Function of an economiser in a boiler system In a fired boiler with heat-recovery sections, the economiser is specifically intended to preheat which stream?

Introduction / Context:
Heat-recovery sections in boilers—economiser and air preheater—capture sensible heat from flue gases. Knowing which stream each device heats is fundamental to understanding boiler efficiency improvements and stack temperature targets.

Given Data / Assumptions:

• Conventional order: boiler bank → superheater (if any) → economiser → air preheater → stack.
• Economiser duty is to raise the temperature of boiler feedwater before it enters the steam drum or economiser outlet header.
• Air preheaters warm combustion air, not water.

Concept / Approach:
Transferring residual flue-gas heat to feedwater increases the average temperature of water entering the boiler, reducing required fuel for a given steam duty and lowering stack losses. This section is therefore called the economiser, reflecting fuel-economy benefits.

Step-by-Step Solution:

Verification / Alternative check:
Boiler heat-balance sheets assign economiser duty to water heating; separate APH duty to air heating, confirming roles.

Why Other Options Are Wrong:

• Combustion air: Heated in the air preheater, not the economiser.
• Pulverised coal / Furnace oil: Fuels may be preheated by separate heaters; not the economiser’s function.
• Cooling water: Not part of the boiler steam-generating path.

Common Pitfalls:
Confusing economiser and air preheater locations; mixing up feedwater heaters in power plants with lube or fuel oil heaters.

Final Answer:
Feedwater