🔍 Key Concepts

• Boiler refractory is usually arranged in layers, from the hot furnace side to the cool steel shell side. • An insulating block is placed on the cooler side to reduce heat loss through the boiler casing. • In sectional drawings, the insulating layer is often shown between the working (hot‑face) firebrick and the outer steel casing.

💭 Think About

• Look at which numbered material is farthest away from the furnace opening and closest to the boiler shell or casing. • Compare the shading/texture: which layer looks like a backing insulation rather than a hard, wear‑resistant firebrick? • Ask yourself: which numbered item is drawn as a broad, continuous layer supporting other refractory pieces rather than a small shaped brick or tile around the burner opening?

✅ Before You Answer

• Identify the furnace hot face (the surface exposed directly to flame) and trace outward toward the boiler shell. • Confirm which numbered item lies between the hot‑face refractory and the outer wall/casing, not at the flame surface or floor. • Make sure the option you choose represents a large background layer of material, not a localized piece like a burner tile or floor brick.