Feedwater heat-recovery placement — where is the economiser located? In a conventional boiler flue-gas path, the economiser is generally installed between which two components?

Introduction / Context:
An economiser recovers sensible heat from flue gases to raise feedwater temperature before it enters the boiler drum or economiser outlet header. Knowing its position in the gas path helps understand temperature profiles and heat-exchanger duty allocation.

Given Data / Assumptions:

• Typical arrangement from furnace outlet: superheater/reheater → economiser → air preheater → chimney.
• Counterflow or mixed-flow heat exchange is assumed.

Concept / Approach:
The economiser must see sufficiently hot gas to be effective, but not so hot as to cause feedwater boiling or excessive metal temperatures—hence it is placed downstream of the superheater/reheater and upstream of the air preheater. The air preheater uses the remaining gas heat to warm combustion air before discharge to the stack.

Step-by-Step Solution:

Verification / Alternative check:
Plant heat balance diagrams consistently show this order; deviations are special cases (e.g., regenerative or condensing air heaters, or split-stream arrangements).

Why Other Options Are Wrong:

• Between ID and FD fans: these are on different sides of the system; not a heat-exchanger placement.
• Air preheater and chimney: would underutilize gas heat for feedwater and overcool gases before air preheat.
• None/furnace–soot blower: not meaningful placements for the economiser unit.

Common Pitfalls:
Confusing economiser with air preheater roles; ignoring dew-point corrosion if gases are cooled too low after air preheater.

Final Answer: