🔍 Key Concepts

• complete vs incomplete combustion and how they affect stack temperature • the effect of excess air on flame temperature and heat transfer • what happens when combustion continues beyond the furnace (secondary combustion)

💭 Think About

• Think about which condition would cause hot gases to still be releasing heat as they travel up the stack rather than in the furnace where it belongs. • Consider whether more or less excess air generally leads to higher flame temperatures and how that affects stack temperature. • Ask yourself which option would reduce heat transfer to the water/steam side so that more heat remains in the flue gas leaving through the stack.

✅ Before You Answer

• For each choice, decide if it makes the flue gas hotter at the point it enters the stack or cooler. • Check which options would cause poor heat absorption by boiler surfaces, leaving higher temperatures in the gas. • Verify which condition is most associated with afterburning/secondary combustion versus normal primary combustion in the furnace.