Economiser is used to heat

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Examoogle · Accepted Answer

Feed heater

Explanation: An economiser is a heat exchange device located in the exhaust flue of a thermal power plant that heats the feed water before it enters the boiler. By utilizing the heat from flue gases that would otherwise be wasted, it significantly increases the overall thermal efficiency of the steam power cycle. Working Principle: The device consists of a series of tubes placed in the path of hot flue gases escaping from the boiler. As feed water at a lower temperature flows through these tubes, it absorbs thermal energy from the high-temperature gases through convection and radiation. This pre-heating process reduces the fuel consumption required in the boiler to reach the saturation temperature, thereby improving the boiler efficiency. Key Formulas: $\eta_{boiler} = \frac{\dot{m}_{s}(h_{out} - h_{in})}{\dot{m}_{f} \cdot CV}$ — where $h_{in}$ increases due to the economiser, reducing the enthalpy required for steam generation.; $Q = U \cdot A \cdot \Delta T_{lm}$ — the heat transfer rate formula where $U$ is the overall heat transfer coefficient. Key Points: Economisers are classified as waste heat recovery units.; They help prevent thermal shock in the boiler by reducing the temperature difference between incoming water and boiler metal.; The use of an economiser typically improves boiler efficiency by 5% to 15%.; It is usually positioned between the superheater and the air preheater in the flue gas path. Advantages: Reduces fuel consumption.; Increases overall steam generation capacity.; Reduces the thermal stress on the boiler drum. Disadvantages: Increases the pressure drop in the flue gas path requiring larger induced draft fans.; Risk of corrosion if flue gas temperature drops below the acid dew point. Applications: Thermal Power Plants.; Industrial Steam Boilers.; HVAC Systems for heat recovery. Additional Info: Option B (Air) is heated by an Air Preheater, not an economiser.. Option C (Flue gases) are the source of heat, not the medium being heated.. Feed water temperature is typically raised to just below the saturation temperature of the boiler pressure. Answer: A is correct — The economiser is specifically designed to pre-heat the feed water before it enters the boiler drum using residual heat from flue gases. Exam Tip: Always remember the order of components in the flue gas path: Superheater -> Economiser -> Air Preheater. The temperature of flue gases progressively decreases as they pass through these units.

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