Boiler accessories: What is the primary purpose of installing an economiser in a steam generator? Choose the option that best reflects its main function in thermal performance.

Introduction:
An economiser is a heat-recovery device located in the boiler flue gas path. By preheating boiler feedwater using exhaust gas sensible heat, it boosts overall efficiency and reduces the specific fuel consumption of the boiler plant.

Given Data / Assumptions:

• Feedwater enters economiser before the boiler drum/evaporator.
• Flue gases upstream of the air preheater still have recoverable sensible heat.
• No phase change occurs in the economiser (ideally subcooled liquid at outlet).

Concept / Approach:
Thermal efficiency of a boiler increases when stack losses are reduced. Preheating feedwater decreases the enthalpy rise required in the evaporator and superheater, thereby reducing fuel input for the same steam generation. Although economisers influence steam conditions indirectly, they do not superheat steam (that is the superheater’s job) nor do they directly raise boiler pressure (that depends on firing rate, controls, and turbine demand).

Step-by-Step Solution:
1) Identify heat source: hot flue gases leaving superheater.2) Transfer heat to cooler feedwater: increase feedwater temperature.3) Reduced heat duty in evaporator: lower fuel rate for same steam flow.4) Net effect: reduced fuel consumption and improved efficiency.

Verification / Alternative check:
Energy balance on boiler island shows stack temperature reduction and lower fuel input at constant steam production when an economiser is added.

Why Other Options Are Wrong:
Superheat the steam: handled by superheater, not economiser.

Increase steam pressure: pressure is a control variable, not a direct economiser outcome.

All of these: incorrect because only fuel reduction is the primary direct effect.

Common Pitfalls:
Allowing economiser outlet to reach saturation causing steaming; misplacing it relative to air preheater and SCR units.

Final Answer:
reduce fuel consumption