In a thermal power plant, heat from the flue gases is recovered in

Question

Examoogle · Accepted Answer

Economizer

Explanation: An economizer is a heat exchange device installed in the flue gas path of a thermal power plant that recovers waste heat to preheat the feedwater. By transferring heat from exiting flue gases to the incoming feedwater, it significantly improves the overall thermal efficiency of the steam power plant.

Working Principle: The economizer consists of a network of tubes through which the feedwater flows before entering the boiler drum. As hot flue gases leave the boiler furnace, they pass over these tubes, heating the water via convective heat transfer. This reduces the heat input required in the boiler furnace, thereby saving fuel and preventing thermal shock to the boiler shell.

Key Formulas: $Q = \dot{m}_w c_{pw} (T_{out} - T_{in})$ — Heat energy gained by feedwater in the economizer; $\eta = 1 - \frac{Q_{lost}}{Q_{in}}$ — Fundamental thermal efficiency relation influenced by heat recovery

Key Points: Economizers increase the steam generation capacity for the same amount of fuel consumption.; The placement of the economizer is between the air preheater and the boiler.; It helps in reducing the temperature of flue gases before they enter the chimney, which protects the chimney lining.; Feedwater temperature must be maintained above the dew point to prevent condensation and subsequent corrosion of tubes.

Advantages: Fuel savings (typically 5-10%); Reduced thermal stress on boiler drums; Increased steam generation capacity

Disadvantages: Increases the pressure drop in the flue gas path; Requires additional pumping power for feedwater circulation; Prone to ash erosion and soot deposition

Applications: Coal-fired power plants; Marine steam boilers; Industrial process steam systems

Additional Info: Chimney is used for natural draft and flue gas dispersion, not heat recovery.. De-superheaters are used for controlling steam temperature to prevent damage to turbines.. Condensers are used at the low-pressure end to convert exhaust steam into liquid water.. The use of an economizer allows a smaller boiler size for a given output.

Answer: C is correct — The economizer extracts waste heat from flue gases to preheat feedwater before it enters the boiler, enhancing overall cycle efficiency.

Exam Tip: Always remember the order of auxiliaries in a thermal plant: Boiler -> Economizer -> Air Preheater -> Dust Collector -> Chimney. Understanding this sequence explains the heat flow path.

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