🔍 Key Concepts

• Heat transfer in boiler tubes – conduction through metal vs. insulation/scale vs. fluid films • Effect of scale or soot deposits on thermal resistance • Relationship between temperature drop and where the ‘bottleneck’ to heat flow is located

💭 Think About

• Look at how the temperature line changes slope from the fireside to waterside. Where is the biggest temperature drop over the shortest distance? • Which labeled region represents a poor heat conductor (insulating layer or stagnant film) rather than the steel tube wall or moving water? • Think about maintenance: what kind of deposit do engineers worry about most because it drastically reduces heat transfer?

✅ Before You Answer

• Identify which letters correspond to metal tube wall, fireside deposits/film, and waterside deposits/film in the sketch before choosing. • Verify in your heat-transfer notes which material (metal, gas, water, scale/insulation) has the highest thermal resistance (lowest conductivity). • Make sure the area you pick matches the steepest portion of the temperature gradient in the illustration, showing where most of the temperature drop occurs.