forgetting that safety valve setting must not exceed the boiler's maximum allowable working pressure (MAWP).
- Confusing superheater attemperation with desuperheating in process steam lines.
- Mixing up gauge glass water level with drum level - the gauge glass shows the actual level but is not the control signal.
Exam strategy
For scenario questions, identify the component, its function, and the consequence of its failure. State the pressure limits and temperature relationships explicitly.