Mechanism of drying thick porous bodies during the falling rate period: I. The pseudo‐wet‐bulb temperature

The drying of two highly porous thick textiles is studied and compared. Extremes are chosen in that one package is composed of a Terylene (British form of a polyester fiber) net fabric of open structure and the other of a woolen flannel of close structure. The cloths are wound as bobbins and dried by hot air streaming in a wind tunnel, the air flowing parallel to the axis of the cylinder of material. The weight of water as drying progresses is measured by a balance, and thermocouples within the bobbin provide a temperature record. On investigation of the thermal conductivity of the dry structure, it is found that whereas the coefficient for the wool‐air mixture is constant throughout, the coefficient for the Terylene‐air mixture applies only in the depths, the apparent thermal conductivity growing larger toward the surface and with increasing air speed, as if the heat transfer through the open structure is assisted by some form of air penetration. As the thick textiles dry, the rate of evaporation falls off, since heat and water vapor have to pass through an increasing layer of dry material. While this is occurring, a constant temperature, the “pseudo‐wet‐bulb temperature,” is established throughout the wet cloth. This state of equilibrium may be expressed as an equation between the rate of heat conduction inward and that required to produce the vapor diffusion outward. From this equation the pseudo‐wet‐bulb temperature can be calculated.