Boiler Fundamentals
Boiler Fundamentals
This domain tests the vocabulary and energy balance behind every operator decision: what a boiler is, how capacity and efficiency are stated, and which pressure parts and accessories do what. Expect definitions, rating conversions, and simple heat balances rather than deep design math.
Core rules and concepts
- A boiler is a closed pressure vessel in which water is heated to produce steam or hot water. Construction rules differ by service; power boilers generally follow ASME BPVC Section I, heating boilers Section IV.
- Capacity may be stated in lb steam/hr, Btu/hr, or boiler horsepower (BHP). One BHP is 33,475 Btu/hr, equivalent to evaporating about 34.5 lb/hr of steam from and at 212°F.
- Efficiency = useful heat output ÷ fuel heat input. From-and-at-212°F is a rating convention, not the actual operating enthalpy rise.
- Major parts include drum or shell, furnace, tubes, headers, economizer, air heater, burner, safety valve, water column, gauge glass, low-water fuel cutoff, and feedwater and blowdown systems.
Worked example
A boiler makes 20,000 lb/hr of saturated steam at 150 psig. Feedwater is 200°F. From steam tables, hg is about 1,195 Btu/lb and hf at 200°F is about 168 Btu/lb, so the enthalpy rise is about 1,027 Btu/lb. Output is about 20,000 × 1,027 = 20.54 MMBtu/hr. At 80% efficiency, fuel input is about 20.54 ÷ 0.80 = 25.7 MMBtu/hr. The common error is to use only the 970 Btu/lb latent heat; that ignores sensible heat in the feedwater and the pressure-dependent steam enthalpy.
Traps that cost marks
- Confusing gauge pressure with absolute pressure when reading steam tables.
- Treating BHP as mechanical horsepower.
- Quoting from-and-at-212°F capacity as if it were actual operating output.
- Mixing higher heating value and lower heating value in an efficiency calculation.