🔍 Key Concepts

• Excess air in a boiler furnace and how it affects flue gas temperature and gas density/velocity • Relationship between gas temperature entering the superheater and heat transfer rate to the steam • Difference between superheater inlet temperature (steam leaving the boiler bank) and superheater outlet temperature (steam going to the turbine)

💭 Think About

• When you add more excess air at constant load and fuel rate, what happens to the flame temperature and the temperature of the gases leaving the furnace? Hotter, cooler, or unchanged? • If the furnace gas temperature changes, how will that affect the temperature of steam entering the superheater versus the steam leaving the superheater? Think about where the heat is coming from. • Consider whether more, cooler gas flowing over the superheater will transfer more total heat, less heat, or the same heat to the steam at constant firing rate.

✅ Before You Answer

• Be clear on the physical meaning of superheater inlet temperature (saturated steam from boiler bank) versus superheater outlet temperature (final superheated steam to main steam line). • Check whether increasing excess air tends to raise or lower furnace/flue gas temperature at the same firing rate. • Verify how a change in flue gas temperature would affect both the rate of heat transfer and each of the listed temperatures before choosing an option.