Water-jacketed exhaust manifolds as used on anchor-handling supply vessel main engines require periodic inspection of the jackets and scale removal as necessary. What would be the impact of a failure to remove scale deposits on a timely basis?
• Heat transfer between hot exhaust gas, metal manifold wall, and cooling water in the jacket • How scale deposits (like lime/rust) affect heat transfer – are they good conductors or insulators? • Relationship between reduced heat transfer and what happens to the metal temperature on the gas side
• Think of scale as a layer on the water side of the manifold: does that make it easier or harder for heat to leave the metal and go into the cooling water? • If heat can’t escape efficiently into the water jacket, what happens to the temperature of the manifold wall that is in contact with the exhaust gas? • Look carefully at the choices: which ones correctly match decrease vs increase in heat transfer with the direction of the temperature change of the inside wall?
• Decide first: scale is an insulator or conductor of heat? Commit to that before reviewing the answers. • Match your decision: if scale insulates, then overall heat transfer between exhaust gas and cooling water will decrease, not increase. • Once you’re sure about increase/decrease in heat transfer, eliminate all options that pair that with an illogical change in inside-wall temperature.
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