🔍 Key Concepts

• Function of downcomers in a water-tube boiler (do they carry hotter mixture up or cooler water down?) • How natural circulation works: density difference between heated risers and cooler downcomers • Visual clues in the illustration: where the furnace/radiant zone is versus where the large downcomer tubes are placed

💭 Think About

• Look at where the waterwall tubes facing the fire are, then compare that to the location of the downcomers. Are the downcomers inside the hot radiant furnace space or outside the main furnace envelope? • Ask yourself: for circulation to work properly, should downcomers stay as cool (and dense) as practical, or should they be strongly heated like riser tubes? What would happen if they were directly exposed to furnace radiation? • Consider whether downcomers need refractory support, or if they are usually free-standing tubes connected between drums/headers outside the furnace walls.

✅ Before You Answer

• Identify in the drawing which tubes are labeled as downcomers and trace whether they pass through the furnace or remain outside it. • Confirm whether the downcomers appear to be shielded from direct radiant heat by casing/walls or located right next to the burner flame zone. • Verify that none of the downcomers are shown as embedded in refractory or directly adjacent to the superheater bundles.