An economiser is installed in a boiler primarily to

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Reduce fuel consumption

Explanation: An economizer is a heat exchanger used in thermal power plants to preheat the feedwater using the waste heat from flue gases before it enters the boiler. By increasing the feedwater temperature, the amount of fuel required to convert the water into steam is significantly reduced, thereby improving the overall plant efficiency.

Working Principle: The economizer is placed in the path of the exhaust flue gases exiting the boiler. As the hot gases flow over the economizer tubes containing incoming feedwater, heat transfer occurs from the gases to the water. This raises the enthalpy of the feedwater, meaning the boiler requires less heat input to reach the evaporation stage, directly leading to fuel savings.

Key Formulas: $\eta_{overall} = \frac{Q_{out}}{Q_{in}}$ — representing the thermal efficiency which increases as $Q_{in}$ (fuel) decreases; $\Delta H = \dot{m} C_p (T_{out} - T_{in})$ — enthalpy gain of feedwater in the economizer

Key Points: Economizers recover waste heat from flue gases that would otherwise be lost to the atmosphere.; Using an economizer improves the thermal efficiency of the Rankine cycle.; It also helps in reducing thermal stress on boiler drums by minimizing the temperature difference between incoming water and boiler water.; Flue gas temperature must stay above the dew point to prevent acid condensation and corrosion.

Advantages: Reduces fuel consumption; Increases overall steam generation efficiency; Reduces thermal stress in boiler components

Disadvantages: Increases the pressure drop in flue gas path; Susceptible to external corrosion if gas temperature drops below the dew point

Applications: Coal-fired thermal power plants; Industrial steam generators; Marine boilers

Additional Info: Superheaters are designed to increase the temperature of saturated steam, not feedwater.. Option A is incorrect because steam pressure is controlled by fuel firing rate and steam demand, not by preheating feedwater.. Option C refers to the function of a superheater, which adds heat to steam, not water.

Answer: B is correct — An economizer preheats the feedwater using waste heat, which directly reduces the fuel input required for steam generation.

Exam Tip: In exams, remember: Economizer = Feedwater (Liquid), Superheater = Steam (Vapor), Air Preheater = Combustion Air (Gas).

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