values of f, and f were determined by Gamson, Thodos, and Haugen (1943) for drying by forcing air through beds of moist spherical or cylindrical pellets. In the transfer of vapor from the surface of the solid to the moving air stream, the vapor must first pass through a laminar layer of moist air and then usually through an adjacent turbulent zone. The change in partial pressure of vapor with distance from the surface is shown schematically in Fig.