🔍 Key Concepts

• Acid dew point of flue gases and what happens when metal surfaces are colder than this temperature • How sulfur compounds and water vapor in exhaust form acidic condensate • Typical damage type in economizers exposed to acidic condensate over time

💭 Think About

• Think about what physically happens when hot, sulfur‑bearing flue gas hits a tube surface that is colder than the acid dew point—does something change from vapor to liquid? • What kind of long‑term damage would repeated exposure to acidic liquid on the outside of tubes most likely cause? • Compare which option is directly related to chemical attack by an acid rather than mechanical or high‑temperature issues.

✅ Before You Answer

• Verify which choice clearly involves chemical corrosion rather than mechanical cracking or thermal stress problems. • Eliminate any options that usually occur at very high metal temperatures rather than low, condensing temperatures. • Confirm that your chosen option fits with damage caused on the gas‑side surface of economizer tubes where acid might condense.