Wetting in Practice

Controlling Wetting in Practice Table 21-17 summarizes typical changes in material and operating variables which improve wetting uniformity. Also listed are appropriate routes to achieve these changes... 

Thus, to encourage wetting, 7sl and 7lv should be made as small as possible. This is done in practice by adding a surfactant to the liquid phase. The surfactant adsorbs to both the liquid-solid and liquid-vapor interfaces, lowering those interfacial tensions. Nonvolatile surfactants do not affect 7sv appreciably (see, however. Section X-7). It might be thought that it would be sufficient merely to lower ytv and that a rather small variety of additives would suffice to meet all needs. Actually it is equally if not more important that the surfactant lower 7sL> and each solid will make its own demands. 

Ambient Wet-Bulb Temperature The temperature in degrees Fahrenheit to which air can be cooled, making it adiabatic to saturation by the addition of water vapor, in practical terms, the wet-bulb temperature is the temperature indicated by a thermometer, the bulb of which is kept moist by a wick and over which air is circulated. 

In practical calculations the. Mollier diagram s constant enthalpy iine can be used as the auxiliary iine for the wet bulb temperature line to a satisfactory... 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

The terms hot corrosion or dry corrosion are normally taken to apply to the reactions taking place between metals and gases at temperatures above 100 C i.e. temperatures at which the presence of liquid water is unusual. The obvious cases of wet corrosion at temperatures above 100 C, i.e. in pressurised boilers or autoclaves, are not considered here. In practice, of course, common metals and alloys used at temperatures above normal do not suffer appreciable attack in the atmosphere until the temperature is considerably above 100 C. Thus iron and low-alloy steels form only the thinnest of interference oxide films at about 200 C, copper shows the first evidence of tarnishing at about 180 C, and while aluminium forms a thin oxide film at room temperature, the rate of growth is extremely slow even near the melting point. 

In practice, the human body can exist in dry bulb temperatures well above blood temperature, providing the wet bulb is low enough to permit evaporation. The limiting factor is therefore one of wet bulb rather than dry bulb temperature, and the closer the upper limits are approached, the less heat can be rejected and so the less work can be done. 

It has been shown that the kinetics of the spreading of a liquid on a rubber is largely dependent on viscoelastic dissipation in the wetting ridge of the substrate near the triple line. This behavior may, in practice, be slightly altered by moderate swelling that modifies the solid/liquid interactions for long contact times. 

There are two processes for manufacturing cements, the dry process and the wet process. The dry process is cheaper than the wet process, but in practice... 

The above discussion is applicable to layers unpeirturbed by the presence of a vapor phase, such as in a sandwich layer tank. In practice, most separations are performed in large volume chanbers in the presence of a vapor phase. It is almost impossible to fully saturate such chanbers so that a temporal and spatial vapor equilibrium is unlikely to. exist. Two opposing phenomena can be expected to influence the rate of solvent migration. Vaporization of solvent from the wetted layer might reasonably be expected to depend on the wetted surface area of the plate and the vapor pressure of the solvent in the tank. The loss of solvent from the layer will result in a reduction of the mobile phase... 

---