Evaporation, thermal equilibrium vapor pressure

Thermal vaporization (PVD technology) The vaporization of a material by raising its temperature. A useful vaporization rate for PVD processing is when the equilibrium vapor pressure is above about 2mTorr. See also Evaporation Sublimation. 

An important extension of this relationship is its application to the evaporation of liquids into a given atmosphere, such as air. Consider the evaporation of water into air (Xo2 = 0.21 and Xy, = 0.79). Suppose the air is at 21 °C. If the water is in thermal equilibrium with the air, also at 21 °C, its vapor at the surface must have a vapor pressure of 0.0247 atm (from standard Steam Tables). However, if the water and the air are at the same temperature, no further heat transfer can occur. Therefore no evaporation can take place. We know this cannot be tme. As discussed, in the phenomenon of evaporative cooling , the surface of the liquid water will have to drop in temperature until a new equilibrium p(T) can satisfy the conservation laws, i.e. 

Within the bubble boiling regime, thermal induced disintegration occurs when the vapor pressure unbalances the equilibrium between surface tension, viscous forces and inertial forces. The nature of this mechanism is different from those observed onto cold surfaces, as it is triggered by combined effects induced by the liquid surface tensirm and the latent heat of evaporation, /ifg, and the analysis requires the use of dimensionless groups complementary to those in Table 8.1. The most important is the Jakob number, defined as/a = Cp(Tw — 7 sat)//tfg where Cp is the specific heat of the liquid. 

In the constant rate period, the rate of evaporation is constant. When evaporation starts, the temperature at the surface of the gel drops because of a loss of heat due to the latent heat of vaporization of the liquid. However, heat flow to the surface from the atmosphere quickly establishes thermal equilibrium where transfer of heat to the surface balances the heat loss due to the latent heat of vaporization. The temperature at the surface becomes steady and is called the wet-bulb temperature (r ,). The surface of the gel is therefore at the wet-bulb temperature during the CRP. The rate of evaporation, Ve is proportional to the difference between the vapor pressure of the liquid at the surface pw and the ambient vapor pressure Pa . 

However in many heat and mass transfer processes in fluids, condensing or boiling at a solid surface play a decisive role. In thermal power plants water at high pressure is vaporized in the boiler and the steam produced is expanded in a turbine, and then liquified again in a condenser. In compression or absorption plants and heat pumps, boilers and condensers are important pieces of equipment in the plant. In the separation of mixtures, the different composition of vapours in equilibrium with their liquids is used. Boiling and condensing are, therefore, characteristic for many separation processes in chemical engineering. As examples of these types of processes, the evaporation, condensation, distillation, rectification and absorption of a fluid should all be mentioned. 

Molecular N ature of Steam. The molecular structure of steam is not as well known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H20 molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been determined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equilibrium constants, Kn> for cluster formation in steam, where n is the number of molecules in a cluster. 

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