E14 Phase change - Evaporation and Condensation

Topic

The phase change between water and steam—whether consuming energy during evaporation or releasing it during condensation—involves a significant amount of energy without altering the temperature of the water or steam itself while the transition occurs. In the kitchen, evaporation takes place on the surface of a roast, a sautéed dish, or food drying in an oven, whereas condensation occurs on a cold pot lid or during the steaming process. To proceed, evaporation requires this substantial amount of latent heat—far exceeding the sensible heat needed to raise the water's temperature by a comparable margin without a phase change. This high demand for latent heat explains why the food's surface can remain near its boiling point while the interior stays cool: the steam forming on the surface acts as a thermal regulator by consuming latent heat rather than allowing the surface temperature to continue rising. Because the latent heat consumed by evaporation is so high, the food's surface remains near the boiling point for a longer period before the interior finishes heating.

The rate of evaporation depends on the vapor density gradient between the food's saturated surface and the surrounding, drier environment. A dry or ventilated environment increases this gradient and accelerates evaporation, creating a crispier crust; conversely, an unventilated oven retains the moisture released by evaporation, reduces the vapor density gradient, and yields a more tender surface texture rather than a crispy crust. If the environment surrounding the food is drier or better ventilated, the vapor density gradient increases, and consequently, evaporation occurs more rapidly.

Thus, this model—based on the concepts of evaporation and vapor density gradients introduced earlier—demonstrates via the Clausius-Clapeyron equation that the saturation vapor pressure of the water rises sharply (increasing more than proportionally) with the water's temperature. Inside a pressure cooker, where the water is subjected to high pressure, its boiling point rises above the standard boiling point at atmospheric pressure, thereby accelerating the cooking process; conversely, at high altitudes, where atmospheric pressure is lower, the water's boiling point drops below the standard level, slowing down the cooking process. Higher pressure on the water results—according to the Clausius-Clapeyron equation—in a higher boiling point and, consequently, faster cooking.

Finally, the model confirms—through the Lewis number for the water vapor-air pair, which is very close to unity—a direct analogy between heat transfer and the mass transfer of the water vapor itself. This proximity of the Lewis number to unity allows the water vapor mass transfer coefficient to be estimated directly from the convective heat transfer coefficient established by another model in the set, without requiring an independent measurement of the mass transfer. The fact that this convective coefficient for the food is higher means—based on this same analogy between heat and mass—that the mass transfer coefficient for the water vapor is consequently also higher, and that evaporation at the food's surface is therefore more rapid.

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