Dynamic Water Vapor and Heat Transport through Layered Fabrics

Abstract

Dynamic water vapor and heat transport in the transient state was investigated for fabrics made of polyester, acrylic, cotton, and wool fibers. The overall dissipation rate of water vapor depends on both the vapor transport rate and the vapor absorption by fibers, which are mutually interrelated. Water vapor transport is governed by the vapor pressure gradient that develops across a fabric layer. When a fabric is subjected to given environmental conditions, the actual water vapor transport rate greatly differs depending on the nature of the fibers, even when other parameters are nearly identical, such as density, porosity, and thickness. The actual differential vapor pressure that develops across a layer depends on the water vapor absorption characteristics of the fibers. The higher the water vapor absorption rate, the lower the differential vapor pressure for water vapor transport and thus the lower the overall water vapor transport rate. However, the water vapor transport rate (when considering the actual differential vapor pressure across a fabric layer) is nearly identical regardless of fiber type, providing the fabric structures are kept nearly the same. Water vapor transport occurs through the open air spaces confined by fibers, and the nature of fibers does not greatly affect the characteristic vapor transport rate through these spaces. The temperature of the air space between two layers of fabric rises when water vapor transport occurs. This rise is nearly proportional to the water vapor absorption rate of a fabric, which is determined by the chemical nature of the constituent fibers.