In an induced-draft (ID) flue-gas system, where is the principal control damper commonly located for regulating furnace draft and gas flow?

Introduction / Context:
Induced-draft (ID) systems draw flue gases from the furnace and push them toward the stack. Dampers or inlet guide vanes are used to control flow and furnace pressure (draft). Correct damper placement improves controllability, protects the fan, and stabilizes furnace operation.

Given Data / Assumptions:

• ID fan handles hot, dust-laden gas; stable control is needed.
• Dampers can be located at inlet or outlet; placement affects fan performance and draft control.
• We seek the most common and effective arrangement.

Concept / Approach:
In practice, primary control is typically achieved with an inlet damper or inlet guide vanes before the ID fan. Inlet control provides good turndown and avoids pressure pulsations. While outlet dampers exist, relying on downstream locations (or near the chimney top) is less common for precise fan flow control and can increase losses.

Step-by-Step Solution:
Recall common design: ID fan with inlet damper/vanes.Draft setpoint is maintained by modulating the inlet device.Hence, “before the ID fan” is the best answer.

Verification / Alternative check:
Fan control literature favors inlet modulation for stable operation and energy advantage; many furnace P&IDs show a damper or guide vanes upstream of the ID fan casing.

Why Other Options Are Wrong:
After the ID fan / near the chimney / anywhere after: Possible in some layouts but not the typical principal control point specified for draft control.

Common Pitfalls:

• Assuming damper placement is arbitrary; it strongly affects controllability and fan operating point.
• Confusing stack cap dampers (coarse adjustment) with main ID control dampers.

Final Answer:
before the I.D fan