Boilers and Combustion Systems
Core concepts
Understand firetube vs. watertube construction, their pressure and capacity ranges, and typical applications. Know the major boiler types (HRT, locomotive, packaged, Stirling, waste-heat) and their characteristic features. Combustion fundamentals: the three T's (temperature, turbulence, time), stoichiometric air, excess air, and the products of complete vs. incomplete combustion. Be able to interpret flue-gas analysis (CO₂, O₂, CO) and relate it to excess air and efficiency. Know draft systems (natural, forced, induced, balanced) and the role of the stack.
Traps to watch
- Confusing firetube and watertube advantages (firetube: high storage, low pressure; watertube: high pressure, fast response).
- Assuming more excess air always improves efficiency - beyond optimum, stack losses rise.
- Mixing up primary and secondary air functions.
- Forgetting that CO in flue gas signals incomplete combustion and wasted fuel.
- Confusing boiler horsepower with kW (1 BHP ≈ 9.81 kW ≈ 33,475 Btu/h).
Worked example / rule of thumb
Theoretical air for methane: CH₄ + 2O₂ → CO₂ + 2H₂O, so 2 mol O₂ per mol CH₄. With air at 21% O₂, that is ~9.52 mol air per mol CH₄, or about 17 kg air per kg fuel. If measured O₂ in flue is 3%, excess air is roughly 15–17%. Rule of thumb: each 1% O₂ in flue gas ≈ 5% excess air for natural gas.