Impermeable surfaces condensation

The insulation on chilled water lines and other impermeable surfaces that are always cold must be wrapped with a vapor barrier jacket, or such cold surfaces must be insulated with a material that is impermeable to moisture. This is because moisture that migrates through the insulation to the cold surface condenses and remains there indefmitely with no possibility of vaporizing and moving back to the outside. The accumulation of moisture-in such cases may render the insulation useless, resulting in excessive energy consumption. 

FIGURE 9.18 Film condensation at a vertical impermeable surface with 8,/d > 1 and 5tg/d > 1. Distributions of saturation, temperature, and liquid phase velocity are also depicted. 

FIGURE 9.19 Prediction and experimental results of Chung et al. [120] for condensation (in a packed bed of spheres) at a bounding vertical impermeable surface. The large Bo results of Cheng [116] are also shown. 

Desert rodents lead the most water-independent life of all vertebrates. Kangaroo rats can so reduce their evaporation that they are able to maintain water balance on only metabolic water. Other species survive on only meiabolic water plus free water in air-dry seeds. Respiratory water loss is reduced by cool nasal mucosal surfaces, which condense water from warm air coming from the lungs, before it can be expired. Skin impermeability involves a physical vapor barrier in the epidermis, pins unknown physiological factors. 

Even relatively low concentrations of noncondensible gases can substantially reduce the condensation rate. The main reason for this is that as vapor condenses, noncondensible gases get carried with the vapor toward the surface of the film. Since the film is impermeable, the concentration and partial pressure of noncondensible gases build up near the film to levels much higher than those far from the film. Since tdbe total pressure is constant, the noncondensible gases suppress the partial pressure of the vapor at thd edge of the film. This reduces the saturation temperature locally at the film/vapor interface as illustrated in Fig. 11.19. In turn, the reduction in the driving temperature difference leads to a reduction in the heat transfer and condensation... 

One-component room temperature-vulcanizing, condensation-curing materials (RTV-1) based on di- or trialkoxysilyl tmninated polymers caimot be used, as the vulcanizate formed on the surface of the applied material is water vapor impermeable. Further difhision of moisture is prevented, and thus deep section cure does not take place (Scheme 2). The solution is to use mono-silanol-terminated polymers. 

FIGURE 9.17 Condensation at an impermeable vertical surface for cases with 8tld < 1,8(/d = 1, and 8fld > 1. 

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