Vaporization

Evaporating the phase transition of a liquid or liquid mixture in the gaseous state.

For the evaporation of a liquid, the enthalpy of vaporization has to be applied. The surroundings and the fluid is withdrawn from heat. No heat is supplied from outside the system, a closed system has reached a certain state of equilibrium, described by the temperature and physical state of the phase fractions of the materials.

Evaporation forms

A distinction is made between two forms upon evaporation

• Boil
• Evaporation

A liquid boils, saturated steam, if the temperature-dependent pressure of the liquid is greater than the pressure of the superposed gas phase.

Upon evaporation a liquid with another gas is superimposed, which is overheated (the state of the superposed gas is in the Ts diagram above the critical point ). The superimposed gas can absorb depends on its temperature a given volume or mole fraction of the liquid. Above a liquid is in equilibrium, the partial pressure of the evaporating liquid, a, corresponding to the saturated state. The ratio of the partial pressure of the vaporized phase to the partial pressure at saturation in the superimposed gas is called the relative humidity φ.

The liquid evaporates, so long as the saturation vapor pressure in the gas phase is not achieved.

Example

On Earth, water evaporates at an external pressure of 1.013 bar up to a temperature of 100 ° C. The evaporation rate is determined by the temperature of the liquid and the degree of saturation of the superposed air with water vapor. With strong air movement (wind) the air over a water container at a high rate is exchanged for unsaturated air; the evaporation rate of water is greater. Furthermore, the evaporation rate is increased when the temperature rises, as at higher temperatures, the air can hold more water ( the saturation partial pressure is higher).

At a temperature of 100 ° C and the external air pressure of 1.013 bar water boils. As long as it is an open system with constant pressure, the water evaporates. The mass flow rate of the boiling liquid is determined by the supply of heat to the liquid.