Condensation surface effects

Condensers The vapor from the last effect of an evaporator is usually removed by a condenser. Surface condensers are employed when mixing of condensate with condenser coohng water is not desired. They are for the most part shell-and-tube condensers with vapor on the shell side and a multipass flow of cooling water on the... 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

The tubes in the condenser required for subcooling steal heat-transfer surface area required for condensation. In effect, the condenser shrinks. This makes it more difficult to liquefy the refrigerant vapor. The vapor is then forced to condense at a higher temperature and pressure. Of course, this raises the compressor discharge pressure. And, as we have seen in the pressure section, this increase in compressor discharge pressure invariably reduces the compressor s capacity and may also increase the horsepower needed to drive the compressor. 

The most important design variable is the terminal temperature difference. This variable has the strongest influence on the condenser surface required in the evaporators and on the heat economy of the plant. The number of stages also has an effect, but it is considerably less than the effect of the terminal temperature difference. Also, the relationships shown in Figure 3 are for a blowdown concentration of twice that of incoming sea water. However, variation in blowdown concentration has only a minor effect on the economics. 

If the evaporating and condensing surfaces were very close together, perhaps an inch or less, some diffusion transfer of water vapor would occur, without physical transport by circulating air. However, limited heat transfer by conduction from basin to cover could also occur. It is possible that the net effect would be a moderate increase in the water yield and a corresponding decrease in heat loss due to air circulation. But the practicality of close positioning of salt water surface and cover is questionable. 

A liquid does not have a fixed shape, so the surface area of a liquid can be easily changed. (The surface area of solids can also be changed by processes such as grinding. However, this requires a considerable amount of energy.) In condensed phases, molecules on the surface have a different environment from molecules in the bulk therefore, a measure of the surface area is necessary to completely define the state of the system. In Chapter 11, we will discuss surface effect in liquids by use of the surface tension, y, which is the extra energy per unit... 

Therefore, it can be concluded that the polymerization takes place at the silica surface, i.e. after adsorption of the aminosilane molecules. The surface effect can be explained by the interaction of the silane NH2 group with the substrate surface. As shown above, in water solvent the hydrolyzed aminosilane molecules are stabilized by internal hydrogen bonding of the amino group to the silane hydroxyls. When the amino group is H-bonded to a surface hydroxyl group this stabilization disappears and the silane silanols can condense to form a siloxane linkage. When the reaction is performed with hydrated silica in a dry solvent (sample 1), the hydrolysis only takes place at the silica surface and can immediately be followed by the condensation reaction. In both cases, structures of type I are formed. 

Homogeneous and Inhomogeneous Contributions. Two other contributions to UPS linewidths for molecular solids have been articulated in a study of isopropyl benzene films at low temperatures ( ). The shape and size of the isopropyl benzene molecule prohibited the explicit observation of the surface effect discussed for anthracene. Isopropyl benzene was of interest as a model molecule for polystyrene, however. The measurements were carried out on condensed molecular-solid films in the temperature range 15°K < T < 150°K. 

Dropwise condensation, characterized by countless droplets of varying diameters on the condensing surface instead of a continuous liquid film, is one of the most effective mechanisms of heat transfer, and extremely large heat transfer coefficients can be achieved with this mechanism (Fig. 10-35). 

Sometimes simple models derived from continuum theory promise better results, but MD or Monte-Carlo (MC) simulations are still the preferable approaches for condensed-matter investigations [3,4], These methods were developed in the early 1950s [3, 4], They include a model-inherent dynamical description in the case of MD, and large samples can be accounted for. These methods allow working scientists to treat more than one molecule and make use of so-called periodic boundary conditions which mimic images around the central cell in such a way that problems due to surface effects can be overcome. 

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