Name the heat-recovery device in boiler practice Which device recovers part of the waste heat of flue gases to preheat the air supplied to the furnace for combustion?

Introduction:
Thermal efficiency of steam plants improves when waste heat is recovered from flue gases. Two common heat exchangers are the economiser and the air preheater. Differentiating their roles is vital for boiler operation and energy audits.

Given Data / Assumptions:

• Conventional coal or oil-fired boiler with flue gas path.
• Air supply system with forced or induced draught fans.

Concept / Approach:
An air preheater transfers heat from hot flue gases to the incoming combustion air, raising its temperature before it enters the furnace. This enhances flame stability, improves fuel-air mixing, increases furnace temperature for a given fuel input, and reduces unburnt losses. An economiser, by contrast, heats feedwater using flue gas waste heat. A superheater raises the temperature of steam beyond saturation; an injector uses steam to force feedwater into the boiler and is not a heat-recovery exchanger.

Step-by-Step Solution:

Verification / Alternative check:
Flue gas temperature profiles across air preheater sections show significant air temperature rise, leading to reduced fuel consumption and improved boiler efficiency.

Why Other Options Are Wrong:

• Superheater: acts on steam, not air.
• Economiser: acts on feedwater, not air.
• Injector/de-aerator: not flue gas heat-recovery devices for air preheating.

Common Pitfalls:
Mixing up economiser and air preheater roles; assuming both heat the same medium; overlooking fouling and corrosion control that impact their performance.

Final Answer: