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The heat exchanger is a crucial component in the operation of both firetube and watertube boilers because it is where the heat transfer occurs between the combustion gases and the water. In firetube boilers, hot gases pass through tubes surrounded by water, effectively heating the water and producing steam. Conversely, in watertube boilers, water flows through tubes surrounded by hot combustion gases. This design facilitates efficient heat transfer and is essential for the generation of steam in both types of boilers.

In firetube boilers, the design is typically simpler, and the gas flows through the tubes, which are submerged in water. In contrast, watertube boilers allow for more rapid heating and better handling of pressure changes, as the water is contained in the tubes. Despite these differences, the fundamental role of the heat exchanger in transferring energy from the combustion gases to the water remains the same, making it a central element common to both types of boilers.

The other options—flue tube, burner assembly, and water chamber—each have specific roles but do not relate to the core function of heat transfer that the heat exchanger provides across different boiler designs.