Which of the tube types listed can be considered to serve as downcomers at low firing rates, and as generating tubes at high firing rates on some boilers?
• Natural circulation in water-tube boilers – difference between downcomers and risers • How firing rate affects circulation and where steam is generated in the boiler • Typical location and function of water wall / water screen / riser tubes in modern boilers
• Think about which tubes are normally exposed to the highest heat flux in the furnace and how that changes their role at different firing rates. • Consider which tubes are usually dedicated downcomers (little or no steaming) versus those that can both carry downward flow at low load and generate steam at high load. • Ask yourself: which of these tube types are placed along the furnace boundaries where heat absorption increases sharply as firing rate rises?
• Be clear on the primary function of each option (screen, wall, superheater support, riser) in a boiler. • Verify which tubes are normally unheated / lightly heated (good downcomers) versus strongly heated (good generators). • Confirm which tube type is specifically known to sometimes reverse flow direction as load increases from low to high firing rates.
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