Moisture vapor transfer

Chem. Desetip Water-based PU coating Uses Urethane for breathable coatings for nylon and polyester fabrics Features Provides high mullens and moisture vapor transfer props. Properties Tens. str. 1200 psi 100% tens. mod. 400 psi elong. 300% hydrostatic resist. (Mullens) 90-100 psi 40 2% solids Solucote 1073 [SoluoQ... 

Chem. Desaip. PU base coat in toluene/IPA Uses Base coat for nylon and polyester fabrics Features Breathable durable provides high hydrostatic resist., moisture vapor transfer, and tapeability to be used with Solucote Top 920 Properties Vise. 40,000 2000 cps tens. str. 2500 psi 100% tens. mod. 2100 psi elong. 110% hydrostatic resist. (Mullens) 100-200 psi 44 2% solids... 

Hong K, Hollies N R S and Spivak S M, Dynamic moisture vapor transfer through textiles. Part I Clothing hygrometry and the influence of fibre type . Textile Res. J., 1988, 58, 697-706. 

Moisture vapor transfer This is the action of water vapor passing in molecular form through an organic substance. Generally, the lower the moisture vapor transfer rate, the better the protection that will be provided by that organic coating. 

Hong,K. HoUies,N. R. S. Spivak, S. M. Dynamic Moisture Vapor Transfer Through Textiles, Part I Clothing Hygrometry and the Influence of Fiber Type, Textile Res. J. 1988,58(12), 697-706. 

If the rate of moisture vaporization is controlled bv the rate of heat transfer to the wet soHd, then for convection dominated heat transfer, h, at the boiling poiat of water, the characteristic time is... 

Description A tray or compartment diyer is an enclosed, insulated housing in which solids are placed upon tiers of trays in the case of particulate solids or stacked in piles or upon shelves in the case of large objects. Heat transfer may be direct from gas to sohds by circulation of large volumes of hot gas or indirect by use of heated shelves, radiator coils, or refractoiy walls inside the housing. In indirec t-heat units, excepting vacuum-shelf equipment, circulation of a small quantity of gas is usually necessary to sweep moisture vapor from the compartment and prevent gas saturation and condensation. Compartment units are employed for the heating and diying of lumber, ceramics, sheet materi s (supported on poles), painted and metal objects, and all forms of particulate solids. 

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).

Salwin and Slawson (1959) found that stability in dehydrated foods was impaired if several products were packaged together. A transfer of water could take place from items of higher moisture-vapor pressure to those of lower moisture-vapor pressure. These authors determined packaging compatibility by examining the respective sorption isotherms. They suggested a formula for calculation of the final equilibrium moisture content of each component from the iso-... 

The ability of wax to prevent the transfer of moisture vapor is of primary concern in the food packaging industry. To maintain the freshness of dry foods, moisture must be kept out of the product, but to maintain the quahty of frozen foods and baked goods the moisture must be kept in the product. This results in two criteria for barrier properties moisture vapor transmission rates (A) at elevated temperatures and high relative humidity and (B) at low temperatures and low relative humidity, for frozen foods. 

Described here is a crystallization apparatus that uses vapor transfer as the method of slowly exchanging the solvents. This technique allows small quantities of air and moisture sensitive organometallic complexes to be crystallized easily under an inert atmosphere to give X-ray diffraction quality crystals. 

A more detailed treatment using a logarithmic driving force for vapor flux and the concept of the humidity potential coefficient O while accounting for the influence of the moisture vapor flux on the transfer of heat to the surface, namely, the Ackermann correction Luikov number Lu, which is essentially the ratio of the Prandtl number Pr to the Schmidt number Sc, has also been introduced. 

Second, in vaporization, drying is carried out by convection, that is, by passing warm air over the product. The air is cooled by the product, and moisture is transferred to the air by the product and carried away. In this case the saturation vapor pressure of the moisture over the solid is less than the atmospheric pressure. 

Falling rate—During this stage, that resembles the falling-rate period observed in food dehydration processes, more internal moisture leaves the food, the internal core temperature rises to the boiling point of the liquid, and the crust layer increases in thickness. After a sufficient time and more removal of moisture, the vapor transfer at the surface decreases. 

Since there is no capillary moisture conduction to the product surface in the frozen product, no first drying stage exists. It starts with the second drying stage. The ice or sublimation front continuously moves inside the product. The required heat for sublimation is transferred to the product by conduction or radiation, or with a combination of both. Moisture vapor diffusion in the dried product follows the Knudsen molecular flow model (see Table 5-2). In the third drying stage residual liquid moisture desorbs to the inner and outer product surface from its bonded state. Since desorption occurs only after complete sublimation of ice, the heat transfer has to be reduced to avoid heating the product over the permissible limit. 

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