🔍 Key Concepts

• boiler combustion efficiency and the role of excess air • effect of stack (exhaust) gas temperature on heat loss • relationship between air–fuel ratio, flame temperature, and heat transfer

💭 Think About

• What happens to the hot gases and their heat content if you supply more air than is needed for complete combustion? • Which option describes a direct mechanism that would cause more heat to leave the boiler instead of being absorbed into the steam/water? • Among the choices, which describes something that is a consistent physical effect of excess air, rather than a condition that might or might not occur?

✅ Before You Answer

• Identify which choice clearly links excess air to increased heat loss up the stack. • Eliminate any options that confuse cause and effect (i.e., things that may happen in boilers but are not directly caused by excess air). • Check which options describe effects that always occur with excess air versus those that are situational or unrelated to air quantity.