🔍 Key Concepts

• Sensible heat carried away by dry flue gases vs latent heat of vaporization in steam/water vapor • Typical composition of boiler flue gas: which component is present in the largest percentage by volume and mass? • Effect of excess air on nitrogen and oxygen content in flue gases and resulting stack losses

💭 Think About

• Which flue gas component leaves the boiler in the greatest quantity and at high temperature, so it can carry away the most energy? • How does the phase of the substance (gas vs vapor with latent heat) affect how much heat is lost when it goes up the stack? • Among the options, which component is mostly a diluent brought in with combustion air rather than a combustion product itself, and how does that affect heat loss?

✅ Before You Answer

• Compare the typical volume percentages in flue gas (which of these is present the most?) • Consider both sensible heat (temperature) and, where applicable, latent heat (phase change) when thinking about heat loss • Ask: if I reduce excess air, which component’s stack loss would be reduced the most, and how does that relate to the choices?