Water vapor transfer

Kayserilioglu, B. S., Bakir, U., Yilmaz, L. Akkas, N. (2003). Use of xylan, an agricultural by-product, in wheat gluten based biodegradable films mechanical, solubility and water vapor transfer rate properties. Bioresource Technology, Vol. 87, 3, (May 2003), pp. (239-246), ISSN 0960-8524... 

K Ito, S-I Kaga, Y Takeya. Studies on hard gelatin capsules I. Water vapor transfer between capsules and powders. Chem Pharm Bull 17 1134—1137, 1969. 

Figure 29 Reciprocal of resistance (conductance) for water vapor transfer through semistopper closures dependence on pressure. (From Ref. 5.)... 

A subtle aspect of stability analysis of freeze-dried products in vials with rubber stoppers is the tendency for water vapor to be transferred from the stopper to the solid during storage. Representative data for residual moisture as a function of time at different temperatures are shown in Figure 11. As expected, the residual moisture level increases more rapidly at higher temperature, but the plateau level is independent of temperature as equilibrium is established between the freeze-dried solid and the stopper. The extent to which this is observed depends on several factors. First, the nature of the rubber stopper formulation affects the diffusivity of water in the rubber. Second, the processing of the stopper can affect the level of residual moisture present. It is not uncommon for extended drying of the stopper to be necessary to minimize residual moisture. Finally, the mass of the freeze-dried solid determines the extent to which the percent residual moisture is affected by water vapor transfer from the stopper, where large cakes may be relatively unaffected by the small amount of water vapor that is... 

Figure 11 Changes in residual moisture during storage of freeze-dried solids due to water vapor transfer from the stopper at different storage temperatures 5°C (diamonds), 25°C (squares), and 40°C (triangles).

Membrane processes termed as osmotic distillation or membrane distillation could be shown to be applications of membrane contactor technology also. Both of these processes are based on gas membranes. Osmotic distillation, sometimes called osmotic evaporation, involves transfer of water vapor across a gas-fiUed membrane, the process is driven by a difference in water vapor pressure maintained across the membrane [58-59] by separate aqueous hquids. Membrane distillation is a process where water vapor transfer is driven solely by a temperature difference across the gas-fiUed membrane [60-61]. Water evaporates from a hot aqueous phase and condenses on a cooler surface. This process may be useful in desalinating water or producing pure water if a good natural source of warm water is available, such as in a geothermal process. 

As the shaft turns at a speed of about 60 r.p.m., a film of feed water is picked up by the disk (5) and passes approximately 1/8 inch from the condenser plates. Water vapor transfers by diffusion through the air from the hot feed water film on the disk surface to the colder condenser surface. The residual water film on the disk is cooled by the evaporation and its salinity is increased. The disk surface then returns to the reservoir, where the cool concentrated film is washed off and the film is renewed with hot feed water. 

Osczevski, R.J., 1996. Water vapor transfer through a hydrophihc film at subzero temperatures. Text. Res. J. 66, 24-29. 

Rossi, R.M., Gross, R., May, H., 2004. Water vapor transfer and condensation effects in multilayer textile combinations. Text. Res. J. 74, 1-6. 

Mahmoud, R., Savello, P.A. Mechanical properties of and water vapor transferability through whey protein films. J. Dairy Sci. 75, 942-946 (1992)... 

Table 7.3 Water vapor permeability (WVP), water vapor transfer rate (WVTR), and strength properties of edible hemicellulose films. ...

Water vapor transfer properties of the textile auxiliary walls were analyzed by the evaporative dish method, using an experimental setup defined by the standard BS 7209 1990 [59]. 

Fukazawa, T., Lee, G., Matsuoka, T., Kano, K., Tochihara, Y., 2004. Heat and water vapor transfer of protective clothing system in a cold environment, measured with a newly developed sweating thermal manikin. Eur. J. Appl. Physiol. 92 (6), 645-648. 

FIGURE 13.4 In zinc/air button cell, water vapor transfer is the dominant form of gas transfer degradation. (Courtesy of Duracell, Inc.)... 

Effect of Water Vapor Transfer on Service Life. The decay mechanism determining the useful service life is water vapor transfer. It occurs when a partial pressure difference exists between the vapor pressure of the electrolyte and the surrounding environment. As indicated previously, a cell with a typical electrolyte, consisting of 30% concentration of potassium hydroxide, will lose water when the humidity at room temperature is below 60% and will gain water at humidities above 60%. Excessive water loss increases the concentration of the electrolyte and can eventually cause the ceU to fail because of inadequate electrolyte to... 

The water vapor transmission test is used to determine values of water vapor transfer through permeable materials. Two basic methods, the desiccant method and the water method, have been developed. The desiccant method generally yields lower results than the water method. 

Meanwhile, measurements of water vapor permeability of woven fabrics have indicated that in the lower ranges of fabric density, the main path of water vapor transfer is through the air spaces between fibers and yams. This covers the densities characteristic of most apparel fabrics made from staple fibers, although filament yam fabrics may be woven to higher densities in which the kind of fiber itself in the passage of water vapor, it is necessary to account for the water vapor passage through air spaces. 

Ea, J. Y. Water Vapor Transfer in Breathable Fabrics for Clothing, PhD thesis. University of Leeds, 1988. 

Spencer-Smith, J. L. The physical Basis of Clothing Comfort, Part 3 Water Vapor Transfer Through Dry Clothing Assembhes, Clothing Res. J. 1977,5, 82-100. Spilman, L. Goren, S. Model for Predicting Pressure Drop and Filtration Efficiency in Fibrous Media, Environ. Sci. Techno. 1968, 2, TV -2%1. 

Prasad, K., W. H. Twilley, and J. R. Lawson, Thermal Performance of Fire Fighters Protective Clothing Numerical Study of Transient Pleat and Water Vapor Transfer. 2002, US Department of Commerce, Technology Administration, National Institute of Standards and Technology. 

For a person wearing clothing, the heat flows and d . depend on the heat transfer resistance and on the water vapor transfer resistance R f. Both parameters can be measured and set according to the requirements. Their combined value should equal the heat production rate Q, . 

Temperature affects the rate of water-vapor transfer into a resinous structure and the hydrolytic degradation. In nature, rh usually decreases as the temperature rises during the day and increases again as the temperature falls. However, under some storage conditions, water may be trapped and the rh remains at almost 100% throughout temperature variations. 

Table 2.1 contains numerical values of relevant observed skin friction coefficients, Cf. The size of the additional resistance for real and model surfaces has been reviewed by Montieth and Unsworth (1990) and is summarized as follows. For water vapor transfer to rigid rough surfaces the following expression applies ... 

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