What does the term 'thermal expansion' refer to in boiler operations?

The term 'thermal expansion' in boiler operations specifically refers to the increase in volume of water as it heats and expands. As water is heated in a boiler, its temperature rises, leading to an increase in kinetic energy among the water molecules. This increased kinetic energy causes the molecules to move apart, resulting in a volumetric expansion. It is crucial for operators to understand this phenomenon because failing to account for the expansion of water can lead to pressure fluctuations and potential safety hazards within the boiler system.

In contrast, the other choices do not accurately represent thermal expansion. The decrease in volume of steam would contradict the principle of thermal expansion, as steam expands when heated. The reduction of pressure when water cools does not align with the concept of thermal expansion, which specifically deals with heating. Lastly, the melting of ice inside the boiler is unrelated, as it involves a phase change that occurs at a temperature below the boiling point of water, rather than expansion due to heating.