Answer: An economizer is a counterflow heat exchanger which raises the temperature of the feedwater leaving the feed pump (or feedwater heater FWH in the regenerative Rankine cycle) to the saturation temperature corresponding to boiler pressure.
The temperature of hot flue gases exiting the system is around 370 to 540 degrees Celsius. This is a huge amount of energy loss from the system to its surroundings. We can not afford to lose such an amount of energy.
Hence, an economizer is used to transfer this heat from flue gases to the feedwater.
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