🔍 Key Concepts

• heat transfer and insulation in boiler furnace walls • the purpose of refractory material around tubes • how furnace wall design affects tube metal temperature and furnace gas temperature

💭 Think About

• Ask yourself: What does adding refractory around water wall tubes actually do to the heat flow from the flame to the steel tube? • Consider which option relates most directly to protecting the tube metal from excessive heat rather than changing tube quantity or steel quality. • Think about whether studded water wall tubes with refractory make the furnace run hotter or cooler at the tube surface itself.

✅ Before You Answer

• Verify which choice is most clearly linked to temperature control at the furnace wall, not to material cost or number of tubes. • Check your understanding of refractory: is its main function structural support, cost reduction, or thermal protection/insulation? • Confirm that the benefit should reasonably follow from adding an insulating layer around the tubes, considering basic heat transfer principles.