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A closed tank is filled with water completely and heated from outside. What will occur boiling or evaporation or both?

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If you fill a tank completely with liquid water (no gas bubble) and start heating it, the liquid water will try to expand faster than the vessel since the coefficient of thermal expansion of water is larger than the corresponding value for typical vessel materials (e.g. steel). Thus the pressure will increase sharply so that no evaporation can occur.

Because of the strong pressure increase, the vessel will probably fail before reaching 100 °C. When the vessel fails, the pressure will quickly drop to ambient pressure. If the temperature is still below 100 °C, no boiling can occur; there will be just slow evaporation from the surface of the warm water.

If the temperature is already above 100 °C when the vessel fails, some flash evaporation will occur when the pressure quickly drops to ambient pressure. A part of the hot liquid water flashes to steam (which can be calculated using the enthalpy balance). The temperature drops to the boiling point (100 °C) at the new pressure. After that, there will be slow evaporation from the surface of the warm water.

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