🔍 Key Concepts

• Flue gas air heater purpose – how does preheating combustion air affect overall boiler efficiency and gas temperatures? • Stack / uptake temperature – what typically happens to the temperature of gases leaving the boiler when heat is recovered into incoming air? • Low-temperature corrosion – how are corrosion rates in uptakes and economizers related to gas temperature and acid dew point?

💭 Think About

• If an air heater is added, is more or less heat extracted from the flue gas before it goes up the uptake? How would that change the gas temperature there compared to a boiler without an air heater? • Consider where cold spots and condensation are most likely to form in a system with and without an air heater. Would routing flue gas through more heat‑exchange surfaces generally make condensation and acid attack better or worse in uptakes/economizers? • Think about boiler thermal efficiency: when you improve efficiency by adding heat‑recovery equipment (like an air heater), what usually happens to stack losses and stack temperature?

✅ Before You Answer

• Be clear on whether recovering heat from flue gas into combustion air raises or lowers stack/uptake gas temperature. • Relate corrosion rate to whether surfaces are above or below the acid dew point of the flue gases. • Check if statement I and statement II can both be true at the same time, based on how an air heater actually changes the energy balance of the boiler.