🔍 Key Concepts

• Heat transfer path in a water-tube boiler from hot gas to water • Relative insulating effect of gas films, metal walls, and deposits/fouling (like soot or scale) • How the overall heat-transfer coefficient is limited by the layer with the poorest conductivity

💭 Think About

• Trace the heat path step-by-step: hot gas → any deposits → gas film → tube metal → water/steam film → water. At which step is heat flow most "choked" under normal operating conditions? • Compare which is a better conductor of heat: clean steel, moving water/steam, or still gas. Then consider what happens when an extra insulating layer builds up. • Ask yourself: in practice, which part do engineers worry about cleaning on the fireside to keep efficiency high, and why?

✅ Before You Answer

• Be clear on the meaning of fireside (gas side) vs waterside (inside the tube). • Identify which option represents a stationary or nearly stationary layer with poor heat conduction, versus a well-mixed moving fluid or metal. • Consider which resistance is present even when the tube is clean, and which appears only when maintenance is neglected.