What defines the last stage of heat recovery in a boiler?

Prepare for the 3rd Class Power Engineering (3B1) Exam. Study with flashcards and multiple choice questions, each question has hints and explanations. Get ready for your exam!

The last stage of heat recovery in a boiler is characterized by an air heater with a heat exchange surface. This component is essential in enhancing the overall efficiency of the boiler system through the recovery of waste heat.

In a boiler system, after the combustion process, the flue gases still contain significant thermal energy. An air heater utilizes this residual heat by transferring it to the incoming combustion air. This process effectively preheats the air before it enters the combustion chamber, which not only improves combustion efficiency by allowing for a more complete burn of the fuel but also helps in reducing fuel consumption and emissions.

The other options, while important components of the thermal efficiency improvements within boiler systems, do not represent the final stage of heat recovery. An economizer primarily focuses on heating the feed water, often as a preceding step to broader heat recovery. A super heater enhances the temperature of the steam but does not directly engage with the waste heat recovery from flue gases. A feedwater heater, while crucial for preheating, also occurs earlier in the heat recovery sequence. Thus, the air heater is the best representation of the final step in this context.

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