ANS:A - It results in corrosion due to the condensation on the flue gas side of the economiser tubes due to handling of low temperature water.

If the feed water temperature is very low in a thermal power plant boiler due to preheaters being out for repairs or the starving of steam supply, the performance of the economiser would typically be affected as follows: It results in corrosion due to the condensation on the flue gas side of the economiser tubes due to handling of low temperature water. Here's why:

1. Corrosion: When cold feed water enters the economiser tubes, it may lead to condensation of moisture on the surface of the tubes exposed to hot flue gases. This condensation can result in localized areas of low pH due to the formation of acidic condensate, especially in the presence of sulfur and other acidic gases in the flue gases. As a result, corrosion of the economiser tubes can occur, leading to reduced tube life and potential failure.
2. Temperature Differential: Cold feed water entering the economiser lowers the overall temperature of the economiser tubes. This temperature differential between the flue gases and the economiser tubes may exacerbate the condensation of moisture on the tube surface, accelerating the corrosion process.
3. Reduced Heat Transfer Efficiency: The low temperature of the feed water can also reduce the overall heat transfer efficiency of the economiser. Cold water entering the economiser extracts less heat from the flue gases compared to warmer water, reducing the amount of heat recovered and transferred to the feed water stream.
4. Increased Risk of Scale Formation: Additionally, the low temperature of the feed water may increase the risk of scale formation on the economiser tubes. Scaling occurs when dissolved minerals in the water precipitate out and deposit on the tube surfaces, reducing heat transfer efficiency and potentially leading to tube blockages and overheating.

Overall, the handling of low-temperature feed water in the economiser can lead to corrosion, reduced heat transfer efficiency, and increased risk of scale formation, negatively impacting the performance and reliability of the economiser in the thermal power plant boiler.