🔍 Key Concepts

• Boiler circulation path from steam drum to lower headers and back through the furnace • Function and location of waterwall tubes in the furnace enclosure • How downcomers are drawn compared to heated tubes in the gas path

💭 Think About

• Look at where arrow "E" is pointing relative to the burner and the main furnace space: is it part of the furnace wall exposed directly to radiant heat, or is it outside the gas path? • Ask yourself which kind of tube would normally form the inner lining of the furnace and protect the casing from flame impingement. • Compare the thickness and routing of the tube (or tube panel) at "E" with the external large piping that usually carries unheated water down from the drum—do they look the same or different?

✅ Before You Answer

• Confirm whether the surface at "E" is clearly forming part of the furnace wall rather than outside structural support. • Check if the tube(s) at "E" appear to be in direct contact with the flame and hot gases (a clue for waterwall duty). • Verify that downcomers in this illustration are shown as larger, more isolated external pipes, not forming a flat wall panel.