Excess Air (from Flue Gas)
Calculate the excess air of a combustion from the oxygen in dry flue gas, EA = O₂ ÷ (20.9 − O₂) × 100%, from the measured O₂ percentage in the stack. The result, in %, shows how much air was supplied beyond stoichiometric — measured by the leftover oxygen in the exhaust gases. Some excess air (10-30%) is needed to ensure complete combustion (avoid CO and soot), but too much wastes energy heating useless air that leaves hot through the stack. Gas analyzers measure O₂ and compute the excess air to optimize combustion efficiency. Enter the O₂ percentage in the gases.
Resultado
—
Excesso de ar (pelos gases)
Na prática, nunca se queima um combustível com a quantidade exata de ar (estequiométrica) — sempre se fornece um pouco a mais, o excesso de ar, para garantir que cada molécula de combustível encontre oxigênio e queime completamente, evitando monóxido de carbono (CO, tóxico e desperdício de energia) e fuligem. A forma mais prática de medir o excesso de ar é analisar o oxigênio que sobra nos gases de exaustão: EA = O₂ ÷ (20,9 − O₂) × 100%, onde O₂ é o percentual medido nos gases secos (20,9% é o O₂ do ar). Se os gases têm 3% de O₂, o excesso de ar é ~17%. Há um equilíbrio delicado. Excesso de ar insuficiente: combustão incompleta, CO, fuligem, perda de energia e poluição. Excesso de ar demais: o ar extra entra frio e sai quente pela chaminé sem participar da queima, roubando calor e reduzindo a eficiência — além de baixar a temperatura de chama. Cada combustível e queimador tem um excesso de ar ótimo (tipicamente 10-30% para gás, mais para sólidos), que minimiza as perdas totais. Por isso analisadores de gases de combustão medem o O₂ continuamente e sistemas de controle ajustam a relação ar-combustível em tempo real (controle de trim de O₂) para manter a eficiência máxima — uma das formas mais eficazes de economia de energia em caldeiras e fornos industriais. Informe o O₂ percentual nos gases.
Related Tools
Stack Heat Loss (Siegert)
Calculate the heat loss through the exhaust gases by the Siegert formula, loss = K × (T_gas − T_air) ÷ CO₂, from the fuel factor K (~0.5 for natural gas, ~0.6 for oil), the gas and combustion air temperatures (°C) and the CO₂ percentage in the gases. The result, in %, is the largest energy loss of a boiler or furnace — the heat escaping hot through the stack. Lowering the gas temperature (with economizers and preheaters) and adjusting the excess air (which dilutes CO₂) minimizes this loss. The combustion efficiency is approximately 100% minus this loss. Enter the K factor, the temperatures and the CO₂.
Acid Dew Point of Flue Gas
Computes the temperature at which sulphuric acid starts to condense on the cold surfaces of a boiler, using the Verhoff and Banchero correlation: the reciprocal of the absolute dew point temperature is a combination of the logarithms of the partial pressures of water vapour and sulphur trioxide in the flue gas, plus the product of those two logarithms. The result is the thermal floor of the design — keeping the stack, the economiser and the air preheater above it is what prevents the acid corrosion that eats steel in a few weeks, and it is why heavy fuel oil boilers throw away up the stack heat they could otherwise recover. SO₃ is what rules here, not humidity, and the reason is the range each one spans: water vapour barely leaves the 5% to 15% band in a flue gas, which accounts for 11 °C end to end, while SO₃ varies by orders of magnitude with the sulphur in the fuel — going from 1 to 10 ppm alone raises the dew point by almost 22 °C. The Verhoff and Banchero correlation was adopted, with partial pressures in millimetres of mercury at atmospheric pressure, as it is the one most used in boiler design, in its original form with the interaction term between the two logarithms; the later Okkes correlation returns 1 to 8 °C lower for the same composition, so treat the value as a reference and not as an exact limit. Enter the water vapour content and the SO₃ content of the flue gas.
Theoretical Maximum CO₂
Calculate the theoretical maximum CO₂ percentage in dry flue gas from complete combustion of a hydrocarbon C_xH_y, CO₂max = x ÷ (x + (x + y/4) × 3.762) × 100%, from the carbon x and hydrogen y atoms. The result, in %, is the CO₂ obtained with perfect stoichiometric combustion (no excess air) — the reference value of gas analyzers. Methane has CO₂max ≈ 11.7%; coal, ~18-20%. Comparing the measured CO₂ with the theoretical maximum indicates the excess air: the lower the measured CO₂ relative to the maximum, the more excess air diluting the gases. Enter x and y.
Actual Air-Fuel Ratio
Calculate the actual air-fuel ratio, AFR_actual = AFR_stoichiometric × (1 + excess air ÷ 100), from the stoichiometric air-fuel ratio and the excess air (%). The result (kg air per kg fuel) is the amount of air actually supplied in practice, always greater than stoichiometric, because real combustion needs excess air to ensure complete burning (the mixture is never perfect). This value sizes the air supply (fans), the exhaust gas flow and influences the flame temperature and efficiency. Enter the stoichiometric air-fuel ratio and the excess air.
Stoichiometric Air-Fuel Ratio
Calculate the stoichiometric air-fuel ratio (AFR) by mass of a hydrocarbon C_xH_y, AFR = (x + y/4) × 137.93 ÷ M, from the number of carbon atoms x, hydrogen atoms y and the fuel molar mass M (g/mol). The result (kg air per kg fuel) is the exact amount of air needed for complete combustion, with no leftover air or fuel. Methane (CH₄) has AFR ≈ 17.2; gasoline ≈ 14.7. The 137.93 constant comes from the air mass per mole of O₂ (32 ÷ 0.232). It is the base parameter of combustion control and mixture in engines and burners. Enter x, y and the fuel molar mass.
Boiler Efficiency
Calculate the thermal efficiency of a boiler by the direct method, η = (m_steam × Δh) ÷ (m_fuel × LHV) × 100%, comparing the useful heat absorbed by the water/steam (steam flow × enthalpy gain) with the energy released by burning the fuel (fuel flow × lower heating value). The result, in %, shows how much fuel energy actually reached the steam; the rest is lost in flue gases, blowdown, radiation and unburnt fuel. Well-run industrial boilers reach 80–90%. Enter the steam flow, enthalpy gain, fuel flow and LHV.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.