Question 39 MODE02 - Assistant MODU Engineer

The Correct Answer is B The greatest resistance to heat transfer from the fireside (hot flue gases) to the waterside (water/steam mixture) of a water-tube boiler takes place in the **gas film layer surrounding the tube**. Heat transfer in the boiler relies on convection from the hot gases to the tube surface, followed by conduction through the tube wall, and finally convection/boiling into the water. The primary mechanism of heat transfer through a fluid (like the flue gases) to a solid surface involves overcoming the thermal resistance of a stagnant boundary layer, or "film," directly adjacent to the surface. Gases, due to their low density and low thermal conductivity (typically $0.02 - 0.1 \mathrm{~W}/(\mathrm{m} \cdot \mathrm{K})$), exhibit significantly higher thermal resistance compared to liquids or solids. This thin, relatively motionless layer of gas provides the dominant barrier to heat flow, often accounting for 80% or more of the total thermal resistance. **Why the other options are incorrect:** * **A) soot buildup directly on the tube exterior:** Soot (fly ash) accumulation significantly increases thermal resistance and is a major operational problem. However, the intrinsic resistance of a clean tube wall system is still dominated by the stationary gas film (B). While soot often has a conductivity higher than the gas film, even a thin layer of soot, when combined with the underlying gas film, increases overall resistance. In a scenario focused on the inherent physics of heat transfer mediums, the gas film itself offers the greatest resistance among the fundamental components when the tube is clean or lightly fouled. If the question implies a heavily fouled operational scenario, the answer might shift, but based on standard heat transfer principles for clean boiler tubes, the low conductivity of the flue gas film is the chief resistance. * **C) moving water and steam inside the tube:** The heat transfer mechanism on the waterside is highly efficient due to forced convection and nucleate boiling (phase change), which rapidly removes heat from the tube wall. Water and steam mixtures have very high heat transfer coefficients compared to the flue gas film, offering minimal resistance. * **D) steel tube wall itself:** Steel is a metal with high thermal conductivity (typically $15 - 50 \mathrm{~W}/(\mathrm{m} \cdot \mathrm{K})$). The tube wall is relatively thin, and the resistance offered by conduction through the steel is negligible compared to the convective resistance of the gas film (B).