In furnace technology, what should the ideal flue-gas outlet temperature at the chimney exit be (to secure adequate natural draft and avoid water-vapor condensation of acidic species)? State the approximate value in °C.

Introduction / Context:
For fired heaters, kilns, and industrial furnaces, the flue-gas temperature at the chimney (stack) exit is a key operating target. Too low, and water vapor condenses with acidic species (such as SO3/H2SO4), causing corrosion; too high, and unnecessary sensible heat is lost up the stack. This question tests practical furnace operation knowledge by asking for the typical “ideal” range and the most cited nominal value.

Given Data / Assumptions:

• Continuous operation of a furnace venting through a chimney.
• Ambient air is cooler than the flue gas, and natural/induced draft must be sustained.
• Corrosion risk increases if the gas cools below the acid dew point.

Concept / Approach:
The acid dew point for common fuel gases typically falls around 120 to 140°C (varies with sulfur content and moisture). To stay safely above this and still limit stack losses, standard practice is to aim near 150°C at the chimney exit. This balances draft reliability, corrosion avoidance, and energy efficiency. Much hotter gas (e.g., 250°C) wastes heat; too cool (50–100°C) invites condensation/corrosion and draft instability.

Step-by-Step Solution:
Identify acid dew point range for typical fuels: about 120–140°C.Ensure stack exit temperature stays above dew point with margin.Select a practical target widely used in operations: about 150°C.

Verification / Alternative check:
Energy audits and furnace best-practice guides recommend stack temperatures moderately above the acid dew point. Operators commonly track stack temperature trends to prevent corrosion and maintain draft, and 150°C is a frequent benchmark value in training problems and audits.

Why Other Options Are Wrong:
50°C and 100°C: Likely below the acid dew point for sulfur-bearing fuels; risk of condensation and severe corrosion.250°C: Avoids condensation but wastes valuable sensible heat and lowers overall efficiency.

Common Pitfalls:

• Assuming “colder is always better” for efficiency; not true if corrosion/draft issues arise.
• Ignoring fuel sulfur content, which shifts the acid dew point and required safety margin.

Final Answer:
150